Carbon-coated lithium iron phosphate anode material and preparation method thereof

Abstract

The application 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 of low electronic conductivity, slow ion diffusion and easy particle aggregation in the cycle of the lithium iron phosphate positive electrode material; the carbon-coated lithium iron phosphate positive electrode material comprises phosphorus-doped modified carbon nanotubes, N-methyl pyrrolidone and lithium iron phosphate powder; the preparation method is that the carbon nanotubes are modified by using a carbon nanotube modifier first, then phosphorus doping is carried out to obtain the phosphorus-doped modified carbon nanotubes, and then the lithium iron phosphate is subjected to carbon coating treatment by using the phosphorus-doped modified carbon nanotubes; the electronic and ion transmission efficiency is remarkably improved, the high-rate discharge performance is optimized, the cycle stability is strengthened, and the volume expansion and side reactions are inhibited.

Description

TECHNICAL FIELD

[0001] The application 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. BACKGROUND

[0002] With the rapid development of new energy vehicles, the requirements for lithium ion batteries are also increasing, therefore, it is of important practical significance to explore lithium ion battery positive electrode materials which are excellent in performance and low in cost. Lithium iron phosphate (LiFePO4) has become a top-notch positive electrode material for lithium batteries due to its excellent safety, low cost and low toxicity. However, the electronic conductivity and lithium ion diffusion coefficient of the material are low, which limits the further improvement of the high-rate performance and cycle life of the material and affects the application of the material in more extensive scenarios. SUMMARY

[0003] In order to overcome the above technical problems, the purpose of the application is to provide a carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof.

[0004] The purpose of the application can be achieved by the following technical solutions.

[0005] In a first aspect, the application provides a carbon-coated lithium iron phosphate positive electrode material, which comprises the following components by mass fraction:

[0006] 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 a positive electrode 001 from Shenzhen Tiancheng and Technology Co., Ltd.; and the CAS of the polyvinylidene fluoride is 24937-79-9.

[0007] The phosphorus-doped modified carbon nanotubes are prepared by the following steps:

[0008] Step a1: p-dichlorobenzene, triphenylphosphine and chloroform are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 65-70 DEG C and a stirring rate of 300-400 r / min for 3-4 h; after the reaction is completed, the reaction product is cooled to room temperature, and then the solvent is removed by rotary evaporation, followed by recrystallization in xylene to obtain a first intermediate;

[0009]

[0010] Step a2: the first intermediate, 2-nitrobenzaldehyde and dichloromethane were added to a three-necked flask equipped with a stirrer, a thermometer and a constant pressure dropping funnel, while stirring at a temperature of 30-35℃ and a stirring rate of 300-400 r / min, sodium hydroxide solution was added dropwise, the dropping rate was controlled at 1-2 drops / s, after the addition was completed, the temperature was raised to 40-45℃, and the stirring reaction was continued for 24-26 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;

[0011]

[0012] Step a3: tin powder, the second intermediate and anhydrous ethanol were added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a constant pressure dropping funnel, and protected by nitrogen, while stirring at a temperature of 0-5℃ and a stirring rate of 300-400 r / min, hydrochloric acid solution was added dropwise, the dropping rate was controlled at 1-2 drops / s, after the addition was completed, the temperature was raised to 25-30℃, and the stirring reaction was continued for 24-26 h, after the reaction was completed, the pH value of the system was adjusted to 7 with sodium bicarbonate aqueous solution, then dichloromethane was used for extraction for 3-4 times, the organic phase was combined and dried with anhydrous magnesium sulfate, then eluted with dichloromethane-petroleum ether solution to obtain the carbon nanotube modifier;

[0013]

[0014] Step a4: multi-walled carbon nanotubes and concentrated nitric acid were added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 115-120℃ and a stirring rate of 300-400 r / min for 3-4 h, after the reaction was completed, the reaction product was cooled to room temperature, transferred to a centrifuge tube, centrifuged for 10-12 min, the supernatant was discarded, then deionized water was added to the centrifuge tube, ultrasonic dispersion was carried out under a power of 300-400 W for 5-7 min, and then centrifuged again, the washing was repeated for 3-4 times until the pH value of the supernatant was 6-8, then the washed multi-walled carbon nanotubes were placed in a vacuum drying oven and dried at a temperature of 75-80℃ for 12-13 h to obtain carboxylated multi-walled carbon nanotubes;

[0015] Step a5: transfer the carboxylated multi-walled carbon nanotubes, anhydrous N,N-dimethylformamide into a three-necked flask, ultrasonic dispersion under the condition of power of 300-400W for 30-35min, then add the carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide into the flask; introduce nitrogen into the three-necked flask, then start magnetic stirring, heat the constant temperature water bath to 55-60℃, and keep the temperature for 23-24h; after the reaction is completed, transfer the mixture into a centrifuge tube, centrifuge for 10-12min, collect the lower solid, then wash with deionized water for 3-4 times, finally wash with anhydrous ethanol once, then place the washed solid in a vacuum drying oven, dry at a temperature of 75-80℃ for 12-13h, and obtain the modified carbon nanotubes;

[0016] Step a6: add the modified carbon nanotubes into the phosphoric acid aqueous solution, ultrasonic dispersion under the condition of power of 300-400W for 30-35min, start magnetic stirring for 2-3h, then place in a vacuum drying oven, dry at a temperature of 75-80℃ for 12-13h, then place in the constant temperature zone of a quartz tube furnace, introduce argon for 30min, then heat to 700-800℃ at a rate of 5℃ / min, keep the temperature for 2-3h, and then naturally cool to room temperature; collect the black powder, add the product into deionized water, ultrasonic washing for 10-12min, then centrifuge for 10-12min, discard the supernatant, wash with deionized water for 3-4 times, then place the washed solid in a vacuum drying oven, dry at a temperature of 75-80℃ for 12-13h, and obtain the phosphorus-doped modified carbon nanotubes.

