Many battery systems are developed to meet the needs of energy supply, such as metal ion batteries, metal air batteries, etc. The development of new energy requires the support of systems with uninterrupted energy storage characteristics to solve the intermittent problems of green energy. One of the most suitable devices to replace the new energy storage system is the electrochemical energy storage system. Choosing a suitable electrochemical energy to replace the new energy storage system is one of the effective means to solve environmental pollution problems and reduce the ecological environment load.
The battery does not produce any polluting gases in the process of converting the stored energy into electrical energy. This process involves first converting the electrical energy into many advantages, such as: higher energy density, especially high voltage, high specific internal energy rechargeable batteries have high safety and low cost. With the increasing application of communication equipment, portable devices, electric vehicles and energy storage devices, lithium-ion batteries, as the leader of secondary batteries, have received more and more attention. How to further improve the energy density of lithium-ion batteries has always been an important topic for scientific researchers.

1 Lithium-ion Battery Overview

Over the past two decades, technology has developed rapidly, and lithium-ion batteries (LIBs) have been widely used in people’s daily life and even in aerospace. Compared with secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries, and secondary alkaline zinc-manganese batteries, LIBs have a series of remarkable characteristics such as high energy density, wide operating temperature range, long battery life, and no memory effect. It is precisely because of these characteristics that the LIBs industry has been able to develop into a global market. With the widespread use of lithium-ion batteries in fields such as mobile phones and the growing demand in the electric vehicle industry, its global market value is expected to be approximately US$92 billion in 2024. Commonly used LIBs today mainly use transition metal lithium intercalation compounds (such as LiCoO₂, LiMn₂O₄, LiFePO₄, etc.) as positive electrodes, carbon materials such as graphite and mesophase carbon microspheres (MCMB) as negative electrodes, and solutions of lithium salts (such as LiPF₆, LiBF₄, LiCIO₄, etc.) in organic carbonate solvents [such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), etc.1] as electrolytes. However, the safety issues of LIBs have seriously hindered their widespread application in the field of electric vehicles. In recent years, there have been many reports on LIBs fires and explosions, including the Samsung Galaxy Note and Tesla “Model S” fires, which have attracted the attention of relevant researchers. ILs as electrolytes have the advantages of low flammability and low volatility, so using ILs as electrolytes can improve the cycle stability of LIBs to a certain extent, thereby improving battery safety and avoiding the occurrence of safety accidents.

1.1 Working Principle

The working principle of LIBs is ion migration, that is, lithium ions migrate reversibly between the positive electrode and the negative electrode, so they are also called “rocking chair batteries.” Figure 1 is a schematic diagram of the working principle of LIBs using commercial lithium cobalt oxide as the positive electrode material and graphite as the negative electrode material.

LIBs with lithium-rich materials as positive electrodes must first be charged. During charging, under the action of external voltage, lithium ions in the positive electrode material escape, move to the negative electrode, pass through the electrolyte and the middle diaphragm, and are embedded in the carbon material of the negative electrode, while the movement direction of electrons in the external circuit is exactly the opposite. After charging is completed, lithium ions migrate from the positive electrode to the negative electrode, the positive electrode is in a lithium-poor state and the negative electrode becomes lithium-rich, and the voltage difference between the positive and negative electrodes reaches the maximum. In the subsequent discharge process, lithium ions will escape from the negative electrode again, pass through the middle diaphragm, and re-embed into the positive electrode material. The repeated embedding and extraction of lithium ions in the positive and negative electrode structures during the charging and discharging process can be regarded as an ideal reversible reaction process. The reaction formula is expressed as:

LIBs are actually concentration difference batteries. The potential difference between the positive and negative electrode materials plays a decisive role in their output voltage. Therefore, when selecting positive and negative electrode materials, the positive electrode material should be selected with a high potential, while the negative electrode material should be selected with a low potential. The redox reaction occurs during the charging and discharging process.

LIBs have problems such as fast aging, high cost, and short life in practical applications, among which the most prominent is the safety of the battery (see Figure 2). With the increasing popularity of electric vehicles, their safety has become the primary concern of producers and consumers. The current safety issues of batteries are mainly concentrated on electrolyte leakage and thermal runaway caused by side reactions and exothermic reactions, which need to be solved urgently. Although thermal runaway is usually caused by short circuits, there is another major problem that cannot be ignored, that is, internal chemical crosstalk can generate a lot of heat. Electrolyte is an indispensable part of LIBs and also the most dangerous part. This is because organic electrolytes are flammable and can easily cause fires once they leak. The electrolytes used in LIBs today are mainly solutions composed of Lip₆ lithium salts and chain or cyclic carbonate (EC, DEC, EMC, etc.) solvents. The presence of organic solvents makes the battery prone to leakage, combustion, thermal runaway, etc. during use. Generally speaking, the operation of high-capacity LIBs involves strong chemical reactions that release a lot of heat, which may cause thermal runaway of the battery or even explosion. In addition, the biggest limitation of traditional LIBs is that the operating voltage is difficult to reach 4.5 V, because the traditional electrolyte will decompose under high operating voltage. Therefore, it is urgent to design a new and safe electrolyte system to solve the safety problem of LIBs.

1.2 Commonly used electrolytes in lithium-ion batteries

Electrolyte is a crucial component of LIBs. It is a bridge connecting the positive and negative electrode materials. It plays the role of transporting lithium ions and conducting current between the positive and negative electrodes of the battery. During the charging and discharging process, the role of the electrolyte is to transfer charge. When using LIBs, there are many factors that affect its performance, such as the compatibility between the electrolyte and the electrode, the rate at which the electrolyte transfers charge, and the matching of the electrolyte and the diaphragm. To a certain extent, different electrolytes will also affect the charging and discharging mechanism of LIBs. Since different electrolytes have different properties and different manufacturing prices, choosing a suitable electrolyte is also very important for LIBs.

In the LIBs system, the performance of the electrolyte components will affect the performance of the battery, such as conductivity, viscosity, thermal stability, redox and polarity, which will have a great impact on the battery life, reversible capacity, rate charge and discharge performance, etc.
The prerequisite for determining the specific capacity of LIBs is the positive and negative electrode materials, but the impact of the electrolyte on the reversible specific capacity of the electrode material should not be underestimated. During the charge and discharge cycle of LIBs, the electrode material will undergo the process of lithium removal and lithium insertion, and the electrolyte will interact with it. The result of the interaction will not only affect the interface properties between the electrolyte and the electrode material, but also have a certain effect on the stability of the internal structure of the electrode, such as the surface passivation of the active substance of the electrode material, the redox decomposition of the electrolyte on the electrode surface, etc., which will also have a certain impact on the lithium removal and insertion capacity of the electrode material. Organic electrolytes also have a significant impact on the compatibility of LIBs electrode materials. For example, the positive electrode material will be dissolved by the organic electrolyte, and a solid electrolyte interface (SEI) film will be formed on the surface of the negative electrode.

Cycle life is an important indicator for evaluating the quality of LIBs. Generally, the storage life of commercial LIBs is mostly 3 years. The service life of LIBs is largely related to the electrolyte. The selection of different electrolytes will change its life. The positive electrode current collector of LIBs is generally aluminum foil. The type of lithium salt will greatly affect the corrosion of aluminum foil, but when ionic liquid is added, the stability of lithium salt (especially imide salt) will be increased, which solves the corrosion problem. By improving the corrosion resistance of the current collector in the electrolyte, the battery life can be greatly improved.

The lithium insertion and deinsertion reaction in LIBs takes place at the interface between the electrolyte and the electrode, so the rate and discharge performance of lithium-ion batteries are related to the conductivity of the electrolyte and the lithium ion mobility at the interface between the electrode and the electrolyte. In addition, different electrolytes will also have a certain degree of influence on the internal resistance of the battery, which includes ohmic internal resistance, charge transfer resistance, and interface resistance between electrodes/electrolytes. The internal resistance of the battery is proportional to the resistance of Li crossing the interface. When the resistance is large, it will lead to poor high-rate charge and discharge performance. By optimizing the electrolyte system, the interface resistance can be reduced, thereby improving the performance of the battery.

In addition to the type of electrode material, the self-discharge of the battery is also affected by the composition of different electrolytes and the different properties of the electrode/electrolyte interface. If the lithium ions in the electrolyte are embedded in the lattice of the positive electrode material, it will cause the positive electrode material to self-discharge. If the lithium ions of the negative electrode material escape and enter the electrolyte, it will cause the negative electrode to self-discharge. When the electrolyte undergoes oxidative decomposition, impurities are generated. The negative impact of these impurities cannot be ignored. It undergoes redox reactions on the surface of the positive and negative electrodes, and the active substances are consumed in this reaction, which can also cause self-discharge.

