A lithium-ion battery cathode composite material of lithium manganese iron phosphate and its preparation method

Abstract

This invention provides a lithium-ion battery cathode material of lithium manganese iron phosphate and its preparation method. The preparation method specifically includes: S1, preparation by dopamine oxidative self-polymerization; S2, preparation of ultrafine LMFP@C composite material by single-point doping; S3, preparation of single-crystal LMFP@C. ‑ Preparation of @C double-coated multi-site doped composite material: LMFP@C ultrafine powder was annealed in a multi-functional sintering furnace at 270°C-650°C for 1-4 hours to form Na + / La 3+ / Cl ‑ LMFP@Cl, a multi-site doped single-crystal LMFP double-shell composite material that partially replaces Li sites, Mn / Fe sites, and PO4 sites, respectively. ‑ @C. This invention addresses the problems of manganese leaching, poor conductivity, and poor high-temperature stability in lithium manganese iron phosphate cathode materials. It synergistically utilizes single crystallization, multi-site multi-component element doping, and surface double coating measures. The modification strategy is achieved through oxidative self-polymerization conformal coating-melt quenching-recrystallization annealing technology. It has the advantages of simple operation, liquid phase synthesis, high yield, and low cost, making it suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and particularly relates to a lithium iron manganese phosphate composite material for a lithium ion battery cathode and a preparation method thereof. Background Art

[0002] With the upgrading of demands in fields such as mobile devices, electric vehicles, and renewable energy storage batteries, the development of high-energy and high-safety batteries is an irresistible trend. Lithium ion batteries are one of the core components in the current new energy field and the electric vehicle industry. The technical level of key materials of lithium ion batteries directly determines the development of new energy vehicles and new energy storage technologies. The proportion and cost of the cathode material of lithium ion batteries are as high as 40% - 45%, which is a decisive factor for the electrochemical performance of lithium ion batteries and determines the energy density, cycle life, and safety of the batteries. At present, cathode materials such as lithium cobalt oxide (LCO), lithium manganese oxide (LMO), ternary material lithium nickel cobalt manganese oxide (NCM), and lithium iron phosphate (LFP) have been industrialized. The commercially available cathode materials are difficult to meet the stringent requirements of power batteries in terms of high specific energy, high safety, long life, low cost, etc. Therefore, it is necessary to develop new cathode materials to meet the requirements of power batteries.

[0003] Olivine-type lithium manganese phosphate (LiMnPO4) has a similar structure to LiFePO4 (LFP), but its electrode potential relative to Li metal is as high as 4.1 V, and the theoretical energy density can reach 701 Wh / kg, which is 1 / 4 higher than that of LFP's 550 Wh / kg. The raw material resources are rich and the price is low. The extremely high energy density makes LiMnPO4 have broad application prospects. The electrochemical activity of pure-phase LiMnPO4 is extremely poor, its electronic conductivity is only 10 -14 S / cm, and the lithium ion diffusion coefficient is also only 10 -14 cm 2 / s. The Jahn-Teller effect of Mn 3+ results in lattice distortion and dissolution of Mn 2+ and other problems, which limit its practical application.

[0004] Lithium iron manganese phosphate (LMFP) is formed by doping Fe 2+ at the Mn site to form a LiMn 1- x Fe x PO4 (0 < x < 1) solid solution. This material is a product of the combination of lithium manganese phosphate and lithium iron phosphate, fully combining the advantages of both, making its working potential, energy density, and low-temperature performance superior to those of lithium iron phosphate. Compared with ternary materials, lithium iron manganese phosphate has higher safety, longer cycle life, and lower cost, and is a very promising new cathode material. [[ID=三十二]] [[ID=三十三]]

