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

Abstract

The invention discloses a uniform carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, and relates to the technical field of lithium iron phosphate positive electrode materials, the uniform carbon-coated lithium iron phosphate positive electrode material 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 modified tantalum carbide nanocrystals; the preparation method of the cerium modified tantalum carbide nanocrystal comprises the following steps: dissolving pentaethoxy tantalum and cerous nitrate in ethylene glycol, adding urea as a precipitator, and synthesizing a Ta-Ce-O precursor through a microwave-assisted solvothermal method; the Ta-Ce-O precursor and phenolic resin are mixed and then subjected to ball milling and sintering, and the cerium modified tantalum carbide nanocrystal is obtained. By introducing a small amount of cerium modified tantalum carbide nanocrystals into the composite carbon layer for many times, uniform coating of the composite carbon layer can be realized, and the electrochemical performance of the lithium iron phosphate positive electrode material is enhanced.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO 4 ) is a substance widely used in the field of cathode materials. Its unique olivine structure endows it with many advantages: high safety, long cycle life, and excellent thermal stability. In addition, since lithium iron phosphate does not contain harmful elements such as nickel, it causes relatively little environmental pollution.

[0003] However, to improve the electrochemical performance of lithium iron phosphate, it is urgent to solve its inherent limitations, including poor electrical conductivity, low tap density, low energy density, and insufficient low-temperature performance. By optimizing the particle size, carbon coating treatment, and ion doping modification, it is of practical significance for optimizing the performance of lithium iron phosphate materials and developing high-energy-density batteries.

[0004] In the prior art, coating lithium iron phosphate materials with conductive substances is a key means to improve their rate and low-temperature performance. Among them, carbon materials are favored as a simple and economical excellent choice. Carbon coating can play multiple roles: carbon can act as a reducing agent to effectively prevent the oxidation of Fe in lithium iron phosphate materials; carbon coating can significantly improve the electronic conductivity of the materials; at the same time, carbon coating can, to a certain extent, block the direct contact between particles and effectively inhibit the excessive growth of particles. 2+ However, the current carbon coating technology in the market mainly relies on organic carbon sources, such as glucose, sucrose, etc., to coat lithium iron phosphate through high-temperature reduction reactions. However, this method is limited by conditions such as reaction temperature and time, making it difficult to precisely control the graphitization degree of the carbon layer, thereby affecting the electronic conductivity of lithium iron phosphate. On the other hand, inorganic carbon sources such as conductive carbon black, graphene, etc., although having a high graphitization degree and a large specific surface area, can directly improve the electronic conductivity, but due to their poor water solubility, the consistency of the precursor slurry formed during the high-temperature solid-phase method processing is poor, resulting in uneven carbon coating of lithium iron phosphate. This non-uniformity cannot effectively control the growth of lithium iron phosphate grains, leading to particle agglomeration phenomena and ultimately reducing the capacity of the battery.

[0005] Therefore, there is an urgent need for a method that can significantly improve the uniform carbon coating of lithium iron phosphate, thereby effectively enhancing the consistency and electronic conductivity of lithium iron phosphate materials.

[0006] Summary of the Invention

[0007] ​The object of the present invention is to provide a uniformly carbon-coated lithium iron phosphate cathode material and a preparation method thereof, and solve the following technical problems: How to improve the uniformity of the carbon coating layer of the lithium iron phosphate cathode material.

[0008] The object of the present invention can be achieved by the following technical solutions: In the first aspect, the present invention discloses a uniformly carbon-coated lithium iron phosphate cathode material, which is characterized in that it includes lithium iron phosphate matrix particles, and the outside of the lithium iron phosphate matrix particles is coated with a composite carbon layer, and cerium (Ce)-modified tantalum carbide (TaC) nanocrystals are doped in the composite carbon layer; 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%; 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%.

[0009] Furthermore, the preparation method of the cerium-modified tantalum carbide nanocrystals is as follows: Step 1: Dissolve pentaethoxytantalum and cerium nitrate in ethylene glycol according to the molar ratio of Ta:Ce = 100:(1-3), add urea as a precipitating agent, and synthesize a Ta-Ce-O precursor by microwave-assisted solvothermal method; Step 2: Mix the Ta-Ce-O precursor and phenolic resin according to the weight ratio of 1:(5-8), then carry out ball milling, and sinter at 1200-1300 °C for 2-4 h to obtain cerium-modified tantalum carbide nanocrystals.

[0010] Furthermore, in Step 1, the ratio of the total mass of pentaethoxytantalum and cerium nitrate to the volume of ethylene glycol is 1 g:(10-20) mL.