[0017] As a preferred embodiment of the present application, the use amount ratio of the p-dichlorobenzene, triphenylphosphine and chloroform solution in step a1 is 1.75-1.85g:5.25-5.75g:50-60mL.

[0018] As a preferred embodiment of the present application, the use amount 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.

[0019] As a preferred embodiment of the present application, the mass fraction of the sodium hydroxide solution in step a2 is 50%.

[0020] As a preferred embodiment of the present application, the use amount ratio of the 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.

[0021] As a preferred embodiment of the present application, the mass fraction of the hydrochloric acid solution in step a3 is 36%; the dichloromethane-petroleum ether solution is a solution prepared by mixing dichloromethane and petroleum ether in a volume ratio of 5:1.

[0022] As a preferred embodiment of the present application, the amount ratio of the multi-walled carbon nanotube and concentrated nitric acid in step a4 is 0.1-0.2g:20-25mL.

[0023] As a preferred embodiment of the present application, the concentration of the concentrated nitric acid in step a4 is 68%; the diameter of the multi-walled carbon nanotube is 10-20nm, and the aspect ratio is 50-80.

[0024] As a preferred embodiment of the present application, the amount ratio of the carboxylated multi-walled carbon nanotube, 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.

[0025] As a preferred embodiment of the present application, the amount ratio of the modified carbon nanotube, aqueous phosphoric acid solution and deionized water in step a6 is 0.1-0.2g:10-12mL:10-12mL.

[0026] As a preferred embodiment of the present application, the mass fraction of the aqueous phosphoric acid solution in step a6 is 85%.

[0027] In a second aspect, the present application provides a preparation method of a carbon-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0028] The phosphorus-doped modified carbon nanotube is added into N-methyl pyrrolidone and ultrasonically dispersed for 30-35min to obtain a uniform carbon nanotube dispersion liquid. Lithium iron phosphate powder is added into the above carbon nanotube dispersion liquid and magnetically stirred for 30-35min. The slurry is transferred to a planetary ball mill, ZrO2 grinding balls with a ball-to-material ratio of 10:1 are added, and ball milling is performed for 2h. After the ball milling is completed, the slurry is transferred to an evaporation dish, evaporated in a 55℃ water bath with stirring, and then placed in a vacuum drying oven for drying at a temperature of 75-80℃ for 12-14h. Then, the slurry is placed in a tube furnace, heated to 300℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, kept for 2h, heated to 500℃ at a heating rate of 5℃ / min, kept for 3h, and then naturally cooled to room temperature. The slurry is ground through a 300-mesh sieve to obtain the carbon-coated lithium iron phosphate positive electrode material.

[0029] The present application has the following advantages:

[0030] The carbon-coated lithium iron phosphate positive electrode material and the preparation method thereof, the carbon nanotube is modified by using a carbon nanotube modifier, then phosphorus doping is performed to obtain phosphorus-doped modified carbon nanotubes, and then the lithium iron phosphate is subjected to carbon coating treatment by using the phosphorus-doped modified carbon nanotubes. The preparation method solves the core pain points of the lithium iron phosphate positive electrode material, such as low electronic conductivity, slow ion diffusion, and easy particle aggregation in the cycle, significantly improves the electronic and ion transmission efficiency, optimizes the high-rate discharge performance, strengthens the cycle stability, and inhibits the volume expansion and side reactions.

[0031] The lithium iron phosphate is a typical "insulator", and the coating layer improves the transmission efficiency in two ways. The first way is an electronic channel. The hollow tubular structure of the multi-walled carbon nanotube forms a continuous conductive network, and the conjugated pi bond of the carbon nanotube modifier further bridges the interface between the carbon nanotube and the lithium iron phosphate particles to avoid "conductive islands". The second way is an ion channel. The phosphorus doping introduces defect active sites in the carbon skeleton to reduce the lithium ion migration energy barrier. Meanwhile, the polar groups of the phosphorus atom and the carbon nanotube modifier can interact with lithium ions to assist ion transmission.

[0032] The lithium iron phosphate is prone to micro-volume expansion due to lithium ion intercalation / deintercalation in long-term charging and discharging, and the surface is prone to side reactions with the electrolyte, resulting in loss of active material. The coating layer can form "physical-chemical double protection". The modified carbon nanotube coating layer doped with phosphorus is tightly attached to the surface of the lithium iron phosphate particles, limiting the aggregation and fragmentation of the particles in the cycle and relieving the cracking of the pole piece caused by volume expansion. The amino group of the carbon nanotube modifier can form a hydrogen bond with the hydroxyl group on the surface of the lithium iron phosphate, enhancing the interfacial bonding force 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 the lithium iron phosphate and the electrolyte and inhibit the excessive growth and damage of the SEI film.

[0033] The reaction principle of the carbon nanotube modifier is as follows. In the first step, the first intermediate is synthesized. The lone pair of electrons of triphenylphosphine attacks the methylene carbon of one chloromethyl in p-dichlorobenzene in the opposite direction of the C-Cl bond to form a five-coordinate transition state. Then the transition state rapidly decomposes, the C-Cl bond is completely broken, and Cl - serves as a leaving group, and at the same time, the P-C bond is completely formed. The phosphorus atom is positively charged, generating a monosubstituted quaternary phosphonium salt, and the remaining other chloromethyl repeats the above process. Another molecule of triphenylphosphine attacks the methylene carbon of the chloromethyl, and after the Sn2 transition state, Cl -The leaving group is finally generated into a double-substituted quaternary phosphonium salt; the second step is the synthesis of a second intermediate, the quaternary phosphonium salt is dehydrogenated by NaOH to generate a phosphorus ylide, the ylide is nucleophilically added to the carbonyl group of o-nitrobenzaldehyde, the Ph3PO is eliminated, the Wittig reaction is formed to form a conjugated double bond, and the double nitro is introduced; the third step is the synthesis of the carbon nanotube modifier, in the Sn / HCl system, the nitro group is gradually reduced to an amino group, and the Sn provides an electron, H + participate in protonation. BRIEF DESCRIPTION OF DRAWINGS

[0034] For the convenience of those skilled in the art to understand, the present application will be further described below in conjunction with the drawings.

