Microporous membrane

A porous polymer membrane is made by dissolving a polymer consisting primarily of vinylidene fluoride in a suitable liquid, casting the polymer solution to form a thin layer on a substrate, and then slowly evaporating the liquid phase (without contacting the layer with any other liquids) so that a substantially uniform microporous membrane of polymer is formed. The liquid may comprise a solvent for the polymer, combined with a small proportion of a non-solvent which dissolves in the solvent. Alternatively the liquid may be a latent solvent. The evaporation process may be performed at a temperature above ambient temperature, and involves exposing the film to dry gas. The microporous membrane or film may be used to form a solid electrolyte for a lithium cell, by soaking it in a non-aqueous liquid electrolyte, the film combining with the electrolyte solution to form a solid electrolyte or a gelled electrolyte.