Lithium iron phosphate composite material and preparation method thereof

Abstract

The invention discloses a lithium iron phosphate composite material and a preparation method thereof, and belongs to the technical field of lithium iron phosphate, the preparation method comprises the following steps: 1, preparing a vinyl siloxane modified lithium iron phosphate precursor; and 2, carrying out polymerization reaction under the action of an initiator by taking the vinyl siloxane modified lithium iron phosphate precursor and vinyl imidazole ionic liquid as raw materials: adding the vinyl siloxane modified lithium iron phosphate precursor and the vinyl imidazole ionic liquid into methanol, adding the initiator, carrying out ultrasonic dispersion in an inert atmosphere, and carrying out room-temperature reaction to obtain the vinyl siloxane modified lithium iron phosphate. An ionic liquid coated lithium iron phosphate precursor is obtained; and 3, roasting in an inert atmosphere to obtain the lithium iron phosphate composite material. Vinyl siloxane is introduced to construct a more complex conductive network, the ionic liquid is used as a carbon source to realize multi-element doping, and the prepared lithium iron phosphate composite material is better in discharge capacity and cycle performance when being used as a battery electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium iron phosphate, and in particular relates to a lithium iron phosphate composite material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate (LIFP) has an olivine-type structure, resulting in a high theoretical specific capacity, excellent structural stability during charge and discharge, and a long cycle life. Furthermore, its raw materials are abundant, inexpensive, and environmentally friendly, meeting the requirements of sustainable development. However, LFP has low electrical conductivity, which makes electron transfer difficult during battery charge and discharge. This, in particular under high current charge and discharge conditions, results in poor rate performance and cannot meet the demands of high power output. Furthermore, low conductivity causes the battery's voltage to drop rapidly during high-rate discharge, affecting its output performance and limiting its widespread application in power-hungry applications such as electric vehicles and energy storage systems.

[0003] At present, in order to improve the conductivity of lithium iron phosphate, the commonly used methods include ion doping, nano-sizing, surface coating and carbon coating. Ion doping is to dope other metal ions (such as Mg) into the lithium iron phosphate lattice. 2+ 、Al 3+ 、Ti 4+ etc.), which can cause defects in the lattice, thereby improving the electrical conductivity and ion diffusion performance of the material; nano-sizing refers to the preparation of lithium iron phosphate particles into nano-scale size, which can significantly shorten the diffusion path of lithium ions, thereby improving the rate performance of the material; the surface coating method is to coat other conductive materials on the surface of lithium iron phosphate to further improve the conductivity of the material, and the battery performance can be optimized by adjusting the type of polymer and the coating method; carbon coating is to coat a layer of carbon on the surface of lithium iron phosphate, which can build a conductive network and increase the conductivity of the material.

[0004] However, the above methods have some drawbacks. For example, ion doping requires precise control of the type and content of the doping element, which is a complex process and may affect other properties of the material. Although nano-scaling can shorten the diffusion path of lithium ions, nanoparticles are prone to agglomeration, and the preparation process is complex and costly. Surface coating is limited by the stability of the coating material and its compatibility with lithium iron phosphate. The current conventional method for coating lithium iron phosphate with carbon is to directly physically mix the carbon source with lithium iron phosphate and then treat it at high temperature. The advantage of this method is that it is suitable for large-scale industrial production. Its disadvantage is that the coating degree of the lithium iron phosphate surface is uneven and the carbon coating is insufficient, which will reduce the electrochemical properties of the lithium iron phosphate material (such as energy density, rate performance, etc.). Summary of the Invention

[0005] The object of the present invention is to provide a lithium iron phosphate composite material and a preparation method thereof, so as to solve the problem of poor electrochemical performance of lithium iron phosphate materials.

[0006] The purpose of the present invention can be achieved through the following technical solutions: In a first aspect, the present application provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The first step is to prepare a vinylsiloxane-modified lithium iron phosphate precursor; In the second step, a polymerization reaction occurs under the action of an initiator using a vinylsiloxane-modified lithium iron phosphate precursor and vinylimidazole ionic liquid as raw materials: A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazole ionic liquid are added to methanol, an initiator is added, and the mixture is ultrasonically dispersed under an inert atmosphere, reacted at room temperature, washed, and freeze-dried to obtain an ionic liquid-coated lithium iron phosphate precursor. The third step is to calcine the ionic liquid-coated lithium iron phosphate precursor under an inert atmosphere to obtain a lithium iron phosphate composite material.

[0007] Furthermore, the anion in the vinyl imidazolium ionic liquid is [BF4] - PF6 - , [CF3SO3] - and [NTf2] - One of them.

