Modified lithium manganese iron phosphate material, preparation method and application thereof

Abstract

The application specifically discloses a modified manganese iron lithium phosphate material and a preparation method and application thereof. The modified manganese iron lithium phosphate material comprises doped manganese iron lithium phosphate and a coating layer covering the doped manganese iron lithium phosphate; the chemical formula of the doped manganese iron lithium phosphate is LiM x Mn y Fe （1‑y） PO4, wherein M comprises at least one element of Gd and Ce, x is the doping amount of M, x = 0.008-0.012, the value of y satisfies 0.5≤y≤0.7, and the coating layer comprises carbon nanocapsules with a core-shell structure. The modified manganese iron lithium phosphate material has fine particles and excellent dispersibility, and has excellent electronic conductivity, lithium ion diffusion rate, structural stability and low ion dissolution amount in electrolyte; while realizing high specific capacity of the material, the rate performance and cycle performance of the material can also be improved.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a modified lithium manganese iron phosphate material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries have received increasing attention in recent years due to their advantages such as high voltage, high energy density, long lifespan, and environmental friendliness. Among them, the cathode material, which is one of the key materials restricting the improvement of its performance, is the cathode material. Currently, widely studied cathode materials include lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate.

[0003] Lithium manganese iron phosphate (LiMn) a Fe 1-a PO4 (LMFP) is used as a cathode material for lithium-ion batteries due to its high theoretical specific capacity (approximately 170 mAh / g) and relatively high operating voltage platform (approximately 4.1 V vs. Li). + Lithium iron phosphate (LiFePO4) has great potential in energy storage and power battery fields, with a significantly improved energy density compared to traditional lithium iron phosphate. However, its intrinsic conductivity is low and the lithium-ion diffusion rate is slow, resulting in poor rate performance. During fast charging or high-current discharge, the battery capacity rapidly decays. At the same time, during multiple charge-discharge cycles, changes in the internal structure of the material, such as manganese dissolution and lattice distortion, lead to poor cycle stability, which limits its large-scale commercial application.

[0004] In summary, existing lithium manganese iron phosphate (LFP) materials cannot effectively solve the problems of ion dissolution, poor rate performance, and unsatisfactory cycle stability. Therefore, improving the overall performance of LFP materials and promoting their widespread application in the lithium-ion battery field is of great significance. Summary of the Invention

[0005] To address the technical problems in the prior art, this application provides a modified lithium manganese iron phosphate material, its preparation method, and its application.

[0006] In a first aspect, this application provides a modified lithium manganese iron phosphate material, which adopts the following technical solution: A modified lithium manganese iron phosphate material includes doped lithium manganese iron phosphate and a coating layer covering the doped lithium manganese iron phosphate; the chemical formula of the doped lithium manganese iron phosphate is LiM. x Mn y Fe （1-y） PO4, wherein M includes at least one element selected from Gd and Ce, x is the doping amount of M, x = 0.008-0.012; the value of y satisfies 0.5 ≤ y ≤ 0.7; the coating layer includes carbon nanocapsules with a core-shell structure.

[0007] The modified lithium manganese iron phosphate material of this application is doped with Gd and / or Ce elements. First, the doping of Gd and / or Ce elements into the lithium manganese iron phosphate material can optimize the crystal structure of lithium manganese iron phosphate through the lattice expansion effect; Ce 3+ The introduction of Mn can activate the quasi-bonding effect, stabilize oxygen vacancies in the lattice, and thus directly suppress the influence of Mn. 3+ The Jahn-Teller effect leads to lattice distortion, while Gd 3+ By optimizing the electron cloud distribution and precisely controlling the crystal field environment, the structural integrity of the material during charging and discharging is further enhanced; this helps reduce ion dissolution in the electrolyte of modified lithium manganese iron phosphate materials. Secondly, Gd and / or Ce doping increases the lattice constant of nano-ferrite (Mg-Ni-Co), optimizes the Li-O bond length through lattice expansion, and increases the interplanar spacing of lithium manganese iron phosphate, thereby expanding the lithium-ion transport and diffusion channels, reducing lithium-ion transport resistance, and improving the rate performance of the material, alleviating the problem of slow intrinsic ion diffusion rate in lithium manganese iron phosphate materials. Thirdly, due to Gd… 3+ Ce 3+ The ionic radii of all are significantly larger than those of Mn. 2+ Fe 2+ The ionic radius of Gd during doping. 3+ Ce 3+ Unable to occupy Li sites, it is instead doped in Mn 2+ / Fe 2+ Site (transition metal site), that is, Gd 3+ Ce 3+ It is easier to suppress the dissolution of manganese ions during charge and discharge by forming stable chemical bonds or occupying specific lattice sites, thereby improving the structural stability and cycle performance of the material. Fourth, both Gd and Ce have high melting points and good oxidation resistance. After being doped into lithium manganese iron phosphate materials, they can form stable lattice structures inside the material, reducing the dissolution of manganese and the degradation of the material structure under high temperature conditions, thereby improving the high-temperature cycle stability of the material. Moreover, Gd and Ce are both rare earth elements, and co-doping can play a synergistic role. On the one hand, it can superimpose the lattice regulation effect and more effectively alleviate the dissolution of Mn. 3+ Jahn-Teller distortion, amplifying Li + On the one hand, the diffusion channels can enhance the thermal stability and oxidation resistance of the material, reduce manganese dissolution and structural degradation at high temperatures, and co-doping can also reduce problems such as excessive lattice distortion caused by excessive doping of a single element, making the overall performance of modified lithium manganese iron phosphate materials, such as cycle stability and rate performance, better than that of single doping.

[0008] Meanwhile, the surface of the modified lithium manganese iron phosphate material of this application is a carbon nanocapsule coating layer with a core-shell structure. The hollow core of the carbon nanocapsule buffers the stress of volume change, and its graphitized carbon shell constructs a highly efficient three-dimensional conductive network. While greatly improving the electronic conduction efficiency of the modified lithium manganese iron phosphate material, it can also provide more channels for lithium-ion transport, thereby significantly improving cycle stability and rate performance.

[0009] Preferably, when M includes two elements, Gd and Ce, the doping ratio of Gd to Ce is (3-5):(1-3).

[0010] Preferably, the particle size of the doped lithium manganese iron phosphate is 20-200 nm; the particle size of the carbon nanocapsules is 50-100 nm.

[0011] Preferably, the coating layer accounts for 1.5%-2.5% of the mass of the modified lithium manganese iron phosphate material.

