A uniform carbon-coated lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a uniform carbon-coated lithium iron phosphate cathode material and a preparation method thereof, relating to the technical field of lithium iron phosphate cathode materials. The material comprises lithium iron phosphate matrix particles, wherein the lithium iron phosphate matrix particles are coated with a composite carbon layer, and the composite carbon layer is doped with cerium-modified tantalum carbide nanocrystals. The preparation method of the cerium-modified tantalum carbide nanocrystals comprises the following steps: dissolving tantalum pentaethoxide and cerium nitrate in ethylene glycol, adding urea as a precipitant, and synthesizing a Ta-Ce-O precursor by a microwave-assisted solvent thermal method; mixing the Ta-Ce-O precursor with a phenolic resin, ball milling the mixture, and sintering the mixture to obtain the cerium-modified tantalum carbide nanocrystals. By introducing the cerium-modified tantalum carbide nanocrystals into the composite carbon layer in small amounts and multiple times, uniform coating of the composite carbon layer can be achieved, and the electrochemical performance of the lithium iron phosphate cathode material can be enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate positive electrode materials, and in particular to a uniformly carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4) is widely used as a cathode material. Its unique olivine structure offers numerous advantages: high safety, long cycle life, and excellent thermal stability. Furthermore, since LiFePO4 does not contain harmful elements such as nickel, it poses relatively low environmental impact.

[0003] However, improving the electrochemical performance of lithium iron phosphate (LFP) requires addressing its inherent limitations, including poor conductivity, low tap density, low energy density, and insufficient low-temperature performance. Optimizing particle size, carbon coating, and ion doping modifications have practical implications for optimizing LFP material performance and developing high-energy-density batteries.

[0004] In the existing technology, coating lithium iron phosphate materials with conductive materials is a key means to improve their rate and low-temperature performance. Among them, carbon materials are favored as the simplest and most economical choice. Carbon coating can play multiple roles: carbon can act as a reducing agent to effectively prevent the Fe in lithium iron phosphate materials from 2+ oxidation; carbon coating can significantly improve the electronic conductivity of the material; at the same time, carbon coating can block the direct contact between particles to a certain extent, effectively inhibiting the excessive growth of particles.

[0005] However, the carbon coating technology currently on the market mainly relies on organic carbon sources, such as glucose, sucrose, etc., to achieve the coating of lithium iron phosphate through high-temperature reduction reaction. However, this method is limited by conditions such as reaction temperature and time, making it difficult to accurately control the degree of graphitization of the carbon layer, which in turn affects the electronic conductivity of lithium iron phosphate. On the other hand, inorganic carbon sources such as conductive carbon black, graphene, etc., although they have a high degree of graphitization and a large specific surface area, can directly improve electronic conductivity, but due to their poor water solubility, the precursor slurry formed during the high-temperature solid-phase processing is not consistent, resulting in uneven carbon coating of lithium iron phosphate. This unevenness cannot effectively control the growth of lithium iron phosphate grains, resulting in particle agglomeration, and ultimately reducing the capacity of the battery.

[0006] Therefore, there is an urgent need for a method that can significantly improve the uniformity of carbon coating on lithium iron phosphate, thereby effectively enhancing the consistency and electronic conductivity of lithium iron phosphate materials. Summary of the Invention

[0007] The purpose of the present invention is to provide a uniform carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, to solve the following technical problems:

[0008] How to improve the uniformity of the carbon coating layer of lithium iron phosphate positive electrode materials.

[0009] The purpose of the present invention can be achieved through the following technical solutions:

[0010] In a first aspect, the present invention discloses a uniform carbon-coated lithium iron phosphate positive electrode material, characterized in that it comprises lithium iron phosphate matrix particles, the lithium iron phosphate matrix particles are coated with a composite carbon layer, and the composite carbon layer is doped with cerium (Ce) modified tantalum carbide (TaC) nanocrystals;

[0011] The mass of the composite carbon layer accounts for 1.5-2.4 wt % of the mass of the lithium iron phosphate matrix particles, and the mass fraction of the cerium-modified tantalum carbide nanocrystals in the composite carbon layer is 1.2-2.8 wt %;

[0012] Preferably, the mass of the composite carbon layer accounts for 2 wt % of the mass of the lithium iron phosphate, and the mass fraction of the cerium-modified tantalum carbide nanocrystals in the composite carbon layer is 2.5 wt %.