[0005] However, existing lithium manganese iron phosphate materials mainly have the following problems: (1) The inherent low electronic conductivity of lithium manganese iron phosphate materials, and the existence of only one-dimensional ion and electron transport channels in the internal structure, result in poor rate performance and high-temperature performance. In addition, lithium manganese iron phosphate is still affected by Mn during charging and discharging. 3+ The influence of the Jiang-Taylor effect and Mn 2+ Ion dissolution causes the formation of a manganese-deficient phase in the positive electrode and deposits on the surface of the negative electrode, destroying the SEI layer structure and ultimately leading to capacity decay and deterioration of cycle performance. (2) The preparation process of lithium manganese iron phosphate materials is not yet unified, and the bottleneck of preparation technology has resulted in few breakthroughs in pure products. The preparation technology of lithium manganese iron phosphate mainly includes solid phase method and liquid phase method. The solid phase method is to prepare lithium manganese iron phosphate by mixing, grinding, drying, sintering and pulverizing solid raw materials. The solid phase method is simple, but it is not easy to mix and has high requirements for subsequent modification. The liquid phase method uses soluble raw materials and prepares lithium manganese iron phosphate by water / solvothermal method, sol-gel method and other methods. Then, it is dried, sintered and pulverized to obtain the product. The liquid phase method is more suitable for the production of lithium manganese iron phosphate than the solid phase method and has better performance. However, the liquid phase method is more complicated and has a higher preparation cost. In particular, the hydrothermal method requires high temperature and high pressure synthesis, which has a high risk factor and is difficult to produce on a large scale.

[0006] To address the key issues of low electronic conductivity and lithium-ion diffusion coefficient, poor J.T. Taylor effect of trivalent manganese, and poor electrochemical activity in lithium iron phosphate cathode materials, researchers typically employ measures such as reducing particle size, coating with conductive materials, bulk ion doping, and material structure design and morphology control to optimize and improve performance.

[0007] Nanostructuring can shorten the lithium-ion diffusion path of lithium manganese iron phosphate, improve lithium-ion migration efficiency, and thus improve the cycle stability of the battery. However, nanostructuring alone reduces the material's packing density, thereby lowering the volumetric energy density. While increased contact with the electrolyte increases the number of active sites, it also exacerbates manganese dissolution and side reactions. Therefore, controlling the particle size of the material can effectively improve its utilization rate.

[0008] Carbon coating can effectively improve the conductivity and cycle performance of lithium manganese iron phosphate (LFP). It enhances the conductivity, structural stability, and electrochemical activity of LFP materials, prevents particle agglomeration, improves uniformity, and inhibits the precipitation of some manganese ions, thus increasing battery cycle life. Due to the barrier effect of the coating layer, the material matrix avoids direct contact with the electrolyte, suppressing interfacial side reactions. Simultaneously, it stabilizes the matrix structure and alleviates phase transition stress during electrochemical processes. The highly conductive coating layer not only promotes the migration of electrons and ions at the interface, but the interconnected coating layers can also form a three-dimensional electron transport network, greatly enhancing the kinetic performance of the material during electrochemical processes. Furthermore, studies have found that doping with nitrogen, sulfur, and boron atoms promotes the graphitization of the carbon layer, and the difference in electronic structure between the dopant elements and carbon leads to structural defects in the carbon layer, providing a potential source of phosphorus for lithium iron phosphate (LiFePO4). + Providing additional channels for transport can effectively improve the conductivity and ion diffusion coefficient of the surface carbon layer, thereby achieving ultra-high power density and energy density of phosphate materials. However, excessive surface coating can reduce the proportion of active material to some extent, thus lowering the specific capacity of the material, and excessively thick surface coating can also limit lithium-ion transport.

[0009] Bulk ion doping: effectively controls the grain size and internal transport properties of materials, suppresses lattice distortion, and improves the ionic and electronic conductivity of lithium manganese iron phosphate, thereby improving rate performance and cycle stability. Finding suitable dopant ions and determining the optimal doping amount are key to ion-doped modified lithium manganese iron phosphate materials.

[0010] Different electronic structures, valence states, or doping sites of doping elements play different roles in electrochemical processes. For example, doping with hypervalent ions can create vacancies in the crystal lattice, thereby enhancing the material's electronic and ion conductivity; while doping with electrochemically inert elements can stabilize the crystal structure during charge and discharge. Controlling the type, quantity, and relative content of doping elements has a significant impact on material properties. Doping sites include Li sites (M1 sites), Mn / Fe sites (M2 sites), and single-site or dual-site doping at the O site of PO4. More research focuses on Fe / Mn site (M2 site) doping. Many reports indicate that Mn, Co, Ni, and Mg doping at the M2 site can enhance the electrochemical performance of LiFePO4, especially Mg. 2+ Doping has proven effective. However, overall, there is currently no well-developed technology on the market to address the issues of manganese leaching and poor high-temperature stability in lithium manganese iron phosphate cathode materials. Summary of the Invention