At present, the most widely studied LIBs electrolytes are mainly divided into two categories: liquid electrolytes and solid electrolytes. Liquid electrolytes include organic liquid electrolytes and room temperature ionic liquid electrolytes, and solid electrolytes can be divided into inorganic solid electrolytes (SSE) and organic polymer electrolytes (SPE). Different types of electrolytes have their own advantages and disadvantages, as shown in Table 1. It is of great significance to study the LIBs electrolyte system to meet different production practices.

1 Organic liquid electrolyte

Traditional organic electrolytes are the most mature and widely used electrolytes in LIBs research. They are mainly composed of high-purity organic solvents, electrolyte lithium salts and functional additives.

Organic solvents are the main part of organic liquid electrolytes. The properties of the solvent greatly affect the total ion transport capacity of the electrolyte, especially its conductivity or mobility (i.e., inversely proportional to viscosity), as well as the thermal stability of the electrolyte in the electrolyte. The organic solvents used in LIBs are a mixture of solvents with high dielectric constants and low viscosities. As organic solvents for LIBs, the following conditions must be met: ① It has a high dielectric constant (e) when ε is greater than 15, it can dissolve lithium salts at a sufficient concentration and also prevent ion pairing; ② It has a low viscosity (n), which is conducive to the migration of lithium ions; ③ It is inert to the cathode and anode surfaces during battery operation; ④ The solvent has a high boiling point (T) and a low melting point (Tm), so that it remains in a liquid state over a wide temperature range; ⑤ It has a high flash point (T), is safe, low toxicity, and is cheap.

Esters and ethers are similar in some aspects as organic solvents. They both have two structures: chain and ring. Among them, esters mainly include alkyl carbonates and carboxylates, and carboxylates have poor stability, so they are not often used as lithium-ion electrolytes. Carbonates have cyclic and chain structures. Cyclic carbonates are mainly ethyl carbonate (EC) and propylene carbonate (PC), and chain carbonates mainly include dimethyl carbonate (DMC), dimethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Ether solvents also have cyclic and chain structures, including cyclic ethers and chain ethers. Ether solvents have the characteristics of low oxidation potential and are generally rarely considered as commercial electrolytes.

To meet the performance requirements of electrolytes, a single solvent has great limitations. For example, the boiling point of a general solvent is proportional to its viscosity, and a high boiling point leads to a high viscosity. Therefore, in order to meet the performance requirements of electrolytes in practical applications, a mixed solvent is generally used as an electrolyte solution. Taking commonly used alkyl carbonates as an example, cyclic esters have high polarity and a large relative dielectric constant, but due to the strong intermolecular force, they have high viscosity, such as propylene carbonate and ethylene carbonate; while straight-chain esters have a small relative dielectric constant because the alkyl group can rotate freely, has low polarity and low viscosity, such as dimethyl carbonate and diethyl carbonate.

The role of electrolyte lithium salt is to provide a transport channel for lithium ions. The choice of lithium salt will affect the chemical/electrochemical stability as well as the ionic stability and conductivity. Under the conditions of optimal salt concentration and suitable temperature, LIBs can obtain excellent ionic conductivity. From the perspective of dissociation and ion migration of electrolyte lithium salts in organic solvents, the larger the anion radius, the better the effect of lithium salts when used as electrolytes. According to the different compositions of ionic liquids, electrolyte lithium salts can be divided into two categories: inorganic anion electrolyte lithium salts and organic anion electrolyte lithium salts. Currently, the more studied inorganic anion electrolyte lithium salts include lithium hexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBE₄), lithium perchlorate (LiCIO₄) and lithium hexafluoroarsenate (LiAsF₆). Researchers have made corresponding comparisons based on the conductivity, thermal stability, and oxidation resistance of electrolyte lithium salts, and the relationship is as follows:

Conductivity: LiAsF₆>LIPsF₆>LiC1O₄>LiBF₄
Oxidation resistance: LiAsF₆>LiPF₆>LiBF₄>LiC1O₄
Thermal stability: LiAsF₆>LiBF₄>LiPF₆

From the above strength relationship, it can be seen that LiAsF₆ has the best comprehensive performance, but the reason why it has not been widely developed and utilized is that it contains toxic arsenic elements; LiCIO₄ cannot exist stably in LIBs due to its extremely strong oxidizing properties, so it cannot be used in LIBs; LiPF₆ is the most widely used inorganic anion lithium salt on the market, but it also has problems such as poor thermal stability and easy hydrolysis. For example, LiPF₆/organic carbonate solvent electrolytes will undergo violent electrolyte decomposition and side reactions between electrodes/electrolytes at 40°C and 60°C, because carbonate solvent electrolytes have low anode stability and poor thermal stability. In order to develop electrolyte lithium salts at a deeper level, it is imperative to find cheap electrolyte lithium salts that can replace LiPE6. Researchers mainly focus on two aspects: ① Lithium compounds complexed with boric acid; ② Lithium compounds complexed with phosphoric acid. Among them, because complexed boric acid lithium compounds have less pollution and are more environmentally friendly, they are favored by researchers in the study of electrolyte lithium salts.

Research on organic anion electrolyte lithium salts mainly focuses on solving the following problems: improving the stability of lithium salts, increasing the radius of anions, delocalizing the charge of anions, thereby reducing lattice energy, reducing the interaction between ions, and improving the solubility and conductivity of electrolytes. In the research of new lithium salts, a series of fluorine-containing organic lithium salts and organic borate lithium salts such as lithium bis(trifluoromethylsulfonyl)imide [LiN(SO₂CF₃)₃, lithium bis(polyfluoroalkoxysulfonyl)imide [LiN(RfOSO2CF₃)₃] and lithium bis(oxalatoborate) (LiBOB) have also received special attention, and these lithium salts have been widely used as additives. In addition, compared with organic electrolytes, as the temperature decreases, the lithium salts in the battery may precipitate crystals, seriously affecting the performance of the battery.

In order to improve the safety performance, high temperature performance, cycle life and other aspects of the battery, researchers have tried to add a small amount of certain substances to the organic electrolyte of LIBs and achieved corresponding results. These small amounts of substances are called additives. In the development of LIBs technology, researchers have been keen on the research and development of new additives. On the basis of basically not increasing the cost of the battery, if you want to significantly improve certain performance of the battery, you can consider choosing a suitable additive. Additives have the characteristics of “small dosage and quick effect” and have a significant positive effect on battery performance. Additives generally should have the following characteristics: ① good compatibility with the internal materials of the battery; ② no obvious side effects on battery performance; ③ relatively low price; ④ no toxicity or low toxicity of LIBs electrolyte additives. At present, researchers are mainly studying the following three aspects: ① SEI film formation additives; ② overcharge protection additives; ③ flame retardants to reduce the flammability of electrolytes.

One of the most important functions of electrolyte additives is to form a solid electrolyte interface (SE) film on the electrode surface to provide protection for the electrode, improve battery stability and extend battery life. In the past decade, boron-based additives, phosphorus-based additives, sulfone-based additives, nitrile-based additives and ionic liquids (ILs) have been developed for use in electrolytes for high-voltage LIBs. For example, adding flame retardants to conventional liquid electrolytes or using non-flammable liquids as additives can reduce their flammability, improve their flame retardancy, and even achieve non-flammability, significantly improving the safety of LIBs. Among these additives, ILs, as common ionic solvents, have a wide electrochemical window and are expected to broaden the voltage of LIBs. Zhang et al. selected CPMIMTFSI as an electrolyte additive through DFT calculations, providing a more effective and economical method for finding suitable additives.

In summary, the high ionic conductivity and low viscosity of organic liquid electrolytes make them the most widely used LIBs electrolytes. The standard electrolyte for LIBs is mainly a mixture of two components: a solvent mixture (e.g., ethylene carbonate and dimethyl carbonate) and LiPF₆ salt. However, this system is not very safe. In addition, LiPF₆ can also hydrolyze to form HF, which is another chemical with better stability besides the above components. In addition, the low thermal stability of the electrolyte means that storage in a high temperature environment will lead to battery degradation. Therefore, improving the safety performance of the electrolyte is the biggest problem that needs to be solved urgently. The development of organic liquid electrolytes mainly focuses on the development of new lithium salts, the composition ratio of the electrolyte, and the development of additives.