[0013] Furthermore, the preparation method of the cerium-modified tantalum carbide nanocrystals is:

[0014] Step 1: dissolving tantalum pentaethoxide and cerium nitrate in ethylene glycol at a molar ratio of Ta:Ce = 100:(1-3), adding urea as a precipitant, and synthesizing a Ta-Ce-O precursor by microwave-assisted solvothermal method;

[0015] Step 2: The Ta-Ce-O precursor and the phenolic resin are mixed in a weight ratio of 1:(5-8) and ball-milled, and then sintered at 1200-1300° C. for 2-4 hours to obtain cerium-modified tantalum carbide nanocrystals.

[0016] Furthermore, in step 1, the ratio of the total mass of the pentaethoxytantalum and cerium nitrate to the volume of ethylene glycol is 1 g:(10-20) mL.

[0017] Furthermore, in step 1, the ratio of the total weight of the pentaethoxytantalum and cerium nitrate to the weight of urea is 1:(1.5-2.5).

[0018] Furthermore, in step 2, the ball mill uses zirconia grinding balls, the ball-to-material ratio is 10:1, the rotation speed is 300-400 rpm, and the ball milling time is 2-4 hours.

[0019] Based on this, a preferred method for preparing cerium-modified tantalum carbide nanocrystals is obtained:

[0020] Step 1: dissolving pentaethoxytantalum and cerium nitrate in ethylene glycol at a molar ratio of Ta:Ce = 100:2, where the ratio of the total mass of the pentaethoxytantalum and cerium nitrate to the volume of ethylene glycol is 1 g:15 mL; then adding urea as a precipitant, where the ratio of the total weight of the pentaethoxytantalum and cerium nitrate to the weight of urea is 1:2; and synthesizing a Ta-Ce-O precursor by microwave-assisted solvothermal method;

[0021] Step 2: The Ta-Ce-O precursor and phenolic resin were mixed in a weight ratio of 1:6, added to a zirconia ball mill, and ball milled at a ball-to-material ratio of 10:1 and a speed of 350 rpm for 3 hours, and then sintered at 1250°C for 2 hours to obtain cerium-modified tantalum carbide nanocrystals.

[0022] Furthermore, the composite carbon layer is a composition of an organic carbon source and an inorganic carbon source in a carbon molar ratio of 1:(1.5-2), wherein the organic carbon source is a composition of any one or more of polyacrylic acid, polyaniline, and glucose in any proportion, and the inorganic carbon source is a composition of any one or two of carbon nanotubes and graphene in any proportion;

[0023] Preferably, the organic carbon source is glucose, and the inorganic carbon source is a mixture of carbon nanotubes and graphene in a 1:1 ratio.

[0024] In a second aspect, the present invention further discloses a method for preparing a lithium iron phosphate positive electrode material, which is used to prepare the uniform carbon-coated lithium iron phosphate positive electrode material as described above, comprising the following steps:

[0025] S1, dissolving a lithium source, an iron source, and a phosphorus source in an ethanol aqueous solution at a molar ratio of Li:Fe:P = (1.05-1.10):1:1 to form a mixed solution with a solid content of 25-35%, adding a dispersant accounting for 0.5-2wt% of the mixed solution to the mixed solution, and stirring uniformly to obtain a precursor solution;

[0026] S2, dissolving an organic carbon source and an inorganic carbon source in a precursor solution, and then ball milling, adding cerium-modified tantalum carbide nanocrystals in small amounts and multiple times during the ball milling process, and obtaining a spray slurry after the ball milling is completed;

[0027] S3. Prepare the spray slurry into precursor powder by spray drying, with the spray drying air inlet temperature being 180-220°C and the air outlet temperature being 80-100°C;

[0028] S4. Place the precursor powder in a tube furnace, heat it to 180-220°C at a rate of 3°C / min under an inert atmosphere, keep it warm for 1-3 hours, then heat it to 750-800°C at a rate of 1°C / min, keep it warm for 2-4 hours, and obtain a carbonized product;

[0029] S5. Grind and refine the carbonized product to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

[0030] Furthermore, in step S1, the lithium source includes a composition consisting of one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate in any proportion; the iron source includes a composition consisting of one or more of iron oxides and iron salts in any proportion; and the phosphorus source includes a composition consisting of one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate in any proportion.

