Ionic Liquid Electrolytes: A Comparative Analysis of Electrochemical Windows for Advanced Applications

Abstract

This article provides a comprehensive comparative analysis of the electrochemical stability windows (ESWs) of ionic liquid (IL) electrolytes, a critical parameter determining their performance and safety in advanced electrochemical devices. Tailored for researchers and drug development professionals, we explore the fundamental principles governing ESW, the profound influence of cationic and anionic molecular structure, and the critical role of the solvation environment. The content details state-of-the-art computational and experimental methodologies for ESW prediction and measurement, addresses key challenges such as viscosity-conductivity trade-offs and interfacial instability, and presents strategies for electrolyte optimization. Through a comparative evaluation of IL families and hybrid systems, this resource aims to guide the selection and design of high-performance, task-specific IL electrolytes for applications ranging from energy storage to biomedical devices.

Understanding Electrochemical Stability Windows: Fundamentals and Governing Factors

Defining the Electrochemical Stability Window (ESW) and Its Critical Role in Device Performance

The Electrochemical Stability Window (ESW) is a fundamental parameter defining the operational limits of any electrochemical device. It refers to the specific voltage range within which the core components of an electrochemical system, particularly the electrolyte, remain stable and do not undergo significant decomposition [1]. In a battery system, the ESW represents the range of voltages between which the electrolyte solvents are neither significantly oxidized at the positive electrode (cathode) nor reduced at the negative electrode (anode) [1]. Operating a device beyond this window inevitably leads to detrimental effects; at high voltages, electrolyte components may oxidize, breaking down to produce gases or other byproducts, while at low voltages, reduction can lead to metal plating or the formation of solid decomposition layers [1]. Consequently, a wide ESW is paramount for developing high-voltage, high-energy-density batteries and ensuring the longevity of other electrochemical devices like supercapacitors and redox flow batteries [1] [2].

The pursuit of higher performance in energy storage systems has intensified the focus on ionic liquids (ILs) as advanced electrolytes. ILs are molten salts composed of organic cations and organic or inorganic anions with melting points typically below 100°C [2]. Those liquid at ambient temperature are known as room-temperature ionic liquids (RTILs) [2]. Their appeal lies in a unique combination of properties, including high ionic conductivity, low vapor pressure, non-flammability, good thermal stability, and an inherently wide ESW, often reported to be between 3–5 V versus Li+/Li, and sometimes extending up to 5–6 V [2]. Furthermore, the physicochemical properties of ILs, including their ESW, can be tuned by carefully selecting the cation-anion combination, making them "designer solvents" for specific electrochemical applications [3] [2].

Experimental Protocols for Determining and Comparing ESW

Accurately determining the ESW is a critical step in evaluating any new electrolyte material. The following section outlines standard experimental methodologies and the tools required to perform these measurements reliably.

Standard Methodologies for ESW Measurement

The most common technique for determining the ESW is linear sweep voltammetry (LSV) or cyclic voltammetry (CV) using a standard three-electrode electrochemical cell [2]. In this setup, the working electrode (often an inert material like platinum or glassy carbon) provides the surface where the oxidation or reduction of the electrolyte is probed. The potential of the working electrode is controlled relative to a stable reference electrode (e.g., Li/Li+ for lithium systems or Ag/AgCl for aqueous systems), while a counter electrode (e.g., platinum wire) completes the circuit.

The experimental workflow for a standard LSV measurement to determine the anodic and cathodic stability limits is illustrated below.

The protocol begins with assembling an electrochemical cell filled with the electrolyte under study. To assess the anodic stability limit, a positive-going potential sweep is applied from the open-circuit potential, and the potential at which the current begins to increase rapidly and steadily due to electrolyte oxidation is identified. Conversely, for the cathodic stability limit, a negative-going potential sweep is applied, and the onset of reduction current is noted. The ESW is calculated as the difference between these two onset potentials [1] [2]. It is critical to conduct these measurements under controlled conditions, as factors such as scan rate, temperature, and electrode material can influence the results. For battery research, long-term cycling tests at high voltages complement the LSV data by providing practical insight into the electrolyte's stability under operating conditions, with analysis of charge endpoint capacity slippage or gas evolution indicating degradation [1].

The Scientist's Toolkit: Essential Research Reagents

Table 1: Key Research Reagents for ESW and Electrolyte Studies.

Item/Reagent	Function in Experimentation
Ionic Liquids (e.g., [C₄mim][NTf₂], BmimCl)	Serves as the primary electrolyte or co-solvent, providing high ionic conductivity and a wide ESW. Their tunable nature allows for property optimization [4] [2].
Lithium Salts (e.g., LiTFSI, LiPF₆)	Provides the charge carriers (Li⁺ ions) in lithium battery electrolytes. LiTFSI is often preferred with ILs for its stability, unlike the moisture-sensitive LiPF₆ [2].
Vanadium Salts (e.g., VOSO₄, VCl₃)	Active material for redox flow battery electrolytes, enabling energy storage through vanadium's multiple oxidation states [4].
Zinc Salts (e.g., ZnSO₄)	Active material for aqueous zinc-ion battery electrolytes. ILs can be added to modify the electrolyte structure and suppress side reactions [3].
Linear Sweep/Cyclic Voltammetry	Core electrochemical technique for directly measuring the anodic and cathodic stability limits of an electrolyte on an inert working electrode [2].
COSMO-RS (Conductor-like Screening Model for Real Solvents)	A computational tool for pre-screening ILs by predicting properties like activity coefficients and sigma profiles, streamlining the selection of optimal cations and anions for a specific application [3].

Comparative Analysis of Ionic Liquid Electrolytes

The ESW of an ionic liquid is not an intrinsic property but is determined by the structure of its constituent ions. The following data provides a comparative overview of ESW performances across different IL systems and device contexts.

ESW Performance Data of Different IL Systems

Table 2: Experimental ESW Data of Ionic Liquid Electrolytes in Various Applications.

Ionic Liquid / Electrolyte System	Device Context	Reported ESW / Stability Window	Key Performance Highlights
Generic ILs (Review)	Lithium-Ion Batteries	3–5 V vs. Li⁺/Li, up to 5–6 V for some [2]	High thermal stability, non-flammability, low volatility.
[C₄mim][NTf₂] / [C₄mpyr][NTf₂] with V(acac)₃	Non-Aqueous Vanadium Redox Flow Battery	Cell potential of 2.2 V [4]	~90% coulombic efficiency over 50 cycles; wider window than aqueous systems.
BmimCl with VCl₃	Aqueous Vanadium Redox Flow Battery	Stable potential window of ~1.8 V [4]	Ionic conductivity of 0.201 S cm⁻¹; >85% capacity retention.
PyrrH⁺CH₃SO₃⁻ (Protic IL) with VOSO₄	Vanadium Redox Flow Battery	Open Circuit Potential (OCP) of 1.39 V [4]	Enabled 6 M VOSO₄ solubility; stable from -20°C to 80°C.
TMAm-HSO₄ with ZnSO₄	Aqueous Zinc-Ion Battery	Not explicitly stated; improved stability [3]	Screened via COSMO-RS; enhances salt solubility and cycling stability.
Self-Adaptive Electrolyte	Zinc/Lithium Metal Batteries	Dynamically expanded window [5]	Phase separation enriches stable solvents at electrodes during charging.

Impact of Cation and Anion Selection on ESW

The data in Table 2 demonstrates that the choice of cation and anion directly dictates the ESW and application suitability. For instance, imidazolium-based ILs (e.g., [C₄mim][NTf₂]) are widely used but can have limited cathodic stability due to the reduction of the cation. In contrast, pyrrolidinium-based cations (e.g., [C₄mpyr][NTf₂]) often exhibit a wider ESW, particularly on the cathodic side, making them more suitable for batteries with high-energy anodes [4] [2]. The anion plays an equally critical role in the anodic stability. anions like [NTf₂]⁻ (bis(trifluoromethylsulfonyl)imide) are known for their excellent oxidative stability, which contributes to the high cell potential of 2.2 V achieved in non-aqueous vanadium flow batteries [4]. The ability to mix and match ions allows researchers to engineer ILs with ESWs tailored to specific electrode materials.

The Critical Role of ESW in Device Performance

The ESW is not merely a number; it is a primary determinant of the energy density, safety, and longevity of electrochemical devices. The relationship between a wide ESW and key device performance metrics is multifaceted, as shown in the following logic diagram.

• Enabling Higher Energy Density: The maximum energy a battery can store is a product of its capacity and operating voltage. A wider ESW allows for the pairing of high-voltage cathodes (e.g., charging above 4.4 V) with high-capacity anodes without causing electrolyte oxidation, directly leading to higher energy density [1] [2]. For example, the push for fast-charging, energy-dense lithium-ion batteries for electric vehicles is a key driver for developing electrolytes with a wider ESW [5].

• Ensuring Device Stability and Longevity: When a device operates within its ESW, the electrolyte remains stable, minimizing parasitic decomposition reactions that lead to capacity fade, gas generation, and increased internal resistance. Ionic liquids, with their wide ESW and non-flammable nature, significantly enhance cycle life and safety compared to conventional organic carbonates [2]. This is crucial for the long-term performance of grid-scale storage like vanadium redox flow batteries, where IL-based electrolytes have demonstrated excellent cyclic stability over hundreds of cycles [4].

• Unlocking New Battery Chemistries: A sufficiently wide ESW is a prerequisite for next-generation battery technologies. For instance, the development of reliable lithium-metal and zinc-metal batteries has been hampered by electrolyte reduction and dendrite formation at the anode. Recent innovations, such as self-adaptive electrolytes that dynamically expand the ESW during charging by segregating stable solvents to the electrode interfaces, demonstrate how mastering the ESW can enable these promising systems [5].

The Electrochemical Stability Window is a foundational concept that governs the performance ceiling of electrochemical devices. As this comparison guide illustrates, ionic liquids, with their tunable chemistry and robust ESW, are at the forefront of electrolyte research, enabling progress toward safer, higher-energy, and more durable energy storage solutions. The continuous refinement of IL structures and the development of novel concepts like self-adaptive electrolytes promise to further push the boundaries of the achievable ESW, paving the way for the next generation of advanced electrochemical technologies.

In the pursuit of next-generation electrochemical devices, from high-voltage batteries to advanced gating transistors, ionic liquids (ILs) have emerged as a cornerstone technology due to their exceptional stability and tunable properties. Their utility hinges on the electrochemical stability window (ESW), the voltage range beyond which the electrolyte decomposes. Unlike molecular solvents, the ESW of an IL is an intrinsic property governed by the constituent ions' molecular structures. This guide provides a comparative analysis of how cationic and anionic structures determine redox limits, synthesizing recent computational and experimental studies to equip researchers with predictive design principles. The central thesis is that the electrochemical window is not a simple sum of individual ion stabilities but is determined by a complex interplay of ion structure, inter-ion interactions, and computational methodology, necessitating a holistic design approach for advanced electrochemical applications.

Computational Prediction of Electrochemical Stability Windows

Predicting ESWs computationally offers a powerful tool for high-throughput screening of new ionic liquids. However, the predictive accuracy is highly dependent on the chosen methodology.

Key Computational Approaches and Their Accuracy

A systematic investigation compared different computational approaches for estimating the ESW of imidazolium and tetra-alkyl ammonium-based ILs [6]. The study evaluated methods based on calculations performed on single ions versus ionic couples, utilizing various functionals including MP2 and B3LYP, and considering both vertical (electronic transition without structural relaxation) and adiabatic transitions (with full structural relaxation) [6].

Table 1: Comparison of Computational Methods for ESW Prediction [6]

Computational Approach	Level of Theory	Transition Type	Key Finding	Agreement with Experiment
Ionic Couples	B3LYP	Vertical/Adiabatic	Fails to reproduce experimental ESW	Poor
Single Ions	MP2 (vacuum)	Vertical	Best quantitative agreement	Excellent
Single Ions	B3LYP (vacuum)	Vertical	Underestimates ESW	Moderate
Single Ions	B3LYP (Polar Medium)	Vertical	Excessively widens ESW	Poor
Single Ions	MP2/B3LYP	Adiabatic	Large shrinkage of ESW	Poor

The results demonstrate that the most reliable approach involves calculations on single ions using the MP2 functional in vacuum and modeling the oxidation and reduction as vertical transitions [6]. Methods that employ ionic couples or include a polarizable medium consistently deviated from experimental data, highlighting the challenge of accurately modeling the bulk liquid environment [6].

Experimental Protocols for ESW Validation

Experimentally, ESW is typically determined using cyclic voltammetry (CV) or linear sweep voltammetry (LSV) in a three-electrode cell. Key protocol details include:

• Electrode Material: The working electrode material (e.g., Pt, glassy carbon, aluminum) significantly influences the observed breakdown potentials and must be carefully selected and reported [6] [7].
• Reference Electrode: A stable reference electrode (e.g., Ag/Ag+) is crucial, with potentials often referenced to the Li+/Li0 couple using a conversion factor (e.g., -1.46 V) for battery applications [6].
• Temperature Control: ESW can vary with temperature; measurements are often conducted at room temperature but may be performed at low temperatures (e.g., -33 °C) to suppress kinetic side reactions and reveal the "effective" stability window [7].
• Data Analysis: The anodic and cathodic limits are typically defined by a current density threshold (e.g., 1 mA/cm²) or the onset of a rapid current increase.

Structural Determinants of Redox Limits

The electrochemical stability of an IL is a direct consequence of the electronic and steric properties of its ions.

Anionic Redox Limits and Molecular Structure

The anion primarily governs the anodic limit (resistance to oxidation) [6]. Key structural principles include:

• Electron Withdrawing Groups: Highly fluorinated anions, such as bis(trifluoromethylsulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI), exhibit high anodic stability due to the electron-withdrawing effect of fluorine atoms, which stabilizes the highest occupied molecular orbital (HOMO) [6] [7].
• Charge Delocalization: Anions with a delocalized negative charge, like TFSI, are more resistant to oxidation than those with localized charge. The imide group facilitates this delocalization across the S-N-S bond framework.

Cationic Redox Limits and Molecular Structure

The cation primarily determines the cathodic limit (resistance to reduction) [7]. Structural trends include:

• Alkyl Chain Substitution: In ammonium-based cations, shorter alkyl chains generally lead to wider anodic stability [7]. For piperidinium cations, increasing the main alkyl chain length can widen the cathodic limit [7].
• Ether Functionalities: Ether chains generally confer lower electrochemical stability compared to alkyl chains of the same length. The oxygen atom, being more electron-withdrawing, can destabilize the cation against reduction [7]. Furthermore, an oxygen atom positioned far from the positively charged nitrogen center can significantly decrease the anodic potential limit [7].
• Asymmetric Design: Ammonium cations with three short chains of different lengths and one long chain have been shown to exhibit larger electrochemical stability compared to those with symmetric chains [7].

Table 2: Impact of Ion Structure on Electrochemical Stability Window

Ion Type	Structural Feature	Effect on Redox Limit	Example	Performance Implication
Anion	Fluorination	↑ Anodic Stability	TFSI, FSI [6]	Enables high-voltage cathodes (>4.5 V vs. Li)
Anion	Charge Delocalization	↑ Anodic Stability	TFSI [7]	Improves oxidative resistance at electrodes
Cation	Short Alkyl Chains	↑ Anodic Stability	N1114+ vs. N1888+ [7]	Wider overall ESW
Cation	Ether Functionality	↓ Cathodic & Anodic Stability	P122(201)+ vs. P1224+ [7]	Narrower ESW; requires careful voltage management
Cation	Asymmetric Substitution	↑ Overall ESW	Ammonium with 3 short/1 long chain [7]	Improved stability for gating and battery applications

The Critical Role of Ion Pairing

A critical finding from recent studies is that cations and anions cannot be considered in isolation [7]. The association between anions and cations plays an essential role in their electrochemical stabilities. The formation of ion pairs or larger correlated ion networks can shift redox potentials by stabilizing or destabilizing the transition states involved in the electron transfer process [8] [7]. This non-ideality means that the ESW of an IL is not a simple additive function of the individual ions' redox limits and underscores the need for experimental validation of computational predictions.

Advanced Concepts: Anionic Redox and Solvent Engineering

Harnessing Anionic Redox in Batteries

Beyond electrolyte stability, anionic redox chemistry is being exploited in high-capacity lithium-rich cathode materials (e.g., Li1.2Ni0.13Mn0.54Co0.13O2). In these systems, oxide ions (O2⁻) can be reversibly oxidized to On⁻ (n<2) upon charging, contributing significantly to capacity [9]. However, this process is often accompanied by sluggish kinetics, large voltage hysteresis, and voltage fade, issues linked to the fundamental differences between cationic and anionic redox [9]. Cationic redox (e.g., Ni2+/4+) is kinetically fast and shows little hysteresis, whereas anionic redox (O2⁻/On⁻) is slower and exhibits different oxidation versus reduction potentials [9].

Engineering the Solvation Environment

Pushing redox potentials to extreme values requires control over the entire ion solvation environment. Recent work has utilized strategically fluorinated benzenes (C6FxH6−x, xFB) as solvents with weakly coordinating anions (WCAs) like [Al(ORF)4]⁻ [10]. These solvent systems feature:

• High Polarity and Weak Coordination: Asymmetric fluorination (e.g., 1,2,3-trifluorobenzene) can yield dielectric constants as high as 22.1, yet the solvents are very weak ligands, minimizing stabilization of oxidized species [10].
• Wide Electrochemical Windows: These xFB/WCA systems have ESWs exceeding 5 V, with positive limits pushed to record values between +1.82 V and +2.67 V vs. Fc+/Fc [10]. This approach demonstrates that tuning solvent-ion interactions is as crucial as ion selection for accessing novel redox chemistry.

The Scientist's Toolkit: Key Reagents and Materials

Table 3: Essential Research Reagents for Ionic Liquid Electrochemistry

Reagent/Material	Function/Application	Key Characteristics	Example Use Case
Bis(trifluoromethylsulfonyl)imide (TFSI) Anion	Anion for high-anodic-stability ILs	High degree of fluorination, charge delocalization [6] [7]	Formulating ILs for high-voltage Li-ion batteries [7]
Tetra-alkyl Ammonium Cations (e.g., N1114+)	Cations for wide ESW	Short, asymmetric alkyl chains enhance stability [7]	Electrolyte for gating applications at low temperatures [7]
Weakly Coordinating Anions (WCAs, e.g., [Al(ORF)4]⁻)	Supporting electrolyte/salt	Extremely low nucleophilicity and high stability [10]	Accessing highly positive redox potentials in fluorobenzene solvents [10]
Fluorobenzene Solvents (xFB)	Inert, polar solvent medium	Tunable polarity via fluorination, weak coordination [10]	Creating electrolytes with >5 V stability windows [10]
Nafion Membrane	Ion-exchange membrane	High proton conductivity; benchmark material [11]	Separating compartments in vanadium redox flow batteries [11]

Signaling Pathways and Workflows

The following diagram synthesizes the core logical relationship between ion structure, its resulting properties, and the ultimate electrochemical performance, providing a conceptual framework for designing new ionic liquids.

Electrochemical stability is a cornerstone property determining the viability and safety of electrolytes in advanced energy storage systems. The electrochemical stability window (ESW), which defines the voltage range within which an electrolyte remains inert, is not merely an intrinsic property of individual ions but a complex characteristic emerging from their interactions. This guide focuses on the critical roles of ion association and ionic synergy in defining the ESW of ionic liquid (IL) electrolytes. Ion association refers to the specific interactions, such as Coulombic forces and hydrogen bonding, between cations and anions that influence their collective behavior. Ionic synergy describes how the combination of specific cation-anion pairs produces electrochemical properties that are not simply the sum of their parts. For researchers developing next-generation batteries and supercapacitors, a deep understanding of these phenomena is essential for the rational design of high-voltage, stable electrolytes. This guide provides a comparative analysis of different IL systems, supported by experimental and computational data, to elucidate how strategic ion pairing can push the boundaries of electrochemical performance.

Fundamental Concepts and Design Strategies

Key Parameters Governing Electrochemical Stability

The ESW of an electrolyte is primarily dictated by its oxidation potential (anodic limit, determined by the anion) and reduction potential (cathodic limit, determined by the cation). A wider ESW allows for higher operating voltages, which directly translates to greater energy density in storage devices [12]. The following parameters are critical in the design of stable IL electrolytes:

• Ion Association Strength: Strong Coulombic interactions between cations and anions can significantly alter the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels of the individual ions. This association can lead to a coordinated stability that resists decomposition at extreme potentials [13] [14].
• Functional Groups: The introduction of specific functional groups, such as ethers or fluorinated chains, can dramatically modify the charge distribution within an ion. For instance, ether functional groups in pyrrolidinium cations act as electron donors, enhancing the overall electrochemical stability of the IL [13].
• Ionic Synergy: The selection of cation-anion pairs is not arbitrary. Complementary pairs that exhibit weak electrostatic interaction can facilitate higher ionic conductivity, while pairs that form a stable interfacial layer can suppress detrimental side reactions at the electrode surface [13] [12].

