Modified lithium iron manganese phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a modified lithium iron manganese phosphate positive electrode material, a preparation method thereof and a lithium ion battery. The modified lithium manganese iron phosphate positive electrode material comprises an inner core, and a shell and a protective layer which are coated on the surface of the inner core, the inner core comprises a lithium manganese iron phosphate material, and the doping concentration of the aluminum element in the shell is distributed in a sectional index increasing mode from inside to outside in the radial direction. According to the invention, the comprehensive performance optimization of the lithium manganese iron phosphate positive electrode material is realized, the doping concentration of the aluminum element in the shell is distributed in a sectional index increasing manner from inside to outside along the radial direction, and the cycle life and mechanical stress adaptability of the material are improved due to the design; meanwhile, the outermost protective layer can effectively reduce the interface impedance and improve the structural stability of the material. In conclusion, the modified lithium iron manganese phosphate positive electrode material realizes comprehensive improvement of material bulk phase stability, interface stability and conductivity, and has excellent comprehensive performance.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a modified lithium manganese iron phosphate cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) has a higher voltage plateau (~3.9-4.1V vs. LiFePO4) compared to lithium iron phosphate (LFP). + With its high energy density and theoretical energy density, LMFP offers better safety and cost advantages than high-nickel ternary materials, making it one of the ideal cathode materials for next-generation high-safety, high-energy-density lithium-ion batteries. However, some inherent drawbacks of LMFP hinder its mass production, such as: 1) Manganese leaching and the Jahn-Teller effect: Mn 3+ Disproportionation reaction (2Mn) is prone to occur during charging and discharging. 3+ →Mn 2+ +Mn 4+ ) leads to Mn 2+ Leaching, disrupting structural stability; Mn 3+ 1) Jahn-Teller distortion leads to lattice distortion, affecting cycling performance. 2) Low electronic / ionic conductivity: Compared with ternary materials, the intrinsic conductivity is lower, affecting rate performance; 3) Slow two-phase reaction kinetics: Fe 2+ / Fe 3+ and Mn 2+ / Mn 3+ Redox reactions involve two-phase transitions and have relatively slow kinetics.

[0003] Existing modification methods include: 1) Bulk doping: such as introducing Mg 2+ Al 3+ Ti 4+ Zr 4+ 、Nb 5+ Doping with these elements can stabilize the structure and improve conductivity, but the doping of these elements may weaken the overall capacity of the material and has limited improvement on surface problems (such as manganese leaching). 2) Surface coating: Carbon coating (graphene, carbon nanotubes, or amorphous carbon, etc.) is the most commonly used method, which can improve electronic conductivity, but has limited effect on inhibiting manganese leaching and improving interfacial ion transport; coating with a single metal oxide (such as Al2O3) can inhibit manganese leaching but may hinder lithium-ion diffusion. 3) Core-shell structure design: such as manganese-rich core / iron-rich shell or doped core / undoped shell, aims to combine high voltage (core) and good kinetics / stability (shell), but the interface of traditional core-shell structures is clear, which may lead to stress concentration and ion migration resistance, and the shell is usually homogeneous in composition, limiting the optimization space.

[0004] Therefore, how to comprehensively improve the bulk stability, interfacial stability, and conductivity of materials to obtain a lithium manganese iron phosphate cathode material with excellent overall performance is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a modified lithium manganese iron phosphate cathode material, its preparation method, and a lithium-ion battery. The present invention designs a unique core-shell structure, achieving comprehensive performance optimization of the lithium manganese iron phosphate cathode material. Specifically, the aluminum doping concentration in the outer shell exhibits a segmented exponential increase from the inside to the outside in the radial direction. This design allows the outer shell region near the core to fill manganese vacancies and stabilize the crystal structure through low-concentration aluminum doping, while the outer shell region away from the core uses high-concentration aluminum doping to enhance electronic conductivity and suppress manganese ion dissolution, thereby synergistically improving the material's cycle life and mechanical stress adaptability. Simultaneously, the outermost protective layer effectively reduces interfacial impedance and enhances the material's structural stability. In summary, through the unique core-shell structure design, the segmented exponential increase in aluminum doping concentration in the outer shell, and the synergistic effect of the protective layer, the modified lithium manganese iron phosphate cathode material achieves comprehensive improvements in bulk stability, interfacial stability, and conductivity, resulting in excellent overall performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a modified lithium manganese iron phosphate cathode material, the modified lithium manganese iron phosphate cathode material comprising a core, and a shell and a protective layer covering the surface of the core.

[0008] The core comprises lithium manganese iron phosphate material, and the aluminum doping concentration in the outer shell is distributed in a segmented exponentially increasing manner from the inside to the outside along the radial direction.

[0009] This invention designs a unique core-shell structure, achieving comprehensive performance optimization of lithium manganese iron phosphate cathode materials. In the outer shell, the aluminum doping concentration exhibits a segmented, exponentially increasing distribution from the inside to the outside in the radial direction. This design allows the outer shell region near the core to fill manganese vacancies and stabilize the crystal structure through low-concentration aluminum doping, while the outer shell region away from the core uses high-concentration aluminum doping to enhance electronic conductivity and suppress manganese ion dissolution, thereby synergistically improving the material's cycle life and mechanical stress adaptability. Simultaneously, the outermost protective layer effectively reduces interfacial impedance and enhances the material's structural stability. In summary, through the unique core-shell structure design, the segmented, exponentially increasing aluminum doping concentration distribution in the outer shell, and the synergistic effect of the protective layer, this modified lithium manganese iron phosphate cathode material achieves comprehensive improvements in bulk stability, interfacial stability, and conductivity, resulting in excellent overall performance.

[0010] Preferably, the lithium manganese iron phosphate material is a pure phase with the general chemical formula LiMn. (1-x) Fe x PO4, x = 0.25-0.35, for example, it can be 0.25, 0.3, or 0.35, etc. For example, the chemical composition of the core can specifically be LiMn. 0.7 Fe 0.3 PO4.

[0011] Preferably, the ratio of the average radius of the core to the average radius of the modified lithium manganese iron phosphate cathode material particles is (0.5-0.55):1, for example, it can be 0.5:1, 0.51:1, 0.52:1, 0.53:1, 0.54:1, or 0.55:1, etc., and preferably the average radius of the core is half the average radius of the modified lithium manganese iron phosphate cathode material particles. For example, when the average radius of the complete particle is designed to be 1500nm, the average radius of the core can be 780nm, and its ratio to the particle radius is approximately 0.52:1. In this case, the outer shell layer thickness is approximately 720nm, and the outermost layer is a metal protective layer with a thickness of 2-5nm.

[0012] The present invention preferably uses pure phase lithium manganese iron phosphate material as the core, and the average radius is about half the average radius of the modified lithium manganese iron phosphate cathode material particles, which can provide a high voltage platform and high theoretical capacity.

[0013] Preferably, the outer shell is made of aluminum-doped lithium manganese iron phosphate.

[0014] Preferably, the outer shell includes an inner shell layer and an outer shell layer in the radial direction, and the concentration distribution of aluminum in the inner shell layer and the outer shell layer respectively follows different exponential growth functions.

[0015] In this invention, by designing the concentration distribution of aluminum in the inner and outer layers of the outer shell to follow different exponential growth functions, a smooth transition and stability of the crystal structure can be achieved in the inner layer of the outer shell, while a rapid improvement in interface dynamics and effective suppression of manganese dissolution can be achieved in the outer layer of the outer shell.

[0016] Preferably, the thickness ratio of the inner layer and the outer layer of the outer shell in the radial direction is (2.5-3.5):(1.5-2.5), wherein the selection range of the inner layer of the outer shell "2.5-3.5" can be, for example, 2.5, 3 or 3.5, etc., and the selection range of the outer layer of the outer shell "1.5-2.5" can be, for example, 1.5, 2 or 2.5, etc.

[0017] In this invention, by designing a certain ratio between the thicknesses of the inner and outer shell layers in the radial direction, the growth space required for the aluminum doping gradient and the lithium ion diffusion path can be synergistically controlled.

[0018] Preferably, the ratio of the thickness of the outer shell to the average radius of the modified lithium manganese iron phosphate cathode material particles is (0.48-0.499):1, for example, it can be 0.48:1, 0.485:1, 0.49:1, 0.495:1, 0.498:1, or 0.499:1, etc. For example, when the average radius of the intact particles is 1500 nm, and the ratio is 0.48:1, the outer shell thickness is approximately 720 nm.

[0019] Preferably, the concentration distribution of aluminum in the inner layer of the outer shell follows the following exponential growth function: C Al =A1×e k1 (r2-r1) Where A1 is 0.3-0.7 at%, for example, it can be 0.3 at%, 0.4 at%, 0.5 at%, 0.6 at%, or 0.7 at%, etc.; k1 is 1.5-3.5 nm. -1 For example, it could be 1.5nm -1 2nm -1 2.5nm -1 3nm -1 Or 3.5nm -1 etc.; r2 is the average radius of the target position in the inner layer of the outer shell, and r1 is the average radius of the core.

[0020] In this invention, the concentration distribution of aluminum in the inner layer of the outer shell follows C Al =A1×e k1(r2-r1) Under the regulation of this function, a gradual exponential increase in aluminum concentration from the core-shell interface can be achieved. This gradual transition can effectively fill manganese vacancies and suppress Jahn-Teller distortion, significantly improving the structural stability of the near-interface region of the core without excessively sacrificing lithium-ion diffusion channels, thus laying the foundation for a long overall cycle life.

[0021] Preferably, the concentration distribution of aluminum in the outer layer of the shell follows the following exponential growth function: C Al =C t +K(1-e -k2(r4-r3) ); where C t K represents the aluminum doping concentration at the boundary between the inner and outer layers of the outer shell, expressed in at%; K is the concentration increment limit, where K > 0, and can be, for example, 1 at%, 2 at%, 3 at%, 4 at%, or 5 at%, etc.; k2 ≥ 4 nm -1 For example, it could be 4nm -1 5nm -1 6nm -1 7nm -1 or 8nm -1 etc.; r4 is the average radius of the target position in the outer shell, and r3 is the maximum radius of the inner shell.

