Carbon-coated lithium iron phosphate of olivine crystal structure and lithium secondary battery using the same

1. Lithium iron phosphate having an olivine crystal structure suitable for lithium secondary battery, wherein the lithium iron phosphate has a composition represented by the following Formula 1 and carbon (C) is coated on the surface of the lithium iron phosphate by chemical bonding via sulfur (S) in form of ‘oxygen-sulfur-carbon’, and

wherein the sulfur (S) is contained in the lithium iron phosphate in an amount of 0.005 to 1% by weight, based on the total weight of the lithium iron phosphate:

Li 1+a Fe 1−x M x (PO 4−b )X b   (1)

wherein

M is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y,

X is at least one selected from F, S and N,

−0.5 ≦a≦+ 0.5, 0 ≦x≦ 0.5, 0 ≦b≦ 0.1.

2. The lithium iron phosphate according to claim 1 , wherein the lithium iron phosphate is LiFePO 4 .

3. The lithium iron phosphate according to claim 1 , wherein the carbon is coated at an amount of 0.01 to 10% by weight, based on the total weight of the lithium iron phosphate.

4. The lithium iron phosphate according to claim 1 , wherein the carbon is coated on the surface of lithium iron phosphate to a thickness of 2 to 10 nm.

5. The lithium iron phosphate according to claim 1 , wherein the sulfur (S) is derived from a precursor for preparation of lithium iron phosphate.

6. The lithium iron phosphate according to claim 1 , wherein the sulfur (S) is incorporated by coating lithium iron phosphate with a sulfur-containing compound.

7. The lithium iron phosphate according to claim 6 , wherein the sulfur-containing compound is one or more selected from sulfides, sulfites and sulfates.

8. The lithium iron phosphate according to claim 1 , wherein the lithium iron phosphate is prepared by a supercritical hydrothermal method.

9. A method for preparing the lithium iron phosphate according to claim 1 comprising:

(a) primarily mixing precursors as starting materials and an alkalinizing agent;

(b) secondarily mixing the mixture obtained in step (a) with supercritical or subcritical water to synthesize lithium iron phosphate;

(c) mixing the synthesized lithium iron phosphate with a carbon precursor and drying the mixture; and

(d) heating the mixture of lithium iron phosphate and the carbon precursor.

10. A method for preparing the lithium iron phosphate according to claim 1 comprising:

(a′) primarily mixing precursors as starting materials and an alkalinizing agent;

(b′) secondarily mixing the mixture obtained in step (a′) with supercritical or subcritical water to synthesize lithium iron phosphate, followed by drying;

(c′) heating the synthesized lithium iron phosphate; and

(d′) milling the lithium iron phosphate and a carbon powder.

11. The method according to claim 9 , wherein the heating is carried out under an inert gas atmosphere.

12. A method for preparing the lithium iron phosphate according to claim 1 comprising:

(a″) synthesizing lithium iron phosphate using precursors as starting materials by a coprecipitation or solid phase method;

(b″) adding the synthesized lithium iron phosphate to a dispersion bath containing a sulfur-containing compound, followed by stirring;

(c″) drying the mixture obtained in step (b″), followed by baking; and

(d″) mixing the dried/baked lithium iron phosphate with a carbon powder, followed by milling, or mixing the dried/baked lithium iron phosphate and carbon precursor with a solvent, followed by drying and baking.

13. A cathode mix comprising the lithium iron phosphate according to claim 1 as a cathode active material.

14. A lithium secondary battery comprising a cathode in which the cathode mix according to claim 13 is applied to a current collector.

15. The lithium secondary battery according to claim 14 , wherein the lithium secondary battery is used as a unit battery of a battery module that is a power of a medium to large device.

16. The lithium secondary battery according to claim 15 , wherein the medium to large device is a power tool, an electric vehicle, a hybrid electric vehicle, or an electric golf cart.

17. The method according to claim 10 , wherein the heating is carried out under an inert gas atmosphere.

18. The lithium iron phosphate according to claim 1 , wherein the sulfur (S) is contained in the lithium iron phosphate in an amount of 0.01 to 0.7% by weight, based on the total weight of the lithium iron phosphate.