The positive electrode piece further comprises a leakage-preventing separating layer, and the leakage-preventing separating layers are provided on the outermost side of the positive electrode piece. The leakage-preventing separating layer has a pore diameter of 10 to 800 μm, and a thickness of 0.01 to 1000 μm, and the through-pore has a porosity of 10% to 90%. The leakage-preventing separating layer has either a single-layer or multi-layer structure. The material, thickness, pore diameter and porosity of the through-pore of each layer in the multiple-layer structure are either the same or different. The leakage-preventing separating layer not only plays a role of isolating electrons, but also plays a role of preventing the electric-conductive cathode particles from leaking from the positive electrode piece. By pasting the leakage-preventing separating layer on the outermost side, the insulation sealing of the positive electrode piece is realized. The material of the leakage-preventing separating layer is electron nonconductive porous polymer material. Or, the material of the leakage-preventing separating layer is an electric-insulating porous material composed of inorganic nonmetal material and organic polymer. Or, the material of the separating layer is an electric-insulating gel polymer electrolyte composite material composed of three parts: a polymer matrix, a liquid organic plasticizer and a lithium salt. Or, the material of the separating layer is the material which infuses ion conductive electrolyte or polymer gel material in the pores of the electron electric-insulating polymer material or in the pores of the porous material composed of the inorganic nonmetal material and organic polymer.