Surface-modified lithium iron phosphate and method for preparing same, cathode material, battery, and application

Abstract

The application relates to the technical field of lithium ion batteries, in particular to surface-modified lithium iron phosphate and a preparation method thereof, a positive electrode material, a battery and application; the application is used for solving the problems of poor conductivity and low lithium ion diffusion coefficient of existing lithium iron phosphate; tin and vanadium are doped into the lithium iron phosphate, and the ion migration resistance is cooperatively reduced through tin-vanadium co-doping, so that the rate performance, electronic conductivity and discharge specific capacity of the material are improved; the lithium iron phosphate is coated with carbon and polypyrrole, and graphene is introduced into the material, so that the electronic conductivity is improved; the graphene is bridged with each other to form an electronic conduction network, and the rate performance of the material is significantly improved; the coating of the polypyrrole reduces the resistance between particles, accelerates the transmission speed of electrons between the particles, improves the electron exchange rate, forms a continuous conductive grid, improves the surface electronic conductivity, and improves the charge-discharge performance and cycle performance of the positive electrode material under high rate.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to surface-modified lithium iron phosphate and its preparation method, cathode material, battery, and applications. Background Technology

[0002] In current lithium-ion battery technology, lithium iron phosphate has received widespread attention as an important cathode material. As an ideal cathode material for the development of lithium-ion power batteries, olivine-structured lithium iron phosphate has attracted widespread attention due to its relatively high specific capacity, safety and stability, and low price. Compared with other common cathode materials such as lithium cobalt oxide and ternary materials, lithium iron phosphate has two prominent problems: poor conductivity and low lithium-ion diffusion coefficient within the material.

[0003] Therefore, it is of great significance to develop a surface-modified lithium iron phosphate, its preparation method, cathode material, battery, and applications. Summary of the Invention

[0004] In order to overcome the above-mentioned technical problems, the present invention aims to provide surface-modified lithium iron phosphate, its preparation method, cathode material, battery and application, which solves the problems of poor conductivity and low lithium-ion diffusion coefficient of existing lithium iron phosphate.

[0005] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this application provides a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step a1: Add iron source, phosphorus source, tin source, vanadium source, ascorbic acid and deionized water to a three-necked flask equipped with a stirrer and thermometer, mix and stir for 30 min, add surfactant and continue stirring at 80℃ for 1-2 h, raise the temperature to 85℃ and react for 8 h, after the reaction is completed, transfer to a vacuum drying oven and dry at 110℃ for 24 h, after drying, grind in a mortar for 15-30 min, place in a crucible, pre-calcine at 300℃ in air atmosphere for 2-3 h, introduce argon gas for protection, raise the temperature to 730℃ at 3℃ / min and hold for 12 h to obtain tin-vanadium co-doped FePO4; Step a2: Add LiOH·H2O and ethylene glycol to a three-necked flask equipped with a stirrer and thermometer, stir for 15-30 min, add graphene, mix and stir for 15 min, and then sonicate at 80W power for 1 h to obtain a brownish-yellow liquid; add tin-vanadium co-doped FePO4, ascorbic acid and diethylene glycol to a beaker, stir for 30 min under argon protection to obtain a precursor; add the precursor to the above brownish-yellow liquid under argon protection, stir magnetically for 30 min, place in an oil bath at 180℃ to react for 16 h, cool naturally to 25℃, filter, wash the filter cake alternately with deionized water and anhydrous ethanol 3-4 times, and dry in a drying oven at 45℃ for 24 h to obtain the intermediate product; Step a3: Add carbon source, NaNO3 and concentrated H2SO4 solution to a three-necked flask equipped with a stirrer and thermometer, sonicate at 0℃ for 1h, then add KMnO4, stir at 5℃ for 2h, heat to 38℃ and sonicate for 30min, add deionized water, stir at 95℃ for 30min, add H2O2 and HCl solution, let stand for 15min and filter, wash the filter cake with deionized water and let stand 3 times, add the intermediate product to the above three-necked flask, sonicate for 1-2h, then place in a drying oven and dry at 120℃ for 12h. After drying, put it in a high-temperature ball mill with a ball-to-material ratio of 15:1 and a heating rate of 5℃ / min, ball mill at 300℃ for 1h under nitrogen atmosphere for pre-sintering, and then ball mill at 630℃ for 9h to obtain carbon-coated lithium iron phosphate. Step a4: Add carbon-coated lithium iron phosphate and deionized water to a three-necked flask equipped with a stirrer and thermometer, add oxidant, mix and stir for 1-2 hours, then add pyrrole, polymerize at 25°C for 18 hours, wash, centrifuge, and vacuum dry at 110°C for 3 hours to obtain surface-modified lithium iron phosphate.

