Carbon-coated lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the field of positive electrode materials, in particular to a carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, which are used for solving the core pain points that the lithium iron phosphate positive electrode material is low in electronic conductivity, slow in ion diffusion and easy to agglomerate particles in circulation. The carbon-coated lithium iron phosphate positive electrode material comprises a phosphorus-doped modified carbon nanotube, N-methyl pyrrolidone and lithium iron phosphate powder, the preparation method comprises the following steps: firstly, modifying a carbon nano tube by using a carbon nano tube modifier, then carrying out phosphorus doping to obtain a phosphorus-doped modified carbon nano tube, and then carrying out carbon coating treatment on lithium iron phosphate by using the phosphorus-doped modified carbon nano tube, the electron and ion transmission efficiency is remarkably improved, the high-rate discharge performance is optimized, the cycle stability is enhanced, and volume expansion and side reaction are inhibited.

Description

Technical Field

[0001] This invention relates to the field of cathode materials, specifically to a carbon-coated lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles, the requirements for lithium-ion batteries are also increasing. Therefore, exploring lithium-ion battery cathode materials that are both high-performance and low-cost is of great practical significance. Lithium iron phosphate (LiFePO4) has become a leader among lithium battery cathode materials due to its excellent safety, low cost, and low toxicity. However, its low electronic conductivity and lithium-ion diffusion coefficient limit further improvements in its high-rate performance and cycle life, affecting its application in a wider range of scenarios. Summary of the Invention

[0003] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a carbon-coated lithium iron phosphate cathode material and its preparation method.

[0004] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this application provides a carbon-coated lithium iron phosphate cathode material, comprising the following components by weight: The composition includes 0.2-0.3 parts of phosphorus-doped modified carbon nanotubes, 10-12 parts of N-methylpyrrolidone, and 10-15 parts of lithium iron phosphate powder; wherein the lithium iron phosphate powder is from Shenzhen Tianchenghe Technology Co., Ltd., and its model is positive electrode 001; the CAS number of the polyvinylidene fluoride is 24937-79-9. The phosphorus-doped modified carbon nanotubes are prepared by the following steps: Step a1: Add benzyl dichloroisocyanurate, triphenylphosphine and chloroform to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Reflux the mixture at 65-70℃ and a stirring rate of 300-400 r / min for 3-4 h. After the reaction is complete, cool the reaction product to room temperature, then remove the solvent by rotary evaporation, and finally recrystallize in xylene to obtain the first intermediate.

[0005] Step a2: Add the first intermediate, 2-nitrobenzaldehyde, and dichloromethane to a three-necked flask equipped with a stirrer, thermometer, and constant-pressure dropping funnel. Under the conditions of 30-35℃ and stirring rate of 300-400 r / min, add sodium hydroxide solution dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, raise the temperature to 40-45℃ and continue stirring for 24-26 h. After the reaction is completed, filter the reaction product, collect the filter cake, and then recrystallize it in anhydrous ethanol to obtain the second intermediate.

[0006] Step a3: Add tin powder, the second intermediate, and anhydrous ethanol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 0-5℃ and stirring rate of 300-400 r / min, add hydrochloric acid solution dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, raise the temperature to 25-30℃ and continue stirring for 24-26 h. After the reaction is complete, adjust the pH of the system to 7 with sodium bicarbonate aqueous solution, then extract with dichloromethane 3-4 times. Combine the organic phases and dry with anhydrous magnesium sulfate. Then elute with dichloromethane-petroleum ether solution to obtain carbon nanotube modifier.

[0007] Step a4: Add multi-walled carbon nanotubes and concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Reflux the mixture at 115-120℃ and a stirring rate of 300-400 r / min for 3-4 h. After the reaction, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 10-12 min, discard the supernatant, add deionized water to the centrifuge tube, and ultrasonically disperse the mixture at 300-400 W for 5-7 min. Centrifuge again and repeat the washing process 3-4 times until the pH of the supernatant is 6-8. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry them at 75-80℃ for 12-13 h to obtain carboxylated multi-walled carbon nanotubes. Step a5: Transfer carboxylated multi-walled carbon nanotubes and anhydrous N,N-dimethylformamide to a three-necked flask and ultrasonically disperse them for 30-35 min at a power of 300-400 W. Then, add carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide to the flask. Purge the three-necked flask with nitrogen, then turn on magnetic stirring. Heat the water bath to 55-60℃ and maintain the temperature for 23-24 h. After the reaction is complete, transfer the mixture to a centrifuge tube and centrifuge for 10-12 min. Collect the lower solid layer and wash it 3-4 times with deionized water. Finally, wash it once with anhydrous ethanol. Place the washed solid in a vacuum drying oven and dry it at 75-80℃ for 12-13 h to obtain modified carbon nanotubes. Step a6: Add the modified carbon nanotubes to an aqueous solution of phosphoric acid and ultrasonically disperse them for 30-35 minutes at a power of 300-400W. Then, turn on magnetic stirring for 2-3 hours. After that, place them in a vacuum drying oven and dry them at a temperature of 75-80℃ for 12-13 hours. Then, place them in the constant temperature zone of a quartz tube furnace and purge them with argon gas for 30 minutes. Then, raise the temperature to 700-800℃ at a rate of 5℃ / min and hold it for 2-3 hours. After that, let them cool naturally to room temperature and collect the black powder. Add the product to deionized water and ultrasonically wash it for 10-12 minutes. Then, centrifuge it for 10-12 minutes, discard the supernatant, and wash it with deionized water 3-4 times. Then, place the washed solid in a vacuum drying oven and dry it at a temperature of 75-80℃ for 12-13 hours to obtain phosphorus-doped modified carbon nanotubes.

