Halogen coordinated rare earth metal nitrogen-doped carbon and carbon-coated modified lithium manganese iron phosphate as well as preparation method and application thereof

Abstract

The invention relates to the technical field of new energy materials, in particular to halogen coordinated rare earth metal nitrogen-doped carbon and carbon-coated modified lithium manganese iron phosphate as well as a preparation method and application thereof. The invention provides halogen coordination rare earth metal nitrogen doped carbon. The carbon material can precisely regulate and control the electronic structure of rare earth metal atoms from the atomic level through halogen coordination, so that the adsorption capacity of the carbon material to lithium ions is enhanced. The carbon material is used as a carbon source, and a carbon-coated modified lithium manganese iron phosphate material with a compact structure is further prepared. The compact carbon-coated structure can effectively relieve the problems of manganese dissolution and active lithium deposition, and the cycling stability and reversible capacity of the material are remarkably improved. A lithium ion battery constructed based on the carbon-coated modified lithium manganese iron phosphate material shows excellent electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy materials. More specifically, it relates to a halogen-coordinated rare earth metal nitrogen-doped carbon-coated modified lithium manganese iron phosphate and its preparation method and application. Background Art

[0002] Lithium-ion batteries have become the mainstream secondary power source in the field of energy storage and power batteries due to their significant advantages such as high energy density, long cycle life, environmental friendliness and no memory effect. Among them, the positive electrode material is the core factor that determines the energy density of lithium-ion batteries, and its performance improvement is crucial to the development of battery technology. As an upgraded positive electrode material of lithium iron phosphate, lithium manganese iron phosphate has outstanding advantages such as high operating voltage, high energy density, abundant raw materials, low cost, high safety and long cycle stability. In recent years, it has become a research hotspot in the field of new energy materials. However, the introduction of manganese elements leads to inherent defects in the material: on the one hand, the electronic conductivity and ionic conductivity are low; on the other hand, the manganese dissolution phenomenon caused by the Jahn-Teller effect is significant. These factors together lead to poor rate performance and cycle stability of the material.

[0003] In order to break through the above technical bottlenecks, the carbon coating strategy has become a key means to optimize the conductivity of lithium manganese iron phosphate positive electrode materials. The uniform carbon coating layer can not only significantly improve the conductivity of the material, but also serve as an elastic buffer layer to effectively alleviate the volume expansion effect during the lithium insertion / delithiation process, thereby avoiding the interface side reaction between the electrolyte and the positive electrode material, and ensuring the structural stability and interface stability of the material. The existing process usually uses glucose, polyethylene glycol, sucrose, etc. as carbon sources to form a conventional carbon coating layer through pyrolysis. Although this type of coating can improve electronic conductivity, increase ion migration rate and inhibit manganese dissolution to a certain extent, due to the lack of lithium-philic active sites in the surface carbon layer, it is difficult to effectively reduce the nucleation overpotential and enhance the adsorption energy between the carbon layer and lithium ions, which makes it difficult to comprehensively improve the electronic / ionic conductivity, rate performance and cycle stability of the material.

[0004] To address the above issues, existing technologies use rare earth metal-doped metal-organic frameworks (MOFs) as novel carbon and nitrogen sources, and achieve element embedding by coating a lithium manganese iron phosphate precursor and sintering it at high temperature. This strategy effectively improves the conductivity and cycle stability of the positive electrode material through the synergistic effect of MOF-derived carbon and rare earth doping. However, the rare earth metal-doped organic metal framework material undergoes pyrolysis during high-temperature sintering, and the gases produced by its decomposition form a large number of microporous structures within the material. Although this porous property can enhance electrolyte wettability, it can also cause some active lithium to irreversibly deposit in the pores, resulting in reversible capacity loss, which in turn limits the electrochemical performance of lithium-ion batteries prepared based on this structure. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of the prior art in which rare earth metal-doped metal-organic framework materials are used as carbon and nitrogen sources, where the porous structure produced by high-temperature sintering causes irreversible deposition of active lithium, thereby limiting the electrochemical performance of the battery; at the same time, the introduction of manganese elements in lithium manganese iron phosphate reduces the rate performance and cycle stability of the material, and provide a preparation method for halogen-coordinated rare earth element nitrogen-doped carbon.

[0006] Another object of the present invention is to provide halogen-coordinated rare earth element nitrogen-doped carbon prepared by the above preparation method.

[0007] Another object of the present invention is to provide the use of the above-mentioned halogen-coordinated rare earth element nitrogen-doped carbon in the preparation of lithium-ion battery positive electrode materials.

[0008] Another object of the present invention is to provide a method for preparing carbon-coated modified lithium manganese iron phosphate.

[0009] Another object of the present invention is to provide carbon-coated modified lithium manganese iron phosphate prepared by the above preparation method.

[0010] Another object of the present invention is to provide a positive electrode for a lithium ion battery.

[0011] Another object of the present invention is to provide a lithium ion battery.

[0012] The above-mentioned purpose of the present invention is achieved through the following technical solutions: The present invention provides a method for preparing halogen-coordinated rare earth metal nitrogen-doped carbon, comprising the following steps: S1. The rare earth metal-doped zinc-based metal organic framework material is fully calcined to obtain rare earth metal nitrogen-doped carbon; S2. The rare earth metal nitrogen-doped carbon obtained in step S1 is dispersed in hydrohalic acid, and after sufficient reaction, centrifuged, the precipitate is washed, dried, and calcined to obtain halogen-coordinated rare earth metal nitrogen-doped carbon.

[0013] This study successfully prepared a halogen-coordinated rare earth metal nitrogen-doped carbon. Specifically, through halogen coordination, this carbon material can precisely control the electronic structure of rare earth metal atoms at the atomic level, thereby enhancing its ability to adsorb lithium ions. This lays a solid foundation for the subsequent preparation of high-performance carbon-coated modified lithium manganese iron phosphate materials.