[0031] Furthermore, in step S1, the dispersant is polyethylene glycol.

[0032] Furthermore, in step S2, the ball mill uses alumina grinding balls, the ball-to-material ratio is 15:1, the rotation speed is 500-600 rpm, and the ball milling time is 1-2 hours.

[0033] Based on this, a preferred method for preparing a uniform carbon-coated lithium iron phosphate positive electrode material is obtained, comprising the following steps:

[0034] S1, lithium carbonate, iron oxide, and phosphoric acid were dissolved in an ethanol aqueous solution at a molar ratio of Li:Fe:P = 1.08:1:1 to form a mixed solution with a solid content of 30%, polyethylene glycol accounting for 1.2 wt% of the mixed solution was added to the mixed solution, and the mixture was stirred to obtain a precursor solution;

[0035] S2. Glucose, carbon nanotubes, and graphene were dissolved in a precursor solution at a carbon molar ratio of 1:0.9:0.9, and then ball-milled using alumina balls at a ball-to-material ratio of 15:1 and a speed of 550 rpm for 1.5 h. During the ball-milling process, an equal amount of cerium-modified tantalum carbide nanocrystals was added three times with an interval of 20 min between each addition. After the ball-milling, a spray slurry was obtained.

[0036] S3, preparing the spray slurry into a precursor powder by spray drying, with the spray drying air inlet temperature being 200°C and the air outlet temperature being 90°C;

[0037] S4. Place the precursor powder in a tube furnace, heat it to 200°C at a rate of 3°C / min under an inert atmosphere, keep it warm for 2 hours, then heat it to 780°C at a rate of 1°C / min, keep it warm for 3 hours, and obtain a carbonized product;

[0038] S5. Grind and refine the carbonized product to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

[0039] Beneficial effects of the present invention:

[0040] 1. Cerium-modified tantalum carbide nanocrystals are introduced into the composite carbon layer of the uniform carbon-coated lithium iron phosphate positive electrode material of the present invention. In terms of the uniformity of the composite carbon layer coating, Ce doping enables TaC to increase its surface Zeta potential, thereby generating electrostatic attraction with the negatively charged lithium iron phosphate matrix, thereby promoting uniform adsorption of carbon slurry; at the same time, cerium-modified tantalum carbide nanocrystals have high surface energy, which can provide preferential nucleation sites for the thermal decomposition of the carbon precursor, thereby increasing the nucleation density of the carbon layer and inhibiting the island growth of the carbon layer; the Ce-O bonds on the surface of the nanocrystals form a chemical anchoring effect with the oxygen-containing functional groups (-OH, -COOH) in the carbon precursor, preventing the migration and agglomeration of carbon particles, thereby improving the uniformity of the composite carbon layer when coated on the surface of the lithium iron phosphate.

[0041] 2. The composite carbon layer of the uniform carbon-coated lithium iron phosphate cathode material of the present invention introduces cerium-modified tantalum carbide nanocrystals. From the perspective of electrochemical performance, TaC has metal-like conductivity, and the delocalized electrons in its conduction band form a three-dimensional electron highway. After cerium doping, Ce 3+ The 4f orbital electrons of Ce hybridize with the d orbital of TaC to form a shallow energy level defect state, which reduces the electron transition barrier; 3+ Replace Ta 5+ The generation of cation vacancies induces local lattice distortion to form oxygen vacancy clusters, which serve as Li + The migration hopping sites can increase the diffusion coefficient and thus improve the electrochemical performance.

