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

Abstract

The application discloses a preparation method of a lithium iron manganese phosphate positive electrode material in the field of lithium ion battery positive electrode materials and the positive electrode material. The method comprises the following steps: dissolving a manganese source, an iron source and a phosphorus source in water, adding ascorbic acid to adjust the pH value to form a uniform solution, and generating a precursor through high-temperature hydrothermal reaction. Then, the precursor is ball-milled and mixed with a pre-prepared yttrium-gadolinium co-doped lithium ion conductor, and middle-temperature sintering is carried out under an inert atmosphere to form a matrix material coated with the ion conductor. Finally, the matrix material is compounded with nitrogen-fluorine co-doped graphene carbon nanotube three-dimensional reinforcing bodies, ball-milling and low-temperature heat treatment are carried out to obtain the final product. Through the synergistic effect of the ion conductor coating and the two-dimensional carbon material enhancement, the ion and electron conduction capacity of the material is significantly improved, and the prepared positive electrode material has high capacity, excellent cycle stability and rate performance.

Description

Technical Field

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

[0002] Lithium-ion batteries, as a new generation of green high-energy chemical power sources, are widely used in portable electronic devices, electric vehicles, and large-scale energy storage systems. Their performance largely depends on the key characteristics of the cathode material. Among numerous cathode materials, olivine-structured phosphate materials have attracted widespread attention due to their excellent thermal stability, cycle life, and safety. Lithium iron phosphate (LFP) has achieved large-scale commercial application, but its energy density is limited by a relatively low operating voltage platform. To further improve energy density, researchers have developed lithium manganese phosphate (LMP), which has a higher operating voltage; however, its poor electronic conductivity and manganese leaching problem severely restrict its practical application. Lithium manganese iron phosphate (LMP), formed by combining iron and manganese in a certain proportion, is expected to combine the advantages of both, possessing both high voltage and high safety characteristics, making it one of the most promising next-generation cathode materials and thus a current research hotspot in industry and academia.

[0003] Despite the promising prospects of lithium manganese iron phosphate (LFP) materials, their industrial application still faces several severe technical challenges. The inherently low electronic conductivity and ion diffusion rate of this material result in poor rate performance, particularly severe capacity decay at high discharge rates. Furthermore, during charge-discharge cycling, especially at high temperatures, manganese ions readily dissolve, leading to crystal structure destruction and rapid capacity decay, making it difficult to meet the cycle stability requirements of practical applications. While traditional carbon coating modification strategies can improve the conductivity of the material to some extent, their effectiveness in suppressing manganese dissolution and stabilizing the interface structure is limited. Existing single modification methods cannot simultaneously address multiple issues such as electronic conductivity, ion conduction, and interface stability. There is an urgent need to develop a more comprehensive and efficient multi-dimensional modification technology to synergistically improve the overall electrochemical performance of the material.

[0004] To overcome the aforementioned technical challenges, this invention aims to provide a method for preparing lithium manganese iron phosphate cathode materials. This method employs a sophisticated combination of material design and processes to synergistically modify the bulk phase, interface, and surface of the material at multiple scales. The core technology involves first synthesizing a structurally regular precursor via a hydrothermal method, followed by introducing a rare-earth-doped lanthanum zirconate layer with high ionic conductivity for interface modification. This aims to effectively suppress the dissolution of transition metal ions and enhance the transport kinetics of lithium ions at the interface. Finally, by combining the precursor with a nitrogen- and fluorine-coated carbon nanocomposite reinforcement, a highly efficient three-dimensional electronic conductive network is constructed, significantly improving the electronic conductivity of the material. This synergistic strategy of ion conductor coating and carbon network reinforcement simultaneously solves the two core problems of poor conductivity and interfacial instability, thereby producing high-performance lithium manganese iron phosphate cathode materials with high capacity, long cycle life, and excellent rate performance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing lithium manganese iron phosphate cathode material and the cathode material itself, which solves the technical problems of poor electronic and ionic conductivity, severe manganese ion dissolution during cycling, and poor cycle life and rate performance 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. First, dissolve manganese sulfate, ferrous sulfate, and ammonium dihydrogen phosphate in deionized water, and add ascorbic acid; then add lithium hydroxide solution until the pH is 6.5-8.0 to form a homogeneous mixed solution; transfer the mixed solution to a high-pressure hydrothermal reactor and react at 198-202℃ to generate lithium manganese iron phosphate precursor;

[0009] S2. After the reaction is completed, cool to room temperature, filter and wash with deionized water, and dry in a vacuum drying oven at 78-82℃ to obtain precursor powder; put the precursor powder and yttrium-gadolinium co-doped lanthanum zirconate lithium ion conductor into a ball mill jar, add anhydrous ethanol, and ball mill; after drying the ball-milled slurry, heat to 620-680℃ in a tube furnace under an argon atmosphere to sinter and form LMFP matrix material coated with ion conductor;

[0010] S3. The sintered ion conductor-coated LMFP matrix material is mixed with nitrogen and fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement, ethanol is added, and the mixture is ball-milled again; finally, it is heat-treated at 545-555℃ under an argon atmosphere.

