Lithium iron phosphate cathode material, method for preparing same, and electrochemical device

Abstract

The application provides a lithium iron phosphate positive electrode material, a preparation method thereof and an electrochemical device. The lithium iron phosphate positive electrode material comprises first particles with a particle size of 1000 nm or more and second particles with a particle size of 500 nm or less. The carbon content of the first particles is 0.4%-0.8%, the specific surface area of the first particles is 4 m 2 / g-8.0 m 2 / g, the carbon content of the second particles is 1.5%-2.0%, and the specific surface area of the second particles is 15 m 2 / g-20 m 2 / g. The first particles and the second particles have carbon content and specific surface area adjusted, and are mixed and graded so that the lithium iron phosphate positive electrode material has high compactness and long cycle performance.

Description

Technical Field

[0001] This application relates to the field of new energy materials technology, specifically to a lithium iron phosphate cathode material, its preparation method, and an electrochemical device. Background Technology

[0002] In recent years, lithium iron phosphate (LFP) cathode materials have experienced rapid development in both the power and energy storage sectors due to their advantages such as low cost, high safety, and long lifespan. The accelerated expansion of the energy storage sector has significantly driven demand for LFP cathode materials. Industry analysis predicts that the demand for LFP cathode materials in the energy storage sector will continue to grow rapidly at a compound annual growth rate of approximately 25%.

[0003] However, existing technologies produce lithium iron phosphate (LFP) products with a wide particle size distribution. Especially in recent years, to pursue higher compaction density LFP and improve energy density, the industry's common method has been to increase compaction density through particle size distribution. To ensure optimal electrical performance, the number of small particles in LFP cathode materials has gradually increased, and the particle size has gradually decreased from 0.4 μm to below 0.3 μm. This has led to increased manufacturing costs and deterioration in cycle performance and processing performance, which does not meet the requirements of the energy storage field for LFP cathode materials. Summary of the Invention

[0004] In view of this, in order to solve at least one of the above technical problems, this application provides a lithium iron phosphate cathode material.

[0005] This application also provides a method for preparing the aforementioned lithium iron phosphate cathode material, and an electrochemical device using the lithium iron phosphate cathode material.

[0006] In a first aspect, embodiments of this application provide a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material comprises a first particle with a particle size of 1000 nm or more and a second particle with a particle size of 500 nm or less, the first particle having a carbon content of 0.4% to 0.8% and a specific surface area of ​​4 m². 2 / g~8.0m 2 / g, the carbon content of the second particle is 1.5%~2.0%, and the specific surface area of ​​the second particle is 15m². 2 / g~20m 2 / g.

[0007] Based on the first aspect, in some possible embodiments, in the lithium iron phosphate cathode material, the volume density of the first particles accounts for 60.0% to 70.0%, and the volume density of the second particles accounts for 12.0% to 20.0%.

[0008] Based on the first aspect, in some possible embodiments, the first particle contains a first metal element, the first metal element including at least one of V, Ti, Nb, W and B; and / or, the second particle contains a second metal element, the second metal element including at least one of V, Ti, Nb, W and B.

[0009] Based on the first aspect, in some possible embodiments, in the lithium iron phosphate cathode material, the volume density of particles with a size of less than 500 nm accounts for 12.0% to 20.0%, the volume density of particles with a size of less than 1000 nm accounts for 30.0% to 40.0%, the volume density of particles with a size of less than 2000 nm accounts for 66.0% to 75.0%, the volume density of particles with a size of less than 3000 nm accounts for 85.0% to 95.0%, and the volume density of particles with a size of greater than 3000 nm accounts for 5.0% to 15.0%.

[0010] Based on the first aspect, in some possible embodiments, the lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) the median particle size Dv50 of the lithium iron phosphate cathode material is 0.8 μm to 1.5 μm; (2) the carbon content of the lithium iron phosphate cathode material is 0.9% to 1.5%; (3) the specific surface area of ​​the lithium iron phosphate cathode material is 10 m². 2 / g~15m 2 / g; (4) The powder compaction density of the lithium iron phosphate cathode material is 2.4 g / cm³. 3 ~2.75g / cm 3 .

[0011] In a first aspect, embodiments of this application provide a method for preparing a lithium iron phosphate cathode material, comprising: preparing lithium iron phosphate small particles, wherein the median particle size Dv50 of the lithium iron phosphate small particles is 0.3 μm to 0.5 μm, the carbon content is 1.5% to 2.2%, and the specific surface area is 15 m². 2 / g~22m 2 / g; Prepare large lithium iron phosphate particles, wherein the median particle size Dv50 of the large lithium iron phosphate particles is 1.0μm~2.0μm, the carbon content is 0.3%~0.8%, and the specific surface area is 3m². 2 / g~8m 2 / g; and mixing, crushing, sieving and removing iron from the lithium iron phosphate small particles and the lithium iron phosphate large particles to obtain lithium iron phosphate cathode material.

[0012] Based on the second aspect, in some possible embodiments, in the preparation method of the lithium iron phosphate cathode material: The method for preparing the lithium iron phosphate small particles includes: mixing a phosphorus source, an iron source, a lithium source, a first carbon source, and a solvent to form a first mixture, and grinding the first mixture to obtain a first slurry, wherein the mass percentage of iron in the first carbon source and the iron source in the first mixture is 14.5%~21.5%, and the particle size Dv50 of the first slurry is 0.5μm~1.5μm; sintering the first slurry to obtain sintered material A; mixing the sintered material A, a second carbon source, and a solvent to form a second mixture, and grinding the second mixture to obtain a second slurry, wherein the mass percentage of the second carbon source and the sintered material A is 6.0%~16.5%, and the particle size Dv50 of the second slurry is 0.3μm~0.4μm; and sintering the second slurry to obtain the lithium iron phosphate small particles. The method for preparing large lithium iron phosphate particles includes: mixing a phosphorus source, an iron source, a lithium source, a third carbon source, and a solvent to form a third mixture, and grinding the third mixture to obtain a third slurry, wherein the mass percentage of iron in the third carbon source and the iron source in the third mixture is 14.5%~21.5%, and the particle size Dv50 of the third slurry is 0.3μm~1.2μm; sintering the third slurry to obtain sintered material B; mixing the sintered material B, a fourth carbon source, and a solvent to form a fourth mixture, and grinding the fourth mixture to obtain a fourth slurry, wherein the mass percentage of the fourth carbon source and the sintered material B is 4%~13%, and the particle size Dv50 of the fourth slurry is 1.0μm~2.0μm; and sintering the fourth slurry to obtain the large lithium iron phosphate particles. The mass ratio of the small lithium iron phosphate particles to the large lithium iron phosphate particles is (0.3~0.7):(0.7~0.3).

[0013] Based on the second aspect, in some possible embodiments, a first additive is further added to the first mixture and / or the second mixture, the first additive comprising at least one of oxalate, phosphate, oxide and carbonate containing a first metal element, the first metal element comprising at least one of V, Ti, Nb, W and B; and / or a second additive is further added to the third mixture and / or the fourth mixture, the second additive comprising at least one of oxalate, phosphate, oxide and carbonate containing a second metal element, the second metal element comprising at least one of V, Ti, Nb, W and B.

