Preparation method of lithium iron manganese phosphate positive electrode material and lithium iron manganese phosphate positive electrode material

Abstract

The application provides a preparation method of a lithium manganese iron phosphate positive electrode material and the lithium manganese iron phosphate positive electrode material. The preparation method comprises the following steps: mixing a lithium manganese iron phosphate precursor and a COF-5 covalent organic framework material to obtain a first mixture, the COF-5 covalent organic framework material has a porous hole structure and contains carbon elements and active functional groups; and performing first sintering on the first mixture, so that the lithium manganese iron phosphate precursor is converted into a lithium manganese iron phosphate matrix, the COF-5 covalent organic framework material is carbonized to form a porous carbon coating layer, and thus the lithium manganese iron phosphate positive electrode material is obtained. The positive electrode material obtained by the preparation method has a uniform surface coated with a porous carbon layer formed by carbonization of COF-5, the coating layer retains an ordered porous structure, can improve the electronic conductivity and lithium ion diffusion capacity of the material, inhibits manganese dissolution and interface side reactions in high-temperature cycles, and improves particle dispersity and particle size consistency.

Description

Technical Field

[0001] This application relates to the field of cathode material technology, and in particular to a method for preparing lithium manganese iron phosphate cathode material and the lithium manganese iron phosphate cathode material itself. Background Technology

[0002] Lithium manganese iron phosphate (LMP) is considered a promising cathode material for lithium-ion batteries due to its high safety and high voltage platform. However, LMP cathode materials suffer from low intrinsic electronic conductivity, slow lithium-ion diffusion rate, and are prone to manganese dissolution during high-temperature cycling, leading to rapid capacity decay and imperfect rate performance, thus hindering their widespread application. To address these issues, existing technologies typically employ carbon coating strategies to improve electronic conductivity and suppress manganese dissolution. However, traditional carbon coating methods (such as direct mixing with a carbon source followed by sintering) often fail to form a uniform, dense carbon layer that is firmly bonded to the substrate. The carbon layer may exhibit localized excessive thickness or incomplete coating, resulting in limited protection for the material. Summary of the Invention

[0003] In view of this, in order to solve at least one of the above technical problems, this application provides a method for preparing lithium manganese iron phosphate cathode material.

[0004] In addition, this application also provides a method for preparing lithium manganese iron phosphate cathode material.

[0005] In a first aspect, this application provides a method for preparing a lithium manganese iron phosphate cathode material, comprising: providing a lithium manganese iron phosphate precursor; mixing the lithium manganese iron phosphate precursor with a covalent organic framework material (COF-5) to obtain a first mixture, wherein the covalent organic framework material has a porous structure and contains carbon elements and active functional groups; and performing a first sintering on the first mixture to transform the lithium manganese iron phosphate precursor into a lithium manganese iron phosphate matrix, and to carbonize the covalent organic framework material to form a porous carbon coating layer, wherein the carbon coating layer coats the surface of the lithium manganese iron phosphate matrix, and the active functional groups provide interaction sites for the lithium manganese iron phosphate matrix and the carbon coating layer during the first sintering process, so that the carbon coating layer bonds with the lithium manganese iron phosphate matrix to obtain the lithium manganese iron phosphate cathode material.

[0006] Based on the first aspect, in some embodiments of this application, the first mixture further includes a first carbon source, which forms the carbon coating layer together with the covalent organic framework material during the first sintering process.

[0007] Based on the first aspect, in some embodiments of this application, the method for preparing the lithium manganese iron phosphate precursor includes: mixing a manganese source, an iron source, a phosphorus source, a lithium source, and a second carbon source to obtain a second mixture; and subjecting the second mixture to a second sintering to obtain the lithium manganese iron phosphate precursor; wherein the manganese source contains manganese element in an unstable valence state, and the second carbon source is used to reduce the manganese element in the unstable valence state during the second sintering process; and / or, the iron source contains iron element in an unstable valence state, and the second carbon source is used to reduce the iron element in the unstable valence state during the second sintering process.

[0008] Based on the first aspect, in some embodiments of this application, the second carbon source includes at least one of sucrose, glucose, citric acid, polypropylene, and polyethylene glycol; and / or, the second sintering is carried out under inert gas protection.

[0009] Based on the first aspect, in some embodiments of this application, the second sintering temperature is 500℃~700℃ and the time is 6h~12h.

[0010] Based on the first aspect, in some embodiments of this application, the amount of the covalent organic framework material added accounts for 1% to 5% of the mass of the lithium manganese iron phosphate precursor.

[0011] Based on the first aspect, in some embodiments of this application, the active functional group includes at least one of C=N bond, ether bond, carboxyl group, hydroxyl group, and aldehyde group.

[0012] Based on the first aspect, in some embodiments of this application, the lithium manganese iron phosphate precursor further includes a doping element, which includes at least one of Ti, Mg, Nb and V.

[0013] Based on the first aspect, in some embodiments of this application, the amount of the dopant element added to the lithium manganese iron phosphate precursor is 0.1% to 1.0%.

[0014] Based on the first aspect, in some embodiments of this application, the first mixture further includes a first carbon source.

[0015] Based on the first aspect, in some embodiments of this application, the first carbon source includes at least one of sucrose, glucose, citric acid, polypropylene, and polyethylene glycol.

[0016] Based on the first aspect, in some embodiments of this application, the temperature of the first sintering is 700°C to 850°C, and the time is 6h to 12h; and / or, the first sintering is carried out under inert gas protection.

[0017] Based on the first aspect, in some embodiments of this application, the heating rate of the first sintering is 2°C / min to 5°C / min.

[0018] Based on the first aspect, in some embodiments of this application, the preparation method of COF-5 includes: dissolving triazine and formaldehyde in a solvent, adding an alkaline catalyst to adjust the pH value to 9-11; performing a solvothermal reaction to cause the triazine and formaldehyde to undergo a condensation reaction, followed by solid-liquid separation, and washing and drying to obtain COF-5.

[0019] Based on the first aspect, in some embodiments of this application, the molar mass ratio of the triazine to formaldehyde is 1~1.05:2~2.05; and / or, the solvent includes dimethylformamide.

[0020] Based on the first aspect, in some embodiments of this application, the temperature in the thermal reaction is 80°C to 100°C, and the time is 12h to 24h.

[0021] Secondly, embodiments of this application provide a lithium manganese iron phosphate cathode material, which is prepared by the aforementioned method for preparing lithium manganese iron phosphate cathode materials.

