Method for preparing lithium manganese iron phosphate by using ferrophosphite and application thereof

Abstract

The application provides a method for preparing lithium manganese iron phosphate by using ferrous manganese phosphate composite ferrous phosphate, sequentially adding an iron source, a phosphorus source, an antioxidant and ammonia water into a reaction kettle, aging after the addition is completed, forming a mixed slurry; adding the mixed slurry into the reaction kettle and heating, then adding a lithium source, a phosphorus source and an alkaline solution, filtering and washing after the reaction to obtain a filter cake; mixing the iron source, the manganese source and the phosphorus source after stirring, then adding the antioxidant to obtain a mixed solution; putting the filter cake into the reaction kettle, adding the antioxidant, then simultaneously adding the mixed solution and an ammonia water solution dropwise, aging after the addition is completed, then washing with water and pressure filtering to obtain a ferrous manganese phosphate composite ferrous phosphate precursor; mixing the ferrous manganese phosphate composite ferrous phosphate precursor, nano-treated lithium phosphate, an organic carbon source, an additive and deionized water to form a mixture, and then performing processes of ball milling, drying, sintering, screening, batching and packaging to obtain a lithium manganese iron phosphate product.

Description

Technical Field

[0001] This invention relates to the technical field of lithium-ion battery cathode material preparation methods, and particularly to a method for preparing lithium manganese iron phosphate using iron manganese phosphate composite 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.

[0004] The basic approach to preparing lithium manganese iron phosphate (LFP) involves directly mixing lithium, manganese, iron, phosphorus, and carbon sources, along with compounds containing dopant elements, followed by grinding, drying, sintering, and pulverizing processes to obtain a doped LFP-carbon composite material. However, this method produces LFP materials with small particle sizes and an unbalanced particle size distribution, making it difficult to improve their compaction density. Summary of the Invention

[0005] In view of the above, the present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite and its application, wherein the lithium manganese iron phosphate has higher compaction density, higher capacity, and better cycle performance.

[0006] Therefore, in a first aspect, embodiments of the present invention provide a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite ferrous phosphate, the method comprising: adding an iron source, a phosphorus source, an antioxidant, and ammonia water sequentially to a reaction vessel under an inert gas atmosphere; aging the mixture for a period of time after the addition is complete to form a mixed slurry; adding the mixed slurry to the reaction vessel and heating it; then adding a lithium source and a phosphorus source in a certain proportion; adding an alkaline solution; filtering and washing after the reaction to obtain a filter cake; and adding an antioxidant after stirring and mixing the iron source, manganese source, and phosphorus source, controlling the pH value of the solution to be between 3.0 and 4.0. A mixed solution is obtained; the filter cake is put into a reaction vessel, and an antioxidant is added under an inert gas atmosphere. Then, the mixed solution and an ammonia solution are added dropwise. After the addition is completed, the mixture is aged for a period of time, washed with water and filtered to obtain a manganese iron phosphate composite ferrous phosphate precursor; the manganese iron phosphate composite ferrous phosphate precursor, nano-treated lithium phosphate, organic carbon source, additives and deionized water are mixed to form a mixture. After ball milling, drying and sintering, a sintered material is obtained; the above sintered material is further processed through screening, batching and packaging to obtain the finished lithium manganese iron phosphate.

[0007] Preferably, the inert gas is nitrogen, the pH of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25-40℃, and the aging time is 1 hour; the iron source can be one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride; the phosphorus source can be one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the antioxidant can be ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the amount of iron source added; the molar ratio of the added iron source to phosphorus source satisfies n(Fe) / n(P) = 1.45-1.46.

[0008] Preferably, the heating temperature is 75-85℃, and the molar ratio of the added lithium source to phosphorus source satisfies: n(Li) / n(P) = 3-3.1; the lithium source can be one or more of lithium hydroxide, lithium oxalate, and lithium acetate, and the phosphorus source can be one or more of phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the alkaline solution is ammonia water, and the pH value of the solution is adjusted to 10-13; the washing can be repeated multiple times, wherein the first washing mainly removes impurities such as magnesium and sulfur, and the last washing adds 1:1 diluted ammonia water to adjust the pH value to 6.5-7.0 to remove SO4. 2- Ions; the molar ratio of each element in the filter cake satisfies n(Li):n(Fe):n(P)=[1.03-1.04]:1:[1.03-1.04].