[0035] Figure 1 The capacity retention rate test results of the carbon-coated lithium iron phosphate positive electrode material of the present application examples 1-3 and comparative examples 1-3 are shown in the figure.

[0036] Figure 2 The discharge specific capacity test results of the carbon-coated lithium iron phosphate positive electrode material of the present application examples 1-3 and comparative examples 1-3 are shown in the figure. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0038] Example 1:

[0039] The present embodiment is a preparation method of a carbon-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0040] Step s1: 1.75g of p-dichlorobenzyl, 5.25g of triphenylphosphine and 50mL of chloroform are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 65℃ and a stirring rate of 300r / min for 3h. After the reaction is completed, the reaction product is cooled to room temperature, and then the solvent is removed by rotary evaporation, and then recrystallized in xylene to obtain a first intermediate;

[0041] 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, a thermometer and a constant-pressure dropping funnel, 1.0 mL of sodium hydroxide solution was added dropwise under the condition of a temperature of 30°C and a stirring rate of 300 r / min, the dropping rate was controlled to be 1 drop / s, after the addition was completed, the temperature was increased to 40°C, and the reaction was continuously 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;

[0042] Step s3: 1.20 g of tin powder, 1.86 g of the second intermediate and 20 mL of anhydrous ethanol were added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a constant-pressure dropping funnel, and protected by nitrogen, 2.0 mL of hydrochloric acid solution was added dropwise under the condition of a temperature of 0°C and a stirring rate of 300 r / min, the dropping rate was controlled to be 1 drop / s, after the addition was completed, the temperature was increased to 25°C, and the reaction was continuously stirred for 24 h, after the reaction was completed, the pH value of the system was adjusted to 7 with a sodium bicarbonate aqueous solution, then the organic phase was extracted with dichloromethane for 3 times, the combined organic phase was dried with anhydrous magnesium sulfate, and then eluted with a dichloromethane-petroleum ether solution to obtain a carbon nanotube modifier;

[0043] Step s4: 0.1 g of multi-walled carbon nanotubes and 20 mL of concentrated nitric acid were added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and the reaction was refluxed under the condition of a temperature of 115°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, transferred to a centrifuge tube, centrifuged for 10 min, and the supernatant was discarded, then deionized water was added to the centrifuge tube, ultrasonic dispersion was performed under the condition of a power of 300 W for 5 min, and then centrifugation was performed again, the washing was repeated for 3 times until the pH value of the supernatant was 6, then the washed multi-walled carbon nanotubes were placed in a vacuum drying oven and dried at a temperature of 75°C for 12 h to obtain carboxylated multi-walled carbon nanotubes;

[0044] Step s5: 0.05 g of carboxylated multi-walled carbon nanotubes and 10 mL of anhydrous N,N-dimethylformamide were transferred to a three-necked flask, and ultrasonic dispersion was performed at a power of 300 W for 30 min. Then, 0.01 g of a carbon nanotube modifier, 0.02 g of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and 0.015 g of N-hydroxysuccinimide were added to the flask. Nitrogen was introduced into the three-necked flask, and then magnetic stirring was started. The temperature of the constant-temperature water bath was raised to 55°C, and the reaction was kept at this temperature for 23 h. After the reaction was completed, the mixture was transferred to a centrifuge tube, and centrifugation was performed for 10 min. The lower solid was collected. Then, the solid was washed with deionized water three times and with anhydrous ethanol once. Finally, the washed solid was placed in a vacuum drying oven, and drying was performed at a temperature of 75°C for 12 h to obtain modified carbon nanotubes.

[0045] Step s6: 0.1 g of the modified carbon nanotubes was added to 10 mL of an aqueous phosphoric acid solution, and ultrasonic dispersion was performed at a power of 300 W for 30 min. Magnetic stirring was started for 2 h, and then the mixture was placed in a vacuum drying oven and dried at a temperature of 75°C for 12 h. Then, the mixture was placed in the constant-temperature zone of a quartz tube furnace, and argon was introduced for 30 min. The temperature was raised to 700°C at a rate of 5°C / min, and the mixture was kept at this temperature for 2 h. After natural cooling to room temperature, a black powder was collected. The product was added to 10 mL of deionized water, and ultrasonic washing was performed for 10 min. Centrifugation was performed for 10 min, and the supernatant was discarded. The solid was washed with deionized water three times. Finally, the washed solid was placed in a vacuum drying oven and dried at a temperature of 75°C for 12 h to obtain phosphorus-doped modified carbon nanotubes.

