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

Abstract

The application discloses a modified manganese iron lithium phosphate positive electrode material and a preparation method thereof, and relates to the technical field of manganese iron lithium phosphate positive electrode materials. The application is characterized in that a multi-stage chemical modification is performed on a multi-walled carbon nanotube to prepare a block amphiphilic grafted carbon nanotube and use the block amphiphilic grafted carbon nanotube for in-situ composite modification. The block amphiphilic grafted carbon nanotube is first oxidized by mixed acid, then is hydroxylated, is initiated and grafted by bromine to graft vinyl pyrrolidone, and is further grafted by acrylic acid to construct an amphiphilic structure; the block amphiphilic grafted carbon nanotube is used as a conductive framework and a dispersant, and is combined with an iron source, a manganese source, a lithium source and phosphoric acid to perform hydrothermal synthesis on a precursor, and then the modified manganese iron lithium phosphate is obtained by ball milling and sintering with glucose. The amphiphilic carbon tube can efficiently disperse metal ions, induce uniform growth of crystal nuclei, and construct a stable three-dimensional conductive network.

Description

Technical Field

[0001] This invention relates to the field of lithium manganese iron phosphate cathode material technology, specifically to a modified lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium manganese iron phosphate (LMFP), as a high-voltage, high-safety cathode material for lithium-ion batteries, has shown great application potential in the fields of power lithium batteries and energy storage batteries due to its high operating voltage, excellent thermal stability, and low raw material cost. Compared with traditional lithium iron phosphate, LMFP can effectively improve battery energy density and is gradually becoming a research hotspot for next-generation high-energy-density cathode materials, with broad industrialization prospects in scenarios such as new energy vehicles and large-scale energy storage power stations.

[0003] However, lithium manganese iron phosphate (LFP) suffers from low electronic conductivity and slow lithium-ion diffusion rate. Furthermore, it is prone to particle agglomeration and crystal inhomogeneity during synthesis, making it difficult to meet practical application requirements for high-rate performance and cycle stability. While commonly used modification methods such as carbon material coating and ion doping can improve conductivity to some extent, ordinary carbon nanotubes are inert, poorly dispersible, and prone to self-stacking. Their weak interfacial bonding with active materials prevents uniform dispersion and directional growth of the precursor, hindering fundamental improvements in rate performance and structural stability and restricting the further commercial application of LFP. Summary of the Invention

[0004] The purpose of this invention is to provide a modified lithium manganese iron phosphate cathode material and its preparation method, so as to solve the problems existing in the prior art.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for preparing a modified lithium manganese iron phosphate cathode material includes the following preparation steps: S1. Under nitrogen protection, polyvinylpyrrolidone-grafted carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine were added to N,N-dimethylformamide at a mass ratio of 10 to 12 times that of the polyvinylpyrrolidone-grafted carbon nanotubes. Acrylic acid was then added dropwise, and the reaction was carried out at 65 to 70°C with stirring for 36 to 38 hours. After the reaction was completed, the mixture was diluted with a 1:1 volume ratio of anhydrous ethanol and deionized water, vacuum filtered, and washed three times with the same mixture. The mixture was then vacuum dried to obtain block amphiphilic grafted carbon nanotubes. S2. Weigh out block amphiphilic grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid according to a mass ratio of 1:13~14:8~8.5:11~12:11.5~12. Under nitrogen protection, add the block amphiphilic grafted carbon nanotubes to 200~300 times their mass of deionized water and ultrasonically disperse for 20~30 min. Then add ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in sequence. Stir at 55~65℃ for 2~3 h. After the reaction is complete, react at 200~220℃ for 10~12 h. After the reaction is complete, cool to 55~65℃, vacuum filter, wash with deionized water 3~5 times, and vacuum dry to obtain the precursor. S3. Weigh the precursor and glucose at a mass ratio of 1:0.2~0.3, mix the precursor and glucose, and ball mill at 600~800 rpm for 6~8 h; under an inert atmosphere, heat to 600~650℃ at 5℃ / min, hold for sintering for 4.5~5.5 h, cool to 150~160℃ and collect the sample to obtain the modified lithium manganese iron phosphate cathode material.

[0006] As an optimization, the preparation steps of the polyvinylpyrrolidone-grafted carbon nanotubes are as follows: Modified carbon nanotubes, cuprous bromide, pentamethyldiethylenetriamine, and vinylpyrrolidone are weighed according to a mass ratio of 1:0.14~0.15:0.16~0.18:11~12. Under nitrogen protection, the modified carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine are added to N,N-dimethylformamide at a mass ratio of 10~12 times that of the modified carbon nanotubes. Then, vinylpyrrolidone is added dropwise. The reaction is stirred at 75~85℃ for 48~50 h. After the reaction is complete, anhydrous ethanol is added for dilution, the mixture is vacuum filtered, washed 3~5 times with anhydrous ethanol, and dried under vacuum to obtain polyvinylpyrrolidone-grafted carbon nanotubes.

