Method for preparing lithium ferric manganese phosphate from ferric hydroxyphosphate and use

Abstract

Provided in the present invention is a method for preparing lithium ferric manganese phosphate from ferric hydroxyphosphate, comprising: purifying ferrous sulfate to form a ferrous sulfate solution; sequentially adding hydrogen peroxide, phosphoric acid, an ammonium dihydrogen phosphate solution, titanium sulfate and aqueous ammonia to same, reacting same to form a mixed slurry, heating the mixed slurry to a certain temperature and then performing heat preservation, and then performing water washing and filter pressing to form ferric hydroxyphosphate precursors having different ferric-to-phosphorus ratios; performing flash evaporation drying and high-temperature sintering and crushing, so as to obtain the ferric hydroxyphosphate precursors having different ferric-to-phosphorus ratios and different specific surface areas; crushing and mixing the ferric hydroxyphosphate precursors to obtain ferric hydroxyphosphate finished products; according to a certain ratio, mixing ferric hydroxyphosphate having a high ferric-to-phosphorus ratio with ferric hydroxyphosphate having a low ferric-to-phosphorus ratio, then proportioning same with a lithium source, a manganese source and a phosphorus source according to a certain ratio, and adding a carbon source and an additive so as to form a mixture; and subjecting the mixture to sand milling, spray drying, sintering, crushing, screening, batch combination, packaging and other steps, so as to obtain a lithium ferric manganese phosphate finished product.

Description

A method for preparing lithium iron manganese phosphate from hydroxyferric phosphate and its application Technical Field

[0001] The present invention relates to the technical field of methods for preparing positive electrode materials for lithium-ion batteries, and in particular to a method for preparing lithium manganese iron phosphate from hydroxyferric phosphate and its application. Background Art

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

[0003] Lithium iron manganese phosphate (LMP) is a cathode active material for lithium-ion batteries. Like LFP, LMP also belongs to the phosphate class of cathode active materials, offering advantages such as good cycling performance, excellent safety, and environmental friendliness. Common synthesis methods for LMP include high-temperature solid-phase, hydrothermal, and sol-gel methods. However, during the synthesis and preparation process, high-quality LMP requires higher control over the type of raw materials, the composition of the precursors, the ratio of key elements, the preparation method, and the process parameters.

[0004] Summary of the Invention

[0005] In view of the above, the present invention aims to address at least one of the technical problems existing in the prior art. To this end, the present invention provides a method for preparing lithium iron manganese phosphate from ferric hydroxyphosphate and its application. This method uses ferrous sulfate as a raw material, adds hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate, titanium sulfate, and ammonia to synthesize ferric hydroxyphosphate, and then adds a manganese source, a lithium source, and a phosphorus source to produce a lithium iron manganese phosphate material with a high compaction density and high capacity.

[0006] To this end, in a first aspect, a first embodiment of the present invention provides a method for preparing lithium manganese iron phosphate from hydroxy iron phosphate, the method comprising: adding a phosphorus source and a precipitant to purify ferrous sulfate, a by-product of titanium dioxide, and filtering and purifying to obtain a ferrous sulfate solution; adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution; adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, titanium sulfate and ammonia water to the ferrous sulfate solution in sequence, reacting for a period of time to form a mixed slurry, heating and keeping the mixed slurry warm for a period of time, and then washing and filtering multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios; flash drying the hydroxy iron phosphate precursor in a flash evaporator, and sintering at high temperature for a certain period of time to obtain hydroxy iron phosphate precursor products with different iron-phosphorus ratios and different specific surface areas; The sintered material is crushed with a mechanical mill and mixed with a ribbon mixer to obtain hydroxy iron phosphate products with different iron-phosphorus ratios and different specific surface areas; the hydroxy iron phosphate with a high iron-phosphorus ratio and high specific surface area is mixed with the hydroxy iron phosphate with a low iron-phosphorus ratio and low specific surface area in a certain proportion, and then proportioned with a lithium source, a manganese source and a phosphorus source in a certain proportion, and a certain amount of carbon source and additives are added to form a mixture; the above mixture is sand-milled to obtain a nano-sized mixture; the nano-sized mixture is spray-dried to obtain a spray material; the above spray material is placed in a box furnace for sintering to obtain a sintered material, and the sintered material is crushed by a jet mill to obtain a pulverized material; the pulverized material is further subjected to screening, batching, packaging and other processes to obtain a finished lithium manganese iron phosphate product.

[0007] Preferably, in the step S1, the mass ratio of the ferrous sulfate: the phosphorus source: the precipitant is 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 h, 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 the step S2, the amount of the phosphoric acid added is according to the molar ratio of n(Fe):n(phosphoric acid)=1:0.15; in the step S3, when the iron-phosphorus feed ratio in the mixed slurry satisfies the iron-phosphorus molar ratio: Fe / P=1.475-1.490, a high iron-phosphorus ratio hydroxy ferric phosphate can be formed; when the iron-phosphorus feed ratio in the mixed slurry satisfies the iron-phosphorus molar ratio: Fe / P=1.460-1.475, a low iron-phosphorus ratio hydroxy ferric phosphate can be generated.

[0008] Preferably, in step S3, the water washing times can be multiple times, the first water washing mainly washes away the impurities of magnesium, manganese and sulfur, and the last water washing is added with 1:1 diluted ammonia water to adjust the pH value to 6.5-7.0 to wash away SO4 2-ions; the concentration of hydrogen peroxide is 30%-60%, and the mixed slurry is heated to 60-80°C and the holding time is 3h.

[0009] Preferably, step S3 includes: adding an excess of hydrogen peroxide to the ferrous sulfate solution and continuing oxidation for a certain time; adding a phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolving ammonium dihydrogen phosphate powder in water to form a 30% concentration of ammonium dihydrogen phosphate solution with a dissolution temperature of 30-40°C, and adding the solution to the oxidized ferrous sulfate solution; then diluting titanium sulfate 100 times and adding it to the oxidized ferrous sulfate solution; adding ammonia water to the ferrous sulfate solution, adjusting 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 warm for a period of time, and then washing and filtering multiple times to form a hydroxy iron phosphate precursor with different iron-phosphorus ratios.

[0010] Preferably, in step S4, the air inlet temperature of the flash evaporator is controlled to be 220±20°C, the air outlet temperature is 110±5°C, the sintering atmosphere is air, the sintering temperature is 535-560°C, and the sintering time is 4-5h; in step S5, the particle size D10 is controlled to be ≥1.0μm, D50: 6-15μm, D90≤60μm, the mixing frequency of the mixer is controlled to be 35±2Hz, and the mixing time is 1-2h; the high iron-phosphorus ratio hydroxy 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=15-20m 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=5-10m 2 / g.

