TECHNOLOGY FOCUS - POLYMERS - Preferred Materials: A fluoropolymer having a reaction temperature similar to that of the fluorine containing copolymer, is used, and a common crosslinking agent is used. Preferably, the fluoropolymer and fluorine containing copolymer have a reacting group (such as an iodine group and/or bromine group) for reacting with a peroxide type crosslinking agent. The fluoropolymer is preferably, polyvinylidene fluoride or vinylidene fluoride-hexafluoro propene copolymer. The fluorine containing copolymer is amorphous vinylidene fluoride copolymer. The crosslinking agent is a polyfunctional unsaturated compound with a peroxide group. EXAMPLE - (In grams) Perfluoro ammonium octanoic acid (10), sodium hydroxide (2) and deionized water (5 l) were prepared, and press-fitted with 1,4-diiodo perfluorobutane (15.0). Vinylidene fluoride (70 mole%) and hexafluoro propene (30 mol.%) monomer gases were mixed and press-fitted. Polymerization was performed by adding water (150 ml) dissolved with ammonium persulfate (5). The obtained aqueous latex was added with potassium alum water (5 weight%) to form copolymer which was coagulated, washed and dried to obtain a fluorine containing copolymer A (2188).    (In weight parts) Polyvinylidene fluoride (7.5), fluorine containing copolymer A (2.5), triaryl isocyanurate (0.1), 2,5-dimethyl-2,5-di(3-butylperoxy) hexane (0.0125) and methyl ethyl ketone (90) were uniformly mixed at 40degreesC to obtain a solution. The solution was coated on a glass plate and dried for 1 hour at 40degreesC. Crosslinking was performed for 20 minutes at 165degreesC to obtain a film thickness of 100 mum. The film was immersed for 2 hours at 70degreesC in a solution comprising lithium phosphorous hexafluoride (LiPF6), and ethylene carbonate-propylene carbonate (1:1) mixed solvent dissolved with 1 mole concentration. The film was removed, a filter paper absorbed a surface electrolyte, and was removed. The obtained electrolyte had excellent tensile strength and electric conductivity.