[0012] Secondly, this application provides a method for preparing modified lithium manganese iron phosphate material, using the following technical solution: A method for preparing a modified lithium manganese iron phosphate material, characterized by comprising the following steps: S1, Weigh lithium source, manganese source, iron source, and phosphorus source according to stoichiometric ratio; Mix the manganese source and the iron source with the polyol solution to obtain solution A; Mix the phosphorus source with the polyol solution to obtain solution B; S2, mix solution B with solution A, adjust the pH value to obtain a mixed solution; S3, the mixed solution is mixed with a complexing agent and a first solvent, stirred to obtain a sol, the first solvent is separated and dried to obtain a manganese iron phosphate precursor; S4, the manganese iron phosphate precursor and the second solvent containing the dopant are mixed and dispersed for the first time. Then, the carbon nanocapsules are added to the resulting system and dispersed for the second time. The second solvent is then separated to obtain carbon nanocapsules-doped manganese iron phosphate. S5, the carbon nanocapsules-doped manganese iron phosphate are mixed with the lithium source and then sintered to obtain the modified lithium manganese iron phosphate material.

[0013] Preferably, the stoichiometric ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is 1:(0.5-0.7):(0.3-0.5):1; the lithium source includes at least one of lithium carbonate, lithium bicarbonate, and lithium acetate; the manganese source includes at least one of manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetroxide; the iron source includes at least one of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, and ferrous sulfate; and the phosphorus source includes at least one of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid.

[0014] Preferably, the polyol solution in S1 comprises a polyol and deionized water in a volume ratio of (3-5):(1-2); the polyol comprises at least one of ethylene glycol, propylene glycol, and butanediol; the concentration of metal ions in solution A is 0.8 mol / L-1.2 mol / L; the concentration of phosphorus in solution B is 0.8 mol / L-1.2 mol / L; the volume ratio of solution B to solution A in S2 is (1-2):1; and the pH value in S2 is adjusted to 7.5-7.9 with ammonia water before obtaining the mixed solution.

[0015] Preferably, the stoichiometric ratio of the mixed solution to the complexing agent in S3 is (5-10):1; the volume ratio of the mixed solution to the first solvent is 1:1-1:3; the complexing agent includes at least one of citric acid, ethylenediaminetetraacetic acid, ammonium gluconate, and ethylenediamine; and the first solvent includes anhydrous ethanol.

[0016] Preferably, the stoichiometric ratio of the dopant added in S4 to the manganese iron phosphate precursor is (0.008-0.012):1; the mass ratio of the manganese iron phosphate precursor to the carbon nanocapsules is (9-11):1; the dopant includes at least one of Gd(NO3)3·6H2O, Ce(NO3)3·6H2O, (NH4)2Ce(NO3)6, and CeCl3·6H2O; and the second solvent includes anhydrous ethanol.

[0017] Preferably, the sintering process in S5 adopts a segmented heating method, specifically including the following steps: first, heating to 350-450℃ at a heating rate of 3-5℃ / min in a mixed atmosphere, and holding at that temperature for 2-3 hours; then heating to 650-750℃ at a heating rate of 5-7℃ / min, and holding at that temperature for 4-5 hours; finally heating to 800-900℃ at a heating rate of 2-3℃ / min, and holding at that temperature for 8-10 hours; wherein, the mixed atmosphere includes argon and hydrogen in a volume ratio of (7-9):(1-3).

[0018] Thirdly, this application provides a lithium-ion battery, which adopts the following technical solution: A lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode comprises the modified lithium manganese iron phosphate material as described above or the modified lithium manganese iron phosphate material prepared by the method described above. Detailed Implementation

[0019] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0021] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0022] Unless otherwise stated, the terms “a,” “an,” “an,” and “the” as used in this specification are intended to include “at least one” or “one or more.” For example, “a component” refers to one or more components, and therefore more than one component may be considered and may be employed or used in the implementation of the described embodiments.

[0023] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0024] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where other unmentioned elements exist in addition to the mentioned elements.

[0025] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0026] To address the problems in the prior art, this application adopts the following technical solution: The first aspect of this application provides a modified lithium manganese iron phosphate material, comprising doped lithium manganese iron phosphate and a coating layer covering the doped lithium manganese iron phosphate; the chemical formula of the doped lithium manganese iron phosphate is LiM xMn y Fe （1-y） PO4, wherein M includes at least one element selected from Gd and Ce, x is the doping amount of M, x = 0.008-0.012; the value of y satisfies 0.5 ≤ y ≤ 0.7; the coating layer includes carbon nanocapsules with a core-shell structure.

[0027] The modified lithium manganese iron phosphate material of this application, doped with Gd and / or Ce elements, can precisely control the crystal field environment by optimizing the electron cloud distribution or optimize the crystal structure of lithium manganese iron phosphate through the lattice expansion effect, thereby directly suppressing the influence of Mn. 3+ The lattice distortion caused by the Jahn-Teller effect helps reduce ion dissolution in the electrolyte of modified lithium manganese iron phosphate materials; furthermore, the above doping can increase the interplanar spacing of lithium manganese iron phosphate, thereby expanding the lithium-ion transport and diffusion channels, reducing lithium-ion transport resistance, and thus improving the rate performance of the material; due to Gd 3+ Ce 3+ The ionic radii of all are significantly larger than those of Mn. 2 + Fe 2+ The ionic radius allows for stable doping of Mn during the doping process. 2+ / Fe 2+ Transition metal sites (Gd sites) can further suppress the dissolution of manganese ions during charging and discharging by forming stable chemical bonds or occupying specific lattice sites, thereby improving the structural stability and cycle performance of the material. Furthermore, Gd and Ce are both rare earth elements, and co-doping can exert a synergistic effect, on the one hand, superimposing lattice modulation effects to more efficiently alleviate the dissolution of Mn ions. 3+ Jahn-Teller distortion, amplifying Li + On the one hand, the diffusion channels can enhance the thermal stability and oxidation resistance of the material, reduce manganese dissolution and structural degradation at high temperatures, and co-doping can also reduce problems such as excessive lattice distortion caused by excessive doping of a single element, making the overall performance of modified lithium manganese iron phosphate materials, such as cycle stability and rate performance, better than that of single doping.

[0028] Meanwhile, the surface of the modified lithium manganese iron phosphate material of this application is a carbon nanocapsule coating layer with a core-shell structure. The hollow core of the carbon nanocapsule buffers the stress of volume change, and its graphitized carbon shell constructs a highly efficient three-dimensional conductive network. While greatly improving the electronic conduction efficiency of the modified lithium manganese iron phosphate material, it can also provide more channels for lithium-ion transport, thereby significantly improving cycle stability and rate performance.

[0029] In one embodiment of this application, when M includes two elements, Gd and Ce, the doping ratio of Gd to Ce is (3-5):(1-3).

[0030] This application addresses the issue of excessive doping of Gd and Ce. While co-doping of Gd and Ce can produce synergistic effects, excessive doping can introduce excessive lattice stress, potentially leading to structural defects or even cracking. Controlling the total doping amount of Gd and Ce to a moderate ratio and fine-tuning their ratio helps to minimize potential negative effects while achieving performance improvements. Excessive Gd doping may limit the material's electronic conductivity, while excessive Ce doping may introduce uncertainties in its valence characteristics during long-term cycling. Therefore, the co-doping of these two elements within the aforementioned ratio range helps to achieve a balance between the stability and electrochemical activity of the modified lithium manganese iron phosphate material.

