Lithium iron phosphate positive electrode material, preparation method and application thereof

Abstract

The application relates to the field of lithium ion battery positive electrode materials, and discloses a lithium iron phosphate positive electrode material and a preparation method and application thereof. The preparation method comprises the following steps: carrying out heat preservation treatment on iron phosphate under inert atmosphere I, and carrying out quenching treatment on the product obtained through the heat preservation treatment to obtain an iron phosphate precursor; carrying out high-energy ball milling on the iron phosphate precursor to obtain an intermediate; in the presence of a solvent, the intermediate is mixed with a carbon source and a lithium source to obtain a mixed solution; and the mixed solution is sequentially subjected to spray drying and sintering. The preparation method of the lithium iron phosphate positive electrode material provided by the application has the advantages that the prepared lithium iron phosphate positive electrode material has high crystallinity and suitable particle size, and when the lithium iron phosphate positive electrode material is applied to a lithium ion battery, the lithium ion battery can have good electrochemical performance and excellent cycle performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode materials, specifically to a lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] In recent years, with the construction of new power systems, industrial and commercial energy storage, grid-side energy storage, and generation-side energy storage have attracted widespread attention from all sectors of society.

[0003] In new energy storage installations, the vast majority of lithium-ion battery energy storage devices use lithium iron phosphate as the cathode material. However, to meet the economic requirements of energy storage devices, lithium-ion batteries often need to have a lifespan of 15 to 20 years, which is difficult to meet even though lithium iron phosphate is known for its structural stability and excellent cycle life.

[0004] The degradation of lithium iron phosphate (LFP) batteries is mainly attributed to the loss of active lithium and Fe dissolution during long-term cycling. During long-term cycling, the stress generated by repeated charge-discharge cycles causes particle cracking, exposing more fresh surface area. This exacerbates side reactions with the electrolyte, leading to the loss of active lithium. Simultaneously, the intensified side reactions also cause more Fe to dissolve in the electrolyte, thereby disrupting the crystal structure stability of the cathode and hindering normal lithium insertion / extraction, resulting in capacity degradation and lifespan decline in the later stages of cycling. Furthermore, the dissolved Fe can precipitate at the anode, easily causing micro-short circuits or thermal runaway, leading to battery failure.

[0005] Currently, preventing particle cracking can be achieved by increasing the sintering temperature and extending the sintering time to improve the crystallinity of the material, thereby enhancing its structural stability. However, excessively high sintering temperatures and excessively long sintering times can lead to excessively large grain growth in lithium iron phosphate materials, resulting in larger particles. Larger particles prolong the lithium-ion transport path, reducing the lithium-ion diffusion coefficient of the material. Furthermore, larger particles exhibit uneven internal stress, making them more prone to cracking.

[0006] Therefore, developing a method to improve the crystallinity of materials without causing excessive particle growth is crucial for the application of lithium iron phosphate materials in long-cycle batteries. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing lithium iron phosphate cathode materials in terms of poor long-cycle performance.

[0008] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0009] Ferric phosphate was subjected to heat treatment under an inert atmosphere I, and the product obtained from the heat treatment was then quenched to obtain the ferric phosphate precursor.

[0010] The iron phosphate precursor was subjected to high-energy ball milling to obtain an intermediate.

[0011] In the presence of a solvent, the intermediate is contacted and mixed with a carbon source and a lithium source to obtain a mixed solution;

[0012] The mixed solution was sequentially spray-dried and sintered;

[0013] The conditions for the heat preservation treatment include: a temperature of 400-600℃ and a time of 0.5-3h.

[0014] The conditions for the quenching treatment include: a cooling rate of 200-1000℃ / s;

[0015] The conditions for high-energy ball milling include: a rotation speed of 300-1200 rpm and a time of 1-24 h;

[0016] The molar ratio of the iron phosphate (calculated as P), the carbon source (calculated as C), and the lithium source (calculated as Li) is 1:0.3-1:1.

[0017] The inventors of this invention discovered during their research that by controlling the conditions of heat preservation, quenching, and high-energy ball milling, a large number of lattice defects can be generated in the iron phosphate lattice during the synthesis of cathode materials. Particle growth is achieved through grain boundary migration; defects in the lattice typically accumulate at grain boundaries. During high-temperature sintering, diffusion at grain boundaries is hindered by these defects, making it difficult for grains to grow. Through the technical solution of this invention, even with increased sintering temperature and extended sintering time, the particles do not grow excessively due to the enrichment of defects at grain boundaries. Therefore, lithium iron phosphate cathode materials with high crystallinity and suitable particle size can be obtained, contributing to improved long-cycle performance of lithium iron phosphate materials.

