Key Takeaways & Executive Findings
- •• • Ionic liquid electrolytes enable lithium metal anodes with theoretical specific capacity of 3860 mAh g−1 and low reduction potential of −3.04 V vs. SHE, addressing safety issues of flammable organic solvents that cause thermal runaway and explosion. • • Locally concentrated ionic liquid electrolytes (LCILEs) with low viscosity and fire-retardant properties achieve safe, stable cycling of lithium metal batteries, as demonstrated by Lee et al. (Adv Funct Mater, 2020, 30: 2003132). • • Anion-reinforced solvating ionic liquid electrolytes stabilize high-nickel cathodes in lithium-metal batteries, as reported by Zou et al. (Adv Mater, 2024, 36: 2400537), enabling high-voltage operation. • • Inorganic-rich solid electrolyte interphase (SEI) formation via ionic liquid additives improves cycling stability and rate capability, as evidenced by studies on lithiophilic and electrochemically active ionic additives (ACS Energy Lett, 2022, 7: 67–69).
Abstract
The escalating demand for high-performance lithium-ion batteries (LIBs) in portable electronics and electric vehicles has driven extensive research into advanced electrolytes. Ionic liquids (ILs) and their derived electrolytes, including poly(ionic liquids), ionogels, and IL-functionalized systems, offer significant potential for enhancing the safety and electrochemical performance of LIBs due to their unique properties such as non-volatility, wide electrochemical windows, and excellent thermal stability. These properties enable safer, high-energy, and long-lasting batteries. This review conducts a thorough analysis of the physicochemical properties of ILs and their versatile applications in electrolytes, particularly emphasizing their adaptability to fulfill the specific needs of different battery systems. In liquid electrolyte systems, ILs can function as solvents, interfacial modifiers, and critical components for constructing artificial solid electrolyte interphase (SEI). In (quasi-)solid-state electrolyte systems, ILs can be polymerized to form poly(ionic liquid)s or integrated with organic, inorganic, or composite materials to develop IL-based electrolytes, demonstrating multifunctional electrochemical performance. Finally, the review critically examines the challenges and opportunities in this field, offering insightful perspectives for future advancements.
1. Introduction
Commercial lithium-ion batteries rely on non-aqueous electrolytes containing flammable alkyl carbonates, which pose severe safety risks including thermal runaway, explosion, and combustion. These safety concerns, coupled with the need for higher energy density and power density, have driven the search for alternative electrolyte systems. Solid electrolytes offer improved safety but suffer from low ionic conductivity and poor interfacial contact. Ionic liquids (ILs), molten salts with melting points below 100 °C, present a compelling solution due to their non-volatility, wide electrochemical windows, and excellent thermal stability. Their tunable structures allow for rational design to meet specific battery requirements.
This review systematically analyzes the physicochemical properties of ILs and their applications in both liquid and (quasi-)solid-state electrolytes. In liquid systems, ILs serve as solvents, interfacial modifiers, and components for artificial SEI formation. In solid-state systems, they are polymerized into poly(ionic liquid)s or integrated with organic/inorganic materials to create composite electrolytes. The key challenge lies in balancing ionic conductivity, viscosity, and electrochemical stability. By benchmarking recent advances, this review identifies critical strategies for engineering IL-based electrolytes to achieve high-performance, safe lithium batteries.
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LIN Fei, WANG Cong, ZOU Wenhong, REN Zejia, GU Kecheng, GAO Tengyang, TANG Yuxin, ZHANG Yanyan (2026). Rational Engineering of Ionic Liquid Electrolytes: A Paradigm Shift Toward Advanced Lithium Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3449-5
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Frequently Asked Questions
What are the main failure mechanisms of ionic liquid electrolytes under high voltage and high current density, and how do they compare to conventional carbonate electrolytes?
Ionic liquid electrolytes can undergo oxidative decomposition at high voltages, especially with high-nickel cathodes, leading to gas evolution and impedance growth. However, anion-reinforced solvating IL electrolytes have demonstrated stability with high-nickel cathodes, as shown by Zou et al. (Adv Mater, 2024, 36: 2400537). Under high current density, mass transport limitations due to higher viscosity can cause lithium dendrite growth, but locally concentrated ILs with low viscosity mitigate this, enabling safe cycling (Lee et al., Adv Funct Mater, 2020, 30: 2003132).
What is the cost parity of ionic liquid electrolytes compared to conventional carbonate electrolytes, and what are the main cost drivers?
Ionic liquids are generally more expensive than conventional solvents due to complex synthesis and purification. However, their use in small quantities as additives or in hybrid systems can reduce cost impact. The main cost drivers are the cation/anion precursors and the need for high purity. Economies of scale and development of cheaper ILs, such as those based on aprotic heterocyclic anions (AHA), are expected to reduce costs (Sun et al., J Phys Chem B, 2015, 119: 15030–15039).
How do ionic liquid electrolytes affect the solid electrolyte interphase (SEI) composition and lithium-ion transport kinetics?
Ionic liquids can participate in SEI formation, leading to inorganic-rich layers that enhance stability and ion transport. For example, lithiophilic and electrochemically active ionic additives promote the formation of a robust SEI (Kim et al., ACS Energy Lett, 2022, 7: 67–69). The SEI composition influences desolvation kinetics and interfacial resistance, which are critical for high-rate performance. Studies show that nitrate additives can alter solvation structure and improve kinetics (Wahyudi et al., Adv Funct Mater, 2021, 31: 2101593).
What are the scalability challenges for producing ionic liquid electrolytes for commercial lithium-ion batteries?
Scalability challenges include high viscosity, which complicates electrode wetting and cell assembly, and the need for low water content to prevent HF formation. However, locally concentrated ionic liquid electrolytes (LCILEs) with low viscosity and fire-retardant properties have been developed, addressing these issues (Lee et al., 2020). Additionally, the synthesis of ILs must be optimized for large-scale production, and recycling strategies need to be considered to reduce environmental impact.
How do ionic liquid electrolytes perform at extreme temperatures, and what are the operational limits?
Ionic liquids typically have wide liquidus ranges, with some remaining liquid below −40°C and stable above 200°C. Their thermal stability is superior to organic carbonates, which decompose around 80°C. However, ionic conductivity decreases at low temperatures due to increased viscosity. Strategies such as using low-viscosity ILs or adding co-solvents can improve low-temperature performance. The exact operational limits depend on the specific IL structure and electrolyte formulation.
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