Long-circulation lithium iron phosphate battery and preparation method thereof

Abstract

The invention discloses a long-circulation lithium iron phosphate battery and a preparation method thereof, a positive plate, a negative plate, a porous diaphragm and an electrolyte are assembled into the battery in a glove box filled with argon, and the long-circulation lithium iron phosphate battery is prepared. A negative plate is a long-circulation lithium iron phosphate battery prepared by mixing polyvinylidene fluoride, a negative electrode material and acetylene and loading the mixture on a copper foil, the negative electrode material contains silicon dioxide, so that the lithium storage performance of the negative electrode material is improved, and meanwhile, high-temperature carbonization treatment is performed, so that an organic framework forms a grid carbon layer to coat the surface of the silicon dioxide, and the lithium storage performance of the lithium iron phosphate battery is improved. The lithium iron phosphate battery negative electrode material prevents silicon dioxide in the negative electrode material from promoting volume expansion due to charging and discharging, so that the stability of the negative electrode material is ensured, the grid carbon layer increases the conductivity of the negative electrode material, and the negative electrode material is integrally coated with the carbon layer, so that the expansion of the negative electrode material is limited, and the service life of the lithium iron phosphate battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery preparation, and in particular to a long-cycle lithium iron phosphate battery and a preparation method thereof. Background Art

[0002] Renewable clean energy such as solar energy, wind energy, and hydropower are abundant on the earth and will not cause adverse effects on the environment during use. Therefore, they are widely studied by countries around the world to replace traditional fossil energy. However, problems such as instability, geographical restrictions, and high preparation costs restrict their practical application in the field of energy storage. As one of the most representative new chemical energy storage and conversion devices, lithium-ion batteries are widely used in various portable electronic devices, new energy vehicles, and high-power devices due to their excellent lithium storage performance, high energy density, high potential, no memory effect, and environmental friendliness. In order to improve the energy storage effect of the battery, the negative electrode material is often replaced with a material with a high lithium storage capacity. After multiple cycles, these materials often expand in volume, resulting in damage to the electrode structure, thereby affecting the cycle stability of the battery, greatly reducing the performance of the battery, and even causing accidents. Summary of the invention

[0003] The purpose of the present invention is to provide a long-cycle lithium iron phosphate battery and a preparation method thereof, which solves the problem of short cycle life of lithium iron phosphate batteries at this stage.

[0004] The purpose of the present invention can be achieved through the following technical solutions:

[0005] A method for preparing a long-cycle lithium iron phosphate battery comprises the following steps:

[0006] Step S1: dissolving polyvinylidene fluoride in N-methylpyrrolidone with a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, uniformly loading on a circular aluminum foil by a doctor blade coating method, and drying to obtain a positive electrode sheet;

[0007] Step S2: dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding negative electrode material and acetylene black and mixing evenly, uniformly loading on copper foil by doctor blade coating method, and drying to obtain a negative electrode sheet;

[0008] Step S3: Assemble the positive electrode sheet, the negative electrode sheet, the porous separator and the electrolyte into a battery in a glove box filled with argon gas to produce a long-cycle lithium iron phosphate battery.

[0009] Furthermore, the negative electrode material is prepared by the following steps:

[0010] Step A1: The modified organic framework, tetraethyl orthosilicate and ethanol are mixed, and ultrasonic treatment is performed for 30-40 minutes at a frequency of 30-50kHz and a temperature of 20-25°C, deionized water and ammonia water are added, the temperature is raised to 50-60°C, and ultrasonic reaction is performed for 20-25 hours to obtain a modified carrier, and the modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, and the speed is 200-300r / min and the temperature is 25-30°C. The mixture is stirred for 20-30 minutes, the temperature is raised to 220-230°C, and the reaction is performed for 20-25 hours to obtain a precursor;

[0011] Step A2: the precursor, sodium hydroxide and ethanol are mixed evenly, and the reaction is carried out for 1-1.5 hours at a speed of 120-150 r / min and a temperature of 25-30° C., and then chloroacetic acid is added, and the temperature is raised to 75-80° C. and the reaction is carried out for 2-3 hours to obtain a functionalized precursor, and the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are mixed evenly, and nitrogen is introduced for protection, and the spinning solution is prepared at a speed of 500-600 r / min and a temperature of 40-50° C., and the spinning solution is prepared;

[0012] Step A3: electrospin the spinning solution at a spinning speed of 1-1.5 mL / h, a spinning voltage of 15-20 kV, and a receiving distance of 15-20 cm to obtain a modified film, and heat-treat the modified film at 150°C, 250°C, and 350°C for 1 hour, and then calcine at 700-800°C in an argon atmosphere for 30-40 minutes to obtain a negative electrode material.