[0046] Step s7: 0.2 parts of the phosphorus-doped modified carbon nanotubes were added to 10 parts of N-methylpyrrolidone, and ultrasonic dispersion was performed at a power of 300 W for 30 min to obtain a uniform carbon nanotube dispersion. Then, 10 parts of lithium iron phosphate powder (lithium iron phosphate powder, model positive electrode 001, Shenzhen Tianchenghe Technology Co., Ltd.) was added to the carbon nanotube dispersion, and magnetic stirring was performed for 30 min. The mixture was transferred to a planetary ball mill, and ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added. Ball milling was performed at a rotation speed of 250 r / min for 2 h. After the ball milling was completed, the slurry was transferred to an evaporation dish, and evaporation was performed in a 55°C water bath with stirring. Then, the mixture was placed in a vacuum drying oven and dried at a temperature of 75°C for 12 h. Finally, the mixture was placed in a tube furnace, and the temperature was raised to 300°C at a rate of 2°C / min under a nitrogen atmosphere. The mixture was kept at this temperature for 2 h. Then, the temperature was raised to 500°C at a rate of 5°C / min, and the mixture was kept at this temperature for 3 h. After natural cooling to room temperature, the mixture was ground through a 300-mesh sieve to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0047] Example 2:

[0048] The embodiment is a preparation method of a carbon-coated lithium iron phosphate positive electrode material, including the following steps.

[0049] Step s1: 1.80g of p-dichlorobenzyl, 5.50g of triphenylphosphine and 55mL of chloroform are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 67℃ and a stirring speed of 350r / min for 3h; after the reaction is completed, the reaction product is cooled to room temperature, and then solvent is removed by rotary evaporation, followed by recrystallization in xylene to obtain a first intermediate;

[0050] Step s2: 1.45g of the first intermediate, 0.93g of 2-nitrobenzaldehyde and 16mL of dichloromethane are added to a three-necked flask equipped with a stirrer, a thermometer and a constant-pressure dropping funnel, and 1.05mL of sodium hydroxide solution is added dropwise under stirring at a temperature of 33℃ and a stirring speed of 350r / min, with the dropping speed controlled at 1 drop / s; after the addition is completed, the temperature is raised to 43℃, and the reaction is continued under stirring for 25h; after the reaction is completed, the reaction product is filtered, and the filter cake is collected and recrystallized in anhydrous ethanol to obtain a second intermediate;

[0051] Step s3: 1.25g of tin powder, 1.90g of the second intermediate and 23mL of anhydrous ethanol are added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a constant-pressure dropping funnel, and 2.2mL of hydrochloric acid solution is added dropwise under stirring at a temperature of 3℃ and a stirring speed of 350r / min, with the dropping speed controlled at 1 drop / s; after the addition is completed, the temperature is raised to 27℃, and the reaction is continued under stirring for 25h; after the reaction is completed, the pH value of the system is adjusted to 7 with a sodium bicarbonate aqueous solution, and then the organic phase is extracted with dichloromethane for 3 times, dried with anhydrous magnesium sulfate, and eluted with a dichloromethane-petroleum ether solution to obtain a carbon nanotube modifier;

[0052] Step s4: 0.15g of multi-walled carbon nanotubes and 23mL of concentrated nitric acid are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 117℃ and a stirring speed of 350r / min for 3h; after the reaction is completed, the reaction product is cooled to room temperature, transferred to a centrifuge tube, centrifuged for 11min, and the supernatant is discarded; then deionized water is added to the centrifuge tube, ultrasonically dispersed for 6min at a power of 350W, and centrifuged again; the washing is repeated for 3 times until the pH value of the supernatant is 7; then the washed multi-walled carbon nanotubes are placed in a vacuum drying oven and dried at a temperature of 77℃ for 12h to obtain carboxylated multi-walled carbon nanotubes;

[0053] 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 ultrasonic dispersion was performed at a power of 350 W for 33 min. Then, 0.02 g of a 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 was introduced into the three-necked flask, and then magnetic stirring was started. The temperature of the constant-temperature water bath was raised to 57 ℃, and the reaction was kept at this temperature for 23 h. After the reaction was completed, the mixture was transferred to a centrifuge tube, and centrifugation was performed for 11 min. The lower solid was collected. Then, the solid was washed with deionized water three times and with anhydrous ethanol once. Finally, the washed solid was placed in a vacuum drying oven, and drying was performed at a temperature of 77 ℃ for 12 h to obtain modified carbon nanotubes.

[0054] Step s6: 0.15 g of the modified carbon nanotubes was added to 11 mL of an aqueous phosphoric acid solution, and ultrasonic dispersion was performed at a power of 350 W for 33 min. Magnetic stirring was started for 2.5 h, and then the mixture was placed in a vacuum drying oven and dried at a temperature of 77 ℃ for 12 h. Then, the mixture was placed in the constant-temperature zone of a quartz tube furnace, and argon was introduced for 30 min. The temperature was raised to 750 ℃ at a rate of 5 ℃ / min, and the mixture was kept at this temperature for 2.5 h. After natural cooling to room temperature, a black powder was collected. The product was added to 11 mL of deionized water, and ultrasonic washing was performed for 11 min. Then, centrifugation was performed for 11 min, and the supernatant was discarded. The solid was washed with deionized water three times. Finally, the washed solid was placed in a vacuum drying oven and dried at a temperature of 77 ℃ for 12 h to obtain phosphorus-doped modified carbon nanotubes.

[0055] Step s7: 0.25 parts of the phosphorus-doped modified carbon nanotubes were added to 11 parts of N-methylpyrrolidone, and ultrasonic dispersion was performed at a power of 350 W for 32 min to obtain a uniform carbon nanotube dispersion. Then, 13 parts of lithium iron phosphate powder (lithium iron phosphate powder, model positive electrode 001, Shenzhen Tianchenghe Technology Co., Ltd.) was added to the carbon nanotube dispersion, and magnetic stirring was performed for 32 min. The mixture was transferred to a planetary ball mill, and ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added. Ball milling was performed at a rotation speed of 270 r / min for 2 h. After the ball milling was completed, the slurry was transferred to an evaporation dish, and evaporation was performed in a 55 ℃ water bath with stirring. Then, the mixture was placed in a vacuum drying oven and dried at a temperature of 77 ℃ for 13 h. After that, the mixture was placed in a tube furnace and heated to 300 ℃ at a rate of 2 ℃ / min under a nitrogen atmosphere. The mixture was kept at this temperature for 2 h. Then, the temperature was raised to 500 ℃ at a rate of 5 ℃ / min, and the mixture was kept at this temperature for 3 h. After natural cooling to room temperature, the mixture was ground through a 300-mesh sieve to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0056] Example 3:

[0057] The embodiment is a preparation method of a carbon-coated lithium iron phosphate positive electrode material, including the following steps.

