High-compaction lithium iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention relates to a high-compaction lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of batteries, the method adopts a two-step synthesis method, the first step adopts ferrous phosphate high-grinding sintering to prepare first-sintering large particles, and adopts in-situ doped iron phosphate low-grinding to prepare first-sintering small particles; 2, performing high grinding on the first-fired large particles, performing low grinding on the first-fired small particles, accurately controlling the sizes of the small particles, and stripping C3N4 through grinding to form g-C3N4; the large and small particles are mixed according to a certain mass ratio, and then secondary sintering is carried out to prepare the high-compaction high-capacity long-circulation lithium iron phosphate material, the advantages of different precursors are utilized, the particle composition is optimized, and the roundness of the large and small particles is better; meanwhile, the nitrogen-doped carbon layer is prepared by taking defect-type graphene and high-molecular organic nitride which are converted from C3N4 in a secondary sintering process as carbon sources, so that the conductivity and the cycle performance of the material are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a high tap density lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery. Background Art

[0002] Lithium ion batteries are widely used due to their advantages such as high energy density, no memory effect, and long cycle life. The cathode material of lithium ion batteries is the key to affecting their performance and application scenarios. Olivine-structured lithium iron phosphate has become a strong competitor for power batteries due to its advantages such as high voltage, high capacity ratio, long cycle life, high safety, and low cost. With the increasing demand for safety by consumers, lithium iron phosphate has once again become the first choice of automobile manufacturers.

[0003] Compared with the traditional two-step process, the same precursor is used for both large and small particle precursors. During the second sintering process, the phenomenon of material fusion and growth is obvious, and it is difficult to achieve the theoretical proportion of large and small particle size distributions; at the same time, the existing second sintering process uses the iron phosphate method or the higher-cost ferrous oxalate method and liquid phase method, resulting in an increase in manufacturing cost. Summary of the Invention

[0004] The present invention provides a high tap density lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery to solve the above-mentioned deficiencies of the prior art. By optimizing the particle size distribution of the two precursors, the obtained two precursors have better particle roundness. In addition, the conductivity and cycle performance of the lithium iron phosphate cathode material are improved by means of re-sintering.

[0005] In order to achieve the purpose of the present invention, the following technologies are proposed: In a first aspect, a preparation method of a high tap density lithium iron phosphate cathode material is provided, including the steps of: Step 01, mixing ferrous phosphate, lithium phosphate, element supplement, a first carbon source, a first dopant, and deionized water, and sequentially performing wet grinding, spray drying, sintering, and air flow pulverization to obtain first granular lithium iron phosphate; Step 02, mixing in-situ doped iron phosphate, lithium carbonate, a second carbon source, and deionized water, and sequentially performing wet grinding, spray drying, sintering, and air flow pulverization to obtain second granular lithium iron phosphate; Wherein, the particle size of the first granular lithium iron phosphate obtained in Step 01 is larger than the particle size of the second granular lithium iron phosphate obtained in Step 02; Step 03, mixing the first granular lithium iron phosphate obtained in Step 01, a third carbon source, and deionized water, and performing wet grinding to obtain a first slurry; Mixing the second granular lithium iron phosphate obtained in Step 02, a fourth carbon source, and deionized water, and performing wet grinding to obtain a second slurry; Step 04: Mix the first slurry and the second slurry obtained in Step 03, and sequentially obtain the lithium iron phosphate cathode material through spray drying, sintering, and air flow pulverization.

[0006] Further, in Step 01: The Fe / P molar ratio in ferrous phosphate is 1.42 to 1.48, and the specific surface area of ferrous phosphate is 20 m 2 / g to 40 m 2 / g; The addition amount of lithium phosphate maintains the Li / Fe molar ratio at 1.03 to 1.07, and the purity of lithium phosphate ≥ 98%; The element supplement is one of lithium carbonate and phosphoric acid. The purity of lithium carbonate ≥ 99.5%, and the purity of phosphoric acid is 85%; The Fe / P molar ratio of the first granular lithium iron phosphate is 0.965; The first carbon source is one or a combination of glucose, sucrose, and polyethylene glycol. The addition amount of the first carbon source accounts for 0.4 wt% to 0.8 wt% of the carbon content in the first granular lithium iron phosphate; The first dopant is one or more of titanium dioxide, ammonium metavanadate, manganese pentoxide, niobium pentoxide, and cerium oxide. The addition amount of the first dopant is determined according to the proportion of one or more of titanium, vanadium, manganese, niobium, and cerium elements in the first dopant in the first granular lithium iron phosphate being 3000 ppm to 5000 ppm.

