LITHIUM IRON PHOSPHATE COMPOSITE MATERIAL, PREPARATION METHOD AND USE

Abstract

The present application provides a lithium iron phosphate composite material, a preparation method and use. The lithium iron phosphate composite material includes a core and a shell coated on the core, in particular, the core is Li.sub.6MnO.sub.4, and the shell is carbon-coated lithium iron phosphate. The lithium iron phosphate composite material provided by the present application adopts Li.sub.6MnO.sub.4 as the positive electrode lithium supplement material, and solves problems of active lithium loss and capacity depletion under high-rate charge and discharge of lithium iron phosphate positive electrode, thereby improving the rate performance of the lithium iron phosphate materials and the cycle life of batteries at high rates.

Claims

1. A preparation method for a lithium iron phosphate composite material, wherein the lithium iron phosphate composite material comprises a core and a shell coated on the core, the core is Li.sub.6MnO.sub.4, and the shell is carbon-coated lithium iron phosphate; the preparation method for the lithium iron phosphate composite material comprises: preparing a carbon-coated lithium iron phosphate precursor sol is prepared by adopting a sol-gel process, mixing a Li.sub.6MnO.sub.4 precursor and the carbon-coated lithium iron phosphate precursor sol under stirring until a solvent is completely evaporated to obtain a wet material, and calcining the wet material to obtain the lithium iron phosphate composite material.

2. The preparation method according to claim 1, wherein a mass ratio of the core to the shell is (0.005-0.05):1.

3. The preparation method according to claim 1, wherein a preparation process for the Li.sub.6MnO.sub.4 precursor comprises: weighing a manganese source and a first lithium source according to a stoichiometric ratio in Li.sub.6MnO.sub.4, respectively, mixing the manganese source and the first lithium source, ball milling and pre-calcining in sequence to obtain the Li.sub.6MnO.sub.4 precursor.

4. The preparation method according to claim 3, wherein the first lithium source comprises any one or a combination of at least two selected from the group consisting of lithium hydroxide, lithium carbonate, lithium acetate, and lithium nitrate.

5. The preparation method according to claim 3, wherein the manganese source comprises any one or a combination of at least two selected from the group consisting of manganese hydroxide, manganese acetate, and manganese monoxide.

6. The preparation method according to claim 3, wherein the ball milling is performed at a rotation speed of 1000-3500 r/min, and the ball milling is performed for a period of 0.5 to 2 hours.

7. The preparation method according to claim 3, wherein the pre-calcining is performed at a temperature of 300-400 C., the pre-calcining is performed for a period of 1 to 3 hours, and the pre-calcining is performed in a nitrogen atmosphere or an argon atmosphere.

8. The preparation method according to claim 1, wherein a preparation process for the carbon-coated lithium iron phosphate precursor sol comprises: mixing an iron source and a solvent, stirring and dispersing to obtain a dispersion solution, adding a carbon source, a chelating agent, a phosphorus source and a second lithium source under stirring, and warming for a period of time, to obtain the carbon-coated lithium iron phosphate precursor sol.

9. The preparation method according to claim 8, wherein the iron source comprises any one or a combination of at least two selected from the group consisting of ferric nitrate, ferrous sulfate and ferrous oxalate.

10. The preparation method according to claim 8, wherein the solvent comprises deionized water.

11. The preparation method according to claim 8, wherein the carbon source comprises any one or a combination of at least two selected from the group consisting of glucose, sucrose, and polyethylene glycol.

12. The preparation method according to claim 8, wherein the chelating agent comprises polyethylene glycol and/or citric acid.

13. The preparation method according to claim 8, wherein the phosphorus source comprises any one or a combination of at least two selected from the group consisting of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

14. The preparation method according to claim 8, wherein the second lithium source comprises any one or a combination of at least two selected from the group consisting of lithium hydroxide, lithium acetate, lithium nitrate, and lithium dihydrogen phosphate.

15. The preparation method according to claim 8, wherein a molar ratio of Li element in the second lithium source, to Fe element in the iron source, to P element in the phosphorus source is (1.001-1.01):1:1.

16. The preparation method according to claim 8, wherein a mass ratio of a total of the second lithium source, the iron source and the phosphorus source to the carbon source is 1:(0.01-0.05).

17. The preparation method according to claim 8, wherein a mass ratio of a total of the second lithium source, the iron source and the phosphorus source to the chelating agent is 1:(0.01-0.05).

