A method for preparing high-compaction lithium iron phosphate by using regenerated lithium dihydrogen phosphate and regenerated ferrous oxalate

Abstract

The application discloses a method for preparing high-compaction lithium iron phosphate by using regenerated lithium dihydrogen phosphate and regenerated ferrous oxalate, and crude regenerated lithium dihydrogen phosphate and crude regenerated ferrous oxalate are refined respectively; wherein the crude regenerated lithium dihydrogen phosphate is subjected to the following steps: dissolving, removing heavy metals by using a chelating resin, removing iron and aluminum by adjusting pH, vacuum concentration, recrystallization and vacuum drying, so as to obtain refined regenerated lithium dihydrogen phosphate; the crude regenerated ferrous oxalate is subjected to the following steps: beating and washing by using a dilute oxalic acid solution and airflow pulverization and grading, so as to obtain refined regenerated ferrous oxalate; compared with outsourcing of raw materials, the use of fully regenerated raw materials can reduce the raw material cost by more than 30%; meanwhile, carbon emission in the process of mining and processing of primary minerals is reduced, the method is in line with the industrial policy guidance and the requirement of circular economy development of recycling of power batteries, and has the value of large-scale industrialization and popularization.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate technology, and in particular to a method for preparing high-pressure lithium iron phosphate using regenerated lithium dihydrogen phosphate and regenerated ferrous oxalate. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) has become the mainstream cathode material system for power batteries of new energy vehicles and large-scale energy storage batteries due to its excellent thermal stability, long cycle life and low cost.

[0003] As the requirements for volumetric energy density of power batteries continue to increase, high-density lithium iron phosphate has become a core direction for technological research and development in the industry. Currently, the mainstream process for preparing high-density lithium iron phosphate in the industry is a solid-state sintering route that combines purchased battery-grade lithium dihydrogen phosphate with purchased battery-grade ferrous oxalate. This route can achieve uniform carbon coating and controllable grain crystallinity, and the compaction density of mass-produced products can reach 2.6~2.7 g / cm³, which can meet the application requirements of mid-to-high-end power batteries.

[0004] At the same time, the rapid development of the new energy vehicle industry has driven the continuous growth of the scale of retired lithium iron phosphate batteries. Regenerated lithium dihydrogen phosphate and regenerated ferrous oxalate can be prepared from retired batteries through processes such as wet recycling and pyrometallurgical recycling, which is the core path to realize the resource recycling of power batteries.

[0005] However, recycled raw materials have many inherent defects, which severely limit their large-scale application in the preparation of high-pressure lithium iron phosphate: Firstly, the impurity content is too high, and residual impurities are commonly found in recycled lithium dihydrogen phosphate. As well as impurities such as Al and Si, regenerated ferrous oxalate is prone to carrying trivalent iron oxides and heavy metal impurities. Direct use in sintering will inhibit normal grain growth, introduce impurity phases, and lead to a decrease in product crystallinity and an increase in internal resistance. Secondly, the morphology and particle size controllability are poor. Recycled raw materials are mostly prepared by precipitation method, resulting in irregular particle morphology and wide particle size distribution, usually with a particle size distribution Span > 1.8. The integrity of primary crystallization is insufficient, and a dense packing structure cannot be formed. After directly using the sintering process of purchased raw materials, the compaction density of the product is usually lower than 2.45 g / cm, which is far lower than the level of products prepared from purchased raw materials. Third, the reaction kinetics are mismatched, the active sites of the recycled raw materials are unevenly distributed, and the conventional two-stage sintering process of the purchased process is prone to problems such as local over-reaction, abnormal grain growth or incomplete reaction. At the same time, the continuity of the carbon coating layer is poor, which ultimately leads to the degradation of the rate performance and cycle performance of the material.