Computational Evaluation of Ionic Liquid Stability

Computational methods are indispensable for predicting ESW and understanding ion association at the molecular level. Density Functional Theory (DFT) is widely used to calculate key properties. The workflow below outlines the primary computational strategy for evaluating the ESW of an ionic liquid.

The most accurate predictions are often achieved through vertical transition calculations on single ions using the MP2 functional in vacuum, which show strong correlation with experimental measurements [14]. In contrast, calculations on neutral ion pairs or those using a polarizable continuum model tend to overestimate the ESW [14].

Comparative Analysis of Ionic Liquid Electrolytes

Performance Comparison of Common IL Systems

The electrochemical stability of an IL is highly dependent on the specific cation-anion pairing. The table below summarizes key data for prevalent IL systems, highlighting the impact of different ion combinations.

Table 1: Electrochemical Stability Windows of Common Ionic Liquid Electrolytes

Cation	Anion	Computational ESW (V)	Experimental ESW (V)	Key Synergistic Effect / Note
N,N-propylmethylpyrrolidinium (P13+) [14]	TFSI− [14]	~5.5 (B3LYP, vertical, ion pair) [14]	>4.5 vs Li/Li+ [14]	Wide ESW suitable for high-voltage applications.
1-butyl-3-methylimidazolium (BMIM+) [14]	TFSI− [14]	~4.5 (B3LYP, vertical, ion pair) [14]	>4.5 vs Li/Li+ (on carbon) [14]	Good anodic stability on carbon composites.
N-methoxyethyl-N-methylpyrrolidinium (Pyr12O1+) [13]	FSI− [13]	N/A	High stability (details inferred) [13]	Ether group enhances stability and reduces Li+ interaction.
N-butyl-N-methylpyrrolidinium (Pyr14+) [13]	FSI− [13]	N/A	Lower stability vs Pyr12O1+ [13]	Lacks ether group, leading to lower stability.
EMIM+ [14]	FSI− [14]	~4.0 (B3LYP, vertical, ion pair) [14]	>4.5 vs Li/Li+ [14]	High anodic stability observed experimentally.
N122(2O1)+ [14]	TFSI− [14]	~5.5 (B3LYP, vertical, ion pair) [14]	>4.5 vs Li/Li+ [14]	Ether-functionalized cation for improved properties.

Impact of Cation and Anion Chemistry

A deeper analysis of the data reveals clear trends based on ion chemistry:

• Cation Effect: Pyrrolidinium-based cations (e.g., P13+, Pyr12O1+) generally offer wider ESWs and greater stability compared to imidazolium-based cations (e.g., BMIM+, EMIM+) [14]. This is attributed to the robust cyclic structure and higher electrochemical stability of the pyrrolidinium ring. Furthermore, ether-functionalization of cations (e.g., Pyr12O1+, N122(2O1)+) introduces an electron-donating group that redistributes charge, weakens the association with the anion, and further boosts stability against reduction [13].
• Anion Effect: The anion primarily governs the anodic limit of the ESW. Highly fluorinated anions like TFSI− and FSI− demonstrate superior oxidative stability, enabling operation at high voltages [14]. The FSI− anion, while conductive, is known to decompose and contribute to the formation of a solid electrolyte interphase (SEI) on reactive electrodes like lithium metal [13].

Table 2: Comparison of Computational Methods for ESW Prediction (Data for EMITFSI) [14]

Computational Method	Transition Type	Calculated ESW (V)	Accuracy / Note
B3LYP (gas phase)	Vertical (single ions)	~4.5	Underestimates experimental ESW [14].
MP2 (gas phase)	Vertical (single ions)	~5.8	Best quantitative agreement with experiments [14].
B3LYP (polar medium)	Vertical (single ions)	~7.5	Excessively overestimates ESW [14].
B3LYP (gas phase)	Adiabatic (single ions)	~2.0	Large shrinkage of ESW vs. vertical transition [14].
B3LYP (gas phase)	Vertical (ion pair)	~4.5	Poor agreement with experimental data [14].

Experimental Protocols and Methodologies

Standard Protocol for ESW Measurement

To ensure the reliability and comparability of ESW data, a standardized experimental protocol is essential. The following workflow details the key steps for determining the ESW of an ionic liquid electrolyte using linear sweep voltammetry (LSV) or cyclic voltammetry (CV).

Detailed Methodology:

• Cell Assembly: A three-electrode system is used. The working electrode is typically an inert material like platinum, glassy carbon, or aluminum, as the material can significantly influence the measured ESW [14]. The counter electrode is often lithium metal or platinum, and the reference electrode is a stable system like Li/Li+.
• Electrolyte Preparation: The ionic liquid must be rigorously purified and dried to remove water and impurities, which can narrow the ESW. The electrolyte is prepared in an argon-filled glovebox (H₂O, O₂ < 1 ppm).
• Instrument Setup: A potentiostat is used to control the voltage and measure the current. The scan rate is typically slow (e.g., 1-5 mV/s) to minimize capacitive effects.
• Potential Scanning:
  • Anodic Scan: The working electrode potential is swept linearly from the open-circuit potential towards positive voltages. The anodic limit is defined as the potential at which the current density exceeds a predetermined threshold (e.g., 0.1 mA/cm²), indicating the onset of anion oxidation [14].
  • Cathodic Scan: Similarly, the potential is swept towards negative voltages to determine the cathodic limit, where cation reduction begins.
• Data Analysis: The ESW is calculated as the difference between the anodic and cathodic limits. It is critical to report the current density threshold and working electrode material for meaningful comparisons [14].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents and Materials for IL Electrolyte Research

Reagent / Material	Function / Role	Example in Context
Pyrrolidinium Salts (e.g., Pyr12O1+, Pyr14+, P13+)	Electrochemically stable cations that provide a wide ESW.	[Pyr12O1][FSI] offers greater interfacial stability than [Pyr14][FSI] due to the ether functional group [13].
Fluorinated Anions (e.g., TFSI−, FSI−, BF₄−)	Anions with high anodic stability, key for high-voltage operation.	TFSI−-based ILs consistently demonstrate anodic stability >4.5 V vs. Li/Li+ [14].
Lithium Salts (e.g., LiTFSI, LiFSI)	Added to ILs to introduce Li⁺ for lithium-ion or lithium-metal battery operation.	A 0.8:0.2 molar mixture of IL with LiTFSI is used to test anodic stability on carbon electrodes [14].
Inert Working Electrodes (Pt, GC, Al)	To measure intrinsic electrolyte stability without electrode participation.	The ESW of EMIFSI was measured on a carbon composite electrode over aluminum [14].
Reference Electrodes (Li/Li+, Ag/Ag+)	Provide a stable, known potential reference for 3-electrode measurements.	Essential for accurately reporting oxidation and reduction potentials vs. a standard [14].

The electrochemical stability of ionic liquid electrolytes is a synergistic property, profoundly influenced by the specific association between cations and anions. This guide demonstrates that pyrrolidinium-based cations paired with fluorinated anions like TFSI− represent a high-performance combination, reliably achieving ESWs exceeding 4.5 V vs. Li/Li+. The strategic introduction of ether functional groups into cations further enhances stability by modifying charge distribution and weakening ion interactions. For accurate a priori design, computational methods, particularly MP2-level calculations of vertical transitions for single ions, provide the most reliable prediction of ESW. Moving forward, the rational design of ILs must move beyond considering ions in isolation and focus on optimizing cation-anion pairs to harness their synergistic potential, paving the way for safer, higher-energy-density electrochemical devices.

In the pursuit of next-generation energy storage and conversion systems, ionic liquids (ILs) have emerged as promising electrolyte candidates due to their remarkable properties, including negligible volatility, non-flammability, high thermal stability, and intrinsic ionic conductivity [15] [16] [17]. While initial research focused primarily on the inherent electrochemical stability of individual ions, a paradigm shift has occurred toward understanding that the macroscopic electrochemical window of an IL-based electrolyte is not merely a sum of its constituent ions' properties. Instead, it is governed by the complex solvation environment and the resulting interfacial effects that occur at electrode-electrolyte interfaces [15] [18] [8].

The electrochemical stability window (ESW), defining the voltage range within which an electrolyte remains stable without decomposing, is a critical parameter determining the energy density and operational voltage of electrochemical devices [16] [19]. Classical approaches to IL electrolyte design often prioritized the selection of chemically stable cations and anions with wide intrinsic ESWs. However, contemporary research reveals that the practical ESW is profoundly influenced by the collective interactions between ions, the formation of distinct solvation nanostructures, and the chemical composition of the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during cycling [15]. This comparative guide examines the fundamental mechanisms through which solvation environment and interfacial chemistry dictate the practical electrochemical stability of ionic liquid electrolytes, providing researchers with a structured framework for evaluating and selecting IL electrolytes for advanced applications.

Fundamental Mechanisms: How Solvation and Interfaces Govern Electrochemical Windows

The Solvation Nanostructure of Ionic Liquids

Ionic liquids are not simple homogeneous solvents but exhibit complex nanostructuring arising from a balance of Coulombic interactions, hydrogen bonding, and van der Waals forces [16] [8]. This nanostructuring creates distinct polar and non-polar domains that influence ion transport and stability. In the context of Li-ion batteries, the solvation structure of Li⁺ ions becomes particularly important. Li⁺ ions can exist in multiple coordination states: as free ions, solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs), or larger aggregates (AGGs) [15]. The distribution of these species is concentration-dependent and critically impacts the electrolyte's reductive and oxidative stability.

In highly concentrated IL electrolytes, the proportion of CIPs and AGGs increases, creating an anion-rich solvation sheath around Li⁺ ions. This anion-rich environment promotes the preferential reduction of anions at the anode surface, leading to the formation of a robust, inorganic-rich SEI predominantly composed of LiF when fluorine-containing anions like TFSI⁻ and FSI⁻ are used [15]. This SEI layer is thin, dense, and effectively suppresses continued electrolyte decomposition and lithium dendrite growth, thereby extending the practical electrochemical window by preventing continuous parasitic reactions at low potentials [15] [17].

Interfacial Phenomena and Interphase Formation

The electrode-electrolyte interface (EEI) is the crucial boundary where electrolyte stability is determined. During initial cycling, especially in lithium metal batteries, the electrolyte components undergo reductive decomposition at the anode surface to form a passivating layer known as the solid electrolyte interphase (SEI). Similarly, oxidative decomposition can occur at the cathode, forming a cathode electrolyte interphase (CEI) [15]. The chemical and mechanical properties of these interphases—whether they are stable and protective or unstable and fragile—dictate the long-term operational voltage window of the device.

Ionic liquids influence interphase formation through their inherent decomposition products. For instance, FSI⁻-based ILs are known to form excellent SEI layers on lithium metal, facilitating stable lithium plating and stripping [15] [16]. However, the organic cations of ILs can also participate in interfacial reactions. The redox activity of organic cations can lead to higher organic content in the EEI, which may reduce interface stability and increase impedance if not properly managed [15]. Therefore, effective IL electrolyte design must consider the combined interfacial behavior of all ions, not just their bulk stability.

Comparative Analysis of Ionic Liquid Electrolyte Systems

Electrochemical Window and Ionic Conductivity Comparison

The following table summarizes key properties of common ionic liquid electrolytes and their combinations, highlighting how different ion structures and electrolyte formulations influence the resulting electrochemical performance.

Table 1: Comparative Electrochemical Properties of Ionic Liquid-Based Electrolytes

Ionic Liquid Electrolyte Composition	Electrochemical Window (V)	Ionic Conductivity (mS/cm)	Key Characteristics and Performance
Pyrrolidinium-based ILs (e.g., PYR₁₄TFSI)	>5.5 [17]	0.1 - 18 (common range for ILs) [16]	Excellent air and thermal stability (up to 300°C); widely studied for LIBs [17].
Imidazolium-based ILs (e.g., [BMIM][BF₄])	~4-6 [19]	1.81 × 10⁻³ [19]	Lower melting points but often reduced cathodic stability due to reactive cations [16].
Dual-Anion IL (0.8Pyr₁₄FSI–0.2LiTFSI)	Effectively wide for NCM88	Not Specified	Synergistic effect of FSI⁻ and TFSI⁻ enables excellent long-cycle stability in Li/NCM88 batteries (88% capacity retention after 1000 cycles) [15].
IL with Ether-Functionalized Cation (e.g., [P₁,₂O₂][FSI])	Not Specified	Not Specified	Ether oxygen atoms and flexible chains suppress crystallization, enhancing low-temperature performance and ion transport [16].
Neat ILEs (High Concentration)	Effectively widened at anode	Lower due to high viscosity	Increased CIPs and AGGs create anion-rich environment, promoting LiF-rich SEI and stabilizing Li metal anode [15].
Locally Concentrated ILEs	Effectively widened	Maintains higher conductivity	Balances high local Li⁺ concentration for good SEI with manageable bulk viscosity for better transport [15].

Impact of Impurities and Additives on Electrochemical Window

A critical aspect often overlooked in fundamental studies is the impact of trace impurities, particularly water, on the electrochemical stability of ILs. The presence of trace water can significantly narrow the ESW of ILs like [EMIM][TFSI] by participating in undesirable redox reactions at the electrode interfaces [18]. However, a sophisticated solution involves engineering the solvation structure to mitigate this effect. Research has demonstrated that adding a proper amount of LiTFSI to wet [EMIM][TFSI] leads to the formation of a Localized Solvation Nanostructure (LSNS) [18].

Table 2: Strategies for Mitigating Impurity Effects and Modulating Interfacial Chemistry

Strategy	Mechanism	Impact on Electrochemical Window
Li-salt addition to form LSNS [18]	Li⁺ ions coordinate with water molecules as the center, surrounded by TFSI⁻ anions and EMIM⁺ cations, sequestering H₂O from the interface.	Recovers the originally decreased ESW by preventing trace water from approaching and reacting at the electrode surface.
Dual-Anion Formulations [15]	Competitive interactions between TFSI⁻ and FSI⁻ anions slow FSI⁻ reaction kinetics, leading to a thin, dense SEI.	Enhances compatibility with Li metal, enabling more uniform Li deposition and a stable cycling voltage range.
Ether-Functionalization of Cations [15] [16]	Introducing ether oxygen atoms into cation side chains modifies cation coordination and reduces nanostructured domain formation.	Improves ion transport, which can support stable operation over a wider range of current densities and temperatures.
Cation Fluorination [15]	Increases the oxidative stability of the organic cation and influences the interphase composition.	Can widen the anodic (oxidative) limit and contribute to a more stable CEI on high-voltage cathodes.

Experimental Protocols for Characterizing Solvation and Interfacial Effects

Probing Solvation Structures

Understanding the solvation structure requires techniques that can probe ion coordination and nanostructuring:

• Raman and FTIR Spectroscopy: These techniques identify the distribution of Li⁺ species (free ions, SSIPs, CIPs, AGGs) by detecting shifts in vibrational peaks of anions (e.g., TFSI⁻, FSI⁻) that coordinate with Li⁺ [15].
• Nuclear Magnetic Resonance (NMR): NMR, particularly pulsed-field gradient NMR, can be used to measure self-diffusion coefficients of different ions, providing insights into ion correlations and transport mechanisms [8].
• X-ray and Neutron Scattering: These methods reveal the characteristic nanoscale polar and non-polar domain structures inherent to many ILs, which underlie their solvation behavior [8].

Evaluating Interfacial Chemistry and Electrochemical Stability

The quality of the interphase and the practical ESW are evaluated using electrochemical and surface analysis techniques:

• Linear Sweep Voltammetry (LSV) or Cyclic Voltammetry (CV): These standard methods are used to determine the electrochemical stability window by scanning the potential until significant current from electrolyte decomposition is observed. The working electrode material (e.g., Pt, glassy carbon) must be noted, as it can influence the results [16] [19].
• Electrochemical Impedance Spectroscopy (EIS): EIS is performed on symmetric cells (e.g., Li||Li) to track the evolution of interfacial resistance during cycling, which is indicative of SEI stability and growth [15].
• X-ray Photoelectron Spectroscopy (XPS): Perhaps the most powerful tool for interfacial analysis, XPS provides quantitative chemical composition of the SEI and CEI layers (e.g., LiF, Li₂O, organic compounds) after cycling, directly linking performance to interfacial chemistry [15].
• In Situ/Operando Microscopy: Techniques like in situ TEM or atomic force microscopy can visually monitor Li deposition morphology (dendritic vs. planar) and interface evolution in real time [15].

Essential Research Reagent Solutions for IL Electrolyte Studies

The following table catalogues key materials and their functions for researchers designing experiments on IL-based electrolytes.

Table 3: Research Reagent Solutions for Ionic Liquid Electrolyte Research

Reagent / Material	Function in Research	Key Considerations
LiTFSI (Lithium bis(trifluoromethanesulfonyl)imide)	Common lithium salt for Li-ion and Li-metal battery electrolytes. Highly dissociable in ILs [15] [17].	Source of Li⁺ ions; TFSI⁻ anion contributes to wide ESW and forms LiF-rich SEI. Must be handled in inert, dry atmosphere.
LiFSI (Lithium bis(fluorosulfonyl)imide)	Alternative lithium salt known for forming excellent SEI on Li metal anodes [15] [16].	FSI⁻ anions have low viscosity and high charge delocalization, but ILs containing them may crystallize at sub-zero temperatures.
Pyrrolidinium-based ILs (e.g., PYR₁₄TFSI)	Serves as a stable, high-voltage electrolyte solvent or co-solvent [17].	Preferred for battery applications due to thermal and electrochemical stability from aliphatic cation structure [16].
Imidazolium-based ILs (e.g., [EMIM][TFSI])	Widely used model IL for fundamental studies and applications like electrochromic devices [18] [19].	Cations have lower stability against reduction, limiting use with Li metal anodes, but offer low viscosity and melting point.
Ether-functionalized ILs (e.g., [P₁,₂O₂][FSI])	Designed to suppress crystallization and improve low-temperature conductivity [16].	Ether oxygen atoms in cation side chains provide flexibility and modify solvation dynamics and nanostructure.
Fluoroethylene Carbonate (FEC)	SEI-forming additive used in combination with ILs to improve passivation and cycle life [17].	Reduces capacity decline and enhances the stability of the interphase formed by the IL itself.

Schematic Workflow: From Ion Selection to Stable Electrochemical Window

The following diagram visualizes the logical workflow and key decision points for designing an IL electrolyte with a stable, wide electrochemical window, integrating the concepts of solvation and interfacial engineering.

Diagram Title: IL Electrolyte Design Workflow

The electrochemical window of an ionic liquid electrolyte is a dynamic property, shaped not in isolation but through the intricate interplay of ions, their coordinated solvation structures, and the passivating interphases they form. As this comparison demonstrates, a superior IL electrolyte is no longer defined solely by the intrinsic stability of its ions but by its ability to form a protective solvation nanostructure and promote the growth of a stable, ionically conductive interphase. Future advancements will likely rely on data-driven approaches and machine learning to navigate the vast combinatorial space of ion pairs and additives, efficiently linking molecular structure to solvation behavior and interfacial outcomes [15] [8]. By moving beyond the simplistic view of isolated ions and embracing the critical roles of the solvation environment and interfacial effects, researchers can unlock the full potential of ionic liquids for safe, high-energy-density electrochemical devices.

Predicting and Measuring ESW: Computational Models and Experimental Techniques

The electrochemical stability window (ESW) of an electrolyte defines the range of voltages within which it remains stable without undergoing decomposition. This property is a cornerstone of modern energy storage, as it directly dictates the operational voltage and, consequently, the energy density of devices like batteries and supercapacitors. For instance, the energy density of a battery is proportional to its operating voltage, while that of a supercapacitor is proportional to the square of the operating voltage [20]. Pushing the boundaries of energy storage technology, therefore, hinges on the ability to design and identify electrolytes with ever-wider ESWs. Computational modeling has emerged as an indispensable tool in this pursuit, providing a robust, instructional, and efficient means to explore electrolyte formulations from the atomic scale upwards [21]. This guide compares the performance of different computational approaches used to predict the ESW, with a specific focus on their application to ionic liquid (IL) electrolytes, which are noted for their intrinsic wide ESW, thermal stability, and non-flammability [7] [17].

