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

Abstract

This application provides a lithium iron phosphate cathode material, its preparation method, and an electrochemical device. The preparation method includes: mixing a phosphorus source, an iron source, a lithium source, a first carbon source, a first additive, and a solvent to form a first mixture and grinding it to obtain a first slurry, wherein the mass percentage of iron in the first carbon source and the iron source is 14.5% to 21.5%, and the particle size of the first slurry is 0.3 μm to 0.8 μm; sintering the first slurry once to obtain a first-sintered material; mixing the first-sintered material, a second carbon source, an additive, and a solvent to form a second mixture and grinding it to obtain a second slurry, wherein the mass percentage of the second carbon source and the first-sintered material is 4% to 13%; and drying the second slurry and then sintering it a second time to obtain a lithium iron phosphate cathode material, wherein the particle size of the lithium iron phosphate cathode material is a large-particle lithium iron phosphate cathode material of 1.0 μm to 2.0 μm.

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 method for preparing lithium iron phosphate cathode material.

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

[0006] In a first aspect, embodiments of this application provide a method for preparing a lithium iron phosphate cathode material, comprising: mixing a phosphorus source, an iron source, a lithium source, a first carbon source, a first additive, 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 is 14.5% to 21.5%, and the particle size Dv50 of the first slurry is 0.3 μm to 0.8 μm; sintering the first slurry once to obtain a first-sintered material; mixing the first-sintered material, 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 first-sintered material is 4% to 13%; and sintering the second slurry a second time to obtain the lithium iron phosphate cathode material with a coating layer, wherein the particle size Dv50 of the lithium iron phosphate cathode material is 1.0 μm to 2.0 μm.

[0007] In some possible embodiments, the carbon content of the lithium iron phosphate cathode material is 0.3% to 1.2%.

[0008] In some possible embodiments, the particle size Dv50 of the primary sintering material is 0.8 μm to 1.2 μm.

[0009] In some possible embodiments, the first additive includes at least one of oxalate, phosphate, oxide and carbonate containing a metal element, wherein the metal element includes at least one of V, Ti, Nb, W, Li, B and Fe; and / or the first additive accounts for 0.1% to 0.6% of the mass percentage of the first mixture.

[0010] In some possible embodiments, the particle size Dv50 of the second slurry is 1.0 μm to 2.0 μm.

[0011] In some possible embodiments, a second additive is also added to the second mixture. The second additive includes at least one of oxalate, phosphate, oxide and carbonate containing a metal element, wherein the metal element includes at least one of V, Ti, Nb, W, Li, B and Fe. The second additive accounts for 0% to 0.3% of the mass percentage of the second mixture.

[0012] In some possible embodiments, the temperature of the first sintering is 600℃~800℃, the time of the first sintering is 3h~10h; and / or the heating rate of the first sintering is 2℃ / min~5℃ / min; and / or the temperature of the second sintering is 600℃~850℃, the time of the second sintering is 3h~10h; and / or the heating rate of the second sintering is 2℃ / min~5℃ / min.

[0013] In some possible embodiments, the specific surface area of ​​the lithium iron phosphate cathode material is 3.0 m². 2 / g~10.0m 2 / g; and / or the tap density of the lithium iron phosphate cathode material is 1.4 g / cm³. 3 ~1.8g / cm 3 .

[0014] Secondly, embodiments of this application provide a lithium iron phosphate cathode material prepared using the aforementioned method for preparing lithium iron phosphate cathode materials.

[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 method for preparing lithium iron phosphate cathode material provided in this application employs a two-stage grinding and two-stage sintering process. By controlling the grinding particle size and adjusting the addition amounts of the first and second carbon sources during the preparation process, the particle size and carbon coating amount of the lithium iron phosphate cathode material are controlled, resulting in lithium iron phosphate cathode material with a large particle size. Specifically, by precisely controlling the addition amounts of the first and second carbon sources during the preparation process, the carbon content in the finished lithium iron phosphate cathode material is reduced, and the coating layer thickness is effectively reduced. This mitigates the hindering effect of the coating layer on lithium-ion transport, facilitating rapid lithium-ion transport in the lithium iron phosphate cathode material.

