Modified lithium manganese iron phosphate material and preparation method thereof

Abstract

The application discloses a modified lithium manganese iron phosphate material and a preparation method thereof, relates to the field of lithium ion batteries, and discloses the following steps: first, preparing a lithium manganese iron phosphate / carbon matrix; then, in-situ polymerizing aniline on the surface of the matrix; and finally, after one-time doping of a conductive enhancer in the polymerization process, and two-time doping of a high-pressure-resistant stabilizer, obtaining the modified lithium manganese iron phosphate material, wherein the conductive enhancer is prepared through a phosphorylation reaction and acidification treatment of diaphosphorus pentoxide and sodium hydroxyethyl sulfonate, and the high-pressure-resistant stabilizer is prepared through a nucleophilic substitution reaction and hydrolysis of 3-iodine-4-methyl thiophene-2-carboxylic acid methyl ester and 5-amino-2-pyridine carbonitrile, so that the modified lithium manganese iron phosphate material has high electron conductivity and high-voltage oxidation resistance stability through the doping of the conductive enhancer and the high-pressure-resistant stabilizer in sequence, and the rate performance and cycle stability of the material are effectively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to modified lithium manganese iron phosphate materials and their preparation methods. Background Technology

[0002] Lithium manganese iron phosphate combines the high safety and structural stability of lithium iron phosphate, utilizing the high redox potential of manganese. At the same time, lithium manganese phosphate materials have advantages such as high specific capacity, high operating voltage, low environmental pollution, and good thermal stability, making them a strong candidate for high-performance lithium-ion battery cathode materials.

[0003] However, while lithium manganese iron phosphate (LFP) exhibits significant performance advantages, it also faces several pressing technical challenges. First, LFP has extremely low intrinsic electronic conductivity, which severely limits its rate performance and the utilization rate of active materials. Furthermore, the slow lithium-ion diffusion kinetics of LFP further restricts its performance under high-current charge-discharge conditions.

[0004] To address the aforementioned problems of lithium manganese iron phosphate (LFP), existing technologies mainly employ three modification methods: carbon coating, ion doping, and material nanosizing. While ion doping can stabilize the crystal structure at the atomic scale and create more lithium vacancies or electronic defects within the material, thereby improving intrinsic electronic conductivity and lithium-ion diffusion coefficient, its improvement on surface properties is limited, and it struggles to resolve interfacial side reactions. Surface coating can effectively isolate the electrolyte from direct contact with the active material, suppressing manganese dissolution and interfacial side reactions. Carbon coating is currently the most widely used method. However, traditional carbon coating only acts on the material surface, and the coating itself is uneven, leaving residues. Carbon coating itself cannot provide capacity or effectively improve the low ionic conductivity of LFP. Furthermore, a single carbon coating layer is prone to cracking due to volume changes during long-term cycling, leading to protection failure. Particle nanosizing shortens the lithium-ion diffusion path by preparing LFP into nanoscale particles, but the high surface energy resulting from nanosizing may exacerbate side reactions, usually requiring synergy with surface coating.

[0005] In recent years, conductive polymer coating has attracted attention as a novel surface modification strategy. Polyaniline, due to its excellent conductivity and good environmental stability, has been used for coating modification of lithium-ion battery cathode materials. However, in existing technologies, polyaniline coating is usually used only as a single conductive layer, failing to simultaneously address the insufficient conductivity of polyaniline and the interfacial instability of lithium manganese iron phosphate under high voltage. Conventional polyaniline coating layers cannot fully leverage their role in promoting electronic conductivity. Summary of the Invention

