Lithium iron manganese phosphate material, preparation method thereof, positive electrode material and secondary battery

Abstract

The application relates to the technical field of battery materials, in particular to a lithium manganese iron phosphate material, a preparation method of the lithium manganese iron phosphate material, a positive electrode material and a secondary battery. The preparation method of the lithium manganese iron phosphate material comprises the following steps: (a) mixing a first solution containing a lithium source and a second solution containing a phosphorus source, adjusting the pH to 6-9 to obtain a mixed solution; the lithium source comprises at least one of lithium carbonate and lithium dihydrogen phosphate; (b) adding a third solution containing a divalent manganese source and a divalent iron source into the mixed solution, and performing reaction at 90-100 DEG C; (c) performing solid-liquid separation on the material after the reaction in step (b), collecting the solid and drying the solid to obtain the lithium manganese iron phosphate material. According to the application, by using suitable reaction raw materials, the synthesis of the lithium manganese iron phosphate material with high crystallinity and without carbon coating can be realized under normal pressure and low temperature conditions, high-pressure equipment is not needed, energy consumption is low, the yield is high, and the application is more suitable for industrial production.

Description

[0001] This disclosure claims priority to Singapore Patent Application No. 10202500212P, filed with the Singapore Patent Office on January 23, 2025, entitled “Lithium manganese iron phosphate material and preparation method thereof, cathode material and secondary battery”, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0002] This application relates to the field of battery materials technology, and in particular to a lithium manganese iron phosphate material, its preparation method, a cathode material, and a secondary battery. Background Technology

[0003] Secondary batteries, characterized by high capacity and long lifespan, are widely used in various electronic devices. With the increasing application and promotion of secondary batteries, the performance requirements for them are gradually increasing. Lithium manganese iron phosphate cathode materials, due to their excellent cycle performance, high energy density, and good low-temperature performance, have become one of the most sought-after cathode active materials.

[0004] Existing processes for preparing lithium manganese iron phosphate mainly include solid-state methods, hydrothermal methods, and sol-gel methods. However, the morphology and size of lithium manganese iron phosphate materials prepared by solid-state methods are difficult to control, resulting in poor uniformity and severe agglomeration, which affects their electrochemical performance. Hydrothermal methods require high temperature and high pressure conditions, while sol-gel methods have long production cycles, neither of which is conducive to large-scale, efficient production. Summary of the Invention

[0005] The first aspect of this application provides a method for preparing lithium manganese iron phosphate material, comprising the following steps:

[0006] (a) A first solution containing a lithium source is mixed with a second solution containing a phosphorus source, and the pH is adjusted to 6-9 to obtain a mixed solution; wherein the lithium source includes at least one of lithium carbonate and lithium dihydrogen phosphate.

[0007] (b) A third solution containing a divalent manganese source and a divalent iron source is added to the mixed solution and the mixture is kept at 90–100°C for reaction.

[0008] (c) The material after the reaction in step (b) is subjected to solid-liquid separation, the solid is collected and dried to obtain the lithium manganese iron phosphate material.

[0009] In some embodiments, the phosphorus source includes at least one of phosphoric acid and lithium dihydrogen phosphate.

[0010] In some embodiments, the heat preservation reaction is carried out at atmospheric pressure.

[0011] In some embodiments, the heat preservation reaction takes 3 to 8 hours. Further, the heat preservation reaction is accompanied by stirring; the stirring speed is 50 to 400 rpm.

[0012] In some embodiments, the solvent used for the first solution, the second solution, and the third solution is water.

[0013] In some embodiments, in step (b), the third solution is added to the mixed solution dropwise; the dropwise addition time is 0.5 to 3 hours.

[0014] In some implementations, Mn 2+ The divalent manganese source and Fe 2+ The total amount of the divalent iron source used is calculated and the amount of Li + The molar ratio of the lithium source used is calculated to be 1:(3-3.3); with Mn 2+ The divalent manganese source and Fe 2+ The total amount of divalent iron source used is calculated and the amount of PO4 used is calculated. 3- The molar ratio of the phosphorus source used is 1:(0.9 to 1.2).

[0015] In some implementations, Mn 2+ The divalent manganese source and Fe 2+ The molar ratio of the divalent iron source is calculated to be 1:9 to 9:1.

[0016] In some embodiments, the molar concentration of the lithium source in the first solution is 1 to 3 mol / L; and the total molar concentration of the divalent manganese source and the divalent iron source in the third solution is 1 to 3 mol / L.

[0017] In some embodiments, the divalent manganese source includes at least one of manganese sulfate, manganese chloride, manganese acetate, and manganese nitrate; the divalent iron source includes at least one of ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous nitrate.

[0018] The second aspect of this application provides a lithium manganese iron phosphate material prepared by the preparation method of the first aspect.

[0019] In some embodiments, the BET specific surface area of ​​the lithium manganese iron phosphate material is 4.1–18.7 m². 2 / g, tap density is 0.92~1.21g / cm³ 3 .