[0006] In a preferred embodiment of the present invention, the ratio of the amount of iron source, phosphorus source, tin source, vanadium source, ascorbic acid, deionized water and surfactant in step a1 is 1mol:1.05mol:0.015mol:0.025mol:0.075mol:200mL:40g.

[0007] In a preferred embodiment of the present invention, the iron source in step a1 is one of ferric nitrate nonahydrate, FeCl3·6H2O, and ferric ammonium citrate; the phosphorus source is one of NH4H2PO4, H3PO4, and (NH4)2HPO4; the tin source is one of SnCl4·5H2O and SnC2O4; and the vanadium source is NH4VO3.

[0008] In a preferred embodiment of the present invention, the surfactant in step a1 is one of polyethylene glycol PEG-400, citric acid, and hexadecyltrimethylammonium bromide.

[0009] In a preferred embodiment of the present invention, the ratio of LiOH·H2O, ethylene glycol, graphene, tin-vanadium co-doped FePO4, ascorbic acid and diethylene glycol in step a2 is 0.015mol:40mL:11.8mg:0.16-0.2g:0.0025mol:16mL.

[0010] In a preferred embodiment of the present invention, the graphene in step a2 has a thickness of 0.55-1.2 nm and a diameter of 0.5-3 μm.

[0011] In a preferred embodiment of the present invention, the ratio of the amount of carbon source, NaNO3, concentrated H2SO4 solution, KMnO4, deionized water, H2O2, HCl solution and intermediate product in step a3 is 1g:0.5g:23mL:3g:160-200mL:25mL:40mL:6-7g.

[0012] In a preferred embodiment of the present invention, the carbon source in step a3 is one of carbon black, acetylene black, carbon nanotubes, sucrose, and glucose; the mass fraction of the concentrated H2SO4 solution is 98%; the mass fraction of the H2O2 solution is 30%; and the mass fraction of the HCl solution is 10%.

[0013] In a preferred embodiment of the present invention, the ratio of carbon-coated lithium iron phosphate, deionized water, oxidant and pyrrole in step a4 is 1-1.5g: 50-100mL: 0.002-0.003g: 0.06-0.18mL.

[0014] In a preferred embodiment of the present invention, the oxidant in step a4 is ferric p-toluenesulfonate.

[0015] Secondly, this application provides surface-modified lithium iron phosphate, which is prepared according to the preparation method of the surface-modified lithium iron phosphate.

[0016] Thirdly, this application provides a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material is prepared from the above-mentioned surface-modified lithium iron phosphate.

[0017] Fourthly, this application provides a lithium-ion battery, wherein the lithium-ion battery comprises the above-mentioned surface-modified lithium iron phosphate.

[0018] Fifthly, this application provides the application of surface-modified lithium iron phosphate in lithium-ion batteries as described in the first aspect.