[0006] To address the shortcomings mentioned in the background section, the present invention aims to provide a modified lithium manganese iron phosphate material and its preparation method. Through the synergistic effect of primary doping with a conductivity enhancer and secondary doping with a high-voltage stabilizer, the modified lithium manganese iron phosphate material possesses both high electronic conductivity and high voltage oxidation resistance, effectively improving the material's rate performance and cycle stability. The objective of this invention can be achieved through the following technical solutions: A method for preparing a modified lithium manganese iron phosphate material includes the following steps: The first step is to mix lithium, manganese, iron, phosphorus and carbon sources, grind them into fine powder, spray dry them, sinter and cool them under a nitrogen atmosphere to obtain lithium manganese iron phosphate / carbon material. The second step involves adding aniline, hydrochloric acid solution, and conductivity enhancer to the reactor, stirring to ensure complete dissolution and uniform mixing, then adding lithium manganese iron phosphate / carbon material, stirring thoroughly to form a uniformly dispersed suspension, and then adding a mixture of ammonium persulfate and hydrochloric acid dropwise to the suspension under stirring conditions at 1-10℃. After the addition is complete, continue stirring in an ice bath for 5-10 hours, and then let it stand for 16-24 hours to obtain the first-doped modified lithium manganese iron phosphate. The third step involves adding the modified lithium manganese iron phosphate, high-pressure stabilizer, and anhydrous ethanol to the reactor, stirring and dispersing them evenly, and then allowing them to stand at room temperature for 16-24 hours to obtain the modified lithium manganese iron phosphate material. The conductivity enhancer is prepared by acidifying a conductivity enhancer precursor obtained by phosphorylation reaction of phosphorus pentoxide and sodium hydroxyethyl sulfonate with dilute sulfuric acid. The high voltage stabilizer is prepared by hydrolyzing a high voltage stabilizer precursor obtained by nucleophilic substitution reaction of methyl 3-iodo-4-methylthiophene-2-carboxylate and 5-amino-2-pyridinecarboxylate.

[0007] More preferably, the lithium source in the first step is one or more of lithium dihydrogen phosphate, lithium phosphate, lithium acetate, lithium carbonate, and lithium hydroxide.

[0008] More preferably, the manganese source in the first step is one or more of manganese carbonate, manganese acetate, manganese oxalate, manganese trioxide, manganese dioxide, and manganese tetroxide.

[0009] More preferably, the iron source in the first step is one or more of ferrous oxalate, ferric phosphate, ferrous acetate, ferric carbonate, ferric hydroxide, ferric oxide, and ferric oxide.

[0010] More preferably, the phosphorus source in the first step is one or more of ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium phosphate.

[0011] More preferably, the carbon source in the first step is one or more of sucrose, glucose, soluble starch, citric acid, polyacrylol, polyethylene glycol, carbon black, acetylene black, and carbon nanotubes.

[0012] More preferably, the molar ratio of lithium source, manganese source, iron source and phosphorus source in the first step is 1-1.1:0.6-1.0:0.1-0.3:1.

[0013] More preferably, the amount of carbon source added in the first step accounts for 3-5% of the total mass of the raw materials.

[0014] More preferably, the particle size after grinding and refining in the first step is 0.2-1 μm.

[0015] More preferably, the molar concentration of the hydrochloric acid solution in the second step is 1-1.2 mol / L.

[0016] More preferably, in the second step, the mass ratio of aniline, conductive enhancer, lithium manganese iron phosphate / carbon material to ammonium persulfate is 4-6:14-20:22-34:6-10.

[0017] More preferably, the mass ratio of the modified lithium manganese iron phosphate to the high-voltage stabilizer in the third step is 18-28:1.0-2.0.

[0018] More preferably, the method for preparing the conductivity enhancer includes the following steps: S1. Add phosphorus pentoxide, sodium hydroxyethyl sulfonate, pyridine and N,N-dimethylformamide to the reactor, and stir the reaction at 50-60℃ for 10-12h to obtain the conductivity enhancer precursor. S2. Dissolve the conductivity enhancer precursor in deionized water, and slowly add dilute sulfuric acid dropwise while stirring for 2-3 hours under ice bath cooling. After complete acidification, add ethanol to precipitate, and then filter and wash with cold water to obtain the conductivity enhancer.

[0019] More preferably, the molar ratio of phosphorus pentoxide and sodium hydroxyethyl sulfonate in step S1 is 1:2-2.2.

[0020] More preferably, the molar concentration of dilute sulfuric acid in step S2 is 1-2 mol / L.