[0058] Step s1: 1.85g of p-dichlorobenzyl, 5.75g of triphenylphosphine and 60mL of chloroform are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 70℃ and a stirring speed of 400r / min for 4h; after the reaction is completed, the reaction product is cooled to room temperature, and then solvent is removed by rotary evaporation, followed by recrystallization in xylene to obtain a first intermediate;

[0059] Step s2: 1.50g of the first intermediate, 0.96g of 2-nitrobenzaldehyde and 17mL of dichloromethane are added to a three-necked flask equipped with a stirrer, a thermometer and a constant-pressure dropping funnel, and 1.1mL of sodium hydroxide solution is added dropwise under stirring at a temperature of 35℃ and a stirring speed of 400r / min, with the dropping speed controlled at 2 drops / s; after the addition is completed, the temperature is raised to 45℃, and the reaction is continued under stirring for 26h; after the reaction is completed, the reaction product is filtered, and the filter cake is collected and recrystallized in anhydrous ethanol to obtain a second intermediate;

[0060] Step s3: 1.30g of tin powder, 1.98g of the second intermediate and 25mL of anhydrous ethanol are added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube and a constant-pressure dropping funnel, and 2.5mL of hydrochloric acid solution is added dropwise under stirring at a temperature of 5℃ and a stirring speed of 400r / min, with the dropping speed controlled at 2 drops / s; after the addition is completed, the temperature is raised to 30℃, and the reaction is continued under stirring for 26h; after the reaction is completed, the pH value of the system is adjusted to 7 with a sodium bicarbonate aqueous solution, and then the organic phase is extracted with dichloromethane four times, dried with anhydrous magnesium sulfate, and eluted with a dichloromethane-petroleum ether solution to obtain a carbon nanotube modifier;

[0061] Step s4: 0.2g of multi-walled carbon nanotubes and 25mL of concentrated nitric acid are added to a three-necked flask equipped with a stirrer, a thermometer and a reflux condenser, and refluxed at a temperature of 120℃ and a stirring speed of 400r / min for 4h; after the reaction is completed, the reaction product is cooled to room temperature, transferred to a centrifuge tube, centrifuged for 12min, and the supernatant is discarded; then deionized water is added to the centrifuge tube, ultrasonically dispersed for 7min at a power of 400W, and then centrifuged again; the washing is repeated four times until the pH value of the supernatant is 8; then the washed multi-walled carbon nanotubes are placed in a vacuum drying oven and dried at a temperature of 80℃ for 13h to obtain carboxylated multi-walled carbon nanotubes;

[0062] Step s5: 0.07 g of carboxylated multi-walled carbon nanotubes, 12 mL of anhydrous N,N-dimethylformamide were transferred to a three-necked flask, and ultrasonic dispersion was performed for 35 min at a power of 400 W. Then, 0.03 g of a carbon nanotube modifier, 0.04 g of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and 0.02 g of N-hydroxysuccinimide were added to the flask. Nitrogen was introduced into the three-necked flask, and then magnetic stirring was started. The temperature of the constant-temperature water bath was raised to 60°C, and the reaction was maintained for 24 h. After the reaction was completed, the mixture was transferred to a centrifuge tube, and centrifugation was performed for 12 min. The lower solid was collected. Then, the solid was washed with deionized water four times and with anhydrous ethanol once. The washed solid was then placed in a vacuum drying oven, and drying was performed at a temperature of 80°C for 13 h to obtain modified carbon nanotubes.

[0063] Step s6: 0.2 g of the modified carbon nanotubes was added to 12 mL of an aqueous phosphoric acid solution, and ultrasonic dispersion was performed for 35 min at a power of 400 W. Magnetic stirring was started for 3 h, and then the mixture was placed in a vacuum drying oven and dried at a temperature of 80°C for 13 h. The mixture was then placed in the constant-temperature zone of a quartz tube furnace, and argon was introduced for 30 min. The temperature was then raised to 800°C at a rate of 5°C / min, and the mixture was maintained at this temperature for 3 h. The mixture was then naturally cooled to room temperature, and a black powder was collected. The product was added to 12 mL of deionized water, and ultrasonic washing was performed for 12 min. Centrifugation was then performed for 12 min, and the supernatant was discarded. The solid was then washed with deionized water four times. The washed solid was then placed in a vacuum drying oven and dried at a temperature of 80°C for 13 h to obtain phosphorus-doped modified carbon nanotubes.

[0064] Step s7: 0.3 parts of the phosphorus-doped modified carbon nanotubes were added to 12 parts of N-methylpyrrolidone, and ultrasonic dispersion was performed for 35 min at a power of 400 W to obtain a uniform carbon nanotube dispersion. 15 parts of lithium iron phosphate powder (lithium iron phosphate powder, model positive electrode 001, Shenzhen Tianchenghe Technology Co., Ltd.) was added to the carbon nanotube dispersion, and magnetic stirring was performed for 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. Ball milling was performed at a rotation speed of 300 r / min for 2 h. After the ball milling was completed, the slurry was transferred to an evaporation dish, and evaporation was performed in a 55°C water bath with stirring. The mixture was then placed in a vacuum drying oven and dried at a temperature of 80°C for 14 h. The mixture was then placed in a tube furnace, and the temperature was raised to 300°C at a rate of 2°C / min under a nitrogen atmosphere. The mixture was maintained at this temperature for 2 h. The temperature was then raised to 500°C at a rate of 5°C / min, and the mixture was maintained at this temperature for 3 h. The mixture was then naturally cooled to room temperature, ground through a 300-mesh sieve, and a carbon-coated lithium iron phosphate positive electrode material was obtained.

