Ultra-high current lithium iron phosphate battery and its preparation method

Abstract

The present invention relates to the technical field of battery packs, and particularly to an ultra-high current lithium iron phosphate battery and a preparation method thereof. The lithium iron phosphate battery in the present invention is first prepared by preparing a positive electrode slurry, coating the obtained positive electrode slurry on the surface of an aluminum foil to obtain a positive electrode sheet, then assembling the positive electrode sheet with a negative electrode and a separator, and injecting an electrolyte and sealing it to obtain the battery. Compared with the prior art, the present invention improves the charge and discharge capacity of the material under high current by coating lithium iron phosphate with modified carbon nanotubes, improves the battery rate performance and cycle performance, and can meet the market demand for high-energy and high-power lithium ion batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of battery packs, and particularly to an ultra-high current lithium iron phosphate battery and a preparation method thereof. Background Art

[0002] As a highly regarded cathode material for lithium-ion batteries, lithium iron phosphate is renowned for its stable olivine structure. The batteries made from this material outperform lithium-ion batteries with other cathode materials in terms of safety and cycle life, and are regarded as a strong candidate to replace traditional petrochemical energy sources. They show great potential especially in the large-scale applications of electric vehicles and energy storage systems.

[0003] CN106876705B provides a preparation method for in-situ synthesizing a carbon / carbon nanotube-coated lithium iron phosphate composite material, which relates to the technical field of battery materials. The preparation method of this invention is as follows: weigh the raw materials of lithium source, iron powder, phosphate, and carbon source; first ball-mill the iron powder and phosphate, add hydrogen peroxide; then add the lithium source and carbon source to obtain a slurry, dry it, and sinter it under the protection of reducing / inert gas. This invention uses an in-situ synthesis method to prepare a carbon / carbon nanotube-coated lithium iron phosphate composite material, with a short heat treatment time; the composite material has a high carbon coating rate, stable electrochemical performance and good consistency, and its cycle performance and rate performance are greatly improved. The entire preparation process flow is simple, and it has the advantages of safety, high efficiency, low cost, and environmental friendliness.

[0004] CN102227024B discloses a lithium iron phosphate cathode material suitable for power lithium-ion batteries and a preparation method thereof. The technical problem to be solved is to obtain a higher capacity. The lithium iron phosphate cathode material suitable for power lithium-ion batteries of this invention has a doped lithium iron phosphate as the matrix, and is coated with carbon outside the matrix. The mass ratio is: lithium iron phosphate 50 - 99%, dopant 0.01 - 49%, and carbon precursor 0.01 - 20%. The preparation method of this invention includes the following steps: mixing, doping, carbon coating, and sintering. Compared with the prior art, the lithium iron phosphate material coated with a carbon source by a multi-component composite method has an electronic conductivity of 6.2×10 -3 Scm -1 , a reversible specific capacity greater than 155 mAh / g, excellent rate performance, a 20C / 1C retention rate greater than 90%, and stable safety performance. It is applicable to power batteries, energy storage batteries, electric tools, and various portable device batteries of lithium-ion batteries.

[0005] However, there are some challenges with lithium iron phosphate materials themselves. Their inherent conductivity is relatively low, and the diffusion coefficient of lithium ions is not high either. This results in unsatisfactory performance of lithium iron phosphate batteries under low-temperature conditions, and mediocre performance in high-rate charge and discharge as well as maximum current. These limitations have hindered their applications in power batteries for electric vehicles (including pure electric vehicles and hybrid vehicles), starting power supplies, and load regulation for peak and valley in smart grids. Summary of the Invention

[0006] In view of the above defects of the prior art, the technical problem to be solved by the present invention is to provide a lithium iron phosphate battery with ultra-high current and a preparation method thereof.

