A separator based on a sheetlike flexible substrate having a multiplicity of openings and having a coating on and in the substrate, the material of the substrate being selected from woven or non-woven electrically nonconductive fibers of glass or ceramic or a combination thereof and the coating being a porous electrically insulating ceramic coating, characterized by a thickness of less than 100 .mu.m; a process for producing the separator; and a battery comprising the separator.