[0065] Comparative Example 1

[0066] The present comparative example is a preparation method of a carbon-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0067] Step s1: 1.75 g of p-dichlorobenzene, 5.25 g of triphenylphosphine, and 50 mL of chloroform were added to a three-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, and refluxed at a temperature of 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, followed by recrystallization in xylene to obtain a first intermediate;

[0068] 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, a thermometer, and a constant-pressure dropping funnel, and 1.0 mL of a sodium hydroxide solution was added dropwise while stirring at a temperature of 30°C and a stirring rate of 300 r / min, with the dropping rate controlled at 1 drop / s. After the addition was completed, the temperature was raised to 40°C, and the reaction was continued for 24 h while stirring. After the reaction was completed, the reaction product was filtered, and the filter cake was collected and recrystallized in anhydrous ethanol to obtain a second intermediate;

[0069] Step s3: 1.20 g of tin powder, 1.86 g of the second intermediate, and 20 mL of anhydrous ethanol were added to a three-necked flask equipped with a stirrer, a thermometer, a gas inlet tube, and a constant-pressure dropping funnel, and 2.0 mL of a hydrochloric acid solution was added dropwise while stirring under nitrogen protection at a temperature of 0°C and a stirring rate of 300 r / min, 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 continued for 24 h while stirring. After the reaction was completed, the pH value of the system was adjusted to 7 with a sodium bicarbonate aqueous solution, and then the organic phase was extracted with dichloromethane three times, dried with anhydrous magnesium sulfate, and eluted with a dichloromethane-petroleum ether solution to obtain a carbon nanotube modifier;

[0070] Step s4: 0.1 g of multi-walled carbon nanotubes and 20 mL of concentrated nitric acid were added to a three-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, and refluxed at a temperature of 115°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, transferred to a centrifuge tube, centrifuged for 10 min, and the supernatant was discarded. Deionized water was then added to the centrifuge tube, and ultrasonic dispersion was performed for 5 min under a power of 300 W, followed by centrifugation. The washing was repeated three times until the pH value of the supernatant was 6. The washed multi-walled carbon nanotubes were then placed in a vacuum drying oven and dried at a temperature of 75°C for 12 h to obtain carboxylated multi-walled carbon nanotubes;

[0071] Step s5: 0.05 g of carboxylated multi-walled carbon nanotubes, 10 mL of anhydrous N,N-dimethylformamide were transferred to a three-necked flask, and ultrasonic dispersion was performed under the condition of a power of 300 W for 30 min. Then, 0.01 g of a carbon nanotube modifier, 0.02 g of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and 0.015 g of N-hydroxysuccinimide were added to the flask. Nitrogen was introduced into the three-necked flask, and then magnetic stirring was started. The constant-temperature water bath was heated to 55°C, and the reaction was kept for 23 h. After the reaction was completed, the mixture was transferred to a centrifuge tube, and centrifugation was performed for 10 min. The lower solid was collected. Then, the solid was washed with deionized water for 3 times and with anhydrous ethanol for 1 time. Then, the washed solid was placed in a vacuum drying box, and drying was performed under the condition of a temperature of 75°C for 12 h to obtain modified carbon nanotubes.

[0072] Step s6: 0.2 parts of phosphorus-doped modified carbon nanotubes were added to 10 parts of N-methylpyrrolidone, and ultrasonic dispersion was performed under the condition of a power of 300 W for 30 min to obtain a uniform carbon nanotube dispersion liquid. 10 parts of lithium iron phosphate powder (lithium iron phosphate powder was produced by Shenzhen Tianchenghe Technology Co., Ltd., and the model was positive electrode 001) was added to the carbon nanotube dispersion liquid, and magnetic stirring was performed for 30 min. Then, the mixture was transferred to a planetary ball mill, and ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added. Ball milling was performed under the condition of a rotation speed of 250 r / min for 2 h. After the ball milling was completed, the slurry was transferred to an evaporation dish, and evaporation was performed in a 55°C water bath with stirring. Then, the evaporation dish was placed in a vacuum drying box, and drying was performed under the condition of a temperature of 75°C for 12 h. Then, the evaporation dish was placed in a tube furnace, and heating was performed at a temperature increasing rate of 2°C / min to 300°C under a nitrogen atmosphere. The temperature was kept for 2 h, and then heating was performed at a temperature increasing rate of 5°C / min to 500°C. The temperature was kept for 3 h, and then the evaporation dish was naturally cooled to room temperature. The mixture was ground through a 300-mesh sieve to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0073] Comparative Example 2

[0074] This comparative example is a method for preparing a carbon-coated lithium iron phosphate positive electrode material, which comprises the following steps:

[0075] Step s1: 0.1 g of multi-walled carbon nanotubes and 20 mL of concentrated nitric acid were added to a three-necked flask equipped with a stirrer, a thermometer, and a reflux condenser. Refluxing was performed under the condition of a temperature of 115°C and a stirring speed of 300 r / min for 3 h. After the reaction was completed, the reaction product was cooled to room temperature. The product was transferred to a centrifuge tube, and centrifugation was performed for 10 min. The supernatant was discarded. Then, deionized water was added to the centrifuge tube, and ultrasonic dispersion was performed under the condition of a power of 300 W for 5 min. Then, the centrifuge tube was centrifuged again. The washing was repeated for 3 times until the pH value of the supernatant was 6. Then, the washed multi-walled carbon nanotubes were placed in a vacuum drying box, and drying was performed under the condition of a temperature of 75°C for 12 h to obtain carboxylated multi-walled carbon nanotubes.

