A preparation method of a lithium iron phosphate composite material

Abstract

A preparation method of a lithium iron phosphate composite material comprises the following steps: adding an appropriate amount of graphene oxide into deionized water to prepare an aqueous graphene oxide solution; adding high-purity fine iron powder and oxalic acid into the aqueous graphene oxide solution, and reacting at a certain pH, reaction temperature, and stirring rate to obtain a mixed solution A; adding a dopant, a phosphorus source, a lithium source, and a carbon source into solution A, and uniformly mixing to obtain a solid-liquid mixture B; spray-drying the solid-liquid mixture B, pre-burning at a certain temperature for a period of time to obtain a solid powder C; refining the solid powder C by a sand mill, and sintering the treated powder at a certain temperature; the finished product obtained after the sintered powder is subjected to air flow pulverization, classification, and iron removal by sieving is the lithium iron phosphate composite material. Through the synergistic effect of the surface carbon layer mesoporous structure and the uniform graphene coating layer in the present invention, the lithium battery prepared by using the lithium iron phosphate composite material has high power, excellent low-temperature discharge performance, and cycling performance.

Description

Technical Field

[0001] The present invention relates to the field of cathode materials for lithium batteries, and specifically relates to a preparation method of a lithium iron phosphate composite material. Background Art

[0002] Currently, cathode materials widely used in lithium-ion batteries include lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), lithium manganese oxide (LiMn2O4), lithium nickel cobalt oxide (LiNixCo1-xO2), lithium nickel cobalt manganese oxide (LiNixCoyMn1-x-yO2), etc. Compared with other cathode materials, LiFePO4 has advantages such as a higher discharge specific capacity, a relatively stable discharge platform, excellent cycle stability, thermal stability, and low price. However, the olivine structure of LiFePO4 itself determines its low electronic conductivity and low lithium ion diffusion rate, resulting in defects such as poor rate performance and poor low-temperature discharge performance of LiFePO4 materials, making it difficult to meet the power characteristic indicators of power lithium-ion batteries.

[0003] Aiming at the defects of poor low-temperature discharge performance and poor power performance of LiFePO4 materials, currently, mainly methods such as particle nanosizing and surface carbon coating are used to improve their low-temperature discharge performance and rate performance. Conventional carbon coating is mainly achieved by introducing an organic carbon source, mixing it evenly with lithium source, iron source, and phosphorus source, and then sintering in an inert atmosphere at a temperature of 600 - 800 °C. The carbon coating is realized by the pyrolysis carbonization of the organic carbon source at high temperature. However, due to the low sintering temperature of LiFePO4 materials, the carbon atoms in the carbon coating layer have low order, and it is difficult for Li ions to diffuse between the carbon coating layers at low temperature or ultra-low temperature.

[0004] Due to its special two-dimensional structure, graphene has excellent electronic conductivity and a high specific surface area. Its composite with LiFePO4 materials can greatly improve the electronic conductivity of LiFePO4 materials and improve the low-temperature performance and power performance of LiFePO4 materials. Based on the above advantages, graphene coating has become a research hotspot for the surface modification of LiFePO4 materials. Currently, the coating of graphene is carried out by mixing graphene with LiFePO4 materials in a liquid phase, adding an organic surface modifier, and then performing high-temperature sintering, such as Chinese patents CN201110222958.1, CN201911318904.8, etc. This will cause voids between the graphene coating layer and the material particle body, and the graphene coating layer is extremely easy to fall off from the material body during the battery cycle process. These will all hinder electron conduction and lithium ion diffusion, deteriorating the battery performance. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a preparation method of a lithium iron phosphate composite material, which uses simple and easily available raw materials, and the synthesis conditions are simple and controllable. The lithium battery made of the lithium iron phosphate composite material of the present invention has high power and excellent low-temperature discharge performance.

