LR03 battery.Research progress on ionic liquid electrolytes for supercapacitors?

　　Research progress on ionic liquid electrolytes for supercapacitors?

　　Room temperature ionic liquids are a type of liquid material composed entirely of ions due to the extreme asymmetry and spatial obstruction of anions and cations, resulting in low ion electrostatic potential. They are referred to as ionic liquids for short. Aluminum trichloride and ethylpyridinium halide ionic liquids are the first generation of room temperature ionic liquids; after S.John et al. synthesized dialkylimidazole cation salts with better electrochemical stability, ionic liquids quickly became a research hotspot.

　　The specific energy of supercapacitors is lower than that of lithium-ion batteries. It is an urgent problem to improve the specific energy while maintaining high specific power. To improve the specific energy of a single supercapacitor, it is necessary to increase the specific capacitance while increasing the operating voltage. The working voltage is related to the decomposition voltage of the electrolyte. At present, there are mainly two types of electrolytes for supercapacitors: aqueous and organic. The aqueous electrolyte is sulfuric acid solution or potassium hydroxide solution, which is highly corrosive, and the working voltage of the prepared single supercapacitor is low (only about 1V). The organic electrolyte is an organic solution of electrolytes such as tetrafluoroborate tetraethylammonium salt. The working voltage of the prepared monomer supercapacitor is above 2.5V; however, the organic solvent is easy to volatilize, and it is difficult to increase the conductivity and working voltage, and has safety issues. hidden dangers and environmental impact.

　　Ionic liquids can be used directly as the liquid electrolyte of supercapacitors, or they can be dissolved in organic solvents as electrolyte salts, and solid polymer electrolytes can also be introduced to improve related properties.

　　1 liquid electrolyte

　　The anions of ionic liquids are mainly composed of bis(trifluoromethylsulfonyl)imide (TFSI-), BF4- and PF6-. The cations of ionic liquids are mainly composed of organic large-volume ions such as imidazoles, pyrroles and short-chain fatty quaternary ammonium salts.

　　1.1 Imidazole ionic liquids

　　Imidazole ionic liquids have low viscosity and high conductivity. Since 1-ethyl-3-methylimidazole tetrafluoroborate (EMIBF4), imidazole ionic liquids have developed rapidly.

　　1-Butyl-3-methylimidazole (BMI+) ionic liquids have been extensively studied due to their low viscosity, relatively high conductivity, and easy synthesis. B.Andrea et al. used 1-butyl-3-methylimidazole hexafluorophosphate (BMIPF6) and 1-butyl 3-butylimidazole tetrafluoroborate (BMIBF4) as activated carbon (AC)/polytrimethyl Electrolyte for thiophene (pMeT) hybrid capacitors. Compared with organic electrolyte (PC-EtNBF4) capacitors, ionic liquid capacitors have higher specific energy, power density and current efficiency at 60°C.

　　High viscosity is one of the main obstacles for the industrial application of ionic liquids. 1-Ethyl-3-methylimidazole fluoride (EMIF 2.3HF), which has quite high conductivity and low viscosity at low temperatures, has been widely studied as a supercapacitor electrolyte. U. Makoto et al. used EMIF 2.3HF as the electrolyte and conducted comparative experiments with 1mol/LEt3MeNBF4/PC electrolyte. At 25°C, the conductivity of the former can reach 100mS/cm, and that of the latter is 13mS/cm. Supercapacitors using EMIF 2.3HF ionic liquid have relatively low internal resistance (between aqueous and organic electrolytes), and their capacitance is higher than that of common EMIBF4 ionic liquid supercapacitors even at low temperatures. The decomposition voltage of EMIF 2.3HF is only about 2V, resulting in too low energy density; above 70°C, the cycle performance and thermal stability (weight loss starts at about 77°C) are not ideal. Coupled with the toxicity of HF, it is not suitable as an industrial electrolyte. Applications are restricted.

