Preparation method of high-compaction high-capacity lithium iron phosphate material

Abstract

The invention provides a preparation method of a high-compaction high-capacity lithium iron phosphate material, which comprises the following steps: (1) mixing polyvinyl alcohol, iron phosphate, a lithium source, a carbon source and water to obtain slurry A; mixing iron phosphate, a lithium source, a carbon source and water to obtain slurry B; (2) performing spray drying on the slurry A, and performing low-temperature sintering to obtain a black material C; performing spray drying on the slurry B, and performing high-temperature sintering to obtain a black material D; and (3) mixing the black material C and the black material D, crushing, adding barium titanate, carrying out ball milling, and drying to obtain the lithium iron phosphate material. The polyvinyl alcohol is partially acetylated modified polyvinyl alcohol. The sintering temperature is reduced by adding the modified polyvinyl alcohol to prepare the compact lithium iron phosphate material with smaller particle size, and the lithium iron phosphate material and the lithium iron phosphate material obtained by high-temperature sintering form grading of large and small particles, so that the compaction density of the lithium iron phosphate is improved.

Description

Technical Field

[0001] This invention relates to the technical field of lithium iron phosphate cathode materials, and specifically to a method for preparing high-pressure, high-capacity lithium iron phosphate cathode materials. Background Technology

[0002] With the rapid development of electric vehicles and energy storage, higher demands are being placed on the performance of lithium-ion batteries. Compared with ternary cathode materials, lithium iron phosphate materials have greater advantages in safety and cycle life. However, the specific capacity of existing products is already close to their theoretical energy density, so it is necessary to increase their volumetric capacity by improving their compaction density. Existing technologies use carbon coating to appropriately improve the volumetric capacity of the material, but excessive carbon coating can also hinder the migration of lithium ions between material particles, thereby reducing electrochemical performance.

[0003] Therefore, it is necessary to provide a new method for preparing high-pressure, high-capacity lithium iron phosphate cathode materials to further improve the electrical performance of lithium iron phosphate cathode materials. Summary of the Invention

[0004] To address the above problems, this invention provides a method for preparing high-pressure, high-capacity lithium iron phosphate materials, comprising the following steps: (1) Mix polyvinyl alcohol, iron phosphate, lithium source, carbon source and water to obtain slurry A; Iron phosphate, lithium source, carbon source and water are mixed to obtain slurry B; (2) Slurry A is spray-dried and then sintered at low temperature to obtain black material C; Slurry B is spray-dried and then sintered at high temperature to obtain black material D; (3) Mix black material C and black material D and crush them. Add barium titanate and ball mill and dry to obtain lithium iron phosphate material.

[0005] Furthermore, the polyvinyl alcohol is a partially acetylated modified polyvinyl alcohol.

[0006] Furthermore, the partial acetylation modification method is as follows: polyvinyl alcohol is dissolved in a solvent, acetoacetate is added, and the mixture is reacted at 60-110℃ for 1-8 hours to obtain modified polyvinyl alcohol.

[0007] Further, the lithium source includes one or more of lithium carbonate or lithium hydroxide; the carbon source includes one or more of glucose, fructose, sucrose or polyvinyl alcohol; the molar ratio of iron in the iron phosphate, lithium in the lithium source, and carbon in the carbon source is 1:(1.02-1.06):(0.3-0.5).

[0008] Furthermore, the low-temperature sintering temperature is 600-720℃, the sintering time is 8-11h, and the sintering is carried out in an argon or nitrogen atmosphere.

[0009] Furthermore, the high-temperature sintering temperature is 740-800℃, the sintering time is 8-11h, and the sintering is carried out in an argon or nitrogen atmosphere.

[0010] Furthermore, the mass ratio of the black material C to the black material D is (6-8):(2-4), and the amount of barium titanate added is 0.4%-0.6% of the total mass of the black materials C and D.

[0011] Furthermore, the ball milling speed is 300-400 r / min, and the ball milling time is 1-2 h; the drying temperature is 80-100℃, and the drying time is 8-10 h.

[0012] A high-density, high-capacity lithium iron phosphate material is prepared using the above-described method.

[0013] The above-mentioned high-pressure, high-capacity lithium iron phosphate material is used in cathode materials.

[0014] The beneficial effects of this invention are as follows: 1. Polyvinyl alcohol can pyrolyze at a lower temperature to produce carbon. The resulting carbon layer coats the surface of lithium iron phosphate particles, inhibiting excessively rapid grain growth and agglomeration. The fine and uniform particles shorten the atomic diffusion distance, making solid-phase reactions easier and thus lowering the sintering temperature. At the same time, the acetylated polyvinyl alcohol has some hydroxyl groups replaced, which helps to reduce the concentrated release of gas caused by the rapid decomposition of polyvinyl alcohol during sintering, thereby further reducing the formation of large pores and promoting the formation of a denser lithium iron phosphate structure.