2 Room temperature ionic liquid electrolyte

In recent years, ionic liquids have attracted much attention from the academic community and the industry as a “new type of solvent”. Compared with traditional organic solvents, it is a completely different new type of substance. In addition, due to the inherent strong electrostatic interaction of ionic liquids, they have extremely low vapor pressure, which means that most ionic liquids are fundamentally difficult to ignite, which makes them very attractive as electrolytes for energy storage applications. So far, ionic liquids have been used in many fields such as organic synthesis, polymer synthesis, separation technology, and various electrochemical devices (including lithium secondary batteries, fuel cells, supercapacitors, etc.). When applied to LIBs electrolytes, ionic liquids can exhibit many excellent physical and chemical properties.

In LIBs, the room temperature ionic liquid electrolytes that have been studied more are mainly divided into two categories. The first category is imidazolium ionic liquid electrolytes, which are also a relatively mature type of ionic liquid electrolytes; the other category is quaternary ammonium salt ionic liquid electrolytes. Since its first application in LIBs in 1980, imidazolium ionic liquid electrolytes have received widespread attention. This is because the 1-ethyl-3-methylimidazolium cation (EMI) has the advantages of high conductivity and good chemical stability. However, this type of electrolyte also has certain disadvantages: for example, the electrochemical window is relatively narrow (about 4.2 V), and the stability is poor when coexisting with the electrode material metal lithium. At the same time, the imidazole ring at the 2 position is connected to the acid proton and it is a strong reducing agent with a reduction potential of about 1 V (vs. Lit/Li). These disadvantages make it impossible to use metal lithium and carbon in the negative electrode, which inevitably becomes the biggest obstacle to its application in lithium ions. The current widely used modification method is to use Li₄Ti₅Or₁₂ as the negative electrode material or replace the acidic proton connected to the carbon atom at the 2 position with an alkyl group.

Compared with imidazolium ionic liquid electrolytes, quaternary ammonium ionic liquids have better stability, with a reduction potential lower than 0V (vs. Li⁺/Li) and an oxidation potential higher than 5 V, which is conducive to its own stability and also prevents oxidative decomposition when it coexists with the positive electrode material. According to the structural formula of quaternary ammonium salts, they can be divided into cyclic quaternary ammonium salts and chain quaternary ammonium salts, and pyrrole (five-membered ring) and piperidine (six-membered ring) ionic liquids belong to the category of dry cyclic quaternary ammonium salts. The melting point of quaternary ammonium salt ionic liquids is low because the negative charge of its main anion TFSI is dispersed from the nitrogen atom to the two S atoms in the adjacent position, weakening the interaction with the cation. Sun et al. 2 studied a variety of quaternary ammonium ionic liquids with TFSF as anion and found that both chain and cyclic quaternary ammonium salts follow the following rules: ionic liquids with small cations and high symmetry will lead to higher melting points, and ionic liquids with large cations and low symmetry will lead to relatively low melting points. At room temperature, the conductivity relationship between pyridinium salt P₁₄, quaternary ammonium salt N₁₁₃₄ and imidazolium salt BMI with the same number of carbon atoms is: σ(N₁₁₃₄) (0.8mS·cm^-1)< σ(P₁₄) (2 mS·cm^-1)<σ(BMI) (38 mS·cm^-1). Cations with a planar structure on the surface can greatly improve the conductivity of ionic liquids.

The chemical properties of the cations strongly affect the reduction stability of ionic liquids, while the oxidation stability of the anions mainly determines the anodic limit of ionic liquids. Ionic liquids are obtained by combining a large number of cations [such as pyrrolidine (RYR), piperidine, imidazole and quaternary ammonium] with anions (such as BF^–₄). TFSI- and FSI- have extremely wide electrochemical windows, and researchers have conducted systematic experimental and theoretical studies on them as electrolytes for LIBs.

Before developing any lithium battery based on ionic liquid electrolytes, four key issues should be considered.

1) High viscosity and slow diffusion

The viscosity of ionic liquids is very important in electrochemical systems (including LIBs4l) because it is related to conductivity, diffusivity and wettability, and is more complicated than the viscosity of conventional fluids, as the addition of lithium salts to an ionic liquid increases its viscosity and reduces conductivity. In fact, the diffusion of ionic liquids is not as slow as assumed. The diffusion coefficient of active species in ionic liquids can be on the order of 10^-7cm²  s^-1exhibiting diffusion-controlled behavior. The problem of slow diffusion of lithium ions in ionic liquids is due to the use of high concentrations of lithium salts in lithium batteries to increase the migration number. In other words, the main factor that slows down the diffusion is not the viscosity, but the active adsorption of lithium ions in the rigid structure of the ionic liquid.

Generally, lithium ions diffuse much faster in electrolytes than in solid electrodes (anode or cathode) (even in viscous ILs). A key issue caused by the high viscosity of ILs is the poor wettability of electrode materials. Because the electrolyte should be distributed in the porous structure of electrode materials, one can consider optimizing the porosity size of electroactive materials to improve the wettability of the materials in specific ILs.

When solid-state diffusion is much slower than the diffusion of lithium ions in ionic liquids, as in the case of lithium iron phosphate, battery performance using ionic liquid ions is much better than that of conventional organic solutions. In the case of fast solid-state diffusion in cathode materials (such as lithium manganese oxide), slow diffusion in ionic liquids is the rate-determining step. When lithium ions have enough time to reach the solid active material under slow charge/discharge, ionic liquids are quite competitive with conventional organic solutions. It is worth noting that although low viscosity is beneficial for the rated capacity, ionic liquids with higher viscosity can show certain advantages in specific capacity.

2) Solvation in ionic liquid electrolytes

The key step of lithium insertion is to release lithium ions at the electrode/electrolyte interface. Lithium solvation in ILs is significantly different from that of polar molecules, where lithium ions should be released from solvation traps. Due to the difference in anions of the ionic liquids, lithium ions are more likely to break out of the solvation traps, leading to better battery performance. When lithium diffusion is mentioned, it is not the free diffusion of single lithium cations but solvated lithium ions, which means that the diffusion process strongly depends on the size and charge of the solvated lithium, which in turn depends significantly on the ionic liquid used.

Lithium coordination in a particular ionic liquid plays an important role in battery performance. Lithium coordination is different in protic and aprotic ionic liquids. Generally, lithium coordination number is lower in protic ionic liquids. Aprotic ionic liquids can achieve more stable and wider electrochemical windows.

3) Double-layer barrier on the electrode surface

In any electrochemical system, the electrode surface can form a double layer through charged species; however, this phenomenon is more serious and complicated in ionic liquids composed entirely of charged ions, which means that lithium cations must pass through this barrier to achieve transfer on the electrode surface (as shown in Figure 3). Surface analysis shows that ionic liquids do not form a classic double layer, but instead form a complex interfacial layer composed of several layers of cations and anions at the electrode/electrolyte interface.

Figure 3 Schematic diagram of the complex interaction between one-dimensional diffusion in conventional electrolytes and diffusion in ionic liquids

The structure of ionic liquids shows a possible asymmetrical arrangement of interacting groups. In addition, ions in ionic liquids can be chemically/physically adsorbed on the electrode surface like Li⁺.

4) Solid Electrolyte Interface (SEI)

For batteries based on ionic liquids, the formation of SEI on the electrode surfaces (anode and cathode) is a key issue. Solid electrolyte interphase layers can be beneficial or detrimental. For example, when bis(trimethylsilyl)-2-fluoro-2-methylmalonic acid is used as a reducing additive, because hydrogen fluoride (HF) is eliminated during the first charge, the remaining organic components can enter the SEI, forming a thinner layer, protecting the battery from harmful side reactions. The formation of a thin permeable film protects the electrodes during battery operation, but the SEI film may also be blocked if it is too thick. In organic solutions, the SEI is usually formed by the decomposition of organic compounds at extreme potentials. However, the SEI in ionic liquids is actually the result of a chemical reaction, because the electrode surface can also be passivated without electroplating. In this case, the initial capacity of ionic liquids is usually low, but the resulting SEI is conducive to excellent cycling performance. Electrochemical reduction of ionic liquid cations can lead to the formation of SEI, but this is not only related to the electrochemical stability of the cations.

The anions of ionic liquid electrolytes are crucial in lithium battery applications. The cathodic instability of ionic liquid electrolytes at negative potentials is caused by cation reduction, but the anions play an important role in electrochemical stability, thus widening the potential window. Battery performance is usually affected by cations rather than anions, because (at least in the common case) cations have a more significant effect on the viscosity of lithium-ion binary electrolytes. The formation of the SEI is the result of the electrochemical reaction of anions and cations at the electrode surface. Therefore, the properties of the SEI are directly related to the interfacial charge transfer at the electrode surface, which leads to film growth. It is still undecided whether ionic liquid electrolytes can form a stable electrochemically active SEI, because the situation can vary greatly depending on the electrode material considered.