Comparing Computational Approaches: A Multi-Scale Perspective

Computational methods for predicting ESWs have evolved significantly, ranging from simple, high-throughput screening tools to complex simulations that account for the intricate electrochemistry at electrode-electrolyte interfaces. The following table summarizes the core characteristics, applications, and limitations of the primary approaches.

Table 1: Comparison of Computational Models for Predicting Electrolyte ESW

Computational Model	Fundamental Approach	Key Performance Metrics	Advantages	Limitations & Challenges
Isolated Molecule/Ion Model [20]	Calculates HOMO/LUMO energies or redox potentials of single ions/molecules in a vacuum.	HOMO (for oxidation), LUMO (for reduction) energy levels; Estimated redox potentials (Vox, Vred).	- High computational speed and low cost.- Ideal for initial high-throughput screening of many candidate molecules or ions.- Provides a clear initial trend for ESW.	- Neglects critical solvation effects and ion correlations.- Can significantly overestimate the practical ESW [20].
Solvation Model [20]	Incorporates the effect of the solvent environment, either implicitly (as a dielectric continuum) or explicitly with a few solvent molecules.	Redox potentials adjusted for solvation free energy.	- Provides a more accurate description of the redox process than isolated models.- Implicit models offer a good balance between accuracy and cost.	- Explicit solvation with a full liquid-phase structure remains challenging.- May still overlook specific electrode surface effects.
Electrode-Electrolyte Interface Model [20] [22]	Simulates the full interfacial region, including the electrode surface and the electrolyte, using DFT or Ab Initio Molecular Dynamics (AIMD).	Decomposition reaction pathways and energy barriers; Predicted stable voltage range based on interfacial reactions.	- Most realistic and accurate method for predicting the functional ESW.- Can reveal decomposition mechanisms and Solid Electrolyte Interphase (SEI) formation [22].	- Extremely computationally expensive.- Complex to set up and requires significant expertise.- Not suitable for high-throughput screening.
Stoichiometry Stability Method (For Solid-State Electrolytes) [23]	A middle-ground approach between HOMO-LUMO and full phase diagrams; assesses stability against electrode materials.	Computed electrochemical stability window relative to decomposition products.	- Represents a bridge between simple and complex methods.- Provides a more realistic ESW for solid-state electrolytes than HOMO-LUMO.	- Primarily developed and applied for solid-state electrolytes like LLZO, LIPON, and LGPS [23].

The relationship between these models and their respective trade-offs between computational cost and predictive accuracy can be visualized as a multi-step workflow in the following diagram.

Experimental Data and Performance of Ionic Liquid Electrolytes

While computational models are essential for prediction, they must be validated against experimental data. Ionic liquids, often termed "designer solvents" due to the tunability of their cation-anion pairs, are a key area of study. The electrochemical stability of ILs is not solely an intrinsic property of the individual ions but is significantly influenced by their synergistic interactions and the experimental conditions, such as temperature and electrode material [7]. Recent studies have systematically explored how molecular structure affects ESW.

Table 2: Experimentally Measured Electrochemical Stability Windows of Selected Ionic Liquids

Ionic Liquid (Cation-Anion)	Experimental ESW (V)	Measurement Conditions	Key Structural Findings & Performance
PYR14TFSI [17]	> 5.5	N/A	Pyrrolidinium (PYR) cations with TFSI anions are widely studied for excellent thermal and electrochemical stability.
DEME Tf2N [7]	Wide window maintained at low T	Low temperature (< -53°C), high vacuum	Used in gating applications; reactions with electrodes are suppressed at low temperatures, effectively widening the ESW.
Ammonium-based ILs with Tf2N− [7]	Up to 5.0 (effective)	Low temperature (-33°C), electrochemical transistors	Cations with short chains of different lengths show larger ESW. The presence of ether functionalities generally lowers electrochemical stability compared to alkyl chains.
Dual-Anion IL (EMIFSI with LiTFSI/LiFSI) [24]	Effective window enabling stable LMB cycling	Li‖LiFePO4 full cell, 1C rate	The dual-anion design (TFSI− and FSI−) enhances interfacial stability, promotes a LiF-rich SEI, and improves ionic conductivity, leading to >99.93% capacity retention after 200 cycles.

The performance of ILs can be further enhanced through innovative electrolyte engineering. For example, the concept of "self-adaptive electrolytes" that dynamically segregate during charging to enrich different solvents at the anode and cathode has been shown to experimentally expand the ESW beyond the limits of conventional formulations [5]. Similarly, competitive solvation in aqueous-based electrolytes, where aprotic solvents like trimethyl phosphite (TMP) displace water molecules from the primary solvation sheath of cations, has been used to achieve an electrochemical window as wide as 3.2 V [25].

Detailed Experimental Protocols for ESW Determination

To ensure the comparability of data, whether computational or experimental, standardized protocols are essential. Below are detailed methodologies for key experiments cited in this field.

• Objective: To experimentally determine the electrochemical stability window of an electrolyte by identifying its anodic (oxidation) and cathodic (reduction) limits.
• Cell Setup: A standard three-electrode electrochemical cell is used, consisting of:
  • Working Electrode: Inert material such as glassy carbon, platinum, or stainless steel.
  • Counter Electrode: Typically platinum wire or foil.
  • Reference Electrode: A stable reference suitable for the electrolyte (e.g., Ag/AgCl for aqueous systems, Li/Li⁺ for non-aqueous).
• Procedure:
  • The electrolyte is purged with an inert gas (e.g., Ar or N₂) to remove dissolved oxygen.
  • The potential is swept linearly from the open-circuit potential towards positive potentials to scan the anodic limit.
  • A separate experiment is performed by sweeping from the open-circuit potential towards negative potentials to scan the cathodic limit.
  • The scan rate is typically slow (e.g., 1-5 mV/s) to approximate quasi-steady-state conditions.
• Data Analysis: The anodic limit ((E{ox})) is identified as the potential at which the current density exceeds a pre-defined threshold (e.g., 0.1-0.5 mA/cm²), indicating the onset of oxidation. The cathodic limit ((E{red})) is identified similarly for reduction. The ESW is calculated as (ESW = E{ox} - E{red}).

• Objective: To predict the ESW and interfacial stability of an electrolyte using a multi-scale computational approach.
• Software/Tools: Density Functional Theory (DFT) codes (e.g., VASP, Quantum ESPRESSO), Molecular Dynamics (MD) software (e.g., LAMMPS, GROMACS).
• Procedure:
  • Isolated Ion Screening (DFT):
    • Geometry optimize the isolated cation and anion.
    • Calculate the HOMO energy of the anion (related to oxidation stability) and the LUMO energy of the cation (related to reduction stability) [20].
  • Solvation Structure Analysis (MD/DFT):
    • Build a simulation box containing multiple ion pairs and/or solvent molecules.
    • Run MD simulations (classical or ab initio) to equilibrate the bulk electrolyte structure.
    • Analyze Radial Distribution Functions (RDFs) to understand the solvation shell structure (e.g., Li⁺-O, Li⁺-F) [24].
  • Interfacial Stability and SEI Formation (AIMD/DFT):
    • Construct a model of the electrode surface (e.g., Li-metal slab, graphite).
    • Place the equilibrated electrolyte structure adjacent to the electrode surface.
    • Perform Ab Initio Molecular Dynamics (AIMD) simulations to observe spontaneous reactions at the interface [22].
    • Use Nudged Elastic Band (NEB) calculations to determine energy barriers for ion migration through interface layers [22].
• Data Analysis:
  • From MD: Quantify ion aggregates (AGGs), contact ion pairs (CIPs), and free ions via Raman spectrum analysis [24].
  • From AIMD: Identify initial decomposition products and predict SEI composition (e.g., LiF, Li₂O).
  • Correlate the computed interfacial stability with experimental cycling performance.

The Scientist's Toolkit: Essential Research Reagents & Materials

The following table details key materials and computational tools frequently used in the research and development of advanced ionic liquid electrolytes.

Table 3: Essential Research Reagents and Tools for Electrolyte Studies

Category	Item / Reagent	Function & Application Note
Common IL Cations	Pyrrolidinium (e.g., PYR14+), Imidazolium, Ammonium	Form the organic cation of the IL. Pyrrolidinium-based ILs are noted for high stability and wide ESW [17].
Common IL Anions	Bis(trifluoromethanesulfonyl)imide (TFSI−), Bis(fluorosulfonyl)imide (FSI−), BF4−, PF6−	Form the anion of the IL. TFSI− offers high conductivity and stability. FSI− is known for facilitating the formation of a LiF-rich SEI [24].
Lithium Salts	LiTFSI, LiFSI	Provide the charge carrier (Li+) in lithium battery electrolytes. Their combination is used in dual-anion electrolyte designs [24].
Aprotic Solvents / Diluents	Propylene Carbonate (PC), Acetonitrile (AN), Trimethyl Phosphite (TMP), Bis(2,2,2-trifluoroethyl) ether (BTFE)	Used as co-solvents or diluents. They can modify viscosity, conductivity, and solvation structure (e.g., TMP competitively solvates cations to exclude water [25]; BTFE creates locally concentrated ionic liquid electrolytes [24]).
Computational Tools	Density Functional Theory (DFT), Ab Initio Molecular Dynamics (AIMD), Classical Molecular Dynamics (MD)	DFT calculates electronic structure properties. AIMD/MD simulates the dynamic behavior of ions and molecules in bulk and at interfaces [21] [22].

Electrochemical characterization techniques are indispensable in modern materials science and energy storage research, providing critical insights into electron and ion transfer processes, reaction kinetics, and interfacial behaviors. Among these techniques, Electrochemical Impedance Spectroscopy (EIS) and Voltammetry stand as powerful, complementary tools for investigating electrochemical systems. While both methods probe electrode-electrolyte interfaces, they offer distinct approaches and types of information. EIS operates in the frequency domain, applying a small amplitude sinusoidal voltage perturbation across a wide frequency range and measuring the system's impedance response [26]. In contrast, voltammetric techniques function in the time domain, applying a controlled potential waveform and measuring the resulting current response [27]. The selection between these techniques depends on the specific research objectives, whether for kinetic characterization, quantitative analysis, or interfacial properties investigation.

Within the context of comparing electrochemical windows of different ionic liquid electrolytes, both EIS and voltammetry provide valuable, complementary data. EIS helps characterize the stability and interfacial resistance at electrode-electrolyte interfaces, while voltammetry directly probes the redox stability limits of the electrolyte components. This guide provides a comprehensive comparison of these techniques, their operational principles, experimental protocols, and applications, with a specific focus on their utility in evaluating ionic liquid electrolytes for advanced energy storage systems.

Theoretical Foundations and Comparative Principles

Electrochemical Impedance Spectroscopy (EIS)

EIS is a non-destructive technique that measures a system's impedance as a function of frequency. The fundamental principle involves applying a small amplitude sinusoidal AC voltage to an electrochemical cell and analyzing the current response, which reveals both amplitude and phase shift information [26]. The impedance (Z) is calculated using Ohm's law and is typically represented using two components: the real (Z') and imaginary (Z") parts, which can be plotted in Nyquist format, or as magnitude and phase angle in Bode format [26].

The strength of EIS lies in its ability to deconvolve complex electrochemical systems into individual processes with different time constants. Fast processes typically appear in the high-frequency region, while slow processes dominate the low-frequency response [28]. This frequency-domain operation simplifies the analysis of complex systems by separating overlapping processes that would be challenging to distinguish in time-domain techniques. For ionic liquid electrolyte characterization, EIS is particularly valuable for investigating interfacial resistance, charge transfer processes, and bulk electrolyte properties.

EIS data is typically interpreted using equivalent circuit models composed of electrical elements such as resistors, capacitors, constant phase elements, and Warburg impedances, each representing specific physical processes [26]. The equivalent circuit model originally proposed by Larminie and Dicks for fuel cells illustrates this approach, featuring components that represent the electrical double layer capacitance, charge transfer resistance, and solution resistance [26].

Voltammetry

Voltammetry encompasses a family of techniques that measure current as a function of applied potential. The fundamental principle involves applying a controlled potential waveform to an working electrode and monitoring the resulting faradaic and non-faradaic currents arising from electrochemical reactions and capacitive charging, respectively [27]. Unlike EIS, voltammetry operates in the time domain, providing direct information about redox potentials, reaction kinetics, and diffusion characteristics.

Various voltammetric techniques have been developed, each with specific advantages. Linear Sweep Voltammetry (LSV) and Cyclic Voltammetry (CV) provide information about redox potentials and reaction reversibility. Pulse techniques, including Differential Pulse Voltammetry (DPV) and Square-Wave Voltammetry (SWV), offer enhanced sensitivity by minimizing charging current contributions [27]. Stripping techniques, such as Anodic Stripping Voltammetry (ASV), provide exceptional sensitivity for trace element analysis through preconcentration steps [27].

For characterizing electrochemical windows of ionic liquid electrolytes, LSV and CV are particularly valuable as they can directly visualize the potential limits where electrolyte decomposition begins, providing crucial information about the operational voltage range for energy storage devices.

Key Differences and Complementary Aspects

Table 1: Fundamental Comparison Between EIS and Voltammetry

Characteristic	Electrochemical Impedance Spectroscopy (EIS)	Voltammetry
Operating Domain	Frequency domain	Time domain
Excitation Signal	Small amplitude sinusoidal AC voltage	Various potential waveforms (linear, pulsed, etc.)
Measured Response	Impedance (magnitude and phase)	Current
Information Obtained	Resistive, capacitive, and diffusive components; time constants	Redox potentials, reaction kinetics, diffusion coefficients
Perturbation Size	Small (typically 5-10 mV) to maintain linearity	Can be large, driving non-linear responses
Primary Applications	Interface characterization, corrosion studies, device performance	Qualitative and quantitative analysis, mechanistic studies
Data Interpretation	Equivalent circuit modeling	Peak position, shape, and current analysis

The complementary nature of EIS and voltammetry makes them particularly powerful when used together. While voltammetry excels at identifying redox activity and decomposition potentials, EIS provides deeper insight into the resistive and capacitive processes occurring at these potentials. For electrochemical window determination, LSV can rapidly identify decomposition onset potentials, while EIS can monitor the evolution of interfacial resistance as the potential approaches these limits.

Experimental Protocols and Methodologies

Standard EIS Measurement Protocol

EIS measurements require careful experimental design to ensure valid data interpretation. The following protocol outlines a standard approach for characterizing ionic liquid electrolytes:

• Cell Assembly and Stabilization: Assemble a symmetric or asymmetric cell with the ionic liquid electrolyte of interest. For electrochemical window determination, a three-electrode configuration with appropriate reference electrode is essential. Allow the cell to stabilize at the measurement temperature until the open circuit potential remains stable [29].

• Initial Potential Measurement: Record the open circuit potential (OCP) to ensure the system is at steady state before impedance measurements.

• Frequency Scan Configuration: Configure the frequency range, typically from 100 kHz to 10 mHz, with an AC amplitude of 5-10 mV to maintain linearity while ensuring sufficient signal-to-noise ratio [28].

• Data Acquisition: Perform the frequency sweep, collecting impedance data at each frequency point. Multiple measurements at each frequency can improve signal quality through averaging.

• Data Validation: Apply Kramers-Kronig relations or other validation methods to ensure data quality and linearity, stability, and causality of the response.

• Equivalent Circuit Modeling: Select an appropriate equivalent circuit based on the physical characteristics of the system and fit it to the experimental data using specialized software (e.g., ZView, NOVA) [28].

For temperature-dependent studies of ionic liquid electrolytes, this protocol should be repeated at various temperatures to investigate the thermal stability of the electrochemical window and the evolution of resistive components.

Voltammetric Techniques for Electrochemical Window Determination

Determining the electrochemical stability window of ionic liquid electrolytes typically employs Linear Sweep Voltammetry (LSV) or Cyclic Voltammetry (CV) with the following protocol:

• Electrode Preparation: Polish working electrodes (e.g., glassy carbon, platinum) to a mirror finish using alumina slurry, followed by thorough rinsing with appropriate solvents. For ionic liquids with strict anhydrous requirements, implement rigorous drying procedures.

• Cell Assembly: Assemble an airtight electrochemical cell in a glove box when working with oxygen- or moisture-sensitive ionic liquids. Use a three-electrode configuration with a suitable reference electrode (e.g., Ag/Ag⁺ for non-aqueous systems).

• Potential Window Exploration: Perform an initial wide potential scan (e.g., -3.0 V to +3.0 V vs. reference) at a moderate scan rate (10-50 mV/s) to identify approximate decomposition potentials [12].

• Definition of Electrochemical Window: Determine the anodic and cathodic limits where the current exceeds a predetermined threshold (typically 0.1-0.5 mA/cm²). Multiple cycles may be necessary to distinguish reversible processes from irreversible decomposition.

• Validation with Different Electrodes: Confirm the electrochemical stability window using different working electrode materials to exclude electrode-specific reactions.

For more sensitive detection of minor redox events or trace impurities that could affect electrolyte stability, Square-Wave Voltammetry (SWV) or Differential Pulse Voltammetry (DPV) may be employed, as these techniques effectively suppress charging currents [27].

Advanced and Hybrid Approaches

Recent methodological advances include Multi-Frequency Electrochemical Faradaic Spectroscopy (MEFS), which combines aspects of SWV and EIS by applying square-wave potential cycles with progressively increasing frequency [30]. This approach enables rapid kinetic characterization with a minimal set of experiments, as the system is interrogated with a range of SW frequencies in a single experiment [30].

Additionally, machine learning approaches are increasingly applied to EIS data analysis, automating the process of equivalent circuit classification and enabling rapid, systematic analysis of complex impedance spectra [28]. These approaches can identify subtle patterns in impedance data that might be overlooked in traditional analysis, potentially revealing early indicators of electrolyte instability before obvious decomposition occurs.

Data Interpretation and Analysis

Interpreting EIS Data for Ionic Liquid Electrolytes

EIS data for ionic liquid electrolytes typically features several characteristic regions in the Nyquist plot. The high-frequency intercept with the real axis represents the ohmic resistance (RΩ), which includes contributions from electrolyte ionic resistance, electrode resistance, and contact resistances [26]. For ionic liquid electrolytes, this parameter is particularly temperature-sensitive due to viscosity changes [12].

Semicircular features in the mid-frequency range often correspond to charge transfer resistance (Rct) in parallel with double layer capacitance (Cdl) [26]. The diameter of the semicircle represents the charge transfer resistance, which may increase as potentials approach the electrochemical window limits due to passivation layer formation.

Low-frequency behavior typically manifests as a Warburg element in diffusion-controlled systems or as a nearly vertical line in capacitor-like systems. The complex behavior of ionic liquids, with their organized interfacial structures, often requires more sophisticated modeling approaches beyond simple equivalent circuits.

Table 2: Key EIS Parameters for Electrolyte Characterization

Parameter	Physical Significance	Information for Ionic Liquid Electrolytes
RΩ (Ohmic Resistance)	Bulk ionic resistance	Ionic conductivity, temperature dependence
Rct (Charge Transfer Resistance)	Kinetics of interfacial charge transfer	Electrode-electrolyte compatibility, passivation effects
Cdl (Double Layer Capacitance)	Interfacial charge storage	Electrode-electrolyte interface structure
W (Warburg Element)	Diffusional limitations	Ion transport properties in concentrated systems
CPE (Constant Phase Element)	Non-ideal capacitive behavior	Electrode surface heterogeneity, roughness

Analyzing Voltammetric Data for Window Determination

In voltammetric determination of electrochemical windows, the onset of significant current increase marks the decomposition potential. For capacitive systems, the background current should remain relatively constant until decomposition begins. The electrochemical window is typically defined as the potential range between the anodic and cathodic decomposition onsets.

For Square-Wave Voltammetry analysis, the net peak current follows a sigmoidal dependence on the dimensionless electrode kinetic parameter κ (κ = ks/√Df), where ks is the standard rate constant, D is the diffusion coefficient, and f is the frequency [30]. This relationship enables quantitative kinetic characterization alongside window determination.

When analyzing voltammetric data for ionic liquids, it is crucial to distinguish between Faradaic processes (electrolyte decomposition) and capacitive charging currents. Multiple scan rates can help differentiate these processes, as capacitive currents scale linearly with scan rate, while Faradaic currents scale with the square root of scan rate for diffusion-controlled processes.

