Long cycle lithium iron phosphate battery and preparation method thereof

Abstract

The application discloses a long-cycle lithium iron phosphate battery and a preparation method thereof. The long-cycle lithium iron phosphate battery is prepared by assembling a positive plate, a negative plate, a porous diaphragm and an electrolyte in an argon-filled glove box. The negative plate is prepared by mixing polyvinylidene fluoride, a negative material and acetylene and loading the mixture on a copper foil. The negative material contains silicon dioxide in the inside, so that the lithium storage performance of the negative material is improved. High-temperature carbonization treatment is performed on the negative material, so that an organic framework forms a grid carbon layer to coat the surface of the silicon dioxide, thereby preventing the silicon dioxide in the negative material from promoting volume expansion due to charging and discharging, and further ensuring the stability of the negative material. The grid carbon layer increases the conductivity of the negative material. The negative material is coated with a carbon layer as a whole on the outside, so that the expansion of the negative material is limited, and the service life of the lithium iron phosphate battery is improved.

Description

TECHNICAL FIELD

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

[0002] Solar energy, wind energy, water energy and other renewable clean energy are very rich in reserves on the earth, and do not cause adverse effects on the environment in the use process, so they are widely researched by countries in the world to replace traditional fossil energy. However, problems such as instability, geographical limitation, high preparation cost and the like restrict the actual application of the renewable clean energy in the energy storage field. As one of the most representative new chemical energy storage and conversion devices, the lithium ion battery is widely used in various portable electronic devices, new energy vehicles and high-power devices due to the excellent lithium storage performance, high energy density, high potential, no memory effect and environmental friendliness and the like. In order to improve the energy storage effect of the battery, the negative electrode material is often replaced by a material with high lithium storage capacity. After multiple cycles, the material often has the phenomenon of volume expansion, which causes the damage of the electrode structure, thereby affecting the cycle stability of the battery, greatly reducing the performance of the battery, and even causing accidents. SUMMARY

[0003] The application aims to provide a long-cycle lithium iron phosphate battery and a preparation method thereof, and solves the problem of short cycle life of the lithium iron phosphate battery at the present stage.

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

[0005] A preparation method of a long-cycle lithium iron phosphate battery, specifically comprising the following steps:

[0006] Step S1: polyvinylidene fluoride is dissolved in N-methylpyrrolidone, the mass fraction of the polyvinylidene fluoride is 5%, lithium iron phosphate and carbon black are added and uniformly mixed, a scraper coating method is used, the circular aluminum foil is uniformly loaded, and a positive electrode sheet is prepared by drying;

[0007] Step S2: polyvinylidene fluoride is dissolved in N-methylpyrrolidone, a negative electrode material and acetylene black are added and uniformly mixed, a scraper coating method is used, the copper foil is uniformly loaded, and a negative electrode sheet is prepared by drying;

[0008] Step S3: the positive electrode sheet, the negative electrode sheet, a porous diaphragm and an electrolyte are assembled into a battery in an argon-filled glove box, and a long-cycle lithium iron phosphate battery is prepared.

[0009] Further, the negative electrode material is prepared by the following steps.

[0010] Step A1: the modified organic framework, tetraethyl orthosilicate and ethanol are mixed, ultrasonic treatment is carried out at a frequency of 30-50 kHz and a temperature of 20-25℃ for 30-40 min, deionized water and ammonia water are added, the temperature is raised to 50-60℃, and ultrasonic reaction is carried out for 20-25 h to obtain a modified carrier; the modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, stirring is carried out at a rotation speed of 200-300 r / min and a temperature of 25-30℃ for 20-30 min, the temperature is raised to 220-230℃, and reaction is carried out for 20-25 h to obtain a precursor;

[0011] Step A2: the precursor, sodium hydroxide and ethanol are uniformly mixed, reaction is carried out at a rotation speed of 120-150 r / min and a temperature of 25-30℃ for 1-1.5 h, chloroacetic acid is added, the temperature is raised to 75-80℃, and reaction is carried out for 2-3 h to obtain a functionalized precursor; the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide are uniformly mixed, nitrogen protection is carried out, stirring treatment is carried out at a rotation speed of 500-600 r / min and a temperature of 40-50℃ for 6-8 h to obtain a spinning solution;

[0012] Step A3: the spinning solution is electrospun under the conditions of 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; the modified film is sequentially treated at a temperature of 150℃, 250℃ and 350℃ for 1 h, and then calcined at a temperature of 700-800℃ in an argon atmosphere for 30-40 min to obtain a negative electrode material.