[0011] Furthermore, in Step 1, the ratio of the total weight of pentaethoxytantalum and cerium nitrate to the weight of urea is 1:(1.5-2.5).

[0012] Furthermore, in Step 2, the ball milling 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 h.

[0013] Based on this, a preferred preparation method of cerium-modified tantalum carbide nanocrystals is obtained: Step 1: Dissolve tantalum pentaethoxide and cerium nitrate in ethylene glycol at a molar ratio of Ta:Ce = 100:2. The ratio of the total mass of tantalum pentaethoxide and cerium nitrate to the volume of ethylene glycol is 1 g:15 mL. Then add urea as a precipitant, and the ratio of the total weight of tantalum pentaethoxide and cerium nitrate to the weight of urea is 1:2. Synthesize the Ta-Ce-O precursor by microwave-assisted solvothermal method. Step 2: Mix the Ta-Ce-O precursor and phenolic resin at a weight ratio of 1:6, add them to a zirconia ball mill, and ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 350 rpm for 3 h. Then sinter at 1250 °C for 2 h to obtain cerium-modified tantalum carbide nanocrystals.

[0014] Further, the composite carbon layer is a composition composed of an organic carbon source and an inorganic carbon source at a carbon molar ratio of 1:(1.5 - 2). Among them, 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. Preferably, the organic carbon source is glucose, and the inorganic carbon source is a mixture composed of carbon nanotubes and graphene at a ratio of 1:1.

[0015] In a second aspect, the present invention also discloses a preparation method of a lithium iron phosphate cathode material for preparing the uniformly carbon-coated lithium iron phosphate cathode material as described above, including the following steps: S1: Dissolve 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%. Add a dispersant accounting for 0.5 - 2 wt% of the mixed solution to the mixed solution, and stir evenly to obtain a precursor solution. S2: Dissolve the organic carbon source and the inorganic carbon source in the precursor solution, and then perform ball milling. Add the cerium-modified tantalum carbide nanocrystals in small amounts and multiple times during the ball milling process. After the ball milling is completed, obtain a spray slurry. S3: Use spray drying to prepare the spray slurry into a precursor powder. The inlet air temperature for spray drying is 180 - 220 °C, and the outlet air temperature is 80 - 100 °C. S4: Place the precursor powder in a tubular furnace. Under an inert atmosphere, first heat it to 180 - 220 °C at a rate of 3 °C / min and keep it warm for 1 - 3 h, then heat it to 750 - 800 °C at a rate of 1 °C / min and keep it warm for 2 - 4 h to obtain a carbonized product. S5: Grind and refine the carbonized product to obtain a uniformly carbon-coated lithium iron phosphate cathode material.

[0016] Further, in step S1, the lithium source includes a composition composed 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 composed of one or more of iron oxides and iron salts in any proportion; the phosphorus source includes a composition composed of one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate in any proportion.

[0017] Further, in step S1, the dispersant is polyethylene glycol.

[0018] Further, 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.

[0019] Based on this, a preferred preparation method of a uniformly carbon-coated lithium iron phosphate cathode material is obtained, including the following steps: S1. Dissolve lithium carbonate, iron oxide, and phosphoric acid in an ethanol aqueous solution according to the molar ratio of Li:Fe:P = 1.08:1:1 to form a mixed solution with a solid content of 30%. Add polyethylene glycol accounting for 1.2 wt% of the mixed solution to the mixed solution, and after stirring evenly, obtain a precursor solution; S2. Dissolve glucose, carbon nanotubes, and graphene in the precursor solution according to the carbon molar ratio of 1:0.9:0.9. Then use alumina grinding balls to ball mill at a ball-to-material ratio of 15:1 and a rotation speed of 550 rpm for 1.5 h. During the ball milling process, add equal amounts of cerium-modified tantalum carbide nanocrystals in three times, and the time interval between each addition is 20 min. After the ball milling is completed, obtain a spray slurry; S3. Use the spray drying method to prepare the spray slurry into a precursor powder. The inlet air temperature for spray drying is 200 °C, and the outlet air temperature is 90 °C; S4. Place the precursor powder in a tubular furnace. Under an inert atmosphere, first heat it to 200 °C at a rate of 3 °C / min and keep it warm for 2 h, then heat it to 780 °C at a rate of 1 °C / min and keep it warm for 3 h to obtain a carbonized product; S5. Grind and refine the carbonized product to obtain a uniformly carbon-coated lithium iron phosphate cathode material.

