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 containing at least nickel and manganese elements, said positive electrode active material comprising primary particles of said composite oxide having a twinning portion, the composite oxide further contains cobalt element, and the nickel, manganese, and cobalt elements are uniformly dispersed at the atomic level,

wherein said composite oxide has a layered crystal structure and the arrangement of oxygen atoms is a cubic close-packed structure,

wherein said composite oxide contains nickel, manganese and cobalt elements at a ratio satisfying Co/(Ni+Mn)≦1; and

the lithium-composite oxide is formed by

(a) coprecipitating nickel, manganese and cobalt in an aqueous solution to form a composite hydroxide containing nickel, manganese and cobalt,

(b) heating the composite hydroxide to form a dry precursor,

(c) mixing the dry precursor with a lithium compound to form a mixture,

(d) rapidly heating the mixture at a rising temperature rate of not less than 7° C./min to a baking temperature of higher than 750° C., and

(e) subsequently quenching the mixture at a cooling rate of not less than 5° C./min from the baking temperature to 700° C. or lower.

2. The positive electrode active material in accordance with claim 1 , wherein said composite oxide has a defected or disordered portion in the crystal lattice thereof.

3. The positive electrode active material in accordance with claim 1 , wherein said composite oxide has a superlattice arrangement of a [√{square root over ( )}3×√3] R30° when assigned as R3-m.

4. The positive electrode active material in accordance with claim 1 , wherein said composite oxide contains nickel and manganese elements at the same ratio.

5. The positive electrode active material in accordance with claim 1 , wherein said composite oxide has an integrated intensity ratio (003)/(104) of the X-ray diffraction peak when assigned as R3-m which satisfies the equation:

(003)/(104)≦1.2.

6. The positive electrode active material in accordance with claim 1 , wherein said composite oxide has an extra spot or streak substantially in every electron beam diffraction pattern indexed when assigned as R3-m.

7. The positive electrode active material in accordance with claim 1 , wherein said primary particles have at least one of spherical and rectangular parallelepiped hexahedron shapes.

8. The positive electrode active material in accordance with claim 1 , wherein said primary particles have a particle size of 0.1 to 2 μm and said primary particles form secondary particles with a particle size of 2 to 20 μm.

9. 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.

10. The positive electrode active material in accordance with claim 1 , wherein said composite oxide is represented by LiCo x/3 Ni ((3-x)/6) Mn ((3-x)/6) O 2 , where x≦1.

11. The positive electrode active material in accordance with claim 1 , wherein said composite oxide contains nickel, manganese and cobalt at a ratio of 1:1:1.

12. The positive electrode active material in accordance with claim 1 , wherein said composite oxide is represented by LiNi 1/3 Mn 1/3 Co 1/3 O 2 .

13. The positive electrode active material in accordance with claim 1 , wherein the baking temperature is in a range of 900 to 1200° C.