Method for preparing lithium manganese iron phosphate from iron hydroxyphosphate and application thereof

Abstract

The application provides a method for preparing lithium manganese iron phosphate from iron hydroxyphosphate, wherein ferrous sulfate is purified to form a ferrous sulfate solution, hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, titanium sulfate and ammonia are sequentially added into the solution to form a mixed slurry after reaction, the mixed slurry is heated to a certain temperature and kept for a certain time, and then water washing and pressure filtration are performed to form iron hydroxyphosphate precursors with different iron-phosphorus ratios; the iron hydroxyphosphate precursors are subjected to flash drying and high-temperature sintering and crushing to obtain iron hydroxyphosphate precursors with different iron-phosphorus ratios and different specific surface areas; the iron hydroxyphosphate precursors are crushed and mixed to obtain iron hydroxyphosphate finished products; high-iron-phosphorus-ratio iron hydroxyphosphate and low-iron-phosphorus-ratio iron hydroxyphosphate are mixed according to a certain proportion, and lithium source, manganese source and phosphorus source are proportioned according to a certain proportion, and carbon source and additives are added to form a mixture; and after processes such as ball milling, sand milling, spray drying, sintering, crushing, screening, batching and packaging are performed on the mixture, lithium manganese iron phosphate finished products are obtained.

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 manganese iron phosphate from hydroxy iron phosphate and its application. Background Technology

[0002] Lithium-ion batteries are one of the most common chemical power sources, with advantages such as high specific energy, high specific power, long cycle life, and no memory effect. They are ideal power sources for electric vehicles, digital products, and various power tools.

[0003] Lithium manganese iron phosphate (LMP) is a cathode active material used in lithium-ion batteries. Like lithium iron phosphate (LFP), LMP belongs to the phosphate-based cathode active materials and boasts advantages such as good cycle performance, excellent safety, and environmental friendliness. Common synthesis methods for LMP include high-temperature solid-state synthesis, hydrothermal synthesis, and sol-gel synthesis. However, producing high-quality LMP requires stricter control over the types of raw materials, precursor composition, key element ratios, preparation methods, and process parameters. Summary of the Invention

[0004] 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 manganese iron phosphate from ferrous hydroxyphosphate and its application. This method uses ferrous sulfate as a material, adds hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate, titanium sulfate, and ammonia to synthesize ferrous hydroxyphosphate, and adds manganese, lithium, and phosphorus sources to prepare high-density, high-capacity lithium manganese iron phosphate material.

[0005] Therefore, in a first aspect, embodiments of the present invention provide a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate, 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, titanium sulfate, 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 them 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 precursors... The finished material is pulverized using a mechanical mill and mixed with a ribbon mixer to obtain hydroxyferric phosphate products with different iron-phosphorus ratios and specific surface areas. 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 lithium, manganese, and phosphorus sources in a certain proportion, along with 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 manganese iron phosphate product.

[0006] Preferably, in step S1, the mass ratio of ferrous sulfate:phosphorus source:precipitant = 1:[0.001-0.005]:[0.005-0.007], the purification reaction temperature is 40℃, the reaction pH is 2.2-2.5, and the reaction time is 1h. 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. 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.

[0007] Preferably, in step S3, the water washing can be 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 value 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.

[0008] 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; then diluting titanium sulfate 100 times and adding it to the oxidized ferrous sulfate solution; adding ammonia 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.

[0009] Preferably, in step S4, the inlet air temperature of the flash evaporator is controlled at 220±20℃, the outlet air temperature 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; the high-iron-phosphorus hydroxyferric 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=15-20m². 2 / g; the low iron-to-phosphorus ratio hydroxyferric phosphate 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.

[0010] Preferably, in step S6, the manganese source is one or more of elemental manganese, manganese monoxide, manganese tetroxide, manganese dioxide, manganese hydroxide, and manganese carbonate; the lithium source is one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium dihydrogen phosphate; the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and phosphoric acid; the molar ratio of the lithium source, the iron source, the manganese source, and the phosphorus source is Li:Fe:Mn:P = [1.03-1.04]:[0.1-0.9]:[0.1-0.9]:[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, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300 and 3000 ppm.