[0031] In one embodiment of this application, the particle size of the doped lithium manganese iron phosphate is 20-200 nm; the particle size of the carbon nanocapsules is 50-100 nm.

[0032] In the modified lithium manganese iron phosphate material prepared in this application, the doped lithium manganese iron phosphate particles are small and have excellent dispersion, which can shorten the lithium ion diffusion path from the source and improve the reaction activity, providing a good foundation for subsequent modification and performance improvement; while the particle size of the carbon nanocapsules is controlled within the above range, which is conducive to forming a uniform coating on the surface of the doped lithium manganese iron phosphate and enhancing the stability of the conductive network structure.

[0033] In one embodiment of this application, the coating layer accounts for 1.5%-2.5% of the mass of the modified lithium manganese iron phosphate material.

[0034] This application, by adjusting the mass ratio of carbon nanocapsules in modified lithium manganese iron phosphate materials, enables a tighter intermaterial composite between carbon nanocapsules and doped lithium manganese iron phosphate, which is beneficial for forming optimal volumetric packing density, improving compaction density and energy density, while optimizing conductive network and structural stability.

[0035] The second aspect of this application provides a method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: S1, Weigh lithium source, manganese source, iron source, and phosphorus source according to stoichiometric ratio; Mix the manganese source and the iron source with the polyol solution to obtain solution A; Mix the phosphorus source with the polyol solution to obtain solution B; S2, mix solution B with solution A, adjust the pH value to obtain a mixed solution; S3, the mixed solution is mixed with a complexing agent and a first solvent, stirred to obtain a sol, the first solvent is separated and dried to obtain a manganese iron phosphate precursor; S4, the manganese iron phosphate precursor and the second solvent containing the dopant are mixed and dispersed for the first time. Then, the carbon nanocapsules are added to the resulting system and dispersed for the second time. The second solvent is then separated to obtain carbon nanocapsules-doped manganese iron phosphate. S5, the carbon nanocapsules-doped manganese iron phosphate are mixed with the lithium source and then sintered to obtain the modified lithium manganese iron phosphate material.

[0036] This application employs a polyol-sol-gel method to prepare nanoscale manganese iron phosphate precursors. The polyol promotes uniform mixing of metal ions and phosphate ions, inhibiting precursor particle growth and aggregation. The sol-gel process further improves the uniformity and reactivity of each component, resulting in a manganese iron phosphate precursor with uniform structure and good crystallinity. This shortens the lithium ion diffusion path from the source, providing a high-quality foundation for subsequent doping, carbon nanocapsule composites, and material performance improvement. It also helps to obtain modified lithium manganese iron phosphate materials with good dispersion performance, small particle size, and high compaction density.

[0037] In one embodiment of this application, the stoichiometric ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is 1:(0.5-0.7):(0.3-0.5):1; the lithium source includes at least one of lithium carbonate, lithium bicarbonate, and lithium acetate; the manganese source includes at least one of manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetroxide; the iron source includes at least one of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, and ferrous sulfate; and the phosphorus source includes at least one of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid.

[0038] In one embodiment of this application, the polyol solution in S1 comprises a polyol and deionized water in a volume ratio of (3-5):(1-2); the polyol comprises at least one of ethylene glycol, propylene glycol, and butanediol; the concentration of metal ions in solution A is 0.8 mol / L-1.2 mol / L; the concentration of phosphorus in solution B is 0.8 mol / L-1.2 mol / L; the volume ratio of solution B to solution A in S2 is (1-2):1; and the pH value in S2 is adjusted to 7.5-7.9 with ammonia water before obtaining the mixed solution.

[0039] By adjusting the volume ratio of polyol to deionized water and the concentration of metal ions, it is possible to promote the uniform mixing of metal ions and phosphate ions and inhibit the growth and aggregation of precursor particles, which helps to improve the uniformity and reactivity of each component.

[0040] In one embodiment of this application, the stoichiometric ratio of the mixed solution to the complexing agent in S3 is (5-10):1; the volume ratio of the mixed solution to the first solvent is 1:1-1:3; the complexing agent includes at least one of citric acid, ethylenediaminetetraacetic acid, ammonium gluconate, and ethylenediamine; and the first solvent includes anhydrous ethanol.

[0041] Adjusting the stoichiometric ratio of the mixed solution to the complexing agent and the volume ratio of the mixed solution to the second solvent helps to form a homogeneous sol, improves the uniformity and crystallinity of the precursor, and lays the foundation for subsequent modification.

[0042] In one embodiment of this application, the stoichiometric ratio of the dopant added in S4 to the ferromanganese phosphate precursor is (0.008-0.012):1; the mass ratio of the ferromanganese phosphate precursor to the carbon nanocapsules is (9-11):1; the dopant includes at least one of Gd(NO3)3·6H2O, Ce(NO3)3·6H2O, (NH4)2Ce(NO3)6, and CeCl3·6H2O; the second solvent includes anhydrous ethanol.

[0043] By adjusting the stoichiometric ratio of manganese iron phosphate precursor to dopant, precise control of the material's crystal structure can be achieved. The heteroelement can most effectively optimize the electron cloud distribution and enhance the stability of the Mn / Fe-O bond, thereby significantly improving the intrinsic electronic conductivity and suppressing the dissolution of transition metals during cycling. When the amount of dopant is too small, the dopant element introduced by the dopant has limited ability to suppress manganese ion dissolution and lattice distortion. However, when the amount of dopant is too large, the excessive dopant element may destroy the lattice integrity and introduce defects, which may lead to a decrease in electronic conductivity and cycling stability. By adjusting the mass ratio of manganese iron phosphate precursor to carbon nanocapsules, the optimal mass proportion of the coating layer in the final modified lithium manganese iron phosphate material was ensured. At this ratio, the carbon nanocapsules can form a continuous, uniform, and efficient three-dimensional conductive network on the surface of the manganese iron phosphate precursor, greatly improving the electron conduction efficiency between particles. If the amount of carbon nanocapsules is too low, the carbon layer coating will be incomplete and the conductivity will not be improved sufficiently. If the amount of carbon nanocapsules is too high, the excessively thick carbon layer will hinder lithium ion migration and reduce the volumetric energy density of the material.

[0044] In one embodiment of this application, the sintering process described in S5 adopts a segmented heating method, specifically including the following steps: first, heating to 350-450°C at a heating rate of 3-5°C / min in a mixed atmosphere, and holding at that temperature for 2-3 hours; then heating to 650-750°C at a heating rate of 5-7°C / min, and holding at that temperature for 4-5 hours; finally, heating to 800-900°C at a heating rate of 2-3°C / min, and holding at that temperature for 8-10 hours; wherein, the mixed atmosphere includes argon and hydrogen in a volume ratio of (7-9):(1-3).

[0045] This application employs a segmented sintering and atmosphere control process, which enables precise control of the material crystallization process and structural evolution, avoiding the adverse effects of high-temperature sintering on the structure and performance of the modified lithium manganese iron phosphate material. Furthermore, the reduction effect of hydrogen can improve the purity and conductivity of the modified lithium manganese iron phosphate material, giving it both good crystallinity and excellent electrochemical performance, resulting in a comprehensive improvement in specific capacity, rate performance, and cycle stability.