[0009] Preferably, in the mixed solution, the molar ratio of the added manganese source to the iron source is between 8:2 and 2:8, and the molar ratio of the sum of the added manganese source and the iron source to the phosphorus source satisfies: n(Fe+Mn) / n(P)=1.41-1.43.

[0010] Preferably, 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 manganese sulfate, manganese nitrate, manganese chloride, and manganese acetate; the phosphorus source is one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the antioxidant is ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the iron source.

[0011] Preferably, the inert gas is nitrogen, the antioxidant is ascorbic acid, and the ammonia solution concentration is 8-10%. When adding the mixed solution and the ammonia solution, the pH of the solution in the reactor is maintained at 7.5-8.0, the reaction temperature is 25-40℃, and the aging time is 1 hour. The water washing can be repeated multiple times. The first water wash mainly removes impurities such as magnesium 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- ion.

[0012] Preferably, the particle size of the nano-treated lithium phosphate satisfies D50 < 50 nm. In the mixture, the molar ratio is n(Li):n(Fe):n(Mn):n(P) = [1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04]. The organic carbon source is one or more of glucose, polyethylene glycol, sucrose, starch, and citric acid. The amount of carbon source added is based on the carbon content in the final product being between 1.2% and 1.6%. The additives are one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300 and 3000 ppm.

[0013] Preferably, in the ball milling process, the ball milling particle size in the mixture is controlled to be between 5-10 μm; in the drying process, the drying method is spray drying, the drying atmosphere is nitrogen, the inlet air temperature is 200-220℃, the outlet air temperature is 80-110℃, the blowing frequency is 80Hz, and the spray particle size in the spray material is controlled to be between D50=20-40μm; in 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.

[0014] Secondly, embodiments of the present invention provide a lithium-ion battery cathode material obtained by processing lithium iron phosphate after applying the method for preparing lithium manganese iron phosphate using the iron manganese phosphate composite iron phosphate provided in the first aspect above.

[0015] Thirdly, embodiments of the present invention provide a lithium-ion battery, including the lithium-ion battery cathode material described in the second aspect above.

[0016] The method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite provided in this invention ultimately synthesizes solid lithium manganese iron phosphate material with a particle size of 1-25 μm, which can significantly improve the compaction performance of the material. The internal structure is lithium iron phosphate, while the surface is lithium manganese iron phosphate, forming a compact core-shell structure. This reduces the lithium ion insertion and extraction pathways in the lithium manganese iron phosphate material, which is beneficial for improving the electrochemical performance of the material. Attached Figure Description

[0017] Figure 1 This is a flowchart of a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composites provided in an embodiment of the present invention.

[0018] Figure 2 This is an electronic image of the manganese phosphate ferric phosphate composite ferrous phosphate precursor prepared in Example 1 of the present invention;

[0019] Figure 3 This is an EDS spot scan of the manganese phosphate ferric phosphate composite ferrous phosphate precursor prepared in Example 1 of the present invention;

[0020] Figure 4 This is a SEM image of lithium phosphate without nano-processing in Example 1 of the present invention;

[0021] Figure 5 This is a SEM image of the nano-processed lithium phosphate in Example 1 of the present invention;

[0022] Figure 6 These are SEM images of the lithium manganese iron phosphate product prepared in Example 1 of this invention at different magnifications;

[0023] Figure 7 This is an EDS elemental distribution diagram of the lithium manganese iron phosphate product prepared in Example 1 of the present invention;

[0024] Figure 8 This is a diagram showing the total EDS elemental distribution of the lithium manganese iron phosphate product prepared in Example 1 of this invention;

[0025] Figure 9 These are SEM images of the lithium manganese iron phosphate product prepared in Comparative Example 1 of this invention at different magnifications.

[0026] Figure 10The 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.

[0027] Figure 11 The charge-discharge curve (0.1C) of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Comparative Example 1 of this invention. Detailed Implementation

[0028] 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.

[0029] 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.

[0030] This invention provides a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composites, which can be used to prepare lithium manganese iron phosphate with high compaction density, high voltage, and high capacity. Figure 1 As shown, this method includes:

[0031] Step S1: Under an inert gas atmosphere, iron source, phosphorus source, antioxidant and ammonia water are added to the reactor in sequence. After the addition is completed, the mixture is aged for a period of time to form a mixed slurry.

