Low specific surface area sodium ion battery hard carbon negative electrode material and preparation method thereof

Abstract

The present invention discloses a low-specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof. The preparation method comprises the following steps: S1, pretreatment: placing an organic precursor in a low-temperature carbonization furnace, and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor; S2, vacuum carbonization: placing the carbon precursor in step S1 in a vacuum atmosphere furnace, and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon; S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace, and performing high-temperature sintering in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material. The low-specific surface area hard carbon negative electrode material for sodium ion batteries of the present invention and the preparation method thereof have the characteristics of low production energy consumption, low cost, small specific surface area, good cycle performance and high first-cycle coulombic efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a hard carbon negative electrode material for sodium ion batteries with a low specific surface area and a preparation method thereof. Background Art

[0002] In recent years, the price of lithium-ion battery raw materials has fluctuated significantly. However, sodium resources are abundant, widely distributed, simple to extract, and inexpensive. Therefore, sodium-ion batteries, as a technological alternative to lithium-ion batteries, have entered a period of rapid industrial development. The anode material of sodium-ion batteries is primarily hard carbon, which largely determines the performance and cost of sodium-ion batteries.

[0003] Hard carbon materials are usually obtained by high-temperature heat treatment of organic precursors in an inert gas atmosphere. During the high-temperature pyrolysis process, the carbon precursor material will undergo a series of complex chemical reactions, accompanied by the rearrangement of the carbon material structure and the volatilization of volatile substances. There are two main types of volatile substances: one is pyrolysis gases such as H2O, CO, CO2, NH3, CH4, and the other is a complex mixture of condensable hydrocarbons, namely tar, the main component of which is larger aromatic substances. Tar is a by-product in the production process of hard carbon materials. On the one hand, it gradually condenses during transportation and easily adheres to the inner wall of the furnace and pipeline, causing equipment corrosion and pipeline blockage, which endangers the safe operation of the equipment; on the other hand, the production of tar reduces the yield of hard carbon, further increasing production costs.

[0004] Currently, the tar waste is usually burned directly by combustion method, which consumes a lot of energy and the exhaust gas generated by combustion causes environmental pollution problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a hard carbon negative electrode material with a low specific surface area for sodium ion batteries, which has the characteristics of low production energy consumption, low cost, small specific surface area, good cycle performance and high first-cycle coulombic efficiency.

[0006] The present invention can be achieved through the following technical solutions:

[0007] The present invention discloses a method for preparing a hard carbon negative electrode material for a sodium ion battery with a low specific surface area, comprising the following steps:

[0008] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0009] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0010] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0011] The preparation method of the present invention can effectively avoid the following defects of the prior art: First, from the equipment level, it can effectively prevent the tar from gradually condensing during pipeline transportation and easily adhering to the furnace and the inner wall of the pipeline, causing equipment corrosion and pipeline blockage, and reducing the harm to the safe operation of the equipment; Second, from the component aspect, combined with the characteristics that the main components of tar are aromatic organic compounds, the production of tar is reduced, the carbonization rate of raw materials is increased, and the increase in production costs is saved; Third, in terms of pollution control, there is no need to use combustion method to treat waste tar, avoiding energy waste and tail gas pollution; Fourth, in terms of hard carbon structure control, reducing tar volatilization leads to the formation of a porous structure on the surface of the hard carbon material, thereby increasing the specific surface area of ​​the hard carbon material, avoiding the display of high irreversible capacity and low first-week coulomb efficiency, thereby affecting the capacity of the sodium ion battery and the cycle life

[0012] Furthermore, in step S1, the pretreatment conditions are: a heating rate of 10-20°C / min, a treatment temperature of 100-600°C, an activation time of 2-5 hours; and the protective gas is nitrogen and / or argon.