[0007] As an optimization, the modified carbon nanotubes include the following preparation steps: hydroxylated carbon nanotubes, 4-dimethylaminopyridine, triethylamine, and 2-bromoisobutylamide bromide are weighed according to a mass ratio of 1:0.06~0.07:0.6~0.8:0.08~0.1; 2-bromoisobutylamide bromide is added to anhydrous chloroform at a mass ratio of 12~15 times that of 2-bromoisobutylamide bromide; after stirring and dissolving, a 2-bromoisobutylamide bromide solution is prepared for later use; under nitrogen protection, the weighed... Hydroxylated carbon nanotubes, 4-dimethylaminopyridine, and triethylamine were added to anhydrous chloroform at a concentration of 20-22 times the mass of the hydroxylated carbon nanotubes. Then, 2-bromoisobutylamide bromide solution was added dropwise. The mixture was stirred and reacted at -5 to 0°C for 2-3 hours, followed by stirring and reacting at 25 to 30°C for 9-10 hours. After the reaction was completed, the mixture was diluted with anhydrous chloroform, vacuum filtered, washed 3-5 times with anhydrous chloroform, and then vacuum dried to obtain modified carbon nanotubes.

[0008] As an optimization, the hydroxylated carbon nanotubes include the following preparation steps: Carboxylated carbon nanotubes, sulfoxide, and N,N-dimethylformamide are weighed according to a mass ratio of 1:120~130:0.02~0.03; under nitrogen protection, the carboxylated carbon nanotubes are added to sulfoxide, followed by dropwise addition of N,N-dimethylformamide. The mixture is refluxed at 60~70℃ for 22~26 h. After the reaction, unreacted material is removed by vacuum distillation, and the mixture is washed 3~5 times with anhydrous chloroform and dried under vacuum. The dried product is then added to anhydrous ethylene glycol at 80~90 times its mass, ultrasonically dispersed, and reacted at 110~130℃ under nitrogen protection for 45~50 h. After the reaction, the mixture is cooled to 25~35℃, diluted with anhydrous ethanol, filtered, and washed alternately with anhydrous ethanol and distilled water 3~5 times. The mixture is then dried under vacuum to obtain hydroxylated carbon nanotubes.

[0009] As an optimization, the carboxylated carbon nanotubes include the following preparation steps: Multi-walled carbon nanotubes are added to a mixed acid solution at a mass ratio of 1:10-12, ultrasonically vibrated for 30-40 min, and refluxed at 110-120℃ for 3-4 h. After the reaction, ice water is added for dilution, and the mixture is allowed to cool to 25-35℃ before vacuum filtration. The filtrate is then washed with deionized water until the pH reaches 6.8-7.2, and the filtered product is vacuum dried. Subsequently, the vacuum-dried filtered product is added to a secondary mixed acid solution at a mass ratio of 1:4-5, ultrasonically dispersed for 20-30 min, and stirred at 65-75℃ for 2-3 h. After the reaction, the mixture is cooled to 25-35℃, diluted with deionized water, and vacuum filtered. The filtrate is washed with deionized water until the pH reaches 6.8-7.2, and the filtered product is vacuum dried to obtain carboxylated carbon nanotubes.

[0010] As an optimization, the mixed acid solution is a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1.

[0011] As an optimization, the secondary mixed acid solution is a mixture of concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 and a mass fraction of 30%.

[0012] A modified lithium manganese iron phosphate cathode material is prepared by any one of the preparation methods described above.

[0013] Compared with the prior art, the beneficial effects achieved by the present invention are: This application employs block-type amphiphilic grafted carbon nanotubes to modify lithium manganese iron phosphate cathode materials, effectively solving the key problems of weak interfacial bonding, poor dispersibility, and unstable conductive network of traditional carbon nanotubes and cathode materials. Due to its unique molecular structure, the amphiphilic modified carbon nanotubes can form chemical bonds and hydrogen bonds with the lithium manganese iron phosphate precursor through surface polar groups, achieving in-situ uniform nucleation and directional growth of the precursor on the carbon nanotube surface, inhibiting the aggregation of active particles. Furthermore, it can construct a three-dimensional conductive network penetrating the electrode, significantly improving the material's electronic conductivity and lithium-ion diffusion rate, while mitigating the volume effect during charge and discharge, enhancing electrode structural stability, and thus improving the rate performance, cycle life, and processing performance of the cathode material, meeting the high requirements of solid-state batteries for interfacial bonding and structural stability.