[0008] In a preferred embodiment of the present invention, the ratio of the amount of dichlorobenzyl, triphenylphosphine and chloroform solution used in step a1 is 1.75-1.85g: 5.25-5.75g: 50-60mL.

[0009] In a preferred embodiment of the present invention, the ratio of the first intermediate, 2-nitrobenzaldehyde, dichloromethane solution and sodium hydroxide solution in step a2 is 1.40-1.50g: 0.91-0.96g: 15-17mL: 1.0-1.1mL.

[0010] In a preferred embodiment of the present invention, the sodium hydroxide solution in step a2 has a mass fraction of 50%.

[0011] In a preferred embodiment of the present invention, the ratio of tin powder, second intermediate, anhydrous ethanol and hydrochloric acid solution in step a3 is 1.20-1.30g: 1.86-1.98g: 20-25mL: 2.0-2.5mL.

[0012] In a preferred embodiment of the present invention, the hydrochloric acid solution in step a3 has a mass fraction of 36%; the dichloromethane-petroleum ether solution is a solution composed of dichloromethane and petroleum ether mixed in a volume ratio of 5:1.

[0013] In a preferred embodiment of the present invention, the ratio of the multi-walled carbon nanotubes to concentrated nitric acid in step a4 is 0.1-0.2g: 20-25mL.

[0014] In a preferred embodiment of the present invention, the concentration of the concentrated nitric acid in step a4 is 68%; the diameter of the multi-walled carbon nanotubes is 10-20 nm, and the aspect ratio is 50-80.

[0015] In a preferred embodiment of the present invention, the ratio of carboxylated multi-walled carbon nanotubes, anhydrous N,N-dimethylformamide, carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide in step a5 is 0.05-0.07g: 10-12mL: 0.01-0.03g: 0.02-0.04g: 0.015-0.02g.

[0016] In a preferred embodiment of the present invention, the ratio of the modified carbon nanotubes, phosphoric acid aqueous solution and deionized water in step a6 is 0.1-0.2g: 10-12mL: 10-12mL.

[0017] In a preferred embodiment of the present invention, the mass fraction of the phosphoric acid aqueous solution in step a6 is 85%.

[0018] Secondly, this application provides a method for preparing carbon-coated lithium iron phosphate cathode material, comprising the following steps: Phosphorus-doped modified carbon nanotubes were added to N-methylpyrrolidone and ultrasonically dispersed for 30-35 min to obtain a uniform carbon nanotube dispersion. Lithium iron phosphate powder was added to the above carbon nanotube dispersion and magnetically stirred for 30-35 min. The mixture was then transferred to a planetary ball mill, and ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added. The mixture was ball-milled for 2 h. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a 55°C water bath. Afterward, it was placed in a vacuum drying oven and dried at 75-80°C for 12-14 h. Then, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 h. Then, it was heated to 500°C at a heating rate of 5°C / min and held for 3 h. Afterward, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0019] The beneficial effects of this invention are: This invention discloses a carbon-coated lithium iron phosphate cathode material and its preparation method. First, carbon nanotubes are modified with a carbon nanotube modifier, then phosphorus-doped to obtain phosphorus-doped modified carbon nanotubes. Finally, the phosphorus-doped modified carbon nanotubes are used to carbon-coat lithium iron phosphate. This preparation method solves the core problems of lithium iron phosphate cathode materials, such as low electronic conductivity, slow ion diffusion, and easy particle aggregation during cycling. It significantly improves electron and ion transport efficiency, optimizes high-rate discharge performance, enhances cycle stability, and suppresses volume expansion and side reactions.