[0020] The third aspect of this application provides a cathode material made from lithium manganese iron phosphate material prepared by the preparation method of the first aspect.

[0021] A fourth aspect of this application provides a secondary battery including a positive electrode sheet, the positive electrode sheet comprising a positive electrode material as described in any embodiment of the third aspect of this application. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 These are scanning electron microscope (SEM) images of lithium manganese iron phosphate materials prepared in different embodiments of this application;

[0024] Figure 2 X-ray diffraction (XRD) patterns of lithium manganese iron phosphate materials prepared in different embodiments of this application. Detailed Implementation

[0025] The technical solution of this application will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are only some embodiments of this application, not all embodiments, and are only used to illustrate this application, and should not be regarded as limiting the scope of this application. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0026] In the description of this application, it should be noted that the terms "first", "second", and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0027] In existing lithium manganese iron phosphate (LFP) preparation processes, although the solid-phase method is simple and has relatively low production costs, the resulting LFP material exhibits poor uniformity, affecting its electrochemical performance. The hydrothermal method, synthesizing LFP material through a hydrothermal or solvothermal reaction under high temperature and pressure, is more advantageous than the solid-phase method in controlling product particle size and uniformity. However, existing hydrothermal methods, when using a pure aqueous system, typically require reaction temperatures above 150°C, inevitably necessitating the use of high-pressure reactors and corresponding equipment, increasing production costs. While adding organic solvents such as ethylene glycol can lower the reaction temperature to approximately 120–130°C, the introduction of organic solvents increases the solvent separation process and also raises environmental concerns.

[0028] In view of this, the inventors of this application have conducted in-depth research on the synthesis process of lithium manganese iron phosphate materials. By controlling and optimizing the reaction raw materials and reaction procedures, the preparation of lithium manganese iron phosphate materials can be achieved under more mild and controllable conditions, while ensuring the uniformity of the lithium manganese iron phosphate materials.

[0029] This application provides a lithium manganese iron phosphate material, its preparation method, a cathode material, and a secondary battery, which can obtain a lithium manganese iron phosphate material that balances high crystallinity and good electrochemical performance, thereby enabling the secondary battery to achieve both good cycle stability and discharge capacity.

[0030] The following is a detailed description of this application.

[0031] In a first aspect, this application provides a method for preparing lithium manganese iron phosphate material, comprising the following steps:

[0032] (a) A first solution containing a lithium source is mixed with a second solution containing a phosphorus source, and the pH is adjusted to 6-9 to obtain a mixed solution; wherein the lithium source includes at least one of lithium carbonate and lithium dihydrogen phosphate.

[0033] (b) A third solution containing a divalent manganese source and a divalent iron source is added to the mixed solution and the mixture is kept at 90–100°C for reaction.

[0034] (c) The material after the reaction in step (b) is subjected to solid-liquid separation, the solid is collected and dried to obtain the lithium manganese iron phosphate material.

[0035] The preparation process of lithium manganese iron phosphate material provided in this application is simple. The reaction is carried out at ≤100℃, allowing the raw material components to undergo a sufficient chemical reaction in a short time. In hydrothermal methods, the materials need to be converted into ionic states in the solution for the reaction to proceed. Taking lithium phosphate as an example, lithium phosphate has very low solubility in water, resulting in extremely low concentrations of lithium and phosphorus ions in the aqueous solution. This makes it difficult for the lithium and phosphorus ions to combine with divalent manganese and divalent iron ions, thus requiring high temperature and high pressure conditions to promote the reaction. The inventors of this application creatively discovered during their research that when lithium carbonate is used as the lithium source, it reacts with the phosphorus source in the system first to form lithium dihydrogen phosphate. Lithium dihydrogen phosphate has significantly higher solubility in water than lithium phosphate and other lithium sources, resulting in a large number of lithium and phosphorus ions in the aqueous solution. This makes it easier for the lithium and phosphorus ions to combine with divalent manganese and divalent iron ions, allowing the reaction to be completed at normal pressure and a relatively low reaction temperature (≤100℃). Furthermore, the preparation method of this application does not require nitrogen protection, further simplifying the reaction process and reducing production costs. When lithium dihydrogen phosphate is used as the lithium source, lithium manganese iron phosphate can be prepared under normal pressure and at a relatively low reaction temperature (≤100℃) by combining it with other lithium sources such as lithium carbonate. However, since the lithium-phosphorus ratio in lithium dihydrogen phosphate is 1:1, it is necessary to further combine it with other lithium sources such as lithium carbonate to make up the amount of lithium required for the preparation of lithium manganese iron phosphate, which increases the variety of raw materials and leads to operational complexity.

[0036] The method for preparing lithium manganese iron phosphate provided in this application meets the needs of industrial-scale production and is easy to scale up. Furthermore, compared with existing hydrothermal methods, the method for preparing lithium manganese iron phosphate provided in this application has a higher yield. Simultaneously, this application eliminates the need for various additives such as complexing agents and precipitants during the reaction process, thereby avoiding the generation of large amounts of waste liquid and making the post-treatment process simpler, greener, and more energy-efficient.