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

[0014] Further, the use amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid in step A2 is 5 g:6 g:150 mL:6 g, and the use amount ratio of the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide is 1 g:25 mmol:20 mmol:20 mL.

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

[0016] Step B1: Heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and DMF are mixed uniformly, and the mixture is reacted at a speed of 150-200 r / min and a temperature of 60-70° C. for 2-4 hours. 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 uniformly, and the mixture is 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: Dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, and DMF are uniformly mixed and reacted at a speed of 120-150 r / min and a temperature of 20-25° C. for 1-1.5 hours. Intermediate 2 is then added and the reaction is continued for 20-25 hours. Potassium hydroxide solution is added and the temperature is raised to 40-50° C. and the reaction is continued for 2-3 hours. Hydrochloric acid solution is added to adjust the pH to acidic to obtain a modified ligand;

[0018] Step B3: Cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water, and DMF were mixed uniformly, stirred at a speed of 150-200 r / min and a temperature of 30-40°C, and the modified ligand was added. The mixture was stirred for 30-40 minutes, and the temperature was raised to 140-145°C. Lactic acid was added and the reaction was carried out for 30-35 hours to 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 amount ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution described in step B2 is 1mmol:2mmo l:5mL:2mmo l:3mL, 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] The application has the beneficial effects that: the long cycle lithium iron phosphate battery prepared by the application assembles the positive plate, the negative plate, the porous diaphragm and the electrolyte in an argon-filled glove box into a battery to obtain the long cycle lithium iron phosphate battery, the negative plate is prepared by mixing polyvinylidene fluoride, a negative material and acetylene and loading on copper foil, the negative material is prepared by ultrasonic treatment of modified organic framework and tetraethyl orthosilicate as raw materials, so that the tetraethyl orthosilicate enters the pores of the modified organic framework, and then hydrolysis under the action of ammonia and deionized water, so that the modified organic framework is filled with nanosilica inside, the 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 on the surface into sodium alcohol, and then reacts with the chlorine atom site on chloroacetic acid to obtain a functionalized precursor, the functionalized precursor, 4,4'-diamino diphenyl ether and pyromellitic dianhydride are reacted to coat polyamide acid on the surface of the functionalized precursor to obtain a spinning solution, the spinning solution is electrospun and treated at high temperature to form polyimide, and then baked in an argon atmosphere to obtain the negative material.

[0023] The modified organic framework is prepared by reacting heptafluorobutanol and epichlorohydrin, so that the hydroxyl on the heptafluorobutanol reacts with the epoxy group on the epichlorohydrin, and then ring closing under the action of sodium hydroxide solution to form a new epoxy group to obtain intermediate 1, the p-nitroaniline and the intermediate 1 are reacted under alkaline conditions to make the amino group on the p-nitroaniline react with the epoxy group on the intermediate 1 to obtain intermediate 2, the 2,5-dihydroxy terephthalic acid dimethyl ester and the intermediate 2 are reacted to make the phenolic hydroxyl on the 2,5-dihydroxy terephthalic acid dimethyl ester react with the nitro group on the intermediate 2, and then hydrolysis under the action of potassium hydroxide to convert the formate into formic acid to obtain the modified ligand, the modified ligand and cobalt nitrate hexahydrate are reacted to form a cobalt organic framework to obtain the modified organic framework.

[0024] The negative material contains silica inside, so that the lithium storage performance of the negative material is improved, and high-temperature carbonization treatment makes the organic framework form a grid carbon layer coated on the surface of the silica to prevent the volume expansion of the silica in the negative material due to charging and discharging, thereby ensuring the stability of the negative material, the grid carbon layer increases the conductivity of the negative material, and the organic framework forms fluorine and nitrogen doped carbon after carbonization, the introduction of fluorine atoms and nitrogen atoms produces defects in the carbon material lattice, and these defects can act as channels for charge transport, thereby improving the overall conductivity of the material, the surface molybdenum disulfide has a large interlayer spacing which is beneficial to the embedding of lithium ions, so that fast de-embedding lithium reaction can be realized, thereby improving the overall lithium storage performance of the lithium ion battery, and the overall carbon layer coated on the outside of the negative material limits the expansion of the negative material, thereby improving the service life of the lithium iron phosphate battery. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0026] Embodiment 1