[0042] 3. Cerium-modified tantalum carbide nanocrystals are introduced into the composite carbon layer of the uniform carbon-coated lithium iron phosphate positive electrode material of the present invention. During the introduction process, a ball milling mixing method of adding small amounts multiple times is adopted to make the distribution in the system more uniform, thereby enhancing the uniform contact with the carbon source, so that in the subsequent sintering process, the cerium-modified tantalum carbide nanocrystals can be fully and evenly doped into the composite carbon layer.

[0043] 4. In the preparation process of the uniform carbon-coated lithium iron phosphate positive electrode material of the present invention, a two-step carbonization process is used in combination with a specific heating program to form a dense carbon layer through the low-temperature stage and enhance the degree of graphitization through the high-temperature stage, thereby enhancing the stability of the carbon layer structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be further described below with reference to the accompanying drawings.

[0045] Figure 1 is a SEM image of the uniform carbon-coated lithium iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0046] Figure 2 is a SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 of the present invention;

[0047] Figure 3 1 is a curve comparison diagram of the impedance test results of the lithium iron phosphate positive electrode materials of Examples 1-5 of the present invention and Comparative Examples 1-3;

[0048] Figure 4 It is a curve comparison diagram of the capacity retention rate test results of the lithium iron phosphate positive electrode materials of Examples 1-5 of the present invention and Comparative Examples 1-3. DETAILED DESCRIPTION

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments 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.

[0050] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0051] Preparation Example 1

[0052] Preparation of cerium-modified tantalum carbide nanocrystals:

[0053] Step 1: Pentaethoxytantalum and cerium nitrate (25.64 g in total) were dissolved in 385 mL of ethylene glycol at a molar ratio of Ta:Ce = 100:2. 51.28 g of urea was then added as a precipitant to synthesize a Ta-Ce-O precursor via a microwave-assisted solvothermal method.

[0054] Step 2: The Ta-Ce-O precursor and phenolic resin were mixed in a weight ratio of 1:6, added to a zirconia ball mill, and ball milled at a ball-to-material ratio of 10:1 and a speed of 350 rpm for 3 hours, and then sintered at 1250°C for 2 hours to obtain cerium-modified tantalum carbide nanocrystals.

[0055] Preparation Example 2

[0056] Preparation of cerium-modified tantalum carbide nanocrystals:

[0057] Step 1: Pentaethoxytantalum and cerium nitrate (20.35 g total) were dissolved in 305 mL of ethylene glycol at a molar ratio of Ta:Ce = 100:1. 30.53 g of urea was then added as a precipitant to synthesize a Ta-Ce-O precursor via a microwave-assisted solvothermal method.

[0058] Step 2: The Ta-Ce-O precursor and phenolic resin were mixed in a weight ratio of 1:5, added to a zirconia ball mill, and ball milled at a ball-to-material ratio of 10:1 and a speed of 350 rpm for 3 hours, and then sintered at 1250°C for 2 hours to obtain cerium-modified tantalum carbide nanocrystals.

[0059] Preparation Example 3

[0060] Step 1: Pentaethoxytantalum and cerium nitrate (30.56 g in total) were dissolved in 458 mL of ethylene glycol at a molar ratio of Ta:Ce = 100:3. 76.4 g of urea was then added as a precipitant to synthesize a Ta-Ce-O precursor via a microwave-assisted solvothermal method.

[0061] Step 2: The Ta-Ce-O precursor and phenolic resin were mixed in a weight ratio of 1:8 and added to a zirconia ball mill at a ball-to-material ratio of 10:1 and a speed of 350 rpm for 3 hours, and then sintered at 1250°C for 2 hours to obtain cerium-modified tantalum carbide nanocrystals.

