A lithium manganese iron phosphate cathode material for lithium secondary batteries and a method for preparing the same

Abstract

The application discloses the lithium ion battery anode material technical field, and relates to a lithium manganese iron phosphate anode material for a lithium secondary battery and a preparation method thereof. The preparation method comprises the following steps: firstly, a lithium manganese iron phosphate precursor containing magnesium and fluorine is synthesized through a hydrothermal method; then, the lithium manganese iron phosphate precursor is mixed with sucrose; through two-step sintering, carbon-coated magnesium-fluorine co-doped lithium manganese iron phosphate powder is obtained; the carbon-coated magnesium-fluorine co-doped lithium manganese iron phosphate powder is mixed with lithium titanate-tantalate powder and sintered, so as to form a composite anode material with a core-shell structure. Finally, the composite material is mixed with polyaniline grafted cyclodextrin diatomite composite conductive gel, conductive carbon black and a binder to prepare electrode slurry; the electrode slurry is coated on an aluminum foil current collector; after drying and tabletting, a positive electrode sheet is prepared. The anode material prepared by the method has high electronic conductivity and ion migration rate, can significantly improve the rate performance and cycle stability of a lithium battery, and is suitable for the field of lithium secondary batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a lithium manganese iron phosphate cathode material for lithium secondary batteries and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, higher requirements have been placed on the energy density, safety performance, and cycle life of lithium-ion batteries. Lithium iron phosphate (LiFePO4) with an olivine structure is widely used due to its excellent safety performance and cycle stability; however, its low operating voltage platform and limited theoretical capacity restrict further improvements in battery energy density. While lithium manganese phosphate (LMP) has a higher operating voltage platform, its electronic conductivity and ion diffusion rate are extremely low, and manganese dissolution is a serious problem. Lithium iron manganese phosphate (LMP) combines the advantages of both, possessing a moderate voltage platform and a relatively high theoretical energy density, making it one of the most promising next-generation cathode materials. However, this material also faces inherent defects such as low electronic conductivity and slow lithium-ion diffusion rate, resulting in unsatisfactory rate performance and capacity.

[0003] To improve the electrochemical performance of lithium manganese iron phosphate (LFP) materials, researchers have proposed various modification strategies. While traditional carbon coating technology can improve electronic conductivity to some extent, excessive carbon content reduces volumetric energy density, and simple carbon coating offers limited improvement in ionic conductivity. Ion doping is another commonly used method, introducing heterovalent ions to increase intrinsic electronic conductivity or expand lithium-ion migration channels, but the effect of single doping is often limited. In recent years, surface coating with fast ion conductor materials has been considered an effective solution, protecting the direct contact between the active material and the electrolyte while providing efficient channels for lithium-ion transport. However, achieving the synergistic effect of multiple modification techniques and precisely controlling the construction of doping, coating, and composite conductive networks at the nanoscale remains a key technical challenge in this field.

[0004] Existing modification methods mostly focus on optimizing a single approach, making it difficult to simultaneously address issues of electronic conductivity, ionic conductivity, and structural stability. This is particularly true for lithium manganese iron phosphate materials with high manganese content, where problems such as low intrinsic conductivity, manganese dissolution due to the Jahn-Teller effect during charge and discharge, and structural distortion are more pronounced. Therefore, there is an urgent need to develop a comprehensive modification strategy that leverages multi-level synergistic effects to comprehensively improve the electrochemical performance of lithium manganese iron phosphate materials while maintaining their advantages in safety and long cycle life, in order to meet the development requirements of high-energy-density lithium-ion batteries. Summary of the Invention

[0005] The purpose of this invention is to provide a lithium manganese iron phosphate cathode material for lithium secondary batteries and its preparation method, which solves the technical problems of low electronic conductivity and ion diffusion rate, poor rate performance and insufficient cycle stability of existing lithium manganese iron phosphate cathode materials.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A method for preparing lithium manganese iron phosphate cathode material, comprising the following steps:

[0008] S1. Weigh lithium carbonate, manganese acetate, ferrous sulfate, ammonium dihydrogen phosphate, magnesium acetate and ammonium fluoride, add them to deionized water and stir to dissolve them, controlling the pH of the solution to 5.5-6.5; then transfer the mixed solution to a microwave hydrothermal reactor and react at 180-220℃ to obtain a magnesium and fluorine-containing LMFP precursor.