[0015] 2. By adding modified polyvinyl alcohol to lower the sintering temperature, a smaller and denser lithium iron phosphate material is prepared, which forms a particle size distribution with the lithium iron phosphate material obtained by high-temperature sintering, thereby improving the compaction density of lithium iron phosphate.

[0016] 3. Barium titanate has a high dielectric constant, which can create an internal electric field within the material, accelerate ion migration and repel anions, thereby increasing the diffusion rate of lithium ions, reducing the polarization of LFP materials, increasing the charge-discharge specific capacity, and improving rate performance and cycle performance. Attached Figure Description

[0017] Figure 1 Example 1: SEM morphology of lithium iron phosphate cathode material at a magnification of 10k. Detailed Implementation

[0018] The embodiments of the present invention will be described in detail below with reference to the examples. The following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention.

[0019] In the following examples, the molar ratio of iron phosphate, lithium carbonate, and glucose refers to the molar ratio of iron in iron phosphate, lithium in lithium source, and carbon in carbon source.

[0020] Example 1 (1) Weigh polyvinyl alcohol 1788 and dimethyl sulfoxide solvent at a mass ratio of 1:10. Heat the solvent to 100°C and slowly add polyvinyl alcohol to the solvent. Stir for 1 hour until completely dissolved. Slowly add acetoacetate (where the molar ratio of acetoacetate to polyvinyl alcohol structural units is 0.3:1) at 100°C and continue stirring for 6 hours until the reaction is complete. Then slowly pour it into a large amount of anhydrous ethanol to obtain a white gel-like precipitate. Filter the white gel-like precipitate and wash it repeatedly with anhydrous ethanol. Place the washed precipitate in an oven at 80°C and dry for 8 hours to obtain partially acetylated modified polyvinyl alcohol.

[0021] (2) Weigh out two portions of ferric phosphate, lithium carbonate, and glucose in a molar ratio of 1:1.02:0.4. Add 1% of the total mass of modified polyvinyl alcohol to one portion and add deionized water. Place the mixture in a sand mill to obtain slurry A, with a solid content controlled at 38%. After sand milling for a certain period of time, the D50 is measured to be 350 nm. Add deionized water directly to the other portion and place it in a sand mill to obtain slurry B, with a solid content controlled at 38%. After sand milling for a certain period of time, the D50 is measured to be 350 nm. Spray dry slurry A and slurry B respectively, with an outlet air temperature of 80℃.

[0022] (3) Weigh 100g of dried A and put it into a tube furnace under nitrogen protection and heat it to 650℃. Keep it at the temperature for 8h to obtain lithium iron phosphate black material C.

[0023] (4) Weigh 100g of dried B and put it into a tube furnace under nitrogen protection. Heat it to 750℃ and keep it at that temperature for 8h to obtain lithium iron phosphate black material D.

[0024] (5) Mix 80g of black material C and 20g of black material D evenly and then crush them. Add them together with 0.5g of barium titanate into anhydrous ethanol and seal and ball mill for 1h. The ball milling speed is 380r / min. After ball milling, place it in a vacuum drying oven and dry at 90℃ for 8h to obtain high-pressure lithium iron phosphate cathode material doped with barium titanate.

[0025] Example 2 The difference from Example 1 is that the reaction time of acetoacetate and polyvinyl alcohol is 8 hours, while the rest is the same as in Example 1, resulting in a high-pressure lithium iron phosphate cathode material doped with barium titanate.

[0026] Example 3 The difference from Example 1 is that the molar ratio of acetoacetate to polyvinyl alcohol structural units is 0.15:1, while the rest is the same as in Example 1, resulting in a high-pressure lithium iron phosphate cathode material doped with barium titanate.

[0027] Example 4 The difference from Example 1 is that the molar ratio of acetoacetate to polyvinyl alcohol structural units is 0.45:1, while the rest is the same as in Example 1, resulting in a high-pressure lithium iron phosphate cathode material doped with barium titanate.

[0028] Example 5 The difference from Example 1 is that the temperature of step (3) is 700°C, and the rest is the same as in Example 1, to obtain a high-pressure lithium iron phosphate cathode material doped with barium titanate.

[0029] Example 6 The difference from Example 1 is that step (1) is cancelled, and the modified polyvinyl alcohol in step (2) is replaced with polyvinyl alcohol. The rest is the same as in Example 1, and a lithium iron phosphate cathode material doped with barium titanate is obtained.

[0030] Example 7 The difference from Example 1 is that barium titanate is not added in step (5), and the rest is the same as in Example 1, to obtain lithium iron phosphate cathode material.

