Carbon-coated lithium iron phosphate / graphene composite positive electrode material and preparation method thereof

Abstract

This invention discloses a carbon-coated lithium iron phosphate / graphene composite cathode material and its preparation method, relating to the field of lithium-ion battery cathode materials. The material uses a three-dimensional graphene network as a framework, with lithium iron phosphate nanoparticles grown in situ on the surface and coated with an amorphous carbon layer, forming a dual conductivity-enhancing structure. This structure significantly improves the material's electronic conductivity and ion diffusion rate, endowing it with excellent high-rate performance and cycle stability. The material preparation process is simple and controllable, making it suitable for power batteries and high-power energy storage applications.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically a carbon-coated lithium iron phosphate / graphene composite cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become an important cathode material for power batteries and energy storage batteries due to its advantages such as high safety, long cycle life, low cost, and environmental friendliness. However, the low intrinsic electronic conductivity and small lithium-ion diffusion coefficient of lithium iron phosphate result in poor rate performance, which limits its application in scenarios requiring rapid charging and discharging (such as electric vehicles, drones, start-stop systems, etc.).

[0003] Commonly used modification methods include carbon coating, nanostructuring, and ion doping. Among these, carbon coating can effectively improve electronic conductivity, but the conductivity of carbon layers formed by traditional carbon sources (such as glucose and sucrose) is limited, and they are prone to material accumulation and prolonged ion transport paths. Graphene has extremely high electronic conductivity and specific surface area, making it an ideal conductive additive. However, simple physical mixing makes it difficult to form a stable three-dimensional conductive network, and graphene is prone to agglomeration, affecting dispersion uniformity.

[0004] Therefore, this invention proposes a lithium iron phosphate composite cathode material with a graphene-supported three-dimensional conductive network and a carbon layer double coating. Summary of the Invention

[0005] This invention provides a carbon-coated lithium iron phosphate / graphene composite cathode material and its preparation method. Through the composite structure design of graphene and lithium iron phosphate and the secondary carbon coating process, this invention achieves efficient synergistic transport of electrons and ions, significantly improving the rate performance and cycle stability of the material.

[0006] A (high-rate) carbon-coated lithium iron phosphate / graphene composite cathode material, which is a core-shell structure composite material, comprising: -Core: A three-dimensional conductive network framework composed of interconnected graphene sheets; - Intermediate layer: Lithium iron phosphate nanoparticles uniformly anchored on the graphene surface, with a particle size of 20-100nm; - Outer shell: An amorphous carbon layer covering the outside of lithium iron phosphate particles, with a carbon layer thickness of 2-10nm.

[0007] The chemical composition of the composite material is: LiFePO4 / graphene@C, wherein the mass content of graphene is 1%-10%, the mass content of the carbon coating layer is 2%-8%, and the balance is lithium iron phosphate.

[0008] The composite material has a porous microsphere structure with a particle size of 5-20 μm and a specific surface area of ​​30-80 m².2 / g.

[0009] This invention also provides a method for preparing the above-mentioned composite cathode material, comprising the following steps: (1) Graphene dispersion and preparation of composite precursor: Graphene oxide was dispersed in a water-ethanol mixed solvent and ultrasonically dispersed evenly; lithium source, iron source, phosphorus source and organic carbon source were added in sequence, stirred and hydrothermally reacted to obtain graphene / lithium iron phosphate precursor composite material. (2) Spray drying granulation: The precursor composite material slurry is spray dried to obtain microspherical precursor powder; (3) High-temperature sintering and carbon coating: The precursor powder is subjected to two-step heat treatment under an inert atmosphere: the first step is to pre-calcine at 300-450℃ for 2-5 hours to remove residual solvent and perform preliminary carbonization; the second step is to sinter at 600-750℃ for 6-12 hours to crystallize lithium iron phosphate, while the organic carbon source is carbonized to form a uniform carbon coating layer. (4) Crushing and sieving: The sintered product is gently crushed and sieved to obtain the target composite material.