[0009] S2. The magnesium and fluorine-containing LMFP precursor was mixed with sucrose, ball-milled, and then subjected to a two-step sintering process under an argon atmosphere: first, pre-sintering at 348-352℃, followed by main sintering at 748-752℃, to obtain C-LMFP-Mg-F powder; the C-LMFP-Mg-F powder was then mixed with LiTa 0.3 Ti 0.7 O3 was mixed, ball-milled, and then sintered at 700-800℃ in air atmosphere to obtain C-LMFP-Mg-F@LTTO composite powder;

[0010] S3. C-LMFP-Mg-F@LTTO composite powder, PANI-g-CD / Diatomite composite conductive gel, conductive carbon black and polyvinylidene fluoride are mixed at high speed in N-methyl-2-pyrrolidone to form a uniform slurry, which is then coated on aluminum foil, vacuum dried at 118-122℃, pressed into tablets and slit.

[0011] In this invention, the preparation reaction mechanism of lithium manganese iron phosphate cathode material mainly involves the synergistic effects of ion doping, carbothermal reduction, crystal growth, and surface modification. In the hydrothermal reaction stage, lithium, manganese, iron, phosphorus, magnesium, and fluorine sources undergo co-precipitation and crystallization under specific temperature and pH conditions. Magnesium ions enter the crystal structure by substituting manganese iron sites, while fluorine ions achieve anion doping by substituting oxygen sites. This anion-cation co-doping effect effectively improves the intrinsic electronic conductivity of the material and broadens the lithium-ion migration channels. In the carbothermal reduction stage, sucrose decomposes under an inert atmosphere to generate a reducing atmosphere, reducing ferric ions to electrochemically active ferrous ions. Simultaneously, pyrolytic carbon uniformly coats the material surface, forming a continuous conductive network. In the surface modification stage, a high-temperature solid-state reaction causes a dense coating layer of lithium titanate fast ion conductors to form on the material surface. This coating layer not only inhibits the corrosion of the active material by the electrolyte but also provides an efficient channel for lithium-ion transport. Finally, during the mixing process with the composite conductive gel, the polyaniline-grafted cyclodextrin diatomaceous earth composite, through its three-dimensional porous structure and abundant functional groups, forms a close contact with the active material, constructing a dual continuous electron and ion transport pathway, thereby significantly improving the rate performance and cycle stability of the material. The entire preparation process achieves atomic-level doping optimization, nanoscale surface modification, and micrometer-level conductive network construction through multi-scale structural control, resulting in a cathode material exhibiting excellent electrochemical performance.

[0012] In this invention, the C-LMFP-Mg-F@LTTO composite powder is a high-performance material constructed through a multi-level modification strategy. Its name intuitively reflects the core composition and multi-layered structure of the material: "C" represents an amorphous carbon coating layer, and "LMFP" is lithium manganese iron phosphate (LiMn). x Fe 1-x "Mg-F" is an abbreviation for PO4, indicating that magnesium ions (Mg) have been incorporated into the material. 2+ ) and fluoride ions (F - The term "LTTO" refers to the co-doping of lithium titanate (LiTa) with "@" indicating the core-shell structure design, and "LTTO" indicating the outermost lithium titanate (LiTa) layer. 0.3 Ti 0.7 O3) fast ion conductor coating. Its core value lies in the synergistic effect of bulk doping, surface carbon coating, and fast ion conductor modification, which systematically solves three key problems of traditional lithium manganese iron phosphate materials: low intrinsic electronic conductivity, slow lithium-ion diffusion rate, and poor electrode / electrolyte interface stability. Magnesium ions are introduced through lattice doping. Due to their similar ionic radii to manganese and iron ions, they can replace some manganese or iron sites in the lattice, generating lattice distortion and defects, widening the lithium-ion migration channels, and potentially introducing a small number of cation vacancies, which helps to improve intrinsic ionic conductivity; fluoride ions partially replace oxygen sites (PO4). 3- O in2- Anion doping enhances the stability of the crystal structure by strengthening the PF bond energy, suppressing lattice volume changes during charge and discharge, and reducing the dissolution of transition metal ions. Fluorine doping also helps to regulate the band structure of the material, improving its electronic conductivity to some extent. The amorphous carbon layer produced by the pyrolysis of sucrose under an inert atmosphere (such as argon) through two steps (first, low-temperature pre-firing to melt and initially carbonize the sucrose, then high-temperature main firing to fully crystallize the crystal and form a continuous conductive network) uniformly coats the surface of the active material, constructing a highly efficient electronic conduction network. This significantly reduces the interfacial charge transfer impedance of the material and inhibits the excessive growth of active nanoparticles during high-temperature sintering. The outermost layer, LTTO (LiTa),... 0.3 Ti 0.7 O3 is a fast ion conductor material with a perovskite structure, possessing good lithium-ion conductivity and a wide electrochemical stability window. This coating layer is formed by subsequent ball milling with LTTO powder and secondary sintering at a low temperature under atmospheric atmosphere. It effectively isolates the active material from direct contact with the electrolyte, reducing the occurrence of side reactions (such as Mn). 3+ The Jahn-Teller effect leads to manganese dissolution and electrolyte oxidative decomposition, while simultaneously providing a high-speed channel for lithium-ion cross-interface transport, reducing interfacial impedance, and its high mechanical strength can, to some extent, inhibit lithium dendrite growth. The preparation method of this material is as follows: First, a magnesium-fluorine co-doped LMFP precursor is synthesized via microwave hydrothermal method. Then, it is mixed with sucrose, dispersed by ball milling, and subjected to a two-step sintering process under an inert atmosphere to achieve carbothermic reduction and carbon coating, obtaining a C-LMFP-Mg-F intermediate. Next, this intermediate is mixed with pre-synthesized LTTO powder through a second ball milling process to achieve uniform mixing and preliminary coating. Finally, a low-temperature sintering is performed in an air atmosphere to strengthen the bond between the LTTO coating layer and the core, ensuring the stability of the LTTO's crystal structure and ionic conductivity. The resulting composite powder significantly improves the specific capacity, rate performance, cycle life, and thermal stability of the lithium manganese iron phosphate cathode material, making it suitable for high-energy-density and high-safety lithium secondary battery systems.

