| 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 which is low-cost, and has both a high energy density and high reliability can be obtained by using a negative electrode active material made of metal composite oxide represented by the formula A2±xB2±yO5±z; (1) (0≰x≰0.1, 0≰y≰0.1, 0≰z≰0.3, A includes one selected from the group consisting of strontium, barium, and magnesium, but does not include manganese and calcium, and B includes at least Fe, but does not include 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.
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