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

Abstract

This invention belongs to the field of cathode material technology, specifically relating to a lithium iron phosphate cathode material, its preparation method, and the cathode sheet. The lithium iron phosphate cathode material includes at least large-particle lithium iron phosphate and small-particle lithium iron phosphate, wherein the titanium doping concentration in the large-particle and small-particle lithium iron phosphate is distributed in a graded manner. By synergistically designing and precisely controlling the physical gradation at the particle size and the spatial distribution of doping elements, characteristic modification of particles of different sizes can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of cathode material technology, specifically relating to a lithium iron phosphate cathode material, its preparation method, and cathode sheet. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Lithium iron phosphate (LFP) has advantages such as high theoretical specific capacity, stable voltage platform, excellent thermal stability, cycle safety, environmental friendliness, and low cost. However, its primary particles usually grow anisotropically, resulting in low secondary particle stacking efficiency. This leads to low compaction density when preparing the positive electrode sheet, which restricts the improvement of battery volumetric energy density. At the same time, LFP has low conductivity, which causes severe polarization of the material during high-rate charge and discharge, resulting in poor capacity utilization and limited power performance.

[0004] In existing technologies, multi-scale particle size distribution is used to improve compaction density. However, the presence of small particles can exacerbate interfacial side reactions between the electrode and electrolyte, while excessively large particles can prolong the lithium-ion diffusion path and deteriorate rate performance. Elemental doping often only improves either compaction density or rate performance. Summary of the Invention

[0005] This invention addresses the contradiction between existing homogenization doping strategies and the difficulty in simultaneously optimizing the compaction density and rate performance of lithium iron phosphate materials. It provides a lithium iron phosphate cathode material, its preparation method, and the cathode sheet. By synergistically designing and precisely controlling the physical gradation at the particle size and the spatial distribution of doping elements, characteristic modification of particles of different sizes can be achieved.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a lithium iron phosphate cathode material, comprising at least large-particle lithium iron phosphate and small-particle lithium iron phosphate, wherein the doping concentration of titanium in the large-particle lithium iron phosphate and small-particle lithium iron phosphate decreases in a stepped manner.

[0007] In some embodiments, the D50 of large-particle lithium iron phosphate is 1.0~3.0 μm, and the titanium doping concentration is 5000~6000 ppm; The D50 of small-particle lithium iron phosphate is 0.1~0.5 μm, and the titanium doping concentration is 1000-2000 ppm.

[0008] Large-particle lithium iron phosphate refers to lithium iron phosphate particles with a diameter greater than 1.0 μm; small-particle lithium iron phosphate refers to lithium iron phosphate particles with a diameter less than 0.5 μm.

[0009] The D50 of large-particle lithium iron phosphate can be 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, or 3.0 μm; the titanium doping concentration can be 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm, or 6000ppm.

[0010] The D50 of small-particle lithium iron phosphate can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm or 0.5 μm; the titanium doping concentration can be 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm or 2000 ppm.

[0011] Large lithium iron phosphate particles are spherical or near-spherical, primarily serving to provide high tap density and good processing performance, forming the skeletal structure. Small lithium iron phosphate particles may contain nanoscale primary particles or micron-sized aggregates, filling the voids between the large lithium iron phosphate particles. Grading two types of lithium iron phosphate particles with different sizes can effectively improve the compaction density of lithium iron phosphate materials.

[0012] Low-dose doping was applied to small-particle lithium iron phosphate (filler particles), with the titanium doping amount (as a weight ratio of TiO2 or Ti to the lithium iron phosphate fraction) controlled at 1000-2000 ppm. The main purpose of this low-dose doping was to utilize the stabilizing effect of titanium on the particle surface / near surface to effectively passivate the active surface of the high specific surface area small particles, suppressing side reactions with the electrolyte, thereby improving the cycling stability and storage performance of the material under high voltage discharge electrode conditions.