[0013] According to a preferred embodiment of the present invention, in step S1, the hydrothermal reaction is carried out at 180-220°C for 8-10 hours.

[0014] According to a preferred embodiment of the present invention, in step S2, the pre-firing is carried out at 348-352℃ for 4-6 hours; the main firing time is 12-14 hours at 748-752℃; and the sintering time is 4-8 hours at 700-800℃.

[0015] According to a preferred embodiment of the present invention, in step S3, the vacuum drying time at 118-122°C is 3-5 hours.

[0016] According to a preferred embodiment of the present invention, the preparation method of the PANI-g-CD / Diatomite composite conductive gel includes: A1, dissolving β-cyclodextrin in sodium hydroxide solution, adding activated porous diatomaceous earth, and stirring at 58-62°C; subsequently adding aniline monomer and ammonium persulfate, and reacting at 0-5°C; A2, finally washing the product with deionized water and ethanol until neutral, and vacuum drying at 58-62°C.

[0017] In this invention, the preparation process of the PANI-g-CD / Diatomite composite conductive gel involves a complex physicochemical mechanism, primarily manifested in two key stages: carrier functionalization modification and in-situ composite of conductive polymers. The first stage's reaction mechanism is based on the synergistic effect of diatomite carrier activation and cyclodextrin modification. Natural diatomite undergoes high-temperature calcination, effectively removing organic impurities and adsorbed water from its surface and pores, fully exposing its inherent porous structure. Subsequent acid washing not only further dissolves the metal oxide impurities in the diatomite, achieving deep purification, but more importantly, the hydrogen ions in the acid react with the silanol groups on the diatomite surface, increasing the density of surface active sites. The activated diatomite possesses a large specific surface area and rich surface chemical properties, providing an ideal three-dimensional framework carrier for the construction of the composite conductive gel. In the subsequent reaction, under alkaline conditions, the hydroxyl groups in the cyclodextrin molecular structure undergo deprotonation, forming more reactive groups. These reactive groups can be firmly anchored to the silanol groups on the diatomaceous earth surface and the inner walls of the pores through various intermolecular forces. The unique conical structure of the cyclodextrin molecule has hydrophobic properties on the inside and hydrophilic properties on the outside. Its hydrophobic cavity provides an ideal host environment for the subsequent pre-assembly of monomers. Through host-guest interactions, aniline monomers are adsorbed and enriched at the reaction interface, creating favorable conditions for the polymerization reaction. The reaction mechanism of the second stage focuses on the in-situ oxidative polymerization of aniline and its composite process with the functionalized carrier. Under low-temperature conditions, the active species generated by the decomposition of the oxidant initiate the polymerization reaction of aniline monomers. Due to the pre-organization and concentration effect of the cyclodextrin cavity on the aniline monomers, the polymerization reaction preferentially occurs on the surface and inside the pores of the cyclodextrin-modified diatomaceous earth. This characteristic allows the polyaniline molecular chains to grow along the pore direction using the three-dimensional porous structure of diatomaceous earth as a template, forming a continuous and stable conductive network. During polymerization, polyaniline chains and cyclodextrin molecules form a stable composite structure through π-π stacking and hydrogen bonding. The cyclodextrin, in turn, is chemically bonded to the diatomaceous earth framework, resulting in a tight composite structure among the three components. This multi-level composite structure retains the high specific surface area and porous characteristics of diatomaceous earth while fully leveraging the high conductivity of polyaniline and the supramolecular recognition ability of cyclodextrin. The resulting composite conductive gel exhibits excellent electronic conductivity and ion transport performance, providing a more efficient and stable conductive network for lithium manganese iron phosphate cathode materials and significantly improving the overall electrochemical performance of the battery.

