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

Abstract

Disclosed is 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.sub.1+aFe.sub.1−xM.sub.x(PO.sub.4−b)X.sub.b  (1) (wherein M, X, a, x, and b are the same as defined in the specification).

Claims

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.sub.1+aFe.sub.1−xM.sub.x(PO.sub.4−b)X.sub.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 wherein the sulfur (S) is contained at an amount of 0.1 to 2% by weight, based on the total weight of the lithium iron phosphate, wherein the sulfur (S) is derived from a precursor for preparation of the lithium iron phosphate, and wherein the sulfur and the carbon are present in the form of a structure in which carbon is coated on the surface of lithium iron phosphate particles and the sulfur is contained inside of lithium iron phosphate particles.

2. The lithium iron phosphate according to claim 1, wherein the lithium iron phosphate is LiFePO.sub.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 the lithium iron phosphate to a thickness of 2 to 50 nm.

5. 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.sub.(1−a−b)Fe.sub.a/2M′.sub.b/2Fe.sub.1−cM″.sub.cP.sub.1−dX.sub.dO.sub.4−eS.sub.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 (S) is contained in an amount of 0.1 to 2% by weight, based on the total weight of the lithium iron phosphate, and is derived from a precursor for preparation of the lithium iron phosphate, and wherein the sulfur and the carbon are present in the form of a structure in which carbon is coated on the surface of lithium iron phosphate particles and the sulfur is contained inside of lithium iron phosphate particles.

6. A cathode mix comprising the lithium iron phosphate according to claim 5 as a cathode active material.

7. A method for preparing the lithium iron phosphate according to claim 1 comprising: (a) primarily mixing a lithium precursor, an iron (Fe) precursor and a phosphorus precursor as starting materials, wherein the lithium precursor is Li.sub.2CO.sub.3, Li(OH), Li(OH)H.sub.2O or LiNO.sub.3, wherein the iron precursor is FeSO.sub.4, FeC.sub.2O.sub.4.2H.sub.2O or FeCl.sub.2, wherein the phosphorus precursor is H.sub.3PO.sub.4, NH.sub.4H.sub.2PO.sub.4, (NH.sub.4).sub.2HPO.sub.4, P.sub.2O.sub.5; (b) secondarily mixing a mixture obtained in step (a) with supercritical or subcritical water to synthesize lithium iron phosphate; (c) mixing a synthesized lithium iron phosphate with a carbon precursor and drying a resulting mixture, wherein the carbon precursor is a polyol-type carbon-containing precursor; and (d) heating the mixture of the lithium iron phosphate and the carbon precursor.

8. A method for preparing the lithium iron phosphate according to claim 1 comprising: (a′) primarily mixing a lithium precursor, an iron (Fe) precursor and a phosphorus precursor as starting materials, wherein the lithium precursor is Li.sub.2CO.sub.3, Li(OH), Li(OH)H.sub.2O or LiNO.sub.3, wherein the iron precursor is FeSO.sub.4, FeC.sub.2O.sub.4.2H.sub.2O or FeCl.sub.2, wherein the phosphorus precursor is H.sub.3PO.sub.4, NH.sub.4H.sub.2PO.sub.4, (NH.sub.4).sub.2HPO.sub.4, P.sub.2O.sub.5; (b′) secondarily mixing the mixture obtained in step (a′) with supercritical or subcritical water to synthesize lithium iron phosphate, followed by drying; (c′) heating a synthesized lithium iron phosphate; and (d′) milling the lithium iron phosphate and a carbon powder.

9. The method according to claim 7, wherein the heating is carried out under an inert gas atmosphere.

10. The method according to claim 7, wherein the synthesis of the lithium iron phosphate is carried out by a continuous reaction process.

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

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

13. A method for preparing the lithium iron phosphate according to claim 1 comprising: (a″) synthesizing lithium iron phosphate using a lithium precursor, an iron (Fe) precursor and a phosphorus precursor as starting materials by a coprecipitation or solid phase reaction, wherein the lithium precursor is Li.sub.2CO.sub.3, Li(OH), Li(OH)H.sub.2O or LiNO.sub.3, wherein the iron precursor is FeSO.sub.4, FeC.sub.2O.sub.4.2H.sub.2O or FeCl.sub.2, wherein the phosphorus precursor is H.sub.3PO.sub.4, NH.sub.4H.sub.2PO.sub.4, (NH.sub.4).sub.2HPO.sub.4, P.sub.2O.sub.5; (b″) adding a synthesized lithium iron phosphate to a dispersion chamber containing a sulfur-containing compound, followed by stirring; (c″) drying a mixture obtained in step (b″), followed by baking; and (d″) mixing a dried/baked lithium iron phosphate with a carbon powder, followed by milling, or mixing a dried/baked lithium iron phosphate and carbon precursor with a solvent, followed by drying and baking, wherein the carbon precursor is a polyol-type carbon-containing precursor.