[0017] Meanwhile, by refining the grain size in the first slurry through grinding, the uniformity of the primary sintering reaction can be improved. Combined with the doping of the first additive, the technical challenge of the significant capacity degradation of lithium iron phosphate cathode material particles with a particle size greater than or equal to 1 μm is overcome. A breakthrough has been achieved in preparing lithium iron phosphate cathode materials with a primary particle size of 1 μm to 2 μm that still maintain high capacity. This can effectively improve the conductivity and tap density of lithium iron phosphate cathode materials, improve the uniformity of particle size distribution and particle roundness, and reduce the specific surface area of ​​lithium iron phosphate cathode materials. This has resulted in a comprehensive improvement in the capacity performance, cycle performance and processing performance of lithium iron phosphate cathode materials. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of a method for preparing lithium iron phosphate cathode material according to an embodiment of this application.

[0019] Figure 2 This is a scanning electron microscope image of the sintered material in Example 8 of this application.

[0020] Figure 3 This is a scanning electron microscope image of the lithium iron phosphate cathode material in Example 8 of this application.

[0021] Figure 4 This is a scanning electron microscope image of the lithium iron phosphate cathode material in Comparison 1 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] Please see Figure 1 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: The phosphorus source, iron source, lithium source, first carbon source, first additive 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 is 14.5%~21.5%, and the particle size Dv50 of the first slurry is 0.3μm~0.8μm.

[0024] In this step, a first mixture is formed by mixing raw materials such as a phosphorus source, an iron source, a lithium source, a first carbon source, a first additive, 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 particles to 0.3 μm to 0.8 μm. This achieves thorough mixing and uniform distribution of the raw materials and enhances the activity and uniformity of the solid-phase reaction during the subsequent primary sintering. The particle size Dv50 of the first slurry particles can, for example, be 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, or any value within the range of any two of the above values.

[0025] In some embodiments, the first carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol.

[0026] In some embodiments, the first additive may include at least one of oxalates, phosphates, oxides, and carbonates containing a metal element, wherein the metal element in the first additive includes at least one of V, Ti, Nb, W, Li, B, and Fe. Doping with the above-mentioned first additive can effectively improve the crystal structure, electronic conductivity, ionic conductivity, and thermal stability of lithium iron phosphate cathode materials, thereby increasing the capacity and rate performance of lithium iron phosphate cathode materials.

[0027] In some embodiments, the first additive may account for 0.1% to 0.6% of the mass percentage of the first mixture. By controlling the amount of the first additive added, the doping amount of the first additive in the lithium iron phosphate cathode material can be adjusted within a suitable range, which is beneficial to balance the doping modification effect of the first additive on the lithium iron phosphate cathode material and the capacity performance of the lithium iron phosphate cathode material. This mass percentage may, exemplarily, be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, or any value within the range of any two of the above values. The mass percentage may further be 0.2% to 0.4%.

[0028] 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.

[0029] In some embodiments, the iron source may include at least one of iron phosphate, oxalate, carbonate, and oxide. The iron source may further be iron phosphate.

[0030] In some embodiments, the lithium source may include at least one of lithium carbonate, lithium phosphate, and lithium dihydrogen phosphate.

[0031] In some embodiments, the mixing method may include mixing with a high-speed mixer, stirring, or grinding. It is understood that the mixing methods include, but are not limited to, the methods described above.

[0032] In some embodiments, grinding the first mixture may include: first grinding (i.e., coarse grinding) the first mixture using grinding beads with a diameter of 0.5 mm to 0.8 mm until the particle size D50 is 1.0 μm to 3.0 μm, rapidly reducing the particle size and improving the mixing uniformity among the raw materials; then grinding (i.e., fine grinding) the first mixture using grinding beads with a diameter of 0.2 mm to 0.4 mm until the particle size D50 is 0.3 μm to 0.8 μm, further refining the particle size, making the particle size distribution of the first mixture more uniform, improving its roundness, and ensuring thorough and uniform mixing. Adopting a step-by-step grinding method, combining coarse and fine grinding, is beneficial for improving grinding efficiency and particle size distribution.

[0033] Step S2: The first slurry is sintered once to obtain a sintered material.

[0034] The first slurry after grinding is sintered once to carry out solid-phase reaction between raw materials. The first carbon source in the first slurry is thermally reduced to form a sintered material with lithium iron phosphate crystal phase. Since the first mixture was fully ground in step S1, the particle size Dv50 of the first slurry was precisely controlled to be 0.3μm~1.5μm, thereby effectively improving the activity and uniformity of the solid-phase reaction during the first sintering, and obtaining a sintered material with high purity lithium iron phosphate phase doped and modified.