[0019] The beneficial effects of this invention are: This invention relates to surface-modified lithium iron phosphate, its preparation method, cathode material, battery, and applications. The process involves mixing and stirring an iron source, a phosphorus source, a tin source, a vanadium source, ascorbic acid, and deionized water; adding a surfactant and continuing the stirring reaction; drying; grinding; and sintering to obtain tin-vanadium co-doped FePO4. Tin has high conductivity and is readily available and inexpensive. Tin doping causes lattice distortion, refines the particle size, and improves the Li-P-FeO content. + De-embedding provides more space, expanding the Li + Channels reduce polarization and accelerate Li + Diffusion improves the material's conductivity; tin stabilizes the crystal lattice and reduces Fe dissolution; vanadium doping helps refine the grains and weaken Li-O bonds, thereby enhancing lithium-ion diffusion and improving conductivity, electrochemical performance, and rate performance; tin-vanadium co-doping synergistically reduces ion migration resistance and improves rate performance; LiOH·H2O, ethylene glycol, and graphene are mixed, stirred, and sonicated; then tin-vanadium co-doped FePO4, ascorbic acid, and diethylene glycol are added; the mixture is magnetically stirred, reacted in an oil bath, and cooled, filtered, washed, and dried to obtain an intermediate product; a carbon source, NaNO3, and concentrated H2SO4 solution are mixed and sonicated; then KMnO4 is added, stirred, and sonicated; finally, deionized water and H2O are added. 2. HCl solution, after standing, filtered, filter cake washed, intermediate product added, ultrasonically dispersed, dried, ball-milled, and sintered to obtain carbon-coated lithium iron phosphate; Graphene has the characteristics of good flexibility, good electrical and thermal conductivity, high Young's modulus, and large specific surface area. Introducing graphene into the material improves electronic conductivity, and the bridging between graphene molecules forms an electronic conductive network, significantly improving the rate performance of the material; Carbon-coated lithium iron phosphate, deionizer, and oxidant are mixed and stirred, pyrrole polymerization is added, washed, centrifuged, and dried to obtain surface-modified lithium iron phosphate; Polypyrrole obtained by pyrrole polymerization is a conductive polymer with a conjugated structure and is a conductive polymer material with ion penetration capability, Li +Polypyrrole can be inserted and extracted, and polypyrrole has a high specific capacity. Polypyrrole itself has high conductivity and can undergo a completely reversible redox reaction during the reaction. Through polypyrrole coating, the resistance between particles is effectively reduced, the electron transport speed between particles is accelerated, and the electron exchange rate is improved. Polypyrrole forms a continuous conductive grid on the surface of lithium iron phosphate, and electrons are transported by jumping through π-π conjugated chains, crossing the insulating region between particles. The polypyrrole layer isolates the electrolyte from direct contact with lithium iron phosphate, reduces the amount of iron dissolution, improves the surface electronic conductivity of lithium iron phosphate particles, and improves the charge-discharge performance and cycle performance of the cathode material at high rates. Tin-vanadium bulk doping, carbon coating, and polypyrrole coating work synergistically to improve its conductivity, electrochemical performance, and lithium-ion diffusion coefficient. Attached Figure Description

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] Figure 1 This is a schematic diagram showing the powder conductivity test results of lithium-ion batteries prepared based on lithium iron phosphate in Examples 1-3 and Comparative Examples 1-4 of the present invention.

[0022] Figure 2 This is a schematic diagram showing the rate performance test results of lithium-ion batteries prepared based on lithium iron phosphate in Examples 1-3 and Comparative Examples 1-4 of the present invention.

[0023] Figure 3 This is a schematic diagram showing the lithium-ion diffusion coefficient test results of lithium-ion batteries prepared based on lithium iron phosphate in Examples 1-3 and Comparative Examples 1-4 of the present invention.