[0022] In this invention, the concentration distribution of aluminum in the outer shell follows C Al =C t +K(1-e -k2(r4-r3) Under the regulation of this function, it helps to achieve a rapid, near-saturation increase in aluminum concentration starting from the inflection point. This design can quickly establish a high-concentration aluminum-doped layer in the near-surface region of the particles, greatly improving the electronic conductivity of the particle surface, while forming a concentration barrier to effectively suppress the dissolution of manganese ions during charging and discharging, and optimizing the interface bonding with the subsequent protective layer.

[0023] Preferably, the C t Satisfies: 2.8at%≤C t ≤3.6at%, for example, it can be 2.8at%, 3at%, 3.2at%, 3.4at%, or 3.6at%, etc.

[0024] In this invention, setting the aluminum doping concentration at the boundary between the inner and outer shell layers within the above range helps to achieve an optimal balance between structural stability and interfacial dynamics. Too low a concentration will result in insufficient performance improvement of the shell layer, while too high a concentration may prematurely affect the diffusion of lithium ions within the shell layer.

[0025] Preferably, k1 and k2 satisfy: k2 / k1≥1.5, for example, it can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5, etc.

[0026] This invention limits k1 and k2 to satisfy: k2 / k1≥1.5, in order to ensure that the concentration gradient of the outer shell is significantly steeper than that of the inner shell. This “gentle inner and steep outer” concentration design is the key to this invention. It creates a synergistic effect of stable internal structure and active external reaction, which is the core of obtaining high stability and high rate performance at the same time.

[0027] Preferably, K satisfies: 3at%≤K≤5at%, for example, it can be 3at%, 3.5at%, 4at%, 4.5at%, or 5at%, etc.

[0028] In this invention, K is the limit value for the increase of aluminum doping concentration. The limit of 3at%≤K≤5at% can ensure that the aluminum doping concentration on the particle surface reaches the ideal range. This not only ensures the sufficiency of the surface modification effect, but also avoids excessively hindering the entry and exit of lithium ions due to excessively high surface aluminum concentration, thus achieving the optimal ratio of capacity and stability.

[0029] Preferably, the protective layer is a copper-based protective layer.

[0030] The present invention uses a copper-based protective layer to cover the surface of the shell, which can effectively reduce the interface impedance; in the field of thermal management, the thermal conductivity of the copper-based protective layer significantly reduces the temperature rise at high rates and increases the thermal runaway temperature.

[0031] Preferably, the thickness of the protective layer is 2-5 nm, for example, it can be 2 nm, 3 nm, 4 nm or 5 nm.

[0032] Preferably, an interface transition layer is formed between the protective layer and the outer shell, and the thickness of the interface transition layer is 0.5-3nm, for example, it can be 0.5nm, 1nm, 1.5nm, 2nm, 2.5nm or 3nm.

[0033] In this invention, the interface transition layer is generated by the in-situ reaction between the copper-based protective layer and the outer shell surface, which can simultaneously suppress manganese leaching and copper oxidation.

[0034] In a second aspect, the present invention provides a method for preparing the modified lithium manganese iron phosphate cathode material as described in the first aspect, the method comprising the following steps:

[0035] Manganese source, iron source, phosphorus source, lithium source and solvent are mixed and co-precipitated to obtain core precursor;

[0036] An aluminum-doped shell solution is prepared, and then the shell solution, manganese source, iron source, lithium source and phosphorus source are added in parallel to the reaction system containing the core precursor to carry out a hydrothermal reaction to obtain the shell precursor;

[0037] The shell precursor is subjected to a crystallization reaction to obtain a shell precursor crystal; in the shell precursor crystal, the concentration of aluminum element is distributed in a segmented exponential increase from the inside to the outside along the radial direction.

[0038] A protective layer is coated onto the surface of the precursor crystal in the outer shell to obtain the modified lithium manganese iron phosphate cathode material.

[0039] Preferably, the mixing method includes:

[0040] (a) The manganese source and the iron source are premixed to obtain a manganese-iron premix.

[0041] (b) The manganese-iron premix, phosphorus source and lithium source are fed into the solvent in parallel.

[0042] Preferably, in step (a), the manganese source is a manganese solution with a concentration of 1-1.5 mol / L, and the concentration may be, for example, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L or 1.5 mol / L; the manganese solution includes any one of manganese acetate solution, manganese sulfate solution or manganese chloride solution.

[0043] Preferably, in step (a), the iron source is a ferrous solution with a concentration of 0.3-0.5 mol / L, such as 0.3 mol / L, 0.4 mol / L, or 0.5 mol / L; the ferrous solution includes ferrous sulfate solution and / or ferrous oxalate solution.

[0044] Preferably, in the manganese-iron premixed solution in step (a), the molar ratio of manganese ions to ferrous ions is (6-8):(2-4), wherein the selection range of manganese ions "6-8" can be, for example, 6, 7 or 8, and the selection range of ferrous ions "2-4" can be, for example, 2, 3 or 4.

[0045] Preferably, an antioxidant is also added during the mixing process. For example, ascorbic acid may be used.

[0046] The purpose of adding antioxidants during the mixing process in this invention is to prevent oxidation.

[0047] Preferably, the temperature of the coprecipitation reaction is 68-75℃, for example, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃ or 75℃, and the reaction time is 2-4h, for example, 2h, 3h or 4h.

[0048] Preferably, during the coprecipitation reaction, the phosphorus source is a phosphorus-containing solution with a concentration of 1.5-2.5 mol / L (e.g., 1.5 mol / L, 1.75 mol / L, 2 mol / L, 2.25 mol / L, or 2.5 mol / L, etc.) and a pH of 3.5-4.5 (e.g., 3.5, 4, or 4.5, etc.). For example, it can be an ammonium dihydrogen phosphate solution, etc.

[0049] Preferably, during the co-precipitation reaction, the lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L (e.g., 2.5 mol / L, 2.75 mol / L, 3 mol / L, 3.25 mol / L, or 3.5 mol / L, etc.). For example, it can be a lithium hydroxide solution.

[0050] Preferably, the aluminum-doped shell solution includes an aluminum source, a phosphorus source, and a surfactant. For example, the aluminum source may be aluminum sulfate.

[0051] Preferably, the concentration of the phosphorus source in the aluminum-doped shell solution is 1.5-2 mol / L, for example, it can be 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, or 2 mol / L. For example, it can be ammonium dihydrogen phosphate.

[0052] Preferably, the aluminum-doped shell solution includes an aluminum-doped inner shell solution and an aluminum-doped outer shell solution, wherein the aluminum doping concentration in the inner shell solution is lower than the aluminum doping concentration in the outer shell solution.

[0053] Preferably, the surfactant in the inner layer solution of the outer shell includes PVP (polyvinylpyrrolidone), and the concentration of the surfactant is 0.03-0.1 wt%, for example, it can be 0.03 wt%, 0.06 wt%, 0.08 wt%, or 0.1 wt%.

[0054] This invention introduces PVP into the inner layer solution of the outer shell, and PVP can react with Al. 3+ The formation of weak coordination bonds slows down the deposition kinetics and preferentially adsorbs on specific crystal planes to regulate growth, thereby achieving a uniform distribution of aluminum ions on the deposition interface and ultimately making the gradient error <±3%.

[0055] Preferably, the surfactant in the outer shell solution includes CTAB, and the concentration of the surfactant is 0.05-0.1wt%, for example, it can be 0.05wt%, 0.06wt%, 0.07wt%, 0.08wt%, 0.09wt%, or 0.1wt%, etc.

[0056] The present invention introduces CTAB into the outer shell solution, which can effectively reduce the surface energy of the particles and increase the density of surface hydroxyl (-OH) groups, which significantly enhances the chemical bonding force between the subsequent copper-based protective layer and the particle matrix.

[0057] Preferably, the specific steps of the parallel flow addition include:

[0058] (i) The inner shell solution, manganese source, iron source, lithium source and phosphorus source are fed into the reaction system where the core precursor is located in parallel to carry out the first hydrothermal deposition reaction to obtain the inner shell precursor.

[0059] (ii) The outer shell solution, manganese source, iron source, lithium source and phosphorus source are fed into the reaction system containing the inner shell precursor in parallel to carry out the second hydrothermal deposition reaction to obtain the outer shell precursor.

[0060] It should be noted that in the first hydrothermal deposition reaction, the reaction process needs to be monitored to ensure that the aluminum concentration follows the expected exponential growth. If monitoring shows that the aluminum concentration deviates from the exponential growth and shows a linear trend, it indicates that the reaction conditions are unbalanced. This can be corrected by appropriately adding PVP (e.g., increasing by 0.02 wt%) and simultaneously fine-tuning the exponential growth rate of the flow rate equation (e.g., reducing it by 10-20%) to ensure that the aluminum concentration follows the expected exponential growth.

[0061] In this invention, the inner and outer shell precursors are prepared by two separate hydrothermal deposition processes to avoid poor interfacial bonding of doped materials, prevent mismatch in core-shell expansion coefficients, and effectively release stress.

[0062] Preferably, in step (i), the manganese source is a manganese solution with a concentration of 0.3-0.5 mol / L (e.g., 0.3 mol / L, 0.4 mol / L, or 0.5 mol / L); the manganese solution includes any one of manganese acetate solution, manganese sulfate solution, or manganese chloride solution.

[0063] Preferably, in step (i), the iron source is a ferrous solution with a concentration of 0.1-0.3 mol / L (e.g., 0.1 mol / L, 0.2 mol / L, or 0.3 mol / L, etc.); the ferrous solution includes ferrous sulfate solution and / or ferrous oxalate solution.

[0064] Preferably, in step (i), the lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L (e.g., 2.5 mol / L, 3 mol / L, or 3.5 mol / L, etc.); the lithium solution includes a lithium hydroxide solution.

[0065] Preferably, in step (i), the phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5 mol / L (e.g., 0.5 mol / L, 1 mol / L, or 1.5 mol / L, etc.); the phosphorus-containing solution includes an ammonium dihydrogen phosphate solution.

[0066] Preferably, in step (i), the feed flow rate of the inner layer solution of the outer shell satisfies V1=8e 0.0105t1 The unit is mL / min; where 0≤t1≤100min, and t1 is the time of the first hydrothermal deposition reaction. For example, it can be 10min, 20min, 30min, 40min, 50min, 60min, 70min, 80min, 90min or 100min, etc.

