| CPC H01M 10/36 (2013.01) [H01M 4/485 (2013.01); H01M 4/525 (2013.01); H01M 2300/0005 (2013.01)] | 10 Claims |

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1. An aqueous sodium ion secondary battery comprising a cathode, an anode, an electrolytic solution and a separator, wherein the cathode comprises a cathode active material containing at least a sodium transition metal polyanion represented by the formula Na3-xMPO4CO3 (M is at least one member selected from the group consisting of Fe, Mn, Ni and Co, and x is 0 or more and 2 or less), and the electrolytic solution is an aqueous electrolytic solution.
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