[0018] Preferably, the particle size D of the iron phosphate is... 50 It is 0.5-5μm.

[0019] Preferably, the inert atmosphere I is argon and / or nitrogen.

[0020] Preferably, the carbon source is selected from at least one of glucose, sucrose, fructose, polyethylene glycol, and polyvinylpyrrolidone.

[0021] Preferably, the lithium source is selected from at least one of lithium hydroxide, lithium carbonate, and lithium acetate.

[0022] Preferably, the concentration of Li in the mixed solution is 0.02-0.3 mol / L.

[0023] Preferably, the contact mixing conditions include a temperature of 23-27°C and a time of 0.5-2 hours.

[0024] Preferably, the conditions for spray drying include: a feed rate of 200-2000 mL / h and an air inlet temperature of 200-300℃.

[0025] Preferably, the sintering conditions include: being carried out in an inert atmosphere ⅠⅠ, with a heating rate of 1-10℃ / min, a temperature of 650-750℃, and a time of 8-24h.

[0026] Preferably, the inert atmosphere ⅠⅠ is nitrogen and / or argon.

[0027] A second aspect of the present invention provides a lithium iron phosphate cathode material prepared by the preparation method described in the first aspect.

[0028] Preferably, the lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix, and the content of the carbon coating layer is 0.5-5 wt% based on the total weight of the lithium iron phosphate cathode material.

[0029] Preferably, the lithium iron phosphate cathode material has a secondary particle structure formed from primary particles.

[0030] Preferably, the particle size D of the lithium iron phosphate cathode material is... 50 The range is 0.7-2.0 μm.

[0031] The third aspect of the present invention provides the application of the lithium iron phosphate cathode material described in the second aspect in lithium-ion batteries.

[0032] The present invention has the following advantages through the above technical solution:

[0033] (1) The method for preparing lithium iron phosphate cathode material provided by the present invention includes a lithium iron phosphate matrix and a carbon coating layer on the surface of the lithium iron phosphate matrix. When the lithium iron phosphate cathode material is applied to lithium-ion batteries, it can ensure good electrochemical performance while having excellent cycle performance.

[0034] (2) The method for preparing lithium iron phosphate cathode material provided by the present invention effectively overcomes the disadvantage of excessive particle growth during high-temperature sintering. The resulting lithium iron phosphate cathode material not only has high crystallinity but also has a suitable particle size.

[0035] (3) The preparation method of lithium iron phosphate cathode material provided by the present invention is simple, easy to implement, low in cost and highly reproducible. Attached Figure Description

[0036] Figure 1 This is the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 of this invention;

[0037] Figure 2 This is a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Example 1 of the present invention;

[0038] Figure 3 This is a transmission electron microscope (TEM) image of the lithium iron phosphate cathode material prepared in Example 1 of this invention. Detailed Implementation

[0039] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0040] It should be noted that in this invention, "PVP" refers to polyvinylpyrrolidone, "PVDF" refers to polyvinylidene fluoride, and "NMP" refers to N-methylpyrrolidone.

[0041] In this invention, D 50 This refers to the particle size at which the cumulative particle size distribution of lithium iron phosphate cathode material reaches 50%, as measured by a Mastersizer 3000 laser particle size analyzer. In other words, 50% of the particles are smaller than (or larger than) this particle size.

[0042] As mentioned above, a first aspect of the present invention provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0043] Ferric phosphate was subjected to heat treatment under an inert atmosphere I, and the product obtained from the heat treatment was then quenched to obtain the ferric phosphate precursor.

[0044] The iron phosphate precursor was subjected to high-energy ball milling to obtain an intermediate.

[0045] In the presence of a solvent, the intermediate is contacted and mixed with a carbon source and a lithium source to obtain a mixed solution;

[0046] The mixed solution was sequentially spray-dried and sintered;

[0047] The conditions for the heat preservation treatment include: a temperature of 400-600℃ and a time of 0.5-3h.