[0046] A third aspect of this application provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode comprises the modified lithium manganese iron phosphate material as described above or the modified lithium manganese iron phosphate material prepared by the method described above.

[0047] The following describes the implementation methods of this application. The implementation methods described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the implementation methods, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents, materials, or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0048] Example 1 1. Preparation of modified lithium manganese iron phosphate materials S1. Weigh the raw materials according to the stoichiometric ratio of lithium source (lithium carbonate Li2CO3), manganese source (manganese sulfate MnSO4.H2O), iron source (ferrous sulfate FeSO4.7H2O), and phosphorus source (ammonium dihydrogen phosphate NH4H3PO4) of 1:0.6:0.4:1. Manganese and iron sources were dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 3:2) to obtain solution A with a total metal ion concentration of 0.8 mol / L; The phosphorus source was dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 3:2) to obtain solution B with a phosphorus concentration of 0.8 mol / L; S2, under continuous stirring and constant temperature of 80℃, solution B is added dropwise to solution A at a volume ratio of 1:1 (7 mL / min) according to solution B to solution A. At the same time, ammonia water is added dropwise to adjust the pH value to 7.5. The reaction is carried out for 5 hours to obtain a mixed solution. S3, add the complexing agent (citric acid) to the mixed solution at a stoichiometric ratio of 8:1, and add the first solvent (anhydrous ethanol) to the mixed solution at a volume ratio of 1:2. Continue stirring for 3 hours to form a sol. Slowly evaporate the first solvent (anhydrous ethanol) from the sol at 60°C to form a gel. Dry the gel under vacuum at 100°C for 24 hours, and then grind it into powder to obtain the manganese iron phosphate precursor. S4. The manganese iron phosphate precursor was dispersed in a second solvent (anhydrous ethanol) containing a dopant (gadolinium nitrate Gd(NO3)3·6H2O) with a solution concentration of 0.8 mol%. The dispersion was ultrasonically dispersed for 2 h. Then, carbon nanocapsules with a particle size of 50-100 nm were added to the resulting system at a mass ratio of manganese iron phosphate precursor to carbon nanocapsules of 9:1. The dispersion was continued ultrasonically for 3 h. The second solvent (anhydrous ethanol) was slowly evaporated under magnetic stirring to obtain carbon nanocapsules-doped manganese iron phosphate.

[0049] S5. Carbon nanocapsules-doped manganese iron phosphate were mixed with lithium carbonate and placed in a tube furnace. Under a mixed atmosphere (argon and hydrogen volume ratio of 9:1), the temperature was first increased to 350℃ at a rate of 3℃ / min and held for 2 hours. Then, the temperature was increased to 650℃ at a rate of 5℃ / min and held for 4 hours. Finally, the temperature was increased to 800℃ at a rate of 2℃ / min and held for 8 hours. After completing the above sintering process, the modified lithium manganese iron phosphate material (chemical formula LiGd) was obtained. 0.008 Mn 0.6 Fe 0.4 PO4 and the carbon coating layer account for 1.8% of the total mass.

[0050] 2. Lithium-ion battery manufacturing 2.1 Preparation of positive electrode sheet The modified lithium manganese iron phosphate material prepared in step 1 was used as the positive electrode material. It was dissolved in N-methylpyrrolidone with conductive carbon and polyvinylidene fluoride at a mass ratio of 8:1:1 and stirred to form a uniform and stable slurry. The slurry was then coated onto the surface of carbon-coated aluminum foil by a scraper method, with a coating thickness of about 100 μm. After drying, it was cut into round pieces with a diameter of 14 mm by a slicing machine to obtain the positive electrode sheet. It was weighed, sealed and placed in a desiccator for later use.

[0051] 2.2 Assemble the battery Using lithium metal sheets as negative electrodes, CR2032 coin cells were assembled with the above-prepared positive electrodes in a glove box filled with high-purity argon atmosphere. The electrolyte is 1 mol / L de1 LiPF6, and the solvent is a mixed solvent composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1. The diaphragm is a PP diaphragm.

[0052] Example 2 The difference between this embodiment and Example 1 is that a change occurred in S4 during the preparation of modified lithium manganese iron phosphate; specifically: S4. The manganese iron phosphate precursor was dispersed in a second solvent (anhydrous ethanol) containing a dopant (gadolinium nitrate Gd(NO3)3·6H2O) at a concentration of 1 mol%. The dispersion was ultrasonically dispersed for 2 h. Then, carbon nanocapsules with a particle size of 50-100 nm were added to the resulting system at a mass ratio of manganese iron phosphate precursor to carbon nanocapsules of 10:1. The dispersion was continued ultrasonically for 3 h. The second solvent (anhydrous ethanol) was slowly evaporated under magnetic stirring to obtain carbon nanocapsules-doped manganese iron phosphate.

[0053] All other steps and parameter settings are consistent with those in Example 1.

[0054] Example 3 The difference between this embodiment and Example 1 is that a change occurred in S4 during the preparation of modified lithium manganese iron phosphate; specifically: S4. The manganese iron phosphate precursor was dispersed in a second solvent (anhydrous ethanol) containing a dopant (gadolinium nitrate Gd(NO3)3·6H2O) at a concentration of 1.2 mol%. The dispersion was ultrasonically dispersed for 2 h. Then, carbon nanocapsules with a particle size of 50-100 nm were added to the resulting system at a mass ratio of manganese iron phosphate precursor to carbon nanocapsules of 11:1. The dispersion was continued ultrasonically for 3 h. The second solvent (anhydrous ethanol) was slowly evaporated under magnetic stirring to obtain carbon nanocapsules-doped manganese iron phosphate.

[0055] All other steps and parameter settings are consistent with those in Example 1.

[0056] Example 4 The difference between this embodiment and Embodiment 2 is that the doping element is Ce; all other steps and parameter settings are the same as in Embodiment 2.

[0057] Example 5 1. Preparation of modified lithium manganese iron phosphate materials S1. Weigh the raw materials according to the stoichiometric ratio of lithium source (lithium bicarbonate LiHCO3), manganese source (manganese chloride MnCl2·4H2O), iron source (ferrous chloride FeCl2·4H2O), and phosphorus source (ammonium phosphate (NH4)3PO4) of 1:0.7:0.3:1. Manganese and iron sources were dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 5:1) to obtain solution A with a total metal ion concentration of 1.2 mol / L. The phosphorus source was dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 5:1) to obtain solution B with a phosphorus concentration of 1.2 mol / L; S2, under continuous stirring and constant temperature of 80℃, solution B is added to solution A at a volume ratio of 2:1 (solution B to solution A) at a rate of 5 mL / min, while ammonia is added dropwise to adjust the pH value to 7.9. The reaction is carried out for 5 hours to obtain a mixed solution. S3, add the complexing agent (citric acid) to the mixed solution at a stoichiometric ratio of 5:1, and add the first solvent (anhydrous ethanol) to the mixed solution at a volume ratio of 1:3. Continue stirring for 3 hours to form a sol. Slowly evaporate the first solvent (anhydrous ethanol) from the sol at 60°C to form a gel. Dry the gel under vacuum at 100°C for 24 hours, and then grind it into powder to obtain the manganese iron phosphate precursor. S4. The manganese iron phosphate precursor was dispersed in a second solvent (anhydrous ethanol) containing a dopant (gadolinium nitrate Gd(NO3)3·6H2O) at a concentration of 1 mol%. The dispersion was ultrasonically dispersed for 2 h. Then, carbon nanocapsules with a particle size of 50-100 nm were added to the resulting system at a mass ratio of manganese iron phosphate precursor to carbon nanocapsules of 10:1. The dispersion was continued ultrasonically for 3 h. The second solvent (anhydrous ethanol) was slowly evaporated under magnetic stirring to obtain carbon nanocapsules-doped manganese iron phosphate.