[0062] Example 1

[0063] Preparation of uniform carbon-coated lithium iron phosphate cathode material:

[0064] Lithium carbonate, iron oxide, and phosphoric acid (51.23 g in total) were dissolved in 119.54 g of a 50% ethanol aqueous solution at a molar ratio of Li:Fe:P = 1.08:1:1 to form a mixed solution with a solid content of 30%. 2.05 g of polyethylene glycol was added to the mixed solution and stirred uniformly to obtain a precursor solution. Glucose, carbon nanotubes, and graphene (0.1 g in total) were dissolved in the precursor solution at a carbon molar ratio of 1:0.9:0.9. The mixture was then ball-milled for 1.5 h using alumina balls at a ball-to-material ratio of 15:1 and a speed of 550 rpm. During the ball milling process, the cerium-modified tantalum carbide nanocrystals prepared in Preparation Example 1 were added three times, with 0. 009g, the time interval between each addition is 20min, and after the ball milling is completed, a spray slurry is obtained; the spray slurry is prepared into a precursor powder by a spray drying method, the spray drying inlet air temperature is 200℃, and the outlet air temperature is 90℃; the precursor powder is placed in a tube furnace, and the temperature is first increased to 200℃ at a rate of 3℃ / min under an inert atmosphere, and kept warm for 2h, and then increased to 780℃ at a rate of 1℃ / min, and kept warm for 3h to obtain a carbonized product; the carbonized product is ground and refined to obtain a uniform carbon-coated lithium iron phosphate positive electrode material; the obtained lithium iron phosphate positive electrode material is subjected to microscopic morphology detection using a microscope, and the following is obtained: Figure 1 The SEM images shown are for Figure 1 Observation shows that the lithium iron phosphate particles are evenly distributed and the composite carbon layer is evenly coated.

[0065] Example 2

[0066] Preparation of uniform carbon-coated lithium iron phosphate cathode material:

[0067] Lithium carbonate, iron oxide, and phosphoric acid were dissolved in 150.63 g of 50% ethanol aqueous solution at a molar ratio of Li:Fe:P = 1.05:1:1 (a total of 50.21 g) to form a mixed solution with a solid content of 25%. 1.004 g of polyethylene glycol was added to the mixed solution and stirred evenly to obtain a precursor solution. Glucose, carbon nanotubes, and graphene were dissolved in the precursor solution at a carbon molar ratio of 1:0.9:0.9 (a total of 0.744 g). Then, aluminum oxide balls were used for ball milling at a ball-to-material ratio of 15:1 and a speed of 550 rpm for 1.5 h. During the ball milling process, cerium prepared in Preparation Example 1 was added three times. Modified tantalum carbide nanocrystals were added at a rate of 0.003 g each time with an interval of 20 minutes between each addition. After ball milling, a spray slurry was obtained. The spray slurry was prepared into a precursor powder by a spray drying method with an inlet air temperature of 180°C and an outlet air temperature of 80°C. The precursor powder was placed in a tubular furnace and heated to 180°C at a rate of 3°C / min under an inert atmosphere for 3 hours, and then heated to 750°C at a rate of 1°C / min for 4 hours to obtain a carbonized product. The carbonized product was ground and refined to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

[0068] Example 3

[0069] Preparation of uniform carbon-coated lithium iron phosphate cathode material:

[0070] Lithium carbonate, iron oxide, and phosphoric acid were dissolved in 101.12 g of 50% ethanol aqueous solution at a molar ratio of Li:Fe:P = 1.1:1:1 (a total of 54.45 g) to form a mixed solution with a solid content of 35%. 1.089 g of polyethylene glycol was added to the mixed solution and stirred evenly to obtain a precursor solution. Glucose, carbon nanotubes, and graphene were dissolved in the precursor solution at a carbon molar ratio of 1:0.9:0.9 (a total of 1.27 g). Alumina balls were then used for ball milling at a ball-to-material ratio of 15:1 and a speed of 550 rpm for 1.5 h. During the ball milling process, the cerium-modified Tantalum carbide nanocrystals were added at a rate of 0.012 g each time with an interval of 20 minutes between each addition. After ball milling, a spray slurry was obtained. The spray slurry was prepared into a precursor powder by a spray drying method with an air inlet temperature of 220°C and an air outlet temperature of 100°C. The precursor powder was placed in a tubular furnace and heated to 220°C at a rate of 3°C / min under an inert atmosphere, kept warm for 1 hour, and then heated to 800°C at a rate of 1°C / min and kept warm for 2 hours to obtain a carbonized product. The carbonized product was ground and refined to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

[0071] Example 4

[0072] Compared with Example 1, the only difference is that the cerium-modified tantalum carbide nanocrystals of Preparation Example 1 are replaced by the cerium-modified tantalum carbide nanocrystals of Preparation Example 2; other conditions and steps remain the same, and finally a uniform carbon-coated lithium iron phosphate positive electrode material is obtained.