[0013] Furthermore, the amount ratio of the modified organic framework, tetraethyl orthosilicate, ethanol, deionized water and ammonia water described in step A1 is 1g:3mL:80mL:5mL:2mL, the mass fraction of ammonia water is 30%, and the amount ratio of the modified carrier, sodium molybdate dihydrate, thiourea and deionized water is 5g:2.5mmo l:5mmo l:40mL.

[0014] Furthermore, the amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid described in step A2 is 5g:6g:150mL:6g, and the amount ratio of the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic acid dianhydride and N,N-dimethylacetamide is 1g:25mmo l:20mmo l:20mL.

[0015] Further, the modified organic framework is prepared by the following steps:

[0016] Step B1: Heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed evenly, reacted at a speed of 150-200 r / min and a temperature of 60-70° C. for 2-4 hours, and then sodium hydroxide solution is added, the temperature is raised to 80-85° C., and the reaction is carried out for 4-6 hours to obtain intermediate 1. Intermediate 1, p-nitroaniline and DMF are mixed evenly, and reacted at a speed of 120-150 r / min, a temperature of 40-50° C. and a pH value of 10-11 for 3-5 hours to obtain intermediate 2;

[0017] Step B2: Mix dimethyl 2,5-dihydroxyterephthalate, potassium carbonate and DMF evenly, react for 1-1.5 hours at a speed of 120-150 r / min and a temperature of 20-25° C., add intermediate 2, continue to react for 20-25 hours, add potassium hydroxide solution, raise the temperature to 40-50° C., react for 2-3 hours, add hydrochloric acid solution to adjust the pH value to acidic, and obtain a modified ligand;

[0018] Step B3: Mix cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF evenly, stir and add the modified ligand at a speed of 150-200 r / min and a temperature of 30-40°C, stir for 30-40 minutes, heat to 140-145°C, add lactic acid, react for 30-35 hours, and obtain a modified organic framework.

[0019] Furthermore, the amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution in step B1 is 10 mmol:10 mmol:0.5 g:30 mL, and the molar ratio of intermediate 1 to p-nitroaniline is 2:1.

[0020] Furthermore, the dosage ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1 mmol:2 mmol:5 mL:2 mmol:3 mL, the mass fraction of potassium hydroxide solution is 15%, and the mass fraction of hydrochloric acid solution is 10%.

[0021] Furthermore, the amount ratio of cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, modified ligand and lactic acid in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0022] Beneficial effects of the present invention: A long-cycle lithium iron phosphate battery prepared by the present invention is prepared by assembling a positive electrode sheet, a negative electrode sheet, a porous diaphragm and an electrolyte into a battery in a glove box filled with argon gas, and preparing a long-cycle lithium iron phosphate battery. The negative electrode sheet is prepared by mixing polyvinylidene fluoride, a negative electrode material and acetylene and loading them on a copper foil. The negative electrode material is ultrasonically treated with a modified organic framework and tetraethyl orthosilicate as raw materials, so that the tetraethyl orthosilicate enters the pores of the modified organic framework, and then hydrolyzed under the action of ammonia water and deionized water, so that the modified organic framework is filled with nano-silicon dioxide. A modified carrier, sodium molybdate dihydrate and thiourea are reacted to load molybdenum disulfide on the pores on the surface of the modified carrier to obtain a precursor, the precursor is treated with sodium hydroxide to convert the alcohol hydroxyl groups on the surface into sodium alcohol, and then react with the chlorine atom sites on chloroacetic acid to obtain a functionalized precursor, the functionalized precursor, 4,4'-diaminodiphenyl ether and pyromellitic dianhydride are reacted to coat the surface of the functionalized precursor with polyamic acid to obtain a spinning solution, the spinning solution is electrospun, and treated at high temperature to form polyimide, and then calcined in an argon atmosphere to obtain a negative electrode material.