[0011] In this invention, the overall preparation of lithium manganese iron phosphate cathode material is a multi-step, multi-scale composite synergistic process. Its reaction mechanism encompasses the crystal growth of the precursor, the interface modification of the ionic conductor, and the network construction of the carbon reinforcement, ultimately achieving synergistic optimization of the bulk phase, interface, and surface. The hydrothermal synthesis step is the foundation for preparing high-quality precursors. In a closed high-pressure reactor under suitable pH conditions, lithium ions, manganese ions, ferrous ions, and phosphate ions undergo hydrothermal crystallization at high temperature in a reducing atmosphere created by ascorbic acid. Ascorbic acid effectively inhibits the oxidation of divalent manganese and divalent iron ions, ensuring that manganese and iron exist in the desired valence states in the product. During this process, ions gradually desolvate and form crystal nuclei, eventually growing into olivine-type lithium manganese iron phosphate crystals with good crystallinity and uniform composition. The unique environment of the hydrothermal method endows the precursor material with the advantages of regular morphology, uniform particle size, and high crystallinity. The subsequent ball milling and medium-temperature sintering process with the ion conductor achieved crucial interface modification. The mechanical force of ball milling caused the ion conductor nanoparticles to uniformly adhere to the surface of the precursor micron particles, forming a preliminary physical coating. During the medium-temperature heat treatment under an inert atmosphere, slight interfacial interdiffusion and chemical reactions may occur between the ion conductor layer and the precursor particle surface, forming a strong chemical bond rather than simple physical adsorption. This robust coating layer constructs an ideal physical barrier between the active material and the electrolyte, effectively preventing the electrolyte from corroding the active material, especially inhibiting the dissolution of manganese ions. At the same time, the inherent lithium-ion conductivity of the layer itself provides a low-impedance channel for lithium ions to cross the interface, significantly improving the ion transport kinetics of the interface. Finally, the integration of the entire material was completed through composite formation with a two-dimensional carbon reinforcement and low-temperature heat treatment. Secondary ball milling uniformly dispersed and wound the two-dimensional carbon reinforcement around the ion-conducting matrix material particles, forming a permeable three-dimensional conductive network. Subsequent low-temperature heat treatment further eliminated defects in the carbon material, enhanced its graphitization, and promoted contact and bonding between the carbon network and the outer coating layer, resulting in a stronger adhesion. The final material is a multi-level composite: the core is the capacity-providing lithium manganese iron phosphate active material; the middle layer is the ion-conducting coating layer that suppresses side reactions and promotes ion transport; and the outermost layer is a nitrogen-fluorine co-doped carbon three-dimensional network that ensures rapid electron transport. This multi-level synergistic "core-shell-network" structure essentially solves the three core problems of lithium manganese iron phosphate materials—low ionic conductivity, poor electronic conductivity, and weak interfacial stability—thus enabling it to exhibit comprehensive electrochemical performance with high specific capacity, excellent rate performance, and ultra-long cycle life.

[0012] According to a preferred embodiment of the present invention, in step S1, the reaction time at 198-202°C is 10-12 h.

[0013] According to a preferred embodiment of the present invention, in steps S2 and S3, the sintering time at 620-680℃ is 8-10h; the heat treatment time at 545-555℃ is 3-6h.

[0014] According to a preferred embodiment of the present invention, the preparation method of the yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor includes: A1, mixing lithium carbonate, lanthanum oxide, yttrium oxide, gadolinium oxide and zirconium dioxide to obtain a mixed powder; placing the mixed powder and anhydrous ethanol into a planetary ball mill jar, ball milling and mixing evenly to obtain a slurry; A2, drying the ball-milled slurry in a vacuum drying oven at 78-82℃; placing the dried powder in an alumina crucible, heating it to 798-802℃ in a muffle furnace for pre-calcination; ball milling the pre-calcined powder again, then sintering it at 1340-1360℃, followed by annealing at 1100-1200℃, and cooling it to room temperature in the furnace to obtain a bulk material; grinding and sieving the sintered bulk material.