Electroactive films become rough due to solid-state diffusion during charge/discharge. While the surface morphology changes dramatically after cycling in conventional electrolytes, electroactive films can maintain their dense structure after cycling in ionic liquid electrolytes. The pores formed after cycling in conventional electrolytes lead to poor mechanical stability.

The electrochemical reactions on the electrode surface are controlled not only by the electrode material but also by its atomic structure. In batteries similar to carbon anodes, those anodes with more edge carbon atoms show higher irreversible capacity due to the presence of side reactions. The dangling carbon atoms on the edge of the graphene plane have been shown to change the charge transfer path in the electrochemical system.
In summary, the application of ionic liquid electrolytes in LIBs still faces many problems that must be solved. First, the biggest problem is that almost all ionic liquids have high viscosity, which increases with the increase of charge carrier (salt) concentration. Secondly, the higher cost associated with ionic liquids (mainly from anions such as TFS- and FSI-) is another limiting factor for their widespread application. In addition, the migration number of Lit in ionic liquids is very low, which limits the capacity of LIBs at high rates. The above characteristics may not apply to all ionic liquids.

3 Inorganic solid electrolyte

All-solid-state batteries are considered to be alternative candidates for the next generation of clean energy storage. The use of inorganic solid electrolytes (SSEs) eliminates the liquid component in all-solid-state LIBs (ASSBs), which increases the possibility of using lithium metal anodes to improve battery energy density. Compared with organic liquid electrolytes, SSEs have the following main advantages: non-flammable, non-volatile, non-leaking, wider battery voltage window, and higher energy density. To date, various types of inorganic solid electrolytes have been reported: Li₄GeO₄, Li₁₀GeP₂S₁₂, Li₇PS₆, Li₆PS₅C₁, Li₇La₃Zr₂O₁₂, LiTi₂(PO₄)₃, Li₇PN4, LiBH₄, Li_0.38Sr_0.44Tao.7Hf_0.3O_0.295F_0.05, LisYCl6. . The low ionic conductivity of solid electrolytes is considered to be the main problem hindering the development of all-solid-state LIBs. The schematic diagram of a typical all-solid-state LIB is shown in Figure 4. The battery components include anode, solid electrolyte, cathode and current collector. During the insertion and deinsertion process, lithium is transported through the anode, the interface between the solid electrolyte and the anode, the solid electrolyte, and the interface between the solid electrolyte and the cathode. The ionic conductivity of solid electrolytes can be greatly improved by element substitution or doping, structural adjustment, and regulation of internal and heterogeneous interfaces (e.g., anode-SSE reaction layer). Wang et al. found that the body-centered cubic anion skeleton can provide a direct lithium conduction path between adjacent tetrahedral sites and is therefore considered to be the best crystal structure for obtaining high ionic conductivity.

Figure 4 Schematic diagram of the structure of all-solid-state LIBs

SSEs have much better ionic conductivity than polymer electrolytes, so they have been sought after by many researchers. In recent years, the most studied inorganic electrolytes are perovskite, NASICON, garnet, LISICON and sulfide materials. Garnet electrolyte is the most representative ceramic oxide, with the following notable characteristics: high lithium ion conductivity, ideal lithium ion migration number close to 1; low electronic conductivity; good compatibility with the cathode; strong tolerance to extremely high voltages; good stability to lithium metal and moisture in the air. Of course, their shortcomings are also obvious: too hard texture can easily lead to poor interface contact with the electrode; relatively fragile properties make it difficult to manufacture and transport; in some cases, lithium may penetrate the garnet electrolyte layer along the grain boundary.

The conductivity of solid electrolytes is closely related to their chemical composition, crystal structure and density. In solid inorganic electrolytes with tetrahedral or octahedral crystal structures, cations with sufficient activation energy move or jump along continuous coordination sites to achieve the purpose of ion conduction. In fact, the favorable coordination environment is mainly generated by point defects in the lattice. The point defect mechanism is mainly divided into three categories: vacancies, direct filling and filling substitution, that is, ions continuously enter adjacent vacancies or replace ions in adjacent lattices. In short, in addition to the characteristics of the key materials themselves, the structure of the crystal determines the ion transport type, activation energy and even ionic conductivity.

The conductivity of solid electrolytes is closely related to their chemical composition, crystal structure and density. In solid inorganic electrolytes with tetrahedral or octahedral crystal structures, cations with sufficient activation energy move or jump along continuous coordination sites to achieve the purpose of ion conduction. In fact, the favorable coordination environment is mainly generated by point defects in the lattice. The point defect mechanism is mainly divided into three categories: vacancies, direct filling and filling substitution, that is, ions continuously enter adjacent vacancies or replace ions in adjacent lattices. In short, in addition to the characteristics of the key materials themselves, the structure of the crystal determines the ion transport type, activation energy and even ionic conductivity.

Oxide SSEs are one of the earliest families of inorganic solid electrolytes that have been widely explored. Perovskite-type Li_3xLa_2/3-x-xTiO₃ (0<x<0.16), NASICON-type Li_1.3Al0.3Ti_1.7(PO₄)₃, and garnet-type Li₇L₃Zr₂O₁₂ are the most common oxides with good ionic conductivity. In addition, sulfides are also one of the research focuses. However, to use sulfide SSEs in 4 V-class cathodes, a protective coating must be introduced. This is because the inherent instability of sulfides at high potentials makes the surface of active materials easily oxidized7. Studies have found that many oxides can be used as potential coating materials, such as LiNbO₃, LiNb_0.5Ta_0.5O₃, Li₄TisO12, Li₃PO₄, Ta₂O₅, Al₂O₃, and Li_3-xB_1-xC_xO₃:. According to the Faian rule, the deformability of chlorides is expected to be even better than that of sulfides, which predicts more covalency of chlorides. Li₃YCl₆ was reported to be a poor ionic conductor (about 10^-7 S·cm^-1 at 110°C), but further investigation of this material showed that it can exhibit moderate room temperature ionic conductivity of 4×10^-5 S·cm^-1 as a well-crystallized material, up to 5.1×10^-4 S·cm^-1 in a low crystalline form. LisErCl. also exhibited similar ionic conductivity (ball milled: 3.3×10^-4 S·cm^-1, followed by annealing: 5×10^-5 S·cm^-1). Li₃InCl₆ can achieve ionic conductivity of 1.49×10^-3 S·cm^-1.

The mechanical properties of solid electrolytes inhibit the growth of lithium dendrites, enhance the compatibility between electrodes and electrolytes, that is, reduce interface resistance and increase interface stability, and even greatly inhibit the generation and expansion of cracks. Specifically, lithium dendrites consume excess lithium ions and accelerate the consumption of active lithium ions in the reduction-oxidation reaction. If the formed lithium dendrites are long enough to penetrate the entire battery, they may even cause internal short circuits in the battery. Interface resistance may play an important role in limiting the performance of all-solid-state LIBs. Most reported solid electrolytes have low wettability with lithium metal anodes, so the electrode-electrolyte interface resistance cannot be ignored, and is even very large in some cases. The Han research group improved the cycling performance of all-solid-state LIBs by adding an intermediate layer, namely Al₂O₃, between the solid electrolyte and lithium metal to increase wettability and reduce the interface resistance from 17102·cm² to 12·cm². Kerman et al. found that the “cracking” phenomenon may be related to the mechanical properties of the solid electrolyte, such as shear modulus, hardness, fracture toughness, etc. However, the mechanism of “cracking” of solid electrolytes is not yet clear.

4 Organic polymer electrolyte

Compared with SSEs, organic polymer electrolytes are mechanically flexible and contain a large amount of lithium salts or liquid ions, which can form ion-conducting solutions with appropriate mechanical stability. In addition, polymers have good dimensional stability, thermal stability, and electrochemical stability, which successfully solves the problem that inorganic solid electrolytes cannot form continuous contact with active materials, that is, the electrochemical interface is discontinuous. The ductile organic polymer electrolyte can fill the gaps between electrodes and adapt to the volume changes during charge/discharge, reducing the strain at the interface between the electrolyte and the electrode material, as shown in Figure 5.

