Electrolytic capacitor Ionic liquids have gradually become the focus of researchers due to their high safety and other advantages. At present, researchers have made initial progress in the study of the physical and chemical properties and structural design of ionic liquid electrolytes for electrolytic capacitors.

This chapter briefly introduces the physicochemical properties of ionic liquids in electrolytic capacitors, the electrochemical properties and structural design of ionic liquid electrolytes.

1. Physicochemical properties of ionic liquids for electrolytic capacitors

1.1 Melting point

Electrolytic capacitor ionic liquids generally require low melting points and are liquid at room temperature. From the melting points of different chlorides, it can be seen that their melting points are affected by the structural characteristics of the cations. The interionic interaction force of the cation is proportional to its structural symmetry. The lower the structural symmetry, the smaller the interaction force. The more uniform the charge distribution of the cation, the lower its melting point. For example, the ionic liquid containing the symmetrical cation [EtEtIm]⁺ has a higher melting point than the ionic liquid containing the asymmetrical cation [EtMeIm]⁺. The longer the alkyl side chain of the cation, the larger the volume, the more dispersed the charge, and the worse the symmetry. The lower the melting point, but when the side chain increases to a certain extent, the melting point increases instead. The optimal number of C atoms in the side chain is n=6, 8; the higher the degree of branching of the alkyl chain in the cation structure, the higher the melting point, such as the melting point of [iPrMeIm]⁺ containing isopropyl is higher than that of [PrMelm]⁺ containing n-propyl; in addition, the replacement of the proton at the 2-position C of the imidazole ring by an alkyl group will also increase the melting point. It can be seen that the low melting point of ionic liquids can be attributed to their low symmetry, weak intermolecular forces and the dispersion of cationic charges.

The increase in the volume of anions will also affect their melting point, but the degree of influence is much smaller than that of cations. In most cases, the larger the volume of anions, the lower the melting point; when a strong electron-withdrawing group (F-) is substituted on an anion, the negative charge is highly dispersed, the interaction between the anion and the cation is weakened, and the melting point is lowered, such as the [N(CF₃SO₂)₂] ^–anion.

As we all know, general ionic compounds are solid at room temperature, and the electrostatic force between anions and cations is strong, so they have high melting points, boiling points and hardness. For example, the melting point of NaCl can reach 804℃. Most ionic liquids can only exist at high temperatures (high-temperature molten salts). Therefore, if the anions and cations are large and the structure is asymmetric, due to the effect of spatial hindrance, the strong electrostatic force cannot make the anions and cations densely stacked at the microscopic level, and the force between ions is significantly reduced, thereby reducing its melting point and becoming liquid at room temperature. In this way, room temperature ionic liquids can be obtained. There are only anions and cations in this liquid, and there are no neutral molecules.

1.2 Electrolytic Capacitor Ionic Liquids Thermal stability

Electrolytic capacitor ionic liquids (ILs) are the safest solvents for various high temperature applications due to their non-flammable properties. To study their thermal stability, thermogravimetric analysis (TGA) is widely used to analyze the kinetics of the thermal decomposition process of ILs.

In ionic liquid electrolyte mixtures, anions and cations can freely combine. It is also feasible to predict the thermal stability of ILs mixtures using TGA data of pure ILs. Compared with monocationic ionic liquids, dicationic ionic liquids (DILs) have better thermal stability, and the effect of their structure on thermal stability is similar to that of monocationic ionic liquids. In addition, due to strong electrostatic interactions, DILs have a higher melting point, and adjusting the length of the connecting chain and the type of anions can obtain DILs with low melting points and high thermal stability.

Compared with most organic solvents and water, ionic liquids have a wider liquid temperature range. The thermal stability of ionic liquids is mainly determined by the forces between heteroatoms and carbon atoms and the forces between heteroatoms and hydrogen bonds, so their thermal stability is closely related to the structure and properties of the anions and cations that make them up. Imidazole ionic liquids generally have high thermal stability. The thermal stability of ionic liquids is better than that of carbonate solvents in organic electrolytes. Figure 2-1 is a comparison of the flammability tests of pure organic electrolytes and electrolytes mixed with ionic liquids. Pure organic electrolyte burns directly with bright flames, while the combustion phenomenon of electrolyte with added electrolytic capacitor ionic liquids is not obvious.

