Positive electrode active material for sodium battery, and method of producing the same

A method of producing a positive electrode active material for a sodium battery, wherein the sodium battery, represented by general formula (1) below:

• NaxMy(AO4)z(P2O7)w (1)
• where M is at least one selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc;
• A is at least one selected from the group consisting of aluminum, silicon, phosphorus, sulfur, titanium, vanadium and tungsten;
• x satisfies a condition 4≧x≧2;
• y satisfies a condition 4≧y≧1, z satisfies a condition 4≧z≧0;
• w satisfies a condition 1≧w≧0;
• and at least one of z and w is 1 or more,
• the method comprising:
• a pre-firing step of firing a starting material mixture which includes at least an Na-containing compound, an M-containing compound containing said M, an A-containing compound containing said A and a P-containing compound in an open-air atmosphere at from 150 to 500° C.;
• and
• a main firing step of firing the thus-obtained pre-fired material in an open-air atmosphere at from 500 to 800° C. after implementing the pre-firing step.