Positive electrode active material, sodium secondary battery, and production method of olivine-type phosphate
Disclosed are a positive electrode active material and a method for producing an olivine-type phosphate. The positive electrode active material comprises an olivine-type phosphate represented by the following formula (I), wherein the maximum peak in an X-ray diffraction pattern obtained using a CuKα ray is the peak of the (031) plane of the olivine-type phosphate and the half-value width of the peak is 1.5° or less: A a M b PO 4 (I), wherein A represents one or more elements selected from among alkali metals; M represents one or more elements selected from among transition metals; a is from 0.5 to 1.5; and b is from 0.5 to 1.5. The method for producing an olivine-type phosphate comprises preparing a raw material comprising element A, element M, and phosphorus (P) so that a A:M:P molar ratio may be a:b:1, preliminary calcining the raw material, and mainly calcining the preliminary calcined raw material.
1. A positive electrode active material comprising an olivine-type phosphate represented by the following formula (I), wherein the maximum peak of the positive electrode active material in an X-ray diffraction pattern obtained using a CuKα ray is the peak of the (031) plane of the olivine-type phosphate, and the half-value width of the peak is 1.5° or less:
A a M b PO 4 (I)
wherein A comprises sodium, M represents one or more elements selected from among transition metals, a is 1, and b is 1.
2. The positive electrode active material according to claim 1 , wherein M comprises at least iron.
3. The positive electrode active material according to claim 2 , wherein M is iron.
4. The positive electrode active material according to claim 3 , wherein the positive electrode active material has a peak at 2θ=34.8° in an X-ray diffraction pattern obtained using a CuKα ray, and an I/I 0 value is 0.5 or less where the signal intensity at said peak is I 0 and the signal intensity at 2θ=34.2° is I.
5. A positive electrode for sodium secondary batteries, comprising the positive electrode active material according to claim 2 .
6. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 5 .
7. The sodium secondary battery according to claim 6 further comprising a negative electrode comprising a mixture of a carbonaceous material and a binder.
8. The sodium secondary battery according to claim 6 further comprising a separator.
9. The sodium secondary battery according to claim 8 , wherein the separator is a separator comprising a porous laminate film in which a heat-resistant porous layer comprising a heat-resistant resin and a porous film comprising a thermoplastic resin are laminated.