Composite cathode active material, cathode and lithium battery containing the same, and method of preparing the composite cathode active material

1. A composite cathode active material comprising a first cathode active material comprising:

a core comprising a first lithium transition metal oxide represented by Formula 3 and having a first layered crystalline phase that belongs to a R-3m space group;

a coating layer disposed on the core and comprising a second lithium transition metal oxide having a plurality of layered crystalline phases, in which each layered crystalline phase of the plurality of layered crystalline phases has a different composition

Li a Ni b CO c Mn d O 2   Formula 3

wherein, in Formula 3, 1.0≤a≤1.03, 0.4<b<1.0, 0<c<0.3, 0<d<0.4, and b+c+d=1; and

a residual lithium compound on a surface of the composite cathode active material,

wherein an amount of the residual lithium compound on the surface of the composite cathode active material is 632 parts per million to 1500 parts per million, based on a total content of the composite cathode active material.

2. The composite cathode active material of claim 1 , wherein the plurality of layered crystalline phases comprises

a second layered crystalline phase that belongs to a C2/m space group; and

a third layered crystalline phase that belongs to a R-3m space group.

3. The composite cathode active material of claim 2 , wherein the second layered crystalline phase has a composition represented by Formula 4:

Li 2 MnO 3 .  Formula 4

4. The composite cathode active material of claim 3 , the third layered crystalline phase has a composition represented by Formula 5:

LiMO 2   Formula 5

wherein, in Formula 5, M comprises at least two elements comprising nickel, manganese, vanadium, chromium, iron, cobalt, zirconium, rhenium, aluminum, boron, germanium, ruthenium, tin, titanium, niobium, molybdenum, platinum, or a combination thereof.

5. The composite cathode active material of claim 1 , wherein the second lithium transition metal oxide is represented by Formula 6:

a Li 2 MnO 3 .(1− a )LiMO 2   Formula 6

wherein, in Formula 6,

0.2<a<1; and

M comprises at least two elements comprising nickel, manganese, vanadium, chromium, iron, cobalt, zirconium, rhenium, aluminum, boron, germanium, ruthenium, tin, titanium, niobium, molybdenum, platinum, or a combination thereof.

6. The composite cathode active material of claim 1 , wherein the second lithium transition metal oxide is represented by Formula 7:

Li x M4 y M5 z Mn w O 2   Formula 7

wherein, in Formula 7,

1.07≤x≤1.20, 0.1≤y<0.8, 0≤z≤0.13, 0.46≤w<0.8, and 0.80≤y+z+w≤0.93; and

M4 and M5 each independently comprise nickel, vanadium, chromium, iron, cobalt, zirconium, rhenium, aluminum, boron, germanium, ruthenium, tin, titanium, niobium, molybdenum, platinum, or a combination thereof.

7. The composite cathode active material of claim 1 , wherein the coating layer comprising the second lithium transition metal oxide is a discontinuous coating on a surface of the core.

8. The composite cathode active material of claim 1 , wherein an average particle diameter of the first cathode active material is in a range of about 10 micrometers to about 20 micrometers.

9. The composite cathode active material of claim 1 further comprising a second cathode active material having an average particle diameter of about 5 micrometers or less.

10. The composite cathode active material of claim 9 , wherein the second cathode active material comprises the second lithium transition metal oxide having an average particle diameter in a range of about 1 micrometer to about 5 micrometers.

11. The composite cathode active material of claim 9 , wherein an amount of the second cathode active material is in a range of about 1 weight percent to about 30 weight percent, based on a total weight of the composite cathode active material.

12. The composite cathode active material of claim 1 , wherein the composite active material has a press density of about 3 grams per cubic centimeter or greater after pressing at a pressure of 2.6 tons/cm′ for 30 seconds.

13. A cathode comprising:

a current collector; and

the composite cathode active material of claim 1 on the current collector.

14. A lithium battery comprising:

the cathode of claim 13 ;

an anode; and

a separator between the cathode and the anode.

15. The lithium battery of claim 13 , wherein the lithium battery is configured to be charged to a voltage of about 4.5 volts or greater with respect to a lithium metal.

16. The composite cathode active material of claim 1 , wherein the amount of the residual lithium compound on the surface of the composite cathode active material is 1175 parts per million to 1500 parts per million, based on the total content of the composite cathode active material.

17. A method of preparing a composite cathode active material, the method comprising:

providing a first lithium transition metal oxide represented by Formula 3 and having a first layered crystalline phase that belongs to a R-3m space group;

combining the first lithium transition metal oxide, a precursor of a second lithium transition metal oxide, and a lithium source to prepare a mixture;

heat-treating the mixture in an oxidation atmosphere to obtain a composite cathode active material

Li a Ni b CO c Mn d O 2   Formula 3

wherein, in Formula 3,

1.0≤a≤1.03, 0.4<b<1.0, 0<c<0.3, and 0<d<1.0.4; and

a residual lithium compound on a surface of the composite cathode active material

wherein an amount of the residual lithium compound on the surface of the composite cathode active material is 632 parts per million to 1500 parts per million, based on a total content of the composite cathode active material.