A method and application of preparing lithium iron manganese phosphate from ferrous phosphate and ferromanganese phosphate

Abstract

The invention provides a method for preparing lithium ferromanganese phosphate from ferrous phosphate composite ferromanganese phosphate. The method comprises the following steps: stirring and mixing an iron source, a manganese source, phosphoric acid, and an inorganic carbon source, and then adding an antioxidant and a phosphorus source to obtain a mixed solution; dripping an ammonia solution into the mixed solution, washing with water, filtering, and co-precipitating to obtain a ferromanganese phosphate precursor; crushing the ferromanganese phosphate precursor, adding it into a reactor and heating it, then adding a phosphoric acid solution, a lithium source, a phosphorus source, and an alkaline solution, reacting, filtering, and washing to obtain a filter cake; adding the filter cake, an antioxidant, a ferrous sulfate solution, phosphoric acid, an ammonium dihydrogen phosphate solution, and ammonia water into the reactor to form a mixed slurry; washing and filtering the mixed slurry with water to form a ferrous phosphate composite ferromanganese phosphate precursor; mixing the ferrous phosphate composite ferromanganese phosphate precursor with nano-sized lithium phosphate, an organic carbon source, an additive, and deionized water to form a mixture, and performing ball milling, drying, sintering, screening, batching, packaging, and other processes to obtain a finished lithium ferromanganese 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 ferrous phosphate and ferromanganese phosphate composites, and its application. Background Art

[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, and sintering processes to obtain a doped LFP-carbon composite material. However, this method has drawbacks. Simultaneous mixing of multiple raw materials makes it difficult to obtain a homogeneous mixture. Manganese, present on the surface of the LFP material, is easily dissolved and leached, leading to poor cycle performance. Furthermore, the redox reaction reduces Mn and Fe to their divalent states, potentially generating impurities and lowering the material's electrochemical performance. 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 and its application using ferrous phosphate composite manganese iron phosphate, 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 from ferrous phosphate and ferromanganese phosphate. The method includes: mixing an iron source, a manganese source, phosphoric acid, and an inorganic carbon source, then adding an antioxidant and a phosphorus source, controlling the pH of the solution between 3.0 and 4.0 to obtain a mixed solution; adding a 16-20% ammonia solution A to a reaction vessel, and simultaneously adding the mixed solution and an 8-10% ammonia solution B under an inert gas atmosphere; aging the mixture for a period of time after the addition is complete to co-precipitate and prepare a ferromanganese phosphate precursor; pulverizing the ferromanganese phosphate precursor into micron-sized spherical particles, adding them to a reaction vessel for heating, and then adding a certain concentration of phosphoric acid for acidification treatment. Then, lithium and phosphorus sources are added in a certain proportion, followed by an alkaline solution. After reaction, the mixture is filtered and washed to obtain a filter cake. Under an inert gas atmosphere, the filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution, and ammonia are added sequentially to the reactor. After the addition is complete, the mixture is aged for a period of time to form a mixed slurry. The mixed slurry is washed with water and filtered to form a ferrous phosphate composite ferromanganese phosphate precursor. The ferrous phosphate composite ferromanganese 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 sintered material is further processed through screening, batching, and packaging to obtain the finished lithium manganese iron phosphate product.

[0007] Preferably, in step S1, 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 iron source to the phosphorus source satisfies: n(Fe+Mn) / n(P) = 1.41-1.43; 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 inorganic carbon source is one or more of graphene, acetylene black, and conductive carbon; 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.

[0008] Preferably, in step S2, the pH value of the solution in the reactor is controlled between 7.5 and 8.0, the reaction temperature is 25-40℃, the aging time is 1h, and the molar ratio of iron, manganese, and phosphorus in the ferrous manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.455.