[0011] Preferably, in step S7, the abrasive particle size in the abrasive slurry is controlled between 0.45-0.75 μm. During spray drying, the inlet air temperature is 200-220℃, the outlet air temperature is 80-110℃, and the blowing frequency is 80Hz. The abrasive particle size in the spray material is controlled between D50 = 20-40 μm. In step S8, 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 0.2-0.4 MPa, the grading frequency is 80-200Hz, and the particle size of the pulverized material satisfies: D10 > 0.35 μm, D50 = 0.7-2.0 μm, D90 < 10 μm, and D100 < 30 μm.

[0012] Secondly, embodiments of the present invention provide a lithium-ion battery cathode material obtained by processing lithium manganese iron phosphate using the method for preparing lithium manganese iron phosphate provided in the first aspect above.

[0013] Thirdly, embodiments of the present invention provide a lithium-ion battery, including the lithium-ion battery cathode material described in the second aspect above.

[0014] The method for preparing lithium manganese iron phosphate from ferrous hydroxyphosphate 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. 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 and low-iron-to-phosphorus-ratio, low-specific-surface-area ferrous phosphates are mixed, and then further mixed with a lithium source, a manganese source, and a phosphorus source 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 manganese iron phosphate. The method for preparing lithium manganese iron phosphate from ferrous hydroxyphosphate provided in this invention also has the following advantages:

[0015] I. This method uses soluble ferrous salts as the iron source, which react with phosphates to form hydroxyferric phosphate. A simple water washing process is then used to reduce the amount of water used for washing. After hydroxyferric phosphate is used as the iron source, it is fully milled, sprayed, sintered and pulverized with other manganese and lithium sources. This process is beneficial for synthesizing lithium manganese iron phosphate materials with a uniform manganese-iron ratio, which is beneficial for improving the compaction density, conductivity and capacity of the material. Moreover, no high-salt wastewater is generated during the preparation process.

[0016] II. Uniform distribution of dopant elements and good doping effect. This method pre-dops ferric hydroxyphosphate in the solution phase. During the reaction, iron and dopant elements form precipitates containing dopant elements, which can achieve pre-mixing of dopant elements and iron elements, thereby avoiding local agglomeration of dopant elements and improving the doping effect. Pre-doping ferric hydroxyphosphate is also beneficial for the dopant elements to enter the crystal lattice of manganese iron phosphate, further improving its electrochemical performance.

[0017] Third, the raw materials are widely available and inexpensive. The raw materials used in this method are all common inorganic chemical raw materials with low and stable market prices. Using these raw materials to prepare pre-doped hydroxy iron phosphate and further prepare doped lithium manganese iron phosphate materials can reduce raw material costs.

[0018] Fourth, hydroxyferric phosphate, as an iron source, exists in an amorphous form. Compared with crystalline iron, manganese, and lithium sources, it requires a lower reaction temperature in the synthesis of lithium manganese iron phosphate, which is more conducive to the formation of lithium manganese iron phosphate. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for preparing lithium manganese iron phosphate from hydroxyferric phosphate according to an embodiment of the present invention;

[0020] Figure 2 A flowchart of step S3 in the preparation of lithium manganese iron phosphate from ferric hydroxyphosphate provided in an embodiment of the present invention;

[0021] 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;

[0022] Figure 4 The image shows the SEM spectrum of the lithium iron phosphate cathode material prepared in Example 1 of this invention.

[0023] Figure 5 The XRD pattern of the high iron-to-phosphorus ratio and high specific surface area hydroxyferric phosphate prepared in Example 1 of this invention;

[0024] Figure 6 The image shows the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1 of this invention.

[0025] Figure 7 The image shows the EDS elemental content test spectrum of the lithium iron manganese phosphate cathode material prepared in Example 1 of this invention.

[0026] Figure 8 The surface scan spectrum of the lithium iron manganese phosphate cathode material EDS prepared in Example 1 of this invention;

[0027] Figure 9The charge-discharge curve (0.1C) of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of this invention;

[0028] Figure 10 The charge-discharge curve (0.2C) of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of this invention;

[0029] Figure 11 The charge-discharge curve (1C) of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of this invention. Detailed Implementation

[0030] 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.