[0022] Thirdly, embodiments of this application provide an electrochemical device, the electrochemical device including a positive electrode sheet, the positive electrode sheet including a positive electrode material, the positive electrode material being a positive electrode material prepared by the aforementioned method for preparing lithium manganese iron phosphate positive electrode material or as described above.

[0023] The method for preparing lithium manganese iron phosphate cathode material provided in this application involves mixing a lithium manganese iron phosphate precursor and a COF-5 covalent organic framework material, followed by sintering to carbonize the COF-5 and form a porous carbon coating layer. This coating layer is uniform and dense, firmly bonded to the substrate, and retains an ordered pore structure. This ordered pore structure can construct a three-dimensional conductive network on the substrate surface, improving electronic conductivity; it also provides a fast diffusion channel for lithium ions, improving rate performance; the uniform and dense coating layer can physically block the contact between the electrolyte and the substrate surface, effectively inhibiting manganese ion dissolution, stabilizing the material structure, and improving cycle capacity retention; furthermore, the coating layer helps improve particle dispersibility and particle size consistency, enhancing processing performance and compaction density. Attached Figure Description

[0024] Figure 1 A process flow diagram of the preparation method of lithium manganese iron phosphate cathode material provided in the embodiments of this application.

[0025] Figure 2 This is a comparison chart showing the capacity retention of lithium manganese iron phosphate cathode materials prepared in Comparative Example 1 and Examples 1 to 3 of this application after 100 cycles of 1C charge-discharge at 45°C.

[0026] Figure 3 The images shown are scanning electron microscope (SEM) images of the lithium manganese iron phosphate cathode materials prepared in Comparative Example 1 and Examples 1 to 3; wherein, Figure 3 Figure (a) in the figure is a scanning electron microscope image of the lithium manganese iron phosphate cathode material prepared in Comparative Example 1; Figure 3 Figure (b) is a scanning electron microscope image of the lithium manganese iron phosphate cathode material prepared in Example 1; Figure 3 Figure (c) is a scanning electron microscope image of the lithium manganese iron phosphate cathode material prepared in Example 2; Figure 3 Figure (d) is a scanning electron microscope image of the lithium manganese iron phosphate cathode material prepared in Example 3. Detailed Implementation

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of this application pertain. The terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the embodiments of this application. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturer of reagents or instruments is not specified, they are all conventional products that can be purchased commercially. The use of terms such as "first" and "second" in this application is only for distinguishing different process steps or components and does not imply any limitation on order, quantity, or importance. For example, "first grinding" and "second grinding" only distinguish two grinding steps and do not mean that the first grinding must be performed before the second grinding, nor does it mean that one grinding step is more important than the other.

[0028] The following describes some embodiments of this application in detail. Unless otherwise specified, the embodiments and features described below can be combined with each other.

[0029] Please see Figure 1 As shown, this application provides a method for preparing lithium manganese iron phosphate cathode material, comprising the following steps: providing a lithium manganese iron phosphate precursor; mixing the lithium manganese iron phosphate precursor and a covalent organic framework material to obtain a first mixture, wherein the covalent organic framework material (COF-5) has a porous structure and contains carbon elements and active functional groups; and performing a first sintering on the first mixture to transform the lithium manganese iron phosphate precursor into a lithium manganese iron phosphate matrix, and carbonizing the covalent organic framework material to form a porous carbon coating layer, wherein the carbon coating layer coats the surface of the lithium manganese iron phosphate matrix, and the active functional groups provide interaction sites for the lithium manganese iron phosphate matrix and the carbon coating layer during the first sintering process, so that the carbon coating layer bonds with the lithium manganese iron phosphate matrix to obtain the lithium manganese iron phosphate cathode material.

[0030] Specifically, the preparation method of the lithium manganese iron phosphate cathode material provided in this application includes: Step S1: Provide lithium manganese iron phosphate precursor.

[0031] In some embodiments, the preparation method of lithium manganese iron phosphate precursor includes: mixing a manganese source, an iron source, a phosphorus source, and a lithium source to obtain a second mixture; and subjecting the second mixture to a second sintering to allow the raw materials to undergo a solid-state reaction, generating a lithium manganese iron phosphate precursor with an olivine structure. Depending on the valence states of the metal elements in the manganese and iron sources, a second carbon source may be selectively added to the second mixture. When the manganese source contains manganese with an unstable valence state, or the iron source contains iron with an unstable valence state, a second carbon source is added to the second mixture. This second carbon source can reduce high-valence manganese and iron to divalent states during the second sintering process, promoting the formation of a pure-phase lithium manganese iron phosphate lattice and avoiding the formation of impurity phases due to residual high-valence metal ions. When both the manganese and iron sources are already divalent, there is no need to add a second carbon source; the raw materials can directly react during the second sintering process to form the lithium manganese iron phosphate precursor. Specifically, the preparation method of this lithium manganese iron phosphate precursor includes: mixing a manganese source, an iron source, a phosphorus source, a lithium source, and a second carbon source to obtain a second mixture; and subjecting the second mixture to a second sintering to obtain the lithium manganese iron phosphate precursor. It should be noted that the second carbon source is essentially consumed during the reduction reaction, therefore almost no carbon remains in the precursor. Through the above processes, the obtained lithium manganese iron phosphate precursor possesses high phase purity, good lattice integrity, and suitable surface conditions, laying the foundation for the subsequent preparation of high-performance lithium manganese iron phosphate cathode materials.

[0032] In some embodiments, the preparation method of lithium manganese iron phosphate precursor further includes a second grinding process, specifically as follows: a manganese source, an iron source, a phosphorus source, a lithium source, and a second carbon source are mixed, and after a second grinding process, a second mixture is obtained. Through the above-mentioned second grinding, each raw material component is refined under mechanical force, resulting in smaller particle size and increased specific surface area, which is beneficial for the full occurrence of solid-phase reactions during subsequent sintering. Simultaneously, the mechanochemical action generated during grinding helps to improve the surface activity of the raw material particles, enabling the metal ions in the manganese and iron sources to be evenly distributed, laying the foundation for the formation of a uniformly distributed lithium manganese iron phosphate lattice. The phosphorus and lithium sources are sufficiently refined during grinding and come into close contact with the transition metal source, which is beneficial for achieving precise elemental proportions and rapid reactions during subsequent sintering. The second carbon source is uniformly dispersed in the mixture during grinding and mainly plays a reducing role during subsequent sintering, reducing the high-valence metal ions in the manganese and iron sources to divalent states, promoting the formation of a pure-phase lithium manganese iron phosphate lattice. The solvent in this step includes at least one of deionized water and pure water.