14. The lithium iron phosphate according to claim 4, wherein the carbon is coated on the particle surface of the lithium iron phosphate to a thickness of 3 to 10 nm.

15. The lithium iron phosphate according to claim 1, wherein the precursor is an iron precursor of FeSO.sub.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is an image showing results after lithium iron phosphate of Example 1 is stirred in a solvent in Test Example 2;

(3) FIG. 2 is an image showing results after lithium iron phosphate of Comparative Example 1 is stirred in a solvent in Test Example 2; and

(4) FIG. 3 shows XRD data of a gray powder obtained after lithium iron phosphate of Comparative Example 1 is stirred in a solvent in Test Example 2.

BEST MODE

(5) Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only to illustrate the present invention and should not be construed as limiting the scope and spirit of the present invention.

Example 1

(6) 42.9 g of LiOH—H.sub.2O, 32.4 g of aqueous ammonia (˜29 wt %), and 924.7 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 14.13 g of sucrose, 57.7 g of phosphoric acid (85 wt %), and 786.87 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of the slurry.

(7) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a obtain LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 15 wt %, and 15 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(8) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(9) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 2.9 wt % and the content of sulfur was found to be 0.92 wt %.

Example 2

(10) 42.9 g of LiOH—H.sub.2O, 38.2 g of aqueous ammonia (˜29 wt %), and 918.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 14.13 g of sucrose, 57.7 g of phosphoric acid (85 wt %), and 793.94 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(11) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 15 wt %, and 9.8 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(12) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(13) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 1.54 wt % and the content of sulfur was found to be 0.89 wt %.

Example 3

(14) 42.9 g of LiOH—H.sub.2O, 44.1 g of aqueous ammonia (˜29 wt %), and 918.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 7.07 g of sucrose, 57.7 g of phosphoric acid (85 wt %), and 793.94 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(15) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor, and 10-fold weight of distilled water of the resulting slurry was added thereto, followed by washing to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 20 wt %, and 12 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(16) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(17) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 2.1 wt % and the content of sulfur was found to be 0.53 wt %.

Example 4

(18) 42.9 g of LiOH—H.sub.2O, 44.1 g of aqueous ammonia (˜29 wt %), and 918.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 7.07 g of sucrose, 57.7 g of phosphoric acid (85 wt %), and 801 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(19) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor, and 10-fold weight of distilled water of the resulting slurry was added thereto, followed by washing to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 10 wt %, and 7 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(20) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(21) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was 1.3 wt % and the content of sulfur was 0.42 wt %.

Example 5

(22) 47.9 g of LiOH—H.sub.2O, 30.4 g of aqueous ammonia (˜29 wt %), and 926.7 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 135.3 g of FeSO.sub.4-7H.sub.2O, 14.13 g of sucrose, 55.7 g of phosphoric acid (85 wt %), and 786.87 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 90 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 13 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(23) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 15 wt %, and 15 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(24) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 6 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(25) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 3.1 wt % and the content of sulfur was found to be 0.92 wt %.

Example 6

(26) 42.9 g of LiOH—H.sub.2O, 38.2 g of aqueous ammonia (˜29 wt %), and 918.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 135.3 g of FeSO.sub.4-7H.sub.2O, 13.13 g of ascorbic acid, 51.7 g of phosphoric acid (85 wt %), and 793.94 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (480° C., 270 bar) was flowed at an elevated temperature and at an elevated pressure at 150 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(27) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 15 wt %, and 9 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(28) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 6 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(29) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 2.34 wt % and the content of sulfur was found to be 0.73 wt %.

Example 7

(30) 42.9 g of LiOH—H.sub.2O, 44.1 g of aqueous ammonia (˜29 wt %), and 918.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 7.07 g of sucrose, 59.7 g of phosphoric acid (85 wt %), and 773.94 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(31) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor, and 10-fold weight of distilled water of the resulting slurry was added thereto, followed by washing to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 20 wt %, and 12 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(32) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(33) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was 2.4 wt % and the content of sulfur was 0.33 wt %.