[0023] Modified organic framework: Heptafluorobutanol and epichlorohydrin are reacted so that the hydroxyl group on the heptafluorobutanol reacts with the epoxy group on the epichlorohydrin, and then the ring is closed under the action of sodium hydroxide solution to form a new epoxy group to obtain intermediate 1; p-nitroaniline and intermediate 1 are reacted under alkaline conditions so that the amino group on the p-nitroaniline reacts with the epoxy group on the intermediate 1 to obtain intermediate 2; dimethyl 2,5-dihydroxyterephthalate and intermediate 2 are reacted so that the phenolic hydroxyl group on the dimethyl 2,5-dihydroxyterephthalate reacts with the nitro group on the intermediate 2; then the reaction is hydrolyzed under the action of potassium hydroxide so that the formate is converted into formic acid to obtain a modified ligand; the modified ligand is reacted with cobalt nitrate hexahydrate to form a cobalt organic framework to obtain a modified organic framework.

[0024] The negative electrode material contains silicon dioxide inside, which improves the lithium storage performance of the negative electrode material. At the same time, the high-temperature carbonization treatment allows the organic framework to form a grid carbon layer coated on the silicon dioxide surface, preventing the silicon dioxide in the negative electrode material from promoting volume expansion due to charging and discharging, thereby ensuring the stability of the negative electrode material. The grid carbon layer increases the conductivity of the negative electrode material. After carbonization, the organic framework forms fluorine and nitrogen-doped carbon. After the introduction of fluorine atoms and nitrogen atoms, defects will be generated in the lattice of the carbon material. These defects can serve as channels for charge transfer, thereby improving the overall conductivity of the material. The molybdenum disulfide on the surface has a large interlayer spacing that is conducive to the embedding of lithium ions, thereby making it possible to achieve rapid lithium embedding and deintercalation reactions and improve the overall lithium storage performance of the lithium-ion battery. The negative electrode material is overall coated with a carbon layer, which limits the expansion of the negative electrode material and thereby improves the service life of the lithium iron phosphate battery. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1

[0027] A method for preparing a long-cycle lithium iron phosphate battery comprises the following steps:

[0028] Step S1: dissolving polyvinylidene fluoride in N-methylpyrrolidone with a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, uniformly loading on a circular aluminum foil by a doctor blade coating method, and drying to obtain a positive electrode sheet;

[0029] Step S2: dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding negative electrode material and acetylene black and mixing evenly, uniformly loading on copper foil by doctor blade coating method, and drying to obtain a negative electrode sheet;

[0030] Step S3: Assemble the positive electrode sheet, the negative electrode sheet, the porous separator and the electrolyte into a battery in a glove box filled with argon gas to produce a long-cycle lithium iron phosphate battery.

[0031] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S1 is 1:20, and the mass ratio of polyvinylidene fluoride, lithium iron phosphate and carbon black is 8:85:7.

[0032] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S2 is 1:20, and the mass ratio of polyvinylidene fluoride, negative electrode material and carbon black is 10:85:5.

[0033] The porous membrane in step S3 is a Celgard 2400 polypropylene porous membrane, and the electrolyte is 1 mol / L LiPF 6 The volume ratio of ethylene carbonate, dimethyl carbonate and diethyl carbonate in the solvent is 1:1:1.

[0034] The negative electrode material is prepared by the following steps:

[0035] Step A1: The modified organic framework, tetraethyl orthosilicate and ethanol are mixed, and ultrasonic treatment is performed for 30 minutes at a frequency of 30kHz and a temperature of 20°C. Deionized water and ammonia water are added, and the temperature is raised to 50°C, and ultrasonic reaction is performed for 20 hours to obtain a modified carrier. The modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, and the temperature is raised to 200r / min and 25°C. The mixture is stirred for 20 minutes, and the temperature is raised to 220°C. The reaction is performed for 20 hours to obtain a precursor;

[0036] Step A2: The precursor, sodium hydroxide and ethanol are mixed evenly, reacted at a speed of 120 r / min and a temperature of 25° C. for 1 hour, chloroacetic acid is added, the temperature is raised to 75° C., and the reaction is carried out for 2 hours to obtain a functionalized precursor, and the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are mixed evenly, nitrogen is introduced for protection, and the spinning solution is prepared at a speed of 500 r / min and a temperature of 40° C. for 6 hours under stirring;

[0037] Step A3: Electrospin the spinning solution at a spinning speed of 1 mL / h, a spinning voltage of 15 kV, and a receiving distance of 15 cm to obtain a modified film. The modified film is heat-treated at 150°C, 250°C, and 350°C for 1 h, and then calcined at 700°C in an argon atmosphere for 30 min to obtain a negative electrode material.