[0021] More preferably, the method for preparing the high-pressure stabilizer includes the following steps: A. Under an argon atmosphere, methyl 3-iodo-4-methylthiophene-2-carboxylate, 5-amino-2-pyridinecarboxylonitrile, 1,10-phenanthroline, cuprous oxide, cesium carbonate and dimethyl sulfoxide are added to a reactor and reacted at 90-100℃ for 36-48h to obtain a high-pressure stabilizer precursor. B. Add the high-pressure stabilizer precursor, lithium iodide and N,N-dimethylformamide to the reactor, reflux at 70-80℃ for 2-4 hours under nitrogen protection, cool to room temperature and dilute with water, extract with ethyl acetate and retain the aqueous phase, acidify the aqueous phase with 1-2% citric acid aqueous solution to pH 2-3 to obtain the high-pressure stabilizer.

[0022] More preferably, in step A, the molar ratio of methyl 3-iodo-4-methylthiophene-2-carboxylate, 5-amino-2-pyridinecarboxylate, 1,10-phenanthroline, cuprous oxide and cesium carbonate is 1:1-1.5:0.1-0.3:0.2-0.3:2-3.

[0023] More preferably, in step B, the molar ratio of the high-pressure stabilizer precursor to lithium iodide is 1:1-1.2.

[0024] More preferably, the molar concentration of the citric acid aqueous solution is 1-1.2 mol / L.

[0025] The beneficial effects of this invention are: In this invention, aniline undergoes in-situ oxidative polymerization on the surface of lithium manganese iron phosphate material to generate a polyaniline coating layer. Its conjugated π-electron system provides a migration channel for hole carriers, constructing a second electron conductive network outside the carbon coating layer. This double-layer conductive structure forms a continuous electron transport path from the inside to the outside of the material, effectively solving the conductivity bottleneck caused by the discontinuity of carbon coating between lithium manganese iron phosphate particles.

[0026] In this invention, the sulfonic acid group in the conductivity enhancer undergoes proton acid doping of polyaniline during polymerization, transforming the insulating polyaniline into a conductive state and endowing the coating layer with intrinsic electronic conductivity. This allows electrons to be rapidly transferred along the polyaniline molecular chain to the surface of the lithium manganese iron phosphate material, laying the foundation for subsequent functionalization modifications. Building upon the conductivity imparted by primary doping, a secondary doping process incorporates a high-voltage stabilizer containing pyridine, thiophene, and cyano groups. The aromatic heterocyclic structure is π-π stacked and anchored with the polyaniline, while the cyano groups further increase the oxidation potential of the polyaniline. This secondary doping enables the already conductive coating layer to simultaneously acquire high-voltage oxidation resistance, achieving a balance between conductivity and voltage withstand capability. This significantly improves the specific capacity and active material utilization of the lithium manganese iron phosphate material at high-rate discharge, effectively curbing structural degradation caused by coating layer failure and extending the cycle life of the material at high cutoff voltages.