[0011] Preferably, in step S6, the manganese source is one or more of elemental manganese, manganese monoxide, manganese trimanganese tetroxide, manganese dioxide, manganese hydroxide, manganese carbonate, and ferromanganous phosphate; 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 hydroxy ferric phosphate with a high iron-phosphorus ratio and a high specific surface area to the hydroxy ferric phosphate with a low iron-phosphorus ratio and a low specific surface area is between 2:8 and 8:2; the lithium source, the iron source, The manganese source and the phosphorus source are added 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]; the carbon source is added in an amount based on a carbon content of 1.2%-1.6% in the final product, and the carbon source is one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol; the additive is selected from one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300-3000 ppm.

[0012] Preferably, in step S7, the sanding particle size in the sanding slurry is controlled between 0.30-0.75 μm, in spray drying, the inlet air temperature is 200-220°C, the outlet air temperature is 80-110°C, the blast frequency is 80 Hz, and the spray 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°C, the heating rate is 3°C / min, the sintering time is 8-12h, and then the sintered material is obtained after natural cooling. During the crushing process, the air pressure is controlled between 0.2-0.4 MPa, the grading frequency is 80-200 Hz, and the particle size of the crushed material satisfies: D10>0.35 μm, D50=0.7-2.0 μm, D90<10 μm, and D100<30 μm.

[0013] Preferably, the step S6 comprises: step S61: adding an antioxidant and a phosphorus source after stirring and mixing an iron source, a manganese source, and phosphoric acid, and controlling the pH value of the solution to be between 3.0 and 4.0 to obtain a mixed solution; step S62: adding an ammonia solution A with a concentration of 16-20% into a reactor, and simultaneously adding the mixed solution and an ammonia solution B with a concentration of 8-10% dropwise under an inert gas atmosphere, controlling the pH value to be between 7.5 and 8.0, and the reaction temperature to be 25-40°C. After the addition is completed, aging for 1 hour, and co-precipitating to prepare phosphoric acid Ferromanganese precursor; step S63: filtering, washing, flash drying, high-temperature sintering, and crushing the ferromanganese phosphate precursor to obtain a ferromanganese phosphate product; step S64: ball-milling ferromanganese phosphate and phosphoric acid in a certain ratio, and then adding lithium phosphate, and wet-grinding to obtain a nano-sized sand-milled slurry; step S65: adding a certain amount of hydroxy ferric phosphate with a high iron-phosphorus ratio and a high specific surface area, hydroxy ferric phosphate with a low iron-phosphorus ratio and a low specific surface area, lithium phosphate, a carbon source, and an additive to the above-mentioned sand-milled slurry to obtain a mixture.

[0014] Preferably, in step S61, the molar ratio of the added manganese source to the iron source is between 8:2 and 2:8, the molar ratio of the sum of the molar amounts of the added manganese source and the iron source to the phosphorus source is [1.41-1.43]:1, the amount of the antioxidant added is 1% of the mass fraction of the iron source, the iron source is one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride, the manganese source is one or more of manganous sulfate, manganous nitrate, manganous chloride, and manganous acetate, and the phosphorus source is phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and dihydrogen phosphate. One or more of ammonium, the antioxidant is ascorbic acid; in the step S64, the molar ratio of Fe:Mn in the ferromanganese phosphate is between 2:8 and 8:2, the amount of phosphoric acid added is between 0.5% and 5% of the mass fraction of the ferromanganese phosphate, and in the sand grinding slurry, according to the molar ratio, Li:Fe:Mn:P=[1.03-1.04]:[0.1-0.9]:[0.1-0.9]:[1.03-1.04], and the sanding particle size in the sand grinding slurry is controlled between 0.60-0.80 μm.

[0015] In a second aspect, an embodiment of the present invention provides a lithium-ion battery positive electrode material obtained by treating the method for preparing lithium manganese iron phosphate using ferric hydroxyphosphate provided in the first aspect.

[0016] In a third aspect, an embodiment of the present invention provides a lithium-ion battery, comprising the lithium-ion battery positive electrode material described in the second aspect.

[0017] The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate provided in an embodiment of the present invention utilizes ferrous sulfate, a byproduct of titanium dioxide, to generate ferric sulfate, which is then added to other materials for reaction to generate hydroxyferric phosphate with different iron-phosphorus ratios. After undergoing different sintering processes, hydroxyferric phosphate products with high iron-phosphorus ratio and high specific surface area and low iron-phosphorus ratio and low specific surface area are obtained. The hydroxyferric phosphate with high iron-phosphorus ratio and high specific surface area and the hydroxyferric phosphate with low iron-phosphorus ratio and low specific surface area are mixed and then mixed with a lithium source, a manganese source, and a phosphorus source in a certain proportion. After adding additives to form a mixture, the mixture is subjected to sand milling, spray drying, sintering, screening, batching, packaging, and other processes to obtain the finished lithium iron manganese phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is a flow chart of a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate according to a first embodiment of the present invention;

[0019] FIG2 is a flow chart of step S3 in the method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate provided in the first embodiment of the present invention;

[0020] FIG3 is a flow chart of a method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate according to a second embodiment of the present invention;

[0021] FIG4 is a flow chart of step S6 in the method for preparing lithium manganese iron phosphate from ferric hydroxyphosphate provided in an embodiment of the second embodiment of the present invention;

[0022] FIG5 is a SEM spectrum of ferric hydroxyphosphate with a high iron-phosphorus ratio and high specific surface area prepared in Example 1 of the present invention;

[0023] FIG6 is a SEM spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0024] FIG7 is an XRD spectrum of ferric hydroxyphosphate with a high iron-phosphorus ratio and high specific surface area prepared in Example 1 of the present invention;

[0025] FIG8 is an XRD spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0026] FIG9 is an EDS element content test spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0027] FIG10 is a surface scan of the EDS spectrum of the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention;

[0028] FIG11 is a SEM spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 3 of the present invention;

[0029] FIG12 is an XRD spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 3 of the present invention;

[0030] FIG13 is a charge-discharge curve (0.1C) of a button-type half-cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention;

[0031] FIG14 is a charge and discharge curve (0.2C) of a button-type half-cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention;

[0032] FIG15 is a charge and discharge curve (1C) of a button-type half-cell assembled with the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention.