[0031] 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, but those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0032] This invention provides a method for preparing lithium manganese iron phosphate from hydroxyapatite, which can be used to prepare lithium manganese iron phosphate materials with high compaction density and high capacity. For example... Figure 1 As shown, this method includes:

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Step S2: Add an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution;

[0037] The amount of phosphoric acid added is based on a molar ratio of n(Fe):n(phosphoric acid) = 1:0.15.

[0038] Step S3: Hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, titanium sulfate 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.

[0039] 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.

[0040] 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.

[0041] In addition, the titanium sulfate content is adjusted to ensure that the titanium doping level in the final product is between 300-500 ppm.

[0042] 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.

[0043] Specifically, in the first embodiment of the present invention, as Figure 2 As shown, step S3 includes:

[0044] Step S31: Add excess hydrogen peroxide to the ferrous sulfate solution and continue oxidation for a certain period of time;

[0045] 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. Then dilute titanium sulfate 100 times and add it to the oxidized ferrous sulfate solution.

[0046] 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.

[0047] The heating temperature of the mixed slurry is 60-80℃, and the holding time is 3h.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] Step S6: Mix hydroxy ferric phosphate with high iron-to-phosphorus ratio and high specific surface area with hydroxy ferric phosphate with low iron-to-phosphorus ratio and low specific surface area in a certain proportion, and then mix it with lithium source, manganese source and phosphorus source in a certain proportion, and add a certain amount of carbon source and additives to form a mixture.

[0054] The manganese source is one or more of elemental manganese, manganese monoxide, manganese tetroxide, manganese dioxide, manganese hydroxide, and manganese carbonate; the lithium source is one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium dihydrogen phosphate; and the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and phosphoric acid.

[0055] In this embodiment of the invention, the molar ratio of high-iron-phosphorus hydroxyferric phosphate to low-iron-phosphorus hydroxyferric phosphate is between 2:8 and 8:2, preferably, the molar ratio of high-iron-phosphorus hydroxyferric phosphate to low-iron-phosphorus hydroxyferric phosphate satisfies 3:7. Furthermore, in the mixture, the amounts of the lithium source, the iron source, the manganese source, and the phosphorus source added are in a molar ratio of Li:Fe:Mn:P = [1.03-1.04]:[0.1-0.9]:[0.1-0.9]:[1.03-1.04].

[0056] 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 can be one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol. The additive can be one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, with the doping amount controlled between 300 and 3000 ppm.

[0057] 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;

[0058] 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.

[0059] 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;

[0060] 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.

[0061] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished product of lithium manganese iron phosphate can be obtained.

[0062] The method for preparing lithium manganese iron phosphate using ferric hydroxyphosphate 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, titanium sulfate, 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, ferric hydroxyphosphate precursors with different iron-to-phosphorus ratios are generated. These precursors are then subjected to different sintering processes to obtain ferric 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 ferric 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 ferric hydroxyphosphate precursor more stable. Furthermore, heating the mixed slurry to 60-80°C and holding it at this temperature results in larger ferric 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 method minimizes impurities such as ions, resulting in a low impurity content and high purity in the final hydroxyferric phosphate product. 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. High iron-to-phosphorus ratio hydroxyferric phosphate particles are smaller, improving the material's discharge capacity; low iron-to-phosphorus ratio hydroxyferric phosphate particles are larger, improving the material's compaction density. In-situ doping of titanium into the hydroxyferric phosphate precursor results in uniform dopant distribution and better doping performance.

[0063] In this method, high-iron-phosphorus ratio hydroxyferric phosphate and low-iron-phosphorus ratio hydroxyferric phosphate are mixed in subsequent steps, and then further mixed with lithium, manganese, and phosphorus sources 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 manganese iron phosphate product. This method requires low reaction temperature, short reaction time, low equipment requirements, and a simple process flow, improving production efficiency and making it suitable for large-scale industrial production.