Figure 2-1 Phenomenon of pure organic electrolyte (left) and 50% organic electrolyte-50% ionic liquid mixture (right) exposed to naked flame

1.3 Electrolytic Capacitor Ionic Liquids  Viscosity

The migration rate of metal ions in ILs is mainly affected by viscosity (also known as dynamic viscosity n). Viscosity is inversely proportional to the ion migration rate. As the viscosity increases, the ion migration rate will gradually decrease. Hydrogen bonding and van der Waals forces are the determining factors of viscosity. Because the Coulomb attraction, van der Waals force and hydrogen bonding interaction between anions and cations in ionic liquids are generally stronger than the intermolecular forces of small molecule solvents, the viscosity of ionic liquids is higher. Therefore, the viscosity of ionic liquids is 1 to 3 orders of magnitude higher than that of traditional organic molecule solvents (as shown in Figure 2-2), which has become one of the limiting factors in the application of ionic liquids. In general, the weaker the hydrogen bond, the lower the viscosity of the ionic liquid, and the stronger the van der Waals force, the higher the viscosity of the ionic liquid. However, when the van der Waals force is strong, ionic liquids may also show lower viscosity. This is mainly because the weak hydrogen bonding force reduces the viscosity of the system by more than the increase in the viscosity of the system due to the van der Waals force. Temperature also has a great influence on the viscosity of ionic liquids. The viscosity of ionic liquids decreases with increasing temperature, generally obeying the VFT (Vogel-Fulcher-Tamman) equation. Usually, the viscosity value decreases with increasing temperature, This may be the result of the breaking of hydrogen bonds between and within the molecules of the ionic liquid of the electrolytic capacitor.

Figure 2-2 Comparison of viscosity between organic electrolyte and electrolyte mixed with ionic liquid

Impurities have a significant effect on viscosity. The presence of trace impurities can cause a sharp drop in viscosity, so the viscosity of ionic liquid mixtures is much lower than that of pure ionic liquids. Common methods for measuring the viscosity of ionic liquids include the rolling ball method, the falling body method, and the moving piston method. Among them, the most commonly used method is the falling body method.

The viscosity of electrolytic capacitor ionic liquids is also significantly affected by the cation structure. When the size of the cation increases (or the alkyl chain grows), the van der Waals force increases and the viscosity increases; when the cation substituents have side chains, the viscosity increases; the unsaturated bonds in the cations disperse the positive charge, reduce the Coulomb attraction, and reduce the viscosity (such as imidazole ionic liquids); the introduction of electron-donating groups in the cation structure can disperse the positive charge, reduce the Coulomb attraction, and reduce the viscosity. On the contrary, the introduction of electron-withdrawing groups increases the viscosity; in addition, the ether group is flexible, which can improve the mobility of ions and reduce viscosity.

Anions also have a certain effect on viscosity. When the cations are the same, the larger the anion size, the greater the van der Waals force and the higher the viscosity, such as CF₃SO^–₃<C₄F₉SO^–₃ or CF₃CO^–₂; <C₃F₇CO^–₂; when the anion has a strong hydrogen bond, the viscosity increases, such as the viscosity of ionic liquids containing BE^–₄or PF^–₆ is higher than that of ionic liquids containing N(CF₃SO₃)^–₂; in addition, the fluorine substituents on the anions have a dispersing effect on the negative charge, which can weaken the interaction with the cations and reduce the viscosity.

Taking ionic liquids using TFSI^– as anion as an example, the viscosity is mainly affected by cations. Because the high symmetry of the spatial structure of the TFSI^–anion itself determines that its overall structure is close to electrical neutrality, the charge is evenly distributed throughout, and the electrostatic force between lithium ions and it is similar to the van der Waals force. The structural influence of cations mainly depends on their composition, that is, the branch chain of cations and their branch space structure have a significant impact on the viscosity of ionic liquids, so usually ionic liquid cations are substituted with methyl and propyl structures (1, 3 structure), or substituted with ethyl and butyl structures (2,4 structure), which can minimize the viscosity of the system and maintain a more ideal structure.

1.4 Volumetric properties

Volumetric properties are one of the basic and important thermodynamic properties. Volumetric properties refer to characteristics that are closely related to the volume change of a substance, such as density, molar volume, partial molar volume, excess molar volume, compressibility, and expansibility. Ionic liquid electrolytes are usually multi-component mixtures containing ionic liquids. They are very complex systems whose properties depend not only on intermolecular interactions, but also on ion-ion and ion-molecule interactions. At the same time, some factors that affect structural effects must also be considered, because the differences in molar volume and free volume between solution components will cause gap regulation.