[0020] The beneficial effects of the present invention: 1. In the composite carbon layer of the uniformly carbon-coated lithium iron phosphate cathode material of the present invention, cerium-modified tantalum carbide nanocrystals are introduced. 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, and further promoting the uniform adsorption of the carbon slurry. At the same time, the cerium-modified tantalum carbide nanocrystals have a high surface energy, which can provide preferential nucleation sites for the pyrolysis of the carbon precursor, increase the nucleation density of the carbon layer, and thus inhibit the island-like 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 aggregation of carbon particles, thereby improving the uniformity of the composite carbon layer coating on the outer surface of lithium iron phosphate.

[0021] 2. In the composite carbon layer of the uniformly carbon-coated lithium iron phosphate cathode material of the present invention, cerium-modified tantalum carbide nanocrystals are introduced. From the perspective of electrochemical performance, TaC has metallic-like conductivity, and the delocalized electrons in its conduction band form a three-dimensional electron highway. After cerium doping, the 4f orbital electrons of Ce 3+ hybridize with the d orbitals of TaC to form shallow energy level defect states, reducing the electron transition barrier; Ce 3+ substitutes Ta 5+ to generate cation vacancies, inducing local lattice distortion to form oxygen vacancy clusters. These vacancies serve as hopping sites for Li + migration, thereby increasing the diffusion coefficient and improving the electrochemical performance.

[0022] 3. In the composite carbon layer of the uniformly carbon-coated lithium iron phosphate cathode material of the present invention, cerium-modified tantalum carbide nanocrystals are introduced. During the introduction process, a ball milling and mixing method of adding in small amounts and multiple times is adopted, which can make their distribution in the system more uniform, and further enhance the uniform contact with the carbon source, so that during the subsequent sintering process, the cerium-modified tantalum carbide nanocrystals can be fully and uniformly doped into the composite carbon layer.

[0023] 4. During the preparation process of the uniformly carbon-coated lithium iron phosphate cathode material of the present invention, a two-step carbonization process combined with a specific heating program is adopted. By forming a dense carbon layer in the low-temperature stage and enhancing the graphitization degree in the high-temperature stage, the stability of the carbon layer structure is enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described below with reference to the accompanying drawings.

[0025] Figure 1 is the SEM image of the uniformly carbon-coated lithium iron phosphate cathode material prepared in Example 1 of the present invention; Figure 2 is the SEM image of the lithium iron phosphate cathode material prepared in Comparative Example 1 of the present invention; Figure 3It is a curve comparison diagram of the impedance test results of the lithium iron phosphate cathode materials of Examples 1-5 and Comparative Examples 1-3 of the present invention; Figure 4 It is a curve comparison diagram of the capacity retention rate test results of the lithium iron phosphate cathode materials of Examples 1-5 and Comparative Examples 1-3 of the present invention. Detailed implementation manners

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0027] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and are carried out according to the technologies or conditions described in the literature in the field or according to the product specifications. The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0028] Preparation Example 1 Prepare cerium-modified tantalum carbide nanocrystals: Step 1: Dissolve pentaethoxytantalum and cerium nitrate in a molar ratio of Ta:Ce = 100:2 (a total of 25.64 g) in 385 mL of ethylene glycol, then add 51.28 g of urea as a precipitating agent, and synthesize a Ta-Ce-O precursor by microwave-assisted solvothermal method; Step 2: Mix the Ta-Ce-O precursor with phenolic resin in a weight ratio of 1:6, add it to a zirconia ball mill, and ball mill for 3 h at a ball-to-material ratio of 10:1 and a rotation speed of 350 rpm, and then sinter at 1250 °C for 2 h to obtain cerium-modified tantalum carbide nanocrystals.

[0029] Preparation Example 2 Prepare cerium-modified tantalum carbide nanocrystals: Step 1: Dissolve pentaethoxytantalum and cerium nitrate in a molar ratio of Ta:Ce = 100:1 (a total of 20.35 g) in 305 mL of ethylene glycol, then add 30.53 g of urea as a precipitating agent, and synthesize a Ta-Ce-O precursor by microwave-assisted solvothermal method; Step 2: Mix the Ta-Ce-O precursor with phenolic resin in a weight ratio of 1:5, add it to a zirconia ball mill, and ball mill for 3 h at a ball-to-material ratio of 10:1 and a rotation speed of 350 rpm, and then sinter at 1250 °C for 2 h to obtain cerium-modified tantalum carbide nanocrystals.