Applications in Ionic Liquid Electrolyte Research

Characterizing Electrochemical Stability Windows

The primary application of these techniques in ionic liquid electrolyte research is determining electrochemical stability windows, a critical parameter for energy storage applications. Research on potassium battery electrolytes demonstrates how ionic liquids like Pyr12O1FSI and Pyr12O1TFSI, combined with appropriate potassium salts (KFSI, KTFSI), can enhance ionic conductivity and electrochemical stability [31]. Voltammetry directly measures the decomposition onset potentials, while EIS monitors interfacial stability during potential holding experiments.

For supercapacitor applications, the electrochemical stability window defines the maximum operational voltage, which directly impacts energy density (E = ½CV²) [12]. EIS helps characterize the equivalent series resistance (ESR), which governs power density, while voltammetry ensures the electrolyte remains stable across the intended operating voltage range.

Interface and Interphase Investigation

EIS is particularly valuable for studying solid electrolyte interphase (SEI) formation in ionic liquid electrolytes. Research on potassium batteries has employed EIS alongside XPS and MAS-ssNMR to investigate the SEI formed on potassium metal surfaces [31]. The evolution of charge transfer resistance provides insights into interphase stability and composition, crucial for long-term cycling performance.

For supercapacitor applications, EIS characterizes the electrode-electrolyte interface, where ionic liquids often form extended double layer structures due to their nanoscale ordering. The frequency response reveals information about ion accessibility in porous electrodes, a critical factor in rate performance, especially at low temperatures where viscosity increases significantly [12].

Temperature-Dependent Performance

Both EIS and voltammetry provide valuable insights into the temperature dependence of ionic liquid electrolyte performance. EIS tracks the evolution of resistive components with temperature, while voltammetry monitors changes in electrochemical window and reaction kinetics.

Research on low-temperature supercapacitors highlights how electrolyte viscosity increases exponentially as temperature decreases (η = η0e^(-Eb/αKBT)), significantly impacting ionic conductivity and device performance [12]. EIS can quantify these changes, guiding the development of optimized ionic liquid formulations for extreme environment operation.

Essential Research Reagents and Materials

Table 3: Key Research Reagents and Materials for Electrochemical Characterization

Reagent/Material	Function/Application	Examples/Specifications
Ionic Liquids	Electrolyte medium	Pyr12O1FSI, Pyr12O1TFSI [31]
Conductive Salts	Ion providers for electrolyte formulation	KFSI, KTFSI for potassium systems [31]
Working Electrodes	Surface for electrochemical reactions	Glassy carbon, platinum, gold electrodes
Reference Electrodes	Stable potential reference	Ag/Ag⁺, Fc/Fc⁺ for non-aqueous systems
Electrode Polishing Materials	Surface preparation	Alumina slurry (0.3, 0.05 µm), diamond paste
Aprotic Solvents	Ionic liquid dilution or cleaning	Acetonitrile, propylene carbonate [12]
Specialized Gases	Atmosphere control	Argon, nitrogen for oxygen-free environments
Equivalent Circuit Modeling Software	EIS data analysis	ZView, NOVA, EC-Lab [28]

Experimental Workflows and Signaling Pathways

The following diagrams illustrate the key experimental workflows and logical relationships for EIS and voltammetry techniques.

Diagram 1: EIS Experimental Workflow

Diagram 2: Voltammetry Technique Classification

Electrochemical Impedance Spectroscopy and Voltammetry offer complementary approaches for characterizing ionic liquid electrolytes. EIS excels at deconvoluting interfacial processes and resistive contributions across frequency domains, while voltammetry provides direct information about redox processes and electrochemical stability limits. For comprehensive characterization of electrochemical windows in ionic liquid electrolytes, researchers should employ both techniques: voltammetry for direct window determination and EIS for investigating interfacial stability and evolution under applied potentials.

The continuing development of hybrid techniques like Multi-Frequency Electrochemical Faradaic Spectroscopy and machine learning-assisted data analysis promises to enhance the efficiency and depth of electrochemical characterization. As ionic liquid electrolytes continue to evolve for advanced energy storage applications, these electrochemical characterization techniques will remain essential tools for understanding and optimizing their performance across diverse operating conditions.

The pursuit of higher performance in electrochemical energy storage systems is fundamentally linked to the electrochemical stability window (ESW) of the electrolyte. The ESW defines the voltage range between which the electrolyte does not undergo decomposition via oxidation at the anode or reduction at the cathode [32]. A wider ESW is a critical enabler for both enhanced energy density and longer cycle life, two parameters that are often in a trade-off relationship. While a wider window allows the use of higher voltage electrodes, increasing the energy density according to the equation E = ½ CV², the intrinsic stability of the electrolyte within this window dictates the long-term degradation kinetics of the cell components [33] [17]. This guide provides an objective comparison of different ionic liquid (IL) electrolytes, framing their performance within the broader thesis that a strategically widened ESW, achieved through specific ion selection and structural design, directly correlates to superior device performance. We summarize key experimental data, detail standard characterization protocols, and provide a toolkit for researchers to navigate this complex field.

Ionic Liquid Electrolytes at a Glance: A Comparative Analysis

Ionic liquids, defined as salts with melting points below 100 °C, are considered "designer solvents" due to the vast tunability of their cations and anions [33] [17]. This tunability directly influences their core properties, including the ESW, viscosity, and ionic conductivity, which in turn dictate the performance of the resulting energy storage device. The following tables offer a comparative overview of common IL components and their reported performance in supercapacitors and lithium-ion batteries.

Table 1: Key Cation and Anion Families in Ionic Liquid Electrolytes and Their Properties

Ion Type	Specific Examples	Key Characteristics	Impact on ESW & Performance
Cations	Imidazolium (e.g., EMI+, BMI+)	Low viscosity, moderate stability	Good conductivity, but cathodic stability can be limited [34] [17].
	Pyrrolidinium (e.g., PYR13+, PYR14+)	High electrochemical stability, higher viscosity	Wider ESW, particularly at the cathode; excellent for high-voltage LIBs [17] [2].
	Ammonium (e.g., N1114+)	Aliphatic structure, reduced ion-ion interactions	Promises high conductivity and stability; e.g., [N1114][NTf2] operates up to 3.6 V [34].
	Phosphonium	High thermal and chemical stability	Emerging option for specialized applications, good tunability [35].
Anions	Bis(trifluoromethanesulfonyl)imide (TFSI-, NTf2-)	High stability, good delocalization of charge	Contributes to a wide ESW (up to 5-6 V) and thermal stability [33] [2].
	Tetrafluoroborate (BF4-)	Smaller size, moderate stability	Good conductivity, but stability window can be narrower than TFSI [34] [17].
	Hexafluorophosphate (PF6-)	Commonly used, but moisture sensitivity	Can form HF upon decomposition, a drawback compared to TFSI [33].

Table 2: Reported Performance Metrics of Energy Storage Devices Using Ionic Liquid Electrolytes

Device Type	Ionic Liquid Electrolyte	Voltage Window	Specific Capacitance / Capacity	Cycle Life Stability	Key Findings
Supercapacitor	EMIMBF4 in graphene-based SC	3.5 V	144.4 F g⁻¹	Information Missing	Achieved high energy density of 60.7 Wh kg⁻¹ [34].
Supercapacitor	[N1114][NTf2] in AC-based SC	Up to 3.6 V	~2000 F g⁻¹ (specific electrode capacitance)	Excellent stability with minimal faradaic reactions	Energy and power densities comparable to LIBs [34].
Lithium-Ion Battery	PYR14TFSI with LiTFSI and PC	> 4.5 V	Stable cycling at 60°C	Good cycling performance at room temp and 60°C	Non-flammable property achieved with 80 wt% IL [17].
Aqueous Li-Ion Battery	21m LiTFSI (Water-in-Salt)	3.0 V (1.9-4.9 V vs Li/Li+)	Information Missing	Allows use of graphite and LiMn2O4	Expanded ESW enables new aqueous battery chemistries [32].

Experimental Protocols for Characterizing ESW and Performance

To ensure the reproducibility and validity of performance comparisons, researchers rely on a set of standardized experimental protocols. The following methodologies are essential for evaluating the ESW of an electrolyte and its correlation to device-level performance.

Electrochemical Stability Window (ESW) Measurement

• Protocol: Linear Sweep Voltammetry (LSV) or Cyclic Voltammetry (CV).
• Setup: A three-electrode system is used, with a Pt working electrode, a Pt counter electrode, and a stable reference electrode (e.g., Ag/AgCl for aqueous systems or Li+/Li for non-aqueous systems) [32] [36].
• Procedure: The potential of the working electrode is scanned at a low, constant rate (e.g., 0.5 mV/s) across a wide voltage range. The current response is monitored.
• Data Analysis: The anodic and cathodic limits of the ESW are typically defined at a preset current density threshold (e.g., 0.1 mA cm⁻² for the anode and 0.05 mA cm⁻² for the cathode), indicating the onset of electrolyte decomposition (oxygen and hydrogen evolution, respectively) [36]. It is crucial to minimize mass transfer effects to obtain unbiased stability limits [32].

Device Fabrication and Cycling

• Supercapacitor Assembly: Symmetric coin cells are commonly fabricated using activated carbon (AC) electrodes on aluminum current collectors, separated by a glass fiber or polymer separator, and filled with the IL electrolyte [34].
• Battery Assembly: For LIBs, half-cells (e.g., LiMn2O4 vs. Li) or full-cells (e.g., LiMn2O4||PTCDI) are assembled in an argon-filled glovebox to prevent moisture contamination [36] [2].
• Performance Testing: Assembled devices undergo galvanostatic charge-discharge (GCD) cycling at specific current rates (C-rates) within the stable voltage window. Cycle life is determined by the number of cycles until a certain percentage of the initial capacity (e.g., 80%) is retained. Electrochemical impedance spectroscopy (EIS) is used to track internal resistance changes over cycling [37].

Correlating ESW with Energy Density and Cycle Life: A Mechanistic Workflow

The relationship between a wide ESW and the resulting device performance is governed by a series of interconnected physical and electrochemical mechanisms. The following diagram and explanation outline this logical pathway.

Wide ESW to Enhanced Energy Density (Orange/Red Path): A fundamentally wider ESW directly allows a device to be charged to a higher operating voltage (V) without causing electrolyte decomposition [32] [2]. Since the energy stored (E) in a device is proportional to the square of its operating voltage (E ∝ V²), even a modest increase in voltage leads to a dramatic boost in energy density [33] [17].

Wide ESW to Extended Cycle Life (Blue/Green Path): The stability window is not merely a voltage limit but a reflection of the electrolyte's kinetic inertia. A wide ESW indicates strong suppression of solvent breakdown and parasitic reactions like hydrogen and oxygen evolution [32] [34]. This suppression, often aided by the formation of a stable solid-electrolyte interphase (SEI) on the electrode surface, directly reduces the rate of electrode degradation and active material loss over hundreds of charge-discharge cycles [37] [2]. This mechanistic link is why ILs with wide ESWs, such as [N1114][NTf2], demonstrate exceptional cycling stability with minimal faradaic reactions [34].

The Scientist's Toolkit: Essential Research Reagents and Materials

To conduct research in this field, several key materials and reagents are essential. The following table lists critical components and their functions in developing and testing IL electrolytes.

Table 3: Essential Research Reagents and Materials for Ionic Liquid Electrolyte Research

Reagent/Material	Function/Explanation	Example from Literature
Lithium Salts (LiTFSI)	Provides Li⁺ ions for conduction in LIBs. LiTFSI is preferred for its stability with ILs and resistance to hydrolysis.	Used in "Water-in-Salt" electrolytes and PYR14TFSI-based LIB electrolytes [32] [17].
IL Solvents (PYR14TFSI, EMIMBF4)	The ionic liquid itself, serving as the primary medium for ion transport. Chosen for wide ESW and non-flammability.	PYR14TFSI offers a cathodic stability >5.5 V; EMIMBF4 is used in high-power supercapacitors [34] [17].
SEI-Forming Additives (e.g., VC, FEC)	Added in small amounts (<5%) to form a stable passivation layer on graphite anodes, preventing continual electrolyte reduction.	Vinylene carbonate (VC) is added to IL-based electrolytes to protect the graphite anode and improve cycle life [17] [2].
Activated Carbon (AC) Electrodes	High-surface-area active material for electric double-layer supercapacitors.	YP80f Kuraray AC is used in symmetric supercapacitor cells to test IL electrolyte performance [34].
High-Voltage Cathodes (e.g., LiNi₀.₅Mn₁.₅O₄)	Electrode materials that operate at high potentials, leveraging the wide ESW of ILs to increase energy density.	LiNi₀.₅Mn₁.₅O₄ is successfully used with water-in-salt and IL electrolytes due to their expanded ESW [32].
Binder (e.g., PVdF)	A polymer used to cohesively bind active electrode particles to a current collector.	Polyvinylidene fluoride (PVdF) is a common binder in electrode slurries for both LIBs and supercapacitors [34] [17].

The direct correlation between a wide electrochemical stability window and the enhancement of both energy density and cycle life is a cornerstone principle in the development of next-generation energy storage devices. As the comparative data and mechanisms outlined in this guide demonstrate, ionic liquid electrolytes, with their tunable structures and inherently stable properties, provide a versatile platform to exploit this correlation. Continued research into novel cation-anion pairs, hybrid electrolyte systems, and their compatibility with advanced electrode materials is essential to fully realize the potential of wide-ESW electrolytes, paving the way for safer, more powerful, and longer-lasting energy storage solutions.

The pursuit of higher energy density in lithium metal batteries (LMBs) necessitates operation at increasingly higher voltages, pushing conventional organic electrolytes beyond their electrochemical stability limits. Ionic liquid electrolytes (ILEs) have emerged as promising candidates due to their intrinsic non-flammability, thermal stability, and wider electrochemical windows compared to organic alternatives [38]. A key parameter for achieving high energy density is the electrochemical stability window (ESW), which directly influences the maximum operating voltage of a cell; a wider window enables the use of high-voltage cathode materials, thereby increasing the overall energy density [12] [39]. However, a significant challenge for conventional ILEs is the trade-off between achieving a wide ESW and maintaining high ionic conductivity, as highly stable ions often exhibit higher viscosity and slower transport kinetics [39].

To overcome these limitations, recent research has focused on multi-anion electrolyte strategies, which synergistically combine the advantageous properties of different anions within a single system. This case study explores the design and performance of dual-anion ILEs, objectively comparing their electrochemical performance against single-anion and conventional electrolytes. We will provide detailed experimental data and methodologies, framing the discussion within the broader thesis of comparing the electrochemical windows of different ionic liquid electrolytes. The subsequent sections will dissect the specific dual-anion formulations, their quantified performance, the mechanisms behind their stability, and the practical experimental protocols for their implementation.

Electrolyte Design & Comparative Performance

Dual-Anion Formulations and Key Characteristics

The dual-anion strategy manifests in two primary forms: liquid dual-anion locally concentrated ionic liquid electrolytes (D-LCILE) and solid-state dual-anion-rich polymer electrolytes. Both approaches aim to create a more favorable solvation structure and a stable electrode-electrolyte interface.

• D-LCILE System: This design utilizes EMI-FSI as the ionic liquid solvent, with LiFSI and LiTFSI as the lithium salts, and bis(2,2,2-trifluoroethyl) ether (BTFE) as a diluent [24]. The combination of FSI⁻ and TFSI⁻ anions is crucial. The FSI⁻ anion facilitates the formation of a LiF-rich Solid Electrolyte Interphase (SEI) on the lithium metal anode, which is critical for suppressing dendrite growth. The larger TFSI⁻ anion, known for its chemical stability, helps reduce overall electrolyte viscosity and enhances lithium-ion mobility. The BTFE diluent further mitigates viscosity issues and improves wettability on electrode and separator surfaces without disrupting the locally concentrated ion environment [40] [24].

• Dual-Anion-Rich Polymer Electrolyte: This solid-state approach incorporates a poly(vinylidene fluoride) (PVDF)-based polymer matrix, dual lithium salts (e.g., LiTFSI), and functional ferroelectric barium titanate (BTO) nanoparticles [41]. The BTO nanoparticles enhance the local built-in electric field within the electrolyte, which promotes the dissolution and dissociation of the lithium salts. This process leads to a dual-anion-rich solvation structure with an enhanced steric effect, significantly improving Li⁺ transport kinetics and electrochemical stability, making it suitable for high-voltage cathodes [41].

Quantitative Performance Comparison

The following tables summarize key performance metrics for dual-anion electrolytes against single-anion and conventional benchmarks.

Table 1: Comparative Physicochemical Properties of Electrolytes

Electrolyte Type	Ionic Conductivity (mS cm⁻¹)	Viscosity (mPa·s)	Li⁺ Transference Number	Electrochemical Window (V)
D-LCILE (LiFSI/LiTFSI)	Details not provided	Details not provided	Details not provided	Stable with Li metal & high-voltage NCM811 [24]
Dual-Anion-Rich Polymer	0.41 [41]	-	0.70 [41]	Stable up to 4.4 V vs. Li/Li⁺ [41]
Conventional ILE ([Pyr₁,₄][Tf₂N])	2.7 [39]	78.0 [39]	-	3.7 [39]
1 M TEABF₄ in PC (Organic)	12.2 [39]	3.72 [39]	-	3.0 [39]

Table 2: Full-Cell Electrochemical Performance Metrics

Electrolyte System	Cell Configuration	Capacity Retention	Cycle Life	Coulombic Efficiency	Reference
D-LCILE	Li‖LiFePO₄ (20 μm Li)	>99.93%	200 cycles at 1C	>99.90% (avg.)	[40] [24]
D-LCILE	Li‖Li Symmetric	Stable Li plating/stripping	>5000 hours	-	[24]
Dual-Anion-Rich Polymer	Li‖LiFePO₄	108.3 mAh g⁻¹	1000 cycles at 2C	-	[41]
Dual-Anion-Rich Polymer	Li‖NCM811	High retention	300 cycles at 1C (4.4 V)	-	[41]
ILCSPE with LiDFOB	Li‖NCM811	>80%	100 cycles at 0.1C (4.5 V)	-	[42]

Experimental Protocols & Methodologies

Electrolyte Preparation and Cell Assembly

Protocol 1: Synthesis of Dual-Anion Locally Concentrated Ionic Liquid Electrolyte (D-LCILE)

• Materials Preparation: The ionic liquid 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI) is used as the primary solvent. Lithium salts LiFSI and LiTFSI are dried under vacuum at elevated temperatures to remove moisture. The hydrofluoroether bis(2,2,2-trifluoroethyl) ether (BTFE) is used as the diluent [24].
• Mixing Procedure: All procedures are conducted in an argon-filled glovebox (H₂O, O₂ < 0.1 ppm). The EMI-FSI ionic liquid is first placed in a vial. The LiFSI and LiTFSI salts are added in predetermined molar ratios (e.g., 0.8:0.2 mol fraction) and stirred magnetically at 50°C until a homogeneous, clear solution is obtained. Subsequently, the BTFE diluent is added dropwise with continuous stirring to achieve the final D-LCIE formulation [24].
• Cell Assembly: For symmetric cell tests, Li chips are used as both working and counter electrodes. For full-cell tests, LiFePO₄ (LFP) or LiNi₀.₈Co₀.₁Mn₀.₁O₂ (NCM811) cathodes are paired with a thin lithium metal anode (20-30 μm). A Celgard separator is typically used, and the assembled coin cells (CR2032) are filled with the prepared D-LCILE [40] [24].

Protocol 2: Preparation of Dual-Anion-Rich Solid Polymer Electrolyte

• Solution Casting: A predetermined amount of PVDF-HFP copolymer is dissolved in butanone by magnetic stirring at 50°C for 30 minutes to form a clear polymer solution [42].
• Slurry Formulation: In an inert atmosphere, LiTFSI salt and ferroelectric BTO nanoparticles are added to the polymer solution. The mixture is stirred vigorously for several hours to ensure a homogeneous dispersion [41].
• Membrane Formation: The resulting slurry is cast onto a glass plate or PTFE mold using a doctor blade to control thickness. The membrane is then dried under vacuum at elevated temperatures (e.g., 60-80°C) for 24 hours to completely remove the solvent, resulting in a free-standing, flexible solid polymer electrolyte film [41] [42].

