AQUEOUS SODIUM ION SECONDARY BATTERY

To provide an aqueous sodium ion secondary battery which can achieve a high electrochemical capacity as compared with a known sodium ion secondary battery having an aqueous electrolytic solution. An aqueous sodium ion secondary battery includes a cathode, an anode, an electrolytic solution and a separator, wherein the cathode has at least a cathode active material containing at least a sodium transition metal polyanion represented by the formula NaMPOCO(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.