Method for preparing lithium iron phosphate from iron hydroxyphosphate and ferrous oxalate and application thereof

Abstract

The application provides a method for preparing lithium iron phosphate from iron hydroxyl phosphate and ferrous oxalate. The ferrous sulfate is purified to form a ferrous sulfate solution. Hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia water are sequentially added to the solution to form a mixed slurry. The mixed slurry is heated and kept warm, and then washed with water and filtered under pressure to form an iron hydroxyl phosphate precursor with different iron and phosphorus ratios. The iron hydroxyl phosphate precursor is dried by flash evaporation and sintered at high temperature and then crushed to obtain an iron hydroxyl phosphate precursor with different iron and phosphorus ratios and different specific surface areas. The iron hydroxyl phosphate precursor is crushed and mixed to obtain an iron hydroxyl phosphate product. The high-iron and low-iron ratio iron hydroxyl phosphate is mixed with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a certain proportion, and a carbon source and an additive are added to form a mixture. The mixture is subjected to a series of processes such as ball milling, sand milling, spray drying, sintering, crushing, screening, batching and packaging to obtain a lithium iron phosphate product.

Description

Technical Field

[0001] This invention relates to the technical field of lithium-ion battery cathode material preparation methods, and particularly to a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, and its application. Background Technology

[0002] Lithium iron phosphate (LFP) cathode material is currently the fastest-growing lithium-ion battery cathode material in China. Its raw materials are widely available and inexpensive, and it is widely used in the domestic battery industry in fields such as automobiles, power tools, energy storage devices, emergency power supplies, and mobile power supplies. New energy electric vehicles are the main application area, accounting for over 45% of the total LFP usage. Compared with other cathode materials, LFP has advantages such as safety, environmental friendliness, low cost, long cycle life, and good high-temperature performance, making it one of the most promising lithium-ion battery cathode materials. Currently, the main methods for preparing LFP include solid-state methods, carbothermal reduction methods, and sol-gel template methods.

[0003] For example, CN105024073A discloses a lithium-ion battery cathode material, hydroxyferric phosphate, and its preparation method, wherein the molecular formula of the lithium-ion battery cathode material is Fe. 2.95 (PO4)2(OH)2; its preparation method is as follows: H3PO4 solution and FeCl3 solid powder are mixed evenly with water, then methyltriethylammonium chloride is added to adjust the pH to 2.0-3.5, and a hydrothermal synthesis reaction is carried out at 150-200℃ for 30 hours to obtain a reaction solution. The reaction solution is then centrifuged, washed, and dried to obtain Fe. 2.95 (PO4)2(OH)2.

[0004] The journal article "Performance Study of Lithium Iron Phosphate Prepared from Hydroxyferric Phosphate as Cathode Material for Lithium-ion Batteries" describes the synthesis of lithium iron phosphate from ferric phosphate waste residue (a byproduct of phosphorus chemical industry), phosphoric acid, and hydrogen peroxide.

[0005] However, the above methods require high reaction temperatures, long reaction times, and harsh reaction conditions, placing high demands on production equipment and resulting in low production efficiency, which does not meet the current market demand for cost reduction in lithium iron phosphate. Furthermore, the raw material costs of these methods are high, and the finished product contains many impurities that are difficult to remove, which will subsequently affect the performance of hydroxyl iron phosphate and, consequently, lithium iron phosphate. Summary of the Invention

[0006] In view of the above, the present invention aims to at least solve one of the technical problems existing in the prior art. Therefore, the present invention proposes a method for preparing lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate, and its application. This method uses ferrous sulfate as a material, adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate, and ammonia to synthesize ferrous hydroxyphosphate, and uses ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate as raw materials to prepare lithium iron phosphate with high compaction density and high capacity. Furthermore, this method has high production efficiency and low production cost, making it suitable for large-scale industrial production.

[0007] Therefore, in a first aspect, embodiments of the present invention provide a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, the method comprising: adding ferrous sulfate, a byproduct of titanium dioxide, to a phosphorus source and a precipitant for purification, and obtaining a ferrous sulfate solution after pressure filtration; adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution; sequentially adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, and ammonia to the ferrous sulfate solution and reacting for a period of time to form a mixed slurry; heating and holding the mixed slurry at a certain temperature for a period of time, and then washing and filtration multiple times with water to form ferric hydroxyphosphate precursors with different iron-phosphorus ratios; flash drying the ferric hydroxyphosphate precursors in a flash evaporator and sintering at high temperature for a certain time to obtain finished ferric hydroxyphosphate precursors with different iron-phosphorus ratios and different specific surface areas; and sintering the sintered material... Hydroxyferric phosphate products with different iron-phosphorus ratios and specific surface areas can be obtained by pulverizing with a mechanical mill and mixing with a ribbon mixer. Hydroxyferric phosphate with a high iron-phosphorus ratio and high specific surface area is mixed with hydroxyferric phosphate with a low iron-phosphorus ratio and low specific surface area in a certain proportion, and then formulated with ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate in a certain proportion, with the addition of a certain amount of carbon source and additives to form a mixture. This mixture is then sand-milled to obtain a nano-sized sand-milled slurry. The nano-sized sand-milled slurry is then spray-dried to obtain a spray-dried material. This spray-dried material is then sintered in a box furnace to obtain a sintered material, which is then pulverized using an air jet mill to obtain a pulverized material. Finally, the pulverized material undergoes further processes such as sieving, batching, and packaging to obtain the finished lithium iron phosphate product.

[0008] Preferably, in step S1, the ferrous sulfate : phosphorus source : precipitant are in a mass ratio of 1 : [0.001-0.005] : [0.005-0.007], the purification reaction temperature is 40°C, the reaction pH is 2.2-2.5, the reaction time is 1 hour, the phosphorus source is one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and sodium phosphate, and the precipitant is one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia water.

[0009] Preferably, in step S2, the amount of phosphoric acid added is in a molar ratio of n(Fe):n(phosphoric acid) = 1:0.15; in step S3, when the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio: Fe / P = 1.475-1.490, high iron-phosphorus ratio hydroxyferric phosphate can be formed; when the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio: Fe / P = 1.460-1.475, low iron-phosphorus ratio hydroxyferric phosphate can be generated.