[0006] In existing technologies, recycled lithium iron phosphate raw materials are mostly mixed with purchased raw materials at a low ratio of 10% to 30%, which cannot achieve full replacement of recycled raw materials. Some processes improve the purity of recycled raw materials by adding purification steps, but the sintering regime and morphology control process are not optimized according to the morphological characteristics and reaction characteristics of recycled raw materials, so it is still difficult to achieve the performance indicators of high-pressure lithium iron phosphate.

[0007] In summary, the industry currently lacks a complete process method that is compatible with the characteristics of recycled lithium dihydrogen phosphate and recycled ferrous oxalate raw materials and can stably prepare high-pressure lithium iron phosphate, which restricts the industrial implementation of high-value closed-loop recycling of all components of retired lithium iron phosphate batteries. Summary of the Invention

[0008] In order to overcome the shortcomings of the prior art, one of the objectives of this invention is to provide a method for preparing high-pressure lithium iron phosphate using regenerated lithium dihydrogen phosphate and regenerated ferrous oxalate.

[0009] One of the objectives of this invention is achieved through the following technical solution: A method for preparing high-pressure lithium iron phosphate using recycled lithium dihydrogen phosphate and recycled ferrous oxalate includes the following steps: (1) Pretreatment of regenerated raw materials by classification and refining: crude regenerated lithium dihydrogen phosphate and crude regenerated ferrous oxalate are refined respectively; the regenerated lithium dihydrogen phosphate is obtained by dissolving, removing heavy metals with chelating resin, removing iron and aluminum by pH adjustment, concentration and recrystallization under reduced pressure and vacuum drying; the regenerated ferrous oxalate is obtained by pulping and washing with dilute oxalic acid solution and classifying by air jet milling. (2) Mixing of dual carbon source materials: Weigh refined regenerated lithium dihydrogen phosphate and refined regenerated ferrous oxalate according to the Li:Fe:P molar ratio of 1.02~1.05:1:1, add a composite carbon source composed of glucose and conductive carbon black, and mix to obtain a mixture; the total amount of the composite carbon source added is 2.5~3.5% of the total mass of the raw materials; (3) Wet ultrafine sand milling homogenization: The mixture is prepared into a slurry with a solid content of 55~65wt% by mixing with deionized water, sodium polyacrylate dispersant is added, and the slurry is fed into a horizontal sand mill for ultrafine sand milling until the slurry particles are uniformly sized. The depth is 0.4~0.8m; (4) Spray granulation and densification: The homogeneous slurry after sand milling is granulated by a centrifugal spray dryer to obtain spherical precursor secondary particles; (5) Three-stage gradient temperature-controlled sintering: The precursor is placed in a high-purity nitrogen protective atmosphere with an oxygen content of 10ppm and sintered in three stages: ① Low-temperature decomposition stage: the temperature is increased to 380~420℃ at a rate of 3~5℃ / min and held for 2~3h; ② Medium-temperature nucleation stage: the temperature is increased to 560~600℃ at a rate of 2~3℃ / min and held for 3~4h; ③ High-temperature densification stage: the temperature is increased to 730~760℃ at a rate of 1~2℃ / min and held for 9~12h; After sintering, the precursor is naturally cooled in the furnace to obtain lithium iron phosphate sintered blocks; (6) Low-temperature shaping and grading post-processing: The sintered block is first mechanically crushed, and then pulverized and graded by low-temperature airflow to obtain high-pressure lithium iron phosphate cathode material.

[0010] Further, the specific refining process of regenerated lithium dihydrogen phosphate in step (1) is as follows: crude regenerated lithium dihydrogen phosphate is dissolved in deionized water at 80-90℃ at a solid-liquid ratio of 1:4 to 1:6 and stirred until completely dissolved; 0.3-0.5 wt% of D401 chelating resin is added and stirred for 30-60 min to remove heavy metal impurities; after filtration, 0.1-0.2 wt% of lithium hydroxide is added to the filtrate to adjust the pH to 5.0-5.5 and precipitate to remove iron and aluminum impurities; after secondary filtration, the filtrate is concentrated by vacuum evaporation to a density of 1.38-1.42 g / cm³, cooled to room temperature at a rate of 2-3℃ / h for recrystallization, centrifuged and then vacuum dried at 110-120℃ to obtain refined regenerated lithium dihydrogen phosphate.