[0007] Lithium iron phosphate has advantages such as low price, excellent cycle performance, and safety and stability, and has become the main force of the cathode material for lithium-ion batteries. However, the arrangement of Fe, O, and P atoms in the bulk structure restricts the + transport of Li + The capacity of the battery decays rapidly at high currents, and the rate performance is poor. Therefore, how to improve the diffusion rate of Li + in LiFePO4 to improve the charge and discharge capacity and discharge current of the battery has become the key. Currently, surface modification and element doping are mainly adopted to improve the conductivity, or the diffusion distance of Li + is shortened by nanoparticle size reduction to improve the migration ability. The most important way of surface modification of lithium iron phosphate is carbon coating. Although carbon materials such as carbon nanotubes and graphene have excellent electrochemical properties, there are problems such as uneven structure coverage and easy agglomeration. Currently, acidification of carbon nanotubes is mostly used to improve the defect of easy agglomeration, but this method will damage the surface structure of carbon nanotubes. Therefore, the present invention provides a method for non-covalent modification of carbon nanotubes. Polyethylene glycol capped with oligofuran is mixed with carbon nanotubes. Since polyethylene glycol capped with oligofuran has amphiphilicity, and furan, as an aromatic ring with π electron cloud, can form π-π interactions including van der Waals forces and electrostatic interactions with the surface of carbon nanotubes. Thus, the oligofuran end can be well adsorbed on the surface of carbon nanotubes, while the hydrophilic end of polyethylene glycol can enhance the dispersion of carbon nanotubes in water. This non-covalent modification method not only improves the defect of easy agglomeration of carbon nanotubes during dispersion, but also enhances the dispersion of carbon nanotubes in solvents. The modified carbon nanotube dispersion liquid is sintered with iron phosphate, lithium carbonate, etc. to obtain the cathode material. The polyethylene glycol capped with oligofuran on the surface of carbon nanotubes is cracked into a carbon network under high temperature, which acts as a conductive layer and a binder. The carbon network covers the surface of lithium iron phosphate, thus forming a network-like fast conductive network, which can well improve the + diffusion rate of Li + in LiFePO4, thereby improving the charge and discharge capacity and discharge current of the battery.

[0008] To achieve the above object, the present invention provides a method for preparing a lithium iron phosphate battery with ultra-large current, comprising the following steps:

[0009] S1. Add lithium iron phosphate cathode material, conductive material, and binder to N-methylpyrrolidone, stir evenly to obtain a slurry, evenly coat it on aluminum foil, and compact it after baking to obtain a positive electrode plate;

[0010] S2. Use the above positive electrode plate as the positive electrode, graphite as the negative electrode, and polypropylene film as the separator. After assembly, inject electrolyte and seal it to obtain the battery.

[0011] Further, the conductive material is acetylene black.

[0012] Further, the binder is polyvinylidene fluoride.

[0013] Further, the mass ratio of the lithium iron phosphate cathode material, conductive material, and binder is 100:3-5:2-6.

[0014] Further, the solid content of the positive electrode slurry is 40 wt%.

[0015] Further, the electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1.

[0016] Preferably, the method for preparing the lithium iron phosphate battery with ultra-large current comprises the following steps:

[0017] S1. Add lithium iron phosphate cathode material, acetylene black, and polyvinylidene fluoride in a mass ratio of 100:3-5:2-6 to N-methylpyrrolidone, stir evenly to obtain a slurry. The solid content of the positive electrode slurry is 40 wt%, evenly coat it on aluminum foil, and compact it after baking to obtain a positive electrode plate;

[0018] S2. Use the above positive electrode plate as the positive electrode, graphite as the negative electrode, and polypropylene film as the separator. After assembly, inject electrolyte and seal it. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1 to obtain the battery.

[0019] The preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0020] X1. Add 3-furanacetic acid to N,N-dimethylformamide, then add N-bromosuccinimide. Stir at room temperature for 16 - 24 h under a nitrogen atmosphere. Dilute with ethyl acetate and extract with water three times. After synthesis, the organic phase is dried and concentrated to obtain a residue, which is recrystallized from ethanol for the next step;

[0021] X2. Mix the product from the previous step with methoxypolyethylene glycol-hydroxyl, heat up to remove water, add dichloromethane and 4-dimethylaminopyridine, then dropwise add a dichloromethane solution of 1 mol / L dicyclohexylcarbodiimide. Stir at room temperature for 40 - 60 h and then filter. When ether is added to the filtrate, a precipitate appears. Filter again. After drying the residue, add it together with 2-(tributylstannyl)furan to N,N-dimethylformamide. Under a nitrogen atmosphere, add bis(triphenylphosphine)palladium dichloride, heat up and stir for 40 - 60 h, then dilute with ether. A precipitate appears. Filter. The residue after drying is the polyethylene glycol capped with oligofuran. Mix the polyethylene glycol capped with oligofuran and multi-walled carbon nanotubes at a mass ratio of 10:1 and add them to water to obtain a 10 wt% modified carbon nanotube dispersion;

[0022] X3. Weigh and mix Li2CO3 and FePO4 according to a molar ratio of 1:1, add 12 wt% glucose, mix evenly by ball milling, then add the modified carbon nanotube dispersion, so that the addition amount of the modified carbon nanotubes is 1 - 5 wt%. After mixing evenly by ball milling, spray dry and then calcine at 700 - 800 °C at a rate of 5 °C / min for 4 - 6 h to obtain the lithium iron phosphate cathode material.

