Composite cathode active material, and cathode and lithium battery including the material

1. A composite cathode active material, comprising:

an overlithiated metal oxide represented by Chemical Formula 1; and

a material having an olivine structure;

wherein the material having an olivine structure includes a coating layer including an inorganic material formed thereon, or the material having an olivine structure is doped with an inorganic material,

wherein the inorganic material includes a nitride or carbide of a non-transition metal,

Li[Li x Me y ]O 2+d ,  <Chemical Formula 1>

where x+y=1, 0<x<1, 0≤d≤0.1, and Me is one or more selected from the group of Mn, V, Cr, Fe, Co, Ni, Zr, Re, Al, B, Ge, Ru, Sn, Ti, Nb, Mo, and Pt,

wherein the overlithiated metal oxide is a solid solution of LiMe′O 3 and LiMe″O 2 ,

Me′ is one or more selected from the group of Mn, Zr and Ti, and

Me″ is one or more selected from the group of Ni, Co, Mn, Cr, Fe, V, Al, Mg and Ti.

2. The composite cathode active material as claimed in claim 1 , wherein a content of the overlithiated metal oxide is in a range of about 5 wt % to about 95 wt % of a total weight of the composite cathode active material.

3. The composite cathode active material as claimed in claim 2 , wherein the content of the overlithiated metal oxide is in a range of about 40 wt % to about 60 wt % of the total weight of the composite cathode active material.

4. The composite cathode active material as claimed in claim 1 , wherein the overlithiated metal oxide is represented by Chemical Formula 2:

Li[Li x Ni a Co b Mn c ]O 2+d ,  <Chemical Formula 2>

where x+a+b+c=1, 0<x<1, 0 a<1, 0<b<1, 0<c<1, and 0≤d≤0.1.

5. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure includes a lithium metal oxide represented by Chemical Formula 3:

Li x M y M′ z PO 4-d X d ,  <Chemical Formula 3>

where 0.9≤x≤1.1, 0≤y≤1, 0≤z≤1, 1.9≤x+y+z≤2.1, 0≤d≤0.2, M is one or more of Fe, Mn, Ni, and Co, M′ is one or more of Mg, Ca, Sr, Ba, Ti, Zr, Nb, Mo, W, Zn, Al, and Si, and X is one or more of S and F.

6. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure includes one or more of LiFePO 4 , LiFe 1-a Mn a PO 4 (0<a<1), and LiMnPO 4 .

7. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure includes a metal with an oxidation number of +2 and a metal with an oxidation number of +3, and a content of the metal with an oxidation number of +3 in the material having an olivine structure is about 5 wt % or less of a total content of the metal with an oxidation number of +2 and the metal with an oxidation number of +3 in the material having an olivine structure.

8. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure exhibits a nitrogen peak having a binding energy range of about 398 eV to about 400 eV and an oxygen peak having a binding energy range of about 530 eV to about 534 eV in an X-ray photoelectron spectrum, and a ratio of an area of the nitrogen peak to an area of the oxygen peak is about 1:100 or more.

9. The composite cathode active material as claimed in claim 8 , wherein the ratio of the area of the nitrogen peak to the area of the oxygen peak is about 1:100 to about 20:100.

10. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure exhibits a nitrogen peak having a binding energy range of about 398 eV to about 400 eV and a phosphorus peak having a binding energy range of about 132 eV to about 136 eV in an X-ray photoelectron spectrum, and a ratio of an area of the nitrogen peak to an area of the phosphorus peak is about 1:100 or more.

11. The composite cathode active material as claimed in claim 10 , wherein the ratio of the area of the nitrogen peak to the area of the phosphorus peak is about 1:100 to about 30:100.

12. The composite cathode active material as claimed in claim 1 , wherein the material having an olivine structure exhibits a nitrogen peak having a binding energy range of about 396 eV to about 398 eV.

13. The composite cathode active material as claimed in claim 1 , further comprising a coating layer including conductive carbon formed on the material having an olivine structure.

14. The composite cathode active material as claimed in claim 1 , wherein the inorganic material is one or more of boron nitride (BN) and boron carbide (B 4 C).

15. The composite cathode active material as claimed in claim 1 , wherein the inorganic material includes one or more of amorphous boron nitride, boron nitride having a cubic crystal structure, boron nitride having a hexagonal crystal structure, and boron nitride having a wurtzite crystal structure.

16. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has one or more forms of a nanoparticle, a nanofiber, a nanomesh, and a nanotube.

17. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has a Mohs hardness higher than a Mohs hardness of the material having an olivine structure.

18. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has a Mohs hardness of about 1 or more.

19. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has an electrical conductivity lower than an electrical conductivity of amorphous carbon.

20. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has an electrical conductivity of about 10 −1 S/cm or less.

21. A cathode including the cathode active material as claimed in claim 1 .

22. A lithium battery including the cathode as claimed in claim 21 .