[0035] Furthermore, the mass percentage of iron in the first carbon source and iron source is 14.5% to 21.5%. This range of first carbon source proportion is beneficial for fully reducing ferric iron in the iron source to ferrous iron during the primary sintering process, reducing the generation of impurity phases, and physically blocking the contact between particles of the primary sintering material, preventing them from becoming too large or agglomerating at high temperatures, thereby obtaining particles with high phase purity and uniform size. The mass percentage of iron in the first carbon source and iron source can, for example, be 14.5%, 15.5%, 16.5%, 17.5%, 18.5%, 19.5%, 20.5%, 21.5%, or any value within the range of any two of the above values. This mass percentage can further be 15.5% to 18.5%.

[0036] In some embodiments, the particle size Dv50 of the material after one sintering can be 0.8 μm to 1.2 μm. Controlling the particle size of the material after one sintering within the above range is beneficial for regulating the particle size of the final lithium iron phosphate cathode material.

[0037] In some embodiments, the primary sintering temperature is 600°C to 800°C. Primary sintering temperatures within this range are beneficial for the formation of the lithium iron phosphate crystalline phase and for controlling the particle size of the primary sintered material. The primary sintering temperature can, exemplarily, be 600°C, 650°C, 700°C, 750°C, 800°C, or any value within the range of any two of the above values. Further, the primary sintering temperature can be 650°C to 750°C.

[0038] In some embodiments, the sintering time can be 3h to 10h, which is beneficial for the complete reaction of the raw materials and further controls the particle size of the sintered material. The sintering time can be exemplarily 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or any value within the range of any two of the above values. The sintering time can further be 4h to 8h.

[0039] In some embodiments, the heating rate of a single sintering step can be 2°C / min to 5°C / min, which is beneficial for the uniformity of the solid-phase reaction and reduces thermal stress and structural defects caused by rapid heating.

[0040] In some embodiments, the atmosphere for primary sintering can be an inert atmosphere to prevent oxidation of metal elements in the primary mixture and improve the integrity of the crystal structure and the stability of the chemical composition of the primary sintered material. Furthermore, the oxygen concentration in the inert atmosphere can be controlled below 10 ppm to reduce metal element oxidation and prevent the formation of impurity phases.

[0041] In some embodiments, the sintering equipment for a single sintering process may include one of a pusher kiln, a roller kiln, and a rotary kiln.

[0042] In some embodiments, before the first slurry is sintered, it can be spray-dried to help quickly convert the liquid first slurry into a uniform powder, which is beneficial to the uniform reaction of the first slurry during the first sintering process. The inlet air temperature of the spray dryer can be 250℃~350℃, and the outlet air temperature can be 85℃~100℃.

[0043] Step S3: Mix the primary sintering material, the second carbon source, and the solvent to form a second mixture, and grind the second mixture to obtain a second slurry. The mass percentage of the second carbon source to the primary sintering material is 2.5% to 8%.

[0044] In this step, the primary sintering material, the second carbon source, and a solvent (e.g., water) are mixed to form a second mixture. In this step, the carbon source (the second carbon source) is added again and mixed with the primary sintering material and then ground. This grinding process breaks up and disperses the agglomerates in the primary sintering material, allowing it to fully contact and mix with the second carbon source.

[0045] In some embodiments, the second carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol.

[0046] In some embodiments, a second additive is also added to the second mixture. The second additive can form a coating layer on the surface of the lithium iron phosphate cathode material together with the second carbon source, thereby improving the conductivity and rate performance of the lithium iron phosphate cathode material.

[0047] Specifically, the second additive may include at least one of oxalates, phosphates, oxides, and carbonates containing a metal element, wherein the metal element in the second additive includes at least one of V, Ti, Nb, W, Li, B, and Fe. The mass percentage of the second additive in the second mixture may be 0.05% to 0.3%. By controlling the amount of the second additive added, the coating amount of the second additive on the lithium iron phosphate cathode material can be adjusted within a suitable range, which is beneficial to balance the coating modification effect of the second additive on the lithium iron phosphate cathode material and the capacity performance of the lithium iron phosphate cathode material. This mass percentage may, for example, be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any value within the range of any two of the above values. The mass percentage may further be 0.1% to 0.25%.