[0023] Further, the molar ratio of 3-furanacetic acid to N-bromosuccinimide is 3 - 5:8.

[0024] Further, the molar ratio of the product from the previous step in step X2 to methoxypolyethylene glycol-hydroxyl, 4-dimethylaminopyridine, and dicyclohexylcarbodiimide is 5 - 15:5 - 10:1:1.

[0025] Further, the temperature range for heating and stirring in step X2 is 50 - 60 °C.

[0026] Further, the molar ratio of the residue in step X2 to 2-(tributylstannyl)furan and bis(triphenylphosphine)palladium dichloride is 1:4 - 5:0.1 - 0.25.

[0027] Preferably, the preparation method of the lithium iron phosphate cathode material includes the following steps:

[0028] X1. Add 3-furan acetic acid to N,N-dimethylformamide, and then add N-bromosuccinimide. The molar ratio of 3-furan acetic acid to N-bromosuccinimide is 3-5:8. Stir at room temperature for 16-24 h under a nitrogen atmosphere. Dilute with ethyl acetate and extract with water three times. After synthesis, the organic phase is dried and concentrated to obtain a residue, which is recrystallized with ethanol for the next step;

[0029] X2. Mix the product of the previous step with methoxypolyethylene glycol-hydroxyl, heat up to remove water, add dichloromethane and 4-dimethylaminopyridine, and then dropwise add a dichloromethane solution of 1 mol / L dicyclohexylcarbodiimide. Stir at room temperature for 40-60 h and then filter. The molar ratio of the product of the previous step to methoxypolyethylene glycol-hydroxyl, 4-dimethylaminopyridine, and dicyclohexylcarbodiimide is 5-15:5-10:1:1. Precipitation appears after adding ether to the filtrate. Filter, dry the residue, and add it together with 2-(tributylstannyl)furan to N,N-dimethylformamide. Add bis(triphenylphosphine)palladium dichloride under a nitrogen atmosphere. The molar ratio of the residue to 2-(tributylstannyl)furan and bis(triphenylphosphine)palladium dichloride is 1:4-5:0.1-0.25. Heat up and stir for 40-60 h, then dilute with ether, precipitation appears, filter, and dry the residue to obtain oligofuran-capped polyethylene glycol. Mix oligofuran-capped polyethylene glycol and multi-walled carbon nanotubes at a mass ratio of 10:1 and add them to water to obtain a 10 wt% modified carbon nanotube dispersion;

[0030] X3. Weigh Li2CO3 and FePO4 according to a molar ratio of 1:1 and mix them, and add 12 wt% glucose. After ball milling and mixing evenly, add the modified carbon nanotube dispersion so that the addition amount of the modified carbon nanotubes is 1-5 wt%. After ball milling evenly, spray dry and then heat up to 700-800 °C at a rate of 5 °C / min and calcine for 4-6 h to obtain the lithium iron phosphate cathode material.

[0031] The present invention also provides an ultra-high current lithium iron phosphate battery, which is prepared by the above method.

[0032] Advantages of the present invention:

[0033] By coating lithium iron phosphate with modified carbon nanotubes, the present invention improves the disadvantages of poor electronic conductivity and poor ionic conductivity of the lithium iron phosphate material, improves the charge and discharge ability of the material under high current, improves the battery rate performance and cycle performance, and can meet the market demand for high-energy and high-power lithium-ion batteries. Detailed implementation method

[0034] Methoxypolyethylene glycol-hydroxyl, product number: YS-M4201, Mw = 2000, Chongqing Yusai Medical Technology.

[0035] Multi-walled carbon nanotubes, length: 10 - 20 μm, diameter: 30 - 60 nm.

