NOVELTY - A coatable composition is prepared and cast to form a layer. The fluids are volatilized from the layer at which solvent volatilizes at faster rate which causes gelling of polymeric material in non-solvent-predominant regions. The non-solvent from the droplets is volatilized, to yield a distribution of mesopore voids and filler particles (24,25) throughout the polymeric matrix (22), to form mesoporous polymeric membrane.    USE - As interelectrode separator in rechargeable battery cell.    ADVANTAGE - The mesoporous matrix structure readily absorbs electrolyte solution, to provide a high level of ionic conductivity in the battery cell. The filler imparts structural strength to the layer in its uniform dispersal, provides support to maintain the open structure of the mesopores after diffusion of the non-solvent vehicle, and also provides additional support during the compressive tendencies of the thermal lamination operation.    DETAILED DESCRIPTION - A coatable composition comprising a polymeric material, a volatile fluid solvent for polymeric material, a fluid (II), and filler particles, is prepared. The fluid (II) is miscible with solvent, and has lesser volatility than the solvent. The fluid (II) is a non-solvent exhibiting no significant solvency for the polymeric material. The filler particles are inert to the remaining components of the composition. The obtained composition is cast to form a layer. The fluids are volatilized from the layers at which the solvent volatilizes at a faster rate than the non-solvent which causes the gelling of polymeric material in the more non-solvent-predominant regions of the layer and isolating non-solvent as droplets (36) distributed uniformly throughout a matrix of the polymeric material. The volatilization of solvent is continued to render membrane self-supporting, and volatilization of non-solvent from the droplets is carried out, to yield a distribution of mesopore voids throughout the membrane matrix and filler particles within the voids throughout the polymeric matrix, to form mesoporous polymeric membrane.    DESCRIPTION OF DRAWING(S) - The figure shows the diagrammatic representation of a segment of separator membrane showing relative dispersion of non-solvent vehicle droplets and inert filler particles during final stage gelling of the membrane polymer.    separator membrane (15)    polymer matrix (22)    filler particles (24,25)    non-solvent droplets (36)