Hard carbon negative electrode material, preparation method thereof and use thereof in sodium ion battery

Abstract

The application provides a hard carbon negative electrode material, a preparation method thereof and an application thereof in a sodium ion battery. The application first improves the density of a hard carbon precursor 1 through a hot-pressing densification treatment, so that the obtained hard carbon precursor 1 has a lower specific surface area; then sulfonated graphene is uniformly and closely coated on the surface of the hard carbon precursor 1 by means of carboxyl and sulfonic acid groups; subsequently, in a pre-carbonization treatment process, the sulfonated graphene coating layer induces the hard carbon precursor to grow along the graphene layer, and a more regular and ordered graphite-like layer microcrystalline structure layer is generated on the surface of the hard carbon precursor; finally, in a high-temperature carbonization treatment process, the carboxyl and sulfonic acid groups on the surface of the sulfonated graphene are reduced by organic acid, graphene is obtained, meanwhile, the defects on the surface of the graphene are reduced, and the first coulomb efficiency and the cycle performance of the hard carbon negative electrode material are improved.

Description

Technical Field

[0001] This invention relates to the field of hard carbon anode material technology, specifically to a hard carbon anode material, its preparation method, and its use in sodium-ion batteries. Background Technology

[0002] With the rapid development of renewable energy, energy storage technology is playing an increasingly prominent role in modern energy systems. Among various energy storage devices, sodium has similar physicochemical properties to lithium, and its abundant and inexpensive resources make sodium-ion batteries an ideal choice for next-generation large-scale energy storage applications. Hard carbon materials have high sodium storage capacity and, due to their low plateau voltage and good cycle stability, are commonly used anode materials for sodium-ion batteries. However, the initial coulombic efficiency and cycle performance of currently used hard carbon anode materials are relatively low.

[0003] Currently, a common approach is to coat hard carbon with asphalt and then carbonize it to improve the initial coulombic efficiency and cycle performance of hard carbon anode materials. For example, most commercially available hard carbon anode materials are produced by mechanically mixing hard carbon and solid asphalt powder, followed by carbonization to obtain amorphous carbon-coated hard carbon. Research has found that the existing asphalt-coated amorphous carbon layer is relatively thick and uneven, easily reducing the compaction density of the hard carbon anode material. Furthermore, the reversible capacity of the surface-coated amorphous carbon is low, leading to capacity loss in the hard carbon anode material. Therefore, further research is needed to improve the initial coulombic efficiency and cycle performance of hard carbon anode materials without reducing their compaction density. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention provides a hard carbon anode material, its preparation method, and its application in sodium-ion batteries. The hard carbon anode material exhibits high initial coulombic efficiency and cycle performance, while also achieving a relatively high compaction density.

[0005] The objective of this invention is achieved through the following technical solution: A method for preparing a hard carbon anode material, the method comprising the following steps: (1) The biomass raw material is placed on a hot press molding machine for hot pressing densification treatment, cooled and crushed to obtain hard carbon precursor 1; (2) Sulfonated graphene was mixed with water and ultrasonically dispersed to obtain a sulfonated graphene dispersion; (3) Add the hard carbon precursor 1 from step (1) to the sulfonated graphene dispersion from step (2), stir, filter, wash and dry to obtain hard carbon precursor 2. (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace for pre-carbonization treatment. After cooling to room temperature, hard carbon precursor 3 is obtained. (5) The hard carbon precursor 3 from step (4) is soaked in an acid solution, washed and dried to obtain hard carbon precursor 4. (6) After the hard carbon precursor 4 from step (5) is mixed evenly with organic acid, it is placed in a high-temperature furnace for high-temperature carbonization treatment. After cooling to room temperature, the hard carbon anode material is obtained.

[0006] According to an embodiment of the present invention, in step (1), the biomass raw material includes at least one of lychee wood, apple wood, poplar wood, bamboo, fruit shell, straw, coconut shell and corn cob.

[0007] According to an embodiment of the present invention, in step (1), the temperature of the hot pressing densification treatment is 150-220°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C or 220°C; the pressure of the hot pressing densification treatment is 4-7 MPa, for example, 4 MPa, 4.5 MPa, 5 MPa, 5.5 MPa, 6 MPa, 6.5 MPa or 7 MPa; the time of the hot pressing densification treatment is 8-15 min, for example, 8 min, 9 min, 10 min, 11 min, 12 min, 13 min, 14 min or 15 min.

[0008] According to an embodiment of the present invention, in step (1), the purpose of the hot pressing densification treatment is to increase the density of the hard carbon precursor 1, so that the obtained hard carbon precursor 1 has a lower specific surface area.