[0009] Preferably, in step S3, the particle size of the pulverized micron-sized ferromanganese phosphate is between 2-5 μm, and the heating temperature is 75-85℃; the added phosphoric acid concentration is 60%-85%, and the molar ratio of the added lithium source to phosphorus source satisfies: n(Li) / n(P) = 3-3.08; the lithium source is lithium hydroxide; the phosphorus source is 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 is adjusted to 10-13; the washing is performed 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(Mn):n(P)=[1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04].

[0010] Preferably, in step S4, 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 is 1 hour; the inert gas is nitrogen, the antioxidant is ascorbic acid, the phosphoric acid solution concentration is 85%, the ammonium dihydrogen phosphate solution is prepared by dissolving ammonium dihydrogen phosphate powder in water to form a 30% concentration ammonium dihydrogen phosphate solution, and the ammonia water concentration is 20%.

[0011] Preferably, in step S5, the water washing is performed multiple times. The first water wash mainly removes impurities such as magnesium and sulfur. In the final water wash, ammonia water diluted 1:1 is added to adjust the pH value to 6.5-7.0 to remove SO4. 2- The ions; the molar ratio of iron, manganese and phosphorus in the ferrous phosphate composite manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.455.

[0012] Preferably, in step S6, 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 additive is one or more of titanium dioxide, ammonium metavanadate, niobium pentoxide, and magnesium acetate, and the doping amount is controlled between 300 and 3000 ppm.

[0013] Preferably, in step S6, 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=20-40μm; 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.

[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 ferrous phosphate composite iron manganese 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 by combining ferrous phosphate and ferromanganese phosphate provided in this invention embodiment finally obtains solid lithium manganese iron phosphate material with a particle size of 1-25μm, which can significantly improve the compaction 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 phosphate and ferromanganese phosphate, as provided in an embodiment of the present invention.

[0018] Figure 2 This is a SEM image of the manganese ferrous phosphate precursor prepared in Example 1 of the present invention;

[0019] Figure 3 The image shows the XRD pattern of the manganese ferrous phosphate precursor prepared in Example 1 of this invention.

[0020] Figure 4 This is a SEM image of the spray material prepared in Example 1 of the present invention;

[0021] Figure 5 This is a SEM image of lithium phosphate without nano-processing in Example 1 of the present invention;

[0022] Figure 6 This is a SEM image of the nano-processed lithium phosphate in Example 1 of the present invention;

[0023] Figure 7 These are SEM images of the lithium manganese iron phosphate product prepared in Example 1 of this invention at different magnifications;

[0024] Figure 8 These are SEM images of the lithium manganese iron phosphate product prepared in Comparative Example 1 of this invention at different magnifications.

[0025] Figure 9 The 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;

[0026] Figure 10 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.

[0027] Figure 11 This is a cycle data graph of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[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 phosphate and ferromanganese phosphate composites, to produce lithium manganese iron phosphate with high compaction density and high capacity. For example... Figure 1 As shown, this method includes:

[0031] Step S1: Mix the iron source, manganese source, phosphoric acid, and inorganic carbon source, then add the antioxidant and phosphorus source, and control the pH of the solution between 3.0 and 4.0 to obtain a mixed solution;

[0032] 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.

[0033] 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 inorganic carbon source can be one or more of graphene, acetylene black, and conductive carbon; 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.

[0034] Step S2: Add a 16-20% ammonia solution A to the reactor. Under an inert gas atmosphere, simultaneously add a mixed solution and an 8-10% ammonia solution B. After the addition is complete, let it age for a period of time and co-precipitate to prepare the manganese ferrous phosphate precursor.

[0035] The pH value of the solution in the reactor is controlled between 7.5 and 8.0, the reaction temperature can be 25-40℃, and the aging time can be 1 hour. The molar ratio of iron, manganese, and phosphorus in the ferrous manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.455.

[0036] Step S3: After crushing the manganese ferrous phosphate precursor into micron-sized spherical particles, add it to the reaction vessel and heat it. Then add a certain concentration of phosphoric acid for acidification. 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.