[0014] Amphiphilic carbon nanotubes were constructed by grafting polyvinylpyrrolidone (PVP) and acrylic acid into two segments, combining the synergistic advantages of both polymers and achieving precise performance control. The PVP segments exhibit excellent dispersibility, complexation, and lipophilicity, enabling efficient adsorption of Fe²⁺. + Mn² + Li + Plasma guides the uniform growth of precursors and improves the dispersibility of carbon nanotubes in organic systems, optimizing the processing performance of electrode slurries. The PAA segments formed by acrylic acid grafting are rich in carboxyl groups, possessing excellent hydrophilicity and reactivity. They can enhance the interfacial bonding force between carbon nanotubes and lithium manganese iron phosphate, accelerate the interfacial transport of lithium ions, and improve the dispersion stability of carbon nanotubes in aqueous systems, avoiding the failure of conductive networks caused by carbon nanotube aggregation. The synergistic effect of both optimizes the amphiphilicity and modification effect of carbon nanotubes, giving full play to their role in dispersion, conductivity, and structural support in cathode materials. Detailed Implementation

[0015] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0016] Example 1 S1. Multi-walled carbon nanotubes were added to a mixed acid solution at a mass ratio of 1:10, ultrasonically vibrated for 30 min, and refluxed at 110℃ for 3 h. After the reaction, ice water was added for dilution, and the mixture was allowed to cool to 25℃ before vacuum filtration. The product was then washed with deionized water until the pH of the filtrate was 6.8, and the filtered product was vacuum dried. Subsequently, the vacuum-dried filtered product was added to a secondary mixed acid solution at a mass ratio of 1:4, ultrasonically dispersed for 20 min, and stirred at 65℃ for 2 h. After the reaction, the mixture was cooled to 25℃, diluted with deionized water, and vacuum filtered. The product was washed with deionized water until the pH of the filtrate was 6.8, and the filtered product was vacuum dried to obtain carboxylated carbon nanotubes. The mixed acid solution was a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1; the secondary mixed acid solution was a mixture of concentrated sulfuric acid and hydrogen peroxide at a volume ratio of 4:1 and a 30% hydrogen peroxide concentration. S2. Weigh carboxylated carbon nanotubes, sulfoxide, and N,N-dimethylformamide according to a mass ratio of 1:120:0.02. Under nitrogen protection, add carboxylated carbon nanotubes to sulfoxide, then add N,N-dimethylformamide dropwise. Reflux at 60°C for 22 h. After the reaction, remove unreacted material by vacuum distillation, wash three times with anhydrous chloroform, and vacuum dry. Then add the dried product to anhydrous ethylene glycol at 80 times the mass of the dried product, disperse by ultrasonication, and react at 110°C under nitrogen protection for 45 h. After the reaction, cool to 25°C, dilute with anhydrous ethanol, filter, and wash three times alternately with anhydrous ethanol and distilled water. Vacuum dry to obtain hydroxylated carbon nanotubes. S3. Weigh hydroxylated carbon nanotubes, 4-dimethylaminopyridine, triethylamine, and 2-bromoisobutylamide bromide according to a mass ratio of 1:0.06:0.6:0.08. Add 2-bromoisobutylamide bromide to anhydrous chloroform at a mass ratio of 12 times that of 2-bromoisobutylamide bromide, stir to dissolve, and prepare a 2-bromoisobutylamide bromide solution for later use. Under nitrogen protection, add the weighed hydroxylated carbon nanotubes, 4-dimethylaminopyridine, and triethylamine to anhydrous chloroform at a mass ratio of 20 times that of hydroxylated carbon nanotubes, then add the 2-bromoisobutylamide bromide solution dropwise. Stir and react at -5℃ for 2 hours, then stir and react at 25℃ for 9 hours. After the reaction is completed, dilute with anhydrous chloroform, filter under vacuum, wash three times with anhydrous chloroform, and dry under vacuum to obtain modified carbon nanotubes. S4. Weigh modified carbon nanotubes, cuprous bromide, pentamethyldiethylenetriamine, and vinylpyrrolidone according to a mass ratio of 1:0.14:0.16:11. Under nitrogen protection, add the modified carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine to N,N-dimethylformamide at a mass ratio of 10 times that of the modified carbon nanotubes. Then add vinylpyrrolidone dropwise. Stir and react at 75°C for 48 hours. After the reaction is complete, dilute with anhydrous ethanol, filter under vacuum, wash three times with anhydrous ethanol, and dry under vacuum to obtain polyvinylpyrrolidone-grafted carbon nanotubes. S5. Under nitrogen protection, polyvinylpyrrolidone-grafted carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine were added to N,N-dimethylformamide at a mass of 10 times that of the polyvinylpyrrolidone-grafted carbon nanotubes. Acrylic acid was then added dropwise, and the mixture was stirred at 65°C for 36 hours. After the reaction was completed, the mixture was diluted with a 1:1 volume ratio of anhydrous ethanol and deionized water, vacuum filtered, and washed three times with the same mixture. The mixture was then vacuum dried to obtain block amphiphilic grafted carbon nanotubes. S6. Weigh out block amphiphilic grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid according to a mass ratio of 1:13:8:11:11.5. Under nitrogen protection, add the block amphiphilic grafted carbon nanotubes to deionized water at a mass of 200 times that of the block amphiphilic grafted carbon nanotubes, and disperse by ultrasonication for 20 min. Then add ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in sequence. Stir at 55℃ for 2 h. After the stirring reaction is completed, react at 200℃ for 10 h. After the reaction is completed, cool to 55℃, vacuum filter, wash three times with deionized water, and vacuum dry to obtain the precursor. S7. Weigh the precursor and glucose at a mass ratio of 1:0.2, mix the precursor and glucose, and ball mill at 600 rpm for 6 hours. Under an inert atmosphere, heat to 600℃ at 5℃ / min, hold for 4.5 hours and sinter. After cooling to 150℃, collect the sample to obtain the modified lithium manganese iron phosphate cathode material.