Figure 5 Schematic diagram of the interface between organic polymer electrolyte, inorganic solid electrolyte and electrode material

There are two common types of organic polymer electrolytes, with poly(ethylene oxide) (PEO) and polycarbonate as matrices. PEO is a semi-crystalline polymer that is widely used as an organic polymer electrolyte after complexing with lithium salts. Its glass transition temperature (Ta) and melting temperature (Tm) are 60°C and 65°C, respectively, and its ether groups show high affinity for lithium ions, allowing lithium salts to be mixed in the polymer matrix. PEO-based solid electrolytes have good compatibility with electrodes and mitigate the growth of lithium dendrites in LIBs. As a semi-crystalline polymer, the ionic conductivity of PEO decreases significantly in large crystalline areas. The mobility of anions along the chain is greater than that of cations, which leads to a decrease in the lithium ion migration number, which can be improved by properly selecting salts with larger organic anions and huge electron delocalization, such as TFSI. Bao et al. proposed a new type of oxygenated ethyl polyionic liquid modified graphene oxide nanoparticles (ox-PIL@GO) for the preparation of PEO-based organic-inorganic composite electrolyte membranes (CPEs). Both experimental and DFT simulation results indicate that the electrostatic interaction between imidazolium cations and TFSI-anions in CPEs significantly enhances the dissociation of LiTFSI. Adding plasticizers or copolymers to PEO-based organic polymer electrolytes can reduce crystallinity and glass transition temperature, increase electrochemical stability and inhibit the formation of lithium dendrites. For example, the conductivity of PEO/PEI blends with lithium chloride is 10^-4S·cm^-1 at 30°C, while PEO/AA blends improve ionic conductivity by blocking anions and increasing the migration number of lithium. PEO/PAN copolymers composited with lithium chloride show an ionic conductivity of 6.79×10^-1 S·cm^-1 at 25°C. So far, many systems of diblock and triblock copolymers of PEO-based organic polymer electrolytes have been developed. The combination of single ion conducting block poly(lithium methacrylate-co-oligoethylene glycol methacrylate) [P(MALi-co-OEGMA)] and polystyrene block enables room temperature ionic conductivity to reach 0.2×10^-4S·cm^-1.

Organic polymer electrolytes have very poor inherent ionic conductivity (10^-7~10^-5S·cm^-1 basically not exceeding 10^-4S·cm^-1, while the ionic conductivity of liquid electrolytes is usually 10^-2 S·cm^-1) due to their relatively small dielectric constant values, which makes it impossible for LIBs to achieve excellent rate performance and high power density. Lithium salts in organic polymer electrolytes based on PEO cannot be effectively dissociated, resulting in a large number of ion aggregates. The polar carbonate groups (-O（C=O)O) in polycarbonate can inhibit the crystallization ability of the main matrix and also increase the dissociation of lithium salts. Aliphatic amorphous polyvinyl carbonate (PEC) mixed with LiFSI as electrolyte significantly reduced the coordination bond between polymer chains and lithium ions, reaching a conductivity of 2.2×10^-4S·cm^-1,When 1% (mass fraction) of titanium dioxide nanoparticles were added, the conductivity increased to 4.3×10^-4 S·cm^-1

The lithium ion transfer number of the PEC-LiTFSI organic polymer electrolyte is 0.4, which is 4 times higher than that obtained by the PEO-based organic polymer electrolyte. When 0.2 mol·L^-1 of LiTFSI is introduced, good electrochemical stability up to 4.5 V (vs. Li⁺/Li) and ionic conductivity of 4.7×10^-4 S·cm^-1 can be obtained. The organic polymer electrolyte based on polyvinyl carbonate (PVCA) has an electrochemical window of 4.5 V (vs. Li⁺/Li) and an ionic conductivity of 9.82×10^-5S·cm^-1 at a temperature of 50°C. The self-supporting polypropylene carbonate (PPC)-based composite material has an ionic conductivity of 5.2×10^-4 S·cm^-1 at 20°C, an electrochemical window of 4.6 V (vs. Li⁺/Li), a lithium ion transfer number of 0.75, and can operate in the temperature range of 0~160°C1110. Organic polymer electrolytes based on PCL and PPC have good electrochemical stability, high ion transference number, good interfacial compatibility, high Coulombic efficiency, long cycle life and ionic conductivity of 10^-4S·cm^-1!.

Polymethyl methacrylate (PMMA) shows high affinity for lithium ions and plasticizing solvents. Polymethyl methacrylate has good compatibility with other polymers and has been mixed with different counterparts to obtain solid electrolytes for LIBs. PVDF has a high dielectric constant, good chemical and electrochemical stability, as well as good thermal stability and excellent mechanical properties. In addition, there are some plastic crystal electrolytes, that is, lithium salt solutions are incorporated into plastic crystals, for example: with polycarbonate (PC) as the polymer electrolyte matrix, the synthesized all-solid-state LIBs can operate at high temperatures. The mechanical ductility of plastic crystals allows them to be used as solid polymer electrolytes for flexible batteries, and they exhibit excellent room temperature ionic conductivity and thermal stability.

Based on the previous single ion conductor theory, Chen et al. synthesized a new lithium ion conductor P(SSPSILi-alt-MA) with a very large anion volume. They mixed PEO as a polymer matrix with this new lithium salt, dissolved it in DMSO, and dried it to obtain a solid polymer electrolyte. The P(SSPSI Li-alt-MA) membrane showed good performance in the experiment, with a lithium ion migration number of 0.97 and a lithium ion conductivity of 3.08×10^-4 S·cm^-1 at 25°C. Xu et al. prepared PEO-LiTFSI electrolyte with Li_3/8Sr_7/16Ta_3/4 Zrv_1/4O₃ filler, which maintained good stability and mechanical strength even at 45°C. A typical example of in-situ preparation of high-performance solid PEO-LiTFSI electrolyte on LFP cathode is as follows: a cross-linked polymer of PEO and tetraglycerol was synthesized, tetraethylene glycol dimethacrylate oligomer was introduced, and the material was cured by ultraviolet irradiation.

Although solid polymer electrolytes (SPEs) have many advantages over inorganic solid electrolytes, their application is limited by the high interfacial impedance of SPEs. There are three common strategies to solve this problem: one is to wet the electrode with liquid electrolytes, plasticizers or ionic liquids; the second is to form a buffer layer between the electrode and the rigid inorganic electrolyte; the third is to mix the active material with a solid electrolyte (including polymer electrolytes and inorganic electrolytes) or functional additives (such as dielectrics and sintering additives), which greatly improves the contact between them and promotes the transfer of lithium ions. Li et al. used n-propyltrimethoxysilane (PTMS), polyethylene glycol methyl ether (MPEG) and polyethylene glycol (PEG) as raw materials to synthesize Si-doped flexible self-supporting comb-like PEG-based copolymers (Si-PEG) through polycondensation reaction.

2 Application of ionic liquids in lithium-ion batteries

Although there are many problems in the application of ionic liquid electrolytes, if these problems are better solved, LIBs will make a big step forward in terms of safety performance and application in high-temperature environments. Therefore, there are still many studies dedicated to optimizing the performance of ionic liquid electrolytes. There are usually three forms of application of ionic liquids in LIBs electrolytes. The first is that pure ionic liquids are directly used as solvents for lithium salts in LIBs. The second is that they are mixed with organic solvents in a certain proportion and applied to LIBs electrolytes. In addition, ionic liquids can also be used as fillers for gel polymer electrolytes, or ionic liquids can be directly polymerized, that is, applied to gel polymer electrolytes in the form of polyionic liquids. The following will introduce the application of ionic liquid electrolytes in LIBs according to these three application forms.

1 Pure ionic liquid electrolyte

Ionic liquids composed of a large number of combinations of cations (such as pyrrolidinium, piperidinium, imidazolium, and quaternary ammonium) and anions (such as BF4 (tetrafluoroborate), TFSI [bis (trifluoromethanesulfonyl) imide], and FSI [bis (fluorosulfonyl) imide]) can achieve very wide electrochemical windows and have been extensively studied experimentally and theoretically as electrolytes for LIBs. As early as 2007, Nakagawa et al. reported that [PP₁₃] [TFSI] was used as an additive to improve the thermal stability of mixed electrolytes, while Wang et al. found that [BMMI] [TFSII, [PP₁₄] [TFSI] and [TMBA] [TFSI] electrolytes were much safer than conventional carbonate-based electrolytes. Srour et al. tested LIBs based on graphite/lithium iron phosphate (C_gr/LFP), Li₄Ti₅O₁₂/lithium iron phosphate (LTO/LFP), and Li₄Ti₅O₁₂/LiNi_1/3Mny_1/3Co_1/3O₂ (LTO/NMC), using [C₁C_nIm][NTf₂] and [C₁C₁C _nIm][NTf₂] (n=4 and 6) based electrolytes dissolved with different lithium salts (LiIN(SO₂CF₃)₂] (LiNTf₂) Li[N(SO₂F)₂] (LiFSI) and LiPF₆), respectively, and compared them with commercial carbonate-based electrolytes [EC:DEC][LiPF₆]. The results showed that the cycling of the C_gr/LFP system only occurred in the presence of carbonate additives; since the reduction potential of the [C₁C₁C₆Im] cation is lower than that of the [C₁C₆Im] cation, the methyl substitution at the C₂ position only has a positive effect on the C_gr/LFP system. For LTO anode, the battery cycle was stable using imidazolium ionic liquid electrolyte. In addition, [C₁C₆oIm][NTf₂] also achieved stable high capacity for LFP and NMC cathodes and showed better performance than commercial electrolytes.

