TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The spinel type lithium manganese oxide is larger in amount than the layered type lithium manganese oxide. The nonaqueous electrolyte contains a boron-containing lithium compound (other than LiBF4) in a nonaqueous solvent obtained by dissolving a lithium salt in a carbonate nonaqueous solvent. The spinel type lithium manganese oxide and/or the layered type lithium manganese oxide have a coating containing boron and manganese. The lithium salt is LiPF6, LiClO4, LiBF4, LiAsF6, LiSbF6, or bis-pentafluoroethane imide lithium sulfonate. The negative electrode active material used for the negative electrode formed on the negative electrode sheet includes an amorphous carbon or graphite.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The carbonate nonaqueous solvent includes ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, or propylene carbonate. A disulfide derivative, cyclohexyl benzene, or vinylene carbonate is added to the nonaqueous electrolyte. EXAMPLE - Li1.02Mn1.98Al0.02O4 having an average grain diameter of 10 microns and a specific surface area of 1 .5 m2/g as the spinel type lithium manganese oxide, Li1.02Ni0.38Co0.3Mn0.3Mg0.01O2 having an average grain diameter of 8 microns and a specific surface area of 1.7 m2/g as the layered type lithium manganese oxide, and acetylene black powder as conductive material, were subjected to dry blending. Polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP) was homogeneously dispersed in the mixture obtained to produce a paste. This paste was applied to both sides of an aluminum foil with a thickness of 20 microns, dried, and pressed to a thickness of 120 microns to produce the positive electrode sheet. The solid content weight ratio within the positive electrode was spinel type lithium manganese oxide:layered type lithium manganese oxide:acetylene black:PVDF=80.8:4.2:10.8:4.3 (spinel type lithium manganese oxide:layered type lithium manganese oxide=95:5).