A high-rate carbon-coated lithium iron phosphate anode material and a preparation method thereof

Abstract

This invention discloses a high-rate carbon-coated lithium iron phosphate cathode material and its preparation method, relating to the field of lithium-ion battery cathode material technology. The material constructs a unique bilayer composite carbon-coated structure on the surface of lithium iron phosphate particles, including an inner continuous dense conductive carbon layer and an outer three-dimensional porous carbon network layer. The inner carbon layer ensures rapid electron conduction, while the outer porous carbon layer maintains electron pathways, provides abundant ion diffusion channels, and buffers volume changes. The preparation method utilizes a stepwise liquid-phase coating and segmented heat treatment process. This invention effectively solves the problems of poor electronic conductivity and slow ion diffusion in lithium iron phosphate materials, significantly improving its high-rate charge-discharge performance and cycle stability. Furthermore, the process is controllable and suitable for large-scale production, showing broad application prospects in power batteries and high-power energy storage.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a carbon-coated lithium iron phosphate (LiFePO4) cathode material with excellent rate performance and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is widely used in power batteries and large-scale energy storage due to its advantages such as high safety, long cycle life, environmental friendliness, and relatively low cost. However, its inherently low electronic conductivity (approximately 10⁻⁶ ppm) is a significant drawback. -9 Its slow lithium-ion diffusion rate (s / cm) results in poor rate performance, especially under high-rate charge and discharge conditions, where capacity decay is severe, limiting its application in scenarios requiring high power output (such as rapid acceleration of electric vehicles, takeoff of drones, and grid frequency regulation).

[0003] Currently, common methods for improving the rate performance of lithium iron phosphate (LFP) lithium iron phosphate include carbon coating, ion doping, and nanostructuring. Among these, carbon coating is an effective means of improving its electronic conductivity. Traditional carbon coating methods (such as ball milling followed by sintering) often struggle to achieve a uniform, continuous carbon layer with an ideal porous structure. An excessively thick or dense coating layer can hinder lithium-ion transport; an excessively thin or discontinuous layer offers limited enhancement of electronic conductivity. Furthermore, a single-structure carbon coating layer cannot simultaneously optimize both electronic conduction and ion diffusion pathways.

[0004] To address the aforementioned issues, this invention proposes a novel multi-layered, porous carbon coating design, aiming to construct a surface modification layer that combines an efficient electron transport network with a fast ion diffusion channel. This significantly improves the high-rate charge-discharge performance of lithium iron phosphate while maintaining its advantages of high safety and long lifespan. Summary of the Invention

[0005] The primary objective of this invention is to provide a carbon-coated lithium iron phosphate cathode material with high rate capability.

[0006] A second objective of the present invention is to provide a lithium-ion battery comprising the above-mentioned positive electrode material.

[0007] A third objective of this invention is to provide a method for preparing the above-mentioned cathode material.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A high-rate carbon-coated lithium iron phosphate cathode material, the core of which lies in: using lithium iron phosphate particles as a matrix, constructing a double-layer composite carbon coating structure on its surface; the double-layer composite carbon coating structure includes: Inner conductive carbon layer: This is a uniform, continuous, and dense carbon layer with a thickness of 2-5nm, which is directly coated on the surface of lithium iron phosphate particles. Its main function is to provide an efficient electron conduction path and reduce the contact resistance between particles.

[0009] Outer porous carbon network layer: This is a carbon layer with a three-dimensional interconnected porous structure, covering the inner conductive carbon layer. It has a thickness of 10-30 nm, a porosity of 40%-60%, and an average pore size of 2-10 nm. While ensuring continuous electron conduction, its abundant nanoscale pores provide short-range channels for electrolyte wetting and rapid lithium-ion diffusion, and can mitigate volume changes during charging and discharging.

[0010] Preferably, the lithium iron phosphate particles have a primary particle size D. 50 For nano or submicron particles in the 50-200 nm range, the tap density is ≥1.2 g / cm³. 3 .

[0011] Preferably, the inner conductive carbon layer is derived from the pyrolytic carbon of glucose, sucrose, or citric acid; the outer porous carbon network layer is derived from porous carbon formed by carbonization of polydopamine, phenolic resin, or polyacrylonitrile through a specific process.

[0012] Preferably, the total carbon content, based on the total mass of the cathode material, is 1.5wt%-3.0wt%, with the inner layer containing 30%-50% carbon and the outer layer containing 50%-70% carbon.