[0014] Based on the second aspect, in some possible embodiments, in the step of sintering the first slurry to obtain sintered material A, the sintering temperature is 300℃~500℃ and the sintering time is 3h~15h; and / or in the step of sintering the second slurry to obtain the lithium iron phosphate small particles, the sintering temperature is 700℃~800℃ and the sintering time is 3h~15h; and / or in the step of sintering the third slurry to obtain sintered material B, the sintering temperature is 600℃~800℃ and the sintering time is 3h~15h; and / or in the step of sintering the fourth slurry to obtain the lithium iron phosphate large particles, the sintering temperature is 600℃~850℃ and the sintering time is 3h~15h.

[0015] Thirdly, embodiments of this application provide an electrochemical device, the electrochemical device including a positive electrode sheet, the positive electrode sheet including a positive electrode active material, the positive electrode active material being the aforementioned lithium iron phosphate positive electrode material.

[0016] Compared to existing technologies, the lithium iron phosphate cathode material provided in this application uses a gradation of a larger first particle and a smaller second particle. The capacity and rate performance of the first particle are improved by adjusting its carbon content and specific surface area. Based on the breakthrough in the performance of the first particle, the cycle performance of the smaller second particle is improved by adjusting its carbon content and specific surface area. The gradation of the first and second particles enables the lithium iron phosphate cathode material to achieve both high density and long cycle performance. Attached Figure Description

[0017] Figure 1 This is a general process flow diagram of the preparation method of lithium iron phosphate cathode material provided in an embodiment of this application.

[0018] Figure 2 This is a schematic diagram of the process flow for preparing lithium iron phosphate cathode material according to an embodiment of this application.

[0019] Figure 3 This is a scanning electron microscope image (magnification 10K) of the lithium iron phosphate cathode material in Example 2 of this application.

[0020] Figure 4 This is a scanning electron microscope image (magnification 5K) of the lithium iron phosphate cathode material in Example 2 of this application.

[0021] Figure 5 This is a particle size and volume distribution diagram of the lithium iron phosphate cathode material in Example 2 of this application. Detailed Implementation

[0022] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0023] This application provides a lithium iron phosphate cathode material, which includes first particles (i.e., large particles) with a particle size of 1000 nm or more and second particles (i.e., small particles) with a particle size of 500 nm or less. The first particles have a carbon content of 0.4% to 0.8% and a specific surface area of ​​4.0 μm². 2 / g~8.0m 2 / g, the carbon content of the second particle is 1.5%~2.0%, and the specific surface area of ​​the second particle is 15m². 2 / g~20m 2 / g.

[0024] In this application, the carbon content in lithium iron phosphate (LFP) originates from the carbon coating layer on its surface. For the first LFP particle, which has a larger particle size (above 1000 nm), the carbon content is reduced from the industry standard of 1.0%~1.5% to a lower range of 0.4%~0.8%. This reduces the amount of carbon coating in the first particle, decreases the carbon layer thickness, and overcomes the obstacle of a thick carbon layer to the lithium-ion diffusion rate. This improves the capacity utilization and rate performance of large LFP particles. Furthermore, reducing the thickness of the low-density carbon coating layer also helps to increase the tap density of the first particle. On the other hand, this application controls the specific surface area of ​​the first particle to be around 4.0 μm². 2 / g~8.0m 2 The low specific surface area ( / g) gives the first particles good flowability and processing performance, significantly improving cycle performance. Through the synergistic effect of low carbon content and low specific surface area, the problems of low capacity and poor rate performance of large-particle lithium iron phosphate have been overcome.

[0025] In some embodiments, the first particle may also contain a first metal element, which may include at least one of V, Ti, V, Nb, W, B, etc. The first metal element may exist inside or on the surface of the first particle, both of which can synergistically reduce carbon content and specific surface area, thereby further improving the rate performance of the first particle.

[0026] Based on the breakthrough in the capacity performance of the first particle, the smaller lithium iron phosphate second particle (with a size below 500nm) can increase its carbon content to a higher range of 1.5% to 2.0%, which can improve the conductivity of the second particle and thus improve the cycle performance of the second particle (small particle).

[0027] In some embodiments, the second particle may also contain a second metal element, which may include at least one of V, Ti, Nb, W, B, etc. The second metal element may exist inside or on the surface of the second particle, which can further improve the cycling performance of the second particle.

[0028] The lithium iron phosphate cathode material provided in this application is obtained by grading the above-mentioned large-sized first particles with high capacity and high rate performance and small-sized second particles with long cycle performance, thus achieving a lithium iron phosphate cathode material that balances high capacity and long cycle performance.

[0029] In some embodiments, in lithium iron phosphate cathode materials, the volumetric density of the first particle can be 60.0%~70.0%, and the volumetric density of the second particle can be 12.0%~20.0%. By adjusting the volumetric density ratio of the two, it is beneficial to further improve the gradation effect of the first and second particles, thereby controlling the overall performance parameters of the lithium iron phosphate cathode material (such as median particle size, specific surface area, compaction density, etc.), and further improving the electrical and processing performance of the lithium iron phosphate cathode material. In addition, a larger proportion of large-sized first particles and a smaller proportion of small-sized second particles is beneficial to reducing the preparation cost (smaller particles require more fine grinding, resulting in higher costs), and further improving the processing performance of the lithium iron phosphate cathode material (an excessively high proportion of small particles is not conducive to improving the flowability of the cathode slurry). Therefore, at this ratio, the lithium iron phosphate cathode material can further balance high capacity, long cycle life, low cost, and excellent processing performance.

[0030] Specifically, in lithium iron phosphate cathode materials, the volumetric density of particles with a size below 500 nm accounts for 12.0%~20.0%, particles with a size below 1000 nm account for 30.0%~40.0%, particles with a size below 2000 nm account for 66.0%~75.0%, particles with a size below 3000 nm account for 85.0%~95.0%, and particles with a size greater than 3000 nm account for 5.0%~15.0%. Figure 5 The volume distribution characteristics of the lithium iron phosphate cathode material show that the graded cathode material has a distinct bimodal characteristic.

[0031] In some embodiments, the median particle size Dv50 of the lithium iron phosphate cathode material can be 0.8 μm to 1.5 μm. A median particle size Dv50 within this range is beneficial for achieving optimal electrical and processing performance of the lithium iron phosphate material. A particle size distribution within this range significantly improves the processing performance of nano-lithium iron phosphate, and while ensuring optimal electrical performance, it can increase the stability of the slurry's solid content and viscosity, thereby improving the batch consistency of the battery gradation.

[0032] In some embodiments, the carbon content of the lithium iron phosphate cathode material can be 0.9% to 1.5%. Within this range, the carbon content can achieve uniform coating of the primary lithium iron phosphate particles, resulting in relatively balanced electronic and ionic conductivity.

[0033] In some embodiments, the specific surface area of ​​the lithium iron phosphate cathode material can be 10 m². 2 / g~15m 2 / g, which helps reduce side reactions between lithium iron phosphate cathode material and electrolyte.

[0034] In some embodiments, the powder compaction density of the lithium iron phosphate cathode material is 2.4 g / cm³. 3 ~2.75g / cm 3 Due to the gradation of the first and second particles, the overall compaction density of the lithium iron phosphate cathode material is increased, thereby improving the energy density of the lithium iron phosphate cathode material.

