Capacitor ionic liquids refer to organic salts composed entirely of positively charged ions and negatively charged ions, usually organic ionic liquids that are liquid at room temperature. Theoretically, there may be trillions of ionic liquids, from which scientists can choose the ionic liquids they need for their work. Ionic liquids generally do not form vapor, can be reused many times, and are easy to use. As early as the 19th century, scientists began to study ionic liquids. It was not until the early 1970s that American scientists used ionic liquids as liquid electrolytes in batteries for military and space exploration. After that, ionic liquids gradually received attention and research. In 1999, “Chem. Rev” published the first review report on ionic liquids with the theme of “Room Temperature Ionic Liquids: Solvents for Synthesis and Catalysis”. After more than 20 years of development, ionic liquids have received widespread attention and application development in many fields.
Capacitor ionic liquids are an excellent solvent. Unlike typical organic solvents, there are no electrically neutral molecules in ionic liquids. They are mostly in liquid state between 100 and 200 °C and have good thermal stability and conductivity. In addition, some ionic liquids also show acidic or even superacidic properties, which makes them not only used as solvents, but also as catalysts for accelerating chemical reactions in the production process of the petrochemical industry, avoiding the disadvantages of using additional toxic catalysts and the large amount of waste that may be generated, and reducing the use of highly volatile organic solvents, significantly reducing environmental pollution. Ionic liquids generally do not produce harmful gases that pollute the atmosphere during chemical experiments. Ionic liquids can be used in vacuum systems without destroying the stability of the vacuum system. Most ionic liquids are stable to water and can be easily prepared in the aqueous phase. Ionic liquids have excellent designability, and ionic liquids with specific functions can be obtained through molecular design. Ionic liquids also have excellent electrochemical properties, such as a wide electrochemical window, high ionic conductivity, and suitable interfacial compatibility with electrode materials, which have a certain positive effect on improving battery performance. In short, ionic liquids have many advantages that other traditional volatile solvents do not have, and are environmentally friendly green solvents. The research and development of ionic liquids conforms to the requirements of clean technology and sustainable development currently advocated, and has been increasingly recognized and accepted by people.
1. Overview of capacitor ionic liquids
1.1 Definition and classification of capacitor ionic liquids
Capacitor ionic liquids have been widely explored and studied due to their advantages such as wide electrochemical window and wide stable temperature range. At present, preliminary progress has been made in the research of ionic liquids. Compared with common salts, ionic liquids have a much lower melting point, so ionic liquids are also called “low-temperature molten salts”.
There are many types of ionic liquids. According to the different combinations of anions and cations, they can be divided into three categories: organic cation-organic anions, organic cation-inorganic anions, and inorganic cation-inorganic anions. The most commonly used organic cations in ionic liquids are usually low in symmetry and large in size, such as imidazolium ions, pyridinium ions, quaternary ammonium ions, etc. substituted with different alkyl groups. Organic or inorganic anions can be divided into two categories: mononuclear and polynuclear. Mononuclear anions are usually alkaline or neutral ions, including BF–4, PF–6, HSO–4, NO–3, CH3COO-, X-, etc. Polynuclear anions are usually unstable to water and air, including Al2Cl–7, AI3Cl–10, Au2Cl–7, Fe2CI–7, Sb2F–11, etc. By varying the combinations of anions and cations, it is possible to design trillions of ionic liquids.
According to the different types of anions, ionic liquids can be divided into two categories. One category is chloroaluminate ionic liquids whose composition can be changed. The anions of this type of ionic liquid are usually AICI–4 and Al2CI–7 (where C1 can also be replaced by Br). For example, [Bmim]Cl/AlCl3 can also be recorded as [Bmim]AlCl4. When the molar fraction of AlCl3 is x=0.5, it is neutral, when x<0.5, it is alkaline, and when x>0.5, it is acidic. Mixing solid halide salts with AlCl3 usually produces liquid ionic liquids, but the heat release during the preparation process is large. Therefore, a small amount of two solids are usually added alternately to the prepared ionic liquid of the same type to facilitate heat dissipation. This type of ionic liquid has many advantages, but it is extremely sensitive to water and needs to be handled and used in a vacuum or inert atmosphere. The presence of protons and oxide impurities will have a decisive influence on the chemical reactions carried out in this type of ionic liquid. In addition, AlCl3 will release HCI gas when it comes into contact with water, which is irritating to the skin. Another type is also called a new type of ionic liquid, which has a fixed composition and can mostly remain stable in air and water. The anions that make up this type of ionic liquid mainly include PF–6, BF–4, CF3SO–3;, (CF3SO3)2N-, AsF–6, etc. In addition, some functional groups with special properties can be introduced into the cations or anions of the ionic liquid through basic organic chemical reactions such as substitution and addition, thereby obtaining functionalized ionic liquids. The introduced functional groups mainly include -NH2, -OH, -SO2H, -CH2OCH3, etc. At the same time, the positions of the cations and anions can be adjusted to obtain new ionic liquids.
According to the different types of cations, ionic liquids can be divided into four categories: quaternary ammonium salts, quaternary phosphonium salts, alkyl-substituted imidazoles and pyridines. Among them, the most commonly used cations are: ① alkyl quaternary ammonium salt cation NR+4; ② alkyl quaternary phosphonium salt cation PR+4; ③ alkyl-substituted imidazolium cations, such as 1-butyl-3-methyl-imidazolium cation (Bmimt+), 1-ethyl-3-methyl-imidazolium cation (Emimt+); ④ alkyl-substituted pyridinium cation Rpy+. The structural formulas of the above four common cations are shown in Figure 1-1.
Capacitor Ionic Liquids Figure 1-1 Structural formulas of four common cations
(a) quaternary ammonium salt cation; (b) quaternary phosphine salt cation; (c) alkyl imidazolium cation; (d) alkyl pyridine
1.2 Composition and structure of capacitor ionic liquids
Capacitor ionic liquids can be classified according to their structure into functionalized ionic liquids, chiral ionic liquids, switchable polar solvent ionic liquids, biological ionic liquids, polyionic liquids, high-energy ionic liquids, neutral ionic liquids, acidic ionic liquids, alkaline ionic liquids, proton ionic liquids, metal ionic liquids and loaded ionic liquids.
1. Functionalized ionic liquids
In the past few decades, ionic liquids have attracted much attention due to their special properties. According to the needs of the application, the synthesis of functionalized ionic liquids can be achieved by adjusting the combination of cations and anions. Wu et al. studied the catalytic properties of ionic liquids and found that ILs cations activated benzyl alcohol through acid/hydrogen bond catalysis to form benzyl cations, while the anions as hydrogen bond acceptors activated the C—H bonds in the benzene ring of the aromatic hydrocarbons, thereby synergistically achieving the benzylation of aromatic hydrocarbons and obtaining diarylmethanes.