[0067] In preparing the inner shell layer precursor, this invention limits the feed flow rate of the inner shell layer solution to satisfy V1=8e. 0.0105t1 Through precise exponential program control, the preset exponential aluminum concentration growth function (C0) can be directly and accurately applied. Al =A1×e k1(r2-r1) This mapping, when applied to actual material growth, ensures high fidelity and repeatability of the inner shell gradient.

[0068] Preferably, in step (i), the manganese source, iron source, lithium source and phosphorus source are all fed at a constant rate.

[0069] Preferably, the temperature of the first hydrothermal deposition reaction is 170-200℃, for example, it can be 170℃, 180℃, 190℃ or 200℃.

[0070] Preferably, in step (ii), the manganese source is a manganese solution with a concentration of 0.3-0.5 mol / L (e.g., 0.3 mol / L, 0.4 mol / L, or 0.5 mol / L); the manganese solution includes any one of manganese acetate solution, manganese sulfate solution, or manganese chloride solution.

[0071] Preferably, in step ii), the iron source is a ferrous solution with a concentration of 0.1-0.3 mol / L (e.g., 0.1 mol / L, 0.2 mol / L, or 0.3 mol / L, etc.); the ferrous solution includes ferrous sulfate solution and / or ferrous oxalate solution.

[0072] Preferably, in step (ii), the lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L (e.g., 2.5 mol / L, 3 mol / L, or 3.5 mol / L, etc.); the lithium solution includes a lithium hydroxide solution.

[0073] Preferably, in step (ii), the phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5 mol / L (e.g., 0.5 mol / L, 1 mol / L, or 1.5 mol / L, etc.); the phosphorus-containing solution includes an ammonium dihydrogen phosphate solution.

[0074] Preferably, in step (ii), the feed flow rate of the outer shell solution satisfies V2=18e 0.018(t2-100) The unit is mL / min; where 100min ≤ t2 ≤ 160min, and t2 is the total time of the first and second hydrothermal deposition reactions. For example, it can be 100min, 110min, 120min, 130min, 140min, 150min, or 160min, etc.

[0075] In preparing the outer shell precursor, this invention limits the feed flow rate of the outer shell solution to satisfy V2=18e. 0.018 (t2-100) Through a higher exponential growth rate, the aluminum concentration is driven to rapidly increase and approach saturation in the near-surface region. This is the key process guarantee for forming a "gentle inner and steep outer" gradient structure and enhancing surface modification.

[0076] Preferably, in step (ii), the feed flow rate of the manganese source satisfies V Mn =(6-0.036(t2-100)), the unit is mL / min.

[0077] Preferably, in step (ii), the feed flow rate of the iron source satisfies VFe =(3-0.018(t2-100)), the unit is mL / min.

[0078] In preparing the outer shell precursor, this invention limits the feed flow rates of the manganese source and the iron source to meet the above linear variation trend. The aim is to actively and synchronously reduce the deposition amount of Mn / Fe to accurately compensate for the overall stoichiometric balance ((Mn+Fe+Al):P≈1:1) that needs to be maintained due to the sharp increase in aluminum concentration, thereby avoiding the formation of impurity phases and ensuring the structural stability of the material.

[0079] Preferably, in step (ii), both the lithium source and the phosphorus source are fed at a constant rate.

[0080] Preferably, the temperature of the second hydrothermal deposition reaction is 180-210°C, for example, it can be 180°C, 190°C, 200°C or 210°C.

[0081] Preferably, the crystallization reaction includes pre-sintering, first sintering, and second sintering performed by sequentially increasing the temperature.

[0082] Preferably, the pre-sintering temperature is 300-400℃, for example, 300℃, 350℃, or 400℃, and the time is 2-3 hours, for example, 2 hours, 2.5 hours, or 3 hours; the first sintering temperature is 550-600℃, for example, 550℃, 560℃, 570℃, 580℃, 590℃, or 600℃, and the time is 3.5-4.5 hours, for example, 3.5 hours, 4 hours, or 4.5 hours; the second sintering temperature is 650-750℃, for example, 650℃, 700℃, or 750℃, and the time is 0.5-1.5 hours, for example, 0.5 hours, 1 hour, or 1.5 hours; the heating rates of the pre-sintering, first sintering, and second sintering increase sequentially.

[0083] Preferably, the pre-sintering atmosphere is an inert atmosphere, for example, nitrogen or argon.

[0084] Preferably, the atmosphere for the first sintering is a mixture of an inert gas and a reducing gas, wherein the inert gas may be, for example, nitrogen or argon, and the reducing gas may be, for example, hydrogen.

[0085] Preferably, the atmosphere for the second sintering is an inert atmosphere, for example, such as nitrogen or argon.

[0086] Preferably, the crystallization reaction is followed by an annealing treatment. The annealing temperature is 450-550℃, for example, 450℃, 500℃ or 550℃, and the time is 1-2 hours, for example, 1 hour, 1.5 hours or 2 hours.

[0087] Preferably, the protective layer is a copper-based protective layer, and the deposition method of the copper-based protective layer includes: firstly, depositing a copper-based protective layer precursor using pulsed laser deposition, and then performing in-situ reduction.

[0088] Preferably, the parameters of the pulsed laser deposition method include: the target material is a copper-based alloy target or a copper metal target; the laser energy is 250-300 mJ / pulse, for example, 250 mJ / pulse, 260 mJ / pulse, 270 mJ / pulse, 280 mJ / pulse, 290 mJ / pulse or 300 mJ / pulse; the pulse frequency is 8-12 Hz, for example, 8 Hz, 9 Hz, 10 Hz, 11 Hz or 12 Hz; and the substrate temperature is 150-250℃, for example, 150℃, 200℃ or 250℃.

[0089] Preferably, the in-situ reduction atmosphere is a mixture of inert gas and reducing gas, wherein the inert gas may be, for example, nitrogen or argon, and the reducing gas may be, for example, hydrogen.

[0090] Preferably, the in-situ reduction temperature is 300-400℃, for example, 300℃, 350℃ or 400℃, and the time is 25-35min, for example, 25min, 30min or 35min.

[0091] Preferably, the preparation method includes the following steps:

[0092] (1) Preparation of the kernel precursor, including the following steps:

[0093] (a) A manganese source and an iron source are premixed to obtain a manganese-iron premixed solution; wherein the manganese source is a manganese solution with a concentration of 1-1.5 mol / L and the iron source is a ferrous solution with a concentration of 0.3-0.5 mol / L; and the molar ratio of manganese ions to ferrous ions in the manganese-iron premixed solution is (6-8):(2-4).

[0094] (b) The manganese-iron premix, phosphorus source, and lithium source are respectively introduced into an aqueous solvent at a constant flow rate at 60-70℃ (e.g., 60℃, 65℃, or 70℃) for co-precipitation reaction. After the reaction, the mixture is washed to obtain the core precursor. An antioxidant is added to the aqueous solvent. The temperature of the co-precipitation reaction is 68-75℃, the reaction time is 2-4 hours, and the reaction atmosphere is an inert atmosphere (e.g., nitrogen or argon). The phosphorus source is a phosphorus-containing solution with a concentration of 1.5-2.5 mol / L and a pH of 3.5-4.5. The lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L.

[0095] (2) Prepare an aluminum-doped inner shell solution and an aluminum-doped outer shell solution respectively; wherein, the inner shell solution includes a first aluminum source, a first phosphorus source for the outer shell, a first surfactant, a pH adjuster, and an antioxidant, wherein the concentration of the first aluminum source is 0.15-0.2 mol / L (e.g., 0.15 mol / L, 0.16 mol / L, 0.17 mol / L, 0.18 mol / L, 0.19 mol / L, or 0.2 mol / L, etc.), the concentration of the first phosphorus source for the outer shell is 1.5-2 mol / L, the concentration of the first surfactant is 0.03-0.1 wt%, the concentration of the pH adjuster is 0.02-0.04 wt% (e.g., 0.02 wt%, 0.03 wt%, or 0.04 wt%, etc.), and the concentration of the antioxidant is 0.008-0.012 wt% (e.g., 0.008 wt%, 0.009 wt%, 0.01 wt%). The outer shell solution comprises a second aluminum source, a second phosphorus source for the outer shell, a second surfactant, a pH adjuster, and a silica dispersion. The concentration of the second aluminum source is 0.5-1 mol / L (e.g., 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, or 1 mol / L, etc.), the concentration of the second phosphorus source for the outer shell is 1.5-2 mol / L, the concentration of the second surfactant is 0.05-0.1 wt%, the concentration of the pH adjuster is 0.08-0.12 wt% (e.g., 0.08 wt%, 0.09 wt%, 0.1 wt%, 0.11 wt%, or 0.12 wt%, etc.), and the concentration of the silica dispersion is 0.04-0.06 wt% (e.g., 0.04 wt%, 0.05 wt%, or 0.06 wt%, etc.).

[0096] The aluminum-doped inner shell solution, the first manganese source, the first iron source, the first lithium source, and the compensating phosphorus source are fed concurrently into the reaction system containing the core precursor. A first hydrothermal deposition reaction is performed under stirring conditions to obtain the inner shell precursor. The reaction system containing the core precursor includes the core precursor, water, and an antioxidant. The solid-liquid ratio of the core precursor to water is 1:(6-10) (e.g., 1:6, 1:7, 1:8, 1:9, or 1:10, etc.). The feed flow rate of the inner shell solution satisfies V1=8e 0.0105t1The unit is mL / min; where 0≤t1≤100min, t1 is the time of the first hydrothermal deposition reaction; the first manganese source, the first iron source, the first lithium source, and the compensating phosphorus source in the outer shell are all fed at a constant rate; the temperature of the first hydrothermal deposition reaction is 170-200℃, and the reaction atmosphere is an inert atmosphere (e.g., nitrogen or argon); the first manganese source in the outer shell is a manganese solution with a concentration of 0.3-0.5mol / L, the first iron source in the outer shell is a ferrous solution with a concentration of 0.1-0.3mol / L, the first lithium source in the outer shell is a lithium solution with a concentration of 2.5-3.5mol / L, and the compensating phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5mol / L.