[0037] The pulverized micron-sized ferromanganese phosphate has a particle size between 2-5 μm. The heating temperature is 75-85℃, and the concentration of added phosphoric acid is 60%-85%. The molar ratio of the added lithium source to phosphorus source satisfies n(Li) / n(P) = 3-3.08. The lithium source can be lithium hydroxide; 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 is adjusted to 10-13. 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(Mn):n(P)=[1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04].

[0038] Step S4: Under an inert gas atmosphere, filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution 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.

[0039] The pH value of the solution in the reaction vessel is controlled between 1.5 and 6.5, the reaction temperature is 25-40℃, and the aging time can be 1 hour. In this embodiment of the invention, the inert gas can be nitrogen, the antioxidant can be ascorbic acid, the phosphoric acid solution concentration is 85%, the ammonium dihydrogen phosphate solution is prepared by dissolving ammonium dihydrogen phosphate powder in water to a 30% concentration, and the ammonia concentration is 20%.

[0040] Step S5: Wash and filter the mixed slurry with water to form ferrous phosphate composite manganese phosphate precursor;

[0041] The process can involve multiple washes. The first wash primarily removes impurities such as magnesium and sulfur. The final wash involves adding a 1:1 diluted ammonia solution to adjust the pH to 6.5-7.0 to remove SO4. 2- Ions. Specifically, the water washing can be performed three times. The first and second water washes mainly remove impurities such as 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- Ions. The molar ratio of iron, manganese, and phosphorus in the ferrous phosphate composite manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.455.

[0042] Step S6: Mix ferrous phosphate composite ferrous manganese phosphate precursor, nano-treated lithium phosphate, organic carbon source, additives and deionized water to form a mixture, and then ball mill, dry and sinter to obtain sintered material.

[0043] 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.

[0044] 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 to obtain the sintered material.

[0045] Step S7: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0046] The method for preparing lithium manganese iron phosphate using ferrous phosphate and manganese iron phosphate provided in this invention involves adding phosphoric acid to lower the pH value of a solution after the iron and manganese sources are prepared, thereby reducing the probability of oxidation of ferrous and manganese ions. 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 rates are of the same order of magnitude, and the resulting manganese ferrophosphate has a dense secondary spherical particle structure with uniform and stable content and ratio of Mn, Fe, and P. Adding an inorganic carbon source to the mixed solution ensures that the dense manganese ferrophosphate material encapsulates the inorganic carbon source during co-precipitation, facilitating the subsequent fabrication of a loose and porous manganese ferrophosphate precursor.

[0047] This invention, through the crushing of spherical ferromanganese phosphate and controlling its size between 2-5 μm, ensures the size of the subsequently synthesized spherical lithium manganese iron phosphate product, thereby improving its electrochemical performance and compaction density. Furthermore, mixing ferromanganese phosphate with a certain concentration of phosphoric acid allows for acidification, resulting in a more porous and less porous internal structure and higher external roundness. The porous internal structure ensures more uniform mixing, further enhancing electrochemical performance; the higher external roundness reduces friction between particles, improving particle flowability and further increasing compaction density. Moreover, by adding a certain proportion of lithium and phosphorus sources, nanoscale lithium phosphate material can be synthesized and evenly distributed within and on the surface of the ferromanganese phosphate material. Mixing nanoscale lithium phosphate material from the inside out significantly improves material consistency.

[0048] This invention uses ferrous manganese phosphate as a substrate, and then synthesizes lithium phosphate through co-precipitation, which is uniformly distributed in the pores and surface of the ferrous manganese phosphate. Ferrous phosphate is then synthesized through co-precipitation and coated on the precursor surface, forming a core-shell structure. The internal Mn, Fe, and P elements are uniformly distributed, and the surface material also has a uniform distribution of Fe and P elements, thereby further preventing the dissolution and leaching of Mn. Furthermore, antioxidants are added to both the ferrous manganese phosphate and ferrous phosphate during the synthesis process to 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.

