Lithium iron phosphate material, preparation method and application thereof

Abstract

The application provides a lithium iron phosphate material and a preparation method and application thereof, and belongs to the technical field of lithium batteries. 2 0.5-1.5 parts, 1.5-2.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 4-6 parts of surface carboxyl modified polyacrylonitrile based carbon fiber. The lithium iron phosphate material of the application effectively improves the conductivity and structural stability of the material through the steps of Mg-Ti co-doping, atomic layer deposition of a Li2SiO3 layer and chemical deposition of a carbon layer, so that the material exhibits excellent charge and discharge performance and cycle performance in the lithium battery positive electrode material, and has a significant advantage in a low temperature environment.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a lithium iron phosphate material, its preparation method and application. Background Technology

[0002] Currently, the development of energy conversion and storage technologies is one of the key factors driving technological progress and industrial upgrading. Among them, lithium battery technology, due to its advantages such as high energy density, long cycle life, and environmental friendliness, has been widely used in portable electronic devices, electric vehicles, and energy storage systems. With the continuous advancement of new energy technologies, the performance requirements for lithium batteries are also increasing.

[0003] Lithium iron phosphate (LFP) batteries are characterized by good safety and high energy density, and have become the mainstream battery type for power batteries. However, at low temperatures, the resistance encountered by lithium ions in the extraction from the cathode material and migration in the electrolyte increases, leading to a sharp decline in the charge-discharge performance and cycle performance of LFP batteries. Therefore, improving the charge-discharge performance and cycle performance of LFP batteries at low temperatures is of great significance. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a lithium iron phosphate material, a method for its preparation, and its application.

[0005] Specifically, the first aspect of this application provides a lithium iron phosphate material comprising the following raw materials in parts by mass: 88-92 parts of LiFePO4 precursor, 2-4 parts of Li2SiO3, 0.5-1.5 parts of MgF2, 1.5-2.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 4-6 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber.

[0006] The Mg-Ti-Zr ternary gradient doped sol is prepared by mixing magnesium nitrate, tetrabutyl titanate, and zirconium oxychloride in a molar ratio of (0.8-1.2):(0.5-0.8):(0.2-0.4), dissolving them in an ethanol-water mixed solvent (volume ratio 7:3) containing 1-3 wt% citric acid and 0.5-1.5 wt% polyethylene glycol, and continuously stirring and refluxing at 60-80℃ for 2-4 hours to form a stable transparent sol.

[0007] Furthermore, the preparation method of the LiFePO4 precursor is to mix ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate in a molar ratio of 2-3:2-3:1, add a chelating agent to dissolve in water, and stir until homogeneous.

[0008] The second aspect of this application provides a method for preparing lithium iron phosphate material, comprising the following steps:

[0009] S1: The LiFePO4 precursor and the Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction to obtain LiFePO4 powder.

[0010] S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0011] S3: LiFePO4@(Li2SiO3-MgF2) powder was mixed with surface carboxylated polyacrylonitrile-based carbon fiber by chemical deposition, and propylene was introduced for carbon deposition to obtain lithium iron phosphate material.

[0012] Furthermore, the temperature of the reaction in the microwave reactor in step S1 is 200-210℃;

[0013] The microwave reaction pressure is 2.0-2.5 MPa;

[0014] The microwave reaction power is 700-800W;

[0015] The microwave reaction time is 30-32 minutes.

[0016] Further, after the reaction in step S1 is completed, the product is cooled to below 80°C by circulating water, centrifuged to separate the product, and then vacuum dried at 80-100°C for 10-14 hours to obtain LiFePO4 powder.

[0017] Furthermore, the Li2SiO3 layer deposition in step S2 is performed sequentially by lithium source pulse, purge, silicon source pulse, purge, and oxidation steps;

[0018] The MgF2 layer deposition process involves sequential magnesium source pulse, purging, fluorine source pulse, and purging.

[0019] Alternating between 2-4 Li2SiO3 layer depositions followed by 1 MgF2 layer deposition, this process is defined as one cycle.

[0020] Furthermore, in step S2, the thickness of the Li2SiO3-MgF2 deposited layer is 3-6 nm, of which the Li2SiO3 layer accounts for 60-80% of the thickness and the MgF2 layer accounts for 20-40%.