2. Chiral ionic liquids
Chiral ionic liquids are one of the most important ionic liquids in the fields of liquid chromatography, stereoselective polymerization, synthesis of potentially active chiral compounds, liquid crystals, nuclear magnetic resonance chiral identification, and many functional activities. This type of ionic liquid is usually used as a catalyst or solvent to promote the asymmetric synthesis of chiral compounds. Their chiral centers can exist in the cations or anions of the ionic liquids. Because of their chiral properties, they are difficult to synthesize. To date, most of the reported ionic liquids are based on chiral cations, and only a very limited number of ionic liquids contain chiral anions. Yu et al. synthesized a series of chiral ionic liquids with novel structures, which have both chiral cations and chiral anions. Among them, the cation is an imidazolyl group, and the anion is a borate ion with a helical structure and chiral substituents. Figure 1-2 shows the general synthesis route of chiral ionic liquids.
Figure 1-2 General synthesis route of chiral ionic liquids
R1: methyl, isopropyl, isobutyl, phenyl, benzyl; R2: ethyl, butyl, (S)-2-methyl-butyl
3. Switchable polar solvent ionic liquid
In the synthesis of switchable polar solvent ionic liquid, a certain activator is used to achieve a balance between the lower and higher ranges of cation polarity. In formula (1-1), a secondary amine is usually used to react with carbon dioxide (as an activator) to form a carbamate to obtain a switchable polar solvent ionic liquid.
（1-1）
Some researchers have found that 1,8-diazabicyclo[5.4.0]dec-7-ene (DBU) and ethanol can be reversibly reacted in a switchable polarity solvent (SPS) to obtain a more polar ionic liquid [DBUH+][RCO3]–. Figure 1-3 shows the reaction formula.
Figure 1-3 Switchable polarity ionic liquid reaction
4. Biological ionic liquids
Bio-ionic liquids can be synthesized using recyclable and sustainable bio-precursors. They are low in toxicity, environmentally friendly, and biodegradable. Amino acids are one of the most abundant, cheapest, and most readily available biomolecules. Due to their non-toxicity, biodegradability, and good biocompatibility, they have been widely used as raw materials for the synthesis of various ionic liquids. These ionic liquids synthesized using amino acids as raw materials are bio-ionic liquids. Aathira et al. synthesized a series of choline-type amino acid ionic liquids and applied them for the first time as environmentally friendly lubricants in PEG 200 base liquid on steel surfaces. The synthesis process of this type of ionic liquid is shown in Figures 1-4. By measuring the physicochemical properties of ionic liquids such as density and viscosity index, it was found that the synthesized ILs have high thermal stability when mixed with PEG200 at different concentrations, and can be used as efficient friction reducers and anti-wear agents.
Figure 1-4 Synthesis process of choline amino acid ionic liquids ([Ch][AA]ILs)
5. Polyionic Liquids
Polyionic liquids (PILs) are the main chains of repeating motifs of each monomer unit of ILs, first forming dimers, trimers, oligomers, and finally forming polymers or copolymers. PILs combine the unique properties of ionic liquids with the flexibility of macromolecular structures to obtain compounds with new functions, including solid ion conductors, strong dispersants, stabilizers, absorbents, carbon material precursors, porous polymers, etc., which have great potential in application fields such as polymer chemistry and materials science. So far, the preparation of PILs based on various forms of ions and anions has mainly focused on conventional free radical polymerization of ILs monomers, examples of which are shown in Figures 1-5
Figure 1-5 General route for the synthesis of PILs from imidazole-containing methacryloyl ILs monomers
R: alkyl chain; X: halide
6. High-energy ionic liquids
With the extensive development of the field of ILs, people’s interest in the field of high-energy ionic liquids (EILs) for capacitors continues to grow. Compared with traditional energetic compounds [such as 2,4,6-trinitrotoluene (TNT), 2,4,6,8,10,12-hexanitro-2,4,6,8, 10,12-hexaazaisowurtzitane (HNIW), 1,3,5, 7-tetranitro-1, 3,5, 7-tetraazaalkane (HMX), 4,4′-dinitro-3,3′-diazafuran (DDF) and nitromethane (such as trinitromethane) (RDX)], EILs have the characteristics of high density, high thermal stability, easy synthesis, negligible vapor pressure and vapor toxicity, safe transportation and wide application. In addition to high energy properties, ideal EILs also need to meet the basic requirements of ILs. Figure 1-6 shows an example of the structure of EILs
Figure 1-6 Structure of a typical EILs
7. Neutral ionic liquids
Room temperature ionic liquids (RTILs) are mainly divided into neutral ionic liquids, acidic ionic liquids and basic ionic liquids. The anions and cations of neutral ionic liquids form extremely weak electrostatic interactions. This type of ionic liquid has good electrochemical stability and thermal stability and is usually used as an inert solvent. Ghorbani et al. synthesized a new type of neutral ionic liquid, 2-ethylimidazole acetate [2-Eim]OAc, which can be used as an efficient, uniform and reusable catalyst. Under solvent-free conditions, a series of 2-amino-4H-chromene derivatives can be synthesized by a one-pot three-component condensation reaction of various aldehydes, malononitrile and resorcinol. In addition, the response surface methodology (RSM) was used to optimize the reaction conditions. The obtained catalyst can be used and recycled at least four times without causing significant activity loss. The synthesis process is shown in Figures 1-7.
Figure 1-7 Synthesis process of 2-ethylimidazole acetate [2-Eim]OAc
8. Acidic ionic liquids
Bronsted acidic ionic liquids (BAILs) are one of the most important subcategories of functionalized ionic liquids (TSILs). BAILs have the properties of both solid acids and mineral acids and can replace traditional liquid acids (such as sulfuric acid and hydrochloric acid) used in chemical processes. Arian et al. prepared some new BAILs based on 1.4-dimethylpiperazine and 1,10-benzotriazine cations and phosphotungstate anion counterparts as acidic catalysts for the synthesis of 3.3-diaryloctanediol, 2H-indazole [2.1-b] phthalazine trione, as well as esterification reactions and selective oxidation of alcohols.
9. Alkaline ionic liquids
Acidic and neutral ionic liquids are now widely recognized and have the potential to be used in many organic transformation reactions. Alkaline ionic liquids replace commonly used inorganic bases and can also be used in organic transformation reactions such as Michael addition reaction, Markovnikov addition reaction, aldol condensation reaction and nitrogen-Michael reaction. In addition, alkaline ionic liquids have potential environmentally friendly catalysis and have more advantages than inorganic bases, such as flexibility, non-corrosiveness, non-volatility and miscibility with many organic solvents. Dutta et al. used alkaline ionic liquids to synthesize 2,3-dihydroquinazolin-4(1H)-one with medicinal value.
