Application of sulfonium-, thiophenium-, and thioxonium-based salts as electric double-layer capacitor electrolytes

The purpose of this paper is to develop novel molten salts based on sulfonium, thiophenium, and thioxonium cations as electrolytes for EDLCs. We evaluated various kinds of the salts, with tetrafluoroborate (BF4) and bis(trifluorometanesulfonyl)amide (TFSA) anions, including several kinds of ionic liquids. The cell using the electrolyte containing diethylmethylsulfonium (DEMS)–BF4 salt had the higher capacitance and the lower DC-IR value than those containing conventional salts such as 1-ethyl-3-methylimidazolium (EMI)–BF4 and N,N,N-triethyl-N-methylammonium (TEMA)–BF4 at 243 K and 2 V. The capacitance and DC-IR at low temperature depended strongly on the structure, particularly on the size of each ion. We also examined the durability of cells by continuous charging at 333 K and 2.5 V. The stabilities of sulfonium-, thiophenium-, and thioxonium-based electrolytes were much inferior to that of EMI–BF4.