The withstand voltage of an electrolyte solution is able to be increased by adding an additive for electrolyte solutions, which is composed of a compound that is configured from a silicon-containing sulfonate anion represented by formula (1) and a cation of, for example, a quaternary ammonium salt or a quaternary phosphonium salt, into the electrolyte solution. In the formula, each R independently represents an alkyl group having 1-8 carbon atoms; and n represents an integer of 2-6).