[0024] Figure 4 This is a schematic diagram showing the capacity retention test results of lithium-ion batteries prepared based on lithium iron phosphate in Examples 1-3 and Comparative Examples 1-4 of the present invention after 2000 cycles at 1C. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1: This embodiment describes a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 1 h, and then heated to 85 °C for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 15 min, placed in a crucible, and pre-calcined at 300 °C in air for 2 h. Argon gas was introduced for protection, and the temperature was increased to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: Add 0.015 mol LiOH·H2O and 40 mL ethylene glycol to a three-necked flask equipped with a stirrer and thermometer, stir for 15 min, add 11.8 mg of graphene with a thickness of 0.55 nm and a diameter of 0.5 μm, mix and stir for 15 min, then sonicate at 80 W for 1 h to obtain a brownish-yellow liquid; add 0.16 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid and 16 mL of diethylene glycol to a beaker, stir under argon protection for 30 min to obtain a precursor; add the precursor to the above brownish-yellow liquid under argon protection, stir magnetically for 30 min, place in an oil bath at 180 °C and react for 16 h, cool naturally to 25 °C, filter, wash the filter cake three times alternately with deionized water and anhydrous ethanol, and dry in a drying oven at 45 °C for 24 h to obtain the intermediate product; Step S3: Add 1g of acetylene black, 0.5g of NaNO3, and 23mL of 98% concentrated H2SO4 solution to a three-necked flask equipped with a stirrer and thermometer. Sonicate at 0℃ for 1h, then add 3g of... KMnO4 was stirred at 5℃ for 2 hours, heated to 38℃ and sonicated for 30 minutes. 160 mL of deionized water was added, and the mixture was stirred at 95℃ for 30 minutes. 25 mL of 30% H2O2 and 40 mL of 10% HCl solution were added. After standing for 15 minutes, the mixture was filtered. The filter cake was washed with deionized water and allowed to stand three times. 6 g of the intermediate product was added to the three-necked flask and sonicated for 1 hour. Then, it was placed in a drying oven and dried at 120℃ for 12 hours. After drying, it was placed in a high-temperature ball mill with a ball-to-material ratio of 15:1 and a heating rate of 5℃ / min. It was ball-milled at 300℃ for 1 hour under a nitrogen atmosphere for pre-sintering, and then ball-milled at 630℃ for 9 hours to obtain carbon-coated lithium iron phosphate. Step S4: Add 1g of carbon-coated lithium iron phosphate and 50mL of deionized water to a three-necked flask equipped with a stirrer and thermometer, add 0.002g of ferric p-toluenesulfonate, mix and stir for 1h, then add 0.06mL of pyrrole, polymerize at 25℃ for 18h, wash, centrifuge, and vacuum dry at 110℃ for 3h to obtain surface-modified lithium iron phosphate.

[0027] Example 2: This embodiment describes a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 1.5 h. The temperature was raised to 85 °C and reacted for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 20 min, placed in a crucible, and pre-calcined at 300 °C in air for 2.5 h. Argon gas was introduced for protection, and the temperature was raised to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: 0.015 mol LiOH·H2O and 40 mL ethylene glycol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 20 min. 11.8 mg of graphene with a thickness of 0.88 nm and a diameter of 1.75 μm was added. After mixing and stirring for 15 min, the mixture was sonicated at 80 W for 1 h to obtain a brownish-yellow liquid. 0.18 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid, and 16 mL of diethylene glycol were added to a beaker. The mixture was stirred for 30 min under argon protection to obtain a precursor. The precursor was added to the above brownish-yellow liquid under argon protection and magnetically stirred for 30 min. The mixture was placed in an oil bath at 180 °C and reacted for 16 h. After naturally cooling to 25 °C, the mixture was filtered. The filter cake was washed four times alternately with deionized water and anhydrous ethanol and dried in a drying oven at 45 °C for 24 h to obtain the intermediate product. Step S3: Add 1g of acetylene black, 0.5g of NaNO3, and 23mL of 98% concentrated H2SO4 solution to a three-necked flask equipped with a stirrer and thermometer. Sonicate at 0℃ for 1h, then add 3g of... KMnO4 was stirred at 5℃ for 2 hours, heated to 38℃ and sonicated for 30 minutes. 180 mL of deionized water was added, and the mixture was stirred at 95℃ for 30 minutes. 25 mL of 30% H2O2 and 40 mL of 10% HCl solution were added. After standing for 15 minutes, the mixture was filtered. The filter cake was washed with deionized water and allowed to stand three times. 6.5 g of the intermediate product was added to the three-necked flask and sonicated for 1.5 hours. Then, it was placed in a drying oven and dried at 120℃ for 12 hours. After drying, it was placed in a high-temperature ball mill with a ball-to-material ratio of 15:1 and a heating rate of 5℃ / min. It was ball-milled at 300℃ for 1 hour under a nitrogen atmosphere for pre-sintering, and then ball-milled at 630℃ for 9 hours to obtain carbon-coated lithium iron phosphate. Step S4: Add 1.25g of carbon-coated lithium iron phosphate and 75mL of deionized water to a three-necked flask equipped with a stirrer and thermometer, add 0.003g of p-toluenesulfonate, mix and stir for 1.5h, then add 0.12mL of pyrrole, polymerize at 25℃ for 18h, wash, centrifuge, and vacuum dry at 110℃ for 3h to obtain surface-modified lithium iron phosphate.

