Lithium iron phosphate containing sulfur compound based upon sulfide bond and lithium secondary battery using the same

1. Lithium iron phosphate having an olivine crystal structure, wherein the lithium iron phosphate has a composition represented by the following Formula 1, a sulfur compound with a sulfide bond is contained, as an impurity, in the lithium iron phosphate particles, and carbon (C) is coated on particle surfaces of the lithium iron phosphate,

wherein the carbon (C) is coated at an amount of 0.01 to 10% by weight, based on the total weight of the lithium iron phosphate, and

wherein the lithium iron phosphate is prepared by a supercritical hydrothermal method,

wherein the sulfur (S) constituting the sulfur compound is contained at in an amount of about 0.1 to 5% 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, and

−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 sulfur compound is at least one selected from the group consisting of Li 2 S, FeS, and (NH 4 ) 2 S.

4. The lithium iron phosphate according to claim 1 , wherein the sulfur compound is derived from a precursor for preparation of the lithium iron phosphate.

5. The lithium iron phosphate according to claim 1 , wherein the lithium iron phosphate particle further comprises iron oxide as an impurity.

6. The lithium iron phosphate according to claim 1 , wherein the carbon is coated on the particle surface of lithium iron phosphate to a thickness of 2 to 50 nm.

7. Lithium iron phosphate having an olivine crystal structure wherein the lithium iron phosphate has a composition represented by the following Formula 2, a sulfur compound with a sulfide bond is contained, as an impurity, in the lithium iron phosphate particles, and carbon (C) is coated on the particle surface of the lithium iron phosphate containing a predetermined amount of sulfur (S) and wherein the carbon (C) is coated at an amount of 0.01 to 10% by weight, based on the total weight of the lithium iron phosphate, and

Li (1-a-b) Fe a/2 M′ b/2 Fe 1-c M″ c PO 4-c S e   (2)

wherein M′ is at least one selected from the group consisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr and Zn;

M″ is at least one selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr and Zn; and

0≦a≦0.6, 0≦b≦0.6, 0≦c≦1, 0≦e≦3.5.

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

(a) primarily mixing precursors as starting materials;

(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 the lithium iron phosphate and the carbon precursor.

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

(a′) primarily mixing precursors as starting materials;

(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.

10. The method according to claim 8 , wherein the heating is carried out under an inert as atmosphere.

11. The method according to claim 8 , wherein the synthesis of the lithium iron phosphate is carried out by a continuous reaction process.

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

13. A lithium secondary battery comprising a cathode in which the cathode mix according to claim 12 is applied to a current collector.

14. The lithium secondary battery according to claim 13 , wherein the lithium secondary battery is a unit battery of a battery module that is a power source of a medium to large device.

15. The lithium secondary battery according to claim 14 , wherein the medium to large device is a power tool, an electric vehicle, a hybrid electric vehicle, or an electric golf cart.

16. A cathode mix comprising the lithium iron phosphate according to claim 7 as a cathode active material.

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

18. The lithium iron phosphate according to claim 7 , wherein the lithium iron phosphate is prepared by a supercritical hydrothermal method.