Positive active material, preparing method thereof, positive electrode for lithium secondary battery including the same, and lithium secondary battery employing the same

1. A positive active material comprising:

a lithium transition metal composite oxide core; and

a coating layer comprising LiF in an amount of about 0.001 parts by weight to about 0.2 parts by weight based on 100 parts by weight of the lithium transition metal composite oxide core on at least one portion of a surface of the lithium transition metal composite oxide core,

wherein the amount of Li-containing impurities that remain on a surface of a lithium transition metal composite oxide core is 0.25 parts by weight or less based on 100 parts by weight of the lithium transition metal composite oxide core.

2. The positive active material of claim 1 , wherein the Li-containing impurities comprise lithium carbonate (Li 2 CO 3 ) and lithium hydroxide (LiOH).

3. The positive active material of claim 1 , wherein the coating layer comprises the surface of the lithium transition metal composite oxide core surface treated with at least one of NH 4 F, NH 4 HF 2 , HF, CF 3 COOH, and anhydrous hydrogen fluoride.

4. The positive active material of claim 1 , wherein the coating layer has LiF particles scattered on the surface of the lithium transition metal composite oxide core.

5. The positive active material of claim 1 , wherein the amount of the LiF is about 0.05 parts by weight to about 0.2 parts by weight, based on 100 parts by weight of the lithium transition metal composite oxide core.

6. The positive active material of claim 1 , wherein the thickness of the coating layer is from about 10 nm to about 500 nm.

7. The positive active material of claim 1 , wherein an amount of nickel with respect to a total amount of transition metals comprised in the lithium transition metal composite oxide core is 40 wt % or greater.

8. The positive active material of claim 1 , wherein the lithium transition metal composite oxide core is a lithium transition metal composite oxide represented by Formula 1 below:

Li a Ni b (M1) c (M2) d O 2   <Formula 1>

in Formula 1,

0.9<a≦1.1, 0.5≦b<0.9, 0.1≦c<0.45, and 0≦d<0.15, 0.1≦c+d<0.6, M1 is at least one of Co and Mn, and M2 is at least one of Al, Ti, Mg, Sc, V, Cr, Fe, Co, Mn, Zn, B, Si, Ga, Ge, N, P, As, Zr, Mo, W, Ru, Rh, Pt, Ag, and Sn.

9. The positive active material of claim 1 , wherein the lithium transition metal composite oxide core is a secondary particle in which a plurality of primary particles are aggregated.

10. The positive active material of claim 9 , wherein an average diameter (D50) of the secondary particle is about 3 μm to about 20 μm.

11. A positive electrode for a lithium secondary battery comprising the positive active material according to claim 1 .

12. A lithium secondary battery comprising:

a negative electrode;

a positive electrode comprising:

a lithium transition metal composite oxide core;

a coating layer comprising LiF in an amount of about 0.001 parts by weight to about 0.2 parts by weight based on 100 parts by weight of the lithium transition metal composite oxide core on at least one portion of a surface of the lithium transition metal composite oxide core, and

wherein the amount of Li-containing impurities that remain on a surface of a lithium transition metal composite oxide core is 0.25 parts by weight or less based on 100 parts by weight of the lithium transition metal composite oxide core.

13. The lithium secondary battery of claim 12 , wherein the Li-containing impurities comprise lithium carbonate (Li 2 CO 3 ) and lithium hydroxide (LiOH).

14. The lithium secondary battery of claim 12 , wherein the coating layer comprises the surface of the lithium transition metal composite oxide core surface treated with at least one of NH 4 F, NH 4 HF 2 , HF, CF 3 COOH, and anhydrous hydrogen fluoride.

15. The lithium secondary battery of claim 12 , wherein the coating layer has LiF particles scattered on the surface of the lithium transition metal composite oxide core.