[0064] In addition, the method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate provided in this embodiment of the invention has the following advantages:

[0065] I. This method uses soluble ferrous salts as the iron source, which react with phosphates to form hydroxyferric phosphate. A simple water washing process is then used to reduce the amount of water used for washing. After hydroxyferric phosphate is used as the iron source, it is fully milled, sprayed, sintered and pulverized with other manganese and lithium sources. This process is beneficial for synthesizing lithium manganese iron phosphate materials with a uniform manganese-iron ratio, which is beneficial for improving the compaction density, conductivity and capacity of the material. Moreover, no high-salt wastewater is generated during the preparation process.

[0066] II. Uniform distribution of dopant elements and good doping effect. This method pre-dops ferric hydroxyphosphate in the solution phase. During the reaction, iron and dopant elements form precipitates containing dopant elements, which can achieve pre-mixing of dopant elements and iron elements, thereby avoiding local agglomeration of dopant elements and improving the doping effect. Pre-doping ferric hydroxyphosphate is also beneficial for the dopant elements to enter the crystal lattice of manganese iron phosphate, further improving its electrochemical performance.

[0067] Third, the raw materials are widely available and inexpensive. The raw materials used in this method are all common inorganic chemical raw materials with low and stable market prices. Using these raw materials to prepare pre-doped hydroxy iron phosphate and further prepare doped lithium manganese iron phosphate materials can reduce raw material costs.

[0068] Fourth, hydroxyferric phosphate, as an iron source, exists in an amorphous form. Compared with crystalline iron, manganese, and lithium sources, it requires a lower reaction temperature in the synthesis of lithium manganese iron phosphate, which is more conducive to the formation of lithium manganese iron phosphate.

[0069] The following detailed description, in conjunction with some specific embodiments, further illustrates the specific process and effects of the method for preparing lithium manganese iron phosphate using hydroxyferric phosphate according to the present invention, but does not limit the scope of protection of the present invention.

[0070] Example 1

[0071] This embodiment provides a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate, including the following steps:

[0072] 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.

[0073] 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;

[0074] 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 titanium sulfate solution diluted 100 times to the ferrous sulfate solution so that the doped titanium content meets 300-500ppm. Then add ammonia water to form a mixed slurry. Heat the mixed slurry to 60℃ 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Step S6: Mix hydroxyferric phosphate with a high iron-to-phosphorus ratio and high specific surface area with hydroxyferric phosphate with a low iron-to-phosphorus ratio and low specific surface area in a 3:7 ratio, and then mix it with manganese monoxide, lithium carbonate, and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:Mn:P = 1.03:0.4:0.6:1.03. Add a carbon source mixture consisting of sucrose and polyethylene glycol to make the carbon content of the finished product 1.35%, and magnesium acetate with a doping amount of 1800 ppm and ammonium metavanadate with 800 ppm to form a mixture.

[0080] 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.

[0081] 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.

[0082] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished product of lithium manganese iron phosphate can be obtained.

[0083] The SEM spectrum of the lithium iron phosphate cathode material prepared according to Example 1 is shown below. Figure 4 As shown.

[0084] The XRD pattern of the lithium iron phosphate cathode material prepared according to Example 1 is shown below. Figure 6 As shown.

[0085] The EDS (electromagnetic dispersive saturation) of the lithium manganese iron phosphate cathode material prepared in Example 1 was tested, and the results are shown in Table 1. The elemental composition of the EDS in the lithium manganese iron phosphate cathode material prepared in Example 1 is shown in the following figure. Figure 7 As shown, the EDS surface scan of the lithium manganese iron phosphate cathode material prepared according to Example 1 is as follows. Figure 8 As shown.

[0086] Table 1. Test items and test results of Example 1

[0087]

[0088] Based on the test results obtained from the testing of the lithium manganese iron phosphate product prepared in Example 1, it can be seen that the elemental ratio of n(Mn):n(Fe) in the lithium manganese iron phosphate prepared in Example 1 is accurate, with a ratio of n(Mn):n(Fe) = 0.6:0.4. Furthermore, the synthesized individual particles are not lithium iron phosphate, nor lithium manganese phosphate, but rather lithium manganese iron phosphate. Additionally, from... Figure 8 It can be seen that Mn, Fe and P are uniformly distributed in lithium manganese iron phosphate, and the doping elements V and Mg are uniformly distributed in the lithium manganese iron phosphate material.