[0035] Based on the same inventive concept, please refer to Figure 1 and Figure 2 As shown in the figure, this application provides a method for preparing lithium iron phosphate cathode material, which specifically includes the following steps: Step S1: Prepare lithium iron phosphate particles with a median particle size Dv50 of 0.3 μm to 0.5 μm, a carbon content of 1.5% to 2.2%, and a specific surface area of ​​15 m². 2 / g~22m 2 / g.

[0036] The specific steps for preparing lithium iron phosphate small particles may include: In step S11, the phosphorus source, iron source, lithium source, first carbon source and solvent are mixed to form a first mixture, and the first mixture is ground to obtain a first slurry. The mass percentage of iron in the first carbon source and iron source in the first mixture is 14.5%~21.5%, and the particle size Dv50 of the first slurry is 0.5μm~1.5μm.

[0037] Step S12: Sinter the first slurry to obtain sintered material A.

[0038] In step S13, the sintering material A, the second carbon source and the solvent are mixed to form a second mixture, and the second mixture is ground to obtain a second slurry. The mass percentage of the second carbon source to the sintering material A is 6.0% to 16.5%, and the particle size Dv50 of the second slurry is 0.3 μm to 0.4 μm.

[0039] Step S14: Sinter the second slurry to obtain lithium iron phosphate particles.

[0040] A first mixture is formed by mixing raw materials, including a phosphorus source, an iron source, a lithium source, a first carbon source, and a solvent (e.g., water), in a certain proportion. The first mixture is then ground to ensure uniform mixing and refine the particle size (Dv50) of the first slurry to 0.5 μm to 1.5 μm, achieving thorough mixing and uniform distribution of the raw materials. The ground first slurry is then sintered to allow for a uniform solid-phase reaction between the raw materials. Under the thermal reduction action of the first carbon source, the first slurry forms a sintered material A with a lithium iron phosphate crystalline phase. The sintered material A, a second carbon source, and a solvent (e.g., water) are then mixed to form a second mixture. In this step, a carbon source (the second carbon source) is added again and mixed with the sintered material A, followed by grinding. This grinding process breaks down and disperses the agglomerates in the sintered material A, ensuring thorough contact and mixing with the second carbon source. The ground second slurry is then sintered, and the second carbon source forms a carbon coating layer on the surface of the lithium iron phosphate, resulting in small lithium iron phosphate particles.

[0041] By performing two grinding and two sintering processes, and adjusting the addition amounts of the first and second carbon sources, a particle size Dv50 of 0.3 μm to 0.5 μm, a carbon content of 1.5% to 2.2%, and a specific surface area of ​​15 m² was obtained. 2 / g~22m 2 / g of lithium iron phosphate particles.

[0042] In some embodiments, the first carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol; the second carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol.

[0043] In some embodiments, a first additive is further added to the first mixture or the second mixture. The first additive includes at least one selected from oxalate, phosphate, oxide, and carbonate containing a first metal element, and the first metal element includes at least one selected from V, Ti, Nb, W, B, etc. Adding the above-mentioned first additive can effectively improve the crystal structure, electronic conductivity, ionic conductivity, and thermal stability of the small particles, further improving the cycling performance of the small particles.

[0044] In some embodiments, the phosphorus source includes at least one selected from iron phosphate, phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, and ammonium dihydrogen phosphate. The phosphorus source may further be iron phosphate. The iron source may include at least one selected from iron phosphate, oxalate, carbonate, and oxide. The iron source may further be iron phosphate. The lithium source may include at least one selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0045] In the step of sintering the first slurry to obtain sintered material A, the sintering temperature can be 300℃~500℃ and the sintering time can be 3h~15h, which is conducive to the formation of lithium iron phosphate crystal phase and to the control of particle size of sintered material A.

[0046] In some embodiments, the particle size Dv50 of the second slurry can be 0.3μm to 0.4μm. By grinding, the agglomerates in the sintered material A are opened and dispersed, and the particle size Dv50 of the second slurry is further adjusted to 0.3μm to 0.4μm, which is beneficial to control the size of the final lithium iron phosphate particles.

[0047] In the step of sintering the second slurry to obtain lithium iron phosphate particles, the sintering temperature can be 700℃~800℃ and the sintering time can be 3h~15h. The sintering temperature and time within the above range are beneficial to control the size of lithium iron phosphate particles and form a stable carbon coating layer, thereby further improving the conductivity, rate performance and cycle performance of lithium iron phosphate particles.

[0048] In some embodiments, the sintering equipment may include one of a pusher kiln, a roller kiln, and a rotary kiln. The kiln speed of the sintering equipment may be 5Hz to 20Hz.

[0049] In some embodiments, before sintering, the first or second slurry can be spray-dried, which helps to quickly convert the liquid slurry into a uniform powder material, thereby improving the uniformity of sintering. The inlet air temperature for spray drying can be 200℃~270℃, and the outlet air temperature can be 85℃~100℃.

[0050] In some embodiments, the first mixture can be ground using a sand mill, specifically, it can be ground once or multiple times. For example, coarse grinding is performed first, using zirconium beads with a diameter of about 0.6 mm, and the coarse grinding time is 10 to 20 minutes; then fine grinding is performed, using zirconium beads with a diameter of about 0.3 mm, and the fine grinding time is 20 to 80 minutes. After multiple grindings, a first slurry with a particle size Dv50 of 0.5 μm to 1.5 μm is obtained.

[0051] In some embodiments, the second mixture can be ground by a sand mill, and the grinding can be performed once or multiple times. For example, a single grinding can be performed directly, using zirconium beads with a diameter of about 0.3 mm and a grinding time of 60 to 300 min. After one grinding, a second slurry with a particle size Dv50 of 0.3 μm to 0.4 μm is obtained.

[0052] Step S2: Prepare large lithium iron phosphate particles with a median particle size Dv50 of 1.0 μm to 2.0 μm, a carbon content of 0.3% to 0.8%, and a specific surface area of ​​3 m². 2 / g~8m 2 / g.

[0053] The specific steps for preparing large lithium iron phosphate particles may include: In step S21, the phosphorus source, iron source, lithium source, third carbon source and solvent are mixed to form a third mixture, and the third mixture is ground to obtain a third slurry. The mass percentage of iron in the third carbon source and iron source in the third mixture is 14.5%~21.5%, and the particle size Dv50 of the third slurry is 0.3μm~1.2μm.

[0054] Step S22: Sinter the third slurry to obtain sintered material B.

[0055] Step S23: The sintering material B, the fourth carbon source and the solvent are mixed to form a fourth mixture, and the fourth mixture is ground to obtain a fourth slurry. The mass percentage of the fourth carbon source to the sintering material B is 4%~13.0%, and the particle size Dv50 of the fourth slurry is 1.0μm~2.0μm.

[0056] Step S24: Sinter the fourth slurry to obtain large lithium iron phosphate particles.

