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 compound with an olivine crystal structure; and

an inorganic material, the inorganic material including a nitride or carbide of at least one element selected from the group of Group 2, Group 13, Group 14, and Group 15 of the periodic table of elements,

wherein the inorganic material includes at least one selected from the group of an amorphous boron nitride, a boron nitride with a cubic crystalline structure, a boron nitride with a hexagonal crystalline structure, and a boron nitride with a wurtzite crystalline structure, the inorganic material is included in the composite cathode active material in an amount of about 0.2 wt % to about 1.0 wt %, based on a total weight of the composite cathode active material, and the inorganic material improves the electrical conductivity of the compound of the olivine crystal structure by being dispersed in the composite cathode active material, and

wherein the compound with the olivine crystal structure is represented by Formula 2 below:

Li x Mn 1-z M′ z PO 4-d X d   Formula 2

wherein, in Formula 2, 0.6≦x≦1.1, 0≦z≦0.2, and 0≦d≦0.2;

M′ includes at least one selected from the group of Fe, Ni, Co, Mg, Ca, Cu, Nd, Sr, Ba, Ti, Zr, Nb, Mo, W, Zn, Al, and Si; and

X includes at least one selected from the group of S and F.

2. The composite cathode active material as claimed in claim 1 , wherein the inorganic material does not include a nitride of a transition metal.

3. The composite cathode active material as claimed in claim 1 , wherein the compound with the olivine crystal structure includes LiMn 0.9 Fe 0.1 PO 4 .

4. The composite cathode active material as claimed in claim 1 , wherein the compound with the olivine crystal structure includes LiFe 1-a Mn a PO 4 (0<a<1).

5. The composite cathode active material as claimed in claim 1 , wherein the inorganic material further includes a nitride or carbide of a metalloid element selected from the group of Group 13, Group 14, and Group 15 of the periodic table of elements.

6. The composite cathode active material as claimed in claim 1 , wherein the inorganic material further includes at least one selected from the group of boron carbide (B 4 C), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), a boron nitride-boron carbide complex, a boron nitride-silicon nitride (Si 3 N 4 ) complex, and a boron nitride-silicon carbide (SiC) complex.

7. The composite cathode active material as claimed in claim 1 , wherein the inorganic material is in the form of at least one selected from the group of a nanoparticle, a nanofiber, a nanomesh, and a nanotube.

8. The composite cathode active material as claimed in claim 1 , wherein the inorganic material has a Mohs hardness greater than a Mohs hardness of the compound with the olivine crystal structure.

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

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

11. 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.

12. The composite cathode active material as claimed in claim 1 , further comprising a carbonaceous material.

13. The composite cathode active material as claimed in claim 12 , wherein the carbonaceous material is included in an amount of about 0.1 wt % to about 30 wt % based on a total weight of the composite cathode active material.

14. A cathode comprising the composite cathode active material as claimed in claim 1 .

15. A lithium battery comprising the cathode as claimed in claim 14 .