[0017] Example 2 S1. Multi-walled carbon nanotubes were added to a mixed acid solution at a mass ratio of 1:11, ultrasonically vibrated for 35 min, and refluxed at 115℃ for 3.5 h. After the reaction, ice water was added for dilution, and the mixture was allowed to cool to 30℃ before vacuum filtration. The product was then washed with deionized water until the pH of the filtrate was 7.0, and the filtered product was vacuum dried. Subsequently, the vacuum-dried filtered product was added to a secondary mixed acid solution at a mass ratio of 1:4.5, ultrasonically dispersed for 25 min, and stirred at 70℃ for 2.5 h. After the reaction, the mixture was cooled to 30℃, diluted with deionized water, and vacuum filtered. The product was washed with deionized water until the pH of the filtrate was 7.0, and the filtered product was vacuum dried to obtain carboxylated carbon nanotubes. The mixed acid solution was a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1; the secondary mixed acid solution was a mixture of concentrated sulfuric acid and hydrogen peroxide at a volume ratio of 4:1 and a 30% hydrogen peroxide concentration. S2. Weigh carboxylated carbon nanotubes, sulfoxide, and N,N-dimethylformamide according to a mass ratio of 1:125:0.025. Under nitrogen protection, add carboxylated carbon nanotubes to sulfoxide, then add N,N-dimethylformamide dropwise. Reflux at 65°C for 24 hours. After the reaction, remove unreacted material by vacuum distillation, wash four times with anhydrous chloroform, and vacuum dry. Then add the dried product to anhydrous ethylene glycol at 85 times the mass of the dried product, disperse ultrasonically, and react at 120°C under nitrogen protection for 47.5 hours. After the reaction, cool to 30°C, dilute with anhydrous ethanol, filter, and wash four times alternately with anhydrous ethanol and distilled water. Vacuum dry to obtain hydroxylated carbon nanotubes. S3. Weigh hydroxylated carbon nanotubes, 4-dimethylaminopyridine, triethylamine, and 2-bromoisobutylamide bromide according to a mass ratio of 1:0.065:0.7:0.09. Add 2-bromoisobutylamide bromide to anhydrous chloroform at a mass ratio of 13.5 times that of 2-bromoisobutylamide bromide, stir to dissolve, and prepare a 2-bromoisobutylamide bromide solution for later use. Under nitrogen protection, add the weighed hydroxylated carbon nanotubes, 4-dimethylaminopyridine, and triethylamine to anhydrous chloroform at a mass ratio of 21 times that of hydroxylated carbon nanotubes, then add the 2-bromoisobutylamide bromide solution dropwise. Stir and react at -2.5℃ for 2.5 h, then stir and react at 27.5℃ for 9.5 h. After the reaction is completed, dilute with anhydrous chloroform, filter under vacuum, wash four times with anhydrous chloroform, and dry under vacuum to obtain modified carbon nanotubes. S4. Weigh modified carbon nanotubes, cuprous bromide, pentamethyldiethylenetriamine, and vinylpyrrolidone according to a mass ratio of 1:0.145:0.17:11.5. Under nitrogen protection, add the modified carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine to N,N-dimethylformamide at a mass ratio of 11 times that of the modified carbon nanotubes. Then add vinylpyrrolidone dropwise. Stir and react at 80°C for 49 hours. After the reaction is complete, dilute with anhydrous ethanol, filter under vacuum, wash four times with anhydrous ethanol, and dry under vacuum to obtain polyvinylpyrrolidone-grafted carbon nanotubes. S5. Under nitrogen protection, polyvinylpyrrolidone-grafted carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine were added to N,N-dimethylformamide at a mass of 11 times that of the polyvinylpyrrolidone-grafted carbon nanotubes. Acrylic acid was then added dropwise, and the reaction was stirred at 67.5℃ for 37 h. After the reaction was completed, the mixture was diluted with a 1:1 volume ratio of anhydrous ethanol and deionized water, vacuum filtered, and washed three times with the same mixture. The mixture was then vacuum dried to obtain block amphiphilic grafted carbon nanotubes. S6. Weigh out block amphiphilic grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in a mass ratio of 1:13.5:8.25:11.5:11.75. Under nitrogen protection, add the block amphiphilic grafted carbon nanotubes to deionized water at a mass of 250 times the mass of the block amphiphilic grafted carbon nanotubes, and ultrasonically disperse for 25 min. Then add ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in sequence. Stir at 60℃ for 2.5 h. After the stirring reaction is completed, react at 210℃ for 11 h. After the reaction is completed, cool to 60℃, vacuum filter, wash 4 times with deionized water, and vacuum dry to obtain the precursor. S7. Weigh the precursor and glucose at a mass ratio of 1:0.25, mix the precursor and glucose, and ball mill at 700 rpm for 7 hours. Under an inert atmosphere, heat to 625℃ at 5℃ / min, hold for sintering for 5 hours, cool to 155℃ and collect the sample to obtain the modified lithium manganese iron phosphate cathode material.