10. Protic Ionic Liquids
The difference between protic and other acidic ILs is the presence of compensating Bronsted acidic protons. Therefore, it can be used as a solvent or catalyst in various reactions such as hydrolysis, dehydration, fuel cell chemistry, etc. Protic ILs can be synthesized by proton transfer between equimolar mixtures of Bronsted acids and Bronsted bases. This method has the advantages of high cost-effectiveness, easy preparation, and no residual products are formed during the synthesis process. Protic ILs can produce hydrogen bonds between acids and bases, and even form extended hydrogen bond networks. Figures 1-8 show some examples of the structures of protic ILs.
Figure 1-8 Structures of some proton ionic liquids
11. Metal ion liquids
Most metal ionic liquids (MILs) are prepared using imidazolylpyridinium cations or direct metal halides. In order to improve their Bronsted and Lewis acidity, bromate or chlorine-containing metal salts (such as [Al2Br7]–, [CuCI3]–, [FeCI4]–, [AICI4]–, [NiCI4]–, [SnCl3]–) are often used as anions of ionic liquids. Metal halide or Lewis acidic ionic liquids have high viscosity compared to other ionic liquids in nature. The structures of some metal halide-based MILs are shown in Figure 1-9.
Figure 1-9 Common structures of metal ionic liquids
12. Loading ionic liquid
The use of pure ILs as solvents or catalysts is costly, and their high viscosity also imposes certain limitations on achieving the maximum product yield. In the case of large-scale production, the main problem is how to dispose of the recycled low-purity ionic liquids.
Sefat et al. fixed 3-sulfobutyl-1-(3-propyltriethoxysilane) imidazole hydrogen sulfate on silica gel by covalent bonds to prepare a supported di-acidic ionic liquid catalyst. Under solvent-free conditions, the new solidified acidic ionic liquid effectively catalyzed the multi-component condensation of aldehydes, 2-naphthols and amides in a one-pot reaction to synthesize amide alkylnaphthols. The catalyst can be recycled 6 times without significantly reducing the catalytic activity. The synthesis process is detailed in Figures 1-10.
Figure 1-10 shows the synthesis process of ions in liquid
1.3 Performance and characteristics of capacitor ionic liquids
Capacitor ionic liquids have the following characteristics: ① composition diversity; ② some properties that are not easy to have in traditional organic solvents; ③ good solubility; ④ wide electrochemical window; ⑤ excellent green solvent; ⑥ because of the weak coordination ions, they often have high polarization ability rather than coordination ability, which allows ionic liquids to dissolve transition metal complexes without coordination with them; ⑦ some ionic liquids show a certain acidity. Usually, ionic liquids containing Lewis acids (such as AlCl3) show Lewis, Bronsted, Franklin acids and even superacids under certain conditions, so this type of ionic liquid can act as a catalyst while serving as a reaction medium.
Due to the characteristics of wide electrochemical window, non-flammability, and basically no volatility, ionic liquids are expected to be developed and applied to energy storage devices such as high energy density batteries and supercapacitors. As lithium-ion battery electrolytes, ionic liquids have the following advantages: ① low vapor pressure and non-volatile; ② non-flammable and safe; ③ good thermal stability, and the decomposition temperature is generally higher than 300℃; ④ good compatibility with lithium salts and organic solvents; ⑤ high conductivity, generally 10-2~10-3 S·cm-1; ⑥ wide electrochemical window, generally higher than 5V. For example: cyanide ionic liquids are the main candidate materials for manufacturing low-cost and high-safety batteries due to their inherent low volatility and lack of expensive fluorides. They make the electrolyte show high ionic conductivity (5mS·cm-1) and good cycle stability at room temperature, and are suitable for low-voltage lithium batteries, such as lithium-sulfur batteries.
1.4 Development History of Ionic Liquids for Capacitors
The earliest capacitor ionic liquids studied did not attract much attention at the time because they could not exist stably in the air and were extremely explosive. Generally speaking, in order for ionic compounds to overcome the constraints of ionic bonds, they must melt into liquids at very high temperatures. This state is called “molten salt”. The ionic bonds in ionic compounds weaken as the radius of the cations increases, and the melting point also decreases. For most substances, the melting point of the mixture is lower than that of the pure substance. If the volume and structural asymmetry of the cations or anions are further increased and the interaction between the anions and cations is weakened, liquid ionic compounds at room temperature can be obtained. Based on this, F.H. Hurley and T.P.Wiler first synthesized ionic liquids that were liquid at ambient temperature in 1951. They chose N-ethylpyridine as the cation and synthesized the ionic liquid using a mixture of n-ethylpyridine bromide and aluminum chloride (the molar ratio of aluminum chloride to n-ethylpyridine bromide was 1:2). However, the liquid temperature range of this ionic liquid is still relatively narrow, and aluminum chloride-based ionic liquids will release hydrogen chloride when in contact with water, which has a certain irritating effect on the skin.
In 1975, Robert used AlCl3/[N-EtPy]Cl to conduct organic electrochemical research and found that this room temperature ionic liquid is a good electrolyte, miscible with organic matter, proton-free, and has a wide electrochemical window. In 1992, Wilkes et al. synthesized 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4), an ionic liquid that can exist stably in water and air. After that, the application research of ionic liquids was widely carried out. Davis and his colleagues used thiazole ionic liquids as catalysts for benzoin condensation as an example and first proposed the concept of designing ionic liquids by interacting with solutes in a specific way. Subsequently, a paper published in Task-Specific Ionic Liquids outlined this concept and introduced the term “functionalized ionic liquids”, which is intended to give ionic liquids specific properties or reactive functional groups. Functionalized ionic liquids can be defined as ionic liquids whose functional groups are covalently attached to the cations or anions (or both) of the ionic liquids, which are considered to be liquid forms of solid-supported catalysts, with advantages such as kinetic fluidity and large operating space.
Figure 1-11 shows the development of capacitor ionic liquids since their research. Nowadays, ionic liquids have received more and more attention, and ionic liquid compositions with specific properties can be synthesized according to people’s needs for physical, chemical and biological properties. For example, covalent organic framework (COF) membranes constrained by ionic liquids can efficiently separate ethylene/ethane. For another example, various types of molecular sieves are designed to perform specific tasks, such as catalysis, organic synthesis, chiral induction, synthesis and stabilization of nanomaterials, electrochemical applications, catalyst surface modification, etc.
Figure 1-11 Development history of ionic liquids
1.5 Application prospects of capacitor ionic liquids
As an emerging green solvent, capacitor ionic liquids have many excellent properties and have attracted the interest of researchers in many fields. Figure 1-12 shows the properties of capacitor ionic liquids and their possible application fields. At present, the excellent properties of ionic liquids make them suitable for more and more fields. With the deepening of research on ionic liquids, the application prospects of ionic liquids are very bright.