[0097] The aluminum-doped outer shell solution, the second manganese source, the second iron source, the second lithium source, and the compensating phosphorus source are fed concurrently into the reaction system containing the inner shell precursor. A second hydrothermal deposition reaction is carried out under stirring conditions to obtain the outer shell precursor. The feed flow rate of the outer shell solution satisfies V² = 18e⁻¹. 0.018(t2-100) The unit is mL / min; where 100min ≤ t2 ≤ 160min, and t2 is the total time of the first and second hydrothermal deposition reactions; the feed flow rate of the manganese source satisfies V Mn =(6-0.036(t2-100)), in mL / min; the feed flow rate of the iron source satisfies V Fe =(3-0.018(t2-100)), in mL / min; both the second lithium source and the compensating phosphorus source in the outer shell are fed at a constant rate; the temperature of the second hydrothermal deposition reaction is 180-210℃, and the reaction atmosphere is an inert atmosphere (e.g., nitrogen or argon); the second manganese source in the outer shell is a manganese solution with a concentration of 0.3-0.5 mol / L, the second iron source in the outer shell is a ferrous solution with a concentration of 0.1-0.3 mol / L, the second lithium source in the outer shell is a lithium solution with a concentration of 2.5-3.5 mol / L, and the compensating phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5 mol / L.

[0098] (3) In an inert atmosphere, the semi-finished product obtained after step (2) is pre-sintered at 300-400℃ at a heating rate of 4-6℃ / min (e.g., 4℃ / min, 5℃ / min or 6℃ / min, etc.); then, in a mixed atmosphere of inert gas and reducing gas, the temperature is raised to 550-600℃ at a heating rate of 4-6℃ / min (e.g., 4℃ / min, 5℃ / min or 6℃ / min, etc.) for 3.5-4 hours. The first sintering is carried out for 5 hours; then, in an inert atmosphere, the temperature is increased to 650-750℃ for 0.5-1.5 hours at a heating rate of 15-25℃ / min (e.g., 15℃ / min, 20℃ / min, or 25℃ / min, etc.), followed by annealing at 450-550℃ for 1-2 hours at a cooling rate of 8-12℃ / min (e.g., 8℃ / min, 10℃ / min, or 12℃ / min, etc.) to obtain the shell precursor crystal.

[0099] (4) The semi-finished product after step (3) is acid-soaked, washed and dried, and then a copper-based protective layer precursor is deposited by pulsed laser deposition. Then, in-situ reduction is performed to obtain a copper-based protective layer, thus completing the preparation of modified lithium manganese iron phosphate cathode material. The parameters of the pulsed laser deposition method include: the target material is a copper-based alloy target or a copper metal target, the laser energy is 250-300mJ / pulse, the pulse frequency is 8-12Hz, and the substrate temperature is 150-250℃. The atmosphere of the in-situ reduction is a mixed atmosphere of inert gas and reducing gas, the temperature of the in-situ reduction is 300-400℃, and the time is 25-35min.

[0100] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode sheet, the positive electrode sheet comprising the modified lithium manganese iron phosphate positive electrode material as described in the first aspect, or comprising the modified lithium manganese iron phosphate positive electrode material prepared by the preparation method described in the second aspect.

[0101] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0102] Compared with the prior art, the present invention has the following beneficial effects:

[0103] This invention designs a unique core-shell structure, achieving comprehensive performance optimization of lithium manganese iron phosphate cathode materials. In the outer shell, the aluminum doping concentration exhibits a segmented, exponentially increasing distribution from the inside to the outside in the radial direction. This design allows the outer shell region near the core to fill manganese vacancies and stabilize the crystal structure through low-concentration aluminum doping, while the outer shell region away from the core uses high-concentration aluminum doping to enhance electronic conductivity and suppress manganese ion dissolution, thereby synergistically improving the material's cycle life and mechanical stress adaptability. Simultaneously, the outermost protective layer effectively reduces interfacial impedance and enhances the material's structural stability. In summary, through the unique core-shell structure design, the segmented, exponentially increasing aluminum doping concentration distribution in the outer shell, and the synergistic effect of the protective layer, this modified lithium manganese iron phosphate cathode material achieves comprehensive improvements in bulk stability, interfacial stability, and conductivity, resulting in excellent overall performance. Attached Figure Description

[0104] Figure 1 This is a schematic diagram of the structure of the modified lithium manganese iron phosphate cathode material prepared in Example 1 of this invention.

[0105] Among them, 1-core; 2-inner shell layer; 3-outer shell layer; 4-copper-based protective layer. Detailed Implementation

[0106] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0107] It should be understood that in the description of this invention, the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0108] It should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "set," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0109] Example 1

[0110] This embodiment provides a modified lithium iron phosphate cathode material, the structural schematic of which is shown below. Figure 1 As shown, the modified lithium manganese iron phosphate cathode material includes a core 1, a shell covering the surface of the core, and a copper-based protective layer 4.

[0111] The core 1 is a pure-phase lithium manganese iron phosphate material with the chemical formula LiMn. 0.7 Fe 0.3 PO4; The average radius of core 1 is half that of the average radius (1500nm) of the modified lithium manganese iron phosphate cathode material particles.

[0112] The outer shell is made of aluminum-doped lithium manganese iron phosphate material, and the ratio of the shell thickness to the average radius of the modified lithium manganese iron phosphate cathode material particles is 0.497:1. The outer shell comprises an inner layer 2 and an outer layer 3 in the radial direction, with a thickness ratio of 3:2 between the inner layer 2 and the outer layer 3. The aluminum concentration distributions in the inner layer 2 and the outer layer 3 follow different exponential growth functions. Specifically, the aluminum concentration distribution in the inner layer 2 follows the following exponential growth function: C Al =A1×e k1(r2-r1) A1 is 0.5 at%, and k1 is 2.5 nm. -1 r2 is the average radius of the target location in the inner layer 2 of the outer shell, and r1 is the average radius of the core 1; the concentration distribution of aluminum in the outer layer 3 of the outer shell follows the following exponential growth function: C Al =C t +K(1-e -k2(r4-r3) ), C t C represents the aluminum doping concentration at the boundary between the inner layer 2 and the outer layer 3 of the outer shell. t =3.2at%, K=4at%, k2=5nm -1 r4 is the average radius of the target position in the outer shell 3, and r3 is the maximum radius of the inner shell 2; k1 and k2 satisfy: k2 / k1=2.

[0113] The thickness of the copper-based protective layer 4 is 3 nm; a Li layer is formed between the copper-based protective layer 4 and the outer shell 3. 0.8 The CuO2 interface transition layer has a thickness of 1.5 nm.

[0114] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0115] (1) Preparation of the kernel precursor, including the following steps:

[0116] (a) A manganese source and an iron source are premixed to obtain a manganese-iron premixed solution; wherein the manganese source is a 1.2 mol / L manganese sulfate solution and the iron source is a 0.4 mol / L ferrous oxalate solution, and the solution is pre-reduced by heating at 300°C for 2 h in a mixed atmosphere; the molar ratio of manganese ions to ferrous ions in the manganese-iron premixed solution is 7:3.

[0117] (b) Inject deionized water into the reactor, heat it to 65°C, and pass nitrogen gas at a flow rate of 3L / min to remove oxygen for 30 minutes, and add 0.1wt% ascorbic acid as an antioxidant.

[0118] A manganese-iron premix solution at a flow rate of 15 mL / min, an ammonium dihydrogen phosphate solution at a flow rate of 30 mL / min, and a lithium hydroxide solution at a flow rate of 10 mL / min were fed concurrently into the reactor. After 120 min, the feed was stopped, and the reactor was allowed to stand at 70 °C for 3 h. After the reaction, the reactor was washed with deionized water at 60 °C to obtain the core precursor. An antioxidant was added to the aqueous solvent. The reaction atmosphere for the coprecipitation reaction was a nitrogen atmosphere. The concentration of the ammonium dihydrogen phosphate solution was 2 mol / L, and its pH was adjusted to 4 with sodium citrate. The concentration of the lithium hydroxide solution was 3 mol / L, and carbonates were filtered out.

[0119] (2) Prepare an aluminum-doped inner shell solution and an aluminum-doped outer shell solution respectively; wherein, the inner shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, PVP, sodium citrate and ascorbic acid, the concentration of aluminum sulfate is 0.18 mol / L, the concentration of ammonium dihydrogen phosphate is 1.8 mol / L, the concentration of PVP is 0.06 wt%, the concentration of sodium citrate is 0.03 wt%, and the concentration of ascorbic acid is 0.01 wt%; the outer shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, CTAB, sodium citrate and nano silica dispersion, the concentration of aluminum sulfate is 0.7 mol / L, the concentration of ammonium dihydrogen phosphate is 1.8 mol / L, the concentration of CTAB is 0.08 wt%, the concentration of sodium citrate is 0.1 wt%, and the concentration of nano silica dispersion is 0.05 wt%.

[0120] The core precursor was dispersed in another reactor, deionized water was added, the solid-liquid ratio was controlled at 1:8, and 0.1 wt% ascorbic acid was added. N2 was introduced at a flow rate of 3 L / min for deoxygenation for 30 min. Finally, the temperature was raised to 185°C and stirring was started.

[0121] The aluminum-doped inner shell solution, a 0.4 mol / L manganese sulfate solution, a 0.2 mol / L ferrous oxalate solution, a 3 mol / L lithium hydroxide solution, and a 1 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the core precursor. A first hydrothermal deposition reaction is carried out under stirring conditions to obtain the inner shell precursor. The feed flow rate of the inner shell solution satisfies V1 = 8e-1. 0.0105t1 The unit is mL / min; where 0≤t1≤100min, t1 is the time of the first hydrothermal deposition reaction; the manganese sulfate solution, ferrous oxalate solution, lithium hydroxide solution and compensating phosphoric acid are all fed at a constant rate, with feed flow rates of 6mL / min, 3mL / min, 9mL / min and 12mL / min, respectively; the temperature of the first hydrothermal deposition reaction is 185℃, and the reaction atmosphere is a nitrogen atmosphere.

