ETHYLENE CARBONATE-BASED ORGANIC ELECTROLYTES FOR ELECTRIC DOUBLE-LAYER CAPACITORS

A new electrolytic solution based on a mixed solvent of ethylene carbonate (EC) with sulfolane (SL) or gamma-butyrolactone (gamma-BL) has been examined as an electric double layer capacitor. Fundamental properties, such as electrolytic conductivity, viscosity, and thermal stability, were measured for solutions containing quaternary alkylammonium salts as the supporting electrolyte. Maximum conductivities were obtained for the solutions with mixed solvent of 20-40 mol % EC in the EC + gamma-BL system: 1.2-1.3 x 10(-2) S cm-1 for EC + gamma-BL dissolving 0.5 M Et4NBF4 (Et = C2H5). The electrochemical and the thermal stabilities of the solution were dependent on the electrolytic salt as well as the solvent composition. A stable discharge capacitance and a high coulombic efficiency were obtained in a model capacitor using carbon fibre electrodes and the organic electrolyte of EC + gamma-BL/Et4NBF4 (or EC + gamma-BL/Et4NPF6).