Key Characterization Techniques

• Electrochemical Impedance Spectroscopy (EIS): Used to measure ionic conductivity and interfacial resistance. Cells are tested over a frequency range (e.g., 1 MHz to 0.1 Hz) at ambient temperature. Ionic conductivity (σ) is calculated from the bulk resistance (R₆), using the formula σ = L / (R₆ × A), where L is the thickness and A is the contact area of the electrolyte [12] [24].
• Linear Sweep Voltammetry (LSV): Employed to determine the electrochemical stability window. A working electrode (e.g., stainless steel) and lithium metal counter/reference electrodes are used. The potential is swept from the open-circuit voltage to a higher potential (e.g., 6 V vs. Li/Li⁺) at a slow scan rate (e.g., 0.1 mV s⁻¹). The anodic limit is identified as the potential where the current density sharply increases due to electrolyte oxidation [39].
• X-ray Photoelectron Spectroscopy (XPS) Depth Profiling: This technique is critical for analyzing the composition and distribution of elements within the SEI and CEI. Sputtering with an Ar⁺ ion beam is used to etch the surface layer by layer, revealing the chemical composition (e.g., LiF, LixBOyFz, C-O, C-F bonds) as a function of depth, confirming the role of dual anions in forming a stable, inorganic-rich interface [24] [42].
• Scanning Electron Microscopy (SEM): Used to examine the morphology of lithium metal deposits after cycling. Samples are retrieved from disassembled cells, washed with a pure solvent to remove residual salts, and dried. SEM images reveal whether the lithium deposition is dense and dendrite-free or mossy and dendritic [40] [24].

Mechanisms of Stability and Performance Enhancement

The superior performance of dual-anion electrolytes, particularly at high voltages, is attributed to synergistic interfacial stabilization and enhanced ion transport.

Anion Synergy at the Electrode-Electrolyte Interface

The core mechanism involves the synergistic decomposition of the two different anions (e.g., FSI⁻ and TFSI⁻, or TFSI⁻ and DFOB⁻) during the initial charging cycles, leading to the in-situ formation of a robust, protective interphase on both electrodes.

• Cathode Electrolyte Interphase (CEI): In high-voltage systems using NCM811 cathodes, the cooperative decomposition of TFSI⁻ and DFOB⁻ anions forms a stable, fluorine-rich CEI layer containing components like LiF and LixBOyFz [42]. This CEI effectively shields the cathode surface from direct contact with the electrolyte, thereby suppressing parasitic reactions, transition metal dissolution, and particle cracking that would otherwise occur at voltages above 4.3 V [42].
• Solid Electrolyte Interphase (SEI): On the lithium metal anode, the FSI⁻ anion is particularly prone to reduction, leading to the incorporation of LiF into the SEI [24]. LiF is known for its high mechanical modulus and ionic conductivity, which promotes uniform lithium ion flux and inhibits dendrite penetration, resulting in a denser, more homogeneous lithium deposition [40] [24].

Solvation Structure and Ion Transport

The presence of two anions with different sizes and chemical properties (e.g., the smaller FSI⁻ and the larger TFSI⁻) disrupts the uniform ionic structures found in single-anion systems. Molecular dynamics (MD) simulations and Raman spectroscopy confirm that this dual-anion design reduces the formation of tight contact ion pairs (CIPs) and increases the proportion of aggregates (AGGs), leading to a more flexible and fluid solvation shell around the Li⁺ ion [24]. This restructuring reduces the energy barrier for Li⁺ desolvation and enhances ionic mobility, directly translating to improved rate capability [41] [24]. In polymer electrolytes, the addition of ferroelectric BTO nanoparticles enhances the local electric field, further facilitating salt dissociation and stabilizing the dual-anion-rich solvation structure [41].

Diagram 1: Mechanism of Interphase Stabilization via Dual-Anion Synergy. The diagram illustrates how different anions preferentially decompose at the electrodes to form stable SEI and CEI layers.

The Scientist's Toolkit: Essential Research Reagents & Materials

Table 3: Key Research Reagents for Dual-Anion Electrolyte Development

Reagent/Material	Function/Role	Example Use Case
LiFSI Salt	Primary lithium salt; contributes to LiF formation in the SEI, enhancing anode stability.	D-LCILE formulation for stable lithium metal cycling [24].
LiTFSI Salt	Co-salt; provides electrochemical stability and helps modulate electrolyte viscosity.	Used in both liquid D-LCILE and solid polymer electrolytes [24] [42].
LiDFOB Salt	Additive salt; undergoes synergistic decomposition with TFSI⁻ to form a stable, protective CEI.	Added in trace amounts to composite cathodes for high-voltage NCM811 [42].
EMIM-FSI Ionic Liquid	Ionic liquid solvent; provides intrinsic safety (non-flammability) and serves as an ion source.	Solvent base for constructing locally concentrated electrolytes [24].
BTFE Diluent	Hydrofluoroether diluent; reduces viscosity and improves wettability without breaking the Li⁺ solvation sheath.	Key component in LCILE systems to enhance ion transport [24].
PVDF-HFP Copolymer	Polymer matrix; provides mechanical integrity for solid-state electrolytes and dissolves lithium salts.	Base polymer for solid polymer and composite electrolytes [41] [42].
BTO Nanoparticles	Ferroelectric filler; enhances the local electric field, promoting salt dissociation and Li⁺ transport.	Additive in PVDF-based SPEs to create a dual-anion-rich solvation structure [41].

This case study demonstrates that dual-anion ionic liquid electrolytes represent a significant advancement over conventional single-anion systems. By strategically combining anions like FSI⁻ and TFSI⁻, researchers can engineer electrolytes that simultaneously achieve high ionic conductivity, superior interfacial stability, and a wide electrochemical window. The data confirms that these systems enable stable cycling of high-energy lithium metal batteries with demanding high-voltage cathodes like NCM811, overcoming critical challenges related to cathode degradation and lithium dendrite growth [41] [24] [42]. The experimental protocols and mechanistic insights provided offer a roadmap for further research and development in this promising field, contributing valuable knowledge to the ongoing thesis of optimizing ionic liquid electrolytes for next-generation energy storage.

Overcoming Limitations: Strategies for Enhancing Ionic Conductivity and Interfacial Stability

Addressing the Viscosity-Conductivity Trade-off in Neat Ionic Liquids

Ionic liquids (ILs), salts in the liquid state at low temperatures, are renowned for their unique properties, including negligible vapor pressure, high thermal stability, and wide electrochemical windows. These characteristics make them premier candidates for applications in energy storage, electrocatalysis, and pharmaceutical sciences [43] [44]. However, a fundamental challenge persists in their practical application: the inherent trade-off between viscosity and ionic conductivity.

High viscosity, often a result of strong electrostatic interactions and hydrogen bonding between ions, typically impedes ion mobility, thereby reducing conductivity [45] [46]. This review objectively compares the performance of different IL systems and the strategies employed to overcome this trade-off, providing researchers with a clear guide based on recent experimental data.

Fundamental Relationships and the Decoupling Phenomenon

The inverse relationship between viscosity and conductivity is a well-established principle in IL chemistry. High viscosity increases resistance to ion flow, thereby limiting conductivity. However, research has revealed that this coupling is not absolute and can be decoupled under specific structural conditions.

Simulations of LiF in BeF₂ mixtures have demonstrated that the conductivity-viscosity relationship changes with composition. In concentrated mixtures, a phenomenon of strongly decoupled Li⁺ migration through a viscous network was observed. This decoupling was associated with the appearance of migration channels in the network structure, leading to cooperative effects in cation migration that facilitate ion transport even in a viscous medium [47]. This breakthrough in understanding provides a theoretical foundation for designing ILs with decoupled properties.

Comparative Performance of Ionic Liquid Systems

The following tables summarize key physicochemical data for various ILs, highlighting the interplay between their structure, viscosity, and conductivity.

Table 1: Physicochemical Properties of Dicyanamide (DCA) Ionic Liquids at 293.15 K - 333.15 K [48]

Ionic Liquid	Cation Type	Viscosity (mPa·s)	Conductivity (mS·cm⁻¹)	Electrochemical Window (V)
[C₄mim][N(CN)₂]	Imidazolium	~19 (at 298 K)	~40 (at 298 K)	~4.0
[C₄m2im][N(CN)₂]	Methylated Imidazolium	~25 (at 298 K)	~28 (at 298 K)	Data not specified
N₄₄₄₂[N(CN)₂]	Ammonium	Lower than imidazolium	Higher than imidazolium	Data not specified
N₈₄₄₄[N(CN)₂]	Ammonium	Higher than N₄₄₄₂	Lower than N₄₄₄₂	Data not specified

Table 2: Properties of Hydroxyl-Functionalized Imidazolium Ionic Liquids (303.15 K) [46]

Ionic Liquid	Anion	Density (g·cm⁻³)	Viscosity (mPa·s)	Conductivity (mS·cm⁻¹)
[HPmim][FA]	Formate	~1.23	~120	~2.5
[HPmim][AC]	Acetate	~1.20	~150	~1.8
[HPmim][Pro]	Propanoate	~1.17	~220	~1.1
[HPmim][Gly]	Glycolate	Data not specified	Data not specified	Data not specified
[HPmim][Ala]	Alaninate	Data not specified	Data not specified	Data not specified

Key Insights from Comparative Data

• Cation Structure Influence: Table 1 shows that ammonium-based DCA ILs (e.g., N₄₄₄₂) generally exhibit lower viscosity and higher conductivity than imidazolium-based ones (e.g., [C₄mim]) due to weaker cation-anion interactions [48]. Methylating the imidazolium ring ([C₄m2im]) increases viscosity and reduces conductivity compared to its non-methylated counterpart.
• Anion Structure and Water Content: Table 2 demonstrates that for a series of ILs with identical cations, the anion significantly impacts properties. Smaller anions like formate yield lower viscosity and higher conductivity than bulkier ones like propanoate [46]. The presence of water can dramatically reduce viscosity and increase conductivity, but this may come at the cost of a narrowed electrochemical window, as noted in imidazolium triflate systems [49].
• The Role of Functional Groups: The hydroxyl-functionalized ILs in Table 2 possess higher viscosity due to enhanced hydrogen bonding, which negatively impacts conductivity [46].

Experimental Protocols for Key Measurements

To ensure reproducibility and provide a clear toolkit for researchers, this section outlines standard experimental methodologies for obtaining the critical data presented in this guide.

• Cation Halide Synthesis: The precursor cation halide (e.g., [C₄mim]Br) is first synthesized via a Menschutkin reaction, involving the alkylation of the corresponding amine or imidazole with an alkyl halide.
• Metathesis Reaction: The cation halide (e.g., 20 mmol) is dissolved in distilled water (50 mL). A slight molar excess of silver dicyanamide (Ag[N(CN)₂], 21 mmol) is added.
• Purification: The reaction mixture is stirred overnight in the dark at room temperature. The precipitated AgBr byproduct is removed by filtration.
• Solvent Removal: The aqueous filtrate is dried under vacuum at elevated temperature (e.g., 373 K) to yield the pure DCA ionic liquid as a colorless or light yellow liquid.
• Characterization: The product is verified using ¹H NMR, ¹³C NMR, IR spectroscopy, and elemental analysis.

• Sample Preparation: IL samples are dried under vacuum to minimize the influence of water. The water content is typically verified by Karl Fischer titration.
• Density and Viscosity: Density (ρ) is measured using a vibrating-tube densimeter. Dynamic viscosity (η) is determined with a rotational viscometer equipped with a concentric cylinder system. Measurements are performed isothermally across a defined temperature range (e.g., 293.15 K to 333.15 K).
• Conductivity: Molar conductivity (Λ) is measured using a conductivity meter with a platinum electrode. The cell constant is determined using a standard KCl solution.
• Data Analysis: The temperature dependence of viscosity and conductivity is often modeled with the Arrhenius equation or the Vogel–Fulcher–Tammann equation. The Walden plot (log Λ vs. log (1/η)) is frequently used to analyze ion mobility and dissociation.

Advanced Strategies for Breaking the Trade-off

Biphasic Solvent Systems

A significant innovation involves creating IL-based biphasic solvents for CO₂ capture, which effectively overcomes the viscosity-loading trade-off. A novel IL, [DETA][3HPyr], was combined with diethylene glycol butyl ether (DGME) and water [45]. Upon CO₂ absorption, the system separates into a CO₂-rich phase and a CO₂-lean phase. Only the rich phase requires regeneration, drastically reducing energy consumption. The strategic composition with DGME and water abated the viscosity of the rich phase while maintaining a high absorption capacity, thereby breaking the trade-off effect [45].

Machine Learning-Guided Formulation

The vast combinatorial space of IL formulations is being navigated with machine learning (ML). A chemical foundation model (SMI-TED-IC) was fine-tuned on a dataset of 13,666 electrolyte formulations to predict ionic conductivity [50]. This model successfully designed novel electrolyte formulations, improving the conductivity of LiDFOB-based electrolytes by 172% over baseline formulations. This ML-guided workflow represents a paradigm shift from trial-and-error to a predictive approach for designing high-performance IL mixtures [50].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagent Solutions for Ionic Liquid Research

Reagent/Material	Function/Application	Key Characteristic
Dicyanamide Anion ([N(CN)₂]⁻)	Anion for low-viscosity, high-conductivity ILs [48]	Halogen-free, low viscous, good ligand ability
Imidazolium Cations (e.g., [C₄mim]⁺)	Versatile, tunable cations for IL synthesis [48]	Strong cation-anion interaction, widely studied
Ammonium Cations (e.g., N₄₄₄₂⁺)	Cations for weaker ion pairing [48]	Often yield lower viscosity than imidazolium
Hydroxyl-Functionalized ILs	Introducing hydrogen bonding for task-specific design [46]	High viscosity, tunable solubility
Diethylene Glycol Butyl Ether (DGME)	Phase-change accelerator in biphasic systems [45]	Regulates proton transfer, helps control viscosity
Silver Dicyanamide (Ag[N(CN)₂])	Reagent for metathesis synthesis of DCA ILs [48]	Used in anion exchange to create pure DCA ILs

Conceptual Workflow and Relationship Diagrams

The following diagram illustrates the strategic approaches and their logical relationships for addressing the viscosity-conductivity trade-off, as discussed in this guide.

The pursuit of ionic liquids that simultaneously offer high ionic conductivity and practical viscosity is a central challenge in materials science. As this comparison guide demonstrates, no single IL is superior in all aspects; the choice depends on the application's specific priorities.

For electrochemical applications requiring a wide potential window and good conductivity, dicyanamide-based ILs, particularly those with ammonium cations, present a strong option [48]. When task-specific functionality like hydrogen bonding is needed, hydroxyl-functionalized ILs are valuable, despite their higher viscosity [46]. The most promising strategies for fundamentally breaking the trade-off involve innovative formulation approaches, such as biphasic systems [45] and machine learning-guided design [50], which move beyond simple cation-anion pairing to create optimized multi-component systems. These advanced approaches enable the precise control of intermolecular interactions and ion transport pathways, paving the way for the next generation of high-performance ionic liquids.

Ionic liquids (ILs) have emerged as a promising class of electrolytes for advanced energy storage systems due to their remarkable properties, including non-flammability, negligible vapor pressure, and wide electrochemical stability windows. These characteristics position ILs as superior alternatives to conventional organic electrolytes in lithium metal batteries (LMBs), which demand enhanced safety and stability. The engineering of IL-based electrolytes has focused on overcoming inherent limitations such as high viscosity and sluggish ion transport through strategic molecular design. This comparison guide objectively evaluates three primary engineering strategies—functionalization, co-solvents, and multi-anion systems—based on recent experimental findings, with particular emphasis on their electrochemical stability and performance in battery applications.

Comparative Analysis of Engineering Strategies

The table below summarizes the key performance metrics of ionic liquid electrolytes employing different engineering strategies, based on recent experimental studies.

Table 1: Performance Comparison of Ionic Liquid Electrolyte Engineering Strategies

Engineering Strategy	Specific System	Electrochemical Window / Stability	Key Performance Findings	Capacity Retention / Cycling Performance	Ionic Conductivity / Viscosity
Ether Functionalization	Py13FSI with FDG cosolvent	Not explicitly quantified in data, but demonstrated stable operation with NMC811 cathode	Significantly reduced overpotential for Li intercalation; formation of favorable cathode interphase [51]	Enhanced cycle life in Li||NMC811 cells [51]	Viscosity: 54 cP at 20°C (reduced from 77 cP for neat Py13FSI) [51]
Co-solvent Addition	Py13FSI with TTE cosolvent	Not explicitly quantified in data	Failed to form stable cathode interphase; poor lithium transport [51]	Inferior cycling performance compared to FDG formulation [51]	Viscosity: 45 cP at 20°C [51]
Multi-Anion System	D-LCILE (TFSI− and FSI−) with BTFE diluent	Enhanced interfacial stability; wider operational window implied	LiF-rich SEI formation; homogeneous lithium deposition [24]	>99.93% after 200 cycles in Li||LFP full cell at 1C [24]	Significantly improved ionic conductivity and reduced viscosity versus single-anion systems [24]

Detailed Experimental Protocols

Co-solvent Strategy Evaluation

The investigation of co-solvents with distinct solvation capabilities followed a systematic experimental protocol to evaluate their impact on IL-based electrolyte performance [51].

Electrolyte Formulation: Researchers dissolved 1.4 m (molality) LiFSI in a liquid phase comprising 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Py13FSI) and selected co-solvents in a volumetric ratio of 4:1. Three co-solvents with distinct solvating capabilities were evaluated: 1,1,1-trifluoro-2-(2-(2-(2,2,2-trifluoroethoxy)ethoxy)ethoxy)ethane (FDG), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE), and diglyme (DG) [51].

Physicochemical Characterization: Viscosity measurements were performed using a rheometer at controlled temperature (20°C). FTIR and Raman spectroscopy were employed to analyze solvation structures, focusing on changes in peak positions and intensities that indicate different coordination environments around Li+ ions [51].

Electrochemical Testing: Li\|\|NMC811 (LiNi0.8Mn0.1Co0.1O2) cells were fabricated using the different electrolyte formulations. Cycling tests were conducted under standardized conditions to evaluate capacity retention and overpotential development. Post-cycling electrode surfaces were examined using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to characterize interphase composition and morphology [51].

Computational Studies: Molecular dynamics (MD) simulations were performed to understand the solvation structures and coordination environments influenced by the different co-solvents at the molecular level [51].

Dual-Anion System Implementation

The development and evaluation of dual-anion ionic liquid electrolytes followed a comprehensive methodology to assess their enhanced properties [24].

Electrolyte Design and Preparation: Dual-anion locally concentrated ionic liquid electrolytes (D-LCILE) were formulated using EMIFSI as solvent, incorporating both LiFSI and LiTFSI salts with bis(2,2,2-trifluoroethyl) ether (BTFE) as diluent. Molar fraction compositions were carefully controlled as detailed in supplementary materials of the original study [24].

Solvation Structure Analysis: Raman spectroscopy was utilized to examine solvation dynamics, specifically differentiating free anions, contact ion pairs (CIPs), and aggregates (AGGs) by analyzing the S–N–S stretching peaks between 700 and 750 cm−1, characteristic of both FSI− and TFSI− anions [24].

Interface Characterization: X-ray photoelectron spectroscopy (XPS) depth profiling was conducted to determine the composition and distribution of elements within the solid electrolyte interphase (SEI). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provided additional molecular-level information about the interphase structure [24].

Electrochemical Performance Testing: Symmetric Li\|\|Li cells were assembled to evaluate lithium plating/stripping behavior and interface stability. Full cells incorporating LFP cathodes and thin lithium metal anodes were cycled at 1C rate to assess long-term performance, coulombic efficiency, and capacity retention [24].

Computational Analysis: Density functional theory (DFT) calculations and molecular dynamics (MD) simulations were employed to investigate the solvation shell structure and ionic transport mechanisms in the dual-anion system [24].

Strategic Implementation Workflows

The following diagram illustrates the logical decision pathway for selecting and implementing appropriate ionic liquid electrolyte engineering strategies based on performance objectives and application requirements.

Diagram 1: Strategic selection pathway for ionic liquid electrolyte engineering approaches

Experimental Workflow for Electrolyte Development

The diagram below outlines a generalized experimental workflow for developing and characterizing advanced ionic liquid electrolytes, incorporating the key methodologies discussed in the research.

Diagram 2: Comprehensive experimental workflow for IL electrolyte development

The Scientist's Toolkit: Essential Research Reagents and Materials

The table below catalogues key reagents, materials, and their functions employed in the development and testing of advanced ionic liquid electrolytes, as referenced in the examined studies.