[0122] The aluminum-doped outer shell solution, a 0.4 mol / L manganese sulfate solution, a 0.2 mol / L ferrous oxalate solution, a 3 mol / L lithium hydroxide solution, and a 1 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the inner shell precursor. A second hydrothermal deposition reaction is carried out under stirring conditions to obtain the outer shell precursor. The feed flow rate of the outer shell solution satisfies V² = 18e⁻¹. 0.018(t2-100) The unit is mL / min; where 100min ≤ t2 ≤ 160min, and t2 is the total time of the first and second hydrothermal deposition reactions; the feed flow rate of the manganese sulfate solution satisfies V Mn =(6-0.036(t2-100)), in mL / min; the feed flow rate of the ferrous oxalate solution satisfies V Fe =(3-0.018(t2-100)), in mL / min; lithium hydroxide solution and compensating phosphoric acid are fed at constant rates of 9 mL / min and 12 mL / min, respectively; the temperature of the second hydrothermal deposition reaction is 195 °C, and the reaction atmosphere is nitrogen atmosphere.

[0123] (3) In a nitrogen atmosphere, the semi-finished product obtained after step (2) is pre-sintered at 350°C at a heating rate of 5°C / min for 2.5h; then in a mixed atmosphere of nitrogen and hydrogen, the temperature is raised to 580°C at a heating rate of 5°C / min for 4h of first sintering; then in an argon atmosphere, the temperature is raised to 700°C at a heating rate of 20°C / min for 1h of second sintering, and then cooled to 500°C at a cooling rate of 10°C / min for 1.5h of annealing to obtain the shell precursor crystal.

[0124] (4) Soak the semi-finished product after step (3) in 0.5wt% hydrofluoric acid solution for 10s, wash with deionized water until neutral, and dry at 120℃ for 4h until the moisture content is ≤100ppm.

[0125] A copper-based protective layer precursor was deposited using pulsed laser deposition, followed by in-situ reduction to obtain the copper-based protective layer, thus completing the preparation of the modified lithium manganese iron phosphate cathode material. The parameters of the pulsed laser deposition method included: the target material was Cu. 97 The laser target is Zr3 alloy, the laser energy is 280 mJ / pulse, the pulse frequency is 10 Hz, the substrate temperature is 200℃, and the base vacuum is 5 × 10⁻⁶. -4 Pa, deposition time is 8 min; the in-situ reduction atmosphere is a mixture of argon and hydrogen with a volume ratio of 95:5, the in-situ reduction temperature is 350℃, and the time is 30 min.

[0126] Example 2

[0127] This embodiment provides a modified lithium manganese iron phosphate cathode material, which includes a core, a shell covering the surface of the core, and a copper-based protective layer.

[0128] The core is a pure-phase lithium manganese iron phosphate material with the chemical formula LiMn. 0.7 Fe 0.3 PO4; the average radius of the core is half that of the average radius (1500 nm) of the modified lithium manganese iron phosphate cathode material particles.

[0129] The outer shell is made of aluminum-doped lithium manganese iron phosphate material, and the ratio of the shell thickness to the average radius of the modified lithium manganese iron phosphate cathode material particles is approximately 0.498:1. The shell comprises an inner shell layer and an outer shell layer in the radial direction, with a thickness ratio of 3:2 between the inner and outer shell layers. The aluminum concentration distributions in the inner and outer shell layers follow different exponential growth functions. Specifically, the aluminum concentration distribution in the inner shell layer follows the following exponential growth function: C Al =A1×e k1(r2-r1)A1 is 0.3 at%, and k1 is 1.5 nm. -1 r2 is the average radius of the target location in the inner layer of the outer shell, and r1 is the average radius of the core; the concentration distribution of aluminum in the outer layer of the outer shell follows the following exponential growth function: C Al =C t +K(1-e -k2(r4-r3) ), C t C represents the aluminum doping concentration at the boundary between the inner and outer layers of the outer shell. t =2.8at%, K=3at%, k2=4nm -1 r4 is the average radius of the target position in the outer shell, and r3 is the maximum radius of the inner shell; k1 and k2 satisfy: k2 / k1=2.67.

[0130] The thickness of the copper-based protective layer is 2 nm; a Li-type protective layer is formed between the copper-based protective layer and the outer shell. 0.7 The CuO2 interface transition layer has a thickness of 0.5 nm.

[0131] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0132] (1) Preparation of the kernel precursor, including the following steps:

[0133] (a) A manganese source and an iron source are premixed to obtain a manganese-iron premixed solution; wherein the manganese source is a 1 mol / L manganese sulfate solution and the iron source is a 0.3 mol / L ferrous oxalate solution, and the solution is pre-reduced by heating at 300°C for 2 h under a mixed atmosphere; the molar ratio of manganese ions to ferrous ions in the manganese-iron premixed solution is 7:3.

[0134] (b) Inject deionized water into the reactor, heat it to 65°C, and pass nitrogen gas at a flow rate of 3L / min to remove oxygen for 30 minutes, and add 0.1wt% ascorbic acid as an antioxidant.

[0135] A manganese-iron premix solution at a flow rate of 15 mL / min, an ammonium dihydrogen phosphate solution at a flow rate of 30 mL / min, and a lithium hydroxide solution at a flow rate of 10 mL / min were fed concurrently into the reactor. After 120 min, the feed was stopped, and the mixture was allowed to stand at 68 °C for 4 h. After the reaction, the mixture was washed with deionized water at 60 °C to obtain the core precursor. An antioxidant was added to the aqueous solvent. The reaction atmosphere for the coprecipitation reaction was a nitrogen atmosphere. The concentration of the ammonium dihydrogen phosphate solution was 1.5 mol / L, and its pH was adjusted to 4 using sodium citrate. The concentration of the lithium hydroxide solution was 2.5 mol / L, and carbonates were filtered out.

[0136] (2) Prepare an aluminum-doped inner shell solution and an aluminum-doped outer shell solution respectively; wherein, the inner shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, PVP, sodium citrate and ascorbic acid, the concentration of aluminum sulfate is 0.15 mol / L, the concentration of ammonium dihydrogen phosphate is 1.5 mol / L, the concentration of PVP is 0.03 wt%, the concentration of sodium citrate is 0.02 wt%, and the concentration of ascorbic acid is 0.008 wt%; the outer shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, CTAB, sodium citrate and nano silica dispersion, the concentration of aluminum sulfate is 0.5 mol / L, the concentration of ammonium dihydrogen phosphate is 1.5 mol / L, the concentration of CTAB is 0.05 wt%, the concentration of sodium citrate is 0.08 wt%, and the concentration of nano silica dispersion is 0.04 wt%.

[0137] The core precursor was dispersed in another reactor, deionized water was added, the solid-liquid ratio was controlled at 1:8, and 0.1 wt% ascorbic acid was added. N2 was introduced at a flow rate of 3 L / min for deoxygenation for 30 min. Finally, the temperature was raised to 170°C and stirring was started.

[0138] The aluminum-doped inner shell solution, a 0.3 mol / L manganese sulfate solution, a 0.1 mol / L ferrous oxalate solution, a 2.5 mol / L lithium hydroxide solution, and a 0.5 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the core precursor. A first hydrothermal deposition reaction is carried out under stirring conditions to obtain the inner shell precursor. The feed flow rate of the inner shell solution satisfies V1 = 8e-1. 0.0105t1 The unit is mL / min; where 0≤t1≤100min, t1 is the time of the first hydrothermal deposition reaction; the manganese sulfate solution, ferrous oxalate solution, lithium hydroxide solution and compensating phosphoric acid are all fed at a constant rate, with feed flow rates of 6mL / min, 3mL / min, 9mL / min and 12mL / min, respectively; the temperature of the first hydrothermal deposition reaction is 170℃, and the reaction atmosphere is a nitrogen atmosphere.

[0139] The aluminum-doped outer shell solution, a 0.3 mol / L manganese sulfate solution, a 0.1 mol / L ferrous oxalate solution, a 2.5 mol / L lithium hydroxide solution, and a 0.5 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the inner shell precursor. A second hydrothermal deposition reaction is carried out under stirring conditions to obtain the outer shell precursor. The feed flow rate of the outer shell solution satisfies V² = 18e⁻¹. 0.018(t2-100)The unit is mL / min; where 100min ≤ t2 ≤ 160min, and t2 is the total time of the first and second hydrothermal deposition reactions; the feed flow rate of the manganese sulfate solution satisfies V Mn =(6-0.036(t2-100)), in mL / min; the feed flow rate of the ferrous oxalate solution satisfies V Fe =(3-0.018(t2-100)), in mL / min; lithium hydroxide solution and compensating phosphoric acid are fed at constant rates of 9 mL / min and 12 mL / min, respectively; the temperature of the second hydrothermal deposition reaction is 180℃, and the reaction atmosphere is nitrogen atmosphere.

[0140] (3) In a nitrogen atmosphere, the semi-finished product obtained after step (2) is pre-sintered at 300°C at a heating rate of 4°C / min for 3 hours; then in a mixed atmosphere of nitrogen and hydrogen, the temperature is raised to 550°C at a heating rate of 4°C / min for 3.5 hours of first sintering; then in an argon atmosphere, the temperature is raised to 650°C at a heating rate of 15°C / min for 1.5 hours of second sintering, and then cooled to 550°C at a cooling rate of 8°C / min for 1 hour of annealing to obtain the shell precursor crystal.

[0141] (4) Soak the semi-finished product after step (3) in 0.5wt% hydrofluoric acid solution for 10s, wash with deionized water until neutral, and dry at 120℃ for 4h until the moisture content is ≤100ppm.

[0142] A copper-based protective layer precursor was deposited using pulsed laser deposition, followed by in-situ reduction to obtain the copper-based protective layer, thus completing the preparation of the modified lithium manganese iron phosphate cathode material. The parameters of the pulsed laser deposition method included: the target material was Cu. 97 The laser target is Zr3 alloy, the laser energy is 250 mJ / pulse, the pulse frequency is 8 Hz, the substrate temperature is 150℃, and the base vacuum is 5 × 10⁻⁶. -4 Pa, deposition time is 6 min; the in-situ reduction atmosphere is a mixture of argon and hydrogen with a volume ratio of 95:5, the in-situ reduction temperature is 300℃, and the time is 35 min.

[0143] Example 3

[0144] This embodiment provides a modified lithium manganese iron phosphate cathode material, which includes a core, a shell covering the surface of the core, and a copper-based protective layer.

[0145] The core is a pure-phase lithium manganese iron phosphate material with the chemical formula LiMn. 0.7 Fe 0.3PO4; the average radius of the core is half that of the average radius (1500 nm) of the modified lithium manganese iron phosphate cathode material particles.