[0030] Preparation Example 3 Step 1: Dissolve tantalum pentaethoxide and cerium nitrate in a molar ratio of Ta:Ce = 100:3 (a total of 30.56 g) in 458 mL of ethylene glycol, then add 76.4 g of urea as a precipitating agent, and synthesize the Ta-Ce-O precursor by microwave-assisted solvothermal method; Step 2: Mix the Ta-Ce-O precursor and phenolic resin in a weight ratio of 1:8, add them to a zirconia ball mill, and ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 350 rpm for 3 h, and then sinter at 1250 °C for 2 h to obtain cerium-modified tantalum carbide nanocrystals.

[0031] Example 1 Prepare a uniformly carbon-coated lithium iron phosphate cathode material: Dissolve lithium carbonate, iron oxide, and phosphoric acid in a molar ratio of Li:Fe:P = 1.08:1:1 (a total of 51.23 g) in 119.54 g of an ethanol aqueous solution with a mass fraction of 50% to form a mixed solution with a solid content of 30%. Add 2.05 g of polyethylene glycol to the mixed solution, stir evenly to obtain a precursor solution; dissolve glucose, carbon nanotubes, and graphene in a carbon molar ratio of 1:0.9:0.9 (a total of 0.1 g) in the precursor solution, and then use alumina grinding balls to ball mill at a ball-to-material ratio of 15:1 and a rotation speed of 550 rpm for 1.5 h. During the ball milling process, add the cerium-modified tantalum carbide nanocrystals prepared in Preparation Example 1 three times, 0.009 g each time, with a time interval of 20 min between each addition. After the ball milling is completed, obtain a spray slurry; use the spray drying method to prepare the spray slurry into a precursor powder, with an inlet temperature of 200 °C and an outlet temperature of 90 °C for spray drying; place the precursor powder in a tube furnace, first heat it to 200 °C at a rate of 3 °C / min in an inert atmosphere, hold for 2 h, and then heat it to 780 °C at a rate of 1 °C / min and hold for 3 h to obtain a carbonized product; grind and refine the carbonized product to obtain a uniformly carbon-coated lithium iron phosphate cathode material; use a microscope to detect the micro-morphology of the obtained lithium iron phosphate cathode material, and obtain the SEM image as shown in Figure 1 shown, and by observing Figure 1 it can be seen that the lithium iron phosphate particles are evenly distributed and the composite carbon layer is evenly coated.

[0032] Example 2 Prepare a uniformly carbon-coated lithium iron phosphate cathode material: Lithium carbonate, iron oxide, and phosphoric acid were dissolved in 150.63 g of an ethanol aqueous solution with a mass fraction of 50% according to 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 after stirring evenly, a precursor solution was obtained; glucose, carbon nanotubes, and graphene were dissolved in the precursor solution according to a carbon molar ratio of 1:0.9:0.9 (a total of 0.744 g). Then, alumina grinding balls were used to ball-mill at a ball-to-material ratio of 15:1 and a rotation 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 in three portions, 0.003 g each time, and the time interval between each addition was 20 min. After the ball-milling was completed, a spray slurry was obtained; the spray slurry was prepared into a precursor powder by spray drying. The inlet air temperature for spray drying was 180 °C, and the outlet air temperature was 80 °C; the precursor powder was placed in a tube furnace, and under an inert atmosphere, it was first heated to 180 °C at a rate of 3 °C / min and held for 3 h, and then heated to 750 °C at a rate of 1 °C / min and held for 4 h to obtain a carbonized product; the carbonized product was ground and refined to obtain a uniformly carbon-coated lithium iron phosphate cathode material.

[0033] Example 3 Preparation of a uniformly carbon-coated lithium iron phosphate cathode material: Lithium carbonate, iron oxide, and phosphoric acid were dissolved in 101.12 g of an ethanol aqueous solution with a mass fraction of 50% according to 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 after stirring evenly, a precursor solution was obtained; glucose, carbon nanotubes, and graphene were dissolved in the precursor solution according to a carbon molar ratio of 1:0.9:0.9 (a total of 1.27 g). Then, alumina grinding balls were used to ball-mill at a ball-to-material ratio of 15:1 and a rotation 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 in three portions, 0.012 g each time, and the time interval between each addition was 20 min. After the ball-milling was completed, a spray slurry was obtained; the spray slurry was prepared into a precursor powder by spray drying. The inlet air temperature for spray drying was 220 °C, and the outlet air temperature was 100 °C; the precursor powder was placed in a tube furnace, and under an inert atmosphere, it was first heated to 220 °C at a rate of 3 °C / min and held for 1 h, and then heated to 800 °C at a rate of 1 °C / min and held for 2 h to obtain a carbonized product; the carbonized product was ground and refined to obtain a uniformly carbon-coated lithium iron phosphate cathode material.