[0058] S5. Carbon nanocapsules-doped manganese iron phosphate were mixed with lithium carbonate and placed in a tube furnace. Under a mixed atmosphere (argon and hydrogen volume ratio of 7:1), the temperature was first increased to 450℃ at a heating rate of 5℃ / min and held for 3 hours. Then, the temperature was increased to 750℃ at a heating rate of 7℃ / min and held for 5 hours. Finally, the temperature was increased to 900℃ at a heating rate of 3℃ / min and held for 10 hours. After completing the above sintering process, the modified lithium manganese iron phosphate material (chemical formula LiGd) was obtained. 0.01 Mn 0.7 Fe 0.3 PO4 and the carbon coating layer account for 1.7% of the total mass.

[0059] All other steps and parameter settings are consistent with those in Example 2.

[0060] Example 6 By changing the subscript of lithium manganese iron phosphate, the parameters can be adjusted, and the raw materials can be selected, with the value taken at or near the end value. 1. Preparation of modified lithium manganese iron phosphate materials S1. Weigh the raw materials according to the stoichiometric ratio of lithium source (lithium carbonate Li2CO3), manganese source (manganese chloride MnCl2·4H2O), iron source (ferrous chloride FeCl2·4H2O), and phosphorus source (ammonium phosphate (NH4)3PO4) of 1:0.5:0.5:1. Manganese and iron sources were dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 3:1) to obtain solution A with a total metal ion concentration of 1 mol / L. The phosphorus source was dissolved in a polyol solution (ethylene glycol to deionized water volume ratio of 3:1) to obtain solution B with a phosphorus concentration of 1 mol / L. S2, under continuous stirring and constant temperature of 80℃, solution B is added to solution A at a volume ratio of 1:1 (1:1) at a rate of 10 mL / min. At the same time, ammonia water is added dropwise to adjust the pH value to 7.6. The reaction is carried out for 5 hours to obtain a mixed solution. S3, add the complexing agent (citric acid) to the mixed solution at a stoichiometric ratio of 10:1, and add the first solvent (anhydrous ethanol) to the mixed solution at a volume ratio of 1:1. Continue stirring for 3 hours to form a sol. Slowly evaporate the first solvent (anhydrous ethanol) from the sol at 60°C to form a gel. Dry the gel under vacuum at 100°C for 24 hours, and then grind it into powder to obtain the manganese iron phosphate precursor. S4. The manganese iron phosphate precursor was dispersed in a second solvent (anhydrous ethanol) containing a dopant (gadolinium nitrate Gd(NO3)3·6H2O) at a concentration of 1.1 mol%. The dispersion was ultrasonically dispersed for 2 h. Then, carbon nanocapsules with a particle size of 50-100 nm were added to the resulting system at a mass ratio of manganese iron phosphate precursor to carbon nanocapsules of 11:1. The dispersion was continued ultrasonically for 3 h. The second solvent (anhydrous ethanol) was slowly evaporated under magnetic stirring to obtain carbon nanocapsules-doped manganese iron phosphate.

[0061] S5. Carbon nanocapsules-doped manganese iron phosphate were mixed with lithium carbonate and placed in a tube furnace. Under a mixed atmosphere (argon and hydrogen volume ratio of 9:2), the temperature was first increased to 400℃ at a heating rate of 4℃ / min and held for 2 hours. Then, the temperature was increased to 720℃ at a heating rate of 6℃ / min and held for 5 hours. Finally, the temperature was increased to 820℃ at a heating rate of 2℃ / min and held for 10 hours. After completing the above sintering process, the modified lithium manganese iron phosphate material (chemical formula LiGd) was obtained.0.011 Mn 0.5 Fe 0.5 PO4 and the carbon coating layer account for 1.6% of the total mass.

[0062] All other steps and parameter settings are consistent with those in Example 2.

[0063] Example 7 The difference between this embodiment and Embodiment 2 is that the modified lithium manganese iron phosphate material prepared is simultaneously doped with Gd and Ce, and the ratio of Gd to Ce doping amounts is 5:3; other steps and parameter settings are consistent with Embodiment 2.

[0064] Example 8 The difference between this embodiment and Embodiment 2 is that the modified lithium manganese iron phosphate material prepared is simultaneously doped with Gd and Ce, and the ratio of Gd to Ce doping amounts is 5:1; other steps and parameter settings are consistent with Embodiment 2.

[0065] Example 9 The difference between this embodiment and Embodiment 2 is that in the modified lithium manganese iron phosphate material, the particle size of the doped lithium manganese iron phosphate is 205-300 nm, and the particle size of the carbon nanocapsules is 20-30 nm. All other steps and parameter settings are consistent with those in Example 2.

[0066] Example 10 The difference between this embodiment and Embodiment 2 is that in the modified lithium manganese iron phosphate material, the particle size of the doped lithium manganese iron phosphate is 205-300 nm, and the particle size of the carbon nanocapsules is 105-150 nm. All other steps and parameter settings are consistent with those in Example 2.

[0067] Example 11 The difference between this embodiment and Embodiment 2 is that the coating layer accounts for 0.5% of the mass of the modified lithium manganese iron phosphate material; all other steps and parameter settings are consistent with Embodiment 2.

[0068] Example 12 The difference between this embodiment and Embodiment 2 is that the coating layer accounts for 5% of the mass of the modified lithium manganese iron phosphate material; all other steps and parameter settings are consistent with Embodiment 2.