Table 2: Essential Research Reagents and Materials for Ionic Liquid Electrolyte Studies

Category	Specific Reagent/Material	Function in Research	Application Context
Ionic Liquids	1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Py13FSI)	Primary IL solvent; provides ionic conductivity and electrochemical stability [51]	Base component for co-solvent studies in LMBs [51]
Co-solvents	1,1,1-trifluoro-2-(2-(2-(2,2,2-trifluoroethoxy)ethoxy)ethoxy)ethane (FDG)	Ether-functionalized fluorinated co-solvent; moderate solvating capability reduces viscosity while maintaining interface stability [51]	Optimization of solvation structure in Py13FSI-based electrolytes [51]
Co-solvents	1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE)	Non-solvating co-solvent; significantly reduces viscosity but does not participate in Li+ coordination [51]	Reference non-solvating additive for comparison studies [51]
Lithium Salts	Lithium bis(fluorosulfonyl)imide (LiFSI)	Lithium salt; provides Li+ ions for conduction; FSI− contributes to LiF-rich SEI formation [51] [24]	Common lithium source in both co-solvent and dual-anion systems [51] [24]
Lithium Salts	Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)	Lithium salt; TFSI− anion offers different coordination behavior and stability versus FSI− [24]	Component in dual-anion systems for optimized solvation and interface properties [24]
Diluents	Bis(2,2,2-trifluoroethyl) ether (BTFE)	Hydrofluoroether diluent; reduces viscosity while maintaining localized high concentration of ions [24]	Formulation of locally concentrated ionic liquid electrolytes (LCILEs) [24]
Electrode Materials	LiNi0.8Mn0.1Co0.1O2 (NMC811)	High-voltage cathode active material; tests electrolyte stability under highly oxidizing conditions [51]	Evaluation of cathode-electrolyte interphase stability [51]
Electrode Materials	Lithium Iron Phosphate (LFP)	Cathode active material with excellent cycling stability; enables assessment of long-term performance [24]	Cycling stability tests in full-cell configurations [24]

The systematic comparison of ionic liquid engineering strategies reveals distinct advantages and optimal application contexts for each approach. Ether-functionalized co-solvents like FDG demonstrate balanced solvation capability, effectively reducing viscosity while promoting stable interface formation. Co-solvent strategies must be carefully optimized, as demonstrated by the superior performance of FDG compared to both strongly-solvating (DG) and non-solvating (TTE) alternatives. The dual-anion system represents a particularly promising approach, leveraging the complementary properties of FSI− and TFSI− anions to achieve exceptional cycling stability exceeding 99.93% capacity retention after 200 cycles, attributed to enhanced ionic conductivity and stabilized electrode-electrolyte interfaces. These engineering strategies collectively address the fundamental challenges in ionic liquid electrolytes, enabling their application in next-generation lithium metal batteries with improved safety and performance characteristics.

Promoting Stable Solid Electrolyte Interphase (SEI) Formation with LiF-Rich Components

The solid electrolyte interphase (SEI) is a critical component formed on the anode surface of lithium-based batteries, acting as a protective layer that prevents continuous electrolyte decomposition while facilitating lithium-ion transport. Among various SEI components, lithium fluoride (LiF) has emerged as a particularly valuable constituent due to its exceptional properties, including high ionic conductivity, significant mechanical strength, and outstanding electrochemical stability. This guide provides a comparative analysis of recent advanced strategies for constructing LiF-rich SEI layers, focusing on their performance metrics, underlying mechanisms, and practical implementation protocols. Within the broader context of comparing electrochemical windows of different ionic liquid electrolytes, we examine how innovative materials and electrolyte engineering approaches can enhance SEI stability and overall battery performance, providing researchers with objective data to inform their experimental designs.

Performance Comparison of LiF-Rich SEI Formation Strategies

The pursuit of stable LiF-rich SEI layers has led to multiple innovative approaches, each with distinct advantages and limitations. The following table summarizes the performance characteristics of four prominent strategies identified in recent literature.

Table 1: Performance Comparison of LiF-Rich SEI Formation Strategies

Strategy	Key Materials/Approach	Reported Performance Metrics	Advantages	Limitations
Molybdenum-based MXenes	Mo₂Ti₂C₃Tx with abundant F-terminations [52]	~99.79% CE over 544 cycles at 3 mA cm⁻²; 70% capacity retention after 100 cycles in NCM622 full-cell [52]	Inherently lithiophilic surface; spontaneous LiF formation; suitable for high-current-density applications	Complex synthesis process; environmental stability concerns
Dual-Anion Ionic Liquid Electrolytes	TFSI⁻ and FSI⁻ anions in locally concentrated IL electrolyte [24]	>99.90% average CE; >99.93% capacity retention after 200 cycles at 1C; 27.62% F-content in SEI [24]	Enhanced ionic conductivity; superior wettability; wide electrochemical window	Requires precise anion ratio optimization; potential viscosity issues
Functional Ionic Liquid Cations	Cation-modified ILs with vinyl or nitrile groups [53]	Improved reduction potential and chemical hardness for targeted SEI formation [53]	"Designer" functionality for controlled polymerization; tunable properties	Complex synthesis; limited commercial availability
Artificial Protective Layers	AlF₃ and PAA composite coating on Si anodes [54]	75% capacity retention after 100 cycles vs. 7.4% for bare Si [54]	Mitigates volume expansion; enhances mechanical stability; compatible with high-capacity anodes	Additional processing step; potential weight/volume penalties

Experimental Protocols for Key Strategies

MXene-Based Electrode Fabrication and Testing

Synthesis of Mo-based MXenes [52]:

• Begin with Mo₂Ga₂C, Mo₂TiAlC₂, or Mo₂Ti₂AlC₃ MAX-phase powders (40µm average particle size)
• Use HF etching solution (48 wt.% in H₂O) to selectively remove Al/Ga layers
• Confirm successful etching by X-ray diffraction (XRD) showing (002) peak shift to lower angles
• For electrode preparation, mix freeze-dried MXene powders with PVDF binder and Super P carbon (8:1:1 mass ratio) in NMP solvent
• Coat slurry on Cu foil (17µm thickness) using doctor blade technique
• Dry in vacuum oven at 80°C for 8 hours to remove residual solvent, achieving ~3µm coating thickness

Electrochemical Testing [52]:

• Assemble 2032-type coin cells in Ar-filled glove box
• For half-cells, use MXene-coated current collector (14mm diameter) with Li foil counter/reference electrode (12mm diameter)
• Employ Celgard 3501 PP separator and 1M LiTFSI in DOL/DME (1:1 v/v) with 5 wt.% LiNO₃ electrolyte
• Conduct cycling tests at 3 mA cm⁻² with 1 mAh cm⁻² capacity
• For full-cells, pair pre-lithiated MXene anode with NCM622 cathode (10 mg cm⁻² loading) using 1M LiPF₆ in EC/DMC electrolyte

Dual-Anion Ionic Liquid Electrolyte Preparation

Electrolyte Formulation [24]:

• Use EMIFSI as base ionic liquid solvent
• Incorporate both LiFSI and LiTFSI salts with BTFE (bis(2,2,2-trifluoroethyl) ether) as diluent
• Employ molar fraction compositions as detailed in original reference (Table S1)
• Characterize solvation structure using Raman spectroscopy (700-750 cm⁻¹ range for S-N-S stretching)
• Identify contact ion pairs (CIPs) and aggregates (AGGs) to verify dual-anion solvation structure

Performance Validation [24]:

• Measure ionic conductivity via electrochemical impedance spectroscopy (EIS) on symmetric cells
• Evaluate interfacial stability through DFT calculations and MD simulations
• Analyze SEI composition using XPS depth profiling to quantify LiF content
• Test rate capability in 300µm Li metal half-cells
• Assess long-term performance in 20µm Li‖LiFePO₄ full cells at 1C rate

Artificial Protective Layer Fabrication

LPL@Si Anode Preparation [54]:

• Mix Si powder, Super P, and 10 wt.% Li-PAA aqueous binder (8:1:1 mass ratio)
• Prepare Li-PAA binder by mixing PAA aqueous solution (Mv ≈ 450,000) with LiOH aqueous solution
• Disperse slurry using planetary mixer (5 min at 2000 rpm)
• Coat on Cu foil current collector, dry at 60°C for 2 hours, and roll-press
• For protective layer, mix AlF₃ and PAA (7:3 mass ratio) in deionized water
• Coat AlF₃-PAA mixture on Si electrode surface, dry at 60°C for 30 minutes
• Final electrode drying at 110°C under vacuum for 4 hours

Mechanisms of LiF-Rich SEI Formation

The formation of a stable LiF-rich SEI follows distinct pathways depending on the approach used. The molecular orbital theory provides the fundamental framework for understanding SEI formation, where electrolyte components undergo reductive decomposition when their lowest unoccupied molecular orbital (LUMO) energy levels align with the anode's electrochemical potential [55].

Diagram: Mechanisms of LiF-Rich SEI Formation

For MXene-based approaches, the abundant surface fluorine terminations on materials like Mo₂Ti₂C₃Tx directly participate in LiF formation during initial cycling [52]. The negatively charged surface groups create a lithiophilic interface that reduces nucleation overpotential and promotes uniform lithium deposition.

In dual-anion ionic liquid systems, the FSI⁻ anion decomposes at higher potentials than conventional carbonate electrolytes, leading to preferential LiF formation before other SEI components [24]. This creates a layered SEI structure with beneficial inorganic components near the electrode surface. The presence of TFSI⁻ contributes to stable solvation shells and further enriches the SEI with fluorine-containing species.

Artificial protective layers utilize compounds like AlF₃ that chemically convert to LiF upon contact with lithium metal or during initial lithiation [54]. The polymer matrix (e.g., PAA) serves as a mechanical framework that accommodates volume changes while maintaining SEI integrity.

Operando X-ray absorption spectroscopy studies have confirmed that LiF formation typically occurs early in the SEI formation process (around 0.6 V vs. Li/Li⁺ in conventional electrolytes), followed by organic component deposition at lower potentials [56]. This sequential formation mechanism naturally creates a layered structure with beneficial inorganic components near the electrode surface.

Essential Research Reagents and Materials

Successful implementation of LiF-rich SEI strategies requires specific materials with carefully considered functions. The following table compiles key reagents referenced in the experimental protocols.

Table 2: Essential Research Reagents for LiF-Rich SEI Studies

Material Category	Specific Examples	Function in SEI Formation	Key Characteristics
MXene Precursors	Mo₂Ga₂C, Mo₂TiAlC₂, Mo₂Ti₂AlC₃ MAX phases [52]	Provide F-terminated 2D surfaces for spontaneous LiF formation	Layer structure with abundant surface termination groups; inherent lithiophilicity
Etching Agents	HF (48 wt.% in H₂O) [52], TMAOH (25 wt.% in H₂O) [52]	Selective removal of Al/Ga layers from MAX phases	Hazardous handling requirements; critical for MXene delamination
Ionic Liquids	EMIFSI [24], LiTFSI, LiFSI salts [24]	Create stable electrolyte matrices with wide electrochemical windows	Low vapor pressure; high thermal stability; tunable properties
Electrolyte Additives	FEC [56], LiNO₃ [52], BTFE diluent [24]	Modify reduction pathways to favor LiF formation	Sacrificial decomposition at specific potentials; concentration-dependent effects
Polymer Binders	PAA [54], Li-PAA [54], PVDF [52]	Provide mechanical stability to accommodate volume changes	Functional groups for cross-linking; compatibility with electrode materials
Fluorine Sources	AlF₃ [54], FEC [56], FSI⁻ anions [24]	Direct contribution to LiF formation through decomposition	Varying reduction potentials determine formation sequence in SEI
Current Collectors	Cu foil (17µm) [52]	Provide conductive substrate for electrode materials	Surface properties influence deposition uniformity; thickness affects energy density

The strategic promotion of LiF-rich SEI layers represents a critical advancement in developing high-performance lithium batteries. Through comparative analysis of multiple approaches, each method demonstrates distinct advantages: MXenes offer inherent functionalization for spontaneous LiF formation, dual-anion ionic liquids provide precisely tunable electrolyte properties, functional IL cations enable controlled polymerization pathways, and artificial protective layers deliver mechanical robustness for volume-changing electrodes. The experimental protocols and reagent information presented herein offer researchers practical guidance for implementing these strategies. As battery technologies evolve toward higher energy densities and improved safety profiles, the controlled formation of LiF-rich SEI layers will remain essential, with the optimal approach depending on specific application requirements including cost constraints, performance targets, and manufacturing considerations.

Guidelines for Selecting Cations and Anions for Maximum ESW and Minimal Viscosity

Ionic liquids (ILs) have emerged as transformative electrolytes for advanced electrochemical energy storage (EES) devices, with their properties primarily determined by cation-anion combinations. This guide provides a structured framework for selecting IL components to achieve two critical objectives: a wide electrochemical stability window (ESW) and low viscosity. The optimal IL formulation balances these often competing properties, enabling the development of safer, high-performance batteries and supercapacitors, particularly for applications requiring high energy density and operational stability across a wide temperature range.

Ionic Liquid Fundamentals and Property Trade-offs

Ionic liquids are molten salts with melting points below 100°C, composed of asymmetric organic cations and organic or inorganic anions [38]. Their modular nature allows for precise tuning of physicochemical properties through strategic cation-anion pairing. For EES applications, two of the most critical properties are the electrochemical stability window (ESW) and viscosity.

The electrochemical stability window (ESW) defines the voltage range within which the electrolyte remains stable without decomposing. A wider ESW is crucial because the energy density of a device is proportional to the square of its operating voltage [20]. The viscosity of an IL directly influences ion transport kinetics; lower viscosity typically results in higher ionic conductivity, better rate capability, and improved wetting of electrode materials [12].

A fundamental trade-off exists between these properties: structural features that often enhance ESW (e.g., robust aromatic systems, strong Coulombic interactions) can increase viscosity by restricting ion mobility. Therefore, selecting IL components requires a balanced approach to optimize both properties simultaneously for specific applications.

Cation and Anion Selection Guidelines

Cation Selection and Optimization

The cation significantly influences the viscosity, conductivity, and cathodic (reduction) stability of the IL. The following table summarizes key cation families and their properties.

Table 1: Comparison of Ionic Liquid Cations for ESW and Viscosity

Cation Family	Example Structures	Impact on ESW	Impact on Viscosity	Key Characteristics
Pyrrolidinium	N‑ethyl‑N‑methylpyrrolidinium ([C₂mpyr]⁺), N‑propyl‑N‑methylpyrrolidinium ([C₃mpyr]⁺)	Wide ESW (~5 V) [57]	Moderate viscosity	High reductive stability; non-aromatic; good thermal stability [58]
Phosphonium	Tributylmethyl phosphonium (P₁₄₄₄⁺), Trimethyl isobutyl phosphonium (P₁₁₁ᵢ₄⁺)	Good electrochemical stability	Can offer high ionic conductivity [58]	Hydrophobic; can improve cycling stability in batteries [58]
Imidazolium	1-ethyl-3-methylimidazolium (EMIM⁺)	Moderate ESW, lower cathodic stability	Generally lower viscosity [59]	Aromatic ring reduces stability against reduction; widely studied [38]
Oxazolidinium	N‑ethyl‑N‑methyloxazolidinium ([C₂moxa]⁺)	Wide ESW	Can exhibit higher ionic conductivity than analogous pyrrolidinium	Ether functionality can enhance ionic conductivities and free volumes [57]

Design Strategies for Cations:

• For Maximum ESW: Prioritize non-aromatic, saturated ring structures like pyrrolidinium or oxazolidinium. These cations resist reduction at negative potentials, contributing to a wider ESW [58] [57].
• For Minimal Viscosity: While imidazolium cations often yield lower viscosities, their narrower ESW is a significant drawback. A superior strategy is to use pyrrolidinium cations with short, linear alkyl chains (e.g., ethyl, propyl) to balance viscosity and ESW [59].
• Advanced Tactic - Cation Mixing: Combining cations in mixtures is a cost-effective method to fine-tune bulk properties and interfacial nanostructuring. For instance, adding a larger phosphonium cation (P₁₄₄₄⁺) to a pyrrolidinium base electrolyte can disrupt interfacial structuring, improve solid electrolyte interphase (SEI) formation kinetics, and enhance cycling stability without drastically increasing viscosity [58].

Anion Selection and Optimization

The anion primarily determines the anodic (oxidation) stability and also plays a critical role in defining the IL's viscosity and its interactions with the cation.

Table 2: Comparison of Ionic Liquid Anions for ESW and Viscosity

Anion Family	Example Structures	Impact on ESW	Impact on Viscosity	Key Characteristics
Fluorosulfonyl Imides	Bis(trifluoromethanesulfonyl)imide ([TFSI]⁻), Bis(fluorosulfonyl)imide ([FSI]⁻)	Wide ESW, excellent stability	Moderate viscosity, high ionic conductivity	Charge delocalization; common in battery applications; [FSI]⁻ may corrode Al [57]
Hückel-type Anions	4,5‑dicyano‑2‑(trifluoromethyl)imidazolide ([TDI]⁻), 4,5‑dicyano‑2‑(pentafluoroethyl)imidazolide ([PDI]⁻)	Wide ESW, high oxidation resistance	Low viscosity, high ionic conductivity	Delocalized charge weakens ion interactions; designed for stability [57]
Halogenated Boron	Tetrafluoroborate ([BF₄]⁻)	Moderate ESW	Moderate viscosity	Common anion; well-studied [60]
Perchlorate	Perchlorate ([ClO₄]⁻)	Functional in SEI formation	Low viscosity (in mixtures)	Often used as an additive [60]

Design Strategies for Anions:

• For Maximum ESW: Select anions with highly delocalized charge, such as * [TFSI]⁻* or * [TDI]⁻*. The delocalization enhances electrochemical stability and weakens Coulombic interactions with the cation, facilitating ion mobility [57].
• For Minimal Viscosity: * [FSI]⁻* and * [TDI]⁻* are excellent choices due to their molecular structure and charge distribution, which lead to lower viscosity ILs [57].
• Advanced Tactic - Anion Mixing: Creating high-entropy solvation structures by mixing multiple anions (e.g., [TFSI]⁻ and [BF₄]⁻) can disrupt ordered solvent structures, significantly widening the operational voltage window in aqueous systems while maintaining good transport properties [61].

Experimental Protocols for Characterization

To validate the properties of selected ILs, researchers employ standardized experimental protocols. The following are key methodologies cited in recent literature.

Electrochemical Stability Window (ESW) Measurement

The ESW is typically determined using linear sweep voltammetry (LSV) or cyclic voltammetry (CV) in a three-electrode cell.

• Cell Setup: A working electrode (e.g., glassy carbon, platinum), a counter electrode (e.g., platinum wire), and a reference electrode (e.g., Ag/Ag⁺) are used [57].
• Procedure: The potential is swept from the open-circuit potential to more positive potentials (for anodic stability) and to more negative potentials (for cathodic stability). The scan rate is typically slow (e.g., 1-10 mV/s) to approximate equilibrium conditions [20].
• Data Analysis: The anodic and cathodic limits are identified at a predetermined current density threshold (e.g., 0.5 mA/cm²). The ESW is calculated as the difference between these two limits [57]. For instance, this method was used to confirm a ~5 V window for [C₂mpyr][TDI]-based salts [57].

Viscosity and Ionic Conductivity Measurement

• Viscosity: Dynamic viscosity is measured using a rotational or oscillating rheometer. The temperature dependence is critical and is often fitted to the Vogel-Fulcher-Tamman (VFT) equation: η = η₀exp(B/(T-T₀)) to understand behavior across a temperature range [59] [57].
• Ionic Conductivity: This is measured using electrochemical impedance spectroscopy (EIS). A sample is placed between two blocking electrodes, and the impedance is measured over a frequency range. The bulk resistance is obtained from the high-frequency intercept on the real axis in a Nyquist plot. The conductivity (σ) is calculated using the cell constant and the measured resistance [57].
• Joint Analysis: The results are often presented as a function of temperature. High ionic conductivity and low viscosity are typically correlated. Machine learning models, such as Random Forest (RF) for pure ILs and CatBoost for mixtures, are now being employed to accurately predict viscosity based on critical properties (temperature, pressure, Tc, Pc) [59].

