Method for producing sodium composite transition metal oxide, positive electrode active material for sodium secondary battery, positive electrode and sodium secondary battery comprising the same

1. A positive electrode active material for a sodium secondary battery, the positive electrode active material comprising a sodium composite transition metal oxide which is represented by Formula 1 and has a P3 crystal structure:

Na x [Li a M 1-a ]O 2   [Formula 1]

wherein, M is a transition metal, 0.64≤x≤0.7, and 0.01≤a≤0.1.

2. The positive electrode active material for a sodium secondary battery of claim 1 , wherein the M comprises manganese and iron.

3. The positive electrode active material for a sodium secondary battery of claim 2 , wherein the manganese and the iron are included in an atomic ratio of 1:1.

4. The positive electrode active material for a sodium secondary battery of claim 1 , wherein the M is Mn 0.5 Fe 0.5 .

5. The positive electrode active material for a sodium secondary battery of claim 1 , wherein the sodium composite transition metal oxide is represented by Formula 2:

Na x [Li a (Mn 0.5 Fe 0.5 ) 1-a ]O 2   [Formula 2]

wherein, 0.64≤x≤0.7, and 0.01≤a≤0.1.

6. The positive electrode active material for a sodium secondary battery of claim 1 , wherein the positive electrode active material has a c-axis length change rate, which is represented by Equation (1), of 10% or less:

c -axis length change rate (%)={(C 0 —C)/C 0 }×100  Equation (1):

wherein, C 0 is a c-axis length of the positive electrode active material in an uncharged state, and C is a c-axis length of the positive electrode active material in a charged state at 4.3 V.

7. A positive electrode for a sodium secondary battery, the positive electrode comprising the positive electrode active material for a sodium secondary battery of claim 1 .

8. A sodium secondary battery comprising the positive electrode for a sodium secondary battery of claim 7 .