[0048] In some embodiments, the mixing method may include mixing with a high-speed mixer, stirring, or grinding. It is understood that the mixing methods include, but are not limited to, the methods described above.

[0049] In some embodiments, the particle size Dv50 of the second slurry can be 1.0 μm to 2.0 μm. By grinding, the agglomerates in the primary sintering material are opened and dispersed, and the particle size Dv50 of the second slurry is further adjusted to 1.0 μm to 2.0 μm, which is beneficial to control the particle size of the final lithium iron phosphate cathode material and obtain a large-particle-size lithium iron phosphate cathode material.

[0050] Step S4: The second slurry is sintered a second time to obtain a lithium iron phosphate cathode material with a coating layer. The particle size Dv50 of the lithium iron phosphate cathode material is 1.0μm~2.0μm.

[0051] The second slurry after grinding was sintered again to obtain a lithium iron phosphate cathode material with carbon element coating modification and a particle size Dv50 of 1.0μm~2.0μm. The large particle size of the lithium iron phosphate cathode material has a low specific surface area. Therefore, the lithium iron phosphate cathode material has good flow, excellent processing performance, and significantly improved cycle performance.

[0052] Furthermore, the mass percentage of the second carbon source to the primary sintering material is 2.5% to 15.0%, which reduces the carbon content in the finished lithium iron phosphate cathode material and effectively reduces the coating thickness, mitigating the hindering effect of the coating on lithium-ion transport and facilitating rapid lithium-ion transport in the lithium iron phosphate cathode material. The aforementioned second carbon source includes one or more combinations of high-molecular-weight organic compounds such as glucose, sucrose, polyethylene glycol, polyvinyl alcohol, and amino acids. In addition, the appropriate amount of the second carbon source also helps to improve the tap density and volumetric energy density of the lithium iron phosphate cathode material. The mass percentage of the second carbon source to the primary sintering material (the ratio of the sum of the mass ratios of multiple carbon sources to the mass ratio of the primary sintering material) can, for example, be 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14.0%, 15%, or any value within the range of any two of the above values. This mass percentage can further be 4% to 13%.

[0053] In some embodiments, the carbon content of the lithium iron phosphate cathode material can be 0.3% to 1.2%. In the embodiments of this application, the carbon content of the lithium iron phosphate cathode material is only 0.3% to 1.2%, which can effectively improve the conductivity of the lithium iron phosphate cathode material and effectively increase the tap density and compaction density of the lithium iron phosphate cathode material. This is beneficial to improving the energy density of the lithium iron phosphate cathode material and to achieving a better balance between high energy density, good processability and excellent electrochemical performance.

[0054] In some embodiments, the secondary sintering temperature is 600℃ to 850℃. A secondary sintering temperature within this range is beneficial for controlling the particle size of the lithium iron phosphate cathode material. The secondary sintering temperature can, for example, be 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, or any value within the range of any two of the above values. Further, the secondary sintering temperature can be 700℃ to 800℃.

[0055] In some embodiments, the secondary sintering time can be 3h to 10h, which is beneficial for forming a stable coating layer, thereby improving the conductivity, rate performance, and cycle performance of the lithium iron phosphate cathode material. The primary sintering time can, exemplarily, be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or any value within the range of any two of the above values. The primary sintering time can further be 5h to 8h.

[0056] In some embodiments, the heating rate for secondary sintering can be 2°C / min to 5°C / min.

[0057] In some embodiments, the atmosphere for secondary sintering can be an inert atmosphere to prevent oxidation of metal elements in the lithium iron phosphate cathode material and improve the crystal structure integrity and chemical composition stability of the lithium iron phosphate cathode material. Furthermore, the oxygen concentration in the inert atmosphere can be controlled below 10 ppm to reduce metal element oxidation and prevent the formation of impurity phases.

[0058] In some embodiments, the sintering equipment for secondary sintering may include one of a pusher kiln, a roller kiln, and a rotary kiln.

[0059] In some embodiments, before secondary sintering, the second slurry can be spray-dried, which helps to quickly convert the liquid second slurry into a uniform powder. This not only improves the uniformity of the reaction of the second slurry during secondary sintering but also allows the second carbon source to initially coat the lithium iron phosphate during spray drying. The inlet air temperature for spray drying can be 250°C to 350°C, and the outlet air temperature can be 85°C to 100°C.