[0049] In this embodiment of the invention, lithium phosphate is pre-processed into nano-sized particles, which are uniform nano-sized particles. During a simple ball milling process, lithium phosphate is fully mixed on the surface of the ferrous manganese phosphate composite ferric manganese phosphate precursor. This prevents the sand milling process from damaging the core-shell structure of the precursor material, reduces the dissolution of Mn element, and thus improves the cycle stability of lithium manganese iron phosphate.

[0050] The ferrous phosphate composite ferromanganese phosphate precursor prepared by the method provided in this invention uses spherical ferromanganese phosphate as a substrate, lithium phosphate as the middle layer, and ferrous phosphate as the coating layer. Solid lithium manganese iron phosphate material with a particle size of 1-25 μm is then prepared through simple ball milling, spraying, and sintering. The surface of the lithium manganese iron phosphate material is protected by a dense, large-particle lithium iron phosphate material layer of 300 nm-5 μm, which improves the structural stability of the material and significantly enhances its compaction performance.

[0051] 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 ferrous phosphate composite ferrous manganese phosphate of the present invention, but does not limit the scope of protection of the present invention.

[0052] Example 1

[0053] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous phosphate and ferromanganese phosphate composites, including the following steps:

[0054] Step S1: After stirring and mixing ferrous sulfate, manganese sulfate, phosphoric acid, and graphene, add ascorbic acid and diammonium hydrogen phosphate, and control the pH value of the solution between 3.0 and 3.5 to obtain a mixed solution. Control the molar ratio of manganese source to iron source in the mixed solution to be 7:3, and the molar ratio of the sum of the molar amounts of manganese source and iron source to phosphorus source to be 1.43:1.

[0055] Step S2: Add a 16% ammonia solution A to the reactor. Under an inert gas atmosphere, simultaneously add a mixed solution and an 8% ammonia solution B. Control the pH of the solution in the reactor between 7.5 and 8.0. The reaction temperature is 25°C. After the addition is complete, age for 1 hour, then wash with water and filter under pressure. Co-precipitate to prepare the manganese ferrous phosphate precursor.

[0056] The SEM and XRD patterns of the manganese ferrous phosphate precursor prepared according to Example 1 are shown below. Figure 2 and Figure 3 As shown.

[0057] Step S3: After crushing the ferrous manganese phosphate precursor into micron-sized spherical particles, add it to the reaction vessel and heat it to 75°C. Then add lithium hydroxide and phosphoric acid in a ratio of n(Li) / n(P) = 3, and then add ammonia solution to adjust the pH value to 10. After the reaction, filter and wash to obtain filter cake.

[0058] Step S4: Under a nitrogen atmosphere, filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia water are added to the reactor in sequence. The pH value of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25℃, and the mixture is aged for 1 hour after the addition is completed to form a mixed slurry.

[0059] Step S5: Wash and filter the mixed slurry with water to form ferrous phosphate composite manganese phosphate precursor;

[0060] Step S6: Ferrous phosphate composite ferrous manganese phosphate precursor and nano-sized lithium phosphate with a D50 < 50 nm 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, with a carbon content of 1.45% in the finished product, is added, along with magnesium acetate (1500 ppm), ammonium metavanadate (1000 ppm), and an appropriate amount of deionized water to form a mixture. The material was ball-milled to control the ball milling particle size in the mixture to 6 μm; the mixture was then spray-dried in a nitrogen atmosphere, with the inlet air temperature controlled at 220℃, the outlet air temperature at 110℃, 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 was then placed in a box furnace and sintered in a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 770℃, and a sintering time of 8h, and then cooled naturally to obtain the sintered material.

[0061] The SEM image of the spray material prepared according to Example 1 is shown below. Figure 4 As shown; SEM image of lithium phosphate without nano-treatment according to Example 1 is shown below. Figure 5 As shown, the SEM image of the nano-treated lithium phosphate used in Example 1 is as follows. Figure 6 As shown.

[0062] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0063] SEM images of the lithium manganese iron phosphate product prepared according to Example 1 at different magnifications are shown below. Figure 7 As shown, where Figure 7 a is a SEM image of the lithium manganese iron phosphate product prepared in Example 1 at a ratio of 1:500 nm; Figure 7 b is a SEM image of the lithium manganese iron phosphate product prepared in Example 1 at a ratio of 1:2 μm.