[0033] FIG16 is a charge-discharge curve (0.1C) of a button-type half-cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 3 of the present invention;

[0034] FIG17 is a charge-discharge curve (0.2C) of a button-type half-cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 3 of the present invention;

[0035] FIG18 is a charge and discharge curve (1C) of a button-type half-cell assembled with the lithium manganese iron phosphate positive electrode material prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0036] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0037] The first embodiment of the present invention provides a method for preparing lithium iron manganese phosphate from hydroxy iron phosphate, which is used to prepare a high compaction density and high capacity lithium iron manganese phosphate material. As shown in Figure 1, the method includes:

[0038] Step S1: adding a phosphorus source and a precipitant to purify ferrous sulfate, a by-product of titanium dioxide, and filtering to obtain a ferrous sulfate solution;

[0039] 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° C., the reaction pH is 2.2-2.5, and the reaction time is 1 h.

[0040] 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.

[0041] Step S2: adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to lower the pH value of the ferrous sulfate solution;

[0042] The amount of phosphoric acid added was based on a molar ratio of n(Fe):n(phosphoric acid)=1:0.15.

[0043] Step S3: adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, titanium sulfate and ammonia water to the ferrous sulfate solution in sequence and reacting for a period of time to form a mixed slurry, heating and keeping the mixed slurry warm for a period of time, washing and filtering multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios;

[0044] In this embodiment, hydrogen peroxide is added first, and then phosphoric acid and ammonium dihydrogen phosphate are added. On the one hand, this can make the generated hydroxyferric phosphate precursor more stable; on the other hand, it can make the generated hydroxyferric phosphate precursor particles larger and easier to filter and wash.

[0045] Among them, when the iron-phosphorus feed 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, and when the iron-phosphorus feed 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.

[0046] In addition, titanium sulfate is added to ensure that the titanium doping content in the final product is between 300-500 ppm.

[0047] In this embodiment, the concentration of hydrogen peroxide is between 30% and 60%, and the mixed slurry is heated to 60-80°C and kept warm for 3 hours. The water washing times can be multiple times, wherein the first water washing mainly washes away impurities such as magnesium, manganese, and sulfur. During the last water washing, 1:1 diluted ammonia water is added to adjust the pH value to 6.5-7.0 to wash away SO4 2- Specifically, the washing times can be 3 times, wherein the first and second washings mainly remove impurities such as manganese, magnesium, and sulfur. During the third washing, 1:1 diluted ammonia water is added to adjust the pH value to 6.5-7.0 to remove SO4 2- ion.

[0048] Specifically, in the first embodiment of the present invention, as shown in FIG2 , step S3 includes:

[0049] Step S31: adding excess hydrogen peroxide to the ferrous sulfate solution and continuing oxidation for a certain period of time;

[0050] Step S32: adding phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolving ammonium dihydrogen phosphate powder in water to prepare a 30% concentration of ammonium dihydrogen phosphate solution at a dissolution temperature of 30-40°C, and adding the solution to the oxidized ferrous sulfate solution. Then, diluting titanium sulfate 100 times, and adding the solution to the oxidized ferrous sulfate solution.

[0051] Step S33: adding ammonia water to the ferrous sulfate solution, adjusting 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 warm for a period of time, and then washing and filtering multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios.

[0052] The mixed slurry is heated at a temperature of 60-80° C. and kept warm for 3 hours.

[0053] Step S4: flash-drying the hydroxy ferric phosphate precursor in a flash evaporator, and sintering at high temperature for a certain period of time to obtain hydroxy ferric phosphate precursor products with different iron-phosphorus ratios and different specific surface areas;

[0054] The hydroxyferric phosphate precursor is flash dried to remove free water. The flash evaporator air inlet temperature is controlled at 220±20°C and the air outlet temperature is controlled at 110±5°C. The sintering atmosphere is air, the sintering temperature can be 535-560°C, and the sintering time can be 4-5 hours.

[0055] Step S5: crushing the sintered material with a mechanical mill and mixing it with a ribbon mixer to obtain finished ferric hydroxyphosphate products with different iron-phosphorus ratios and different specific surface areas;

[0056] During the pulverization process, the particle size D10 is controlled to be ≥1.0 μm, D50 is 6-15 μm, and D90 is ≤60 μm. The mixing frequency of the mixer is controlled at 35±2 Hz, and the mixing time can be 1-2 hours.

[0057] In this embodiment, the high iron-phosphorus ratio hydroxy 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=15-20m 2 / g; The low iron-phosphorus ratio hydroxyferric phosphate precursor 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=5-10m 2 / g.

[0058] Step S6: mixing hydroxy ferric phosphate with a high iron-phosphorus ratio and a high specific surface area with hydroxy ferric phosphate with a low iron-phosphorus ratio and a low specific surface area in a certain proportion, and then proportioning them with a lithium source, a manganese source and a phosphorus source in a certain proportion, and adding a certain amount of carbon source and additives to form a mixture;

[0059] The manganese source is one or more of elemental manganese, manganese monoxide, manganese trimanganese tetroxide, manganese dioxide, manganese hydroxide, manganese carbonate, and ferromanganous phosphate; 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.

[0060] In this embodiment, the molar ratio of the high iron-phosphorus ratio and high specific surface area ferric hydroxyphosphate to the low iron-phosphorus ratio and low specific surface area ferric hydroxyphosphate is between 2:8 and 8:2. Preferably, the molar ratio of the high iron-phosphorus ratio and high specific surface area ferric hydroxyphosphate to the low iron-phosphorus ratio and low specific surface area ferric hydroxyphosphate is 3:7. In addition, the lithium source, the iron source, the manganese source, and the phosphorus source are added to the mixture 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].

[0061] The amount of carbon source added is based on a carbon content of 1.2% to 1.6% in the final product. 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.

[0062] Step S7: sand-grinding the mixture to obtain a nano-sized mixture; spray-drying the nano-sized mixture to obtain a spray material;

[0063] The sanding particle size of the sanding slurry is controlled to be between 0.30-0.75 μm. During spray drying, the air inlet temperature can be 200-220°C, the air outlet temperature can be 80-110°C, the blast frequency can be 80 Hz, and the spray particle size of the final spray material is controlled to be between D50=20-40 μm.