[0033] In some embodiments, the endpoint particle size Dv50 of the second grinding is 0.50 μm to 1.0 μm. Controlling the endpoint particle size within this range is beneficial for obtaining precursor particles with a more uniform particle size distribution, allowing for more complete subsequent reactions, and improving the processing performance of the powder. The endpoint particle size Dv50 of the second grinding can, exemplarily, be 0.50 μm, 0.60 μm, 0.70 μm, 0.80 μm, 0.90 μm, and 1.0 μm, or any value within the range of any two of the above values. The grinding speed of the second grinding is 800 rpm to 1200 rpm. Controlling the grinding speed within this range is beneficial for obtaining higher grinding efficiency, while also making the equipment operation more stable and the mixing temperature more controllable. The grinding speed can, exemplarily, be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, and 1200 rpm, or any value within the range of any two of the above values.

[0034] In some embodiments, the mass ratio of manganese source, iron source, phosphorus source, lithium source and carbon source is 4.7~5.0: 9.7~10.0: 6.7~7.0: 2.3~2.6: 1.0~1.2.

[0035] In some embodiments, the manganese source includes at least one of manganese carbonate, manganese oxalate, manganese phosphate, manganese dioxide, and manganese tetroxide; the iron source includes at least one of ferric oxide, ferrous oxalate, ferric carbonate, and ferric phosphate; the phosphorus source includes at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ferric phosphate, and manganese phosphate; the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, and lithium acetate; and the carbon source includes at least one of sucrose, glucose, citric acid, polypropylene, and polyethylene glycol.

[0036] In some embodiments, the lithium manganese iron phosphate precursor further includes a doping element, which includes at least one of Ti, Mg, Nb and V.

[0037] In some embodiments, the amount of dopant added to the lithium manganese iron phosphate precursor is 0.1% to 1%.

[0038] In some embodiments, before the second sintering of the second mixture, the second mixture is further dried to obtain a dried second mixture. Removing the solvent through drying is beneficial for obtaining a second mixture with regular morphology and good flowability, facilitating subsequent sintering. The drying in step S1.2 includes spray drying, with an inlet air temperature of 200°C to 260°C and an outlet air temperature of 90°C to 120°C. Spray drying is used in this application to obtain precursor powder with good sphericity and uniform particle size distribution. The inlet air temperature can, exemplarily, be 200°C, 220°C, 240°C, and 260°C, or any value within the range of any two of the above values. The outlet air temperature can, exemplarily, be 90°C, 100°C, 110°C, and 120°C, or any value within the range of any two of the above values.

[0039] In some embodiments, the second sintering temperature is 500℃~700℃, and the time is 6h~12h. Controlling the sintering temperature and time within the above range is beneficial for the precursor to fully crystallize, forming a lithium manganese iron phosphate material with high crystallinity and stable structure, thereby obtaining better electrochemical performance. The second sintering temperature can be, for example, 500℃, 550℃, 600℃, 650℃, and 700℃, or any value within the range of any two of the above values; the second sintering time can be, for example, 6h, 8h, 10h, and 12h, or any value within the range of any two of the above values. The second sintering can be carried out under inert gas protection, which helps prevent the material from being oxidized at high temperatures, ensuring the purity and crystal quality of the product; the inert gas introduction rate is 150L / min~250L / min. Selecting an introduction rate within this range helps maintain a stable and sufficient protective atmosphere during the sintering process, ensuring a uniform and controllable sintering environment. The inlet rate can be, for example, 150 L / min, 180 L / min, 200 L / min, 220 L / min, and 250 L / min, or any value within the range of any two of the above values. The inert gas includes one or more of nitrogen, argon, and helium. Inert gas protection prevents the precursor from oxidizing with oxygen in the air during high-temperature sintering, ensuring that the transition metal elements in the material maintain the required valence state, while preventing the carbon coating layer from being oxidized and consumed, thereby guaranteeing the electrochemical performance of the material.

[0040] Step S2: The lithium manganese iron phosphate precursor and COF-5 (a covalent organic framework material) are mixed to obtain a first mixture. COF-5 has a porous structure and contains carbon elements and active functional groups; COF-5 is a covalent organic framework material with an ordered pore structure and containing nitrogen elements. Through this step, COF-5 can be uniformly dispersed and attached to the surface of the lithium manganese iron phosphate precursor particles. Due to the ordered pore structure and active functional groups of COF-5, its ordered pore structure can serve as a template to guide the orderly deposition of carbonization products during subsequent sintering, while the active functional groups provide chemical bonding sites between the carbon coating layer and the substrate, which is beneficial for forming a porous carbon coating layer that is firmly bonded to the substrate and has a complete pore structure. At the same time, the nitrogen elements contained in COF-5 can be incorporated into the carbon network during the carbonization process to form a nitrogen-doped structure, further optimizing the electronic conductivity of the carbon layer. Through the above effects, this step lays a good foundation for the subsequent sintering to form a uniform, dense, firmly bonded porous carbon coating layer with a three-dimensional conductive network.

[0041] In some embodiments, step S2 further includes a first grinding process, specifically as follows: mixing the lithium manganese iron phosphate precursor and COF-5, and then grinding the mixture to obtain a first mixture. This first grinding process helps to uniformly disperse and adhere COF-5 to the surface of the lithium manganese iron phosphate precursor particles, laying the foundation for the formation of a uniform and dense nitrogen-doped porous carbon coating layer by COF-5 carbonization during subsequent sintering.

[0042] In step S2, the amount of COF-5 added accounts for 1% to 5% of the mass of the lithium manganese iron phosphate precursor. Controlling the amount added within the above range is beneficial for forming a nitrogen-doped porous carbon coating layer with moderate thickness and good coating degree, so that the coating layer can effectively play a protective and conductive role, while maintaining good lithium-ion transport performance.

