An energy storage device comprises two electrodes; and a separator disposed between the electrodes; wherein at least one of the electrodes and the separator comprises a copolymer, which serves as a non-aqueous binder and/or solid electrolyte for the electrodes and the separator of the energy storage device, and the copolymer is a copolymerized product or its derivative formed by the polymerization reaction of acrylonitrile and vinyl acetate. Therefore, the charge and discharge properties of the energy storage device using the copolymer can be improved, thereby effectively extending the efficiency and lifetime of the energy storage device.