A polymer battery which includes a cell assembly having a positive electrode, a negative electrode, and a separator composed primarily of a fluoropolymer is manufactured by impregnating the cell assembly with an electrolyte composition containing (A) an ion-conductive salt, (B) a solvent in which the ion-conductive salt is soluble and (C) a compound having at least two reactive double bonds per molecule, then reacting the component C compound to form a three-dimensional network structure. The polymer battery has a high safety, a good thermal cycling resistance and robust characteristics even when held at an elevated temperature, making it particularly suitable for use as a lithium secondary cell or a lithium ion secondary cell.