[0036] Example 1

[0037] A preparation method of an ultra-high current lithium iron phosphate battery, comprising the following steps:

[0038] S1. Add 100 g of lithium iron phosphate cathode material, 4 g of acetylene black, and 5 g of polyvinylidene fluoride to N-methylpyrrolidone and stir evenly to obtain a slurry. The solid content of the positive electrode slurry is 40 wt%, evenly coated on aluminum foil, and compacted after baking to obtain a positive electrode sheet;

[0039] S2. Using the above positive electrode sheet as the positive electrode, graphite as the negative electrode, and polypropylene film as the separator, assemble and inject the electrolyte and then seal. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1, thus obtaining the battery.

[0040] The preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0041] X1. Add 5 g of 3-furanoic acid to 50 mL of N,N-dimethylformamide, then add 14.24 g of N-bromosuccinimide, stir at room temperature for 20 h under a nitrogen atmosphere, dilute with ethyl acetate and extract with water 3 times. After the organic phase is synthesized, it is dried and concentrated to obtain a residue, which is recrystallized from ethanol for the next step;

[0042] X2. Mix 3.2 g of the product from the previous step with 15 g of methoxypolyethylene glycol-hydroxyl, raise the temperature to 80 °C to remove moisture, add 100 mL of dichloromethane and 0.13 g of 4-dimethylaminopyridine, then dropwise add 10.6 mL of a dichloromethane solution of 1 mol / L dicyclohexylcarbodiimide, stir at room temperature for 50 h and then filter. Precipitation appears after adding ether to the filtrate, filter again. After the residue is dried, it is added to 50 mL of N,N-dimethylformamide together with 6 g of 2-(tributylstannyl)furan. Under a nitrogen atmosphere, add 0.5 g of bis(triphenylphosphine)palladium dichloride, raise the temperature to 60 °C and stir for 50 h, then dilute with ether, precipitation appears, filter. After the residue is dried, the oligofuran-capped polyethylene glycol is obtained. The oligofuran-capped polyethylene glycol and multi-walled carbon nanotubes are mixed at a mass ratio of 10:1 and then added to water to obtain a 10 wt% modified carbon nanotube dispersion;

[0043] X3. Mix 73.89 g of Li2CO3 and 150.79 g of FePO4, add 26.96 g of glucose, and after ball-milling and mixing evenly, add the modified carbon nanotube dispersion so that the addition amount of the modified carbon nanotubes is 4 wt%. After ball-milling evenly, spray-dry and then heat at a rate of 5 °C / min to 750 °C and calcine for 5 h to obtain the lithium iron phosphate cathode material.

[0044] Example 2

[0045] It is basically the same as Example 1, and the only difference is that the addition amount of the modified carbon nanotubes is 1 wt%.

[0046] Example 3

[0047] It is basically the same as Example 1, and the only difference is that the addition amount of the modified carbon nanotubes is 2 wt%.

[0048] Example 4

[0049] It is basically the same as Example 1, and the only difference is that the addition amount of the modified carbon nanotubes is 3 wt%.

[0050] Example 5

[0051] It is basically the same as Example 1, and the only difference is that the addition amount of the modified carbon nanotubes is 5 wt%.

[0052] Control Example 1

[0053] A preparation method of an ultra-high current lithium iron phosphate battery includes the following steps:

[0054] S1. Add 100 g of the lithium iron phosphate cathode material, 4 g of acetylene black, and 5 g of polyvinylidene fluoride to N-methylpyrrolidone, stir evenly to obtain a slurry, the solid content of the positive electrode slurry is 40 wt%, evenly coat it on the aluminum foil, and after baking, press it to obtain the positive electrode plate;

[0055] S2. Using the above positive electrode plate as the positive electrode, graphite as the negative electrode, and polypropylene film as the separator, assemble and then inject the electrolyte and seal it. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, the concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1, thus obtaining the battery.

[0056] The preparation method of the lithium iron phosphate cathode material includes the following steps:

[0057] Mix 73.89 g of Li2CO3 and 150.79 g of FePO4, add 26.96 g of glucose, and after ball-milling and mixing evenly, heat at a rate of 5 °C / min to 750 °C and calcine for 5 h to obtain the lithium iron phosphate cathode material.