[0020] Lithium iron phosphate itself is a typical "insulator," while the coating layer synergistically enhances transport efficiency in two ways: the first is as an electron channel, where the hollow tubular structure of multi-walled carbon nanotubes forms a continuous conductive network, and the conjugated π bonds of the carbon nanotube modifier further bridge the interface between the carbon nanotubes and lithium iron phosphate particles, avoiding "conductive islands"; the second is as an ion channel, where phosphorus doping introduces defective active sites into the carbon framework, reducing the lithium ion migration barrier, while the polar groups of phosphorus atoms and carbon nanotube modifiers can interact weakly with lithium ions, assisting ion transport.

[0021] Lithium iron phosphate (LFP) is prone to slight volume expansion during long-term charge-discharge cycles due to lithium-ion insertion / deintercalation, and its surface is susceptible to side reactions with the electrolyte, leading to the loss of active materials. The coating layer can form a "physical-chemical dual protection." The phosphorus-doped modified carbon nanotube coating layer adheres tightly to the surface of LFP particles, limiting particle aggregation and breakage during cycling and alleviating electrode cracking caused by volume expansion. The amino groups of the carbon nanotube modifier can form hydrogen bonds with the hydroxyl groups on the surface of LFP, enhancing the interfacial bonding between the coating layer and the active material and preventing the coating layer from falling off. At the same time, the carbon-based coating layer can reduce the direct contact between LFP and the electrolyte, inhibiting the excessive growth and damage of the SEI film.

[0022] The reaction principle of carbon nanotube modifiers: The first step is the synthesis of the first intermediate. The lone pair electrons of triphenylphosphine attack the methylene carbon of the chloromethyl group in p-dichlorobenzyl along the C-Cl bond in the opposite direction, forming a five-coordinate transition state. The transition state then rapidly decomposes, the C-Cl bond is completely broken, and Cl... - As a leaving group, it detaches, while the PC bond is fully formed, the phosphorus atom becomes positively charged, and a monosubstituted quaternary phosphonium salt is generated. The remaining chloromethyl group repeats the above process. Another triphenylphosphine molecule nucleophilically attacks the methylene carbon of this chloromethyl group. After passing through the Sn2 transition state, Cl... - The process begins with the formation of a disubstituted quaternary phosphonium salt. The second step involves the synthesis of a second intermediate, where the quaternary phosphonium salt undergoes dehydrogenation with NaOH to generate a phosphorus ylide. The ylide then nucleophilically adds to the carbonyl group of o-nitrobenzaldehyde, eliminating Ph3PO and triggering a Wittig reaction to form a conjugated double bond, introducing a dinitro group. The third step involves the synthesis of a carbon nanotube modifier. In the Sn / HCl system, the nitro group is gradually reduced to an amino group, and through nitroso and hydroxylamine intermediates, Sn provides electrons, and H... + Participating in protonation. Attached Figure Description

[0023] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0024] Figure 1 This is a schematic diagram showing the capacity retention test results of carbon-coated lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention.

[0025] Figure 2 This is a schematic diagram showing the discharge specific capacity test results of carbon-coated lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0026] 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.