[0021] Furthermore, in step S3, the mass ratio of LiFePO4@(Li2SiO3-MgF2) powder to surface carboxylated modified polyacrylonitrile-based carbon fiber is 10-15:1.

[0022] The flow rate of propylene is 20-25 mL / min;

[0023] Carbon deposition time is 1.5-2.5 hours;

[0024] The carbon deposition temperature is 640-660℃.

[0025] A third aspect of this application provides the application of the lithium iron phosphate material described above in lithium battery cathode materials.

[0026] The present invention has the following beneficial effects:

[0027] The lithium iron phosphate material of this invention optimizes bulk conductivity through Mg-Ti-Zr co-doping, reduces interfacial impedance through a Li2SiO3-MgF2 coating layer, and constructs a continuous conductive network using porous, surface-carboxylated polyacrylonitrile-based carbon fibers, synergistically improving the conductivity and structural stability of the lithium iron phosphate material. At low temperatures, this lithium iron phosphate material exhibits excellent charge-discharge and cycle performance, significantly improving the energy density and lifespan of lithium iron phosphate batteries. Practical examples show that the obtained material retains 96.5% capacity after 2000 cycles at 1C and 81.3% capacity at -30℃, demonstrating a significant performance improvement over traditional lithium iron phosphate materials. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0029] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0030] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0031] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0032] An embodiment of the first aspect of this application provides a lithium iron phosphate material comprising, by mass parts: 88-92 parts of LiFePO4 precursor, 2-4 parts of Li2SiO3, 0.5-1.5 parts of MgF2, 1.5-2.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 4-6 parts of surface carboxylation modified polyacrylonitrile-based carbon fiber.

[0033] Specifically, the preparation method of the LiFePO4 precursor is to ball-mill ferrous oxalate, ammonium dihydrogen phosphate and lithium carbonate in a molar ratio of 2-3: 2-3: 1 at a speed of 800 rpm, add chelating agent citric acid dissolved in deionized water and stir until a transparent yellow-green solution is formed.

[0034] Specifically, the Mg-Ti-Zr ternary gradient doped sol is prepared by mixing magnesium nitrate, tetrabutyl titanate, and zirconium oxychloride in a molar ratio of (0.8-1.2):(0.5-0.8):(0.2-0.4), dissolving them in an ethanol-water mixed solvent (volume ratio 7:3) containing 1-3 wt% citric acid and 0.5-1.5 wt% polyethylene glycol, and continuously stirring and refluxing at 60-80℃ for 2-4 hours to form a stable transparent sol.

[0035] Zr in Mg-Ti-Zr ternary gradient doped sol 4+ Ionic radius and Fe 2+ / Fe 3+ The greater the difference, the more effectively Li can be expanded. +Migration pathways; strong Zr-O bond energy contributes to improved structural stability, especially under high-voltage cycling. Sol-gel preparation processes (specific chelating agents, reflux) encourage dopant elements to form concentration gradients within LiFePO4 particles during subsequent ball milling and reactions (e.g., Mg biased towards the bulk phase, Ti in the middle, Zr biased towards the surface), optimizing bulk ion conduction and surface stability. Citric acid strongly chelates metal ions to ensure uniformity, while PEG-400 adjusts sol viscosity to prevent agglomeration during subsequent ball milling and may form a weak adsorption layer on the grain surface, affecting grain growth. The sol state is more uniform than simple solution mixing, exhibiting better dispersion and more uniform and deep doping when ball-milled with the LiFePO4 precursor.

[0036] The mass fraction of the LiFePO4 precursor is any value or combination of 88, 89, 90, 91, and 92 parts. A LiFePO4 precursor content below 88 parts negatively impacts the overall performance of the lithium iron phosphate material, while a content above 92 parts may increase preparation costs without significant performance improvement. The mass fraction of the Li2SiO3 is any value or combination of 2, 2.5, 3, 3.5, and 4 parts. Insufficient Li2SiO3 content may result in incomplete coating, while excessive content may affect the bulk properties of the lithium iron phosphate material. The Mg-Ti-Zr trioxide... The mass fraction of the ternary gradient doped sol is any value or combination of 1.5 parts, 1.8 parts, 2 parts, 2.2 parts, and 2.5 parts. Too little Mg-Ti-Zr ternary gradient doped sol may result in poor doping effect, while too much may cause changes in the structure of the lithium iron phosphate material. The mass fraction of the surface carboxylated modified polyacrylonitrile-based carbon fiber is any value or combination of 4 parts, 4.5 parts, 5 parts, 5.5 parts, and 6 parts. Too little surface carboxylated modified polyacrylonitrile-based carbon fiber may result in an incomplete conductive network, while too much may affect the tap density of the lithium iron phosphate material.

