The concentration of ion-conductive salt in the solvent is preferably about 0.5 to about 1.5 mol/L. [0253] The separator is composed of a base material, illustrative, non-limiting examples of which include fluoropolymers, polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, polyacrylonitrile, polyvinylidene chloride, polymethyl methacry