A polyurethane-coated lithium iron phosphate composite material, its preparation method and application

Abstract

This invention relates to the field of lithium battery materials technology, and discloses a polyurethane-coated lithium iron phosphate composite material, its preparation method, and its applications. The general chemical formula of the material is LiFe. 1‑x‑ y Ti x Mg y PO4 / C, where 0.01≤x≤0.05, 0.01≤y≤0.05, 0.02≤x+y≤0.1. The preparation method includes the following steps: Step 1, hydrothermal reaction of NH4H2PO4, Fe(NO3)3·9H2O, and a surfactant to obtain an iron phosphate precursor; Step 2, mixing the iron phosphate precursor, lithium source, titanium source, and magnesium source to obtain LiFe. 1‑x‑y Ti x Mg y PO4; Step 3, LiFe 1‑x‑y Ti x Mg y The lithium iron phosphate composite material is obtained by ball milling and calcining a mixture of PO4 and polyurethane or modified polyurethane. In this invention, the method of adding surfactants and polyurethane or modified polyester as carbon sources during the hydrothermal reaction improves the agglomeration problem of iron phosphate particles, and the doping with titanium and magnesium metal ions yields an electrochemical cathode material with high conductivity, excellent rate performance, and excellent cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, specifically to a polyurethane-coated lithium iron phosphate composite material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is an electrode material for lithium-ion batteries and is mainly used in various lithium-ion batteries. Since NTT of Japan first revealed the olivine-structured lithium battery cathode material of LiFeCoPO4 in 1996, a research group led by John B. Goodenough at the University of Texas in the United States also reported the reversible lithium migration and extraction characteristics of LiFePO4 in 1997.

[0003] Patent document CN105355859A discloses an iron- and barium-doped lithium iron phosphate cathode material for lithium-ion batteries and its preparation method: lithium hydroxide, ammonium dihydrogen phosphate, barium carbonate, ferrous oxalate, and dysprosium oxide are mixed and ball-milled to obtain a nano-precursor; iron salt sol is formed by dissolving iron propylene glycol in toluene as a co-solvent to obtain an iron salt sol coating solution; the above precursor powder is added to the coating phase ethyl ketone and mixed; the above iron salt sol coating solution is added and ball-milled; after drying, it is sintered to obtain the iron- and barium-doped lithium iron phosphate cathode material.

[0004] For example, patent document CN114335478A discloses an iron-doped lithium iron phosphate / carbon composite microsphere with high tap density, its preparation method and application, which belongs to the field of lithium battery technology. The preparation steps include: (1) weighing appropriate amounts of iron source, phosphorus source, lithium source, iron hydroxide, PEG-400 and carbon source A, mixing them in a solid phase to obtain a mixture, then adding the mixture to deionized water containing zirconium sand for ball milling, and filtering the zirconium sand with a sieve to obtain a slurry; (2) spray drying the slurry obtained in step (1) to obtain a yellow-brown precursor powder; (3) placing the yellow-brown precursor powder obtained in step (2) in a tube furnace rich in inert gas for high-temperature sintering to obtain iron-doped lithium iron phosphate / carbon composite microspheres with high tap density.

[0005] For example, patent document CN108520948A discloses a method for preparing iron oxide-coated lithium iron phosphate carbon composite material using the sol-gel method. The method combines the metal oxide iron oxide with lithium iron phosphate carbon using the sol-gel method and calcines it in a muffle furnace at 600-750°C for 3-6 hours to obtain the iron oxide-coated lithium iron phosphate carbon composite material.

[0006] Existing methods for preparing lithium iron phosphate have not yet yielded high-performance products, and the morphology of the precursor directly determines the performance of the final product. Among numerous preparation methods, the liquid-phase method has significant advantages over the solid-phase reaction method in terms of particle uniformity and molecular-level dispersibility. Lithium iron phosphate precursors and lithium iron phosphate prepared by the liquid-phase method have advantages such as small particle size, uniform dispersion, and low energy consumption.