[0014] Furthermore, in step S1, the method for preparing the rare earth metal-doped zinc-based metal-organic framework material comprises the following steps: SI. The zinc salt and the rare earth metal salt are dissolved in a solvent to obtain a mixed salt solution; SII. Adding the nitrogen-containing organic ligand to the mixed salt solution obtained in step SI, fully reacting, and post-processing to obtain a rare earth metal-doped zinc-based metal-organic framework material.

[0015] Furthermore, in step S1, the zinc salt includes zinc nitrate, zinc acetate, zinc chloride, zinc sulfate or a hydrate of any of the above zinc salts.

[0016] Furthermore, in step S1, the rare earth metal salt includes one or more of lanthanum salts, cerium salts, praseodymium salts, neodymium salts, samarium salts, europium salts, gadolinium salts, terbium salts, dysprosium salts, holmium salts, erbium salts, ytterbium salts, and lutetium salts.

[0017] Furthermore, the lanthanum salt includes lanthanum nitrate, lanthanum acetate, lanthanum chloride and lanthanum sulfate or a hydrate of any of the above lanthanum salts.

[0018] Furthermore, the cerium salt includes cerium nitrate, cerium acetate, cerium chloride and cerium sulfate or a hydrate of any of the above cerium salts.

[0019] Furthermore, the praseodymium salt includes praseodymium nitrate, praseodymium acetate, praseodymium chloride and praseodymium sulfate or a hydrate of any of the above praseodymium salts.

[0020] Furthermore, the neodymium salt includes neodymium nitrate, neodymium acetate, neodymium chloride and neodymium sulfate or a hydrate of any of the above neodymium salts.

[0021] Furthermore, the samarium salt includes samarium nitrate, samarium acetate, samarium chloride and samarium sulfate or a hydrate of any of the above samarium salts.

[0022] Furthermore, the europium salt includes europium nitrate, europium acetate, europium chloride and europium sulfate or a hydrate of any of the above europium salts.

[0023] Furthermore, the gadolinium salt includes gadolinium nitrate, gadolinium acetate, gadolinium chloride and gadolinium sulfate or a hydrate of any of the above gadolinium salts.

[0024] Furthermore, the terbium salt includes terbium nitrate, terbium acetate, terbium chloride and terbium sulfate or a hydrate of any of the above terbium salts.

[0025] Furthermore, the dysprosium salt includes dysprosium nitrate, dysprosium acetate, dysprosium chloride and dysprosium sulfate or a hydrate of any of the above dysprosium salts.

[0026] Furthermore, the holmium salt includes holmium nitrate, holmium acetate, holmium chloride and holmium sulfate or a hydrate of any of the above holmium salts.

[0027] Furthermore, the erbium salt includes erbium nitrate, erbium acetate, erbium chloride and erbium sulfate or a hydrate of any of the above erbium salts.

[0028] Furthermore, the ytterbium salt includes ytterbium nitrate, ytterbium acetate, ytterbium chloride and ytterbium sulfate or a hydrate of any of the above ytterbium salts.

[0029] Furthermore, the lutetium salt includes lutetium nitrate, lutetium acetate, lutetium chloride and lutetium sulfate or a hydrate of any of the above lutetium salts.

[0030] Preferably, in step S1, the molar ratio of the rare earth metal salt to the zinc salt is 1:(5~30).

[0031] More preferably, in step S1, the molar ratio of the rare earth metal salt to the zinc salt is 1:(15~25).

[0032] Furthermore, in step S1, the solvent includes one or more of water, methanol, and ethanol.

[0033] Preferably, in step S1, the concentration of the mixed salt solution is 0.1~3 mol / L.

[0034] More preferably, in step S1, the concentration of the mixed salt solution is 0.8~1.5 mol / L.

[0035] Furthermore, in step SII, the nitrogen-containing organic ligand includes one or more of a nitrogen-containing heterocyclic ligand, a nitrogen-containing heterocyclic and carboxylic acid mixed ligand, and a nitrogen-containing non-heterocyclic ligand.

[0036] Furthermore, the nitrogen-containing heterocyclic ligand includes one or more of pyridine compounds, imidazole compounds, pyrazole compounds, and triazole compounds.

[0037] Furthermore, the pyridine compound is 2,2'-bipyridine and / or 4,4'-bipyridine.

[0038] Furthermore, the imidazole compound includes one or more of 2-methylimidazole, 4-methylimidazole, 1,4-bis(imidazol-1-yl)benzene, 2-methylbenzimidazole, 5-methylbenzimidazole, and 5,6-dimethylbenzimidazole.

[0039] Furthermore, the pyrazole compound is benzopyrazole.

[0040] Furthermore, the triazole compound is 1,2,3-triazole and / or 1,2,4-triazole.

[0041] Furthermore, the nitrogen-containing heterocycle and carboxylic acid mixed ligand includes one or more of phthalic acid compounds, aminobenzoic acid compounds, and bipyridine dicarboxylic acid compounds.

[0042] Furthermore, the phthalic acid compound includes one or more of 2-aminoterephthalic acid, 5-aminoisophthalic acid, 5-nitroisophthalic acid, and nitroterephthalic acid.

[0043] Furthermore, the aminobenzoic acid compound is 4-aminobenzoic acid.

[0044] Furthermore, the bipyridine phthalic acid compound is 2,2'-bipyridine-5,5'-dicarboxylic acid and / or 4,4'-bipyridine-2,2'-dicarboxylic acid.

[0045] Furthermore, the nitrogen-containing non-heterocyclic ligand is one or more of cyanamide compounds, triazine compounds, and carbamate compounds.

[0046] Furthermore, the cyanamide compound is dicyandiamide.

[0047] Furthermore, the triazine compound is melamine.

[0048] Furthermore, the carbamate compound is urea.