[0089] Example 2

[0090] This embodiment provides a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate, including the following steps:

[0091] 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.

[0092] 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;

[0093] 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 titanium sulfate solution diluted 100 times to the ferrous sulfate solution so that the doped titanium content meets 300-500ppm. Then add ammonia water to form a mixed slurry. Heat the mixed slurry to 60℃, keep it at the temperature for 3h, and then wash and filter it multiple times with water to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0094] 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.

[0095] 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.

[0096] Step S6: Mix hydroxyferric phosphate with a high iron-to-phosphorus ratio and high specific surface area with hydroxyferric phosphate with a low iron-to-phosphorus ratio and low specific surface area in a 4:6 ratio, and then mix it with manganese tetroxide, lithium hydroxide, and diammonium hydrogen phosphate in a molar ratio of Li:Fe:Mn:P = 1.03:0.5:0.5:1.03. 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.

[0097] 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.

[0098] 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.

[0099] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished product of lithium manganese iron phosphate can be obtained.

[0100] Example 3

[0101] This embodiment provides a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate, including the following steps:

[0102] 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.

[0103] 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;

[0104] 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 ratio: Fe / P = 1.485 and Fe / P = 1.465. Then add titanium sulfate solution diluted 100 times to the ferrous sulfate solution to make the titanium doping content meet 300-500ppm. Then add ammonia water to form a mixed slurry. Heat the mixed slurry to 60℃, keep it at the temperature for 3 hours, and then wash and filter it with water multiple times to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

[0105] 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.

[0106] 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.

[0107] Step S6: Mix hydroxyferric phosphate with a high iron-to-phosphorus ratio and high specific surface area with hydroxyferric phosphate with a low iron-to-phosphorus ratio and low specific surface area in a 3:7 ratio, and then mix it with manganese carbonate and lithium dihydrogen phosphate in a molar ratio of Li:Fe:Mn:P = 1.03:0.6:0.4:1.03. 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.

[0108] 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.

[0109] 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.

[0110] Step S9: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished product of lithium manganese iron phosphate can be obtained.

[0111] Comparative Example 1

[0112] This embodiment provides a method for preparing lithium manganese iron phosphate, including the following steps:

[0113] Step S1: Iron oxide, manganese monoxide, lithium carbonate, and ammonium dihydrogen phosphate are mixed in a molar ratio of Li:Fe:Mn:P = 1.03:0.4:0.6:1.03. A carbon source mixture consisting of sucrose and polyethylene glycol, which makes the carbon content of the finished product 1.10%, and titanium dioxide with a doping amount of 3000ppm are added to form a mixture.

[0114] Step S2: 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.

[0115] Step S3: 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.

[0116] Step S4: After further processing the above-mentioned pulverized material through screening, batching, and packaging, the finished product of lithium manganese iron phosphate can be obtained.

[0117] To verify the quality of the lithium manganese iron phosphate cathode material prepared by the method for preparing lithium manganese iron phosphate using hydroxyl iron phosphate provided in this invention, the lithium manganese 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 onto 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. All components were assembled into a coin cell. The test voltage range was 2.5V-4.5V. The battery was charged to 4.5V using a constant current and constant voltage charging method and discharged to 2.5V using a constant current discharging method. The charge / discharge currents were 0.1C for two cycles, 0.2C for two cycles, and 1C for two cycles. The test results are shown in Table 2. The charge-discharge curve (0.1C) of the coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention is as follows: Figure 9 As shown, the charge-discharge curve (0.2C) of the coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention is as follows. Figure 10 As shown. The charge-discharge curve (1C) of the coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention is shown. Figure 11 As shown.

[0118] Table 2. Test items and test results for Examples 1-3 and Comparative Example 1

[0119]

[0120]

[0121] Based on the above examples and comparative examples, and the test results obtained from the tests, the coin cells prepared with lithium manganese iron phosphate cathode material in Examples 1-3 showed a significant improvement in discharge specific capacity at 0.1C, 0.2C, and 1C compared with Comparative Example 1.