[0007] Liquid-phase methods mainly include hydrothermal (solvothermal) methods and coprecipitation methods. However, lithium iron phosphate particles prepared by the hydrothermal method are small, but agglomeration is relatively severe. Materials prepared by the coprecipitation method have poor performance due to particle agglomeration. Summary of the Invention

[0008] This invention addresses the problem of easy agglomeration of lithium iron phosphate particles in the prior art by providing a polyurethane-coated lithium iron phosphate composite material. This material has a uniform particle size distribution, good dispersibility, and is an electrochemical cathode material with high conductivity, excellent rate performance, and excellent cycle stability.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A polyurethane-coated lithium iron phosphate composite material, wherein the general chemical formula of the lithium iron phosphate composite material is LiFe 1-x-y Ti x Mg y PO4 / C, where 0.01≤x≤0.05, 0.01≤y≤0.05, and 0.02≤x+y≤0.1.

[0011] The present invention also provides a method for preparing the polyurethane-coated lithium iron phosphate composite material, comprising the steps of:

[0012] Step 1: NH4H2PO4, Fe(NO3)3·9H2O and surfactant are mixed in water, and the pH is adjusted to 0.9-1.6 using ammonia. The mixed solution is then transferred to a sealed reactor for hydrothermal reaction. The product is separated, washed and dried to obtain the iron phosphate precursor.

[0013] Step 2: The iron phosphate precursor, lithium source, titanium source, and magnesium source prepared in Step 1 are mixed in a liquid medium, filtered to separate the precipitate, and dried to obtain the titanium-magnesium co-doped lithium iron phosphate precursor LiFe. 1-x-y Ti x Mg y PO4 mixture;

[0014] Step 3, LiFe 1-x-y Ti x Mg y The lithium iron phosphate composite material is obtained by ball milling and calcining a mixture of PO4 and a carbon source; the carbon source is polyurethane or modified polyurethane.

[0015] This invention addresses two main issues. First, by incorporating suitable surfactants to regulate the morphology of the precursor and mitigate particle agglomeration. Second, by coating the surface of lithium iron phosphate particles with polyurethane or modified polyurethane, a conductive network is formed after calcination. This network not only improves conductivity but also isolates particles, preventing agglomeration and inhibiting particle growth. Furthermore, the carbon layer of the polyurethane facilitates electrolyte wetting, ensuring sufficient contact between the electrolyte and the cathode material, which positively promotes lithium-ion diffusion and thus enhances the electrochemical performance of lithium iron phosphate. The resulting lithium iron phosphate composite material exhibits uniform particle size, prevents agglomeration, and possesses high conductivity, excellent rate performance, and superior cycle stability, making it an excellent electrochemical cathode material.

[0016] Preferably, the carbon source is modified polyurethane.

[0017] The preparation method of the modified polyurethane includes the following steps:

[0018] Polyurethane acrylate was dispersed in a solvent, and iron acrylate, 1-allyl-3-methylimidazolium tetrafluoroborate and potassium persulfate were added. The grafting reaction was carried out under an inert atmosphere. The product was distilled to remove the solvent and dried to obtain modified polyurethane.

[0019] This modified polyurethane, containing boron, fluorine, and iron, serves as a carbon source. The boron encapsulation within the carbon layer not only increases conductivity but also, due to its position on the electron conduction pathway, enhances the lithium-ion insertion / extraction rate, thereby improving battery performance. Furthermore, the grafting of iron and fluorine onto the polyurethane has a synergistic effect on the charge-discharge performance of LiFePO4, significantly improving the material's rate performance and cycle life. Therefore, using modified polyurethane as a carbon source can further enhance the electrochemical performance of the material.

[0020] Preferably, the amounts of each raw material input in the preparation process of modified polyurethane, based on parts by mass, are as follows:

[0021] The composition includes 30-60 parts polyurethane acrylate, 200-300 parts solvent, 0.02-0.4 parts iron acrylate, 0.002-0.05 parts 1-allyl-3-methylimidazolium tetrafluoroborate, and 2-4 parts potassium persulfate. The olefinic bonds in iron acrylate and 1-allyl-3-methylimidazolium tetrafluoroborate react with the carbon-carbon double bonds in the polyurethane acrylate, grafting small molecules containing iron, boron, and fluorine elements onto the polyurethane matrix.