[0146] The outer shell is made of aluminum-doped lithium manganese iron phosphate material, and the ratio of the shell thickness to the average radius of the modified lithium manganese iron phosphate cathode material particles is 0.495:1. The shell comprises an inner layer and an outer layer in the radial direction, with a thickness ratio of 3:2 between the inner and outer layers. The aluminum concentration distributions in the inner and outer layers follow different exponential growth functions. Specifically, the aluminum concentration distribution in the inner layer follows the following exponential growth function: C Al =A1×e k1(r2-r1) A1 is 0.7 at%, and k1 is 3.5 nm. -1 r2 is the average radius of the target location in the inner layer of the outer shell, and r1 is the average radius of the core; the concentration distribution of aluminum in the outer layer of the outer shell follows the following exponential growth function: C Al =C t +K(1-e -k2(r4-r3) ), C t C represents the aluminum doping concentration at the boundary between the inner and outer layers of the outer shell. t =3.6at%, K=5at%, k2=5.25nm -1 r4 is the average radius of the target position in the outer shell, and r3 is the maximum radius of the inner shell; k1 and k2 satisfy: k2 / k1=1.5.

[0147] The thickness of the copper-based protective layer is 5 nm; a Li-based protective layer is formed between the copper-based protective layer and the outer shell. 0.9 The CuO2 interface transition layer has a thickness of 3 nm.

[0148] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0149] (1) Preparation of the kernel precursor, including the following steps:

[0150] (a) A manganese source and an iron source are premixed to obtain a manganese-iron premixed solution; wherein the manganese source is a 1.5 mol / L manganese sulfate solution and the iron source is a 0.5 mol / L ferrous oxalate solution, and the solution is pre-reduced by heating at 300°C for 2 h in a mixed atmosphere; the molar ratio of manganese ions to ferrous ions in the manganese-iron premixed solution is 7:3.

[0151] (b) Inject deionized water into the reactor, heat it to 65°C, and pass nitrogen gas at a flow rate of 3L / min to remove oxygen for 30 minutes, and add 0.1wt% ascorbic acid as an antioxidant.

[0152] A manganese-iron premix solution at a flow rate of 15 mL / min, an ammonium dihydrogen phosphate solution at a flow rate of 30 mL / min, and a lithium hydroxide solution at a flow rate of 10 mL / min were fed concurrently into the reactor. After 120 min, the feed was stopped, and the mixture was allowed to stand at 75 °C for 2 h. After the reaction, the mixture was washed with deionized water at 60 °C to obtain the core precursor. An antioxidant was added to the aqueous solvent. The reaction atmosphere for the coprecipitation reaction was a nitrogen atmosphere. The concentration of the ammonium dihydrogen phosphate solution was 2.5 mol / L, and its pH was adjusted to 4 using sodium citrate. The concentration of the lithium hydroxide solution was 3.5 mol / L, and carbonates were filtered out.

[0153] (2) Prepare an aluminum-doped inner shell solution and an aluminum-doped outer shell solution respectively; wherein, the inner shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, PVP, sodium citrate and ascorbic acid, the concentration of aluminum sulfate is 0.2 mol / L, the concentration of ammonium dihydrogen phosphate is 2 mol / L, the concentration of PVP is 0.1 wt%, the concentration of sodium citrate is 0.04 wt%, and the concentration of ascorbic acid is 0.012 wt%; the outer shell solution includes aluminum sulfate, ammonium dihydrogen phosphate, CTAB, sodium citrate and nano silica dispersion, the concentration of aluminum sulfate is 1 mol / L, the concentration of ammonium dihydrogen phosphate is 2 mol / L, the concentration of CTAB is 0.1 wt%, the concentration of sodium citrate is 0.12 wt%, and the concentration of nano silica dispersion is 0.06 wt%.

[0154] The core precursor was dispersed in another reactor, deionized water was added, the solid-liquid ratio was controlled at 1:8, and 0.1 wt% ascorbic acid was added. N2 was introduced at a flow rate of 3 L / min for deoxygenation for 30 min. Finally, the temperature was raised to 200°C and stirring was started.

[0155] The aluminum-doped inner shell solution, a 0.5 mol / L manganese sulfate solution, a 0.3 mol / L ferrous oxalate solution, a 3.5 mol / L lithium hydroxide solution, and a 1.5 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the core precursor. A first hydrothermal deposition reaction is carried out under stirring conditions to obtain the inner shell precursor. The feed flow rate of the inner shell solution satisfies V1 = 8e-1. 0.0105t1 The unit is mL / min; where 0≤t1≤100min, t1 is the time of the first hydrothermal deposition reaction; the manganese sulfate solution, ferrous oxalate solution, lithium hydroxide solution and compensating phosphoric acid are all fed at a constant rate, with feed flow rates of 6mL / min, 3mL / min, 9mL / min and 12mL / min, respectively; the temperature of the first hydrothermal deposition reaction is 200℃, and the reaction atmosphere is a nitrogen atmosphere.

[0156] The aluminum-doped outer shell solution, a 0.5 mol / L manganese sulfate solution, a 0.3 mol / L ferrous oxalate solution, a 3.5 mol / L lithium hydroxide solution, and a 1.5 mol / L compensating phosphoric acid solution are fed concurrently into the reaction system containing the inner shell precursor. A second hydrothermal deposition reaction is carried out under stirring conditions to obtain the outer shell precursor. The feed flow rate of the outer shell solution satisfies V² = 18e⁻¹. 0.018(t2-100) The unit is mL / min; where 100min ≤ t2 ≤ 160min, and t2 is the total time of the first and second hydrothermal deposition reactions; the feed flow rate of the manganese sulfate solution satisfies V Mn =(6-0.036(t2-100)), in mL / min; the feed flow rate of the ferrous oxalate solution satisfies V Fe =(3-0.018(t2-100)), in mL / min; lithium hydroxide solution and compensating phosphoric acid are fed at constant rates of 9 mL / min and 12 mL / min, respectively; the temperature of the second hydrothermal deposition reaction is 210 °C, and the reaction atmosphere is nitrogen atmosphere.

[0157] (3) In a nitrogen atmosphere, the semi-finished product obtained after step (2) is pre-sintered at 400°C at a heating rate of 6°C / min for 2 hours; then in a mixed atmosphere of nitrogen and hydrogen, the temperature is raised to 600°C at a heating rate of 6°C / min for 4.5 hours of first sintering; then in an argon atmosphere, the temperature is raised to 750°C at a heating rate of 25°C / min for 0.5 hours of second sintering, and then cooled to 450°C at a cooling rate of 12°C / min for 2 hours of annealing to obtain the shell precursor crystal.

[0158] (4) Soak the semi-finished product after step (3) in 0.5wt% hydrofluoric acid solution for 10s, wash with deionized water until neutral, and dry at 120℃ for 4h until the moisture content is ≤100ppm.

[0159] A copper-based protective layer precursor was deposited using pulsed laser deposition, followed by in-situ reduction to obtain the copper-based protective layer, thus completing the preparation of the modified lithium manganese iron phosphate cathode material. The parameters of the pulsed laser deposition method included: the target material was Cu. 97 Zr3 alloy target, laser energy 300 mJ / pulse, pulse frequency 12 Hz, substrate temperature 250℃, base vacuum level 5 × 10⁻⁶. -4 Pa, deposition time is 14 min; the in-situ reduction atmosphere is a mixture of argon and hydrogen with a volume ratio of 95:5, the in-situ reduction temperature is 400℃, and the time is 25 min.

[0160] Example 4

[0161] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the inner layer of the outer shell follows an exponential growth function: C Al =A1×e k1(r2-r1) In this context, k1 represents 1 nm. -1 .

[0162] The remaining preparation methods and parameters are consistent with those in Example 1.

[0163] Example 5

[0164] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the inner layer of the outer shell follows an exponential growth function: C Al =A1×e k1(r2-r1) In this context, k1 is 4nm. -1 .

[0165] The remaining preparation methods and parameters are consistent with those in Example 1.

[0166] Example 6

[0167] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the outer shell layer follows an exponential growth function: C Al =C t +K(1-e -k2(r4-r3) In ), C t =2.5at%.

[0168] The remaining preparation methods and parameters are consistent with those in Example 1.

[0169] Example 7

[0170] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the outer shell layer follows an exponential growth function: C Al =C t +K(1-e -k2(r4-r3) In ), C t =4at.

[0171] The remaining preparation methods and parameters are consistent with those in Example 1.

[0172] Example 8

[0173] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the outer shell layer follows an exponential growth function: C Al =C t +K(1-e -k2(r4-r3) In this case, K = 1 at%.

[0174] The remaining preparation methods and parameters are consistent with those in Example 1.

[0175] Example 9

[0176] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the outer shell layer follows an exponential growth function: C Al =C t +K(1-e -k2(r4-r3) In this case, K = 6at%.

[0177] The remaining preparation methods and parameters are consistent with those in Example 1.

[0178] Example 10

[0179] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the outer shell layer follows an exponential growth function: C Al =C t +K(1-e -k2(r4-r3) In the given information, k2 = 3nm -1 .

[0180] The remaining preparation methods and parameters are consistent with those in Example 1.

[0181] Example 11

[0182] The difference between this embodiment and Embodiment 1 is that the concentration distribution of aluminum in the inner layer of the outer shell follows an exponential growth function, and the concentration distribution of aluminum in the outer layer of the outer shell follows an exponential growth function in which k2 / k1=1.

[0183] The remaining preparation methods and parameters are consistent with those in Example 1.

[0184] Comparative Example 1

[0185] The difference between this comparative example and Example 1 is that the concentration distribution of aluminum in the inner layer of the outer shell follows the same exponential growth function as the concentration distribution of aluminum in the outer layer of the outer shell, which is C. Al =A1×e k1(r2-r1) .

[0186] The remaining preparation methods and parameters are consistent with those in Example 1.