[0064] Step S8: placing the sprayed material in a box furnace to sinter to obtain a sintered material, and crushing the sintered material by a jet mill to obtain a pulverized material;

[0065] During the sintering process, the sintering atmosphere is nitrogen, the sintering temperature is 750-780°C, the heating rate is 3°C / min, the sintering time is 8-12 hours, and then the sintered material is obtained through natural cooling. During the crushing process, the air pressure is controlled between 0.2-0.4 MPa, the grading frequency is 80-200 Hz, and the particle size of the final crushed material meets the following requirements: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, and D100<30μm.

[0066] Step S9: The crushed material is further subjected to screening, batching, packaging and other processes to obtain the finished product of lithium manganese iron phosphate.

[0067] The method for preparing lithium iron manganese phosphate from hydroxy iron phosphate provided in the embodiment of the first scheme of the present invention produces a hydroxy iron phosphate product with low impurity content and high purity, and the iron-phosphorus ratio and specific surface area of ​​the generated hydroxy iron phosphate are adjustable, and hydroxy iron phosphate with different iron-phosphorus ratios can be generated as needed. Among them, the high iron-phosphorus ratio hydroxy iron phosphate particles are smaller, which can increase the discharge capacity of the material; the low iron-phosphorus ratio hydroxy iron phosphate particles are larger, which can increase the compaction density of the material. Titanium is in situ doped into the hydroxy iron phosphate precursor, and the doping element is evenly distributed, and the doping effect is better. This method uses a soluble divalent iron salt as an iron source, which reacts with phosphate to produce hydroxy iron phosphate, and then a simple water washing process is used to reduce the amount of water washing; after hydroxy iron phosphate is fully sand-milled, sprayed, sintered, and crushed with other manganese sources and lithium sources, it is beneficial to synthesize a lithium iron manganese phosphate material with a uniform manganese-iron ratio, which is beneficial to improving the compaction density, electrical conductivity and capacity of the material. This method pre-dopes hydroxy ferric phosphate in the solution phase. During the reaction, the iron element and the doping element form a precipitate containing the doping element, which can achieve pre-mixing of the doping element and the iron element, thereby avoiding local agglomeration of the doping element and improving the doping effect; pre-doping hydroxy ferric phosphate is conducive to the doping element entering the crystal lattice of manganese iron phosphate, further improving its electrochemical performance. The raw materials used in this method are all common inorganic chemical raw materials, and the market price is low and stable. The preparation of lithium manganese iron phosphate materials with these raw materials can reduce the cost of raw materials. This method uses hydroxy ferric phosphate as an iron source and exists in an amorphous form. Compared with crystalline iron source, manganese source, and lithium source, in the process of synthesizing lithium manganese iron phosphate, only a lower reaction temperature is required, which is more conducive to the formation of lithium manganese iron phosphate.

[0068] The second embodiment of the present invention provides a method for preparing lithium iron manganese phosphate from hydroxy iron phosphate, which is used to prepare a lithium iron manganese phosphate material with high compaction density and high capacity.

[0069] As shown in Figure 3, in this embodiment, step S6 includes: ball-milling ferromanganese phosphate and phosphoric acid in a certain ratio, then adding lithium phosphate, and wet-grinding to obtain a nano-sized sand-milling slurry, and adding a certain amount of hydroxyferric phosphate with a high iron-phosphorus ratio and a high specific surface area, hydroxyferric phosphate with a low iron-phosphorus ratio and a low specific surface area, lithium phosphate, a carbon source, and an additive to the above-mentioned sand-milling slurry to obtain a mixture.

[0070] Except for step S6, which is different from the first embodiment, the remaining steps are the same as those in the first embodiment, so they are not repeated here. As shown in FIG4 , step S6 further includes:

[0071] Step S61: After the iron source, manganese source, and phosphoric acid are stirred and mixed, an antioxidant and a phosphorus source are added, and the pH value of the solution is controlled to be between 3.0 and 4.0 to obtain a mixed solution;

[0072] The molar ratio of the added manganese source to the iron source is between 8:2 and 2:8, the molar ratio of the sum of the molar amounts of the added manganese source and the iron source to the phosphorus source is [1.41-1.43]:1, and the amount of the antioxidant added is 1% of the mass fraction of the iron source.

[0073] In this embodiment, the iron source may be one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride; the manganese source may be one or more of manganous sulfate, manganous nitrate, manganous chloride, and manganous acetate; the phosphorus source may be one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; and the antioxidant may be ascorbic acid.

[0074] Step S62: Adding a 16-20% ammonia aqueous solution A to a reactor, and simultaneously adding the mixed solution and an 8-10% ammonia aqueous solution B dropwise under an inert gas atmosphere, controlling the pH value to be between 7.5 and 8.0, and the reaction temperature to be between 25 and 40° C. After the addition is completed, aging is carried out for 1 hour to obtain a ferromanganese phosphate precursor by coprecipitation;

[0075] The inert gas may be one or more of nitrogen, argon, and helium, and the addition time is 0.5-6 hours.

[0076] Step S63: filtering, washing, flash drying, high-temperature sintering, crushing and mixing the ferromanganese phosphate precursor to obtain a ferromanganese phosphate product;

[0077] Step S64: ball-milling ferromanganese phosphate and phosphoric acid according to a certain ratio, and then adding lithium phosphate and wet-grinding to obtain a nano-sized sand-milled slurry;

[0078] Step S65: adding a certain amount of hydroxy ferric phosphate with high iron-phosphorus ratio and high specific surface area, hydroxy ferric phosphate with low iron-phosphorus ratio and low specific surface area, lithium phosphate, a carbon source and additives to the above-mentioned sand-milling slurry to obtain a mixture.

[0079] The molar ratio of Fe:Mn in ferromanganese phosphate is between 2:8 and 8:2, the amount of phosphoric acid added is 0.5%-5% of the mass fraction of ferromanganese phosphate, and in the sand grinding slurry, the molar ratio of Li:Fe:Mn:P is [1.03-1.04]: [0.2-0.8]: [0.2-0.8]: [1.03-1.04], and the sanding particle size in the sand grinding slurry is controlled between 0.60-0.80 μm.

[0080] In this embodiment, the molar ratio of Fe:Mn in ferromanganese phosphate is between 2:8 and 8:2. Preferably, the molar ratio of Fe:Mn in ferromanganese phosphate is between 3:7 and 7:3.

