Ionic liquid-solvent mixtures as supercapacitor electrolytes for extreme temperature operation

Ionic liquid (IL)-based electrolytes containing molecular solvents were shown to be attractive for extreme temperature applications in electric double layer capacitors (EDLCs). In particular, the IL-butyronitrile (BuCN) mixture provides high capacitance (around 125 F g(-1) at 500 mA g(-1)) independent of testing temperature, and superior performance at high current rates (reduced current dependence at high rates). Importantly, the IL-BuCN electrolyte can safely operate between -20 and + 80 degrees C, which overcomes the high temperature limitations of current commercial EDLCs. An additional advantage of IL-solvent mixtures is that the higher concentration of IL ions in the mixtures allows a greater specific capacitance (F g(-1)) to be achieved. The conductivity of the ionic liquid N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide (PYR14TFSI) could be increased from 2.48 mS cm(-1) up to 45 mS cm(-1) by mixing with an appropriate solvent. Importantly, these solvent mixtures also retain a wide electrochemical voltage window, in the range 4-6 V.