[0037] The polyacrylonitrile-based carbon fibers are subjected to plasma treatment or concentrated nitric acid oxidation to enrich their surface with carboxyl (-COOH) functional groups, forming surface carboxylated polyacrylonitrile-based carbon fibers. These carboxyl functional groups can form hydrogen bonds or undergo esterification reactions with the hydroxyl (-OH) groups on the surface of LiFePO4 particles or the Si-OH groups in the Li2SiO3 layer, significantly improving the bonding force between the carbon fibers and the active material / coating layer. Simultaneously, carboxylation enhances the hydrophilicity of the carbon fibers, resulting in more uniform dispersion in the slurry. Furthermore, stronger interfacial bonding means tighter contact and lower internal resistance, improving overall electronic conductivity.

[0038] The second aspect of this application provides a method for preparing lithium iron phosphate material, comprising the following steps:

[0039] S1: The LiFePO4 precursor and the Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction to obtain LiFePO4 powder.

[0040] S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0041] S3: LiFePO4@(Li2SiO3-MgF2) powder was mixed with surface carboxylated polyacrylonitrile-based carbon fiber by chemical deposition, and propylene was introduced for carbon deposition to obtain lithium iron phosphate material.

[0042] In this embodiment, the reaction temperature in the microwave reactor in step S1 is 200-210℃, the microwave reaction pressure is 2.0-2.5MPa, the microwave reaction power is 700-800W, and the microwave reaction time is 30-32min. Further, the preferred reaction temperature in the microwave reactor in step S1 is 205℃, the preferred microwave reaction pressure is 2.2MPa, the preferred microwave reaction power is 750W, and the preferred microwave reaction time is 30min. Under these conditions, the LiFePO4 precursor and the Mg-Ti-Zr ternary gradient doped sol can be fully reacted to form high-performance LiFePO4 powder.

[0043] The non-thermal effect of microwaves accelerates ion migration, forming highly crystalline nanoscale LiFePO4 powder.

[0044] In this embodiment, after the reaction in step S1 is completed, the temperature is cooled to below 80°C by circulating water. A high-speed centrifuge is used at a speed of 8000 rpm for 10 min to separate the precipitate. The precipitate is washed three times with deionized water, then twice with anhydrous ethanol, and once with acetone. The product is then vacuum filtered and dried at 80-100°C under vacuum for 10-14 h to obtain LiFePO4 powder.

[0045] In this embodiment, the Li2SiO3 layer deposition in step S2 is performed by sequentially performing lithium source pulse, purge, silicon source pulse, purge, and oxidation steps.

[0046] Atomic layer deposition was performed using an ALD (Atomic Layer Deposition) system equipped with infrared thermography and mass spectrometry monitoring. Precursor source:

[0047] Lithium source: Lithium bis(trimethylsilyl)amino (LiHMDS);

[0048] Silicon source: Tetraethoxysilane (TEOS);

[0049] Oxidizing agent: Ozone (O3, concentration 200ppm);

[0050] The cyclic deposition parameters are shown in Table 1.

[0051] Table 1 Cyclic Deposition Parameters

[0052]

[0053] The MgF2 layer deposition process involves sequential magnesium source pulse, purging, fluorine source pulse, and purging. By alternately introducing magnesium- and fluorine-containing precursor gases, the MgF2 layer deposition is completed on the substrate surface.

[0054] Magnesium source pulse: Di(cyclopentadienyl)magnesium (Mg(Cp)2) is used as a precursor to provide magnesium atoms;

[0055] Fluorine source pulse: Use hexafluoroacetylacetone (HFAC) or titanium tetrafluoride (TiF4) as precursors to provide fluorine atoms.

[0056] Alternating between 2-4 Li2SiO3 layer depositions followed by 1 MgF2 layer deposition, this process is defined as one cycle.

