A composite lithium iron phosphate positive electrode material and its preparation method and application

Abstract

The present invention provides a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. The preparation method comprises the following steps: (1) mixing an iron source and a phosphorus source to obtain a mixed solution, adding ammonia water to adjust the pH, performing a microwave hydrothermal reaction, and calcining the obtained product to obtain polycrystalline flower-shaped iron phosphate; (2) mixing and grinding the polycrystalline flower-shaped iron phosphate, a lithium source and a reducing agent, performing a pre-calcination treatment, adding a carbon source and performing a sintering treatment to obtain the composite lithium iron phosphate positive electrode material. The present invention can quickly prepare polycrystalline flower-shaped iron phosphate with a narrow particle size distribution and uniform morphology. The lithium iron phosphate prepared using the polycrystalline flower-shaped iron phosphate has good electrochemical properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] In lithium battery systems, the performance of the positive electrode material plays a decisive role in the overall performance of the battery. LiFePO4, as the positive electrode material with the highest market share and the most extensive application, still has great market potential. Therefore, it is crucial to explore methods for preparing lithium iron phosphate with high electrochemical performance to meet market demand.

[0003] The iron phosphate method has gradually become the primary route for producing high-performance lithium iron phosphate (LFP) due to its advantages such as simplicity, controllable conditions, and excellent electrochemical performance. FePO₄ accounts for over 30% of the total cost of LFP. The purity, morphology, particle size, and specific surface area of ​​FePO₄ directly determine the electrochemical performance of the final LFP electrode material. Therefore, the development and preparation of high-quality FePO₄ precursors has become a key technology in the high-performance LFP battery industry and the core of the entire value chain.

[0004] CN110316712A discloses a method for preparing lithium iron phosphate using nano-sized iron phosphate, comprising the following steps: synthesizing iron phosphate with crystal water; heating the iron phosphate raw material containing crystal water to 500-850°C, and then mixing it with a lithium source to prepare lithium iron phosphate.

[0005] CN114772571A discloses a method for preparing anhydrous iron phosphate and a method for preparing a lithium iron phosphate carbon composite material, which uses green vanadium products purified from titanium dioxide to synthesize iron phosphate products, and uses the above-mentioned anhydrous iron phosphate to perform composite carbon source and composite carbon coating through a rheological phase method to prepare a lithium iron phosphate carbon composite material.

[0006] The lithium iron phosphate cathode material prepared by the above scheme has the problems of low conductivity and poor diffusion of lithium ions therein. Summary of the Invention

[0007] The purpose of the present invention is to provide a composite lithium iron phosphate positive electrode material and its preparation method and application. The present invention can quickly prepare polycrystalline flower-shaped iron phosphate with narrow particle size distribution and uniform morphology. The use of the polycrystalline flower-shaped iron phosphate can significantly improve the electrochemical performance of LiFePO4.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing a composite lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0010] (1) mixing an iron source and a phosphorus source to obtain a mixed solution, adding ammonia water to adjust the pH, and then performing a microwave hydrothermal reaction, and calcining the obtained product to obtain polycrystalline flower-shaped iron phosphate;

[0011] (2) The polycrystalline flower-shaped iron phosphate, lithium source and reducing agent are mixed and ground, and then pre-sintered. A carbon source is added and sintered to obtain the composite lithium iron phosphate positive electrode material.

[0012] The present invention utilizes a microwave hydrothermal method to synthesize FePO4 with a polycrystalline flower-shaped structure in one step. The iron phosphate particles synthesized by the microwave hydrothermal method have a good degree of crystallization and a corresponding sharp peak shape. No other impurities are generated during the synthesis process. The synthesized iron phosphate particles have the best crystal form and few cracks on the crystal faces. The single crystal particles that make up the flower-shaped polycrystalline iron phosphate have the smallest particle size and are uniform in size. The polycrystalline particles have good dispersibility. The morphology of the iron phosphate determines the morphology of the lithium iron phosphate and plays a decisive role in its charge and discharge capacity and stability.