[0027] A preparation method of a long-cycle lithium iron phosphate battery, specifically comprising the following steps:

[0028] Step S1: polyvinylidene fluoride is dissolved in N-methyl pyrrolidone, the mass fraction of polyvinylidene fluoride is 5%, lithium iron phosphate and carbon black are added and uniformly mixed, a doctor blade coating method is used to uniformly load the circular aluminum foil, and a positive electrode sheet is prepared by drying;

[0029] Step S2: polyvinylidene fluoride is dissolved in N-methyl pyrrolidone, a negative electrode material and acetylene black are added and uniformly mixed, a doctor blade coating method is used to uniformly load the copper foil, and a negative electrode sheet is prepared by drying;

[0030] Step S3: the positive electrode sheet, the negative electrode sheet, a porous diaphragm and an electrolyte are assembled into a battery in an argon-filled glove box to prepare a long-cycle lithium iron phosphate battery.

[0031] The mass ratio of polyvinylidene fluoride to N-methyl pyrrolidone 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-methyl pyrrolidone in step S2 is 1:20, and the mass ratio of polyvinylidene fluoride, the negative electrode material and carbon black is 10:85:5.

[0033] The porous diaphragm in step S3 is a Celgard 2400 polypropylene porous membrane, and the electrolyte is a 1 mol / L LiPF6 solution, and 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: modified organic framework, tetraethyl orthosilicate and ethanol are mixed, ultrasonic treatment is performed at a frequency of 30 kHz and a temperature of 20℃ for 30 min, deionized water and ammonia water are added, the temperature is raised to 50℃, and ultrasonic reaction is performed for 20 h to prepare a modified carrier, the modified carrier, sodium molybdate dihydrate, thiourea and deionized water are mixed, stirring is performed at a rotation speed of 200 r / min and a temperature of 25℃ for 20 min, the temperature is raised to 220℃, and reaction is performed for 20 h to prepare a precursor;

[0036] Step A2: the precursor, sodium hydroxide and ethanol were mixed uniformly, and after reaction at a rotation speed of 120 r / min and a temperature of 25℃ for 1h, chloroacetic acid was added, and the temperature was raised to 75℃ and reacted for 2h to obtain a functionalized precursor; the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide were mixed uniformly, and then nitrogen was introduced for protection, and the mixture was stirred at a rotation speed of 500 r / min and a temperature of 40℃ for 6h to obtain a spinning solution;

[0037] Step A3: the spinning solution was electrospun at a spinning speed of 1mL / h, a spinning voltage of 15kV and a receiving distance of 15cm to obtain a modified film; the modified film was sequentially treated at a temperature of 150℃, 250℃ and 350℃ for 1h, and then calcined at a temperature of 700℃ in an argon atmosphere for 30min to obtain the negative electrode material.

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

[0039] The use amount ratio of the precursor, sodium hydroxide, ethanol and chloroacetic acid in step A2 was 5g:6g:150mL:6g; the use amount ratio of the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide was 1g:25mmol:20mmol:20mL.

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

[0041] Step B1: heptafluorobutanol, epichlorohydrin, boron trifluoride ether and DMF were mixed uniformly, and after reaction at a rotation speed of 150 r / min and a temperature of 60℃ for 2h, sodium hydroxide solution was added, and the temperature was raised to 80℃ and reacted for 4h to obtain intermediate 1; intermediate 1, p-nitroaniline and DMF were mixed uniformly, and after reaction at a rotation speed of 120 r / min, a temperature of 40℃ and a pH value of 10 for 3h, intermediate 2 was obtained;

[0042] Step B2: 2,5-dihydroxy terephthalic acid dimethyl ester, potassium carbonate and DMF were mixed uniformly, and after reaction at a rotation speed of 120 r / min and a temperature of 20℃ for 1h, intermediate 2 was added and the reaction was continued for 20h; potassium hydroxide solution was added, the temperature was raised to 40℃ and the reaction was continued for 2h; hydrochloric acid solution was added to adjust the pH value to be acidic to obtain a modified ligand;

[0043] Step B3: The cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water and DMF were mixed uniformly, stirred at a speed of 150 r / min and a temperature of 30℃, and the modified ligand was added, stirred for 30 min, heated to 140℃, and lactic acid was added and reacted for 30 h to obtain the modified organic framework.