[0081] The method for preparing lithium manganese iron phosphate from hydroxy iron phosphate provided in the second embodiment of the present invention can provide a phosphorus source and utilize NH 4+ The coordination effect of Mn 2+ 、Fe 2+ The precipitation rate in the phosphate system is of the same order of magnitude, and the manganese ferrophosphate finally obtained has a dense secondary spherical particle structure, and the content of the three elements Mn, Fe, and P therein is uniform and the ratio is stable. In this method, hydroxy ferrophosphate is used as the iron source to provide a large-particle lithium manganese iron phosphate material, which is beneficial to improving the compaction density, electrical conductivity and capacity of the material; ferromanganese phosphate is used as the iron source and manganese source to synthesize the lithium manganese iron phosphate material, which is beneficial to improving the discharge voltage of the material. In addition, this method can ensure that in the sand-milling slurry, the sand-milling time of the manganese ferrophosphate precursor is much greater than the sand-milling time of hydroxy ferrophosphate, ensuring that the manganese ferrophosphate particles are smaller than hydroxy ferrophosphate, so that the subsequently formed lithium manganese iron phosphate has a better particle grading, and further improves the compaction performance of the material on the basis of ensuring the electrochemical performance.

[0082] The specific process and effects of the method for preparing lithium manganese iron phosphate using hydroxy ferric phosphate of the present invention are further described in detail below in conjunction with some specific examples, but are not limited to the scope of protection of the present invention.

[0083] Example 1

[0084] This embodiment provides a method for preparing lithium manganese iron phosphate from hydroxy iron phosphate, comprising the following steps:

[0085] Step S1: adding 4‰ mass fraction of phosphoric acid and 5‰ mass fraction of sodium hydroxide solution to purify the ferrous sulfate produced as a byproduct of titanium dioxide, and filtering and purifying to obtain a ferrous sulfate solution;

[0086] Step S2: adding phosphoric acid to the ferrous sulfate solution according to a molar ratio of n(Fe):n(phosphoric acid)=1:0.15 to reduce the pH value of the ferrous sulfate solution;

[0087] Step S3: adding an excess of 40% hydrogen peroxide to the ferrous sulfate solution, then adding a phosphoric acid solution and a 30% concentration of ammonium dihydrogen phosphate solution to the ferrous sulfate solution so that the iron-phosphorus ratio in the mixed slurry successively satisfies the iron-phosphorus molar ratio: Fe / P=1.490 and Fe / P=1.460, then adding a 100-fold diluted titanium sulfate solution to the ferrous sulfate solution so that the doped titanium content satisfies 300 ppm, and then adding ammonia water to form a mixed slurry, heating the mixed slurry to 60° C., holding it for 3 hours, and then washing and filtering it multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios;

[0088] Step S4: flash drying the hydroxyferric phosphate precursor in a flash evaporator, controlling the air inlet temperature of the flash evaporator to 200° C., and sintering in an air atmosphere at high temperatures of 535° C. and 560° C. for 5 hours;

[0089] Step S5: The sintered material is crushed by a mechanical mill to control the particle size D10 ≥ 1.0 μm, D50: 6-15 μm, and D90 ≤ 60 μm, and mixed by a ribbon mixer at a frequency of 35 Hz for 1 hour to obtain hydroxy iron phosphate with a high iron-phosphorus ratio and a high specific surface area and hydroxy iron phosphate with a low iron-phosphorus ratio and a low specific surface area;

[0090] The SEM spectrum of the hydroxyferric phosphate with high iron-phosphorus ratio and high specific surface area prepared according to Example 1 is shown in FIG5 .

[0091] The XRD spectrum of the hydroxyferric phosphate with high iron-phosphorus ratio and high specific surface area prepared according to Example 1 is shown in FIG7 .

[0092] Step S6: mixing hydroxyferric phosphate with a high iron-phosphorus ratio and a high specific surface area with hydroxyferric phosphate with a low iron-phosphorus ratio and a low specific surface area in a ratio of 3:7, and then mixing 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, and adding a carbon source mixture consisting of sucrose and polyethylene glycol to achieve a carbon content of 1.35% in the finished product, and a doping amount of 1800 ppm of magnesium acetate and 800 ppm of ammonium metavanadate to form a mixture;

[0093] Step S7: sand-milling the mixture to a particle size of 0.62 μm to obtain a nano-sized sand-milled slurry; spray-drying the nano-sized sand-milled slurry to obtain a spray material with a particle size D50 of 20-40 μm by controlling the air inlet temperature to 220° C., the air outlet temperature to 100° C., and the air frequency to 80 Hz;

[0094] Step S8: placing the spray material in a box furnace and sintering it under a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 765°C, and a sintering time of 10 hours, and then cooling it naturally to obtain a sintered material. The sintered material is crushed by a jet mill with a controlled air pressure of 0.3 MPa and a classification frequency of 130 Hz to obtain a crushed material with a particle size of D10>0.35 μm, D50=0.8-1.8 μm, D90<10 μm, and D100<30 μm;

[0095] Step S9: The crushed material is further subjected to screening, batching, packaging and other processes to obtain the finished product of lithium manganese iron phosphate.

[0096] The SEM spectrum of the lithium manganese iron phosphate positive electrode material prepared according to Example 1 is shown in FIG6 .

[0097] The XRD spectrum of the lithium manganese iron phosphate positive electrode material prepared according to Example 1 is shown in FIG8 .

[0098] The EDS of the lithium iron manganese phosphate cathode material prepared in Example 1 was tested, and the test results are shown in Table 1. The EDS element content test spectrum of the lithium iron manganese phosphate cathode material prepared in Example 1 is shown in FIG9 , and the EDS surface scan of the lithium iron manganese phosphate cathode material prepared in Example 1 is shown in FIG10 .

[0099] Table 1. Test items and test results of Example 1

[0100] The test results obtained from the finished product of lithium manganese iron phosphate prepared in Example 1 above indicate that the ratio of n(Mn):n(Fe) in the lithium manganese iron phosphate prepared in Example 1 is 0.6:0.4, indicating a precise elemental ratio. Furthermore, the synthesized individual particles are not lithium iron phosphate or lithium manganese phosphate, but rather lithium manganese iron phosphate. Furthermore, Figure 10 shows that the three elements Mn, Fe, and P are evenly distributed in the lithium manganese iron phosphate, and the doping elements V and Mg are evenly distributed throughout the lithium manganese iron phosphate material.