[0015] In this invention, the preparation process of yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor is a typical high-temperature solid-state reaction and structure regulation process. Its core mechanism lies in inducing lattice structure formation and doping modification through precise component design and high-temperature heat treatment, thereby obtaining a garnet-type structure with high ionic conductivity. In the initial mixing stage, raw materials such as lithium carbonate, lanthanum oxide, yttrium oxide, gadolinium oxide, and zirconium dioxide undergo preliminary mechanochemical activation during ball milling, refining the particles and achieving highly uniform nanoscale mixing, providing thermodynamic driving force for the subsequent high-temperature reaction. The pre-sintering process is the crucial first stage of phase formation. At this temperature, lithium carbonate first decomposes to generate highly reactive lithium oxide. These newly formed oxides undergo solid-phase reactions with surrounding lanthanum oxide, zirconium dioxide, etc., to initially form the rudimentary structure of lithium lanthanum zirconate. Simultaneously, yttrium and gadolinium ions begin to substitute and dope the lanthanum sites. Due to the difference in ionic radii between yttrium and gadolinium ions and lanthanum ions, this substitution will produce certain lattice distortions and defects within the crystal. These defects create favorable conditions for the subsequent rapid migration of lithium ions. The subsequent high-temperature sintering is the key step in perfecting and densifying the crystal structure. At this extremely high temperature, atoms gain sufficient energy for long-range migration, causing grains to grow and form a dense sintered body, ultimately forming a well-crystallized and structurally stable garnet-type cubic phase structure with three-dimensionally interconnected lithium ion migration channels. The co-doping of yttrium and gadolinium plays a key role in stabilizing the cubic phase and suppressing the formation of impurity phases, while also optimizing lattice parameters and significantly reducing the energy barrier for lithium ion migration. The final annealing process is an important structural relaxation process that can eliminate internal stress and lattice defects generated during high-temperature rapid cooling, further optimizing the occupancy and transport path of lithium ions. The powder obtained after grinding is composed of a large number of micron- or submicron-sized particles. These particles have a continuous three-dimensional lithium-ion conductive network inside, which lays a solid material foundation for them to act as an efficient interfacial ion transport layer in cathode materials.

[0016] According to a preferred embodiment of the present invention, in step A1, the ball milling speed is 400-600 rpm; the ball milling time is 6-8 h.

[0017] According to a preferred embodiment of the present invention, in step A2, the pre-firing time at 798-802℃ is 6-8h; and the sintering time at 1340-1360℃ is 12-14h.

[0018] According to a preferred embodiment of the present invention, the preparation method of the nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement includes: B1, refluxing multi-walled carbon nanotubes in a mixture of concentrated sulfuric acid and concentrated nitric acid at 60-80°C, washing and drying to obtain carboxylated multi-walled carbon nanotubes; dispersing graphene oxide and carboxylated multi-walled carbon nanotubes in deionized water to obtain a mixed solution, and ultrasonically treating the mixed solution to form a uniform dispersion; transferring the dispersion to a polytetrafluoroethylene-lined hydrothermal reactor and reacting at 180-185°C to obtain a hydrothermal product; B2, freezing the hydrothermal product with liquid nitrogen and then placing it in a freeze dryer to dry at -48~-52°C to obtain an aerogel; grinding and mixing the aerogel with urea and polytetrafluoroethylene in an agate mortar to obtain a mixture; annealing the mixture in a tube furnace at 898-902°C under an argon atmosphere, and then naturally cooling to room temperature.