[0011] Furthermore, the specific refining process of regenerated ferrous oxalate in step (1) is as follows: the crude regenerated ferrous oxalate is slurried and washed with a 2-3% (w / w) dilute oxalic acid solution at a liquid-to-solid ratio of 5:1, the washing temperature is 40-50℃, and the stirring time is 30-45 min; after filtration and washing until the filtrate is neutral, it is subjected to air jet milling and classification to control the primary particle size distribution. The particle size was 2-4 μm with a particle size distribution of Span 1.2, and refined regenerated ferrous oxalate was obtained.

[0012] Furthermore, in step (2), the mass ratio of glucose to conductive carbon black in the composite carbon source is 7:3 to 8:2.

[0013] Furthermore, in step (3), the amount of sodium polyacrylate dispersant added is 0.1~0.3wt% of the total mass of the mixture; the grinding media used in the sand mill is 0.3~0.5mm zirconium beads.

[0014] Furthermore, the process parameters for centrifugal spray drying in step (4) are: inlet air temperature 190~210℃, outlet air temperature 95~105℃, and centrifugal atomizer speed 12000~15000rpm; the resulting precursor secondary particles The diameter is 8~12m, the sphericity is 0.85, and the tap density is 1.25g / cm³.

[0015] Further, step (6) specifically involves: first mechanically crushing the sintered block to below 200 mesh, then using an air jet mill for low-temperature shaping and crushing, controlling the particle size by adjusting the grading speed; the final product is lithium iron phosphate. It is 10~14m, D 10 1.5m, sphericity retention rate 90%, compaction density 2.65g / cm.

[0016] Furthermore, the cooling rate for recrystallization of regenerated lithium dihydrogen phosphate in step (1) is 2 °C / h.

[0017] Furthermore, in step (2), the mass ratio of glucose to conductive carbon black in the composite carbon source is 7.5:2.5, and the total amount added is 3.0% of the total mass of the raw materials.

[0018] Furthermore, in step (5), the sintering temperature of the high-temperature densification section is 745℃, and the holding time is 10h.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention, through a systematic process adaptation of raw material grading and refining, morphology reconstruction and control, and gradient sintering optimization, completely breaks through the technical bottleneck of preparing high-compact lithium iron phosphate from recycled raw materials. The lithium iron phosphate product prepared using fully recycled raw materials has a stable compaction density of 2.65 g / cm³, with an optimal density of 2.72 g / cm³. The material has a 0.1C discharge specific capacity of 155 mAh / g, a 1C discharge specific capacity of 145 mAh / g, and a capacity retention rate of 95% after 1000 cycles at 1C. Its comprehensive electrochemical performance is comparable to that of high-compact products prepared by combining purchased lithium dihydrogen phosphate with purchased ferrous oxalate, truly realizing the closed-loop high-value recycling of retired lithium iron phosphate.

[0020] 2. To address the impurity problem in recycled lithium dihydrogen phosphate, this invention employs a combined refining process of chelating resin impurity removal, pH-controlled precipitation, and cooling recrystallization, which can control the impurity content to the level of battery-grade premium products. To address the uneven morphology problem in recycled ferrous oxalate, a method of washing and removing impurities with dilute oxalic acid and airflow classification shaping is used to precisely control the particle size distribution. Subsequently, spray granulation is used to reconstruct the particle morphology, eliminating the negative impact of the recycled raw material itself on the compaction density from the root.