[0007] Further, in Step 01: During wet grinding, it is carried out at 10°C to 45°C, the sand grinding particle size D50 is 0.7 μm to 0.9 μm, and the solid content is 30 wt% to 50 wt%; During spray drying, the feeding temperature is 180°C to 240°C, the discharging temperature is 80°C to 140°C, the particle size D50 of the intermediate product obtained after spray drying is 20 μm to 60 μm, and the water content ≤ 1.5%; During sintering, it is carried out in a nitrogen atmosphere, the sintering temperature is 780°C to 810°C, and the sintering time is 6 h to 10 h; The particle size D50 of the first granular lithium iron phosphate is 2.0 μm to 4.0 μm, and the average particle diameter of the first granular lithium iron phosphate is 2000 nm to 3000 nm.

[0008] Further, in Step 02: The Fe / P molar ratio in in-situ doped ferrous phosphate is 0.970 to 0.980, the specific surface area of in-situ doped ferrous phosphate is 15 m 2 / g to 25 m 2 / g, and the primary particle diameter of in-situ doped ferrous phosphate is 40 nm to 80 nm; The doping element is Ti, and the in-situ doping amount of Ti is from 5000 ppm to 8000 ppm; The purity of lithium carbonate is ≥99.5%, and the Li / Fe molar ratio of lithium carbonate is from 1.01 to 1.04; The second carbon source is a mixture of one or more of glucose, sucrose, polyethylene glycol, citric acid and one of urea, melamine, dicyandiamide, thiourea, and the addition amount of the second carbon source accounts for 0.8 wt% to 1.2 wt% of the carbon content in the second granular lithium iron phosphate; Among them, urea, melamine, dicyandiamide, and thiourea are all C3N4 raw materials, and the carbon content in C3N4 accounts for 0.4 wt% of the carbon content in the second granular lithium iron phosphate.

[0009] Furthermore, in step 02: Wet grinding is carried out at 10°C to 45°C, the sand grinding particle size D50 is from 0.15 μm to 0.25 μm, and the solid content is from 30 wt% to 50 wt%; The feed temperature of spray drying is from 180°C to 240°C, the discharge temperature is from 80°C to 140°C, the particle size D50 of the intermediate product obtained by spray drying is from 20 μm to 60 μm, and the water content ≤1.5%; Sintering is carried out in a nitrogen atmosphere, the sintering temperature is from 500°C to 600°C, and the sintering time is from 3 h to 7 h; The particle size D50 of the second granular lithium iron phosphate obtained after airflow pulverization is from 0.4 μm to 0.6 μm, and the average particle size of the second granular lithium iron phosphate is from 80 nm to 160 nm.

[0010] Furthermore, in step 03: Both the third carbon source and the fourth carbon source are polyvinylpyrrolidone, and the addition amounts of the third carbon source and the fourth carbon source account for 1.1 wt% to 1.5 wt% of the carbon content in the lithium iron phosphate cathode material; The wet grinding of the first slurry and the second slurry is carried out at 10°C to 45°C, and the solid content is from 30 wt% to 50 wt%. The sand grinding particle size D50 of the first slurry is from 0.7 μm to 1.0 μm, and the sand grinding particle size D50 of the second slurry is from 0.10 μm to 0.20 μm.

[0011] Furthermore, in step 04, the mass ratio of the first slurry to the second slurry is 5 to 7:3 to 5.

[0012] Furthermore, in step 04: The feed temperature of spray drying is from 180°C to 240°C, the discharge temperature is from 80°C to 140°C, the particle size of the obtained intermediate product is from 20 μm to 60 μm, and the water content ≤1.5%; The sintering process is carried out in a nitrogen atmosphere, with the sintering temperature ranging from 760°C to 780°C and the sintering time ranging from 6h to 8h; The particle size D50 of the lithium iron phosphate cathode material obtained after airflow pulverization is 0.9μm to 1.2μm.

