Preparation method and application of ultrahigh pressure solid type lithium iron phosphate material

Abstract

The invention provides a preparation method and application of an ultra-high-pressure solid-state lithium iron phosphate material, and belongs to the field of lithium batteries. The method comprises the following steps: respectively mixing iron phosphate with a high iron-phosphorus ratio and iron phosphate with a low iron-phosphorus ratio with a lithium source, an additive and a carbon source, carrying out sanding, spray granulation and sintering treatment to obtain a precursor A and a precursor B, and carrying out secondary sintering on the precursor B to obtain a precursor C; and finally, mixing the precursor A and the precursor C in proportion, and crushing to obtain the ultra-high pressure solid type lithium iron phosphate positive electrode material. According to the invention, the coordination of high compaction density and excellent electrochemical performance of the lithium iron phosphate positive electrode material is realized by constructing a grading structure of a large-particle skeleton and small-particle filling and optimizing a crystal structure by combining a two-step sintering process, and the lithium iron phosphate positive electrode material is suitable for the field of high-capacity lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium batteries, specifically to a method for preparing and applying ultra-high pressure lithium iron phosphate material. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become an important cathode material in power lithium-ion batteries and energy storage systems due to its high safety, long cycle life, and low cost, and is widely used in various scenarios such as new energy vehicles, energy storage power stations, and power tools. However, compared with ternary materials, its lower compaction density limits the further improvement of battery volumetric energy density, making it difficult to meet the urgent needs of high-end electric vehicles and compact energy storage devices for high energy-to-volume ratio, thus limiting its further application.

[0003] Currently, common methods for improving the compaction density of lithium iron phosphate (LFP) mainly focus on particle morphology design, particle size distribution optimization, and carbon coating process improvement. Existing technologies mostly rely on a single precursor synthesis route, supplemented by carbon source composites and high-temperature solid-state reactions, but these methods often have certain limitations. For example, particles obtained by direct ball milling or spray granulation have a concentrated particle size distribution, resulting in low particle packing efficiency and many residual pores during electrode compaction; while the use of excessive carbon sources is beneficial for conductive network construction, it sacrifices the material's tap density and processing performance; and traditional sintering processes lack sufficient control over crystal growth behavior, which, although it can improve particle packing to some extent, easily leads to abnormal grain growth and internal defects, affecting the material's mechanical strength and lithium-ion transport kinetics. Some studies have also proposed optimizing filling performance by physically mixing LFP particles of different sizes, but poor interfacial compatibility between heterogeneous particles and insufficient coating layer continuity not only reduce material stability but also make the material more susceptible to external environmental influences during cycling, thus causing a decrease in cycle stability and shortening battery life.

[0004] Therefore, there is an urgent need to develop a preparation process that can take into account particle size distribution design, carbon structure uniformity and crystal morphology integrity, so as to achieve a synergistic improvement in the high compaction density and electrochemical performance of lithium iron phosphate materials. Summary of the Invention

[0005] The main objective of this invention is to provide a method for preparing ultra-high pressure lithium iron phosphate materials and their applications, thereby solving the problems mentioned in the background art.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for preparing ultra-high pressure lithium iron phosphate material includes the following steps: S1. The first iron phosphate, the first lithium source, the first additive and the first carbon source are mixed evenly in pure water, and after the first sand milling, the first spray granulation and the first sintering treatment, the precursor A is obtained. S2. The second iron phosphate, the second lithium source, the second additive and the second carbon source are mixed evenly in pure water, and after a second sand milling, a second spray granulation and a second sintering treatment, precursor B is obtained. S3. Precursor B is subjected to a third sintering process to obtain precursor C; S4. After mixing precursor A and precursor C in a certain proportion, the mixture is pulverized to obtain ultra-high pressure compacted lithium iron phosphate cathode material.

[0007] Preferably, the first lithium source and the second lithium source are each independently selected from at least one of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, and lithium hydroxide; The first additive and the second additive are each independently selected from at least one of titanium dioxide, amino chelated titanium, ammonium metavanadate, vanadium pentoxide, and niobium pentoxide; The first carbon source and the second carbon source are each independently selected from at least one of sucrose, glucose, citric acid, starch, and polyethylene glycol.