[0038] The amount ratio of the modified organic framework, tetraethyl orthosilicate, ethanol, deionized water and ammonia water described in step A1 is 1g:3mL:80mL:5mL:2mL, the mass fraction of ammonia water is 30%, and the amount ratio of the modified carrier, sodium molybdate dihydrate, thiourea and deionized water is 5g:2.5mmo l:5mmo l:40mL.

[0039] The amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid described in step A2 is 5g:6g:150mL:6g, and the amount ratio of the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide is 1g:25mmol:20mmo l:20mL.

[0040] The modified organic framework is prepared by the following steps:

[0041] Step B1: Heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed evenly, reacted at a speed of 150 r / min and a temperature of 60° C. for 2 h, then a sodium hydroxide solution is added, the temperature is raised to 80° C., and the reaction is carried out for 4 h to obtain intermediate 1; intermediate 1, p-nitroaniline and DMF are mixed evenly, and reacted at a speed of 120 r / min, a temperature of 40° C. and a pH value of 10 for 3 h to obtain intermediate 2;

[0042] Step B2: dimethyl 2,5-dihydroxyterephthalate, potassium carbonate and DMF are mixed evenly, reacted at a speed of 120 r / min and a temperature of 20° C. for 1 h, intermediate 2 is added, the reaction is continued for 20 h, potassium hydroxide solution is added, the temperature is raised to 40° C., the reaction is carried out for 2 h, and hydrochloric acid solution is added to adjust the pH value to acidic to obtain a modified ligand;

[0043] Step B3: Mix cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF evenly, stir and add the modified ligand at a speed of 150 r / min and a temperature of 30°C, stir for 30 minutes, heat to 140°C, add lactic acid, and react for 30 hours to obtain a modified organic framework.

[0044] The amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution in step B1 is 10 mmol:10 mmol:0.5 g:30 mL, and the molar ratio of intermediate 1 to p-nitroaniline is 2:1.

[0045] The amount ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1 mmol:2 mmol:5 mL:2 mmol:3 mL, the mass fraction of potassium hydroxide solution is 15%, and the mass fraction of hydrochloric acid solution is 10%.

[0046] The amount ratio of cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, modified ligand and lactic acid described in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0047] Example 2

[0048] A method for preparing a long-cycle lithium iron phosphate battery comprises the following steps:

[0049] Step S1: dissolving polyvinylidene fluoride in N-methylpyrrolidone with a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, uniformly loading on a circular aluminum foil by a doctor blade coating method, and drying to obtain a positive electrode sheet;

[0050] Step S2: dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding negative electrode material and acetylene black and mixing evenly, uniformly loading on copper foil by doctor blade coating method, and drying to obtain a negative electrode sheet;

[0051] Step S3: Assemble the positive electrode sheet, the negative electrode sheet, the porous separator and the electrolyte into a battery in a glove box filled with argon gas to produce a long-cycle lithium iron phosphate battery.

[0052] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S1 is 1:20, and the mass ratio of polyvinylidene fluoride, lithium iron phosphate and carbon black is 8:85:7.

[0053] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S2 is 1:20, and the mass ratio of polyvinylidene fluoride, negative electrode material and carbon black is 10:85:5.

[0054] The porous membrane in step S3 is a Celgard 2400 polypropylene porous membrane, and the electrolyte is 1 mol / L LiPF 6 The volume ratio of ethylene carbonate, dimethyl carbonate and diethyl carbonate in the solvent is 1:1:1.