[0032] The inert gas can be nitrogen, the pH value of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25-40℃, and the aging time can be 1 hour.

[0033] In this embodiment of the invention, the iron source can be one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride. The phosphorus source can be one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. The antioxidant can be ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the amount of iron source added. The molar ratio of the added iron source to the phosphorus source satisfies n(Fe) / n(P) = 1.45-1.46.

[0034] Step S2: Add the mixed slurry to the reaction vessel and heat it. Then add lithium source and phosphorus source in a certain proportion, and then add alkaline solution. After reaction, filter and wash to obtain filter cake.

[0035] The heating temperature can be 75-85℃, and the molar ratio of the added lithium source to phosphorus source satisfies n(Li) / n(P) = 3-3.1. The lithium source can be one or more of lithium hydroxide, lithium oxalate, and lithium acetate; the phosphorus source can be one or more of phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. The alkaline solution can be ammonia water, and the pH value of the solution is adjusted to between 10 and 13. The washing can be performed multiple times, with the first wash primarily removing impurities such as magnesium and sulfur. In the final wash, a 1:1 diluted ammonia water is added to adjust the pH value to 6.5-7.0 to remove SO4. 2- Ions. Specifically, the washing process can be repeated three times. The first and second washes primarily remove impurities such as magnesium and sulfur. In the third wash, a 1:1 diluted ammonia solution is added to adjust the pH to 6.5-7.0 to remove SO42-. 2- Ions. The molar ratio of each element in the filter cake satisfies n(Li):n(Fe):n(P)=[1.03-1.04]:1:[1.03-1.04].

[0036] Step S3: After mixing the iron source, manganese source, and phosphorus source, add the antioxidant and control the pH value of the solution between 3.0 and 4.0 to obtain a mixed solution;

[0037] In the mixed solution, the molar ratio of the added manganese source to the iron source is between 8:2 and 2:8, and the molar ratio of the sum of the added manganese source and the iron source to the phosphorus source satisfies: n(Fe+Mn) / n(P)=1.41-1.43.

[0038] In this embodiment, the iron source can be one or more of ferrous sulfate, hydrated ferrous sulfate, ferrous nitrate, and ferrous chloride; the manganese source can be one or more of manganese sulfate, manganese nitrate, manganese chloride, and manganese acetate; the phosphorus source can be one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the antioxidant can be ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the iron source.

[0039] Step S4: The filter cake is put into the reaction vessel, and an antioxidant is added under an inert gas atmosphere. Then, a mixed solution and an ammonia solution are added dropwise. After the addition is completed, the mixture is aged for a period of time, washed with water and filtered to obtain the ferrous manganese phosphate composite ferrous phosphate precursor.

[0040] The inert gas can be nitrogen, the antioxidant can be ascorbic acid, and the ammonia solution concentration is 8-10%. When adding the mixed solution and the ammonia solution, maintain the pH of the solution in the reactor at 7.5-8.0, the reaction temperature at 25-40℃, and the aging time at 1 hour. Multiple water washes are possible; the first wash primarily removes impurities such as magnesium and sulfur, while the final wash involves adding 1:1 diluted ammonia 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 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.

[0041] Step S5: Mix the manganese phosphate ferrous phosphate composite ferrous phosphate precursor, nano-treated lithium phosphate, organic carbon source, additives and deionized water to form a mixture, and obtain a sintered material after ball milling, drying and sintering.

[0042] The nano-sized lithium phosphate particles have a particle size that satisfies D50 < 50 nm. In the mixture, the molar ratio is n(Li):n(Fe):n(Mn):n(P) = [1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04]. The organic carbon source can be one or more of glucose, polyethylene glycol, sucrose, starch, and citric acid. The amount of carbon source added is based on a carbon content of 1.2%-1.6% in the final product. The additives are one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, with the doping amount controlled between 300-3000 ppm.

[0043] In this embodiment, during the ball milling process, the ball milling particle size in the mixture is controlled to be between 5-10 μm; during the drying process, the drying method can be spray drying, the drying atmosphere is nitrogen, the inlet air temperature can be 200-220℃, the outlet air temperature can be 80-110℃, the blowing frequency can be 80Hz, and the spray particle size in the resulting spray material is controlled to be between D50=20-40μm; 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.