[0018] Example 3 S1. Multi-walled carbon nanotubes were added to a mixed acid solution at a mass ratio of 1:12, ultrasonically vibrated for 40 min, and refluxed at 120℃ for 4 h. After the reaction, ice water was added for dilution, and the mixture was allowed to cool to 35℃ before vacuum filtration. The product was then washed with deionized water until the pH of the filtrate was 7.2, and the filtered product was vacuum dried. Subsequently, the vacuum-dried filtered product was added to a secondary mixed acid solution at a mass ratio of 1:5, ultrasonically dispersed for 30 min, and stirred at 75℃ for 3 h. After the reaction, the mixture was cooled to 35℃, diluted with deionized water, and vacuum filtered. The product was washed with deionized water until the pH of the filtrate was 7.2, and the filtered product was vacuum dried to obtain carboxylated carbon nanotubes. The mixed acid solution was a mixture of concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1; the secondary mixed acid solution was a mixture of concentrated sulfuric acid and hydrogen peroxide at a volume ratio of 4:1 and a 30% hydrogen peroxide concentration. S2. Weigh carboxylated carbon nanotubes, sulfoxide, and N,N-dimethylformamide according to a mass ratio of 1:130:0.03. Under nitrogen protection, add carboxylated carbon nanotubes to sulfoxide, then add N,N-dimethylformamide dropwise. Reflux at 70°C for 26 hours. After the reaction, remove unreacted material by vacuum distillation, wash five times with anhydrous chloroform, and vacuum dry. Then add the dried product to anhydrous ethylene glycol at 90 times the mass of the dried product, disperse by ultrasonication, and react at 130°C under nitrogen protection for 50 hours. After the reaction, cool to 35°C, dilute with anhydrous ethanol, filter, and wash five times alternately with anhydrous ethanol and distilled water. Vacuum dry to obtain hydroxylated carbon nanotubes. S3. Weigh hydroxylated carbon nanotubes, 4-dimethylaminopyridine, triethylamine, and 2-bromoisobutylamide bromide according to a mass ratio of 1:0.07:0.8:0.1. Add 2-bromoisobutylamide bromide to anhydrous chloroform at a mass ratio of 15 times that of 2-bromoisobutylamide bromide, stir to dissolve, and prepare a 2-bromoisobutylamide bromide solution for later use. Under nitrogen protection, add the weighed hydroxylated carbon nanotubes, 4-dimethylaminopyridine, and triethylamine to anhydrous chloroform at a mass ratio of 22 times that of hydroxylated carbon nanotubes, then add the 2-bromoisobutylamide bromide solution dropwise. Stir the reaction at 0℃ for 3 hours, then stir the reaction at 30℃ for 10 hours. After the reaction is completed, dilute with anhydrous chloroform, filter under vacuum, wash 5 times with anhydrous chloroform, and dry under vacuum to obtain modified carbon nanotubes. S4. Weigh modified carbon nanotubes, cuprous bromide, pentamethyldiethylenetriamine, and vinylpyrrolidone according to a mass ratio of 1:0.15:0.18:12. Under nitrogen protection, add the modified carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine to N,N-dimethylformamide at a mass ratio of 12 times that of the modified carbon nanotubes. Then add vinylpyrrolidone dropwise. Stir and react at 85°C for 50 hours. After the reaction is complete, dilute with anhydrous ethanol, filter under vacuum, wash five times with anhydrous ethanol, and dry under vacuum to obtain polyvinylpyrrolidone-grafted carbon nanotubes. S5. Under nitrogen protection, polyvinylpyrrolidone-grafted carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine were added to N,N-dimethylformamide at a mass of 12 times that of the polyvinylpyrrolidone-grafted carbon nanotubes. Acrylic acid was then added dropwise, and the reaction was stirred at 70°C for 38 hours. After the reaction was completed, the mixture was diluted with a 1:1 volume ratio of anhydrous ethanol and deionized water, vacuum filtered, and washed three times with the same mixture. The mixture was then vacuum dried to obtain block amphiphilic grafted carbon nanotubes. S6. Weigh out block amphiphilic grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid according to a mass ratio of 1:14:8.5:12:12. Under nitrogen protection, add the block amphiphilic grafted carbon nanotubes to deionized water at a mass of 300 times that of the block amphiphilic grafted carbon nanotubes, and disperse by ultrasonication for 30 min. Then add ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in sequence. Stir at 65℃ for 3 h. After the stirring reaction is completed, react at 220℃ for 12 h. After the reaction is completed, cool to 65℃, vacuum filter, wash 5 times with deionized water, and vacuum dry to obtain the precursor. S7. Weigh the precursor and glucose at a mass ratio of 1:0.3, mix the precursor and glucose, and ball mill at 800 rpm for 8 hours. Under an inert atmosphere, heat to 650℃ at 5℃ / min, hold for sintering for 5.5 hours, cool to 160℃ and collect the sample to obtain the modified lithium manganese iron phosphate cathode material.

