Electrolyte Systems for High Withstand Voltage and Durability I. Linear Sulfones for Electric Double-Layer Capacitors

This study reports on new electrolyte systems utilizing linear sulfones. The characteristics of linear sulfones have been evaluated with the primary focus on higher withstand voltage. Investigations have been made on eight different types of linear sulfones with relatively low molecular weights. They were subjected to screening with regard to the melting and boiling point, dielectric constant, viscosity and the solubility of electrolyte salts. The results of the investigations were that Ethyl isopropyl sulfone (EiPS) and Ethyl isobutyl sulfone (EiBS) emerged as solvents with great potential. The EiPS has a relatively low melting point (−8°C) and a high boiling point (265°C), higher than that (242°C) of propylene carbonate (PC). Also, it was possible to dissolve the electrolyte salt in EiPS at 1.5 mol l−1 or higher. The EiPS system showed a high withstand voltage (3.3–3.7 V), exceeding that of PC (2.5–2.7 V). This high withstand voltage was found to have been caused by the high stability of EiPS at the interface between the activated carbon electrode and the electrolyte. By taking into consideration the degradation mechanisms of the EiPS system, we were able to clarify that one of the reasons for the high stability was the low reactivity between EiPS and H2O.