[0049] Preferably, in step SII, the nitrogen-containing organic ligand is an imidazole compound.

[0050] More preferably, in step SII, the nitrogen-containing organic ligand is 2-methylimidazole.

[0051] Preferably, the molar ratio of the zinc salt to the nitrogen-containing organic ligand is 1:(1.5-3).

[0052] Furthermore, in step SII, the temperature for the sufficient reaction is room temperature.

[0053] Preferably, in step SII, the sufficient reaction time is 6 to 24 hours.

[0054] Furthermore, in step SII, the post-treatment includes centrifugation, washing, and drying.

[0055] Furthermore, the centrifugation is to centrifuge the reaction solution and collect the precipitate.

[0056] Furthermore, the washing is washing the collected precipitate with methanol 3 to 5 times.

[0057] Furthermore, the drying is to dry the washed precipitate at 50-80° C. for 6-24 h.

[0058] Specifically, in step SII, the post-treatment includes centrifuging the reaction solution, collecting the precipitate, washing the collected precipitate with methanol for 3 to 5 times, and drying the washed precipitate at 50 to 80° C. for 6 to 24 hours.

[0059] Furthermore, in step S1, the temperature of the sufficient calcination is 900-1100°C.

[0060] Preferably, in step S1, the time for sufficient calcination is 1 to 4 hours.

[0061] Preferably, in step S1, the heating rate of the sufficient calcination is 1-10°C / min.

[0062] Furthermore, in step S1, the sufficient calcination further includes a cooling step.

[0063] Furthermore, in step S2, the hydrohalic acid includes one or more of hydrochloric acid, hydrobromic acid, hydroiodic acid, and hydrofluoric acid.

[0064] Furthermore, in step S2, the molar ratio of the rare earth metal nitrogen-doped carbon to the hydrohalic acid is (0.5-3):1.

[0065] Preferably, in step S2, the molar ratio of the rare earth metal nitrogen-doped carbon to the hydrohalic acid is (0.6-1.5):1.

[0066] Furthermore, in step S2, the temperature for the full reaction is 70-100°C.

[0067] Preferably, in step S2, the reflux reaction time is 12 to 30 h.

[0068] Furthermore, in step S2, the centrifugation is to centrifuge the reaction solution to collect the precipitate.

[0069] Furthermore, in step S2, the washing is to wash the collected precipitate with methanol 3 to 5 times.

[0070] Furthermore, in step S2, the drying is to dry the washed precipitate at 50-80° C. for 6-24 h.

[0071] Preferably, in step S2, the calcination temperature is 900-1000°C.

[0072] Preferably, in step S2, the calcination time is 1 to 4 hours.

[0073] Preferably, in step S2, the heating rate of the calcination is 1-10°C / min.

[0074] Furthermore, in step S2, the calcination further includes a cooling step.

[0075] The present invention protects the halogen-coordinated rare earth metal nitrogen-doped carbon prepared by the above preparation method.

[0076] The present invention protects the use of the halogen coordinated rare earth metal nitrogen-doped carbon in the preparation of positive electrode materials for lithium ion batteries.

[0077] The present invention provides a method for preparing carbon-coated modified lithium manganese iron phosphate, comprising the following steps: Si. The lithium salt, manganese source, phosphate source, iron source, the aforementioned halogen-coordinated rare earth metal nitrogen-doped carbon are thoroughly mixed in a solvent to obtain a slurry, the resulting slurry is ground and dried to obtain a powder; Sii. Under a protective gas atmosphere, the powder obtained in step Si is fully calcined and post-treated to obtain carbon-coated modified lithium manganese iron phosphate.

[0078] The present invention uses halogen-coated rare earth metal nitrogen-doped carbon as a carbon source to successfully prepare a carbon-coated modified lithium manganese iron phosphate material. The significant advantage of this material is that during the high-temperature calcination process, the carbon material can effectively inhibit gas generation and avoid the formation of pore defects inside the material, thereby ensuring that the final carbon-coated modified lithium manganese iron phosphate material has a dense structure. This dense carbon coating structure can effectively alleviate the problem of manganese dissolution by forming a stable protective layer on the surface of the material, reducing the direct contact between manganese ions and the electrolyte, thereby reducing the dissolution rate of manganese ions. At the same time, the carbon material can also improve the deposition behavior of lithium ions on the electrode surface and reduce the active lithium deposition phenomenon. These improvements significantly improve the cycle stability and reversible capacity of the carbon-coated modified lithium manganese iron phosphate material.

[0079] Furthermore, in step Si, the lithium salt includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydroxide, lithium nitrate, and lithium acetate.

[0080] Furthermore, in step Si, the manganese source includes one or more of manganese tetraoxide, manganese oxide, manganese dioxide, manganous chloride, manganous sulfate, manganese nitrate, manganese carbonate, manganese acetate or manganese oxalate.

[0081] Furthermore, in step S1, the phosphate source includes one or more of lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, ammonium hypophosphite, and ammonium polyphosphate.

[0082] Furthermore, in step Si, the iron source includes one or more of ferric phosphate, ferric oxide, ferrous oxide, ferrous oxide, ferrous chloride, ferric chloride, ferrous sulfate, ferrous nitrate, ferric citrate, ferric acetate, ammonium ferric sulfate or ferrous oxalate.

[0083] Preferably, in step Si, the molar ratio of the lithium salt to the manganese source is 1:(0.05-0.95).

[0084] Preferably, in step Si, the molar ratio of the lithium salt to the phosphoric acid source is 1:(0.9-1.1).

[0085] Preferably, in step Si, the molar ratio of the lithium salt to the iron source is 1:(0.05-0.95).

[0086] Furthermore, in step Si, the amount of the halogen-coordinated rare earth metal-doped carbon added is such that the mass ratio of the theoretically generated lithium manganese iron phosphate to the halogen-coordinated rare earth metal-doped carbon is 1:(0.005-0.1).