[0058] Comparative Example 2

[0059] A method for preparing a lithium iron phosphate battery with an ultra-large current, comprising the following steps:

[0060] S1. Add 100 g of lithium iron phosphate cathode material, 4 g of acetylene black, and 5 g of polyvinylidene fluoride to N-methylpyrrolidone, stir evenly to obtain a slurry. The solid content of the cathode slurry is 40 wt%, evenly coat it on aluminum foil, and after baking, compact it to obtain a cathode plate;

[0061] S2. Use the above cathode plate as the cathode, graphite as the anode, and polypropylene film as the separator. After assembly, inject the electrolyte and seal it. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1, thus obtaining the battery.

[0062] The preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0063] Mix 73.89 g of Li2CO3 and 150.79 g of FePO4, add 26.96 g of glucose, mix evenly by ball milling, then add 10 wt% carbon nanotube dispersion liquid so that the addition amount of carbon nanotubes is 4 wt%. After ball milling evenly, spray dry and then heat it at a rate of 5 °C / min to 750 °C and calcine for 5 h to obtain the lithium iron phosphate cathode material.

[0064] Comparative Example 3

[0065] A method for preparing a lithium iron phosphate battery with an ultra-large current, comprising the following steps:

[0066] S1. Add 100 g of lithium iron phosphate cathode material, 4 g of acetylene black, and 5 g of polyvinylidene fluoride to N-methylpyrrolidone, stir evenly to obtain a slurry. The solid content of the cathode slurry is 40 wt%, evenly coat it on aluminum foil, and after baking, compact it to obtain a cathode plate;

[0067] S2. Use the above cathode plate as the cathode, graphite as the anode, and polypropylene film as the separator. After assembly, inject the electrolyte and seal it. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The concentration of lithium hexafluorophosphate is 1 moL / L, and the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1, thus obtaining the battery.

[0068] The preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0069] X1. Add 2 g of multi-walled carbon nanotubes to 100 mL of concentrated mixed acid (concentrated sulfuric acid: concentrated nitric acid = 3:1, v:v), stir evenly, perform ultrasonic dispersion at 400 W and 45 kHz for 6 h at 80 °C, filter, wash until the filtrate is neutral, and dry the residue to obtain acidified carbon nanotubes;

[0070] X2. Mix 73.89 g of Li2CO3 and 150.79 g of FePO4, add 26.96 g of glucose, after ball-milling and mixing evenly, add a 10 wt% acidified carbon nanotube dispersion solution so that the addition amount of acidified carbon nanotubes is 4 wt%, after ball-milling evenly, spray-dry and then heat up to 750 °C at a rate of 5 °C / min and calcine for 5 h to obtain a lithium iron phosphate cathode material.

[0071] Test Example 1

[0072] Measure the initial discharge specific capacity of each battery respectively. The measurement results are shown in Table 1. The measurement method is as follows: The nominal specific capacity is 170 mAh / g. Constant current charge at a rate of 0.1C to 4.2V, stand still for 2 min, and then constant current discharge at a rate of 0.1C to 2.5V. Stop the test and record the value.

[0073] Table 1 Charge and discharge test results of the battery

[0074] Experimental Scheme Initial Discharge Specific Capacity mAh / g Example 1 165.4 Example 2 140.2 Example 3 144.2 Example 4 140.2 Example 5 158.3 Control Example 1 130.1 Control Example 2 135.5 Control Example 3 138.4

