Ionic Liquids For Supercapacitors

Ionic liquid is a kind of liquid material composed entirely of ions due to the asymmetry of anion and cation and spatial obstruction, resulting in low electrostatic potential of ions. Aluminum trichloride and ethyl pyridine halide ionic liquid are the first generation of room temperature ionic liquid. After S. John et al. synthesized dialkyl imidazole cationic salt with better electrochemical stability, ionic liquid quickly became a research hotspot. The specific energy of supercapacitors is lower than that of lithium-ion batteries and it is an urgent problem to improve the specific energy while maintaining high specific power. To improve the specific energy of supercapacitor, it is necessary to increase the working voltage and the specific capacitance. The operating voltage is related to the decomposition voltage of the electrolyte. At present, there are two kinds of electrolytes for ultracapacitors: water system and organic system. The water electrolyte is sulfuric acid solution or potassium hydroxide solution, which is highly corrosive, and the working voltage of the prepared monomer supercapacitor is low. The organic electrolyte is an organic solution of tetraethyl ammonium salt of tetrafluoroborate, and the working voltage of the monomer supercapacitor is above 2.5V. However, there are some problems, such as volatile organic solvent, difficult to improve the conductivity and working voltage, potential safety risks and environmental impact. Ionic liquids can be directly used as liquid electrolytes for supercapacitors, and can also be dissolved in organic solvents as electrolyte salts. Solid polymer electrolytes can also be introduced to improve the related properties.

Classification

• Liquid electrolyte: The anions of ionic liquid are mainly composed of bis (trifluoromethyl sulfonyl) imine (TFSI^-), BF_4^- and PF_6^-. The cations of the ionic liquids are mainly composed of large volumes of organic ions such as imidazole, pyrrole and short-chain fatty quaternary amine salts. Imidazole ionic liquids have high electrical conductivity but high viscosity, which is not conducive to large-scale industrial production. In order to further improve the conductivity of imidazole ionic liquids electrolyte, reduce the viscosity, and maintain a high electrochemical window, the combination of imidazole ionic liquids with hydrophobic organic solvents PC and EC as mixed electrolytes has been studied. Pyrrolidine ionic liquids belong to cyclic quaternary ammonium salts, which have low melting point and high conductivity due to the asymmetry of the cation substitution of pyrrolidine. The high energy density and power density of pyrrolidines ionic liquids can improve the voltage window and cycle life of supercapacitors at high temperature. The best advantage of short-chain fatty quaternary amine ionic liquids is that they are more stable to activated carbon with high specific surface area than imidazole and pyrrolidines ionic liquids. This ionic liquid can improve the high temperature safety performance of supercapacitors.

Figure 1. An example of ionic liquids used as liquid electrolyte for supercapacitors.

• Polymer solid electrolyte: Ionic liquid polymer electrolyte has the advantages of good mechanical properties of polymer and high conductivity of ionic liquid, and improves the safety and stability of supercapacitors. General ionic liquid polymer electrolytes can be divided into ionic liquid containing polymer electrolytes and ionic liquid/polymer electrolyte. Moreover, ionic liquid/polymer electrolyte is obtained by introducing ionic liquid structure on polymer molecules. The polymer matrix is mainly composed of polyoxyethylene (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) and polyvinyl alcohol (PVA).

Reference

• Dou Q, Liu L, Yang B, et al. Silica-grafted ionic liquids for revealing the respective charging behaviors of cations and anions in supercapacitors[J]. Nature Communications, 2017, 8(1):2188.