[0037] The lithium manganese iron phosphate material obtained by the preparation method of this application has advantages such as controllable morphology, adjustable particle size, good uniformity, high batch stability and consistency, and the obtained lithium manganese iron phosphate has higher crystallinity and no impurity phase.

[0038] In some embodiments, adjusting the pH to 6-9 can specifically involve adjusting the pH to a range of any value from 6, 6.5, 7, 7.5, 8, 9, or higher. In the reaction system of this application, adjusting the pH alters the growth rate and orientation of lithium manganese iron phosphate crystals, thereby changing their morphology and size. For example, by adjusting the pH to the range of 6-9, this application can obtain lithium manganese iron phosphate materials with uniform morphology and particle size distribution, such as rhombic flakes, nano-cubes, octahedrons, micron-sized blocks, and dense spheres, to meet different application requirements.

[0039] In some embodiments, the reaction is carried out in an air atmosphere.

[0040] In some embodiments, the reagents used to adjust the pH may be selected from conventional acids and conventional bases, wherein the acid may include at least one of sulfuric acid, hydrochloric acid, and nitric acid; and the base may include at least one of sodium hydroxide and ammonia. Sulfuric acid, hydrochloric acid, nitric acid, and ammonia may be of industrial grade concentration or appropriately diluted with water; sodium hydroxide may be added in solid form, but is not limited thereto, and may also be added in an aqueous solution.

[0041] In some embodiments, the phosphorus source includes at least one of phosphoric acid and lithium dihydrogen phosphate. Using phosphoric acid as the phosphorus source is more conducive to the reaction with lithium carbonate to form lithium dihydrogen phosphate. Optionally, the second solution is an aqueous solution of phosphoric acid with a mass fraction of 70% to 90%, such as industrial-grade phosphoric acid with a mass fraction of 85%, thereby facilitating industrial production.

[0042] In some embodiments, the temperature of the heat-preserving reaction can be any range consisting of 90°C, 92°C, 95°C, 98°C, 100°C, or higher. In some embodiments, the heat-preserving reaction is carried out at atmospheric pressure. When the temperature of the heat-preserving reaction is <90°C, lithium manganese iron phosphate material cannot be obtained; the temperature of the heat-preserving reaction in this application is controlled below 100°C. While ensuring the smooth synthesis of lithium manganese iron phosphate material, it avoids the need for special equipment such as high-pressure reactors, allowing the reaction to be carried out in a conventional reactor, and the reactor does not need to be sealed, as long as the reaction temperature meets the above conditions. This not only makes the reaction easier to control but also reduces energy consumption and facilitates scale-up production. The atmospheric pressure can be 0.1013 MPa ± 0.05 MPa. Atmospheric pressure usually refers to standard atmospheric pressure, but it is understood that pressure fluctuations due to operation, equipment, or environmental factors are still within the scope of protection of this application.

[0043] In some embodiments, the mixing of the first solution and the second solution in step (a) can be carried out in a conventional reaction vessel at room temperature. Specifically, the first solution can be added to the second solution for mixing. It is understood that the mixing process may be accompanied by stirring.

[0044] In some embodiments, the heat preservation reaction time is 3 to 8 hours, for example, it can be any range consisting of 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, or more. This helps to promote the complete reaction of the raw materials of lithium manganese iron phosphate. Furthermore, the heat preservation reaction time is 3 to 4 hours, which can further improve production efficiency, allowing the entire preparation process to be completed within about 8 hours, significantly improving production efficiency.

[0045] In some embodiments, the heat preservation reaction is accompanied by stirring; the stirring speed is 50 to 400 rpm, for example, it can be any range of 50 rpm, 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm or above, to promote the uniformity of mixing and reaction of the components in the system.

[0046] In some embodiments, in step (b), the third solution is added to the mixed solution dropwise; the dropwise addition time is 0.5 to 3 hours, for example, it can be a range consisting of 0.5 hours, 1 hour, 2 hours, 3 hours, or any value above that above. Adding the third solution dropwise helps to balance improving the uniformity of the reaction and production efficiency. It avoids the possibility of excessively rapid local reactions and poor product uniformity that may result from adding the solution too quickly, and also avoids the possibility of reduced production efficiency due to adding the solution too slowly.

[0047] In some embodiments, Mn 2+ The divalent manganese source and Fe 2+ The total amount of the divalent iron source used is calculated and the amount of Li + The molar ratio of the lithium source used is 1:(3 to 3.3), for example, it can be a range consisting of any values ​​such as 1:3, 1:3.1, 1:3.2, 1:3.3 or above; with Mn 2+ The divalent manganese source and Fe 2+ The total amount of divalent iron source used is calculated and the amount of PO4 used is calculated. 3- The molar ratio of the phosphorus source used is 1:(0.9 to 1.2), for example, it can be a range consisting of 1:0.9, 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2 or any of the above values.