Density is one of the volume properties. Some volume properties are calculated based on density. Therefore, there are many reports and studies on density, including mixtures of organic solvents and ionic liquids, mixtures of gases and ionic liquids, mixtures of solids and ionic liquids, etc. In general, the measured density of ionic liquid mixtures depends on temperature, pressure and composition. For the entire composition range, the density of ionic liquid mixtures at different temperatures can be related by using linear equations or second-order polynomials. In general, there is a significant nonlinear relationship between density and the composition of mixtures containing ionic liquids, which is different from general organic solvent mixtures. By comparing the density of chloroaluminates containing imidazolium cations with different substituents, it is found that the density is linearly related to the length of the N-alkyl chain on the imidazolium cation. When the organic cation becomes larger, the density of the ionic liquid becomes smaller, so the density of the ionic liquid can be adjusted by slightly adjusting the cation structure. The effect of anions on density is more obvious. Generally speaking, the larger the anion, the greater the density of the ionic liquid. Therefore, to design ionic liquids of different densities, the corresponding anions should be selected first to determine the approximate range, and then the cations should be selected to fine-tune the density.

1.5 Solubility

Electrolytic Capacitor Ionic Liquids can be used as good solvents for many chemical reactions, and different substances such as organic matter, inorganic matter and polymers can be well dissolved in it. The characteristics of cations and anions will affect the solubility of ionic liquids, with cations having the most significant impact. From the dissolution results of n-octene in the ionic liquid containing the same tosylate anion quaternary ammonium salt, it can be seen that as the quaternary ammonium cation side chain in the ionic liquid becomes larger, that is, the non-polar characteristics increase, the solubility of n-octene It also grew bigger. It can be seen that changing the alkyl group of the cation can adjust the solubility of the ionic liquid. The dissolution results of water in ionic liquids containing different [Bmim]⁺cations can confirm that anions can also have a great impact on the solubility of ionic liquids.

1.6 Polarity

The polarity of ionic liquids for electrolytic capacitors is usually difficult to characterize using traditional methods. so researchers have made a lot of efforts to characterize them, such as solvent colorimetry, fluorescent probe dye method, distribution coefficient method, gas chromatography stationary phase method, etc., but the scale of polarity also changes depending on the method used. Most studies on the polarity of ionic liquids are conducted using polarity-sensitive dyes and fluorescent probes. The polarity of ionic liquids obtained by most probes is higher than that observed by dielectric constants. Some researchers believe that the polarity of ionic liquids is close to that of short-chain alcohols, such as methanol (ε=32.5), but studies on dielectric constants show that the polarity of ionic liquids is equivalent to medium-chain alcohols, such as n-propanol (ε=15.1) and n-octanol (ε=8.8). Most studies have shown that the polarity of ionic liquids is mainly related to the properties of cations, while studies on dielectric constants show that it is mainly related to anions.

Nile Red (Nilered) molecule is the earliest compound used to measure the polarity of ionic liquids. By measuring the size of the electron transition energy (Er) of Nile Red molecules in a series of imidazole ionic liquids with different anion coordination, it was found that the E value did not change much, indicating that ionic liquids are medium-polar solvents, which is consistent with the polarity range of dielectric constant reactions. The most widely used empirical solvent polarity scale Ey is obtained by measuring the maximum absorption wavelength of Reichardt dye. Reichardt dye is a commercial probe molecule with a maximum absorption wavelength of about 375nm. Its absorption wavelength position is determined by the dipole of the solvent, hydrogen bonding, Lewis acidity and hydrogen bond donor ability.The solvent polarity value determined by Reichardt dye can be expressed as E^N_{T.Where E³⁰T is the polarity value determined by Reichardt using betaine 30.E^NT is a normalized value based on E³⁰T to describe the polarity of the solvent.The E^NT value of water is set to 1.The E^NT value of tetramethylsilane is 0.The polarity of all measured solvents is a relative value.E^NTcan be obtained by formula (2-1) and formula (2-2).}

The influence of cations on the E of ionic liquids is quite obvious, and the general order is C_nNH⁺₃;C_nC_nim>C_nC₁C²₁im>(C_n)P⁺, which is basically consistent with the order of the hydrogen bonding ability of cations. The maximum polarity of ionic liquids measured by this method is close to water, and the minimum is close to polar aprotic solvents. Increasing the length of the alkyl chain on the cation will lead to a decrease in the EN value, but the decrease is not obvious, while the functionalized alkyl chain will increase the E value of the ionic liquid. The effect of anions on the E value of ionic liquids is relatively small, mainly because Reichardt dye is a hydrogen bond acceptor rather than a hydrogen bond donor, and it cannot reflect the hydrogen bond acceptance ability of the solvent. The effect of anions on the EN of ionic liquids is also related to the type of cations.