[0019] In this invention, the preparation of a nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement is a complex process involving chemical modification, self-assembly thermal reduction, and in-situ doping. The mechanism aims to construct a three-dimensional carbon network possessing high conductivity, high specific surface area, and strong interfacial bonding. First, the carboxylation treatment of multi-walled carbon nanotubes is a crucial activation step. Under reflux conditions, a concentrated acid mixture strongly erodes the structure of the carbon nanotubes, introducing oxygen-containing functional groups such as carboxyl and hydroxyl groups into their walls and ends. These hydrophilic functional groups not only significantly improve the dispersibility of carbon nanotubes in water but, more importantly, provide abundant reaction sites for their subsequent bonding with graphene oxide. The subsequent hydrothermal reaction with graphene oxide is a crucial self-assembly and reduction process. Under high temperature and pressure, oxygen-containing functional groups on the graphene oxide sheets are partially removed, sp2 carbon domains are gradually restored, and its π-π conjugated system is partially repaired. Simultaneously, activated carbon nanotubes and partially reduced graphene oxide sheets are bonded through strong π-π interactions, hydrogen bonds, and possible covalent bonds. The carbon nanotubes, acting as one-dimensional linear spacers, are inserted between the two-dimensional graphene oxide sheets, effectively preventing the graphene sheets from re-stacking, ultimately forming a hydrothermal gel with a three-dimensional porous network structure. Freeze-drying technology utilizes the template effect of ice crystals to perfectly solidify this hydrogel network structure into an aerogel, maximizing the preservation of its porous properties. The final high-temperature annealing is the core step in achieving heteroatom doping and graphitization of carbon materials. Urea decomposes at high temperatures to produce nitrogen-containing free radicals and small molecules, such as ammonia. These nitrogen-containing species attack defect sites in the carbon skeleton and embed themselves in the carbon lattice in the form of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. Polytetrafluoroethylene (PTFE) is simultaneously pyrolyzed, contributing amorphous carbon as a carbon source and releasing fluorine-containing active species as a fluorine source. These fluorine atoms, with their extremely high electronegativity, replace carbon atoms in the carbon network or adsorb at the edges in the form of semi-ionic bonds. The co-doping of nitrogen and fluorine produces a unique synergistic effect. Nitrogen doping provides excess electron carriers, while the highly electronegative fluorine atoms change the charge distribution on the surface of the carbon material through the electron-withdrawing effect, greatly enhancing the electronic conductivity and electrochemical activity of the carbon material. The final result is a three-dimensional reinforcement with nitrogen and fluorine co-doping and tightly interwoven graphene and carbon nanotubes, which provides excellent electronic conduction pathways and structural support for the composite material.

[0020] According to a preferred embodiment of the present invention, in step B1, the reaction time at 180-185°C is 12-14 hours.

[0021] According to a preferred embodiment of the present invention, in step B2, the annealing time at 898-902°C is 2-4 hours.

[0022] The present invention also provides a lithium manganese iron phosphate cathode material prepared according to the preparation method of the lithium manganese iron phosphate cathode material.

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

[0024] The method for preparing lithium manganese iron phosphate cathode material provided by this invention exhibits significant technical effects and performance advantages through multi-level structural design and synergistic modification. Firstly, this method employs a strategy combining hydrothermal synthesis and segmented heat treatment to successfully prepare lithium manganese iron phosphate matrix material with regular morphology and high crystallinity. By precisely controlling the pH, temperature, and duration of the hydrothermal reaction, the uniform distribution of metal elements such as iron and manganese and the complete formation of the olivine structure are ensured, laying a solid structural foundation for the material's high specific capacity and stable cycling. The coating of ion conductors and the introduction of carbon nanotube reinforcement constitute a multi-dimensional synergistic modification system, fundamentally improving the intrinsic defects of the material.

[0025] Secondly, a stable interfacial protective layer was constructed on the material surface by introducing a yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor coating. This coating not only effectively blocked the electrolyte from corroding the active material and significantly suppressed the dissolution of manganese ions and the degradation of the crystal structure during cycling, but also provided excellent ion transport channels, greatly improving the migration rate of lithium ions at the interface. The dense coating structure formed after medium-temperature sintering enabled the material to maintain extremely high structural integrity and interfacial stability under high temperature and long-cycle conditions, thus exhibiting excellent cycle life and capacity retention.

[0026] Finally, the introduction of nitrogen-fluorine co-doped graphene carbon nanotube three-dimensional reinforcement constructs a highly efficient three-dimensional conductive network on the outside of the material. Heteroatom doping not only significantly improves the electronic conductivity of the carbon material but also increases its binding force with the active material, ensuring rapid electron transport within the electrode. This reinforcement, in conjunction with the ion-conducting coating layer, forms a synergistic enhancement mechanism of "internal ion conduction and external electron transport," enabling the material to possess both high ionic conductivity and high electronic conductivity. The resulting composite cathode material combines the advantages of high tap density, high specific capacity, excellent rate performance, and ultra-long cycle life, fully meeting the application requirements of high-performance lithium-ion batteries. 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 manganese sulfate was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0030] The ferrous sulfate was purchased from Xilong Scientific Co., Ltd.

[0031] The ammonium dihydrogen phosphate was purchased from Aladdin Biochemical Technology Co., Ltd.

[0032] The ascorbic acid was purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0033] The lithium hydroxide was purchased from Chengdu Likai Chiral Technology Co., Ltd.

[0034] The argon gas was purchased from Chengdu Kelin Gas Co., Ltd.