[0009] According to an embodiment of the present invention, in step (1), the pulverization method is not specifically defined, as long as a hard carbon precursor 1 with a suitable particle size can be obtained; for example, the average particle size of the hard carbon precursor 1 is 5-10µm.

[0010] According to an embodiment of the present invention, in step (2), the source of the sulfonated graphene is not specifically defined. It can be prepared by methods known in the art or obtained through commercial purchase.

[0011] According to an embodiment of the present invention, in step (2), the surface of the sulfonated graphene contains carboxyl groups and sulfonic acid groups, and the carboxyl groups and sulfonic acid groups can be used to tightly and uniformly coat the surface of the hard carbon precursor 1 with the sulfonated graphene.

[0012] According to an embodiment of the present invention, in step (2), the mass of sulfonated graphene added per milliliter of water is 1.0-1.5 mg; for example, 1.0 mg, 1.1 mg, 1.2 mg, 1.3 mg, 1.4 mg or 1.5 mg.

[0013] According to an embodiment of the present invention, in step (2), the power and time of ultrasonic dispersion are not particularly limited, as long as the uniform dispersion of sulfonated graphene can be ensured. For example, sulfonated graphene is mixed with water and ultrasonically dispersed for 1-10 hours to obtain a sulfonated graphene dispersion.

[0014] According to an embodiment of the present invention, in step (3), the mass ratio of the sulfonated graphene to the hard carbon precursor 1 is 0.5-3:100; for example, 0.5:100, 1:100, 1.5:100, 2:100, 2.5:100 or 3:100.

[0015] According to an embodiment of the present invention, in step (3), the stirring time is 0.5-12 hours; the stirring temperature is room temperature. During the stirring process, the carboxyl and sulfonic acid groups on the surface of sulfonated graphene can be used to tightly and uniformly coat the surface of the hard carbon precursor 1.

[0016] According to an embodiment of the present invention, in step (3), the washing is performed with deionized water; the drying is performed by freeze drying or spray drying.

[0017] According to an embodiment of the present invention, in step (4), the pre-carbonization treatment is carried out under a protective atmosphere, such as nitrogen or argon. The temperature of the pre-carbonization treatment is 500-800°C, for example, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, or 800°C; the time of the pre-carbonization treatment is 1-12 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, or 8 hours.

[0018] According to an embodiment of the present invention, in step (4), during the pre-carbonization process, the sulfonated graphene coating induces the hard carbon precursor to grow along the graphene layer, and a more regular and ordered graphite-like microcrystalline structure layer is generated on the surface of the hard carbon precursor, further reducing the specific surface area of ​​the hard carbon precursor.

[0019] According to an embodiment of the present invention, in step (5), the acid solution is at least one of hydrochloric acid aqueous solution, sulfuric acid aqueous solution, nitric acid aqueous solution and hydrofluoric acid aqueous solution.

[0020] According to an embodiment of the present invention, in step (5), the pH value of the acid solution is 1-4, for example, 1, 1.5, 2, 2.5, 3, 3.5 or 4.

[0021] According to an embodiment of the present invention, in step (5), the soaking time is 0.5-6 hours; the soaking temperature is room temperature; and the soaking is carried out under stirring conditions.

[0022] According to an embodiment of the present invention, in step (5), the purpose of soaking is to remove impurities and ash from the hard carbon precursor 3.

[0023] According to an embodiment of the present invention, in step (5), the washing is performed with deionized water; the drying is performed in an oven at 80-100°C for 4-12 hours.

[0024] According to an embodiment of the present invention, in step (6), the organic acid is selected from at least one of oxalic acid, malic acid, citric acid, tartaric acid and ascorbic acid.

[0025] According to a preferred embodiment of the present invention, in step (6), the mass ratio of the organic acid to the hard carbon precursor 4 is 2-6:100; for example, 2:100, 3:100, 4:100, 5:100 or 6:100.

[0026] According to a preferred embodiment of the present invention, in step (6), the high-temperature carbonization treatment is carried out under a protective atmosphere, such as nitrogen or argon. The temperature of the high-temperature carbonization treatment is 1000-1400℃, for example, 1000℃, 1050℃, 1100℃, 1150℃, 1200℃, 1250℃, 1300℃, 1350℃ or 1400℃; the time of the high-temperature carbonization treatment is 1-12 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours or 8 hours.