Moreno et al. reported a ternary mixture (LiTFSI-Pyr3TFSI-Pyr₁₃FSI) based on bis(trifluoromethanesulfonyl)imide lithium salt and two ionic liquids sharing the same cation (N-methyl-N-propylpyrrolidine, Pyr₁₃) but different anions. The study found that the mixture can dissociate ions and exhibit better ion transport performance than each single component (conductivity is about 10^-3S·cm^-1 at 20°C). It was observed in the carbon working electrode that the 0.1LiTFSI-0.3Pyr₁₃TFSI-0.6Pvr₁₃FSI ionic liquid electrolyte can achieve an electrochemical stability window of 5V, and preliminary electrochemical performance tests confirmed the good performance of the ternary electrolyte. As shown in Figure 6, the reversible capacities of NMC[Li(Ni_0.33Mn_0.33Co_0.33)O₂] and graphite electrodes are 135 mAh·g^-1 and 220 mAh·g^-1, respectively, which are equivalent to 87% and 60% of the reversible capacities in traditional alkyl carbonate solutions.

Figure 6 Voltage and capacity curves of NMC and graphite electrodes in 0.1LiTFSI-0.3Pyr₁₃TFSI-0.6Pyr₁₃FSI ionic liquid electrolyte at 0.1C rate
Inset is the initial cycle performance diagram

Oltean et al. reported a full-cell based on ionic liquid electrolyte that can be stably cycled at 80°C. The battery consists of a titanium dioxide negative electrode and a LiFePO₄positive electrode, and Li_0.2(Pyr₁₃)_0.8TFSI and 1mol·L^-1 LiTFSI/PC are used as electrolytes for comparison. The electrochemical test results show (Figure 7). The battery containing ionic liquid electrolyte can be cycled for 100 weeks at 80°C without obvious capacity loss, but the use of conventional organic liquid electrolytes will have obvious capacity decay, which is attributed to the oxidative degradation of organic electrolytes, resulting in low coulombic efficiency. The full battery based on ionic liquid still shows excellent performance at a rate of up to 2C.

Figure 7 Cycling performance (a) and rate performance (b) of a battery assembled with titanium dioxide as negative electrode, LiFePO₄ as positive electrode, and Li_0.2(Pyr₁₃)_0.8TFSI ionic liquid electrolyte
The inset in (a) is the cycling performance diagram of a battery using 1mol·L^-1LiTFSIPC organic electrolyte

NaKagawa and Aihara et al. applied 1 mol·L^-1 LiBF₄/EMIBF₄ electrolyte to LiCoO₂/Li₄Ti₅O₁₂ batteries. At a rate of 0.2C, the battery had a first-week discharge capacity of 120mAh·g^-1, a Coulombic efficiency of 71.4%, and a capacity retention rate of 93.8% after 50 cycles. Seki et al. investigated the electrochemical performance of two room-temperature ionic liquid electrolytes, EmimTFSI and DMPImTFSI, in LiCoO₂ electrodes. As shown in Figure 8, LiTFSVDMPImTFSI electrolyte showed better cycle performance in LiCoO₂, with a first-week discharge capacity of 130mAh·g^-1, a capacity retention of 93% after 50 cycles, and a Coulombic efficiency of more than 99.5%. Therefore, modifying the structure of imidazole ionic liquids can significantly improve their compatibility with positive electrode materials.

Figure 8. (a) Discharge capacity and (b) Coulombic efficiency of LiCoO₂-emimIFSI-LiTFSI mixed electrolyte or DMPImTESI-LiTFSI mixed electrolyte [lithium metal anode battery

Seki et al. applied the electrolyte 0.32 mol·kg^-1LiTFSI/DEMETFSI composed of ether-containing quaternary ammonium salt ionic liquid DEMETFSI to LiCoO₂/Li batteries and found that under 1/8 rate charge and discharge, the discharge capacity in the first week reached 145 mAh·g^-1, and after 100 cycles, it still maintained 81% of the capacity.

2.2 Organic solvent/ionic liquid electrolyte

Pure ionic liquid electrolytes have the disadvantages of low lithium salt concentration and poor wettability with electrode materials, which in turn affects the battery’s cycle performance and Coulombic efficiency. In recent years, people have added certain organic electrolyte components to ionic liquids to modify them. Mixing ionic liquids with organic carbonates can combine some of the advantages of both. But this does not meet the ultimate goal of replacing existing organic solutions, because their coexistence may cause safety problems in the battery. However, if the content of ionic liquid in the mixture is high enough, the electrolyte is actually non-flammable. Adding 50% ionic liquid to conventional electrolytes can significantly reduce the flammability of organic electrolytes, while the battery performance remains unchanged. But mixing ionic liquids and organic carbonates can obtain better viscosity and conductivity, and the ionic conductivity of the resulting mixture can even be higher than that of organic carbonates.

Mixed electrolytes do not necessarily require the volume ratio of the two components to be equal, and one of the components can be used as an additive. For example, a small amount of organic carbonate (such as 5%) can significantly improve the performance of the battery. Adding 10% to 20% of organic carbonate can increase the viscosity and ionic conductivity of the ionic liquid, while the electrolyte remains non-flammable. Molecular dynamics simulations show that organic additives increase ion mobility by reducing lithium coordination. In addition to adding organic carbonates to improve the battery performance of ionic liquids, it is also a practical method to use ionic liquids instead of traditional organic electrolytes as liquids. In these cases, ionic liquids are not used as liquid electrolytes, but as organic ion salts to introduce large ions into the electrolyte matrix. At this time, the role of the ionic liquid is to act as a flame retardant for the organic electrolyte. Ionic liquid flame retardant additives can significantly reduce the flammability of conventional organic carbonate electrolytes.

Compared with pure ionic liquids, the presence of ionic liquids in organic carbonates reduces the electrochemical stability window, which has been confirmed in a large number of relevant literature. At the same time, the compatibility of the electrolyte with the electrode improves the anode stability. For example, 0.5 mol·L^-1 LiNO₃/PyrNO₃ electrolyte exhibits good conductivity and electrochemical stability window, which is large enough to ensure the safe extraction and insertion of lithium in lithium iron phosphate. Figure 9 shows a comparison of the conductivity properties of different types of electrolytes used in LIBs. In the conductivity diagram of ionic liquids, the conductivity of the original ionic liquid is three orders of magnitude higher than that of traditional organic liquid electrolytes and gel polymer electrolytes, which is due to the higher ion mobility of ionic liquids. In addition, ionic liquids act as co-solvents in the preparation of electrolytes, which can avoid solvent effects in electrolyte membranes.

Figure 9 Ionic conductivity of ceramics, solid electrolytes, organic liquid electrolytes, polymer electrolytes, and ionic liquid gel electrolytes

Ionic liquid electrolytes containing anions FSI- have higher ionic conductivity and lower viscosity due to the smaller size of FSI compared to TFSI- anions. In some cases, ionic liquids with carbonate baths show higher viscosity and lower conductivity, which leads to poorer cycling performance than organic electrolytes (LiPF₆ in organic carbonates), especially at high rate cycling, due to the formation of random SEI films on the negative electrode and slow ion migration.

To improve the safety performance of LIBs at high temperatures, the properties of hybrid organic electrolytes (i.e., organic carbonates with ionic liquids added) such as flammability and volatility can be improved by changing the organic solvent content. At room temperature, a hybrid organic electrolyte containing Pyr₁₃TFSI:LiTFSI:(EC:DEC, 1:1, mole fraction) (60:10:30) showed performance similar to that of commercial liquid electrolytes in Li/Li₄Ti₄O₁₂ and Li/lithium iron phosphate (LFP) half-cells. Similarly, the highest discharge capacity of 0.3 mol·L^-1 LiTFSI with Pyr₁₃TFSI:VC:(EC:DMC, 1:1, mass fraction) (65:5:30. volume fraction) electrolyte was 150 mAh·g^-1 in Li/lithium iron phosphate half-cells at 75 °C.