[0013] The lithium-ion diffusion path of small-particle lithium iron phosphate (LFP) is very short, and its intrinsic lithium-ion diffusion restriction is relatively weak. High titanium doping mainly improves bulk ionic conductivity, and the benefits for small-particle LFP are not as high as those for large-particle LFP. Using large amounts of expensive titanium in small-particle LFP, which does not require high ionic conductivity gains, not only reduces charge / discharge specific capacity but also has extremely low cost-effectiveness. Gradual doping of titanium into LFP with particle size distribution is a key measure to achieve both material performance and cost control.

[0014] Large-particle lithium iron phosphate (matrix particles) are subjected to high-dose doping, with the titanium doping amount (as a weight ratio of TiO2 or Ti to the lithium iron phosphate fraction) controlled at 5000-6000 ppm. This relatively high doping dose aims to deeply modify the bulk lattice structure of the large particles. 4+ By substituting Fe sites into the LiFePO4 lattice, the diffusion barrier of lithium ions in the one-dimensional channel is lowered, significantly improving the diffusion rate of lithium ions in the bulk phase of larger particles. This compensates for the kinetic loss caused by the increase in size, and thus helps to improve capacity release and rate performance.

[0015] Excess titanium (Ti) 4+ Substituting Fe sites may lead to lattice distortion, introducing excessive structural defects, reducing material crystallinity, and hindering lithium-ion diffusion channels, resulting in deteriorated rate performance; although appropriate amounts of Ti... 4+ It can reduce the lithium-ion diffusion barrier, but high concentration doping may induce lattice stress, leading to a decrease in the diffusion coefficient and offsetting the advantages of large particle bulk phase modification; excessive titanium doping may promote the formation of impurity phases, destroy the single-phase structure of lithium iron phosphate, and lead to accelerated capacity decay during cycling.

[0016] Therefore, by using different titanium concentrations to dope the large and small lithium iron phosphate particles involved in the gradation, and by controlling the concentration appropriately, the compaction density and rate performance of the material can be improved simultaneously.

[0017] In some embodiments, the mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is 2~9:1~8.

[0018] Preferably, the mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is 2~3:1.

[0019] In some embodiments, the surfaces of large and small lithium iron phosphate particles are coated with a carbon layer.

[0020] Preferably, the carbon layer accounts for 0.5% to 2% of the mass of lithium iron phosphate, ensuring a good electronic conductivity network between particles.

[0021] Secondly, the present invention provides a method for preparing the lithium iron phosphate cathode material, comprising the following steps: Iron phosphate, lithium source, carbon source and titanium source are mixed in a set ratio to prepare a first slurry and a second slurry, wherein the concentration of titanium source in the first slurry is greater than that in the second slurry; After grinding the first slurry and the second slurry, nano-sized first grinding slurry and second grinding slurry are obtained respectively. The particle D50 in the first grinding slurry is larger than that in the second grinding slurry. The abrasive slurry is spray-dried to obtain the first spray powder and the second spray powder, respectively. The first spray powder and the second spray powder are mixed, sintered, crushed, sieved and mixed in a certain proportion to obtain the final product.

[0022] By controlling the iron-to-phosphorus ratio of the iron phosphate precursor in the grinding slurry, lithium iron phosphate particles of different sizes were obtained after spray sintering of different grinding slurries. Specifically, in the first grinding slurry, the iron-to-phosphorus ratio of the iron phosphate precursor was lower, and larger lithium iron phosphate particles were more easily obtained after spray sintering and pulverization. In the second grinding slurry, the iron-to-phosphorus ratio of the iron phosphate precursor was higher, and smaller lithium iron phosphate particles were more easily obtained after spray sintering and pulverization.

[0023] When the obtained lithium iron phosphate particles are sieved and mixed, it is easier to obtain the desired lithium iron phosphate material.

[0024] Mixing the first and second sprayed powders before sintering ensures uniform dispersion of both types of particles during sintering, forming a tightly packed structure of "large particle skeleton - small particle filler". If they are sintered separately and then mixed, differences in particle density may lead to stratification or agglomeration, disrupting the gradation effect and reducing compaction density. During mixed sintering, the two types of particles share a carbon source, forming a continuous carbon network that covers both the surface of large particles and the gaps between small particles, improving overall conductivity. Sintering them separately may result in uneven carbon layer thickness (excessively thick carbon layers for large particles and insufficient carbon layers for small particles), affecting electron transport efficiency.

