Mixed metal oxide and sodium secondary battery
The present invention provides a sodium secondary battery capable of reducing the amount of lithium used, and ensuring a larger discharge capacity maintenance rate when having repeated a charge and discharge, as compared with conventional techniques; and a mixed metal oxide usable as the positive electrode active material therefor. A mixed metal oxide of the present invention comprises Na and M 1 wherein M 1 represents three or more elements selected from the group consisting of Mn, Fe, Co and Ni with an Na:M 1 molar ratio being a:1 wherein a is a value falling within the range of more than 0.5 and less than 1. Also, a mixed metal oxide of the present invention is represented by the following formula (1): Na a M 1 O 2 (1) wherein M 1 and a each have the same meaning as above. The positive electrode active material for secondary batteries of the present invention comprises the mixed metal oxide above.
1. A mixed metal oxide represented by the following formula (1):
Na a M 1 O 2 (1)
wherein M 1 represents Mn, Fe and Ni, and a is a value falling within the range of from 0.7 to 0.9, and wherein a M 1 :Mn molar ratio is 1:b, and b is a value not less than 0.333 and 0.46 or less,
wherein a compositional ratio by mol of Mn:Fe:Ni is 1:x:y, wherein x is 1 and y is 0.17 to 1.
2. A positive electrode active material for sodium secondary batteries which comprises the mixed metal oxide according to claim 1 .
3. A positive electrode for sodium secondary batteries which comprises the positive electrode active material according to claim 2 .
4. A sodium secondary battery having the positive electrode according to claim 3 .
5. The sodium secondary battery according to claim 4 further having a separator.
6. The sodium secondary battery according to claim 5 , wherein the separator is a separator having a porous laminated film in which a heat-resistant porous layer comprising a heat-resistant resin and porous film comprising a thermoplastic resin are stacked to each other.