[0013] In a second aspect, a lithium iron phosphate cathode material is provided, which is obtained by the preparation method of the high-compactness lithium iron phosphate cathode material.

[0014] In a third aspect, a lithium-ion battery is provided, which includes: a battery cathode made of the lithium iron phosphate cathode material.

[0015] The advantages of the above technical solutions are as follows: Firstly, the present invention combines the advantages of the ferrous phosphate process and the iron phosphate process, and at the same time uses C3N4 to coat the second granular lithium iron phosphate, which can effectively reduce particle agglomeration, make the grading of the finished lithium iron phosphate cathode material better, and the particle roundness better.

[0016] Secondly, during the secondary grinding process of the second granular lithium iron phosphate in the present invention, the massive C3N4 is exfoliated into g-C3N4, and at the same time, through the second sintering, g-C3N4 is converted into nitrogen-doped defective graphene, thereby improving the conductivity of the material.

[0017] Finally, the present invention uses a polymer nitrogen-containing organic compound as a carbon source to form a nitrogen-doped carbon layer, improve the density of the surface-coated carbon layer, inhibit side reactions, and improve the cycle performance. Description of the Drawings

[0018] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings.

[0019] Figure 1 It is the SEM diagram of the lithium iron phosphate cathode material prepared in Example 1 of the present invention.

[0020] Figure 2 It is the charge-discharge performance curve diagram of the lithium iron phosphate cathode material prepared in Example 1 of the present invention. Detailed Embodiments

[0021] Example 1 A preparation method of a high-compactness lithium iron phosphate cathode material includes the steps: Step 01, 1000g of Fe / P molar ratio of 1.45 and specific surface area of 30 m 2Mix 241.0 g of lithium phosphate, 2.24 g of lithium carbonate, and 241.0 g of ferrous phosphate per gram, and simultaneously add 44.37 g of sucrose, 5.03 g of titanium dioxide, 2.46 g of ammonium metavanadate, and 1855 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with a temperature controlled at 25°C for grinding. The sand particle size D50 is 0.8 μm, and the solid content is 40 wt% to obtain a slurry. Spray dry the slurry, control the inlet air temperature at 210°C and the outlet air temperature at 110°C. After granulation, obtain a precursor powder with a particle size D50 of 40 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 795°C for 5 h, and select nitrogen as the protective atmosphere. Crush the sintered material by air flow, control the particle size D50 at 3.0 μm, and control the average particle size at 2500 nm during crushing to obtain the first granular lithium iron phosphate.

[0022] Step 02: Mix 1000 g of ferric phosphate with an Fe / P molar ratio of 0.975, a specific surface area of 20.0 m 2 / g, an in-situ Ti doping amount of 6000 ppm, and a primary particle size of 60 nm, and 252.35 g of lithium carbonate. Simultaneously add 50 g of glucose, 60 g of citric acid, 80 g of melamine, and 1878 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with a temperature controlled at 25°C for grinding. The particle size D50 is 0.20 μm, and the solid content is 40 wt% to obtain a slurry. Spray dry the slurry, control the inlet air temperature at 210°C and the outlet air temperature at 110°C. After granulation, obtain a precursor powder with a particle size D50 of 40 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 550°C for 4 h, and select nitrogen as the protective atmosphere. Crush the sintered material by air flow, control the particle size D50 at 0.5 μm to obtain the second granular lithium iron phosphate.

[0023] Step 03: Take 1000 g of the first granular lithium iron phosphate prepared in Step 01, add 20 g of polyvinylpyrrolidone and 1500 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with a temperature controlled at 25°C for grinding. The sand grinding particle size D50 is 0.85 μm, and the solid content is 40 wt% to obtain the first slurry. Take 1000 g of the second granular lithium iron phosphate prepared in Step 02, add 20 g of polyvinylpyrrolidone and 1500 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with a temperature controlled at 25°C for grinding. The sand grinding particle size D50 is 0.15 μm, and the solid content is 40 wt% to obtain the second slurry.