[0006] The technical solution of the present invention is as follows: A preparation method of a lithium iron phosphate composite material, the steps are as follows:

[0007] (1) Add an appropriate amount of graphene oxide to deionized water to prepare an aqueous graphene oxide solution;

[0008] (2) Then add high-purity fine iron powder and an appropriate amount of oxalic acid to the graphene oxide solution, and react for a period of time under certain pH, reaction temperature, and stirring rate to obtain a mixed solution A;

[0009] (3) Add an appropriate amount of dopant, phosphorus source, lithium source, and carbon source to solution A, and after mixing evenly, obtain a solid-liquid mixture B;

[0010] (4) After spray-drying the solid-liquid mixture B, pre-burn it at a certain temperature for a period of time to obtain a solid powder C;

[0011] (5) Refine the solid powder C by a sand mill, and sinter the treated powder at a certain temperature;

[0012] (6) The finished product obtained after the sintered powder is subjected to air flow crushing, classification, and iron removal by sieving is the lithium iron phosphate composite material of the present invention.

[0013] The present invention uses high-purity fine iron powder as the iron source, and at the same time, the high-purity fine iron powder is used as the reducing agent of graphene oxide. During the whole reduction process, graphene oxide obtains hydrogen ions from the carboxylic acid functional group of oxalic acid and electrons from the reaction of iron powder and oxalic acid. Therefore, graphene oxide is gradually reduced to graphene by hydrogen ions and electrons. After the iron powder reduces graphene oxide, it will gradually become divalent or trivalent iron ions. The divalent or trivalent iron ions have an electrostatic adsorption effect with negatively charged graphene, and the iron ions adhere to the surface of graphene and become the nucleation points of lithium iron phosphate during the subsequent sintering process. The main body of the lithium iron phosphate material is in close contact with the graphene coating layer. At the same time, since the iron ions are evenly distributed on the surface of graphene, there is no need to add additional dispersants or surfactants, and graphene can evenly coat the surface of the lithium iron phosphate material. During the pre-burning process of the carbon source, while non-carbon elements volatilize, a certain mesoporous structure will be formed. The mesoporous structure can enhance the ability of the material to absorb the electrolyte and increase the lithium ion diffusion coefficient on the surface of the material. Under the synergistic effect of the mesoporous structure of the surface carbon layer and the uniform graphene coating layer, the prepared lithium iron phosphate material has excellent low-temperature discharge performance, power performance, and cycle performance.

[0014] Further, the graphene oxide in step (1) is prepared by the Hummers method. The diameter size of the graphene oxide nanosheets is 200 - 800 nm, the number of layers of each graphene oxide sheet is 1 - 3 layers, the concentration of the graphene oxide aqueous solution is 10 - 50 g / L, and the I D / I G intensity ratio is 0.8 - 1.2 (I D 、I G are the intensities of the D peak and the G peak in the Raman spectrum, and the larger the I D / I G value, the more the number of oxygen-containing functional groups in the graphene oxide).

[0015] Further, the Fe content of the high-purity fine iron powder in step (2) is greater than 99.9%; the particle size of the high-purity fine iron powder is less than 500 mesh.

[0016] Further, the mass ratio of the graphene oxide in step (1) to the high-purity fine iron powder in step (2) is 1:7 - 18.

[0017] Further, the pH in step (2) is 2 - 5, the reaction temperature is 70 - 100 °C, the stirring rate is 400 - 1000 rpm, and the reaction time is 30 - 120 min.

[0018] Further, the dopant in step (3) is one or more of TiO2, MgO, ZrO2, and Al2O3; the phosphorus source is one of (NH4)3PO4, NH4H2PO4, and (NH4)2HPO4; the lithium source is one of Li2CO3, LiOH, and LiNO3; the carbon source is one of glucose, sucrose, and starch.

[0019] Further, in the solid-liquid mixture B in step (3), the molar ratio of P:Li:Fe is 1.03 - 1.06:1.02 - 1.1:1, the dopant addition amount is 0.2% - 2% of the total mass sum of the high-purity fine iron powder, the phosphorus source, and the lithium source; the carbon source addition amount is 10% - 30% of the total mass sum of the high-purity fine iron powder, the phosphorus source, and the lithium source.

[0020] Further, the pre-burning temperature in step (4) is 200 °C - 300 °C, and the pre-burning time is 1 - 4 h.

[0021] Further, after the refinement treatment in step (5), the D50 of the powder is 0.2 - 2 μm, the powder sintering temperature is 600 - 800 °C, and the sintering time is 6 - 12 h.