18. The preparation method according to claim 8, wherein a temperature for the warming is 50-70 C., and the period of time for the warming is 5 to 10 hours.

19. The preparation method according to claim 1, wherein the mixing and stirring are performed at a temperature of 70-90 C.

20. The preparation method according to claim 1, wherein the calcining is a gradient calcining, a process of the gradient calcining comprises: heating materials to a temperature of T.sub.1 and maintaining the temperature of T.sub.1 for a period of time H.sub.1, and then heating materials to a temperature of T.sub.2 and maintaining the temperature of T.sub.2 for a period of time H.sub.2, wherein the T.sub.1 is 350-390 C. the H.sub.1 is 12 to 25 minutes, the T.sub.2 is 600-770 C. and the H.sub.2 is 0.5-5h.

Description

BRIEF DESCRIPTION OF DRA WINGS

[0051] FIG. 1 is an SEM image of the lithium iron phosphate composite material prepared in Example 1;

[0052] FIG. 2 is a TEM image of the lithium iron phosphate composite material prepared in Example 1;

[0053] FIG. 3 is a graph showing the discharge capacity retention rate of Example 1 and Comparative example under 25 C. and 3C rate condition; and

[0054] FIG. 4 is a discharge capacity diagram of Example 1 and Comparative example at 10 C. and different rate conditions.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0055] The technical solution of the present application will be further described below with reference to specific embodiments.

Example 1

[0056] This example provides a preparation method for a lithium iron phosphate composite material. The preparation method includes: [0057] (1) Preparation of Li.sub.6MnO.sub.4 precursor: lithium hydroxide and manganese monoxide with molar ratio Li:Mn=6:1 were weighed, mixed and then ball-milled with a high-speed ball mill at 3500 r/min for 0.5 h, pre-sintered at 400 C. for 1 h to obtain the Li.sub.6MnO.sub.4 precursor, ready for use; [0058] (2) Preparation of carbon-coated lithium iron phosphate precursor sol: ferrous oxalate was added to deionized water and stirred for dispersion to obtain a dispersion. Under stirring conditions, sucrose, polyethylene glycol, ammonium dihydrogen phosphate and lithium hydroxide were added in sequence, and the dispersion was kept at a temperature of 70 C. for 5 h to obtain a carbon-coated lithium iron phosphate precursor sol, in which the molar ratio of Li:Fe:P in lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate was 1.01:1:1, the mass ratio of the total mass of lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate to glucose is 1:0.05, and the mass ratio of the total mass of lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate to polyethylene glycol is 1:0.05; and [0059] (3) Preparation of lithium iron phosphate composite material: the Li.sub.6MnO.sub.4 precursor prepared in step (1) was added to the carbon-coated lithium iron phosphate precursor sol prepared in step (2), and the solution was kept at a temperature of 90 C., and the stirring was continued until the solvent was sufficiently evaporated to obtain a wet material; a gradient calcination was performed on the wet material: the wet material was heated to 390 C. in a high-purity nitrogen atmosphere, and kept the temperature for 12 minutes; then heated to 770 C., and kept the temperature for 0.5h, to obtain the lithium iron phosphate composite material.

[0060] The lithium iron phosphate composite material has a Li.sub.6MnO.sub.4 core and a carbon-coated lithium iron phosphate outer coating. The mass ratio of Li.sub.6MnO.sub.4 to carbon-coated lithium iron phosphate is 0.05:1. In particular, FIG. 1 is an SEM image of the prepared lithium iron phosphate composite material. It can be seen from FIG. 1 that the particle size of the composite material is relatively uniform, and the particle size is basically within 1 m. FIG. 2 is a TEM image of the prepared lithium iron phosphate composite material. It can be seen from FIG. 2 that the thickness of the outer coating of carbon-coated lithium iron phosphate is about 1.5 to 2 nm, and the coating layer is relatively uniform.