[0010] Preferably, in step S3, the water washing is performed at least multiple times. The first water wash mainly removes impurities such as magnesium, manganese, and sulfur. In the final water wash, a 1:1 diluted ammonia solution is added to adjust the pH to 6.5-7.0 to remove SO4. 2- Ions; the hydrogen peroxide concentration is 30%-60%, and the mixed slurry is heated to 60-80℃ and kept at that temperature for 3 hours.

[0011] Preferably, step S3 includes: adding excess hydrogen peroxide to the ferrous sulfate solution and continuing oxidation for a certain period of time; adding phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolving ammonium dihydrogen phosphate powder in water to prepare a 30% concentration ammonium dihydrogen phosphate solution at a dissolution temperature of 30-40℃, and adding it to the oxidized ferrous sulfate solution; adding ammonia water to the ferrous sulfate solution to adjust the pH value of the solution to 3.00±0.02, reacting for a period of time to form a mixed slurry, heating and keeping the mixed slurry at a certain temperature for a period of time, and then washing and filtering it multiple times with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0012] Preferably, in step S4, the inlet air temperature of the flash evaporator is controlled at 220±20℃, the outlet air temperature is controlled at 110±5℃, the sintering atmosphere is air, the sintering temperature is 535-560℃, and the sintering time is 4-5h; in step S5, the particle size is controlled at D10≥1.0μm, D50: 6-15μm, D90≤60μm, the mixing frequency of the mixer is controlled at 35±2Hz, and the mixing time is 1-2h.

[0013] Preferably, in step S5, the high-iron-phosphorus ferric phosphate has a high specific surface area, and its iron-phosphorus molar ratio satisfies: Fe / P = 1.460-1.480, and its specific surface area satisfies: BET = 15-20 m². 2 / g; the low iron-phosphorus ratio hydroxyferric phosphate has a low specific surface area, and its iron-phosphorus molar ratio satisfies: Fe / P=1.440-1.460, and its specific surface area satisfies: BET=5-10m². 2 / g.

[0014] Preferably, in step S6, according to the molar ratio Li:Fe:P = [1.03-1.04]:1:[1.03-1.04], the amount of carbon source added is based on the carbon content in the final product being between 1.2% and 1.6%. The carbon source is one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol. The additive is one or more of titanium dioxide, ammonium metavanadate, and niobium pentoxide, and the doping amount is controlled between 300 and 3000 ppm. In step S7, the abrasive particle size in the abrasive slurry is controlled between 0.45 and 0.75 μm, and in spray drying, the inlet air temperature is 200-220°C. The temperature is 0℃, the outlet air temperature is 80-110℃, the blowing frequency is 80Hz, and the spray particle size in the spray material is controlled between D50=20-40μm; in step S8, the sintering atmosphere is nitrogen, the sintering temperature is 750-780℃, the heating rate is 3℃ / min, the sintering time is 8-12h, and after natural cooling, the sintered material can be obtained. During the crushing process, the air pressure is controlled between 0.2-0.4Mpa, the grading frequency is 80-200Hz, and the particle size of the crushed material meets the following requirements: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, D100<30μm.

[0015] Secondly, embodiments of the present invention provide a lithium-ion battery cathode material obtained by processing lithium iron phosphate prepared by the method for preparing lithium iron phosphate using ferrous hydroxyphosphate and ferrous oxalate provided in the first aspect above.

[0016] Thirdly, embodiments of the present invention provide a lithium-ion battery, including the lithium-ion battery cathode material described in the second aspect above.

[0017] The method for preparing lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate provided in this invention utilizes ferrous sulfate, a byproduct of titanium dioxide production, to generate ferric sulfate. After adding other materials and reacting, ferrous hydroxyphosphate with different iron-to-phosphorus ratios is generated. Different sintering processes are then used to obtain ferrous hydroxyphosphate products with high iron-to-phosphorus ratio and high specific surface area, and low iron-to-phosphorus ratio and low specific surface area. The high-iron-to-phosphorus-ratio, high-specific-surface-area ferrous phosphate and the low-iron-to-phosphorus-ratio, low-specific-surface-area ferrous phosphate are mixed, and then mixed with ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate in a certain proportion. Additives are added to form a mixture, which is then subjected to sand milling, spray drying, sintering, sieving, batching, and packaging to obtain the finished lithium iron phosphate product. This method uses a mixture of high and low iron-to-phosphorus ratio hydroxyferric phosphate and introduces four materials: ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate. The decomposition of ferrous oxalate, lithium carbonate, and ammonium dihydrogen phosphate generates gases that easily reduce the agglomeration of lithium iron phosphate particles, improving their roundness and thus increasing the bulk density of the lithium iron phosphate material, further enhancing compaction density and electrochemical performance. Furthermore, using ferrous oxalate as the iron source and ammonium dihydrogen phosphate as the phosphorus source, the addition of ferrous oxalate and ammonium dihydrogen phosphate to the slurry system lowers the slurry pH, preventing it from becoming alkaline, thus preventing the dissolution of iron from the iron source materials, reducing the formation of harmful substances such as ferric hydroxide, and improving the stability of the iron source. Additionally, oxalate ions have coordination and reducing properties, which can reduce the microscopic size of reactant particles; moreover, oxalate ions are converted into conductive carbon and carbon dioxide gases, further increasing the conductivity of the material. In addition, this method requires low reaction temperature and short reaction time, has low equipment requirements, and a simple process flow, improving production efficiency and making it suitable for large-scale industrial production. Attached Figure Description

[0018] Figure 1 This is a flowchart of a method for preparing lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate, provided in an embodiment of the present invention.

[0019] Figure 2 A flowchart illustrating step S3 in the preparation of lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate, as provided in this embodiment of the invention;

[0020] Figure 3 This is the SEM image of the high iron-to-phosphorus ratio and high specific surface area ferric hydroxyphosphate prepared in Example 1 of the present invention;

[0021] Figure 4 The image shows the SEM spectrum of the lithium iron phosphate cathode material prepared in Example 1 of this invention.