[0043] In some embodiments, the preparation method of COF-5 includes: Method 1: dissolving triazine and formaldehyde in a solvent, adding an alkaline catalyst to adjust the pH value to 9-11, and carrying out a solvothermal reaction to allow the triazine and formaldehyde to undergo a condensation reaction to form COF-5; and obtaining the COF-5 after solid-liquid separation, washing, and drying. Preparing COF-5 using the above method is beneficial for obtaining a covalent organic framework material with an ordered structure, high specific surface area, and good thermal stability, providing a high-quality precursor for the subsequent formation of nitrogen-doped porous carbon coating layers. Specifically, under alkaline conditions, the amino groups in the triazine undergo a nucleophilic addition-condensation reaction with the aldehyde groups in the formaldehyde to form a covalent organic framework structure, thereby constructing COF-5 with regular channels and a periodic skeleton. The solvothermal reaction provides a uniform heating environment and sufficient reaction time, which is beneficial for the formation and growth of COF-5 crystal nuclei, resulting in a product with high crystallinity and an ordered pore structure. After the reaction, unreacted monomers and byproducts are removed by washing, and drying preserves the pore structure of COF-5, ultimately yielding a COF-5 material with high specific surface area, regular pores, and good thermal stability. This COF-5 serves as a precursor for subsequent coating layers. Its ordered porous structure forms a nitrogen-doped porous carbon coating layer with a three-dimensional conductive network and fast ion transport channels during carbonization, thereby improving the electrochemical performance of the lithium manganese iron phosphate cathode material. This method is simple, low-cost, and suitable for industrial applications.

[0044] Method 2: In addition to the methods described above for preparing COF-5, COF-5 can also be prepared using the following method 2. This includes: Step A1, Basic Synthesis Process: 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and terephthalic acid (PDA) were dissolved in a mixed solvent of 1,3,5-trimethylbenzene and 1,4-dioxane at a molar ratio of 1:2, with an appropriate amount of acetic acid added as a catalyst. The mixed solution was transferred to a Pyrex glass tube and subjected to three freeze-evacuation-thawing cycles to remove oxygen from the system. The glass tube was then sealed with a flame torch. The sealed reaction tube was placed in a constant temperature oven and reacted at 120°C for 72 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature, and the crude gray-purple COF-5 product was obtained by centrifugation.

[0045] Step A2, Product Purification and Post-treatment: The crude product was washed 3-5 times sequentially with acetone and methanol to remove unreacted monomers and residual solvents. The washed product was then placed in a Soxhlet extractor and extracted continuously for 24 hours using tetrahydrofuran as the extractant to further purify the product. Finally, the purified COF-5 powder was placed in a vacuum drying oven and dried at 80°C for 12 hours to obtain crystalline COF-5 material. However, this method involves cumbersome synthesis steps and high costs, making it unsuitable for industrial applications.

[0046] Method 3: In addition to the methods described above for preparing COF-5, COF-5 can also be prepared using the following method 3: Under solvent-free conditions at room temperature, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and terephthalic acid (PDA) are placed in a mortar at a molar ratio of 1:2, with a small amount of acetic acid added as a grinding aid. The mixture is then manually or mechanically ground for 5-30 minutes. During grinding, the mechanical force induces a condensation reaction in the monomer molecules, directly generating the COF-5 product. Although this process eliminates the need for organic solvents, offers mild reaction conditions, and has low energy consumption, making it suitable for industrial production, the reaction is uneven, resulting in lower purity and crystallinity of the synthesized product, limiting its applications.

[0047] Therefore, this application primarily utilizes Method 1 for generation. In some embodiments, the solvothermal reaction temperature is 80℃~100℃, and the time is 12h~24h. Controlling the temperature and time of the solvothermal reaction within the above range is beneficial for providing suitable thermodynamic conditions for the nucleation and growth of COF-5, allowing the reactants to fully react and form a covalent organic framework material with high crystallinity and regular pore structure, thereby obtaining a COF-5 product with ordered structure, high specific surface area, and good thermal stability. The heating temperature can be, for example, 80°C, 85°C, 90°C, 95°C, and 100°C, or any value within the range of any two of the above values; the reaction time can be, for example, 12h, 16h, 20h, and 24h, or any value within the range of any two of the above values; and / or, the molar ratio of triazine to formaldehyde is 1~1.05:2~2.05. Controlling the molar ratio within the above range helps ensure stoichiometric balance of reactants and promotes efficient synthesis of COF-5. The molar ratio of triazine to formaldehyde can be, for example, 1:2, 1.02:2.02, and 1.05:2.05, or any value within the range of any two of the above values; the solvent includes dimethylformamide, which facilitates the complete dissolution of reactants and the smooth progress of the polycondensation reaction. The centrifugation speed in this step is 800rpm-1200rpm, and the time is 5min-10min, which facilitates the rapid collection of the COF-5 precipitate generated in the reaction. The solvent includes dimethylformamide or deionized water. The washing process is repeated 3 to 5 times to effectively remove unreacted monomers and solvent residues, thereby improving product purity. The washing frequency can be 3, 4, or 5 times, or any value within the range of any two of the above values. The washed product is then dried under vacuum at a temperature of 60°C to 80°C for 12 to 24 hours. Selecting this range of drying temperature and time helps avoid high-temperature oxidation and obtains a structurally stable and well-dispersed COF-5 powder. The drying temperature can be 60°C, 65°C, 70°C, 75°C, or 80°C, or any value within the range of any two of the above values. The drying time can be 12 hours, 16 hours, 20 hours, or 24 hours, or any value within the range of any two of the above values.

[0048] In some embodiments, the first mixture further includes a first carbon source. The preparation method of the first mixture is as follows: a lithium manganese iron phosphate precursor, a first carbon source, and COF-5 are mixed and subjected to a first grinding process to obtain the first mixture, wherein COF-5 is a covalent organic framework material with an ordered porous structure and containing nitrogen. The use of the first carbon source and COF-5 in this step is mainly to maintain the excellent performance brought about by the formation of a nitrogen-doped porous carbon coating layer by COF-5, while partially replacing the high-cost COF-5 with the relatively low-cost first carbon source. This allows the carbon layer formed by the carbonization of the first carbon source and the ordered carbon layer derived from COF-5 to cross-composite and jointly construct a more complete conductive network, effectively reducing the material preparation cost while ensuring the coating effect and electrochemical performance.

[0049] The first carbon source in step S2 includes at least one of sucrose, glucose, citric acid, polypropylene, and polyethylene glycol. The amount of the first carbon source in step S2 is 3% to 10% of the mass of the lithium manganese iron phosphate precursor. The final particle size Dv50 of the first grinding is 0.27 μm to 0.32 μm. Controlling the final particle size within the above range is beneficial for obtaining uniformly fine mixed particles, allowing COF-5 and the second carbon source to be more fully dispersed and adhered to the surface of the lithium manganese iron phosphate precursor, thereby providing a good foundation for the subsequent sintering to form a uniform and dense nitrogen-doped porous carbon coating layer. The final particle size Dv50 can be, for example, 0.27 μm, 0.28 μm, 0.29 μm, 0.30 μm, 0.31 μm, and 0.32 μm, or any value within the range of any two of the above values. The initial grinding speed is 800 rpm to 1200 rpm. Selecting a speed within this range helps to obtain appropriate shear force while ensuring grinding efficiency, allowing COF-5 and the carbon source to be uniformly dispersed in the mixture, thereby obtaining a mixed system with better dispersion and ensuring the uniformity of the coating layer. The speed can be, for example, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, and 1200 rpm, or any value within the range of any two of the above values.