[0057] This invention involves the deposition of both Li₂SiO₃ and MgF₂ layers, combining the ionic conductivity of Li₂SiO₃ with the extreme stability of MgF₂ to provide dual protection. Li₂SiO₃ provides Li + In terms of conductivity, MgF2 has excellent chemical inertness and electrochemical stability, which can more effectively isolate the contact between the electrolyte and the active material, significantly inhibit interfacial side reactions (HF corrosion, Mn / Ni / Co dissolution), reduce gas production, improve high-temperature cycle life and storage performance, and the combination of the two forms a more robust and stable interfacial layer.

[0058] Furthermore, in step S2, the thickness of the Li2SiO3-MgF2 deposition layer is 3-6 nm, with the Li2SiO3 layer accounting for 60-80% of the thickness and the MgF2 layer accounting for 20-40% of the thickness, to avoid excessive deposition leading to pore blockage.

[0059] In an embodiment of the present invention, the mass ratio of LiFePO4@(Li2SiO3-MgF2) powder to surface carboxylated modified polyacrylonitrile-based carbon fiber in step S3 is 10-15:1; the surface carboxylated modified polyacrylonitrile-based carbon fiber needs to be pretreated: soaked in KOH solution (6mol / L) for 24h, and carbonized at 900℃ for 2h under nitrogen protection, so that the specific surface area is >1500m². 2 / g.

[0060] During carbon deposition, the carrier gas is Ar / H2, with a volume ratio of Ar:H2 of 95:5. The carbon source is propylene, with a flow rate of 20-25 mL / min. Specifically, during carbon deposition, LiFePO4@(Li2SiO3-MgF2) powder and surface carboxylated modified polyacrylonitrile-based carbon fibers are mixed at a mass ratio of 9:1 and placed in a quartz boat. The boat is then pushed into a constant-temperature zone, where the temperature is increased to 640-660℃ at a rate of 10℃ / min for pre-carbonization for 30 min. Carbon deposition is then carried out at 650℃ for 1.5-2.5 h. After cooling to 200℃ in the furnace, the carbon layer is removed, resulting in a carbon layer thickness of 5-8 nm. This forms a continuous and stable conductive network, further improving the conductivity and structural stability of the lithium iron phosphate material.

[0061] An embodiment of the third aspect of this application provides an application of the lithium iron phosphate material described above in lithium battery cathode materials.

[0062] The lithium iron phosphate material of the present invention can be used in lithium battery cathode materials. Especially in low-temperature environments, the lithium iron phosphate material exhibits excellent charge-discharge performance and cycle performance, significantly improving the energy density and lifespan of lithium iron phosphate batteries, and has broad application prospects.

[0063] Example 1

[0064] A lithium iron phosphate material comprises the following raw materials in parts by mass: 90 parts of LiFePO4 precursor, 3 parts of Li2SiO3, 1 part of MgF2, 2 parts of Mg-Ti-Zr ternary gradient doped sol, and 5 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber.

[0065] Example 2

[0066] A lithium iron phosphate material comprises the following raw materials in parts by mass: 88 parts of LiFePO4 precursor, 3.5 parts of Li2SiO3, 0.8 parts of MgF2, 1.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 6 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber.

[0067] Example 3

[0068] A lithium iron phosphate material comprises the following raw materials in parts by mass: 89 parts of LiFePO4 precursor, 2 parts of Li2SiO3, 1.5 parts of MgF2, 2.2 parts of Mg-Ti-Zr ternary gradient doped sol, and 4.5 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber.

[0069] Example 4

[0070] A lithium iron phosphate material comprises the following raw materials in parts by mass: 92 parts of LiFePO4 precursor, 4 parts of Li2SiO3, 0.5 parts of MgF2, 2.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 4 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber.

[0071] Example 5

[0072] A method for preparing lithium iron phosphate material, using the lithium iron phosphate material components in Example 1, includes the following steps:

[0073] S1: The LiFePO4 precursor and Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction. The reaction temperature was 200℃, the microwave reaction pressure was 2.2MPa, the microwave reaction power was 780W, and the microwave reaction time was 31min. After the reaction, the mixture was cooled to below 80℃ by circulating water. The product was separated by centrifugation and then vacuum dried at 90℃ for 12h to obtain LiFePO4 powder.