[0008] Furthermore, the vinylsiloxane-modified lithium iron phosphate precursor includes the following steps: The lithium iron phosphate precursor is dispersed in a methanol aqueous solution, lithium hydroxide is added to adjust the pH value to 8-9, vinyl siloxane is added, and after ultrasonic dispersion under an inert atmosphere, aging is carried out at room temperature for 20-24 hours. During this period, intermittent shaking is allowed to occur, and the solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. The usage ratio of the lithium iron phosphate precursor and the vinyl siloxane is 3g:2-3mL.

[0009] Furthermore, the calcination conditions are as follows: nitrogen is used as a protective gas, the temperature is raised from 30° C. to 300-350° C. at a heating rate of 5-10° C. / min, kept at that temperature for 2-3 hours, and then continued to be raised to 700° C. at the same rate, and kept at that temperature for 10-12 hours.

[0010] Furthermore, the mass ratio of the vinylsiloxane-modified lithium iron phosphate precursor to the vinyl imidazolium ionic liquid is 3:0.9-1.2; and the added amount of the initiator is 1% to 2% of the mass of the vinyl imidazolium ionic liquid.

[0011] Furthermore, the vinyl siloxane is one of vinyl triethoxysilane, vinyl trimethoxysilane and methacryloxypropyl trimethoxysilane.

[0012] Furthermore, the lithium iron phosphate precursor is synthesized by a hydrothermal method.

[0013] Furthermore, the lithium iron phosphate precursor is prepared by the following steps: The lithium source, iron source and phosphorus source are mixed according to the molar ratio of lithium ions, ferrous ions and phosphate ions of 1-4:1:1, heated to 140-200°C under an inert atmosphere for reaction for 9-12 hours, and after the reaction, cooled, washed, filtered and vacuum dried at 110°C for 10-12 hours to obtain a lithium iron phosphate precursor.

[0014] Furthermore, the lithium source is one of lithium hydroxide, lithium carbonate and lithium acetate; the iron source is one of ferrous sulfate, ferrous oxalate, ferrous phosphate, ferrous acetate and ferrous chloride; and the phosphorus source is one of phosphoric acid and ammonium dihydrogen phosphate.

[0015] A lithium iron phosphate composite material is prepared by the above preparation method.

[0016] Beneficial effects of the present invention: The present invention provides a method for preparing a lithium iron phosphate composite material, which includes preparing a vinylsiloxane-modified lithium iron phosphate precursor, using the vinylsiloxane-modified lithium iron phosphate precursor and vinylimidazole ionic liquid as raw materials, causing a polymerization reaction under the action of an initiator, and finally preparing the lithium iron phosphate composite material by in-situ polymerization and sintering. The introduction of vinylsiloxane can construct a more complex conductive network, provide more channels for the diffusion of lithium ions, reduce the diffusion resistance of lithium ions, and thus improve the rate performance of the battery. In addition, silicon doping can enhance the bonding force between the carbon layer and the lithium iron phosphate particles, improve the structural stability of the material, reduce the volume change during the charge and discharge process, and help extend the cycle life of the battery; ionic liquid as a carbon source can achieve multi-element doping, thereby improving the cycle stability, rate energy and charge and discharge performance of the lithium iron phosphate composite material as an electrode material. DETAILED DESCRIPTION

[0017] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0018] The following is a detailed description of a lithium iron phosphate composite material and a preparation method thereof according to an embodiment of the present application.

[0019] The following is a detailed description with reference to the embodiments.

[0020] Example 1 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: Ferrous sulfate and ascorbic acid are dissolved in water to obtain a mixed solution, phosphoric acid is dissolved in water to obtain a phosphoric acid solution, and lithium hydroxide is dissolved in water to obtain a lithium hydroxide solution. The phosphoric acid solution and the mixed solution are sequentially added to the lithium hydroxide solution to generate a precipitate, and ammonia water or dilute hydrochloric acid is added to adjust the pH value to 2-3. The mixture is heated to 180° C. under an inert atmosphere for 10 hours. After the reaction is completed, the mixture is cooled, washed, filtered, and vacuum-dried at 110° C. for 10 hours to obtain a lithium iron phosphate precursor. The molar ratio of lithium hydroxide, ferrous sulfate, and phosphoric acid is 3:1:1, and the molar ratio of ascorbic acid to ferrous sulfate is 0.3:1.

[0021] A lithium iron phosphate precursor is dispersed in a methanol aqueous solution, lithium hydroxide is added to adjust the pH value to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours, with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the ratio of the lithium iron phosphate precursor to the vinyl siloxane is 3g:2mL; the vinyl siloxane is vinyltriethoxysilane.