[0087] Preferably, in step Si, the amount of the halogen-coordinated rare earth metal-doped carbon added is such that the mass ratio of the theoretically generated lithium manganese iron phosphate to the halogen-coordinated rare earth metal-doped carbon is 1:(0.01-0.025).

[0088] Furthermore, in step Si, the solvent includes one or more of water, methanol, and ethanol.

[0089] Preferably, in step S1, the time for sufficient mixing is 0.5 to 3 h.

[0090] Furthermore, in step Si, the solid content of the slurry is 35-45%.

[0091] Furthermore, in step Si, the particle size of the ground slurry is 0.37 μm≤D 50 ≤0.44 μm.

[0092] Furthermore, in step Si, the drying is spray drying.

[0093] Furthermore, in step Sii, the protective gas includes one or more of nitrogen, helium, and argon.

[0094] Furthermore, in step Sii, the calcining equipment is a box furnace, a roller kiln or a tube furnace, preferably a box furnace.

[0095] Furthermore, in step Sii, the calcination temperature is 650-900°C.

[0096] Furthermore, in step Sii, the heating rate of the calcination is 1-10°C / min.

[0097] Preferably, in step Sii, the time for sufficient calcination is 8 to 20 hours.

[0098] Furthermore, in step Sii, the post-processing includes cooling, crushing and screening.

[0099] Furthermore, the cooling is to cool the calcined product to room temperature.

[0100] Furthermore, the pulverization is performed by using air flow pulverization.

[0101] Furthermore, the mesh size of the sieve is 100 mesh or 120 mesh.

[0102] Specifically, in step Sii, the post-treatment includes cooling the calcined product to room temperature, then pulverizing the cooled product using air flow, and then sieving it using a 100-mesh or 120-mesh sieve.

[0103] The present invention protects the carbon-coated modified lithium manganese iron phosphate prepared by the above preparation method.

[0104] The present invention protects a lithium-ion battery positive electrode, comprising a current collector and a positive electrode active material loaded on the current collector, wherein the positive electrode active material comprises the aforementioned carbon-coated modified lithium manganese iron phosphate.

[0105] Preferably, the current collector is aluminum foil or carbon-coated aluminum foil.

[0106] Furthermore, as a preferred manner, the method for preparing the positive electrode of the lithium-ion battery comprises the following steps: dispersing carbon-coated modified lithium manganese iron phosphate, conductive agent carbon black, and binder polyvinylidene fluoride in N-methylpyrrolidone dispersant to obtain a positive electrode slurry, then coating the slurry on aluminum foil, and vacuum drying at 80-140°C for 12-24 hours to obtain the positive electrode.

[0107] Furthermore, the mass ratio of the carbon-coated modified lithium manganese iron phosphate and the conductive agent carbon black is (70-95): (3-20).

[0108] Furthermore, the mass ratio of the carbon-coated modified lithium manganese iron phosphate to the binder polyvinylidene fluoride is (70-95): (2-10).

[0109] The present invention protects a lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode is the positive electrode of the lithium ion battery mentioned above.

[0110] The lithium ion battery prepared by the present invention exhibits excellent charge and discharge capacity, capacity retention rate and cycle stability.

[0111] Furthermore, as a preferred embodiment, the positive electrode of the lithium-ion battery is the positive electrode of the aforementioned lithium-ion battery, the negative electrode is a metal lithium sheet, the separator is a polyethylene film, and the electrolyte is prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0112] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a halogen-coordinated rare earth metal nitrogen-doped carbon. Through halogen coordination, this carbon material can precisely control the electronic structure of rare earth metal atoms at the atomic level, thereby enhancing its adsorption capacity for lithium ions. Using this carbon material as a carbon source, a densely structured carbon-coated modified lithium manganese iron phosphate material is further prepared. This dense carbon-coated structure can effectively alleviate the problems of manganese dissolution and active lithium deposition, significantly improving the material's cyclic stability and reversible capacity. Lithium-ion batteries constructed based on this carbon-coated modified lithium manganese iron phosphate material exhibit excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0113] Figure 1 These are SEM images (A~B) of the carbon-coated modified lithium manganese iron phosphate in Example 1 under different magnifications. DETAILED DESCRIPTION

[0114] The present invention is further described below with reference to specific examples, which, however, are not intended to limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0115] Unless otherwise specified, the reagents and materials used in the following examples were commercially available.

[0116] Example 1 A halogen-coordinated rare earth metal nitrogen-doped carbon, carbon-coated modified lithium manganese iron phosphate, lithium-ion battery and preparation method thereof 1. Preparation of halogen-coordinated rare earth metal nitrogen-doped carbon Zn(NO₃)₂·6H₂O and La(NO₃)₃·6H₂O were dissolved in methanol at a molar ratio of 20:1 to form a 1 mol / L mixed salt solution. 2 mol of 2-methylimidazole was slowly added to the mixed salt solution and stirred continuously at room temperature for 12 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried in a vacuum oven at 60°C for 12 h to obtain Zn-La-MOF. Subsequently, the Zn-La-MOF was placed in a nitrogen atmosphere tube furnace and heated to 900°C at a rate of 3°C / min. The temperature was maintained for 2 h, and then cooled to room temperature to obtain lanthanum-nitrogen-doped carbon. 0.8 mol of the lanthanum-nitrogen-doped carbon obtained above was dispersed in 1 mol of hydrochloric acid and refluxed at 80°C for 24 h. The precipitate was collected by centrifugation, washed three times with deionized water, and dried in a vacuum oven at 60°C for 12 h to obtain a chlorine-coordinated lanthanum-nitrogen-doped carbon precursor powder. The chlorine-coordinated lanthanum nitrogen-doped carbon precursor powder was placed in a nitrogen atmosphere tube furnace and heated to 900 °C at a heating rate of 3 °C / min and kept at this temperature for 2 h. After naturally cooling to room temperature, chlorine-coordinated lanthanum nitrogen-doped carbon was obtained.