[0013] The key to the preparation method of this invention lies in the stepwise construction of a double-layer carbon coating structure: Step 1: Preparation of the inner conductive carbon layer: The lithium iron phosphate precursor (or commercial lithium iron phosphate powder) and the inner carbon source (such as glucose) are uniformly mixed in a solvent. After preliminary drying, the first stage of low-temperature heat treatment (e.g., 300-500℃) is carried out under an inert atmosphere to pyrolyze the carbon source and form a uniform primary carbon layer on the surface of the lithium iron phosphate particles.

[0014] Step 2: Construction of the outer porous carbon network layer: The product obtained in Step 1 is subjected to a surface polymerization reaction with an outer carbon source precursor (such as dopamine hydrochloride, which can undergo self-polymerization in buffer to form a polydopamine coating layer) in a liquid environment, so that the outer carbon source precursor uniformly coats the surface of the particles that already have an inner carbon layer. Subsequently, the product is subjected to a second-stage high-temperature heat treatment (e.g., 600-750℃) under an inert atmosphere. In this stage, the outer carbon source carbonizes to form a porous carbon network, while the inner carbon layer is further graphitized to enhance conductivity, ultimately forming the aforementioned bilayer composite carbon coating structure.

[0015] Preferably, the first stage heat treatment time is 2-4 hours, and the second stage heat treatment time is 4-8 hours.

[0016] Preferably, the porosity and pore size distribution of the outer porous carbon network layer can be controlled by adjusting the concentration of the outer carbon source precursor, the polymerization conditions, and the heating rate of the second-stage heat treatment.

[0017] Compared with the prior art, the advantages of the present invention are as follows: (1) Significantly improves rate performance: The unique double-layer carbon coating structure optimizes both electron conduction and ion diffusion, enabling the material to maintain high capacity while possessing excellent high-rate discharge capability (such as a significant improvement in retention rate above 5C).

[0018] (2) Improved cycle stability: The porous carbon network of the outer layer is elastic and can effectively buffer the volume strain of lithium iron phosphate during cycling, improve structural stability, and extend cycle life.

[0019] (3) The process is controllable and easy to promote: The preparation method is based on liquid phase mixing and segmented heat treatment. The process parameters are easy to adjust, which is suitable for large-scale production and has good economic benefits.

[0020] (4) Maintaining inherent material safety: Based on lithium iron phosphate matrix, combined with appropriate carbon coating, no harmful elements are introduced, thus maintaining the inherent high thermal stability and safety of the material. Attached Figure Description

[0021] Figure 1 This is an electron microscope image of lithium iron phosphate particles with a double-layer composite carbon coating structure in this invention; Figure 2 The figures show a comparison curve of the discharge capacity of batteries assembled using the materials of Example 1 of the present invention and the comparative materials at different discharge rates. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0023] The implementation process includes the following two aspects: In a first aspect, the present invention provides a method for preparing high-rate carbon-coated lithium iron phosphate cathode materials.

[0024] A method for preparing a high-rate carbon-coated lithium iron phosphate cathode material includes the following steps: S1. A certain amount of commercial nano-sized lithium iron phosphate (D 50 =100nm) dispersed in deionized water, glucose with a mass ratio of 0.6% to lithium iron phosphate was added, and ultrasonic stirring was performed to form a uniform suspension.

[0025] S2. Spray dry the suspension to obtain precursor powder.

[0026] S3. Place the precursor powder in a tube furnace, heat it to 450°C at 5°C / min under an argon atmosphere, hold it at that temperature for 3 hours, and then cool it naturally to obtain an intermediate product with an inner carbon coating.

[0027] S4. Disperse the above intermediate product in Tris-HCl buffer (pH=8.5), add dopamine hydrochloride (1.5% of the mass of the intermediate product), and magnetically stir at room temperature for 24 hours to ensure uniform coating of polydopamine.

[0028] S5. The solid product was collected by centrifugation, washed, dried, and then placed in a tube furnace. Under an argon atmosphere, the temperature was increased to 700°C at 3°C / min and held for 6 hours. After natural cooling, it was ground to obtain the double-layer carbon-coated lithium iron phosphate cathode material. The total carbon content was calculated to be approximately 2.2%.

[0029] Secondly, the present invention provides the preparation and performance testing of battery electrodes.

[0030] The positive electrode material prepared according to this invention, conductive carbon black (SuperP), and binder (PVDF) were mixed uniformly in N-methylpyrrolidone (NMP) at a mass ratio of 92:4:4 to form a slurry. The slurry was coated onto aluminum foil, and after drying, rolling, and stamping, a positive electrode sheet was obtained. Using a lithium metal sheet as the negative electrode, a CR2032 coin cell was assembled, and electrochemical tests were conducted at room temperature.