[0022] A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid (VII) were added to methanol, along with azobisisobutyronitrile (ABI) as an initiator. The mixture was ultrasonically dispersed under an inert atmosphere for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The VII ionic liquid was 1-vinyl-3-ethylimidazolium tetrafluoroborate. The mass ratio of the VII to the VII was 3:0.9. The initiator was added at 2% of the mass of the VII ionic liquid.

[0023] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0024] Example 2 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:2mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid (VII) were added to methanol, along with azobisisobutyronitrile (ABI) as an initiator. The mixture was ultrasonically dispersed under an inert atmosphere for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The VII ionic liquid was 1-vinyl-3-ethylimidazolium tetrafluoroborate. The mass ratio of the VII to the VII was 3:1.2. The initiator was added at 2% of the mass of the VII ionic liquid.

[0025] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0026] Example 3 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:3mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid (VII) were added to methanol, along with azobisisobutyronitrile (ABI) as an initiator. The mixture was ultrasonically dispersed under an inert atmosphere for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The VII ionic liquid was 1-vinyl-3-ethylimidazolium tetrafluoroborate. The mass ratio of the VII to the VII was 3:0.9. The initiator was added at 2% of the mass of the VII ionic liquid.

[0027] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0028] Example 4 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:3mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid (VII) were added to methanol, along with azobisisobutyronitrile (ABI) as an initiator. The mixture was ultrasonically dispersed under an inert atmosphere for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The VII ionic liquid was 1-vinyl-3-ethylimidazolium tetrafluoroborate. The mass ratio of the VII to the VII was 3:1.2. The initiator was added at 2% of the mass of the VII ionic liquid.

[0029] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0030] Example 5 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:2mL; the vinyl siloxane is methacryloxypropyltrimethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid (VII) were added to methanol, along with azobisisobutyronitrile (ABI) as an initiator. The mixture was ultrasonically dispersed under an inert atmosphere for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The VII ionic liquid was 1-vinyl-3-ethylimidazolium tetrafluoroborate. The mass ratio of the VII to the VII was 3:0.9. The initiator was added at 2% of the mass of the VII ionic liquid.

[0031] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0032] Example 6 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:2mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid were added to methanol, along with azobisisobutyronitrile as an initiator. Under an inert atmosphere, the mixture was ultrasonically dispersed for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The vinyl imidazolium ionic liquid was 1-vinyl-3-ethylimidazole hexafluorophosphate. The mass ratio of the vinylsiloxane-modified lithium iron phosphate precursor to the vinyl imidazolium ionic liquid was 3:0.9. The initiator was added at 2% of the mass of the vinyl imidazolium ionic liquid.

[0033] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0034] Example 7 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:2mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid were added to methanol, along with azobisisobutyronitrile as an initiator. Under an inert atmosphere, the mixture was ultrasonically dispersed for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The vinyl imidazolium ionic liquid was 1-vinyl-3-ethylimidazole bis(trifluoromethanesulfonyl)imide salt. The mass ratio of the vinylsiloxane-modified lithium iron phosphate precursor to the vinyl imidazolium ionic liquid was 3:0.9. The initiator was added at 2% of the mass of the vinyl imidazolium ionic liquid.

[0035] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0036] Example 8 This embodiment provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor is dispersed in a methanol-water solution, lithium hydroxide is added to adjust the pH to 9, vinyl siloxane is added, and ultrasonic dispersion is carried out under an inert atmosphere for 10 minutes. The mixture is aged at room temperature for 24 hours with intermittent shaking during the period. The solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. In some specific embodiments, the lithium iron phosphate precursor and vinyl siloxane are used in a ratio of 3g:2mL; the vinyl siloxane is vinyltriethoxysilane. The lithium iron phosphate precursor is the same as in Example 1. A vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazolium ionic liquid were added to methanol, along with azobisisobutyronitrile as an initiator. Under an inert atmosphere, the mixture was ultrasonically dispersed for 10 minutes, mechanically stirred at room temperature for 1 hour, and then heated to the boiling point of methanol to remove the methanol. After the reaction, the mixture was washed with methanol and freeze-dried at -70°C to obtain an ionic liquid-coated lithium iron phosphate precursor. The vinyl imidazolium ionic liquid was 1-vinyl-3-ethylimidazole bis(trifluoromethanesulfonyl)imide salt. The mass ratio of the vinylsiloxane-modified lithium iron phosphate precursor to the vinyl imidazolium ionic liquid was 3:0.9. The initiator was added at 2% of the mass of the vinyl imidazolium ionic liquid.