[0018] According to a preferred embodiment of the present invention, in step A1, the reaction time at 0-5°C is 12-14 hours.

[0019] According to a preferred embodiment of the present invention, in step A1, the treatment steps of the activated porous diatomaceous earth include: calcining the diatomaceous earth in a muffle furnace at 500-600°C; then mixing the calcined diatomaceous earth with hydrochloric acid solution and stirring continuously at 60-80°C; then filtering the acid-washed mixture and repeatedly washing the filter cake with deionized water until the filtrate is neutral; finally, drying the washed diatomaceous earth in a vacuum drying oven at 105-120°C.

[0020] According to a preferred embodiment of the present invention, in step A2, the vacuum drying time at 58-62°C is 24-30 hours.

[0021] The present invention also provides a lithium manganese iron phosphate cathode material, which is prepared according to the preparation method of the lithium manganese iron phosphate cathode material.

[0022] The present invention also provides an application of the aforementioned lithium manganese iron phosphate cathode material in lithium secondary batteries.

[0023] The beneficial effects of this invention are as follows:

[0024] The lithium manganese iron phosphate cathode material prepared by this invention exhibits excellent comprehensive electrochemical performance. Through the synergistic doping of magnesium and fluorine, the intrinsic electronic conductivity of the material is effectively improved and the lithium-ion migration channels are broadened, enabling the material to maintain a high specific capacity even under high-speed charge-discharge conditions. The dual protective structure formed by the carbon coating layer and the lithium titanate fast-ion conductor layer not only significantly enhances the electron transport capability of the particle surface but also greatly increases the migration rate of lithium ions at the interface, resulting in excellent capacity retention under various rate conditions. Testing shows that this material maintains stable cycling performance under both high and low temperature environments and retains a high reversible capacity even after long-term cycling, effectively solving the problem of rapid capacity decay in traditional cathode materials under high-voltage operating conditions.

[0025] The preparation method of this invention has multiple technical advantages. The microwave hydrothermal synthesis method enables uniform mixing at the molecular level, ensuring a uniform distribution of dopant elements at the atomic scale, laying the foundation for consistent electrochemical performance. The stepwise sintering process, through precise temperature control, achieves simultaneous optimization of the carbon coating layer and crystal structure, ensuring effective carbonization of the carbon source while avoiding the decomposition of active materials. The unique composite conductive gel constructs a three-dimensional conductive network, providing a more continuous and efficient electron transport path compared to traditional single conductive agents. The entire process flow is rationally designed, with precise parameter control, exhibiting good repeatability and scalability, providing reliable technical support for industrial production.

[0026] This cathode material exhibits significant comprehensive advantages in practical applications. Its high operating voltage platform significantly improves battery energy density while maintaining the inherent safety characteristics of the phosphate system, effectively reducing the risk of thermal runaway. Excellent rate performance enables it to meet the demands of fast-charging applications, greatly shortening charging time. Its long cycle life is particularly suitable for energy storage applications requiring long-term use, reducing the total cost of ownership over its entire lifecycle. Furthermore, the raw materials used in its preparation are widely available, cost-effective, and environmentally friendly, aligning with the development concept of green manufacturing. It has broad application prospects in electric vehicles, large-scale energy storage power stations, and portable electronic devices. Detailed Implementation

[0027] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0028] The following is information on domestic suppliers of key related equipment and materials:

[0029] The lithium carbonate was purchased from Sichuan Guoli Lithium Materials Co., Ltd.

[0030] The manganese acetate was purchased from Sichuan Kunlun Chemical Co., Ltd.

[0031] The ferrous sulfate was purchased from Hunan Yongqing Environmental Protection Co., Ltd.

[0032] The ammonium dihydrogen phosphate was purchased from Sichuan Longmang Phosphate Chemical Co., Ltd.

[0033] The magnesium acetate was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0034] The ammonium fluoride was purchased from Polyfluoride New Materials Co., Ltd.

[0035] The argon gas was purchased from Wuhan Iron and Steel Group Gas Co., Ltd.