[0122] In summary, the method for preparing lithium manganese iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, utilizes ferrous sulfate to generate ferric sulfate. After adding other materials and reacting, ferrous phosphate with different iron-to-phosphorus ratios is generated. Then, through different sintering processes, ferrous phosphate 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 a lithium source, a manganese source, and a phosphorus source in a certain proportion. Additives are added to form a mixture, which is then subjected to processes such as sand milling, spray drying, sintering, sieving, batching, and packaging to obtain the finished lithium manganese iron phosphate.

[0123] The coin cells assembled from the lithium manganese 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.

[0124] 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.

[0125] 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 manganese iron phosphate from ferric hydroxyphosphate, 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, titanium sulfate, 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 hydroxyferric phosphate precursors with different iron-phosphorus ratios. When the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio: Fe / P=[1.475-1.490], a high iron-phosphorus ratio hydroxyferric phosphate precursor is formed; when the iron-phosphorus feeding ratio in the mixed slurry meets the iron-phosphorus molar ratio: Fe / P=[1.460-1.465], a low iron-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 from S4 is pulverized using a mechanical mill and mixed using a ribbon mixer to obtain hydroxy ferric phosphate products with high iron-to-phosphorus ratio and high specific surface area, and hydroxy ferric phosphate products with low iron-to-phosphorus ratio and low specific surface area. Step S6: Mix hydroxy ferric phosphate with high iron-to-phosphorus ratio and high specific surface area with hydroxy ferric phosphate with low iron-to-phosphorus ratio and low specific surface area in a certain proportion, and then mix it with lithium source, manganese source and phosphorus source 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 product of lithium manganese iron phosphate can be obtained.

2. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S1, the mass ratio of ferrous sulfate:phosphorus source:precipitant is 1:[0.001-0.005]:[0.005-0.007]. The purification reaction temperature is 40℃, the reaction pH is 2.2-2.5, and the reaction time is 1h. 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. In step S2, the amount of phosphoric acid added is in a molar ratio of n(Fe):n(phosphoric acid) = 1:0.

15.

3. The method for preparing lithium manganese iron phosphate 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.

4. The method for preparing lithium manganese iron phosphate 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. Then dilute titanium sulfate 100 times 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.

5. The method for preparing lithium manganese iron phosphate 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 at 110±5℃, the sintering atmosphere is air, the sintering temperature is 535-560℃, and the sintering time is 4-5 hours. In step S5, the particle size is controlled as follows: D10≥1.0μm, D50: 6-15μm, D90≤60μm. The mixing frequency of the mixer is controlled at 35±2Hz, and the mixing time is 1-2 hours. The high-iron phosphorus hydroxyferric phosphate has a high specific surface area, which satisfies the following condition: BET=15-20m². 2 / g; The low-iron-phosphorus ferric phosphate has a low specific surface area, which satisfies: BET=5-10m². 2 / g.

6. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S6, the manganese source is one or more of elemental manganese, manganese monoxide, manganese tetroxide, manganese dioxide, manganese hydroxide, and manganese carbonate; the lithium source is one or more of lithium phosphate, lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium dihydrogen phosphate; the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and phosphoric acid; the molar ratio of the lithium source, the hydroxyferric phosphate, the manganese source, and the phosphorus source is Li:Fe:Mn:P = [1.03-1.04]:[0.1-0.9]:[0.1-0.9]:[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, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300-3000 ppm.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S7, the particle size of the sand-milled slurry is controlled between 0.45-0.75 μm. During spray drying, the inlet air temperature is 200-220℃, the outlet air temperature is 80-110℃, and the blowing frequency is 80Hz. The spray particle size of the sprayed 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, 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 0.2-0.4 MPa, the grading frequency is 80-200Hz, and the particle size of the pulverized material meets the following requirements: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, D100<30μm.

8. A lithium-ion battery cathode material, characterized in that, The lithium-ion battery cathode material uses lithium manganese iron phosphate prepared by the method for preparing lithium manganese iron phosphate as described in any one of claims 1-7.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium-ion battery cathode material as described in claim 8.

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