[0019] Example 4 The difference from Example 2 lies only in step S6: Polyvinylpyrrolidone-grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid were weighed according to a mass ratio of 1:13.5:8.25:11.5:11.75. Under nitrogen protection, the polyvinylpyrrolidone-grafted carbon nanotubes were added to deionized water at a mass of 250 times the mass of the polyvinylpyrrolidone-grafted carbon nanotubes and ultrasonically dispersed for 25 minutes. Then, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid were added sequentially. The mixture was stirred at 60°C for 2.5 hours. After the reaction was completed, the mixture was reacted at 210°C for 11 hours. After the reaction was completed, the mixture was cooled to 60°C, vacuum filtered, washed four times with deionized water, and vacuum dried to obtain the precursor.

[0020] Example 5 The difference from Example 2 lies only in step S6: Modified carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are weighed according to a mass ratio of 1:13.5:8.25:11.5:11.75; under nitrogen protection, the modified carbon nanotubes are added to deionized water at a mass ratio of 250 times that of the modified carbon nanotubes, and ultrasonically dispersed for 25 min. Then, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are added sequentially. The mixture is stirred at 60°C for 2.5 h. After the stirring reaction is completed, the mixture is reacted at 210°C for 11 h. After the reaction is completed, the mixture is cooled to 60°C, vacuum filtered, washed four times with deionized water, and vacuum dried to obtain the precursor.

[0021] Example 6 The difference from Example 2 lies only in step S6: hydroxylated carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are weighed according to a mass ratio of 1:13.5:8.25:11.5:11.75; under nitrogen protection, hydroxylated carbon nanotubes are added to deionized water at a mass of 250 times the mass of the hydroxylated carbon nanotubes, and ultrasonically dispersed for 25 min; then ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are added sequentially; the mixture is stirred at 60°C for 2.5 h; after the stirring reaction is completed, the mixture is reacted at 210°C for 11 h; after the reaction is completed, the mixture is cooled to 60°C, vacuum filtered, washed four times with deionized water, and vacuum dried to obtain the precursor.

[0022] Example 7 The difference from Example 2 lies only in step S6: Carboxylated carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are weighed according to a mass ratio of 1:13.5:8.25:11.5:11.75; under nitrogen protection, carboxylated carbon nanotubes are added to deionized water at a mass of 250 times that of carboxylated carbon nanotubes, and ultrasonically dispersed for 25 min. Then, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are added sequentially. The mixture is stirred at 60°C for 2.5 h. After the stirring reaction is completed, the mixture is reacted at 210°C for 11 h. After the reaction is completed, the mixture is cooled to 60°C, vacuum filtered, washed 4 times with deionized water, and vacuum dried to obtain the precursor.

[0023] Example 8 The difference from Example 2 lies only in step S6: Multi-walled carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are weighed according to a mass ratio of 1:13.5:8.25:11.5:11.75; under nitrogen protection, multi-walled carbon nanotubes are added to deionized water at a mass of 250 times that of multi-walled carbon nanotubes, and ultrasonically dispersed for 25 min. Then, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid are added sequentially. The mixture is stirred at 60°C for 2.5 h. After the stirring reaction is completed, the mixture is reacted at 210°C for 11 h. After the reaction is completed, the mixture is cooled to 60°C, vacuum filtered, washed 4 times with deionized water, and vacuum dried to obtain the precursor.

[0024] Button battery assembly Modified lithium manganese iron phosphate cathode material, conductive carbon black, and PVDF were dispersed in NMP solvent at a mass ratio of 8:1:1 for 6 to 8 hours to form a uniform slurry. This slurry was then coated onto aluminum using a blade coating method and vacuum dried at 80°C for 10 hours. The dried electrode sheet was then rolled and sliced ​​to form small circular electrode sheets with a diameter of 12 mm and an active material areal density of 1.0 - 2.0 mg / cm². These sheets were then vacuum dried at 80°C for at least 12 hours. A 14 mm diameter lithium metal sheet was used as both the counter and reference electrodes. The electrolyte was 1 mol / L LiPF6 / EC + DMC + DEC (volume ratio 1:1:1). A Celgard 2500 separator was selected. Button batteries were assembled in a vacuum glove box with water and oxygen content below 0.1 ppm.