[0013] Preferably, the iron source in step (1) comprises Fe(NO3)3 solution.

[0014] Preferably, the molar concentration of the Fe(NO3)3 solution is 0.02 to 0.1 mol / L, for example, 0.02 mol / L, 0.04 mol / L, 0.06 mol / L, 0.08 mol / L or 0.1 mol / L.

[0015] Preferably, the phosphorus source comprises NH4H2PO4 solution.

[0016] Preferably, the molar ratio of NH4H2PO4 in the NH4H2PO4 solution and Fe(NO3)3 in the Fe(NO3)3 solution is (1.5-1.8):1, for example: 1.5:1, 1.55:1, 1.6:1, 1.7:1 or 1.8:1, etc.

[0017] Preferably, the mixing method comprises adding NH4H2PO4 solution dropwise to the stirred Fe(NO3)3 solution.

[0018] Preferably, the dropping speed of the NH4H2PO4 solution is 2-4 ml / min, for example, 2 ml / min, 2.5 ml / min, 3 ml / min, 3.5 ml / min or 4 ml / min.

[0019] Preferably, the pH in step (1) is 2 to 2.2, for example, 2, 2.05, 2.1, 2.16 or 2.2.

[0020] Preferably, stirring is performed after adjusting the pH.

[0021] Preferably, the stirring time is 0.5 to 1 h, for example, 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h or 1 h.

[0022] Preferably, the temperature of the microwave hydrothermal reaction is 120-180°C, for example, 120°C, 130°C, 140°C, 150°C or 180°C.

[0023] Preferably, the microwave hydrothermal reaction time is 70 to 100 min, for example, 70 min, 75 min, 80 min, 90 min or 100 min.

[0024] Preferably, before the calcination treatment in step (1), the product is cooled to 40-60°C (for example, 40°C, 45°C, 50°C, 55°C or 60°C) and then washed and dried.

[0025] Preferably, the washing detergent comprises deionized water and anhydrous ethanol.

[0026] Preferably, the calcination temperature is 500-700°C, for example, 500°C, 550°C, 600°C, 650°C or 700°C.

[0027] Preferably, the calcination treatment time is 8 to 12 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours.

[0028] Preferably, the lithium source in step (2) comprises lithium hydroxide.

[0029] Preferably, the reducing agent comprises ascorbic acid.

[0030] Preferably, the molar ratio of lithium element to polycrystalline flower-shaped iron phosphate in the lithium source is (1-1.2):1, for example: 1:1, 1.05:1, 1.1:1, 1.15:1 or 1.2:1.

[0031] Preferably, the mass of the reducing agent is 8-12% of the total mass of the polycrystalline flower-shaped iron phosphate and the lithium source, for example, 8%, 9%, 10%, 11% or 12%.

[0032] Preferably, the mixing and grinding time in step (2) is 20 to 40 minutes, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes or 40 minutes.

[0033] Preferably, the temperature of the pre-calcination treatment is 300-400°C, for example, 300°C, 320°C, 350°C, 380°C or 400°C.

[0034] Preferably, the pre-firing treatment time is 3 to 6 hours, for example, 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

[0035] Preferably, the carbon source in step (2) includes glucose.

[0036] Preferably, the mass of the carbon source is 8-12% of the total mass of the polycrystalline flower-shaped iron phosphate and the lithium source, for example, 8%, 9%, 10%, 11% or 12%.

[0037] Preferably, the sintering temperature is 600-700°C, for example, 600°C, 620°C, 650°C, 680°C or 700°C.

[0038] Preferably, the sintering treatment time is 8 to 12 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours.

[0039] In a second aspect, the present invention provides a composite lithium iron phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0040] In a third aspect, the present invention provides a positive electrode plate, which comprises the composite lithium iron phosphate positive electrode material as described in the second aspect, and the composite lithium iron phosphate positive electrode material has a polycrystalline flower-like structure.