One of the main functions of ionic liquids is to absorb CO2. In general, traditional ionic liquids absorb CO2 through physical effects, while functionalized ionic liquids absorb CO2 through chemical effects. By adjusting the structure of functional groups, the interaction between functional sites and CO2 can be regulated, thereby achieving high-capacity absorption and low-energy desorption of CO2 and recycling of functionalized ionic liquids. However, the actual application of ionic liquids in industrial processes to absorb CO2 still faces the following challenges:
(1) Compared with traditional absorbents such as aqueous alcohol amine solutions, the viscosity and production cost of functionalized ionic liquids are relatively high;
(2) There is still a need to continuously enrich and improve the interaction mechanism between functionalized ionic liquids and CO2;
Figure 1-12 Properties of capacitor ionic liquids (left) and their possible applications (right)
(3) Research on the absorption of CO2 by functionalized ionic liquids is generally conducted at room temperature and pressure, and there is still a large gap between laboratory research and industrial application;
(4) The influence of other components in flue gas such as H2O, O2), NOx, SO2) on the CO2 absorption performance cannot be ignored
Nanocellulose can be prepared using ionic liquids. Traditional nanocellulose preparation methods have many limitations, such as large acid dosage, easy corrosion of equipment, environmental pollution, difficult reagent recovery, and high energy consumption. Ionic liquids can not only regulate the performance of nanocellulose by designing the type of ionic liquids, but also achieve cost-effectiveness through the effective recovery of ionic liquids.
The physical and chemical properties of capacitor ionic liquids, such as good thermal stability, low volatility and designability, have opened up a new field for chemical research. With the in-depth study of the structure and function of ionic liquids, the application of ionic liquids in the field of chemistry has continued to deepen, but the following aspects still need to be explored in depth:
(1) Synthesis and application of functional ionic liquids. Functional modification of anions and cations of ionic liquids is carried out to achieve the purpose of chemical analysis and synthesis.
(2) Exploration of the relevant action mechanism of ionic liquids. Research on the relationship between structure and function, two-phase distribution, solvent extraction and separation mechanisms is conducive to finding the theoretical basis for the application of ionic liquids from the essence.
(3) Accumulation of the physicochemical properties of ionic liquids. Due to the large number of ionic liquids and the short application time, there is a lack of corresponding accumulation of information on their complete physicochemical properties and structural parameters. The performance parameters of many new ionic liquids (such as comprehensive toxicity data, thermodynamic data, kinetic data, etc.) still need to be further understood.
Ionic liquids have been widely studied in the field of catalysis. By changing the structural parameters of the cations and anions of ionic liquids, using ionic liquids or ionic liquids combined with inorganic acids as homogeneous catalysts can adjust the acid strength and the solubility of the reactants. The immobilization of capacitor ionic liquids is an effective method to adjust the surface adsorption/desorption properties and acid strength distribution of solid acid catalysts. This method has great potential to reduce catalyst deactivation and improve the activity of solid acid catalysts. Sun and his colleagues used a high-temperature solvent thermal synthesis method based on the ionic liquid 1-butyl-3-methylimidazole to obtain nitrogen-, phosphorus- and fluorine-doped CoFe2O4 with excellent properties in alkaline solution. These ionic liquid-derived materials showed larger surface areas and more oxygen vacancies than those synthesized in the absence of ionic liquids. Therefore, the ionic liquid and electrodeposition conditions play a key role in controlling the surface area and activity of the resulting electrocatalysts.
Ionic liquids also play a great role in the synthesis of nanomaterials. The synthesis of metal nanoparticles in ionic liquid media has been extremely successful because their monodisperse and non-agglomerated behavior can be achieved by stabilizing the metal nanoparticles with cations and/or anions of the ionic liquids. The physicochemical properties of ILs greatly affect the size distribution of metal nanoparticles. For example, ionic liquids with longer side chains are conducive to the realization of metal nanoparticles with smaller diameters and narrower size distributions. The increase in the length of the side chains leads to changes in the physicochemical properties of the ionic liquids and the degree of interaction between the ionic liquids and the nanoparticles. In general, the larger size of the nanoparticles can be attributed to the aggregation of unstable metal nanoparticles with the smaller anions of the ionic liquids through strong Coulomb attraction. The combination of less coordinated anions in the ionic liquids with the metal nanoparticles leads to larger and non-uniform distribution of the metal nanoparticles. Ionic liquids also show outstanding advantages in the field of electrodeposition, which makes ionic liquids useful in separation chemistry, metallurgy, environmental pollution control, material preparation, and electrochemistry. For example, replacing the aqueous solution system with an ionic liquid system during electroplating can reduce the waste of precious metals; toxic precious metals and semiconductor materials can be deposited from polluted water sources: the electrochemical performance of energy storage batteries can be improved by selecting appropriate anions and cations to be added to the electrolyte.
Capacitor ionic liquids have shown unique advantages in the application of electrochemistry. At present, ionic liquids such as quaternary ammonium salts, quaternary salts, alkyl substituted imidazoles and pyridines have been further developed and have been widely used in the field of electrochemistry. The performance of any electrochemical device is essentially based on the characteristics of the interface. The characteristics of the ionic liquid/electrode interface are mainly related to three characteristics of ionic liquids:
(1) Conductivity of ionic liquids: The conductivity of electrochemical devices is based on free charges, ions and their mobility. In general, ionic liquids have excellent ionic conductivity because they are completely ionic.
(2) Viscosity of ionic liquids: Ionic liquids have the characteristic of high viscosity, which may be due to the fact that they contain ions of different sizes and the interaction between cations and anions.
(3) Electrochemical potential of ionic liquids: The nature of the electrochemical potential mainly depends on the oxidation and reduction potentials of the ionic liquids. Ionic liquids have a wide electrochemical stability window with or without additives.
1.6 Key scientific issues of capacitor ionic liquids
There are several key scientific issues in the study of the physicochemical properties of capacitor ionic liquids:
(1) The determination of the properties of any substance assumes that its purity is 100%, but in fact, the purity of ionic liquids has always been a problem that has troubled scientists. The non-volatility of ionic liquids makes it difficult to purify them by distillation as solvents, and because they are liquids, it is also difficult to purify them by crystallization. Therefore, the purification method of ionic liquids requires standardization and convenience to facilitate the reproducibility of physicochemical property determination. Many physicochemical properties of ionic liquids are closely related to their purity and have a significant impact. Therefore, the purity of ionic liquids is the fundamental guarantee for the reliability of physicochemical property data.
(2) Since many ionic liquids are hydrophilic, and even if they are not hydrophilic, their hygroscopicity in the air is much higher than that of many organic solvents. Therefore, when determining ionic liquids, the impact of the environment is very important, and their determination and use must be carried out in a dry environment or in a glove box.