[0027] According to a preferred embodiment of the present invention, in step (6), during the high-temperature carbonization process, organic acids reduce the carboxyl groups and sulfonic acid groups on the surface of sulfonated graphene, so that the non-carbon atoms (such as carboxyl groups and sulfonic acid groups) on the sulfonated graphene are completely removed to obtain graphene, while reducing the defects on the graphene surface and improving the first coulombic efficiency of the hard carbon anode material.

[0028] The present invention also provides a hard carbon anode material prepared by the above method.

[0029] According to an embodiment of the present invention, the hard carbon anode material has a core-shell structure, including a core and a shell; the shell is graphene, and the core is hard carbon.

[0030] According to an embodiment of the present invention, the thickness of the shell layer is 2nm-10nm, for example, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm.

[0031] According to an embodiment of the present invention, the average particle size of the hard carbon anode material is 5-10µm.

[0032] The present invention also provides the use of the above-mentioned hard carbon anode material for use as the anode of sodium-ion batteries.

[0033] The beneficial effects of this invention are: This invention first increases the density of the hard carbon precursor 1 through hot-pressing densification, resulting in a lower specific surface area. Then, sulfonated graphene is uniformly and tightly coated onto the surface of the hard carbon precursor 1 by carboxyl and sulfonic acid groups. Subsequently, during pre-carbonization, the sulfonated graphene coating induces the hard carbon precursor to grow along the graphene layers, forming a more regular and ordered graphite-like microcrystalline structure on the surface of the hard carbon precursor, further reducing its specific surface area. Finally, during high-temperature carbonization, organic acids reduce the carboxyl and sulfonic acid groups on the surface of the sulfonated graphene, completely removing non-carbon atoms (such as carboxyl and sulfonic acid groups) to obtain graphene. This process also reduces surface defects and improves the initial coulombic efficiency of the hard carbon anode material. The graphene possesses high elasticity and high stiffness, which can also mitigate the volume expansion of the hard carbon anode material during cycling, effectively improving its cycling performance. Furthermore, since the thickness of the graphene coating is only 2-10 nm, it will not reduce the compaction density of the hard carbon anode material. Detailed Implementation