[0008] Preferably, the iron-to-phosphorus ratio of the first ferric phosphate is 0.965~0.985; The molar ratio of Li in the first lithium source to Fe in the first iron phosphate is (1.04~1.06):1; The amount of the first carbon source added makes the carbon content of precursor A 1.2%~1.5%; The mass of the metal cation in the first additive accounts for 4000~6000 ppm of the mass of precursor A.

[0009] Preferably, the particle size D50 after the first sand milling is 0.3~0.5μm; the inlet air temperature of the first spray drying is 200~230℃, the outlet air temperature of the first spray drying is 85~110℃, and the granulation particle size D50 is 20~45μm; the first sintering temperature is 700~780℃, the sintering time is 5~15h, and the particle size of the precursor A is 0.6~0.9μm.

[0010] Preferably, the iron-to-phosphorus ratio of the second ferric phosphate is 0.950~0.965; The molar ratio of Li in the second lithium source to Fe in the iron phosphate is (1.02~1.04):1; The addition of a second carbon source resulted in a precursor B carbon content of 1.3% to 1.5%. The mass of the metal cation in the second additive is 1000~4000 ppm of the mass of precursor B.

[0011] Preferably, the particle size D50 after the second sand milling is 0.5~0.8μm; the inlet air temperature of the second spray drying is 200~230℃, the outlet air temperature of the second spray drying is 85~110℃, and the granulation particle size D50 is 20~45μm; the second sintering temperature is 700~800℃, the sintering time is 5~15h, and the particle size of the obtained precursor B is 0.9~1.3μm.

[0012] Preferably, the third sintering temperature is 760~820℃, the sintering time is 5~15h, and the resulting precursor C particle size is 1.0~1.4μm.

[0013] Preferably, the mass ratio of precursor A to precursor C is (10~40):(60~90). The particle size of lithium iron phosphate cathode material is 0.8~1.3μm.

[0014] The second aspect of this invention provides an ultra-high pressure lithium iron phosphate cathode material, which is prepared by the above-described preparation method.

[0015] The third aspect of this invention provides the application of the ultra-high pressure lithium iron phosphate cathode material in lithium-ion batteries.

[0016] This invention optimizes particle size distribution design, enabling close packing of particles of different sizes, reducing porosity, and significantly improving the compaction density of the material. By optimizing the carbon source ratio, a uniform carbon structure is formed, achieving good conductivity while avoiding density reduction and deterioration in processing performance caused by excessive carbon source, thus creating conditions for achieving high compaction density. The prepared lithium iron phosphate material has a complete crystal morphology, which not only enhances the material's mechanical strength, making it less prone to breakage during compaction and maintaining structural stability, but also contributes to obtaining higher compaction density. Furthermore, high compaction density provides the battery with higher volumetric energy density. The uniform carbon coating layer and the complete crystal structure work together to optimize lithium-ion transport kinetics, enabling rapid electron transfer between particles; the complete crystal structure reduces obstacles in the lithium-ion transport path, accelerating the insertion and extraction of lithium ions within the material, thereby improving the battery's charge / discharge efficiency and rate performance.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention constructs a small-particle-size precursor A with a high iron-to-phosphorus ratio and a large-particle-size precursor C with a low iron-to-phosphorus ratio, which is then sintered twice. This achieves the optimal gradation of "homogeneous heterogeneous" particles, allowing large particles to form the framework and small particles to effectively fill the pores, significantly improving the compaction density and avoiding the interfacial impedance problem of heterogeneous mixing. Through differentiated lithium ratios and optimization of the crystal structure by secondary sintering, the excellent ionic / electronic conductivity and structural stability of the material are simultaneously ensured.

[0018] 2. The lithium iron phosphate cathode material of the present invention has an extremely high compaction density (>2.72 g / cm³). 3 While possessing excellent electrochemical performance (0.1C discharge capacity ≥160mAh / g, 1C capacity ≥141mAh / g), it is suitable for the field of high-capacity lithium-ion batteries and has significant industrial application value. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings: Figure 1 This is a SEM image of the ultra-high pressure lithium iron phosphate material prepared in Example 1 of this invention; Figure 2 This is the XRD pattern of the ultra-high pressure lithium iron phosphate material prepared in Example 1 of this invention; Figure 3 This is a SEM image of the lithium iron phosphate material prepared in Comparative Example 1 of this invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] Example 1: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: S1. Iron phosphate (iron-to-phosphorus ratio 0.975), lithium carbonate (Li to Fe molar ratio 1.05:1), titanium dioxide (added at 5000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 750℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. S2. Iron phosphate (iron-to-phosphorus ratio 0.960), lithium carbonate (Li to Fe molar ratio 1.03:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. S3. The precursor B obtained in step S2 is sintered at 760°C for 10 hours under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2 μm. S4. Precursor A and precursor C are mixed at a mass ratio of 4:6, and then lightly pulverized and mixed using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0022] SEM and XRD images of the lithium iron phosphate cathode material are shown below. Figure 1-2 As shown.