[0011] To address the shortcomings of existing technologies, this invention proposes a lithium iron phosphate (LFP) composite material for lithium-ion battery cathodes and its preparation method. Addressing the issues of poor conductivity, manganese leaching, and poor high-temperature stability in LFP cathode materials, this invention synergistically utilizes single-crystallization, multi-site multi-component element doping, and surface double coating. A modification strategy is achieved through oxidative self-polymerization conformal coating-melting quenching-recrystallization annealing technology. This method is simple to operate, possesses the advantages of liquid-phase synthesis, and offers high yield and low cost, making it suitable for large-scale production.

[0012] To achieve the above technical solution, the present invention provides a method for preparing a lithium-ion battery cathode composite material of lithium manganese iron phosphate, specifically including the following steps:

[0013] S1, preparation by dopamine oxidative self-polymerization Take a certain amount of FePO4 and The mixture was mechanically ball-milled according to an iron to manganese ion molar ratio of 4:6, added to a 10 mM trihydroxyaminomethane solution, and ultrasonically dispersed for 30 min. A certain amount of dopamine was weighed and then added according to the mass ratio. Add DA in a ratio of 1:0.5 to 1:2 to the above solution and then ultrasonically disperse for 30 min. Adjust the pH of the solution to 8.0-8.5 using 1M HCl and NaOH solutions. Under strong mechanical stirring, allow the DA to undergo oxidative self-polymerization for 24-36 h. Filter, wash, and vacuum dry to obtain... Coated polydopamine ;

[0014] S2. Preparation of unit-point doped ultrafine LMFP@C composite materials: ... When mixed with Li source and dopant source lanthanum salt and molybdenum salt in stoichiometric ratio, the general formula LiMn is obtained. 1-x-y Fe x Me y PO4@C, where 0.2≤x≤0.5, 0.01≤y≤0.1, is melted at 800-1000℃ for 0.5h-2h under a nitrogen protective atmosphere and then rapidly quenched into a 0~10% NaCl aqueous solution to form Mn / Fe site doped LMFP@C ultrafine powder;

[0015] S3, Single-crystal LMFP@Cl - Preparation of @C double-coated multi-site doped composite material: LMFP@C ultrafine powder was annealed in a multi-functional sintering furnace at 270°C-650°C for 1-4 hours. NaCl adsorbed on the surface of LMFP@C contained Na + and Cl - The LMFP diffuses into the lattice of the powder surface, while the LMFP particles further grow and develop, forming Na. + / La 3+ / Cl -LMFP@Cl, a multi-site doped single-crystal LMFP double-shell composite material that partially replaces Li sites, Mn / Fe sites, and PO4 sites, respectively. - @C.

[0016] Preferably, in step S2, the Li source is Li2CO3.

[0017] Preferably, in step S2, the lanthanum salt source for the doping element is La(NO3)3, and the molybdenum salt is ammonium paramolybdate.

[0018] Preferably, in step S2, the quenching cooling medium is an aqueous NaCl solution, which also serves as a source of doping elements for Li and O sites.

[0019] Preferably, in step S3, the annealing temperature is 450°C.

[0020] The present invention also provides a lithium iron phosphate composite material for lithium-ion battery cathodes, which is prepared by the above method.

[0021] Preferably, the composite material is Na synthesized by an oxidative self-polymerization coating-melt quenching-recrystallization annealing technique. + / La 3+ / Cl - The structure consists of single-crystal lithium manganese iron phosphate with multi-site, multi-component synergistic doping at Li, Mn / Fe, and PO4 O sites, respectively, and a double coating layer. The double coating layer consists of an N-doped carbon outer layer and an outer LMFP-doped Cl layer. - The inner layer.