[0024] Step 04: Mix the first slurry and the second slurry prepared in Step 03 according to a mass ratio of 6:4. Spray-dry the mixed slurry, control the inlet air temperature at 210 °C and the outlet air temperature at 110 °C to granulate and obtain a secondary calcination precursor powder with a particle size D50 of 40 μm and a moisture content of ≤1.5%. Calcinate the obtained secondary calcination precursor powder at 770 °C for 7 h, and select nitrogen as the protective atmosphere. Crush the sintered material and control the particle size D50 at 1.1 μm to obtain a lithium iron phosphate cathode material.

[0025] Observe the microstructure morphology of the prepared lithium iron phosphate cathode material, as Figure 1 shown. It can be easily seen from the figure that the first granular lithium iron phosphate and the second granular lithium iron phosphate are evenly distributed, and the roundness of the particles is excellent. Exploring the mechanism, it is because this example uses a polymer nitrogen-containing organic compound and g-C3N4 as carbon sources to form a nitrogen-doped carbon layer and a defective graphene carbon layer, improving the density of the surface-coated carbon layer, inhibiting side reactions, reducing the interface resistance, and significantly improving the cycling performance.

[0026] In addition, in order to further verify its application performance in the battery, the charge-discharge performance of the battery prepared in Example 1 was tested. The test curve graph is as Figure 2 shown. It can be seen from the figure that the main discharge platform of this battery is about 3.4 V, and the capacity in the platform area exceeds 150 mAh / g. Therefore, it shows that the battery processed from the lithium iron phosphate cathode material prepared in this example has a stable two-phase reaction mechanism. The main reason for this result is that the lithium iron phosphate cathode material has a uniform particle structure, thus reducing local polarization. In addition, since the end point of the curve is 160 mAh / g, it can be known that the utilization rate of this lithium iron phosphate cathode material is relatively high. And since the average voltage in the platform area of this battery is about 3.4 V, it can be known that the energy density is approximately equal to 3.4 V × 160 mAh / g = 544 Wh / kg, so it can be seen that its energy density is relatively high. In addition, since the voltage fluctuation in the platform area of this battery is small (ΔV < 0.2 V), it indicates a low impedance at the electrode / electrolyte interface.

[0027] Example 2 A preparation method of a high tap density lithium iron phosphate cathode material, comprising the steps: Step 01: Take 1000 g of Fe / P molar ratio of 1.48 and a specific surface area of 40 m 2Mix 243.64 g of lithium phosphate, 11.93 g of phosphoric acid, and iron phosphate at a ratio of 1000 g per gram with 37.2 g of sucrose, 3.34 g of titanium dioxide, 2.51 g of ammonium metavanadate, and 2900 g of water in a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 10°C for grinding. The grinding particle size D50 is 0.7 μm, and the solid content is 30 wt% to obtain a slurry. Spray-dry the slurry, control the inlet air temperature at 180°C and the outlet air temperature at 80°C for granulation to obtain a precursor powder with a particle size D50 of 20 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 780°C for 10 h, and select nitrogen as the protective atmosphere. Crush the sintered material, control the particle size D50 at 2.0 μm, and the average particle size of the primary particles at 2000 nm to obtain the first granular lithium iron phosphate.

[0028] Step 02: Mix 1000 g of iron phosphate with an Fe / P molar ratio of 0.980, a specific surface area of 25.0 m 2 / g, an in-situ Ti doping amount of 5000 ppm, and a primary particle size of 40 nm, and 248.66 g of lithium carbonate. At the same time, add 30 g of glucose, 38 g of polyethylene glycol, 80 g of melamine, and 2900 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 25°C for grinding. The grinding particle size D50 is 0.15 μm, and the solid content is 30 wt% to obtain a slurry. Spray-dry the slurry, control the inlet air temperature at 180°C and the outlet air temperature at 80°C for granulation to obtain a precursor powder with a particle size D50 of 20 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 600°C for 3 h, and select nitrogen as the protective atmosphere. Crush the sintered material, control the particle size D50 at 0.6 μm to obtain the second granular lithium iron phosphate.

[0029] Step 03: Take 1000 g of the first granular lithium iron phosphate prepared in Step 01, add 20 g of polyvinylpyrrolidone and 2300 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 10°C for grinding. The grinding particle size D50 is 0.70 μm, and the solid content is 30 wt% to obtain the first slurry. Take 1000 g of the second granular lithium iron phosphate prepared in Step 02, add 20 g of polyvinylpyrrolidone and 2300 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 10°C for grinding. The grinding particle size D50 is 0.10 μm, and the solid content is 30 wt% to obtain the second slurry.