[0025] Electrochemical performance testing The batteries prepared in the above embodiments were subjected to charge-discharge performance tests. Constant current was applied at room temperature, and the rate performance test voltage range was 2.5V~4.5V. The charge-discharge rates were: 0.1C for 3 cycles, 0.2C, 0.5C, 1.0C, 2.0C, 5.0C, and 10.0C for 5 cycles each, and finally 0.1C for 5 cycles. Specific test results are shown in Table 1 below. Table 1 A comparison of the experimental data from Examples 1 to 3 in Table 1 reveals that the lithium manganese iron phosphate cathode material prepared by this invention exhibits excellent rate discharge performance and structural stability. It maintains a high discharge specific capacity over a wide rate range from 0.1C to 10.0C, and the capacity decays slowly with increasing rate. This indicates that the constructed block amphiphilic grafted carbon nanotubes can effectively improve the electron conduction and lithium-ion migration rate of the material, enabling the lithium manganese iron phosphate cathode material to possess good electrochemical performance. Furthermore, it can maintain a stable structure and interface even after continuous high-rate charge and discharge, demonstrating excellent cycle reliability.

[0026] The batteries prepared in the above embodiments were subjected to cycle stability tests, and were cycled 100 times at 5.0C and 10.0C at room temperature. The capacity retention rate was calculated. The specific test results are shown in Table 2 below. Table 2 In Example 4, the electrochemical performance of the electrode decreased to a certain extent compared to Examples 1-3. The main reason is speculated to be that the system only uses polyvinylpyrrolidone-grafted carbon nanotubes. Although this improves the dispersibility and system compatibility compared to unmodified carbon nanotubes, the lack of acrylic acid segments to provide abundant carboxyl sites makes it difficult to achieve efficient complexation and directional induction of metal ions. This results in defects in the uniformity and crystallinity of the final lithium manganese iron phosphate particles, thus affecting the rate performance. However, since it still has a polymer grafting structure, the overall structural stability of the material is still excellent, and the capacity decay after multiple cycles remains at a low level.

[0027] In Examples 5 and 6, the electrochemical performance declined more significantly. The main reason is that the modified carbon nanotubes and hydroxylated carbon nanotubes used only underwent surface functionalization treatment and did not construct an amphiphilic block polymer structure. On the one hand, their ability to adsorb and disperse metal ions is limited, which easily leads to local aggregation of precursors. On the other hand, the lack of steric hindrance and interfacial buffering effect of long-chain polymers makes carbon nanotubes prone to self-stacking and aggregation, and unable to form a continuous and stable three-dimensional conductive network. Ultimately, this leads to a significant decrease in the rate performance and cycle stability of the material.

[0028] In Examples 7 and 8, the electrochemical performance of the materials decreased most significantly when carboxylated carbon nanotubes and original multi-walled carbon nanotubes were used, respectively. This is because carboxylated carbon nanotubes only introduce polar groups and lack effective dispersion and induced growth capabilities; while original multi-walled carbon nanotubes have strong surface inertness, poor compatibility, and are prone to aggregation. This not only fails to improve the conductivity of lithium manganese iron phosphate but also exacerbates particle aggregation and grain boundary impedance, resulting in a significant capacity decay at high rates and a marked reduction in cycle retention. This further confirms the crucial role of block amphiphilic graft structures in improving the overall performance of cathode materials.

[0029] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for preparing a modified lithium manganese iron phosphate cathode material, characterized in that, The preparation steps include the following: S1. Under nitrogen protection, polyvinylpyrrolidone-grafted carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine were added to N,N-dimethylformamide at a mass ratio of 10 to 12 times that of the polyvinylpyrrolidone-grafted carbon nanotubes. Acrylic acid was then added dropwise, and the reaction was carried out at 65 to 70°C with stirring for 36 to 38 hours. After the reaction was completed, the mixture was diluted with a 1:1 volume ratio of anhydrous ethanol and deionized water, vacuum filtered, and washed three times with the same mixture. The mixture was then vacuum dried to obtain block amphiphilic grafted carbon nanotubes. S2. Weigh out block amphiphilic grafted carbon nanotubes, ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid according to a mass ratio of 1:13~14:8~8.5:11~12:11.5~12. Under nitrogen protection, add the block amphiphilic grafted carbon nanotubes to 200~300 times their mass of deionized water and ultrasonically disperse for 20~30 min. Then add ferrous sulfate heptahydrate, manganese sulfate monohydrate, lithium hydroxide monohydrate, and phosphoric acid in sequence. Stir at 55~65℃ for 2~3 h. After the reaction is complete, react at 200~220℃ for 10~12 h. After the reaction is complete, cool to 55~65℃, vacuum filter, wash with deionized water 3~5 times, and vacuum dry to obtain the precursor. S3. Weigh the precursor and glucose at a mass ratio of 1:0.2~0.3, mix the precursor and glucose, and ball mill at 600~800 rpm for 6~8 h; under an inert atmosphere, heat to 600~650℃ at 5℃ / min, hold for sintering for 4.5~5.5 h, cool to 150~160℃ and collect the sample to obtain the modified lithium manganese iron phosphate cathode material.

2. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation steps of the polyvinylpyrrolidone-grafted carbon nanotubes are as follows: Modified carbon nanotubes, cuprous bromide, pentamethyldiethylenetriamine, and vinylpyrrolidone are weighed according to a mass ratio of 1:0.14~0.15:0.16~0.18:11~12. Under nitrogen protection, the modified carbon nanotubes, cuprous bromide, and pentamethyldiethylenetriamine are added to N,N-dimethylformamide at a mass ratio of 10~12 times that of the modified carbon nanotubes. Then, vinylpyrrolidone is added dropwise. The reaction is stirred at 75~85℃ for 48~50 h. After the reaction is complete, anhydrous ethanol is added for dilution, the mixture is vacuum filtered, washed 3~5 times with anhydrous ethanol, and then vacuum dried to obtain polyvinylpyrrolidone-grafted carbon nanotubes.

3. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 2, characterized in that, The modified carbon nanotubes include the following preparation steps: Hydroxylated carbon nanotubes, 4-dimethylaminopyridine, triethylamine, and 2-bromoisobutylamide bromide are weighed according to a mass ratio of 1:0.06~0.07:0.6~0.8:0.08~0.

1. The 2-bromoisobutylamide bromide is added to anhydrous chloroform at a mass ratio of 12~15 times that of the 2-bromoisobutylamide bromide, and after stirring and dissolving, a 2-bromoisobutylamide bromide solution is prepared for later use. Under nitrogen protection, the weighed... Hydroxylated carbon nanotubes, 4-dimethylaminopyridine, and triethylamine were added to anhydrous chloroform at a concentration of 20-22 times the mass of the hydroxylated carbon nanotubes. Then, 2-bromoisobutylamide bromide solution was added dropwise. The mixture was stirred and reacted at -5 to 0°C for 2-3 hours, followed by stirring and reacting at 25 to 30°C for 9-10 hours. After the reaction was completed, the mixture was diluted with anhydrous chloroform, vacuum filtered, washed 3-5 times with anhydrous chloroform, and then vacuum dried to obtain modified carbon nanotubes.

4. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 3, characterized in that, The hydroxylated carbon nanotubes include the following preparation steps: weigh carboxylated carbon nanotubes, sulfoxide and N,N-dimethylformamide according to a mass ratio of 1:120~130:0.02~0.03; Under nitrogen protection, carboxylated carbon nanotubes were added to thionyl chloride, followed by the dropwise addition of N,N-dimethylformamide. The mixture was refluxed at 60–70 °C for 22–26 h. After the reaction, unreacted material was removed by vacuum distillation, and the mixture was washed 3–5 times with anhydrous chloroform and dried under vacuum. The dried product was then added to anhydrous ethylene glycol at 80–90 times its weight and ultrasonically dispersed. The mixture was reacted under nitrogen protection at 110–130 °C for 45–50 h. After the reaction, the mixture was cooled to 25–35 °C, diluted with anhydrous ethanol, filtered, and washed 3–5 times alternately with anhydrous ethanol and distilled water. The mixture was then dried under vacuum to obtain hydroxylated carbon nanotubes.

5. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 4, characterized in that, The carboxylated carbon nanotubes are prepared by the following steps: Multi-walled carbon nanotubes are added to a mixed acid solution at a mass ratio of 1:10-12, ultrasonically vibrated for 30-40 minutes, and refluxed at 110-120°C for 3-4 hours. After the reaction, ice water is added for dilution, and the mixture is allowed to cool to 25-35°C. The mixture is then vacuum filtered and washed with deionized water until the pH of the filtrate is 6.8-7.

2. The filtered product is then vacuum dried. Subsequently, the vacuum-dried filtered product is added to a secondary mixed acid solution at a mass ratio of 1:4-5, ultrasonically dispersed for 20-30 minutes, and stirred at 65-75°C for 2-3 hours. After the reaction, the mixture is cooled to 25-35°C, diluted with deionized water, and vacuum filtered. The mixture is then washed with deionized water until the pH of the filtrate is 6.8-7.

2. The filtered product is then vacuum dried to obtain carboxylated carbon nanotubes.

6. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that, The mixed acid solution is a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:

1.

7. The method for preparing the modified lithium manganese iron phosphate cathode material according to claim 5, characterized in that, The secondary mixed acid solution is a mixture of concentrated sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 and a mass fraction of 30%.

8. A modified lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

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