[0027] This invention introduces a hydrophobic functional layer containing thiophene and methyl groups onto the surface of polyaniline through secondary doping. This barrier effectively isolates the electrolyte from the surface of the lithium manganese iron phosphate material, blocking manganese ion dissolution channels. The sulfonic acid groups introduced by primary doping initially provide lithium ion affinity sites, while the pyridine nitrogen and cyano nitrogen introduced by secondary coating further enrich the heteroatom coordination environment. Together, they construct a fast lithium ion channel, promote interfacial desolvation, reduce the interfacial impedance of the lithium manganese iron phosphate material, and improve its high-current charge-discharge capability. Simultaneously, the flexible polyaniline layer buffers volumetric stress, and the rigid aromatic heterocyclic rings enhance mechanical strength, preventing the coating layer from cracking during long-term cycling, thus enabling the lithium manganese iron phosphate material to achieve better performance compatibility between high rates and long cycles. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: A method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: Step 1: Mix 70g lithium phosphate, 100g manganese carbonate, 30g ferrous oxalate, 110g ammonium phosphate and 10g sucrose, grind to a particle size of 0.2μm, spray dry, cool to room temperature, and then sinter at 700℃ for 5h under a nitrogen atmosphere. After sintering, cool to room temperature to obtain lithium manganese iron phosphate / carbon material. Step 2: Add 4g aniline, 50mL of 1mol / L hydrochloric acid solution and 14g conductive enhancer to the reactor, stir to dissolve and mix them evenly, then add 22g lithium manganese iron phosphate / carbon material, stir thoroughly to form a uniformly dispersed suspension, and add a mixture of 6g ammonium persulfate and 5mL of 1mol / L hydrochloric acid dropwise at a rate of 5mL / min under stirring conditions at 1℃. After the addition is complete, continue stirring in an ice bath for 5h, and then let it stand for 16h. After the reaction is complete, filter the product and retain the solid. Wash the solid with deionized water and anhydrous ethanol to obtain the first-doped modified lithium manganese iron phosphate. The third step involves adding 18g of the first-doped modified lithium manganese iron phosphate, 1.0g of high-pressure stabilizer, and 150mL of anhydrous ethanol to the reactor. After stirring and dispersing evenly, the mixture is allowed to stand at room temperature for 16 hours to allow for a second doping reaction. The product is then filtered, and the resulting solid is washed with deionized water and anhydrous ethanol, respectively. The precipitate is dried in an oven at 60°C for 8 hours to obtain the modified lithium manganese iron phosphate material. The preparation method of the conductivity enhancer includes the following steps: S1. Add 12g of phosphorus pentoxide, 24g of sodium hydroxyethyl sulfonate, 12mL of pyridine and 50mL of N,N-dimethylformamide to a reactor, and stir the reaction at 50℃ for 10h to obtain a conductivity enhancer precursor. S2. Dissolve 25g of the conductivity enhancer precursor in deionized water, and slowly add 100mL of 1mol / L dilute sulfuric acid dropwise while cooling in an ice bath. Stir the mixture while adding the solution for 2 hours. After complete acidification, add ethanol to precipitate the solution. Then filter the solution and wash with cold water to obtain the conductivity enhancer. The preparation method of the high-pressure stabilizer includes the following steps: A. Under an argon atmosphere, 2g of methyl 3-iodo-4-methylthiophene-2-carboxylate, 1.3g of 5-amino-2-pyridinecarboxylonitrile, 0.3g of 1,10-phenanthroline, 0.1g of cuprous oxide, 7.2g of cesium carbonate, and 20mL of dimethyl sulfoxide were added to a reactor and reacted at 90℃ for 36h. After the reaction was completed and naturally cooled to room temperature, the reaction solution was extracted with ethyl acetate and the organic phase was retained. The organic phase was then washed with saturated brine and further dried and purified to obtain a high-pressure stabilizer precursor. B. Add 2g of high-pressure stabilizer precursor, 1.0g of lithium iodide and 10mL of N,N-dimethylformamide to the reactor. Reflux at 70℃ for 2h under nitrogen protection. After cooling to room temperature, dilute with water, extract with ethyl acetate and retain the aqueous phase. Acidify the aqueous phase with 1% citric acid aqueous solution to pH 2, extract with ethyl acetate again, combine the organic phases, and wash, dry and purify the organic phase to obtain the high-pressure stabilizer.