[0064] Example 2

[0065] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous phosphate and ferromanganese phosphate composites, including the following steps:

[0066] Step S1: After stirring and mixing ferrous sulfate, manganese sulfate, phosphoric acid, and conductive carbon, add ascorbic acid and diammonium hydrogen phosphate, and control the pH of the solution between 3.0 and 3.5 to obtain a mixed solution. Control the molar ratio of manganese source to iron source in the mixed solution to be 7:3, and the molar ratio of the sum of the molar amounts of manganese source and iron source to phosphorus source to be 1.43:1.

[0067] Step S2: Add a 16% ammonia solution A to the reactor. Under an inert gas atmosphere, simultaneously add a mixed solution and an 8% ammonia solution B. Control the pH of the solution in the reactor between 7.5 and 8.0. The reaction temperature is 25°C. After the addition is complete, age for 1 hour, then wash with water and filter under pressure. Co-precipitate to prepare the manganese ferrous phosphate precursor.

[0068] Step S3: After crushing the manganese ferrous phosphate precursor into micron-sized spherical particles, add it to the reaction vessel and heat it to 75°C. Then, add lithium hydroxide and phosphoric acid in a ratio of n(Li) / n(P) = 3.01, and add ammonia solution to adjust the pH value to 10.5. After the reaction, filter and wash to obtain filter cake.

[0069] Step S4: Under a nitrogen atmosphere, filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia water are added to the reactor in sequence. The pH value of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25℃, and the mixture is aged for 1 hour after the addition is completed to form a mixed slurry.

[0070] Step S5: Wash and filter the mixed slurry with water to form ferrous phosphate composite manganese phosphate precursor;

[0071] Step S6: Ferrous phosphate composite ferrous manganese phosphate precursor and nano-sized lithium phosphate with a D50 < 50 nm 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 achieve a carbon content of 1.50% in the finished product. Magnesium acetate with a doping amount of 1200 ppm and ammonium metavanadate with a doping amount of 1200 ppm, along with an appropriate amount of deionized water, are then added to form a mixture. The material was ball-milled to control the ball milling particle size in the mixture to 6 μm; the mixture was then spray-dried in a nitrogen atmosphere, with the inlet air temperature controlled at 220℃, the outlet air temperature at 100℃, 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 was 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, and then cooled naturally to obtain the sintered material.

[0072] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0073] Example 3

[0074] This embodiment provides a method for preparing lithium manganese iron phosphate using ferrous phosphate and ferromanganese phosphate composites, including the following steps:

[0075] Step S1: After stirring and mixing ferrous sulfate, manganese sulfate, phosphoric acid, and graphene, add ascorbic acid and diammonium hydrogen phosphate, and control the pH of the solution between 3.0 and 3.5 to obtain a mixed solution. Control the molar ratio of manganese source to iron source in the mixed solution to be 6:4, and the molar ratio of the sum of the molar amounts of manganese source and iron source to phosphorus source to be 1.425:1.

[0076] Step S2: Add a 16% ammonia solution A to the reactor. Under an inert gas atmosphere, simultaneously add a mixed solution and an 8% ammonia solution B. Control the pH of the solution in the reactor between 7.5 and 8.0. The reaction temperature is 25°C. After the addition is complete, age for 1 hour, then wash with water and filter under pressure. Co-precipitate to prepare the manganese ferrous phosphate precursor.

[0077] Step S3: After crushing the ferrous manganese phosphate precursor into micron-sized spherical particles, add it to the reaction vessel and heat it to 75°C. Then add lithium hydroxide and phosphoric acid in a ratio of n(Li) / n(P) = 3, and then add ammonia solution to adjust the pH value to 11. After the reaction, filter and wash to obtain filter cake.