(3) Currently, there are a large number of literatures reporting on the physicochemical properties of ionic liquids, but the systematic research on the physicochemical properties of ionic liquids is still very limited, especially the determination of special properties for some specific reaction needs. From the perspective of large-scale production or industrial background, there are still a lot of knowledge areas about the nature of ionic liquids that need to be further explored. For example, the toxicity of ionic liquids has not been studied until recent years; there is no unified applicable standard for the study of the polarity of ionic liquids; the transport phenomenon has only been simply measured in one or two ionic liquids; the solubility/solubility information has only been measured for a few substances and is almost not quantitative.
(4) The ultimate goal of ionic liquid physical property research will be to be able to select ideal ionic liquids based on simple experimental data or using convenient calculation methods.
1.2 Overview of Electrolytes
In the early 1970s, lithium batteries used lithium salts as electrolytes, mainly including LiCIO4, LiAlC14, LiBF4, LiPF6 and LiAsF6. At that time, LiPF6 could not achieve the best lithium metal dissolution/deposition efficiency, and its circulation for carbon electrodes was also problematic. It was not until carbon replaced metallic lithium (later replaced by graphite) that LiPF6 became the main lithium salt for lithium battery electrolytes. After optimizing the solvent, Sony first used this lithium salt as an electrolyte in commercial lithium-ion batteries in 1991.
The electrolyte interacts with the electrode material, and it also has decomposition reactions itself, participating in almost all reaction processes occurring inside the battery. At present, the electrolyte contained in lithium-ion batteries is mostly an organic system. Under the abuse conditions of overcharge, overdischarge, short circuit and thermal shock, the battery temperature rises rapidly, often causing the battery to catch fire or even explode due to the flammable nature of the electrolyte. At present, the most prominent obstacle to the commercialization of high-capacity power lithium-ion batteries is safety issues. Therefore, choosing a suitable capacitor electrolyte system is conducive to obtaining lithium secondary batteries with high energy, long cycle life and high safety performance.
1. Application requirements of capacitor electrolytes
The capacitor electrolyte is a key part necessary for the normal operation of the battery. It plays the role of conducting ions between the positive and negative electrodes of the battery and is a bridge connecting the positive and negative electrode materials. The electrolyte determines the working mechanism of the battery to a large extent, affecting the battery’s safety, rate performance, specific energy, cycle performance, etc. Therefore, the following conditions must be met when selecting the electrolyte: ① Maintain high ionic conductivity over a wide temperature range; ② High thermal stability, not easy to decompose over a wide temperature range (such as low melting point, high boiling point); ③ There must be a wide electrochemical sub-graph to ensure that the electrolyte does not have obvious side reactions between the positive and negative electrodes: ④ Do not react with the internal components of the battery; ⑤ Good safety, not easy to burn or high flash point; ⑥ Low price and cost; ⑦ Non-toxic and non-polluting.
Taking lithium-ion batteries as an example, the electrolytes currently used in lithium-ion batteries are mainly liquid electrolytes, which are prepared by dissolving lithium salts in organic solvents. Lithium salts and solvents are its two basic components, so the properties and ratio of lithium salts and solvents have a great influence on the performance of the battery. Electrolyte salts must meet a series of characteristics, among which the more important ones are the following:
(1) Ionic conductivity. The choice of lithium salt anions greatly affects the conductivity of the electrolyte, which is attributed to the change in the solvation of Lit and the ion association, which is caused by the difference in anion structure and coordination strength.
(2) Lithium salt solubility/crystalline solvate phase. Obtaining a fairly high lithium salt solubility in the electrolyte solvent is necessary to provide sufficient charge carriers for rapid ion conduction and to prevent salting out (i.e., precipitation). Salts are highly soluble, but they are also prone to forming crystalline solvate phases with high melting points, thereby forming solids in the electrolyte.
(3) Stability. The electrolyte must remain stable within the electrochemical potential window of the battery charge and discharge reaction and at high temperatures, that is, it must not react with other battery components to achieve thousands of charge/discharge cycles with low capacity loss (attenuation).
(4) Formation of a solid electrolyte interface (SEI) film at the electrode interface. Ideally, the formation of the SEI film prevents further electrode-electrolyte reactions and allows only Lit transport between the electrode and the electrolyte. The presence of lithium salts in the electrolyte, either as bulk salts or additives, significantly affects the composition, properties, and stability of the SEI. (5) Avoiding aluminum corrosion.
(6) Hydrolysis stability. Many anions undergo hydrolysis when exposed to water, especially at high temperatures, which often leads to the formation of hydrogen fluoride, which seriously affects the cycling performance and life of the battery, especially when cycling at high temperatures and/or high potentials (>4.8 V). The formation of hydrogen fluoride can also be due to the reaction of anions with solvent molecules (capture of hydrogen ions).
Other ideal characteristics of the electrolyte include low cost and low toxicity. Failure to meet one or more of these criteria will hinder the practical use of the electrolyte in lithium-ion batteries. These properties are strongly dependent on the electrolyte formulation, for example, the solvent used, salt concentration, type and concentration of additives.
Over the past three decades, researchers have prepared a wide variety of anions and used lithium salts in dry lithium battery electrolytes. In general, the selection of fluorinated anions reduces the interaction between the anions and Lit, thereby increasing the conductivity of the electrolyte solution with the corresponding lithium salt. Anion fluorination also tends to improve the anodic stability of the anions for oxidation at high potentials, which is an important consideration for preparing electrolytes suitable for high-voltage cathodes. Commonly used inorganic and organic lithium salts include LiPF6, LiClO4, LiBF4, LiAsF6, LiB(C2H5)3(C4H4N), LiB(C6F5)3(CF3), LiCFSO3, etc. LiCIO4 is a strong oxidant and is only used in experiments, but is rarely used in industry; LiPF6 is the most commonly used lithium salt with good ionic conductivity and electrochemical stability, but its heat resistance and hydrolysis resistance are poor, and it is easily hydrolyzed to form HF, so the electrolyte is required to contain as little impurity water as possible. In recent years, nitrile groups have also been used to replace fluorination, but in some cases the stability of such salts is poor. Novel salts remain one of the keys to developing advanced electrolyte formulations.
Electrolyte solutions generally use organic mixed solvents, which are composed of at least one organic solvent with low volatility and high dielectric constant (such as EC, PC) and one low-viscosity, volatile organic solvent (such as DMC, DME, DEC, THF). The resulting electrolyte solution has lower viscosity, higher dielectric constant and lower volatility. Compared with a single solvent, mixed solvents will make the ionic conductivity of liquid electrolytes and other battery properties more advantageous. In addition, elements such as F, B, P, and S can be introduced into the solvent to improve certain properties of the electrolyte.