　　In order to further improve the conductivity of imidazole ionic liquid electrolytes and reduce the viscosity while maintaining a high electrochemical window, imidazole ionic liquids combined with aprotic organic solvents PC and EC have been studied extensively as mixed electrolytes. A.B.McEwen et al. dissolved 2mol/L EMIPF6 in AN and used it as a supercapacitor electrolyte with a maximum conductivity of 60mS/cm.

　　In addition to the selection of anions and cations in imidazole ionic liquids, the substitution of cations and the fluorination of anions have also been studied to a certain extent. From the perspective of cation substitution, the H at position 2 on the EMI+ imidazole ring is relatively active. When H is replaced by a more stable alkyl group, the stability of the ionic liquid is also enhanced. Z. Zhou et al. used perfluorinated ionic liquid [EMI] RfBF3 as the electrolyte of supercapacitor and found that the stability and cycle performance were poor, especially the cycle performance loss was large (50% loss in 2d), which limited practical applications. J.Barisci et al. used ionic liquid electrolytes to study carbon nanotube (CNT) electrodes and found that CNTs have better activity and specific capacitance.

　　L. Kavan et al. used BMIBF4 as the electrolyte to study the electrochemical properties of single-wall CNT, double-wall CNT and fullerene electrodes. The results showed that these electrode materials have obvious supercapacitor characteristics. H.T. Liu et al. studied a hybrid capacitor using BMIPF6 as the electrolyte, mesoporous nickel-based mixed rare earth oxide as the anode material, and AC as the negative electrode material. The capacitor showed high specific power (458W/kg) and specific energy (50Wh /kg), after 500 cycles, the capacitance has no obvious attenuation.

　　Ionic liquids are also used in the research of synthesizing polymer electrode materials for supercapacitors. C. Arbizzani et al. prepared P-type doped polymer pMeT using a constant current polarization method. The solution in the reaction tank was EMITFSI. By adding HTFSI without consuming the ionic liquid, the H+ was reduced to H2, and (MeT0 .3+TFSI-0.3)n polymer. A hybrid capacitor using this polymer as the electrode material and EMITFSI as the electrolyte exhibits a high specific capacitance of 250F/g.

　　1.2 Pyrrolidine ionic liquids

　　Pyrrolidine ionic liquids are cyclic quaternary ammonium salts, which have a lower melting point and higher conductivity due to the asymmetry of pyrrolidine cation substitution.

　　N-butyl-N-methylpyrrole bis(trifluoromethylsulfonyl)imide salt (PYR14 TFSI) has received widespread attention due to its excellent electrochemical and thermal stability at high temperatures. A. Balducci et al. used PYR14TFSI ionic liquid as the electrolyte of AC/pMeT hybrid supercapacitor. After the capacitor was charged and discharged 16,000 times under the conditions of 60°C, 10mA/cm2 and 1.5~3.6V, the overall performance was good, especially the high temperature capacitance retention ability. .

　　The energy density and power density of ionic liquids are high, indicating that pyrrolidine ionic liquids can improve the voltage window and cycle life of hybrid capacitors at high temperatures (60°C). A. Balducci et al. studied the electrolyte of a microporous activated carbon symmetric capacitor using the ionic liquid PYR14TFSI. The resistance of the capacitor basically did not change (9Ωcm2) after 40,000 cycles. The voltage window at 60°C was 3.5V. The ratio of the electrode material The capacitance is 60F/g. This supercapacitor can be used in practice as a high-temperature capacitor.

　　M. Lazzari et al. studied the role of the interface between ionic liquid electrolytes PYR14TFSI and EMITFSI and AC, and found that when the cathode is charged, the capacitance of the carbon electrode largely depends on the polarization of the ionic liquid cations, that is, it depends on the dielectric that affects the double electric layer. The electrical properties and types of cations; the porosity of carbon and the chemical properties of the interface are also important factors affecting the conductivity and polarizability of ionic liquids.