[0048] In some embodiments, Mn 2+ The divalent manganese source and Fe 2+ The molar ratio of the divalent iron source is calculated to be 1:9 to 9:1, for example, it can be any range consisting of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, or more. The specific amounts of divalent manganese and divalent iron sources added can be calculated based on the stoichiometric ratio of the target lithium manganese iron phosphate.

[0049] In some embodiments, the molar concentration of the lithium source in the first solution is 1 to 3 mol / L, for example, it can be any range consisting of 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L or more, optionally 2.5 to 3 mol / L; the total molar concentration of the divalent manganese source and the divalent iron source in the third solution is 1 to 3 mol / L, for example, it can be any range consisting of 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L or more, optionally 2.5 to 3 mol / L.

[0050] In the reaction system of this application, as the reaction proceeds, the lithium source gradually exists in the form of lithium dihydrogen phosphate with high solubility. Therefore, in the preparation method of this application, a first solution containing a lithium source, a second solution containing a phosphorus source, and a third solution containing divalent manganese and divalent iron sources with high molar concentrations can be used. This not only ensures the stable synthesis of lithium manganese iron phosphate material, but also significantly increases the solid content of the reaction system. That is, in the same volume of the reaction system, the yield of lithium manganese iron phosphate material prepared by the method of this application is significantly increased, breaking through the yield limitation of traditional liquid-phase synthesis of lithium manganese iron phosphate material, which is more conducive to industrial production.

[0051] In some embodiments, the divalent manganese source includes at least one selected from manganese sulfate, manganese chloride, manganese acetate, and manganese nitrate; the divalent iron source includes at least one selected from ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous nitrate. In the preparation method of this application, unless otherwise specified, all raw materials can be directly purchased, and each raw material may or may not contain water of crystallization.

[0052] In some embodiments, the solvent used in the first solution, the second solution, and the third solution is water. It is understood that the term "solution" in this application is not a limitation on the dissolution state of the solute therein. Taking the first solution as an example, the lithium source therein can be dispersed in water, or it can be partially dissolved and partially dispersed in water, existing in the form of a suspension. When the lithium source is dispersed in water in a suspended form, the molar concentration of the first solution refers to the ratio of the total amount of lithium source in the suspension to the volume of the suspension.

[0053] In some embodiments, the solid-liquid separation includes any one of pressure filtration, vacuum filtration, and centrifugation; the drying includes any one of spray drying, flash evaporation, microwave drying, and oven drying. Optionally, the solid is washed before drying. Taking oven drying as an example, it can be carried out at 80-90°C for more than 5 hours, but is not limited to this, as long as the moisture content of the lithium manganese iron phosphate material meets the requirements of the cathode material for lithium manganese iron phosphate raw materials.

[0054] In some embodiments, the temperature of the materials in the reaction system is reduced to below 70°C before the solid-liquid separation is performed.

[0055] The second aspect of this application provides a lithium manganese iron phosphate material prepared by the preparation method of the first aspect. The lithium manganese iron phosphate material of this application has the chemical formula LiMn0. x Fe 1-x PO4, 0.1≤x≤0.9.

[0056] The lithium manganese iron phosphate provided in this application has the characteristics of regular morphology, uniform particle size, narrow particle size distribution, uniform elemental distribution, high crystallinity, and high batch stability and consistency.

[0057] In some embodiments, the morphology of the lithium manganese iron phosphate material includes at least one of rhombic flakes, nanocubes, octahedrons, micron-sized blocks, and spheres. Optionally, the primary particle size of the lithium manganese iron phosphate material is 0.1–10 μm, for example, it can be a range consisting of 0.1 μm, 0.2 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, or any value above.

[0058] In some embodiments, the BET specific surface area of ​​the lithium manganese iron phosphate material is 4.1–18.7 m². 2 / g, for example, can be 4.1m 2 / g、5m 2 / g、7m 2 / g、9m 2 / g、12m 2 / g、14m 2 / g, 16m 2 / g, 18.7m 2 The range consisting of any value of / g or above; tap density of 0.92–1.21 g / cm³. 3 For example, it can be 0.92 g / cm³. 3 0.95g / cm 3 1g / cm 3 1.05g / cm 3 1.1g / cm 3 1.15g / cm 3 1.21 g / cm 3 Or any range of the above values. Lithium manganese iron phosphate materials with a certain morphology, combined with suitable specific surface area and tap density, are more conducive to improving the dispersion and compaction density of the cathode material, thereby further contributing to the full realization of its electrochemical performance.

[0059] The third aspect of this application provides a cathode material made from lithium manganese iron phosphate material prepared by the preparation method of the first aspect.

[0060] In some embodiments, the D50 particle size of the positive electrode material is 0.89 to 1.13 μm, for example, it can be a range consisting of 0.89 μm, 0.95 μm, 1 μm, 1.05 μm, 1.1 μm, 1.13 μm or any value above.