[0030] Step 04: Mix the first slurry and the second slurry obtained in Step 03 according to a mass ratio of 7:3. Spray-dry the mixed slurry, control the inlet air temperature at 180 °C and the outlet air temperature at 80 °C to granulate and obtain a second calcination precursor powder with a particle size D50 of 20 μm and a moisture content of ≤1.5%. Calcinate the obtained second calcination precursor powder at 780 °C for 6 h, and select nitrogen as the protective atmosphere. Subject the sintered material to air-flow pulverization, control the particle size D50 at 0.9 μm, and prepare a lithium iron phosphate cathode material.

[0031] Example 3 A preparation method of a high tap-density lithium iron phosphate cathode material, comprising the steps of: Step 01: Mix 1000 g of ferrous phosphate with an Fe / P molar ratio of 1.42 and a specific surface area of 20 m 2 / g, 226.05 g of lithium phosphate, 15.83 g of lithium carbonate, and at the same time add 51.3 g of sucrose, 6.66 g of titanium dioxide, 2.42 g of ammonium metavanadate, and 1220 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 40 °C for grinding, with a particle size D50 of 0.9 μm and a solid content of 50 wt% to obtain a slurry. Spray-dry the slurry, control the inlet air temperature at 240 °C and the outlet air temperature at 140 °C to granulate and obtain a precursor powder with a particle size D50 of 60 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 810 °C for 6 h, and select nitrogen as the protective atmosphere. Pulverize the sintered material, control the particle size D50 at 4.0 μm, and the average particle size of the primary particles at 3000 nm to prepare a first granular lithium iron phosphate.

[0032] Step 02: Mix 1000 g of iron phosphate with an Fe / P molar ratio of 0.980, a specific surface area of 15.0 m 2 / g, an in-situ Ti doping amount of 8000 ppm, and a primary particle size of 80 nm, and 256.05 g of lithium carbonate, and at the same time add 52 g of sucrose, 35 g of polyethylene glycol, 80 g of melamine, and 1300 g of water to a ball mill to form a dispersion. Transfer the dispersion to a sand mill with the temperature controlled at 40 °C for grinding, with a sand grinding particle size D50 of 0.25 μm and a solid content of 30 wt% to obtain a slurry. Spray-dry the slurry, control the inlet air temperature at 240 °C and the outlet air temperature at 140 °C to granulate and obtain a precursor powder with a particle size D50 of 60 μm and a moisture content of ≤1.5%. Calcinate the obtained precursor powder at 500 °C for 7 h, and select nitrogen as the protective atmosphere. Pulverize the sintered material, control the particle size D50 at 0.4 μm, and prepare a second granular lithium iron phosphate.

[0033] Step 03: Take 1000 g of the first granular lithium iron phosphate prepared in Step 01, add 20 g of polyvinylpyrrolidone and 1000 g of water, and mix them in a ball mill to form a dispersion; transfer the dispersion to a sand mill with a temperature controlled at 40°C for grinding. The grinding particle size D50 is 1.0 μm, and the solid content is 50 wt%, obtaining the first slurry. Take 1000 g of the second granular lithium iron phosphate prepared in Step 02, add 20 g of polyvinylpyrrolidone and 1000 g of water, and mix them in a ball mill to form a dispersion. Transfer the dispersion to a sand mill with a temperature controlled at 40°C for grinding. The grinding particle size D50 is 0.20 μm, and the solid content is 50 wt%, obtaining the second slurry.

[0034] Step 04: Mix the first slurry and the second slurry according to a mass ratio of 5:5, and perform spray drying on the mixed slurry. Control the inlet air temperature at 240°C and the outlet air temperature at 140°C to granulate and obtain a secondary calcination precursor powder with a particle size D50 of 60 μm and a moisture content ≤ 1.5%. Calcinate the obtained secondary calcination precursor powder at 760°C for 8 h, and select nitrogen as the protective atmosphere. Crush the sintered material and control the particle size D50 at 1.2 μm to obtain the lithium iron phosphate cathode material.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that in Step 01, the raw materials ferrous phosphate and lithium phosphate are replaced with iron phosphate and lithium carbonate with an Fe / P molar ratio of 0.965, and the grinding D50 particle size is controlled at 0.4 μm.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that in Step 02, the C3N4 raw material (melamine) is not added, and only 70 g of glucose and 65 g of citric acid are added as the second carbon source.