[0030] Example 2: A method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: Step 1: Mix 75g lithium acetate, 105g manganese acetate, 35g iron phosphate, 115g ammonium dihydrogen phosphate and 11g glucose, grind to a particle size of 0.6μm, spray dry, cool to room temperature, and then sinter at 715℃ for 6h under a nitrogen atmosphere. After sintering, cool to room temperature to obtain lithium manganese iron phosphate / carbon material. Step 2: Add 5g aniline, 65mL of 1.1mol / L hydrochloric acid solution and 17g conductivity enhancer to the reactor, stir to dissolve and mix thoroughly, then add 28g lithium manganese iron phosphate / carbon material, stir thoroughly to form a uniformly dispersed suspension, and add a mixture of 8g ammonium persulfate and 7.5mL of 1.1mol / L hydrochloric acid dropwise at a rate of 6.5mL / min under stirring at 5.5℃. After the addition is complete, continue stirring in an ice bath for 7.5h, then let stand for 20h. After the reaction is complete, filter the product and retain the solid. Wash the solid with deionized water and anhydrous ethanol to obtain the first-doped modified lithium manganese iron phosphate. The third step involves adding 23g of the first-doped modified lithium manganese iron phosphate, 1.5g of high-pressure stabilizer, and 200mL of anhydrous ethanol to the reactor. After stirring and dispersing evenly, the mixture is left to stand at room temperature for 20 hours to allow for a second doping reaction. The product is then filtered, and the resulting solid is washed with deionized water and anhydrous ethanol, respectively. The precipitate is then dried in an oven at 75°C for 10 hours to obtain the modified lithium manganese iron phosphate material. The preparation method of the conductivity enhancer includes the following steps: S1. Add 14.5g of phosphorus pentoxide, 30g of sodium hydroxyethyl sulfonate, 15mL of pyridine and 85mL of N,N-dimethylformamide to a reactor, and stir the reaction at 55℃ for 11h to obtain a conductivity enhancer precursor. S2. Dissolve 35g of the conductivity enhancer precursor in deionized water, and slowly add 130mL of 1.5mol / L dilute sulfuric acid dropwise while cooling in an ice bath. Stir the mixture while adding the solution for 2.5h. After complete acidification, add ethanol to precipitate the product, and then filter and wash with cold water to obtain the conductivity enhancer. The preparation method of the high-pressure stabilizer includes the following steps: A. Under an argon atmosphere, 3g of methyl 3-iodo-4-methylthiophene-2-carboxylate, 1.95g of 5-amino-2-pyridinecarboxylonitrile, 0.4g of 1,10-phenanthroline, 0.15g of cuprous oxide, 10.8g of cesium carbonate, and 30mL of dimethyl sulfoxide were added to a reactor and reacted at 95℃ for 42h. After the reaction was completed and naturally cooled to room temperature, the reaction solution was extracted with ethyl acetate and the organic phase was retained. The organic phase was then washed with saturated brine and further dried and purified to obtain a high-pressure stabilizer precursor. B. Add 3g of high-pressure stabilizer precursor, 1.5g of lithium iodide and 20mL of N,N-dimethylformamide to the reactor. Reflux at 75℃ for 3h under nitrogen protection. After cooling to room temperature, dilute with water, extract with ethyl acetate and retain the aqueous phase. Acidify the aqueous phase to pH 2.5 with 1.1% citric acid aqueous solution, extract again with ethyl acetate, combine the organic phases, and wash, dry and purify the organic phase to obtain the high-pressure stabilizer.