[0023] Depend on Figure 1 It can be seen that small particles A fill the pores between large particles C, forming a skeleton-filled gradation structure, which greatly improves the compactness of particle packing and achieves ultra-high pressure density of lithium iron phosphate cathode material; moreover, the particles are evenly dispersed, avoiding the increase in porosity caused by particle agglomeration, ensuring the continuity of electron / ion conduction, and enabling the excellent electrochemical performance of lithium iron phosphate cathode material.

[0024] Depend on Figure 2 As can be seen, the diffraction peaks in the XRD pattern are sharp and have high intensity, with no obvious impurity peaks, indicating that the prepared lithium iron phosphate cathode material has a complete crystal structure and high crystallinity, which facilitates the insertion and extraction of lithium ions in the crystal, thereby affecting the electrochemical performance of the material.

[0025] Example 2: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: S1. Iron phosphate (iron-to-phosphorus ratio 0.975), lithium carbonate (Li to Fe molar ratio 1.05:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under a nitrogen atmosphere, the mixture was sintered at 750℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. S2. Iron phosphate (iron-to-phosphorus ratio 0.960), lithium carbonate (Li to Fe molar ratio 1.03:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. S3. The precursor B obtained in step S2 is sintered at 760°C for 10 hours under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2 μm. S4. Precursor A and precursor C are mixed at a mass ratio of 3:7 and lightly pulverized and mixed using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0026] Example 3: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: Step S1: Iron phosphate (iron-to-phosphorus ratio of 0.985), lithium carbonate (Li to Fe molar ratio of 1.05:1), titanium dioxide (added at 5000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 750℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. Step S2: Iron phosphate (iron-to-phosphorus ratio of 0.955), lithium carbonate (Li to Fe molar ratio of 1.03:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed with an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 750℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. Step S3: The precursor B obtained in step S2 is sintered at 780°C for 10 hours under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2 μm. Step S4: Mix precursor A and precursor C at a mass ratio of 2:8, and then perform light pulverization and mixing using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0027] Example 4: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: Step S1: Iron phosphate (iron-to-phosphorus ratio 0.985), lithium carbonate (Li to Fe molar ratio 1.05:1), titanium dioxide (added at 5000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed with an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. Step S2: Iron phosphate (iron-to-phosphorus ratio 0.965), lithium carbonate (Li to Fe molar ratio 1.03:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 760℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. Step S3: The precursor B obtained in step S2 is sintered at 800℃ for 10h under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2μm. Step S4: Mix precursor A and precursor C at a mass ratio of 3:7, and then perform light pulverization and mixing using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0028] Example 5: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: Step S1: Iron phosphate (iron-to-phosphorus ratio 0.975), lithium carbonate (Li to Fe molar ratio 1.05:1), titanium dioxide (added at 5000 ppm based on Ti), sucrose, and PEG (8% sucrose and 6% PEG relative to iron phosphate by mass) were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed with an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. Step S2: Iron phosphate (iron-to-phosphorus ratio 0.955), lithium carbonate (Li to Fe molar ratio 1.03:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under a nitrogen atmosphere, the mixture was sintered at 730℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. Step S3: The precursor B obtained in step S2 is sintered at 800℃ for 10h under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2μm. Step S4: Mix precursor A and precursor C at a mass ratio of 3:7, and then perform light pulverization and mixing using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0029] Example 6: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: Step S1: Iron phosphate (iron-to-phosphorus ratio 0.975), lithium carbonate (Li to Fe molar ratio 1.06:1), titanium dioxide (added at 5000 ppm based on Ti), sucrose, and PEG (8% sucrose and 6% PEG relative to iron phosphate by mass) were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed with an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. Step S2: Iron phosphate (iron-to-phosphorus ratio 0.955), lithium carbonate (Li to Fe molar ratio 1.04:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.7 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 730℃ for 10 h, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained. Step S3: The precursor B obtained in step S2 is sintered at 800℃ for 10h under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2μm. Step S4: Mix precursor A and precursor C at a mass ratio of 3:7, and then perform light pulverization and mixing using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0030] Example 7: This embodiment provides a method for preparing ultra-high pressure solid lithium iron phosphate cathode material, specifically including the following steps: Step S1: Iron phosphate (iron-to-phosphorus ratio of 0.985), lithium carbonate (Li to Fe molar ratio of 1.04:1), titanium dioxide (added at 5000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water and then milled using a sand mill to control the slurry particle size D50 to 0.4 μm. Subsequently, spray drying was performed at an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm. Finally, under nitrogen atmosphere protection, the mixture was sintered at 780℃ for 10 h, and after crushing and sieving, precursor A with a particle size D50 of 0.8 μm was obtained. Step S2: Iron phosphate (iron-to-phosphorus ratio 0.965), lithium carbonate (Li to Fe molar ratio 1.02:1), titanium dioxide (added at 3000 ppm based on Ti), 8% sucrose (relative to iron phosphate mass), and 6% PEG were mixed in pure water; the mixture was then sand-milled to control the slurry particle size D50 to 0.7 μm; subsequently, spray drying was performed with an inlet air temperature of 220℃ and an outlet air temperature of 85-90℃ to obtain granulated particles with a D50 of 20-30 μm; finally, the mixture was sintered at 760℃ for 10 h under nitrogen atmosphere protection, and after crushing and sieving, precursor B with a particle size D50 of 1.1 μm was obtained; Step S3: The precursor B obtained in step S2 is sintered at 800℃ for 10h under a nitrogen atmosphere to obtain precursor C with a particle size D50 of 1.2μm. Step S4: Mix precursor A and precursor C at a mass ratio of 3:7, and then perform light pulverization and mixing using a high-speed pulverizer to ensure uniform dispersion, thereby obtaining the final ultra-high pressure lithium iron phosphate cathode material with a particle size D50 of 1.0±0.2μm.