[0061] In some embodiments, the BET specific surface area of ​​the positive electrode material is 12.8–17.3 m². 2 / g, for example, can be 12.8m 2 / g, 13.5m 2 / g、14m 2 / g, 15m 2 / g, 16m 2 / g、17m 2 / g, 17.3m 2 The range consisting of / g or any value above.

[0062] In some embodiments, the raw materials for the cathode material include lithium manganese iron phosphate material, lithium source, phosphorus source, carbon source and doped metal source in a molar ratio of 1:(0.02~0.04):(0.03~0.05):(0.04~0.08):(0.01~0.03).

[0063] In some embodiments, the method for preparing the cathode material may include: uniformly mixing the lithium manganese iron phosphate material prepared by the method of the first aspect of this application with a lithium source, a phosphorus source, a carbon source and an optional doping metal source to obtain a mixed raw material; sintering the mixed raw material to obtain the cathode material; wherein, the doping metal source includes, but is not limited to, at least one of Co, Mg, Zn, Ca, Ti, V, Ni and Cr.

[0064] In some embodiments, the sintering process includes calcining at 700–750°C for 8–12 hours under a protective atmosphere. The protective atmosphere may be provided by nitrogen and / or an inert gas.

[0065] In some embodiments, the doped metal source may include at least one selected from oxides, carbonates, acetates, hydrochlorides, and nitrates of a doped metal. Taking a magnesium salt as an example, the magnesium source may include at least one selected from magnesium oxide, magnesium carbonate, and magnesium acetate.

[0066] In some embodiments, the carbon source includes, but is not limited to, at least one of glucose, sucrose, starch, fructose, and polyvinyl alcohol.

[0067] In some embodiments, a solvent, such as water, may be added during the mixing process. Furthermore, when the mixed raw materials contain water, the raw materials are pre-dried before the sintering treatment. The specific drying method is not limited; for example, spray drying can be used, which further helps to ensure the dimensional uniformity of the cathode material.

[0068] A fourth aspect of this application provides a secondary battery including a positive electrode sheet, the positive electrode sheet comprising a positive electrode material as described in any embodiment of the third aspect of this application.

[0069] In some embodiments, the positive electrode sheet of this application includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the positive electrode film layer includes the positive electrode material described in any embodiment of the third aspect of this application. The positive current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0070] In some embodiments, based on the total weight of the positive electrode film, the content of the positive electrode active material in the positive electrode film is ≥50wt%, optionally 85wt% to 99wt%, for example 90wt% to 99wt%.

[0071] In some embodiments, the positive electrode film may optionally include a conductive agent and / or a binder. For example, the conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0072] This application enables the synthesis of highly crystalline, carbon-free lithium manganese iron phosphate materials under normal pressure and low temperature conditions by using suitable reaction raw materials. It eliminates the need for high-pressure equipment, resulting in low energy consumption, high yield, and greater suitability for industrial production.

[0073] The preparation method of this application can be carried out in a pure aqueous system, which omits the solvent separation step, conforms to the concept of green chemistry, and simplifies the process.

[0074] This application allows for the control of the morphology and particle size of lithium manganese iron phosphate materials to obtain cathode material products with different morphologies, sizes, and properties, thereby meeting different application requirements.

[0075] Example 1

[0076] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0077] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add industrial grade ammonia (25wt%) to adjust the pH to 8.5.

[0078] (2) Weigh 16.9 kg of manganese sulfate monohydrate and 250.2 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 1 h, with a stirring speed of 100 rpm; after the dropwise addition is completed, heat to 95 °C and keep the temperature for 4 h.

[0079] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (a), the obtained lithium manganese iron phosphate material is a rhomboid plate-like particle with a size of approximately 300–500 nm.

[0080] The preparation method of the positive electrode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid, glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, mix with pure water (the amount of pure water is twice the mass of the solid material), and add phosphoric acid in the form of industrial-grade 85% phosphoric acid; then, mill at 2000 rpm for 1 hour, and spray dry to obtain dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the positive electrode material Li. 1.04 (Mn 0.1 Fe 0.9 ) 0.98 Mg 0.02 PO4.

[0081] Example 2

[0082] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0083] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add industrial grade ammonia (25wt%) to adjust the pH to 7.5.

[0084] (2) Weigh 50.7 kg of manganese sulfate monohydrate and 194.6 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve the manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 1.5 h, with a stirring speed of 100 rpm; after the dropwise addition is completed, heat to 95 °C and keep the temperature for 3 h.

[0085] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (b), the obtained lithium manganese iron phosphate material is a rhomboid elongated sheet-like particle with a size of approximately 700–900 nm.

[0086] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.3 Fe 0.7 ) 0.98 Mg 0.02 PO4.

[0087] Example 3

[0088] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0089] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add industrial grade ammonia (25wt%) to adjust the pH to 9.

[0090] (2) Weigh 84.5 kg of manganese sulfate monohydrate and 139 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 0.5 h, with a stirring speed of 50 rpm; after the dropwise addition is completed, heat to 90 °C and keep the temperature for 3 h.