[0041] In a fourth aspect, the present invention provides a lithium-ion battery, comprising the positive electrode sheet as described in the third aspect.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) The iron phosphate particles synthesized by the microwave hydrothermal method of the present invention have a good degree of crystallization. The morphology of the iron phosphate determines the morphology of the lithium iron phosphate and plays a decisive role in its charge and discharge capacity and stability. While reducing the reaction time, the cycle performance and rate performance of the lithium iron phosphate positive electrode material can be significantly improved.

[0044] (2) The 0.2C first discharge specific capacity of the battery made of the composite lithium iron phosphate positive electrode material of the present invention can reach more than 134.2mAh / g, and the capacity retention rate after 100 cycles can reach more than 98.7%. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is an SEM image of the composite lithium iron phosphate positive electrode material described in Example 1.

[0046] Figure 2 This is an enlarged SEM image of the composite lithium iron phosphate positive electrode material described in Example 1.

[0047] Figure 3 This is a rate performance diagram of the composite lithium iron phosphate positive electrode material described in Example 1.

[0048] Figure 4 This is a cycle performance diagram of the composite lithium iron phosphate positive electrode material described in Example 1. DETAILED DESCRIPTION

[0049] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0050] Example 1

[0051] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0052] (1) Prepare 100 ml of each 0.05 mol / L Fe(NO3)3 solution and 100 ml of NH4H2PO4 solution with a phosphorus to iron molar ratio of 1.6:1, and stir for 10 minutes. Add the NH4H2PO4 solution to the stirred Fe(NO3)3 solution at a rate of 3 ml / min through a burette and stir for 5 minutes. When the solution is evenly mixed, add ammonia water to the solution at a rate of 4 ml / min to adjust the pH to 2.15. After stirring the solution for 0.6 hours, add it to a microwave hydrothermal reactor, keep it at 150°C for 90 minutes, cool it to 50°C, filter it, wash it with deionized water and anhydrous ethanol, and dry it at 100°C for 36 hours to obtain a light green powder. Then, calcine the product at 600°C in air at 5°C / min for 10 hours to obtain polycrystalline flower-shaped anhydrous FePO4.

[0053] (2) At a molar ratio of Li:F:P=1:1:1, 0.5 g of the prepared FePO4 precursor, 0.1391 g of LiOH·H2O and 0.06391 g of the reducing agent ascorbic acid (10 wt% of the total mass of LiOH·H2O and composite FePO4) were weighed and placed in an agate mortar. After grinding outside an infrared oven for 30 min, the sample was pre-sintered at 350°C in a tube furnace under an argon atmosphere at a heating rate of 6°C / min for 5 h. After taking out the sample, 0.06391 g of glucose (10 wt% of the total mass of LiOH·H2O and FePO4) was added. After grinding in an agate mortar for 20 min, the sample was placed in a tube furnace under an argon atmosphere and calcined at 650°C at a heating rate of 6°C / min for 10 h to obtain the composite lithium iron phosphate positive electrode material. The SEM image of the composite lithium iron phosphate positive electrode material is shown in FIG. Figure 1-2 As shown, the rate performance diagram and cycle performance diagram of the composite lithium iron phosphate positive electrode material are respectively as shown in Figure 3-4 shown.

[0054] Example 2

[0055] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0056] (1) Prepare 100 ml of each 0.06 mol / L Fe(NO3)3 solution and 100 ml of NH4H2PO4 solution with a phosphorus to iron molar ratio of 1.6:1, and stir for 10 minutes. Add the NH4H2PO4 solution to the stirred Fe(NO3)3 solution at a rate of 3.2 ml / min through a burette and stir for 5 minutes. When the solution is evenly mixed, add ammonia water to the solution at a rate of 4 ml / min to adjust the pH to 2.18. After stirring the solution for 0.6 hours, add it to a microwave hydrothermal reactor, keep it at 155°C for 95 minutes, cool it to 52°C, filter it, wash it with deionized water and anhydrous ethanol, and dry it at 100°C for 36 hours to obtain a light green powder. Then, calcine the product at 620°C in air at 5°C / min for 10 hours to obtain polycrystalline flower-shaped anhydrous FePO4.