[0048] The conditions for the quenching treatment include: a cooling rate of 200-1000℃ / s;

[0049] The conditions for high-energy ball milling include: a rotation speed of 300-1200 rpm and a time of 1-24 h;

[0050] The molar ratio of the iron phosphate (calculated as P), the carbon source (calculated as C), and the lithium source (calculated as Li) is 1:0.3-1:1.

[0051] The inventors discovered that by quenching iron phosphate at high temperatures followed by high-energy ball milling, while controlling other conditions, a lithium iron phosphate cathode material can be prepared that solves the problem of poor cycle performance in lithium iron phosphate. Analysis revealed that the rapid cooling during quenching causes numerous defects to appear in the crystal lattice. Subsequent high-temperature ball milling, under strong mechanical forces, causes continuous collisions, friction, and compression between particles, impacting the crystal structure and causing lattice displacement, thus forming defects. These defects accumulate at grain boundaries, hindering grain boundary diffusion during high-temperature sintering. Therefore, high crystallinity and structural stability can be achieved at high temperatures while maintaining a suitable particle size.

[0052] The inventors further discovered that the lithium iron phosphate cathode material obtained by the preparation method provided by this invention has high structural stability, reduces Fe dissolution and active lithium loss during long-term cycling, and improves cycle performance.

[0053] The inventors continued their research and discovered that during the preparation of lithium iron phosphate cathode materials, if the carbon content is too low, the electronic conductivity cannot be improved; if the carbon content is too high, the specific capacity will be lost and the compaction will be reduced. If the lithium iron phosphate particle size is too large, it will hinder electron and ion transport; if the lithium iron phosphate particle size is too small, it will easily agglomerate, which is not conducive to electrolyte wetting.

[0054] Preferably, the particle size D of the iron phosphate is... 50 The thickness is 0.5-5 μm, more preferably 1-4 μm. Under this preferred condition, the lithium iron phosphate cathode material provided by the present invention has higher structural stability.

[0055] Preferably, the inert atmosphere I is argon and / or nitrogen.

[0056] According to a preferred embodiment, the conditions for the heat preservation treatment include: a temperature of 450-600℃ and a time of 0.5-1h.

[0057] According to a preferred embodiment, the quenching conditions include a cooling rate of 300-1000℃ / s.

[0058] Preferably, the conditions for high-energy ball milling include: a rotation speed of 400-1200 rpm and a time of 4-12 h.

[0059] According to a preferred embodiment, the molar ratio of the iron phosphate (calculated as P), the carbon source (calculated as C), and the lithium source (calculated as Li) is 1:0.38-0.65:1.

[0060] Preferably, the concentration of Li in the mixed solution is 0.02-0.3 mol / L, more preferably 0.02-0.2 mol / L. Under this preferred condition, it is more likely to obtain a lithium iron phosphate cathode material with a suitable carbon content.

[0061] Preferably, the solvent is deionized water.

[0062] Preferably, the carbon source is selected from at least one of glucose, sucrose, fructose, polyethylene glycol, and polyvinylpyrrolidone, more preferably from at least one of glucose, polyethylene glycol, and polyvinylpyrrolidone.

[0063] Preferably, the lithium source is selected from at least one of lithium hydroxide, lithium carbonate, and lithium acetate, and more preferably lithium hydroxide and / or lithium acetate.

[0064] In a preferred embodiment, the contact mixing conditions include a temperature of 23-27°C and a time of 0.5-2 hours; more preferably, the contact mixing conditions include a temperature of 23-27°C and a time of 0.5-1 hours.

[0065] Preferably, the spray drying conditions include: a feed rate of 200-2000 mL / h and an inlet air temperature of 200-300℃; more preferably, the spray drying conditions include: a feed rate of 400-1500 mL / h and an inlet air temperature of 240-280℃.

[0066] Preferably, the sintering conditions include: sintering in an inert atmosphere ⅠⅠ, a heating rate of 1-10℃ / min, a temperature of 650-750℃, and a time of 8-24h; more preferably, the sintering conditions include: sintering in an inert atmosphere ⅠⅠ, a heating rate of 5-10℃ / min, a temperature of 700-750℃, and a time of 12-24h. Under this preferred condition, the lithium iron phosphate cathode material provided by the present invention exhibits higher crystallinity.

[0067] In a preferred embodiment, the inert atmosphere ⅠⅠ is nitrogen and / or argon.

[0068] As previously stated, a second aspect of the present invention provides a lithium iron phosphate cathode material prepared by the preparation method described in the first aspect.

