Abstract: A solid polymer electrolyte is in the form of a gel comprising a complex of a solid polymer (e.g. poly(ethene oxide) and an alkali metal salt (e.g. LiClO.sub.4), the polymer being cross-linked to inhibit its transformation from an amorphous form to a crystalline form thereby to improve the alkali metal ion conductivity of the electrolyte at lower temperatures. The electrolyte may be made by irradiating (e.g. by gamma radiation or an electron beam) a solution, preferably aqueous, of the complex to cause the polymer to cross-link and form a gel. The electrolyte may be used as a component in a solid state electrochemical cell (e.g. for a rechargeable lithium battery), where the gel form of the electrolyte assists fabrication.

Abstract: A method of treating a hydrophilic polymer film is described in which the film is immersed at a temperature greater than 80.degree. C., and preferably greater than the crystalline melting point of the polymer, in a liquid capable of swelling the polymer. Polymers to which this method can be applied include graft copolymers of for example polyethylene, polypropylene, polytetrafluoroethylene, and nylon with acrylic or methacrylic acid. The films when treated have decreased electrical resistance and increased hydroxyl ion permeability and are then useful as battery separators and in other semi permeable membrane applications.