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, wherein the lithium iron phosphate has a composition represented by the following Formula 1 and carbon (C) is coated on the particle surface of the lithium iron phosphate containing a predetermined amount of sulfur (S):

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 front F, S and N, and

0.5≦a≦+0.5, 0≦x≦0.5, 0≦b≦0.1

wherein the sulfur (S) is contained at an amount of 0.05 to 5% by weight, based on the total weight of the lithium iron phosphate wherein the sulfur and the carbon are present in the form of a structure selected from the group consisting of (i) a structure in which carbon is coated on the surface of lithium iron phosphate particles in a state in which a predetermined amount of sulfur is contained on the surface of lithium iron phosphate particles, (ii) a structure in which both sulfur and carbon are coated on the surface of lithium iron phosphate particles, (iii) a structure in which a composite of sulfur and carbon is coated on the surface of lithium iron phosphate particles, (iv) a structure in which carbon is bonded through sulfur to lithium iron phosphate particles, and combinations thereof.

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 (C) 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 particle surface of lithium iron phosphate to a thickness of 2 to 50 nm.

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

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

7. The lithium iron phosphate according to claim 6 , wherein the sulfur-containing compound is at least one 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. Lithium iron phosphate having an olivine crystal structure wherein the lithium iron phosphate has a composition represented by the following Formula 2 and carbon (C) is coated on the particle surface of the lithium iron phosphate containing a predetermined amount of sulfur (S):

Li (1−a−b) Fe a/2 M′ b/2 Fe 1−c M″ c P 1−d X d O 4−e 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;

X is at least one selected from the group consisting of As, Sb, Bi, Mo, V, Nb and Te; and

0≦a≦0.6, 0≦b≦0.6, 0≦c≦1, 0≦d≦0.05, 0≦e≦3.5 wherein the sulfur and the carbon are present in the form of a structure selected from the group consisting of (i) a structure in which carbon is coated on the surface of lithium iron phosphate particles in a state in which a predetermined amount of sulfur is contained on the surface of lithium iron phosphate particles, (ii) a structure in which both sulfur and carbon are coated on the surface of lithium iron phosphate particles, (iii) a structure in which a composite of sulfur and carbon is coated on the surface of lithium iron phosphate particles, (iv) a structure in which carbon is bonded through sulfur to lithium iron phosphate particles, and combinations thereof.

10. A cathode mix comprising the lithium iron phosphate according to claim 9 as a cathode active material.