Cation-Anion Selection Workflow

The following diagram outlines a systematic decision-making process for selecting ionic liquid components based on the guidelines above.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagents and Materials for Ionic Liquid Electrolyte Research

Reagent/Material	Function/Application	Examples / Notes
Pyrrolidinium Salts	Base cation for high-stability electrolytes	[C₂mpyr][X], [C₃mpyr][X]; provides wide ESW and good transport properties [58] [57]
Imide Salts	Anions for low viscosity & high stability	LiTFSI, NaFSI, [EMIM][TFSI]; offer delocalized charge and high conductivity [61] [57]
Hückel-type Salts	Novel anions for optimized performance	LiTDI, LiPDI; designed for high oxidation resistance and low viscosity [57]
Co-solvents / Additives	Modifying bulk properties & interfacial structure	Organic carbonates (PC, DMC), DMSO, CH₃CN; reduce viscosity, widen ESW [12] [60]
Mixure Components	Fine-tuning properties via entropy engineering	P₁₄₄₄FSI (cation mixture), EMIMBF₄ (anion mixture); disrupt structure for better performance [58] [61]

Benchmarking Performance: A Comparative Analysis of IL Electrolyte Families

Ionic liquids (ILs) have emerged as a cornerstone for advanced electrochemical applications, prized for their exceptional thermal stability, negligible vapor pressure, and high ionic conductivity. A critical parameter determining their suitability in energy storage devices, such as batteries and supercapacitors, is the electrochemical stability window (ESW)—the voltage range within which the electrolyte remains inert. The chemical structure of the cationic core of the IL is a primary factor influencing this window. This guide provides a head-to-head comparison of four prominent IL classes—Imidazolium, Pyrrolidinium, Ammonium, and Piperidinium—evaluating their electrochemical performance, stability, and practical applications to inform material selection for researchers and scientists.

Comparative Analysis of Ionic Liquid Cations

The choice of cationic core significantly impacts the thermal and electrochemical properties of ionic liquids. Saturated, non-aromatic cations like Pyrrolidinium, Piperidinium, and Ammonium generally offer wider electrochemical windows and greater reductive stability compared to aromatic cations like Imidazolium.

• Imidazolium-based ILs feature a delocalized positive charge over an aromatic ring. While this contributes to high conductivity and low viscosity, the acidic protons on the ring, particularly at the C(2) position, are susceptible to reduction at relatively high potentials (around 1.0 V vs. Li/Li⁺), limiting their stability with lithium metal anodes [62] [63].
• Pyrrolidinium-based ILs, such as 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PIP₁₄NTf₂), are characterized by a localized positive charge on the nitrogen atom. This structure results in high thermal and electrochemical stability, low toxicity, and a wide ESW, making them exceptionally suitable for high-voltage applications [64] [63] [65].
• Piperidinium-based ILs share a similar localized charge and cyclic quaternary ammonium structure with pyrrolidinium ILs. They also demonstrate wide ESWs, high thermal stability, and are particularly promising for 4V-class rechargeable lithium batteries and supercapacitors due to their low cathodic potential [63].
• Ammonium-based ILs encompass a broad range of structures, from simple tetraalkylammonium to more complex cations. Their properties are highly tunable based on the alkyl chain substituents. A key advantage is their water immiscibility in many configurations, and like other saturated cations, they exhibit a wide ESW, which is valuable for various electrochemical devices [63].

Table 1: Comparative Overview of Ionic Liquid Cation Families

Cation Family	Charge Localization	Key Advantages	Key Limitations	Exemplary ESW
Imidazolium	Delocalized over aromatic ring	High conductivity, low viscosity, low melting point [62] [63]	Moderate electrochemical stability; acidic protons prone to reduction [62] [63]	~4.2 V (functionalized) [62]
Pyrrolidinium	Localized on nitrogen	High thermal & electrochemical stability, low toxicity [64] [63]	Can have higher viscosity than imidazolium ILs [62]	> 4.5 V (with [TFSI]⁻) [63]
Piperidinium	Localized on nitrogen	Wide ESW, high thermal stability, suitable for high-voltage Li batteries [63]	Can have higher viscosity than imidazolium ILs [63]	~4.0 V (with [TFSI]⁻) [63]
Ammonium	Localized on nitrogen	Highly tunable, wide ESW, often water-immiscible [63]	Properties highly dependent on alkyl chain substitutions [63]	Varies with structure [63]

Electrochemical Window and Stability Performance

The experimental data reveals a clear performance hierarchy among these cations concerning electrochemical stability. Pyrrolidinium and Piperidinium-based ILs consistently achieve the widest operational voltage windows, a critical factor for high-energy-density devices.

• Pyrrolidinium-based Protic Ionic Liquids (PILs) have been successfully deployed in RuO₂ micro-supercapacitors, enabling pseudocapacitive charge storage at a cell voltage of 1.5 V, outperforming typical aqueous electrolytes. Devices utilizing [Pyr₁H]⁺[BF₄]⁻ demonstrated superior power retention due to lower equivalent series resistance [64].
• Imidazolium-based ILs face inherent limitations due to their reducibility. However, structural functionalization can mitigate this. For instance, ether-functionalization of imidazolium cations shifted the reduction potential from ~1.0 V to ~0.6 V vs. Li/Li⁺ [62]. Similarly, methyl substitution at the C(2) position produced an IL ([C₂M₂IM][TFSI]) with an electrochemical window of 4.5 V, enabling stable operation in Li-O₂ batteries [62].
• Anion and Electrode Dependence: The ESW is not solely determined by the cation. The anion plays a crucial role; [TFSI]⁻ and [TPTP]⁻ based ILs generally exhibit higher stability and wider ESWs than [BF₄]⁻-based ILs [66]. Furthermore, the ESW is a function of the working electrode material, with the stability sequence for a given IL adhering to orders like Au ≈ Glassy Carbon > Pt [66].

Table 2: Experimentally Determined Electrochemical Performance Data

Ionic Liquid	Application/Test	Key Performance Metric	Reported Value	Citation
[Pyr₃H]⁺[TFA]⁻	RuO₂ Micro-supercapacitor	Areal Capacitance	106 mF cm⁻² @ 5 mV s⁻¹	[64]
[Pyr₃H]⁺[TFA]⁻	RuO₂ Micro-supercapacitor	Energy Density	15.5 µWh cm⁻² @ 0.73 mW cm⁻²	[64]
3% [C₂M₂IM][TFSI] in TEGDME	Li-O₂ Battery Electrolyte	Electrochemical Window	4.5 V	[62]
PIP₁₄NTf₂	General Electrolyte	Electrochemical Window	~4.0 V	[63]
EmimCl (in 3.5m aq. sol.)	Aqueous Ammonium-Ion Battery	Performance enhancement via suppressed HER & electrode swelling	Improved cycle performance	[67]

Essential Research Reagents and Methodologies

The Scientist's Toolkit: Key Research Reagents

The following table outlines essential materials and their functions as derived from the cited research protocols.

Table 3: Essential Reagents for Ionic Liquid Electrolyte Research

Reagent / Material	Function in Research	Example from Literature
Bis(trifluoromethylsulfonyl)imide (LiTFSI / [TFSI]⁻)	Lithium salt / anion; provides high electrochemical and thermal stability, low coordination [62] [63]	Used in PIP₁₄NTf₂ and [C₂M₂IM][TFSI] for Li batteries [62] [63]
Tetraethylene Glycol Dimethyl Ether (TEGDME)	Aprotic solvent; co-solvent in electrolyte formulations to reduce viscosity and enhance ion transport [62]	Used as base electrolyte with LiTFSI and [C₂M₂IM][TFSI] additive [62]
Glass Fiber (GF) Separator	Physical barrier between electrodes; prevents short-circuiting while allowing ion transport [62]	Used in Li-O₂ battery cell assembly [62]
Multi-wall Carbon Nanotubes (MWCNT)	Conductive additive/catalyst support; enhances electron conduction in composite electrodes [62]	Used in air cathode for Li-O₂ batteries [62]
RuO₂	Pseudocapacitive active material; stores charge via surface-controlled proton-coupled electron transfer [64]	Porous electrode material for micro-supercapacitors [64]

Core Experimental Protocols

Standardized experimental protocols are critical for the accurate and comparable evaluation of IL electrolytes across different studies.

1. Protocol for Determining Electrochemical Stability Window (ESW) The ESW is typically determined using Cyclic Voltammetry (CV) or Linear Sweep Voltammetry (LSV) [66].

• Working Electrodes: Common materials include Glassy Carbon (GC), Gold (Au), or Platinum (Pt), with the choice impacting the observed ESW limits [66].
• Cell Setup: A standard three-electrode configuration is used, with the IL as the electrolyte.
• Procedure: The potential is scanned from the open-circuit potential toward the cathodic and anodic limits. The scan is reversed once a predefined cut-off current density (typically between 0.1 and 1.0 mA/cm²) is reached [66].
• Data Analysis: The cathodic limit (E~CL~) and anodic limit (E~AL~) are identified at the potential where the current density equals the cut-off value. The ESW is calculated as E~AL~ - E~CL~ [66].
• Critical Consideration: ILs must be thoroughly dried before measurement, as water content dramatically narrows the ESW by promoting water electrolysis [66].

2. Protocol for Fabricating and Testing RuO₂ Micro-Supercapacitors

• Electrode Fabrication: A Ti/Au thin layer is first patterned onto a substrate using photolithography and lift-off techniques to create interdigitated current collectors. Porous RuO₂ is then electrodeposited onto these structures [64].
• Electrolyte Preparation: PILs are synthesized via acid-base reactions and used directly or incorporated into ionogels [64].
• Electrochemical Testing: Devices are characterized using Cyclic Voltammetry (CV) and Galvanostatic Charge-Discharge (GCD) to determine capacitance, energy density, power density, and long-term cycling stability [64].

3. Protocol for Molecular Dynamics (MD) Simulations MD simulations provide atomic-level insights into ion behavior and interfacial structures.

• Software: Packages like GROMACS are commonly used [67].
• Force Fields: Reactive Force Fields (ReaxFF) are employed to model bond formation and breaking, providing insights into viscosity trends, radial distribution functions (RDFs), and ion diffusion mechanisms [64].
• System Setup: Simulation boxes containing IL cations, anions, water, and electrode surfaces are constructed [67].
• Analysis: Key outputs include ion number density profiles near electrodes, conformational changes of ions and solvents, and interaction energy calculations, which help explain macroscopic experimental observations [67].

The following diagram illustrates the logical workflow for the comparative evaluation of ionic liquid electrolytes, integrating both experimental and simulation approaches.

This comparative analysis demonstrates that the selection of an ionic liquid cation is a fundamental trade-off governed by application-specific requirements.

• For applications demanding the highest energy density and voltage stability, such as lithium-metal batteries or high-voltage supercapacitors, Pyrrolidinium and Piperidinium-based ILs are the leading candidates due to their wide ESW and robust reductive stability.
• Where high conductivity and low viscosity are prioritized, and the operational voltage is constrained, functionalized Imidazolium-based ILs (e.g., with C(2) methyl or ether groups) present a viable option, overcoming the inherent instability of traditional imidazolium cations.
• Ammonium-based ILs offer immense tunability for specialized applications, including those requiring water-immiscible electrolytes.

The integration of experimental data with computational modeling, as outlined in the provided protocols, creates a powerful framework for the rational design and selection of next-generation IL electrolytes, pushing the boundaries of electrochemical energy storage.

The electrochemical window (ESW) of an electrolyte, defined as the voltage range between its oxidation and reduction potentials, is a critical determinant in the development of advanced energy storage systems. A wider ESW indicates greater electrochemical stability, which is essential for supporting high-voltage battery chemistries and preventing deleterious side reactions at the electrode interfaces. Among various electrolyte candidates, ionic liquid electrolytes (ILEs) have garnered significant interest due to their intrinsic thermal stability, non-flammability, and tunable electrochemical properties [24]. However, accurately predicting and validating the ESW of ILEs remains a central challenge. This guide provides a systematic comparison of methodologies for correlating computational predictions of ESW with experimental data, offering researchers a framework for robust electrolyte validation.

Computational Prediction of the Electrochemical Window

Computational models are indispensable for the high-throughput screening of ILEs, enabling researchers to prioritize promising candidates before resource-intensive experimental characterization.

First-Principles Calculations

Density Functional Theory (DFT) is a foundational computational tool for estimating the ESW from first principles. The methodology typically involves:

• Geometry Optimization and Energy Calculation: Determining the stable molecular structures and energies of the neutral, oxidized, and reduced states of the ionic liquid ions.
• HOMO-LUMO Gap as a Proxy: The energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) is often used as an initial, qualitative indicator of electrochemical stability. A larger HOMO-LUMO gap suggests a wider potential ESW.
• Explicit Redox Potential Calculation: A more accurate approach involves directly calculating the Gibbs free energy change for the oxidation (HOMO energy) and reduction (LUMO energy) processes, which can then be referenced against standard electrodes to predict oxidation and reduction potentials [24].

Emerging Machine Learning (ML) Approaches

To overcome the computational cost of high-level quantum chemistry, data-driven ML models are being rapidly developed.

• Unified Predictive Frameworks: Recent works demonstrate frameworks that integrate molecular structure information to predict key electrolyte properties. These models often use Graph Neural Networks (GNNs) to create molecular embeddings, which are then aggregated in a permutation-invariant manner to handle multi-component formulations [68].
• Physics-Informed Architectures: Leading models incorporate physical priors, such as empirical dependencies on temperature and salt concentration, to enhance prediction accuracy and ensure results remain within physically plausible bounds [68].
• Addressing Data Scarcity: Sparse modeling techniques, which extract significant descriptors from small datasets, are particularly valuable for ESW prediction where experimental data is limited. The Sparse Modeling for Small data (SpM-S) approach combines machine learning with domain knowledge to construct interpretable, linear regression models for property prediction [69].

Experimental Validation of Electrochemical Stability

Computational predictions must be rigorously validated against experimental measurements. The following table summarizes the primary electrochemical techniques used for ESW determination.

Table 1: Key Experimental Techniques for ESW Validation

Technique	Measured Parameter	Function in ESW Validation	Key Advantages
Cyclic Voltammetry (CV)	Current response as a function of applied potential	Directly identifies oxidation and reduction onset potentials, defining the practical ESW limits.	Provides rapid assessment; reveals reaction reversibility.
Linear Sweep Voltammetry (LSV)	Current during a single, linear potential sweep	Determines the anodic and cathodic stability limits at a selected sweep rate.	High sensitivity for detecting low-magnitude decomposition currents.
Chronoamperometry	Current decay over time at a fixed potential	Probes long-term stability at a voltage within the presumed ESW.	Assesses passivation layer formation or continuous decomposition.
Electrochemical Impedance Spectroscopy (EIS)	Impedance across a frequency spectrum	Characterizes interfacial stability and resistance changes after polarization.	Can detect subtle changes in the electrode-electrolyte interface [24].

Complementary Characterization Techniques

To gain a holistic understanding of ESW and degradation mechanisms, electrochemical data should be correlated with analytical characterization.

• X-ray Photoelectron Spectroscopy (XPS): This technique is crucial for analyzing the chemical composition of the solid electrolyte interphase (SEI) or cathode electrolyte interphase (CEI) formed on electrode surfaces. For instance, a high LiF content in the SEI, often promoted by specific anions like FSI⁻, is associated with enhanced interfacial stability and a wider practical ESW [24].
• Raman Spectroscopy: Used to investigate the solvation structure of ions in the electrolyte (e.g., contact ion pairs CIPs, aggregates AGGs). Changes in solvation structure, as influenced by a dual-anion design, can lead to a more stable interface and directly impact the observed ESW [24].
• Scanning Electron Microscopy (SEM): Visualizes the morphology of electrode surfaces after cycling. It is used to confirm the deposition of uniform, dendrite-free lithium layers, which indicates stable operation within the electrolyte's ESW [24].

Comparative Analysis: Ionic Liquid Electrolyte Systems

The following table compares the predicted and experimentally validated performance of different ionic liquid electrolyte strategies, highlighting the critical role of anion chemistry.

Table 2: Comparison of Ionic Liquid Electrolyte Strategies for ESW

Electrolyte System	Computational Prediction Focus	Key Experimental Findings	Correlation & Validation Insights
Conventional Single-Anion ILEs	HOMO-LUMO levels of individual ions (e.g., TFSI⁻, BF₄⁻).	Limited practical ESW due to viscosity-conductivity trade-offs and interfacial instability at extremes [24].	DFT predictions often overestimate stability; experimental ESW is narrower due to kinetic and interfacial effects.
Locally Concentrated ILEs (LCILEs)	Molecular dynamics (MD) simulations of localized ion clusters and Li⁺ solvation structure.	Improved ionic conductivity and initial ESW, but challenges with wettability and high-rate viscosity persist [24].	MD simulations help rationalize improved Li⁺ transport, but full ESW validation requires complementary LSV/CV experiments.
Dual-Anion ILEs (D-LCILE)	DFT and MD used to model solvation shell structure and predict synergistic anion effects (e.g., TFSI⁻ + FSI⁻) [24].	>99.93% capacity retention after 200 cycles in Li‖LiFePO₄ cells; XPS depth profiling showed LiF content increased from 11.56% to 27.62% [24].	Computational models correctly predicted a more stable solvation structure, which was experimentally validated by superior interfacial stability and a de facto wider operational ESW.
High-Entropy Electrolytes	Machine learning models (e.g., invariant neural networks) predict properties like conductivity from complex component embeddings [68].	Formulations with high ionic conductivity and tailored solvation structure (e.g., high anion coordination) can be identified [68].	Generative ML models can navigate the vast design space to propose multi-component formulations that meet specific ESW-related property targets.

Integrated Workflow for ESW Prediction and Validation

A robust validation pipeline integrates computational and experimental efforts. The following diagram outlines a systematic workflow for ESW correlation.

Workflow for ESW Prediction and Validation

The Scientist's Toolkit: Essential Reagents and Materials

Successful ESW validation relies on specific research-grade materials and instruments.

Table 3: Essential Research Reagents and Materials for ESW Studies

Item	Function / Rationale	Example Application
High-Purity Ionic Liquids & Salts	Foundation of the electrolyte. Purity is critical to avoid side reactions that narrow the ESW.	EMIM-TFSI, LiTFSI, LiFSI [24].
Aprotic Solvent Diluents	Modulate viscosity and ionic conductivity, which can indirectly affect the practical ESW.	Bis(2,2,2-trifluoroethyl) ether (BTFE) [24].
Inert Atmosphere Glovebox	Prevents contamination of moisture- and oxygen-sensitive ILEs and electrodes.	Essential for all electrolyte preparation and cell assembly [24] [68].
Electrochemical Cell (e.g., Swagelok, Coin Cell)	Standardized platform for conducting CV, LSV, and long-term cycling tests.	Li‖Stainless Steel cells for LSV; Li‖LiFePO₄ for full-cell validation [24].
Potentiostat/Galvanostat	Core instrument for applying controlled potentials/currents and measuring electrochemical response.	Used for CV, LSV, EIS, and chronoamperometry measurements.
Reference Electrodes	Provides a stable, known potential reference for accurate voltage measurement during CV/LSV.	Li/Li⁺ is common for lithium battery research.

The accurate validation of an ionic liquid's electrochemical window is a multi-faceted process that hinges on the strong correlation between computational predictions and experimental data. While DFT provides a foundational understanding, advanced methods like MD and machine learning offer powerful pathways to navigate the complex formulation space and predict key properties. Experimentally, electrochemical techniques like LSV and CV must be complemented with interface-sensitive analysis like XPS to fully understand the practical ESW. The emerging paradigm of dual-anion systems and data-driven generative models demonstrates that a synergistic approach, which uses computation to guide targeted experiments, is the most effective strategy for developing next-generation ILEs with validated, high-voltage stability.

The development of advanced energy storage systems, particularly those utilizing ionic liquid electrolytes (ILEs), hinges on the critical assessment of three core performance metrics: capacity retention, which measures a battery's ability to maintain its energy storage capacity over repeated charge-discharge cycles; Coulombic efficiency (CE), defined as the ratio of discharge capacity to charge capacity in each cycle, reflecting the reversibility of electrochemical reactions; and cycling stability, which indicates the system's ability to maintain performance over extended cycling. These metrics are profoundly influenced by the electrochemical window of the electrolyte—the voltage range within which it remains electrochemically stable without decomposing. A wider electrochemical window enables the use of high-voltage cathode materials, which is essential for developing next-generation high-energy-density batteries, while also directly impacting long-term metric stability by reducing detrimental side reactions at both electrodes.

The fundamental relationship between electrolyte chemistry and these performance metrics manifests through several key mechanisms. Electrolytes with wider electrochemical windows suppress decomposition at high voltages, leading to more stable capacity retention. The formation of a stable solid electrolyte interphase (SEI) at the anode, heavily influenced by electrolyte composition, directly determines Coulombic efficiency by preventing continuous lithium and electrolyte consumption. Furthermore, electrolytes that facilitate uniform ion deposition significantly enhance cycling stability by inhibiting dendrite formation. Ionic liquids have emerged as particularly promising candidates for optimizing these metrics due to their unique properties, including non-flammability, negligible vapor pressure, and intrinsic thermal stability, which collectively address safety concerns prevalent in conventional organic carbonate-based systems.

Experimental Protocols for Metric Evaluation

Standardized experimental protocols are essential for the accurate and comparable evaluation of ionic liquid electrolyte performance across research studies. These methodologies provide the foundational framework for quantifying the key metrics of capacity retention, Coulombic efficiency, and cycling stability under controlled, reproducible conditions.