1.7 Acidity and alkalinity

The acidity and alkalinity of ionic liquids for electrolytic capacitors is one of the focuses of attention. Ionic liquids are the products of the combination of anions and cations. Therefore, ionic liquids synthesized in different forms and structures may be Lewis acids or Lewis bases under certain conditions. But the acidity and alkalinity of ionic liquids are actually determined by the nature of the anions. For example, when Lewis acid (such as AlCl₃) is added to the ionic liquid [Bmim]Cl, when the molar fraction of AlCl₃ x(AlCl₃) <0.5, the ionic liquid is alkaline; when x(AlCl₃) = 0.5, the ionic liquid is neutral, and the anion is only AlCI; when x(AlCl₃) > 0.5, as AlCl₃ increases, there will be anions such as Al₂CI^–₇ and Al₃Cl^–₁₀, and the ionic liquid exhibits strong acidity.
When studying the acidity and alkalinity of ionic liquids, it is necessary to pay attention to their “potential acidity” and “superacidity”. Compared with traditional superacid systems, superacidic ionic liquids are safer to handle.

1.8 Surface tension

The resistance of a liquid substance under a certain force is called surface tension, which is one of the most important characteristics of ionic liquids. The surface tension value of ionic liquids mainly depends on the structure of cations and anions on the surface of ionic liquids).

(1) The commonly used relationship for calculating the surface tension of ionic liquids is shown in formula (2-3):

Y=a-bT         (2-3)

Where: y is the surface tension of the ionic liquid; a and b are the parameters for fitting the experimental data; T is the experimental temperature.

(2) The variation of the surface tension of ionic liquids with temperature can be calculated by the Eotvos equation, as shown in formula (2-4):

Y^V2/3_m=K(Tc-T)            (2-4)

Where: y is the surface tension of the ionic liquid; V_m is the molar volume of the ionic liquid; k is an empirical constant; T_c is the critical temperature.
There are interactions such as van der Waals force, Coulomb force, and hydrogen bond inside the ionic liquid, which will affect the surface tension of the ionic liquid. Arjmand et al. used an approximate method based on the pseudolattice theory to obtain a simple linear relationship for the surface tension of ionic liquids. The parameters of this linear law can be used to estimate the surface tension over a wide temperature range and to predict the surface tension of binary mixtures of ionic liquids.

1.9 Refractive index

Polarity is related to the symmetry of the ionic liquid. As the symmetry of the ionic liquid decreases, its refractive index increases. Generally, pyridinium salts have a higher refractive index than imidazolium salts, which is why pyridinium salts are more polar than imidazolium salts.

Under the same external conditions and the same anions, the refractive index of ionic liquids increases with the increase of the chain length of the cation substituents. This may be because the polarizability of ionic liquids increases with the increase of chain length. When the cations are the same, the refractive index of ionic liquids is negatively correlated with the size of the anions; at different temperatures, the refractive index tends to decrease with the increase of external temperature. When the anions are constant, the refractive index is basically linear with the chain length of the cation substituents.

The various physical and chemical properties of ionic liquids may affect the performance of the prepared electrolytes. Therefore, the study of the physical and chemical properties of ionic liquids is of great significance. Trace amounts of water or any organic solvents present as impurities have a significant impact on the physical and chemical properties of ionic liquids. Trace amounts of water in ionic liquids affect their acidity, density, viscosity, conductivity, enthalpy, surface and interfacial tension, etc. For example, the addition of a small amount of water can lead to a large change in viscosity or enthalpy.

2 Electrochemical properties of ionic liquids for electrolytic capacitors

2.1 Electrolytic Capacitor Ionic Liquids Conductivity

When ionic liquids are used in battery electrolytes, their conductivity performance is particularly important. Ion mobility and current carriers determine the conductivity of ionic liquids. Although ionic liquids are completely composed of anions and cations and have sufficient charge carriers, the association between some ions will lead to a decrease in the number of carriers and a decrease in conductivity. Conductivity is also closely related to viscosity, and the conductivity of ionic liquids is greatly affected by viscosity. Generally speaking, the higher the viscosity of the ionic liquid, the more difficult it is for lithium ions to migrate in it. Among the many ionic liquids, EMITFSI has a higher conductivity (also has a lower viscosity), and its room temperature conductivity is close to 10^-2S·cm^-1‘. The conductivity of the ionic liquid system is also related to the concentration of lithium salts. Adding a certain amount of organic electrolyte to the ionic liquid system can significantly reduce its viscosity and increase its conductivity. As the volume fraction of the organic electrolyte increases, the viscosity of the system gradually decreases. However, the conductivity shows a different change pattern. The conductivity of the ionic liquid system shows a trend of first increasing and then decreasing with the increase of the organic electrolyte content. This shows that for the ionic liquid system, its conductivity is greatly affected by the viscosity and lithium salt concentration.