[0057] A third mixture is formed by mixing raw materials, including a phosphorus source, an iron source, a lithium source, a third carbon source, and a solvent (e.g., water), in a certain proportion. This third mixture is then ground to ensure uniform mixing and refine the particle size (Dv50) of the third slurry to 0.5 μm–1.5 μm, achieving thorough mixing and uniform distribution of the raw materials. The ground third slurry is then sintered to allow for a uniform solid-phase reaction between the raw materials. Under the thermal reduction effect of the third carbon source, the third slurry forms a sintered material B with a lithium iron phosphate crystalline phase. The sintered material B, a fourth carbon source, and a solvent (e.g., water) are then mixed to form a fourth mixture. In this step, a carbon source (the fourth carbon source) is added again and mixed with the sintered material B, followed by grinding. This grinding process breaks down and disperses the agglomerates in the sintered material B, ensuring thorough contact and mixing with the fourth carbon source. The ground fourth slurry is then sintered, and the fourth carbon source forms a carbon coating layer on the surface of the lithium iron phosphate, resulting in large lithium iron phosphate particles.

[0058] By performing two grinding and two sintering processes, and adjusting the addition amounts of the third and fourth carbon sources, a particle size Dv50 of 1.0 μm to 2.0 μm, a carbon content of 0.3% to 0.8%, and a specific surface area of ​​3 m² were obtained. 2 / g~8m 2 / g of large lithium iron phosphate particles.

[0059] In some embodiments, the third carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol; the fourth carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol.

[0060] In some embodiments, a second additive is further added to the third or fourth mixture. The second additive includes at least one selected from oxalate, phosphate, oxide, and carbonate containing a second metal element, and the second metal element includes at least one selected from V, Ti, Nb, W, B, etc. Adding the above-mentioned second additive can effectively improve the crystal structure, electronic conductivity, ionic conductivity, and thermal stability of the small particles, further enhancing their cycling performance.

[0061] In some embodiments, the phosphorus source includes at least one selected from iron phosphate, phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, and ammonium dihydrogen phosphate. The phosphorus source may further be iron phosphate. The iron source may include at least one selected from iron phosphate, oxalate, carbonate, and oxide. The iron source may further be iron phosphate. The lithium source may include at least one selected from lithium carbonate, lithium phosphate, and lithium dihydrogen phosphate.

[0062] In the step of sintering the third slurry to obtain sintered material B, the sintering temperature can be 600℃~800℃ and the sintering time can be 3h~15h, which is conducive to the formation of lithium iron phosphate crystal phase and to the control of particle size of sintered material B.

[0063] In some embodiments, the particle size Dv50 of the fourth slurry can be 1.0 μm to 2.0 μm. By grinding, the agglomerates in the sintered material A are opened and dispersed, and the particle size Dv50 of the second slurry is further adjusted to 0.3 μm to 0.4 μm, which is beneficial to control the size of the final lithium iron phosphate particles.

[0064] In the step of sintering the fourth slurry to obtain large lithium iron phosphate particles, the sintering temperature can be 600℃~850℃ and the sintering time can be 3h~15h. The sintering temperature and time within the above range are beneficial to control the size of the large lithium iron phosphate particles and form a stable carbon coating layer, thereby further improving the capacity, rate performance, cycle performance and processing performance of the large lithium iron phosphate particles.

[0065] In some embodiments, the sintering equipment may include one of a pusher kiln, a roller kiln, and a rotary kiln. The kiln speed of the sintering equipment may be 5Hz to 20Hz.

[0066] In some embodiments, before sintering, the third or fourth slurry can be spray-dried, which helps to quickly convert the liquid slurry into a uniform powder, thereby improving the uniformity of sintering. The inlet air temperature for spray drying can be 200℃~270℃, and the outlet air temperature can be 85℃~100℃.

[0067] In some embodiments, the third mixture can be ground using a sand mill, specifically, one or more grinding operations can be performed. For example, coarse grinding is performed first, using zirconium beads with a diameter of about 0.6 mm, for 10 to 30 minutes; then fine grinding is performed, using zirconium beads with a diameter of about 0.3 mm, for 80 to 300 minutes. After multiple grinding operations, a third slurry with a particle size Dv50 of 0.3 μm to 1.2 μm is obtained.

[0068] In some embodiments, the fourth mixture can be ground by a sand mill, and the grinding can be performed once or multiple times. For example, it can be ground directly once, using zirconium beads with a diameter of about 0.3 mm, and the grinding time is 10 to 80 minutes. After one grinding, a fourth slurry with a particle size Dv50 of 1.0 μm to 2.0 μm is obtained.

[0069] Step S3: Mix and crush small lithium iron phosphate particles with large lithium iron phosphate particles to obtain lithium iron phosphate cathode material.

[0070] Specifically, small lithium iron phosphate particles and large lithium iron phosphate particles are mixed in a certain proportion using a high-speed mixer. The mixed material is then pulverized to break up the agglomerates and fully disperse them, yielding the finished lithium iron phosphate cathode material. This lithium iron phosphate cathode material includes first particles with a particle size greater than 1000 nm and second particles with a particle size less than 500 nm. The carbon content of the first particles is 0.4%~0.8%, and the specific surface area of ​​the first particles is 4.0 m². 2 / g~8.0m 2 / g, the carbon content of the second particle is 1.5%~2.0%, and the specific surface area of ​​the second particle is 15m². 2 / g~20m 2 / g.

[0071] In some embodiments, the mass ratio of small lithium iron phosphate particles to large lithium iron phosphate particles can be (0.3~0.7):(0.7~0.3) or (30%~70%):(70%~30%). Mixing at this ratio facilitates better particle size distribution, allowing the lithium iron phosphate cathode material to better balance high capacity, long cycle life, low cost, and excellent processing performance, while also improving the compaction density of the lithium iron phosphate cathode material. This mass ratio can be exemplarily 0.3:0.7, 0.4:0.6, 0.5:0.5, 0.6:0.4, 0.7:0.3, or any value within the range of any two of the above values.

[0072] In some embodiments, the mixing speed can be around 1000 rpm and the mixing time can be around 30 minutes, which is beneficial for thorough and uniform mixing.

[0073] In some embodiments, the mixed material is pulverized using an air jet mill to a particle size Dv50 of 0.8 μm to 1.5 μm, that is, the median particle size Dv50 of the lithium iron phosphate cathode material is 0.8 μm to 1.5 μm.

[0074] By controlling the process parameters of airflow pulverization, agglomerates can be broken up, achieving uniform dispersion of particles of different sizes, regulating the median particle size of the cathode material, and simultaneously preserving the original performance of particles of different sizes as much as possible. Specifically, by controlling the pulverizing air pressure at 0.3~0.5 MPa and the classifier wheel frequency at 25~35 Hz, secondary agglomerates can be broken down to achieve monodisperse, single-crystal lithium iron phosphate products.

[0075] The grinding air pressure is controlled between 0.3 and 0.5 MPa. Within this range, the grinding air pressure is moderate, which can fully break up agglomerated particles without causing excessive particle breakage and cracking due to strong collisions caused by excessive grinding air pressure, or insufficient particle collisions due to insufficient grinding pressure, resulting in particles not being dispersed in the first stage. For example, the grinding air pressure can be controlled at 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa, or any value within the range of any two of the above values.

[0076] The frequency control of the grading wheel in the crushing process directly determines the size of the largest particles in the crushed material. A higher frequency results in more thorough crushing. Generally, for a 10kg / h machine, the grading wheel frequency is controlled at 60%~90% (25~35Hz) under full load conditions. This ensures that the material is basically completely crushed, preventing the escape of many large, uncrushed particles that would lead to incomplete crushing. For example, the frequency of the crushing grading wheel can be controlled at 25Hz, 27Hz, 29Hz, 31Hz, 33Hz, 35Hz, or any value within the range of any two of these values.