[0069] Preferably, the lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix, and the content of the carbon coating layer is 0.5-5 wt% based on the total weight of the lithium iron phosphate cathode material.

[0070] Preferably, the lithium iron phosphate cathode material has a secondary particle structure formed from primary particles.

[0071] In a preferred embodiment, the particle size D of the lithium iron phosphate cathode material is... 50 The range is 0.7-2.0 μm.

[0072] As previously stated, the third aspect of the present invention provides the application of the lithium iron phosphate cathode material described in the second aspect in lithium-ion batteries.

[0073] The present invention will be described in detail below through examples.

[0074] Unless otherwise specified, the raw materials and equipment used in the following examples are all commercially available products.

[0075] In the following examples, the relevant characteristic parameters were measured using the following methods:

[0076] (1) The content of carbon coating layer: was obtained by testing with a SYNSE HCS-800 infrared carbon-sulfur analyzer.

[0077] (2) Microstructure: The microstructure was obtained using a JSM-7900F scanning electron microscope (SEM) from Nippon Electronics Co., Ltd. and a Talos F200s transmission electron microscope (TEM) from FEI Corporation, USA.

[0078] (3) Crystallization properties: The results were obtained using a Smart-lab XRD-X-ray powder diffractometer from Rigaku, Japan.

[0079] (4) Specific capacity: The specific capacity was obtained by charging and discharging the battery using a 3001A model charge and discharge tester from Wuhan Landian Electronics Co., Ltd.

[0080] In the following examples, unless otherwise specified, the amount of ferric phosphate used in each example is 3g.

[0081] Example 1

[0082] (1) Ferric phosphate was heat-treated under an inert atmosphere I and the product obtained by heat treatment was quenched to obtain ferric phosphate precursor.

[0083] (2) The above-mentioned iron phosphate precursor was subjected to high-energy ball milling to obtain an intermediate;

[0084] (3) In the presence of deionized water, the above intermediate is contacted and mixed with a carbon source and a lithium source to obtain a mixed solution;

[0085] (4) The above mixed solution is spray-dried and sintered in sequence to obtain lithium iron phosphate cathode material.

[0086] The raw material ratios and specific process conditions are shown in Table 1.

[0087] Unless otherwise specified, the remaining embodiments and comparative examples follow the same process as in Example 1, except for the different raw material ratios and specific process conditions, as detailed in Table 1.

[0088] Comparative Example 1

[0089] The method is the same as in Example 1, except that the quenching conditions in this comparative example are different. Specifically, the quenching conditions are: a cooling rate of 122°C / s.

[0090] Comparative Example 2

[0091] The method is the same as in Example 1, except that the high-energy ball milling conditions are different in this comparative example. Specifically, the high-energy ball milling conditions are: a rotation speed of 200 rpm and a time of 0.5 h.

[0092] Comparative Example 3

[0093] The method is the same as in Example 1, except that the molar ratios of iron phosphate (calculated as P), carbon source (calculated as C), and lithium source (calculated as Li) are different in this comparative example.

[0094] Specifically, the molar ratio of iron phosphate (calculated as P), carbon source (calculated as C), and lithium source (calculated as Li) is 1:0.28:1.

[0095] Table 1

[0096]

[0097] Note: Particle size D 50 1 The particle size D of ferric phosphate 50 N 葡萄糖 :N 聚乙二醇 The molar ratio of glucose to polyethylene glycol; N 葡萄糖 :N PVP The molar ratio of glucose to PVP is given; the molar ratio is the ratio of the iron phosphate (calculated as P), the carbon source (calculated as C), and the lithium source (calculated as Li).

[0098] Test case

[0099] The performance of the lithium iron phosphate cathode materials prepared in the examples was tested, including: carbon coating content, particle size D. 50 Crystallization properties and microstructure were analyzed, and the results are shown in Table 2.

[0100] The lithium iron phosphate cathode material prepared in the examples was applied to assemble button cells in lithium-ion batteries. The assembled button cells were tested for their first-cycle 0.1C discharge specific capacity and capacity retention after 1000 cycles at 1C rate. The results are shown in Table 2. The specific assembly method for the button cells is as follows:

[0101] Weigh 0.08g of lithium iron phosphate cathode material and place it in a mortar. Add 0.01g of PVDF and 0.01g of acetylene black, and grind until uniform in color and without obvious particle texture. Add 0.5mL of NMP and grind until uniform, so that PVDF is fully dissolved in NMP to obtain cathode slurry. Coat the cathode slurry onto aluminum foil and dry it in a vacuum oven at 120℃ for 12 hours. Then place it in a glove box, add a separator, lithium sheet and electrolyte, and assemble to obtain CR2032 button battery.