[0091] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (c), the obtained lithium manganese iron phosphate material is a secondary spherical particle formed by the agglomeration of sheet-like particles, with a size of approximately 40 μm.

[0092] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.5 Fe 0.5 ) 0.98 Mg 0.02 PO4.

[0093] Example 4

[0094] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0095] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add industrial grade ammonia (25wt%) to adjust the pH to 8.

[0096] (2) Weigh 92.95 kg of manganese sulfate monohydrate and 125.1 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 1 h, with a stirring speed of 100 rpm; after the dropwise addition is completed, heat to 90 °C and keep the temperature for 4 h.

[0097] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (d), the obtained lithium manganese iron phosphate material is a micron-sized sheet-like particle with a size of about 1 to 2 μm.

[0098] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.55 Fe 0.45 ) 0.98 Mg 0.02 PO4.

[0099] Example 5

[0100] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0101] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add industrial grade ammonia (25wt%) to adjust the pH to 7.6.

[0102] (2) Weigh 101.4 kg of manganese sulfate monohydrate and 111.2 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve the manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 1.5 h, with a stirring speed of 150 rpm; after the dropwise addition is completed, heat to 95 °C and keep the temperature for 5 h.

[0103] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (e), the obtained lithium manganese iron phosphate material is a nanoscale bulk particle with a size of approximately 100–200 nm.

[0104] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.6 Fe 0.4) 0.98 Mg 0.02 PO4.

[0105] Example 6

[0106] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0107] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add sulfuric acid (98wt%) to adjust the pH to 7.

[0108] (2) Weigh 109.85 kg of manganese sulfate monohydrate and 97.3 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve the manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 2 h, with a stirring speed of 200 rpm; after the dropwise addition is completed, heat to 95 °C and keep the reaction at this temperature for 6 h.

[0109] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (f), the obtained lithium manganese iron phosphate material is a nanoscale sheet-like particle with a size of approximately 200–300 nm.

[0110] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.65 Fe 0.35 ) 0.98 Mg 0.02 PO4.

[0111] Example 7

[0112] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0113] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add sulfuric acid (98wt%) to adjust the pH to 6.5.

[0114] (2) Weigh 118.3 kg of manganese sulfate monohydrate and 83.4 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 2.5 h, with a stirring speed of 250 rpm; after the dropwise addition is completed, heat to 100 °C and keep the temperature for 7 h.

[0115] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (g), the obtained lithium manganese iron phosphate material is an octahedral particle with a size of approximately 700–900 nm.

[0116] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.7 Fe 0.3 ) 0.98 Mg 0.02 PO4.

[0117] Example 8

[0118] The preparation method of lithium manganese iron phosphate material includes the following steps:

[0119] (1) In a 1500L reactor, add 110kg of industrial grade concentrated phosphoric acid (85wt%); weigh 120kg of lithium carbonate and add it to 600L of pure water to obtain a lithium carbonate suspension; add the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 7.2. Add sulfuric acid (98wt%) to adjust the pH to 6.

[0120] (2) Weigh 126.75 kg of manganese sulfate monohydrate and 69.5 kg of ferrous sulfate heptahydrate, add 350 L of water to completely dissolve the manganese sulfate monohydrate and ferrous sulfate heptahydrate to obtain a metal solution; then add the metal solution dropwise to the reaction vessel of step (1) within 3 h, with a stirring speed of 300 rpm; after the dropwise addition is completed, heat to 100 °C and keep the temperature for 8 h.

[0121] (3) After the reaction is complete, wait for the temperature of the reactor to drop below 70°C, filter and wash the product, collect it, and dry it in a forced-air oven at 80°C for 10 hours. The SEM image of the lithium manganese iron phosphate material prepared in this embodiment is shown below. Figure 1 As shown in (h), the obtained lithium manganese iron phosphate material is a long rod-shaped particle with a size of approximately 10 μm.

[0122] The preparation method of the cathode material includes the following steps: Weigh the above-mentioned lithium manganese iron phosphate material, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid materials; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.75 Fe 0.25 ) 0.98 Mg 0.02 PO4.

[0123] Example 9

[0124] This embodiment refers to the preparation method of lithium manganese iron phosphate material and cathode material in Example 5. The only difference is that in step (1), 99.16 kg of lithium dihydrogen phosphate is used to replace 110 kg of industrial grade concentrated phosphoric acid (85 wt%) in Example 5, and the amount of lithium carbonate is adjusted from 120 kg to 80 kg. The other steps remain unchanged.

[0125] Comparative Example 1

[0126] The preparation method of the cathode material includes the following steps: Weigh lithium manganese iron phosphate, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide in a molar ratio of 1:0.03:0.04:0.06:0.02, and mix with pure water, the amount of pure water being twice the mass of the solid materials; then, mill at 2000 rpm for 1 hour, and spray dry to obtain dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.8 Fe 0.2 ) 0.98 Mg 0.02 PO4.