[0077] In some embodiments, the pulverized lithium iron phosphate cathode material can also be sieved, iron removed, and packaged, wherein sieving is used to remove excessively large particles.

[0078] Compared with the prior art, the method for preparing lithium iron phosphate cathode material provided in this application has the following beneficial effects: 1. This application utilizes the synergistic effect of multiple factors, including controlling the amount of carbon source added, two grinding processes, and two sintering processes, to prepare large particles of lithium iron phosphate cathode material with specific carbon content and specific surface area. These large particles overcome the technical challenge of significant capacity degradation in lithium iron phosphate cathode material with particle sizes greater than or equal to 1 μm. This breakthrough yields lithium iron phosphate cathode material with primary particle sizes of 1 μm to 2 μm that still maintain high capacity. Furthermore, it effectively improves the conductivity and tap density of the lithium iron phosphate cathode material, enhances particle size distribution uniformity and particle roundness, and reduces the specific surface area, thus achieving a combined improvement in capacity performance, cycle performance, and processing performance.

[0079] 2. Based on the breakthrough in the performance of large lithium iron phosphate particles, the particle size of small lithium iron phosphate particles used in the preparation of high-density lithium iron phosphate cathode material gradation can be increased from below 0.3μm to 0.3μm~0.5μm. By controlling the amount of carbon source added, two grinding and two sintering, small lithium iron phosphate cathode material particles with specific carbon content and specific surface area were prepared by utilizing the synergistic effect of multiple factors, thereby improving cycle performance.

[0080] 3. By grading large and small lithium iron phosphate particles, lithium iron phosphate cathode materials that balance high compaction density, high capacity, long cycle performance, high rate performance, low cost, and excellent processing performance can be obtained.

[0081] This application also provides an electrochemical device (e.g., a battery) that includes a positive electrode, wherein the positive electrode includes a positive active material, and the positive active material is the lithium iron phosphate positive electrode material as described above.

[0082] The electrochemical device prepared using the aforementioned lithium iron phosphate cathode material has advantages such as high capacity, high rate performance and good cycle performance.

[0083] The following specific examples further illustrate the aforementioned lithium iron phosphate cathode material, its preparation method, and electrochemical device.

[0084] Preparation of lithium iron phosphate small particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (first carbon source), 0.66 kg of titanium dioxide and 135 kg of water to form a first mixture. Then, introduce the first mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm, to obtain the first slurry. The mass percentage of the first carbon source glucose and the iron element in the ferric phosphate in the first mixture is about 16%, and the particle size Dv50 of the first slurry is 0.5 μm.

[0085] Step 2: First, spray dry the first slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 500℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled to be below 10ppm to obtain sintered material A. Step 3: Mix 100kg of sintered material A, 6.7kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.15kg of titanium dioxide, and 110kg of water to form a second mixture. Then, introduce the second mixture into a sand mill for grinding. The diameter of the zirconium beads in one grinding is 0.3mm, the grinding time is 250min, and the median particle size Dv50 is 0.35μm to obtain the second slurry. That is, the mass percentage of the second carbon source to sintered material A is about 9.7%, and the particle size Dv50 of the second slurry is about 0.35μm.

[0086] Step 4: First, spray dry the second slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain lithium iron phosphate small particles with a median particle size Dv50 of about 0.4μm.

[0087] Preparation of large lithium iron phosphate particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (third carbon source), 0.66 kg of titanium dioxide, and 135 kg of water to form a third mixture. Then, introduce the third mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 150 min) until the median particle size Dv50 is 0.35 μm, to obtain the third slurry. That is, the mass percentage of the third carbon source and iron element in the ferric phosphate in the third mixture is about 16%, and the particle size Dv50 of the third slurry is 0.35 μm.

[0088] Step 2: First, spray dry the third slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm to obtain sintered material B. Step 3: Mix 100kg of sintered material B, 2.5kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a fourth mixture. Then, introduce the fourth mixture into a sand mill for grinding. The diameter of the zirconium beads in one grinding is 0.6mm, the grinding time is 30min, and the median particle size Dv50 is 1.5μm to obtain the fourth slurry. That is, the mass percentage of the fourth carbon source to sintered material B is about 5.5%, and the particle size Dv50 of the fourth slurry is about 1.5μm.

[0089] Step 4: First, spray dry the fourth slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 780℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, the powder is passed through an air jet mill, sieved, iron removed, and packaged to obtain large lithium iron phosphate particles with a median particle size Dv50 of about 1.5μm.

[0090] Example 1 Step 1: Mix the above-mentioned small lithium iron phosphate particles and large lithium iron phosphate particles at a mass ratio of 50%:50% using a high-speed mixer at 100 rpm for 30 min, then crush them using an air jet mill (crushing air pressure 0.35 MPa, classifying wheel frequency 30 Hz), sieve, remove iron, and package to obtain lithium iron phosphate cathode material.

[0091] Example 2 Step 1: Mix the above-mentioned small lithium iron phosphate particles and large lithium iron phosphate particles at a mass ratio of 40%:60% using a high-speed mixer at 100 rpm for 30 min, then crush them using an air jet mill (crushing air pressure controlled at 0.35 MPa, classifying wheel frequency at 30 Hz), sieve, remove iron, and package to obtain lithium iron phosphate cathode material.

[0092] Example 3 Step 1: Mix the above-mentioned small lithium iron phosphate particles and large lithium iron phosphate particles at a mass ratio of 30%:70% using a high-speed mixer at 100 rpm for 30 min, then crush them using an air jet mill (crushing air pressure 0.35 MPa, classifying wheel frequency 30 Hz), sieve, remove iron, and package to obtain lithium iron phosphate cathode material.

[0093] Example 4 Step 1: Mix the above-mentioned small lithium iron phosphate particles and large lithium iron phosphate particles at a mass ratio of 60%:40% using a high-speed mixer at 100 rpm for 30 min, then crush them using an air jet mill (crushing air pressure 0.35 MPa, classifying wheel frequency 30 Hz), sieve, remove iron, and package to obtain lithium iron phosphate cathode material.

[0094] Example 5 Step 1: Mix the above-mentioned small lithium iron phosphate particles and large lithium iron phosphate particles at a mass ratio of 70%:30% using a high-speed mixer at 100 rpm for 30 min, then crush them using an air jet mill (crushing air pressure 0.35 MPa, classifying wheel frequency 30 Hz), sieve, remove iron, and package to obtain lithium iron phosphate cathode material.

[0095] Example 6 Preparation of lithium iron phosphate small particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (first carbon source), 0.66 kg of titanium dioxide and 135 kg of water to form a first mixture. Then, introduce the first mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm, to obtain the first slurry. The mass percentage of the first carbon source glucose and the iron element in the ferric phosphate in the first mixture is about 16%, and the particle size Dv50 of the first slurry is 0.5 μm.

[0096] Step 2: First, spray dry the first slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 500℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled to be below 10ppm to obtain sintered material A. Step 3: Mix 100kg of sintered material A, 6.7kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a second mixture. Then, introduce the second mixture into a sand mill for grinding. The diameter of the zirconium beads in one grinding is 0.3mm, the grinding time is 290min, and the median particle size Dv50 is 0.3μm to obtain the second slurry. That is, the mass percentage of the second carbon source to sintered material A is about 9.7%, and the particle size Dv50 of the second slurry is about 0.3μm.