[0034] The preparation method of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0035] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0036] Example 1 (1) Place 1000g of coconut shell on a hot press molding machine and perform hot pressing densification treatment (temperature is 180℃, pressure is 6MPa, hot pressing time is 15min). After cooling, crush to obtain hard carbon precursor 1. (2) Mix 1g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 5 hours to obtain a sulfonated graphene dispersion; (3) Add 100g of hard carbon precursor 1 to the sulfonated graphene dispersion in step (2), stir, filter, wash, freeze dry to obtain hard carbon precursor 2; (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace and heated to 700°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 3. (5) Immerse the hard carbon precursor 3 from step (4) in hydrochloric acid solution (pH 1.5) and stir for 2 hours. Then wash it with deionized water 3 times until the pH of the filtrate is 6.8. After filtration, dry it in an oven at 100°C for 6 hours to obtain hard carbon precursor 4. (6) Mix 100g of the hard carbon precursor 4 from step (5) with 2g of oxalic acid, place it in a high-temperature furnace, heat it to 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at that temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0037] Example 2 (1) Place 1000g of fruit shells on a hot press molding machine and perform hot pressing densification treatment (temperature is 200℃, pressure is 4MPa, hot pressing time is 10min). After cooling, crush to obtain hard carbon precursor 1. (2) Mix 1.2g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 5 hours to obtain a sulfonated graphene dispersion; (3) Add 100g of hard carbon precursor 1 to the sulfonated graphene dispersion in step (2), stir, filter, wash, freeze dry to obtain hard carbon precursor 2; (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace and heated to 750°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 3. (5) Immerse the hard carbon precursor 3 from step (4) in hydrochloric acid solution (pH 2) and stir for 4 hours. Then wash it 3 times with deionized water until the pH of the filtrate is 6.7. After filtration, dry it in an oven at 100°C for 6 hours to obtain hard carbon precursor 4. (6) Mix 100g of the hard carbon precursor 4 from step (5) with 3g of citric acid, place it in a high-temperature furnace, heat it to 1280°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at that temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0038] Example 3 (1) Place 1000g of coconut shell on a hot press molding machine and perform hot pressing densification treatment (temperature is 180℃, pressure is 5MPa, hot pressing time is 15min). After cooling, crush to obtain hard carbon precursor 1. (2) Mix 1.3g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 5 hours to obtain a sulfonated graphene dispersion; (3) Add 100g of hard carbon precursor 1 to the sulfonated graphene dispersion in step (2), stir, filter, wash, freeze dry to obtain hard carbon precursor 2; (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace and heated to 700°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 3. (5) Immerse the hard carbon precursor 3 from step (4) in hydrochloric acid solution (pH 1.5) and stir for 2 hours. Then wash it with deionized water 3 times until the pH of the filtrate is 6.8. After filtration, dry it in an oven at 100°C for 6 hours to obtain hard carbon precursor 4. (6) Mix 100g of the hard carbon precursor 4 from step (5) with 4g of oxalic acid, place it in a high-temperature furnace, heat it to 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at the temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0039] Example 4 (1) Place 1000g of coconut shell on a hot press molding machine and perform hot pressing densification treatment (temperature is 160℃, pressure is 6MPa, hot pressing time is 15min). After cooling, crush to obtain hard carbon precursor 1. (2) Mix 1.4g of sulfonated graphene with 1 liter of water and ultrasonically disperse for 8 hours to obtain a sulfonated graphene dispersion; (3) Add 100g of hard carbon precursor 1 to the sulfonated graphene dispersion in step (2), stir, filter, wash, freeze dry to obtain hard carbon precursor 2; (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace and heated to 700°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 3. (5) Immerse the hard carbon precursor 3 from step (4) in hydrochloric acid solution (pH 1.5) and stir for 2 hours. Then wash it with deionized water 3 times until the pH of the filtrate is 6.8. After filtration, dry it in an oven at 100°C for 6 hours to obtain hard carbon precursor 4. (6) Mix 100g of the hard carbon precursor 4 from step (5) with 4g of oxalic acid, place it in a high-temperature furnace, heat it to 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at the temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0040] Comparative Example 1 (1) Place 1000g of coconut shell in a high-temperature furnace, heat it to 180°C in air, keep it at that temperature for 2 hours, cool it down and then crush it to obtain hard carbon precursor 1; (2) The hard carbon precursor 1 from step (1) is placed in a high-temperature furnace and heated to 700°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 2. (3) The hard carbon precursor 2 from step (2) was immersed in hydrochloric acid solution (pH 1.5) and stirred for 2 hours. Then it was washed 3 times with deionized water until the pH of the filtrate was 6.8. After filtration, it was dried in an oven at 100°C for 6 hours to obtain hard carbon precursor 3. (4) Place 100g of the hard carbon precursor 3 from step (3) in a high-temperature furnace, heat it to 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at that temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0041] Comparative Example 2 (1) Place 1000g of coconut shell in a high-temperature furnace, heat it to 180°C in air, keep it at that temperature for 2 hours, cool it down and then crush it to obtain hard carbon precursor 1; (2) The hard carbon precursor 1 from step (1) is placed in a high-temperature furnace and heated to 700°C at a heating rate of 5°C / min in a nitrogen atmosphere. After holding at the temperature for 2 hours in a nitrogen atmosphere, it is cooled to room temperature to obtain hard carbon precursor 2. (3) The hard carbon precursor 2 from step (2) was immersed in hydrochloric acid solution (pH 1.5) and stirred for 2 hours. Then it was washed 3 times with deionized water until the pH of the filtrate was 6.8. After filtration, it was dried in an oven at 100°C for 6 hours to obtain hard carbon precursor 3. (4) Mix 100g of the hard carbon precursor 3 from step (3) with 2g of asphalt evenly, place it in a high-temperature furnace, heat it to 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere, and keep it at the temperature for 4 hours in a nitrogen atmosphere before cooling it to room temperature to obtain the hard carbon anode material.

[0042] Electrochemical performance testing The hard carbon anode materials prepared in Examples 1-4 and Comparative Examples 1-2 were used as anode materials for sodium-ion batteries, and their electrochemical performance was tested using the following methods: The hard carbon anode material, conductive agent Super P, binder sodium carboxymethyl cellulose (CMC), and aqueous binder (SBR) of the above examples were weighed in a mass ratio of 95:2:1.5:1.5. After thorough grinding in an agate mortar, a small amount of deionized water was added to form a uniform black paste. This paste was coated onto a copper foil current collector as a test electrode, and a sodium metal sheet was used as a control electrode to assemble a coin cell. The electrolyte was a 1M sodium hexafluorophosphate solution dissolved in a 1:1 volume ratio mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), with 5 wt.% fluoroethylene carbonate (FEC) added as an additive. A glass fiber separator and a CR2032 stainless steel casing were used to assemble the coin cell. The resulting coin cells were tested for initial discharge capacity and initial coulombic efficiency at a constant rate of 0.1C within a voltage range of 0.01-3.0V.