[0022] Figure 5 The XRD pattern of the high ferric phosphorus ratio and high specific surface area hydroxyferric phosphate prepared in Example 1 of this invention;

[0023] Figure 6The image shows the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 of this invention.

[0024] Figure 7 The charge-discharge curve (0.1C) of a coin cell assembled with the lithium iron phosphate cathode material prepared in Example 1 of this invention;

[0025] Figure 8 The charge-discharge curve (1C) of a coin cell assembled with the lithium iron phosphate cathode material prepared in Example 1 of this invention. Detailed Implementation

[0026] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0027] The following disclosure provides numerous different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples. Such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. Additionally, examples of various specific processes and materials are provided in this invention; however, those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0028] This invention provides a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, which can be used to prepare lithium iron phosphate with high compaction density and high capacity. Figure 1 As shown, this method includes:

[0029] Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a phosphorus source and a precipitant for purification. After pressure filtration and purification, a ferrous sulfate solution is obtained.

[0030] The reaction was carried out in the following mass ratio: ferrous sulfate: phosphorus source: precipitant = 1: [0.001-0.005]: [0.005-0.007]. The purification reaction temperature was 40℃, the reaction pH was 2.2-2.5, and the reaction time was 1h.

[0031] In this embodiment, the phosphorus source can be one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, sodium phosphate, etc., and the precipitant can be one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia water, etc.

[0032] Step S2: Add an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution;

[0033] The amount of phosphoric acid added is based on a molar ratio of n(Fe):n(phosphoric acid) = 1:0.15.

[0034] Step S3: Hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia are added to ferrous sulfate solution in sequence and reacted for a period of time to form a mixed slurry. After heating and keeping the mixed slurry at a certain temperature for a period of time, it is washed with water and filtered repeatedly to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0035] In this embodiment, hydrogen peroxide is first added to fully oxidize ferrous ions into ferric ions, and then phosphoric acid and ammonium dihydrogen phosphate are added to adjust the solution ions to a suitable iron-phosphorus molar ratio. On the one hand, this can make the generated ferric hydroxyphosphate precursor more stable; on the other hand, it can make the generated ferric hydroxyphosphate precursor particles larger and easier to filter and wash.

[0036] When the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio of Fe / P = 1.475-1.490, high iron-phosphorus ratio hydroxyferric phosphate can be formed; when the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio of Fe / P = 1.460-1.475, low iron-phosphorus ratio hydroxyferric phosphate can be generated.

[0037] In this embodiment, the hydrogen peroxide concentration is between 30% and 60%, and the mixed slurry is heated to 60-80°C and held at that temperature for 3 hours. The washing process can be repeated multiple times. The first wash primarily removes impurities such as magnesium, manganese, and sulfur. The final wash involves adding a 1:1 diluted ammonia solution to adjust the pH to 6.5-7.0 to remove SO4. 2- Ions. Specifically, the water washing can be performed three times. The first and second water washes mainly remove impurities such as manganese, magnesium, and sulfur. In the third water wash, a 1:1 diluted ammonia solution is added to adjust the pH to 6.5-7.0 to remove SO4. 2- ion.

[0038] Specifically, in the first embodiment of the present invention, as Figure 2 As shown, step S3 includes:

[0039] Step S31: Add excess hydrogen peroxide to the ferrous sulfate solution and continue oxidation for a certain period of time;

[0040] Step S32: Add the phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolve the ammonium dihydrogen phosphate powder in water to prepare a 30% concentration ammonium dihydrogen phosphate solution at a dissolution temperature of 30-40℃, and add it to the oxidized ferrous sulfate solution.

[0041] Step S33: Add ammonia to the ferrous sulfate solution to adjust the pH of the solution to 3.00±0.02. After reacting for a period of time, a mixed slurry is formed. The mixed slurry is heated and kept at a certain temperature for a period of time, and then washed and filtered repeatedly with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0042] The heating temperature of the mixed slurry is 60-80℃, and the holding time is 3h.

[0043] Step S4: Flash dry the hydroxyferric phosphate precursor in a flash evaporator and sinter it at high temperature for a certain time to obtain hydroxyferric phosphate precursor products with different iron-phosphorus ratios and different specific surface areas.

[0044] The flash drying of the ferric hydroxyphosphate precursor is to remove free water. The inlet air temperature of the flash evaporator is controlled at 220±20℃, and the outlet air temperature is controlled at 110±5℃. The sintering atmosphere is air, the sintering temperature can be 535-560℃, and the sintering time can be 4-5 hours.

[0045] Step S5: The sintered material is crushed by a mechanical mill and mixed by a ribbon mixer to obtain hydroxy ferric phosphate products with different iron-phosphorus ratios and different specific surface areas.

[0046] During the pulverization process, the particle size is controlled as follows: D10 ≥ 1.0 μm, D50 6-15 μm, and D90 ≤ 60 μm. The mixing frequency of the mixer is controlled at 35 ± 2 Hz, and the mixing time can be 1-2 hours.

[0047] In this embodiment of the invention, the high-iron-phosphorus ferric phosphate has a high specific surface area, and its iron-phosphorus molar ratio satisfies: Fe / P = 1.460-1.480, and its specific surface area satisfies: BET = 15-20 m². 2 / g; The low iron-to-phosphorus ratio hydroxyferric phosphate precursor has a low specific surface area, and its iron-to-phosphorus molar ratio satisfies: Fe / P=1.440-1.460, and its specific surface area satisfies: BET=5-10m². 2 / g.

[0048] Step S6: Mix high-iron-phosphorus ratio hydroxyferric phosphate and low-iron-phosphorus ratio hydroxyferric phosphate in a certain proportion, then mix them with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a certain proportion, and add a certain amount of carbon source and additives to form a mixture.