[0035] The lithium carbonate was purchased from Tianqi Lithium Corporation.

[0036] The lanthanum oxide was purchased from the Hunan Rare Earth Metal Materials Research Institute.

[0037] The yttrium oxide was purchased from China Northern Rare Earth (Group) High-Tech Co., Ltd.

[0038] The gadolinium oxide was purchased from Guangdong Pearl River Rare Earth Co., Ltd.

[0039] The zirconium dioxide was purchased from Dongfang Zirconium Industry Technology Co., Ltd.

[0040] The multi-walled carbon nanotubes were purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0041] The concentrated sulfuric acid was purchased from Chengdu Kelong Chemical Co., Ltd.

[0042] The concentrated nitric acid was purchased from Chongqing Chuandong Chemical (Group) Co., Ltd.

[0043] The graphene oxide was purchased from Suzhou CarbonFeng Technology Co., Ltd.

[0044] The urea was purchased from Henan Xinlianxin Chemical Industry Group Co., Ltd.

[0045] The polytetrafluoroethylene was purchased from Zhonghao Chenguang Chemical Research Institute Co., Ltd.

[0046] The tubular furnace was purchased from Hefei Kejing Materials Technology Co., Ltd.

[0047] Example 1

[0048] First, 10.0 g of manganese sulfate, 15.0 g of ferrous sulfate, and 25.0 g of ammonium dihydrogen phosphate were dissolved in 200 ml of deionized water, and 1.5 g of ascorbic acid was added. Then, 1.0 mol / L lithium hydroxide solution was slowly added dropwise until the pH reached 7.0, forming a homogeneous mixture. The mixture was transferred to a 100 ml high-pressure hydrothermal reactor and reacted at 200 °C for 11 h to generate lithium manganese iron phosphate precursor. After the reaction was completed, the mixture was naturally cooled to room temperature, filtered, washed three times with deionized water, and dried in a vacuum drying oven at 80 °C for 12 h to obtain precursor powder. Preparation of yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor: 2.20 g lithium carbonate, 11.70 g lanthanum oxide, 0.27 g yttrium oxide, 0.38 g g gadolinium oxide, and 3.66 g zirconium dioxide were mixed; the mixed powder was placed in a planetary ball mill jar with 100 ml anhydrous ethanol and ball-milled at 500 rpm for 7 h; the ball-milled slurry was dried in a vacuum drying oven at 80 °C for 12 h; the dried powder was placed in an alumina crucible and pre-calcined in a muffle furnace at 800 °C for 7 h with a heating rate of 5 °C / min; the pre-calcined powder was ball-milled again for 4 h, then sintered at 1350 °C for 13 h, followed by annealing at 1150 °C for 5 h, and cooled to room temperature with the furnace; the sintered bulk material was ground and passed through a 400-mesh sieve to obtain yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor powder. 10.0 g of precursor powder and 1.0 g of the above-mentioned yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor powder were placed in a ball mill jar, 50 ml of anhydrous ethanol was added, and the mixture was ball-milled at 400 rpm for 4 h. The ball-milled slurry was dried at 80 °C for 12 h, and then sintered in a tube furnace under an argon atmosphere at a temperature increase of 2 °C / min to 650 °C for 9 h to form an ion conductor-coated LMFP matrix material. Preparation of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement: 1.0 g of multi-walled carbon nanotubes were refluxed at 70 °C for 6 h in a mixture of 120 ml concentrated sulfuric acid and 40 ml concentrated nitric acid, washed with deionized water until neutral, and then dried. 1.0 g of graphene oxide and 0.5 g of carboxylated multi-walled carbon nanotubes were dispersed in 200 ml of deionized water, and the mixture was ultrasonically treated for 2 h to form a uniform dispersion. The dispersion was transferred to a hydrothermal reaction chamber lined with polytetrafluoroethylene. The reaction was carried out in a reactor at 182℃ for 13 hours. The hydrothermal product was rapidly frozen with liquid nitrogen and then placed in a freeze dryer and dried at -50℃ for 48 hours to obtain an aerogel. 1.0 g of aerogel was ground and mixed with 5.0 g of urea and 0.5 g of polytetrafluoroethylene in an agate mortar for 30 minutes. The mixture was annealed in a tube furnace at 900℃ for 3 hours under an argon atmosphere at a rate of 5℃ / min, and then naturally cooled to room temperature to obtain nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforced powder.10.0g of LMFP matrix material coated with ion conductor was mixed with 0.5g of the above nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement powder, 40ml of ethanol was added, and the mixture was ball-milled at 350rpm for 3h. Finally, the mixture was heat-treated at 550℃ for 4h under argon atmosphere at 3℃ / min, and then cooled in the furnace to obtain the final product.