[0022] Preferably, the polyurethane acrylate is dispersed in the solvent by stirring at 50-60°C for 30-90 minutes;

[0023] The grafting reaction temperature is 60-70℃, and the reaction time is 3-6 hours.

[0024] The solvent includes any one or more of ethanol, deionized water, and acetone;

[0025] The inert gas includes either nitrogen or argon.

[0026] In step 1, the molar ratio of NH4H2PO4 to Fe(NO3)3·9H2O is (0.97-1.15):(0.98-1.3).

[0027] In step 1, the hydrothermal reaction temperature is 80-120℃, and the reaction time is 6-12 hours. Both reaction temperature and reaction time restrict particle growth and morphology. As the temperature increases, the solubility of the material increases, the supersaturation decreases, and the solution viscosity decreases. At higher temperatures, the reaction rate accelerates, more primary crystal nuclei form in the solution, and the particle size decreases. However, when the temperature exceeds a certain critical point, due to the decrease in size, the surface energy of the particles increases, and excessive crystals can cause particle agglomeration. At lower temperatures, the solution saturation is higher, the reaction rate is slower, and the nucleation rate is also slower.

[0028] If the reaction time is too short, the crystals may not complete the nucleation and growth process, resulting in a disordered particle structure. Conversely, if the reaction time is too long, the particles will inevitably become larger. Excessively large particles negatively impact the electrochemical performance of the material; therefore, the hydrothermal reaction time and temperature are crucial to the electrochemical performance of the synthesized material.

[0029] Preferably, at a temperature of 100°C and after 8 hours of hydrothermal reaction, the particles exhibit good morphology and uniform particle size. The addition of a cationic surfactant in this invention effectively disperses and controls the particle morphology.

[0030] The surfactant includes one or more of cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP); preferably, the surfactant is CTAB, which is a cationic surfactant with a significantly better dispersing effect than the other two.

[0031] Preferably, in step 1, NH4H2PO4, Fe(NO3)3·9H2O and the surfactant are first prepared into aqueous solutions. The aqueous solutions should be prepared in a way that can evenly disperse the raw materials. Preparing the aqueous solutions in advance is more conducive to the full mixing of the raw materials.

[0032] More preferably, the molar concentration of the surfactant in the pre-prepared aqueous solution is 0.01-0.2 mol / L;

[0033] In step 1, the amount of surfactant added is 0.9-1.9% of the molar amount of Fe(NO3)3·9H2O.

[0034] Preferably, in step 1, the product separation after the hydrothermal reaction is carried out by centrifugation; the product is washed with deionized water and anhydrous ethanol, and unreacted raw materials and impurities are removed by multiple washings.

[0035] In step 2, the molar ratio of lithium in the iron phosphate precursor and the lithium source is 0.96–1.05: 0.98–1.02;

[0036] In step 2, the molar ratio of titanium in the iron phosphate precursor and the titanium source is 1:0.01–0.05; the molar ratio of magnesium in the iron phosphate precursor and the magnesium source is 1:0.01–0.05. Co-doping with metal ions can enhance the material's conductivity, rate performance, and cycle stability. Ti ion doping mainly causes lattice defects in LiFePO4, while Mg... 2+ Ion doping can weaken the Li-O bond energy and increase the Li-O bond energy. + The mobility and diffusion coefficient are improved, thereby synergistically enhancing the rate performance of the material and facilitating the preparation of cathode materials with superior electrochemical performance.

[0037] The lithium source includes any one or more of lithium hydroxide, lithium carbonate, lithium acetate, and lithium nitrate.

[0038] The titanium source includes one or more of titanium dioxide, tetramethyl titanate, and titanium tetrachloride.

[0039] The magnesium source includes one or more of magnesium oxalate, magnesium sulfate, magnesium carbonate, magnesium nitrate, and magnesium hydroxide.

[0040] In step 2, the liquid medium is any one or more of water, ethanol, and acetone;

[0041] In step 2, the substances are mixed at room temperature for 1-3 hours.