[0102] The present invention provides, by way of example, XRD pattern, scanning electron microscope (SEM) image, and transmission electron microscope (TEM) image of the lithium iron phosphate cathode material prepared in Example 1, as shown below. Figure 1 , Figure 2 and Figure 3 As shown. From Figure 1 As can be seen from the above, the preparation method provided by this invention successfully synthesized lithium iron phosphate cathode material, which exhibits high crystallinity and is free of impurity peaks; Figure 2 and Figure 3 It is known that the lithium iron phosphate cathode material obtained by the present invention includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix, and the cathode material has a secondary particle structure formed by primary particles.

[0103] Table 2

[0104]

[0105]

[0106] Note: Particle size D 50 2 The particle size D of lithium iron phosphate cathode material 50 The peak intensity of the highest peak is the peak intensity of the diffraction peak of lithium iron phosphate in the XRD pattern.

[0107] The results above show that the lithium iron phosphate cathode material prepared by the method of the present invention includes a lithium iron phosphate matrix and a carbon coating layer on the surface of the lithium iron phosphate matrix. This lithium iron phosphate cathode material not only has high crystallinity, but also has a suitable particle size. When applied to lithium-ion batteries, it can ensure good electrochemical performance while having excellent cycle performance.

[0108] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: Ferric phosphate was subjected to heat treatment under an inert atmosphere I, and the product obtained from the heat treatment was then quenched to obtain the ferric phosphate precursor. The iron phosphate precursor was subjected to high-energy ball milling to obtain an intermediate. In the presence of a solvent, the intermediate is contacted and mixed with a carbon source and a lithium source to obtain a mixed solution; The mixed solution was sequentially spray-dried and sintered; The conditions for the heat preservation treatment include: a temperature of 400-600℃ and a time of 0.5-3h. The conditions for the quenching treatment include: a cooling rate of 200-1000℃ / s; The conditions for high-energy ball milling include: a rotation speed of 300-1200 rpm and a time of 1-24 h; The molar ratio of the iron phosphate (calculated as P), the carbon source (calculated as C), and the lithium source (calculated as Li) is 1:0.3-1:

1.

2. The preparation method according to claim 1, wherein, The particle size D of the ferric phosphate 50 It is 0.5-5μm; And / or, the inert atmosphere I is argon and / or nitrogen.

3. The preparation method according to claim 1, wherein, The concentration of Li in the mixed solution is 0.02-0.3 mol / L.

4. The preparation method according to any one of claims 1-3, wherein, The carbon source is selected from at least one of glucose, sucrose, fructose, polyethylene glycol, and polyvinylpyrrolidone; And / or, the lithium source is selected from at least one of lithium hydroxide, lithium carbonate, and lithium acetate.

5. The preparation method according to any one of claims 1-3, wherein, The conditions for contact mixing include: a temperature of 23-27°C and a time of 0.5-2 hours.

6. The preparation method according to any one of claims 1-3, wherein, The conditions for spray drying include: a feed rate of 200-2000 mL / h and an inlet air temperature of 200-300℃.

7. The preparation method according to any one of claims 1-3, wherein, The sintering conditions include: being carried out in an inert atmosphere ⅠⅠ, with a heating rate of 1-10℃ / min, a temperature of 650-750℃, and a time of 8-24h; And / or, the inert atmosphere ⅠⅠ is nitrogen and / or argon.

8. A lithium iron phosphate cathode material prepared by any one of claims 1-7.

9. The lithium iron phosphate cathode material according to claim 8, wherein, The lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix, and the content of the carbon coating layer is 0.5-5 wt% based on the total weight of the lithium iron phosphate cathode material. And / or, the lithium iron phosphate cathode material has a secondary particle structure formed from primary particles; And / or, the particle size D of the lithium iron phosphate cathode material 50 The range is 0.7-2.0 μm.

10. The application of the lithium iron phosphate cathode material according to claim 8 or 9 in lithium-ion batteries.

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