Non-aqueous electrolyte secondary battery

1. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a non-aqueous electrolyte having lithium ion conductivity, wherein a mixture of a lithium-containing transition metal oxide Li a Ni x Mn y O 2 , wherein a, x, and y respectively satisfy the following relations: 1<a≦1.5; 0.5≦x+y≦1; 0<x<1; and 0<y<1, containing nickel and manganese as transition metals in a molar ratio (x/y) of Ni/Mn in the lithium-containing transition metal oxide of less than 1 while satisfying x+y<1 and having a layered rock-salt type crystal structure of a hexagonal system belonging to space group R-3m, and a lithium manganese oxide Li (1+b) Mn (2−b) O 4 , wherein b satisfies 0≦b≦0.33, having a spinel structure, is used as the positive electrode active material.

2. The non-aqueous electrolyte secondary battery according to claim 1 , wherein the lithium-containing transition metal oxide and the lithium manganese oxide are mixed at a ratio (lithium-containing transition metal oxide:lithium manganese oxide) by weight in a range from (1:9) to (9:1).

3. The non-aqueous electrolyte secondary battery according to claim 1 , wherein Li is contained in the 3b transition metal site of the lithium-containing transition metal oxide having the layered rock-salt type crystal structure of a hexagonal system belonging to space group R-3m.