Positive electrode active material for lithium secondary cell and lithium secondary cell

1. A positive active material for lithium secondary batteries, comprising:

a composite oxide comprising:

an oxide which is represented by the composite formula Li x Mn a Ni b Co c O 2 and has an α-NaFeO 2 structure; and

an impurity phase comprising Li 2 MnO 3 ,

wherein a, b, and c are values within such a range that in a ternary phase diagram showing the relationship among these, (a, b, c) is present on the perimeter of or inside the quadrilateral ABCD defined by point A (0.5, 0.5, 0), point B(0.55, 0.45, 0), point C (0.55, 0.15, 0.30), and point D(0.15, 0.15, 0.7) as vertexes, and 0.95<x/(a+b+c)<1.35.

2. The positive active material for lithium secondary batteries of claim 1 , wherin the composite oxide in X-ray powder diffractometry with CuK a ray, gives a diffraction diagram in which the s/m ratio is less than 0.04, where s is the intensity of the diffracted ray at 2θ=21±1.5°, and m is the intensity of the diffracted ray at 2θ=18.6±0.3°.

3. The positive active material for lithium secondary batteries of claim 1 , wherein the composite oxide has a BET specific surface area as measured by the N 2 adsorption method of 0.3 m 2 /g or greater.

4. The positive active material for lithium secondary batteries of claim 1 , wherein the composite oxide comprises particles which are secondary particles formed by the aggregation of primary particles, the primary particles having the shape of nearly a rectangular parallelopiped in which the ratio of the dimension of the longest side to that of the shortest side is 2 or less, and the primary particles having an average diameter of 0.6 μm or greater.

5. The positive active material for lithium secondary batteries of claim 1 , wherein the composite oxide is one obtained by mixing a precursor with a lithium compound and heat-treating the mixture, the precursor mainly comprising a compound which has manganese and nickel as transition metal elements and has a β-Ni(OH) 2 crystal structure.

6. The positive active material for lithium secondary batteries of claim 1 , wherein the composite oxide is one obtained by mixing a precursor with a lithium compound and heat-treating the mixture, the precursor mainly comprising a compound which has manganese, nickel and cobalt as transition metal elements and has a β-Ni(OH) 2 crystal structure.

7. The positive active material for lithium secondary batteries of claim 5 , wherein the lithium compound comprises LiOH.

8. A lithium secondary battery comprising:

a positive electrode comprising the positive active material for lithium secondary batteries of claim 1 ; and

a negative electrode comprising a negative-electrode material capable of doping/undoping lithium ions.

9. The positive active material for lithium secondary batteries of claim 6 , wherein the lithium compound comprises LiOH.

10. The positive active material for lithium secondary batteries of claim 2 , wherein the composite oxide has a BET specific area as measured by the N 2 adsorption method of 0.3 m 2 /g or greater.

11. The positive active material for lithium secondary batteries of claim 2 , wherein the composite oxide comprises particles which are secondary particles formed by the aggregation of primary particles, the primary particles having the shape of nearly a rectangular parallelopiped in which the ratio of the dimension of the longest side to that of the shortest side is 2 or less, and the primary particles having an average diameter of 0.6 μm or greater.

12. The positive active material for lithium secondary batteries of claim 2 , wherein the composite oxide is one obtained by mixing a precursor with a lithium compound and heat-treating the mixture, the precursor mainly comprising a compound which has manganese and nickel as transition metal elements and has a β-Ni(OH) 2 crystal structure.

13. The positive active material for lithium secondary batteries of claim 2 , wherein the composite oxide is one obtained by mixing a precursor with a lithium compound and heat-treating the mixture, the precursor mainly comprising a compound which has manganese, nickel and cobalt as transition metal elements and has a β-Ni(OH) 2 crystal structure.

14. The positive active material for lithium secondary batteries of claim 2 , wherin the composite oxide is one obtained by mixing a precursor mainly comprising a compound which has manganese and nickel as transition metal elements and has a β-(OH) 2 crystal structure, with a lithium compound and with a precursor comprising a cobalt compound and heat-treating the mixture.

15. The positive active material for lithium secondary batteries of claim 1 , wherein the composite oxide is one obtained by mixing a precursor mainly comprising a compound which has manganese and nickel as transition metal elements and has a β-Ni(OH) 2 crystal structure, with a lithium compound and with a precursor comprising a cobalt compound and heat-treating the mixture.

16. The positive active material for lithium secondary batteries of claim 15 , wherein the precursor comprising a cobalt compound comprises one of an oxide and a carbonation product salt.

17. The positive active material for lithium secondary batteries of claim 15 , wherein the lithium compound comprises LiOH.

18. The positive active material for lithium secondary batteries of claim 14 , wherein the precursor comprising a cobalt compound comprises one of an oxide and a carbonation product salt.

19. The positive active material for lithium secondary batteries of claim 1 , wherein said Li 2 MnO 3 has a structure in which a lithium phase and an LiMn 2 phase are alternately superposed through oxygen atoms to have the same structure as said α-NaFeO 2 structure.

20. The positive active material for lithium secondary batteries of claim 1 , wherein said Li 2 MnO 3 and said oxide coexist in a solid solution state.