[0187] Comparative Example 2

[0188] The difference between this comparative example and Example 1 is that the concentration distribution of aluminum in the inner layer of the outer shell and the concentration distribution of aluminum in the outer layer of the outer shell both follow linear functions. Specifically, the concentration distribution of aluminum in the inner and outer layers of the outer shell follows the same linear function: C Al (r)=C0+m×(rRc ), where C0 is the initial concentration at the core / shell interface, set to 0.5 at%; m is a constant slope, set to 0.0043 at% / nm; R c Let r be the core radius and r be the radial coordinate. Based on this calculation, at r = 1200 nm (the inflection point between the inner and outer layers in the original Example 1), the concentration is 0.5 + 0.0043 × (1200 - 750) ≈ 2.44 at; at the particle surface (r = 1500 nm), the concentration is 0.5 + 0.0043 × (1500 - 750) ≈ 3.73 at.

[0189] The remaining preparation methods and parameters are consistent with those in Example 1.

[0190] Performance testing

[0191] The CR2032 coin cell was prepared based on the modified lithium manganese iron phosphate cathode material provided in the above embodiments and comparative examples. The specific steps included: 1) The modified lithium manganese iron phosphate cathode material, conductive agent (acetylene black), and binder (polyvinylidene fluoride, PVDF) were thoroughly mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 8:1:1 to form a slurry; the slurry was uniformly coated on an aluminum foil current collector, and after vacuum drying at 120°C for 12 hours, it was cut into cathode sheets. 2) In a glove box filled with argon (H2O<0.1ppm, O2<0.1ppm), a lithium metal sheet was used as the negative electrode, a 1mol / L LiPF6 EC / DMC (volume ratio 1:1) solution was used as the electrolyte, and a Celgard 2400 polypropylene membrane was used as the separator to assemble a CR2032 coin cell.

[0192] The electrochemical performance of the CR2032 coin cell was tested under a constant temperature environment of 25°C, including:

[0193] i) Initial charge and discharge efficiency test: Within the voltage range of 2.8-4.3V, the first charge and discharge is performed at a low rate of 0.1C (1C=170mAh / g), and the initial discharge specific capacity and initial charge and discharge efficiency (initial discharge capacity / initial charge capacity × 100%) are recorded; the low-rate first charge can more accurately reflect the intrinsic capacity of the material.

[0194] ii) Rate performance test: Within the voltage range of 2.8-4.3V, charge and discharge cycles were performed at rates of 0.2C, 0.5C, 1C, 2C, and 5C, with 5 cycles at each rate. The average discharge specific capacity at each rate was recorded. This test is used to evaluate the high rate performance of the material.

[0195] iii) Long-cycle performance test: Constant current charge and discharge cycles were performed at a rate of 1C within a voltage range of 2.8–4.3V. The discharge capacity at the 500th cycle was recorded to calculate the capacity retention relative to the initial discharge capacity.

[0196] The results are shown in Table 1.

[0197] Table 1

[0198]

[0199] analyze:

[0200] This invention designs a unique core-shell structure, achieving comprehensive performance optimization of lithium manganese iron phosphate cathode materials. In the outer shell, the aluminum doping concentration exhibits a segmented, exponentially increasing distribution from the inside to the outside in the radial direction. This design allows the outer shell region near the core to fill manganese vacancies and stabilize the crystal structure through low-concentration aluminum doping, while the outer shell region away from the core uses high-concentration aluminum doping to enhance electronic conductivity and suppress manganese ion dissolution, thereby synergistically improving the material's cycle life and mechanical stress adaptability. Simultaneously, the outermost protective layer effectively reduces interfacial impedance and enhances the material's structural stability. In summary, through the unique core-shell structure design, the segmented, exponentially increasing aluminum doping concentration distribution in the outer shell, and the synergistic effect of the protective layer, this modified lithium manganese iron phosphate cathode material achieves comprehensive improvements in bulk stability, interfacial stability, and conductivity, resulting in excellent overall performance.

[0201] As can be seen from the comparison between Example 1 and Examples 4-5, if the concentration distribution of aluminum in the inner shell follows an exponential growth function with k1 being too small, the stabilizing effect on the core lattice will be insufficient, resulting in a significant decrease in the material's initial discharge capacity and cycle stability. If the concentration distribution of aluminum in the inner shell follows an exponential growth function with k1 being too large, the concentration in the inner shell will rise too quickly, which will prematurely hinder the diffusion of lithium ions inside the shell, seriously damaging the material's high-rate performance and long-term cycle life.

[0202] A comparison of Examples 1 and 6-7 shows that if the concentration distribution of aluminum in the outer shell follows an exponential growth function, C t If the concentration is too small, the transition from "structural stability as the primary factor" to "interface strengthening as the primary factor" occurs too early, resulting in insufficient improvement in the overall electronic conductivity of the outer shell, which fails to effectively suppress manganese dissolution and deteriorates cycle stability. Furthermore, if the concentration distribution of aluminum in the outer shell follows an exponential growth function, C... t If the inner shell is too thick, the high-resistance region will expand, which is also detrimental to lithium-ion conduction, resulting in a significant reduction in rate performance and capacity retention.

[0203] As can be seen from the comparison between Example 1 and Examples 8-9, if K is too small in the exponential growth function followed by the concentration distribution of aluminum in the outer shell, the aluminum doping concentration on the particle surface is insufficient, the surface modification effect is weak, and the improvement of electronic conductivity and the suppression of interfacial side reactions are limited, resulting in poor capacity and cycle performance. If K is too large in the exponential growth function followed by the concentration distribution of aluminum in the outer shell, the aluminum concentration on the particle surface is too high, which will seriously block the channels for lithium ions to enter and exit the particles, resulting in a sharp drop in the first charge and discharge efficiency and rate performance.

[0204] A comparison of Examples 1 and 10-11 shows that if k2 is too small in the exponential growth function of the aluminum concentration distribution in the outer shell, the concentration growth of the outer shell layer is weak, and an effective, highly conductive aluminum-doped reinforcement layer cannot be formed near the surface. The rate performance and surface stability of the material are limited. If k2 / k1 = 1 in both the exponential growth function of the aluminum concentration distribution in the inner shell and the exponential growth function of the aluminum concentration distribution in the outer shell, it means that the gradient design of "gentle inside and steep outside" is lost. The structural stability and interface dynamics of the material cannot be optimized in a coordinated manner, and its overall performance, especially long-term cycling stability, will decline significantly.

[0205] As can be seen from the comparison between Example 1 and Comparative Example 1, if the concentration distribution of aluminum in the inner layer of the outer shell follows the same exponential growth function as the concentration distribution of aluminum in the outer layer of the outer shell, then the material only has a single concentration change rate and cannot take into account both the lattice stability near the core and the interface strengthening of the particle surface. Its overall performance, especially the cycle life, is significantly inferior to the segmented design of the present invention.

[0206] As can be seen from the comparison between Example 1 and Comparative Example 2, if the concentration distribution of aluminum in the inner layer of the shell follows a linear function and the concentration distribution of aluminum in the outer layer of the shell follows a linear function, the concentration change rate is constant, and it is impossible to make targeted control in key areas (such as the core interface and particle surface). This results in a comprehensive deficiency in the structural stability, electronic conductivity and manganese dissolution inhibition ability of the material, and all electrochemical performances are far lower than those of the present invention.

[0207] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A modified lithium manganese iron phosphate cathode material, characterized in that, The modified manganese iron phosphate positive electrode material comprises an inner core, an outer shell and a protective layer coated on the surface of the inner core. The inner core comprises a manganese iron phosphate material, and the doping concentration of aluminum elements in the outer shell is in a segmented exponential increasing distribution along the radial direction from inside to outside.

2. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, The lithium manganese iron phosphate material is a pure phase, and the chemical general formula is LiMn (1-x) Fe x PO4, x = 0.25-0.35; The ratio of the average radius of the inner core to the average radius of the modified manganese iron phosphate positive electrode material particle is (0.5-0.55):1, preferably the average radius of the inner core is half of the average radius of the modified manganese iron phosphate positive electrode material particle.

3. The modified lithium iron manganese phosphate cathode material according to claim 1 or 2, characterized in that The material of the outer shell is an aluminum-doped manganese iron phosphate material. The outer shell comprises an inner layer and an outer layer along the radial direction, and the concentration distribution of aluminum elements in the inner layer and the outer layer respectively follows different exponential growth functions. The thickness ratio of the inner layer to the outer layer in the radial direction is (2.5-3.5):(1.5-2.5). The ratio of the thickness of the outer shell to the average radius of the modified manganese iron phosphate positive electrode material particle is (0.48-0.499):

1.

4. The modified lithium iron manganese phosphate cathode material of claim 3, wherein, The concentration distribution of the aluminum element in the inner layer of the shell follows the following exponential growth function: C Al =A1×e k1(r2-r1) ; wherein A1 is 0.3-0.7 at%; k1 is 1.5-3.5 nm -1 ; r2 is the average radius of the target position in the inner layer of the shell, and r1 is the average radius of the inner core. And / or, the concentration distribution of aluminum element in the outer layer of the shell follows the following exponential growth function: C Al =C t +K(1-e -k2(r4-r3) ); wherein, C t is the doping concentration of aluminum at the boundary between the inner layer and the outer layer of the shell, in at%; K is the limit value of concentration increment, K>0; k2≥4nm -1 ; r4 is the average radius of the target position in the outer layer of the shell, and r3 is the maximum radius of the inner layer of the shell.

5. The modified lithium iron manganese phosphate cathode material of claim 4, wherein, The C t satisfies: 2.8 at% ≤ C t ≤ 3.6 at%; The k1 and k2 satisfy: k2 / k1≥1.

5. The K satisfies: 3at%≤K≤5at%.

6. The modified lithium iron manganese phosphate cathode material according to any one of claims 1 to 3, characterized in that The protective layer is a copper-based protective layer. The thickness of the protective layer is 2-5nm. An interface transition layer is formed between the protective layer and the outer layer, and the thickness of the interface transition layer is 0.5-3nm.

7. A method for preparing the modified lithium manganese iron phosphate cathode material according to any one of claims 1-4, characterized in that, The preparation method comprises the following steps: Mixing a manganese source, an iron source, a phosphorus source, a lithium source and a solvent to perform a co-precipitation reaction to obtain an inner core precursor; An aluminum-doped outer shell solution is prepared, and then the outer shell solution, a manganese source, an iron source, a lithium source and a phosphorus source are added into the reaction system where the inner core precursor is located in a concurrent manner to perform a hydrothermal reaction to obtain an outer shell precursor; A crystallization reaction is performed on the outer shell precursor to obtain an outer shell precursor crystal, and the concentration of aluminum elements in the outer shell precursor crystal is in a segmented exponential increasing distribution along the radial direction from inside to outside. A protective layer is coated on the surface of the outer shell precursor crystal to obtain the modified manganese iron phosphate positive electrode material.