[0069] Example 13 The difference between this embodiment and Embodiment 2 is that a different method is used to prepare the modified lithium manganese iron phosphate material; specifically, the modified lithium manganese iron phosphate material is prepared using the following hydrothermal-solid phase method: (1) Weigh the raw materials according to the stoichiometric ratio of lithium source (lithium carbonate Li2CO3), manganese source (manganese sulfate MnSO4.H2O), iron source (ferrous sulfate FeSO4.7H2O), and phosphorus source (ammonium dihydrogen phosphate NH4H3PO4) of 1:0.6:0.4:1; (2) Dissolve the manganese source and the iron source in a polyol solution (the volume ratio of ethylene glycol to deionized water is 3:2) to obtain a solution A with a total metal ion concentration of 0.8 mol / L; (3) Dissolve the phosphorus source in a polyol solution (the volume ratio of ethylene glycol to deionized water is 3:2) to obtain a solution B with a phosphorus concentration of 0.8 mol / L; (4) Under continuous stirring and constant temperature of 80℃, add solution B to solution A at a volume ratio of 1:1 of solution B to solution A at a rate of 5-10 mL / min, and simultaneously add ammonia water to adjust the pH value to 7.5. After reacting for 5 hours, a mixed solution is obtained. (5) Transfer the mixed solution into a hydrothermal reactor, keep it at 180℃ for 12h, cool it naturally to room temperature, centrifuge the product at 300rpm, wash it three times each with deionized water and anhydrous ethanol, dry it under vacuum at 100℃ for 24h, and grind it to obtain the manganese iron phosphate precursor.

[0070] (6) Mix the precursor with lithium carbonate according to the stoichiometric ratio, add citric acid, and wet ball mill with anhydrous ethanol for 6 hours (300 rpm); dry at 60°C after ball milling, and grind evenly to obtain mixed powder; pre-calcine the mixed powder at 350°C for 4 hours under nitrogen / argon protection; sinter at 750°C for 8 hours, and grind after natural cooling to obtain modified lithium manganese iron phosphate material.

[0071] All other steps and parameter settings are consistent with those in Example 2.

[0072] Example 14 The difference between this embodiment and Embodiment 2 lies in the temperature control during the sintering process. Specifically, the sintering process includes the following steps: Carbon nanocapsules-doped manganese iron phosphate were mixed with lithium carbonate and placed in a tube furnace. Under a mixed atmosphere (argon and hydrogen volume ratio of 9:1), the temperature was increased to 800℃ at a heating rate of 2℃ / min and held for 8 hours to complete the above sintering process, thus obtaining the modified lithium manganese iron phosphate material (chemical formula LiGd). 0.008 Mn 0.6 Fe 0.4 PO4); All other steps and parameter settings are consistent with those in Example 2.

[0073] Comparative Example 1 The difference between this comparative example and Example 2 is that an equal weight of glucose was used instead of carbon nanocapsules in the preparation of the modified lithium manganese iron phosphate material; all other steps and parameter settings were the same as in Example 2.

[0074] Comparative Example 2 The difference between this comparative example and Example 2 is that Gd doping is not performed during the preparation of the modified lithium manganese iron phosphate material; all other steps and parameter settings are consistent with Example 2.

[0075] Comparative Example 3 The difference between this comparative example and Example 2 is that the doping element in the modified lithium manganese iron phosphate material is Mg; all other steps and parameter settings are consistent with Example 2.

[0076] Comparative Example 4 The difference between this comparative example and Example 7 is that the modified lithium manganese iron phosphate material is doped with Mg and Ce, and the ratio of Mg to Ce doping amounts is 5:3; other steps and parameter settings are consistent with Example 7.

[0077] Comparative Example 5 The difference between this comparative example and Example 2 is that the lithium manganese iron phosphate material is prepared without Gd doping and does not include the coating layer formed by carbon nanocapsules; all other steps and parameter settings are consistent with Example 2.

[0078] Comparative Example 6 The difference between this comparative example and Example 2 is that the Gd doping amount in the prepared modified lithium manganese iron phosphate material is 1.4 mol%, and the mass ratio of the coating layer is 1.3%; other steps and parameter settings are consistent with Example 2.

[0079] Test methods I. ICP Testing After cycling the ML 2032 coin cell 200 times at 1C, the negative electrode was removed, and ICP testing was performed on the negative electrode using an iCAP 7200ICP-OES plasma spectrometer. The results are shown in Table 1.

[0080] The assembled CR2032 coin cell was subjected to constant current charge / discharge tests using the LAND CT2001A battery testing system, with a charge / discharge voltage window of 2.0–4.3V.

[0081] II. Discharge Capacity Test Test the lithium battery according to the following steps: (1) Preparation stage: Measure the battery mass m (g) and record the nominal capacity Cn (e.g., 50mAh); Calculate the current: 0.1C = 0.1 × Cn, 1C = 1 × Cn, 5C = 5 × Cn (mA); Charging: 0.1C constant current to 4.25V, then constant voltage until current ≤0.02C; Discharge cut-off voltage 2.0V; (2) 0.1C discharge: After full charge, let stand for 30 minutes, discharge at a constant current of 0.1C to 2.0V, record the discharge capacity Q1 (mAh), and let stand for 30 minutes; (3) 1C discharge: Recharge fully, let stand for 30 minutes, then discharge at a constant current of 1C to 2.0V, record the discharge capacity Q2 (mAh), and let stand for 30 minutes; (4) 5C discharge: Recharge fully, let stand for 30 minutes, discharge at a constant current of 5C to 2.0V, and record Q3. Let stand for 30 minutes; (5) Calculate the specific capacity: Specific capacity (mAh / g) = discharge capacity Q ÷ battery mass m, calculate the corresponding values ​​for 0.1C, 1C and 5C respectively; The results are shown in Table 2.

[0082] III. Cyclic Performance Testing (1) Test conditions: Environment: Constant temperature of 25±1℃; Cut-off voltage: 3.3V for charging, 2.0V for discharging; 1C current = nominal capacity C_n (mA, e.g., 50mAh, then 1C = 50mA) (2) Cyclic test steps (single cycle, repeated 500 times) Step 1: 1C constant current charging; charging at a 1C current, automatically switching to constant voltage when the voltage rises to 4.25V; Step 2: 4.25V constant voltage charging; maintain 4.25V, stop charging when the current drops to ≤0.02C; Step 3: After charging, let it stand; let it stand with the circuit open for 30 minutes and record the open-circuit voltage; Step 4: 1C constant current discharge; discharge at a current of 1C until the voltage drops to 2.0V, then stop discharging and record the discharge capacity Qn; Step 5: Let stand after discharging; let stand for 30 minutes with the circuit open.

[0083] Step 6: Loop counting; after completing one lap, it automatically starts the next lap, accumulating to 500 laps. (3) Data recording and calculation Record the following for each revolution: number of revolutions n, discharge capacity Qn (mAh), charge / discharge time, and highest temperature; Capacity retention rate (%) = (Discharge capacity Qn in the nth cycle ÷ Discharge capacity Q1 in the 1st cycle) × 100%; Battery capacity (mAh / g) = Discharge capacity Q ÷ Battery mass m; The results are shown in Table 2.

[0084] IV. Ratio Performance Test Test the lithium battery according to the following steps: (1) Pretreatment activation: 0.1C CC-CV is fully charged, 0.1C constant current is discharged to the cutoff voltage, cycle for 3 rounds, and the discharge capacity of the 3rd round is recorded as the initial capacity C0; (2) Rate performance test: Charge and discharge at 0.2C, 0.5C, 1C, 2C, 5C and 10C respectively, and record the discharge capacity at each rate; (3) 10C long cycle 200 times: Charge with 10C CC-CV and discharge with 10C CC, set to cycle 200 times, and continuously test; (4) Capacity check after cycling: After 200 cycles, charge and discharge once at 0.1C and record the discharge capacity C. 200 .