[0022] Further, the D50 of the lithium iron phosphate composite material described in step (6) is 0.5 μm to 2.5 μm, the specific surface area is greater than 20 m² / g, and the discharge specific capacity at 10C is greater than 138 mAh / g.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The raw materials are simple and easy to obtain, and the uniform coating of graphene can be completed without adding additional dispersants or surfactants. 2. The high-purity fine iron powder can be used as both an iron source and a reducing agent for graphene oxide, without introducing additional reducing agents. 3. After the iron powder reduces graphene oxide, it will gradually become divalent or trivalent iron ions, and the divalent or trivalent iron ions will have an electrostatic adsorption effect with the negatively charged graphene. The iron ions adhere to the surface of the graphene and become the nucleation points of lithium iron phosphate during the subsequent sintering process. The lithium iron phosphate material body is in close contact with the graphene coating layer. 4. The lithium battery prepared from the lithium iron phosphate material of the present invention has high power, excellent low-temperature discharge performance and cycling performance. Description of the Drawings

[0024] Figure 1 SEM image of the lithium iron phosphate composite material prepared according to Example 1 of the present invention.

[0025] Figure 2 TEM image of the lithium iron phosphate composite material prepared according to Example 1 of the present invention.

[0026] Figure 3 XRD pattern of the lithium iron phosphate composite material prepared according to Example 1 of the present invention.

[0027] Figure 4 Low-temperature pulse discharge performance comparison chart of lithium batteries made of the lithium iron phosphate composite materials prepared in Comparative Example 1 and Example 1.

[0028] Figure 5 10C cycling performance comparison of lithium batteries made of the lithium iron phosphate composite materials prepared in Comparative Example 1 and Example 1. Detailed Embodiments

[0029] The present invention will be further described in detail below with reference to specific embodiments, so that those skilled in the art can understand the present invention more clearly.

[0030] The following embodiments are only used to illustrate the present invention, but not to limit the scope of the present invention. Based on the specific embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0031] In the embodiments of the present invention, unless otherwise specified, all raw material components are commercially available products well-known to those skilled in the art; in the embodiments of the present invention, unless specifically specified, the technical means used are conventional means well-known to those skilled in the art.

[0032] Example 1

[0033] The preparation steps are as follows:

[0034] (1) Weigh 14 g of graphene oxide dry powder, add 1000 g of deionized water, and obtain a graphene oxide aqueous solution after high-speed stirring for 60 min;

[0035] (2) Weigh 140 g of high-purity fine iron powder, add the iron powder to the graphene oxide aqueous solution, then add an appropriate amount of oxalic acid, adjust the pH of the solution to 3.5, and stir the solution at 90 °C and a stirring speed of 800 rpm for 60 min to obtain a reduced graphene oxide solution;

[0036] (3) Weigh 230 g of NH4H2PO4, 77.7 g of Li2CO3, 98.4 g of sucrose, and 5.28 g of nano-sized MgO. Add the weighed materials to the above-mentioned reduced graphene oxide solution and mix evenly at room temperature to obtain a mixed material;

[0037] (4) After spray-drying the mixed material in step (3), pre-bake it at 260 °C for 2 h;

[0038] (5) Subject the pre-baked product in step (4) to sanding and refining treatment, and control the D50 of the refined material at 0.8 μm; after sintering the refined material at 700 °C for 8 h, naturally cool it to room temperature;

[0039] (6) After subjecting the material in step (5) to air-flow pulverization and iron removal by sieving, the lithium iron phosphate composite material can be obtained. The prepared lithium iron phosphate composite material has D50 = 0.9 μm, a specific surface area of 22 m² / g, and a discharge specific capacity of 141 mAh / g at 10C.

[0040] Figure 1 is the SEM image of the lithium iron phosphate composite material prepared according to Example 1 of the present invention. It can be seen from Figure 1 that the lithium iron phosphate composite material prepared by the present invention is of nano-scale size, with uniform particle size and good particle dispersibility.

[0041] Figure 2 is the TEM image of the lithium iron phosphate composite material prepared according to Example 1 of the present invention. It can be seen from Figure 2 that the lithium iron phosphate composite material prepared by the present invention has a core-shell structure, with an amorphous carbon layer coated on the body of the lithium iron phosphate material, and a graphene layer uniformly coated on the outermost layer.