[0050] In some embodiments, the first carbon source includes at least one of sucrose, glucose, citric acid, polypropylene, and polyethylene glycol.

[0051] In some embodiments, the mass ratio of the first carbon source to COF-5 is 5:1 to 1:5.

[0052] In some embodiments, the active functional group includes at least one of C=N bond, ether bond, carboxyl group, hydroxyl group, and aldehyde group.

[0053] Step S3: The first mixture is sintered to transform the lithium manganese iron phosphate precursor into a lithium manganese iron phosphate matrix, and the COF-5 covalent organic framework material is carbonized to form a porous carbon coating layer. This carbon coating layer coats the surface of the lithium manganese iron phosphate matrix. During sintering, the active functional groups provide interaction sites for the lithium manganese iron phosphate matrix and the carbon coating layer, enabling the carbon coating layer to bond with the lithium manganese iron phosphate matrix, thus obtaining the lithium manganese iron phosphate cathode material. Through the first sintering, the lithium manganese iron phosphate precursor undergoes a crystal transformation at high temperature, forming an olivine-structured lithium manganese iron phosphate matrix. Simultaneously, COF-5 undergoes pyrolysis and carbonization at high temperature. Because COF-5 itself has an ordered pore structure and contains nitrogen, its ordered framework structure is preserved during carbonization, and carbon elements are deposited in the original framework positions, forming a nitrogen-doped porous carbon coating layer. The nitrogen-doped porous carbon coating enhances the performance of lithium manganese iron phosphate (LFP) cathode materials primarily through the following mechanisms: First, the ordered pore structure retained after COF-5 carbonization constructs a three-dimensional conductive network on the LFP matrix surface, composed of interconnected interwoven carbon nanofibers. When multiple LFP matrix particles are coated with this three-dimensional conductive network, the conductive layers on the particle surfaces contact or overlap, forming a continuous conductive pathway throughout the entire electrode. Electrons can rapidly transport between particles through this pathway, effectively reducing the contact resistance between particles and improving the overall electronic conductivity of the electrode. Simultaneously, nitrogen elements in the COF-5 molecular structure are incorporated into the carbon network during carbonization, forming active sites such as C=N. These nitrogen-doped sites optimize the electronic structure of the carbon layer, increasing the carrier concentration and mobility of the conductive network, making electron transport in the three-dimensional conductive network more efficient, and further reducing the resistivity of the material. Second, the ordered pores retained in the nitrogen-doped porous carbon coating provide rapid diffusion channels for lithium ions. During charging and discharging, lithium ions can rapidly migrate within the electrode material through these ordered channels, reducing ion transport resistance and thus improving the material's rate performance and capacity utilization. Simultaneously, the uniform, dense, and firmly bonded nitrogen-doped porous carbon coating acts as a physical barrier, effectively preventing direct contact between the electrolyte and the lithium manganese iron phosphate matrix surface, reducing surface side reactions, and inhibiting manganese ion dissolution. This enhances the material's structural stability, allowing it to maintain good capacity retention during long-term cycling. Furthermore, the presence of the nitrogen-doped porous carbon coating helps improve the dispersibility of lithium manganese iron phosphate particles, preventing agglomeration during grinding and sintering, resulting in a more uniform primary particle size distribution, reducing micropowder generation, and improving particle size consistency. This, in turn, improves the material's processing performance and compaction density. Through these synergistic effects, the lithium manganese iron phosphate cathode material prepared in this application exhibits higher electronic conductivity, superior lithium-ion diffusion capability, stronger structural stability, and better processing performance, thereby achieving higher discharge capacity, better rate performance, and longer cycle life.

[0054] In some embodiments, before the first sintering in step S3, a drying step is further included. Drying includes spray drying, which is beneficial for obtaining powder particles with good sphericity and uniform particle size distribution, providing a good foundation for the subsequent sintering to form a uniform coating layer. The inlet air temperature of the spray dryer is 200℃~260℃, and the outlet air temperature is 90℃~120℃. Selecting inlet and outlet air temperatures within this range is beneficial for controlling the drying rate and particle morphology, obtaining well-dispersed dried powder. The inlet air temperature can, for example, be 200℃, 220℃, 240℃, and 260℃, or any value within the range of any two of the above values; and / or, the outlet air temperature can, for example, be 90℃, 100℃, 110℃, and 120℃, or any value within the range of any two of the above values.

[0055] In some embodiments, the first sintering temperature is 700℃~850℃, and the time is 6h~12h. Controlling the sintering temperature and time within the above range is beneficial for the transformation of the lithium manganese iron phosphate precursor into an olivine-structured lithium manganese iron phosphate matrix, obtaining a target product with high crystallinity and a complete crystal lattice. Simultaneously, under these temperature conditions, COF-5 can undergo sufficient pyrolysis and carbonization, preserving its ordered pore structure during the carbonization process. Carbon elements are deposited in the original framework positions, and nitrogen elements in the COF-5 molecular structure are incorporated into the carbon network during carbonization, forming a porous carbon coating layer with a complete structure, ordered pores, and uniform nitrogen doping. This provides a good foundation for the subsequent construction of a three-dimensional conductive network and a rapid lithium-ion transport channel. The first sintering temperature can, for example, be 700℃, 750℃, 800℃, and 850℃, or any value within the range of any two of the above values; the first sintering time can, for example, be 6h, 8h, 10h, and 12h, or any value within the range of any two of the above values.

[0056] In some embodiments, the heating rate of the first sintering is 2°C / min to 5°C / min. Selecting a heating rate within this range helps control the uniformity of the carbonization process, resulting in a more structurally complete nitrogen-doped porous carbon coating. The heating rate can, for example, be 2°C / min, 3°C / min, 4°C / min, and 5°C / min, or any value within the range of any two of the above values. And / or, the first sintering is carried out under an inert gas atmosphere. This helps prevent the material from oxidizing at high temperatures, ensuring the structural integrity of the coating and the substrate. And / or, the inert gas introduction rate is 150L / min to 250L / min. Selecting an introduction rate within this range helps maintain a stable and sufficient protective atmosphere, ensuring a uniform and controllable sintering environment. The introduction rate can, for example, be 150L / min, 180L / min, 200L / min, 220L / min, and 250L / min, or any value within the range of any two of the above values.