[0049] In this embodiment of the invention, ferrous oxalate is used as the iron source and ammonium dihydrogen phosphate as the phosphorus source. This is because ferrous oxalate is soluble in water and exists as iron ions and oxalate ions; ammonium dihydrogen phosphate is also soluble in water and exists as phosphate ions, allowing for more uniform mixing of the materials. Furthermore, the dissolution of ferrous oxalate and ammonium dihydrogen phosphate in water results in the presence of iron ions and phosphate ions in the solution; lithium carbonate and lithium phosphate are also slightly soluble in water, resulting in the presence of a small amount of lithium ions in the solution. Since lithium ions, iron ions, and phosphate ions—the three main elements for synthesizing lithium iron phosphate—all exist in ionic form in the solution, it facilitates the synthesis of small-particle lithium iron phosphate with a particle size of less than 10 nm. In addition, using ferrous oxalate as the iron source and ammonium dihydrogen phosphate as the phosphorus source, adding ferrous oxalate and ammonium dihydrogen phosphate to the slurry system can lower the pH value of the slurry, preventing it from becoming alkaline, preventing the dissolution of iron from the iron source material, reducing the formation of harmful substances such as ferric hydroxide, and improving the stability of the iron source.

[0050] In this embodiment of the invention, the ratio of high-iron-phosphorus hydroxyferric phosphate to low-iron-phosphorus hydroxyferric phosphate is between 2:8 and 8:2, preferably, the ratio of high-iron-phosphorus hydroxyferric phosphate to low-iron-phosphorus hydroxyferric phosphate is 3:7. Furthermore, in the mixture, the molar ratio is Li:Fe:P = [1.03-1.04]:1:[1.03-1.04]. The amount of carbon source added is based on a carbon content of 1.2%-1.6% in the final product.

[0051] In this embodiment of the invention, the carbon source can be one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol, and the additive can be one or more of titanium dioxide, ammonium metavanadate, and niobium pentoxide, with the doping amount controlled between 300-3000 ppm.

[0052] Step S7: The above mixture is milled to obtain a nano-sized milled slurry; the nano-sized milled slurry is spray-dried to obtain a spray-dried material;

[0053] Specifically, the abrasive particle size in the abrasive slurry is controlled between 0.45 and 0.75 μm. In spray drying, the inlet air temperature can be 200-220℃, the outlet air temperature can be 80-110℃, the blowing frequency can be 80Hz, and the final spray particle size in the sprayed material is controlled between D50 and 20-40 μm.

[0054] Step S8: The above sprayed material is placed in a box furnace for sintering to obtain sintered material, and the sintered material is pulverized by an air jet mill to obtain pulverized material;

[0055] During the sintering process, the sintering atmosphere is nitrogen, the sintering temperature is 750-780℃, the heating rate is 3℃ / min, and the sintering time is 8-12h. After natural cooling, the sintered material is obtained. During the pulverization process, the gas pressure is controlled between 0.2-0.4Mpa, the grading frequency is 80-200Hz, and the particle size of the final pulverized material meets the following requirements: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, D100<30μm.

[0056] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished lithium iron phosphate product can be obtained.

[0057] The method for preparing lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate provided in this invention involves purifying ferrous sulfate, a byproduct of titanium dioxide production, to obtain a ferrous sulfate solution. Hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, and ammonia are then added sequentially to the ferrous sulfate solution, and the mixture is reacted for a period of time to form a mixed slurry. After multiple water washings and pressure filtrations, ferrous hydroxyphosphate precursors with different iron-to-phosphorus ratios are generated. These precursors are then subjected to different sintering processes to obtain ferrous hydroxyphosphate products with high iron-to-phosphorus ratios and high specific surface areas, and low iron-to-phosphorus ratios and low specific surface areas. The ferrous hydroxyphosphate prepared by this method involves first adding hydrogen peroxide to fully oxidize ferrous ions to ferric ions, then adding phosphoric acid and ammonium dihydrogen phosphate to adjust the solution ions to a suitable iron-to-phosphorus molar ratio. This process makes the generated ferrous hydroxyphosphate precursor more stable. Furthermore, heating the mixed slurry to 60-80°C and holding it for 3 hours results in larger ferrous hydroxyphosphate precursor particles, which are easier to filter and wash. During the water washing and purification stage, impurities are less likely to be trapped inside the crystals. After multiple water washings, impurities such as magnesium, manganese, sulfur, and SO4 are mainly removed. 2- The product contains few ions and other impurities, resulting in low impurity content and high purity. Furthermore, the iron-to-phosphorus ratio and specific surface area of ​​the hydroxyferric phosphate produced by this method are adjustable, allowing for the generation of hydroxyferric phosphates with different iron-to-phosphorus ratios as needed. Specifically, high iron-to-phosphorus ratio hydroxyferric phosphate particles are smaller, which can improve the material's discharge capacity; low iron-to-phosphorus ratio hydroxyferric phosphate particles are larger, which can improve the material's compaction density.

[0058] In subsequent steps, this method mixes high-iron-phosphorus ratio hydroxyferric phosphate and low-iron-phosphorus ratio hydroxyferric phosphate, then mixes them with ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate in a certain proportion. After adding additives to form a mixture, the mixture undergoes sand milling, spray drying, sintering, sieving, batching, and packaging to obtain the finished lithium iron phosphate product. This method uses a mixture of high- and low-iron-phosphorus ratio hydroxyferric phosphate and introduces four materials: ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate. The decomposition of ferrous oxalate, lithium carbonate, and ammonium dihydrogen phosphate produces gases, which easily reduce the agglomeration of lithium iron phosphate particles, improve the roundness of lithium iron phosphate particles, thereby increasing the bulk density of lithium iron phosphate material and further improving compaction density and electrochemical performance. Specifically, ferrous oxalate is used as the iron source and ammonium dihydrogen phosphate as the phosphorus source. Firstly, ferrous oxalate is soluble in water and exists as iron and oxalate ions; ammonium dihydrogen phosphate is also soluble in water and exists as phosphate ions, allowing for more uniform mixing of the materials. Secondly, the dissolution of ferrous oxalate and ammonium dihydrogen phosphate in water results in the presence of iron and phosphate ions in the solution; lithium carbonate and lithium phosphate are also slightly soluble in water, resulting in the presence of small amounts of lithium ions in the solution. The three main elements for synthesizing lithium iron phosphate—lithium ions, iron ions, and phosphate ions—all exist in ionic form in the solution, facilitating the synthesis of small-particle lithium iron phosphate with a particle size of less than 10 nm. Furthermore, using ferrous oxalate as the iron source and ammonium dihydrogen phosphate as the phosphorus source, adding ferrous oxalate and ammonium dihydrogen phosphate to the slurry system can lower the pH value of the slurry, preventing it from becoming alkaline, preventing the dissolution of iron from the iron source material, reducing the formation of harmful substances such as ferric hydroxide, and improving the stability of the iron source. Furthermore, oxalate ions possess coordination and reducing properties, which can reduce the microscopic size of reactant particles; moreover, oxalate ions are converted into conductive carbon and carbon dioxide gases, further increasing the conductivity of the material. In addition, this method requires low reaction temperatures and short reaction times, has low equipment requirements, and a simple process flow, thus improving production efficiency and making it suitable for large-scale industrial production.

