Abstract: Based on the discovery that the melting point and solubility of onium salts are affected by the asymmetry of the substitution on cation, and that the branched substituents effectively shield onium cations from electrochemical reduction, new onium salts are synthesized and high performance electrolytes based on these salts for electrochemical capacitor are provided. The composition of the new electrolyte comprises an onium salt or mixture of such onium salts dissolved in aprotic, non-aqueous solvents or mixture of such solvents. The electrolyte is able to perform at high rate of charge/discharge, at low ambient temperatures, and within wide operating voltage, due to the high solubility, low melting temperature, and the improved reduction stability of the new onium cations, respectively.

Abstract: Based on the discovery that the melting point and solubility of onium salts are affected by the asymmetry of the substitution on cation, and that the branched substituents effectively shield onium cations from electrochemical reduction, new onium salts are synthesized and high performance electrolytes based on these salts for electrochemical capacitor are provided. The composition of the new electrolyte comprises an onium salt or mixture of such onium salts dissolved in aprotic, non-aqueous solvents or mixture of such solvents. The electrolyte is able to perform at high rate of charge/discharge, at low ambient temperatures, and within wide operating voltage, due to the high solubility, low melting temperature, and the improved reduction stability of the new onium cations, respectively.