[0074] S2: LiFePO4 powder is placed in the atomic layer deposition reaction chamber, and Li2SiO3 layer and MgF2 layer are deposited sequentially using a dual-cycle alternating deposition process. This process is repeated for 15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0075] S3: LiFePO4@(Li2SiO3-MgF2) powder and surface carboxylated modified polyacrylonitrile-based carbon fiber were mixed at a mass ratio of 12:1 by chemical deposition. Propylene was introduced for carbon deposition at a flow rate of 22 mL / min and a carbon deposition time of 2 h. The carbon deposition temperature was 650℃ to obtain lithium iron phosphate material.

[0076] Example 6

[0077] A method for preparing lithium iron phosphate material, using the lithium iron phosphate material components in Example 2, includes the following steps:

[0078] S1: The LiFePO4 precursor and Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction. The reaction temperature was 200℃, the microwave reaction pressure was 2.5MPa, the microwave reaction power was 750W, and the microwave reaction time was 32min. After the reaction, the mixture was cooled to below 80℃ by circulating water. The product was separated by centrifugation and then vacuum dried at 100℃ for 14h to obtain LiFePO4 powder.

[0079] S2: LiFePO4 powder is placed in the atomic layer deposition reaction chamber, and Li2SiO3 layer and MgF2 layer are deposited sequentially using a dual-cycle alternating deposition process. This process is repeated for 10 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0080] S3: LiFePO4@(Li2SiO3-MgF2) powder and surface carboxylated modified polyacrylonitrile-based carbon fiber were mixed at a mass ratio of 10:1 by chemical deposition. Propylene was introduced for carbon deposition at a flow rate of 20 mL / min and a carbon deposition time of 2.5 h. The carbon deposition temperature was 660 °C to obtain lithium iron phosphate material.

[0081] Example 7

[0082] A method for preparing lithium iron phosphate material, using the lithium iron phosphate material components in Example 3, includes the following steps:

[0083] S1: The LiFePO4 precursor and Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction. The reaction temperature was 210℃, the microwave reaction pressure was 2.4MPa, the microwave reaction power was 700W, and the microwave reaction time was 31min. After the reaction, the mixture was cooled to below 80℃ by circulating water. The product was separated by centrifugation and dried under vacuum at 95℃ for 13h to obtain LiFePO4 powder.

[0084] S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0085] S3: LiFePO4@(Li2SiO3-MgF2) powder and surface carboxylated modified polyacrylonitrile-based carbon fiber were mixed at a mass ratio of 8:1 by chemical deposition. Propylene was introduced for carbon deposition at a flow rate of 25 mL / min and a carbon deposition time of 2 h. The carbon deposition temperature was 650℃ to obtain lithium iron phosphate material.

[0086] Example 8

[0087] A method for preparing lithium iron phosphate material, using the lithium iron phosphate material components in Example 4, includes the following steps:

[0088] S1: The LiFePO4 precursor and Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction. The reaction temperature was 200℃, the microwave reaction pressure was 2.0MPa, the microwave reaction power was 800W, and the microwave reaction time was 30min. After the reaction was completed, the mixture was cooled to below 80℃ by circulating water. The product was separated by centrifugation and then vacuum dried at 80℃ for 10h to obtain LiFePO4 powder.

[0089] S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder.

[0090] S3: LiFePO4@(Li2SiO3-MgF2) powder and surface carboxylated modified polyacrylonitrile-based carbon fiber were mixed at a mass ratio of 10:1 by chemical deposition. Propylene was introduced for carbon deposition at a flow rate of 23 mL / min and a carbon deposition time of 1.5 h. The carbon deposition temperature was 640 °C to obtain lithium iron phosphate material.

[0091] Comparative Example 1

[0092] This comparative example is basically the same as Example 5, except that Mg-Ti-Zr co-doping is not performed.

[0093] Comparative Example 2

[0094] This comparative example is basically the same as Example 5, except that only Mg doping is performed.

[0095] Comparative Example 3

[0096] This comparative example is basically the same as Example 5, except that carbon deposition is not performed.

[0097] The lithium iron phosphate materials prepared in Examples 5-8 and Comparative Examples 1-3 were used as positive electrode materials in batteries, wherein the positive electrode consisted of 94% of the above-mentioned lithium iron phosphate material, 3% of polyvinylidene fluoride-hexafluoropropylene copolymer, and 3% of carbon nanotubes, coated on a honeycomb aluminum foil sheet.