[0049] Example 2

[0050] The specific implementation method is the same as in Example 1, except that: firstly, 12.0 g of manganese sulfate, 13.0 g of ferrous sulfate, and 26.0 g of ammonium dihydrogen phosphate are dissolved in 200 ml of deionized water, and 1.2 g of ascorbic acid is added; then, 1.0 mol / L lithium hydroxide solution is added until the pH reaches 7.5, forming a homogeneous mixed solution; the mixed solution is transferred to a high-pressure hydrothermal reactor and reacted at 200 °C for 10 h to generate lithium manganese iron phosphate precursor. After the reaction is completed, the mixture is cooled to room temperature, filtered, washed with deionized water, and dried in a vacuum drying oven at 80 °C to obtain precursor powder. Preparation of yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor: 2.25 g lithium carbonate, 12.00 g lanthanum oxide, 0.30 g yttrium oxide, 0.35 g gadolinium oxide, and 3.70 g zirconium dioxide were mixed; ball milled for 6 h; dried and pre-calcined at 800 °C for 6 h; then sintered at 1350 °C for 12 h, and annealed at 1150 °C; ground and sieved. 10.0 g precursor powder and 1.5 g yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor powder were ball milled and mixed; sintered at 620 °C for 10 h under an argon atmosphere. Preparation of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement: 1.2 g of multi-walled carbon nanotubes were carboxylated; 1.2 g of graphene oxide and 0.6 g of carbon nanotubes were hydrothermally reacted at 182 °C for 12 h; after freeze-drying, 1.2 g of aerogel was mixed with 6.0 g of urea and 0.6 g of polytetrafluoroethylene; annealed at 900 °C for 2 h under argon atmosphere. 10.0 g of the sintered material was ball-milled with 0.3 g of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement powder; finally, it was heat-treated at 550 °C for 3 h under argon atmosphere.

[0051] Example 3

[0052] The specific implementation method is the same as in Example 1, except that: firstly, 9.0 g of manganese sulfate, 16.0 g of ferrous sulfate, and 24.0 g of ammonium dihydrogen phosphate are dissolved in 200 ml of deionized water, and 1.8 g of ascorbic acid is added; then, 1.0 mol / L lithium hydroxide solution is added until the pH reaches 6.5, forming a homogeneous mixed solution; the mixed solution is transferred to a high-pressure hydrothermal reactor and reacted at 200 °C for 12 h to generate lithium manganese iron phosphate precursor. After the reaction is completed, the mixture is cooled to room temperature, filtered, washed with deionized water, and dried in a vacuum drying oven at 80 °C to obtain precursor powder. Preparation of yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor: 2.15 g lithium carbonate, 11.50 g lanthanum oxide, 0.25 g yttrium oxide, 0.40 g gadolinium oxide, and 3.60 g zirconium dioxide were mixed; ball milled for 8 h; dried and pre-calcined at 800 °C for 8 h; then sintered at 1350 °C for 14 h, and annealed at 1150 °C; ground and sieved. 10.0 g precursor powder and 0.5 g yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor powder were ball milled and mixed; sintered at 680 °C for 8 h under an argon atmosphere. Preparation of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement: 0.8 g of multi-walled carbon nanotubes were carboxylated; 0.8 g of graphene oxide and 0.4 g of carbon nanotubes were hydrothermally reacted at 182 °C for 14 h; after freeze-drying, 0.8 g of aerogel was mixed with 4.0 g of urea and 0.4 g of polytetrafluoroethylene; annealed at 900 °C for 4 h under argon atmosphere. 10.0 g of the sintered material was ball-milled and mixed with 0.8 g of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement powder; finally, it was heat-treated at 550 °C for 6 h under argon atmosphere.

[0053] Comparative Example 1

[0054] The specific implementation method is the same as in Example 1, except that: firstly, 10.0g of manganese sulfate, 15.0g of ferrous sulfate, and 25.0g of ammonium dihydrogen phosphate are dissolved in 200ml of deionized water, and 1.5g of ascorbic acid is added; then, 1.0mol / L lithium hydroxide solution is added until the pH reaches 7.0, forming a homogeneous mixed solution; the mixed solution is transferred to a high-pressure hydrothermal reactor and reacted at 200℃ for 11h to generate lithium manganese iron phosphate precursor. After the reaction, the mixture is cooled to room temperature, filtered, washed with deionized water, and dried in a vacuum drying oven at 80℃ to obtain precursor powder. The precursor powder is directly placed in a tube furnace and sintered at 650℃ for 9h under an argon atmosphere. 10.0g of the sintered material is mixed with 0.5g of conductive carbon black, 40ml of ethanol is added, and the mixture is ball-milled for 3h; finally, it is heat-treated at 550℃ for 4h under an argon atmosphere.