1.2 Current Status of Capacitor Electrolyte Development
After 2000, the development of secondary batteries entered a new stage, and the research on electrode materials became more and more mature. Taking lithium-ion batteries as an example, the electrolyte currently used in lithium-ion batteries is mainly organic electrolytes. When lithium-ion batteries based on traditional electrolytes are charged and discharged, lithium ions move back and forth in the electrolyte to complete the conduction process. However, organic electrolytes have some disadvantages that restrict the development of lithium-ion batteries. First, the electrical conductivity of organic electrolytes is low, which will form a large impedance inside the lithium-ion battery; second, during the charge and discharge process, the problem of lithium dendrites often occurs in metallic lithium. This defect often punctures the diaphragm and causes the lithium-ion battery to short-circuit, causing a large amount of heat to be generated inside the battery. Under extreme conditions of high temperature, low-viscosity dimethyl carbonate (DMC) and diethyl carbonate (DEC) can cause the battery to burn, posing a serious safety hazard; finally, from a cost perspective, organic electrolytes are relatively expensive, resulting in a high production cost for lithium-ion batteries. In addition, since moisture has a great influence on the performance of lithium-ion batteries, organic lithium-ion batteries must be isolated from moisture during the manufacturing and production process, which further increases costs.
In polymer lithium-ion batteries, the capacitor electrolyte system consists of two parts: polymer and lithium salt. The prerequisite for the polymer electrolyte to be conductive is that the polymer has the ability to couple with lithium salt. The polar groups in the polymer (-0, =0, -N, -P, -S, C=O, C=N, etc.) can be complexed with Li+ to form a polymer-lithium salt complex. The essence of the conductivity of the polymer electrolyte is the process of continuous coupling-decoupling of Li+ and the polar groups in the polymer under the action of the electric field. When the polar groups and Li+ are in a certain ratio, the polymer chain will form a helical structure. The cations couple and migrate inside the helical structure, while the anions are free on the outside of the helical structure, so the anions and cations exist independently and will not interfere with each other to affect migration. When the degree of dissociation of anions and cations is higher, the number of free-moving cations is greater, the charge carried by a single ion increases, the ion mobility is better, and the ion conductivity is higher. The cation migration number and ion conductivity are important criteria for judging the conductive properties of polymer electrolytes. However, in many electrochemical devices, anions tend to accumulate on the surface of the electrode/electrolyte during multiple charge and discharge cycles, resulting in concentration polarization. A certain concentration difference is generated between the ion concentration on the electrode surface and the electrolyte body, which affects the electrode potential and may generate a polarization voltage opposite to the external electric field, hindering the migration of Li+ and reducing the battery’s service life and performance.
In 1994, Canadian researchers began to try to replace organic solutions with aqueous solutions. Dahn’s research group proposed the concept of “aqueous lithium-ion battery”. The research group used 5 mol·L-1‘LiNO3 aqueous solution as the electrolyte solution of the battery, and LiMn2O4 and VO2 as the positive and negative electrode materials of the battery respectively. This research pioneered the research era of aqueous lithium-ion batteries. However, the working voltage of aqueous electrolytes is only about 1.23V, and its stable electrochemical window is very narrow. Once the range of this potential window is exceeded, the aqueous solution will undergo hydrogen evolution or oxygen absorption reactions, resulting in side reactions of the electrode materials with water or oxygen during the charge and discharge process. When traditional lithium batteries use organic electrolytes for charge and discharge, a layer of SEI film will be formed on the surface of the electrode material, which plays a role in protecting the electrode material, thereby reducing the occurrence of side reactions and ensuring that the lithium-ion battery can work stably for a long time. However, aqueous electrolytes have strong polarity and cannot form SEI films on the surface of electrode materials. Therefore, the electrochemical performance of aqueous lithium-ion batteries is not ideal. Currently, the most widely studied modification method for aqueous lithium-ion batteries is to coat the surface of the electrode material with materials such as graphene or conductive polymers.
In 1999, the concept of “functional electrolyte” was created and developed based on controlling the thickness of the SEI film and adding trace amounts of various additives to high-purity electrolytes to improve battery performance, such as adding anode additives, cathode additives, and overcharge prevention additives to the electrolyte.
In the past two decades, ionic liquids have been increasingly studied in the field of batteries, thanks to the ease of preparation and processing of ionic liquids under atmospheric conditions. Figure 1-13 is a schematic diagram of the structure of ionic liquids and their derivative materials and their possible application battery components.
Figure 1-13 Schematic diagram of the structure of capacitor ionic liquids and their derivative materials and their possible application in battery components
1.3 Overview of ionic liquid electrolytes
1.1 Advantages of capacitor ionic liquid electrolytes
Ionic liquid electrolytes have many advantages that conventional electrolytes cannot match: 1. Very small vapor pressure, almost non-volatile; 2. Good thermal stability, the liquid temperature range reaches 300℃; 3. High chemical and electrochemical stability, wide electrochemical stability window, such as [Emim] (CF3SO2) 2N The electrochemical window is greater than 4V, and it can remain stable in the air at 400℃; 4. Strong solubility, and no solvation phenomenon, can dissolve many organic substances, organic metal compounds, inorganic metal compounds and polymer materials; 5. Good electrical conductivity, such as 2mol·L-1‘[Emim] PF6 acrylonitrile The conductivity is as high as 5mS·cm-1‘; 6. Good designability. By designing the structure of anions and cations, the viscosity, solubility, conductivity and electrochemical activity of ionic liquids can be controlled. At the same time, polymers with high conductivity can be obtained by copolymerizing and grafting ionic liquids with polymers; 7. Environmentally friendly. The above advantages of ionic liquid electrolytes lay a favorable foundation for their future development and give them broad application prospects.
1.2 Classification of capacitor ionic liquid electrolytes
There are many types of ionic liquid electrolytes being studied, such as ionic liquid electrolytes containing Li/Na salts, hybrid electrolytes of ionic liquids and molecular solvents, ionic liquid-nanoparticle hybrid electrolytes, solvated ionic liquid electrolytes, etc. In addition, there are some new types of ionic liquid electrolytes, such as polyionic liquid electrolytes, ion gel electrolytes, organic ionic plastic crystal electrolytes, and artificial solid electrolyte interfaces.
1. Ionic liquid electrolyte containing Li/Na salts
Ionic liquid electrolytes containing lithium/sodium salts can meet different requirements such as high ionic conductivity, wide electrochemical stability window, and stable interface properties by regulating the anion and cation structure. In fact, the types of ionic liquids that can be used as electrolytes are very limited. The types of cations mainly include non-functionalized imidazole, pyrrolidine, phosphorus-containing cations, etc., while the anions are mainly high-stability, low-viscosity species such as [BF4]–, [TFSI]–, [FSI]–, and [PF6]–. The electrochemical properties of ionic liquid electrolytes are closely related to the structure of anions and cations. For example, the viscosity of the electrolyte increases with the increase of the radius of the cation, while the ionic conductivity decreases accordingly; the introduction of ether-containing functional groups in organic cations will obtain ionic liquids with lower viscosity.
