Introduction

Room temperature ionic liquids (RTILs) are molten salts with melting points well below 100°C. Most RTILs are organic salts with high variability in molecular design . Ionic liquids (ILs) possess many useful properties, including low vapor pressure, broad liquid-state temperature window, high chemical and thermal stability, broad electrochemical voltage window, flame retardancy, high ionic conductivity, and the ability to react in various organic or good solubility in inorganic materials [1]. The unique characteristics of ILs also make their application range very wide [1] .

In recent years, the potential application of ILs as novel electrolytes for various secondary rechargeable batteries has attracted great attention. For example, ILs have been used to accelerate the development of Li -ion batteries, to reduce the flammability of Li-ion batteries [2-5], and to improve the cycle stability and Coulombic efficiency of dual graphite batteries [6-8]. Li-ion secondary batteries composed of graphite or pure lithium anodes and organic solvent electrolytes have the potential for high operating voltage, high energy density, and good cycle stability, but the electrolyte has the disadvantage of being flammable, which may lead to safety hazards [2-5]. In recent years, ILs have also been studied to improve the safety of Li- ion batteries. Dual-graphite cells using low-cost graphite as anode and cathode, and using nonflammable ionic liquid electrolytes, may bring environmental benefits, improved safety, and cost savings [6-8].

This article briefly reviews the use of ionic liquid electrolytes in lithium-ion batteries. It also outlines key questions explored to identify future directions for IL development.

Synthesis of Ionic Liquids

ILs (ionic liquids) consist of bulky and asymmetric cations (classes such as imidazole, pyrrolidine, pyridine, piperidine, ammonium, and ammonium phosphate) and different inorganic or organic anions, including halides (chloride [Cl–], bromide [Br–], iodide [I–]), acetate [AcO–], tetrafluoroborate [BF4–], hexafluorophosphate [PF6–], tetrachloroaluminate [AlCl4–], bistrifluoro methane sulfonimide [TFSI–], ethyl sulfate [EtSO4–], dicyanamide [N(CN)2–] and thiocyanate [SCN–]. Figure 1 shows the molecular structures of cations and anions of several normal-temperature ionic liquids commonly used in lithium-ion batteries.

Figure 1: Schematic diagram of the molecular structure of cations and anions in ionic liquids commonly used in lithium-ion batteries. Among them, A) imidazole cation, B) pyrrolidine cation, C) piperidine cation, D) ammonium cation, E) hexafluorophosphate anion, F) dicyanamide anion, G) tetrachloroaluminum acid salt anion, H) bis(trifluoromethyl) sulfonamide anion.

ILs can usually be prepared through a single-step or two- step synthetic process. For example, imidazolium salts can be prepared by simple alkylation of 1-methylimidazole (Project No. M109227) with alkyl halides (halide anion (X–): Cl–, Br- or I–, etc.) salt (imidazolium halide salt) (Figure 2). The resulting imidazolium hydrochloride can be used directly as an ionic liquid or to generate the imidazolium salt of the desired anion in a subsequent pairing reaction. First, imidazole hydrochloride is mixed with M+A- metal salts (M+: Ag+, Na+ or K+, etc.; A-: B4−, PF6−, TFSI−, etc.). Then, the halide anion was replaced with the desired A - anion to obtain the imidazolium A- salt. However, in some cases, it is difficult to obtain high-purity ionic liquids through metal salt pairing reactions because M+X– (as an impurity) is soluble in ionic liquids. The presence of residual halogen contaminants in ionic liquids may affect their physical properties.

In addition, high-purity ionic liquids can also be obtained through a metal-salt-free process. For example, 1-alkyl-3-methylimidazolium formate can be prepared by alkylating carbonate with 1-alkylimidazolium in a PTFE-coated autoclave at a reaction temperature of 210°C and a reaction time of more than 2 hours. By further neutralizing the solution with acid, an ionic liquid can be obtained, with insoluble by-products being methanol and CO gas. These insoluble by-products can be easily removed by vacuum and heat treatment.

Figure 2 : A) Synthesis principle of imidazolium salt with anion; B) Synthesis principle of 1-ethyl-3-methylimidazolium acetate

Ionic Liquid Electrolytes in Li- ion Batteries

High thermal stability is a desired property of RTIL lithium-ion batteries. The Lombardo group at the University of Rome in Italy reported that the addition of N-butyl-N-methyl pyrrolidinium bis (trifluoro methane sulfonyl) imide to commercial carbonate-based electrolytes can significantly improve the resistance of lithium -ion battery electrolytes. Combustion characteristics (see Figure 3) [2]. Sakaebe et al. pointed out that N-methyl-N-propyl piperazinium bis (trifluoro methane sulfonyl) imide is the most promising candidate for lithium-ion battery electrolytes. In this electrolyte, Li/LiCoO2 cells exhibit good performance, with LiCoO2 having stable capacity and good Coulombic efficiency (>97% at C/10 rate) that is stable with the number of cycles. After fully charged, the battery can be opened in the air, and it will not ignite when a fire comes into contact with the battery [3] .

Figure 3: Experimental comparison diagram of each component in different time periods of ignition

In addition, some scholars have studied the cycle stability of lithium-based batteries using various ionic liquid electrolytes. Holzapfel et al. showed that when 1 M LiPF6 with 5% vinyl carbonate was added in 1-ethyl-3-methylimidazole-bis(trifluoromethylsulfonyl ) imide (EMI-TFSI, Product ID: E465636) excellent charge capacity retention over 300 cycles was observed when cycling LiCoO2 positive electrodes in the electrolyte and Li4Ti5O12 as the negative electrode . However, the reduction stability of pure EMI-based electrolytes seems to be insufficient in lithium battery systems (i.e., the negative electrode is Li metal) [4]. Elia et al. demonstrated a kind of Advanced long-life lithium-ion battery [5]. The battery underwent long-term cycling at 40°C, showing a stable capacity of about 140 mAhg-1 and maintaining a retention rate of more than 99% after more than 400 cycles (Figure 4).

Figure 4: Cycle performance diagram of MTO-TFSI, 1M LiPF6, 5% ethylene carbonate

outlook

Although ionic liquid electrolytes exhibit high conductivity and low viscosity in Li-ion batteries, insufficient cycling stability limits their practicality when using pure Li battery anodes. Pyrrolidine salts have also been mixed with traditional alkyl carbonate-based electrolytes to improve the flame retardancy of Li-ion batteries. On the other hand, Li-ion batteries using pure piperidinium-based ionic liquid electrolytes do exhibit lower flammability and good cycle stability. To compete with commercial Li- ion batteries, long-term cycle stability (e.g., 80% capacity retention after 2000 cycles) will be a major obstacle for the application of ionic liquid electrolytes in Li-ion batteries.

References

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7.Carlin RT, De Long HC, Fuller J, Trulove PC. Dual Intercalating Molten Electrolyte Batteries. J. Electrochem. Soc. 141(7): L73-L76. https://doi.org/10.1149/1.2055041

8.Rothermel S, Meister P, Schmuelling G, Fromm O, Meyer H, Nowak S, Winter M, Placke T. Dual-graphite cells based on the reversible intercalation of bis (trifluoro methane sulfonyl) imide anions from an ionic liquid electrolyte. Energy Environ. Sci. 7(10):3412-3423. https://doi.org/10.1039/c4ee01873g