[0057] (2) With a molar ratio of Li:F:P=1:1:1, 0.5g of the prepared FePO4 precursor, 0.1391g of LiOH·H2O and 0.06391g of the reducing agent ascorbic acid (10wt% of the total mass of LiOH·H2O and composite FePO4) were weighed and placed in an agate mortar. After grinding outside an infrared oven for 30min, the sample was pre-sintered at 340℃ in a tube furnace under argon atmosphere at a heating rate of 6℃ / min for 5h. After taking out the sample, 0.06391g of glucose (10wt% of the total mass of LiOH·H2O and FePO4) was added. After grinding in an agate mortar for 20min, it was placed in a tube furnace under argon atmosphere and calcined at 660℃ at a heating rate of 6℃ / min for 10h to obtain the composite lithium iron phosphate positive electrode material.

[0058] Example 3

[0059] The only difference between this embodiment and embodiment 1 is that the pH in step (1) is 1.8, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0060] Example 4

[0061] The only difference between this embodiment and embodiment 1 is that the pH in step (1) is 2.3, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0062] Example 5

[0063] The only difference between this embodiment and embodiment 1 is that the time of the microwave hydrothermal reaction in step (1) is 60 minutes, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0064] Example 6

[0065] The only difference between this embodiment and embodiment 1 is that the time of the microwave hydrothermal reaction in step (1) is 120 min, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0066] Comparative Example 1

[0067] The only difference between this comparative example and Example 1 is that commercial iron phosphate is used to prepare the lithium iron phosphate positive electrode material, and the other conditions and parameters are exactly the same as those in Example 1.

[0068] Performance testing:

[0069] The lithium iron phosphate positive electrode material prepared in the Example and the Comparative Example was mixed with a conductive carbon black Super P and polyvinylidene fluoride at a mass ratio of 8:1:1 and an appropriate amount of NMP solvent to prepare an electrode slurry. The slurry was then evenly coated on an aluminum foil and then dried overnight in a vacuum drying oven at 80-120°C. A slicer was used to prepare an electrode sheet with a diameter of 12 mm. Lithium half-cells were assembled in a vacuum glove box filled with argon. The lithium half-cells were subjected to rate and cycle tests. The test results are shown in Table 1:

[0070] Table 1

[0071] 0.2C first discharge specific capacity (mAh / g) Capacity retention rate after 100 cycles (%) Example 1 135.1 98.8 Example 2 134.2 98.7 Example 3 130.2 97.5 Example 4 128.8 98.0 Example 5 121.2 97.1 Example 6 124.9 96.8 Comparative Example 1 120.1 96.2

[0072] As can be seen from Table 1, from Examples 1-2, the 0.2C first discharge specific capacity of the battery made of the composite lithium iron phosphate positive electrode material of the present invention can reach more than 134.2 mAh / g, and the capacity retention rate after 100 cycles can reach more than 98.7%.

[0073] By comparing Example 1 with Examples 3-4, it can be seen that during the preparation of the composite lithium iron phosphate positive electrode material of the present invention, the pH of the reaction will affect the morphology of the obtained iron phosphate, and thus affect the performance of the prepared lithium iron phosphate positive electrode material. When the pH is controlled at 2 to 2.2, the performance of the obtained lithium iron phosphate is better. If the pH is too low, the iron phosphate particles are mostly aggregated, the dispersion is poor, and the single crystal effect is not obvious. If the pH is too high, the flower-like particles formed by the uneven aggregation of single crystal particles have different morphologies.

[0074] By comparing Example 1 with Examples 5-6, it can be seen that in the preparation process of the composite lithium iron phosphate positive electrode material described in the present invention, the time of the hydrothermal reaction will affect the morphology of the obtained iron phosphate, and thus affect the performance of the prepared lithium iron phosphate positive electrode material. When the time of the hydrothermal reaction is controlled within 70 to 100 minutes, the performance of the lithium iron phosphate obtained is better. If the time of the hydrothermal reaction is too short, the precursor iron phosphate formed fails to form single crystal particles or aggregates, and the advantages of microwave hydrothermal reaction cannot be reflected. If the time of the hydrothermal reaction is too long, the flower-like particles formed will break and the morphology will be uneven.