8. The preparation method according to claim 7, characterized in that, The mixing method comprises: (a) Pre-mixing a manganese source and an iron source to obtain a manganese-iron pre-mixed material; (b) Passing the manganese-iron pre-mixed material, a phosphorus source and a lithium source into a solvent in a concurrent manner; And / or, an antioxidant is added during the mixing process; And / or, the temperature of the co-precipitation reaction is 68-75℃, and the reaction time is 2-4h; And / or, the aluminum-doped outer shell solution comprises an aluminum source, a phosphorus source and a surfactant; And / or, the aluminum-doped outer shell solution comprises an aluminum-doped inner layer solution and an aluminum-doped outer layer solution, and the aluminum doping concentration in the inner layer solution is less than that in the outer layer solution.

9. The production method according to claim 8, characterized by, In the inner layer solution, the surfactant comprises PVP, and the concentration of the surfactant is 0.03-0.1wt%; And / or, in the outer layer solution, the surfactant comprises CTAB, and the concentration of the surfactant is 0.05-0.1wt%; And / or, the specific steps of concurrent addition comprise: (i) introducing the shell inner layer solution, manganese source, iron source, lithium source and phosphorus source into the reaction system where the inner core precursor is located in a concurrent flow, and performing a first hydrothermal deposition reaction to obtain a shell inner layer precursor; (ii) introducing the shell outer layer solution, manganese source, iron source, lithium source and phosphorus source into the reaction system where the shell inner layer precursor is located in a concurrent flow, and performing a second hydrothermal deposition reaction to obtain a shell outer layer precursor.

10. The method of claim 9, wherein, In step (i), the feed flow rate of the shell inner layer solution satisfies V1=8e 0.0105t1 , units: mL / min; wherein 0≤t1≤100 min, t1 is the time of the first hydrothermal deposition reaction; In step (i), the manganese source, iron source, lithium source and phosphorus source are all constant speed feedings; And / or, the temperature of the first hydrothermal deposition reaction is 170-200℃; And / or, in step (ii), the feed flow rate of the shell outer layer solution satisfies V2 = 18e 0.018(t2-100) , unit: mL / min; wherein 100 min≤t2≤160 min, t2 is the total time of the first hydrothermal deposition reaction and the second hydrothermal deposition reaction; In step (ii), the feed flow rate of the manganese source satisfies V Mn = (6 - 0.036(t2-100)), in mL / min; In step (ii), the feed flow rate of the iron source satisfies V Fe = (3 - 0.018(t2-100)), in mL / min; In step (ii), the lithium source and phosphorus source are both constant speed feedings; And / or, the temperature of the second hydrothermal deposition reaction is 180-210℃; And / or, the crystallization reaction includes pre-sintering, first sintering and second sintering performed in sequence with increasing temperature; Wherein, the temperature of the pre-sintering is 300-400℃, and the time is 2-3h; the temperature of the first sintering is 550-600℃, and the time is 3.5-4.5h; the temperature of the second sintering is 650-750℃, and the time is 0.5-1.5h; the temperature increasing rates of the pre-sintering, first sintering and second sintering are increasing in sequence; And / or, the crystallization reaction is further subjected to annealing treatment, and the temperature of the annealing treatment is 450-550℃, and the time is 1-2h; And / or, the protective layer is a copper-based protective layer, and the deposition method of the copper-based protective layer includes: first depositing a copper-based protective layer precursor by using a pulsed laser deposition method, and then performing in-situ reduction. Wherein, the parameters of the pulsed laser deposition method include: the target material is a copper-based alloy target material or a copper metal target material, the laser energy is 250-300mJ / pulse, the pulse frequency is 8-12Hz, and the substrate temperature is 150-250℃.

11. The method of any one of claims 7-9, wherein, The preparation method includes the following steps: (1) preparing an inner core precursor, the steps including: (a) pre-mixing a manganese source and an iron source to obtain a manganese-iron pre-mixed solution; wherein the manganese source is a manganese solution with a concentration of 1-1.5mol / L, and the iron source is a ferrous solution with a concentration of 0.3-0.5mol / L; in the manganese-iron pre-mixed solution, the molar ratio of manganese ions to ferrous ions is (6-8):(2-4); (b) introducing the manganese-iron pre-mixed solution, a phosphorus source and a lithium source into a water solvent at 60-70℃ in a concurrent flow at a constant flow rate to perform a co-precipitation reaction, and after the reaction is completed, washing is performed to obtain an inner core precursor; wherein an antioxidant is added to the water solvent; the temperature of the co-precipitation reaction is 68-75℃, the reaction time is 2-4h, and the reaction atmosphere is an inert atmosphere; the phosphorus source is a phosphorus-containing solution with a concentration of 1.5-2.5mol / L and a pH of 3.5-4.5; the lithium source is a lithium solution with a concentration of 2.5-3.5mol / L; (2) respectively configure an aluminum-doped inner shell solution and an aluminum-doped outer shell solution; wherein the inner shell solution comprises a first aluminum source, a first shell phosphorus source, a first surfactant, a pH adjuster and an antioxidant, the concentration of the first aluminum source is 0.15-0.2 mol / L, the concentration of the first shell phosphorus source is 1.5-2 mol / L, the concentration of the first surfactant is 0.03-0.1 wt%, the concentration of the pH adjuster is 0.02-0.04 wt%, and the concentration of the antioxidant is 0.008-0.012 wt%; the outer shell solution comprises a second aluminum source, a second shell phosphorus source, a second surfactant, a pH adjuster and a silica dispersion, the concentration of the second aluminum source is 0.5-1 mol / L, the concentration of the second shell phosphorus source is 1.5-2 mol / L, the concentration of the second surfactant is 0.05-0.1 wt%, the concentration of the pH adjuster is 0.08-0.12 wt%, and the concentration of the silica dispersion is 0.04-0.06 wt%; The aluminum-doped shell inner layer solution, the shell first manganese source, the shell first iron source, the shell first lithium source and the compensatory phosphorus source are fed into the reaction system in which the core precursor is located in parallel, and a first hydrothermal deposition reaction is carried out under stirring to obtain a shell inner layer precursor; wherein the reaction system in which the core precursor is located includes the core precursor, water and an antioxidant, and the solid-liquid ratio of the core precursor and water is 1:(6-10); the feeding flow rate of the shell inner layer solution satisfies V1=8e 0.0105t1 , unit: mL / min; wherein 0≤t1≤100 min, t1 is the time of the first hydrothermal deposition reaction; the shell first manganese source, the shell first iron source, the shell first lithium source and the compensatory phosphorus source are all constant-speed feeding; the temperature of the first hydrothermal deposition reaction is 170-200 ℃, and the reaction atmosphere is inert atmosphere; the shell first manganese source is a manganese solution with a concentration of 0.3-0.5 mol / L, the shell first iron source is a ferrous solution with a concentration of 0.1-0.3 mol / L, the shell first lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L, and the compensatory phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5 mol / L; The aluminum-doped shell outer layer solution, the shell second manganese source, the shell second iron source, the shell second lithium source and the compensatory phosphorus source are fed into a reaction system in which the shell inner layer precursor is located, and a second hydrothermal deposition reaction is carried out under stirring to obtain a shell outer layer precursor; wherein the feeding flow rate of the shell outer layer solution satisfies V2 = 18e 0.018(t2-100) , unit: mL / min; wherein 100 min≤t2≤160 min, t2 is the total time of the first hydrothermal deposition reaction and the second hydrothermal deposition reaction; the feeding flow rate of the manganese source satisfies V Mn =(6-0.036(t2-100)), unit: mL / min; the feeding flow rate of the iron source satisfies V Fe =(3-0.018(t2-100)), unit: mL / min; the shell second lithium source and the compensatory phosphorus source are both constant-speed feeding; the temperature of the second hydrothermal deposition reaction is 180-210 ℃, and the reaction atmosphere is inert atmosphere; the shell second manganese source is a manganese solution with a concentration of 0.3-0.5 mol / L, the shell second iron source is a ferrous solution with a concentration of 0.1-0.3 mol / L, the shell second lithium source is a lithium solution with a concentration of 2.5-3.5 mol / L, and the compensatory phosphorus source is a phosphorus-containing solution with a concentration of 0.5-1.5 mol / L; (3) pre-sinter the semi-finished product obtained after step (2) at a temperature rising rate of 4-6 ℃ / min to 300-400 ℃ for 2-3 h in an inert atmosphere; then first sinter at a temperature rising rate of 4-6 ℃ / min to 550-600 ℃ for 3.5-4.5 h in a mixed atmosphere of inert gas and reducing gas; then second sinter at a temperature rising rate of 15-25 ℃ / min to 650-750 ℃ for 0.5-1.5 h in an inert atmosphere, and then anneal at a temperature falling rate of 8-12 ℃ / min to 450-550 ℃ for 1-2 h, to obtain a shell precursor crystal; (4) perform acid immersion, washing and drying on the semi-finished product after step (3), then deposit a copper-based protective layer precursor by a pulsed laser deposition method, and then perform in-situ reduction to obtain a copper-based protective layer, thereby completing the preparation of the modified lithium manganese iron phosphate positive electrode material; wherein the acid immersion step comprises soaking in a hydrofluoric acid solution with a concentration of 0.4-0.6 wt% for 5-15 s; the parameters of the pulsed laser deposition method comprise: the target material is a copper-based alloy target or a copper metal target, the laser energy is 250-300 mJ / pulse, the pulse frequency is 8-12 Hz, and the substrate temperature is 150-250 ℃; the in-situ reduction atmosphere is a mixed atmosphere of inert gas and reducing gas, and the in-situ reduction temperature is 300-400 ℃ and the time is 25-35 min.

12. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode sheet, and the positive electrode sheet comprises the modified lithium manganese iron phosphate positive electrode material according to any one of claims 1-4 or the modified lithium manganese iron phosphate positive electrode material prepared by the preparation method according to any one of claims 6-9.

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