[0031] Comparative Example 1: Ultra-high pressure lithium iron phosphate cathode material was prepared using the method of Example 2, the difference being that step S3 was omitted, i.e., precursor A and precursor B were directly mixed in a 3:7 ratio.

[0032] The SEM image of the lithium iron phosphate cathode material is as follows: Figure 3 As shown. By Figure 3 It is known that mixing large and small particles cannot efficiently fill the pores of large particles, resulting in the failure to form an ordered skeleton and a filled gradation structure. Small particles exhibit agglomeration, and there are many irregular pores between particles, failing to achieve close packing, leading to a low compaction density.

[0033] Comparative Example 2: Ultra-high pressure lithium iron phosphate cathode material was prepared using the method of Example 2, the difference being that in step S4, precursor A and precursor C were mixed at a mass ratio of 5:5.

[0034] Comparative Example 3: Ultra-high pressure lithium iron phosphate cathode material was prepared using the method in Example 2, with the difference being: In step S1, the molar ratio of Li to Fe is 1.07:1; In step S2, the molar ratio of Li to Fe is 1.05:1; In step S4, precursor A and precursor C are mixed at a mass ratio of 5:5.

[0035] The performance of the ultra-high pressure lithium iron phosphate materials prepared in the above embodiments and comparative examples was tested, and the test methods are as follows: (1) The compaction density was tested using a compaction density tester with a test pressure of 3T; (2) The prepared ultra-high pressure co-doped lithium iron phosphate material, PVDF, and Super-p were homogenized in a ratio of 90:5:5 with a solid content of 30%. Then, the mixture was coated, dried, and punched to obtain circular electrodes. Finally, the circular electrodes, separator, and lithium sheet were assembled into coin cells in a glove box. The coin cells were charged and discharged in a voltage range of 2.0~3.6V. The results are shown in Table 1.

[0036] Table 1

[0037] As shown in Table 1, the compaction density of the lithium iron phosphate cathode materials prepared in the examples is all higher than 2.72 g / cm³. 3 It is significantly better than the comparative example (2.58-2.66 g / cm³). 3 This is because the embodiments employ a gradation structure where small particles fill the pores of large particles, effectively improving the compactness of particle packing and achieving ultra-high compaction density; among them, Embodiment 3 exhibits the best performance, with a compaction density of 2.774 g / cm³. 3 Comparative Example 1, lacking secondary sintering and the synergistic effect of large particle skeleton and small particle filling, had the lowest compaction density (2.60 g / cm³). 3 In Comparative Example 2, the gradation ratio was unreasonable, and too many small particles damaged the gradation structure; in Comparative Example 3, the lithium content was excessive and improper, failing to form an efficient stacking structure, resulting in low compaction density.