[0076] Step s2: 0.1 g of carboxylated multi-walled carbon nanotubes was added to 10 mL of aqueous phosphoric acid solution, and ultrasonic dispersion was performed at a power of 300 W for 30 min, magnetic stirring was started for 2 h, and then it was placed in a vacuum drying oven and dried at a temperature of 75 °C for 12 h, then it was placed in the constant temperature zone of a quartz tube furnace, argon was introduced for 30 min, then it was heated to 700 °C at a rate of 5 °C / min, and after holding for 2 h, it was naturally cooled to room temperature, the black powder was collected, 10 mL of deionized water was added, ultrasonic washing was performed for 10 min, then centrifugation was performed for 10 min, the supernatant was discarded, and then the washed solid was placed in a vacuum drying oven and dried at a temperature of 75 °C for 12 h, thereby obtaining phosphorus-doped carbon nanotubes;

[0077] Step s3: 0.2 parts of phosphorus-doped modified carbon nanotubes were added to 10 parts of N-methylpyrrolidone, and ultrasonic dispersion was performed at a power of 300 W for 30 min to obtain a uniform carbon nanotube dispersion liquid, 10 parts of lithium iron phosphate powder (lithium iron phosphate powder, model: positive electrode 001, Shenzhen Tianchenghe Technology Co., Ltd.) was added to the above carbon nanotube dispersion liquid, and magnetic stirring was performed for 30 min, then it was transferred to a planetary ball mill, ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added, and ball milling was performed at a rotation speed of 250 r / min for 2 h, after ball milling, the slurry was transferred to an evaporating dish, and evaporation was performed in a 55 °C water bath with stirring, then it was placed in a vacuum drying oven and dried at a temperature of 75 °C for 12 h, then it was placed in a tube furnace and heated to 300 °C at a rate of 2 °C / min under a nitrogen atmosphere, held for 2 h, then heated to 500 °C at a rate of 5 °C / min, held for 3 h, then naturally cooled to room temperature, ground through a 300 mesh sieve, and thereby carbon-coated lithium iron phosphate positive electrode material was obtained.

[0078] Comparative Example 3:

[0079] This comparative example is a method for preparing a carbon-coated lithium iron phosphate positive electrode material, which comprises the following steps:

[0080] Step s1: 0.1 g of multi-walled carbon nanotubes and 20 mL of concentrated nitric acid were added to a three-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, and reflux reaction was performed at a temperature of 115 °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, transferred to a centrifuge tube, centrifuged for 10 min, and the supernatant was discarded, then deionized water was added to the centrifuge tube, ultrasonic dispersion was performed at a power of 300 W for 5 min, and then centrifugation was repeated, the washing was repeated 3 times, until the pH value of the supernatant was 6, then the washed multi-walled carbon nanotubes were placed in a vacuum drying oven and dried at a temperature of 75 °C for 12 h, thereby obtaining carboxylated multi-walled carbon nanotubes;

[0081] Step s2: 0.2 parts of carboxylated multi-walled carbon nanotubes were added to 10 parts of N-methylpyrrolidone and ultrasonically dispersed for 30 min at a power of 300 W to obtain a uniform carbon nanotube dispersion. 10 parts of lithium iron phosphate powder (lithium iron phosphate powder was from Shenzhen Tianchenghe Technology Co., Ltd., model: positive electrode 001) were added to the carbon nanotube dispersion, and magnetic stirring was performed for 30 min. The slurry was transferred to a planetary ball mill, ZrO2 grinding balls with a ball-to-material ratio of 10:1 were added, and ball milling was performed at a rotation speed of 250 r / min for 2 h. After ball milling, the slurry was transferred to an evaporating dish, evaporated in a water bath at 55°C with stirring, and then placed in a vacuum drying oven for drying at a temperature of 75°C for 12 h. Subsequently, the sample was placed in a tube furnace, heated to 300°C at a heating rate of 2°C / min under a nitrogen atmosphere, kept at 300°C for 2 h, then heated to 500°C at a heating rate of 5°C / min, kept at 500°C for 3 h, and then naturally cooled to room temperature. The sample was ground through a 300-mesh sieve to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0082] Preparation of a battery positive electrode:

[0083] 79 g of the carbon-coated lithium iron phosphate positive electrode material, 9 g of conductive carbon black, 9 g of polyvinylidene fluoride, and 160 mL of N-methylpyrrolidone were added to a beaker, stirred at a rotation speed of 300 r / min for 3 h by a magnetic stirrer to prepare a slurry. The slurry was coated on an aluminum foil, and then transferred to a vacuum drying oven for drying at 60°C for 4 h. A roller press was used to roll the electrode sheet, and the rolling pressure was controlled at 8 MPa. After rolling, the electrode sheet was cut into a positive electrode round sheet with a diameter of 10 mm to obtain a positive electrode.

[0084] Preparation of a battery:

[0085] A lithium sheet was used as a negative electrode, Celgard 2400 polypropylene microporous film was used as a separator, and an 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 a water and oxygen content of less than 1 ppm. After assembly, the battery was allowed to stand for 24 h to obtain a battery.

[0086] Performance test:

[0087] A carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof according to Examples 1-3 and Comparative Examples 1-3 were tested for capacity retention rate after 500 cycles at 1C and 10C charge-discharge rates. The test results are shown in Table 1. Figure 1 The discharge specific capacity of the carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof according to Examples 1-3 and Comparative Examples 1-3 were tested at 0.2C and 5C rates. The test results are shown in Table 2. Figure 2

[0088] Referring to Figures 1-2 ​As shown, according to the comparison between examples 1-3 and comparative examples 1-3, it can be known that the modification and doping of P on the lithium iron phosphate can significantly improve the electron and ion transmission efficiency, optimize the high-rate discharge performance, and strengthen the cycle stability.