Ionic liquid electrolytes containing lithium/sodium salts can meet different requirements such as high ionic conductivity, wide electrochemical stability window, and stable interface properties by regulating the anion and cation structure. For Li/Na ion batteries, ionic liquid electrolytes can give batteries higher safety, higher capacity utilization and coulombic efficiency, which is particularly prominent under high voltage and high temperature working conditions. Ionic liquid electrolytes are conducive to the formation of a stable SEI film, which is usually a factor that needs to be considered to improve the long-term cycle stability of batteries. This is particularly significant in electrode materials with severe volume expansion such as silicon negative electrodes. The small amount of decomposition of ionic liquids at the electrode interface generates a stable and strong SEI film on the surface of the silicon negative electrode, which greatly improves the Coulomb efficiency of the battery during the charge and discharge process. Similarly, the decomposition of ionic liquids at high potentials can also form a stable protective layer of positive electrode materials at the positive electrode-electrolyte interface, so that the positive electrode material can maintain structural stability and reduce the occurrence of side reactions. The dual-ion batteries that have attracted much attention in recent years also show excellent electrochemical performance when using ionic liquid electrolytes. This is because the introduction of ionic liquids improves the antioxidant stability of the electrolyte and circumvents the solvent co-embedding problem that is widely present in organic solvent electrolytes. Figure 1-14 is a schematic diagram of the charge and discharge process of a dual-ion battery based on an ionic liquid electrolyte, with aluminum foil as the anode and mesophase carbon microspheres (MCMB) as the cathode. In dual-ion batteries, both cations and anions participate in charge transport and storage. During charging, cations are embedded in the negative electrode (usually lithium metal, Li4Ti5O12 or carbon) and anions are embedded in the positive electrode (usually carbon), while conventional batteries only involve cation transfer.
Figure 1-14 Schematic diagram of the charging and discharging process of a dual-ion battery
Li-S battery is a new type of electrochemical energy storage device with high specific energy, but its development is troubled by the shuttling effect of polysulfide. The use of ionic liquid electrolytes in S-C composite cathode materials can achieve better electrochemical performance than traditional organic solvent electrolytes, because ionic liquid electrolytes inhibit the dissolution of polysulfide intermediates and can induce rapid charge transfer at the electrode-electrolyte interface. The inhibitory effect of ionic liquids on the dissolution of polysulfide is actually related to the electron donating ability of its anions: anions with weaker electron donating ability can effectively inhibit the shuttling effect of polysulfide intermediates. The charge-localized[TFSI]–anions with weak Lewis alkalinity can not only alleviate the shuttling problem of polysulfide in Li-S batteries, but also produce a stable and strong SEI film on the negative electrode interface. Figure 1-15 is a schematic diagram of the mechanism of dissolution and diffusion of polysulfide in ether (left) or [PP13[TFSII-based (right) solvents. The results show that [PP13][TFSII can significantly inhibit the rapid shuttling process of polysulfide.
Figure 1-15 Schematic diagram of the dissolution and diffusion mechanism of polysulfide in ether (left) or [PP13][TFSI1-based (right) solvents
2. Hybrid electrolyte of capacitor ionic liquid and molecular solvent
Due to the strong ion-ion interaction and the huge ion volume, pure ionic liquids often have too high a viscosity, resulting in low ionic conductivity, which limits the application of ionic liquid electrolytes in high-capacity and high-rate electrochemical systems. To solve this problem, organic carbonate solvents are mixed with ionic liquids to form hybrid electrolytes. Hybrid electrolytes have lower viscosity and higher ionic conductivity than pure ionic liquids, and are also safer than pure carbonate electrolytes. In addition, hybrid ionic liquid-molecular solvent electrolytes often bring about an optimization effect on the electrode-electrolyte interface. Figure 1-16 is a schematic diagram of SiO2-IL-TFSIVPC in a 1 mol·L-1ILiTFSI hybrid electrolyte system. The results show that SiO2-PP-TFSI has higher conductivity than SiO2-IM-TFSI and is more effective in promoting uniform lithium deposition and inhibiting battery short circuits, which may be due to the weaker binding of the [PP13][TFSI] ion pair.
Figure 1-16 Schematic diagram of SiO2-IL-TFSVPC in 1mol·L-1ILiTFSI hybrid electrolyte system
3. Capacitor Ionic Liquid-Nanoparticle Hybrid Electrolyte
The practical application of lithium metal batteries is limited by uneven electrochemical deposition and dendrite growth. The use of electrolytes with weak ion mobility has a better effect on inhibiting dendrite growth. In recent years, Lu et al. developed ionic liquid-supported nanoparticle hybrid electrolytes and systematically studied their application in lithium metal anodes. In this hybrid electrolyte, capacitor ionic liquids are bound to the surface of nanoparticles (such as SiO2, TiO2, ZrO2, etc.), and then the nanoparticles are mixed with lithium salts or molecular solvents containing lithium salts. Ionic liquid-nanoparticle hybrid electrolytes can effectively inhibit the growth of lithium dendrites, which is mainly attributed to two aspects: ① The bound ion pairs can dissociate anions in the solvent, inhibiting the generation of space charge layers on the negative electrode surface; ② The impermeability and mechanical strength of nanoparticles can slow down the penetration of nucleated dendrites. Figure 1-17 is a transmission electron microscope image of an electrolyte with a nanoparticle volume fraction of 0.23. The ionic liquid component acts as an almost immobile, undissociated ion source in the entire electrode space, which can quickly replenish ions when the battery is polarized.
4. Solvated ionic liquid capacitor electrolyte
In recent years, high concentrations of lithium or sodium salts dissolved in ethylene glycol dimethyl ether can form molten solvated ionic liquid capacitor electrolytes, which have been widely studied as a new type of electrolyte system. There is a lithium atom in the cation structure of the solvated ionic liquid that can strongly coordinate with the ligand in the solvent.
The mixture of equimolar amounts of triglyme (G3) and LiTFSI [Li(G3)][TFSI] is one of the typical representatives of solvated ionic liquid electrolytes. The lithium atom in the [Li(Ga3)] cation is coordinated with the cyclic ether through four oxygen atoms. The resulting weak Lewis acidity and weak Lewis basicity of anions and cations play a significant role in inhibiting the shuttle effect of polysulfides in the Li-S pool. Ueno’s group studied the structure of lithium ions in equimolar mixtures to distinguish between solvated ionic liquids and concentrated solutions (Figure 1-18). The results confirmed that [Li(glyme)][TFSA] and [Li(glyme)][BETI] (glyme=G3 or G4, G4 is tetraglyme) are considered to be almost free solvated ionic liquids. Due to the presence of large amounts of free glycine in the solvent, complexes containing other anions are considered concentrated solutions because the interaction of the strong Lewis basic anion with Lit+ is much stronger than the interaction of Li+ with glycine.