[0027] Example 1: This embodiment describes a method for preparing carbon-coated lithium iron phosphate cathode material, including the following steps: Step s1: 1.75 g of p-dichlorobenzyl, 5.25 g of triphenylphosphine and 50 mL of chloroform were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was refluxed at 65 °C and a stirring rate of 300 r / min for 3 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the solvent was removed by rotary evaporation. The product was then recrystallized in xylene to obtain the first intermediate. Step s2: 1.40 g of the first intermediate, 0.91 g of 2-nitrobenzaldehyde and 15 mL of dichloromethane were added to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Under the conditions of 30 °C and stirring rate of 300 r / min, 1.0 mL of sodium hydroxide solution was added dropwise while stirring, and the dropping rate was controlled at 1 drop / s. After the addition was completed, the temperature was raised to 40 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was filtered, the filter cake was collected, and then recrystallized in anhydrous ethanol to obtain the second intermediate. Step s3: 1.20 g tin powder, 1.86 g second intermediate and 20 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. Under the conditions of 0 °C and stirring rate of 300 r / min, 2.0 mL hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 25 °C and the reaction was stirred for 24 h. After the reaction was completed, the pH of the system was adjusted to 7 with sodium bicarbonate aqueous solution. Then, it was extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate. Then, it was eluted with dichloromethane-petroleum ether solution to obtain carbon nanotube modifier. Step s4: Add 0.1g of multi-walled carbon nanotubes and 20mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Reflux the mixture at 115℃ and a stirring rate of 300r / min for 3h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 10min, discard the supernatant, add deionized water to the centrifuge tube, sonicate for 5min at 300W, centrifuge again, and repeat the washing process 3 times until the pH of the supernatant is 6. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry them at 75℃ for 12h to obtain carboxylated multi-walled carbon nanotubes. Step s5: Transfer 0.05g of carboxylated multi-walled carbon nanotubes and 10mL of anhydrous N,N-dimethylformamide to a three-necked flask and sonicate for 30min at 300W. Then add 0.01g of carbon nanotube modifier, 0.02g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and 0.015g of N-hydroxysuccinimide to the flask. Purge the three-necked flask with nitrogen, then turn on magnetic stirring. Heat the mixture in a constant temperature water bath to 55℃ and maintain the temperature for 23h. After the reaction is complete, transfer the mixture to a centrifuge tube and centrifuge for 10min. Collect the lower solid layer. Wash the solid layer three times with deionized water and once with anhydrous ethanol. Place the washed solid in a vacuum drying oven and dry it at 75℃ for 12h to obtain modified carbon nanotubes. Step s6: Add 0.1g of modified carbon nanotubes to 10mL of phosphoric acid aqueous solution, and ultrasonically disperse for 30min at a power of 300W. Then turn on magnetic stirring for 2h, and place it in a vacuum drying oven and dry at 75℃ for 12h. Then place it in the constant temperature zone of a quartz tube furnace and purge with argon gas for 30min. Then raise the temperature to 700℃ at a rate of 5℃ / min, hold for 2h, and then cool naturally to room temperature. Collect the black powder, add 10mL of deionized water to the product, ultrasonically wash for 10min, centrifuge for 10min, discard the supernatant, and wash with deionized water 3 times. Then place the washed solid in a vacuum drying oven and dry at 75℃ for 12h to obtain phosphorus-doped modified carbon nanotubes. Step s7: Add 0.2 parts of phosphorus-doped modified carbon nanotubes to 10 parts of N-methylpyrrolidone, and ultrasonically disperse for 30 min at a power of 300W to obtain a uniform carbon nanotube dispersion. Add 10 parts of lithium iron phosphate powder (lithium iron phosphate powder from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 30 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 250 r / min. Under the condition of n, the slurry was ball-milled for 2 hours. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a water bath at 55°C. Then it was placed in a vacuum drying oven and dried at 75°C for 12 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0028] Example 2: This embodiment describes a method for preparing carbon-coated lithium iron phosphate cathode material, including the following steps: Step s1: 1.80 g of p-dichlorobenzyl, 5.50 g of triphenylphosphine and 55 mL of chloroform were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was refluxed at 67 °C and a stirring rate of 350 r / min for 3 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the solvent was removed by rotary evaporation. The product was then recrystallized in xylene to obtain the first intermediate. Step s2: 1.45g of the first intermediate, 0.93g of 2-nitrobenzaldehyde and 16mL of dichloromethane were added to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Under the conditions of 33℃ and stirring rate of 350r / min, 1.05mL of sodium hydroxide solution was added dropwise while stirring, and the dropping rate was controlled at 1 drop / s. After the addition was completed, the temperature was raised to 43℃ and the reaction was stirred for 25h. After the reaction was completed, the reaction product was filtered, the filter cake was collected, and then recrystallized in anhydrous ethanol to obtain the second intermediate. Step s3: 1.25g tin powder, 1.90g second intermediate and 23mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. Under the conditions of 3℃ and stirring rate of 350r / min, 2.2mL hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 27℃ and the reaction was stirred for 25h. After the reaction was completed, the pH of the system was adjusted to 7 with sodium bicarbonate aqueous solution. Then, the system was extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate. Then, the system was eluted with dichloromethane-petroleum ether solution to obtain carbon nanotube modifier. Step s4: Add 0.15g of multi-walled carbon nanotubes and 23mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Reflux the mixture at 117℃ and a stirring rate of 350r / min for 3h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 11min, discard the supernatant, add deionized water to the centrifuge tube, sonicate at 350W for 6min, centrifuge again, repeat the washing process 3 times until the pH of the supernatant is 7. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry at 77℃ for 12h to obtain carboxylated multi-walled carbon nanotubes. Step s5: 0.06 g of carboxylated multi-walled carbon nanotubes and 11 mL of anhydrous N,N-dimethylformamide were transferred to a three-necked flask and ultrasonically dispersed for 33 min at 350 W. Then, 0.02 g of carbon nanotube modifier, 0.03 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and 0.018 g of N-hydroxysuccinimide were added to the flask. Nitrogen gas was introduced into the three-necked flask, and then magnetic stirring was turned on. The temperature was raised to 57 °C in a constant temperature water bath and the reaction was maintained for 23 h. After the reaction was completed, the mixture was transferred to a centrifuge tube and centrifuged for 11 min. The lower solid layer was collected. Then, it was washed three times with deionized water and finally washed once with anhydrous ethanol. The washed solid was then placed in a vacuum drying oven and dried at 77 °C for 12 h to obtain modified carbon nanotubes. Step s6: Add 0.15g of modified carbon nanotubes to 11mL of phosphoric acid aqueous solution, and ultrasonically disperse for 33min at a power of 350W. Then, turn on magnetic stirring for 2.5h. After that, place it in a vacuum drying oven and dry at 77℃ for 12h. Then, place it in the constant temperature zone of a quartz tube furnace and purge with argon gas for 30min. Then, raise the temperature to 750℃ at a rate of 5℃ / min, hold for 2.5h, and then cool naturally to room temperature. Collect the black powder, add 11mL of deionized water to the product, ultrasonically wash for 11min, centrifuge for 11min, discard the supernatant, and wash three times with deionized water. Then, place the washed solid in a vacuum drying oven and dry at 77℃ for 12h to obtain phosphorus-doped modified carbon nanotubes. Step s7: Add 0.25 parts of phosphorus-doped modified carbon nanotubes to 11 parts of N-methylpyrrolidone, and ultrasonically disperse for 32 min at a power of 350W to obtain a uniform carbon nanotube dispersion. Add 13 parts of lithium iron phosphate powder (lithium iron phosphate powder from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 32 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 270 r / min. Ball milling for 2 hours under the specified conditions. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a 55°C water bath. Then, it was placed in a vacuum drying oven and dried at 77°C for 13 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then, it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0029] Example 3: This embodiment describes a method for preparing carbon-coated lithium iron phosphate cathode material, including the following steps: Step s1: 1.85 g of p-dichlorobenzyl, 5.75 g of triphenylphosphine and 60 mL of chloroform were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was refluxed at 70 °C and a stirring rate of 400 r / min for 4 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the solvent was removed by rotary evaporation. The product was then recrystallized in xylene to obtain the first intermediate. Step s2: 1.50 g of the first intermediate, 0.96 g of 2-nitrobenzaldehyde, and 17 mL of dichloromethane were added to a three-necked flask equipped with a stirrer, thermometer, and constant pressure dropping funnel. Under the conditions of 35 °C and a stirring rate of 400 r / min, 1.1 mL of sodium hydroxide solution was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was completed, the temperature was raised to 45 °C and the reaction was continued for 26 h. After the reaction was completed, the reaction product was filtered, the filter cake was collected, and then recrystallized in anhydrous ethanol to obtain the second intermediate. Step s3: 1.30 g tin powder, 1.98 g second intermediate and 25 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. Under the conditions of 5 °C and stirring rate of 400 r / min, 2.5 mL hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 2 drops / s. After the addition was completed, the temperature was raised to 30 °C and the reaction was stirred for 26 h. After the reaction was completed, the pH of the system was adjusted to 7 with sodium bicarbonate aqueous solution. Then, it was extracted 4 times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate. Then, it was eluted with dichloromethane-petroleum ether solution to obtain carbon nanotube modifier. Step s4: Add 0.2g of multi-walled carbon nanotubes and 25mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Reflux the mixture at 120℃ and a stirring rate of 400r / min for 4h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 12min, discard the supernatant, add deionized water to the centrifuge tube, sonicate at 400W for 7min, centrifuge again, repeat the washing process 4 times until the pH of the supernatant is 8. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry at 80℃ for 13h to obtain carboxylated multi-walled carbon nanotubes. Step s5: Transfer 0.07g of carboxylated multi-walled carbon nanotubes and 12mL of anhydrous N,N-dimethylformamide to a three-necked flask and sonicate for 35min at 400W. Then add 0.03g of carbon nanotube modifier, 0.04g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and 0.02g of N-hydroxysuccinimide to the flask. Purge the three-necked flask with nitrogen, then turn on magnetic stirring. Heat the mixture in a constant temperature water bath to 60℃ and maintain the temperature for 24h. After the reaction is complete, transfer the mixture to a centrifuge tube and centrifuge for 12min. Collect the lower solid layer. Wash the solid layer four times with deionized water and once with anhydrous ethanol. Place the washed solid in a vacuum drying oven and dry it at 80℃ for 13h to obtain modified carbon nanotubes. Step s6: Add 0.2g of modified carbon nanotubes to 12mL of phosphoric acid aqueous solution, and ultrasonically disperse for 35min at a power of 400W. Then turn on magnetic stirring for 3h, and place it in a vacuum drying oven and dry at 80℃ for 13h. Then place it in the constant temperature zone of a quartz tube furnace and purge with argon gas for 30min. Then raise the temperature to 800℃ at a rate of 5℃ / min, hold for 3h, and then cool naturally to room temperature. Collect the black powder, add 12mL of deionized water to the product, ultrasonically wash for 12min, centrifuge for 12min, discard the supernatant, and wash with deionized water 4 times. Then place the washed solid in a vacuum drying oven and dry at 80℃ for 13h to obtain phosphorus-doped modified carbon nanotubes. Step s7: Add 0.3 parts of phosphorus-doped modified carbon nanotubes to 12 parts of N-methylpyrrolidone, and ultrasonically disperse for 35 min at a power of 400W to obtain a uniform carbon nanotube dispersion. Add 15 parts of lithium iron phosphate powder (lithium iron phosphate powder from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 35 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 300 r / min. Ball milling for 2 hours under n conditions. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a 55°C water bath. Then it was placed in a vacuum drying oven and dried at 80°C for 14 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0030] Comparative Example 1: This comparative example illustrates a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps: Step s1: 1.75 g of p-dichlorobenzyl, 5.25 g of triphenylphosphine and 50 mL of chloroform were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was refluxed at 65 °C and a stirring rate of 300 r / min for 3 h. After the reaction was completed, the reaction product was cooled to room temperature, and then the solvent was removed by rotary evaporation. The product was then recrystallized in xylene to obtain the first intermediate. Step s2: 1.40 g of the first intermediate, 0.91 g of 2-nitrobenzaldehyde and 15 mL of dichloromethane were added to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Under the conditions of 30 °C and stirring rate of 300 r / min, 1.0 mL of sodium hydroxide solution was added dropwise while stirring, and the dropping rate was controlled at 1 drop / s. After the addition was completed, the temperature was raised to 40 °C and the reaction was stirred for 24 h. After the reaction was completed, the reaction product was filtered, the filter cake was collected, and then recrystallized in anhydrous ethanol to obtain the second intermediate. Step s3: 1.20 g tin powder, 1.86 g second intermediate and 20 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel. Nitrogen gas was introduced for protection. Under the conditions of 0 °C and stirring rate of 300 r / min, 2.0 mL hydrochloric acid solution was added dropwise while stirring, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 25 °C and the reaction was stirred for 24 h. After the reaction was completed, the pH of the system was adjusted to 7 with sodium bicarbonate aqueous solution. Then, it was extracted three times with dichloromethane. The organic phases were combined and dried with anhydrous magnesium sulfate. Then, it was eluted with dichloromethane-petroleum ether solution to obtain carbon nanotube modifier. Step s4: Add 0.1g of multi-walled carbon nanotubes and 20mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Reflux the mixture at 115℃ and a stirring rate of 300r / min for 3h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 10min, discard the supernatant, add deionized water to the centrifuge tube, sonicate for 5min at 300W, centrifuge again, and repeat the washing process 3 times until the pH of the supernatant is 6. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry them at 75℃ for 12h to obtain carboxylated multi-walled carbon nanotubes. Step s5: Transfer 0.05g of carboxylated multi-walled carbon nanotubes and 10mL of anhydrous N,N-dimethylformamide to a three-necked flask and sonicate for 30min at 300W. Then add 0.01g of carbon nanotube modifier, 0.02g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and 0.015g of N-hydroxysuccinimide to the flask. Purge the three-necked flask with nitrogen, then turn on magnetic stirring. Heat the mixture in a constant temperature water bath to 55℃ and maintain the temperature for 23h. After the reaction is complete, transfer the mixture to a centrifuge tube and centrifuge for 10min. Collect the lower solid layer. Wash the solid layer three times with deionized water and once with anhydrous ethanol. Place the washed solid in a vacuum drying oven and dry it at 75℃ for 12h to obtain modified carbon nanotubes. Step s6: Add 0.2 parts of phosphorus-doped modified carbon nanotubes to 10 parts of N-methylpyrrolidone, and ultrasonically disperse for 30 min at a power of 300W to obtain a uniform carbon nanotube dispersion. Add 10 parts of lithium iron phosphate powder (lithium iron phosphate powder is from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 30 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 250 r / min. Under the condition of n, the slurry was ball-milled for 2 hours. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a water bath at 55°C. Then it was placed in a vacuum drying oven and dried at 75°C for 12 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0031] Comparative Example 2: This comparative example illustrates a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps: Step s1: Add 0.1g of multi-walled carbon nanotubes and 20mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Reflux the mixture at 115℃ and a stirring rate of 300r / min for 3h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 10min, discard the supernatant, add deionized water to the centrifuge tube, sonicate for 5min at 300W, centrifuge again, repeat the washing process 3 times until the pH of the supernatant is 6. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry at 75℃ for 12h to obtain carboxylated multi-walled carbon nanotubes. Step s2: 0.1g of carboxylated multi-walled carbon nanotubes were added to 10mL of phosphoric acid aqueous solution and ultrasonically dispersed for 30min at a power of 300W. Magnetic stirring was then turned on for 2h. After that, the mixture was placed in a vacuum drying oven and dried at 75℃ for 12h. Then, it was placed in the isothermal zone of a quartz tube furnace and argon gas was introduced for 30min. The temperature was then increased to 700℃ at a rate of 5℃ / min and held for 2h. After that, it was naturally cooled to room temperature. The black powder was collected, and the product was added to 10mL of deionized water and ultrasonically washed for 10min. After centrifugation for 10min, the supernatant was discarded, and the mixture was washed three times with deionized water. The washed solid was then placed in a vacuum drying oven and dried at 75℃ for 12h to obtain phosphorus-doped carbon nanotubes. Step s3: Add 0.2 parts of phosphorus-doped modified carbon nanotubes to 10 parts of N-methylpyrrolidone, and ultrasonically disperse for 30 min at a power of 300W to obtain a uniform carbon nanotube dispersion. Add 10 parts of lithium iron phosphate powder (lithium iron phosphate powder from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 30 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 250 r / min. Under the condition of n, the slurry was ball-milled for 2 hours. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a water bath at 55°C. Then it was placed in a vacuum drying oven and dried at 75°C for 12 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0032] Comparative Example 3: This comparative example illustrates a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps: Step s1: Add 0.1g of multi-walled carbon nanotubes and 20mL of concentrated nitric acid to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. Reflux the mixture at 115℃ and a stirring rate of 300r / min for 3h. After the reaction is complete, cool the product to room temperature, transfer the product to a centrifuge tube, centrifuge for 10min, discard the supernatant, add deionized water to the centrifuge tube, sonicate for 5min at 300W, centrifuge again, repeat the washing process 3 times until the pH of the supernatant is 6. Then place the washed multi-walled carbon nanotubes in a vacuum drying oven and dry at 75℃ for 12h to obtain carboxylated multi-walled carbon nanotubes. Step s2: Add 0.2 parts of carboxylated multi-walled carbon nanotubes to 10 parts of N-methylpyrrolidone, and ultrasonically disperse for 30 min at a power of 300W to obtain a uniform carbon nanotube dispersion. Add 10 parts of lithium iron phosphate powder (lithium iron phosphate powder from Shenzhen Tianchenghe Technology Co., Ltd., model 001) to the above carbon nanotube dispersion, and magnetically stir for 30 min. Transfer to a planetary ball mill, add ZrO2 grinding balls with a ball-to-material ratio of 10:1, and mill at a speed of 250 r / min. Under the condition of n, the slurry was ball-milled for 2 hours. After ball milling, the slurry was transferred to an evaporating dish and stirred and evaporated in a water bath at 55°C. Then it was placed in a vacuum drying oven and dried at 75°C for 12 hours. After that, it was placed in a tube furnace and heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 2 hours. Then it was heated to 500°C at a heating rate of 5°C / min and held for 3 hours. After that, it was naturally cooled to room temperature and ground through a 300-mesh sieve to obtain carbon-coated lithium iron phosphate cathode material.