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

[0045] The use amount ratio of 2,5-dihydroxy terephthalic acid dimethyl ester, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution 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 use amount ratio of cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water, modified ligand and lactic acid in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0047] Example 2

[0048] A preparation method of a long-cycle lithium iron phosphate battery, specifically comprising the following steps:

[0049] Step S1: Polyvinylidene fluoride was dissolved in N-methyl pyrrolidone, the mass fraction of polyvinylidene fluoride was 5%, lithium iron phosphate and carbon black were added and uniformly mixed, a circular aluminum foil was uniformly loaded by using a doctor blade coating method, and the positive electrode sheet was dried to obtain a positive electrode sheet;

[0050] Step S2: Polyvinylidene fluoride was dissolved in N-methyl pyrrolidone, the negative electrode material and acetylene black were added and uniformly mixed, a copper foil was uniformly loaded by using a doctor blade coating method, and the negative electrode sheet was dried to obtain a negative electrode sheet;

[0051] Step S3: The positive electrode sheet, the negative electrode sheet, the porous diaphragm and the electrolyte were assembled into a battery in an argon-filled glove box to obtain a long-cycle lithium iron phosphate battery.

[0052] The mass ratio of polyvinylidene fluoride and N-methyl pyrrolidone 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 and N-methyl pyrrolidone in step S2 is 1:20, and the mass ratio of polyvinylidene fluoride, the negative electrode material and carbon black is 10:85:5.

[0054] The porous diaphragm in step S3 is Celgard 2400 polypropylene porous film, and the electrolyte is 1 mol / L LiPF6 solution, and 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: mix the modified organic framework, tetraethyl orthosilicate and ethanol, ultrasonic treatment for 35 min under the condition of a frequency of 40 kHz and a temperature of 20℃, add deionized water and ammonia water, heat to 50-60℃, and ultrasonic reaction for 25 h to prepare a modified carrier; mix the modified carrier, sodium molybdate dihydrate, thiourea and deionized water, stir for 25 min under the condition of a rotation speed of 200 r / min and a temperature of 30℃, heat to 225℃, and react for 25 h to prepare a precursor;

[0057] Step A2: mix the precursor, sodium hydroxide and ethanol uniformly, react for 1.5 h under the condition of a rotation speed of 150 r / min and a temperature of 25℃, add chloroacetic acid, heat to 78℃, and react for 2.5 h to prepare a functionalized precursor; mix the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide uniformly, protect under nitrogen, stir for 7 h under the condition of a rotation speed of 600 r / min and a temperature of 45℃ to prepare a spinning solution;

[0058] Step A3: electrospun the spinning solution under the condition of a spinning speed of 1.5 mL / h, a spinning voltage of 18 kV and a receiving distance of 20 cm to prepare a modified film; heat the modified film under the condition of a temperature of 150℃, 250℃ and 350℃ for 1 h respectively, and then calcine the modified film under the condition of a temperature of 750℃ and an argon atmosphere for 35 min to prepare the negative electrode material.

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

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

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

[0062] Step B1: Seven fluorobutanol, epichlorohydrin, boron trifluoride ether and DMF were mixed uniformly, and after reaction for 2-4 h at a rotation speed of 150 r / min and a temperature of 65 DEG C, sodium hydroxide solution was added, and the temperature was raised to 80 DEG C for reaction for 5 h to prepare intermediate 1; intermediate 1, p-nitroaniline and DMF were mixed uniformly, and after reaction for 4 h at a rotation speed of 120 r / min, a temperature of 45 DEG C and a pH value of 11, intermediate 2 was prepared;

[0063] Step B2: 2,5-dihydroxy terephthalic acid dimethyl ester, potassium carbonate and DMF were mixed uniformly, and after reaction for 1 h at a rotation speed of 120 r / min and a temperature of 25 DEG C, intermediate 2 was added, and the reaction was continued for 25 h; potassium hydroxide solution was added, the temperature was raised to 45 DEG C, and the reaction was carried out for 3 h; hydrochloric acid solution was added to adjust the pH value to be acidic to prepare the modified ligand;

[0064] Step B3: Cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water and DMF were mixed uniformly, and after stirring at a rotation speed of 150 r / min and a temperature of 35 DEG C, the modified ligand was added, and stirring was carried out for 35 min; the temperature was raised to 145 DEG C, and lactic acid was added, and the reaction was carried out for 30 h to prepare the modified organic framework.