[0021] 3. Unlike the conventional two-stage sintering process using purchased raw materials, this invention adds a medium-temperature nucleation stage to address the uneven reactivity of recycled raw materials, ensuring uniformity of solid-phase nucleation. Simultaneously, each stage employs a gradually decreasing heating rate to prevent rapid decomposition of ferrous oxalate leading to gas production and particle breakage. The final product achieves a sphericity retention rate >90% and a sintering yield of 92%, significantly higher than the yield of directly using purchased recycled raw materials. This improves product performance while ensuring economic efficiency in production.

[0022] 4. Using fully recycled raw materials can reduce raw material costs by more than 30% compared to purchasing virgin raw materials; at the same time, it reduces carbon emissions from the mining and processing of virgin minerals, which aligns with the industrial policy guidance for the recycling of power batteries and the requirements for the development of a circular economy, and has the value for large-scale industrial promotion.

[0023] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0024] Figure 1 This is a flowchart of the preparation method in this embodiment. Detailed Implementation

[0025] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0026] It should be noted that when a component is said to be fixed to another component, it can be directly on the other component or it may have a component in between. When a component is said to be connected to another component, it can be directly connected to the other component or it may have a component in between. When a component is said to be set to another component, it can be directly set to the other component or it may have a component in between. The terms vertical, horizontal, left, right, and similar expressions used in this document are for illustrative purposes only.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The terminology used herein includes, and / or encompasses, any and all combinations of one or more of the associated listed items.

[0028] Example 1 This embodiment corresponds to all preferred technical solutions in the claims, and provides a method for preparing high-pressure lithium iron phosphate using recycled lithium dihydrogen phosphate and recycled ferrous oxalate. The specific steps are as follows: Graded Refining Pretreatment of Recycled Raw Materials (1) Refining of regenerated lithium dihydrogen phosphate: Take crude regenerated lithium dihydrogen phosphate and add it to deionized water at 85°C at a solid-liquid ratio of 1:5. Stir until completely dissolved. Add 0.4wt% of D401 chelating resin to the solution and keep it warm and stirred for 45 min to remove heavy metal impurities such as Cu, Ni, and Mn. After filtering to remove the resin, add lithium hydroxide to the filtrate to adjust the pH to 5.2 and precipitate to remove Fe and Al impurities. Then filter again. Evaporate the obtained filtrate under reduced pressure to concentrate it to a solution density of 1.40 g / cm³. Cool it slowly to room temperature at a rate of 2°C / h for recrystallization. After centrifugation, dry it in a vacuum drying oven at 115°C to obtain refined regenerated lithium dihydrogen phosphate.

[0029] Testing revealed that the refined recycled lithium dihydrogen phosphate had a main content of 99.92% and a sodium content of 32 ppm. The content is 45ppm, and the Fe content is 5ppm, which meets the battery-grade index requirements defined in the claims.

[0030] (2) Refining of regenerated ferrous oxalate: Take crude regenerated ferrous oxalate and wash it with a 2.5% dilute oxalic acid solution at a liquid-to-solid ratio of 5:1. Control the washing temperature at 45℃ and stir for 35 minutes to selectively dissolve and remove the trivalent iron oxide and soluble impurities on the surface. After filtration, wash with deionized water until the filtrate is neutral. After drying the filter cake, process it with an air jet mill and a classifier. Control the primary particle size D50 to be 3.2m, the particle size distribution Span to be 1.1, and the proportion of spherical particles to be 83% to obtain refined regenerated ferrous oxalate.

[0031] Testing revealed that the refined regenerated ferrous oxalate contained 99.6% ferrous oxalate. The content is 0.08%, which meets the purity index defined in the claims.

[0032] Dual carbon source ingredient mixing Using 100% recycled refined lithium dihydrogen phosphate and refined recycled ferrous oxalate as the sole lithium, iron, and phosphorus sources, the materials were weighed according to a Li:Fe:P molar ratio of 1.03:1:1. A composite carbon source was added, with the total amount of composite carbon source added being 3.0% of the total mass of the raw materials. The composite carbon source was composed of glucose and conductive carbon black in a mass ratio of 7.5:2.5, where glucose served as the organic carbon source to form a uniform and continuous carbon coating layer, and conductive carbon black served as the inorganic carbon skeleton to construct the interparticle conductive network. The weighed materials were pre-dry-mixed evenly to obtain a mixture.