Generally speaking, viscosity, temperature, ion size, etc. will affect ionic conductivity. Among them, viscosity and conductivity are closely related, mainly because ion mobility is related to viscosity. The higher the viscosity, the larger the ion radius and the lower the conductivity. Ionic conductivity is also affected by temperature. The higher the temperature, the higher the ionic conductivity, and the lower the temperature, the lower the ionic conductivity. In the lower temperature range, the relationship between ionic conductivity and temperature roughly obeys the Arrhenius equation. In a wider range, the relationship can be described by the VFT equation [Equation (2-5)]:

Where: A, B, To are all fitting parameters.

In addition to viscosity, the density and ion size of ionic liquids also affect conductivity. The effect of density on conductivity is opposite to that of viscosity, that is, density and conductivity are directly proportional. When density and viscosity are similar, the size of the ion radius plays a major role. The smaller the ion radius, the higher the conductivity.

2.2 Electrochemical window

The electrochemical window is one of the important parameters for selecting ionic liquids as battery electrolytes. The electrochemical stability window is the potential difference at which the ionic liquid begins to undergo oxidation and reduction reactions, and its size is determined by the electrochemical stability of the anion and cation. Usually, the anion determines the oxidation potential of the ionic liquid. At present, the anion of the ionic liquid usually adopts TFSI^– (which can make the system obtain a larger electrochemical window, and the general oxidation potential is greater than 4.5V), while the cation determines the reduction potential of the ionic liquid. The electrochemical window of the ionic liquid narrows with the increase of temperature. This is because the reduction limit potential of the cation increases with the increase of temperature, while the oxidation limit potential of the anion decreases. Most ionic liquids have an electrochemical window of more than 4V, which is wider than that of organic solvents. This is because the characteristics of the anions and cations that make up the ionic liquids cause the reduction of the cations and the oxidation of the anions to generally occur at more negative and more positive potentials. The electrochemical window of ionic liquids can usually be determined by linear sweep test or cyclic voltammetry test.

2.3 Ion migration number

Electrolyte solutions conduct electricity by relying on the directional migration of ions. In order for electric current to pass through an electrolyte solution, two conductors as electrodes need to be immersed in the solution so that the electrodes are in direct contact with the solution. When electric current passes through an electrolyte solution, the positive and negative ions in the solution migrate to the cathode and anode respectively, and redox reactions occur on the electrodes at the same time. According to Faraday’s law, the amount of change in the amount of substance on the electrode is proportional to the amount of electricity passed. The amount of electricity passing through the solution is equal to the sum of the amount of electricity migrated by the positive and negative ions. Since the migration rates of various ions are different, the amount of electricity generated by each ion must also be different. The ratio of the amount of electricity generated by each ion to the total amount of electricity passing through the solution is called the migration number of the ion in this solution, represented by the symbol R, where R is a dimensionless quantity.

Ionic liquids can show high ion migration numbers due to their good electrical conductivity. Zhang et al. systematically studied the ion migration number of polyionic liquids. The results showed that the ion migration number is largely affected by the contribution of cations to conductivity. The kinetic correlation between different cations plays an important role in affecting the real conductivity of polyionic liquids.

3 Electrolytic Capacitor Ionic Liquids Electrolyte structure design

In recent years, the application of new electrolytic capacitor ionic liquids electrolytes in batteries has been widely studied, but there are still many problems with the use of ionic liquids in electrolytes. First, compared with traditional organic electrolyte systems, ionic liquid electrolytes have higher viscosity; second, ionic liquids have poor wettability on some materials and it is difficult to form a good SEI film; finally, although ionic liquids have a wide electrochemical window, when the voltage reaches above 4V, the electrochemical properties of ionic liquids become unstable, and they will react with lithium when used in lithium-ion batteries. The structure of ionic liquids used as electrolytes is usually designed to improve these problems to a certain extent.