[0034] Example 4 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.

[0035] Example 5 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.

[0036] Comparative Example 1 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 obtained lithium iron phosphate positive electrode material is subjected to microscopic morphology detection using a microscope, and the following is obtained: Figure 2 The SEM images shown are 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 uneven thickness.

[0037] Comparative Example 2 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.

[0038] Comparative Example 3 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), and then ball-milled for 1.5 h using alumina grinding balls at a ball-to-material ratio of 15:1 and a rotation speed of 550 rpm. During the ball-milling process, The cerium-modified tantalum carbide nanocrystals of Preparation Example 1 were added three times, 0.009 g each time, and the time interval between each addition was 20 minutes. After the ball milling was completed, a spray slurry was obtained; the spray slurry was prepared into a precursor powder by a spray drying method, the spray drying inlet air temperature was 200°C, and the outlet air temperature was 90°C; the precursor powder was placed in a tubular furnace, and the temperature was increased to 780°C at a rate of 3°C / min under an inert atmosphere, and the temperature was kept 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.

[0039] The lithium iron phosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-3 were respectively made into batteries for testing their electrochemical performance. The manufacturing method is as follows: 90 g of lithium iron phosphate cathode material, 5 g of Super-P, and 5 g of polyvinylidene fluoride were mixed evenly, and then 200 g of N-methylpyrrolidone solvent was added and mixed evenly to obtain a black slurry. The aluminum foil was placed in a coating machine, the black slurry was poured in, and after evenly coating, it was transferred to an oven and dried at 40 °C for 4 h. Then the dried aluminum foil was placed in a punching machine to punch several electrode sheets containing the active material, weighed, and the electrode sheets were placed in a drying bottle and transferred to a vacuum drying oven and dried at 110 °C for 6 h. Finally, it was assembled into a button battery in a vacuum glove box.

[0040] Then, the electrochemical performance of the obtained button batteries was tested. The charge transfer resistance was tested using an electrochemical workstation, and the capacity retention rate was tested using the constant current-constant voltage charge-discharge method to obtain Figures 3-4 a curve comparison diagram, and the test results are listed in Table 1 as follows:

[0041] By analyzing the data in Table 1, it can be known that compared with Comparative Examples 1-3, the lithium iron phosphate cathode materials prepared in Examples 1-4 have significantly lower charge transfer resistance and significantly higher capacity retention rate. This shows that the uniformly carbon-coated lithium iron phosphate cathode material of the present invention has stronger electrochemical performance.

[0042] The above has described in detail an embodiment of the present invention, but the content described is only a preferred embodiment of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the application of the present invention should still fall within the scope covered by the patent of the present invention.

Claims

1. 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 on the outside, and the composite carbon layer is doped with cerium-modified tantalum carbide nanocrystals; The mass of the composite carbon layer accounts for 1.5-2.4wt% of the mass of the lithium iron phosphate matrix particles, and the mass fraction of cerium-modified tantalum carbide nanocrystals in the composite carbon layer is 1.2-2.8wt%.

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 a microwave-assisted solvothermal method; Step 2: Mix the Ta-Ce-O precursor and the phenolic resin in a weight ratio of 1:(5-8), perform ball milling, and then sinter 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 300-400rpm, and the ball milling time is 2-4h.

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 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.

7. A method for preparing a lithium iron phosphate positive electrode material, for preparing the uniform carbon-coated lithium iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The steps include: 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, stirring evenly, 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, preparing the spray slurry into precursor powder by spray drying, the spray drying air inlet temperature is 180-220°C, and the air outlet temperature is 80-100°C; S4, placing the precursor powder in a tube furnace, heating it to 180-220°C at a rate of 3°C / min under an inert atmosphere, keeping it warm for 1-3h, then heating it to 750-800°C at a rate of 1°C / min, keeping it warm for 2-4h, to obtain a carbonized product; S5. Grind and refine the carbonized product to obtain a uniformly carbon-coated lithium iron phosphate positive electrode material.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 7, 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; the phosphorus source includes a composition consisting of one or more of phosphoric acid, ammonium phosphate, and diammonium phosphate in any proportion.

9. The method for preparing a lithium iron phosphate positive electrode material according to claim 7, characterized in that: In step S1, the dispersant is polyethylene glycol.

10. The method for preparing a lithium iron phosphate positive electrode material according to claim 7, characterized in that: 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.

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