　　1.3 Short-chain fatty quaternary ammonium salt ionic liquids

　　The biggest advantage of short-chain fatty quaternary ammonium salt ionic liquids is that they are stable to high specific surface area activated carbon, and have higher stability than imidazole and pyrrolidine ionic liquids.

　　T.Sato et al. studied the synthesis of N,N-dimethyl-N-ethyl-N-2-methoxyethylammonium di(trifluoromethylsulfonyl)imide salt (DEMENTf2) as a supercapacitor electrolyte. performance. EMENTf2 exhibits a wide liquid range. The cyclic voltammetry curve shows that the voltage window can reach 6V (platinum electrode), the conductivity at room temperature is 4.8mS/cm, and compared with traditional organic electrolytes, the specific capacitance and Coulomb efficiency are higher. high. Y. Kanako et al. found that DEMEBF4 and MEMPBF4 (the cation is N-methyl-N-2-methoxyethylpyrrole) electrolytes have better stability, high and low temperature performance, and higher conductivity. This ionic liquid and ionic liquids synthesized from the same cations and TFSI-anions can improve the high-temperature safety performance of supercapacitors.

　　2Polymer solid electrolyte

　　Ionic liquid polymer electrolytes combine the advantages of good mechanical properties of polymers and high conductivity of ionic liquids, while improving the safety and stability of capacitors. Generally, ionic liquid polymer electrolytes can be divided into two categories: ① polymer electrolytes containing ionic liquids; ② introducing ionic liquid structures into polymer molecules to obtain ionic liquid/polymer electrolytes.

　　The polymer matrix is mainly composed of polyoxyethylene (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) and polyvinyl alcohol (PVA). A. Lewandowske et al. studied the electrochemical properties of EMIBF4, EMINTf2, BMIBF4 and BMIPF6 plasma liquid-polymer electrolytes and found that when high specific surface area activated carbon materials are used, the specific capacitance is 45~180F/g.

　　A. Lewandowske et al. used the above ionic liquid as the supercapacitor electrolyte, and improved the conductivity of the electrolyte by adding sulfolane (TMS) as a plasticizer and ionic liquid diluent. Among them, the conductivity of PAN-EMIBF4-TMS was 15mS/cm (Under the same conditions, the conductivity of pure EMIBF4 ionic liquid is 13.8mS/cm). A. Lewandowske et al. used PYR14TFSI, EMIBF4 and BMIPF6 as ion sources and introduced them into PAN, PEO and PVA polymer matrices respectively to make ternary solid electrolytes. At 25., the conductivity of polymer systems with different proportions can reach up to 15mS/cm, and the electrochemical window is 3V.

　　A. Lewandowske et al. introduced the ionic liquid 1-methyl-3-ethylimidazole trifluoromethanesulfonic acid (EMImTf) into different matrices. At 25°C, the highest conductivity value was 16.2mS/cm. J.Reiter et al. studied two polymer electrolytes: poly2-ethoxyethyl-methacrylate (PEOEMA)-PC-BMIPF6 and PEOEMA-PC/EC-BMIPF6. The conductivity of these two polymer electrolytes has a higher voltage window of 4.3~4.4V on the glassy carbon electrode, and the thermal stability temperature can reach above 150°C, showing good thermal stability.

　　3Conclusion

　　Imidazole ionic liquids have high conductivity and low viscosity, and have been extensively studied in supercapacitor electrolytes. However, the stability and cycle performance of some ionic liquids still need to be improved; pyrrole ionic liquids have excellent electrochemical properties and can be used at high temperatures. It has excellent cycle performance and thermal stability and can be used in high-temperature capacitors; short-chain fatty quaternary ammonium salt ionic liquids are stable to large specific surface area activated carbon, but generally cannot form ionic liquids at room temperature. By attaching oxygen-containing alkyl groups Group (such as methoxyethyl) can lower its melting point. This type of ionic liquid has a wide voltage window, high conductivity, good stability, and high and low temperature performance, which can improve the high-temperature safety performance and stability of supercapacitors.

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