[0033] Wet ultrafine grinding homogenization The mixture was mixed with deionized water to prepare a slurry with a solid content of 60 wt%. Sodium polyacrylate dispersant with a mass of 0.2 wt% of the total mass of the mixture was added. The slurry was then fed into a horizontal sand mill for ultrafine sand milling. 0.4 mm zirconium beads were used as the grinding media. The slurry was sand milled until the primary particle size D50 of the slurry was 0.6 m, resulting in a homogeneous slurry.

[0034] Spray granulation for dense shaping The homogeneous slurry was fed into a centrifugal spray dryer for granulation, with the inlet air temperature controlled at 200℃, the outlet air temperature at 100℃, and the centrifugal atomizer speed at 13000rpm; spherical precursor secondary particles were obtained.

[0035] The precursor was tested and found to have a D50 of 10.2 μm, a sphericity of 0.88, and a tap density of 1.29 g / cm³, which meet the precursor specifications defined in the claims.

[0036] Three-stage gradient temperature-controlled sintering The granulation precursor was placed in a pusher kiln and subjected to three-stage gradient sintering under high-purity nitrogen protection (oxygen content 10 ppm): ① Low-temperature decomposition section: The temperature is increased to 400℃ at a rate of 4℃ / min and held for 2.5h to allow ferrous oxalate to decompose slowly and organic carbon source to pre-carbonize, thus avoiding rapid gas production that could lead to particle breakage. ②Medium-temperature nucleation stage: The temperature is increased to 580℃ at a rate of 2.5℃ / min and held for 3.5h to complete the uniform solid-phase reaction nucleation and generate lithium iron phosphate microcrystals; ③ High-temperature densification stage: The temperature is increased to 745℃ at a rate of 1.5℃ / min and held for 10 hours to promote grain growth and lattice perfection. At the same time, the carbon layer crystallizes to form a continuous conductive network.

[0037] After sintering, the lithium iron phosphate sintered blocks are naturally cooled in the furnace to obtain lithium iron phosphate sintered blocks.

[0038] Low-temperature shaping and grading post-processing The sintered blocks are first mechanically crushed to below 200 mesh, and then subjected to low-temperature shaping and crushing using an air jet mill. The particle size distribution is controlled by adjusting the grading speed, and finally the finished lithium iron phosphate cathode material is obtained.

[0039] The finished product was tested and found to have a D50 of 11.8 μm, a D10 of 1.8 μm, a sphericity retention rate of 92%, and a compaction density of 2.71 g / cm³. The 0.1C discharge specific capacity was 156.2 mAh / g, the 1C discharge specific capacity was 146.8 mAh / g, and the capacity retention rate after 1000 cycles at 1C was 95.8%. All indicators met the performance requirements of the claims.

[0040] Example 2 The preparation method in this embodiment is basically the same as that in Example 1, except that in the high-temperature densification section of step 5, the sintering temperature is adjusted to 730℃, the holding time is adjusted to 12h, and the heating rate is maintained at 1.5℃ / min.

[0041] The lithium iron phosphate cathode material obtained in this embodiment has a compaction density of 2.67 g / cm³, a 0.1C discharge specific capacity of 155.7 mAh / g, a 1C discharge specific capacity of 145.9 mAh / g, and a capacity retention rate of 96.2% after 1000 cycles at 1C, which meets the performance range of the claims.

[0042] Example 3 The preparation method in this embodiment is basically the same as that in Example 1, except that in the low-temperature decomposition section of step 5, the temperature is increased to 380°C at a rate of 3°C / min and held for 3 hours.