[0028] Example 3: This embodiment describes a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 2 h, and then heated to 85 °C for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 30 min, placed in a crucible, and pre-calcined at 300 °C in air for 3 h. Argon gas was introduced for protection, and the temperature was increased to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: 0.015 mol LiOH·H2O and 40 mL ethylene glycol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 30 min. 11.8 mg of graphene with a thickness of 1.2 nm and a diameter of 3 μm was added. After mixing and stirring for 15 min, the mixture was sonicated at 80 W for 1 h to obtain a brownish-yellow liquid. 0.2 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid, and 16 mL of diethylene glycol were added to a beaker. The mixture was stirred for 30 min under argon protection to obtain a precursor. The precursor was added to the above brownish-yellow liquid under argon protection and magnetically stirred for 30 min. The mixture was placed in an oil bath at 180 °C and reacted for 16 h. After naturally cooling to 25 °C, the mixture was filtered. The filter cake was washed four times alternately with deionized water and anhydrous ethanol and dried in a drying oven at 45 °C for 24 h to obtain the intermediate product. Step S3: Add 1g of acetylene black, 0.5g of NaNO3, and 23mL of 98% concentrated H2SO4 solution to a three-necked flask equipped with a stirrer and thermometer. Sonicate at 0℃ for 1h, then add 3g of... KMnO4 was stirred at 5℃ for 2 hours, heated to 38℃ and sonicated for 30 minutes. 200 mL of deionized water was added, and the mixture was stirred at 95℃ for 30 minutes. 25 mL of 30% H2O2 and 40 mL of 10% HCl solution were added. After standing for 15 minutes, the mixture was filtered. The filter cake was washed with deionized water and allowed to stand three times. 7 g of the intermediate product was added to the above three-necked flask and sonicated for 2 hours. Then, it was placed in a drying oven and dried at 120℃ for 12 hours. After drying, it was placed in a high-temperature ball mill with a ball-to-material ratio of 15:1 and a heating rate of 5℃ / min. It was ball-milled at 300℃ for 1 hour under a nitrogen atmosphere for pre-sintering, and then ball-milled at 630℃ for 9 hours to obtain carbon-coated lithium iron phosphate. Step S4: Add 1.5g of carbon-coated lithium iron phosphate and 100mL of deionized water to a three-necked flask equipped with a stirrer and thermometer, add 0.003g of p-toluenesulfonate, mix and stir for 2h, then add 0.18mL of pyrrole, polymerize at 25℃ for 18h, wash, centrifuge, and vacuum dry at 110℃ for 3h to obtain surface-modified lithium iron phosphate.

[0029] Comparative Example 1: This comparative example illustrates a method for preparing lithium iron phosphate, comprising the following steps: Step S1: Add 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.075 mol of ascorbic acid, and 200 mL of deionized water to a three-necked flask equipped with a stirrer and thermometer. Mix and stir for 30 min. Add 40 g of hexadecyltrimethylammonium bromide and continue stirring at 80 °C for 1.5 h. Increase the temperature to 85 °C and react for 8 h. After the reaction is complete, transfer to a vacuum drying oven and dry at 110 °C for 24 h. After drying, grind in a mortar for 20 min, place in a crucible, pre-calcine at 300 °C in air atmosphere for 2.5 h, introduce argon gas for protection, increase the temperature to 730 °C at 3 °C / min, and hold for 12 h to obtain FePO4. Step S2: Add 0.015 mol LiOH·H2O and 40 mL ethylene glycol to a three-necked flask equipped with a stirrer and thermometer, stir for 20 min, add 11.8 mg of graphene with a thickness of 0.88 nm and a diameter of 1.75 μm, mix and stir for 15 min, then sonicate at 80 W for 1 h to obtain a brownish-yellow liquid; add 0.18 g FePO4, 0.0025 mol ascorbic acid and 16 mL diethylene glycol to a beaker, stir for 30 min under argon protection to obtain a precursor; add the precursor to the above brownish-yellow liquid under argon protection, stir magnetically for 30 min, place in an oil bath at 180 °C and react for 16 h, cool naturally to 25 °C, filter, wash the filter cake alternately with deionized water and anhydrous ethanol 4 times, and dry in a drying oven at 45 °C for 24 h to obtain lithium iron phosphate.