3.1 Introduction of functional substituent side chains

The biggest feature of electrolytic capacitor ionic liquids is that they can be freely selected and matched from a large number of anions and cations to obtain ionic liquids with specific physical and chemical properties that meet the needs. Introducing functional substituted side chains into conventional ionic liquids as needed is conducive to improving the performance of ionic liquid electrolytes. The Ferrari team found that among many ionic liquids, 1-methyl-4-butyl-pyrrolidone imine (trifluoromethanesulfonyl imide) (Pyr14-TFSI) is considered to be a very promising electrolyte in lithium batteries due to its hydrophobicity, wide electrochemical window and high ionic conductivity in the molten phase. But it is easy to crystallize below 0°C, which obviously causes a sharp drop in the conductivity of the battery within the operating temperature range. In addition, its viscosity is still too high compared to standard organic electrolytes. In order to explore other potential ionic liquids as liquid electrolytes, the team proposed a structure based on methylpyrrolidone (Pyr_1，oR-TFSI), which comes from ethoxy-containing side chains. The team also proposed two systems with different substituent chain lengths and oxygen unit numbers, namely Pyr₁._2O1-TFSI and Pyr₁,20201-TFSI. The study observed that the presence of ether groups inhibited crystallization below room temperature, which may be due to the interaction between the oxygen atoms of the functionalized group and the adjacent imide anions. In addition, oxygen can also reduce the viscosity of ionic liquid electrolytes, thereby expected to improve ionic conductivity. These properties have also been observed in other cyclic quaternary ammonium salts, such as pyrazole, piperidine, morpholine and oxazole quaternary ammonium salts.

Iwasaki et al. studied the physicochemical properties and electrochemical behavior of 1-butyl-3-methylimidazolium aryl trifluoroborate ([C4mim][ArBF₃), which introduced different substituents such as methoxy, fluorine, trifluoromethyl and cyano groups in the phenyl part of the anion. In addition, several positional isomers of the [ArBF₃]-anion were prepared to further understand the influence of the position. The conductivity and electrochemical stability of the room temperature ionic liquid [C₄mim][ArBF₃] were qualitatively discussed from the results of quantum chemical calculations of cation-anion interactions and the highest occupied molecular orbital (HOMO) energy levels of the anion. Interestingly, [C₄mim][ArBF₃] with electron-withdrawing groups in the phenyl part can form an ion-selective membrane on a platinum electrode, and only neutral and cationic species can pass through the membrane. Figure 2-3 lists the optimized structure of the [C₄mim][ArBF₃] ion pair and compares it with the [C₄mim][BF₄] ion pair.

Figure 2-3 Optimized structures of ion pairs consisting of [C₄mim] and aryl trifluoroborate with various substituents. The geometry was optimized at the HF/6-311G (d.p) level.

It was found that almost all optimized ion pair structures have similar conformations. In the optimized structure of the [C₄mim][PhBF₃] ion pair, the BF3 group in the [PhBF₃]- anion interacts with the hydrogen at the C2 position of the imidazolium cation, which is the same as the [C₄mim][BF₄ ]ion pair. The benzene ring is tilted relative to the plane of the imidazole ring. The only exception is the [C₄mim][ArBF₃]ion pair, which is because their anions have -F or -OMe in the ortho-substituent.In the optimized structures of [C₄mim][o-FC₆H₄BF₃] and [C₄mim][o-OMeC₆H₄BF₃]ion pairs, the benzene ring turns to the plane of the imidazolium cation. The steric hindrance between the butyl group on the cation and the ortho-group on the anion is the reason why the optimized structure has different conformations. Studies have shown that the physicochemical properties of the prepared various room temperature ionic liquid electrolytes change with the different substituents introduced on the groups. Among them, [C₄miml[m-FC₆H₄BF₃]shows good fluidity and better ionic conductivity than the unsubstituted [C₄miml[PhBF₃].

Functionalized ionic liquid electrolytes have been designed to address the shortcomings of ionic liquid electrolytes during use. Using functional substituents to improve the physicochemical and electrochemical properties of ionic liquid electrolytes is a very promising research direction.

3.2 Adjusting the Electrolytic Capacitor Ionic Liquids Electrolyte Concentration

Adjusting the electrolyte concentration is one of the key strategies to achieve functional design. The concentration of the ionic liquid electrolyte salt used in the study is usually 1mmol·L-‘. At present, there have been some studies on improving the conductivity of ionic liquid electrolytes by adjusting the salt concentration in ionic liquid electrolytes.

In 2017, Wang et al. applied concentrated NaFSA/TMP and LiFSA/TMP electrolytes to model systems of sodium-ion and lithium-ion batteries based on the concept of concentrated salt electrolytes. The conceptual diagram of the model is shown in Figure 2-4. (a) is a schematic diagram of the explosion of the battery caused by the ejection of heated flammable electrolyte vapor after thermal runaway of the battery system. (b) is a schematic diagram of the behavior of cations (red spheres) embedded in carbon anodes in various electrolytes. Traditional electrolytes passivate the anode by preferentially reducing the electrolytic solvent (yellow ellipsoid) rather than the anion (green ellipsoid), but their high flammability poses serious safety risks. Electrolytes containing non-flammable solvents (blue ellipsoids) usually cannot passivate the anode, resulting in continuous solvent decomposition or solvent co-intercalation. Concentrated electrolytes containing non-flammable solvents can effectively passivate the anode by forming a salt-derived SEI film (green film). Concentrated salt electrolytes can not only act as strong fire extinguishing agents to ensure the safety performance of batteries, but also enable hard carbon or graphite anodes to achieve long-term stable charge-discharge cycles with negligible degradation and better performance than conventional flammable carbonate electrolytes. Typically, increasing the concentration of ionic liquid electrolytes undoubtedly increases the viscosity and cost of the electrolyte and reduces the conductivity of the electrolyte. The excellent cycle stability is achieved by the spontaneously formed salt-derived inorganic SEI membrane, which is superior to the traditional solvent-derived organic-inorganic hybrid SEI membrane. This design strategy is extremely simple and flexible, and can be applied to various combinations of non-flammable or flame-retardant solvents and alkaline salts, which can promote the development of safe and high-performance rechargeable batteries.