[0075] From the data in the table, it can be seen that when carbon nanotubes are added to coat lithium iron phosphate, the discharge specific capacity increases significantly. This is because after coating, the carbon nanotubes can form a three-dimensional conductive network structure on the surface of LiFePO4 particles. The excellent conductivity of the carbon nanotubes improves the electron transfer rate on the surface of LiFePO4 particles, the electronic conductivity of LiFePO4 is improved, the conductivity of the composite material is significantly increased, and the electrochemical reaction can proceed more rapidly. Comparing with Comparative Examples 1-3, it can illustrate that modifying the carbon nanotubes can further enhance the conductivity. This may be because the modification can improve the dispersibility of the carbon nanotubes, so that they can better wrap on the surface of LiFePO4 when mixed with it and are not prone to agglomeration. Comparing Comparative Example 3 with Example 1, the electrochemical performance of the battery in Example 1 is significantly better. This may be because acidifying the carbon nanotubes will damage the surface structure of the carbon nanotubes, while in Example 1, a non-covalent modification method of carbon nanotubes is adopted, mixing oligofuran-capped polyethylene glycol with carbon nanotubes. Since oligofuran-capped polyethylene glycol has amphiphilicity, and furan, as an aromatic ring with π electron cloud, can form π-π interactions including van der Waals forces and electrostatic interactions with the surface of the carbon nanotubes. Therefore, the oligofuran end can be well adsorbed on the surface of the carbon nanotubes, and the hydrophilic end of polyethylene glycol can enhance the dispersibility of the carbon nanotubes in water. This non-covalent modification method not only improves the defect that the carbon nanotubes are prone to agglomeration during dispersion, but also enhances the dispersibility of the carbon nanotubes in the solvent. The modified carbon nanotube dispersion liquid is sintered with iron phosphate, lithium carbonate, etc. to obtain the cathode material. The oligofuran-capped polyethylene glycol on the surface of the carbon nanotubes is cracked into a carbon network under high temperature, which plays the role of a conductive layer and a binder. The carbon network covers the surface of lithium iron phosphate to form a reticular fast conductive network, which can better improve the diffusion rate of Li + in LiFePO4. Therefore, the battery performance in Example 1 is the best. Comparing Example 1 with Examples 2-5, the addition amount of the modified carbon nanotubes is different. From the test results, it can be found that the performance of Example 1 is relatively the best. This may be because when the addition amount is too small when the carbon nanotubes form a three-dimensional conductive network structure on the surface of LiFePO4 particles, the network structure will not be dense enough, while when the addition amount is too large, it will instead affect the transmission of Li + . Therefore, the addition amount in Example 1 is the best.

[0076] Test Example 2

[0077] At 45 °C, the lithium iron phosphate battery is charged at a constant current of 1C to 4.35V, then charged at a constant voltage until the current reaches 0.05C, and then discharged at a constant current of 1C to 3.0V. This is the first cycle. Charge / discharge is carried out 300 times according to the above conditions. The capacity retention rate of the lithium iron phosphate battery after 300 cycles at 45 °C = (discharge capacity after 300 cycles / discharge capacity of the first cycle) × 100%. At 25 °C, the lithium iron phosphate battery is charged at a constant current of 1C to 4.35V, then charged at a constant voltage of 4.35V until the current ≤ 0.05C. After standing for 5 minutes, it is discharged at a constant current of 0.2C to the cut-off voltage of 3V. At this time, the actual discharge capacity is recorded as D0. Then it is charged at a constant current of 1C to 4.35V, then charged at a constant voltage of 4.35V until the current ≤ 0.05C, and finally discharged at 2C to the cut-off voltage of 3V. The actual discharge capacity at this time is recorded as D1. The rate performance = [(D1 - D0) / D0] × 100%. The specific results are shown in Table 2.

[0078] Table 2 Rate performance and cycle test results of the battery

[0079] Experimental Scheme Capacity Retention Rate / % Rate Performance / % Example 1 99.1 95.3 Example 2 90.2 85.4 Example 3 92.3 88.1 Example 4 95.0 90.2 Example 5 96.7 91.1 Control Example 1 82.1 76.7 Control Example 2 85.3 80.2 Control Example 3 88.4 83.2

[0080] As can be seen from Table 2, compared with the control example and the examples, when carbon nanotubes are added to coat lithium iron phosphate, the electrochemical performance of the battery is obviously better. This is because the carbon nanotubes coated on the surface of LiFePO4 form a conductive network, thus improving the charge transfer rate of the positive electrode material. Therefore, both the rate performance and the cycle performance are improved. In Example 1, polyethylene glycol capped with oligofuran is mixed with carbon nanotubes. Since polyethylene glycol capped with oligofuran has amphiphilicity, and furan, as an aromatic ring with π electron clouds, can form π-π interactions with the surface of carbon nanotubes, including van der Waals forces and electrostatic interactions. Thus, the oligofuran end can be well adsorbed on the surface of carbon nanotubes, while the hydrophilic end of polyethylene glycol can enhance the dispersion of carbon nanotubes in water. This non-covalent modification method not only improves the defect that carbon nanotubes are prone to agglomeration during dispersion, but also enhances the dispersion of carbon nanotubes in the solvent. The modified carbon nanotube dispersion liquid is sintered with iron phosphate, lithium carbonate, etc. to obtain the positive electrode material. The polyethylene glycol capped with oligofuran on the surface of carbon nanotubes is cracked into a carbon network under high temperature, which plays the role of a conductive layer and a binder. The carbon network covers the surface of lithium iron phosphate, thus forming a reticular fast conductive network, which can better improve the + diffusion rate of Li in LiFePO4. Therefore, the battery in Example 1 shows the best performance in all aspects. It improves the disadvantages of poor electronic conductivity and poor ionic conductivity of lithium iron phosphate materials, improves the charge and discharge ability of the materials under high current, improves the rate performance and cycle performance of the battery, and can meet the development needs of high-energy and high-power lithium-ion batteries in the market.