[0030] Comparative Example 2: This comparative example illustrates a method for preparing modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 1.5 h. The temperature was raised to 85 °C and reacted for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 20 min, placed in a crucible, and pre-calcined at 300 °C in air for 2.5 h. Argon gas was introduced for protection, and the temperature was raised to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: Add 0.015 mol LiOH·H2O and 40 mL ethylene glycol to a three-necked flask equipped with a stirrer and thermometer, stir for 20 min, add 11.8 mg of graphene with a thickness of 0.88 nm and a diameter of 1.75 μm, mix and stir for 15 min, and then sonicate at 80 W power for 1 h to obtain a brownish-yellow liquid; add 0.18 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid and 16 mL of diethylene glycol to a beaker, stir for 30 min under argon protection to obtain a precursor; add the precursor to the above brownish-yellow liquid under argon protection, stir magnetically for 30 min, place in an oil bath at 180 °C and react for 16 h, cool naturally to 25 °C, filter, wash the filter cake alternately with deionized water and anhydrous ethanol 4 times, and dry in a drying oven at 45 °C for 24 h to obtain modified lithium iron phosphate.

[0031] Comparative Example 3: This comparative example illustrates a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 1.5 h. The temperature was raised to 85 °C and reacted for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 20 min, placed in a crucible, and pre-calcined at 300 °C in air for 2.5 h. Argon gas was introduced for protection, and the temperature was raised to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: 0.015 mol LiOH·H2O and 40 mL ethylene glycol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 20 min. 11.8 mg of graphene with a thickness of 0.88 nm and a diameter of 1.75 μm was added. After mixing and stirring for 15 min, the mixture was sonicated at 80 W for 1 h to obtain a brownish-yellow liquid. 0.18 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid, and 16 mL of diethylene glycol were added to a beaker. The mixture was stirred for 30 min under argon protection to obtain a precursor. The precursor was added to the above brownish-yellow liquid under argon protection and magnetically stirred for 30 min. The mixture was placed in an oil bath at 180 °C and reacted for 16 h. After naturally cooling to 25 °C, the mixture was filtered. The filter cake was washed four times alternately with deionized water and anhydrous ethanol and dried in a drying oven at 45 °C for 24 h to obtain the intermediate product. Step S3: Add 1g of acetylene black, 0.5g of NaNO3, and 23mL of 98% concentrated H2SO4 solution to a three-necked flask equipped with a stirrer and thermometer. Sonicate at 0℃ for 1h, then add 3g of... KMnO4 was stirred at 5℃ for 2 hours, heated to 38℃ and sonicated for 30 minutes. 180 mL of deionized water was added, and the mixture was stirred at 95℃ for 30 minutes. 25 mL of 30% H2O2 and 40 mL of 10% HCl solution were added. After standing for 15 minutes, the mixture was filtered. The filter cake was washed with deionized water and allowed to stand three times. 6.5 g of the intermediate product was added to the three-necked flask and sonicated for 1.5 hours. Then, it was placed in a drying oven and dried at 120℃ for 12 hours. After drying, it was placed in a high-temperature ball mill with a ball-to-material ratio of 15:1 and a heating rate of 5℃ / min. It was ball-milled at 300℃ for 1 hour under a nitrogen atmosphere for pre-sintering, and then ball-milled at 630℃ for 9 hours to obtain surface-modified lithium iron phosphate.