[0073] Example 5

[0074] Compared with Example 1, the only difference is that the cerium-modified tantalum carbide nanocrystals of Preparation Example 1 are replaced by the cerium-modified tantalum carbide nanocrystals of Preparation Example 3; other conditions and steps remain the same, and finally a uniform carbon-coated lithium iron phosphate positive electrode material is obtained.

[0075] Comparative Example 1

[0076] Compared with Example 1, the only difference is that no cerium-modified tantalum carbide nanocrystals are added during the ball milling process; other conditions and steps remain the same, and finally a uniform carbon-coated lithium iron phosphate positive electrode material is obtained; the micromorphology of the obtained lithium iron phosphate positive electrode material is detected by microscope, and the following is obtained: Figure 2 The SEM images shown are for Figure 2 Observation shows that the distribution of lithium iron phosphate particles is still uniform, but the coating of the composite carbon layer is not uniform enough, showing incomplete coating and varying thickness.

[0077] Comparative Example 2

[0078] Compared with Example 1, the only difference is that the cerium-modified tantalum carbide nanocrystals are added at one time during the ball milling process; the other conditions and steps remain the same, and finally a uniform carbon-coated lithium iron phosphate positive electrode material is obtained.

[0079] Comparative Example 3

[0080] Lithium carbonate, iron oxide, and phosphoric acid were dissolved in 119.54 g of 50% ethanol aqueous solution at a molar ratio of Li:Fe:P = 1.08:1:1 (a total of 51.23 g) to form a mixed solution with a solid content of 30%. 2.05 g of polyethylene glycol was added to the mixed solution and stirred evenly to obtain a precursor solution. Glucose, carbon nanotubes, and graphene were dissolved in the precursor solution at a carbon molar ratio of 1:0.9:0.9 (a total of 0.1 g). Then, alumina balls were used for ball milling at a ball-to-material ratio of 15:1 and a speed of 550 rpm for 1.5 h. During the ball milling process, The cerium-modified tantalum carbide nanocrystals of Preparation Example 1 were added three times, 0.009 g each time, with an interval of 20 minutes between each addition. After ball milling, a spray slurry was obtained. The spray slurry was prepared into a precursor powder by a spray drying method, with an inlet air temperature of 200°C and an outlet air temperature of 90°C for spray drying. The precursor powder was placed in a tubular furnace, and the temperature was raised to 780°C at a rate of 3°C / min under an inert atmosphere, and kept warm for 5 hours to obtain a carbonized product. The carbonized product was ground and refined to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

[0081] The lithium iron phosphate positive electrode materials obtained in Examples 1-5 and Comparative Examples 1-3 were respectively made into batteries for electrochemical performance testing. The preparation method was as follows: 90g of lithium iron phosphate positive electrode material, 5g of Super-P, and 5g of polyvinylidene fluoride were mixed evenly, and then 200g of N-methylpyrrolidone solvent was added and mixed evenly to obtain a black slurry. The aluminum foil was placed in a coating machine, and the black slurry was poured in. After uniform coating, it was transferred to an oven and dried at 40°C for 4h. The dried aluminum foil was then placed in a punching machine, and several electrodes containing active substances were punched out. The electrodes were weighed, and the electrodes were placed in a drying bottle. The electrodes were transferred to a vacuum drying oven and dried at 110°C for 6h. Finally, they were assembled into button batteries in a vacuum glove box.

[0082] Then the prepared button battery was subjected to electrochemical performance tests, the charge transfer resistance was tested using an electrochemical workstation, and the capacity retention rate was tested using a constant current-constant voltage charge-discharge method. Figure 3-4 The curve comparison chart is shown in Table 1, and the test results are listed in Table 1. Table 1 is as follows:

[0083]

[0084] By analyzing the data in Table 1, it can be seen that compared with Comparative Examples 1-3, the lithium iron phosphate positive electrode materials prepared in Examples 1-4 have significantly lower charge transfer resistance and significantly higher capacity retention rate, which shows that the uniform carbon-coated lithium iron phosphate positive electrode material of the present invention has stronger electrochemical properties.