[0044] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0045] The method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite phosphate provided in this invention involves adding phosphoric acid to lower the pH value of the solution after preparing solutions from iron and manganese sources, thereby reducing the probability of ferrous ion oxidation. Adding an antioxidant makes the mixed solution less susceptible to oxidation during preparation and feeding. Adding ammonium phosphate to the mixed solution provides both a phosphorus source and allows the utilization of NH4+. 4+ The coordination of Mn with ammonia water enables Mn to... 2+ Fe 2+ In the phosphate system, the precipitation rate is on the same order of magnitude, and the final obtained manganese ferrous phosphate has a dense secondary spherical particle structure, and the contents of Mn, Fe and P are uniform and the ratio is stable.

[0046] The manganese ferrophosphate composite ferrophosphate precursor synthesized in this invention uses ferrophosphate as a base, and then co-precipitates the synthesized manganese ferrophosphate onto the surface of the ferrophosphate to form a core-shell structure. Fe and P elements are uniformly distributed internally, while Fe, Mn, and P elements are distributed on the surface. By pre-treating lithium phosphate into nanoparticles, the lithium phosphate can be fully and uniformly mixed with the manganese ferrophosphate composite ferrophosphate precursor during ball milling. Then, through a carbothermic reduction reaction during sintering, the synthesized sintered material consists of lithium iron phosphate material internally and lithium manganese iron phosphate material coated on the surface. Using highly conductive lithium iron phosphate material as the core and less conductive lithium manganese iron phosphate material as the shell ensures consistent overall conductivity of the spherical core-shell structure of lithium manganese iron phosphate material. In particular, the contact distance between the surface lithium iron phosphate material and the electrolyte is shortened, reducing the distance of lithium ion insertion channels and further improving rate performance.

[0047] The manganese ferrous phosphate composite ferrous phosphate precursors prepared in the embodiments of this invention were all synthesized under inert gas protection, which can ensure the Fe 2+ Mn 2+ It exists in the precursor and does not require a redox reaction in the subsequent carbon reduction process, thus preventing side reactions from occurring.

[0048] The lithium manganese iron phosphate composite ferrous phosphate precursor prepared by the method provided in this invention uses spherical ferrous phosphate as the core and ferrous manganese phosphate as the shell, which is then reacted with nano-sized lithium phosphate to synthesize solid lithium manganese iron phosphate material with a particle size of 1-25 μm, which can significantly improve the compaction performance of the material. Moreover, the formation of a compact core-shell structure on the surface of lithium manganese iron phosphate reduces the lithium ion insertion and extraction pathways in the lithium manganese iron phosphate material, which is beneficial to improving the electrochemical performance of the material.

[0049] 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 the ferrous manganese phosphate composite of the present invention, but does not limit the scope of protection of the present invention.

[0050] Example 1

[0051] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite ferrous phosphate, including the following steps:

[0052] Step S1: Under a nitrogen atmosphere, ferrous sulfate, phosphoric acid, ascorbic acid and ammonia water are added sequentially to the reactor and mixed. The pH value of the solution in the reactor is controlled to be 2.0 and the reaction temperature is 25℃. After the addition is completed, the mixture is aged for 1 hour to form a mixed slurry.

[0053] Step S2: Add the mixed slurry to the reactor and heat it to 75°C. Then add lithium hydroxide and phosphoric acid in a ratio of n(Li) / n(P) = 3. Then add ammonia to adjust the pH of the solution to 10. After the reaction, filter and wash to obtain a filter cake. The molar ratio of each element in the filter cake satisfies n(Li):n(Fe):n(P) = 1.03:1:1.03.

[0054] Step S3: After stirring and mixing ferrous sulfate, manganese sulfate, and phosphoric acid, add ascorbic acid and control the pH of the solution between 3.0 and 4.0 to obtain a mixed solution. Control the molar ratio of manganese source to iron source in the mixed solution to be 7:3. The sum of the molar amounts of manganese source and iron source and the molar ratio of phosphorus source satisfy: n(Fe+Mn) / n(P)=1.43;

[0055] Step S4: The filter cake is put into the reaction vessel, and ascorbic acid is added under a nitrogen atmosphere. Then, a mixed solution and an 8% ammonia solution are added dropwise. The pH value of the solution in the reaction vessel is maintained at 7.5-8.0, the reaction temperature is 25℃, and after the addition is completed, it is aged for 1 hour. After washing with water and filtration, the ferrous manganese phosphate composite ferrous phosphate precursor is obtained.