[0059] The following detailed description, in conjunction with some specific embodiments, further illustrates the specific process and effects of the method for preparing lithium iron phosphate using ferric hydroxyphosphate and ferrous oxalate of the present invention, but does not limit the scope of protection of the present invention.

[0060] Example 1

[0061] This embodiment provides a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, including the following steps:

[0062] Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a solution containing 4‰ phosphoric acid and 5‰ sodium hydroxide for purification. After pressure filtration, a ferrous sulfate solution is obtained.

[0063] Step S2: Add phosphoric acid to the ferrous sulfate solution according to the molar ratio n(Fe):n(phosphoric acid) = 1:0.15 to lower the pH value of the ferrous sulfate solution;

[0064] Step S3: Add excess 40% hydrogen peroxide to the ferrous sulfate solution, then add phosphoric acid solution and 30% ammonium dihydrogen phosphate solution to the ferrous sulfate solution so that the iron-phosphorus feeding ratio in the mixed slurry successively meets the iron-phosphorus molar ratio: Fe / P = 1.490 and Fe / P = 1.460. Then add ammonia water to the ferrous sulfate solution to form a mixed slurry. Heat the mixed slurry to 60°C and keep it at that temperature for 3 hours. Then wash and filter it multiple times with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0065] Step S4: The hydroxyferric phosphate precursor is flash-dried in a flash evaporator, with the flash evaporator inlet air temperature controlled at 200°C, and sintered in air atmosphere at 535°C and 560°C for 5 hours.

[0066] Step S5: The sintered material is crushed by mechanical mill, and the particle size is controlled as follows: D10≥1.0μm, D50:6-15μm, D90≤60μm. The material is then mixed by ribbon mixer at 35Hz for 1 hour to obtain hydroxy ferric phosphate with high iron-phosphorus ratio and high specific surface area and hydroxy ferric phosphate with low iron-phosphorus ratio and low specific surface area.

[0067] The SEM image of the high ferric phosphorus ratio and high specific surface area ferric phosphate prepared according to Example 1 is shown below. Figure 3 As shown.

[0068] The XRD pattern of the high ferric phosphorus ratio and high specific surface area ferric phosphate prepared according to Example 1 is shown below. Figure 5 As shown.

[0069] Step S6: Mix ferric hydroxyphosphate with a high iron-to-phosphorus ratio and high specific surface area with ferric hydroxyphosphate with a low iron-to-phosphorus ratio and low specific surface area in a 3:7 ratio, and then mix it with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:P = 1.03:1:1.03. Add a carbon source mixture consisting of sucrose and polyethylene glycol to make the carbon content of the finished product 1.35%, and titanium dioxide with a doping amount of 2200ppm to form a mixture.

[0070] Step S7: The above mixture is sand-milled, and the sand-milling particle size is controlled to be 0.62μm to obtain a nano-sized sand-milling slurry; the nano-sized sand-milling slurry is spray-dried, and the inlet air temperature is controlled to be 220℃, the outlet air temperature is controlled to be 100℃, and the blowing frequency is 80Hz to obtain a spray material with a spray particle size D50 = 20-40μm.

[0071] Step S8: The above sprayed material is placed in a box furnace and sintered under a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 765℃, and a sintering time of 10h. After natural cooling, the sintered material is obtained. The sintered material is then pulverized by an air jet mill, with the air pressure controlled at 0.3Mpa and the grading frequency at 130Hz, to obtain pulverized material with particle sizes of D10>0.35μm, D50=0.8-1.8μm, D90<10μm, and D100<30μm.

[0072] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished lithium iron phosphate product can be obtained.

[0073] The SEM spectrum of the lithium iron phosphate cathode material prepared according to Example 1 is shown below. Figure 4 As shown.

[0074] The XRD pattern of the lithium iron phosphate cathode material prepared according to Example 1 is shown below. Figure 6 As shown.

[0075] Example 2

[0076] This embodiment provides a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, including the following steps:

[0077] Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a solution of phosphoric acid with a mass fraction of 3‰ and sodium hydroxide with a mass fraction of 5‰ for purification. After purification by pressure filtration, a ferrous sulfate solution is obtained.

[0078] Step S2: Add phosphoric acid to the ferrous sulfate solution according to the molar ratio n(Fe):n(phosphoric acid) = 1:0.15 to lower the pH value of the ferrous sulfate solution;

[0079] Step S3: Add excess 50% hydrogen peroxide to the ferrous sulfate solution, add phosphoric acid solution and 30% ammonium dihydrogen phosphate solution to the ferrous sulfate solution so that the iron-phosphorus feeding ratio in the mixed slurry successively meets the iron-phosphorus molar ratio: Fe / P = 1.490 and Fe / P = 1.460. Then add ammonia water to the ferrous sulfate solution to form a mixed slurry. Heat the mixed slurry to 60℃, keep it at that temperature for 3 hours, and then wash and filter it multiple times with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0080] Step S4: The hydroxyferric phosphate precursor is flash-dried in a flash evaporator, with the flash evaporator inlet air temperature controlled at 200°C, and sintered in air atmosphere at 545°C and 560°C for 4 hours.

[0081] Step S5: The sintered material is crushed by mechanical mill, and the particle size is controlled as follows: D10≥1.0μm, D50:6-15μm, D90≤60μm. The material is then mixed by ribbon mixer at 35Hz for 1 hour to obtain hydroxy ferric phosphate with high iron-phosphorus ratio and high specific surface area and hydroxy ferric phosphate with low iron-phosphorus ratio and low specific surface area.