[0043] The lithium iron phosphate cathode material obtained in this embodiment has a compaction density of 2.66 g / cm³, a 0.1C discharge specific capacity of 155.1 mAh / g, a 1C discharge specific capacity of 145.3 mAh / g, and a capacity retention rate of 95.2% after 1000 cycles at 1C, which meets the performance range of the claims.

[0044] Example 4 The preparation method in this embodiment is basically the same as that in Example 1, except that in the intermediate temperature nucleation stage of step 5, the temperature is increased to 560°C at a rate of 2°C / min and held for 4 hours.

[0045] The lithium iron phosphate cathode material obtained in this embodiment has a compaction density of 2.65 g / cm³, a 0.1C discharge specific capacity of 154.8 mAh / g, a 1C discharge specific capacity of 145.1 mAh / g, and a capacity retention rate of 95.0% after 1000 cycles at 1C, which meets the performance range of the claims.

[0046] Example 5 The preparation method in this embodiment is basically the same as that in Example 1, except that the total amount of composite carbon source added in step 2 is adjusted to 2.5% of the total mass of raw materials, wherein the mass ratio of glucose to conductive carbon black is 8:2.

[0047] The lithium iron phosphate cathode material obtained in this embodiment has a compaction density of 2.68 g / cm³, a 0.1C discharge specific capacity of 155.3 mAh / g, a 1C discharge specific capacity of 145.7 mAh / g, and a capacity retention rate of 95.1% after 1000 cycles at 1C, which meets the performance range of the claims.

[0048] Comparative Example 1 (Purchased raw materials + Invention process) This comparative example uses purchased virgin lithium dihydrogen phosphate and purchased virgin ferrous oxalate that meet battery-grade standards to replace the refined recycled raw materials in Example 1. The remaining preparation process parameters are completely consistent with those in Example 1.

[0049] The tested lithium iron phosphate cathode material obtained in this comparative example has a compaction density of 2.70 g / cm³, a 0.1C discharge specific capacity of 156.5 mAh / g, a 1C discharge specific capacity of 147.1 mAh / g, and a capacity retention rate of 96.1% after 1000 cycles at 1C. The results indicate that the lithium iron phosphate prepared from fully recycled raw materials using the process of this invention has performance essentially equivalent to that of purchased raw materials, achieving an equivalent substitution.

[0050] Comparative Example 2 (Recycled raw materials + conventional two-stage sintering process) This comparative example uses the refined recycled raw materials from Example 1. The sintering process adopts the industry-standard two-stage sintering process: heating to 400℃ at 4℃ / min and holding for 3 hours, then heating to 745℃ at 2℃ / min and holding for 10 hours, without setting a medium-temperature nucleation stage; the remaining preparation steps and parameters are the same as in Example 1.

[0051] Testing revealed that the compaction density of the lithium iron phosphate cathode material obtained in this comparative example was only 2.48 g / cm³; the 0.1C discharge specific capacity was 152.1 mAh / g, the 1C discharge specific capacity was 138.6 mAh / g, and the capacity retention rate after 1000 cycles at 1C was 88.3%. These results indicate that directly applying the conventional two-stage sintering process cannot adapt to the reaction characteristics of the recycled raw materials, leading to a significant decrease in product performance. This demonstrates the necessity of the three-stage gradient sintering process of this invention.

[0052] Comparative Example 3 (Unrefined recycled raw materials + the process of this invention) This comparative example uses crude regenerated lithium dihydrogen phosphate and crude regenerated ferrous oxalate as raw materials without any refining process. The other preparation process parameters are completely consistent with those in Example 1.

[0053] The tested lithium iron phosphate cathode material obtained in this comparative example has a compaction density of 2.52 g / cm³, a 0.1C discharge specific capacity of 150.8 mAh / g, a 1C discharge specific capacity of 137.5 mAh / g, and a capacity retention rate of 86.9% after 1000 cycles at 1C. The impurity phase content in the material is significantly increased, and the cycle decay is accelerated.