Example 8

(34) 40.9 g of LiOH—H.sub.2O, 44.1 g of aqueous ammonia (˜29 wt %), and 919.9 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 145.3 g of FeSO.sub.4-7H.sub.2O, 7.07 g of glucose, 59.7 g of phosphoric acid (85 wt %), and 800 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 10 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry.

(35) The LiFePO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor, and 10-fold weight of distilled water of the resulting slurry was added thereto, followed by washing to obtain a LiFePO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 10 wt %, and 7 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFePO.sub.4 powder.

(36) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated LiFePO.sub.4 powder. As a result of XRD-Rietveld analysis, it could be seen that the powder was a LiFePO.sub.4 crystal.

(37) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 1.3 wt % and the content of sulfur was found to be 0.32 wt %.

Example 9

(38) 41.9 g of LiOH—H.sub.2O, 32.4 g of aqueous ammonia (˜29 wt %), and 924.7 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 113.3 g of FeSO.sub.4-7H.sub.2O, 10.7 g of MnSO.sub.4—H.sub.2O, 14.13 g of sucrose, 61.7 g of phosphoric acid (85 wt %), and 786.87 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry to prepare LiFe.sub.0.82Mn.sub.0.18PO.sub.4.

(39) The LiFe.sub.0.82Mn.sub.0.18PO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a LiFe.sub.0.82Mn.sub.0.18PO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 15 wt %, and 15 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFe.sub.0.82Mn.sub.0.18PO.sub.4 powder.

(40) The powder thus obtained was heated under a nitrogen atmosphere at about 750° C. for 10 hours to obtain a final carbon-coated LiFe.sub.0.82Mn.sub.0.18PO.sub.4 powder.

(41) The LiFe.sub.0.82Mn.sub.0.18PO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 3.2 wt % and the content of sulfur was found to be 1.1 wt %.

Example 10

(42) 42.9 g of LiOH—H.sub.2O, 32.4 g of aqueous ammonia (˜29 wt %), and 924.7 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 141.3 g of FeSO.sub.4-7H.sub.2O, 14.13 g of sucrose, 54.8 g of phosphoric acid (85 wt %), 2.1 g of H.sub.3AsO.sub.4-0.5H.sub.2O and 795.87 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry to prepare LiFeP.sub.0.95As.sub.0.05O.sub.4.

(43) The LiFeP.sub.0.95As.sub.0.05O.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a obtain LiFeP.sub.0.95As.sub.0.05O.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 10 wt %, and 10 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated LiFeP.sub.0.95As.sub.0.05O.sub.4 powder.

(44) The powder thus obtained was heated under a nitrogen atmosphere at about 750° C. for 10 hours to obtain a final carbon-coated LiFeP.sub.0.95As.sub.0.05O.sub.4 powder.

(45) The LiFeP.sub.0.95As.sub.0.05O.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 2.1 wt % and the content of sulfur was found to be 0.9 wt %.

Example 11

(46) 41 g of LiOH—H.sub.2O, 32.4 g of aqueous ammonia (˜29 wt %), and 924.7 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution A. In the same manner as above, 117.1 g of FeSO.sub.4-7H.sub.2O, 10.2 g of MgSO.sub.4-7H.sub.2O, 14.13 g of sucrose, 57.7 g of phosphoric acid (85 wt %), and 791.3 g of distilled water were mixed with one another and dissolved to prepare an aqueous solution B. Supercritical water (450° C., 250 bar) was flowed at an elevated temperature and at an elevated pressure at 100 g/min into a continuous tubular reactor, and the aqueous solution A and the aqueous solution B were flowed at a flow rate of 15 g/min and brought in contact with the supercritical water for several seconds and mixed to induce reaction. At this time, the aqueous solution A first contacted the aqueous solution B to produce a slurry and was then reacted with the supercritical water. The aqueous solution A was reacted with supercritical water as soon as possible after production of slurry to prepare Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4.

(47) The Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4 reaction solution thus obtained was cooled and filtered at the end of the tubular reactor to obtain a Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4 slurry. A controlled concentration of water was added to the slurry to obtain a slurry having a solid content of 10 wt %, and 8 wt % of sucrose based on the solid was added thereto, followed by dissolution. The slurry thus obtained was spray-dried to obtain a sucrose-coated Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4 powder.

(48) The powder thus obtained was heated under a nitrogen atmosphere at about 700° C. for 10 hours to obtain a final carbon-coated Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4 powder.

