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 °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.