[0075] From the comparison between Example 1 and Comparative Example 1, it can be seen that the iron phosphate particles synthesized by the microwave hydrothermal method of the present invention have a good degree of crystallization. The morphology of iron phosphate determines the morphology of lithium iron phosphate and has a decisive effect on its charge and discharge capacity and stability. While reducing the reaction time, the cycle performance and rate performance of the lithium iron phosphate positive electrode material can be significantly improved.

[0076] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a composite lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: (1) mixing an iron source and a phosphorus source to obtain a mixed solution, adding ammonia water to adjust the pH to 2-2.2, and then performing a microwave hydrothermal reaction. The obtained product is calcined at 500-700° C. to obtain polycrystalline flower-shaped iron phosphate; The microwave hydrothermal reaction time is 90 to 100 minutes, and the microwave hydrothermal reaction temperature is 150 to 180°C; The iron source includes a Fe(NO3)3 solution, the phosphorus source includes an NH4H2PO4 solution, and the molar ratio of NH4H2PO4 in the NH4H2PO4 solution to Fe(NO3)3 in the Fe(NO3)3 solution is (1.5-1.8):1; (2) mixing and grinding the polycrystalline flower-shaped iron phosphate, a lithium source, and a reducing agent, and then pre-calcining the mixture. Then, adding a carbon source and sintering the mixture to obtain the composite lithium iron phosphate positive electrode material. The reducing agent includes ascorbic acid.

2. The preparation method according to claim 1, wherein The molar concentration of the Fe(NO3)3 solution is 0.02-0.1 mol / L.

3. The preparation method according to claim 1, wherein The mixing method in step (1) includes adding NH4H2PO4 solution dropwise to the stirred Fe(NO3)3 solution.

4. The preparation method according to claim 3, wherein The dropping speed of the NH4H2PO4 solution is 2-4 ml / min.

5. The preparation method according to claim 1, wherein The pH is adjusted and then stirred.

6. The preparation method according to claim 5, wherein The stirring time is 0.5 to 1 hour.

7. The preparation method according to claim 1, wherein Before the calcination treatment in step (1), the product is cooled to 40-60° C. and then washed and dried.

8. The preparation method according to claim 7, wherein The washing detergent includes deionized water and anhydrous ethanol.

9. The preparation method according to claim 1, wherein The calcination treatment time is 8 to 12 hours.

10. The preparation method according to claim 1, wherein The lithium source in step (2) includes lithium hydroxide.

11. The preparation method according to claim 1, wherein The molar ratio of lithium element to polycrystalline flower-shaped iron phosphate in the lithium source is (1-1.2):

1.

12. The preparation method according to claim 1, wherein The mass of the reducing agent is 8-12% of the total mass of the polycrystalline flower-shaped iron phosphate and the lithium source.

13. The preparation method according to claim 1, wherein The mixing and grinding time in step (2) is 20 to 40 minutes.

14. The preparation method according to claim 1, wherein The temperature of the pre-firing treatment is 300-400°C.

15. The preparation method according to claim 1, wherein The pre-firing treatment time is 3 to 6 hours.

16. The preparation method according to claim 1, wherein The carbon source in step (2) includes glucose.

17. The preparation method according to claim 1, wherein The mass of the carbon source is 8-12% of the total mass of the polycrystalline flower-shaped iron phosphate and the lithium source.

18. The preparation method according to claim 1, wherein The sintering temperature is 600-700°C.

19. The preparation method according to claim 1, wherein The sintering treatment time is 8 to 12 hours.

20. A composite lithium iron phosphate positive electrode material, characterized in that: The composite lithium iron phosphate positive electrode material is prepared by the method according to any one of claims 1 to 19.

21. A positive electrode plate, characterized in that: The positive electrode plate comprises the composite lithium iron phosphate positive electrode material as claimed in claim 20, and the composite lithium iron phosphate positive electrode material has a polycrystalline flower-like structure.

22. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 21.

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