[0060] In some embodiments, the specific surface area of ​​the lithium iron phosphate cathode material is 3.0 m². 2 / g~8.0m 2 The relatively low specific surface area ( / g) gives lithium iron phosphate cathode materials good flowability and excellent processing performance, which is beneficial for improving the cycle performance of lithium iron phosphate cathode materials. The specific surface area of ​​lithium iron phosphate cathode materials can be exemplarily 3.0 m² / g. 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g or any value within the range of any two of the above values. The specific surface area of ​​the lithium iron phosphate cathode material can further be 5m². 2 / g~8m 2 / g.

[0061] In some embodiments, the tap density of the lithium iron phosphate cathode material is 1.4 g / cm³. 3 ~1.8g / cm 3 This effectively improves the capacity performance and energy density of lithium iron phosphate cathode materials. The tap density of the lithium iron phosphate cathode material is, for example, 1.4 g / cm³. 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 1.8g / cm 3 Or any value within the range of any two of the above values. The tap density of the lithium iron phosphate cathode material can further be 1.5 g / cm³. 3 ~1.65g / cm 3 .

[0062] In some embodiments, the lithium iron phosphate cathode material has a uniform particle size distribution with a particle size distribution width of less than 2.0, indicating that the lithium iron phosphate cathode material has uniform particle size and high consistency.

[0063] 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 overcomes the technical challenge of severe capacity degradation of lithium iron phosphate cathode materials when the particle size is greater than or equal to 1 μm by controlling the addition amount of the first carbon source and the second carbon source, doping with the first additive, and precisely refining the particle size of the first slurry, and by utilizing the synergistic effect of multiple factors. It has made a breakthrough in preparing lithium iron phosphate cathode materials with a primary particle size of 1 μm to 2 μm that can still maintain high capacity. It can also effectively improve the conductivity and tap density of lithium iron phosphate cathode materials, improve the uniformity of particle size distribution and particle roundness, and reduce the specific surface area of ​​lithium iron phosphate cathode materials, thereby achieving a joint improvement in the capacity performance, cycle performance and processing performance of lithium iron phosphate cathode materials.

[0064] 2. This preparation method is simple, and since it does not require grinding to reduce the particle size to a smaller size (e.g., below 0.3 μm) to prepare large-particle-size lithium iron phosphate cathode materials, it significantly reduces the energy consumption and cost of preparation, which is conducive to the large-scale promotion of lithium iron phosphate cathode materials.

[0065] This application also provides a lithium iron phosphate cathode material prepared using the aforementioned method for preparing lithium iron phosphate cathode materials.

[0066] Compared with existing technologies, the lithium iron phosphate cathode material provided in this application can maintain high capacity while having a large particle size, as well as good conductivity and tap density, thus achieving a joint improvement in capacity performance, cycle performance and processing performance of lithium iron phosphate cathode material.

[0067] 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.

[0068] 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.

[0069] The following specific examples further illustrate the aforementioned lithium iron phosphate cathode material, its preparation method, and electrochemical device.

[0070] Example 1 Step S1: Mix 24.8 kg of lithium carbonate, 100 kg of ferric phosphate, 6.0 kg of glucose, 0.66 kg of titanium dioxide, and 135 kg of water to form a first mixture. Grind the first mixture by first feeding it into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) until the particle size D50 is 1.5 μm. Then feed it into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) until the particle size D50 is 0.60 μm. The first slurry is obtained, in which the mass percentage of glucose (first carbon source) and iron in ferric phosphate is 16.2%, and the particle size Dv50 of the first slurry is 0.60 μm.

[0071] Step S2: Spray dry the first slurry. The inlet air temperature of the spray dryer is 250℃ and the outlet air temperature is 90℃ to obtain dried powder. Then, the powder is sintered once in a rotary kiln under inert atmosphere protection. The temperature is increased to 700℃ at a rate of 3℃ / min, and then sintered for 7 hours. The kiln speed is 5 Hz, and the oxygen concentration in the atmosphere is controlled to be less than 10 ppm to obtain a sintered material with a particle size Dv50 of about 1μm.

[0072] Step S3: Mix 100kg of primary sintering material, 4.2kg of glucose, 6.0kg of polyethylene glycol and 110kg of water to form a second mixture, and then grind the second mixture into a grinder to 1.5μm to obtain a second slurry, namely, the mass percentage of glucose and polyethylene glycol (second carbon source) to primary sintering material is 10.2%.