[0057] In some embodiments, the doping element accounts for 0.1% to 1.0% of the mass ratio of the lithium manganese iron phosphate precursor.

[0058] In some embodiments, the first sintering is carried out under an inert atmosphere, the inert gas including at least one of nitrogen, argon, and helium. Using an inert atmosphere prevents the carbon coating layer from oxidizing at high temperatures, ensuring the smooth progress of the carbonization reaction, and simultaneously prevents the manganese and iron elements in the lithium manganese iron phosphate matrix from being oxidized to higher valence states, thereby obtaining a cathode material with stable structure and good electrochemical performance.

[0059] In some embodiments, the thickness of the nitrogen-doped porous carbon coating layer is 1 nm to 6 nm. Controlling the coating layer thickness within this range is beneficial for forming a uniform and dense protective layer, which can effectively improve conductivity and suppress manganese dissolution, without hindering the rapid transport of lithium ions due to excessive coating layer thickness. Thus, while improving capacity, good rate performance is also achieved. The coating layer thickness can be 2 nm, 3 nm, 4 nm, 5 nm, and 6 nm, or any value within the range of any two of the above values.

[0060] Compared with the prior art, the preparation method of lithium manganese iron phosphate cathode material provided in this application has the following beneficial effects: 1. In the preparation of lithium manganese iron phosphate (LFP) cathode materials, the LFP precursor is mixed with COF-5, and after a first sintering, the COF-5 is carbonized to form a nitrogen-doped porous carbon coating layer that coats the surface of the LFP substrate. Since COF-5 itself has an ordered pore structure and contains nitrogen and carbon elements, its ordered pore structure is retained after carbonization. Simultaneously, nitrogen is incorporated into the carbon network, forming a structurally complete, ordered, and uniformly nitrogen-doped porous carbon coating layer. This coating layer, on the one hand, constructs a three-dimensional conductive network on the substrate surface through its ordered pore structure, reducing the contact resistance between particles and improving electronic conductivity. On the other hand, the retained ordered pores provide a rapid diffusion channel for lithium ions, reducing lithium ion migration resistance, thereby improving the material's rate performance and capacity utilization. Furthermore, the uniform and dense nitrogen-doped porous carbon coating layer acts as a physical barrier, preventing direct contact between the electrolyte and the substrate surface, inhibiting the dissolution of manganese ions, enhancing the material's structural stability, and extending cycle life. Meanwhile, the coating layer also helps improve particle dispersibility, prevent particle agglomeration, enhance particle size uniformity and compaction density, and enable the material to maintain good electrochemical performance under high compaction conditions.

[0061] 2. In the preparation of lithium manganese iron phosphate cathode material, a first carbon source is added in the first grinding step and used together with COF-5. The carbon layer formed by the carbonization of the first carbon source during sintering and the ordered carbon layer derived from COF-5 cross-composite to jointly construct a better conductive network. Since the cost of the first carbon source is relatively low, it partially replaces the high-cost COF-5 while maintaining the excellent performance brought by the formation of nitrogen-doped porous carbon coating layer. This effectively reduces the material preparation cost while ensuring the coating effect and electrochemical performance, thus improving the economy and scalability of the process.

[0062] 3. In the preparation of lithium manganese iron phosphate precursor, by introducing doping elements, the doping elements enter the precursor lattice to achieve uniform substitution, changing the electronic structure inside the lattice, which is beneficial to improving the intrinsic conductivity of the material and improving the rate performance. At the same time, the uniform distribution of doping elements in the lattice helps to enhance the stability of the crystal structure, suppress volume changes during charging and discharging, reduce particle cracking and structural damage, thereby further extending the cycle life of the material.

[0063] 4. The preparation method provided in this application has a clear process route, is simple to operate, requires no complex equipment or harsh reaction conditions, and is easy to scale up for production. By controlling process parameters such as sintering temperature, time, and raw material ratio, lithium manganese iron phosphate cathode materials with high crystallinity, complete structure, and uniform coating can be stably obtained. This material combines high electronic conductivity, high lithium-ion diffusion rate, good structural stability, and excellent processing performance, providing a new technical path for developing high-capacity, long-life, and high-safety lithium-ion battery cathode materials.

[0064] This application provides a lithium manganese iron phosphate cathode material prepared by the aforementioned method for preparing lithium manganese iron phosphate cathode material. Since this lithium manganese iron phosphate cathode material is prepared by mixing a lithium manganese iron phosphate precursor with COF-5 and then sintering, the COF-5 carbonizes to form a nitrogen-doped porous carbon coating layer that coats the surface of the lithium manganese iron phosphate matrix. Therefore, this material possesses a three-dimensional conductive network constructed with an ordered pore structure and a nitrogen-doped optimized electronic structure, which is beneficial for improving electronic conductivity and lithium-ion diffusion capability. Simultaneously, the uniform and dense coating layer can effectively suppress manganese ion dissolution, enhance structural stability, and improve particle dispersion and compaction density.

[0065] This application also provides an electrochemical device, including a positive electrode, a negative electrode, and an electrolyte layer located between the positive and negative electrodes. The positive electrode is prepared by the aforementioned method for preparing lithium manganese iron phosphate positive electrode material or by using the aforementioned lithium manganese iron phosphate positive electrode material. Because this electrochemical device uses a lithium manganese iron phosphate positive electrode material with a nitrogen-doped porous carbon coating, the ordered pore structure of this coating is beneficial for constructing a three-dimensional conductive network, improving electronic conductivity, and providing a fast diffusion channel for lithium ions, thereby achieving higher discharge capacity and better rate performance. Furthermore, the uniform and dense coating can effectively suppress manganese ion dissolution, enhance structural stability, improve cycle capacity retention, and improve particle dispersion and compaction density, giving the battery excellent rate performance, cycle stability, and energy density.

[0066] The present application will be further described below with reference to specific embodiments and comparative examples.

