TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The monomer (a) and oligomer (b) are in the mass ratio of 99/1-40/60. The electrolyte film further contains polymer (c) of phosphoric acid group-containing unsaturated monomer, and conjugated diene group liquid oligomer (d), in the mass ratio of 99/1-40/60. The oligomer (d) is formed by mixing butadiene oligomer having ethylenically unsaturated bonds(s), isoprene oligomer and their derivatives.Preferred Process: The electrolyte film is formed by copolymerizing the phosphoric acid group-containing unsaturated monomer, the conjugated diene group liquid oligomer and another unsaturated copolymerizable monomer. The phosphoric acid group-containing unsaturated monomer copolymerizes the conjugated diene group liquid oligomer and the unsaturated copolymerizable monomer.Preferred Compounds: The phosphoric acid group-containing unsaturated monomer is expressed with formula (I). The conjugated diene group liquid oligomer is expressed with formula (II). The phosphoric acid group-containing unsaturated monomer is phosphoric acid group-containing diester group unsaturated monomer of formula (III).R1,R7,R10=hydrogen or alkyl group, preferably hydrogen or methyl group;R2,R8,R9=hydrogen or (un)substituted alkyl group, preferably hydrogen, methyl or chloromethyl group;n=1-6;R3,R4=hydrocarbon group optionally having another atom, and has ethylenically unsaturated bond(s);R5,R6=hydrogen or methyl group;m=degree of polymerization;andk,p=1-6.At least one of R5 and R6 is hydrogen. The unsaturated copolymerizable monomer is chosen from unsaturated monomer (e) having ethylenically unsaturated bond(s) and functional group(s) in one molecule, and/or unsaturated monomer (f) which does not have functional group. The functional group of (e) is chosen from sulfonic acid group, carboxylic acid group, alcoholic hydroxyl group and/or amino group. The unsaturated monomer (f) is chosen from (meth)acrylonitrile, (meth)acrylate, (un)substituted styrene, vinyl, or fluorine group-containing unsaturated monomer having fluorine group(s) and ethylenically unsaturated bond(s).