[0042] In step 3, the mass of the carbon source is LiFe. 1-x-y Ti x Mg y The coating amount is 1-5 wt% of the PO4 mixture; too much coating will reduce the lithium-ion transport rate, while too little coating will affect its cycle performance.

[0043] In step 3, the calcination temperature is 400-700℃ and the calcination time is 5-7h.

[0044] The present invention also provides the application of the polyurethane-coated lithium iron phosphate composite material in lithium-ion batteries.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) In this invention, the method of adding surfactants and polyurethane or modified polyester as carbon sources during hydrothermal reaction improves the problem of iron phosphate particle agglomeration, and the doping of titanium and magnesium metal ions yields an electrochemical cathode material with high conductivity, excellent rate performance and cycle stability.

[0047] (2) In this invention, modified polyurethane grafted with iron, fluorine and boron is used as a carbon source to coat the surface of titanium-magnesium co-doped lithium iron phosphate precursor. By utilizing the dual-site structure of iron and the excellent properties of boron such as conductivity and stability, the conductivity of the material is greatly improved. When the material is used in batteries, it can greatly enhance the performance and stability of lithium-ion batteries, and at the same time improve the safety of batteries.

[0048] (3) The composite material prepared by the present invention has the characteristics of adjustable morphology and uniform particle size, excellent electrochemical performance, simple preparation method and easy industrial production application. Attached Figure Description

[0049] Figure 1 The image shows a SEM image of the polyurethane-coated lithium iron phosphate composite material prepared in Example 1. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Modifications or equivalent substitutions made by those skilled in the art based on their understanding of the technical solutions of this invention, without departing from the spirit and scope of the invention, should be covered within the protection scope of this invention.

[0051] All raw materials used in the following specific embodiments were purchased commercially. The performance evaluation of the composite materials prepared in each embodiment was conducted using CR2032 button half-cells. The specific preparation process of the positive electrode sheet is as follows:

[0052] The composite material is used as the positive electrode. The composite material, acetylene black, and binder PVDF (polyvinylidene fluoride) are weighed in a mass ratio of 80:10:10 and added to an agate mortar. They are then ground thoroughly until they are uniformly mixed. An appropriate amount of N-methylpyrrolidone is added dropwise to the mixture, and it is ground again until a uniform slurry of a certain viscosity is formed. The slurry is coated onto dry carbon-coated aluminum foil using a scraper. It is first dried in a forced-air drying oven at 70°C for 3 hours, and then dried in a vacuum environment at 120°C for 12 hours. The electrode is removed when the temperature drops below 50°C. The removed electrode is then compacted using a roller press and cut into circular positive electrode sheets with a diameter of 12 mm using a punching machine for later use.

[0053] Lithium foil was selected as the negative electrode, and the electrolyte was 1.0 mol / L LiPF6 / (EC+EMC+DMC) (where LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, EMC is ethyl methyl carbonate, DMC is dimethyl carbonate, and the volume ratio of EC, EMC, and DMC is 1:1:1). Celgard 2400 polypropylene microporous membrane was used as the separator. All materials, including the positive electrode, negative electrode, electrolyte, and separator, were assembled into a CR2032 coin cell in a glove box filled with high-purity argon gas (H2O < 0.1 ppm, O2 < 0.1 ppm). The coin cell was then sealed using a sealing machine. Finally, the assembled CR2032 coin cell was allowed to stand at room temperature for 12 hours for activation before use.

[0054] Example 1

[0055] Step 1: Select 1 mol each of NH4H2PO4 and Fe(NO3)3·9H2O as phosphorus and iron sources, respectively, and dissolve them sequentially in 4 L of deionized water, stirring evenly with a magnetic stirrer. Slowly add the two fully dissolved solutions dropwise to an empty beaker while stirring, and mix thoroughly for 5 min. Add 0.013 mol of the surfactant hexadecyltrimethylammonium bromide (CTAB), mix thoroughly, adjust the pH of the solution to approximately 1.2 with ammonia water, and continue stirring for 20 min. Preheat the oil bath. Transfer the solution to a sealed high-pressure reactor lined with polytetrafluoroethylene for hydrothermal reaction, heating in an oil bath at 100°C for 8 h. After the reaction, separate the solution using a centrifuge, wash repeatedly with deionized water and anhydrous ethanol, and dry to obtain the iron phosphate precursor.