[0082] Step S6: Mix ferric hydroxyphosphate with a high iron-to-phosphorus ratio and high specific surface area with ferric hydroxyphosphate with a low iron-to-phosphorus ratio and low specific surface area in a 4:6 ratio, and then mix it with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:P = 1.03:1:1.03. Add a carbon source mixture consisting of glucose and polyethylene glycol to make the carbon content of the finished product 1.45%, and ammonium metavanadate with a doping amount of 3000ppm to form a mixture.

[0083] Step S7: The above mixture is sand-milled, and the sand-milling particle size is controlled to be 0.55μm to obtain a nano-sized sand-milling slurry; the nano-sized sand-milling slurry is spray-dried, and the inlet air temperature is controlled to be 220℃, the outlet air temperature is controlled to be 100℃, and the blowing frequency is 80Hz to obtain a spray material with a spray particle size D50 = 20-40μm.

[0084] Step S8: The above sprayed material is placed in a box furnace and sintered under a nitrogen atmosphere. The heating rate is 3℃ / min, the sintering temperature is 770℃, and the sintering time is 8h. After natural cooling, the sintered material is obtained. The sintered material is then pulverized by an air jet mill, with the air pressure controlled at 0.3Mpa and the classification frequency at 130Hz, to obtain pulverized material with particle sizes of D10>0.35μm, D50=0.8-1.8μm, D90<10μm, and D100<30μm.

[0085] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished lithium iron phosphate product can be obtained.

[0086] Example 3

[0087] This embodiment provides a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, including the following steps:

[0088] Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a solution containing 2‰ phosphoric acid and 4‰ sodium hydroxide for purification. After pressure filtration, a ferrous sulfate solution is obtained.

[0089] Step S2: Add phosphoric acid to the ferrous sulfate solution according to the molar ratio n(Fe):n(phosphoric acid) = 1:0.15 to lower the pH value of the ferrous sulfate solution;

[0090] Step S3: Add excess 40% hydrogen peroxide to the ferrous sulfate solution, add phosphoric acid solution and 30% ammonium dihydrogen phosphate solution to the ferrous sulfate solution to make the iron-phosphorus feeding ratio in the mixed slurry meet the iron-phosphorus molar ratios of Fe / P = 1.485 and Fe / P = 1.465, then add ammonia to the ferrous sulfate solution to form a mixed slurry. Heat the mixed slurry to 60°C, keep it at that temperature for 3 hours, and then wash and filter it multiple times with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0091] Step S4: The hydroxyferric phosphate precursor is flash-dried in a flash evaporator, with the flash evaporator inlet air temperature controlled at 200°C, and sintered in air atmosphere at 540°C and 555°C for 4 hours.

[0092] Step S5: The sintered material is crushed by mechanical mill, and the particle size is controlled as follows: D10≥1.0μm, D50:6-15μm, D90≤60μm. The material is then mixed with a ribbon mixer at 35Hz for 1 hour to obtain hydroxy ferric phosphate with high iron-phosphorus ratio and high specific surface area and hydroxy ferric phosphate with low iron-phosphorus ratio and low specific surface area.

[0093] Step S6: Mix ferric hydroxyphosphate with a high iron-to-phosphorus ratio and high specific surface area with ferric hydroxyphosphate with a low iron-to-phosphorus ratio and low specific surface area in a 5:5 ratio, and then mix it with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:P = 1.03:1:1.03. Add a carbon source mixture consisting of sucrose, polyethylene glycol and citric acid to make the carbon content of the finished product 1.5%, and titanium dioxide with a doping amount of 3000ppm to form a mixture.

[0094] Step S7: The above mixture is sand-milled, and the sand-milling particle size is controlled to be 0.65μm to obtain a nano-sized sand-milling slurry; the nano-sized sand-milling slurry is spray-dried, and the inlet air temperature is controlled to be 210℃, the outlet air temperature is controlled to be 100℃, and the blowing frequency is 80Hz to obtain a spray material with a spray particle size D50 = 20-40μm.

[0095] Step S8: The above sprayed material is placed in a box furnace and sintered under a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 765℃, and a sintering time of 10h. After natural cooling, the sintered material is obtained. The sintered material is then pulverized by an air jet mill, with the air pressure controlled at 0.35Mpa and the grading frequency at 140Hz, to obtain pulverized material with particle sizes of D10>0.35μm, D50=0.8-1.8μm, D90<10μm, and D100<30μm.

[0096] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished lithium iron phosphate product can be obtained.

[0097] Comparative Example 1

[0098] This embodiment provides a method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, including the following steps:

[0099] Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a solution containing 6‰ phosphoric acid and 4‰ sodium hydroxide for purification. After purification by pressure filtration, a ferrous sulfate solution is obtained.

[0100] Step S2: Add phosphoric acid to the ferrous sulfate solution according to the molar ratio n(Fe):n(phosphoric acid) = 1:0.15 to lower the pH value of the ferrous sulfate solution;

[0101] Step S3: Add excess 10% hydrogen peroxide to the ferrous sulfate solution, add phosphoric acid solution and 30% ammonium dihydrogen phosphate solution to the ferrous sulfate solution to make the iron-phosphorus feeding ratio meet the iron-phosphorus molar ratio: Fe / P = 1.44, then add ammonia water to the ferrous sulfate solution to form a mixed slurry, heat the mixed slurry to 40°C, keep it at the temperature for 2 hours, and then wash and filter it multiple times with water to form the hydroxyferric phosphate precursor;

[0102] Step S4: The hydroxyferric phosphate precursor is flash-dried in a flash evaporator, with the flash evaporator inlet air temperature controlled at 200°C, and sintered in air atmosphere at 525°C for 2 hours.

[0103] Step S5: The sintered material is crushed by mechanical mill, and the particle size is controlled as D10≥1.0μm, D50:6-15μm, D90≤60μm. The material is then mixed with a ribbon mixer at 35Hz for 1 hour to obtain hydroxyferric phosphate product with a single iron-phosphorus ratio and a single specific surface area.