(49) The Li.sub.0.92Mg.sub.0.04Fe.sub.0.84Mg.sub.0.16PO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 1.72 wt % and the content of sulfur was found to be 0.5 wt %.

Comparative Example 1

(50) LiOH—H.sub.2O, Fe(C.sub.2O.sub.4)-2H.sub.2O, and H.sub.3PO.sub.4 were placed as raw materials at a molar ratio of 3:1:1 in an autoclave batch reactor, and the materials were reacted with one another at an elevated internal temperature of the reactor of 250° C. for about 10 hours to synthesize LiFePO.sub.4.

(51) Sucrose was added to the slurry containing LiFePO.sub.4 thus obtained in the same manner as Example 1, followed by spray-drying and baking under a nitrogen atmosphere to obtain a LiFePO.sub.4 powder as a final product.

(52) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 2.5 wt % and the content of sulfur was found to be 0.01 wt %.

Comparative Example 2

(53) LiOH—H.sub.2O, FeSO.sub.4-7H.sub.2O, and H.sub.3PO.sub.4 were placed as raw materials at a molar ratio of 3:1:1 in an autoclave batch reactor, and the materials were reacted with one another at an elevated internal temperature of the reactor of 220° C. for 12 hours to synthesize LiFePO.sub.4. The product was washed several times to remove remaining sulfur.

(54) Sucrose was added to the slurry containing LiFePO.sub.4 thus obtained, in the same manner as Example 1, followed by spray-drying and baking under a nitrogen atmosphere to obtain a LiFePO.sub.4 powder as a final product.

(55) The LiFePO.sub.4 powder thus obtained was subjected to C&S analysis to measure contents of carbon and sulfur. As a result, the content of carbon was found to be 1.1 wt % and the content of sulfur was found to be 0.08 wt %.

Test Example 1

(56) Coin cells including cathodes, Li metal anodes and separators using LiFePO.sub.4 powders prepared in Examples 1 to 11 and Comparative Examples 1 and 2 as cathode active materials were produced. The coin cells thus produced were subjected to a rate-limiting property test (2 C/0.1 C, %). The results are shown in the following Table 1.

(57) TABLE-US-00001 TABLE 1 Rate-limiting property (2C/0.1C, %) Ex. 1 93 Ex. 2 91 Ex. 3 92 Ex. 4 91 Ex. 5 94 Ex. 6 93 Ex. 7 92 Ex. 8 91 Ex. 9 91 Ex. 10 92 Ex. 11 91 Comp. Ex. 1 89 Comp. Ex. 2 88

(58) As can be seen from Table 1 above, LiFePO.sub.4 powders of Examples 2 to 4, 6 to 8 and 10 to 11 exhibited superior electrochemical properties in spite of small amount of carbon, as compared to the LiFePO.sub.4 powder of Comparative Example 1. In addition, the LiFePO.sub.4 powder of Comparative Example 2 exhibited similar electrochemical properties in spite of small amount of carbon, as compared to the LiFePO.sub.4 powder of Comparative Example 1, but the electrochemical properties thereof were slightly deteriorated, as compared to those of Examples.

Test Example 2

(59) The LiFePO.sub.4 powders prepared in Example 1 and Comparative Example 1 were added to a 500 mL beaker, 200 mL of water was added thereto, the mixture was vigorously stirred for 5 minutes and allowed to stand for 10 minutes, and variation in state was observed. FIGS. 1 and 2 show images showing the variations.

(60) As can be seen from FIG. 1, the LiFePO.sub.4 powder of Example 1 according to the present invention had a structure in which carbon was strongly coated on the surface of LiFePO.sub.4 particles, which means that LiFePO.sub.4 particles and carbon were not separated. On the other hand, as shown in FIG. 2, it could be confirmed by XRD analysis that the LiFePO.sub.4 powder of Comparative Example 1 has a structure in which LiFePO.sub.4 particles were completely separated from carbon and gray LiFePO.sub.4 was present in the form of a powder. XRD analysis data is shown in FIG. 3.

(61) Accordingly, the LiFePO.sub.4 powder of the present invention exhibited considerably superior electrochemical properties, although it contains the same amount of carbon and higher carbon coating strength, thus contributing to superior cycle characteristics during battery production.

(62) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

(63) As apparent from the afore-going, the olivine-type lithium iron phosphate according to the present invention has a structure in which carbon is coated on lithium iron phosphate containing sulfur, thus advantageously enabling formation of a uniform thin film coating on the surface of the lithium iron phosphate, being not readily separated in the process of fabricating electrodes and exhibiting superior conductivity and density.