[0036] The LiTa 0.3 Ti 0.7 O3 was purchased from Guangdong Oriental Zirconium Industry Technology Co., Ltd.

[0037] The conductive carbon black was purchased from Shanghai Cabot Chemical Co., Ltd.

[0038] The polyvinylidene fluoride was purchased from Shanghai Sanai New Materials Co., Ltd.

[0039] The N-methyl-2-pyrrolidone was purchased from Shandong Mait New Material Technology Co., Ltd.

[0040] The aluminum foil was purchased from Xinjiang Zhonghe Co., Ltd.

[0041] The β-cyclodextrin was purchased from Shandong Xinda Biotechnology Co., Ltd.

[0042] The sodium hydroxide solution was purchased from Tianjin Bohua Yongli Chemical Co., Ltd.

[0043] The aniline monomer was purchased from Jiangsu Yangnong Chemical Group Co., Ltd.

[0044] The ammonium persulfate was purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0045] The diatomite was purchased from Jilin Yuantong Mining Co., Ltd.

[0046] The hydrochloric acid was purchased from Shanghai Huayi Group Chemical Industry Co., Ltd.

[0047] Example 1

[0048] First, accurately weigh 73.8g of lithium carbonate, 107.9g of manganese acetate, 83.4g of ferrous sulfate, 114.9g of ammonium dihydrogen phosphate, 2.1g of magnesium acetate, and 1.5g of ammonium fluoride. Add these raw materials sequentially to 2000mL of deionized water and stir with a magnetic stirrer at 500rpm for 30min until completely dissolved. Adjust the pH of the solution dropwise with ammonia water to 6.0 and maintain stability. Transfer the entire mixture to a 1000mL microwave hydrothermal reactor lined with polytetrafluoroethylene (PTFE). Seal the reactor and place it in a microwave hydrothermal synthesizer. Set the reaction temperature to 200℃ and the heating rate to 10℃ / min. Maintain the reaction at the target temperature for 9 hours. After the reaction, allow it to cool naturally to room temperature. Open the reactor and remove the precipitate. Wash the precipitate three times with deionized water and anhydrous ethanol by alternating centrifugation. Dry under vacuum at 80℃ for 12 hours to obtain a magnesium and fluorine-containing LMFP precursor.

[0049] 100g of the precursor and 9.8g of sucrose were placed together in a planetary ball mill jar, and zirconia grinding balls were added at a ball-to-material ratio of 10:1. The mixture was ball-milled at 300 rpm for 4 hours to obtain a uniformly mixed powder. The mixed powder was placed in an alumina crucible and placed in a tube furnace. Argon gas was introduced to purge air at a flow rate of 50 mL / min. The temperature was first increased to 350℃ at 5℃ / min and held for 5 hours to complete the pre-carbonization of sucrose. Then, the temperature was increased to 750℃ at 2℃ / min and held for 13 hours to complete crystallization and carbon coating. After natural cooling, C-LMFP powder was obtained. 100g of C-LMFP powder and 8g of LiTa were then mixed with... 0.3 Ti 0.7O3 powder was placed in a ball mill jar, and anhydrous ethanol was added as a dispersion medium. After ball milling for 6 hours, the powder was removed, dried at 80°C to remove the ethanol, and the mixed powder was pressed into tablets and placed in a muffle furnace. The temperature was increased to 750°C at 3°C / min under an air atmosphere and sintered for 6 hours. After natural cooling, the powder was ground through a 400-mesh sieve to obtain C-LMFP@LTTO composite powder.

[0050] Take another 100g of diatomaceous earth and place it in a corundum crucible. Heat the crucible to 550℃ in a muffle furnace at 2℃ / min and calcine for 3h. After natural cooling, mix it with 500mL of 4mol / L hydrochloric acid solution in a round-bottom flask. Install a reflux condenser and heat in an oil bath at 70℃ with magnetic stirring for 6h. After the reaction is complete, filter the solution and wash it with deionized water until the pH of the filtrate is neutral. Dry the solution under vacuum at 110℃ for 18h to obtain activated diatomaceous earth. 10 g of β-cyclodextrin was added to 200 mL of 1 mol / L sodium hydroxide solution and stirred in a water bath at 40 °C until completely dissolved. 5 g of activated diatomaceous earth was added and stirred at 60 °C for 2 h. Then, 10 mL of freshly distilled aniline monomer was added, and the mixture was cooled to 3 °C in an ice-water bath. 22.8 g of ammonium persulfate (dissolved in 50 mL of deionized water) was slowly added and the reaction was maintained at 3 °C for 13 h. After the reaction was completed, the mixture was filtered and washed successively with deionized water, ethanol, and acetone until the filtrate was colorless. The filtrate was dried under vacuum at 60 °C for 28 h to obtain the PANI-g-CD / Diatomite composite conductive gel.