[0022] The beneficial effects of the lithium iron phosphate composite material for lithium-ion battery cathode and its preparation method provided by this invention are as follows:

[0023] (1) This invention uses dopamine (DA) oxidation self-polymerization coating-quenching-recrystallization technology to synthesize Na + / La 3+ / Cl - Single-crystal LMFP@Cl were co-doped at Li sites, Mn / Fe sites, and PO4 O sites, respectively. - @C double-coated composite cathode material. Carbonization, doping, and lithiation processes are carried out simultaneously in a one-pot synthesis. This molten state synthesis method combines ideal liquid-phase reaction kinetics, short reaction time, and the advantages of preparing multi-component materials, making it a scalable synthesis method that can replace existing methods. The liquid melting stage allows for uniform mixing of multiple components, which is beneficial for doping, while the subsequent rapid cooling of the melt easily yields ultrafine composite materials with supersaturated vacancies. Subsequent recrystallization annealing controls the grain size and morphology to submicron to several micron single crystals. This method is simple to operate, combines the advantages of liquid-phase synthesis, has high yield, and low cost, making it suitable for large-scale production.

[0024] (2) This invention employs a combined technique of melt-quench recrystallization, multi-site multi-component bulk phase doping, and double coating to synthesize high-energy-density, high-safety, long-life, and low-cost single-crystal LMFP@Cl in a single batch. - @C composite cathode material. This cathode composite material has a single crystal morphology and improves internal ion transport characteristics, suppresses lattice distortion, improves ion / electron conductivity, and improves rate performance and cycle stability. This lithium manganese iron phosphate cathode material has the following characteristics: (1) medium particle size D50 is 0.5-5 μm, (2) tap density ≥ 0.8 g / cm³ 3 (3) Specific surface area ≤ 30 m² 2 / g, (4) Discharge specific capacity ≥ 150 mAh / g (0.1C), (5) First efficiency ≥ 90% (0.1C), (6) Rate performance: 1C discharge capacity retention ≥ 88%.

[0025] (3) The actual energy density of lithium manganese iron phosphate batteries using the lithium manganese iron phosphate composite material obtained in this invention as the positive electrode is increased by 15%–20% compared to lithium iron phosphate, and the battery cost is reduced by 5%–10% for the same capacity. In terms of safety, lithium manganese iron phosphate exhibits thermal runaway performance comparable to lithium iron phosphate; and it has better low-temperature performance than lithium iron phosphate. At -20℃, lithium manganese iron phosphate can retain nearly 80% of its capacity, which can further broaden the application range of phosphate batteries. Compared to lithium nickel cobalt manganese oxide batteries, lithium manganese iron phosphate batteries have higher safety, longer cycle life, and lower cost. Attached Figure Description

[0026] Figure 1 This is the single-crystal LMFP@Cl in Example 1 of the present invention. - @C multi-site double-coated composite cathode material preparation process flow chart.

[0027] Figure 2 LMFP@Cl prepared at different annealing temperatures - XRD pattern of @C multi-site double-coated composite cathode material.

[0028] Figure 3 This invention relates to Example 1, which describes the synthesis of single-crystal LMFP@Cl through melt quenching and rapid cooling followed by annealing at 450°C. - SEM morphology of the C composite material.

[0029] Figure 4 The LMFP@Cl prepared in Example 1 of this invention - Cyclic performance diagram of a half-cell assembled with @C cathode material.

[0030] Figure 5 The LMFP@Cl prepared in Example 1 of this invention- Rate performance of a half-cell assembled with @C cathode material.

[0031] Figure 6 The LMFP@Cl prepared in Example 1 of this invention - Impedance spectrum of a half-cell assembled with @C cathode material. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0033] Example 1

[0034] Reference Figure 1 As shown, a lithium-ion battery cathode material of lithium manganese iron phosphate and its preparation method specifically include the following steps:

[0035] S1, preparation by dopamine oxidative self-polymerization Take a certain amount of FePO4 and Mix the ingredients by mechanical ball milling at a molar ratio of 4:6, add to a 10 mM (millomoles / liter) trihydroxyaminomethane solution, and sonicate for 30 min. Weigh a certain amount of dopamine and then, according to the mass ratio... DA was added to the above solution at a ratio of 1:1 and then ultrasonically dispersed for 30 min. The pH of the solution was adjusted to 8.0 using 1M (mol / L) HCl and NaOH solutions. Under strong mechanical stirring, the DA oxidative self-polymerization reaction was carried out for 24 h. The product was then filtered, washed, and vacuum dried to obtain... Coated polydopamine Polydopamine coating was designed using dopamine oxidative self-polymerization. The composite powder, after dopamine oxidation and self-polymerization pyrolysis, forms a complete and continuous nitrogen-doped carbon coating layer on the LMFP surface, preventing particle agglomeration. Furthermore, the heterovalent N doping and the resulting differences in electronic structure lead to structural defects in the carbon layer. This layer not only improves the material's electronic conductivity and lithium-ion migration rate but also acts as a protective layer, preventing direct contact between the cathode material and the electrolyte, thereby reducing side reactions and improving the battery's chemical stability and cycle performance.

[0036] S2. Preparation of unit-point doped ultrafine LMFP@C composite materials: When mixed with Li source and doped element source rare earth compounds (such as La(NO3)3, etc.) in stoichiometric ratio, the general formula LiMn is obtained. 1-x-y Fe x Me y PO4@C, where 0.2≤x≤0.5 and 0.01≤y≤0.1, was melted at 800℃ for 1 hour under a nitrogen protective atmosphere and then rapidly quenched into a 10% NaCl aqueous solution to synthesize Mn / Fe site-doped LMFP@C ultrafine powder.

[0037] S3, Single-crystal LMFP@Cl - Preparation of @C double-coated multi-site doped composite material: LMFP@C ultrafine powder was annealed at 450°C for 2 h in a multi-functional sintering furnace, and NaCl adsorbed on the surface of LMFP@C was absorbed. + and Cl - The LMFP diffuses into the lattice of the powder surface, while the LMFP particles further grow and develop, forming Na. + / La 3+ / Cl - LMFP@Cl, a multi-site doped single-crystal LMFP double-shell composite material that partially replaces Li sites, Mn / Fe sites, and PO4 sites, respectively. - @C.

[0038] LMFP@Cl at different annealing temperatures - @See the XRD pattern of the C material. Figure 2 The diffraction peaks of the materials annealed at three different temperatures were consistent with the LMFP standard PDF card (JCPDS: No. 77-0178), showing sharp peaks and good crystallinity. Annealing at 270℃ resulted in a larger FWHM value, indicating smaller grains. Grain size gradually increased with increasing temperature. After annealing at 450℃, LMFP@)Cl - @C material single crystal morphology see Figure 3 The single-crystal LMFP particles have a size ranging from submicron to several micrometers, with smooth surfaces and well-developed grains. The Na substitution at the Li sites is provided by NaCl from the quenching medium in a 10% NaCl aqueous solution; simultaneously, Cl... - It can also be doped into the LMFP lattice on the outer surface to form an inner coating layer, further suppressing the dissolution of manganese.

[0039] The melting process yields lithium manganese iron phosphate-coated carbon composite material (LMFP@C), with carbon pyrolysis, lithiation, and doping occurring simultaneously in a one-pot synthesis. The melt is then rapidly quenched in a 10% NaCl aqueous solution to synthesize ultrafine LMFP@C powders and even nanocrystals. The 10% NaCl aqueous solution has a faster cooling capacity than water, resulting in greater supercooling and finer particles. Subsequent recrystallization annealing removes Na+ from the NaCl adsorbed on the LMFP@C surface. +and Cl - The LMFP diffuses into the lattice of the powder surface, while the LMFP particles further grow and develop, forming Na. + / La 3+ / Cl - LMFP@Cl, a multi-site doped single-crystal LMFP double-coating composite material that partially replaces Li sites, Mn / Fe sites, and PO4 sites, respectively. - @C. Cl - The LMFP that enters the surface plays a secondary coating role, further preventing the dissolution of manganese ions and alleviating the Taylor effect.

[0040] This invention proposes an oxidative self-polymerization coating-quenching-recrystallization annealing technique to prepare double-coated multi-site doped single-crystal LMFP@Cl - @C lithium-ion battery composite cathode material. This composite material has high tap density, with single-crystal LMFP particles ranging from submicron to several micrometers in size. It possesses excellent internal ion transport characteristics, effectively suppresses lattice distortion, improves ionic / electron conductivity, enhances electrode rate performance and cycle stability, and exhibits excellent discharge specific capacity (>150 mAh / g). LMFP@C is prepared via DA oxidation self-polymerization coating-melt quenching technology, with carbonization, doping, and lithiation processes occurring simultaneously in a one-pot synthesis. This molten state synthesis method combines ideal liquid-phase reaction kinetics, short delay time, and the ability to produce high-purity materials, making it a scalable synthesis method that can replace existing methods.