[0127] The lithium manganese iron phosphate material was prepared according to Example 1 in Chinese patent application publication number CN115180607A.

[0128] Comparative Example 2

[0129] The preparation method of the cathode material includes the following steps: Manganese tetroxide, iron phosphate, lithium carbonate, phosphoric acid (industrial grade 85% phosphoric acid), glucose, and magnesium oxide are weighed in a molar ratio of 0.2:0.4:0.502:1.03:0.08:0.02, and mixed with pure water, the amount of pure water being twice the mass of the solid materials; then, the mixture is milled at 2000 rpm for 2 hours, followed by spray drying to obtain a dry material; the dry material is calcined at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.6 Fe 0.4 ) 0.98 Mg 0.02 PO4.

[0130] Comparative Example 3

[0131] The preparation method of the cathode material includes the following steps: Weigh manganese ferric oxalate (manganese-iron molar ratio of 6:4), lithium carbonate, phosphoric acid, glucose, and magnesium oxide in a molar ratio of 1:0.502:1.03:0.065:0.02, and mix them with pure water, the amount of pure water being twice the mass of the solid material; then, mill the mixture at 2000 rpm for 1 hour, and spray dry it to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.6 Fe 0.4 ) 0.98 Mg 0.02 PO4.

[0132] Comparative Example 4

[0133] The preparation method of the cathode material includes the following steps: Weigh ferromanganese carbonate (manganese-iron molar ratio of 6:4), lithium carbonate, glucose, and magnesium oxide in a molar ratio of 1:0.502:0.08:0.02, and mix with pure water, the amount of pure water being twice the mass of the solid material; then, mill at 2000 rpm for 1 hour, and spray dry to obtain a dry material; calcine the dry material at 720℃ for 10 hours under a nitrogen atmosphere to obtain the cathode material Li. 1.04 (Mn 0.6 Fe 0.4 ) 0.98 Mg 0.02 PO4.

[0134] Comparative Example 5

[0135] The preparation method of lithium manganese iron phosphate material in Comparative Example 5 is the same as that in Example 1, except that step (1) is different.

[0136] Step (1) of Comparative Example 5 includes: weighing 122 kg of lithium phosphate and adding it to 600 L of pure water to obtain a lithium phosphate suspension; adding the lithium phosphate suspension to a 1500 L reactor at room temperature, and then adding 10 kg of industrial grade concentrated phosphoric acid (85 wt%) to obtain a mixed solution of lithium salt and phosphate salt. The pH of the mixed solution is 6.2. Adding industrial grade ammonia water (25 wt%) and adjusting the pH value to 8.5.

[0137] The remaining steps are the same, but this comparative ratio cannot obtain lithium manganese iron phosphate. The product obtained contains lithium phosphate, hydrated ferrous phosphate, ferric pyrophosphate and other crystalline phases, and is a mixture containing lithium, iron, manganese and phosphorus elements.

[0138] Comparative Example 6

[0139] The preparation method of lithium manganese iron phosphate material in Comparative Example 6 is the same as that in Example 1, except that step (1) is different.

[0140] Step (1) of Comparative Example 6 includes: adding 110 kg of industrial grade concentrated phosphoric acid (85 wt%) to a 1500 L reactor; weighing 120 kg of lithium carbonate and adding it to 600 L of pure water to obtain a lithium carbonate suspension; adding the lithium carbonate suspension to the reactor at room temperature to obtain a mixed solution of lithium salt and phosphate salt, the pH of the mixed solution is 7.2, and adding sulfuric acid (98 wt%) to adjust the pH to 4.

[0141] The remaining steps are the same, but this comparative ratio cannot obtain lithium manganese iron phosphate. The product obtained contains lithium phosphate, hydrated ferrous phosphate and other crystalline phases, and is a mixture containing lithium, iron, manganese and phosphorus elements.

[0142] Experimental Example

[0143] The BET specific surface area and tap density of lithium manganese iron phosphate materials prepared in different embodiments were tested, and the test results are shown in Table 1. The carbon content, D50 particle size, BET specific surface area, and tap density of the cathode materials prepared in different embodiments and comparative examples were tested, and the test results are shown in Table 2. Batteries were prepared using the cathode materials prepared in different embodiments and comparative examples, and their electrochemical performance was tested. The test results are shown in Table 3.

[0144] Specifically, the preparation method and testing method of the test battery are as follows:

[0145] Preparation of positive electrode sheet: The positive electrode material, conductive carbon nanotube, and binder PVDF prepared in the above examples and comparative examples were added to N-methylpyrrolidone (NMP) at a mass ratio of 90:5:5 and mixed. The mixture was homogenized, coated on aluminum foil, rolled, and sliced ​​to obtain the positive electrode sheet.

[0146] Battery assembly: Using lithium metal sheets as the negative electrode and a 25μm thick polyethylene porous membrane as the separator, a solution of 1mol / L LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 was selected as the electrolyte. The positive electrode, separator, negative electrode and electrolyte were assembled into a 2032 coin cell in an argon glove box with water content and oxygen content of less than 1ppm.

