POSITIVE ELECTRODE ACTIVE MATERIAL FOR SODIUM ION SECONDARY BATTERY

Provided is a Ni-based positive electrode active material for a sodium ion secondary battery having an excellent discharge capacity. A positive electrode active material for a sodium ion secondary battery, the positive electrode active material being composed of crystals represented by a general formula Na(NiM)PO(where M represents at least one transition metal element selected from the group consisting of Fe, Cr, Mn, and Co and the following are satisfied: 0.6≤x≤4, 0.3≤y≤2.7, 0≤a≤0.9, and 6≤z