[0055] The negative electrode material is prepared by the following steps:

[0056] Step A1: The modified organic framework, tetraethyl orthosilicate and ethanol are mixed, and ultrasonic treatment is performed for 35 minutes at a frequency of 40kHz and a temperature of 20°C. Deionized water and ammonia water are added, and the temperature is raised to 50-60°C, and the ultrasonic reaction is performed for 25 hours to obtain a modified carrier. The modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, and the temperature is raised to 200r / min and 30°C. The mixture is stirred for 25 minutes, and the temperature is raised to 225°C. The reaction is performed for 25 hours to obtain a precursor;

[0057] Step A2: The precursor, sodium hydroxide and ethanol are mixed evenly, reacted at a speed of 150 r / min and a temperature of 25° C. for 1.5 hours, chloroacetic acid is added, the temperature is raised to 78° C., and the reaction is carried out for 2.5 hours to obtain a functionalized precursor, and the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are mixed evenly, nitrogen is introduced for protection, and the spinning solution is obtained by stirring for 7 hours at a speed of 600 r / min and a temperature of 45° C.;

[0058] Step A3: Electrospin the spinning solution at a spinning speed of 1.5 mL / h, a spinning voltage of 18 kV, and a receiving distance of 20 cm to obtain a modified film. The modified film is heat-treated at 150°C, 250°C, and 350°C for 1 h, and then calcined at 750°C in an argon atmosphere for 35 min to obtain a negative electrode material.

[0059] The amount ratio of the modified organic framework, tetraethyl orthosilicate, ethanol, deionized water and ammonia water described in step A1 is 1g:3mL:80mL:5mL:2mL, the mass fraction of ammonia water is 30%, and the amount ratio of the modified carrier, sodium molybdate dihydrate, thiourea and deionized water is 5g:2.5mmo l:5mmo l:40mL.

[0060] The amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid described in step A2 is 5g:6g:150mL:6g, and the amount ratio of the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide is 1g:25mmol:20mmo l:20mL.

[0061] The modified organic framework is prepared by the following steps:

[0062] Step B1: Heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed evenly, reacted at a speed of 150 r / min and a temperature of 65° C. for 2-4 hours, and then sodium hydroxide solution is added, the temperature is raised to 80° C., and the reaction is carried out for 5 hours to obtain intermediate 1. Intermediate 1, p-nitroaniline and DMF are mixed evenly, and reacted at a speed of 120 r / min, a temperature of 45° C. and a pH value of 11 for 4 hours to obtain intermediate 2;

[0063] Step B2: dimethyl 2,5-dihydroxyterephthalate, potassium carbonate and DMF are mixed evenly, reacted at a speed of 120 r / min and a temperature of 25° C. for 1 h, intermediate 2 is added, the reaction is continued for 25 h, potassium hydroxide solution is added, the temperature is raised to 45° C., the reaction is carried out for 3 h, and hydrochloric acid solution is added to adjust the pH value to acidic to obtain a modified ligand;

[0064] Step B3: Mix cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF evenly, stir and add the modified ligand at a speed of 150 r / min and a temperature of 35°C, stir for 35 minutes, heat to 145°C, add lactic acid, react for 30 hours, and obtain a modified organic framework.

[0065] The amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution in step B1 is 10 mmol:10 mmol:0.5 g:30 mL, and the molar ratio of intermediate 1 to p-nitroaniline is 2:1.

[0066] The amount ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1 mmol:2 mmol:5 mL:2 mmol:3 mL, the mass fraction of potassium hydroxide solution is 15%, and the mass fraction of hydrochloric acid solution is 10%.

[0067] The amount ratio of cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, modified ligand and lactic acid described in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0068] Example 3

[0069] A method for preparing a long-cycle lithium iron phosphate battery comprises the following steps:

[0070] Step S1: dissolving polyvinylidene fluoride in N-methylpyrrolidone with a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, uniformly loading on a circular aluminum foil by a doctor blade coating method, and drying to obtain a positive electrode sheet;

[0071] Step S2: dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding negative electrode material and acetylene black and mixing evenly, uniformly loading on copper foil by doctor blade coating method, and drying to obtain a negative electrode sheet;

[0072] Step S3: Assemble the positive electrode sheet, the negative electrode sheet, the porous separator and the electrolyte into a battery in a glove box filled with argon gas to produce a long-cycle lithium iron phosphate battery.

[0073] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S1 is 1:20, and the mass ratio of polyvinylidene fluoride, lithium iron phosphate and carbon black is 8:85:7.

[0074] The mass ratio of polyvinylidene fluoride to N-methylpyrrolidone in step S2 is 1:20, and the mass ratio of polyvinylidene fluoride, negative electrode material and carbon black is 10:85:5.