[0067] Example 1 Step S1: Add 4.89 kg of manganese tetroxide, 9.88 kg of iron phosphate, 6.78 kg of lithium dihydrogen phosphate, 2.45 kg of lithium carbonate, 1.02 kg of glucose, 69.28 g of titanium dioxide, and 216.08 g of magnesium carbonate to 30 kg of water and mix evenly; put it into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), and then put it into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) until the final particle size Dv50 is 0.65 μm to obtain the second mixture.

[0068] Step S2: The second mixture obtained in step S1 is spray-dried with an inlet air temperature of 240°C and an outlet air temperature of 100°C. Then, it is sintered under an inert atmosphere (specifically nitrogen) with a heating rate of 3°C / min to 650°C, held for 7 hours, and cooled to obtain the lithium manganese iron phosphate precursor.

[0069] Step S3: Take 10 kg of the above-mentioned lithium manganese iron phosphate precursor, 0.26 kg of glucose and 0.13 kg of COF-5, add them to 15 kg of water and mix evenly; put them into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), and then put them into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) until the final particle size Dv50 is 0.32 μm, to obtain the first mixture.

[0070] Step S4: The first mixture obtained in step S3 is spray-dried at an inlet air temperature of 240°C and an outlet air temperature of 100°C. Then, it is sintered under an inert atmosphere, heated to 750°C at a heating rate of 3°C / min, held for 7 hours, cooled, and then pulverized by airflow to obtain the lithium manganese iron phosphate cathode material of Example 1 (denoted as LMFCF@NC-1).

[0071] Example 2 The difference between this embodiment and Embodiment 1 is that the amount of glucose added in step S3 is 0.13 kg, and the amount of COF-5 added is 0.26 kg. The remaining steps and parameters are the same as in Embodiment 1, resulting in the lithium manganese iron phosphate cathode material of Embodiment 2 (denoted as LMFCF@NC-2).

[0072] Example 3 The difference between this embodiment and Embodiment 1 is that glucose is not added in step S3, and the amount of COF-5 added is 0.39 kg. The remaining steps and parameters are the same as in Embodiment 1, and the lithium manganese iron phosphate cathode material of Embodiment 3 (denoted as LMFCF@NC-3) is obtained.

[0073] Comparative Example 1 The difference between this comparative example and Example 1 is that COF-5 was not added in step S3, and the amount of glucose added was 0.38 kg. The remaining steps and parameters are the same as in Example 1.

[0074] Experimental testing: The lithium iron phosphate cathode materials obtained in the comparative examples and embodiments were subjected to the following performance tests, and the specific test methods are as follows: I. Battery Preparation: The lithium manganese iron phosphate cathode materials obtained in each example and comparative example were used to make cathode sheets, and lithium metal was used as the anode material to assemble coin cells for electrical performance testing.

[0075] Positive electrode preparation: Powdered positive electrode materials: conductive carbon black: PVDF were added to 160g of NMP solvent at a mass ratio of 90:5:5 and stirred at 850 rpm for 10 minutes. The mixture was then coated, dried at 80℃ for 2 hours, and finally rolled to achieve an electrode surface density of 2.0 g / cm³. 3 ~2.2g / cm 3 .

[0076] Button cell assembly: Positive electrode shell - Electrode (electrolyte) - Separator (electrolyte) - Lithium sheet - Nickel foam - Negative electrode shell, sealing pressure 7 kg / cm² 2 .

[0077] II. Performance Testing: The above-mentioned lithium manganese iron phosphate cathode material and the assembled coin cells were tested in the following aspects, as detailed below: (1) pH value detection: The pH value of the lithium manganese iron phosphate cathode material in water was tested according to GB / T24533-2019 standard.

[0078] (2) Specific surface area detection: The nitrogen adsorption-desorption method was adopted. At liquid nitrogen temperature, the correlation between the equilibrium adsorption amount of nitrogen on the material surface and its specific surface area and other properties was determined. Combined with the law of the change of adsorption amount with relative pressure during the adsorption process, the specific surface area was tested.

[0079] (3) Detection of carbon content: The carbon content of lithium manganese iron phosphate cathode material was tested by infrared analysis. Using a carbon-sulfur analyzer, the sample was burned in a high-temperature oxygen-rich state to oxidize the carbon elements contained therein into carbon dioxide, which entered the infrared detector with the carrier gas. The carbon content was calculated by quantitatively analyzing the changes in the intensity of the infrared absorption wavelength of the carbon dioxide signal.

[0080] (4) Testing of powder compaction density: The test was conducted in accordance with the standard "Determination of compaction density of lithium-ion battery cathode material powder" drafted by the National Technical Committee on Standardization of Nonferrous Metals.

[0081] (5) Test of tap density: Refer to GB / T1033-2008 standard to test the tap density of lithium manganese iron phosphate cathode material.

[0082] (6) Detection of powder resistivity: The test was conducted in accordance with the standard "Determination of Powder Resistivity of Cathode Material for Lithium-ion Batteries" drafted by the National Technical Committee on Standardization of Nonferrous Metals.

[0083] (7) Detection of manganese leaching: The test shall be conducted in accordance with GB / T37211-2018 "Test Method for Leaching of Metal Impurities in Cathode Materials of Lithium-ion Batteries".

[0084] (8) Detection of electrochemical performance: ① Initial charge / discharge efficiency and 0.1C specific capacity test: Under normal temperature conditions of 25℃, the assembled coin cell battery was charged at a constant current of 0.1C (nominal capacity 150mAh / g) to 4.5V, and then charged at a constant voltage until the current dropped to 0.05C. Charging was then stopped, and the initial charge capacity was recorded. The battery was then discharged at 0.1C to the cutoff voltage of 2.5V, and the initial discharge capacity was recorded. The 0.1C specific capacity of the positive electrode material was calculated based on the initial discharge capacity, and the initial charge / discharge efficiency (first-time efficiency) of the battery was calculated according to the following formula: Initial charge / discharge efficiency = (initial discharge capacity / initial charge capacity) × 100%.

[0085] Repeat the above charge and discharge process once, record the second charge capacity and the second discharge capacity, and calculate the 0.1C specific capacity of the lithium manganese iron phosphate cathode material based on the second discharge capacity.

[0086] ②1C specific capacity test: Repeat the above charge and discharge process once at a 1C current, record the second charge capacity and the second discharge capacity, and calculate the 1C specific capacity of the lithium manganese iron phosphate cathode material based on the second discharge capacity.

[0087] ③ High-temperature cycle performance test: The battery capacity retention rate was tested after 100 cycles at 45℃ with a 1C charge / discharge current within a voltage range of 2.5V~4.4V. The results are as follows: Figure 1 As shown.