[0056] Step 2: Iron phosphate precursor, Li₂CO₃, TiO₂, and Mg(OH)₂ are mixed in deionized water at room temperature for 3 hours at a molar ratio of iron phosphate precursor to lithium, titanium, and magnesium of 1:0.96:0.015:0.025. The precipitate is then separated by filtration and dried to obtain LiFe. 0.96 Ti 0.015 Mg 0.025 PO4 mixture.

[0057] Step 3, in LiFe 0.96 Ti 0.015 Mg 0.025 LiFe added to PO4 mixture material 0.96 Ti 0.015 Mg 0.025 A mixture of 2 wt% polyurethane (by weight of PO4) was ball-milled and calcined at 480°C for 5 hours to obtain the product LiFe. 0.96 Ti 0.015 Mg 0.025 PO4 / C.

[0058] The SEM image of the material is as follows: Figure 1 As shown, there is no obvious agglomeration between particles, indicating good dispersibility. Its discharge capacity at 0.2C is 156.5 mAh / g. After 50 cycles at 0.2C, the retention rate remains at 99.5%.

[0059] Example 2

[0060] Following the preparation process of Example 1, the following process was substituted, while other processes remained unchanged:

[0061] In step 1, the surfactant used is polyethylene glycol, and the molar ratio of NH4H2PO4, Fe(NO3)3·9H2O and polyethylene glycol is adjusted to 1.03:1.09:0.015; the hydrothermal reaction is carried out under oil bath heating at 80℃ for 10 hours.

[0062] In step 2, tetramethyl titanate is used as the titanium source, magnesium oxalate is used as the magnesium source, and the molar ratio of iron phosphate precursor, Li element, titanium element and magnesium element is 1:0.98:0.015:0.005.

[0063] In step 3, the carbon source dosage was 1.5 wt%, the calcination temperature was 600℃, and the calcination time was 6 h. The resulting composite material had an initial charge capacity of 153.8 mAh / g, and retained 98.2% of its capacity after 50 cycles at 0.2C.

[0064] Example 3

[0065] Following the preparation process of Example 1, the following process was substituted, while other processes remained unchanged:

[0066] In step 1, the surfactant used is polyvinylpyrrolidone, and the molar ratio of NH4H2PO4, Fe(NO3)3·9H2O and polyvinylpyrrolidone is adjusted to 1.1:1.2:0.016; the hydrothermal reaction is carried out under oil bath heating at 120℃ for 6 hours.

[0067] In step 2, titanium tetrachloride is used as the titanium source, magnesium sulfate is used as the magnesium source, and the molar ratio of iron phosphate precursor, Li element, titanium element and magnesium element is 1:0.975:0.015:0.01;

[0068] In step 3, the carbon source dosage was 3 wt%, the calcination temperature was 550℃, and the calcination time was 7 h. The resulting composite material had an initial charge capacity of 150.5 mAh / g, and retained 97.9% of its capacity after 50 cycles at 0.2C.

[0069] Example 4

[0070] Following the preparation process of Example 1, the polyurethane was replaced with modified polyurethane prepared by the following method, while other conditions remained unchanged.

[0071] The specific preparation process of modified polyurethane is as follows:

[0072] S1: Add 200g of ethanol and 30g of polyurethane acrylate to a three-necked flask, stir magnetically, and disperse at 50°C for 30 minutes.

[0073] S2: Add 0.02g of ferric acrylate, 0.002g of 1-allyl-3-methylimidazolium tetrafluoroborate, and 2g of potassium persulfate to the mixture in S1. Purge with nitrogen gas and react at 60°C for 3 hours. After the reaction, distill off the solvent from the product and dry it to obtain the iron / fluorine-boron grafted polyurethane.