[0097] Step 4: First, spray dry the second slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain lithium iron phosphate small particles with a median particle size Dv50 of about 0.4μm.

[0098] Preparation of large lithium iron phosphate particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (third carbon source), 0.81 kg of titanium dioxide, and 135 kg of water to form a third mixture. Then, introduce the third mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm, to obtain the third slurry. That is, the mass percentage of the third carbon source and iron element in the ferric phosphate in the third mixture is about 16%, and the particle size Dv50 of the third slurry is 0.5 μm.

[0099] Step 2: First, spray dry the third slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm to obtain sintered material B. Step 3: Mix 100kg of sintered material B, 2.5kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a fourth mixture. Then, introduce the fourth mixture into a sand mill for grinding. The diameter of the zirconium beads in one grinding is 0.3mm, the grinding time is 20min, and the median particle size Dv50 is 1.5μm to obtain the fourth slurry. That is, the mass percentage of the fourth carbon source to sintered material B is about 5.5%, and the particle size Dv50 of the fourth slurry is about 1.5μm.

[0100] Step 4: First, spray dry the fourth slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 780℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain large lithium iron phosphate particles with a median particle size Dv50 of about 1.5μm.

[0101] Preparation of lithium iron phosphate cathode materials: In this embodiment, small lithium iron phosphate particles and large lithium iron phosphate particles were mixed at a mass ratio of 50%:50% using a high-speed mixer at 100 rpm for 30 min, then crushed by air jet milling (milling pressure 0.35 MPa, classifier wheel frequency 30 Hz), sieved, iron removed, and packaged to obtain lithium iron phosphate cathode material.

[0102] Example 7 Preparation of lithium iron phosphate small particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (first carbon source), 0.66 kg of titanium dioxide and 135 kg of water to form a first mixture. Then, introduce the first mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm, to obtain the first slurry. The mass percentage of the first carbon source glucose and the iron element in the ferric phosphate in the first mixture is about 16%, and the particle size Dv50 of the first slurry is 0.5 μm.

[0103] Step 2: First, spray dry the first slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 500℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled to be below 10ppm to obtain sintered material A. Step 3: Mix 100kg of sintered material A, 7.6kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a second mixture. Then, introduce the second mixture into a sand mill for grinding. The diameter of the zirconium beads in one grinding is 0.3mm, the grinding time is 290min, and the median particle size Dv50 is 0.3μm to obtain the second slurry. That is, the mass percentage of the second carbon source to sintered material A is about 9.7%, and the particle size Dv50 of the second slurry is about 0.3μm.

[0104] Step 4: First, spray dry the second slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain lithium iron phosphate small particles with a median particle size Dv50 of about 0.3μm.

[0105] Preparation of large lithium iron phosphate particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (third carbon source), 0.66 kg of titanium dioxide, and 135 kg of water to form a third mixture. Then, introduce the third mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm. The third slurry is obtained. The mass percentage of the third carbon source and iron in the ferric phosphate in the third mixture is about 16%, and the particle size Dv50 of the third slurry is 0.5 μm.

[0106] Step 2: First, spray dry the third slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm to obtain sintered material B. Step 3: Mix 100kg of sintered material B, 2.5kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a fourth mixture. Then, introduce the fourth mixture into a sand mill for grinding. The diameter of the zircon beads in one grinding is 0.6mm, the grinding time is 15min, and the median particle size Dv50 is 2.0μm to obtain the fourth slurry. That is, the mass percentage of the fourth carbon source to sintered material B is about 5.5%, and the particle size Dv50 of the fourth slurry is about 2.0μm.

[0107] Step 4: First, spray dry the fourth slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 780℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain large lithium iron phosphate particles with a median particle size Dv50 of about 2.0μm.

[0108] Preparation of lithium iron phosphate cathode materials: In this embodiment, small lithium iron phosphate particles and large lithium iron phosphate particles were mixed at a mass ratio of 50%:50% using a high-speed mixer at 100 rpm for 30 min, then crushed by air jet milling (milling pressure 0.35 MPa, classifier wheel frequency 30 Hz), sieved, iron removed, and packaged to obtain lithium iron phosphate cathode material.

[0109] Example 8 Preparation of lithium iron phosphate small particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (first carbon source), 0.66 kg of titanium dioxide and 135 kg of water to form a first mixture. Then, introduce the first mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 70 min) until the median particle size Dv50 is 0.5 μm, to obtain the first slurry. The mass percentage of the first carbon source glucose and the iron element in the ferric phosphate in the first mixture is about 16%, and the particle size Dv50 of the first slurry is 0.5 μm.

[0110] Step 2: First, spray dry the first slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 500℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5 Hz and the oxygen concentration is controlled below 10 ppm to obtain sintered material A. Step 3: Mix 100kg of sintered material A, 6.0kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a second mixture. Then, introduce the second mixture into a sand mill for grinding. The diameter of the zircon beads in one grinding is 0.3mm, the grinding time is 200min, and the grinding is carried out until the median particle size Dv50 is 0.50μm to obtain the second slurry. That is, the mass percentage of the second carbon source to sintered material A is about 9.7%, and the particle size Dv50 of the second slurry is about 0.5μm.

[0111] Step 4: First, spray dry the second slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5 Hz and the oxygen concentration is controlled below 10 ppm. After sintering, crush, sieve and remove iron to obtain lithium iron phosphate small particles with a median particle size Dv50 of about 0.5μm.

[0112] Preparation of large lithium iron phosphate particles: Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose (third carbon source), 0.66 kg of titanium dioxide, and 135 kg of water to form a third mixture. Then, introduce the third mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 150 min) until the median particle size Dv50 is 0.35 μm, to obtain the third slurry. That is, the mass percentage of the third carbon source and iron element in the ferric phosphate in the third mixture is about 16%, and the particle size Dv50 of the third slurry is 0.35 μm.

[0113] Step 2: First, spray dry the third slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is raised to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm to obtain sintered material B. Step 3: Mix 100kg of sintered material B, 2.5kg of glucose, 3kg of polyethylene glycol (second carbon source), 0.417kg of titanium dioxide, and 110kg of water to form a fourth mixture. Then, introduce the fourth mixture into a sand mill for grinding. The diameter of the zircon beads in one grinding is 0.6mm, the grinding time is 50min, and the median particle size Dv50 is 1.0μm to obtain the fourth slurry. That is, the mass percentage of the fourth carbon source to sintered material B is about 5.5%, and the particle size Dv50 of the fourth slurry is about 1.0μm.

[0114] Step 4: First, spray dry the fourth slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 780℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, crush, sieve and remove iron to obtain large lithium iron phosphate particles with a median particle size Dv50 of about 1.0μm.

[0115] Preparation of lithium iron phosphate cathode materials: In this embodiment, small lithium iron phosphate particles and large lithium iron phosphate particles were mixed at a mass ratio of 50%:50% using a high-speed mixer at 100 rpm for 30 min, then pulverized using an air jet mill (pulverizing air pressure 0.35 MPa, classifying wheel frequency 30 Hz), and after sieving, iron removal and packaging, lithium iron phosphate cathode material was obtained.