[0088] The test results for the above test items are summarized in Table 1.

[0089] Table 1 Experimental Results Analysis: Based on the test results in Table 1, it can be seen that compared with Comparative Example 1, the lithium manganese iron phosphate cathode materials in each embodiment showed varying degrees of improvement in charge-discharge performance. The difference between Example 1 and Comparative Example 1 lies in the introduction of COF-5 during the coating process and the corresponding adjustment of the carbon source amount. The test results show that the introduction of COF-5 improved the electrochemical performance of the material. This improvement is attributed to the nitrogen-doped porous carbon coating layer formed after COF-5 carbonization, which is beneficial for improving electronic conductivity and lithium-ion diffusion capability.

[0090] Further comparison of Examples 1, 2, and 3 reveals that the charge-discharge performance of the material continuously improves with increasing COF-5 content. Example 3 differs from Example 1 in that COF-5 completely replaces glucose as the carbon source, and the highest COF-5 content is achieved, resulting in optimal electrochemical performance. This indicates that appropriately increasing the COF-5 ratio is beneficial for fully utilizing the conductive network and porous structure advantages of the nitrogen-doped porous carbon coating layer formed after carbonization. Examples 1 and 2, by introducing glucose as the primary carbon source in conjunction with COF-5, achieve better electrochemical performance while partially replacing the high-cost COF-5, making material preparation more cost-effective.

[0091] Regarding high-temperature cycling stability: Figure 2 The high-temperature cycling performance curves shown indicate that the capacity retention of each embodiment after 100 cycles at 45°C is superior to that of the comparative example. The difference between Example 2 and Example 1 lies in the increased ratio of COF-5 to glucose, which further enhances the high-temperature cycling stability. Example 3, unlike Example 1, does not add glucose and uses only COF-5 as the carbon source for coating, resulting in the most outstanding high-temperature cycling performance. This demonstrates that the nitrogen-doped porous carbon coating layer formed after COF-5 carbonization acts as a physical barrier, effectively isolating the electrolyte from the active material, suppressing manganese dissolution and interfacial side reactions during high-temperature cycling, and buffering volume changes, thereby improving the material's cycling stability.

[0092] Comparative analysis of particle morphology and particle size distribution: combined with Figure 3 Electron microscopy images show that, compared to Comparative Example 1, the lithium manganese iron phosphate particles in all embodiments exhibit a nanosphere morphology. However, after being coated with a nitrogen-doped porous carbon coating layer formed by COF-5 carbonization, the primary particle size distribution is more uniform, and the micropowder content is reduced. Comparing Examples 1, 2, and 3, it can be found that with the increase of COF-5 content, the particle uniformity is further improved, and the micropowder phenomenon is better suppressed. The difference between Example 3 and Example 1 is that Example 3 uses COF-5 entirely as the coating layer precursor, resulting in the most regular particle morphology and the most concentrated particle size distribution. This morphology improvement helps reduce particle agglomeration and breakage, improves the structural consistency of the material, and thus enhances the electrode's processing performance and cycle stability.

[0093] In summary, this application modifies lithium manganese iron phosphate cathode materials by forming a nitrogen-doped porous carbon coating layer through COF-5 carbonization. The lithium manganese iron phosphate cathode materials exhibit significant improvements in electrochemical performance, high-temperature cycle stability, and particle morphology. Furthermore, the modification effect becomes more pronounced with an appropriate increase in the amount of COF-5, achieving a synergistic improvement in conductivity, structural stability, and particle size uniformity. The scheme of introducing glucose as the primary carbon source in conjunction with COF-5 achieves both excellent performance and cost-effectiveness in material preparation.

[0094] The above description describes some specific embodiments of this application, but in actual applications, the application should not be limited to these embodiments. For those skilled in the art, other modifications and alterations made based on the technical concept of this application should fall within the protection scope of this application.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, include: Provide lithium manganese iron phosphate precursor; The lithium manganese iron phosphate precursor and the covalent organic framework material are mixed to obtain a first mixture, wherein the covalent organic framework material has a porous structure and contains carbon elements and active functional groups. as well as The first mixture is subjected to a first sintering process to transform the lithium manganese iron phosphate precursor into a lithium manganese iron phosphate matrix, and the covalent organic framework material is carbonized to form a porous carbon coating layer. The carbon coating layer coats the surface of the lithium manganese iron phosphate matrix. The active functional groups provide interaction sites for the lithium manganese iron phosphate matrix and the carbon coating layer during the first sintering process, so that the carbon coating layer bonds with the lithium manganese iron phosphate matrix to obtain the lithium manganese iron phosphate cathode material.

2. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The first mixture also includes a first carbon source, which forms the carbon coating layer together with the covalent organic framework material during the first sintering process.

3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation method of the lithium manganese iron phosphate precursor includes: A second mixture is obtained by mixing a manganese source, an iron source, a phosphorus source, a lithium source, and a second carbon source; and The second mixture is subjected to a second sintering to obtain the lithium manganese iron phosphate precursor; Wherein, the manganese source contains manganese in an unstable valence state, and the second carbon source is used to reduce the manganese in the unstable valence state during the second sintering process; and / or, The iron source contains iron in an unstable valence state, and the second carbon source is used to reduce the iron in the unstable valence state during the second sintering process.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The amount of the covalent organic framework material added accounts for 1% to 5% of the mass of the lithium manganese iron phosphate precursor.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The active functional group includes at least one of the following: C=N bond, ether bond, carboxyl group, hydroxyl group, and aldehyde group.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The lithium manganese iron phosphate precursor also includes doping elements, which include at least one of Ti, Mg, Nb and V.

7. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The amount of the dopant element added to the lithium manganese iron phosphate precursor is 0.1% to 1.0%.

8. The method for preparing the lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The method for preparing the covalent organic framework material includes: Triazine and formaldehyde are dissolved in a solvent, and an alkaline catalyst is added to adjust the pH value to 9-11. A solvothermal reaction is carried out to cause the triazine and formaldehyde to undergo a condensation reaction. After solid-liquid separation, washing, and drying, the covalent organic framework material is obtained.

9. A lithium iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material is prepared by the method for preparing lithium manganese iron phosphate cathode material as described in any one of claims 1 to 8.

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode sheet, the positive electrode sheet includes a positive electrode material, the positive electrode material being a positive electrode material prepared by the method for preparing lithium manganese iron phosphate positive electrode material as described in any one of claims 1 to 8 or the lithium manganese iron phosphate positive electrode material as described in claim 9.

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