[0010] Preferably, the organic carbon source in step (1) is one or more of glucose, citric acid, polyethylene glycol or ascorbic acid; the hydrothermal reaction temperature is 160-200℃ and the time is 6-12 hours.

[0011] Preferably, in step (2), the spray drying inlet temperature is 180-220℃ and the outlet temperature is 80-120℃.

[0012] Preferably, the inert atmosphere in step (3) is argon or nitrogen, and the heating rate is 2-5℃ / min.

[0013] The present invention also provides a lithium-ion battery, wherein the positive electrode comprises the above-mentioned carbon-coated lithium iron phosphate / graphene composite positive electrode material.

[0014] Compared with the prior art, the advantages of the present invention are as follows: (1) Graphene constructs a three-dimensional continuous conductive network, which significantly improves the overall electronic conductivity; (2) Lithium iron phosphate nanoparticles are grown in situ on the surface of graphene, which shortens the lithium ion diffusion path; (3) The secondary carbon layer coating further stabilizes the particle interface and enhances structural stability; (4) The material has a porous microsphere structure, which is conducive to electrolyte penetration and improves reaction kinetics; (5) The preparation process is simple and controllable, and is suitable for large-scale production. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the composite material of the present invention; Figure 2 A scanning electron microscope image of the composite material prepared in Example 1; Figure 3 The discharge curves of the battery assembled from the composite material in Example 1 at different rates are shown. Detailed Implementation

[0016] 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.

[0017] Example 1: Preparation of LiFePO4 / graphene@C (3% graphene, 5% carbon) 100 mg of graphene oxide was dispersed in 200 mL of a water-ethanol mixture (volume ratio 1:1) and sonicated for 2 hours. LiOH·H₂O, FeSO₄·7H₂O, H₃PO₄ (molar ratio Li:Fe:P = 1.05:1:1), and glucose (5% of the theoretical product mass) were added sequentially. After stirring for 2 hours, the mixture was transferred to a hydrothermal reactor and reacted at 180 °C for 10 hours. The product was washed, dried, and redispersed in water, then spray-dried (inlet 200 °C, outlet 100 °C). The resulting powder was pre-calcined at 400 °C for 3 hours under an argon atmosphere, and then sintered at 650 °C for 10 hours with a heating rate of 3 °C / min to obtain a black powder material.

[0018] Example 2: Preparation of LiFePO4 / graphene@C (5% graphene, 3% carbon) The steps are the same as in Example 1, except that the amount of graphene oxide is adjusted to 5%, the amount of glucose is adjusted to 3%, and the sintering temperature is 700℃.

[0019] Comparative example: Traditional carbon-coated lithium iron phosphate (without graphene) The solid-state method was used to mix Li2CO3, FeC2O4·2H2O, NH4H2PO4 and 10wt% glucose by ball milling, and then sintered at 700℃ for 12 hours in argon atmosphere.

[0020] Electrochemical performance testing The materials from Examples 1 and 2, as well as the comparative examples, were mixed with conductive carbon black and PVDF at a mass ratio of 8:1:1 to form a slurry. This slurry was then coated onto aluminum foil and assembled into CR2032 coin cells. The test results are as follows: Material 0.2C discharge specific capacity (mAh / g) 5C discharge specific capacity (mAh / g) 5C / 0.2C capacity retention 1C cycle 500 cycles capacity retention Example 1: Graphene 3%, Carbon 5% 158 142 89.9% 95.2% Example 2: 5% graphene, 3% carbon 155 138 89.0% 94.5% Comparative example: Traditional carbon coating (without graphene) 148 108 73.0% 88.0% Example 3: Preparation of LiFePO4 / graphene@C (1% graphene, 8% carbon) 100 mg of graphene oxide was dispersed in 200 mL of a water-ethanol mixture (volume ratio 1:1) and sonicated for 2 hours. LiOH·H₂O, FeSO₄·7H₂O, H₃PO₄ (molar ratio Li:Fe:P = 1.05:1:1), and glucose (8% of the theoretical product mass) were added sequentially. After stirring for 2 hours, the mixture was transferred to a hydrothermal reactor and reacted at 160 °C for 12 hours. The product was washed, dried, and redispersed in water, then spray-dried (inlet 180 °C, outlet 120 °C). The resulting powder was pre-calcined at 450 °C for 2 hours under an argon atmosphere, then sintered at 750 °C for 6 hours with a heating rate of 2 °C / min to obtain a black powder material.