[0116] Comparative Example 1 Step 1: Mix 24.8 kg of phosphorus source, 100 kg of ferric phosphate, 6.0 kg of glucose, 0.81 kg of titanium dioxide, and 135 kg of water to form a primary mixture. Then, introduce the primary mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6 mm, grinding time 15 min) until the median particle size Dv50 is 1.5 μm, and then fine grinding (zirconium bead diameter 0.3 mm, grinding time 150 min) until the median particle size Dv50 is 0.35 μm, to obtain a primary slurry. The mass percentage of iron in glucose and ferric phosphate in the primary mixture is approximately 16%, and the particle size Dv50 of the primary slurry is 0.35 μm.

[0117] Step 2: First, spray dry the primary slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 750℃ at a rate of 3℃ / min. Then, feed and sinter for 7 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm to obtain the primary sintered material. Step 3: Mix 100kg of primary sintering material, 4.6kg of glucose, 3kg of polyethylene glycol, 0.417kg of titanium dioxide, and 110kg of water to form a secondary mixture. Then, introduce the secondary mixture into a sand mill for grinding. The grinding includes coarse grinding (zirconium bead diameter 0.6mm, grinding time 30min) until the median particle size Dv50 is 1.5μm. Take out 100kg to obtain slurry A. The remaining slurry is then finely ground (zirconium bead diameter 0.3mm, grinding time 200min) until the median particle size Dv50 is 0.35μm to obtain slurry B. The mass percentage of glucose and polyethylene glycol to primary sintering material is approximately 7.6%.

[0118] Step 4: Add 100kg of slurry A and 100kg of slurry B into a mixing tank and mix for 30 minutes to obtain a mixed slurry.

[0119] Step 5: First, spray dry the mixed slurry. The inlet air temperature for spray drying is 250℃ and the outlet air temperature is 90℃. Then, sinter the spray-dried powder in a rotary kiln under an inert atmosphere. The temperature is increased to 780℃ at a rate of 3℃ / min. Then, feed and sinter for 8 hours. The kiln speed is 5Hz and the oxygen concentration is controlled below 10ppm. After sintering, pulverize (pulverizing air pressure 0.35Mpa, classifying wheel frequency 30Hz), sieve, remove iron and package to obtain lithium iron phosphate cathode material.

[0120] The lithium iron phosphate cathode materials obtained in Examples 1-8 and Comparative Example 1 were tested accordingly.

[0121] Test method: 1. Field emission scanning electron microscopy (SEM) test. Thermo Fisher Scientific's AxiaChemiSEMHiVac model scanning electron microscope was used. This model has high-resolution imaging capabilities, which can clearly observe the microstructure and structural features of the cathode material. The accelerating voltage is 10.00kV, the working distance is 10mm, and the magnification is 10000x.

[0122] 2. Carbon content determination: The carbon content of LFP cathode material is tested by infrared analysis. A carbon-sulfur analyzer is used. The sample is burned in a high-temperature oxygen-rich state. The carbon element is oxidized into carbon dioxide and enters the infrared detector with the carrier gas. The carbon content is quantitatively calculated by statistically analyzing the change in the intensity of the infrared absorption wavelength of the carbon dioxide signal.

[0123] 3. Compacted density determination: The test was conducted in accordance with the method specified in the standard "Determination of compacted density of lithium-ion battery cathode material powder" drafted by the National Technical Committee for Standardization of Nonferrous Metals.

[0124] 4. Resistivity Measurement: The test was conducted in accordance with the method specified in the standard "Determination of Resistivity of Powdered Cathode Material for Lithium-ion Batteries" drafted by the National Technical Committee for Standardization of Nonferrous Metals.

[0125] 5. Electrochemical Performance Testing: Electrochemical performance testing was conducted using coin cells. The above-mentioned positive electrode material, polyvinylidene fluoride (PVDF), and conductive agent (such as acetylene black or conductive carbon black) were mixed in a mass ratio of 93:3.5:3.5, and an appropriate amount of NMP was added to prepare a slurry. Next, the slurry was uniformly coated onto aluminum foil and vacuum dried at 115℃±5℃ / 8h, then compacted and cut into circular pieces. Simultaneously, lithium metal sheets were used as the negative electrode material. Finally, all materials were transferred to a glove box and assembled into CR2025 specification coin cells. Constant current charge-discharge testing was performed using the Xinwei Battery Testing System.

[0126] (a) Rate performance test: The test operating voltage range is 2.0V~3.75V, the temperature is 25℃, and the 0.1C capacity, initial efficiency and 1.0C capacity of the button cell are measured.

[0127] (ii) Cyclic performance test: The test operating voltage range is 2.0V~3.75V, the temperature is 25℃, and the capacity retention rate of the button cell is measured after 100 cycles of 0.1C / 1C.

[0128] 6. Particle size determination: The particle size distribution of the material was measured using a Malvern 3000 particle size analyzer after ultrasonic dispersion for 5 min, including Dmin, Dv10, Dv50, Dv90, Dv99 and Dmax.

[0129] 7. Powder resistivity determination: The test was conducted according to the method specified in the standard "Determination of Powder Resistivity of Cathode Materials for Lithium-ion Batteries" drafted by the National Technical Committee on Standardization of Nonferrous Metals.

[0130] 8. Tap density determination: Refer to GB / T 30835-2014 "Carbon composite lithium iron phosphate cathode material for lithium-ion batteries".

[0131] 9. Specific surface area test: Using the nitrogen adsorption-desorption method, at liquid nitrogen temperature, the equilibrium adsorption amount of nitrogen on the surface of an object is related to its specific surface area and other characteristics. Combined with the law of the change of adsorption amount with relative pressure during the adsorption process, the specific surface area can be tested.

[0132] 10. Processing performance test: The lithium iron phosphate cathode material, polyvinylidene fluoride (PVDF), and conductive agent (such as acetylene black or conductive carbon black) are mixed in a mass ratio of 93:3.5:3.5. An appropriate amount of NMP is added to prepare a slurry with a solid content of 70%. The viscosity of the slurry is tested. The viscosity test method refers to the national standard "Test Method for Cathode Materials of Lithium-ion Batteries - Determination of Slurry Viscosity" (Project No.: 20240765-T-610).

[0133] The relevant process parameters and test results of Examples 1-8 and Comparative Example 1 are shown in Tables 1 to 3. Figures 3 to 5 As shown.

[0134] Table 1 Table 2 Table 3 The above results show that: like Figure 3 and Figure 4 As shown, it can be observed that the lithium iron phosphate cathode material in Example 2 exhibits typical characteristics of a mixture of particles of varying sizes, and the distribution of these particles is uniform. Furthermore, combined with... Figure 5 In Example 2, the lithium iron phosphate cathode material exhibits a typical bimodal particle size distribution, reflecting the effect of particle size gradation in Example 2 of this application. Specifically, the volume density of particles smaller than 500 nm accounts for 12.0%–20.0%, particles smaller than 1000 nm account for 30.0%–40.0%, particles smaller than 2000 nm account for 66.0%–75.0%, particles smaller than 3000 nm account for 85.0%–95.0%, and particles larger than 3000 nm account for 10.0%–20.0%. Furthermore, as shown in Table 3, tests in Examples 1–8 revealed that the carbon content of particles larger than 1000 nm in the lithium iron phosphate cathode materials was 0.4%–0.8%, and the specific surface area was 4.0 m². 2 / g~8.0m 2 / g, particles smaller than 500nm have a carbon content of 1.5%~2.0% and a specific surface area of ​​15m². 2 / g~20m 2 / g.