[0031] Example 3: A method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: Step 1: Mix 80g lithium dihydrogen phosphate, 110g manganese oxalate, 40g ferrous acetate, 120g lithium dihydrogen phosphate and 12g carbon black, grind to a particle size of 1μm, spray dry, cool to room temperature, and then sinter at 730℃ for 7h under a nitrogen atmosphere. After sintering, cool to room temperature to obtain lithium manganese iron phosphate / carbon material. Step 2: Add 6g of aniline, 80mL of 1.2mol / L hydrochloric acid solution and 20g of conductivity enhancer to the reactor, stir to dissolve and mix thoroughly, then add 34g of lithium manganese iron phosphate / carbon material, stir thoroughly to form a uniformly dispersed suspension, and add a mixture of 10g of ammonium persulfate and 10mL of 1.2mol / L hydrochloric acid dropwise at a rate of 8mL / min under stirring at 10℃. After the addition is complete, continue stirring in an ice bath for 10h, then let stand for 24h. After the reaction is complete, filter the product and retain the solid. Wash the solid with deionized water and anhydrous ethanol to obtain the first-doped modified lithium manganese iron phosphate. The third step involves adding 28g of the first-doped modified lithium manganese iron phosphate, 2.0g of high-pressure stabilizer, and 250mL of anhydrous ethanol to the reactor. After stirring and dispersing evenly, the mixture is left to stand at room temperature for 24 hours to allow for a second doping reaction. The product is then filtered, and the resulting solid is washed with deionized water and anhydrous ethanol, respectively. The precipitate is then dried in an oven at 90℃ for 12 hours to obtain the modified lithium manganese iron phosphate material. The preparation method of the conductivity enhancer includes the following steps: S1. Add 17g of phosphorus pentoxide, 36g of sodium hydroxyethyl sulfonate, 18mL of pyridine and 120mL of N,N-dimethylformamide to a reactor, and stir the reaction at 60℃ for 12h to obtain a conductivity enhancer precursor. S2. Dissolve 45g of the conductivity enhancer precursor in deionized water, and slowly add 160mL of 2mol / L dilute sulfuric acid dropwise while cooling in an ice bath. Stir the mixture while adding the solution for 3 hours. After complete acidification, add ethanol to precipitate the solution, and then filter and wash with cold water to obtain the conductivity enhancer. The preparation method of the high-pressure stabilizer includes the following steps: A. Under an argon atmosphere, 4g of methyl 3-iodo-4-methylthiophene-2-carboxylate, 2.6g of 5-amino-2-pyridinecarboxylonitrile, 0.5g of 1,10-phenanthroline, 0.2g of cuprous oxide, 14.4g of cesium carbonate, and 40mL of dimethyl sulfoxide were added to a reactor and reacted at 100℃ for 48h. After the reaction was completed and naturally cooled to room temperature, the reaction solution was extracted with ethyl acetate and the organic phase was retained. The organic phase was then washed with saturated brine and further dried and purified to obtain a high-pressure stabilizer precursor. B. Add 4g of high-pressure stabilizer precursor, 2.0g of lithium iodide and 30mL of N,N-dimethylformamide to the reactor. Reflux at 80℃ for 4h under nitrogen protection. After cooling to room temperature, dilute with water, extract with ethyl acetate and retain the aqueous phase. Acidify the aqueous phase to pH 3 with 1.2% citric acid aqueous solution, extract again with ethyl acetate, combine the organic phases, and wash, dry and purify the organic phase to obtain the high-pressure stabilizer.

[0032] Comparative Example 1: A method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: Step 1: Mix 80g lithium dihydrogen phosphate, 110g manganese oxalate, 40g ferrous acetate, 120g lithium dihydrogen phosphate and 12g carbon black, grind to a particle size of 1μm, spray dry, cool to room temperature, and then sinter at 730℃ for 7h under a nitrogen atmosphere. After sintering, cool to room temperature to obtain lithium manganese iron phosphate / carbon material. Step 2: Add 6g aniline, 80mL of 1.2mol / L hydrochloric acid solution and 20g conductivity enhancer to the reactor, stir to dissolve and mix thoroughly, then add 34g lithium manganese iron phosphate / carbon material, stir thoroughly to form a uniformly dispersed suspension, and add a mixture of 10g ammonium persulfate and 10mL of 1.2mol / L hydrochloric acid dropwise at a rate of 8mL / min under stirring at 10℃. After the addition is complete, continue stirring in an ice bath for 10h, then let stand for 24h. After the reaction is complete, filter the product and retain the solid, then wash with deionized water and anhydrous ethanol respectively, and dry the precipitate in an oven at 90℃ for 12h to obtain the modified lithium manganese iron phosphate material. The preparation method of the conductivity enhancer includes the following steps: S1. Add 17g of phosphorus pentoxide, 36g of sodium hydroxyethyl sulfonate, 18mL of pyridine and 120mL of N,N-dimethylformamide to a reactor, and stir the reaction at 60℃ for 12h to obtain a conductivity enhancer precursor. S2. Dissolve 45g of the conductivity enhancer precursor in deionized water, and slowly add 160mL of 2mol / L dilute sulfuric acid dropwise while cooling in an ice bath. Stir the mixture while adding the solution for 3 hours. After complete acidification, add ethanol to precipitate the product, and then filter and wash with cold water to obtain the conductivity enhancer.

[0033] The difference between this comparative example and Example 3 is that only the conductivity enhancer is doped, while the rest of the preparation process is the same as in Example 3.