[0055] Comparative Example 2

[0056] The specific implementation method is the same as in Example 1, except that 10.0g of precursor powder and 1.0g of yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor powder are ball-milled and sintered at 650℃. 10.0g of the sintered material is mixed with 0.5g of conductive carbon black, 40ml of ethanol is added, and the mixture is ball-milled for 3h; finally, it is heat-treated at 550℃ for 4h under an argon atmosphere.

[0057] Comparative Example 3

[0058] The specific implementation method is the same as in Example 1, except that after the reaction is completed, the mixture is cooled to room temperature, filtered, washed with deionized water, and dried in a vacuum drying oven at 80°C to obtain precursor powder. The precursor powder is directly placed in a tube furnace and sintered at 650°C for 9 hours under an argon atmosphere. 10.0 g of the sintered material is mixed with 0.5 g of nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement powder, 40 ml of ethanol is added, and the mixture is ball-milled again for 3 hours; finally, it is heat-treated at 550°C for 4 hours under an argon atmosphere.

[0059] Performance testing

[0060] The lithium iron phosphate cathode materials prepared according to Examples 1-3 and Comparative Examples 1-3 were tested according to the following performance testing methods:

[0061] Electrochemical performance tests were conducted on the prepared positive electrode material using coin cell half-cells. First, the active material, conductive agent acetylene black, and binder polyvinylidene fluoride were mixed in an 8:1:1 mass ratio in N-methylpyrrolidone solvent and stirred for 12 hours to form a uniform slurry. This slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 120°C for 12 hours. The resulting slurry was then stamped into 12mm diameter discs as the positive electrode. Using lithium metal sheets as the counter and reference electrodes, a Celgard 2400 microporous polypropylene membrane as the separator, and 1 mol / L LiPF6 dissolved in a 1:1:1 volume ratio mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate as the electrolyte, CR2032 coin cells were assembled in an argon-protected glove box. All cells were allowed to stand for 12 hours before testing. Charge-discharge tests were performed at 25°C using a Newway battery testing system, with a voltage range of 2.5-4.5V (vs. Li). + / Li). Rate performance testing was conducted at different current densities, from 0.1C, 0.2C, 0.5C, 1C to 2C, with 5 cycles at each rate, finally returning to 0.1C to test capacity recovery. Cyclic performance testing was conducted at 1C, with capacity retention calculated after 200 cycles. Cyclic voltammetry was performed using a CHI760E electrochemical workstation at 25°C, with a scan rate of 0.1 mV / s and a voltage range of 2.5–4.5 V. Electrochemical impedance spectroscopy was performed at open-circuit potential, scanning within a frequency range of 100 kHz to 0.01 Hz, with a perturbation amplitude of 5 mV.

[0062] Performance test results:

[0063] Table 1: Performance test results of each embodiment and comparative example

[0064]

[0065] As shown in Table 1, the test results indicate that Examples 1-3 effectively solved the key technical bottlenecks of existing lithium manganese iron phosphate cathode materials through a unique dual modification strategy. Compared with the unmodified Comparative Example 1, Example 1 showed an 11.2% higher initial discharge specific capacity, a 4.3 percentage point higher coulombic efficiency, a significantly higher capacity retention rate after 200 cycles (19.5 percentage points higher), a 33.9 mAh / g increase in 2C rate discharge capacity, an approximately 1.6-fold higher ion diffusion coefficient, and an 87.1 Ohm reduction in charge transfer resistance, demonstrating a fundamental improvement in the material's electronic and ionic conductivity. Comparative Example 2, using only ion conductor modification, improved ionic conductivity to some extent (ion diffusion coefficient higher than Comparative Example 1), but due to the lack of an effective electronic conductivity network, its rate performance (2C capacity 115.8 mAh / g) and cycle stability (85.3% retention rate) were still significantly lower than any of the examples. Comparative Example 3, using only carbon material modification, showed improved electronic conductivity (charge transfer resistance of 92.5 Ohms, lower than Comparative Example 1), but due to the lack of an ion conductor protective layer, manganese ion dissolution was not effectively suppressed, resulting in significantly worse cycle performance (80.1% retention) compared to the examples. Examples 1-3 demonstrated the most significant synergistic effect: the yttrium-gadolinium co-doped lanthanum zirconate lithium ion conductor coating not only provided a high-speed lithium ion transport channel, but its stable fluorite structure also effectively suppressed manganese ion dissolution and the Jahn-Teller effect during cycling; while the nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement constructed a three-dimensional electronic conductivity network, and the introduction of nitrogen and fluorine heteroatoms enhanced the material interface stability. This construction of a dual continuous ion / electron conduction network enabled Examples 1-3 to achieve excellent rate performance and cycle life while maintaining high specific capacity, completely solving the technical problems of poor conductivity and severe manganese dissolution in traditional lithium manganese iron phosphate materials.