[0038] The discharge capacities at 0.1C, 0.5C, and 1C rates (141.8-161.3 mAh / g) in Examples 1-3 are not significantly different from those in the comparative examples (144.2-162.1 mAh / g), indicating that the high-compaction structure constructed in this invention does not affect electrochemical performance. In Examples 1-3, the compaction density gradually increases with the increase of the proportion of large-particle C (2.722-2.741-2.774 g / cm³). 3 However, the discharge capacity decreased slightly; this indicates that increasing the proportion of large particles C is beneficial to compaction density, but a certain proportion of small particles should be reasonably retained to maintain a good ion / electron conduction network and electrochemical reversibility.

[0039] In summary, this invention successfully achieved ultra-high pressure density of lithium iron phosphate cathode material while maintaining high specific capacity by constructing a graded structure of large particle skeleton and small particle filling, combined with a two-step sintering process to optimize the crystal structure.

[0040] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-high pressure compacted lithium iron phosphate material, characterized in that: Includes the following steps: S1. The first iron phosphate, the first lithium source, the first additive and the first carbon source are mixed evenly in pure water, and after the first sand milling, the first spray granulation and the first sintering treatment, the precursor A is obtained. S2. The second iron phosphate, the second lithium source, the second additive and the second carbon source are mixed evenly in pure water, and after a second sand milling, a second spray granulation and a second sintering treatment, precursor B is obtained. S3. Precursor B is subjected to a third sintering process to obtain precursor C; S4. After mixing precursor A and precursor C in a certain proportion, the mixture is pulverized to obtain ultra-high pressure compacted lithium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that: The first lithium source and the second lithium source are each independently selected from at least one of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, and lithium hydroxide; The first additive and the second additive are each independently selected from at least one of titanium dioxide, amino chelated titanium, ammonium metavanadate, vanadium pentoxide, and niobium pentoxide; The first carbon source and the second carbon source are each independently selected from at least one of sucrose, glucose, citric acid, starch, and polyethylene glycol.

3. The preparation method according to claim 1, characterized in that: The molar ratio of Li in the first lithium source to Fe in the first iron phosphate is (1.04~1.06):1; The amount of the first carbon source added makes the carbon content of precursor A 1.2%~1.5%; The mass of the metal cation in the first additive accounts for 4000~6000 ppm of the mass of precursor A.

4. The preparation method according to claim 1, characterized in that: The particle size D50 after the first sand milling is 0.3~0.5μm; the inlet air temperature of the first spray drying is 200~230℃, the outlet air temperature of the first spray drying is 85~110℃, and the granulation particle size D50 is 20~45μm. The first sintering temperature is 700~780℃, the sintering time is 5~15h, and the particle size of the precursor A is 0.6~0.9μm.

5. The preparation method according to claim 1, characterized in that: The iron-to-phosphorus ratio of the second iron phosphate is 0.950~0.965; The molar ratio of Li in the second lithium source to Fe in the iron phosphate is (1.02~1.04):1; The addition of a second carbon source resulted in a precursor B carbon content of 1.3% to 1.5%. The mass of the metal cation in the second additive is 1000~4000 ppm of the mass of precursor B.

6. The preparation method according to claim 1, characterized in that: The particle size D50 after the second sand milling is 0.5~0.8μm; the inlet air temperature of the second spray drying is 200~230℃, the outlet air temperature of the second spray drying is 85~110℃, and the granulation particle size D50 is 20~45μm. The second sintering temperature is 700~800℃, the sintering time is 5~15h, and the particle size of the resulting precursor B is 0.9~1.3μm.

7. The preparation method according to claim 1, characterized in that: The third sintering temperature was 760~820℃, the sintering time was 5~15h, and the particle size of the resulting precursor C was 1.0~1.4μm.

8. The preparation method according to claim 1, characterized in that: The mass ratio of precursor A to precursor C is (10~40):(60~90). The particle size of lithium iron phosphate cathode material is 0.8~1.3μm.

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

10. The application of the ultra-high pressure lithium iron phosphate cathode material according to claim 9 in lithium-ion batteries.

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