[0025] In some embodiments, the titanium source is titanium dioxide, and the mass ratio of titanium dioxide to iron phosphate in the first slurry is 5000~6000 ppm; In the second slurry, the mass ratio of titanium dioxide to iron phosphate is 1000~2000 ppm.

[0026] In some embodiments, the particle size D50 in the first abrasive slurry is 0.5~1μm; the particle size D50 in the second abrasive slurry is 0.2~0.4μm.

[0027] In some embodiments, the inlet air temperature of the spray dryer is 200~300℃ and the outlet air temperature is 80~120℃.

[0028] Preferably, the atomization frequency of the spray drying of the first abrasive slurry is 25~30Hz; and the atomization frequency of the spray drying of the second abrasive slurry is 40~50Hz.

[0029] In some embodiments, the mass ratio of the first spray powder to the second spray powder is 2.5 to 3:1.

[0030] In some embodiments, the sintering temperature is 750~800℃ and the sintering time is 5-10 h.

[0031] Thirdly, the present invention provides a positive electrode sheet, comprising a current collector and a positive electrode active material attached to the surface of the current collector, wherein the positive electrode active material is the lithium iron phosphate positive electrode material.

[0032] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: Grading two different particle sizes of lithium iron phosphate can effectively improve the compaction density of lithium iron phosphate materials. Low-dose doping is applied to the small-particle lithium iron phosphate (filler particles), with the titanium doping amount controlled at 1000-2000 ppm. The main purpose of this low-dose doping is to utilize the stabilizing effect of titanium on the particle surface / near surface, effectively passivating the active surface of small particles with high specific surface area, suppressing side reactions with the electrolyte, thereby improving the cycling stability and storage performance of the material under high compaction electrode conditions.

[0033] Large-particle lithium iron phosphate (matrix particles) were subjected to high-dose doping, with the titanium doping amount (as a percentage of TiO2 or Ti by weight in this portion of lithium iron phosphate) controlled at 5000-6000 ppm. This relatively high doping dose aimed to deeply modify the bulk lattice structure of the large particles. 4+ By substituting Fe sites into the LiFePO4 lattice, lattice defects are introduced or lithium-ion diffusion channels are expanded, significantly improving the diffusion rate of lithium ions inside larger particles. This compensates for the kinetic losses caused by the increase in size, and thus helps to improve rate performance. Attached Figure Description

[0034] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0035] Figure 1 This is a SEM image of the multi-scale graded high-pressure lithium iron phosphate prepared in Example 1 of the present invention. Detailed Implementation

[0036] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0037] The present invention will be further described below with reference to the embodiments.

[0038] Example 1 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6000 ppm.

[0039] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0040] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0041] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0042] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0043] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃, held for 2 hours, then heated at a rate of 5 ℃ / min to 790 ℃ for 9 hours. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill at a controlled pressure of 0.32 MPa and a grading frequency of 30 Hz to obtain pulverized material with particle sizes of D10 of 0.34 μm, D50 of 0.98 μm, D90 of 2.46 μm, and D100 of 6.81 μm. Finally, the material was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0044] Comparative Example 1 The difference from Example 1 is that titanium dioxide is omitted in both steps 1) and 2), while everything else is the same as in Example 1. Specifically: A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0045] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0046] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0047] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃, held for 2 hours, then heated at a rate of 5 ℃ / min to 790 ℃ for 9 hours. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill at a controlled pressure of 0.32 MPa and a grading frequency of 30 Hz to obtain pulverized material with particle sizes of D10 of 0.31 μm, D50 of 0.93 μm, D90 of 2.56 μm, and D100 of 7.11 μm. Finally, the material was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0048] Comparative Example 2 The difference from Example 1 is that the content of titanium dioxide in the mixture is the same in steps 1) and 2), which is 1500 ppm. All other aspects are the same as in Example 1, as detailed below: A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0049] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0050] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0051] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0052] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0053] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃, held for 2 hours, then heated at a rate of 5 ℃ / min to 790 ℃ for 9 hours. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill at a controlled pressure of 0.32 MPa and a grading frequency of 30 Hz to obtain pulverized material with particle sizes of D10 of 0.40 μm, D50 of 0.93 μm, D90 of 2.53 μm, and D100 of 5.92 μm. Finally, the material was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0054] Comparative Example 3 The difference from Example 1 is that the content of titanium dioxide in the mixture is the same in steps 1) and 2), which is 6000 ppm. All other aspects are the same as in Example 1, as detailed below: A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6000 ppm.