[0073] Step S4: The second slurry is spray-dried at an inlet air temperature of 280°C and an outlet air temperature of 90°C to obtain dried powder. The powder is then sintered a second time in a rotary kiln under an inert atmosphere, with the temperature increased to 780°C at a rate of 3°C / min and sintered for 7 hours at a kiln speed of 5 Hz. The oxygen concentration in the atmosphere is controlled to be less than 10 ppm. After crushing, sieving, and iron removal, a coated lithium iron phosphate cathode material with a particle size Dv50 of 1.51 μm is obtained.

[0074] Example 2 The difference from Example 1 is that in step S1, the first mixture is ground. First, the first mixture is fed into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) until the particle size D50 is 1.5 μm. Then, it is fed into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) until the particle size D50 is 0.45 μm, resulting in a first slurry with a particle size Dv50 of 0.45 μm. Other steps are basically the same as in Example 1; please refer to Example 1.

[0075] Example 3 The difference from Example 1 is that in step S1, the first mixture is ground. First, the first mixture is fed into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) until the particle size D50 is 1.5 μm. Then, it is fed into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) until the particle size D50 is 0.35 μm, resulting in a first slurry with a particle size Dv50 of 0.35 μm. Other steps are basically the same as in Example 1; please refer to Example 1.

[0076] Example 4 The difference from Example 3 is that 0.166 kg of titanium dioxide additive is added in step S3, and the mass ratio of titanium element in the added titanium source to the mass ratio of the primary sintering material is 0.1%. The other steps are basically the same as in Example 3, please refer to Example 3.

[0077] Example 5 The difference from Example 3 is that 0.292 kg of titanium dioxide additive is added in step S3, and the mass ratio of titanium element in the added titanium source to the mass of the primary sintering material is 0.175%. The other steps are basically the same as in Example 3, please refer to Example 3.

[0078] Example 6 The difference from Example 3 is that 0.417 kg of titanium dioxide additive is added in step S3, and the mass ratio of titanium element in the added titanium source to the mass ratio of the primary sintering material is 0.25%. The other steps are basically the same as in Example 3, please refer to Example 3.

[0079] Example 7 The difference from Example 5 is that the amount of glucose added in step S3 is 3.5 kg. The other steps are basically the same as in Example 5; please refer to Example 5.

[0080] Example 8 The difference from Example 5 is that the weight of glucose in step S3 is 2.8 kg. The other steps are basically the same as in Example 5; please refer to Example 5.

[0081] Example 9 The difference from Example 5 is that the weight of glucose in step S3 is 2.1 kg. The other steps are basically the same as in Example 5; please refer to Example 5.

[0082] Comparative Example 1 Step S1: Mix 24.8 kg of lithium carbonate, 100 kg of ferric phosphate, 6.0 kg of glucose, 0.66 kg of titanium dioxide, and 135 kg of water to form a first mixture. Grind the first mixture and introduce it into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) until the particle size D50 is 1.5 μm. Then transfer it to a fine grinding system (zirconium bead diameter 0.3 mm) and grind it until the particle size D50 is 0.9 μm to obtain the first slurry. The mass percentage of glucose (first carbon source) and iron in ferric phosphate is 16.2%, and the particle size Dv50 of the first slurry is 0.9 μm.

[0083] Step S2: Spray dry the first slurry. The inlet air temperature of the spray dryer is 280℃ and the outlet air temperature is 90℃ to obtain dried powder. Then, the powder is sintered once in a rotary kiln under inert atmosphere protection. The temperature is increased to 700℃ at a rate of 3℃ / min, and then sintered for 7 hours. The kiln speed is 5 Hz, and the oxygen concentration in the atmosphere is controlled to be less than 10 ppm to obtain a sintered material with a particle size Dv50 of 1.0 μm.

[0084] Step S3: Mix 100kg of primary sintering material, 4.2kg of glucose, 6.0kg of polyethylene glycol and 110kg of water to form a second mixture, and then grind the second mixture into a grinder to 0.5μm to obtain a second slurry, that is, the mass percentage of glucose and polyethylene glycol (second carbon source) to primary sintering material is 10.2%.