[0051] Accurately weigh 90g of C-LMFP@LTTO composite powder, 5g of composite conductive gel, 3g of conductive carbon black and 2g of polyvinylidene fluoride, add 100mL of N-methyl-2-pyrrolidone as solvent, and stir at 10000rpm for 2h using a high-speed shear emulsifier to prepare a uniform slurry. Use an automatic coating machine to coat it onto 16μm aluminum foil with a thickness of 100μm, and vacuum dry at 120℃ for 4h. Then use a roller mill to press it into sheets with a pressure of 10MPa, and finally cut it into 50×50mm positive electrode sheets.

[0052] Example 2

[0053] The specific implementation method is the same as in Example 1, except that: firstly, 75.2g of lithium carbonate, 110.3g of manganese acetate, 79.8g of ferrous sulfate, 117.6g of ammonium dihydrogen phosphate, 1.8g of magnesium acetate, and 1.3g of ammonium fluoride are weighed and added to 2000mL of deionized water and stirred to dissolve. The pH of the solution is adjusted to 5.8 with ammonia. The mixed solution is transferred to a microwave hydrothermal reactor and reacted at 190℃ for 10h to obtain a magnesium and fluorine-containing LMFP precursor. This precursor is mixed with 8.5g of sucrose, ball-milled for 5h, and then subjected to two-step sintering under an argon atmosphere: first pre-calcined at 348℃ for 6h, and then main calcined at 748℃ for 14h to obtain C-LMFP powder. 100g of C-LMFP powder and 5g of LiTa 0.3 Ti0.7 O3 powder was mixed and ball-milled for 5 hours, then sintered at 700℃ for 8 hours in air to obtain C-LMFP@LTTO composite powder. Separately, 100g of diatomaceous earth was placed in a muffle furnace and calcined at 500℃ for 4 hours. The calcined diatomaceous earth was mixed with 500mL of 3mol / L hydrochloric acid solution and stirred continuously at 60℃ for 8 hours. The acid-washed mixture was then filtered, and the filter cake was repeatedly washed with deionized water until the filtrate was neutral. Finally, the washed diatomaceous earth was placed in a vacuum drying oven and dried at 105℃ for 24 hours to obtain activated diatomaceous earth. 10g of β-cyclodextrin was dissolved in 200mL of 1mol / L sodium hydroxide solution, and 3g of activated diatomaceous earth was added. The mixture was stirred at 58℃ for 3h. Subsequently, 8mL of aniline monomer and 20.5g of ammonium persulfate were added, and the reaction was carried out at 0℃ for 14h. Finally, the product was washed with deionized water and ethanol until neutral, and then vacuum dried at 58℃ for 30h to obtain PANI-g-CD / Diatomite composite conductive gel. 85g of C-LMFP@LTTO composite powder, 8g of composite conductive gel, 4g of conductive carbon black, and 3g of polyvinylidene fluoride were mixed at high speed in 100mL of N-methyl-2-pyrrolidone to form a uniform slurry. This slurry was coated onto aluminum foil, vacuum dried at 118℃ for 5h, pressed, and slit to obtain the positive electrode sheet.

[0054] Example 3

[0055] The specific implementation method is the same as in Example 1, except that: firstly, 72.5g of lithium carbonate, 105.2g of manganese acetate, 86.7g of ferrous sulfate, 112.3g of ammonium dihydrogen phosphate, 2.4g of magnesium acetate, and 1.7g of ammonium fluoride are weighed and added to 2000mL of deionized water and stirred to dissolve. The pH of the solution is adjusted to 6.2 with ammonia. The mixed solution is transferred to a microwave hydrothermal reactor and reacted at 210℃ for 8 hours to obtain a magnesium and fluorine-containing LMFP precursor. This precursor is mixed with 11.2g of sucrose, ball-milled for 3 hours, and then sintered in two steps under an argon atmosphere: first pre-calcined at 352℃ for 4 hours, and then main-calcined at 752℃ for 12 hours to obtain C-LMFP powder. 100g of C-LMFP powder and 12g of LiTa 0.3 Ti 0.7O3 powder was mixed and ball-milled for 7 hours, then sintered at 800℃ for 4 hours in air to obtain C-LMFP@LTTO composite powder. Separately, 100g of diatomaceous earth was placed in a muffle furnace and calcined at 600℃ for 2 hours. The calcined diatomaceous earth was mixed with 500mL of 6mol / L hydrochloric acid solution and stirred continuously at 80℃ for 4 hours. The acid-washed mixture was then filtered, and the filter cake was repeatedly washed with deionized water until the filtrate was neutral. Finally, the washed diatomaceous earth was placed in a vacuum drying oven and dried at 120℃ for 12 hours to obtain activated diatomaceous earth. 10g of β-cyclodextrin was dissolved in 200mL of 1mol / L sodium hydroxide solution, and 8g of activated diatomaceous earth was added. The mixture was stirred at 62℃ for 1h. Subsequently, 12mL of aniline monomer and 25.6g of ammonium persulfate were added, and the mixture was reacted at 5℃ for 12h. Finally, the product was washed with deionized water and ethanol until neutral, and then vacuum dried at 62℃ for 24h to obtain PANI-g-CD / Diatomite composite conductive gel. 93g of C-LMFP@LTTO composite powder, 2g of composite conductive gel, 2g of conductive carbon black, and 3g of polyvinylidene fluoride were mixed at high speed in 100mL of N-methyl-2-pyrrolidone to form a uniform slurry. This slurry was coated onto aluminum foil, vacuum dried at 122℃ for 3h, pressed, and slit to obtain the positive electrode sheet.