[0061] The physical and chemical indicators of the prepared lithium iron phosphate composite material were tested by conventional technical means. The results are shown in Table 1 below:

TABLE-US-00001 TABLE 1 Carbon Indicators D10 D50 D90 Dmax content Numeral 0.13 m 0.25 m 0.42 m 0.59 m 1.52% pH SSA TD Compaction Size of the Lattice density of the grain volume powder 9.25 13.95 0.83 2.39 g/cc 102.7 nm 290.65 .sup.{circumflex over ()}3 m.sup.2/g g/cm.sup.3

Example 2

[0062] This example provides a preparation method for a lithium iron phosphate composite material. The preparation method includes: [0063] (1) Preparation of Li.sub.6MnO.sub.4 precursor: lithium hydroxide and manganese hydroxide with molar ratio Li:Mn=6:1 were weighed, mixed and then ball-milled with a high-speed ball mill at 1000 r/min for 2 h, pre-sintered at 300 C. for 3 h to obtain the Li.sub.6MnO.sub.4 precursor, ready for use; [0064] (2) Preparation of carbon-coated lithium iron phosphate precursor sol: iron nitrate was added to deionized water and stirred for dispersion to obtain a dispersion. Under stirring conditions, sucrose, polyethylene glycol, ammonium dihydrogen phosphate and lithium dihydrogen phosphate were added in sequence, and the dispersion was kept at a temperature of 50 C. for 10 hours to obtain a carbon-coated lithium iron phosphate precursor sol, in which the molar ratio of Li:Fe:P in lithium dihydrogen phosphate, iron nitrate and ammonium dihydrogen phosphate was 1.001:1:1, the mass ratio of the total mass of lithium dihydrogen phosphate, iron nitrate and ammonium dihydrogen phosphate to glucose is 1:0.01, and the mass ratio of the total mass of lithium dihydrogen phosphate, iron nitrate, and ammonium dihydrogen phosphate to polyethylene glycol is 1:0.01; [0065] (3) Preparation of lithium iron phosphate composite material: the Li.sub.6MnO.sub.4 precursor prepared in step (1) was added to the carbon-coated lithium iron phosphate precursor sol prepared in step (2), and the solution was kept at a temperature of 70 C., and the stirring was continued until the solvent was sufficiently evaporated to obtain a wet material; a gradient calcination was performed on the wet material: the wet material was heated to 350 C. in a high-purity nitrogen atmosphere, and kept the temperature for 25 minutes; then heated to 600 C., and kept the temperature for 5h, to obtain the lithium iron phosphate composite material.

[0066] The lithium iron phosphate composite material has a Li.sub.6MnO.sub.4 core and a carbon-coated lithium iron phosphate outer coating. The mass ratio of Li.sub.6MnO.sub.4 to carbon-coated lithium iron phosphate is 0.005:1.

Example 3

[0067] This example provides a preparation method for a lithium iron phosphate composite material. The preparation method includes: [0068] (1) Preparation of Li.sub.6MnO.sub.4 precursor: lithium hydroxide and manganese monoxide with molar ratio Li:Mn=6:1 were weighed, mixed and then ball-milled with a high-speed ball mill at 2400 r/min for 1 h, pre-sintered at 360 C. for 2 h to obtain the Li.sub.6MnO.sub.4 precursor, ready for use; [0069] (2) Preparation of carbon-coated lithium iron phosphate precursor sol: ferrous oxalate was added to deionized water and stirred for dispersion to obtain a dispersion. Under stirring conditions, sucrose, citric acid, ammonium dihydrogen phosphate and lithium nitrate were added in sequence, and the dispersion was kept at a temperature of 60 C. for 7 hours to obtain a carbon-coated lithium iron phosphate precursor sol, in which the molar ratio of Li:Fe:P in lithium nitrate, ferrous oxalate and ammonium dihydrogen phosphate is 1.005:1:1, the mass ratio of the total mass of ammonium dihydrogen phosphate, ferrous oxalate and ammonium dihydrogen phosphate to sucrose is 1:0.02, and the mass ratio of the total mass of ammonium dihydrogen phosphate, ferrous oxalate and ammonium dihydrogen phosphate to polyethylene glycol is 1:0.03; [0070] (3) Preparation of lithium iron phosphate composite material: the Li.sub.6MnO.sub.4 precursor prepared in step (1) was added to the carbon-coated lithium iron phosphate precursor sol prepared in step (2), and the solution was kept at a temperature of 80 C., and the stirring was continued until the solvent was sufficiently evaporated to obtain a wet material; a gradient calcination was performed on the wet material: the wet material was heated to 370 C. in a high-purity nitrogen atmosphere, and kept the temperature for 20 minutes; then heated to 700 C., and kept the temperature for 2h, to obtain the lithium iron phosphate composite material.

[0071] The lithium iron phosphate composite material has a Li.sub.6MnO.sub.4 core and a carbon-coated lithium iron phosphate outer coating. The mass ratio of Li.sub.6MnO.sub.4 to carbon-coated lithium iron phosphate is 0.01:1.