[0031] To verify the electrochemical performance of the lithium iron phosphate cathode material obtained in the above embodiments, the obtained cathode material was mixed in N-methylpyrrolidone at a mass ratio of LiFePO4:Super P:PVDF = 8:1:1 to prepare a slurry. The slurry was coated on aluminum foil and vacuum dried at 100°C for 12 h. Then, circular electrodes were formed by punching. The circular cathode electrodes were assembled with a separator, electrolyte, and lithium metal sheets to form a coin cell. Charge-discharge cycle tests were conducted at 25°C within a voltage window of 2.0~3.75V at 0.1C. The test results are shown in Table 1.

[0032] Table 1

[0033] The results above show that: (1) The compaction and electrical properties of the finished product increase with the degree of acetylation modification. When the molar ratio of acetoacetate to polyvinyl alcohol structural units exceeds 0.3:1, the properties of the finished product remain basically unchanged.

[0034] (2) The partially acetylated polyvinyl alcohol (PVA) material exhibits superior compaction density and electrochemical performance compared to the unmodified material. This is because the acetylated PVA, with some hydroxyl groups replaced, helps reduce the concentrated release of gas caused by the rapid decomposition of PVA during sintering, thereby reducing the formation of large pores and promoting the formation of a denser lithium iron phosphate structure, which in turn helps improve the compaction density of the graded material. Simultaneously, PVA can pyrolyze at lower temperatures to produce carbon, which coats the surface of the lithium iron phosphate particles, inhibiting excessively rapid grain growth and agglomeration, forming fine and uniform particles, shortening the lithium ion transport path, and thus improving the material's electrical properties.

[0035] (3) The performance of the finished product with the addition of modified polyvinyl alcohol is not significantly different under sintering conditions of 700℃ and 650℃. This is because the addition of modified polyvinyl alcohol can promote fine and uniform particles, shorten the atomic diffusion distance, and make the solid-phase reaction easier to carry out, thereby reducing the sintering temperature.

[0036] (4) The discharge capacity of the material is significantly improved after the addition of barium titanate. This is because barium titanate, as a material with a high dielectric constant, can build a built-in electric field inside the material, improve the migration rate of ions and repel anions, thereby improving the diffusion efficiency of lithium ions and improving the electrical performance of lithium iron phosphate materials.

Claims

1. A method for preparing a high-capacity lithium iron phosphate material with high pressure and high density, characterized in that, Includes the following steps: (1) Mix polyvinyl alcohol, iron phosphate, lithium source, carbon source and water to obtain slurry A; Iron phosphate, lithium source, carbon source and water are mixed to obtain slurry B; (2) Slurry A is spray-dried and then sintered at low temperature to obtain black material C; Slurry B is spray-dried and then sintered at high temperature to obtain black material D; (3) Mix black material C and black material D and crush them. Add barium titanate and ball mill and dry to obtain lithium iron phosphate material.

2. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 1, characterized in that, The polyvinyl alcohol is a partially acetylated modified polyvinyl alcohol.

3. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 2, characterized in that, The partial acetylation modification method is as follows: polyvinyl alcohol is dissolved in a solvent, acetoacetate is added, and the mixture is reacted at 60-110℃ for 1-8 hours to obtain modified polyvinyl alcohol.

4. The method for preparing a high-capacity, high-pressure lithium iron phosphate material according to claim 1, characterized in that, The lithium source includes one or more of lithium carbonate or lithium hydroxide; the carbon source includes one or more of glucose, fructose, sucrose or polyvinyl alcohol; the molar ratio of iron in the iron phosphate, lithium in the lithium source, and carbon in the carbon source is 1:(1.02-1.06):(0.3-0.5).

5. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 1, characterized in that, The low-temperature sintering temperature is 600-720℃, the sintering time is 8-11h, and the sintering is carried out in an argon or nitrogen atmosphere.

6. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 1, characterized in that, The high-temperature sintering temperature is 740-800℃, the sintering time is 8-11h, and the sintering is carried out in an argon or nitrogen atmosphere.

7. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 1, characterized in that, The mass ratio of black material C to black material D is (6-8):(2-4), and the amount of barium titanate added is 0.4%-0.6% of the total mass of black material C and black material D.

8. The method for preparing a high-pressure, high-capacity lithium iron phosphate material according to claim 1, characterized in that, The ball milling speed is 300-400 r / min, and the ball milling time is 1-2 h; the drying temperature is 80-100℃, and the drying time is 8-10 h.

9. A high-pressure, high-capacity lithium iron phosphate material, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. The application of the high-pressure, high-capacity lithium iron phosphate material as described in claim 9 in cathode materials.

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