[0104] Step S6: Mix ferric hydroxyphosphate with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:P = 1.03:1:1.03, and add a carbon source mixture consisting of sucrose and polyethylene glycol to make the carbon content of the finished product 1.10% and titanium dioxide with a doping amount of 3000ppm to form a mixture.

[0105] Step S7: The above mixture is sand-milled, and the sand-milling particle size is controlled to be 0.65μm to obtain a nano-sized sand-milling slurry; the nano-sized sand-milling slurry is spray-dried, and the inlet air temperature is controlled to be 220℃, the outlet air temperature is 100℃, and the blowing frequency is 80Hz to obtain a spray material with a spray particle size D50 = 20-40μm.

[0106] Step S8: The above sprayed material is placed in a box furnace and sintered under a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 765℃, and a sintering time of 10h. After natural cooling, the sintered material is obtained. The sintered material is then pulverized by an air jet mill, with the air pressure controlled at 0.35Mpa and the grading frequency at 140Hz, to obtain pulverized material with particle sizes of D10>0.35μm, D50=0.8-1.8μm, D90<10μm, and D100<30μm.

[0107] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished lithium iron phosphate product can be obtained.

[0108] To verify the quality of the lithium iron phosphate cathode material prepared by the method for preparing lithium iron phosphate using ferrous hydroxyphosphate and ferrous oxalate provided in the embodiments of the present invention, the lithium iron phosphate cathode material prepared in Examples 1-3 and Comparative Example 1 was dispersed in N-methylpyrrolidone with conductive agent carbon black and binder polyvinylidene fluoride at a mass ratio of 90:5:5. After ball milling and uniform dispersion, the material was coated on aluminum foil and vacuum dried to obtain the cathode sheet. The electrolyte was 1 mol / L LiPF6 with a solvent volume ratio of EC:DMC:EMC = 1:1:1. The separator was a Celgard polypropylene membrane, and the lithium metal sheet was used as the negative electrode. The materials were assembled into a coin cell. The test voltage range was 2.0V-3.75V. The material was charged to 3.75V using a constant current and constant voltage charging method and discharged to 2.0V using a constant current discharging method. The charge and discharge currents were 0.1C for two cycles and 1C for two cycles. The test results are shown in Table 1. The charge-discharge curve (0.1C) of the coin cell assembled with the lithium iron phosphate cathode material prepared in Example 1 of the present invention is shown below. Figure 7 As shown, the charge-discharge curve (1C) of the coin cell assembled with the lithium iron phosphate cathode material prepared in Example 1 of the present invention is as follows. Figure 8 As shown.

[0109] Table 1. Test items and test results for Examples 1-3 and Comparative Example 1

[0110]

[0111] Based on the above examples and comparative examples, and the test results obtained from the tests, the coin cells prepared with lithium iron phosphate cathode materials in Examples 1-3 showed significant improvements in both the initial charge-discharge specific capacity at 0.1C and the discharge specific capacity at 1C compared to Comparative Example 1.

[0112] In summary, the method for preparing lithium iron phosphate from ferrous hydroxyphosphate and ferrous oxalate provided in this invention utilizes ferrous sulfate, a byproduct of titanium dioxide production, to generate ferric sulfate. After adding other materials and reacting, ferrous hydroxyphosphate with different iron-to-phosphorus ratios is generated. Then, through different sintering processes, ferrous hydroxyphosphate products with high iron-to-phosphorus ratio and high specific surface area, and those with low iron-to-phosphorus ratio and low specific surface area, are obtained. The high-iron-to-phosphorus-ratio, high-specific-surface-area ferrous phosphate and the low-iron-to-phosphorus-ratio, low-specific-surface-area ferrous phosphate are mixed, and then further mixed with ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate in a certain proportion. Additives are added to form a mixture, which is then subjected to sand milling, spray drying, sintering, sieving, batching, and packaging to obtain the finished lithium iron phosphate product.

[0113] This method uses a mixture of high and low iron-to-phosphorus ratio hydroxyferric phosphate and introduces four materials: ferrous oxalate, lithium phosphate, lithium carbonate, and ammonium dihydrogen phosphate. The decomposition of ferrous oxalate, lithium carbonate, and ammonium dihydrogen phosphate produces gases, which easily reduce the agglomeration of lithium iron phosphate particles, improve their roundness, further increase the bulk density of the lithium iron phosphate material, and enhance its compaction density and electrochemical performance. Specifically, ferrous oxalate is used as the iron source, and ammonium dihydrogen phosphate as the phosphorus source. This is because ferrous oxalate is soluble in water and exists in the form of iron ions and oxalate ions; ammonium dihydrogen phosphate is also soluble in water and exists in the form of phosphate ions, which allows for a more uniform mixing of the materials. On the other hand, the dissolution of ferrous oxalate and ammonium dihydrogen phosphate in water results in the presence of iron and phosphate ions in the solution; lithium carbonate and lithium phosphate are also slightly soluble in water, resulting in the presence of a small amount of lithium ions in the solution. The three main elements for synthesizing lithium iron phosphate—lithium ions, iron ions, and phosphate ions—all exist in ionic form in the solution, facilitating the synthesis of small-particle lithium iron phosphate with a particle size of less than 10 nm. Furthermore, using ferrous oxalate as the iron source and ammonium dihydrogen phosphate as the phosphorus source, adding ferrous oxalate and ammonium dihydrogen phosphate to the slurry system can lower the pH value of the slurry, preventing it from becoming alkaline, preventing the dissolution of iron from the iron source material, reducing the formation of harmful substances such as ferric hydroxide, and improving the stability of the iron source. Additionally, oxalate ions have coordination and reducing properties, which can reduce the microscopic size of reactant particles; moreover, oxalate ions are converted into conductive carbon and carbon dioxide gases, further increasing the conductivity of the material.

[0114] The coin cells assembled from the lithium iron phosphate cathode material prepared by this method exhibit good stability and electrochemical performance. Furthermore, this method requires low reaction temperature and short reaction time, has low equipment requirements, and a simple process flow, thus improving production efficiency and making it suitable for large-scale industrial production.