Li et al. took advantage of the low viscosity of dilute electrolytes and the organic-dominated solid electrolyte interface that can be formed, and proposed an unconventional ultra-low concentration electrolyte (0.3 mol·L^-1) for sodium ion batteries to further reduce costs and expand the operating temperature range. Figure 2-5 shows the changes in the physicochemical properties of the electrolyte from high concentration to low concentration, the interactions between molecules/ions, and the interfacial components. In this ultra-low concentration electrolyte (0.3 mol·L^-1), nanoparticles can work well. The diluted concentration not only significantly reduces the cost, but also expands the operating temperature range of safety-enhanced nanoparticles (30~55℃). In addition, the stable organic-dominated SEI/CEI formed has excellent kinetic properties, allowing the nanoparticles to operate for a long time at extreme temperatures.

Figure 2-5 The changes in the physicochemical properties of electrolytes, the interactions between molecules/ions, and the interfacial components from high to low concentrations

3.3 Preparation of electrolytic capacitor Ionic liquids composite solid electrolyte

Polymer electrolytes form an important bridge between conventional solvent-based liquid electrolytes and solvent-free ceramic or molten electrolytes and are the basis for new practical power sources that do not have the problems associated with the escape of corrosive, flammable or toxic liquids, and they also offer opportunities for new continuous manufacturing processes based on thin film lamination techniques.

Polymer electrolytes must have a series of key properties, including: ① sufficiently high ionic conductivity and high resistivity; ② high cation mobility; ③ good mechanical properties; ④ the ability to form good interfacial contact with electrodes; ⑤ a wide electrochemical stability window; ⑥ easy to handle; 7 high chemical and thermal stability; ⑧ high safety.

Gel electrolytes can show the combined excellent properties of liquid and solid electrolyte systems. The commonly used gel electrolytes in the early years were usually a mixture of low molecular weight organic plasticizers (such as propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, etc.) and conventional organic carbonates. However, electrolytes containing carbonates can degrade or cause device temperatures to rise due to the formation of inorganic salts (such as LiF, Li₂O and Li₂CO₃). In order to overcome the above problems, organic plasticizers are replaced by room temperature ionic liquids. Room temperature ionic liquids can be captured by inorganic salts into the main polymer matrix to form “ionic liquid-based gel polymer electrolytes”. Ionic liquid gel electrolytes are a hybrid electrolyte system that shows the combined properties of liquid and solid electrolytes. Compared with organic liquid electrolytes, ionic liquid electrolytes overcome some of the problems of organic electrolytes and also have other advantages such as improved mechanical/thermal stability, enhanced ionic conductivity, flexibility, safety, leakproof performance and reduced interface resistance. In addition, electrolytic capacitor ionic liquids as plasticizers can take advantage of the amorphous nature of solid polymer electrolytes to enhance ionic conduction.

Polyionic liquids (PILs) are polyelectrolytes containing repeating units of ionic liquids. As a polymeric form of ionic liquids, polyionic liquids not only have the advantages of ionic liquids, such as non-flammability, the ability to transport ions, and good thermal stability, but also show other advantages over ionic liquids, such as excellent processability and controllability. Due to the good affinity between polyionic liquids and liquid electrolytes, polyionic liquids can be a good choice for high-performance polyionic liquid matrices. Generally, the glass transition temperature (T_g) is a key factor affecting ionic conductivity. Polyionic liquids can achieve higher ionic conductivity with lower glass transition temperatures, but these materials usually do not have enough mechanical strength to maintain a free-standing membrane state. For example, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) can be used as a flexible gel polymer electrolyte (GPE) matrix due to its good mechanical properties, high dielectric constant, electrochemical stability, and good heat resistance.