[0089] According to the comparison between example 1 and comparative example 1, it can be known that the lithium iron phosphate with doping P and modification has better performance than the lithium iron phosphate with only modification but without doping P.

[0090] According to the comparison between example 1 and comparative example 2, it can be known that the lithium iron phosphate with doping P and modification has better performance than the lithium iron phosphate with only doping P but without modification.

[0091] According to the comparison between example 1 and comparative example 3, it can be known that the lithium iron phosphate with doping P and modification has better performance than the lithium iron phosphate without doping and modification.

[0092] In the description of the present specification, the description referring to the terms "one embodiment", "an example", "a specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0093] The above is only an example and description of the present application, and those skilled in the art can make various modifications or supplements or use similar ways to replace the described specific embodiments, as long as they do not deviate from the invention or exceed the scope defined in the present application, which shall belong to the protection scope of the present application.

Claims

1. A carbon-coated lithium iron phosphate positive electrode material, characterized in that, The following components are included by mass parts: 0.2-0.3 parts of phosphorus-doped modified carbon nanotubes, 10-12 parts of N-methyl pyrrolidone, and 10-15 parts of lithium iron phosphate powder; The phosphorus-doped modified carbon nanotubes are prepared by the following steps: Step a1: reflux reaction of p-dichlorobenzyl, triphenylphosphine and chloroform, after the reaction is completed, the reaction product is cooled, then rotary evaporation, then recrystallization, to obtain a first intermediate; Step a2: the first intermediate, 2-nitrobenzaldehyde and dichloromethane are added to a three-necked flask, sodium hydroxide solution is added, after the reaction is completed, filtration is performed, the filter cake is collected, then recrystallization is performed, to obtain a second intermediate; Step a3: tin powder, the second intermediate and anhydrous ethanol are added to a three-necked flask, hydrochloric acid solution is added, stirring reaction is continued, the pH value is adjusted to 7, then extraction is performed, the organic phase is combined and dried, then elution is performed, to obtain a carbon nanotube modifier; Step a4: multi-walled carbon nanotubes and concentrated nitric acid are added to a three-necked flask for reflux reaction, after the reaction is completed, cooling is performed, the product is centrifuged, the supernatant is discarded, ultrasonic dispersion is performed again, then centrifugation is performed, then drying is performed, to obtain carboxylated multi-walled carbon nanotubes; Step a5: the carboxylated multi-walled carbon nanotubes and anhydrous N, N-dimethylformamide are transferred to a three-necked flask, ultrasonic dispersion is performed, then the carbon nanotube modifier, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide are added; incubation reaction is performed, the lower solid is collected by centrifugation, then washing and drying are performed, to obtain modified carbon nanotubes; Step a6: the modified carbon nanotubes are added to an aqueous phosphoric acid solution, ultrasonic dispersion is performed, magnetic stirring is started, then drying is performed, then the product is placed in a constant temperature zone of a quartz tube furnace, after calcination and cooling, the black powder is collected for ultrasonic washing, then centrifugation is performed, deionized water is used for washing, then drying is performed, to obtain phosphorus-doped modified carbon nanotubes.

2. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material is characterized by: The use amount ratio of the p-dichlorobenzyl, triphenylphosphine and chloroform solution in step a1 is 1.75-1.85 g: 5.25-5.75 g: 50-60 mL.

3. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material is characterized by: The use amount ratio of the first intermediate, 2-nitrobenzaldehyde, dichloromethane solution and sodium hydroxide solution in step a2 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 positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material is characterized by: The use amount ratio of the tin powder, second intermediate, anhydrous ethanol and hydrochloric acid solution in step a3 is 1.20-1.30 g: 1.86-1.98 g: 20-25 mL: 2.0-2.5 mL.

5. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material has a carbon content of 1-5 wt%. The use amount ratio of the multi-walled carbon nanotubes and concentrated nitric acid in step a4 is 0.1-0.2 g: 20-25 mL.

6. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material has a carbon content of 1-5 wt%. The use amount ratio of the 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 anode material of claim 1, wherein the carbon-coated lithium iron phosphate anode material has a carbon content of 1-10 wt%. The ratio of the modified carbon nanotube, the phosphoric acid aqueous solution and the deionized water in step a6 is 0.1-0.2g:10-12mL:10-12mL.

8. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material has a carbon content of 0.5 to 5 wt%. The mass fraction of the sodium hydroxide solution in step a2 is 50%; the mass fraction of the hydrochloric acid solution in step a3 is 36%; the dichloromethane-petroleum ether solution is a solution prepared by mixing dichloromethane and petroleum ether in a volume ratio of 5:

1.

9. The carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the carbon-coated lithium iron phosphate positive electrode material has a carbon content of 0.5 to 5 wt%. The concentration of the concentrated nitric acid in step a4 is 68%; the diameter of the multi-walled carbon nanotube in step a4 is 10-20nm, 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, A method for preparing the carbon-coated lithium iron phosphate positive electrode material according to any one of claims 1-9 comprises the following steps: The phosphorus-doped modified carbon nanotube is added into N-methyl pyrrolidone and ultrasonically dispersed to obtain a uniform carbon nanotube dispersion liquid, lithium iron phosphate powder is added into the carbon nanotube dispersion liquid, and magnetic stirring is performed, then the slurry is transferred to a planetary ball mill for ball milling, after the ball milling is completed, the slurry is transferred to an evaporation dish, and stirring evaporation, drying, sintering in a tube furnace, cooling, grinding and sieving are performed in sequence to obtain the carbon-coated lithium iron phosphate positive electrode material.

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