[0098] Negative electrode: Silicon-carbon composite material, combined with an MXene conductive layer;

[0099] Electrolyte: 1 mol / L LiPF6 / EC+DEC+DMC, wherein the weight ratio of LiPF6 / EC:DEC:DMC is 1:1:1;

[0100] Separator: Ceramic-coated polyolefin separator.

[0101] The batteries were assembled and their performance was tested. The test results are shown in Table 2.

[0102] Table 2 Test results of Examples 5-8 and Comparative Examples 1-3

[0103]

[0104] As shown in Table 2, Examples 5-8, the batteries prepared with the lithium iron phosphate material of the present invention exhibit excellent performance in terms of initial capacity at 0.1C, discharge capacity at 10C, cycle retention rate, and low-temperature capacity retention rate. Especially at low temperatures, the lithium iron phosphate materials prepared in Examples 5-8 demonstrate higher capacity retention and superior cycle performance compared to Comparative Examples 1-3. This is mainly due to the introduction of Mg-Ti-Zr co-doping and surface carboxylation-modified polyacrylonitrile-based carbon fibers, which effectively improve the conductivity and structural stability of the lithium iron phosphate material. Furthermore, the present invention employs Li2SiO3 layer deposition and MgF2 layer deposition, combining the ionic conductivity of Li2SiO3 and the extreme stability of MgF2, providing dual protection. Li2SiO3 provides Li + In terms of conductivity, MgF2 has excellent chemical inertness and electrochemical stability, which can more effectively isolate the contact between the electrolyte and the active material, significantly inhibit interfacial side reactions (HF corrosion, Mn / Ni / Co dissolution), reduce gas production, improve high-temperature cycle life and storage performance, and the combination of the two forms a more robust and stable interfacial layer.

[0105] As shown in Comparative Example 1, the battery performance of lithium iron phosphate material without Mg-Ti-Zr co-doping is significantly reduced, especially in terms of low-temperature capacity retention, which is only 52.7%, far lower than the 81.3% in Example 5. This may be because the doping of Mg, Ti, and Zr introduces more defects and vacancies, promoting lithium-ion diffusion and transport, while simultaneously improving the structural and thermal stability of the material, thus significantly improving the low-temperature performance of lithium iron phosphate material.

[0106] As can be seen from Comparative Example 2, although the lithium iron phosphate material doped with only Mg shows some improvement compared to Comparative Example 1 without doping, its performance in various indicators is still lower than that of Example 5, especially in terms of cycle retention and low-temperature capacity retention. This indicates that Mg-Ti-Zr co-doping has a more significant effect than single Mg doping. This may be because Mg doping alone cannot fully introduce the advantages brought by Ti element. For example, Ti doping can further improve the electronic conductivity and ionic conductivity of the material. At the same time, the synergistic effect of Mg, Ti and Zr can more effectively stabilize the structure of the material, thereby improving its cycle performance and low-temperature performance.

[0107] As can be seen from Comparative Example 3, the battery performance of lithium iron phosphate materials without carbon deposition also decreases significantly, especially in terms of cycle retention and low-temperature capacity retention. This is mainly because the carbon deposition process can form a continuous and stable conductive network, which can effectively mitigate volume changes during charge and discharge, thereby improving the material's cycle stability and low-temperature performance.

[0108] In summary, the lithium iron phosphate material of the present invention effectively improves the conductivity and structural stability of the material through steps such as Mg-Ti co-doping, atomic layer deposition of Li2SiO3 layer and chemical deposition of carbon layer, thereby exhibiting excellent charge-discharge performance and cycle performance in lithium battery cathode materials, especially with significant advantages in low-temperature environments.