Hu et al. reported a new type of polyionic liquid GPE, which was derived into a porous PIL/(PVDF-HFP) composite membrane through a simple and scalable phase inversion method (Figure 2-6). The ion-dipole interaction between PIL/(PVDF-HFP) gives the PIL/(PVDF-HFP) membrane significantly enhanced mechanical properties and thermal stability. After the liquid electrolyte is absorbed into the PIL/(PVDF-HFP) membrane, a flexible gel can be obtained. Compared with the liquid electrolyte absorbed in the commercial Celgard2325 separator, the PIL/(PVDF-HFP) gel polymer electrolyte shows much higher ionic conductivity (1.78×10^-3S·cm^-1). The lithium/lithium iron phosphate battery assembled based on this electrolyte exhibits a high reversible capacity of 99.2 mAh·g^-1 at a 12C rate, and a high discharge capacity of 138.4 mAh·g-1 and 125.4 mAh·g^-1! at 1C and 4C after 200 cycles of charge and discharge, respectively. More importantly, the gel polymer electrolyte PIL/(PVDF-HFP) also exhibits excellent fire resistance, thanks to the non-flammability of PIL.

Figure 2-6 Schematic diagram of the structure of the porous PIL/(PVDF-HFP) electrolyte (upper left) and the flame retardancy test (upper right) and the cycling performance

Singh et al. prepared self-supporting, transparent and flexible ionic liquid gel polymers using PVDF-HFP polymer as the matrix, LiTFSI and different mass fractions of Pyr₁₃FSI ionic liquids. The cleanliness and flammability experiments of the obtained products are shown in Figures 2-7. The crystallinity of the polymer decreases with the increase of the mass percentage of the ionic liquid. This is because after the addition of the ionic liquid, the segment movement in the polymer chain is enhanced, becoming more flexible and amorphous. When the mass fraction of the added ionic liquid is 70%, Electrolytic capacitor Ionic liquids electrolytes exhibit the highest ionic conductivity, and samples show high safety in flammability tests.

Figure 2-7 (a) Crystallinity percentage (with error bars) and melting temperature; (b) Flammability test phenomenon with different mass fractions of ionic liquids added

The thermal stability and mechanical strength of polymer electrolytes are still insufficient to meet the requirements. High-performance solid composite electrolytes require fast lithium ion conduction, good interfacial contact with electrodes, high thermal stability, wide electrochemical window and sufficient mechanical strength, while using non-toxic and environmentally friendly materials. Nanocomposite electrolytes composed of non-volatile ionic liquids, organic metal salts and porous inorganic materials are a promising choice as solid electrolyte materials. Solid composite electrolytes with nanoporous inorganic matrices prepared by sol-gel methods can obtain high ionic conductivity. These composite electrolytes are also called “ion gels”. Due to the non-volatility, thermal stability and electrochemical stability of ionic liquids and inorganic components, such electrolytes can improve battery safety. In addition, the porous matrix with large porosity but rigidity is conducive to achieving good lithium ion conductivity and mechanical properties. The low-viscosity liquid sol-gel precursor can be impregnated into dense and porous powder electrodes, where it is further gelled, resulting in good interfacial contact between active electrode particles and electrolyte.

Sagara et al. proposed a silica gel solid nanocomposite electrolyte for use in solid batteries. The nanocomposite electrolyte composed of 1-ethyl-3-methylimidazole bis(fluorosulfonyl)imide (EMI-FSI) and LiFSI is confined in a mesoporous silica matrix (Figure 2-8). The ionic conductivity is 6.2 mS·cm^-1 at room temperature. The capacity of lithium/lithium iron phosphate batteries can reach 150mAh·g^-1‘ at 0.1C and 113mAh·g^-1 at 1C. The rate performance of the prepared solid battery is comparable to that of batteries using conventional lithium hexafluorophosphate (LiPF₆) electrolytes.

Figure 2-8 Schematic diagram of composite electrode manufacturing process

(a) LFP powder electrode foil; (b) drop casting of solid composite electrolyte sol-gel precursor (liquid sol-gel precursor infiltrates into the powder electrode, filling the gap between the active material and the gel); (c) gelation 6 days after drop casting (nanoparticles form around the active material, and the composite electrode densifies during the process of gel drying and solidification into nanoparticles)

Research has shown that both polyionic liquid electrolytes and electrolytic capacitor ionic liquids/solid composite electrolytes exhibit excellent electrochemical properties. Similar to the existence of a solid state, these two types of electrolytes effectively improve mechanical strength, avoid leakage in applications, and help improve the safety performance of battery systems. They still maintain the properties of liquid electrolytes, Achieving good conductivity and electrochemical performance of the battery system points out a bright direction for the structural design and development of ionic liquid electrolytes for future electrolytic capacitors

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