[0055] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6000 ppm.

[0056] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0057] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0058] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0059] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃, held for 2 hours, then heated at a rate of 5 ℃ / min to 790 ℃ for 9 hours. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill at a controlled pressure of 0.32 MPa and a grading frequency of 30 Hz to obtain pulverized material with particle sizes of D10 of 0.38 μm, D50 of 0.99 μm, D90 of 2.59 μm, and D100 of 6.35 μm. Finally, the material was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0060] Comparative Example 4 The difference from Example 1 is that the content of titanium dioxide in the mixture is the same in steps 1) and 2), which is 1200 ppm. All other aspects are the same as in Example 1, as detailed below: A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1200 ppm.

[0061] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1200 ppm.

[0062] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0063] 4) The slurry B is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.40μm. The nano-sized milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 60μm.

[0064] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0065] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0066] Comparative Example 5 The difference from Example 1 is that the content of titanium dioxide in the mixture is the same in both steps 1) and 2), which is 5500 ppm. All other aspects are the same as in Example 1, as detailed below: A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 5500 ppm.

[0067] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 5500 ppm.

[0068] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0069] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0070] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0071] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0072] Example 2 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 5500 ppm.

[0073] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0074] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0075] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0076] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0077] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0078] Example 3 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 5300 ppm.

[0079] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0080] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0081] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 34μm.

[0082] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0083] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0084] Example 4 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6200 ppm.

[0085] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1500 ppm.

[0086] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0087] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0088] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0089] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0090] Example 5 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6000 ppm.

[0091] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1200 ppm.

[0092] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0093] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.40μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0094] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 3:1 using a ribbon mixer to obtain spray powder E.

[0095] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0096] Example 6 A method for preparing a lithium iron phosphate cathode material includes the following steps: 1) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture one. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry A with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.965:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 7% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 6000 ppm.

[0097] 2) Mix ferric phosphate, lithium carbonate, glucose and polyethylene glycol in a certain proportion to obtain mixture two. Add water to the mixture and stir in a reactor for 90 minutes to obtain slurry B with a solid content of 40%. In the iron phosphate, the molar ratio of iron to phosphorus is 0.985:1; The molar ratio of iron phosphate to lithium carbonate is 1:1.02; The glucose content of ferric phosphate is 9% by mass; The mass percentage of polyethylene glycol in ferric phosphate is 5%; The mass ratio of titanium dioxide to iron phosphate is 1000 ppm.

[0098] 3) The slurry A is milled to obtain a nano-sized milled slurry with a particle size D50 of 0.75μm; the nano-sized milled slurry is spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 25Hz to obtain a spray powder C with a spray particle size D50 of 60μm.

[0099] 4) The slurry B is sand-milled to obtain a nano-sized sand-milled slurry with a particle size D50 of 0.4μm. The nano-sized sand-milled slurry is then spray-dried with an inlet air temperature of 240℃, an outlet air temperature of 90℃, and an atomization frequency of 40Hz to obtain a spray powder D with a spray particle size D50 of 30μm.

[0100] 5) Spray powder C and spray powder D are thoroughly mixed at a mass ratio of 2.5:1 using a ribbon mixer to obtain spray powder E.