[0147] Electrochemical performance testing: The batteries were charged and discharged using the LANDCT 2001A charge / discharge tester from Wuhan Lanbo Electronics Co., Ltd. The charge / discharge voltage range was 2.0 to 4.35V. The specific capacity of the assembled lithium-ion batteries was tested at 0.1C and 1C rates, and the cycle performance was tested at 1C rate.

[0148] Table 1. Test results of lithium manganese iron phosphate materials from different embodiments.

[0149]

[0150] Figure 2 The XRD patterns of lithium manganese iron phosphate materials prepared in Examples 1, 5, and 8 are shown. Figure 2 It is evident that the lithium manganese iron phosphate material prepared by the method of this application has high purity and is free of impurities.

[0151] Table 2. Test results of cathode materials from different embodiments and comparative examples.

[0152]

[0153]

[0154]

[0155] Table 3. Test results of batteries made from cathode materials of different embodiments and comparative examples.

[0156]

[0157] The test results above show that the lithium manganese iron phosphate material prepared by the method of this application, when further processed into a cathode material, can significantly improve the discharge capacity and cycle stability of lithium-ion batteries. Therefore, the preparation method of lithium manganese iron phosphate material of this application is more suitable for industrial production and can improve the electrochemical performance of lithium-ion batteries.

[0158] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0159] Industrial applicability

[0160] This application enables the synthesis of highly crystalline, carbon-free lithium manganese iron phosphate materials under normal pressure and low temperature conditions, eliminating the need for high-pressure equipment, resulting in low energy consumption, high yield, and greater suitability for industrial production. Furthermore, the preparation method described in this application can be carried out in a pure aqueous system, omitting the solvent separation step, which aligns with the principles of green chemistry and simplifies the process.

Claims

1. A method for preparing lithium manganese iron phosphate materials, characterized in that, Includes the following steps: (a) A first solution containing a lithium source is mixed with a second solution containing a phosphorus source, and the pH is adjusted to 6-9 to obtain a mixed solution; wherein the lithium source includes at least one of lithium carbonate and lithium dihydrogen phosphate. (b) A third solution containing a divalent manganese source and a divalent iron source is added to the mixed solution and the mixture is kept at 90–100°C for reaction. (c) The material after the reaction in step (b) is subjected to solid-liquid separation, the solid is collected and dried to obtain the lithium manganese iron phosphate material.

2. The preparation method according to claim 1, characterized in that, The phosphorus source includes at least one of phosphoric acid and lithium dihydrogen phosphate.

3. The preparation method according to claim 1 or 2, characterized in that, The heat preservation reaction time is 3 to 8 hours.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The heat preservation reaction is carried out under normal pressure.

5. The preparation method according to any one of claims 1 to 4, characterized in that, The heat preservation reaction is accompanied by stirring; the stirring speed is 50-400 rpm.

6. The preparation method according to any one of claims 1 to 5, characterized in that, The solvent used in the first solution, the second solution, and the third solution is water.

7. The preparation method according to any one of claims 1 to 6, characterized in that, In step (b), the third solution is added to the mixed solution dropwise; the dropwise addition time is 0.5 to 3 hours.

8. The preparation method according to any one of claims 1 to 7, characterized in that, With Mn 2+ The divalent manganese source and Fe 2+ The total amount of the divalent iron source used is calculated and the amount of Li + The molar ratio of the lithium source used is 1:(3~3.3); With Mn 2+ The divalent manganese source and Fe 2+ The total amount of divalent iron source used is calculated and the amount of PO4 used is calculated. 3- The molar ratio of the phosphorus source used is 1:(0.9 to 1.2).

9. The preparation method according to any one of claims 1 to 8, characterized in that, With Mn 2+ The divalent manganese source and Fe 2+ The molar ratio of the divalent iron source is calculated to be 1:9 to 9:

1.

10. The preparation method according to any one of claims 1 to 9, characterized in that, In the first solution, the molar concentration of the lithium source is 1–3 mol / L; In the third solution, the total molar concentration of the divalent manganese source and the divalent iron source is 1–3 mol / L.

11. The preparation method according to any one of claims 1 to 10, characterized in that, The divalent manganese source includes at least one of manganese sulfate, manganese chloride, manganese acetate, and manganese nitrate; the divalent iron source includes at least one of ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous nitrate.

12. Lithium manganese iron phosphate material prepared by any one of claims 1 to 11.

13. The lithium manganese iron phosphate material according to claim 12, characterized in that, The BET specific surface area of ​​the lithium manganese iron phosphate material is 4.1–18.7 m². 2 / g, tap density is 0.92~1.21g / cm³ 3 .

14. A positive electrode material, characterized in that, This includes lithium manganese iron phosphate materials prepared by the preparation method according to any one of claims 1 to 11.

15. A secondary battery, characterized in that, It includes a positive electrode sheet, which includes the positive electrode material as described in claim 14.

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