[0075] The porous membrane in step S3 is a Celgard 2400 polypropylene porous membrane, and the electrolyte is 1 mol / L LiPF 6 The volume ratio of ethylene carbonate, dimethyl carbonate and diethyl carbonate in the solvent is 1:1:1.

[0076] The negative electrode material is prepared by the following steps:

[0077] Step A1: The modified organic framework, tetraethyl orthosilicate and ethanol are mixed, and ultrasonic treatment is performed for 40 minutes at a frequency of 50 kHz and a temperature of 25°C, deionized water and ammonia water are added, the temperature is raised to 60°C, and ultrasonic reaction is performed for 25 hours to obtain a modified carrier, and the modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, and the temperature is 300 r / min and 30°C. The mixture is stirred for 30 minutes, the temperature is raised to 230°C, and the reaction is performed for 25 hours to obtain a precursor;

[0078] Step A2: The precursor, sodium hydroxide and ethanol are mixed evenly, and the reaction is carried out for 1.5 hours at a speed of 150 r / min and a temperature of 30° C., and then chloroacetic acid is added, and the temperature is raised to 80° C. and the reaction is carried out for 3 hours to obtain a functionalized precursor, and the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are mixed evenly, and nitrogen is introduced for protection, and the spinning solution is prepared at a speed of 600 r / min and a temperature of 50° C. and stirred for 8 hours;

[0079] Step A3: Electrospin the spinning solution at a spinning speed of 1.5 mL / h, a spinning voltage of 20 kV, and a receiving distance of 20 cm to obtain a modified film. The modified film is heat-treated at 150°C, 250°C, and 350°C for 1 h, and then calcined at 800°C in an argon atmosphere for 40 min to obtain a negative electrode material.

[0080] The amount ratio of the modified organic framework, tetraethyl orthosilicate, ethanol, deionized water and ammonia water described in step A1 is 1g:3mL:80mL:5mL:2mL, the mass fraction of ammonia water is 30%, and the amount ratio of the modified carrier, sodium molybdate dihydrate, thiourea and deionized water is 5g:2.5mmo l:5mmo l:40mL.

[0081] The amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid described in step A2 is 5g:6g:150mL:6g, and the amount ratio of the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide is 1g:25mmol:20mmo l:20mL.

[0082] The modified organic framework is prepared by the following steps:

[0083] Step B1: Heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed evenly, reacted at a speed of 200 r / min and a temperature of 70° C. for 4 hours, and then sodium hydroxide solution is added, the temperature is raised to 85° C., and the reaction is carried out for 6 hours to obtain intermediate 1. Intermediate 1, p-nitroaniline and DMF are mixed evenly, and reacted at a speed of 150 r / min, a temperature of 50° C. and a pH value of 11 for 5 hours to obtain intermediate 2;

[0084] Step B2: dimethyl 2,5-dihydroxyterephthalate, potassium carbonate and DMF were mixed evenly, reacted at a speed of 150 r / min and a temperature of 25° C. for 1.5 h, then intermediate 2 was added, the reaction was continued for 25 h, potassium hydroxide solution was added, the temperature was raised to 50° C., the reaction was continued for 3 h, and hydrochloric acid solution was added to adjust the pH value to acidic to obtain a modified ligand;

[0085] Step B3: Mix cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF evenly, stir and add the modified ligand at a speed of 200 r / min and a temperature of 40°C, stir for 40 minutes, heat to 145°C, add lactic acid, react for 35 hours, and obtain a modified organic framework.

[0086] The amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution in step B1 is 10 mmol:10 mmol:0.5 g:30 mL, and the molar ratio of intermediate 1 to p-nitroaniline is 2:1.

[0087] The amount ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1 mmol:2 mmol:5 mL:2 mmol:3 mL, the mass fraction of potassium hydroxide solution is 15%, and the mass fraction of hydrochloric acid solution is 10%.

[0088] The amount ratio of cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, modified ligand and lactic acid described in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0089] Comparative Example 1

[0090] Compared with Example 1, this comparative example uses a precursor instead of a negative electrode material, and the remaining steps are the same.

[0091] Comparative Example 2

[0092] Compared with Example 1, this comparative example uses a modified carrier instead of a precursor, and the remaining steps are the same.

[0093] Comparative Example 3

[0094] Compared with Example 1, this comparative example uses terephthalic acid instead of the modified ligand, and the remaining steps are the same.