[0041] Example 2

[0042] S1, preparation by dopamine oxidative self-polymerization Take a certain amount of FePO4 and The mixture was mechanically ball-milled at a molar ratio of 4:6, added to a 10 mM trihydroxyaminomethane solution, and ultrasonically dispersed for 30 min. A certain amount of dopamine was weighed and then added according to the mass ratio. A DA ratio of 1:0.5 was added to the above solution and ultrasonically dispersed for 30 min. The pH of the solution was adjusted to 8.5 using 1M HCl and NaOH solutions. Under strong mechanical stirring, the DA oxidative self-polymerization reaction was carried out for 36 h. The product was then filtered, washed, and vacuum dried to obtain... Coated polydopamine ;

[0043] S2. Preparation of unit-point doped ultrafine LMFP@C composite materials: ... LiMn is mixed with a Li source (Li2CO3) and a dopant rare earth compound (La(NO3)3) as the M2 site in stoichiometric ratio. 1-x-y Fe x Me yPO4@C, where 0.2≤x≤0.5 and 0.01≤y≤0.1, was melted at 1000℃ for 0.5h under a nitrogen protective atmosphere and then rapidly quenched into a 10% NaCl aqueous solution to synthesize Mn / Fe site-doped LMFP@C ultrafine powder.

[0044] S3, Single-crystal LMFP@Cl - Preparation of @C double-coated multi-site doped composite material: Annealing at 600°C for 1 h in a multi-functional sintering furnace, NaCl adsorbed on the surface of LMFP@C was used to prepare the composite material. + and Cl - The LMFP diffuses into the lattice of the powder surface, while the LMFP particles further grow and develop, forming Na. + / La 3+ / Cl - LMFP@Cl, a multi-site doped single-crystal LMFP double-shell composite material that partially replaces Li sites, Mn / Fe sites, and PO4 sites, respectively. - @C.

[0045] Single-crystal LMFP@Cl - Electrochemical performance testing of the C-coated multi-site doped composite electrode (In this invention, battery assembly was carried out in an argon-filled glove box, with moisture controlled at 0.1 ppm and oxygen controlled below 0.1 ppm. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention can be purchased commercially or prepared by existing methods):

[0046] The LMFP@Cl prepared through Example 1 of this embodiment - @C, carbon black (Super P), and polyvinylidene fluoride (PVDF) were thoroughly mixed in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 to obtain the positive electrode slurry. After stirring and mixing in an agate mortar, the positive electrode slurry was coated onto aluminum foil and dried in a vacuum drying oven for 24 h. The positive electrode loading was calculated by weighing. The electrolyte was a mixture of 1 mol / L LiPF6 dissolved in ethylene carbonate and ethyl methyl carbonate (EC / EMC volume ratio of 1:1). CR2032 button half-cells were assembled in an argon-filled glove box using lithium metal sheets, electrolyte, positive electrode sheets, and a polypropylene separator (Celgard 2400). Constant current charge / discharge curves were tested on a LANDCT2001A battery tester at 25 °C with a voltage window of 2.0 V to 4.5 V.

[0047] Detection of LMFP@Cl at different annealing temperatures - @C cathode material cycle performance (e.g. Figure 4(As shown in the figure). Cyclic performance graphs at different annealing temperatures reveal that the material annealed at 450℃ exhibits higher cycling stability. After 100 cycles at a current density of 0.5C, the capacity retention remains above 88%, higher than materials prepared at 270℃ and 600℃. This indicates that annealing temperature significantly affects electrochemical performance. By modifying the grain structure and size morphology to control electrode performance, 450℃ was determined to be the optimal annealing temperature.

