Surface modified lithium-containing composite oxide for cathode active material for lithium ion secondary battery and its production process
To provide a surface modified lithium-containing composite oxide having excellent discharge capacity, volume capacity density, safety, durability for charge and discharge cycles, and high rate property. A surface modified lithium-containing composite oxide, comprising particles of a lithium-containing composite oxide having a predetermined composition and a lithium titanium composite oxide containing lithium, titanium and element Q (wherein Q is at least one element selected from the group consisting of B, Al, Sc, Y and In) contained in the surface layer of the particles, wherein the lithium titanium composite oxide is contained in the surface layer of the particles in a proportion of the total amount of titanium and element Q in the lithium titanium composite oxide contained in the surface layer to the lithium-containing composite oxide particles is from 0.01 to 2 mol %, and the lithium titanium composite oxide has a peak at a diffraction angle 2θ within a range of 43.8±0.5° in powder X-ray diffraction measurement in which CuKα rays are used.
1. A surface modified lithium-containing composite oxide, comprising:
a lithium-containing composite oxide particle represented by the formula:
Li w N x M y O z F a ; and
a surface layer surrounding the particle, said surface layer comprising a lithium titanium composite oxide comprising lithium, titanium and an element Q,
wherein:
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, Zn, Sn, an alkaline earth metal element and a transition metal element other than Co, Mn and Ni;
w, x, y, z and a satisfy: 0.9≦w≦1.3, 0.9≦x≦1.0, 0≦y≦0.1, 1.9≦z≦2.1, and 0≦a≦0.05;
the element Q is at least one selected from the group consisting of B, Al, Sc, Y and In;
a proportion of a total amount of the titanium and the element Q in the lithium titanium composite oxide relative to the lithium-containing composite oxide particles is from 0.01 to 2 mol %;
a powder X-ray diffraction measurement of the lithium titanium composite oxide with CuKα rays exhibits a peak at a diffraction angle 2θ within a range of 43.8±0.5°;
an atomic ratio (lithium/(element Q+titanium)) of the lithium to the total amount of the element Q and the titanium contained in the lithium titanium composite oxide satisfies:
0.55≦lithium/(element Q+titanium)≦1.8;
an atomic ratio (element Q/titanium) of the element Q to the titanium contained in the lithium titanium composite oxide satisfies:
0.01≦(element Q/titanium)≦0.5; and
the lithium titanium composite oxide has a crystal structure of space group Fm-3m.
2. The surface modified lithium-containing composite oxide according to claim 1 , wherein the atomic ratio (element Q/titanium) of the element Q to the titanium contained in the lithium titanium composite oxide satisfies:
0.05≦(element Q/titanium)≦0.25
3. The surface modified lithium-containing composite oxide according to claim 1 , wherein the atomic ratio (lithium/(element Q+titanium)) of the lithium to the total amount of the element Q and the titanium contained in the lithium titanium composite oxide satisfies:
0.85≦lithium/(element Q+titanium)≦1.8.
4. The surface modified lithium-containing composite oxide according to claim 1 , wherein M is at least one element selected from the group consisting of Al, Ti, Zr, Hf, Nb, Ta, Mg, Sn and Zn.
5. The surface modified lithium-containing composite oxide according to claim 1 , wherein the powder X-ray diffraction measurement of the lithium titanium composite oxide with CuKα rays exhibits a peak at a diffraction angle 2θ within a range of 63.1±0.5°.
6. A positive electrode, comprising:
a cathode active material;
an electroconductive material; and
a binder,
wherein the cathode active material comprises the surface modified lithium-containing composite oxide of claim 1 .
7. A lithium ion secondary battery, comprising:
the positive electrode of claim 6 ;
a negative electrode; and
an electrolytic solution.
8. A process for producing the surface modified lithium-containing composite oxide of claim 1 , the process comprising impregnating a powder comprising the lithium-containing composite oxide particle represented by the formula: Li w N x M y O z F a with a solution comprising a lithium source, a titanium source and an element Q source to obtain lithium source-titanium source-element Q source-impregnated particles, and heat treating the lithium source-titanium source-element Q source-impregnated particles at from 500 to 1,000° C. to obtain a surface modified lithium-containing composite oxide.
9. The process according to claim 8 , wherein the atomic ratio (element Q/titanium) of element Q to titanium contained in the solution satisfies 0.05≦(element Q/titanium)≦0.25.
10. The process according to claim 8 , wherein an atomic ratio (lithium/(element Q+titanium)) of lithium to the total amount of element Q and titanium contained in the solution satisfies 0.85≦lithium/(element Q+titanium)≦1.8.
11. The process according to claim 8 , wherein the solution further comprises a carboxylic acid comprising at least 2 carboxy groups, or at least 2 in total of carboxy groups and hydroxy groups or carbonyl groups.
12. The process according to claim 8 , wherein the solution has a pH of from 1 to 7.
13. The process according to claim 8 , wherein the titanium source is titanium lactate.
14. The process according to claim 8 , wherein the element Q comprises boron oxide.
15. The process according to claim 8 , wherein the element Q comprises basic aluminum lactate.
16. The process according to claim 8 , wherein the element Q comprises indium acetate.
17. The process according to claim 8 , wherein the solution is an aqueous solution.
18. The process according to claim 8 , wherein the lithium source is lithium carbonate.
19. The process according to claim 8 , wherein the powder X-ray diffraction measurement of the lithium titanium composite oxide exhibits a peak at a diffraction angle 2θ within a range of 63.1±0.5°.