[0135] Referring to Tables 1 and 2, compared to Comparative Example 1, Examples 1-8, by separately preparing large and small lithium iron phosphate particles and simultaneously controlling the amount of carbon added and particle size, obtained large and small lithium iron phosphate particles with specific carbon content and specific surface area. This improved the capacity and rate performance of the large lithium iron phosphate particles and the capacity and cycle performance of the small lithium iron phosphate particles. Furthermore, the gradation process of large and small particles increased the compaction density of the lithium iron phosphate cathode material, resulting in a lithium iron phosphate cathode material that balances high capacity and long cycle life. Therefore, the powder resistivity of Comparative Example 1, which did not separately control the carbon content and specific surface area of ​​the large and small particles, was as high as 15.4 Ω·cm, the 0.1C discharge capacity was only 158.8 mAh / g, the 1C discharge capacity was only 138.8 mAh / g, and the capacity retention rate after 100 cycles was only 97.3%. In Examples 1-8, the resistivity of the lithium iron phosphate cathode materials was low, and the electrical performance was improved to varying degrees compared with Comparative Example 1. Among them, the resistivity of the powder in Example 1 was only 12.23 Ω·cm, the 0.1C discharge capacity was as high as 161.0 mAh / g, the 1C discharge capacity was as high as 146.3 mAh / g, and the capacity retention rate after 100 cycles was 98.3%.

[0136] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material comprises a first particle with a particle size greater than 1000 nm and a second particle with a particle size less than 500 nm. The carbon content of the first particle is 0.4% to 0.8%, and the specific surface area of ​​the first particle is 4.0 m². 2 / g~8.0m 2 / g, the carbon content of the second particle is 1.5%~2.0%, and the specific surface area of ​​the second particle is 15m². 2 / g~20m 2 / g.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, In the lithium iron phosphate cathode material, the volume density of the first particle accounts for 60.0%~70.0%, and the volume density of the second particle accounts for 12.0%~20.0%.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The first particle contains a first metallic element, which includes at least one of V, Ti, Nb, W, and B; and / or The second particle contains a second metallic element, which includes at least one of V, Ti, Nb, W and B.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that, In the lithium iron phosphate cathode material, the volume density of particles with a size of less than 500 nm accounts for 12.0% to 20.0%, the volume density of particles with a size of less than 1000 nm accounts for 30.0% to 40.0%, the volume density of particles with a size of less than 2000 nm accounts for 66.0% to 75.0%, the volume density of particles with a size of less than 3000 nm accounts for 85.0% to 95.0%, and the volume density of particles with a size of more than 3000 nm accounts for 5.0% to 15.0%.

5. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following conditions: (1) The median particle size Dv50 of the lithium iron phosphate cathode material is 0.8 μm to 1.5 μm; (2) The carbon content of the lithium iron phosphate cathode material is 0.9%~1.5%; (3) The specific surface area of ​​the lithium iron phosphate cathode material is 10 m². 2 / g~15m 2 / g; (4) The compacted density of the lithium iron phosphate cathode material is 2.4 g / cm³. 3 ~2.75g / cm 3 .

6. A method for preparing a lithium iron phosphate cathode material, characterized in that, include: Lithium iron phosphate (LFP) particles were prepared, wherein the median particle size (Dv50) of the LFP particles was 0.3 μm to 0.5 μm, the carbon content was 1.5% to 2.2%, and the specific surface area was 15 m². 2 / g~22m 2 / g; Large lithium iron phosphate particles were prepared, wherein the median particle size Dv50 of the large lithium iron phosphate particles was 1.0 μm to 2.0 μm, the carbon content was 0.3% to 0.8%, and the specific surface area was 3 m². 2 / g~8m 2 / g; and The lithium iron phosphate small particles are mixed with the lithium iron phosphate large particles and then crushed to obtain the lithium iron phosphate cathode material.

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In the preparation method of the lithium iron phosphate cathode material: The method for preparing the lithium iron phosphate particles includes: A phosphorus source, an iron source, a lithium source, a first carbon source, and a solvent are mixed to form a first mixture, and the first mixture is ground to obtain a first slurry. The mass percentage of iron in the first carbon source and the iron source in the first mixture is 14.5% to 21.5%, and the particle size Dv50 of the first slurry is 0.5 μm to 1.5 μm. The first slurry is sintered to obtain sintered material A; The sintering material A, the second carbon source, and the solvent are mixed to form a second mixture, and the second mixture is ground to obtain a second slurry. The mass percentage of the second carbon source to the sintering material A is 6.0%~16.5%, and the particle size Dv50 of the second slurry is 0.3μm~0.4μm. The second slurry is sintered to obtain the lithium iron phosphate particles; The method for preparing the large lithium iron phosphate particles includes: A phosphorus source, an iron source, a lithium source, a third carbon source, and a solvent are mixed to form a third mixture, and the third mixture is ground to obtain a third slurry. The mass percentage of iron in the third carbon source and the iron source in the third mixture is 14.5% to 21.5%, and the particle size Dv50 of the third slurry is 0.3 μm to 1.2 μm. The third slurry is sintered to obtain sintered material B; The sintering material B, the fourth carbon source, and the solvent are mixed to form a fourth mixture, and the fourth mixture is ground to obtain a fourth slurry. The mass percentage of the fourth carbon source to the sintering material B is 4.0%~13%, and the particle size Dv50 of the fourth slurry is 1.0μm~2.0μm. The fourth slurry is sintered to obtain the large lithium iron phosphate particles; The mass ratio of the small lithium iron phosphate particles to the large lithium iron phosphate particles is (0.3~0.7):(0.7~0.3).

8. The method for preparing the lithium iron phosphate cathode material according to claim 7, characterized in that, A first additive is further added to the first mixture and / or the second mixture, the first additive comprising at least one of oxalate, phosphate, oxide, and carbonate containing a first metal element, the first metal element comprising at least one of V, Ti, Nb, W, and B; and / or A second additive is also added to the third mixture and / or the fourth mixture, the second additive comprising at least one of oxalate, phosphate, oxide and carbonate containing a second metal element, the second metal element comprising at least one of V, Ti, Nb, W and B.

9. The method for preparing the lithium iron phosphate cathode material according to claim 7, characterized in that, In the step of sintering the first slurry to obtain sintered material A, the sintering temperature is 300℃~500℃, and the sintering time is 3h~15h; and / or In the step of sintering the second slurry to obtain the lithium iron phosphate particles, the sintering temperature is 700℃~800℃, and the sintering time is 3h~15h; and / or In the step of sintering the third slurry to obtain sintered material B, the sintering temperature is 600℃~800℃, and the sintering time is 3h~15h; and / or In the step of sintering the fourth slurry to obtain the large lithium iron phosphate particles, the sintering temperature is 600℃~850℃ and the sintering time is 3h~15h.

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode, wherein the positive electrode includes a positive active material, and the positive active material is a lithium iron phosphate positive electrode material as described in any one of claims 1 to 5 or is prepared by the method of preparing lithium iron phosphate positive electrode material as described in any one of claims 6 to 9.

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