[0037] Comparative Example 3 The difference between this comparative example and Example 1 is that in Step 03, the third carbon source is 20 g of glucose + 23 g of polyethylene glycol.

[0038] Comparative Example 4 The difference between this comparative example and Example 1 is that in Step 04, the first slurry and the second slurry are mixed according to a ratio of 9:1.

[0039] In order to further verify the lithium iron phosphate cathode materials obtained in the above Examples 1 to 3 and Comparative Examples 1 to 4, tests were carried out.

[0040] First, the lithium iron phosphate cathode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were used to photograph the microstructural morphology at a magnification of 5k by scanning electron microscopy.

[0041] Secondly, weigh 1 g of the lithium iron phosphate cathode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 4, put the samples into a compaction mold, place the mold in a compaction device to start the test, and take the powder compaction density result under a pressure of 30 KN.

[0042] Then, disperse the lithium iron phosphate cathode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 4, Super P, and PVDF in NMP according to a mass ratio of 80:10:10. After ball milling and dispersing evenly, coat them on an aluminum foil and dry them in vacuum to obtain a positive electrode sheet. The electrolyte is 1 mol / L LiPF6, where the volume ratio of the solvents is EC:DMC:EMC = 1:1:1 (volume ratio), the separator is a Celgard polypropylene membrane, and the lithium metal sheet is the negative electrode. Assemble them together into a coin cell half-cell. The test voltage range is 2.0 V - 3.75 V. Charge it to 3.75 V in a constant current and constant voltage charging mode, and discharge it to 2.0 V in a constant current discharge mode. The charge and discharge current is 0.1C for 2 cycles. Then cycle it at a 1C charge and discharge current for 2 cycles, and the cut-off voltage condition is the same as that at 0.1C; Prepare the lithium iron phosphate materials obtained in Examples 1 - 3 and Comparative Examples 1 - 4 into full cells and test their cycling performance at room temperature. The test results are shown in Table 1 below.

[0043] Table 1 Test Results It can be seen from the above results that the lithium iron phosphate cathode materials prepared in the examples of the present invention have higher powder compaction and better capacity.

[0044] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. A method for preparing a high-density lithium iron phosphate positive electrode material, characterized in that: Includes steps: Step 01, mixing ferrous phosphate, lithium phosphate, an element supplement, a first carbon source, a first dopant and deionized water, and sequentially performing wet grinding, spray drying, sintering and air flow milling to obtain first granular lithium iron phosphate; Step 02, mixing in-situ doped iron phosphate, lithium carbonate, a second carbon source and deionized water, and sequentially performing wet grinding, spray drying, sintering and air flow milling to obtain second granular lithium iron phosphate; Wherein, the particle size of the first granular lithium iron phosphate obtained in step 01 is greater than the particle size of the second granular lithium iron phosphate obtained in step 02; Step 03, mixing the first granular lithium iron phosphate obtained in step 01, the third carbon source and deionized water, and obtaining a first slurry by wet grinding; The second granular lithium iron phosphate obtained in step 02, the fourth carbon source and deionized water are mixed to obtain a second slurry by wet grinding; Step 04, mixing the first slurry and the second slurry obtained in step 03, and sequentially performing spray drying, sintering and air flow milling to obtain a lithium iron phosphate positive electrode material.

2. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 01: The Fe / P molar ratio in ferrous phosphate is 1.42 to 1.48, and the specific surface area of ​​ferrous phosphate is 20 m 2 / g to 40m 2 / g; The amount of lithium phosphate added ensures that the Li / Fe molar ratio is 1.03 to 1.07, and the purity of lithium phosphate is ≥98%; The element supplement is one of lithium carbonate and phosphoric acid, the purity of lithium carbonate is ≥99.5%, and the purity of phosphoric acid is 85%; The Fe / P molar ratio of the first granular lithium iron phosphate is 0.965; The first carbon source is one or more of glucose, sucrose, and polyethylene glycol, and the amount of the first carbon source added accounts for 0.4wt% to 0.8wt% of the carbon content in the first granular lithium iron phosphate; The first dopant is one or more of titanium dioxide, ammonium metavanadate, manganese pentoxide, niobium pentoxide and cerium oxide. The amount of the first dopant added is determined according to the proportion of one or more of titanium, vanadium, manganese, niobium and cerium elements in the first dopant in the first granular lithium iron phosphate being 3000ppm to 5000ppm.

3. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 01: When wet grinding is carried out at 10°C to 45°C, the sand grinding particle size D50 is 0.7μm to 0.9μm, and the solid content is 30wt% to 50wt%; During spray drying, the feed temperature is 180°C to 240°C, the discharge temperature is 80°C to 140°C, the particle size D50 of the intermediate product obtained after spray drying is 20 μm to 60 μm, and the water content is ≤1.5%; During sintering, the sintering is carried out in a nitrogen atmosphere, the sintering temperature is 780°C to 810°C, and the sintering time is 6h to 10h; The particle size D50 of the first granular lithium iron phosphate is 2.0 μm to 4.0 μm, and the average particle size of the first granular lithium iron phosphate is 2000 nm to 3000 nm.

4. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 02: The molar ratio of Fe / P in the in-situ doped iron phosphate is 0.970 to 0.980, and the specific surface area of ​​the in-situ doped iron phosphate is 15m 2 / g to 25m 2 / g, the primary particle size of the in-situ doped iron phosphate is 40nm to 80nm; The doping element is Ti, and the in-situ doping amount of Ti is 5000ppm to 8000ppm; The purity of lithium carbonate is ≥99.5%, and the Li / Fe molar ratio of lithium carbonate is 1.01 to 1.04; The second carbon source is a mixture of one or more of glucose, sucrose, polyethylene glycol, citric acid and one of urea, melamine, dicyandiamide, and thiourea, and the amount of the second carbon source added accounts for 0.8wt% to 1.2wt% of the carbon content in the second granular lithium iron phosphate; Urea, melamine, dicyandiamide and thiourea are all raw materials of C3N4, and the carbon content in C3N4 accounts for 0.4wt% of the carbon content in the second granular lithium iron phosphate.

5. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 02: Wet grinding is carried out at 10°C to 45°C, the sand grinding particle size D50 is 0.15μm to 0.25μm, and the solid content is 30wt% to 50wt%; The feed temperature of the spray drying is 180°C to 240°C, the discharge temperature is 80°C to 140°C, the particle size D50 of the intermediate product obtained by the spray drying is 20 μm to 60 μm, and the water content is ≤1.5%; The sintering is carried out in a nitrogen atmosphere, the sintering temperature is between 500° C. and 600° C., and the sintering time is between 3 h and 7 h; The particle size D50 of the second granular lithium iron phosphate obtained after air flow pulverization is 0.4 μm to 0.6 μm, and the average particle size of the second granular lithium iron phosphate is 80 nm to 160 nm.

6. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 03: The third carbon source and the fourth carbon source are both polyvinyl pyrrolidone, and the addition amount of the third carbon source and the fourth carbon source accounts for 1.1wt% to 1.5wt% of the carbon content in the lithium iron phosphate positive electrode material; The wet grinding of the first slurry and the second slurry is performed at 10°C to 45°C with a solid content of 30wt% to 50wt%, a sand grinding particle size D50 of the first slurry of 0.7μm to 1.0μm, and a sand grinding particle size D50 of the second slurry of 0.10μm to 0.20μm.

7. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 04, the mass ratio of the first slurry to the second slurry is 5-7:3-5.

8. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step 04: The feed temperature of the spray drying is 180°C to 240°C, the discharge temperature is 80°C to 140°C, the particle size of the intermediate product is 20μm to 60μm, and the water content is ≤1.5%; The sintering process is carried out in a nitrogen atmosphere, the sintering temperature is between 760°C and 780°C, and the sintering time is between 6h and 8h; The particle size D50 of the lithium iron phosphate positive electrode material obtained after air flow milling is 0.9 μm to 1.2 μm.

9. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the preparation method of the high-density lithium iron phosphate positive electrode material according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises: a battery positive electrode made from the lithium iron phosphate positive electrode material according to claim 9.

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