Patent ID: 8658125

Claim:
A positive electrode active material comprising a lithium-containing composite oxide having a layered structure and containing nickel with an oxidation state of 2.0 to 2.5, manganese with an oxidation state of 3.5 to 4.0, and M being one or more elements except for nickel and manganese, said oxidation state determined by shifts of energy at which absorption maximum is observed in the X-ray absorption near-K-edge structures, wherein the nickel, manganese, and M are uniformly dispersed in the nano level forming a solid solution, wherein the positive electrode active material comprises a mixture of crystal particles of said lithium-containing composite oxide having primary particles with an average particle size of 0.1 to 2 μm and secondary particles having an average particle size of 2 to 20 μm wherein said mixture of crystal particles of said lithium-containing composite oxide comprises primary and secondary particles, wherein the secondary particles are sintered primary particles, and wherein values of nickel and manganese determined by the shifts of energy at which absorption maximum is observed in the X-ray absorption near-K-edge structures are bivalent and tetravalent respectively, before charging and when a voltage relative to lithium metal is 2.5 to 3.5 V.