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

[0066] The use amount ratio of 2,5-dihydroxy terephthalic acid dimethyl ester, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution 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 use amount ratio of cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water, modified ligand and lactic acid in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0068] Example 3

[0069] A preparation method of a long-cycle lithium iron phosphate battery, specifically comprising the following steps:

[0070] Step S1: Polyvinylidene fluoride was dissolved in N-methyl pyrrolidone, the mass fraction of polyvinylidene fluoride was 5%, lithium iron phosphate and carbon black were mixed uniformly, and a circular aluminum foil was uniformly loaded by using a doctor blade coating method, and dried to prepare a positive electrode sheet;

[0071] Step S2: Dissolve polyvinylidene fluoride in N-methyl pyrrolidone, add negative electrode material and acetylene black, mix uniformly, use doctor blade coating method, uniformly load on copper foil, dry to obtain negative electrode sheet;

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

[0073] The mass ratio of polyvinylidene fluoride and N-methyl pyrrolidone 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 and N-methyl pyrrolidone 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 separator in step S3 is Celgard 2400 polypropylene porous membrane, and the electrolyte is 1 mol / L LiPF6 solution, and 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: Mix modified organic framework, tetraethyl orthosilicate and ethanol, ultrasonic treatment for 40 min under the condition of frequency 50 kHz and temperature 25℃, add deionized water and ammonia water, heat to 60℃, ultrasonic reaction for 25 h, prepare modified carrier, mix modified carrier, sodium molybdate dihydrate, thiourea and deionized water, stirring for 30 min under the condition of rotation speed 300 r / min and temperature 30℃, heat to 230℃, reaction for 25 h, prepare precursor;

[0078] Step A2: Mix the precursor, sodium hydroxide and ethanol uniformly, react for 1.5 h under the condition of rotation speed 150 r / min and temperature 30℃, add chloroacetic acid, heat to 80℃, react for 3 h, prepare functionalized precursor, mix the functionalized precursor, 4,4'-diamino diphenyl ether, pyromellitic dianhydride and N,N-dimethylacetamide uniformly, protect with nitrogen, stirring treatment for 8 h under the condition of rotation speed 600 r / min and temperature 50℃, prepare spinning solution;

[0079] Step A3: Electrospinning the spinning solution under the condition of spinning speed 1.5 mL / h, spinning voltage 20 kV and receiving distance 20 cm, prepare modified film, heat treatment for 1 h under the condition of temperature 150℃, 250℃ and 350℃ in sequence, then calcine under the condition of temperature 800℃ and argon atmosphere for 40 min, prepare negative electrode material.

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

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

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

[0083] Step B1: seven fluorobutanol, epichlorohydrin, boron trifluoride ether and DMF are uniformly mixed, and after reaction at a rotation speed of 200r / min and a temperature of 70℃ for 4h, sodium hydroxide solution is added, the temperature is raised to 85℃, and reaction is carried out for 6h to prepare intermediate 1; intermediate 1, p-nitroaniline and DMF are uniformly mixed, and after reaction at a rotation speed of 150r / min, a temperature of 50℃ and a pH value of 11 for 5h, intermediate 2 is prepared;

[0084] Step B2: 2,5-dihydroxy terephthalic acid dimethyl ester, potassium carbonate and DMF are uniformly mixed, and after reaction at a rotation speed of 150r / min and a temperature of 25℃ for 1.5h, intermediate 2 is added, and reaction is continued for 25h; potassium hydroxide solution is added, the temperature is raised to 50℃, and reaction is carried out for 3h; hydrochloric acid solution is added to adjust the pH value to be acidic, and the modified ligand is prepared;

[0085] Step B3: cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water and DMF are uniformly mixed, and after stirring at a rotation speed of 200r / min and a temperature of 40℃ for 40min, the modified ligand is added, the temperature is raised to 145℃, lactic acid is added, and reaction is carried out for 35h to prepare the modified organic framework.

