Patent ID: 8241790

Claim:
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.