[0101] Example 2

[0102] This embodiment provides a method for preparing lithium manganese iron phosphate from hydroxy iron phosphate, comprising the following steps:

[0103] Step S1: adding 3‰ mass fraction of phosphoric acid and 5‰ mass fraction of sodium hydroxide solution to purify the ferrous sulfate produced as a byproduct of titanium dioxide, and filtering and purifying to obtain a ferrous sulfate solution;

[0104] Step S2: adding phosphoric acid to the ferrous sulfate solution according to a molar ratio of n(Fe):n(phosphoric acid)=1:0.15 to reduce the pH value of the ferrous sulfate solution;

[0105] Step S3: adding an excess of 50% hydrogen peroxide to the ferrous sulfate solution, adding a phosphoric acid solution and a 30% concentration of ammonium dihydrogen phosphate solution to the ferrous sulfate solution so that the iron-phosphorus ratio in the mixed slurry successively satisfies the iron-phosphorus molar ratio: Fe / P=1.490 and Fe / P=1.460, then adding a 100-fold diluted titanium sulfate solution to the ferrous sulfate solution so that the doped titanium content satisfies 300 ppm, and then adding ammonia water to form a mixed slurry, heating the mixed slurry to 60° C., holding it for 3 hours, and then washing and filtering it multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios;

[0106] Step S4: flash drying the hydroxyferric phosphate precursor in a flash evaporator, controlling the air inlet temperature of the flash evaporator to 200° C., and sintering in an air atmosphere at high temperatures of 545° C. and 560° C. for 4 hours;

[0107] Step S5: The sintered material is crushed by a mechanical mill to control the particle size D10 ≥ 1.0 μm, D50: 6-15 μm, and D90 ≤ 60 μm, and mixed by a ribbon mixer at a frequency of 35 Hz for 1 hour to obtain hydroxy iron phosphate with a high iron-phosphorus ratio and a high specific surface area and hydroxy iron phosphate with a low iron-phosphorus ratio and a low specific surface area;

[0108] Step S6: mixing ferric hydroxyphosphate having a high iron-phosphorus ratio and a high specific surface area with ferric hydroxyphosphate having a low iron-phosphorus ratio and a low specific surface area in a ratio of 4:6, and then mixing the mixture with manganese tetraoxide, lithium hydroxide, and diammonium hydrogen phosphate in a molar ratio of Li:Fe:Mn:P=1.03:0.5:0.5:1.03, and adding a carbon source mixture consisting of glucose and polyethylene glycol so that the carbon content of the finished product is 1.45% and ammonium metavanadate doped in an amount of 3000 ppm to form a mixture;

[0109] Step S7: sand-grinding the mixture to a particle size of 0.55 μm to obtain a nano-sized mixture; spray-drying the nano-sized mixture to obtain a spray material with a particle size D50 of 20-40 μm by controlling the air inlet temperature to 220° C., the air outlet temperature to 100° C., and the air frequency to 80 Hz;

[0110] Step S8: placing the spray material in a box furnace and sintering it under a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 770°C, and a sintering time of 8 hours, and then cooling it naturally to obtain a sintered material. The sintered material is crushed by a jet mill at a controlled air pressure of 0.3 MPa and a classification frequency of 130 Hz to obtain a crushed material with a particle size of D10>0.35 μm, D50=0.8-1.8 μm, D90<10 μm, and D100<30 μm;

[0111] Step S9: The crushed material is further subjected to screening, batching, packaging and other processes to obtain the finished product of lithium manganese iron phosphate.

[0112] Example 3

[0113] This embodiment provides a method for preparing lithium manganese iron phosphate from hydroxy iron phosphate, comprising the following steps:

[0114] Step S1: adding 4‰ mass fraction of phosphoric acid and 5‰ mass fraction of sodium hydroxide solution to purify the ferrous sulfate produced as a byproduct of titanium dioxide, and filtering and purifying to obtain a ferrous sulfate solution;

[0115] Step S2: adding phosphoric acid to the ferrous sulfate solution according to a molar ratio of n(Fe):n(phosphoric acid)=1:0.15 to reduce the pH value of the ferrous sulfate solution;

[0116] Step S3: adding an excess of 40% hydrogen peroxide to the ferrous sulfate solution, then adding a phosphoric acid solution and a 30% concentration of ammonium dihydrogen phosphate solution to the ferrous sulfate solution so that the iron-phosphorus ratio in the mixed slurry successively satisfies the iron-phosphorus molar ratio: Fe / P=1.490 and Fe / P=1.460, then adding a 100-fold diluted titanium sulfate solution to the ferrous sulfate solution so that the doped titanium content satisfies 300 ppm, and then adding ammonia water to the ferrous sulfate solution to form a mixed slurry, heating the mixed slurry to 60° C., holding it for 3 hours, and then washing and filtering it multiple times to form hydroxy iron phosphate precursors with different iron-phosphorus ratios;

[0117] Step S4: flash drying the hydroxyferric phosphate precursor in a flash evaporator, controlling the air inlet temperature of the flash evaporator to 200° C., and sintering in an air atmosphere at high temperatures of 535° C. and 560° C. for 5 hours;

[0118] Step S5: The sintered material is crushed by a mechanical mill to control the particle size D10 ≥ 1.0 μm, D50: 6-15 μm, and D90 ≤ 60 μm, and mixed by a ribbon mixer at a frequency of 35 Hz for 1 hour to obtain hydroxy iron phosphate with a high iron-phosphorus ratio and a high specific surface area and hydroxy iron phosphate with a low iron-phosphorus ratio and a low specific surface area;

[0119] Step S6: adding 85% phosphoric acid to the ferromanganese phosphate precursor at a mass ratio of 1:200, ball milling for 60 minutes, and then adding lithium phosphate for wet grinding to obtain a nano-sized sand-milled slurry, wherein the molar ratio of Fe:Mn in the ferromanganese phosphate is 3:7, and the sand-milled slurry is proportioned according to a molar ratio of Li:Fe:Mn:P=1.03:0.3:0.7:1.03; controlling the sand-milled particle size D50=0.60-0.62 μm; the sand-milled slurry is proportioned with hydroxyferric phosphate with a high iron-phosphorus ratio and high specific surface area, hydroxyferric phosphate with a low iron-phosphorus ratio and low specific surface area, and lithium phosphate at a molar ratio of Li:Fe:Mn:P=1.03:0.4:0.6:1.03, and adding a carbon source mixture consisting of sucrose and polyethylene glycol to make the carbon content of the finished product 1.35% and ammonium metavanadate with a doping amount of 2200 ppm to obtain a mixture;

[0120] Step S7: sand-grinding the mixture to a particle size of 0.50 μm to obtain a nano-sized mixture; spray-drying the nano-sized mixture to obtain a spray material with a particle size of D50 of 20-40 μm by controlling the air inlet temperature to 220° C., the air outlet temperature to 100° C., and the air frequency to 80 Hz;

[0121] Step S8: placing the spray material in a box furnace and sintering it under a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 765°C, and a sintering time of 10 hours, and then cooling it naturally to obtain a sintered material. The sintered material is crushed by a jet mill with a controlled air pressure of 0.3 MPa and a classification frequency of 130 Hz to obtain a crushed material with a particle size of D10>0.35 μm, D50=0.8-1.8 μm, D90<10 μm, and D100<30 μm;

[0122] Step S9: The crushed material is further subjected to screening, batching, packaging and other processes to obtain the finished product of lithium manganese iron phosphate.