Electrolyte Preparation and Cell Assembly

The synthesis of high-purity ILEs begins with the meticulous drying of ionic liquid solvents and lithium salts. For instance, LiFSI salt is typically dried at 110°C for 24 hours under vacuum, while solvents such as Pyr13FSI are dried at 70°C for 12 hours in vacuum conditions. Co-solvents like ethyl methyl carbonate (EMC) and fluorinated ethylene carbonate (FEC) require drying with 4Å molecular sieves for 72 hours [70]. All electrolyte preparation must be conducted in an argon-filled glove box with both moisture and oxygen content maintained below 1 ppm to prevent contamination. Electrolyte formulations are then created by dissolving lithium salts in mixtures of ionic liquids and molecular co-solvents at specific molar or volume ratios, such as 1M LiFSI in a mixture of Pyr13FSI, EMC, and FEC with a volume ratio of 3:4.9:2.1 [70].

For performance evaluation, researchers typically assemble CR2032-type coin cells using predetermined electrode configurations. Common test systems include Li/Li symmetric cells for assessing lithium metal anode stability, and Li/LFP (LiFePO4) half-cells for evaluating full-cell performance potential. These cells incorporate separators (e.g., Celgard 2400) and are assembled with precisely controlled amounts of electrolyte, typically around 80 μL [70]. This standardized assembly process ensures that performance comparisons between different ILE formulations are valid and reproducible.

Electrochemical Testing Procedures

A comprehensive suite of electrochemical characterization techniques is employed to evaluate ILE performance:

• Galvanostatic Charge-Discharge Cycling: Cells are cycled at specific current rates (e.g., 0.2C, 0.5C, 1C) between predetermined voltage limits (e.g., 2.5-4.0V for LFP cathodes) using constant current-constant voltage (CC-CV) protocols at controlled temperatures (e.g., 30°C) [70] [24]. Capacity retention is calculated as the percentage of initial capacity remaining after a specified number of cycles, while Coulombic efficiency is determined as the ratio of discharge to charge capacity for each cycle.
• Cycling Stability Assessment: Long-term cycling tests are conducted over hundreds of cycles to evaluate capacity fade rates and polarization changes. For high-temperature applications, cells may be tested at elevated temperatures up to 100°C to assess thermal stability and performance under extreme conditions [71].
• Electrochemical Impedance Spectroscopy (EIS): Measurements are typically performed over a frequency range from 1 MHz to 0.1 Hz with an amplitude of 10 mV to analyze interfacial resistance and ionic conductivity [70] [24]. Ionic conductivity values are derived from EIS data obtained from stainless steel symmetric cells.
• Linear Sweep Voltammetry (LSV): This technique determines the electrochemical stability window by scanning from open-circuit voltage toward anodic directions at a slow scan rate (e.g., 1 mV/s) [70].

Figure 1: Experimental workflow for evaluating ionic liquid electrolyte performance metrics.

Material Characterization Techniques

Post-cycling analysis provides critical insights into the structural and chemical changes underlying performance metrics:

• X-ray Photoelectron Spectroscopy (XPS): Depth profiling analysis identifies the chemical composition of the solid electrolyte interphase (SEI), with particular focus on beneficial components such as LiF, which significantly influences Coulombic efficiency [24].
• Scanning Electron Microscopy (SEM): This technique characterizes the morphology of lithium deposits after cycling, revealing dendrite formation or uniform plating, which directly correlates with cycling stability [70] [24].
• Raman Spectroscopy: Combined with molecular dynamics (MD) simulations, this method analyzes solvation structures and identifies the presence of contact ion pairs (CIPs) and aggregates (AGGs), which impact ionic conductivity and transport mechanisms [24].

Comparative Performance of Ionic Liquid Electrolyte Systems

Single-Anion versus Dual-Anion Formulations

The strategic design of ionic liquid electrolytes has progressed from single-anion to advanced dual-anion systems, with significant implications for all three key performance metrics.

Single-anion ILEs based on Pyr13FSI with carbonate co-solvents demonstrate robust performance characteristics. One optimized formulation (1M LiFSI in Pyr13FSI:EMC:FEC, 3:4.9:2.1 by volume) achieved a high ionic conductivity of 8.9 mS/cm at 30°C—approximately double that of pure ionic liquid systems—due to reduced viscosity from carbonate additives [70]. This formulation enabled a Coulombic efficiency of 96.3% over 250 cycles in Li/Li symmetric cells and outstanding capacity retention of 93.2% after 150 cycles in Li/LFP cells. The performance enhancement mechanism involves FEC indirectly increasing coordination between Li+ and FSI− anions, leading to synergistic decomposition that forms a robust, LiF-rich SEI on the lithium metal surface [70].

Dual-anion locally concentrated ionic liquid electrolytes (D-LCILEs) represent a significant advancement, incorporating both FSI− and TFSI− anions with a diluent such as bis(2,2,2-trifluoroethyl) ether (BTFE). This innovative approach creates a more diverse solvation environment, reducing viscosity while maintaining high lithium-ion mobility [24]. The dual-anion structure facilitates a more stable solvation shell, promoting the formation of a uniform SEI with increased LiF content (fluorine content increased from 11.56% to 27.62% according to XPS depth profiling) [24]. This translated to exceptional full-cell performance with average Coulombic efficiency exceeding 99.90% and remarkable capacity retention >99.93% after 200 cycles at 1C in a 20 μm-thick Li‖LFP configuration [24].

Table 1: Performance comparison of single-anion and dual-anion ionic liquid electrolytes

Electrolyte Formulation	Ionic Conductivity (mS/cm)	Coulombic Efficiency (%)	Capacity Retention (%)	Cycle Life	Key Advantages
Single-anion ILE (Pyr13FSI-EMC-FEC) [70]	8.9 @ 30°C	96.3 (Li/Li, 250 cycles)	93.2 (Li/LFP, 150 cycles)	150-250 cycles	High conductivity, stable SEI formation
Dual-anion D-LCILE (FSI−-TFSI−-BTFE) [24]	Data not specified	>99.90 (Li/LFP, 200 cycles)	>99.93 (Li/LFP, 200 cycles)	200+ cycles	Superior interfacial stability, LiF-rich SEI
Phosphonium-based ILE (P4444IM14-LiTFSI) [71]	~10⁻² @ 100°C	Data not specified	>94 (Li/LFP, 0.5C, 100°C)	Limited data	Exceptional high-temperature operation

High-Temperature Ionic Liquid Electrolytes

Performance metrics under extreme conditions represent a critical evaluation parameter for advanced energy storage applications. Phosphonium-based ionic liquids utilizing tetrabutylphosphonium cations coupled with per(fluoroalkylsulfonyl)imide anions demonstrate remarkable thermal robustness exceeding 150°C, maintaining anodic stability above 4.5 V even at 100°C [71]. These systems delivered outstanding capacity retention exceeding 94% of theoretical capacity at 0.5C rate and 100°C in Li/LFP cells, highlighting their potential for high-temperature applications where conventional electrolytes would rapidly degrade [71]. The ion conduction values for these systems range from 10⁻³ to 10⁻² S cm⁻¹ at 100°C, enabled by the selected IL matrices that maintain sufficient ionic mobility at elevated temperatures while resisting decomposition [71].

Research Reagent Solutions for ILE Development

Table 2: Essential research reagents for ionic liquid electrolyte development and testing

Reagent Category	Specific Examples	Function in ILE Development	Performance Impact
Ionic Liquid Solvents	N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI); Tetrabutylphosphonium cations (P4444+); 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) [70] [71]	Primary ionic conductive medium providing wide electrochemical window and thermal stability	Determines fundamental stability, operating voltage range, and temperature performance
Lithium Salts	LiFSI; LiTFSI; LiDFOB [70] [24] [50]	Lithium-ion source for conductivity; influences SEI composition	Impacts ionic conductivity, SEI quality, and anodic stability
Co-solvents & Diluents	Ethyl methyl carbonate (EMC); Bis(2,2,2-trifluoroethyl) ether (BTFE) [70] [24]	Reduce viscosity and enhance wettability while maintaining localized high concentration	Increases ionic conductivity and improves electrode-electrolyte contact
SEI-Forming Additives	Fluorinated ethylene carbonate (FEC) [70]	Promotes formation of LiF-rich stable interphase on electrode surfaces	Enhances Coulombic efficiency and cycling stability by suppressing dendrite growth
Polymer Matrix Components	Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF); Polypropylene carbonate (PPC) [72]	Provide mechanical stability and enable novel battery architectures	Improves safety and enables membrane-free designs in specialized applications

The comparative analysis of ionic liquid electrolytes reveals inherent trade-offs between key performance metrics that inform their application-specific optimization. Formulations based on Pyr13FSI with carbonate co-solvents achieve an effective balance between high ionic conductivity and good cycling stability, making them promising for room-temperature applications. In contrast, advanced dual-anion systems demonstrate exceptional interfacial stability and near-perfect Coulombic efficiency, albeit potentially at higher complexity and cost. Phosphonium-based ILEs excel in high-temperature environments where conventional electrolytes fail, showcasing the critical importance of matching electrolyte formulation to operational requirements.

Future research directions will likely focus on overcoming remaining challenges through several key approaches: First, developing multi-anion strategies that optimize the synergistic effects between different anions to simultaneously enhance conductivity, electrochemical window, and SEI stability. Second, advancing machine learning-guided formulation design to efficiently navigate the vast combinatorial space of ionic liquids, lithium salts, and additives, accelerating the discovery of optimized formulations with tailored property profiles [50]. Third, creating polymer-ionogel hybrid electrolytes that combine the safety advantages of solid-state systems with the high conductivity of ionic liquids, potentially enabling novel membrane-free battery architectures [72]. As these developments progress, the fundamental relationship between electrochemical window breadth and the triad of performance metrics—capacity retention, Coulombic efficiency, and cycling stability—will continue to guide the rational design of next-generation energy storage systems.

Ionic liquids (ILs), a class of materials composed entirely of ions that are liquid below 100°C, have emerged as a transformative presence in electrochemical research. Their unique physicochemical properties, including low volatility, high thermal stability, and tunable solubility, make them particularly valuable as electrolytes in advanced energy storage systems [73]. The evolution of ILs has progressed through four generations, from initial applications as green solvents to current focus on sustainability, biodegradability, and multifunctionality [73]. Within electrochemistry, the electrochemical stability window (ESW)—the voltage range within which an electrolyte remains electrochemically inert—serves as a critical performance parameter, as it directly determines the energy density of devices according to the relationship E=½CV² [74].

This review provides a systematic comparison of hybrid ionic liquid electrolytes across three principal matrices: aqueous, organic, and polymer-based systems. By examining their respective electrochemical windows, conductivity properties, and practical applications, we aim to establish structure-property relationships that can guide the rational design of next-generation electrochemical systems for researchers and scientists engaged in advanced materials development.

Fundamental Properties and Electrochemical Windows of Ionic Liquid Electrolytes

The electrochemical stability of an electrolyte is fundamentally dictated by its decomposition potential, which varies significantly across different solvent systems. Ionic liquids exhibit widened electrochemical stability windows (~4.5 V) compared to both aqueous (~1.23 V) and conventional organic electrolytes (~2.5-3.5 V) [74]. This exceptional stability stems from the unique coordination environments, inter-ion interactions, and the discrete anion-cation structures that characterize ILs [19].

Table 1: Comparative Analysis of Electrolyte Systems with Ionic Liquids

Electrolyte System	Typical Electrochemical Window (V)	Key Advantages	Principal Limitations	Ideal Application Contexts
Aqueous IL Hybrids	1.0 - 2.5	High ionic conductivity, non-flammable, low cost, environmentally benign	Limited by water decomposition potential	Biomedical devices, low-cost supercapacitors, safe batteries
Organic IL Hybrids	3.0 - 5.5	Wider operational voltage, enhanced thermal stability, good electrode wettability	Higher viscosity, toxicity concerns, sensitivity to moisture	High-energy density batteries, supercapacitors, industrial electrochemistry
Polymer IL Hybrids	2.5 - 4.5	Leakage-free operation, mechanical stability, flexibility	Lower ionic conductivity at ambient temperatures	Flexible/wearable electronics, solid-state batteries, smart displays

The electrochemical window represents the voltage range between cathode and anode stability limits before electrolyte decomposition occurs. For ionic liquids, this window is primarily determined by the redox stability of both cations and anions, which can be systematically tuned through molecular design [74]. Aprotic ILs, particularly those based on imidazolium and pyrrolidinium cations, demonstrate the widest electrochemical windows, making them suitable for high-voltage applications such as lithium-metal batteries and advanced supercapacitors [74].

Experimental Protocols for Electrolyte Characterization

Electrolyte Preparation Protocols

Aqueous IL Hybrids: Prepare by dissolving hydrophilic ILs (e.g., [BMIM][BF₄]) in deionized water at concentrations ranging from 0.5-2.0 M. Sonicate for 30 minutes to ensure complete dissolution and homogeneity [19].

Organic IL Hybrids: Formulate by mixing appropriate ILs with organic carbonates (ethylene carbonate, diethyl carbonate) in an argon-filled glove box (O₂, H₂O < 0.1 ppm). Standard compositions include 1:1 (v/v) IL:organic solvent mixtures with 1 M lithium salts (LiTFSI, LiPF₆) [75] [19].

Polymer IL Hybrids (Ionogels): Synthesize via in-situ polymerization of polymer matrices (PMMA, PEGDA) in the presence of ILs (20-60 wt%). Initiate polymerization thermally (60-80°C) or photochemically (UV light, 365 nm) for 2-4 hours to form crosslinked networks [76] [19].

Electrochemical Testing Methodologies

Cyclic Voltammetry (CV): Employ a standard three-electrode configuration with glassy carbon working electrode, platinum counter electrode, and Ag/Ag⁺ reference electrode. Scan rates typically range from 1-100 mV/s. The electrochemical stability window is determined as the potential range where the current density remains below 0.5 mA/cm² [74].

Electrochemical Impedance Spectroscopy (EIS): Measure electrolyte resistance over frequency range 1 MHz to 0.1 Hz with amplitude of 10 mV. Calculate ionic conductivity using the formula σ = L/(R×A), where L is thickness, R is bulk resistance, and A is electrode area [75] [19].

Galvanostatic Charge-Discharge (GCD): Evaluate electrochemical capacitors using symmetric carbon electrodes (activated carbon, graphene) at current densities from 0.1-10 A/g. Calculate specific capacitance from discharge curves: C = 2I×(Δt)/(m×ΔV), where I is current, Δt is discharge time, m is active mass, and ΔV is voltage window [74].

Performance Comparison and Analysis

Electrochemical Window and Conductivity

The search for high-voltage electrolytes has driven the development of IL-based hybrid systems with progressively wider electrochemical windows. Organic IL hybrids consistently achieve the highest operational voltages (up to 5.5 V), particularly those incorporating pyrrolidinium-based cations (e.g., Pyr₁₄FSI) and fluorinated anions, which resist oxidation at high potentials [74]. This makes them indispensable for high-energy-density applications.

Polymer IL hybrids (ionogels) demonstrate intermediate electrochemical windows (2.5-4.5 V) while providing significant mechanical stability. Recent advances in IL-MOF (Metal-Organic Framework) hybrid electrolytes have shown particular promise, with these materials serving as either the main ionic conduction body or as active fillers in composite polymer electrolytes [75]. The confinement of ILs within MOF pores creates unique ion transport pathways that enhance both stability and conductivity.

Aqueous IL hybrids face fundamental limitations due to the narrow thermodynamic stability window of water (1.23 V), though practical devices can achieve up to 2.5 V through overpotential effects and carefully selected electrode materials [19].

The relationship between ionic conductivity and electrochemical window presents a significant trade-off in electrolyte design. Aqueous IL hybrids achieve the highest ionic conductivity (10⁻³~10⁻² S/cm) but suffer from the most limited voltage window [19]. Conversely, organic IL hybrids offer wider windows but typically exhibit lower conductivity, particularly at reduced temperatures due to higher viscosity [74]. Polymer IL hybrids occupy an intermediate position, with recent advances in IL-MOF composites achieving conductivities of 4.4×10⁻⁴ S/cm at room temperature while maintaining wide electrochemical windows [75].

Thermal and Mechanical Stability

Thermal stability represents a critical safety parameter, particularly for high-energy-density devices. IL-based electrolytes demonstrate superior thermal stability (200-500°C) compared to conventional organic electrolytes, with organic IL hybrids maintaining operation from -50°C to 100°C [19] [74]. Eutectic IL mixtures (e.g., 1:1 Pip₁₃FSI:Pyr₁₄FSI) have enabled supercapacitor operation across an exceptionally wide temperature range (-50°C to 100°C) [74].

Polymer IL hybrids provide enhanced mechanical stability, effectively suppressing lithium dendrite formation in metal anode batteries through their high shear modulus [75]. The incorporation of ILs into polymer matrices such as PMMA or PVDF creates semi-solid gel polymer electrolytes that combine the mechanical integrity of solids with the ionic transport properties of liquids [19].

Table 2: Experimental Performance Data for IL-Based Hybrid Electrolytes

Electrolyte Composition	Ionic Conductivity (S/cm)	Electrochemical Window (V)	Thermal Stability (°C)	Application Performance
[BMIM][BF₄] in H₂O	1.81×10⁻³	2.1	200	Capacitance retention: >90% (10,000 cycles) [19]
LiTFSI in PC	~10⁻³	~3.5	150-200	Energy density: ~40 Wh/kg [19]
PEGDA/PMMA/LiTFSI/PC	2.6×10⁻⁴	3.8	250	Flexible supercapacitors [19]
PMMA/LiClO₄/[Emim]BF₄	2.9×10⁻³	4.0	>200	Electrochromic devices [19]
ILs@MOFs composite	4.4×10⁻⁴	4.5	300	Solid-state lithium batteries [75]

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Research Reagents for IL Hybrid Electrolyte Development

Reagent Category	Specific Examples	Function & Application Notes
IL Cations	Imidazolium, Pyrrolidinium, Ammonium, Phosphonium	Determine electrochemical stability, viscosity; Pyrrolidinium offers wide windows [74]
IL Anions	BF₄⁻, PF₆⁻, TFSI⁻, FSI⁻	Influence conductivity, stability; Fluorinated anions enhance oxidative stability [75]
Polymer Matrices	PMMA, PVDF, PEO, PEGDA	Provide mechanical framework for ionogels; impact crystallinity and ion transport [76]
MOF Materials	ZIF-8, MIL-100, UiO-66	Create nano-confinement for ILs; provide ordered pores for selective ion transport [75]
Lithium Salts	LiTFSI, LiPF₆, LiClO₄, LiBF₄	Provide Li⁺ ions for battery applications; LiTFSI offers high solubility and stability [19]
Organic Solvents	Propylene Carbonate, Ethylene Carbonate	Co-solvents for organic IL hybrids; reduce viscosity, enhance wettability [19]

The comparative analysis presented herein demonstrates that each IL hybrid electrolyte system offers distinct advantages tailored to specific electrochemical applications. Organic IL hybrids provide the highest voltage operation for maximum energy density, while aqueous IL systems deliver superior safety and conductivity for cost-sensitive applications. Polymer IL hybrids present an optimal balance of flexibility, safety, and performance for emerging technologies in flexible electronics and solid-state batteries.

Future development directions will likely focus on overcoming existing limitations, particularly through the design of biodegradable fourth-generation ILs [73], optimization of IL-MOF composites for enhanced conduction pathways [75], and the refinement of machine learning models for accurate prediction of IL properties such as viscosity [59]. As research progresses, the rational design of task-specific IL hybrid electrolytes will continue to enable advances across energy storage, smart materials, and biomedical applications.

Conclusion

The electrochemical stability window of an ionic liquid electrolyte is not an intrinsic property but a tunable characteristic, profoundly influenced by a synergy of molecular structure, ion interactions, and the operational environment. This analysis demonstrates that strategic molecular design—such as employing ammonium cations with short, asymmetric alkyl chains or combining anions like FSI− and TFSI−—can effectively widen the ESW, enhance ionic conductivity, and stabilize electrode interfaces. The future of IL electrolytes lies in the development of smart, multifunctional, and biodegradable systems, guided by advanced computational models that accelerate the discovery of optimal formulations. For biomedical and clinical research, these advancements promise safer, more efficient bio-electronic devices, improved drug delivery systems with enhanced stability, and novel diagnostic platforms, ultimately positioning ILs as key enablers of next-generation sustainable and biocompatible technologies.

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