[0033] Preparation of the positive electrode of the battery: 79g of carbon-coated lithium iron phosphate cathode material, 9g of conductive carbon black, 9g of polyvinylidene fluoride, and 160mL of N-methylpyrrolidone were added to a beaker and stirred at 300r / min for 3h using a magnetic stirrer to prepare a slurry. The slurry was coated onto aluminum foil and then transferred to a vacuum drying oven and dried at 60℃ for 4h. The electrode sheet was then rolled using a roller press with the rolling pressure controlled at 8MPa. After rolling, the electrode sheet was cut into 10mm diameter cathode discs to obtain the cathode.

[0034] Battery manufacturing: Using lithium foil as the negative electrode and Celgard 2400 polypropylene microporous membrane as the separator, the electrolyte was 1 mol / L LiPF6 (EC:DEC=1:1, v / v). The positive electrode, negative electrode, separator and electrolyte were assembled in an argon glove box with water and oxygen content of less than 1 ppm. After assembly, the battery was left to stand for 24 hours to obtain the battery.

[0035] Performance testing: The carbon-coated lithium iron phosphate cathode materials and their preparation methods from Examples 1-3 and Comparative Examples 1-3 were tested for capacity retention after 500 cycles at 1C and 10C charge-discharge rates. The test results are as follows: Figure 1 As shown; the discharge specific capacity at 0.2C and 5C rates was tested, and the test results are as follows. Figure 2 As shown.

[0036] See Figure 1-2As shown, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that modifying lithium iron phosphate and doping with P can significantly improve electron and ion transport efficiency, optimize high-rate discharge performance, and enhance cycle stability.

[0037] Based on the comparison between Example 1 and Comparative Example 1, it can be seen that lithium iron phosphate doped with P and modified has better performance than lithium iron phosphate without P and only modified. Based on the comparison between Example 1 and Comparative Example 2, it can be seen that lithium iron phosphate doped with P and modified has better performance than lithium iron phosphate doped with P only without modification. Based on the comparison between Example 1 and Comparative Example 3, it can be seen that the P-doped and modified lithium iron phosphate performs better than the undoped and unmodified lithium iron phosphate.