[0074] The material has an initial charge capacity of 167.7 mAh / g and retains 99.7% of its capacity after 50 cycles at 0.2C.

[0075] Example 5

[0076] Following the preparation process of Example 2, the polyurethane was replaced with modified polyurethane prepared by the following method, while other conditions remained unchanged.

[0077] The specific preparation process of modified polyurethane is as follows:

[0078] S1: Add 230g of ethanol and 40g of polyurethane acrylate to a three-necked flask, stir magnetically, and disperse at 55°C for 60 minutes.

[0079] S2: Add 0.08g of ferric acrylate, 0.004g of 1-allyl-3-methylimidazolium tetrafluoroborate, and 2.6g of potassium persulfate to the mixture in S1. Purge with nitrogen gas and react at 64℃ for 4 hours. After the reaction, distill off the solvent from the product and dry it to obtain iron / fluorine-boron grafted polyurethane.

[0080] The material has an initial charge capacity of 161.5 mAh / g and retains 98.9% of its capacity after 50 cycles at 0.2C.

[0081] Example 6

[0082] Following the preparation process of Example 3, the polyurethane was replaced with modified polyurethane prepared by the following method, while other conditions remained unchanged.

[0083] The specific preparation process of modified polyurethane is as follows:

[0084] S1: Add 300g of ethanol and 60g of polyurethane acrylate to a three-necked flask, stir magnetically, and disperse at 60°C for 90 minutes.

[0085] S2: Add 0.4g of iron acrylate, 0.05g of 1-allyl-3-methylimidazolium tetrafluoroborate, and 2.4g of potassium persulfate to the mixture in S1. Purge with nitrogen gas and react at 70°C for 6 hours. After the reaction, distill off the solvent from the product and dry it to obtain iron / fluorine-boron grafted polyurethane.

[0086] The material has an initial charge capacity of 160.8 mAh / g and retains 98.8% of its capacity after 50 cycles at 0.2C.

[0087] Comparative Example 1

[0088] Following the preparation process of Example 1, no titanium or magnesium source was added in step 2, while other conditions remained unchanged. The resulting composite material exhibited a discharge capacity of 150.5 mAh / g at 0.2C. After 50 cycles at 0.2C, the capacity retention was still 95.5%.

[0089] Comparative Example 2

[0090] Following the preparation process of Example 1, polyurethane was not added in step 3; the material was directly ball-milled and then calcined, with other conditions remaining unchanged. The resulting composite material exhibited a discharge capacity of 148.5 mAh / g at 0.2C. After 50 cycles at 0.2C, the discharge capacity remained at 95%.

Claims

1. A polyurethane-coated lithium iron phosphate composite material, characterized in that, The general chemical formula of the lithium iron phosphate composite material is LiFe 1-x-y Ti x Mg y PO4 / C, where 0.01≤x≤0.05, 0.01≤y≤0.05, 0.02≤x+y≤0.1; The preparation method of the polyurethane-coated lithium iron phosphate composite material includes the following steps: Step 1: NH4H2PO4, Fe(NO3)3·9H2O and surfactant are mixed in water, and the pH is adjusted to 0.9-1.6 using ammonia. The mixed solution is then transferred to a sealed reactor for hydrothermal reaction. The product is separated, washed and dried to obtain the iron phosphate precursor. Step 2: The iron phosphate precursor, lithium source, titanium source, and magnesium source prepared in Step 1 are mixed in a liquid medium, filtered to separate the precipitate, and dried to obtain the titanium-magnesium co-doped lithium iron phosphate precursor LiFe. 1-x-y Ti x Mg y PO4 mixture; Step 3, LiFe 1-x-y Ti x Mg y The lithium iron phosphate composite material was obtained by ball milling and calcining a mixture of PO4 and a carbon source; the carbon source was a modified polyurethane. The preparation method of the modified polyurethane includes the following steps: Polyurethane acrylate was dispersed in a solvent, and iron acrylate, 1-allyl-3-methylimidazolium tetrafluoroborate and potassium persulfate were added. The grafting reaction was carried out under an inert atmosphere. The product was distilled to remove the solvent and dried to obtain modified polyurethane. Based on parts by mass, the input amounts of each raw material in the preparation process of modified polyurethane are as follows: The components are: 30-60 parts polyurethane acrylate, 200-300 parts solvent, 0.02-0.4 parts iron acrylate, 0.002-0.05 parts 1-allyl-3-methylimidazolium tetrafluoroborate, and 2-4 parts potassium persulfate.

2. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 1, characterized in that, Including the following steps: Step 1: NH4H2PO4, Fe(NO3)3·9H2O and surfactant are mixed in water, and the pH is adjusted to 0.9-1.6 using ammonia. The mixed solution is then transferred to a sealed reactor for hydrothermal reaction. The product is separated, washed and dried to obtain the iron phosphate precursor. Step 2: The iron phosphate precursor, lithium source, titanium source, and magnesium source prepared in Step 1 are mixed in a liquid medium, filtered to separate the precipitate, and dried to obtain the titanium-magnesium co-doped lithium iron phosphate precursor LiFe. 1-x-y Ti x Mg y PO4 mixture; Step 3, LiFe 1-x-y Ti x Mg y The lithium iron phosphate composite material was obtained by ball milling and calcining a mixture of PO4 and a carbon source; the carbon source was a modified polyurethane. The preparation method of the modified polyurethane includes the following steps: Polyurethane acrylate was dispersed in a solvent, and iron acrylate, 1-allyl-3-methylimidazolium tetrafluoroborate and potassium persulfate were added. The grafting reaction was carried out under an inert atmosphere. The product was distilled to remove the solvent and dried to obtain modified polyurethane. Based on parts by mass, the input amounts of each raw material in the preparation process of modified polyurethane are as follows: The components are: 30-60 parts polyurethane acrylate, 200-300 parts solvent, 0.02-0.4 parts iron acrylate, 0.002-0.05 parts 1-allyl-3-methylimidazolium tetrafluoroborate, and 2-4 parts potassium persulfate.

3. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 2, characterized in that, Polyurethane acrylate is dispersed in a solvent by stirring at 50-60℃ for 30-90 min. And / or, the grafting reaction temperature is 60-70℃, and the reaction time is 3-6h; And / or, the solvent includes any one or more of ethanol, deionized water, and acetone; And / or, the inert atmosphere includes either nitrogen or argon.

4. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 2, characterized in that, In step 1, the molar ratio of NH4H2PO4 to Fe(NO3)3·9H2O is (0.97-1.15):(0.98-1.3). And / or, in step 1, the hydrothermal reaction temperature is 80-120℃ and the reaction time is 6-12h.

5. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 2, characterized in that, The surfactant includes one or more of cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP); And / or, in step 1, the amount of surfactant aqueous solution added is 0.9-1.9% of the molar amount of Fe(NO3)3·9H2O.

6. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 2, characterized in that, In step 2, the molar ratio of lithium in the iron phosphate precursor and the lithium source is 0.96~1.05:0.98~1.02; And / or, the molar ratio of titanium in the iron phosphate precursor and the titanium source is 1:0.01~0.05; And / or, the molar ratio of magnesium in the iron phosphate precursor and the magnesium source is 1:0.01~0.05; And / or, the lithium source includes any one or more of lithium hydroxide, lithium carbonate, lithium acetate, and lithium nitrate; And / or, the titanium source includes one or more of titanium dioxide, tetramethyl titanate, and titanium tetrachloride; And / or, the magnesium source includes one or more of magnesium oxalate, magnesium sulfate, magnesium carbonate, magnesium nitrate, and magnesium hydroxide.

7. The method for preparing polyurethane-coated lithium iron phosphate composite material according to claim 2, characterized in that, In step 3, the mass of the carbon source is LiFe. 1-x-y Ti x Mg y 1-5 wt% of the PO4 mixture; And / or, in step 3, the calcination temperature is 400-700℃ and the calcination time is 5-7h.

8. The application of the polyurethane-coated lithium iron phosphate composite material according to claim 1 in lithium-ion batteries.

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