[0042] Example 2

[0043] The preparation steps are as follows:

[0044] (1) Weigh 25 g of graphene oxide dry powder, add 1500 g of deionized water, and obtain a graphene oxide aqueous solution after high-speed stirring for 90 min;

[0045] (2) Weigh 200 g of high-purity fine iron powder, add the iron powder into the graphene oxide aqueous solution, then add an appropriate amount of oxalic acid to adjust the pH of the solution to 3, and stir the solution at 90 °C and a stirring speed of 900 rpm for 90 min to obtain a reduced graphene oxide solution;

[0046] (3) Weigh 414.8 g of (NH4)2HPO4, 38 g of LiOH, 178 g of starch, 5.42 g of nano-TiO2, and 4.14 g of nano-ZrO2. Add the weighed materials into the above-mentioned reduced graphene oxide solution and mix evenly at room temperature to obtain a mixed material;

[0047] (4) After spray-drying the mixed material in step (3), pre-burn it at 220 °C for 2 h;

[0048] (5) Grind the pre-burned product in step (4) by sanding, and control the D50 of the material after grinding to 0.6 μm; after sintering the material after grinding at 650 °C for 10 h, naturally cool it to room temperature;

[0049] (6) After air-flow pulverizing and iron removal by sieving the material in step (5), the lithium iron phosphate composite material can be obtained. The prepared lithium iron phosphate composite material has D50 = 0.72 μm, a specific surface area of 32 m² / g, and a discharge specific capacity of 145 mAh / g at 10C.

[0050] Example 3

[0051] The preparation steps are as follows:

[0052] (1) Weigh 40 g of graphene oxide dry powder, add 2000 g of deionized water, and obtain a graphene oxide aqueous solution after high-speed stirring for 60 min;

[0053] (2) Weigh 400 g of high-purity fine iron powder, add the iron powder into the graphene oxide aqueous solution, then add an appropriate amount of oxalic acid to adjust the pH of the solution to 3, and stir the solution at 90 °C and a stirring speed of 900 rpm for 120 min to obtain a reduced graphene oxide solution;

[0054] (3) Weigh 912.6 g of (NH4)3PO4, 291.2 g of LiNO3, 178 g of glucose, 6.74 g of nano-TiO2, and 8.78 g of nano-Al2O3. Add the weighed materials into the above-mentioned reduced graphene oxide solution and mix evenly at room temperature to obtain a mixed material;

[0055] (4) After spray-drying the mixed materials in step (3), pre-calcine them at 280 °C for 3 h;

[0056] (5) Subject the pre-calcined product in step (4) to fine grinding treatment by sand grinding, and control the D50 of the material after the fine grinding treatment to be 0.8 μm; after sintering the material after the fine grinding treatment at 750 °C for 8 h, naturally cool it to room temperature;

[0057] (6) After subjecting the material in step (5) to air-flow pulverization and sieving to remove iron, the lithium iron phosphate composite material can be obtained. The prepared lithium iron phosphate composite material has D50 = 0.85 μm, a specific surface area of 24 m² / g, and a discharge specific capacity of 142 mAh / g at 10C.

[0058] Comparative Example 1

[0059] The preparation steps are as follows:

[0060] (1) Weigh anhydrous iron phosphate, lithium carbonate, and glucose in a mass ratio of 1:0.25:0.11, and then weigh nano-MgO at 1.5% of the total mass of anhydrous iron phosphate and lithium carbonate, and add an appropriate proportion of water and stir to disperse evenly;

[0061] (2) Add graphene oxide at 3% of the total mass of anhydrous iron phosphate and lithium carbonate to the slurry in step (1), and stir to disperse for 90 min;

[0062] (3) Grind with a sand mill, and control the grinding particle size D50 of the slurry to be 0.45 μm;

[0063] (4) Use a spray dryer for spray drying;

[0064] (5) Sinter the dried powder at 780 °C for 8 hours, naturally cool it to room temperature, and after subjecting the powder to air-flow pulverization and sieving to remove iron, a conventional graphene-coated lithium iron phosphate powder is obtained, with a particle size D50 of 0.8 μm, a specific surface area of 12 m² / g, and a discharge specific capacity of 115 mAh / g at 10C.