[0032] Comparative Example 4: This comparative example illustrates a method for preparing surface-modified lithium iron phosphate, comprising the following steps: Step S1: 1 mol of ferric nitrate nonahydrate, 1.05 mol of NH4H2PO4, 0.015 mol of SnC2O4, 0.025 mol of NH4VO3, 0.075 mol of ascorbic acid, and 200 mL of deionized water were added to a three-necked flask equipped with a stirrer and a thermometer. The mixture was stirred for 30 min, and 40 g of hexadecyltrimethylammonium bromide was added. The mixture was stirred at 80 °C for 1.5 h. The temperature was raised to 85 °C and reacted for 8 h. After the reaction was completed, the mixture was transferred to a vacuum drying oven and dried at 110 °C for 24 h. After drying, the mixture was ground in a mortar for 20 min, placed in a crucible, and pre-calcined at 300 °C in air for 2.5 h. Argon gas was introduced for protection, and the temperature was raised to 730 °C at 3 °C / min and held for 12 h to obtain tin-vanadium co-doped FePO4. Step S2: 0.015 mol LiOH·H2O and 40 mL ethylene glycol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 20 min. 11.8 mg of graphene with a thickness of 0.88 nm and a diameter of 1.75 μm was added. After mixing and stirring for 15 min, the mixture was sonicated at 80 W for 1 h to obtain a brownish-yellow liquid. 0.18 g of tin-vanadium co-doped FePO4, 0.0025 mol ascorbic acid, and 16 mL of diethylene glycol were added to a beaker. The mixture was stirred for 30 min under argon protection to obtain a precursor. The precursor was added to the above brownish-yellow liquid under argon protection and magnetically stirred for 30 min. The mixture was placed in an oil bath at 180 °C and reacted for 16 h. After naturally cooling to 25 °C, the mixture was filtered. The filter cake was washed four times alternately with deionized water and anhydrous ethanol and dried in a drying oven at 45 °C for 24 h to obtain the intermediate product. Step S4: Add 1.25g of intermediate product and 75mL of deionized water to a three-necked flask equipped with a stirrer and thermometer, add 0.003g of p-toluenesulfonate, mix and stir for 1.5h, then add 0.12mL of pyrrole, polymerize at 25℃ for 18h, wash, centrifuge, and vacuum dry at 110℃ for 3h to obtain surface-modified lithium iron phosphate.

[0033] Lithium iron phosphate prepared in Examples 1-3 and Comparative Examples 1-4 was used as the positive electrode material. It was then mixed with carbon nanotubes (2 nm outer diameter, 30 μm length) as a conductive agent and PVDF FL2000 as a binder in a ratio of 90:5:5 to prepare a positive electrode slurry. The slurry was uniformly coated on both sides of an aluminum foil with a thickness of 13 μm. The areal density was controlled to be 20 mg / cm³ during the coating process. 2 Meanwhile, a 20mm tab area is reserved on both sides; the coated electrode sheet is dried in a 20m long oven at 100℃ at a speed of 3m / s, and then cut with a mold to obtain the positive electrode sheet; the positive electrode sheet serves as the positive electrode of the battery; graphite, acetylene black, 2% sodium carboxymethyl cellulose solution, and styrene-butadiene rubber YH-791E are mixed evenly in a mass ratio of 95:0.5:1.5:3 to form the negative electrode slurry, which is then uniformly coated on both sides of a 12μm thick copper foil with a coating density of 15mg / cm³. 2 The coated copper foil was dried in a 20m long oven at 100℃ at a speed of 3m / s, and then cut into negative electrode sheets using a mold. LiPF6 was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:1 to obtain an electrolyte with a concentration of 1mol / L. A battery was fabricated using the positive electrode sheet, negative electrode sheet, UTEC 4041 polyethylene separator, and electrolyte. Electrochemical performance was tested according to GB / T 42260-2022 standard. The powder conductivity of the positive electrode material was determined using the four-probe method according to GB / T 30835-2014 standard. The test results are attached. Figures 1-4 As shown: See Figure 1 , Figure 2 , Figure 3 as well as Figure 4As shown, a comparison between Example 2 and Comparative Example 1 reveals that Comparative Example 1 did not involve doping or coating modification, and the conductivity, electrochemical performance, and lithium-ion diffusion coefficient of Example 2 are significantly better than those of Comparative Example 1. A comparison between Example 2 and Comparative Example 2 reveals that Comparative Example 2 did not involve coating modification, and the lithium-ion diffusion coefficient of Comparative Example 2 is significantly greater than that of Comparative Example 1 but less than that of Example 2. A comparison between Example 2 and Comparative Example 3 reveals that Comparative Example 3 did not involve polypyrrole coating modification, and the conductivity and electrochemical performance of Example 2 are better than those of Comparative Example 3, but the conductivity and electrochemical performance of Comparative Example 3 are better than both Comparative Example 1 and Comparative Example 2. A comparison between Example 2 and Comparative Example 4 reveals that Comparative Example 4 did not involve carbon coating modification, and its conductivity and lithium-ion diffusion coefficient are significantly lower than those of Example 2. The comparison shows that tin-vanadium bulk doping, carbon coating, and polypyrrole coating work synergistically to improve conductivity, electrochemical performance, and lithium-ion diffusion coefficient.