[0081] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative efforts. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field based on the concept of the present invention through logical analysis, reasoning, or limited experiments on the basis of the prior art should fall within the protection scope determined by the claims.

Claims

1. A method for preparing an ultra-high current lithium iron phosphate battery, characterized in that: The steps include: S1. Add lithium iron phosphate positive electrode material, conductive material and binder into N-methylpyrrolidone and stir evenly to obtain slurry, evenly coat it on aluminum foil, bake and compact it to obtain positive electrode sheet; S2. Use the above-mentioned positive electrode sheet as the positive electrode, graphite as the negative electrode, and a polypropylene film as a separator. After assembling, inject the electrolyte and seal to obtain a battery; The method for preparing the lithium iron phosphate positive electrode material comprises the following steps: X1. Add 3-furanacetic acid to N,N-dimethylformamide, and then add N-bromosuccinimide, the molar ratio of 3-furanacetic acid to N-bromosuccinimide is 3-5:8, stir at room temperature for 16-24 hours under nitrogen atmosphere, add ethyl acetate to dilute, and extract with water three times. After the organic phase is synthesized, dry and concentrate to obtain a residue, which is recrystallized from ethanol for the next step; X2. After mixing the product of the previous step with methoxy polyethylene glycol-hydroxy, the temperature was raised to remove moisture, dichloromethane and 4-dimethylaminopyridine were added, and then a 1 mol / L dicyclohexylcarbodiimide dichloromethane solution was added dropwise, and the mixture was stirred at room temperature for 40 to 60 hours and filtered. The molar ratio of the product of the previous step to methoxy polyethylene glycol-hydroxy, 4-dimethylaminopyridine, and dicyclohexylcarbodiimide was 5 to 15:5 to 10:1:

1. After adding ether to the filtrate, a precipitate appeared, which was filtered. The residue was dried and then mixed with 2-(tributyltinyl)furan. Add to N,N-dimethylformamide, add bistriphenylphosphine palladium dichloride under nitrogen atmosphere, the molar ratio of the residue to 2-(tributyltinyl)furan and bistriphenylphosphine palladium dichloride is 1:4-5:0.1-0.25, heat and stir for 40-60 hours, then add ether to dilute, precipitate appears, filter, and dry the residue to obtain oligofuran-terminated polyethylene glycol, mix the oligofuran-terminated polyethylene glycol and multi-walled carbon nanotubes in a mass ratio of 10:1, and then add to water to obtain a 10wt% modified carbon nanotube dispersion; X3. Li2CO3 and FePO4 are weighed and mixed in a molar ratio of 1:1, and 12wt% glucose is added. After ball milling and mixing evenly, a modified carbon nanotube dispersion is added to make the added amount of the modified carbon nanotubes 1-5wt%. After ball milling and spray drying, the mixture is heated to 700-800°C at a rate of 5°C / min and calcined for 4-6h to obtain a lithium iron phosphate positive electrode material.

2. The method for preparing the ultra-high current lithium iron phosphate battery according to claim 1, characterized in that: The conductive material is acetylene black.

3. The method for preparing the ultra-high current lithium iron phosphate battery according to claim 1, characterized in that: The binder is polyvinylidene fluoride.

4. The method for preparing the ultra-high current lithium iron phosphate battery according to claim 1, characterized in that: The mass ratio of the lithium iron phosphate positive electrode material to the conductive material and the binder is 100:3-5:2-6.

5. The method for preparing the ultra-high current lithium iron phosphate battery according to claim 1, characterized in that: The solid content of the positive electrode slurry is 40 wt %.

6. The method for preparing the ultra-high current lithium iron phosphate battery according to claim 1, characterized in that: The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the concentration of lithium hexafluorophosphate is 1 mol / L, and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 1:1:

1.

7. An ultra-high current lithium iron phosphate battery, characterized in that: Prepared by the method according to any one of claims 1 to 6.