[0065] Figure 4 It is a comparison chart of the low-temperature pulse discharge performance of lithium batteries made of the lithium iron phosphate composite materials prepared in Comparative Example 1 and Example 1. From Figure 4 It can be seen that the cut-off voltage of the low-temperature pulse discharge performance of the lithium battery made of the lithium iron phosphate material prepared in Comparative Example 1 is the lowest, and the power performance under low-temperature conditions is the worst; the low-temperature pulse discharge performance of Example 1 is very excellent.

[0066] Figure 5 It is a comparison of the 10C cycle performance of lithium batteries made of the lithium iron phosphate composite materials prepared in Comparative Example 1 and Example 1. From Figure 5It can be seen that the lithium battery made of the lithium iron phosphate material prepared in Comparative Example 1 shows a "capacity dive" phenomenon around 1200 cycles, and the 10C / 1C cycle life of Example 1 exceeds 2000 cycles.

Claims

1. A preparation method of a lithium iron phosphate composite material, characterized in that: The steps are as follows: (1) Add an appropriate amount of graphene oxide into deionized water to prepare an aqueous graphene oxide solution; (2) Add an appropriate amount of high-purity fine iron powder and an appropriate amount of oxalic acid into the aqueous graphene oxide solution, and react for a period of time under certain pH, reaction temperature, and stirring rate to obtain a mixed solution A; the Fe content of the high-purity fine iron powder is greater than 99.9%; the particle size of the high-purity fine iron powder is less than 500 mesh; the mass ratio of the high-purity fine iron powder to graphene oxide is 7-18:1; the pH is 2-5; (3) Add an appropriate amount of dopant, phosphorus source, lithium source, and carbon source into solution A, and mix evenly to obtain a solid-liquid mixture B; (4) After spray-drying the solid-liquid mixture B, pre-burn it at 200°C - 280°C for a period of time to obtain a solid powder C; (5) Refine the solid powder C by a sand mill, and sinter the treated powder at a certain temperature; (6) The finished product obtained after air-flow crushing, classification, and iron removal by sieving of the sintered powder is the lithium iron phosphate composite material.

2. The preparation method of the lithium iron phosphate composite material according to claim 1, characterized in that: In step (1), the sheet diameter size of the graphene oxide is 200-800 nm, the number of layers of each graphene oxide sheet is 1-3 layers, the concentration of the graphene oxide aqueous solution is 10-50 g / L, and the intensity ratio (I D / I G ) of the D peak to the G peak in the Raman spectrum of the graphene oxide is 0.8-1.

2.

3. The preparation method of the lithium iron phosphate composite material according to claim 1, characterized in that: In step (2), the reaction temperature is 70 - 100°C, the stirring rate is 400 - 1000 rpm, and the reaction time is 30 - 120 min.

4. The preparation method of the lithium iron phosphate composite material according to claim 1, wherein: The dopant is one or more of TiO2, MgO, ZrO2, and Al2O3; the phosphorus source is one of (NH4)3PO4, NH4H2PO4, and (NH4)2HPO4; the lithium source is one of Li2CO3, LiOH, and LiNO3; the carbon source is one of glucose, sucrose, and starch.

5. The preparation method of the lithium iron phosphate composite material according to claim 1, characterized in that: In step (3), the addition amount of the dopant is 0.2% - 2% of the total mass of the high-purity fine iron powder, phosphorus source, and lithium source, and the addition amount of the carbon source is 10% - 30% of the total mass of the high-purity fine iron powder, phosphorus source, and lithium source; the molar ratio of P:Li:Fe in the solid-liquid mixture B is 1.03 - 1.06:1.02 - 1.1:

1.

6. The preparation method of the lithium iron phosphate composite material according to claim 1, wherein: In step (4), the pre-burning time is 1 - 4 h.

7. The preparation method of the lithium iron phosphate composite material according to claim 1, characterized in that: After the refinement treatment in step (5), the D50 of the powder is 0.2 - 2 μm, the powder sintering temperature is 600 - 800°C, and the sintering time is 6 - 12 h.

8. The preparation method of the lithium iron phosphate composite material according to claim 1, wherein: In step (6), the D50 of the lithium iron phosphate composite material is 0.5 μm - 2.5 μm, the specific surface area is greater than 20 m² / g, and the discharge specific capacity at 10C is greater than 138 mAh / g.

9. A lithium iron phosphate composite material prepared by the preparation method of the lithium iron phosphate composite material according to any one of claims 1 - 8.

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