[0056] The electron image of the manganese ferrous phosphate composite ferrous phosphate precursor prepared according to Example 1 is shown below. Figure 2 As shown; the EDS spot scan image of the manganese phosphate ferrous phosphate composite ferrous phosphate precursor prepared according to Example 1 is shown below. Figure 3 As shown.

[0057] Step S5: The manganese ferrous phosphate composite ferrous phosphate precursor and nano-sized lithium phosphate with a D50 < 50 nm are mixed in a molar ratio of n(Li):n(Fe):n(Mn):n(P) = 1.03:0.6:0.4:1.03. A carbon source mixture consisting of sucrose and polyethylene glycol, with a carbon content of 1.45% in the finished product, is added, along with ammonium metavanadate (600 ppm), magnesium acetate (1500 ppm), and an appropriate amount of deionized water to form a mixture. The mixture is ball-milled to control the ball milling particle size to be 6-8 μm; the mixture is then spray-dried in a nitrogen atmosphere, with the inlet air temperature controlled at 220°C, the outlet air temperature at 110°C, and the blowing frequency at 80 Hz, to obtain a spray material with a spray particle size D50 of 20-40 μm; the spray material is then placed in a box furnace and sintered in a nitrogen atmosphere at a heating rate of 3°C / min, a sintering temperature of 770°C, and a sintering time of 8 h, followed by natural cooling to obtain the sintered material;

[0058] Among them, the SEM image of lithium phosphate without nano-treatment according to Example 1 is as follows: Figure 4 As shown, the SEM image of the nano-treated lithium phosphate used in Example 1 is as follows. Figure 5 As shown.

[0059] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0060] SEM image of the lithium manganese iron phosphate product prepared according to Example 1 is shown below. Figure 6 As shown, where Figure 6 a is a SEM image of the lithium manganese iron phosphate product prepared in Example 1 at a ratio of 1:500 nm; Figure 6 b is the SEM image of the lithium manganese iron phosphate product prepared in Example 1 at a ratio of 1:2 μm. The EDS elemental distribution map and total EDS elemental distribution map of the lithium manganese iron phosphate product prepared in Example 1 are shown below. Figure 7 and Figure 8 As shown.

[0061] Example 2

[0062] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite ferrous phosphate, including the following steps:

[0063] Step S1: Under a nitrogen atmosphere, ferrous sulfate, phosphoric acid, ascorbic acid and ammonia water are added sequentially to the reactor and mixed. The pH value of the solution in the reactor is controlled to be 2.0 and the reaction temperature is 25℃. After the addition is completed, the mixture is aged for 1 hour to form a mixed slurry.

[0064] Step S2: Add the mixed slurry to the reactor and heat to 75°C. Then add lithium oxalate and phosphoric acid in a ratio of n(Li) / n(P) = 3.02. Add ammonia to adjust the pH of the solution to 11. After the reaction, filter and wash to obtain a filter cake. The molar ratio of each element in the filter cake satisfies n(Li):n(Fe):n(P) = 1.03:1:1.03.

[0065] Step S3: After stirring and mixing ferrous sulfate, manganese sulfate, phosphoric acid and ammonium dihydrogen phosphate, ascorbic acid is added. The pH of the solution is controlled between 3.0 and 4.0 to obtain a mixed solution. The molar ratio of manganese source to iron source in the mixed solution is controlled to be 6:4. The sum of the molar amounts of manganese source and iron source and the molar ratio of phosphorus source satisfy: n(Fe+Mn) / n(P)=1.43;

[0066] Step S4: The filter cake is put into the reaction vessel, and ascorbic acid is added under a nitrogen atmosphere. Then, a mixed solution and an 8% ammonia solution are added dropwise. The pH value of the solution in the reaction vessel is maintained at 7.5-8.0, the reaction temperature is 25℃, and after the addition is completed, it is aged for 1 hour. After washing with water and filtration, the ferrous manganese phosphate composite ferrous phosphate precursor is obtained.