[0095] Comparative Example 4

[0096] Compared with Example 1, this comparative example uses a modified organic framework instead of a modified carrier, and the remaining steps are the same.

[0097] The capacity retention rates of the lithium iron phosphate batteries prepared in Examples 1-3 and Comparative Examples 1-4 were tested for 500 cycles at 0.5C, 1C and 2C in the voltage range of 2.7-3.8V. The test results are shown in the following table.

[0098]

[0099] It can be seen from the above table that this application has a very good cycle life.

[0100] The above contents are merely examples and explanations of the concept of the present invention. The technicians in this technical field may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a long cycle lithium iron phosphate battery, characterized in that: The specific steps include: Step S1: dissolving polyvinylidene fluoride in N-methylpyrrolidone with a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, uniformly loading on a circular aluminum foil by a doctor blade coating method, and drying to obtain a positive electrode sheet; Step S2: dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding negative electrode material and acetylene black and mixing evenly, uniformly loading on copper foil by doctor blade coating method, and drying to obtain a negative electrode sheet; Step S3: Assemble the positive electrode sheet, the negative electrode sheet, the porous separator and the electrolyte into a battery in a glove box filled with argon gas to produce a long-cycle lithium iron phosphate battery.

2. The method for preparing a long cycle lithium iron phosphate battery according to claim 1, characterized in that: The negative electrode material is prepared by the following steps: Step A1: mixing a modified organic framework, tetraethyl orthosilicate and ethanol, ultrasonically treating the mixture, adding deionized water and ammonia water, heating the mixture to obtain a modified carrier, and mixing the modified carrier, sodium molybdate dihydrate, thiourea and deionized water to obtain a precursor; Step A2: After the precursor, sodium hydroxide and ethanol are mixed for reaction, chloroacetic acid is added, and the temperature is raised to react to obtain a functionalized precursor, and the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are mixed evenly, nitrogen is introduced for protection, and stirring is performed to obtain a spinning solution; Step A3: electrospinning the spinning solution to obtain a modified film, preserving the modified film at high temperature, and then calcining the film to obtain a negative electrode material.

3. The method for preparing a long cycle lithium iron phosphate battery according to claim 2, characterized in that: The amount ratio of the modified organic framework, tetraethyl orthosilicate, ethanol, deionized water and ammonia water described in step A1 is 1g:3mL:80mL:5mL:2mL, and the amount ratio of the modified carrier, sodium molybdate dihydrate, thiourea and deionized water is 5g:2.5mmol:5mmol:40mL.

4. The method for preparing a long cycle lithium iron phosphate battery according to claim 2, characterized in that: The amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid described in step A2 is 5g:6g:150mL:6g, and the amount ratio of the functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide is 1g:25mmol:20mmol:20mL.

5. The method for preparing a long cycle lithium iron phosphate battery according to claim 2, characterized in that: The modified organic framework is prepared by the following steps: Step B1: After heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed for reaction, sodium hydroxide solution is added, and the temperature is raised to react to obtain intermediate 1; intermediate 1, p-nitroaniline and DMF are mixed for reaction to obtain intermediate 2; Step B2: After dimethyl 2,5-dihydroxyterephthalate, potassium carbonate and DMF are mixed for reaction, intermediate 2 is added, the reaction is continued, potassium hydroxide solution is added, the temperature is increased for reaction, and hydrochloric acid solution is added to adjust the pH value to acidic, thereby obtaining a modified ligand; Step B3: cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF are mixed and stirred, and a modified ligand is added, stirred, heated and lactic acid is added to react to obtain a modified organic framework.

6. The method for preparing a long cycle lithium iron phosphate battery according to claim 5, characterized in that: The amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution in step B1 is 10mmol:10mmol:0.5g:30mL, and the molar ratio of intermediate 1 to p-nitroaniline is 2:

1.

7. The method for preparing a long cycle lithium iron phosphate battery according to claim 5, characterized in that: The amount ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1 mmol: 2 mmol: 5 mL: 2 mmol: 3 mL.

8. The method for preparing a long cycle lithium iron phosphate battery according to claim 5, characterized in that: The amount ratio of cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, modified ligand and lactic acid described in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

9. A long cycle lithium iron phosphate battery, characterized in that: Prepared according to any one of the preparation methods described in claims 1-8.

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