[0086] The use amount ratio of seven fluorobutanol, epichlorohydrin, boron trifluoride ether 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.

[0087] The use ratio of dimethyl 2,5-dihydroxyterephthalate, potassium carbonate, DMF, intermediate 2 and potassium hydroxide solution 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 use ratio of cobalt nitrate hexahydrate, cetyltrimethylammonium bromide, deionized water, modified ligand and lactic acid in step B3 is 50 mg: 1 mg: 5 mL: 70 mg: 1 μL.

[0089] Comparative Example 1

[0090] The present comparative example is compared with example 1 by using a precursor instead of a negative electrode material, and the remaining steps are the same.

[0091] Comparative Example 2

[0092] The present comparative example is compared with example 1 by using a modified carrier instead of a precursor, and the remaining steps are the same.

[0093] Comparative Example 3

[0094] The present comparative example is compared with example 1 by using terephthalic acid instead of a modified ligand, and the remaining steps are the same.

[0095] Comparative Example 4

[0096] The present comparative example is compared with example 1 by using a modified organic framework instead of a modified carrier, and the remaining steps are the same.

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

[0098]

[0099] From the above table, it can be seen that the present application has a good cycle life.

[0100] The above is only an example and description of the present application, and those skilled in the art can make various modifications or supplements to the described specific examples or use similar ways to replace, as long as the modifications or supplements do not deviate from the concept of the present application or exceed the scope defined by the present claims, and should belong to the protection scope of the present application.

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 to a mass fraction of 5%, adding lithium iron phosphate and carbon black and mixing evenly, and evenly loading the mixture onto a circular aluminum foil using 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 the mixture on copper foil using a doctor blade coating method, and drying to obtain a negative electrode sheet; Step S3: assembling 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 prepare a long-cycle lithium iron phosphate battery; The negative electrode material is prepared by the following steps: Step A1: A modified organic framework, tetraethyl orthosilicate, and ethanol are mixed and ultrasonically treated, deionized water and ammonia are added, and the temperature is increased to react so that nano-silica is filled inside the modified organic framework to prepare a modified support. The modified support, sodium molybdate dihydrate, thiourea, and deionized water are mixed and reacted to load molybdenum disulfide on the pores on the surface of the modified support to prepare a precursor; Step A2: After the precursor, sodium hydroxide, and ethanol are mixed and reacted, chloroacetic acid is added and the temperature is raised to react to obtain a functionalized precursor. The functionalized precursor, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride, and N,N-dimethylacetamide are uniformly mixed, nitrogen is introduced for protection, and the mixture is stirred to coat the surface of the functionalized precursor with polyamic acid to obtain a spinning solution. Step A3: electrospinning the spinning solution to obtain a modified film, preserving the modified film at a high temperature to form polyimide, and then calcining the film to obtain a negative electrode material; 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, and hydrochloric acid solution is added to adjust the pH value to acidic to obtain a modified ligand; Step B3: Cobalt nitrate hexahydrate, hexadecyltrimethylammonium bromide, deionized water and DMF are mixed and stirred, and a modified ligand is added thereto. The mixture is stirred, heated, and lactic acid is added thereto for reaction to obtain a modified organic framework.

2. The method for preparing a long-cycle lithium iron phosphate battery according to claim 1, wherein: The modified organic framework, ethyl orthosilicate, ethanol, deionized water and ammonia described in step A1 are used in a ratio of 1 g: 3 mL: 80 mL: 5 mL: 2 mL, and the modified support, sodium molybdate dihydrate, thiourea and deionized water are used in a ratio of 5 g: 2.5 mmol: 5 mmol: 40 mL.

3. The method for preparing a long-cycle lithium iron phosphate battery according to claim 1, wherein: 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.

4. The method for preparing a long-cycle lithium iron phosphate battery according to claim 1, wherein: The amount ratio of heptafluorobutanol, epichlorohydrin, boron trifluoride etherate and sodium hydroxide solution described 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.

5. The method for preparing a long-cycle lithium iron phosphate battery according to claim 1, wherein: 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.

6. The method for preparing a long-cycle lithium iron phosphate battery according to claim 1, wherein: 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.

7. A long cycle lithium iron phosphate battery, characterized in that: Prepared according to any one of claims 1 to 6.

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