[0034] Comparative Example 2: A method for preparing a modified lithium manganese iron phosphate material, comprising the following steps: Step 1: Mix 80g lithium dihydrogen phosphate, 110g manganese oxalate, 40g ferrous acetate, 120g lithium dihydrogen phosphate and 12g carbon black, grind to a particle size of 1μm, spray dry, cool to room temperature, and then sinter at 730℃ for 7h under a nitrogen atmosphere. After sintering, cool to room temperature to obtain lithium manganese iron phosphate / carbon material. Step 2: Add 6g of aniline and 80mL of 1.2mol / L hydrochloric acid solution to the reactor, stir to dissolve and mix thoroughly, then add 34g of lithium manganese iron phosphate / carbon material, stir thoroughly to form a uniformly dispersed suspension. Under stirring conditions at 10℃, add a mixture of 10g of ammonium persulfate and 10mL of 1.2mol / L hydrochloric acid dropwise at a rate of 8mL / min. After the addition is complete, continue stirring in an ice bath for 10h, then let it stand for 24h. After the reaction is complete, filter the product and retain the solid, then wash with deionized water and anhydrous ethanol respectively, and then dry in an oven at 90℃ for 12h to obtain the modified lithium manganese iron phosphate material.

[0035] The difference between this comparative example and Example 3 is that only polyaniline coating is performed, while the rest of the preparation process is the same as in Example 3.

[0036] Performance testing (1) Conductivity and cycle stability test: The conductivity of each group of samples was tested according to GB / T 30835-2014, and the capacity retention rate of each group of samples after 100 cycles at 1C rate was tested according to GB / T 36276-2023. The results of conductivity and cycle stability test are shown in Table 1.

[0037] Table 1: Statistical Table of Conductivity and Cyclic Stability Test Results ; As can be seen from Table 1, the conductivity and capacity retention of Examples 1-3 are better than those of Comparative Examples 1 and 2. This indicates that the present invention constructs a composite coating layer with both high conductivity and high voltage resistance stability through the synergistic effect of primary doping of conductive enhancer and secondary doping of high voltage stabilizer, thereby improving the high current charge-discharge capability and long cycle stability of modified lithium manganese iron phosphate material.

[0038] Capacity and rate performance testing: The discharge specific capacity of each group of samples at 0.1C, 2C and 5C rates was tested in accordance with GB / T 42161-2022. The rate performance was characterized by the ratio of the discharge specific capacity at 5C rate to that at 0.1C rate. The capacity and rate performance test results are shown in Table 2.

[0039] Table 2: Statistical Table of Capacity and Rate Performance Test Results ;

[0040] As can be seen from Table 2, the discharge specific capacity and 5C / 0.1C capacity retention rate of Examples 1-3 at 0.1C, 2C, and 5C rates are all higher than those of Comparative Examples 1 and 2. This shows that the dual conductive network constructed in this invention effectively improves the rate performance of the modified lithium manganese iron phosphate material and significantly enhances its high-current charge and discharge capability.

[0041] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for preparing a modified lithium manganese iron phosphate material, characterized in that, Includes the following steps: The first step is to mix lithium, manganese, iron, phosphorus and carbon sources, grind them into fine powder, spray dry them, sinter and cool them under a nitrogen atmosphere to obtain lithium manganese iron phosphate / carbon material. The second step involves adding aniline, hydrochloric acid solution, and conductivity enhancer to the reactor, stirring to ensure complete dissolution and uniform mixing, then adding lithium manganese iron phosphate / carbon material, stirring thoroughly to form a uniformly dispersed suspension, and then adding a mixture of ammonium persulfate and hydrochloric acid dropwise to the suspension under stirring conditions at 1-10℃. After the addition is complete, continue stirring in an ice bath for 5-10 hours, and then let it stand for 16-24 hours to obtain the first-doped modified lithium manganese iron phosphate. The third step involves adding the modified lithium manganese iron phosphate, high-pressure stabilizer, and anhydrous ethanol to the reactor, stirring and dispersing them evenly, and then allowing them to stand at room temperature for 16-24 hours to obtain the modified lithium manganese iron phosphate material. The conductivity enhancer is prepared by acidifying a conductivity enhancer precursor obtained by phosphorylation reaction of phosphorus pentoxide and sodium hydroxyethyl sulfonate with dilute sulfuric acid. The high voltage stabilizer is prepared by hydrolyzing a high voltage stabilizer precursor obtained by nucleophilic substitution reaction of methyl 3-iodo-4-methylthiophene-2-carboxylate and 5-amino-2-pyridinecarboxylate.

2. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, The lithium source in the first step is one or more of lithium dihydrogen phosphate, lithium phosphate, lithium acetate, lithium carbonate, and lithium hydroxide; the manganese source is one or more of manganese carbonate, manganese acetate, manganese oxalate, manganese trioxide, manganese dioxide, and manganese tetroxide; the iron source is one or more of ferrous oxalate, ferric phosphate, ferrous acetate, ferric carbonate, ferric hydroxide, ferric oxide, and ferric oxide; the phosphorus source is one or more of ammonium phosphate, ammonium dihydrogen phosphate, lithium dihydrogen phosphate, and lithium phosphate; and the carbon source is one or more of sucrose, glucose, soluble starch, citric acid, polyacryl alcohol, polyethylene glycol, carbon black, acetylene black, and carbon nanotubes.

3. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, The molar ratio of lithium source, manganese source, iron source and phosphorus source in the first step is 1-1.1:0.6-1.0:0.1-0.3:1, the amount of carbon source added accounts for 3-5% of the total mass of raw materials, and the particle size after grinding and refining is 0.2-1μm.

4. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, In the second step, the concentration of the hydrochloric acid solution is 1-1.2 mol / L, and the mass ratio of aniline, conductive enhancer, lithium manganese iron phosphate / carbon material to ammonium persulfate is 4-6:14-20:22-34:6-10.

5. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, In the third step, the mass ratio of the modified lithium manganese iron phosphate to the high-voltage stabilizer is 18-28:1.0-2.

0.

6. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, The preparation method of the conductivity enhancer includes the following steps: S1. Add phosphorus pentoxide, sodium hydroxyethyl sulfonate, pyridine and N,N-dimethylformamide to the reactor, and stir the reaction at 50-60℃ for 10-12h to obtain the conductivity enhancer precursor. S2. Dissolve the conductivity enhancer precursor in deionized water, and slowly add dilute sulfuric acid dropwise while stirring for 2-3 hours under ice bath cooling. After complete acidification, add ethanol to precipitate, and then filter and wash with cold water to obtain the conductivity enhancer.

7. The method for preparing a modified lithium manganese iron phosphate material according to claim 6, characterized in that, In step S1, the molar ratio of phosphorus pentoxide to sodium hydroxyethyl sulfonate is 1:2-2.2, and in step S2, the molar concentration of dilute sulfuric acid is 1-2 mol / L.

8. The method for preparing a modified lithium manganese iron phosphate material according to claim 1, characterized in that, The preparation method of the high-pressure stabilizer includes the following steps: A. Under an argon atmosphere, methyl 3-iodo-4-methylthiophene-2-carboxylate, 5-amino-2-pyridinecarboxylonitrile, 1,10-phenanthroline, cuprous oxide, cesium carbonate and dimethyl sulfoxide are added to a reactor and reacted at 90-100℃ for 36-48h to obtain a high-pressure stabilizer precursor. B. Add the high-pressure stabilizer precursor, lithium iodide and N,N-dimethylformamide to the reactor, reflux at 70-80℃ for 2-4 hours under nitrogen protection, cool to room temperature and dilute with water, extract with ethyl acetate and retain the aqueous phase, acidify the aqueous phase with 1-2% citric acid aqueous solution to pH 2-3 to obtain the high-pressure stabilizer.

9. The method for preparing a modified lithium manganese iron phosphate material according to claim 8, characterized in that, In step A, the molar ratio of methyl 3-iodo-4-methylthiophene-2-carboxylate, 5-amino-2-pyridinecarboxylonitrile, 1,10-phenanthroline, cuprous oxide, and cesium carbonate is 1:1-1.5:0.1-0.3:0.2-0.3:2-3. In step B, the molar ratio of the high-pressure stabilizer precursor and lithium iodide is 1:1-1.

2. The molar concentration of the citric acid aqueous solution is 1-1.2 mol / L.

10. A modified lithium manganese iron phosphate material, characterized in that, It is prepared by the method described in any one of claims 1-9.

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