[0117] 2. Preparation of carbon-coated modified lithium manganese iron phosphate According to the quantitative relationship of the element molar ratio of lithium: manganese: iron: phosphorus of 1.03:0.8:0.2:1.05, the corresponding lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate were weighed respectively. Based on the mass of lithium manganese iron phosphate theoretically generated from these raw materials, 1.35wt% of the chlorine-coordinated lanthanum nitrogen-doped carbon obtained in step 1 was added, and the above substances were dispersed in deionized water for 2h to obtain a slurry with a solid content of 38%. The above slurry was coarsely ground and finely ground using a horizontal sand mill system to a particle size of D 50 When the particle size reached 0.38 μm, it was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen atmosphere, the spray powder was heated to 780°C in an atmosphere box furnace at a heating rate of 3°C / min and held at this temperature for 12 hours. After the atmosphere box furnace cooled naturally to room temperature, the sintered material was jet milled and sieved through a 100-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0118] 3. Preparation of lithium-ion batteries The carbon-coated modified lithium manganese iron phosphate obtained in step 2 was dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry. The slurry was then coated on aluminum foil and vacuum dried at 120°C for 12 h to obtain a lithium-ion battery positive electrode material sheet.

[0119] CR2032 button lithium-ion batteries were assembled in a glove box under an atmosphere of O2≤0.01 ppm and H2O≤0.01 ppm. The positive electrode was the positive electrode sheet of the above-mentioned lithium-ion battery, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0120] Example 2 A halogen-coordinated rare earth metal nitrogen-doped carbon, carbon-coated modified lithium manganese iron phosphate, lithium-ion battery and preparation method thereof 1. Preparation of halogen-coordinated rare earth metal nitrogen-doped carbon Zn(NO₃)₂·6H₂O and Ce(NO₃)₃·6H₂O were dissolved in methanol at a molar ratio of 20:1 to form a 1 mol / L mixed salt solution. 2 mol of 2-methylimidazole was slowly added to the mixed salt solution and stirred continuously at room temperature for 12 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried in a vacuum oven at 60°C for 12 h to obtain Zn-Ce-MOF. Subsequently, the Zn-Ce-MOF was placed in a nitrogen atmosphere tube furnace and heated to 900°C at a rate of 3°C / min. The temperature was maintained for 2 h, and the mixture was cooled to room temperature to obtain cerium-nitrogen-doped carbon. 0.8 mol of the obtained cerium-nitrogen-doped carbon was dispersed in 1 mol of hydrobromic acid and refluxed at 80°C for 24 h. The precipitate was collected by centrifugation, washed three times with deionized water, and dried in a vacuum oven at 60°C for 12 h to obtain bromine-coordinated cerium-nitrogen-doped carbon precursor powder. The bromine-coordinated cerium-nitrogen doped carbon precursor powder was placed in a nitrogen atmosphere tube furnace and heated to 900 °C at a heating rate of 3 °C / min and kept at this temperature for 2 h. After naturally cooling to room temperature, bromine-coordinated cerium-nitrogen doped carbon was obtained.

[0121] 2. Preparation of carbon-coated modified lithium manganese iron phosphate According to the quantitative relationship of lithium: manganese: iron: phosphorus molar ratio of 1.03:0.8:0.2:1.05, the corresponding lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate were weighed respectively. Based on the mass of lithium manganese iron phosphate theoretically generated from these raw materials, 1.35wt% of the bromine-coordinated cerium nitrogen-doped carbon obtained in step 1 was added. The above substances were dispersed in deionized water for 2 hours to obtain a slurry with a solid content of 38%. The above slurry was coarsely and finely ground using a horizontal sand mill system to a particle size of D 50 When the particle size reached 0.39 μm, it was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen atmosphere, the spray powder was heated to 780°C in an atmosphere box furnace at a heating rate of 3°C / min and held at this temperature for 12 hours. After the atmosphere box furnace cooled naturally to room temperature, the sintered material was jet milled and sieved through a 100-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0122] 3. Preparation of lithium-ion batteries The carbon-coated modified lithium manganese iron phosphate obtained in step 2 was dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry. The slurry was then coated on aluminum foil and vacuum dried at 120°C for 12 h to obtain a lithium-ion battery positive electrode material sheet.

[0123] CR2032 button lithium-ion batteries were assembled in a glove box under an atmosphere of O2≤0.01 ppm and H2O≤0.01 ppm. The positive electrode was the positive electrode sheet of the above-mentioned lithium-ion battery, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0124] Example 3 A halogen-coordinated rare earth metal nitrogen-doped carbon, carbon-coated modified lithium manganese iron phosphate, lithium-ion battery and preparation method thereof 1. Preparation of halogen-coordinated rare earth metal nitrogen-doped carbon Zn(NO₃)₂·6H₂O and La(NO₃)₃·6H₂O were dissolved in methanol at a molar ratio of 20:1 to form a 1 mol / L mixed salt solution. 2 mol of 2-methylimidazole was slowly added to the mixed salt solution and stirred continuously at room temperature for 12 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried in a vacuum oven at 60°C for 12 h to obtain Zn-La-MOF. Subsequently, the Zn-La-MOF was placed in a nitrogen atmosphere tube furnace and heated to 920°C at a rate of 5°C / min. The temperature was maintained for 2 h, and then cooled to room temperature to obtain lanthanum-nitrogen-doped carbon. 0.8 mol of the lanthanum-nitrogen-doped carbon obtained above was dispersed in 1 mol of hydrochloric acid and refluxed at 80°C for 24 h. The precipitate was collected by centrifugation, washed three times with deionized water, and dried in a vacuum oven at 60°C for 12 h to obtain a chlorine-coordinated lanthanum-nitrogen-doped carbon precursor powder. The chlorine-coordinated lanthanum nitrogen-doped carbon precursor powder was placed in a nitrogen atmosphere tube furnace and heated to 920 °C at a heating rate of 5 °C / min and kept at this temperature for 2 h. After naturally cooling to room temperature, chlorine-coordinated lanthanum nitrogen-doped carbon was obtained.

