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 x (Ni 1−a M a ) y P 2 O z (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<7.5).
1. 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 x (Ni 1−a M a ) y P 2 O z where M represents at least one transition metal element selected from the group consisting of Cr, Mn, and Co and the following are satisfied: 0.6≤x≤4, 0.3≤y≤1.3, 0≤a≤0.9, and 6≤z<7.5).
2. The positive electrode active material according to claim 1 , wherein the crystals have a structure belonging to a triclinic space group P-1.
3. The positive electrode active material according to claim 1 , wherein the crystals have a structure belonging to an orthorhombic space group Pcca.
4. A positive electrode comprising the positive electrode active material according to claim 1 .
5. The positive electrode according to claim 4 , further comprising a sodium ion-conductive solid electrolyte.
6. The positive electrode according to claim 5 , wherein the sodium ion-conductive solid electrolyte contains beta-alumina or NASICON crystals.
7. The positive electrode matcrial according to claim 4 comprising 30 to 100 mass % of the positive electrode active material, 0 to 70 mass % of a sodium ion-conductive solid electrolyte, and 0 to 20 mass % of a conductive agent.
8. A sodium ion secondary battery comprising the positive electrode according to claim 4 .