TECHNOLOGY FOCUS - POLYMERS - Preferred Method: The method comprises selecting a lower alcohol as fluid (II), and polymeric material comprising a polymer or copolymer. The fluids from the layer are volatized, to yield a spatial density of the filler particles within the voids that is greater than the spatial density of the particles distributed within the polymeric matrix. The mesoporous separator membrane is contacted with a fluid electrolyte, to distribute the electrolyte within the membrane by adsorbing portion(s) of the voids, and thereby rendering the membrane ionically-conductive. The non-solvent comprised in the layer, is distributed as droplets throughout the matrix of the polymeric material, and a filler particles are distributed both in non-solvent droplets and in the matrix of the polymeric material.Preferred Monomer: The polymer or copolymer comprising monomer chosen from vinyl chloride, acrylonitrile, vinyl acetate, vinyl fluoride, vinylidene chloride, vinylidene fluoride and/or hexafluoropropylene. EXAMPLE - A coating composition of a separator membrane (15) was prepared by dissolving 88:12 VdF:HFP copolymer (3g) having 380x10 power 3 MW in 20 ml acetone. The solution was stirred with 10 ml ethanol in which the copolymer was insoluble, and 2.0 g of fumed silica (SiO2), to form a composition of clear, smooth consistency. The obtained composition was cast at a wet thickness of about 250 micrometers upon the surface of abherent polyethylene terephthalate film and allowed to dry by evaporation of coating vehicle components in moderately circulating air at about 30degreesC. The resulting separator membrane of about 50 micrometers thickness was self-supporting and appeared homogenous when stripped from the casting substrate. Under photomicrographic examination the membrane exhibited uniform distributions of circular voids and silica particles throughout the polymer matrix with a greater density of such particles within the confines of the voids than in the matrix in general.