[0043] The hard carbon anode materials prepared in Examples 1-4 and Comparative Examples 1-2 were used as anodes, sodium nickel iron manganese oxide as cathodes, and 1M NaPF6+EC:DEC:DMC (volume ratio 1:1:1) solution as electrolyte. The cells were assembled into full cells in a stacked manner and tested at room temperature at a 1C rate. The voltage range was 1.5-3.9V.

[0044] Table 1. Electrochemical performance of hard carbon anode materials in Examples 1-4 and Comparative Examples 1-2

[0045] As can be seen from the test results in Table 1, the hard carbon anode material of the present invention has high initial coulombic efficiency and cycle performance, while also maintaining high compaction density.

[0046] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a hard carbon anode material, wherein, The method includes the following steps: (1) The biomass raw material is placed on a hot press molding machine for hot pressing densification treatment, cooled and crushed to obtain hard carbon precursor 1; (2) Sulfonated graphene was mixed with water and ultrasonically dispersed to obtain a sulfonated graphene dispersion; (3) Add the hard carbon precursor 1 from step (1) to the sulfonated graphene dispersion from step (2), stir, filter, wash and dry to obtain hard carbon precursor 2. (4) The hard carbon precursor 2 from step (3) is placed in a high-temperature furnace for pre-carbonization treatment. After cooling to room temperature, hard carbon precursor 3 is obtained. (5) The hard carbon precursor 3 from step (4) is soaked in an acid solution, washed and dried to obtain hard carbon precursor 4. (6) After the hard carbon precursor 4 from step (5) is mixed evenly with organic acid, it is placed in a high-temperature furnace for high-temperature carbonization treatment. After cooling to room temperature, the hard carbon anode material is obtained.

2. The method for preparing the hard carbon anode material according to claim 1, wherein, In step (1), the biomass raw materials include at least one of lychee wood, apple wood, poplar wood, bamboo, fruit shells, straw, coconut shells and corn cobs; And / or, in step (1), the temperature of the hot pressing densification treatment is 150-220℃; the pressure of the hot pressing densification treatment is 4-7MPa; and the time of the hot pressing densification treatment is 8-15min. And / or, in step (1), the average particle size of the hard carbon precursor 1 is 5-10µm.

3. The method for preparing the hard carbon anode material according to claim 1 or 2, wherein, In step (2), the mass of sulfonated graphene added per milliliter of water is 1.0-1.5 mg; And / or, in step (2), sulfonated graphene is mixed with water and ultrasonically dispersed for 1-10 hours to obtain a sulfonated graphene dispersion.

4. The method for preparing the hard carbon anode material according to any one of claims 1-3, wherein, In step (3), the mass ratio of the sulfonated graphene to the hard carbon precursor 1 is 0.5-3:100; And / or, in step (3), the stirring time is 0.5-12 hours; the stirring temperature is room temperature.

5. The method for preparing the hard carbon anode material according to any one of claims 1-4, wherein, In step (4), the pre-carbonization treatment is carried out under a protective atmosphere, which is nitrogen or argon; the temperature of the pre-carbonization treatment is 500-800℃; and the time of the pre-carbonization treatment is 1-12 hours.

6. The method for preparing the hard carbon anode material according to any one of claims 1-5, wherein, In step (5), the acid solution is at least one of hydrochloric acid aqueous solution, sulfuric acid aqueous solution, nitric acid aqueous solution and hydrofluoric acid aqueous solution; And / or, in step (5), the pH value of the acid solution is 1-4; And / or, in step (5), the soaking time is 0.5-6 hours; the soaking temperature is room temperature; the soaking is carried out under stirring conditions.

7. The method for preparing the hard carbon anode material according to any one of claims 1-6, wherein, In step (6), the organic acid is selected from at least one of oxalic acid, malic acid, citric acid, tartaric acid and ascorbic acid; And / or, in step (6), the mass ratio of the organic acid to the hard carbon precursor 4 is 2-6:100; And / or, in step (6), the high-temperature carbonization treatment is carried out under a protective atmosphere, which is nitrogen or argon; the temperature of the high-temperature carbonization treatment is 1000-1400℃; and the time of the high-temperature carbonization treatment is 1-12 hours.

8. A hard carbon anode material prepared by the method according to any one of claims 1-7.

9. The hard carbon anode material according to claim 8, wherein, The hard carbon anode material has a core-shell structure, including a core and a shell; the shell is graphene, and the core is hard carbon. Preferably, the thickness of the shell layer is 2nm-10nm; Preferably, the hard carbon anode material has an average particle size of 5-10µm.

10. Use of the hard carbon anode material according to claim 8 or 9, for use as the anode of a sodium-ion battery.

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