[0067] Step S5: The manganese ferrous phosphate composite ferrous phosphate precursor and nano-sized lithium phosphate with a D50 < 50 nm are mixed in a molar ratio of n(Li):n(Fe):n(Mn):n(P) = 1.03:0.5:0.5:1.03. A carbon source mixture consisting of sucrose and polyethylene glycol, with a carbon content of 1.55% in the finished product, is added, along with 1000 ppm of ammonium metavanadate, 1000 ppm of magnesium acetate, and an appropriate amount of deionized water to form a mixture. The mixture is ball-milled to control the ball milling particle size to be 7-9 μm; the mixture is then spray-dried in a nitrogen atmosphere, with the inlet air temperature controlled at 210℃, the outlet air temperature at 105℃, and the blowing frequency at 80Hz, to obtain a spray material with a spray particle size D50 of 20-40 μm; the above spray material is then placed in a box furnace and sintered in a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 765℃, and a sintering time of 9h, followed by natural cooling to obtain the sintered material;

[0068] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0069] Comparative Example 1

[0070] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite ferrous phosphate, including the following steps:

[0071] Step S1: Manganese dioxide, iron oxide, phosphoric acid, and lithium carbonate are mixed in a molar ratio of Li:Fe:Mn:P = 1.03:0.6:0.4:1.03. A carbon source mixture consisting of sucrose and polyethylene glycol is added to make the carbon content of the finished product 1.50%, along with magnesium acetate (1000 ppm), ammonium metavanadate (500 ppm), and an appropriate amount of deionized water to form a mixture.

[0072] Step S2: The mixture is ball-milled to control the ball milling particle size in the mixture to be 5-10 μm; then it is sand-milled to a sand milling particle size D50 = 0.45-0.65 μm. The sand milling material is then spray-dried in an air-filled atmosphere, with the inlet air temperature controlled at 220℃, the outlet air temperature at 105℃, and the blower frequency at 80Hz, to obtain a spray material with a spray particle size D50 = 20-40 μm.

[0073] 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 775℃, and a sintering time of 8h. After natural cooling, the sintered material is obtained. The sintered material is then pulverized using 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=0.8-3.0μm, D90<10μm, and D100<30μm.

[0074] 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.

[0075] The SEM image of the lithium manganese iron phosphate product prepared according to Comparative Example 1 is shown below. Figure 9 As shown, where Figure 9 a is a SEM image of the lithium manganese iron phosphate product prepared in Comparative Example 1 at a ratio of 1:500 nm. Figure 9 b is a SEM image of the lithium manganese iron phosphate product prepared in Comparative Example 1 at a ratio of 1:2 μm.

[0076] To verify the quality of the lithium manganese iron phosphate cathode material prepared by the method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composite ferrous phosphate provided in this invention, the lithium manganese iron phosphate cathode material prepared in Examples 1-2 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 dispersion was coated on aluminum foil and vacuum dried to obtain the cathode sheet. The electrolyte was 1 mol / L LiPF6, with a solvent volume ratio of EC:DMC:EMC = 1:1:1. The separator was Celgard polypropylene membrane, and the lithium metal sheet was used as the negative electrode. Together, they 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 current was 0.1C for two cycles, 0.2C for two cycles, and 1C for two cycles. The test results are shown in Table 1.

[0077] Among them, the charge-discharge curves (0.1C) of the coin half-cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 and Comparative Example 1 according to the present invention are as follows: Figure 10 He Ru Figure 11 As shown.

[0078] Table 1. Test items and test results for Examples 1-2 and Comparative Example 1

[0079] Example number 0.1 C initial charge specific capacity (mAh / g) 0.1 C initial discharge specific capacity (mAh / g) Compacted density Example 1 162.02 159.82 2.502 Example 2 164.15 158.45 2.476 Comparative Example 1 148.05 137.04 2.253

[0080] 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 1-2 showed significantly improved specific capacity and compaction density during the first charge-discharge cycle at 0.1C compared to Comparative Example 1.

[0081] In summary, the method for preparing lithium manganese iron phosphate using ferrous manganese phosphate composites provided in this invention ultimately yields solid lithium manganese iron phosphate material with a particle size of 1-25 μm, which significantly improves the material's compaction performance. Furthermore, the formation of a compact core-shell structure on the surface of the lithium manganese iron phosphate reduces the lithium ion insertion and extraction pathways within the material, thus enhancing its electrochemical performance.

[0082] 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 using ferrous manganese phosphate and ferrous manganese phosphate composites, characterized in that, The method includes: Step S1: Under an inert gas atmosphere, iron source, phosphorus source, antioxidant and ammonia water are added sequentially to the reactor. The pH value of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25-40℃, and the aging time after the addition is 1 hour to form a mixed slurry. The inert gas is nitrogen, and the molar ratio of the added iron source to phosphorus source satisfies n(Fe) / n(P) = 1.45-1.