Figure 1-18 Percentage of free triethylene glycol dimethyl ether (cr/ca) in an equimolar molten mixture [Li(glyme)]X at 30°C
Inset: hierarchical optimized geometry of [Li(G4)+in B3LYP/6-311+G**
5. Polyionic liquid capacitor electrolyte
Compared with ionic liquids, polyionic liquids have improved processability, impermeability and mechanical stability, and their ionic part space is also adjustable, so they are regarded as potential polymer electrolytes. Unlike ordinary polymer electrolytes, polyionic liquid electrolytes also have ionic conductivity even without adding lithium salts.
According to different synthesis methods, polyionic liquid electrolytes can be divided into two types: preparing polyionic liquids by direct polymerization of monomers and chemically modifying existing polymer electrolytes with ionic liquids. Polyionic liquid electrolytes prepared by direct polymerization of ionic liquid monomers have abundant lithium ion channels, stable mechanical properties and a wide electrochemical stability window. Some natural polymers can be chemically modified with ionic liquids to produce free anions or cations in their pore structure to improve ionic conductivity. This type of polyionic liquid electrolyte can effectively maintain the excellent mechanical properties of the original polymer electrolyte. As shown in Figure 1-19, 3-(ethyl propyleneoxy)propyltrimethoxysilane in situ sol-gel generated [Pyr13][TFSI immobilized biomimetic ant nest SiO2 matrix, which has many polar groups, such as methacryloyl. In the interconnected channels, the bionic anthill can conduct lithium ions and promote their transport. The bionic anthill structure also inhibits the growth of lithium dendrites and ensures the reversible extraction and insertion of lithium ions at high rates.
Figure 1-19 (a) SEM image of the ant-hive-shaped X-SiO2 ionic liquid skeleton; (b) Reaction mechanism of immobilized ionic liquid and trimethylsilane coupling agent; (c) Schematic diagram of biomimetic ant-hive ionic liquid gel electrolyte and its interaction with Lit; (d) Schematic diagram of Li+ conduction
6. Ion gel capacitor electrolyte
As a gel electrolyte, ionic liquid acts as both a plasticizer and a carrier in ionic gel, so the ionic conductivity of ionic gel far exceeds that of general all-solid electrolytes. In addition, ionic gel has good flexibility and high compatibility with electrode interfaces. The types of reported ionic gel electrolytes are relatively limited, mainly composed of some fluorine-containing anions and imidazole and pyridine cations. However, the matrix types of ionic gel electrolytes are relatively rich, including inorganic matrix, linear polymer matrix, block copolymer matrix and hybrid matrix.
Inorganic ionic gel matrix has advantages in thermal stability, mechanical properties, safety and other aspects. At present, the most commonly used inorganic matrix for confined ionic liquids is silica. The preparation process of silica matrix is simple and the mesh size and pore size are adjustable, but its low dielectric constant limits its ionic conductivity. Various polymer matrices have been widely studied in recent years. The electrically neutral nature of the polymer matrix makes its interaction with the internal ionic liquid weak, which is not only not conducive to the enhancement of ionic conductivity, but also easy to cause phase separation inside the gel. The solution to this problem is usually to introduce polyionic liquids into the gel to enhance ion-ion interactions. There is often a contradiction between mechanical properties and ionic conductivity in the polymer ion gel matrix. Increasing the content of ionic liquid in the gel will increase the ionic conductivity, but at the same time it will sacrifice a certain amount of mechanical strength. In scientific research, chemically cross-linked high-strength polymer matrices are often added to the gel to balance this contradiction.
7. Organic ion plastic crystal electrolyte capacitor
Organic ion plastic crystals are composed of room temperature ionic liquids and inorganic salts composed entirely of anions and cations. The biggest difference between organic ion plastic crystals and ordinary ionic liquid crystals is their plasticity, which reflects the conformation and rotational movement of ions in a certain lattice under low stress. The ion movement and ion conductivity in organic ion plastic crystals are attributed to the vacancies and defects in the material. The properties of anions and cations have an impact on the ionic conductivity of plastic crystals. Zhou et al. proposed an interesting strategy to solve the contradiction between ionic conductivity and mechanical strength. The key lies in the in-situ synthesis technology, which completes the preparation of composite polymer electrolytes and the assembly of high-safety polymer lithium/sodium ion batteries in one step. Figure 1-20 is a schematic diagram of the preparation of PIL-based solid electrolytes (HPILSE) by in-situ polymerization. This in-situ synthesis method not only simplifies the assembly process of polymer lithium/sodium ion batteries, but also ensures that the interface contact between electrodes in high-performance batteries is stable and the resistance is low to resist the electrode volume change during charge/discharge cycles.
Figure 1-20 Schematic diagram of in-situ polymerization preparation of HPILSE
8. Artificial solid electrolyte interface
Alkali metal anodes have problems such as unstable SEI film, dendrite growth, and volume expansion. In addition to optimizing the electrolyte components, it is also a common method to use ionic liquids to protect and regulate the anode interface and construct a metal current collector with a special structure. On the one hand, ionic liquids can participate in the reaction with metal anodes as electrolyte components to generate functional SEI films containing specific elements (such as F, N, etc.); on the other hand, polymers derived from ionic liquids can directly form films on the electrode surface to protect the anode. In addition, ionic liquids can also be used as wetting agents to stabilize the interface properties between inorganic solid electrolytes and metal anodes or cathodes. As shown in Figure 1-21, in order to increase the charge transfer rate at the cathode/electrolyte interface in a solid-state battery composed of Na3V2(PO4)3/solid electrolyte/sodium, the researchers used a small amount of non-flammable and non-volatile ionic liquid as a wetting agent on the cathode side and improved the interface dynamics, that is, N-methyl-N-propylpiperidinium-bis(fluorosulfonyl)imide ([PP13][FSIl) was added. This strategy effectively improves the coulombic efficiency, rate capability, and cycling stability of the battery without compromising the safety of the all-solid-state battery. The excellent performance of the battery is attributed to the multiple roles of ionic liquids in the formation of ionic and electronic conductive networks, including enhancing solid/solid interface contacts, providing ion migration channels, and providing a “soft” buffer space for volume expansion during cycling.
Summary:
Capacitor ionic liquid is a liquid with special properties. It has attracted widespread attention because of its wide electrochemical window, wide stable temperature range and melting point much lower than common salts (called “low-temperature molten salt”). It can be classified in various ways. According to the combination of anions and cations, it can be divided into three categories: organic cations-organic anions, organic cations-inorganic anions, and inorganic cations-inorganic anions; according to the type of anions, it can be divided into chloroaluminate ionic liquids (the composition can be changed, it is sensitive to water, and the preparation has special requirements) and new ionic liquids (fixed composition, relatively stable in air and water).
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