Ababtain et al. developed a new hybrid ionic liquid/organic electrolyte for high temperature applications using a Pip₁₂TFSI-based ionic liquid electrolyte. They used a three-dimensional nanosilicon electrode as the working electrode and metallic lithium as the reference electrode and counter electrode. At current densities of 0.41 mA·g^-1 and 0.52 mA·g^-1, the specific capacity was 1912 mAh·g^-1and 2230 mAh·g^-1, respectively. Cao et al. found that at high temperatures, LNMO/Li₄Ti₅O₁₂ (LTO) batteries containing LiTFSI/PyrTFSI electrolytes had significant improvements in capacity retention and coulombic efficiency. This work studied two LiTFSI/pyrrolidine bis (trifluoromethanesulfonyl) imide room temperature ionic liquid (RTIL)-based electrolytes with inherent thermal stability. Compared with the most advanced LiPF6/organic carbonate-based electrolytes, the linear sweep voltammetry (LSV) curves of the studied LiTFSI/RTIL electrolytes showed higher oxidation stability. The cycling performance of LNMO/Li₄Ti₅O₁₂(LTO) full cells with LiTFSI/RTIL electrolytes showed significant improvements in capacity retention and coulombic efficiency, as shown in Figure 10. Scanning electron microscopy (Figure 11) and X-ray diffraction (XRD) characterization of LNMO particles after 50 cycles at 0.5 C revealed that the LNMO particles still retained their original morphology and structure in the cells using ionic liquid electrolytes. As shown in Figure 10, the LiTFSI/RTIL-based electrolytes studied outperformed the lithium hexafluoride/organic carbonate-based electrolytes in terms of cycling performance of LNMO/LTO full cells at high temperatures.

Figure 10 (a, c) specific discharge capacity and (b, d) coulombic efficiency of LNMO/LTO full cells at 0.5C rate using the studied electrolyte after two formations at 0.2C at (a, b) 40℃ and (c, d) 60℃. The specific capacity is calculated based on the active mass of the LNMO cathode material.

Figure 11 Scanning electron microscope image

(a) Original LNMO electrode and LNMO electrode after cycling in LNMO/LTO full cells with the studied electrolytes:
(b, e) 1 mol·L^-1LiPF₆ EC/DMC (1/1, mass ratio); (c, f) 0.7 mol·L^-1 LiTFSI Pyra₁₄TFSI; (d, g) 0.7 mol·L^-1LiTFSi MMMPyrTFSI. The test temperatures were (b~d) 40℃ and (e~g) 60℃

2.3 Ionic liquid composite solid electrolytes

Liquid electrolytes commonly used in LIBs have the advantages of high ionic conductivity and stable electrochemical performance, but there are also risks of leakage, spontaneous combustion and explosion. The most serious disadvantage of LIBs based on liquid electrolytes is safety. All-solid-state batteries have obvious advantages in this regard. Although solid electrolytes have good safety, their poor contact with electrodes leads to high interface impedance, and most solid electrolytes have low room temperature ionic conductivity, which cannot meet the needs of LIBs. The biggest problem with solid electrolytes is the formation of a “high resistance layer” at the electrode/electrolyte interface. The contact area between the electrode surface and the solid electrolyte is very limited, and there is little chance of self-repair, which is why it is crucial to soften the electrode/electrolyte interface in all-solid-state batteries. The application of ionic liquids in polymer electrolytes usually takes two forms. One is as a plasticizer, filled in a polymer matrix to obtain a gel polymer electrolyte. Compared with traditional liquid electrolytes, gel electrolyte batteries are more flexible and safer. The other is that the anionic and cationic groups of the ionic liquid itself are combined with the polymer main chain to obtain a polyionic liquid, which is combined with the free ionic liquid to form an ionic liquid polymer electrolyte.

Gel polymer electrolytes (GPE) have the advantages of high ionic conductivity, wettability, good interface contact, good compatibility with electrodes, wide electrochemical window, no leakage, better thermal stability and appropriate mechanical stability. In addition, they have two main characteristics: first, they are “solid-like” soft matter electrolytes, and their physical and chemical properties are better than those of ionic liquid electrolytes without polymer hosts; second, by using polymers as one of the media, the components of lithium batteries only involve solid materials. Due to the advantages of ionic liquids such as good thermal and chemical stability and non-flammability, the introduction of ionic liquids into gel polymer electrolytes in future practical applications has a good application prospect. After swelling, the polymer electrolyte matrix has more non-directionality. By adding ionic liquids within a certain range, the T value of the polymer electrolyte can be reduced. In addition, GPE can replace liquid electrolytes and separators. The performance of the separator will affect the safety and thermal stability of the battery, and GPE can avoid these problems. GPE should meet certain requirements: high lithium ion conductivity (>10^-3S·cm^-1) at room temperature, high transfer number, electrolyte absorption rate, good thermal stability, electrochemical properties and mechanical properties to resist the impact during battery assembly and the destruction of lithium dendrites during use.

Due to the unique properties of ionic liquids such as non-volatility, non-flammability and negligible vapor pressure, ion gels have been widely studied as ideal flexible and stretchable materials. Li et al. reported a high-performance silicon hybrid ion gel electrolyte. The electrolyte promoted uniform lithium deposition and suppressed the growth and proliferation of lithium dendrites, which allowed the battery to stably cycle for more than 500 hours at a current density of 0.1 mA·cm^-1. Guo and colleagues reported the preparation of ionic liquid-based polymer gel electrolytes using core-shell structured silica nanoparticles as functional fillers. Figure 12 is a schematic diagram of the synthesis reaction of silica-polyacrylate lithium (SiO2-PAA@Li). The shell of SiO-PAA@Li increases the lithium ion migration number and ionic conductivity of the electrolyte. The silicon core structure improves thermal stability and increases the compatibility between the electrolyte and the lithium electrode. This stable solid electrolyte can inhibit the growth of lithium dendrites during cycling. Figure 13 is a schematic diagram of the synthesis of ILGPE-SiO₂.PAA@Li gel polymer electrolyte. The gel polymer electrolyte exhibits an ionic conductivity of 0.74×10³ S·cm^-1at room temperature. The battery based on this electrolyte has an initial specific capacity of 138 mAh g^-1 at 0.05C, and the capacity loss after 100 cycles is only 13%.

Figure 6-12 Schematic diagram of the synthesis of silicon dioxide-lithium polyacrylate

Figure 13 Schematic diagram of the preparation of ILGPE-SiO₂-PAA@Li gel polymer electrolyte

Figure 14 shows the GPE formed using lithium ion conducting salts, ionic liquids and polymethyl methacrylate polymer hosts. The formation of polymethyl methacrylate-ILs-TFSI-based electrolytes completely follows the percolation threshold mechanism of the polymer. At ambient temperature, the concentration of amorphous regions is far below the percolation threshold, which leads to lower ionic conductivity. Conversely, near the melting temperature (Tm), it exhibits higher ionic conductivity. At 50 °C, the ionic conductivity of the electrolyte can reach (3.47±10^-3) S·cm^-1, which is attributed to the ionic liquid layer on the surface of polymethyl methacrylate nanoparticles. The immobilized anion ionic liquid groups on the particle surface can achieve a high degree of lithium ion dissociation, which further improves the mobility of lithium ions. In addition, within a specific nanoparticle concentration range, the ionic conductivity increases with the increase of nanoparticle loading because the interaction between lithium ions in the solution and anions fixed on the surface of nanoparticles weakens the ion pair interaction in the electrolyte, which in turn enhances the ion mobility.

Figure 14 Steps for preparing PMMA-ILs-TFSI/LiTFSI mixture

By incorporating Al₂O₃ into polymethyl methacrylate hosts [called quaternary polymer electrolytes (QPEs)], the thermal and cycling stability of GPEs are greatly improved by crosslinking with the polymer hosts, thereby reducing the transport distance of lithium ions (Li⁺) and forming more lithium ion conduction paths. Li et al. prepared a quaternary polymer electrolyte using 1g₁₃TFSI ionic liquid, LiTFSI, SiO₂, and PMMA as precursors. Due to the addition of nano-silica and 1g₁₃TFSI, the prepared electrolyte has thermal curability (300℃), stable electrochemical window (4.0V), fast ion migration performance, interfacial stability, and excellent charge/discharge performance. In addition, with the introduction of binary polyionic liquids in the polymer matrix, a thick passivation layer is formed on the anode surface. Therefore, in order to improve the stability of the anode, it is necessary to minimize the formation of the passivation layer on the anode. As can be seen from Figure 15(a), the electrolyte consists of two important phases: the polymer-rich phase and the liquid-rich phase. Figure 15(a) shows a carbonate solvent-based gel electrolyte, which increases the lithium ion concentration and deposits on the metal anode surface in an uneven manner to form a porous structure. As shown in Figure 15(b), the ionic liquid polydimethylsiloxane introduced in the gel matrix occupies the interface of the polymer-rich medium and the liquid electrolyte-rich medium, which in turn reduces the size of the dispersed liquid electrolyte and increases the amount of the electrolyte-rich phase. This behavior may increase the lithium ion concentration in the liquid electrolyte, allowing lithium to be uniformly deposited on the electrode surface in a granular structure and regulating free dendrites at the anode interface. Park et al. observed similar behavior in their study of Pyr₁₄TFSI ionic liquid electrolyte and single-layer graphene-coated silicon anode (FLGSi) electrode, as shown in Figure 16.