[0056] Comparative Example 1

[0057] The specific implementation method is the same as in Example 1, except that magnesium acetate and ammonium fluoride are not added.

[0058] Comparative Example 2

[0059] The specific implementation method is the same as in Example 1, except that the carbon coating step is not performed.

[0060] Comparative Example 3

[0061] The specific implementation method is the same as in Example 1, except that LiTa is not performed. 0.3 Ti 0.7 O3 coating steps.

[0062] Performance testing

[0063] The positive electrode sheets prepared in Examples 1-3 and Comparative Examples 1-3 were punched into 12 mm diameter discs as working electrodes. The active material was accurately weighed using a 0.01 g / L analytical balance. A lithium metal sheet (14 mm diameter, 0.45 mm thickness) was used as the counter electrode and reference electrode. A Celgard 2400 polypropylene membrane (16 mm diameter) was used as the separator. The electrolyte was a 1 mol / L LiPF6 EC / DMC / EMC solution (volume ratio 1:1:1). 80 μL of electrolyte was added to each battery. The battery casing was made of CR2032 stainless steel. All assembly processes were carried out in an argon-protected glove box (water content <0.1 ppm, oxygen content <0.1 ppm). The batteries were sealed using a battery assembly machine under 5 kN pressure. After assembly, the batteries were allowed to stand at a constant temperature of 25°C for 12 h before electrochemical testing. Charge-discharge tests were conducted using a Neware BTS-4008 battery testing system, with a test voltage range of 2.5-4.3V. First, three activation cycles were performed at a 0.1C rate (1C=170mA / g). Each cycle included constant current charging to 4.3V, followed by constant voltage charging until the current dropped to 0.05C, a 5-minute rest period, and then constant current discharging to 2.5V. Next, rate performance tests were performed, with five cycles each at 0.2C, 0.5C, 1C, 2C, and 5C rates. Finally, long-cycle performance tests were conducted, with 200 cycles at a 1C rate, and the discharge specific capacity was recorded for each cycle. Electrochemical impedance spectroscopy (EIS) was performed using a Bio-Logic VMP-300 electrochemical workstation, with a test frequency range of 100kHz-10MHz. A 5mV sinusoidal wave perturbation was applied, and the tests were conducted at 50% state of charge (SOC). Cyclic voltammetry tests were performed within a voltage range of 2.5–4.3 V at a scan rate of 0.1 mV / s for three cycles. All tests were conducted in a constant temperature chamber at 25 ± 0.1 °C, and the average value of three parallel cells was taken for each sample.

[0064] Performance test results:

[0065] Table 1: Performance test results of each embodiment and comparative example

[0066]