[0101] 6) The spray-dried powder E was placed in a roller kiln and sintered under a nitrogen atmosphere at a heating rate of 3 ℃ / min to 400 ℃ and held for 2 hours. Then, the heating rate was increased to 5 ℃ / min, the sintering temperature was 790 ℃, and the sintering time was 9 h. The sintered material was then cooled by water cooling. The sintered material was then pulverized by an air jet mill with the air pressure controlled at 0.32 MPa and the grading frequency at 30 Hz to obtain pulverized material. Finally, it was further sieved, mixed, and packaged to obtain the finished lithium iron phosphate product.

[0102] Table 1 Performance Comparison of Examples and Comparative Examples

[0103] This invention creatively combines "particle size distribution" with "gradient element doping" to achieve synergistic optimization from both physical and chemical dimensions, successfully breaking through the performance barrier between high compaction and high rate of traditional homogenized materials, and providing a new solution for the preparation of next-generation high-energy-density lithium iron phosphate cathode materials.

[0104] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that: It includes at least large-particle lithium iron phosphate and small-particle lithium iron phosphate, wherein the titanium doping concentration in the large-particle lithium iron phosphate and small-particle lithium iron phosphate exhibits a stepped decrease distribution.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that: The D50 of large-particle lithium iron phosphate is 1.0~3.0μm, and the titanium doping concentration is 5000~6000 ppm; The D50 of small-particle lithium iron phosphate is 0.1~0.5μm, and the titanium doping concentration is 1000-2000 ppm; The mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is 2~9:1~8.

3. The lithium iron phosphate cathode material according to claim 2, characterized in that: The mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is 2~3:

1.

4. The lithium iron phosphate cathode material according to claim 2, characterized in that: Both large and small lithium iron phosphate particles are coated with a carbon layer.

5. The lithium iron phosphate cathode material according to claim 4, characterized in that: The carbon layer accounts for 0.5-2% of the mass of lithium iron phosphate.

6. The method for preparing the lithium iron phosphate cathode material according to any one of claims 1-5, characterized in that: Includes the following steps: Iron source, phosphorus source, lithium source, carbon source and titanium source are mixed in a set ratio to prepare a first slurry and a second slurry, wherein the concentration of titanium source in the first slurry is greater than that in the second slurry; After grinding the first slurry and the second slurry, nano-sized first grinding slurry and second grinding slurry are obtained respectively. The particle D50 in the first grinding slurry is larger than that in the second grinding slurry. The abrasive slurry is spray-dried to obtain the first spray powder and the second spray powder, respectively. The first spray powder and the second spray powder are mixed, sintered, crushed, sieved and mixed in a certain proportion to obtain the final product.

7. The method for preparing the lithium iron phosphate cathode material according to claim 6, characterized in that: The titanium source is titanium dioxide. In the first slurry, the mass ratio of titanium dioxide to iron phosphate is 5000~6000ppm; in the second slurry, the mass ratio of titanium dioxide to iron phosphate is 1000~2000ppm. Preferably, the particle size D50 of the first abrasive slurry is 0.5~1μm; and the particle size D50 of the second abrasive slurry is 0.2~0.4μm.

8. The method for preparing the lithium iron phosphate cathode material according to claim 7, characterized in that: The inlet air temperature of the spray dryer is 200~300℃, and the outlet air temperature is 80~120℃; Preferably, the atomization frequency of the spray drying of the first abrasive slurry is 25~30Hz; and the atomization frequency of the spray drying of the second abrasive slurry is 40~50Hz.

9. The method for preparing the lithium iron phosphate cathode material according to claim 6, characterized in that: The mass ratio of the first spray powder to the second spray powder is 2.5~3:1; Preferably, the sintering temperature is 750~800 ℃ and the sintering time is 5~10 h.

10. A positive electrode plate, characterized in that: It includes a current collector and a positive electrode active material attached to the surface of the current collector, wherein the positive electrode active material is the lithium iron phosphate positive electrode material according to any one of claims 1-5.

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