[0048] The effect of detection site doping on the rate performance of single-crystal LMFP@C electrodes, undoped LMFP@C (LMFP-0), La 3+ Unit-point doping of LMFP@C (LMFP-1), Na + / La 3+ / Cl - Multisite doping LMFP@C@Cl - (LMFP-2) Cathode rate performance is shown in [link to relevant documentation]. Figure 5 .

[0049] La 3+ Unit-point doping of LMFP@C (LMFP-1), Na + / La 3+ / Cl - Multisite doping LMFP@C@Cl - (LMFP-2) exhibits similar rate performance, but LMFP-2 demonstrates a higher initial discharge specific capacity. Both are significantly superior to the performance of undoped LMFP@C (LMFP-0). This indicates that doping improves the electrochemical performance of LMFP@C. Using 10% NaCl as the quenching medium provides Na substitution at Li sites and accelerates the cooling rate, resulting in greater supercooling and grain refinement. Simultaneously, it forms Cl-containing... - The inner coating barrier further alleviates Mn 2+ Dissolution.

[0050] Comparison of multi-site doped LMFP@Cl - The impedance of @C and the undoped LMFP@C positive electrode, such as Figure 6 As shown in the figure. It can be seen that compared with the undoped LMFP@C cathode, the LMFP@C cathode with multiple doping sites... - The @C cathode exhibits lower impedance, with a charge transfer impedance Rct value (185Ω) significantly smaller than that of the undoped cathode (320Ω), indicating that the doped material possesses a higher electron transfer rate and favorable Li... + transmission.

[0051] This invention first designs polydopamine coating through dopamine oxidative self-polymerization. The composite powder, after dopamine oxidation and self-polymerization pyrolysis, can form a complete conformal coating of nitrogen-doped carbon layers in LMFP, avoiding particle agglomeration. Furthermore, the heterovalent N doping and the difference in electronic structure lead to structural defects in the carbon layer, providing Li… + The carbon layers provide additional channels for transport, and can also form a connected three-dimensional conductive network, improving electronic conductivity and Li. + This improves power density and cycle life of the electrodes.

[0052] Secondly, after mixing lithium sources and rare earth compounds (such as La(NO3)3 (rare earth salts not only serve as doping sources for M2 sites but also act as rare earth modifiers to refine grains), melt pyrolysis yields lithium manganese iron phosphate-coated carbon composite material (LMFP@C). The melt is then rapidly quenched in a 10% NaCl aqueous solution to synthesize ultrafine powders or amorphous particles. Subsequent recrystallization annealing controls the crystal structure and size of LMFP. The NaCl adsorbed on the surface after quenching diffuses through the annealing process. + Replace Li + To achieve lithium doping, Cl - Cl enters the LMFP lattice on the surface and forms - The doped LMFP inner coating further prevents manganese ion dissolution and alleviates the Taylor effect. Additionally, a 10% NaCl aqueous solution has a faster cooling capacity than water, resulting in greater supercooling and finer particles. Recrystallization annealing regulates the grain structure and morphology, developing it into submicron to several-micron single crystals, forming LMFP@Cl. - @C double-coated composite material.

[0053] Furthermore, a dopamine oxidation self-polymerization complete coating-quenching-recrystallization technique was used to prepare single-crystal lithium manganese iron phosphate (LMFP) lithium-ion battery cathode material, improving tap density and Na+. + / La 3+ / Cl - Multi-site, multi-component synergistic doping was performed at Li sites, Mn / Fe sites, and O sites of PO4 to modulate internal ion transport properties, suppress lattice distortion, improve ionic / electronic conductivity, and enhance rate performance and cycling stability. This resulted in single-crystal LMFP particles with submicron to several micrometer sizes and a specific capacity >150 mAh / g.

[0054] This invention employs a synergistic combination of melt-quench recrystallization, multi-site multi-component bulk phase doping, and double coating technology to synthesize high-energy-density, high-safety, long-life, and low-cost single-crystal LMFP@Cl in a single batch. -@C composite cathode material. This cathode composite material has a single crystal morphology and improves internal ion transport characteristics, suppresses lattice distortion, improves ion / electron conductivity, and improves rate performance and cycle stability. This lithium manganese iron phosphate cathode material has the following characteristics: (1) medium particle size D50 is 0.5-5 μm, (2) tap density ≥ 0.8 g / cm³ 3 (3) Specific surface area ≤ 30 m² 2 / g, (4) Discharge specific capacity ≥ 150 mAh / g (0.1C), (5) First efficiency ≥ 90% (0.1C), (6) Rate performance: 1C discharge capacity retention ≥ 88%.