[0067] As shown in Table 1, the test results indicate that the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 are significantly superior to those in Comparative Examples 1-3 in all electrochemical performance indicators, fully demonstrating that the present invention effectively solves the technical problems existing in lithium manganese iron phosphate materials through a multi-modification strategy. Specifically, Example 1 exhibits the best comprehensive performance, with an initial discharge specific capacity of 158.2 mAh / g, which is 14.2%, 11.2%, and 9.0% higher than that of Comparative Example 1 (undoped), Comparative Example 2 (without carbon coating), and Comparative Example 3 (without LTTO coating), respectively. This indicates that the synergistic effect of magnesium-fluorine co-doping, carbon coating, and LTTO coating significantly improves the reversible capacity of the material. In terms of rate performance, Example 1 retains 89.5% of its capacity at a high rate of 5C, which is much higher than that of Comparative Example 1 (68.9%), Comparative Example 2 (72.6%), and Comparative Example 3 (75.3%). This proves that the triple modification strategy effectively improves the electronic conductivity and ion diffusion rate of the material, enabling the material to maintain a high capacity even at high rates. Regarding cycle stability, Example 1 maintained a capacity retention of 94.2% after 200 cycles, while Comparative Examples 1-3 only maintained 78.3%, 82.4%, and 83.7%, respectively. This demonstrates a significant improvement in the structural stability of the modified material, particularly the effective suppression of electrolyte corrosion and transition metal dissolution by the LTTO coating. Electrochemical impedance spectroscopy further corroborates these conclusions. The charge transfer impedance of Example 1 was only 42.6 Ω, far lower than the 92.4 Ω, 85.7 Ω, and 78.9 Ω of Comparative Examples 1-3, indicating that the modified material exhibits faster charge transport kinetics. In summary, this invention fundamentally solves the technical problems of low electronic conductivity and ion diffusion rate, poor rate performance, and insufficient cycle stability in lithium manganese iron phosphate cathode materials through magnesium-fluorine co-doping to optimize the bulk structure, carbon coating to construct the electronic conduction network, and LTTO coating to promote ion transport and stabilize the interface.

[0068] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron manganese phosphate cathode material, characterized in that the steps of Comprising: S1, take lithium carbonate, manganese acetate, ferrous sulfate, ammonium dihydrogen phosphate, magnesium acetate and ammonium fluoride, add deionized water and stir to dissolve, control the pH value of the solution to be 5.5-6.5; then transfer the mixed solution to a microwave hydrothermal reaction kettle, react at 180-220℃ to obtain a LMFP precursor containing magnesium and fluorine; S2, mixing the LMFP precursor containing magnesium and fluorine with sucrose, ball milling, and then two-step sintering under argon atmosphere: pre-sintering at 348-352℃, and main sintering at 748-752℃, to obtain C-LMFP-Mg-F powder; mixing the C-LMFP-Mg-F powder with LiTa 0.3 Ti 0.7 O3, ball milling, and then sintering at 700-800℃ under air atmosphere, to obtain C-LMFP-Mg-F@LTTO composite powder; S3, the C-LMFP-Mg-F@LTTO composite powder is mixed with PANI-g-CD / Diatomite conductive gel, conductive carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone to form a uniform slurry, which is coated on an aluminum foil, vacuum dried at 118-122℃, pressed into a sheet and cut.

2. The method of claim 1, wherein the lithium iron manganese phosphate cathode material is prepared by the steps of: In step S1, the hydrothermal reaction is carried out at 180-220℃ for 8-10h. ​ 3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S2, pre-burning is carried out at 348-352℃ for 4-6h, main burning is carried out at 748-752℃ for 12-14h, and sintering is carried out at 700-800℃ for 4-8h.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S3, the vacuum drying is carried out at 118-122℃ for 3-5h.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation method of the PANI-g-CD / Diatomite composite conductive gel comprises: A1, dissolving β-cyclodextrin in sodium hydroxide solution, adding activated porous diatomite, stirring at 58-62℃; then adding aniline monomer and ammonium persulfate, reacting at 0-5℃; A2, finally washing the product with deionized water and ethanol to neutral, vacuum drying at 58-62℃.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 5, characterized in that, In step A1, the reaction is carried out at 0-5℃ for 12-14h.

7. The method of claim 5, wherein the lithium iron manganese phosphate cathode material is prepared by the steps of: mixing a lithium source, a manganese source, and an iron source; and heating the mixture to a temperature of 600-800°C for 6-24 hours. In step A1, the treatment steps of the activated porous diatomite include: placing diatomite in a muffle furnace and calcining at 500-600℃; then mixing the calcined diatomite with hydrochloric acid solution and continuously stirring at 60-80℃; then filtering the mixture after acid washing, repeatedly washing the filter cake with deionized water until the filtrate is neutral; finally drying the washed diatomite in a vacuum drying oven at 105-120℃.

8. The method for preparing lithium manganese iron phosphate cathode material according to claim 5, characterized in that, In step A2, the vacuum drying is carried out at 58-62℃ for 24-30h.

9. A lithium iron manganese phosphate cathode material, characterized in that, The lithium manganese iron phosphate positive electrode material is prepared according to the preparation method of the lithium manganese iron phosphate positive electrode material according to any one of claims 1-8.

10. Use of the lithium manganese iron phosphate positive electrode material according to claim 9 in a lithium secondary battery.

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