[0078] Step S4: Under a nitrogen atmosphere, filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia water are added to the reactor in sequence. The pH value of the solution in the reactor is controlled between 1.5 and 6.5, the reaction temperature is 25℃, and the mixture is aged for 1 hour after the addition is completed to form a mixed slurry.

[0079] Step S5: Wash and filter the mixed slurry with water to form ferrous phosphate composite manganese phosphate precursor;

[0080] Step S6: Ferrous phosphate composite ferrous manganese phosphate precursor and nano-treated lithium phosphate with D50 < 50 nm are mixed in a molar ratio of Li:Fe:Mn:P = 1.03:0.5:0.5:1.03. 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 with a doping amount of 1500 ppm, ammonium metavanadate with 800 ppm, and an appropriate amount of deionized water to form a mixture. The mixture is ball-milled to control the ball milling particle size in the mixture to 6 μm. The mixture is then spray-dried with an inlet air temperature of 220℃, an outlet air temperature of 105℃, and a blower frequency of 80Hz to obtain a spray material with a spray particle size of D50 = 20-40 μm. The above spray material is placed in a box furnace and sintered under a nitrogen atmosphere at a heating rate of 3℃ / min, a sintering temperature of 760℃, and a sintering time of 10 h. After natural cooling, the sintered material is obtained.

[0081] Step S6: After further processing the above sintered material through screening, batching, and packaging, the finished lithium manganese iron phosphate product can be obtained.

[0082] Comparative Example 1

[0083] This embodiment provides a method for preparing lithium manganese iron phosphate, including the following steps:

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] Among them, the SEM images of the lithium manganese iron phosphate product prepared according to Comparative Example 1 at different magnifications are as follows: Figure 8 As shown. Among them Figure 8 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 8 b is a SEM image of the lithium manganese iron phosphate product prepared in Comparative Example 1 at a ratio of 1:2 μm.

[0089] To verify the quality of the lithium iron phosphate cathode material prepared by the method for preparing lithium iron phosphate using ferrous phosphate composite ferrous manganese phosphate provided in this invention, the lithium iron phosphate cathode material prepared in Examples 1-3 and Comparative Example 1 was dispersed in N-methylpyrrolidone with conductive agent carbon black and binder polyvinylidene fluoride at a mass ratio of 90:5:5. After ball milling and uniform dispersion, the 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. All components were assembled into a coin cell. The test voltage range was 2.5V-4.5V. The battery was charged to 4.5V using a constant current and constant voltage charging method and discharged to 2.5V using a constant current discharging method. The charge / discharge currents were 0.1C for two cycles, 0.2C for two cycles, and 1C for two cycles. The test results are shown in Table 1.

[0090] 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 9 and Figure 10 As shown, the cycle data of a coin cell assembled with the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention is shown in the figure. Figure 11 As shown.

[0091] Table 1. Test items and test results for Examples 1-3 and Comparative Example 1

[0092] Example No. 0.1C initial charge capacity (mAh / g) 0.1C initial discharge specific capacity (mAh / g) compaction density Example 1 163.09 155.89 2.482 Example 2 163.08 157.58 2.472 Example 3 162.30 156.72 2.484 Comparative Example 1 148.05 137.04 2.253

[0093] Based on the above examples and comparative examples, and the test results obtained from the tests, the coin cells prepared with lithium manganese iron phosphate cathode material in Examples 1-3 showed significantly improved specific capacity and compaction density during the first charge-discharge cycle at 0.1C compared to Comparative Example 1.

[0094] In summary, the method for preparing lithium manganese iron phosphate using ferrous phosphate composite ferromanganese phosphate provided in this invention significantly improves the compaction performance of the material. This method utilizes a composite precursor of spherical ferromanganese phosphate as a substrate and ferrous phosphate as an outer layer. Subsequent simple ball milling, spraying, and sintering then produce solid lithium manganese iron phosphate material with a particle size of 1-25 μm. Furthermore, the content of Mn, Fe, and P elements is uniform and the ratio is stable.

