| 摘要 |
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 A2±xB2±yO5±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. |