[0066] 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 manganese iron phosphate cathode material, characterized in that the steps include: include: S1. First, dissolve manganese sulfate, ferrous sulfate and ammonium dihydrogen phosphate in deionized water, and add ascorbic acid; Subsequently, lithium hydroxide solution was added until the pH reached 6.5-8.0 to form a homogeneous mixed solution; the mixed solution was then transferred to a high-pressure hydrothermal reactor and reacted at 198-202℃ to generate lithium manganese iron phosphate precursor; S2. After the reaction is completed, cool to room temperature, filter and wash with deionized water, and dry in a vacuum drying oven at 78-82℃ to obtain precursor powder; put the precursor powder and yttrium-gadolinium co-doped lanthanum zirconate lithium ion conductor into a ball mill jar, add anhydrous ethanol, and ball mill; after drying the ball-milled slurry, heat to 620-680℃ in a tube furnace under an argon atmosphere to sinter and form LMFP matrix material coated with ion conductor; S3. The sintered ion conductor-coated LMFP matrix material is mixed with nitrogen and fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement, ethanol is added, and the mixture is ball-milled again; finally, it is heat-treated at 545-555℃ under an argon atmosphere. The preparation method of the nitrogen-fluorine co-doped graphene-carbon nanotube three-dimensional reinforcement includes: B1, refluxing multi-walled carbon nanotubes in a mixture of concentrated sulfuric acid and concentrated nitric acid at 60-80℃, washing and drying to obtain carboxylated multi-walled carbon nanotubes; dispersing graphene oxide and carboxylated multi-walled carbon nanotubes in deionized water to obtain a mixed solution, ultrasonically treating the mixed solution to form a uniform dispersion; transferring the dispersion to a polytetrafluoroethylene-lined hydrothermal reactor and reacting at 180-185℃ to obtain a hydrothermal product; B2, freezing the hydrothermal product with liquid nitrogen and then placing it in a freeze dryer to dry at -48~-52℃ to obtain an aerogel; grinding and mixing the aerogel with urea and polytetrafluoroethylene in an agate mortar to obtain a mixture; annealing the mixture in a tube furnace at 898-902℃ under an argon atmosphere, and then naturally cooling to room temperature.

2. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S1, the mass ratio of ammonium dihydrogen phosphate, manganese sulfate, ferrous sulfate, and ascorbic acid is 1:(0.3-0.5):(0.4-0.7):(0.04-0.08); the reaction time is 10-12 h at 198-202℃.

3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In steps S2 and S3, the sintering time is 8-10 hours at 620-680℃; the heat treatment time is 3-6 hours at 545-555℃.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation method of the yttrium-gadolinium co-doped lanthanum zirconate lithium-ion conductor includes: A1, mixing lithium carbonate, lanthanum oxide, yttrium oxide, gadolinium oxide and zirconium dioxide to obtain a mixed powder; placing the mixed powder and anhydrous ethanol into a planetary ball mill jar, ball milling and mixing evenly to obtain a slurry; A2, drying the ball-milled slurry in a vacuum drying oven at 78-82℃; placing the dried powder in an alumina crucible, heating it to 798-802℃ in a muffle furnace for pre-calcination; ball milling the pre-calcined powder again, then sintering it at 1340-1360℃, followed by annealing at 1100-1200℃, and cooling it to room temperature in the furnace to obtain a bulk material; grinding and sieving the sintered bulk material.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 4, characterized in that, In step A1, the ball milling speed is 400-600 rpm; the ball milling time is 6-8 hours.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 4, characterized in that, In step A2, the pre-firing time at 798-802℃ is 6-8 hours; the sintering time at 1340-1360℃ is 12-14 hours.

7. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step B1, the reaction time is 12-14 hours at 180-185℃.

8. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step B2, the annealing time is 2-4 hours at 898-902℃.

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

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