[0085] (5) Calculate the capacity retention rate: Capacity retention rate = C 200 / C0×100%; (6) Equipment settings: Test temperature 25℃, turn on overvoltage and overcurrent protection, and run according to the voltage cut-off step.

[0086] The results are shown in Table 2.

[0087] Table 1

[0088] Table 2

[0089] Based on Examples 1-4, Comparative Examples 1-2, 5-6, and Table 1-2, it can be seen that the modified lithium manganese iron phosphate material of this application, by doping with Gd or Ce elements and compositing them with carbon nanocapsules to form a carbon coating layer on its surface, can significantly alleviate the ion dissolution phenomenon during application and improve the discharge specific capacity and cycle capacity retention of lithium batteries. As shown in Table 1, the manganese ion dissolution amount (11-15 μg / g) and iron ion dissolution amount (40-55 μg / g) of Examples 1-4 are significantly lower than those of Comparative Examples 1-2 and 5-6 (Mn: 26-36 μg / g, Fe: 57-68 μg / g). This is because the lattice distortion caused by Gd or Ce doping can optimize the lithium-ion transport channels and improve the lithium-ion transport rate. Furthermore, Gd or Ce doping can occupy transition metal sites and suppress ion dissolution by forming stable chemical bonds or occupying specific lattice sites. In addition, the carbon nanocapsule core-shell structure can not only buffer the stress caused by volume changes, but also construct a three-dimensional conductive network. The combined effect of these two factors significantly improves the stability of the modified lithium manganese iron phosphate material and enhances the discharge capacity, cycle performance, and rate performance of lithium batteries.

[0090] When glucose, a traditional carbon-coated material, is combined with a precursor of manganese iron phosphate (Comparative Example 1), the amount of manganese ion leaching and iron ion leaching are both higher than in Examples 1-4. Furthermore, the rate performance and capacity retention after 500 cycles of the lithium battery are reduced. This is because, firstly, glucose is difficult to disperse uniformly on the surface of lithium manganese iron phosphate particles during high-temperature carbonization, resulting in significant non-uniformity of the coating layer; secondly, the amorphous carbon formed after glucose carbonization has poor conductivity, typically lower than that of nanocapsules; and finally, the glucose-coated lithium manganese iron phosphate material is prone to manganese ion leaching and lattice structure destruction during cycling.

[0091] When only the manganese iron phosphate precursor was combined with carbon nanocapsules without gadolinium doping (Comparative Example 2), the manganese and iron ion dissolution rates were significantly higher than in Example 2, resulting in a certain decrease in the battery's rate performance and cycle stability. This is because the in-situ doping of the rare earth element gadolinium precisely controls the material's crystal structure, optimizes the electron cloud distribution, significantly improves electronic conductivity, enhances the material's structural stability during charge and discharge, effectively suppresses manganese dissolution and lattice distortion, and significantly improves cycle stability.

[0092] When lithium manganese iron phosphate material was prepared using only the polyol-sol-gel method (Comparative Example 5) without gadolinium doping and carbon nanocapsule composite, its manganese ion dissolution and iron ion dissolution were the highest among all samples. Although the lithium manganese iron phosphate material itself has a small particle size and high compaction density, the rate performance and cycle stability of the lithium battery using it are worse than those of Example 2.

[0093] By adjusting the doping amount of Gd or Ce and controlling the mass ratio of the carbon coating layer in the modified lithium manganese iron phosphate material, the ion dissolution rate of the modified lithium manganese iron phosphate can be reduced, thereby improving the specific capacity, cycle capacity retention, and rate performance of the lithium battery. As shown in Example 2, when the doping amount is the median of 1.0 mol% and the mass ratio of precursor to carbon nanocapsules is 10:1, the manganese ion dissolution and iron ion dissolution are both the lowest. The modified lithium manganese iron phosphate material exhibits the best overall electrochemical performance, with the highest discharge specific capacity at 0.1C, 1C, and 5C rates. Furthermore, after 500 cycles at 1C, the capacity retention rate reaches 97.2%, which is significantly better than Comparative Example 6, whose parameters deviate from this range. Comparative Example 6, due to excessive doping, suffers from lattice distortion and damage to the coordination stability of Mn ions. Simultaneously, charge imbalance forms Mn ion migration channels, exacerbating ion dissolution and significantly reducing its electrochemical performance. This indicates that controlling the doping amount of Gd or Ce elements and the mass ratio of the carbon coating layer are key to achieving a balance between high capacity, high rate capability, and long cycle life in lithium batteries.

[0094] Based on Examples 2, 7-8, Comparative Examples 3-4, and Table 1-2, it can be seen that the modified lithium manganese iron phosphate material prepared by co-doping Gd and Ce elements in this application exhibits superior overall performance compared to single doping (as in Example 2) or Mg doping (as in Comparative Examples 3-4). The discharge specific capacity and 500-cycle capacity retention of Examples 7-8 at 1C rate are superior to those of Examples 2 and Comparative Examples 3-4. This indicates that co-doping can more significantly suppress Jahn-Teller distortion and reduce lattice stress, thereby effectively suppressing Mn dissolution; simultaneously, the electronic conductivity of the material is comparable to that of Li. + The diffusion coefficients were all improved, resulting in better rate performance. Furthermore, co-doping also helps improve the cycle life of lithium batteries, suppress oxygen loss, and reduce interfacial side reactions.

[0095] Based on Examples 2, 9-10, and Table 2, it can be seen that the particle size of the modified lithium manganese iron phosphate material or the particle size of the carbon nanocapsules in Examples 9-10 are either too large or too small compared to the modified lithium manganese iron phosphate material or the carbon nanocapsules in Example 2, which will affect the capacity utilization and cycle performance of the lithium battery. This is because when the particle size of the doped lithium manganese iron phosphate is too high and the particle size of the carbon nanocapsules is too low, the small carbon nanocapsules cannot completely encapsulate the large particles of doped lithium manganese iron phosphate, easily resulting in local exposure and uneven contact, leading to interruption of the electron transport path. At the same time, the large particles of doped lithium manganese iron phosphate themselves have slow ion diffusion, ultimately resulting in decreased rate performance, increased polarization during cycling, and easy structural damage. When the particle size of both the doped lithium manganese iron phosphate and the carbon nanocapsules is too high, the overall electrode compaction density decreases, the ion transport distance becomes significantly longer, the conductive network becomes loose, and the mass transfer resistance increases. This not only limits the capacity utilization but also causes significant voltage decay at high rates and deteriorates long-term cycle stability.