[0115] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0116] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate, characterized in that, The method includes: Step S1: Add ferrous sulfate, a byproduct of titanium dioxide, to a phosphorus source and a precipitant for purification. After pressure filtration and purification, a ferrous sulfate solution is obtained. Step S2: Add an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution; Step S3: Hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, and ammonia are added sequentially to ferrous sulfate solution and reacted for a period of time to form a mixed slurry. The mixed slurry is heated and kept at a certain temperature for a period of time, and then washed and filtered repeatedly with water to form a high iron-to-phosphorus ratio hydroxyferric phosphate precursor and a low iron-to-phosphorus ratio hydroxyferric phosphate precursor. Specifically, when the iron-to-phosphorus ratio in the mixed slurry meets the iron-to-phosphorus molar ratio: Fe / P=1.475-1.490, a high iron-to-phosphorus ratio hydroxyferric phosphate precursor is formed; when the iron-to-phosphorus ratio in the mixed slurry meets the iron-to-phosphorus molar ratio: Fe / P=1.460-1.465, a low iron-to-phosphorus ratio hydroxyferric phosphate precursor is generated. Step S4: High iron-to-phosphorus ratio hydroxyferric phosphate precursor and low iron-to-phosphorus ratio hydroxyferric phosphate precursor are flash-dried in a flash evaporator and sintered at high temperature for a certain time to obtain hydroxyferric phosphate precursor products with different iron-to-phosphorus ratios and different specific surface areas. Step S5: The sintered material is crushed by a mechanical mill and mixed by a ribbon mixer to obtain hydroxy ferric phosphate with high iron-to-phosphorus ratio and high specific surface area and hydroxy ferric phosphate with low iron-to-phosphorus ratio and low specific surface area. Step S6: Mix the high-iron-phosphorus-ratio, high-specific-surface-area hydroxyferric phosphate product with the low-iron-phosphorus-ratio, low-specific-surface-area hydroxyferric phosphate product in a certain proportion, and then mix it with ferrous oxalate, lithium phosphate, lithium carbonate and ammonium dihydrogen phosphate in a certain proportion, and add a certain amount of carbon source and additives to form a mixture. Step S7: The above mixture is milled to obtain a nano-sized milled slurry; the nano-sized milled slurry is spray-dried to obtain a spray material; Step S8: The above sprayed material is placed in a box furnace for sintering to obtain sintered material, and the sintered material is pulverized by an air jet mill to obtain pulverized material; Step S9: After further sieving, batching and packaging of the above-mentioned pulverized material, the finished lithium iron phosphate product can be obtained.

2. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S1, the ferrous sulfate : phosphorus source : precipitant are in a mass ratio of 1 : [0.001-0.005] : [0.005-0.007]. The purification reaction temperature is 40°C, the reaction pH is 2.2-2.5, and the reaction time is 1 hour. The phosphorus source is one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and sodium phosphate. The precipitant is one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia water.

3. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S2, the amount of phosphoric acid added is in a molar ratio of n(Fe):n(phosphoric acid) = 1:0.

15.

4. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S3, the water washing is performed multiple times. The first water wash mainly removes impurities such as magnesium, manganese, and sulfur. In the final water wash, a 1:1 diluted ammonia solution is added to adjust the pH to 6.5-7.0 to remove SO4. 2- Ions; the hydrogen peroxide concentration is 30%-60%, and the mixed slurry is heated to 60-80℃ and kept at that temperature for 3 hours.

5. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, Step S3 includes: Step S31: Add excess hydrogen peroxide to the ferrous sulfate solution and continue oxidation for a certain period of time; Step S32: Add the phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolve the ammonium dihydrogen phosphate powder in water to prepare a 30% concentration ammonium dihydrogen phosphate solution at a dissolution temperature of 30-40℃, and add it to the oxidized ferrous sulfate solution. Step S33: Add ammonia to the ferrous sulfate solution to adjust the pH of the solution to 3.00±0.

02. After reacting for a period of time, a mixed slurry is formed. The mixed slurry is heated and kept at a certain temperature for a period of time, and then washed and filtered repeatedly with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

6. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S4, the inlet air temperature of the flash evaporator is controlled at 220±20℃, the outlet air temperature is controlled at 110±5℃, the sintering atmosphere is air, the sintering temperature is 535-560℃, and the sintering time is 4-5h. In step S5, the particle size is controlled at D10≥1.0μm, D50: 6-15μm, D90≤60μm, the mixing frequency of the mixer is controlled at 35±2Hz, and the mixing time is 1-2h.

7. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S5, the ferric hydroxyphosphate product with a high ferric phosphorus ratio and high specific surface area has a specific surface area that satisfies: BET = 15-20m². 2 / g; Low iron-to-phosphorus ratio and low specific surface area of ​​hydroxyferric phosphate product, with a specific surface area satisfying: BET=5-10m 2 / g.

8. The method for preparing lithium iron phosphate from ferric hydroxyphosphate and ferrous oxalate according to claim 1, characterized in that, In step S6, according to the molar ratio Li:Fe:P = [1.03-1.04]:1:[1.03-1.04], the amount of carbon source added is based on the carbon content in the final product being between 1.2% and 1.6%. The carbon source is one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol. The additive is one or more of titanium dioxide, ammonium metavanadate, and niobium pentoxide, and the doping amount is controlled between 300 and 3000 ppm. In step S7, the particle size of the sand-milling slurry is controlled between 0.45 and 0.75 μm, and the inlet air temperature is 200-220℃ during spray drying. The outlet air temperature is 80-110℃, the blowing frequency is 80Hz, and the spray particle size in the spray material is controlled between D50=20-40μm; in step S8, the sintering atmosphere is nitrogen, the sintering temperature is 750-780℃, the heating rate is 3℃ / min, the sintering time is 8-12h, and after natural cooling, the sintered material can be obtained. During the crushing process, the gas pressure is controlled between 0.2-0.4Mpa, the grading frequency is 80-200Hz, and the particle size of the crushed material meets the following requirements: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, D100<30μm.

9. A lithium-ion battery cathode material, characterized in that, The positive electrode material of the lithium-ion battery uses lithium iron phosphate prepared by the method for preparing lithium iron phosphate by reacting ferrous hydroxyphosphate and ferrous oxalate as described in any one of claims 1-8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium-ion battery cathode material as described in claim 9.

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