[0123] The SEM spectrum of the lithium manganese iron phosphate positive electrode material prepared according to Example 3 is shown in FIG11 .

[0124] The XRD spectrum of the lithium manganese iron phosphate positive electrode material prepared according to Example 3 is shown in FIG12 .

[0125] Comparative Example 1

[0126] This embodiment provides a method for preparing lithium manganese iron phosphate, comprising the following steps:

[0127] Step S1: Iron oxide is mixed 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, and a carbon source mixture consisting of sucrose and polyethylene glycol with a carbon content of 1.10% and titanium dioxide doped in an amount of 3000 ppm are added to form a mixture;

[0128] Step S2: sand-milling the mixture to a particle size of 0.65 μm to obtain a nano-sized sand-milled slurry; spray-drying the nano-sized sand-milled slurry to obtain a spray material with a particle size D50 of 20-40 μm by controlling the air inlet temperature to 220° C., the air outlet temperature to 100° C., and the air frequency to 80 Hz;

[0129] Step S3: placing the spray material in a box furnace and sintering it under a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 765°C, and a sintering time of 10 hours, and then cooling it naturally to obtain a sintered material. The sintered material is crushed by a jet mill at a controlled air pressure of 0.35 MPa and a classification frequency of 140 Hz to obtain a crushed material with a particle size of D10>0.35 μm, D50=0.8-1.8 μm, D90<10 μm, and D100<30 μm;

[0130] Step S4: The crushed material is further subjected to screening, batching, packaging and other processes to obtain the finished product of lithium manganese iron phosphate.

[0131] The lithium manganese iron phosphate positive electrode material prepared in Examples 1-3 and Comparative Example 1 was dispersed in N-methylpyrrolidone at a mass ratio of 90:5:5 with the conductive agent carbon black and the binder polyvinylidene fluoride. After ball milling to disperse uniformly, the material was coated on aluminum foil and vacuum dried to produce a positive electrode sheet. The electrolyte was 1 mol / L LiPF6, with the solvent volume ratio of EC:DMC:EMC = 1:1:1. The separator was Celgard polypropylene film, and the metal lithium sheet served as the negative electrode. The cells were assembled into a button-type half-cell. The test voltage range was 2.5V-4.5V. The cells were charged to 4.5V using a constant current constant voltage charging method and discharged to 2.5V using a constant current discharge method. The charge and 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. Among them, the charge and discharge curves (0.1C), (0.2C), and (1C) of the button-type half-cell assembled with the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention are shown in Figures 13, 14, and 15, respectively. The charge and discharge curves (0.1C), (0.2C), and (1C) of the button-type half-cell assembled with the lithium manganese iron phosphate positive electrode material prepared in Example 3 of the present invention are shown in Figures 16, 17, and 18, respectively.

[0132] Table 2. Test items and test results of Examples 1-3 and Comparative Example 1

[0133] Based on the above examples and control examples, as well as the test results obtained from these examples, the coin-type half-cells prepared with the lithium manganese iron phosphate cathode materials in Examples 1-3 exhibited significantly improved discharge capacities at 0.1C, 0.2C, and 1C compared to Comparative Example 1. In summary, the coin-type half-cells assembled with the lithium manganese iron phosphate cathode materials prepared by this method exhibited excellent stability and electrochemical performance.

[0134] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the claims and their equivalents.

Claims

1. A method for preparing lithium iron manganese phosphate from hydroxyferric phosphate, characterized in that: The method comprises: Step S1: adding a phosphorus source and a precipitant to purify ferrous sulfate, a byproduct of titanium dioxide, and filtering and purifying to obtain a ferrous sulfate solution; Step S2: adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to reduce the pH value of the ferrous sulfate solution; Step S3: adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, titanium sulfate and ammonia water to the ferrous sulfate solution in sequence and reacting for a period of time to form a mixed slurry, heating and keeping the mixed slurry warm for a period of time, and then washing and filtering for multiple times to form hydroxyferric phosphate precursors with different iron-phosphorus ratios; Step S4: flash drying the hydroxyferric phosphate precursor in a flash evaporator, and sintering at high temperature for a certain period of time to obtain hydroxyferric phosphate precursor finished products with different iron-phosphorus ratios and different specific surface areas; Step S5: crushing the sintered material with a mechanical mill and mixing it with a ribbon mixer to obtain finished hydroxy iron phosphate products with different iron-phosphorus ratios and different specific surface areas; Step S6: mixing hydroxy ferric phosphate with a high iron-phosphorus ratio and a high specific surface area with hydroxy ferric phosphate with a low iron-phosphorus ratio and a low specific surface area in a certain proportion, blending with a lithium source, a manganese source and a phosphorus source in a certain proportion, and adding a certain amount of a carbon source and additives to form a mixture; Step S7: sand-grinding the mixture to obtain a nano-sized mixture; spray-drying the nano-sized mixture to obtain a spray material; Step S8: placing the spray material in a box furnace to sinter to obtain a sintered material, and crushing the sintered material by a jet mill to obtain a crushed material; Step S9: The above crushed materials are further subjected to screening, batching, packaging and other processes to obtain the finished lithium manganese iron phosphate product.

2. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 1, characterized in that: In the step S1, according to the mass ratio, the ferrous sulfate: the phosphorus source: the precipitant = 1: [0.001-0.005]: [0.005-0.007], the purification reaction temperature is 40° C., the reaction pH value 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; in the step S2, the amount of the phosphoric acid added is according to the molar ratio of n(Fe): n(phosphoric acid) = 1: 0.15; in the step S3, when the iron-phosphorus feed ratio in the mixed slurry satisfies the iron-phosphorus molar ratio: Fe / P = 1.475-1.490, a high iron-phosphorus ratio hydroxy ferric phosphate can be formed; when the iron-phosphorus feed ratio in the mixed slurry satisfies the iron-phosphorus molar ratio: Fe / P = 1.460-1.475, a low iron-phosphorus ratio hydroxy ferric phosphate can be generated.

3. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 1, characterized in that: In step S3, the water washing times can be multiple times. The first water washing mainly washes away the impurities of magnesium, manganese and sulfur. In the last water washing, 1:1 diluted ammonia water is added to adjust the pH value to 6.5-7.0 to wash away SO4 2- ions; the concentration of the hydrogen peroxide is 30%-60%, and the mixed slurry is heated to 60-80°C and the insulation time is 3h.

4. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 1, characterized in that: The step S3 comprises: Step S31: adding an excess of hydrogen peroxide to the ferrous sulfate solution and continuing oxidation for a certain period of time; Step S32: adding phosphoric acid solution to the oxidized ferrous sulfate solution, then dissolving ammonium dihydrogen phosphate powder in water to prepare a 30% concentration of ammonium dihydrogen phosphate solution at a dissolution temperature of 30-40°C, and adding the solution to the oxidized ferrous sulfate solution, then diluting titanium sulfate by 100 times and adding the solution to the oxidized ferrous sulfate solution; Step S33: adding ammonia water to the ferrous sulfate solution, adjusting 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 warm for a period of time, and then washing and filtering for multiple times to form hydroxyferric phosphate precursors with different iron-phosphorus ratios.

5. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 1, characterized in that: In the step S4, the air inlet temperature of the flash evaporator is controlled to be 220±20°C, the air outlet temperature is 110±5°C, the sintering atmosphere is air, the sintering temperature is 535-560°C, and the sintering time is 4-5h; in the step S5, the particle size D10 is controlled to be ≥1.0μm, D50: 6-15μm, D90≤60μm, the mixing frequency of the mixer is controlled to be 35±2Hz, and the mixing time is 1-2h; the high iron-phosphorus ratio hydroxy 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=15-20m 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=5-10m 2 / g.

6. The method for preparing lithium iron manganese phosphate from hydroxy iron phosphate according to claim 1, characterized in that: In the step S6, the manganese source is one or more of elemental manganese, manganese monoxide, manganese tetraoxide, manganese dioxide, manganese hydroxide, manganese carbonate, and ferromanganous phosphate; 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 hydroxy ferric phosphate with a high iron-phosphorus ratio and a high specific surface area to the hydroxy ferric phosphate with a low iron-phosphorus ratio and a low specific surface area is between 2:8 and 8:2; the lithium source, the iron source, and the The amount of manganese source and the phosphorus source added is according to the molar ratio of 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%, and the carbon source is one or more of sucrose, glucose, citric acid, starch, and polyethylene glycol; the additive is selected from one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300-3000ppm.

7. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 1, characterized in that: In the step S7, the sanding particle size in the sanding slurry is controlled between 0.30-0.75 μm. In the spray drying, the air inlet temperature is 200-220°C, the air outlet temperature is 80-110°C, the blast frequency is 80 Hz, and the spray particle size in the spray material is controlled between D50=20-40 μm; in the step S8, the sintering atmosphere is nitrogen, the sintering temperature is 750-780°C, the heating rate is 3°C / min, the sintering time is 8-12h, and then the sintering material is obtained by natural cooling. During the crushing process, the air pressure is controlled between 0.2-0.4Mpa, the classification frequency is 80-200Hz, and the particle size of the crushed material satisfies: D10>0.35μm, D50=0.7-2.0μm, D90<10μm, D100<30μm.

8. The method for preparing lithium iron manganese phosphate from hydroxy iron phosphate according to claim 1, characterized in that: The step S6 comprises: Step S61: After the iron source, manganese source and phosphoric acid are stirred and mixed, an antioxidant and a phosphorus source are added, and the pH value of the solution is controlled to be between 3.0 and 4.0 to obtain a mixed solution; Step S62: adding a 16-20% ammonia solution A into a reaction kettle, and simultaneously adding a mixed solution and a 8-10% ammonia solution B dropwise under an inert gas atmosphere, controlling the pH value to be between 7.5 and 8.0, and the reaction temperature to be 25-40° C., aging for 1 hour after the addition is completed, and co-precipitating to prepare a ferromanganese phosphate precursor; Step S63: filtering, washing, flash drying, high-temperature sintering, crushing and mixing the ferromanganese phosphate precursor to obtain a ferromanganese phosphate product; Step S64: ball-milling ferromanganese phosphate and phosphoric acid according to a certain ratio, and then adding lithium phosphate and wet-grinding to obtain a nano-sized sand-milled slurry; Step S65: Add a certain amount of hydroxy ferric phosphate with high iron-phosphorus ratio and high specific surface area, hydroxy ferric phosphate with low iron-phosphorus ratio and low specific surface area, lithium phosphate, a carbon source and additives to the above-mentioned sand-milled slurry to obtain a mixture.

9. The method for preparing lithium iron manganese phosphate from hydroxyferric phosphate according to claim 8, characterized in that: In the step S61, the molar ratio of the added manganese source to the iron source is between 8:2 and 2:8, the molar ratio of the sum of the molar amounts of the added manganese source and the iron source to the phosphorus source is [1.41-1.43]:1, the amount of the antioxidant added is 1% of the mass fraction of the iron source, the iron source is one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride, the manganese source is one or more of manganous sulfate, manganous nitrate, manganous chloride, and manganous acetate, and the phosphorus source is one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. One or more of, the antioxidant is ascorbic acid; in the step S64, the molar ratio of Fe:Mn in the ferromanganese phosphate is between 2:8 and 8:2, the amount of phosphoric acid added is between 0.5%-5% of the mass fraction of ferromanganese phosphate, and in the sand grinding slurry, according to the molar ratio, Li:Fe:Mn:P=[1.03-1.04]:[0.1-0.9]:[0.1-0.9]:[1.03-1.04], and the sand grinding particle size in the sand grinding slurry is controlled between 0.60-0.80μm.

10. A positive electrode material for a lithium ion battery, characterized in that: The lithium-ion battery positive electrode material uses a method for preparing lithium iron manganese phosphate using hydroxy iron phosphate as described in any one of claims 1 to 9.

11. A lithium ion battery, characterized in that: The lithium-ion battery comprises the lithium-ion battery positive electrode material as claimed in claim 10.

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