[0096] Combining Examples 2, 11-12, and Table 2, it can be seen that the proportion of the protective layer in the modified lithium manganese iron phosphate material in Examples 11-12 is either too high or too low compared to the proportion of the coating layer in Example 2, which will affect the rate and cycle performance of the lithium battery. This is because when the proportion of the coating layer in the modified lithium manganese iron phosphate material is too low, a continuous and complete coating film cannot be formed, and the material surface is prone to direct side reactions with the electrolyte, resulting in Mn... 2+ Poor dissolution suppression and discontinuous electronic conductivity network result in limited improvement in rate and cycle stability. When the coating layer accounts for too high a mass proportion in the modified lithium manganese iron phosphate material, it will crowd out the proportion of active material, leading to a significant decrease in the overall specific capacity of the battery. At the same time, an excessively thick coating layer will increase the lithium-ion diffusion resistance, intensify polarization, and deteriorate high-rate performance.

[0097] Combining Examples 2 and 13 with Table 2, it can be seen that the modified lithium manganese iron phosphate material prepared by the sol-gel method in Example 2 has more uniform particle size, more consistent element doping distribution, and better rate performance and cycle stability than the modified lithium manganese iron phosphate material prepared in Example 13. This is because the sol-gel method can achieve uniform mixing of Gd and Ce doping elements with lithium manganese iron phosphate precursor at the molecular level, reducing component segregation. At the same time, the product particles are small and have regular morphology, which is beneficial to shortening the lithium ion transport path and improving the continuity of the conductive network.

[0098] Combining Examples 2 and 14 with Table 2, it can be seen that when a gradient heating process is not used in the sintering process, the final modified lithium manganese iron phosphate material exhibits poor crystallinity, severe particle agglomeration, and uneven distribution of doping elements. Simultaneously, surface defects and oxygen vacancies increase, resulting in lower discharge specific capacity, poorer rate performance, and faster capacity decay during cycling of the lithium-ion battery. This may be because high-temperature sintering damages the structure of the modified lithium manganese iron phosphate material, and rapid heating easily leads to incomplete thermal decomposition of the precursor and exacerbates lattice distortion, while simultaneously intensifying Mn... 3+ The Jiang-Taylor effect and manganese ion dissolution further deteriorate the electrode interface stability and ion transport efficiency.

[0099] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application, but such modifications or substitutions are all within the scope of protection of this application.

Claims

1. A modified lithium manganese iron phosphate material, characterized in that: Includes doped lithium manganese iron phosphate and a coating layer covering the doped lithium manganese iron phosphate; The chemical formula of the doped lithium manganese iron phosphate is LiM x Mn y Fe （1-y） PO4, wherein M includes at least one element selected from Gd and Ce, x is the doping amount of M, x = 0.008-0.012; and the value of y satisfies 0.5 ≤ y ≤ 0.

7. The coating layer comprises carbon nanocapsules with a core-shell structure.

2. The modified lithium manganese iron phosphate material according to claim 1, characterized in that: The doped lithium manganese iron phosphate has a particle size of 20-200 nm; the carbon nanocapsules have a particle size of 50-100 nm.

3. The modified lithium manganese iron phosphate material according to claim 1, characterized in that: The coating layer accounts for 1.5%-2.5% of the mass of the modified lithium manganese iron phosphate material.

4. The method for preparing the modified lithium manganese iron phosphate material according to any one of claims 1-3, characterized in that: Includes the following steps: S1, Weigh lithium source, manganese source, iron source, and phosphorus source according to stoichiometric ratio; Mix the manganese source and the iron source with the polyol solution to obtain solution A; Mix the phosphorus source with the polyol solution to obtain solution B; S2, mix solution B with solution A, adjust the pH value to obtain a mixed solution; S3, the mixed solution is mixed with a complexing agent and a first solvent, stirred to obtain a sol, the first solvent is separated and dried to obtain a manganese iron phosphate precursor; S4, the manganese iron phosphate precursor and the second solvent containing the dopant are mixed and dispersed for the first time. Then, the carbon nanocapsules are added to the resulting system and dispersed for the second time. The second solvent is then separated to obtain carbon nanocapsules-doped manganese iron phosphate. S5, the carbon nanocapsules-doped manganese iron phosphate are mixed with the lithium source and then sintered to obtain the modified lithium manganese iron phosphate material.

5. The method for preparing the modified lithium manganese iron phosphate material according to claim 4, characterized in that: The stoichiometric ratio of the lithium source, the manganese source, the iron source, and the phosphorus source is 1:(0.5-0.7):(0.3-0.5):1; The lithium source includes at least one of lithium carbonate, lithium bicarbonate, and lithium acetate; the manganese source includes at least one of manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetroxide; the iron source includes at least one of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, and ferrous sulfate; and the phosphorus source includes at least one of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid.

6. The method for preparing the modified lithium manganese iron phosphate material according to claim 4, characterized in that: The polyol solution in S1 comprises a polyol and deionized water in a volume ratio of (3-5):(1-2); the polyol comprises at least one of ethylene glycol, propylene glycol, and butanediol. The concentration of metal ions in solution A is 0.8 mol / L to 1.2 mol / L; the concentration of phosphorus in solution B is 0.8 mol / L to 1.2 mol / L. The volume ratio of solution B to solution A in S2 is (1-2):1; before obtaining the mixed solution in S2, the pH value is adjusted to 7.5-7.9 using ammonia water.

7. The method for preparing the modified lithium manganese iron phosphate material according to claim 4, characterized in that: The stoichiometric ratio of the mixed solution to the complexing agent in S3 is (5-10):1; The volume ratio of the mixed solution to the first solvent is 1:1 to 1:3; The complexing agent includes at least one of citric acid, ethylenediaminetetraacetic acid, ammonium gluconate, and ethylenediamine. The first solvent includes anhydrous ethanol.

8. The method for preparing the modified lithium manganese iron phosphate material according to claim 4, characterized in that: The stoichiometric ratio of the dopant dosage in S4 to the ferromanganese phosphate precursor is (0.008-0.012):1; The mass ratio of the manganese iron phosphate precursor to the carbon nanocapsule is (9-11):1; The dopant includes at least one of Gd(NO3)3·6H2O, Ce(NO3)3·6H2O, (NH4)2Ce(NO3)6, and CeCl3·6H2O; The second solvent includes anhydrous ethanol.

9. The method for preparing the modified lithium manganese iron phosphate material according to claim 4, characterized in that: The sintering process described in S5 employs a segmented heating method, specifically including the following steps: In a mixed atmosphere, first raise the temperature to 350-450℃ at a heating rate of 3-5℃ / min and hold for 2-3 hours; then raise the temperature to 650-750℃ at a heating rate of 5-7℃ / min and hold for 4-5 hours; finally raise the temperature to 800-900℃ at a heating rate of 2-3℃ / min and hold for 8-10 hours. The mixed atmosphere comprises argon and hydrogen in a volume ratio of (7-9):(1-3).

10. A lithium-ion battery, characterized in that: It includes a positive electrode, a negative electrode, an electrolyte, and a separator; wherein the positive electrode includes the modified lithium manganese iron phosphate material as described in any one of claims 1-3 or the modified lithium manganese iron phosphate material prepared by the method described in any one of claims 4-9.

Images