[0095] 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 phosphate and manganese iron phosphate composites, characterized in that, The method includes: Step S1: Mix the iron source, manganese source, phosphoric acid, and inorganic carbon source, then add the antioxidant and phosphorus source, and control the pH of the solution between 3.0 and 4.0 to obtain a mixed solution; Step S2: Add a 16-20% ammonia solution A to the reactor. Under an inert gas atmosphere, simultaneously add a mixed solution and an 8-10% ammonia solution B. After the addition is complete, let it age for a period of time and co-precipitate to prepare the manganese ferrous phosphate precursor. Step S3: After crushing the manganese ferrous phosphate precursor into micron-sized spherical particles, add it to the reaction vessel and heat it. Then add a certain concentration of phosphoric acid for acidification. Then add lithium source and phosphorus source in a certain proportion. Then add alkaline solution to adjust the pH value. After the reaction, filter and wash to obtain filter cake. Step S4: Under an inert gas atmosphere, filter cake, antioxidant, ferrous sulfate solution, phosphoric acid, ammonium dihydrogen phosphate solution 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. Step S5: Wash and filter the mixed slurry with water to form ferrous phosphate composite manganese phosphate precursor; Step S6: Mix ferrous phosphate composite ferrous manganese phosphate precursor, nano-treated lithium phosphate, organic carbon source, additives and deionized water to form a mixture, and then ball mill, dry and sinter to obtain sintered material. Step S7: 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 by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S1, the molar ratio of manganese source to iron source added to the mixed solution is between 8:2 and 2:8, and the molar ratio of the sum of the molar amounts of manganese source and iron source to the molar ratio of phosphorus source satisfies: n(Fe+Mn) / n(P) = 1.41-1.43; 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 inorganic carbon source is one or more of graphene, acetylene black, and conductive carbon; 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.

3. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S2, the pH value of the solution in the reactor is controlled between 7.5 and 8.0, the reaction temperature is 25-40℃, the aging time is 1h, and the molar ratio of iron, manganese and phosphorus in the iron manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.

455.

4. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S3, the particle size of the pulverized micron-sized ferromanganese phosphate is between 2-5 μm. The concentration of added phosphoric acid is 60%-85%, and the heating temperature is 75-85℃. The molar ratio of the added lithium source to phosphorus source satisfies: n(Li) / n(P) = 3-3.

08. The lithium source is lithium hydroxide; the phosphorus source is 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 is adjusted to 10-13. The washing is performed multiple times, with the first wash mainly removing impurities such as magnesium and sulfur. In the last wash, a 1:1 diluted ammonia water is added 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(Mn):n(P)=[1.03-1.04]:[0.2-0.8]:[0.2-0.8]:[1.03-1.04].

5. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S4, 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 is 1 hour; the inert gas is nitrogen, the antioxidant is ascorbic acid, the phosphoric acid solution concentration is 85%, the ammonium dihydrogen phosphate solution is prepared by dissolving ammonium dihydrogen phosphate powder in water to form a 30% concentration ammonium dihydrogen phosphate solution, and the ammonia water concentration is 20%.

6. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S5, the water washing is performed multiple times. The first water wash mainly removes impurities such as magnesium and sulfur. In the final water wash, ammonia water diluted 1:1 is added to adjust the pH to 6.5-7.0 to remove SO4. 2- The ions; the molar ratio of iron, manganese and phosphorus in the ferrous phosphate composite manganese phosphate precursor satisfies: n(Fe+Mn) / n(P)=1.440-1.

455.

7. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S6, 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.

8. The method for preparing lithium manganese iron phosphate by combining ferrous phosphate and manganese iron phosphate according to claim 1, characterized in that, In step S6, 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=20-40μm; 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.

9. A lithium-ion battery cathode material, characterized in that, The lithium-ion battery cathode material uses a method for preparing lithium manganese iron phosphate, including the method described in any one of claims 1-8, for the preparation of lithium manganese iron phosphate using ferrous phosphate composite ferrous manganese phosphate.

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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