[0085] Step S4: The second slurry is spray-dried at an inlet air temperature of 250°C and an outlet air temperature of 90°C to obtain dried powder. The powder is then sintered a second time in a rotary kiln under an inert atmosphere, with the temperature increased to 780°C at a rate of 3°C / min and sintered for 8 hours at a kiln speed of 5 Hz. The oxygen concentration in the atmosphere is controlled to be less than 10 ppm. After crushing, sieving, and iron removal, a coated lithium iron phosphate cathode material with a particle size Dv50 of 0.85 μm is obtained.

[0086] Comparative Example 2 Step S1: Mix 24.8 kg of lithium carbonate, 100 kg of ferric phosphate, 6.0 kg of glucose, 0.66 kg of titanium dioxide, and 135 kg of water to form a first mixture. Grind the first mixture and introduce it into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) until the particle size D50 is 1.5 μm. Then transfer it to a fine grinding system (zirconium bead diameter 0.3 mm) and grind it until the particle size D50 is 0.9 μm to obtain the first slurry. The mass percentage of glucose (first carbon source) and iron in ferric phosphate is 16.2%, and the particle size Dv50 of the first slurry is 0.9 μm.

[0087] Step S2: Spray dry the first slurry. The inlet air temperature of the spray dryer is 280℃ and the outlet air temperature is 90℃ to obtain dried powder. Then, the powder is sintered once in a rotary kiln under inert atmosphere protection. The temperature is increased to 700℃ at a rate of 3℃ / min, and then sintered for 7 hours. The kiln speed is 5 Hz, and the oxygen concentration in the atmosphere is controlled to be less than 10 ppm to obtain a sintered material with a particle size Dv50 of 1.0 μm.

[0088] Step S3: Mix 100kg of primary sintering material, 4.2kg of glucose, 6.0kg of polyethylene glycol and 110kg of water to form a second mixture, and then grind the second mixture into a grinder to 1.5μm to obtain a second slurry, namely, the mass percentage of glucose and polyethylene glycol (second carbon source) to primary sintering material is 10.2%.

[0089] Step S4: The second slurry is spray-dried at an inlet air temperature of 250°C and an outlet air temperature of 90°C to obtain dried powder. The powder is then sintered a second time in a rotary kiln under an inert atmosphere, with the temperature increased to 780°C at a rate of 3°C / min and sintered for 8 hours at a kiln speed of 5 Hz. The oxygen concentration in the atmosphere is controlled to be less than 10 ppm. After crushing, sieving, and iron removal, a coated lithium iron phosphate cathode material with a particle size Dv50 of 1.45 μm is obtained.

[0090] The lithium iron phosphate cathode materials obtained in Examples 1-9 and Comparative Examples 1-2 were tested accordingly.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] (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.

[0097] (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.

[0098] 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.

[0099] 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 for Standardization of Nonferrous Metals.

[0100] 8. Tap density determination: Refer to GB / T 30835-2014 "Carbon composite lithium iron phosphate cathode material for lithium-ion batteries".

[0101] 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.

[0102] 10. Processing performance test: Mix lithium iron phosphate cathode material with polyvinylidene fluoride (PVDF) and conductive agent (such as acetylene black or conductive carbon black) at a mass ratio of 93:3.5:3.5, add an appropriate amount of NMP to prepare a slurry with a solid content of 70%, and test the viscosity of the slurry. 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).

[0103] The relevant process parameters and test results of Examples 1-9 and Comparative Examples 1-2 are shown in Table 1-2 below.

[0104] Table 1 Table 2 The above results show that: The scanning electron microscope results of the primary sintered material in Example 8 are as follows: Figure 2 As shown, through the formulation design and grinding treatment of the first mixture combined with the control of the primary sintering process, the particles in the primary sintered material are uniformly distributed and the particle size Dv50 is approximately 1 μm. Figure 3 As shown, in Example 8, the primary sintering material was mixed with an appropriate amount of a second carbon source and a second additive, and then subjected to grinding, drying, and secondary sintering processes to obtain lithium iron phosphate cathode material with a relatively large particle size (Dv50 approximately 1.53 μm). Figure 4 As shown, the particle size of the lithium iron phosphate cathode material in Comparative Example 1 is approximately 0.85 μm, which is typical for small-particle-size lithium iron phosphate cathode materials. The particle size and morphology of the lithium iron phosphate cathode materials provided in Examples 1-9 of this application are significantly different from those in Comparative Example 1. The lithium iron phosphate cathode materials in Examples 1-9 are uniformly dispersed single-crystal products with concentrated particle size distribution, larger and more rounded particles.