[0125] 2. Preparation of carbon-coated modified lithium manganese iron phosphate According to the quantitative relationship of the element molar ratio of lithium: manganese: iron: phosphorus of 1.03:0.6:0.4:1.05, the corresponding lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate were weighed respectively. Based on the mass of lithium manganese iron phosphate theoretically generated from these raw materials, 1.35wt% of the chlorine-coordinated lanthanum nitrogen-doped carbon obtained in step 1 was added, and the above substances were dispersed in deionized water for 2h to obtain a slurry with a solid content of 38%. The above slurry was coarsely ground and finely ground to a particle size of D using a horizontal sand mill system. 50When the particle size reached 0.41 μm, it was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen atmosphere, the spray powder was heated to 780°C in an atmosphere box furnace at a heating rate of 3°C / min and held at this temperature for 12 hours. After the atmosphere box furnace cooled naturally to room temperature, the sintered material was jet milled and sieved through a 100-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0126] 3. Preparation of lithium-ion batteries The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 120°C for 12 h to obtain a lithium-ion battery positive electrode material sheet.

[0127] CR2032 button lithium-ion batteries were assembled in a glove box under an atmosphere of O2≤0.01 ppm and H2O≤0.01 ppm. The positive electrode was the positive electrode sheet of the above-mentioned lithium-ion battery, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0128] Comparative Example 1 A rare earth metal nitrogen-doped carbon, carbon-coated modified lithium manganese iron phosphate, lithium ion battery and preparation method thereof The difference from Example 1 is that in step 1, halogen coordination is not performed on the rare earth metal nitrogen-doped carbon. The specific preparation steps are as follows: 1. Preparation of rare earth metal nitrogen-doped carbon Zn(NO₃)₂·6H₂O and La(NO₃)₃·6H₂O were dissolved in methanol at a molar ratio of 20:1 to form a 1 mol / L mixed salt solution. To this mixed salt solution, 2 mol of 2-methylimidazole was slowly added and stirred continuously at room temperature for 12 hours. The precipitate was collected by centrifugation, washed three times with methanol, and dried in a vacuum drying oven at 60°C for 12 hours to obtain Zn-La-MOF. Subsequently, the Zn-La-MOF was placed in a nitrogen atmosphere tube furnace and heated to 900°C at a rate of 3°C / min. The temperature was maintained for 2 hours, and then cooled naturally to room temperature to obtain lanthanum-nitrogen-doped carbon.

[0129] 2. Preparation of carbon-coated modified lithium manganese iron phosphate According to the quantitative relationship of lithium: manganese: iron: phosphorus molar ratio of 1.03:0.8:0.2:1.05, the corresponding lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate were weighed respectively. Based on the mass of lithium manganese iron phosphate theoretically generated from these raw materials, 1.35wt% of the lanthanum nitrogen-doped carbon obtained in step 1 was added, and the above substances were dispersed in deionized water for 2 hours to obtain a slurry with a solid content of 38%. The above slurry was coarsely and finely ground using a horizontal sand mill system to a particle size of D 50 When the particle size reached 0.38 μm, it was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen atmosphere, the spray powder was heated to 780°C in an atmosphere box furnace at a heating rate of 3°C / min and held at this temperature for 12 hours. After the atmosphere box furnace cooled naturally to room temperature, the sintered material was jet milled and sieved through a 100-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0130] 3. Preparation of lithium-ion batteries The carbon-coated modified lithium manganese iron phosphate obtained in step 2 was dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry. The slurry was then coated on aluminum foil and vacuum dried at 120°C for 12 h to obtain a lithium-ion battery positive electrode material sheet.

[0131] CR2032 button lithium-ion batteries were assembled in a glove box under an atmosphere of O2≤0.01 ppm and H2O≤0.01 ppm. The positive electrode was the positive electrode sheet of the above-mentioned lithium-ion battery, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0132] Comparative Example 2 A nitrogen-doped carbon, carbon-coated modified lithium manganese iron phosphate, lithium ion battery and preparation method thereof The difference from Example 1 is that in step 1, no rare earth metal is used to dope the zinc salt. The specific preparation steps are as follows: 1. Preparation of Nitrogen-doped Carbon Zn(NO₃)₂·6H₂O was dissolved in methanol to form a 1 mol / L mixed salt solution. 2 mol of 2-methylimidazole was slowly added to the mixed salt solution and stirred continuously at room temperature for 12 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried in a vacuum drying oven at 60°C for 12 h to obtain Zn-MOF. Subsequently, the Zn-MOF was placed in a nitrogen atmosphere tube furnace and heated to 900°C at a rate of 3°C / min. The temperature was maintained for 2 h, and then cooled to room temperature to obtain nitrogen-doped carbon. 0.8 mol of the obtained nitrogen-doped carbon was dispersed in 1 mol of hydrochloric acid and refluxed at 80°C for 24 h. The precipitate was collected by centrifugation, washed three times with deionized water, and dried in a vacuum drying oven at 60°C for 12 h to obtain nitrogen-doped carbon precursor powder. The nitrogen-doped carbon precursor powder was placed in a nitrogen atmosphere tube furnace and heated to 900°C at a heating rate of 3°C / min and kept at this temperature for 2 h. After naturally cooling to room temperature, the nitrogen-doped carbon after secondary calcination was obtained.