[0037] The ionic liquid-coated lithium iron phosphate precursor was calcined under an inert atmosphere: nitrogen was used as the protective gas, the temperature was raised from 30°C to 300°C at a rate of 5°C / min, kept warm for 3 hours, and then continued to be heated to 700°C at the same rate and kept warm for 12 hours to obtain a lithium iron phosphate composite material.

[0038] Comparative Example 1 This comparative example provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor, vinyl triethoxysilane, and 1-vinyl-3-ethylimidazolium tetrafluoroborate were mixed in a ratio of 3 g:2 mL:0.9 g, ground evenly, and then sintered under nitrogen as a protective atmosphere. The sintering process and lithium iron phosphate precursor were the same as in Example 1.

[0039] Comparative Example 2 This comparative example provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: The lithium iron phosphate precursor and 1-vinyl-3-ethylimidazolium tetrafluoroborate were mixed in a ratio of 3 g:0.9 g, ground evenly, and then sintered under nitrogen as a protective atmosphere. The sintering process and the lithium iron phosphate precursor were the same as those in Example 1.

[0040] Comparative Example 3 This comparative example provides a method for preparing a lithium iron phosphate composite material, comprising the following steps: Ferrous sulfate and ascorbic acid are dissolved in water to obtain a mixed solution, phosphoric acid is dissolved in water to obtain a phosphoric acid solution, and lithium hydroxide is dissolved in water to obtain a lithium hydroxide solution. Glucose, phosphoric acid solution, and the mixed solution are sequentially added to the lithium hydroxide solution to produce a precipitate. Ammonia or dilute hydrochloric acid is added to adjust the pH to 2-3. The mixture is heated to 180°C under an inert atmosphere for 10 hours. After the reaction, the mixture is cooled, washed, filtered, and vacuum-dried at 110°C for 10 hours to obtain a lithium iron phosphate composite material. The molar ratio of lithium hydroxide, ferrous sulfate, and phosphoric acid is 3:1:1; the molar ratio of ascorbic acid to ferrous sulfate is 0.3:1; and the ratio of glucose to ferrous sulfate is 0.9 g:0.02 mol. Glucose serves as a carbon source.

[0041] Performance testing The performance of lithium iron phosphate composite materials prepared in Examples 1 to 8 and Comparative Examples 1 to 3 was tested. Sample preparation: The lithium iron phosphate composite material prepared in Examples 1 to 8 and Comparative Examples 1 to 3 was mixed with PVDF (polyvinylidene fluoride) and acetylene black in a mass ratio of 8:1:1 and ground evenly. N-methylpyrrolidone solvent was added dropwise and grinding was continued. The resulting slurry was smeared on aluminum foil with a thickness of 50 μm and vacuum dried at 110° C. for 10-12 h to obtain a positive electrode sheet.

[0042] The experimental battery used 99.9% metallic lithium sheet as the negative electrode and Celgard2400 (microporous polyethylene membrane) as the separator. It was assembled into a simulated battery in a vacuum glove box. A 2032-type button cell was prepared with lithium sheet as the negative electrode material, polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 as the electrolyte.

[0043] Performance tests were performed on Examples 1 to 8 and Comparative Examples 1 to 3: Specific capacity test: In the voltage range of 2.0~3.8V, at a constant temperature of 25℃, the first discharge capacity of the sample battery is tested at 0.1C and 10C rates respectively; Cycling performance test: The sample battery is subjected to charge and discharge cycle test in the voltage range of 2.0~3.8V, constant temperature condition of 25℃ and 0.1C rate, and the discharge capacity after 100 cycles is recorded.

[0044] The results are shown in Table 1: Table 1

[0045] In combination with Example 1 and Comparative Examples 1 to 3, different preparation methods will affect the performance of the battery. In Example 1, the in-situ polymerization and sintering method is used to prepare the lithium iron phosphate composite material. Compared with the grinding and mixing method in Comparative Examples 1 to 2 and the in-situ coating of the conventional carbon source in Comparative Example 3, the method of Example 1 can prepare a uniform and tightly coated carbon layer. Moreover, compared with Comparative Examples 2 and 3, the silane coupling agent selected in Example 1 can construct a more complex conductive network, provide more channels for the diffusion of lithium ions, reduce the diffusion resistance of lithium ions, and thus improve the rate performance of the battery. In addition, silicon doping can enhance the bonding force between the carbon layer and the lithium iron phosphate particles, improve the structural stability of the material, reduce the volume change during the charge and discharge process, and help extend the cycle life of the battery.