[0105] Referring to Tables 1 and 2, compared to Comparative Example 2, in Examples 1-3, by controlling the primary grinding particle size during the synthesis of lithium iron phosphate, reducing the primary grinding particle size from 0.8 μm to 0.3 μm, the 0.1C specific capacity of the synthesized lithium iron phosphate increased from 153.3 mAh / g to 157.9 mAh / g, and the powder compaction density increased from 2.43 g / cc to 2.47 g / cc. Therefore, by reducing the primary grinding particle size, the fusion growth between particles can be promoted, the uniformity of element distribution in the material can be effectively improved, and the capacity and compaction density of the product can be increased.

[0106] Combining Tables 1 and 2, compared with Example 3, the addition of titanium in the secondary grinding of lithium iron phosphate in Examples 4-6 by controlling the amount of titanium added during synthesis shows that by adding 0.175% titanium, the specific capacity of the material increased from 157.9 mAh / g to 159.5 mAh / g, which is basically comparable to the capacity of conventional lithium iron phosphate products (Comparative Example 1). This indicates that the coating of titanium effectively improves the diffusion rate of lithium ions.

[0107] Combining Tables 1 and 2, compared with Example 5, in Examples 7-9, by controlling the amount of carbon source added during the synthesis of lithium iron phosphate, the carbon content of the finished product was reduced from about 1.2% to about 0.6%. In Example 8, when the carbon content was about 0.8%, the specific capacity was increased to >160mAh / g, and the electrical performance was further improved. It can be seen that by reducing the carbon content, the thickness of the carbon coating layer was reduced, which further improved the lithium-ion conduction rate and the electrical performance was significantly improved.

[0108] The above embodiments significantly improve the tap density of the product by increasing the primary particle size to about 1.5μm. The corresponding solid content of the battery slurry can be increased to more than 70%, which significantly reduces the amount of NMP used in the slurry preparation process and reduces the cost of the cell material preparation process. In addition, the increase in primary particle size significantly improves the cycle performance of the material. After 100 cycles of coin cell testing, the capacity can still be maintained at more than 98%, which is more than 2% higher than the retention rate of Comparative Example 1.

[0109] 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 method for preparing a lithium iron phosphate cathode material, characterized in that, include: A phosphorus source, an iron source, a lithium source, a first carbon source, a first additive, 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 is 14.5% to 21.5%, and the particle size Dv50 of the first slurry is 0.3 μm to 0.8 μm. The first slurry is sintered once to obtain a sintered material; The primary sintering material, 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, wherein the mass percentage of the second carbon source to the primary sintering material is 4% to 13%; and The second slurry is sintered a second time to obtain the lithium iron phosphate cathode material with a coating layer, wherein the particle size Dv50 of the lithium iron phosphate cathode material is 1.0 μm to 2.0 μm.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The carbon content of the lithium iron phosphate cathode material is 0.3% to 1.2%.

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The particle size Dv50 of the primary sintering material is 0.8μm~1.2μm.

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The first additive comprises at least one of oxalates, phosphates, oxides, and carbonates containing a metal element, wherein the metal element comprises at least one of V, Ti, Nb, W, Li, B, and Fe; and / or The first additive accounts for 0.1% to 0.6% of the mass of the first mixture.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The particle size Dv50 of the second slurry is 1.0μm~2.0μm.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The second additive is also added to the second mixture. The second additive includes at least one of oxalate, phosphate, oxide and carbonate containing a metal element. The metal element includes at least one of V, Ti, Nb, W, Li, B and Fe. The second additive accounts for 0% to 0.3% of the mass percentage of the second mixture.

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The temperature of the first sintering is 600℃~800℃, and the sintering time is 3h~10h; and / or The heating rate for the first sintering is 2℃ / min to 5℃ / min; and / or The secondary sintering temperature is 600℃~850℃, and the secondary sintering time is 3h~10h; and / or The heating rate for the secondary sintering is 2℃ / min to 5℃ / min.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The specific surface area of ​​the lithium iron phosphate cathode material is 3.0 m². 2 / g~10.0m 2 / g; and / or The tap density of the lithium iron phosphate cathode material is 1.4 g / cm³. 3 ~1.8g / cm 3 .

9. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared using the method for preparing lithium iron phosphate cathode material as described in any one of claims 1 to 8.

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 the lithium iron phosphate positive electrode material as described in claim 9.

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