| abstract |
A negative electrode active material which is low-cost and has a high energy density, and a lithium ion secondary battery using such a negative electrode active material are provided. The lithium ion secondary battery uses, as the negative electrode active material, an orthorhombic-system metal composite oxide represented by the formula A 2±x B 2±y O 5±z ; (0≦x≦0.1, 0≦y≦0.1, 0≦z≦0.3, A includes at least one element selected from the group consisting of alkaline earths and transition metals except for manganese, and B includes at least manganese), where the formal oxidation number of A is +2, and the formal oxidation number of B is greater than or equal to +2.5 and less than or equal to +3.3. |