Comparative Example

[0072] This comparative example provides a preparation method for a lithium iron phosphate composite material. The preparation method includes: [0073] (1) Preparation of carbon-coated lithium iron phosphate precursor sol: ferrous oxalate was added to deionized water and stirred for dispersion to obtain a dispersion. Under stirring conditions, sucrose, polyethylene glycol, ammonium dihydrogen phosphate and lithium hydroxide were added in sequence, and the dispersion was kept at a temperature of 50 C. for 5 h to obtain a carbon-coated lithium iron phosphate precursor sol, in which the molar ratio of Li:Fe:P in lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate was 1.001:1:1, the mass ratio of the total mass of lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate to glucose is 0.01:1, and the mass ratio of the total mass of lithium hydroxide, iron nitrate and ammonium dihydrogen phosphate to polyethylene glycol is 0.01:1; and [0074] (2) the carbon-coated lithium iron phosphate precursor prepared in step (1) was stirred continuously at 70 C. until the solvent was sufficiently evaporated to obtain a wet material; a gradient calcination was performed on the wet material: the wet material was heated to 350 C. in a high-purity nitrogen atmosphere, and kept the temperature for 12 minutes; then heated to 600 C., and kept the temperature for 0.5h, to obtain the carbon-coated lithium iron phosphate material.

[0075] The lithium iron phosphate materials prepared in Examples 1-3 and Comparative example were tested for electrochemical performance in lithium ion batteries. The results are shown in Table 2. FIG. 3 is a comparison graph showing the discharge capacity retention rate of Example 1 and Comparative example under 25 C. and 3C rate condition. FIG. 4 is a comparison discharge capacity diagram of Example 1 and Comparative example at 10 C. and different rate conditions.

TABLE-US-00002 TABLE 2 Discharge capacity retention rate for Discharge Discharge Discharge 400 cycles under capacity capacity capacity 25 C. and 3 C at 10 C. at 10 C. at 10 C. Examples (%) 1 C (mAh) 2 C (mAh) 3 C (mAh) Example 1 95.71 108.16 107.75 104.65 Example 2 95.9 106.14 105.93 102.18 Example 3 95.3 108.02 106.95 103.97 Comparative 91.82 97.36 92.97 85.05 example

[0076] It can be seen from the data in Table 1 that the capacity retention rate and discharge capacity at different discharge rates of the carbon-coated lithium iron phosphate positive electrode material prepared in Comparative example are smaller than those in the Examples.

[0077] As can be seen from FIG. 3, under 3C (25 C. 2.0-3.65V) condition, as to the battery performance in Example 1, the discharge capacity retention rate for 400 cycles is 95.71%. However, as to the battery performance in the lithium iron phosphate material of the Comparative example, the discharge capacity retention rate for 400 cycles is only 91.82%. As can be seen from FIG. 4, under 10 C. (1C/2C/3C 2.0-3.65V) condition, as to the battery performance in Example 1, the discharge capacity at 1C is 108.16 mAh, the discharge capacity at 2C is 107.75 mAh, and the discharge capacity at 3C is 104.65 mAh. However, as to the battery performance in the lithium iron phosphate material of the Comparative example, the discharge capacity at 1C is 97.36 mAh, the discharge capacity at 2C is 92.97 mAh, and the discharge capacity at 3C is 85.05 mAh, indicating that the lithium iron phosphate composite material prepared in Example 1 has good rate performance and cycle performance.

[0078] In summary, in the preparation method of the present application, nanoscale lithium iron phosphate precursor is prepared by the sol-gel method, nanoscale lithium iron phosphate is evenly coated on Li.sub.6MnO.sub.4 as the shell. Adopting Li.sub.6MnO.sub.4 as the positive electrode lithium supplement material to solve problems of active lithium loss and capacity depletion under high-rate charge and discharge of lithium iron phosphate positive electrode, thereby improving the rate performance of the lithium iron phosphate materials and the cycle life of batteries at high rates. Moreover, by adopting the preparation method of the present application, the temperature and period required for sintering of lithium iron phosphate materials can be effectively reduced, which decreases the production energy consumption and production cycle.

[0079] It is declared by the applicant that the above are only specific embodiments of the present application, the protection scope of the present application is not limited thereto. Those skilled in the art should understand that any changes or substitutions that easily occurs to any person skilled in the art in the technical scope of the present application fall within the protection scope and disclosure scope of the present application.