allow the molecules of the electrolyte to pass therethrough (e.g., a zeolite); a method in which use is made of particles of a material (e.g., polyacrylonitrile, poly(methyl methacrylate) and poly(vinylidene fluoride) which is swellable with an electrolytic liquid, wherein the swellable material is at least one insulating substance, a substance other than the insulating and the other substance, t