[0021] Example 4: Preparation of LiFePO4 / graphene@C (10% graphene, 2% carbon) 100 mg of graphene oxide was dispersed in 200 mL of a water-ethanol mixture (volume ratio 1:1) and sonicated for 2 hours. LiOH·H₂O, FeSO₄·7H₂O, H₃PO₄ (molar ratio Li:Fe:P = 1.05:1:1), and glucose (2% of the theoretical product mass) were added sequentially. After stirring for 2 hours, the mixture was transferred to a hydrothermal reactor and reacted at 200 °C for 6 hours. The product was washed, dried, and redispersed in water, then spray-dried (inlet 220 °C, outlet 80 °C). The resulting powder was pre-calcined at 300 °C for 5 hours under a nitrogen atmosphere, and then sintered at 600 °C for 12 hours with a heating rate of 5 °C / min to obtain a black powder material.

[0022] 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 carbon-coated lithium iron phosphate / graphene composite cathode material, characterized in that, It is a core-shell structured composite material, including: Core: A three-dimensional conductive network framework composed of graphene; Intermediate layer: Lithium iron phosphate nanoparticles uniformly anchored on the graphene surface; Outer shell: An amorphous carbon layer covering the outside of lithium iron phosphate particles; The graphene content is 1%-10%, the carbon layer content is 2%-8%, and the remainder is lithium iron phosphate.

2. The carbon-coated lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that, The lithium iron phosphate nanoparticles have a particle size of 20-100 nm; the amorphous carbon layer has a thickness of 2-10 nm.

3. The carbon-coated lithium iron phosphate / graphene composite cathode material according to claim 1, characterized in that, The composite material has a porous microsphere structure with a particle size of 5-20 μm and a specific surface area of ​​30-80 m². 2 / g.

4. A method for preparing the carbon-coated lithium iron phosphate / graphene composite cathode material according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Graphene oxide was dispersed in a water-ethanol mixed solvent and ultrasonically dispersed evenly; lithium source, iron source, phosphorus source and organic carbon source were added in sequence, stirred and hydrothermally reacted to obtain graphene / lithium iron phosphate precursor composite material. (2) Spray dry the precursor composite slurry to obtain microspherical precursor powder; (3) The precursor powder was subjected to two-step heat treatment under an inert atmosphere to obtain the sintered product; (4) The sintered product is gently crushed and sieved to obtain the target composite cathode material.

5. The method for preparing the carbon-coated lithium iron phosphate / graphene composite cathode material according to claim 4, characterized in that, In step (1), the hydrothermal reaction temperature is 160-200℃ and the reaction time is 6-12 hours; the organic carbon source is one or more of glucose, citric acid, polyethylene glycol or ascorbic acid.

6. The method for preparing the carbon-coated lithium iron phosphate / graphene composite cathode material according to claim 4, characterized in that, In step (2), the spray drying inlet temperature is 180-220℃ and the outlet temperature is 80-120℃.

7. The method for preparing the carbon-coated lithium iron phosphate / graphene composite cathode material according to claim 4, characterized in that, In step (3), the first step of the two-step heat treatment is a pre-firing temperature of 300-450℃ for 2-5 hours; the second step is a sintering temperature of 600-750℃ for 6-12 hours; the inert atmosphere is argon or nitrogen, and the heating rate is 2-5℃ / min.

8. A lithium-ion battery, characterized in that, Its positive electrode comprises the carbon-coated lithium iron phosphate / graphene composite positive electrode material as described in any one of claims 1-3.

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