[0109] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium iron phosphate material, characterized in that, The raw materials include the following by mass parts: 88-92 parts of LiFePO4 precursor, 2-4 parts of Li2SiO3, 0.5-1.5 parts of MgF2, 1.5-2.5 parts of Mg-Ti-Zr ternary gradient doped sol, and 4-6 parts of surface carboxylated modified polyacrylonitrile-based carbon fiber. The preparation method of the Mg-Ti-Zr ternary gradient doped sol is as follows: magnesium nitrate, tetrabutyl titanate, and zirconium oxychloride are mixed in a molar ratio of (0.8-1.2): (0.5-0.8): (0.2-0.4), dissolved in an ethanol-water mixed solvent containing citric acid and polyethylene glycol, and stirred and refluxed at 60-80℃ for 2-4 hours to form a stable transparent sol; The preparation method of the lithium iron phosphate material includes the following steps: S1: The LiFePO4 precursor and the Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction to obtain LiFePO4 powder. S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder. S3: LiFePO4@(Li2SiO3-MgF2) powder was mixed with surface carboxylated polyacrylonitrile-based carbon fiber by chemical deposition, and propylene was introduced for carbon deposition to obtain lithium iron phosphate material.

2. The lithium iron phosphate material of claim 1, wherein, The LiFePO4 precursor is prepared by mixing ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate in a molar ratio of 2-3:2-3:1, adding a chelating agent to dissolve in water, and stirring until homogeneous.

3. A method of producing a lithium iron phosphate material, characterized by, Includes the following steps: S1: The LiFePO4 precursor and the Mg-Ti-Zr ternary gradient doped sol were ball-milled to form a uniform slurry. The slurry was then transferred to a microwave reactor for reaction to obtain LiFePO4 powder. The preparation method of the Mg-Ti-Zr ternary gradient doped sol is as follows: magnesium nitrate, tetrabutyl titanate, and zirconium oxychloride are mixed in a molar ratio of (0.8-1.2): (0.5-0.8): (0.2-0.4), dissolved in an ethanol-water mixed solvent containing citric acid and polyethylene glycol, and stirred continuously at 60-80℃ and refluxed for 2-4 hours to form a stable transparent sol; S2: Place LiFePO4 powder in the atomic layer deposition reaction chamber, and use a dual-cycle alternating deposition process to deposit Li2SiO3 layer and MgF2 layer sequentially, repeating 10-15 cycles to generate a Li2SiO3-MgF2 composite coating layer on the surface of LiFePO4 powder, thus obtaining LiFePO4@(Li2SiO3-MgF2) powder. S3: LiFePO4@(Li2SiO3-MgF2) powder was mixed with surface carboxylated polyacrylonitrile-based carbon fiber by chemical deposition, and propylene was introduced for carbon deposition to obtain lithium iron phosphate material.

4. The method for preparing lithium iron phosphate material according to claim 3, characterized in that, In step S1, the temperature of the reaction in the microwave reactor is 200-210℃; And / or, the microwave reaction pressure is 2.0-2.5 MPa; And / or, the microwave reaction power is 700-800W; And / or, the microwave reaction time is 30-32 min.

5. The method for preparing lithium iron phosphate material according to claim 3, characterized in that, After the reaction in step S1 is completed, the product is cooled to below 80°C by circulating water, centrifuged to separate the product, and then vacuum dried at 80-100°C for 10-14 hours to obtain LiFePO4 powder.

6. The method for preparing lithium iron phosphate material according to claim 3, characterized in that, In step S2, the Li2SiO3 layer deposition process involves sequential steps of lithium source pulse, purge, silicon source pulse, purge, and oxidation. The MgF2 layer deposition process involves sequential magnesium source pulse, purging, fluorine source pulse, and purging. Alternating between 2-4 Li2SiO3 layer depositions followed by 1 MgF2 layer deposition, this process is defined as one cycle.

7. The method of claim 3, wherein the lithium iron phosphate material is prepared by the steps of: In step S2, the thickness of the Li2SiO3-MgF2 deposition layer is 3-6 nm, of which the Li2SiO3 layer accounts for 60-80% of the thickness and the MgF2 layer accounts for 20-40%. ​ 8. The method of claim 3, wherein the lithium iron phosphate material is prepared by the steps of: In step S3, the mass ratio of LiFePO4@(Li2SiO3-MgF2) powder to surface carboxylated modified polyacrylonitrile-based carbon fiber is 10-15:

1. ​ And / or, the flow rate of propylene is 20-25 mL / min; And / or, the carbon deposition time is 1.5-2.5 h; And / or, the carbon deposition temperature is 640-660℃.

9. The application of the lithium iron phosphate material as described in any one of claims 1-2 in the cathode material of lithium batteries.

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