Lithium-containing composite oxide and its production method

1. A lithium-containing composite oxide represented by the general formula Li p N x M y O z F a (where N is at least one element selected from the group consisting of Co, Mn and Ni, M is at least one element selected from the group consisting of Al, Sn, alkaline earth metal elements and transition metal elements other than Co, Mn and Ni, 0.9≦p≦1.2, 0.965≦x<2.00, 0<y≦0.035, 1.9≦z≦4.2, and 0≦a≦0.05, wherein when a powder of the lithium-containing composite oxide is classified into small particles with an average particle size Ds 50 of 2 μm≦Ds 50 ≦8 μm and large particles with an average particle size Dl 50 of 10 μm≦Dl 50 ≦25 μm, the content of the small particles is from 15 to 40% by weight, and the content of the large particles is from 60 to 85% by weight, and 0.01≦ys≦0.06, 0.001≦y≦0.02 and 0.01≦yl/ys≦0.7, where ys is a proportion of the M element in the general formula in the small particles, and yl is a proportion of the M element in the general formula in the large particles.

2. The lithium-containing composite oxide according to claim 1 , wherein a specific surface area of the small particles is from 0.5 to 1.5 m 2 /g, a specific surface area of the large particles is from 0.1 to 0.5 m 2 /g, and a specific surface area of the sum of the small particles and large particles is from 0.2 to 0.5 m 2 /g.

3. The lithium-containing composite oxide according to claim 1 , wherein the M element comprises Al and at least one element selected from the group consisting of Mg, Zr, Ti, Hf, Nb, Ta, Sn and Zn.

4. The lithium-containing composite oxide according to claim 1 , wherein the M element comprises at least Al or Mg, and wherein a total amount of Al and Mg in the small particles is larger than that in the large particles.

5. The lithium-containing composite oxide according to claim 1 , wherein the M element comprises at least Al and Mg.

6. The lithium-containing composite oxide according to claim 1 , wherein the M element comprises Al and at least one of Zr and Ti, and an atomic ratio of Al/(total of Zr and Ti) is from 1/20 to 40/1.

7. A process for producing the lithium-containing composite oxide as defined in claim 1 , which comprises mixing a mixture containing from 15 to 40% by weight of a small-particle-size precursor with an average particle size of 2 μm≦D 50 ≦8 μm in which from 1 to 6 atomic % of the M element based on a total of the M and N elements, is supported on an N element source, and from 60 to 85% by weight of a large-particle-size precursor with an average particle size of 10 μm≦D 50 ≦25 μm in which at most 2 atomic % of the M element based on a total of the M and N elements, is supported on the N element source, a lithium source, and, if necessary, a fluorine source; and firing the resultant mixture at a temperature of from 700° C. to 1,100° C. in an oxygen-containing atmosphere.

8. The process for producing the lithium-containing composite oxide according to claim 7 , wherein the mixture of the small-particle-size precursor and the large-particle-size precursor in which the M element is supported on the N element source, is an N element-M element coprecipitated product.

9. A process for producing the lithium-containing composite oxide as defined in claim 1 , which comprises mixing a lithium source and, if necessary, a fluorine source in a precursor in which the M element is supported on an N element source, obtained by mixing the N element source containing from 15 to 40% by weight of a small-particle-size N element source with an average particle size of 2 μm≦D 50 ≦8 μm and from 60 to 85% by weight of a large-particle-size N element source with an average particle size of 10 μm≦D 50 ≦25 μm and an M element source comprising an aqueous solution of an M element-containing carboxylate; and firing the resultant mixture at from 700° C. to 1,100° C. in an oxygen-containing atmosphere.

10. The process for producing the lithium-containing composite oxide according to claim 9 , wherein the M element-containing carboxylate is a salt of at least one acid selected from the group consisting of citric acid, maleic acid, lactic acid and tartaric acid.

11. The process for producing the lithium-containing composite oxide according to claim 9 , wherein the aqueous solution of the M element-containing carboxylate has a pH of from 2 to 12.

12. The process for producing the lithium-containing composite oxide according to claim 7 , wherein the N element source is at least one member selected from the group consisting of a nickel salt, a cobalt salt, a nickel-cobalt coprecipitated product and a nickel-cobalt-manganese coprecipitated product.

13. The process for producing the lithium-containing composite oxide according to claim 7 , wherein the N element source is at least one member selected from the group consisting of cobalt hydroxide, cobalt oxyhydroxide, tricobalt tetroxide and cobalt carbonate.

14. The lithium-containing composite oxide produced by the process as defined in claim 7 .

15. A positive electrode for a lithium secondary battery comprising a cathode active material, a binder and an electroconductive material, wherein the cathode active material comprises the lithium-containing composite oxide as defined in claim 1 .

16. A lithium secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte and an electrolytic solution, wherein the positive electrode is as defined in claim 15 .