Composite metal oxide, process for producing the composite metal oxide, positive active material for sodium secondary battery, positive electrode for sodium secondary battery, and sodium secondary battery

1. A composite metal oxide represented by the following formula (I):

Na x Fe y Mn 1-y O 2   (I)

wherein the value of x is ⅔ or more and less than 1, and the value of y is ⅓ or more and less than ⅔,

wherein the composite metal oxide has an x-ray diffraction peak near 48 degree, and

wherein the composite metal oxide is constituted of an oxide having a P2 structure and a lamellar oxide, said lamellar oxide containing some extent of stacking faults, having and having an octahedral structure and/or a triangular-prism structure.

2. The composite metal oxide according to claim 1 , wherein the lamellar oxide is an oxide having an O3 structure.

3. The composite metal oxide according to claim 1 , wherein a ratio of the lamellar oxide present in the composite metal oxide is 3 to 15% by volume.

4. A process for producing the composite metal oxide according to claim 1 , the process comprising calcination of a mixture of a sodium compound, a manganese compound, and an iron compound under a temperature of 800 to 1000° C. for 2 to 24 hours.

5. A positive active material for a sodium secondary battery, the positive active material comprising the composite metal oxide according to claim 1 .

6. A positive electrode for a sodium secondary battery, the positive electrode comprising the positive active material according to claim 5 .

7. A sodium secondary battery comprising the positive electrode according to claim 6 .