[0085] The above is a detailed description of an embodiment of the present invention. However, the content described is only a preferred embodiment of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A uniform carbon-coated lithium iron phosphate positive electrode material, characterized in that: The invention comprises lithium iron phosphate matrix particles, wherein the lithium iron phosphate matrix particles are coated with a composite carbon layer, and the composite carbon layer is doped with cerium-modified tantalum carbide nanocrystals; The mass of the composite carbon layer accounts for 1.5-2.4 wt % of the mass of the lithium iron phosphate matrix particles, and the mass fraction of the cerium-modified tantalum carbide nanocrystals in the composite carbon layer is 1.2-2.8 wt %; The method for preparing the uniform carbon-coated lithium iron phosphate positive electrode material comprises the following steps: S1, dissolving a lithium source, an iron source, and a phosphorus source in an ethanol aqueous solution at a molar ratio of Li:Fe:P=(1.05-1.10):1:1 to form a mixed solution with a solid content of 25-35%, adding a dispersant accounting for 0.5-2wt% of the mixed solution to the mixed solution, and stirring uniformly to obtain a precursor solution; S2, dissolving an organic carbon source and an inorganic carbon source in a precursor solution, and then ball milling, adding cerium-modified tantalum carbide nanocrystals in small amounts and multiple times during the ball milling process, and obtaining a spray slurry after the ball milling is completed; S3. Prepare the spray slurry into precursor powder by spray drying, with the spray drying air inlet temperature being 180-220°C and the air outlet temperature being 80-100°C; S4. Place the precursor powder in a tube furnace, heat it to 180-220°C at a rate of 3°C / min under an inert atmosphere, keep it warm for 1-3 hours, then heat it to 750-800°C at a rate of 1°C / min, keep it warm for 2-4 hours, and obtain a carbonized product; S5. Grind and refine the carbonized product to obtain a uniform carbon-coated lithium iron phosphate positive electrode material.

2. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The preparation method of the cerium-modified tantalum carbide nanocrystals is: Step 1: dissolving tantalum pentaethoxide and cerium nitrate in ethylene glycol at a molar ratio of Ta:Ce=100:(1-3), adding urea as a precipitant, and synthesizing a Ta-Ce-O precursor by microwave-assisted solvothermal method; Step 2: The Ta-Ce-O precursor and the phenolic resin are mixed in a weight ratio of 1:(5-8) and ball-milled, and then sintered at 1200-1300° C. for 2-4 hours to obtain cerium-modified tantalum carbide nanocrystals.

3. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step 1, the ratio of the total mass of the pentaethoxytantalum and cerium nitrate to the volume of ethylene glycol is 1 g:(10-20) mL.

4. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step 1, the ratio of the total weight of the pentaethoxytantalum and cerium nitrate to the weight of urea is 1:(1.5-2.5).

5. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: In step 2, the ball mill uses zirconia grinding balls, the ball-to-material ratio is 10:1, the rotation speed is 300-400 rpm, and the ball milling time is 2-4 hours.

6. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The composite carbon layer is a composition composed of an organic carbon source and an inorganic carbon source in a carbon molar ratio of 1:(1.5-2), wherein the organic carbon source is a composition composed of any one or more of polyacrylic acid, polyaniline, and glucose in any proportion, and the inorganic carbon source is a composition composed of any one or two of carbon nanotubes and graphene in any proportion.

7. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the lithium source includes a composition consisting of one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium dihydrogen phosphate in any proportion; the iron source includes a composition consisting of one or more of iron oxides and iron salts in any proportion; and the phosphorus source includes a composition consisting of one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate in any proportion.

8. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the dispersant is polyethylene glycol.

9. The uniform carbon-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, alumina grinding balls are used for ball milling, the ball-to-material ratio is 15:1, the rotation speed is 500-600 rpm, and the ball milling time is 1-2 h.

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