[0046] Table 1, combined with Examples 1 and 2-5, shows no significant differences in discharge capacity and cycle capacity retention between Examples 2-5 and Example 1 at different rates, indicating no significant differences in capacity and cycle performance between Examples 2-5 and Example 1. This is because the changes in Examples 2-5 compared to Example 1 are solely changes in the raw materials and their ratios within the required range, which have minimal impact on the resulting battery performance. Compared to Example 1, Examples 6-8 vary the type of ionic liquid, demonstrating that different ionic liquids can affect the capacity and cycle performance of the material to a certain extent. In particular, the ionic liquid selected in Example 6, 1-vinyl-3-ethylimidazolium hexafluorophosphate, exhibits the most pronounced improvement. The ionic liquid in this invention acts as a carbon source, forming a heteroatom-doped carbon source that in situ coats the lithium iron phosphate. F doping reduces lithium-iron antisite defects in the lithium iron phosphate, reducing the transport resistance of electrons and lithium ions and accelerating lithium ion migration. Compared to N and B, P has a higher electron-donating capacity, thereby promoting electronic conductivity and improving bonding stability. At the same time, the high specific surface area generated by the PC bonds formed after the P-doped carbon layer helps to improve the graphitization level of the carbon layer, and the battery performance is further improved by co-doping F and P.

[0047] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0048] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a lithium iron phosphate composite material, characterized in that: The steps include: The first step is to prepare a vinylsiloxane-modified lithium iron phosphate precursor; In the second step, a vinylsiloxane-modified lithium iron phosphate precursor and a vinyl imidazole ionic liquid are used as raw materials, and an in-situ polymerization reaction is carried out under the action of an initiator to obtain an ionic liquid-coated lithium iron phosphate precursor; The third step is to calcine the ionic liquid-coated lithium iron phosphate precursor under an inert atmosphere to obtain a lithium iron phosphate composite material.

2. The method for preparing a lithium iron phosphate composite material according to claim 1, wherein: The anion in the vinyl imidazolium ionic liquid is [BF4] - PF6 - , [CF3SO3] - and [NTf2] - One of them.

3. The method for preparing a lithium iron phosphate composite material according to claim 1, wherein: The vinylsiloxane-modified lithium iron phosphate precursor includes the following steps: The lithium iron phosphate precursor is dispersed in a methanol aqueous solution, lithium hydroxide is added to adjust the pH value to 8-9, vinyl siloxane is added, and after ultrasonic dispersion under an inert atmosphere, aging is carried out at room temperature for 20-24 hours, and the solvent is removed to obtain a vinyl siloxane-modified lithium iron phosphate precursor. The amount ratio of the lithium iron phosphate precursor to the vinyl siloxane is 3g:2-3mL.

4. The method for preparing a lithium iron phosphate composite material according to claim 1, wherein: Calcination conditions: nitrogen as protective gas, heating rate of 5-10℃ / min from 30℃ to 300-350℃, keeping warm for 2-3h, and then continue heating to 700℃ at the same rate, keeping warm for 10-12h.

5. The method for preparing a lithium iron phosphate composite material according to claim 1, wherein: The mass ratio of the vinylsiloxane-modified lithium iron phosphate precursor to the vinylimidazole ionic liquid is 3:0.9-1.2; the added amount of the initiator is 1% to 2% of the mass of the vinylimidazole ionic liquid.

6. The method for preparing a lithium iron phosphate composite material according to claim 3, characterized in that: The vinyl siloxane is one of vinyl triethoxysilane, vinyl trimethoxysilane and methacryloxypropyl trimethoxysilane.

7. The method for preparing a lithium iron phosphate composite material according to claim 1, characterized in that: The lithium iron phosphate precursor is synthesized by a hydrothermal method.

8. The method for preparing a lithium iron phosphate composite material according to claim 1, characterized in that: The lithium iron phosphate precursor is prepared by the following steps: The lithium source, iron source and phosphorus source are mixed according to the molar ratio of lithium ions, ferrous ions and phosphate ions of 1-4:1:1, heated to 140-200°C under an inert atmosphere for reaction for 9-12 hours, and after the reaction, cooled, washed, filtered and vacuum dried at 110°C for 10-12 hours to obtain a lithium iron phosphate precursor.

9. The method for preparing a lithium iron phosphate composite material according to claim 8, characterized in that: The lithium source is one of lithium hydroxide, lithium carbonate and lithium acetate; the iron source is one of ferrous sulfate, ferrous oxalate, ferrous phosphate, ferrous acetate and ferrous chloride; the phosphoric acid is one of phosphoric acid and ammonium dihydrogen phosphate.

10. A lithium iron phosphate composite material, characterized in that: The method is prepared by the method according to any one of claims 1 to 9.

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