[0054] The results show that the raw material grading and refining process is the foundation for ensuring high compaction and high performance of the product, and unrefined recycled raw materials cannot achieve the target performance indicators.

[0055] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing high-pressure lithium iron phosphate using recycled lithium dihydrogen phosphate and recycled ferrous oxalate, characterized in that, Using 100% recycled refined lithium dihydrogen phosphate and refined recycled ferrous oxalate as the sole lithium, iron, and phosphorus sources, a lithium iron phosphate cathode material with a compaction density of 2.65 g / cm³ was prepared through dual-carbon source formulation, wet homogenization, spray shaping, gradient sintering, and low-temperature post-treatment. The specific steps include the following: (1) Pretreatment of recycled raw materials for graded refining: Differential refining was performed on the crude recycled lithium dihydrogen phosphate and crude recycled ferrous oxalate respectively; the recycled lithium dihydrogen phosphate was dissolved in hot water, selectively adsorbed with D401 chelating resin to remove heavy metals, precisely adjusted the pH of lithium hydroxide to precipitate to remove iron and aluminum, concentrated under reduced pressure, slowly cooled and recrystallized and vacuum dried to obtain battery-grade refined recycled lithium dihydrogen phosphate; the recycled ferrous oxalate was washed with a weakly acidic dioxic acid solution to remove the trivalent iron oxide on the surface, and then graded by airflow pulverization to narrow the particle size distribution to obtain high-purity narrow-distribution refined recycled ferrous oxalate. (2) Mixing of dual carbon source materials: Weigh refined regenerated lithium dihydrogen phosphate and refined regenerated ferrous oxalate according to the Li:Fe:P molar ratio of 1.02~1.05:1:1, add a composite carbon source composed of glucose and conductive carbon black in a mass ratio of 7:3~8:2, and dry mix evenly to obtain a mixture; the total amount of the composite carbon source added is 2.5~3.5% of the total mass of the raw materials; (3) Wet ultrafine sand milling homogenization: The mixture is prepared with deionized water to form a slurry with a solid content of 55~65wt%, sodium polyacrylate dispersant is added, and it is fed into a horizontal sand mill with 0.3~0.5mm zirconium beads as grinding media for ultrafine sand milling until the primary particle size D50 of the slurry is 0.4~0.8m, and a homogeneous slurry is obtained; (4) Spray granulation and densification: The homogeneous slurry after sand milling is granulated by a centrifugal spray dryer. The inlet air temperature is controlled at 190~210℃, the outlet air temperature at 95~105℃, and the centrifugal atomizer speed at 12000~15000rpm to obtain spherical precursor secondary particles with D50 of 8~12m, sphericity of 0.85, and tap density of 1.25g / cm. (5) Three-stage gradient temperature-controlled sintering: The precursor is placed in a high-purity nitrogen protective atmosphere with an oxygen content of 10ppm and sintered in three stages according to the rule of gradually slowing down the heating rate: ① Low temperature decomposition stage: The temperature is raised to 380~420℃ at a rate of 3~5℃ / min and held for 2~3h to allow ferrous oxalate to decompose slowly and organic carbon source to pre-carbonize; ② Medium temperature nucleation stage: The temperature is raised to 560~600℃ at a rate of 2~3℃ / min and held for 3~4h to complete the uniform solid phase reaction nucleation; ③ High temperature densification stage: The temperature is raised to 730~760℃ at a rate of 1~2℃ / min and held for 9~12h to promote grain growth and lattice perfection. After sintering, the lithium iron phosphate sintered blocks are naturally cooled in the furnace to obtain the sintered blocks. (6) Low-temperature shaping and grading post-processing: The sintered block is first mechanically crushed to below 200 mesh, and then pulverized and graded by low-temperature airflow to obtain high-pressure lithium iron phosphate cathode material; the sphericity retention rate of the finished product is 90%, and the sintering yield is 92%.

2. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, The specific refining process of regenerated lithium dihydrogen phosphate in step (1) is as follows: crude regenerated lithium dihydrogen phosphate is dissolved in deionized water at 80-90℃ at a solid-liquid ratio of 1:4 to 1:6 and stirred until completely dissolved; 0.3-0.5 wt% of D401 chelating resin is added and stirred for 30-60 min to remove Cu, Ni, and Mn heavy metal impurities; after filtration, 0.1-0.2 wt% of lithium hydroxide is added to the filtrate to adjust the pH to 5.0-5.5 and precipitate to remove Fe and Al impurities; after secondary filtration, the filtrate is concentrated by vacuum evaporation to a density of 1.38-1.42 g / cm³, cooled to room temperature at a rate of 2-3℃ / h for recrystallization, centrifuged and then vacuum dried at 110-120℃ to obtain refined regenerated lithium dihydrogen phosphate.

3. The method for preparing high-pressure lithium iron phosphate according to claim 2, characterized in that, In step (1), the cooling rate for recrystallization of regenerated lithium dihydrogen phosphate is 2℃ / h; after purification, the main content of regenerated lithium dihydrogen phosphate is 99.9%, and the Na content is 35ppm. Content 50ppm, Fe content 8ppm.

4. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, The specific refining process of regenerated ferrous oxalate in step (1) is as follows: crude regenerated ferrous oxalate is washed with a 2-3% mass fraction dilute oxalic acid solution at a liquid-solid ratio of 5:

1. The washing temperature is 40-50℃ and the stirring time is 30-45min. The surface trivalent iron oxide and soluble impurities are selectively dissolved and removed. After filtration and washing until the filtrate is neutral, air jet milling and classification are performed to control the primary particle size D50 to be 2-4 μm and the particle size distribution Span 1.2, thus obtaining refined regenerated ferrous oxalate.

5. The method for preparing high-pressure lithium iron phosphate according to claim 4, characterized in that, The refined regenerated ferrous oxalate has a main content of 99.5%. Content 0.1%, with 80% being spherical particles.

6. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, In step (2), the mass ratio of glucose to conductive carbon black in the composite carbon source is 7.5:2.5, and the total amount added is 3.0% of the total mass of the raw materials; wherein glucose is an organic carbon source used to form a uniform and continuous carbon coating layer, and conductive carbon black is an inorganic carbon skeleton used to construct an interparticle conductive network.

7. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, In step (3), the amount of sodium polyacrylate dispersant added is 0.1~0.3wt% of the total mass of the mixture; the particle size D50 of the slurry after sand milling is 0.5~0.7m.

8. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, The precursor secondary particles obtained in step (4) have a D50 of 9~11m, a sphericity of 0.87, and a tap density of 1.28g / cm.

9. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, The specific parameters for the three sintering stages in step (5) are as follows: ① Low-temperature decomposition stage: heating to 400℃ at a rate of 4℃ / min and holding for 2.5h; ② Medium-temperature nucleation stage: heating to 580℃ at a rate of 2.5℃ / min and holding for 3.5h; ③ High-temperature densification stage: heating to 745℃ at a rate of 1.5℃ / min and holding for 10h.

10. The method for preparing high-pressure lithium iron phosphate according to claim 1, characterized in that, Step (6) specifically involves: first mechanically crushing the sintered block to below 200 mesh, then using an airflow mill for low-temperature shaping and crushing, controlling the particle size distribution by adjusting the grading speed; the final lithium iron phosphate product has a D50 of 10~14m, a D10 of 1.5m, a sphericity retention rate of 92%, and a compaction density of 2.70g / cm; the obtained lithium iron phosphate cathode material has a 0.1C discharge specific capacity of 155mAh / g, a 1C discharge specific capacity of 145mAh / g, and a capacity retention rate of 95% after 1000 cycles at 1C rate.

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