[0055] The lithium manganese iron phosphate (LFP) battery using the LFP composite material obtained in this invention as the positive electrode exhibits a 15%-20% higher actual energy density compared to LFP, while reducing battery cost by 5%-10% for the same capacity. In terms of safety, LFP shows comparable thermal runaway performance to LFP, and demonstrates better low-temperature performance. At -20°C, LFP retains nearly 80% of its capacity, further expanding the application range of phosphate batteries. Compared to lithium nickel manganese cobalt oxide (LCO) batteries, LFP batteries offer higher safety, longer cycle life, and lower cost.

[0056] The above description is only a preferred embodiment of the present invention, but the present invention should not be limited to the content disclosed in the embodiments and drawings. Therefore, any equivalent or modified embodiments made without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium-ion battery cathode composite material of lithium manganese iron phosphate, characterized in that... Specifically, the steps include the following: S1, preparation by dopamine oxidative self-polymerization Take a certain amount and The mixture was mechanically ball-milled according to an iron to manganese ion molar ratio of 4:6, added to a 10 mM trihydroxyaminomethane solution, and ultrasonically dispersed for 30 min. A certain amount of dopamine was weighed and then added according to the mass ratio. The solution was added to the above solution and ultrasonically dispersed for 30 min. The pH of the solution was adjusted to 8.0-8.5 using 1M HCl and NaOH solutions. Under strong mechanical stirring, the DA oxidative self-polymerization reaction was carried out for 24-36 h. After filtration, washing, and vacuum drying, the product was obtained. Coated polydopamine ; S2. Preparation of unit-point doped ultrafine LMFP@C composite materials: ... Mixed with Li source and dopant source lanthanum or molybdenum salt in stoichiometric ratio, general formula Where 0.2≤x≤0.5, 0.01≤y≤0.1, melted at 800-1000℃ for 0.5h-2h under nitrogen protection atmosphere, and rapidly quenched into NaCl aqueous solution with a concentration greater than 0% and less than or equal to 10% to form Mn / Fe site doped LMFP@C ultrafine powder; S3, Single Crystal Preparation of double-coated multi-site doped composite materials: LMFP@C ultrafine powder was annealed in a multi-functional sintering furnace at 270°C-650°C for 1-4 hours, and NaCl adsorbed on the surface of LMFP@C was... and The LMFP diffuses into the lattice of the powder surface, while the LMFP particles further grow and develop, forming... Partial substitution of Li sites, Mn / Fe sites and O-site multi-site doped single-crystal LMFP double-coated shell composite material .

2. The preparation method of the lithium-ion battery cathode lithium iron phosphate composite material as described in claim 1, characterized in that: In step S2, the Li source is .

3. The method for preparing the lithium-ion battery cathode lithium iron phosphate composite material as described in claim 1, characterized in that: In step S2, the dopant source lanthanum salt is... The molybdenum salt is ammonium paramolybdate.

4. The method for preparing the lithium-ion battery cathode lithium iron phosphate composite material as described in claim 1, characterized in that: In step S2, the quenching cooling medium is an aqueous NaCl solution, which also serves as a source of doping elements for Li and O sites.

5. The method for preparing the lithium-ion battery cathode lithium iron phosphate composite material as described in claim 1, characterized in that: In step S3, the annealing temperature is 450°C.

6. A lithium-ion battery cathode composite material of lithium manganese iron phosphate, characterized in that: Prepared by the method described in any one of claims 1-5.

7. The lithium iron phosphate composite material for the positive electrode of a lithium-ion battery as described in claim 6, characterized in that: The composite material was synthesized using an oxidative self-polymerization coating-quenching-recrystallization annealing technique. At Li sites, Mn / Fe sites and It consists of a single-crystal lithium manganese iron phosphate with multi-site and multi-component synergistic doping at the O site and a double coating layer, wherein the double coating layer is an N-doped carbon coating outer layer and a double coating layer. Doped surface LMFP inner layer.

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