CR1225 battery.Latest 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 there are 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 (EMIF2.3HF), which has quite high conductivity and low viscosity at low temperatures, has been widely studied as a supercapacitor electrolyte. U. Makoto et al. used EMIF2.3HF 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 EMIF2.3HF 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 EMIF2.3HF 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.

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