[0031] Using the process described in the above embodiments, the products of Examples 1-3 were prepared: Example 1: Materials were prepared according to the specific implementation process described above.

[0032] Example 2: The amount of dopamine hydrochloride added in S4 was changed to 2.5% of the intermediate product, while other aspects remained the same as in Example 1. The total carbon content was approximately 2.8%.

[0033] Example 3: Citric acid was used instead of glucose in S1, and the amount added was 0.8% of lithium iron phosphate; other aspects were the same as in Example 1. The total carbon content was approximately 2.5%.

[0034] Comparative example: The traditional one-step carbon coating method was used. Lithium iron phosphate was directly mixed with glucose of equal total carbon content (2.2%), ball-milled, and then sintered at 700°C for 6 hours in argon atmosphere.

[0035] The performance test results are shown in the table below: Table 1. Material physical properties and performance at 0.2C / 5C rates sample Specific surface area (m² / g) Tap density (g / cm³) 0.2C discharge specific capacity (mAh / g) 5C discharge specific capacity (mAh / g) 5C / 0.2C capacity retention Example 1 25.3 1.35 155.2 138.5 89.2% Example 2 32.1 1.28 153.8 136.0 88.4% Example 3 23.8 1.38 154.5 137.2 88.8% Comparative Example 18.5 1.40 152.0 110.3 72.6% As shown in Table 1, the materials in the embodiments of the present invention maintain a high tap density and close to the theoretical capacity at a 0.2C rate, while their 5C high-rate discharge capacity and capacity retention rate are significantly higher than those of the comparative materials with traditional one-step carbon coating, demonstrating the significant advantages of the double-layer carbon coating structure of the present invention in improving rate performance.

[0036] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in this application, based on the technical solution and concept of this application, should be included within the scope of protection of this application.

Claims

1. A high-rate carbon-coated lithium iron phosphate cathode material, characterized in that: It includes a lithium iron phosphate particle matrix and a double-layer composite carbon coating structure covering its surface; the double-layer composite carbon coating structure includes: (1) The inner conductive carbon layer is a continuous and dense carbon layer with a thickness of 2-5 nm; (2) An outer porous carbon network layer, which covers the inner conductive carbon layer, is a carbon layer with a three-dimensional interconnected porous structure, with a thickness of 10-30 nm, a porosity of 40%-60%, and an average pore size of 2-10 nm.

2. The high-rate carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that: The primary particle size D of the lithium iron phosphate particles in the lithium iron phosphate particle matrix 50 The wavelength is 50-200nm.

3. The high-rate carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that: Based on the total mass of the cathode material, the total carbon content is 1.5wt%-3.0wt%, of which the inner layer carbon content accounts for 30%-50% of the total carbon content and the outer layer carbon content accounts for 50%-70% of the total carbon content.

4. The high-rate carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that: The inner conductive carbon layer is derived from the pyrolytic carbon of glucose, sucrose, or citric acid; the outer porous carbon network layer is derived from the carbonization products of polydopamine, phenolic resin, or polyacrylonitrile.

5. A lithium-ion battery, characterized in that: Its positive electrode comprises a high-rate carbon-coated lithium iron phosphate positive electrode material as described in any one of claims 1-4.

6. A method for preparing a high-rate carbon-coated lithium iron phosphate cathode material as described in any one of claims 1-4, characterized in that: Includes the following steps: (1) Mix lithium iron phosphate precursor or lithium iron phosphate powder with inner carbon source in solvent, dry, and then perform first-stage heat treatment under inert atmosphere to form inner conductive carbon layer intermediate. (2) The intermediate obtained in step (1) is reacted with the outer carbon source precursor in a liquid environment to make the outer carbon source precursor coat the surface of the intermediate. (3) The product obtained in step (2) is subjected to a second stage of heat treatment in an inert atmosphere to carbonize and form an outer porous carbon network layer, while the inner carbon layer is further graphitized to obtain a high-rate carbon-coated lithium iron phosphate cathode material.

7. The method for preparing high-rate carbon-coated lithium iron phosphate cathode material according to claim 6, characterized in that: The first stage of heat treatment is performed at a temperature of 300-500℃ for 2-4 hours; the second stage of heat treatment is performed at a temperature of 600-750℃ for 4-8 hours.

8. The method for preparing high-rate carbon-coated lithium iron phosphate cathode material according to claim 6, characterized in that: In step (2), the surface reaction is a self-polymerization reaction carried out in a buffer solution.

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