Positive electrode active material and non-aqueous electrolyte secondary battery containing the same

1. 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.

2. The positive electrode active material in accordance with claim 1 , wherein said lithium-containing composite oxide contains nickel and manganese substantially at the same ratio.

3. A non-aqueous electrolyte secondary battery, comprising: a negative electrode containing, as a negative electrode active material, a material capable of absorbing and desorbing lithium ions and/or metal lithium; a positive electrode containing the positive electrode active material in accordance with claim 1 ; and an electrolyte.

4. The positive electrode active material in accordance with claim 1 , wherein said lithium-containing composite oxide is represented by the formula Li[Co x (Ni δ Mn γ ) 1-x ]O 2 ,

0 <x≦ 0.5, δ=0.5±0.1, and γ=0.5±0.1.

5. 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, wherein said lithium-containing composite oxide is represented by the formula Li[M x (Ni δ Mn γ ) 1-x ]O 2 ,

wherein M is one or more of elements except for nickel and manganese, 0<x≦0.3, δ=0.5+0.1, and γ=0.5±0.1, and wherein an oxidation state of said M is trivalent.

6. The positive electrode active material in accordance with claim 5 , wherein said M contains at least one of aluminum and cobalt.

7. A non-aqueous electrolyte secondary battery, comprising: a negative electrode containing, as a negative electrode active material, a material capable of absorbing and desorbing lithium ions and/or metal lithium; a positive electrode containing the positive electrode active material in accordance with claim 5 ; and an electrolyte.