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

1. A composite cathode active material comprising:

a first metal oxide comprising a plurality of layered crystalline phases comprising a first layered crystalline phase and a second layered crystalline phase, wherein the first and second layered crystalline phases have a different compositions than each other; and

a second metal oxide comprising a composite crystalline phase, wherein the composite crystalline phase is different from at least one of the plurality of layered crystalline phases of the first metal oxide,

wherein the second metal oxide is represented by Formula 1,

wherein at least a portion of the second metal oxide is disposed on a first layered crystalline phase of the plurality of layered crystalline phases of the first metal oxide, and

wherein the first layered crystalline phase is in a space group of R-3m:

Li x M y O z   Formula 1

wherein, in Formula 1, 0≤x≤3, 1≤y≤3, and 2≤z≤8, and

M is at least one selected from a Group 4 element to a Group 13 element.

2. The composite cathode active material of claim 1 , wherein the second metal oxide comprising the composite crystalline phase is disposed on a surface of a particle ofthe composite cathode active material.

3. The composite cathode active material of claim 2 , wherein the composite crystalline phase has a dimension of less than or equal to about 100 nanometers.

4. The composite cathode active material of claim 1 , wherein the second metal oxide comprising the composite crystalline phase is disposed at a grain boundary of the first metal oxide and inside the composite cathode active material.

5. The composite cathode active material of claim 1 , wherein the composite cathode active material comprises a pore within the composite cathode active material.

6. The composite cathode active material of claim 5 , wherein each pore has a maximum diameter of about 500 nanometers.

7. The composite cathode active material of claim 5 , wherein the second metal oxide comprising the composite crystalline phase is disposed in the pore.

8. The composite cathode active material of claim 1 , wherein the composite cathode active material comprises lithium in an amount of less than or equal to about 3,000 parts per million.

9. The composite cathode active material of claim 1 , wherein an amount of metal in the second metal oxide is in a range of about 0.01 mole percent to about 3 mole percent, based on a total amount of a metal in the first metal oxide.

10. The composite cathode active material of claim 1 , wherein the first metal oxide is represented by Formula 2:

Li a Ni b Co c Mn d O 2   Formula 2

wherein, in Formula 2, 1.0≤a≤1.2, 0.7<b<1.0, 0<c<0.3, and 0<d<0.3.

11. The composite cathode active material of claim 1 , wherein the plurality of layered crystalline phases of the first metal oxide comprises a second layered crystalline phase in a space group of C2/m.

12. The composite cathode active material of claim 1 , wherein the plurality of layered crystalline phases of the first metal oxide comprises a second layered crystalline phase having a composition represented by Formula 3:

Li 2 MnO 3 .  Formula 3

13. The composite cathode active material of claim 12 , wherein an amount of the second layered crystalline phase is greater than or equal to about 3 weight percent, based on a total weight of the composite cathode active material.

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

Li x M′ y O z   Formula 5

wherein, in Formula 5, 0≤x≤3, 1≤y≤3, and 2≤z≤8, and

M′ is at least one selected from manganese, vanadium, chromium, iron, cobalt, nickel, zirconium, rhenium, aluminum, boron, germanium, ruthenium, tin, titanium, niobium, molybdenum, and platinum.

15. The composite cathode active material of claim 1 , wherein the second metal oxide is at least one selected from Li 3 VO 8 , V 2 O 5 , LiV 3 O 8 , LiMn 2 O 4 , Li 2 MnO 3 , LiNi 0.5 Mn 1.5 O 4 , LiCo 1-x Ma x O 2 where 0≤x<1 and Ma is a transition metal, LiNi 1-x Ma x O 2 where 0≤x<1 and Ma is a transition metal, and LiMn 1-x Ma x O 2 where 0≤x<1 and Ma is a transition metal.

16. A cathode comprising a composite cathode active material of claim 1 .

17. A lithium battery comprising the cathode of claim 16 .

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

providing a first metal oxide comprising a plurality of layered crystalline phases, wherein each layered crystalline phase of the plurality of layered crystalline phases has a different composition than any other layered crystalline phase of the plurality of layered crystalline phases;

combining the first metal oxide and a precursor of a second metal oxide to form a mixture, wherein the second metal oxide is represented by Formula 1;

drying the mixture; and

heat-treating the dried mixture to form the composite cathode active material:

Li x M y O z   Formula 1

wherein, in Formula 1, 0<x<3, 1<y<3, and 2<z<8, and

M is at least one selected from a Group 4 element to a Group 13 element.

19. The method of claim 18 , wherein the precursor of the second metal oxide comprises a material which does not comprise lithium.

20. The method of claim 18 , wherein the drying or heat-treating of the mixture is performed in an oxygen atmosphere at a temperature in a range of about 200° C. to about 800° C. for about 2 hours to about 40 hours.