Figure 15 Schematic diagram of the effect of ionic liquid on lithium deposition

(a) Deposition in carbonate electrolyte; (b) Deposition in ionic liquid gel polymer electrolyte

Figure 16 Schematic diagram of the effect of gel polymer electrolyte and carbonate electrolyte on lithium deposition

The structural configuration in which polyionic liquids are directly used in the gel skeleton matrix and another ionic liquid is used as a plasticizer to fill it retains some excellent characteristics of ionic liquids and general characteristics of macromolecules, and can also obtain new properties and functions. Compared with ionic liquids, polyionic liquids have processability, leakage prevention, enhanced mechanical stability, durability and spatial controllability of ionic parts, so polyionic liquids are considered to be promising solid polymer electrolytes. Similar to traditional solid polymer electrolytes, the potential of polyionic liquid-based electrolytes to produce thin films and flexible batteries without the need for additional separators. But the difference is that polyionic liquids are ion-conductive materials even without the addition of salts. However, compared with liquid electrolytes, the ionic conductivity of polyionic liquids and their mixtures with lithium/sodium salts is still low, so polyionic liquids are often used as polymer matrices for gel polymer electrolytes to limit the molecular solvents or ionic liquids that dissolve salts. In general, the synthesis of polyionic liquids involves two basic strategies: direct polymerization of ionic liquid monomers or chemical modification of existing polymers, which can be obtained through different polymerization techniques, including conventional free radical polymerization, reversible addition fragmentation chain transfer (RAFT) polymerization, step-growth polymerization, etc.

Polyionic liquids synthesized with ILs as monomers can provide abundant lithium-rich channels for the transport of lithium ions. For example, the direct polymerization of 1, 3-di-4, 6-dinitrobenzene and imidazole synthesized poly(chloroimidazole-4,6-dinitrobenzene-1,3-diyl) ionic liquid, which is stable below 500°C and has a much higher ionic conductivity (2.88×10^-5S·cm^-1 than commercially available polyaniline (16.7×10^-6S·cm^-1) at room temperature. In addition to good conductivity and a wide electrochemical window, polyimides synthesized by polymerization of ILs monomers also have good mechanical properties. Adding succinonitrile (SN) and LiTFSI to PILs can obtain a composite polymer electrolyte composed of 40% PIL-40% SN-20% LiTFSI. The electrolyte showed an ionic conductivity of 5.74×10^-4S·cm^-1, a wide electrochemical window (5.5V) and good mechanical strength (Young’s modulus 4.9 MPa). In addition, the addition of polycarbonate can further improve the conductivity. The lithium/lithium iron phosphate half-cell using polyionic liquid/30% polycarbonate membrane electrolyte showed an initial discharge capacity of 120.4 mAh·g^-1‘ and a Coulombic efficiency of 89.1%.

ang et al. designed two methods to improve the lithium ion transport performance of PVDF-based solid electrolytes. First, a nickel oxalate composite solid electrolyte was synthesized, and the electrolyte exhibited excellent impedance performance and lithium ion migration performance by changing the surface morphology of PVDF. Second, a composite polymer solid electrolyte composed of ZIF-8 and PVDF was prepared, and the resulting solid electrolyte had a high lithium ion migration number (0.833) and ionic conductivity (1.5×10^-4S·cm^-1) at room temperature. The results show that the dodecahedral structure of ZIF-8 provides a stable channel for the transmission of lithium ions in PVDF-based solid electrolyte materials. The two improved composite solid electrolytes enable the battery to exhibit excellent rate performance and cycle stability at room temperature.

2.4 Other functions of ionic liquid electrolytes

The application of ILs in lithium batteries is not limited to electrolytes. The ionic forces of ionic liquids can form connections between electroactive particles. Ionic liquids are composed of ions, which can adsorb on the surface of particles depending on the chemical composition. One research example is the functionalization of carbon surfaces, where charged ions interact with the free p electrons of the carbon surface and then form a gel-like material. This work has opened up the possibility of using ionic liquids as binders for electrodes. The application of ionic liquid binders is not just to mechanically mix the electroactive materials with the binder. In addition, carbon materials can be used to improve the conductivity of cathode materials (especially materials with poor ionic conductivity, such as lithium iron phosphate).

ILs are often used as solvents to improve the performance of LIBs. On this basis, Cai et al. proposed a new ionic liquid 1-(3-amino-3-oxypropyl)-3-vinylimidazole bis(trifluoromethylsulfonyl)amide ([PIVM][TFSA1) with two functional groups: vinyl and amide. Vinyl has an unsaturated carbon-carbon double bond, and amide is alkaline. They added two anions PF₆^– and TFSA^–  to this electrolyte. TFSA^–  helps to improve the stability of the electrochemical reaction, and the ILs additive acts as an inhibitor, reducing the content of HF and water, thereby protecting the Si-based electrode from damage. Experimental results confirm that the maximum capacity of the battery containing 0.5% (mass fraction) [PIVM][TFSA] in the electrolyte is 679.7mAh·g^-1. The capacity retention rate of the battery based on ILs electrolyte is higher than that of the battery without ILs, and increases with the increase of [PIVM][TFSA] content. When the mass fraction of ILs is 3%, the battery capacity retention rate is 94.7%.

In ILs-functionalized SWNTs, the free side chains of ILs can help separate SWNTs, similar to the Scotch Tape method of separating graphene from graphite structures. ILs not only wrap the separated carbon layers in the electroactive nanoparticles, but also act as a binder between the nanoparticles and the carbon coating. Therefore, lithium ions can move freely along the ILs layer. This creates a shortcut for the diffusion path because lithium ions do not need to diffuse vertically to the particles to shorten the solid-state diffusion length. Due to this feature, ILs are widely used as binders for carbon paste electrodes. The key role of binders is to keep the active materials and conductive agents in close contact with the current collector, thereby maintaining the mechanical integrity of the electrode network. Since the organic electrolytes used in LIBs are less conductive than aqueous electrolytes, the electrodes are made very thin in order to obtain high current output. This requires higher standards for the binders used in the preparation of LIBs electrodes. Therefore, binders make a significant contribution to the actual battery capacity, rate capability, and cycle life. With the help of molecular dynamics simulations, Lee and colleagues proposed that the mechanical strength of the interface between the binder and the active material will be weaker than that of the intermediate binder or intermediate active material. After mixing the binder with the active electrode components, the solvent molecules will carry the binder and diffuse it into the tissue material of the active electrode components. Later, during the drying process, the binder will hold the adjacent particles together by pulling each other. By adjusting the electrode manufacturing method, the interfacial contact between the active material and the binder can be increased to ensure the mechanical interlocking effect.

The main reason for the poor low-temperature performance of LIBs is the low ionic conductivity of the electrolyte and the solid electrolyte interface (SEI) on the graphite surface. Current solutions mainly focus on the research of new electrolytes. However, the performance of LIBs also strongly depends on the composition and structure of the electrodes. Binder, as an important component in LIBs electrodes, not only adsorbs active materials and conductive additives on the current collector, but also has a great influence on the processing and electrochemical properties of the electrodes.

Summary:

Ionic liquid electrolytes are widely used in lithium-ion batteries (LIBs). They can be used directly as lithium salt solvents, mixed with organic solvents, or used in gel polymer electrolytes. Pure ionic liquid electrolytes have a wide electrochemical window, and ternary mixtures can improve ion transport performance. Organic solvent/ionic liquid electrolytes combine the advantages of both parties to improve flammability, stability, etc. Ionic liquid composite solid electrolytes are safe and flexible, and gel polymer electrolytes have many advantages. In addition, ionic liquids can also be used as electrode binders to improve electrode performance and solve problems such as poor low-temperature performance of LIBs.

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