[0133] 2. Preparation of carbon-coated modified lithium manganese iron phosphate According to the quantitative relationship of the element molar ratio of lithium: manganese: iron: phosphorus of 1.03:0.8:0.2:1.05, the corresponding lithium carbonate, manganese tetraoxide, iron phosphate, and lithium dihydrogen phosphate were weighed respectively. Based on the mass of lithium manganese iron phosphate theoretically generated from these raw materials, 1.35wt% of the nitrogen-doped carbon obtained after secondary calcination in step 1 was added, and the above substances were dispersed in deionized water for 2 hours to obtain a slurry with a solid content of 38%. The above slurry was coarsely and finely ground using a horizontal sand mill system to a particle size of D 50 When the particle size reached 0.38 μm, it was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen atmosphere, the spray powder was heated to 780°C in an atmosphere box furnace at a heating rate of 3°C / min and held at this temperature for 12 hours. After the atmosphere box furnace cooled naturally to room temperature, the sintered material was jet milled and sieved through a 100-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0134] 3. Preparation of lithium-ion batteries The carbon-coated modified lithium manganese iron phosphate obtained in step 2 was dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry. The slurry was then coated on aluminum foil and vacuum dried at 120°C for 12 h to obtain a lithium-ion battery positive electrode material sheet.

[0135] CR2032 button lithium-ion batteries were assembled in a glove box under an atmosphere of O2≤0.01 ppm and H2O≤0.01 ppm. The positive electrode was the positive electrode sheet of the above-mentioned lithium-ion battery, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was prepared by dissolving 1 mol / L LiPF6 in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 1:1:1.

[0136] Experimental Example 1 Composition Characterization of Nitrogen-Doped Carbon Materials and Carbon-Coated Lithium Manganese Iron Phosphate (1) SEM measurement The carbon-coated lithium manganese iron phosphate prepared in Example 1 was characterized using an emission scanning electron microscope. Figure 1 As shown in (A~B), the carbon-coated lithium manganese iron phosphate material in Example 1 presents a uniform nano-granular morphology under different magnifications, and no obvious free carbon is observed, which indicates that the halogen-coated lanthanum nitrogen-doped carbon in the present invention is uniformly coated on the surface of the lithium manganese iron phosphate.

[0137] (2) Determination of halogen content and rare earth metal content The halogen content and rare earth metal content of the nitrogen-doped carbon materials obtained in Examples 1-3 and Comparative Examples 1-2 were measured using an inductively coupled plasma emission spectrometer; the test data are shown in Table 1.

[0138] Table 1 Summary of halogen content and rare earth metal content in nitrogen-doped carbon materials

[0139] ND means that the content of the corresponding substance was not detected.

[0140] From the ICP test results in Table 1, it can be seen that in Examples 1 to 3, for the halogen-coordinated rare earth metal nitrogen-doped carbon, the molar numbers of each were calculated by dividing the contents of halogen and rare earth metal by their relative atomic masses. The results show that the molar ratio of halogen to rare earth metal is close to 1:1. This shows that the hydrohalic acid treatment successfully achieved the coordination of halogen in rare earth metal nitrogen-doped carbon, confirming that the carbon material prepared by the rare earth metal-doped zinc-based metal organic framework has the expected structural characteristics. In contrast, in Comparative Example 1, since the lanthanum nitrogen-doped carbon was not subjected to halogen coordination treatment, the halogen content could not be detected. In Comparative Example 2, no halogen component was detected in the nitrogen-doped carbon after secondary calcination because the sample was not pre-doped with rare earth metals, and the chlorine in the hydrochloric acid could not be doped into the nitrogen-doped carbon due to the lack of rare earth metal sites available for coordination.

[0141] Experimental Example 2 Physical Properties of Carbon-Coated Modified Lithium Manganese Iron Phosphate Cathode Material and Physical and Chemical Properties of Lithium-Ion Batteries Constructed Based on Such Materials (1) Electrochemical performance test The carbon-coated modified lithium manganese iron phosphate materials obtained in Examples 1-3 and Comparative Examples 1-2 were used as the positive electrode active material. A uniformly dispersed positive electrode slurry consisting of active material: conductive agent carbon black: binder polyvinylidene fluoride (PVDF) was weighed and dispersed in N-methylpyrrolidone at a mass ratio of 94:4:2. The slurry was then coated onto aluminum foil and vacuum-dried at 120°C for 12 hours before punching and weighing to produce circular electrode sheets. A lithium metal sheet was used as the negative electrode, the prepared electrode sheet as the positive electrode, a polyethylene film as the separator, and an electrolyte consisting of 1 mol / L LiPF₆ dissolved in a 1:1:1 (volume) mixture of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylene carbonate (EC). CR2032 lithium-ion button cells were assembled in a Mikrona glove box (O₂ ≤ 0.01 ppm, H₂O ≤ 0.01 ppm) and allowed to stand for 12 hours before electrochemical performance testing at room temperature. The test voltage range is set to 2~4.5 V, and the charge and discharge test is performed at a current rate of 0.1 C / 1 C. The calculation formula involved is as follows: First cycle discharge efficiency = first cycle discharge specific capacity / first cycle charge specific capacity × 100%; Capacity retention rate = 100th cycle discharge capacity / first cycle discharge capacity × 100%.

[0142] (2) Powder resistance test method An automatic powder resistivity tester was used to apply a specific excitation current to the carbon-coated modified lithium manganese iron phosphate positive electrode material powders prepared in Examples 1 to 3 and Comparative Examples 1 to 2, and the corresponding voltage value was accurately measured to obtain the surface resistivity and resistance data of the material.