[0034] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0035] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing surface-modified lithium iron phosphate, characterized in that, Includes the following steps: Step a1: Mix and stir the iron source, phosphorus source, tin source, vanadium source, ascorbic acid and deionized water, add surfactant and continue stirring, heat and react, dry, grind and sinter after the reaction is completed, introduce argon gas for protection, heat to 730℃ and hold to obtain tin-vanadium co-doped FePO4. Step a2: Mix LiOH·H2O and ethylene glycol, add graphene, mix and stir, then sonicate to obtain a brownish-yellow liquid; stir tin-vanadium co-doped FePO4, ascorbic acid and diethylene glycol under argon protection to obtain a precursor; add the precursor to the above brownish-yellow liquid under argon protection, stir magnetically, react in an oil bath, cool naturally, filter, wash and dry to obtain an intermediate product; Step a3: Add carbon source, NaNO3 and concentrated H2SO4 solution to a three-necked flask, sonicate, add KMnO4, stir, sonicate at 38°C, add deionized water, stir, add H2O2 and HCl solution, let stand, filter, wash, add intermediate product to the above three-necked flask, sonicate, dry, ball mill, pre-sinter under nitrogen atmosphere, and then ball mill at 630°C to obtain carbon-coated lithium iron phosphate; Step a4: Mix and stir carbon-coated lithium iron phosphate, deionized water and oxidant, add pyrrole, polymerize, wash, centrifuge and dry to obtain surface-modified lithium iron phosphate.

2. The method for preparing surface-modified lithium iron phosphate according to claim 1, characterized in that, The ratio of iron source, phosphorus source, tin source, vanadium source, ascorbic acid, deionized water, and surfactant used in step a1 is 1 mol: 1.05mol: 0.015mol: 0.025mol: 0.075mol: 200mL: 40g; The surfactant is one of polyethylene glycol, citric acid, and hexadecyltrimethylammonium bromide.

3. The method for preparing surface-modified lithium iron phosphate according to claim 1, characterized in that, The iron source mentioned in step a1 is one of ferric nitrate nonahydrate, FeCl3·6H2O, and ferric ammonium citrate; the phosphorus source is one of NH4H2PO4, H3PO4, and (NH4)2HPO4; the tin source is one of SnCl4·5H2O and SnC2O4; and the vanadium source is NH4VO3.

4. The method for preparing surface-modified lithium iron phosphate according to claim 1, characterized in that, In step a2, the ratio of LiOH·H2O, ethylene glycol, graphene, tin-vanadium co-doped FePO4, ascorbic acid, and diethylene glycol is 0.015mol:40mL:11.8mg:0.16-0.2g:0.0025mol:16mL; the graphene has a thickness of 0.55-1.2nm and a diameter of 0.5-3μm.

5. The method for preparing surface-modified lithium iron phosphate according to claim 1, characterized in that, The ratio of carbon source, NaNO3, concentrated H2SO4 solution, KMnO4, deionized water, H2O2, HCl solution, and intermediate product used in step a3 is 1g:0.5g:23mL: 3g: 160-200mL: 25mL: 40mL: 6-7g; the carbon source is one of carbon black, acetylene black, carbon nanotubes, sucrose, and glucose; the mass fraction of the concentrated H2SO4 solution is 98%; the mass fraction of H2O2 is 30%; and the mass fraction of the HCl solution is 10%.

6. The method for preparing surface-modified lithium iron phosphate according to claim 1, characterized in that, In step a4, the ratio of carbon-coated lithium iron phosphate, deionized water, oxidant, and pyrrole is 1-1.5g: 50-100mL: 0.002-0.003g: 0.06-0.18mL; the oxidant is ferric p-toluenesulfonate.

7. Surface-modified lithium iron phosphate, characterized in that, The surface-modified lithium iron phosphate is prepared by the method according to any one of claims 1-6.

8. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material includes the surface-modified lithium iron phosphate as described in claim 7.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium iron phosphate cathode material as described in claim 8.

10. The application of surface-modified lithium iron phosphate prepared by the method of any one of claims 1-6 in lithium-ion batteries.

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