46. Step S2: Add the mixed slurry to the reaction vessel and heat it to 75-85℃. Then add lithium source and phosphorus source in a certain proportion, add alkaline solution and adjust the pH value of the solution to 10-13. After reaction, filter and wash to obtain filter cake. The molar ratio of each element in the filter cake satisfies n(Li):n(Fe):n(P)=[1.03-1.04]:1:[1.03-1.04]. The molar ratio of the added lithium source to phosphorus source satisfies n(Li) / n(P)=3-3.

1. The alkaline solution is ammonia. Step S3: After mixing the iron source, manganese source, and phosphorus source, add the antioxidant and control the pH value of the solution between 3.0 and 4.0 to obtain a mixed solution; Step S4: The filter cake is put into the reaction vessel. Under an inert gas atmosphere, an antioxidant is added, and then a mixed solution and an ammonia solution are added dropwise. The pH value of the solution in the reaction vessel is maintained at 7.5-8.0, and the reaction temperature is 25-40℃. After the addition is completed, the mixture is aged for a period of time. After washing with water and filtration, the ferrous manganese phosphate composite ferrous phosphate precursor is obtained. Step S5: Mix the manganese ferrous phosphate composite ferrous phosphate precursor, nano-treated lithium phosphate, organic carbon source, additives and deionized water to form a mixture. After ball milling, drying and sintering, a sintered material is obtained. The particle size of the nano-treated lithium phosphate satisfies D50 < 50 nm. Step S6: After further screening, batching and packaging of the above sintered material, the finished lithium manganese iron phosphate product can be obtained.

2. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S1, the iron source is one or more of ferrous sulfate, ferrous nitrate, and ferrous chloride; the phosphorus source is one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the antioxidant is ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the amount of iron source added.

3. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S2, the lithium source is one or more of lithium hydroxide, lithium oxalate, and lithium acetate; the phosphorus source is one or more of phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the washing is performed multiple times, with the first wash primarily removing impurities such as magnesium and sulfur, and the final wash adding 1:1 diluted ammonia to adjust the pH to 6.5-7.0 to remove SO4. 2- ion.

4. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S3, the molar ratio of the added manganese source to the iron source in the mixed solution is between 8:2 and 2:8, and the molar ratio of the sum of the added manganese source and the iron source to the phosphorus source satisfies: n(Fe+Mn) / n(P) = 1.41-1.

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5. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S3, the iron source is one or more of ferrous sulfate, ferrous nitrate, and ferrous chloride; the manganese source is one or more of manganese sulfate, manganese nitrate, manganese chloride, and manganese acetate; the phosphorus source is one or more of phosphoric acid, sodium phosphate, potassium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the antioxidant is ascorbic acid, and the amount of antioxidant added is 1% of the mass fraction of the iron source.

6. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S4, the inert gas is nitrogen, the antioxidant is ascorbic acid, the ammonia solution concentration is 8-10%, and the aging time is 1 hour. The washing is performed multiple times, with the first wash primarily removing magnesium and sulfur impurities. In the final wash, a 1:1 diluted ammonia solution is added to adjust the pH to 6.5-7.0 to remove SO4. 2- ion.

7. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S5, in the mixture, the molar ratio is n(Li):n(Fe):n(Mn):n(P) = [1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04]; the organic carbon source is one or more of glucose, polyethylene glycol, sucrose, starch and citric acid, and the amount of carbon source added is based on the carbon content in the final product being between 1.2% and 1.6%; 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.

8. The method for preparing lithium manganese iron phosphate according to claim 1, characterized in that, In step S5, during the ball milling process, the ball milling particle size in the mixture is controlled to be between 5-10 μm; during the drying process, the drying method is spray drying, the drying atmosphere is nitrogen, the inlet air temperature is 200-220℃, the outlet air temperature is 80-110℃, the blowing frequency is 80Hz, and the spray particle size in the spray material is controlled to be between D50=20-40 μm; 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.

9. A lithium-ion battery cathode material, characterized in that, The lithium-ion battery cathode material is obtained by processing lithium manganese iron phosphate using a method comprising the preparation of lithium manganese iron phosphate as described in any one of claims 1-8.

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

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