[0038] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0039] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A carbon-coated lithium iron phosphate cathode material, characterized in that, Includes the following components by weight: 0.2-0.3 parts of phosphorus-doped modified carbon nanotubes, 10-12 parts of N-methylpyrrolidone, and 10-15 parts of lithium iron phosphate powder; The phosphorus-doped modified carbon nanotubes are prepared by the following steps: Step a1: Reflux dichlorobenzyl, triphenylphosphine and chloroform. After the reaction is complete, the reaction product is cooled, then rotary evaporated, and then recrystallized to obtain the first intermediate. Step a2: Add the first intermediate, 2-nitrobenzaldehyde and dichloromethane to a three-necked flask, add sodium hydroxide solution, filter after the reaction is complete, collect the filter cake, and then recrystallize to obtain the second intermediate; Step a3: Add tin powder, the second intermediate and anhydrous ethanol to a three-necked flask, add hydrochloric acid solution, continue stirring the reaction, adjust the pH value to 7, then extract, combine the organic phases and dry, then elute to obtain carbon nanotube modifier. Step a4: Add multi-walled carbon nanotubes and concentrated nitric acid to a three-necked flask and reflux. After the reaction is complete, cool the mixture, centrifuge the product, discard the supernatant, disperse it by sonication, centrifuge again, and then dry it to obtain carboxylated multi-walled carbon nanotubes. Step a5: Carboxylated multi-walled carbon nanotubes and anhydrous N,N-dimethylformamide were transferred to a three-necked flask and ultrasonically dispersed. Then, carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and N-hydroxysuccinimide were added. The reaction was kept at a constant temperature, and the lower solid was collected by centrifugation. After washing and drying, modified carbon nanotubes were obtained. Step a6: Add the modified carbon nanotubes to the phosphoric acid aqueous solution, perform ultrasonic dispersion, turn on magnetic stirring, then dry, then place in the constant temperature zone of a quartz tube furnace, calcine and cool, collect the black powder, perform ultrasonic washing, then centrifuge, wash with deionized water, and then dry to obtain phosphorus-doped modified carbon nanotubes.

2. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The ratio of the amount of dichlorobenzyl, triphenylphosphine and chloroform solution used in step a1 is 1.75-1.85g: 5.25-5.75g: 50-60mL.

3. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, In step a2, the ratio of the first intermediate, 2-nitrobenzaldehyde, dichloromethane solution, and sodium hydroxide solution is 1.40-1.50 g : 0.91-0.96 g : 15-17 mL : 1.0-1.1 mL.

4. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The ratio of tin powder, second intermediate, anhydrous ethanol and hydrochloric acid solution used in step a3 is 1.20-1.30g: 1.86-1.98g: 20-25mL: 2.0-2.5mL.

5. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, In step a4, the ratio of multi-walled carbon nanotubes to concentrated nitric acid is 0.1-0.2g: 20-25mL.

6. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The ratio of carboxylated multi-walled carbon nanotubes, anhydrous N,N-dimethylformamide, carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and N-hydroxysuccinimide in step a5 is 0.05-0.07 g : 10-12 mL : 0.01-0.03 g : 0.02-0.04 g : 0.015-0.02 g.

7. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The ratio of the modified carbon nanotubes, phosphoric acid aqueous solution, and deionized water used in step a6 is 0.1-0.2g: 10-12mL: 10-12mL.

8. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The sodium hydroxide solution in step a2 has a mass fraction of 50%; the hydrochloric acid solution in step a3 has a mass fraction of 36%; the dichloromethane-petroleum ether solution is a solution composed of dichloromethane and petroleum ether mixed in a volume ratio of 5:

1.

9. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The concentration of the concentrated nitric acid in step a4 is 68%; the diameter of the multi-walled carbon nanotubes in step a4 is 10-20 nm and the aspect ratio is 50-80; the mass fraction of the phosphoric acid aqueous solution in step a6 is 85%.

10. A method for preparing a carbon-coated lithium iron phosphate cathode material, characterized in that, The preparation of the carbon-coated lithium iron phosphate cathode material as described in any one of claims 1-9 includes the following steps: Phosphorus-doped modified carbon nanotubes were added to N-methylpyrrolidone and ultrasonically dispersed to obtain a uniform carbon nanotube dispersion. Lithium iron phosphate powder was added to the above carbon nanotube dispersion, magnetically stirred, and transferred to a planetary ball mill for ball milling. After ball milling, the slurry was transferred to an evaporating dish, stirred and evaporated, dried, and then placed in a tube furnace for sintering, cooling, grinding and sieving to obtain carbon-coated lithium iron phosphate cathode material.

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