[0143] The test results of (1) to (2) are shown in Table 2. Table 2 Physical properties of carbon-coated modified lithium manganese iron phosphate cathode materials and physical and chemical properties of lithium-ion batteries based on these materials

[0144] As shown in Table 2, the powder resistance of the carbon-coated modified lithium manganese iron phosphate materials prepared in Examples 1-3 is all below 20 Ω·cm, demonstrating excellent electrical conductivity. This is attributed to the introduction of halogen-coordinated rare earth metal nitrogen-doped carbon. The mechanism of action is as follows: halogen coordination precisely regulates the electronic structure of rare earth metal atoms at the atomic level, enhancing their ability to adsorb lithium ions and thereby improving the electrochemical performance of the material. In contrast, in Comparative Example 1, the lack of halogen coordination on the rare earth metal nitrogen-doped carbon prevents the electronic structure of the rare earth metal atoms from being modulated, and thus the lithium ion adsorption capacity from being effectively enhanced, resulting in an increase in the powder resistance of the material to 77.28 Ω·cm. In Comparative Example 2, the lack of rare earth metal coordination on the nitrogen-doped carbon prevents the chlorine in the hydrochloric acid from entering the nitrogen-doped carbon material. This prevents the π electrons on the nitrogen-doped carbon ring from fully utilizing their conjugated advantages and optimizing the material's electronic structure, resulting in a powder resistance as high as 168.20 Ω·cm.

[0145] Electrochemical performance tests further show that the lithium-ion batteries constructed based on carbon-coated modified lithium manganese iron phosphate in Examples 1 to 3 exhibit excellent performance: first charge specific capacity >160 mAh / g, first discharge efficiency >98%, cycle stability >98%, and manganese dissolution <100 ppm. These excellent performances are due to the fact that during the high-temperature calcination of the carbon-coated modified lithium manganese iron phosphate, the aforementioned carbon material can effectively suppress gas generation and avoid the formation of pore defects within the material, thereby ensuring that the final carbon-coated modified lithium manganese iron phosphate material has a dense structure. This dense carbon coating structure can effectively alleviate the problem of manganese dissolution by forming a stable protective layer on the surface of the material, reducing direct contact between manganese ions and the electrolyte, thereby reducing the dissolution rate of manganese ions. At the same time, the halogen coordination of the carbon material can precisely control the electronic structure of rare earth metal atoms at the atomic level, thereby enhancing the adsorption capacity of lithium ions. This improves the deposition behavior of lithium ions on the electrode surface and reduces the phenomenon of active lithium deposition. The above improvements significantly improve the cycle stability and reversible capacity of the carbon-coated modified lithium manganese iron phosphate material. In contrast, in Comparative Examples 1 to 2, due to the lack of halogen coordination and rare earth metal doping, respectively, the electrophilicity of the positively charged central rare earth metal ions cannot be effectively utilized to regulate the electronic structure of the material, which not only weakens the adsorption capacity of lithium ions, resulting in increased active lithium deposition, but also reduces the conductivity of the material. Ultimately, these factors together lead to a decrease in the specific capacity and cycle stability of the material, and aggravate the dissolution of manganese. Therefore, under the combined action of these factors, the electrochemical performance of the batteries prepared in Comparative Examples 1 to 2 is significantly inferior to the performance of the batteries prepared by halogen-coordinated rare earth metal-doped carbon in Examples 1 to 3.

[0146] The above embodiments are preferred implementations of the present invention, but the implementations of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing halogen-coordinated rare earth metal nitrogen-doped carbon, characterized in that: The following steps are involved: S1. The rare earth metal-doped zinc-based metal organic framework material is fully calcined to obtain rare earth metal nitrogen-doped carbon; S2. The rare earth metal nitrogen-doped carbon obtained in step S1 is dispersed in hydrohalic acid, and after sufficient reaction, centrifuged, the precipitate is washed, dried, and calcined to obtain halogen-coordinated rare earth metal nitrogen-doped carbon.

2. The preparation method according to claim 1, characterized in that In step S2, the hydrohalic acid includes one or more of hydrochloric acid, hydrobromic acid, hydroiodic acid, and hydrofluoric acid.

3. The preparation method according to claim 1, characterized in that: In step S1, the method for preparing the rare earth metal-doped zinc-based metal-organic framework material comprises the following steps: SI. The zinc salt and the rare earth metal salt are dissolved in a solvent to obtain a mixed salt solution; SII. Adding the nitrogen-containing organic ligand to the mixed salt solution obtained in step SI, fully reacting, and post-processing to obtain a rare earth metal-doped zinc-based metal-organic framework material.

4. The preparation method according to claim 3, characterized in that In step S1, the rare earth metal salt includes one or more of lanthanum salts, cerium salts, praseodymium salts, neodymium salts, samarium salts, europium salts, gadolinium salts, terbium salts, dysprosium salts, holmium salts, erbium salts, ytterbium salts, and lutetium salts.

5. Halogen-coordinated rare earth metal nitrogen-doped carbon prepared by the preparation method according to any one of claims 1 to 4.

6. Use of the halogen-coordinated rare earth metal nitrogen-doped carbon according to claim 5 in the preparation of positive electrode materials for lithium-ion batteries.

7. A method for preparing carbon-coated modified lithium manganese iron phosphate, characterized in that: The following steps are involved: Si. The lithium salt, manganese source, phosphate source, iron source, the halogen-coordinated rare earth metal nitrogen-doped carbon according to claim 5 are thoroughly mixed in a solvent to obtain a slurry, the obtained slurry is ground and dried to obtain a powder; Sii. Under a protective gas atmosphere, the powder obtained in step Si is fully calcined and post-treated to obtain carbon-coated modified lithium manganese iron phosphate.

8. The carbon-coated modified lithium manganese iron phosphate prepared by the preparation method according to claim 7.

9. A positive electrode material for a lithium ion battery, characterized in that: The invention comprises a current collector and a positive electrode active material loaded on the current collector, wherein the positive electrode active material comprises the carbon-coated modified lithium manganese iron phosphate according to claim 8.

10. A lithium ion battery, characterized in that: The invention comprises an electrolyte, a separator, a positive electrode and a negative electrode, wherein the positive electrode comprises the positive electrode of the lithium-ion battery according to claim 9.

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