[0013] During the pretreatment process in step S1, the organic precursor gradually decomposes to produce volatile gases such as H2O, CO, and CO2. In the present invention, the pretreatment temperature can affect the effectiveness of the present invention. Specifically, if the pretreatment temperature is too high, reaching a temperature at which tar is produced, the objectives of the present invention cannot be achieved. If the pretreatment temperature is too low, the amount of gas generated during the subsequent vacuum carbonization process is excessive, causing the pressure in the vacuum atmosphere furnace to be too high, exceeding the specified value of the equipment and causing damage to the equipment.

[0014] Furthermore, in step S1, the organic precursor is one or more of sawdust powder, walnut shell powder, coffee shell powder, nut shell powder, wheat straw powder, phenolic resin, epoxy resin, furfural resin, glucose, sucrose, and starch.

[0015] Furthermore, in step S2, the vacuum condition is 0.01-0.5 MPa, the carbonization temperature is 700-1000°C, and the carbonization time is 1-5 hours. In the present invention, the vacuum carbonization temperature is the key to the effect of the present invention. Specifically, if the vacuum carbonization temperature is too low, the tar cannot be completely produced in the process, and is then produced in the subsequent high-temperature carbonization process, and the purpose of the present invention cannot be achieved; if the vacuum carbonization temperature is too high, the tar volatilized in the hard carbon material will be cracked in the gas phase and cannot be deposited on the surface of the hard carbon material, and the effect of coating the hard carbon cannot be achieved.

[0016] Furthermore, in step S3, the high-temperature sintering conditions are: a heating rate of 0.5-5°C / min, a carbonization temperature of 1000-1600°C, and a carbonization time of 2-10 hours. In the present invention, the high-temperature sintering temperature also affects the structure and properties of the hard carbon material. Specifically, if the high-temperature sintering temperature is insufficient, the degree of graphitization of the hard carbon material is low, making it unable to effectively store sodium ions. Excessively high carbonization temperatures will result in an excessively narrow interlayer spacing within the hard carbon material, hindering the migration of sodium ions within the hard carbon material and affecting its electrochemical properties.

[0017] Another aspect of the present invention is to protect the hard carbon negative electrode material of the sodium ion battery, which is prepared by the above-mentioned preparation method.

[0018] Another aspect of the present invention is to protect sodium ion batteries. Specifically, the sodium ion battery uses the hard carbon material prepared by the above preparation method as the negative electrode material.

[0019] The present invention provides a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, which has the following beneficial effects:

[0020] From a performance perspective, this invention addresses the drawbacks of conventional hard carbon material production, where tar volatilization leaves behind porous structures, resulting in a large specific surface area, which increases irreversible capacity and reduces initial coulombic efficiency. By decomposing the tar and depositing it on the carbon material, the deposited carbon layer coats the surface, reducing the specific surface area and thereby improving the initial coulombic efficiency of the hard carbon material.

[0021] From the aspect of process feasibility, the process of the present invention is stable, does not produce tar that is easily attached to the furnace and the inner wall of the pipeline, does not cause equipment corrosion and pipeline blockage, and ensures the safe operation of the equipment.

[0022] From the perspective of energy consumption and environmental protection, the present invention reduces production costs by increasing the yield of hard carbon materials; and the method does not require additional energy consumption to process waste tar, further reducing production costs. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention is further described in detail below with reference to embodiments.

[0024] The present invention discloses a method for preparing a hard carbon negative electrode material for a sodium ion battery with a low specific surface area, comprising the following steps:

[0025] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0026] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0027] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0028] Furthermore, in step S1, the pretreatment conditions are: a heating rate of 10-20°C / min, a treatment temperature of 100-600°C, an activation time of 2-5 hours; and the protective gas is nitrogen and / or argon.

[0029] Furthermore, in step S1, the organic precursor is one or more of sawdust powder, walnut shell powder, coffee shell powder, nut shell powder, wheat straw powder, phenolic resin, epoxy resin, furfural resin, glucose, sucrose, and starch.

[0030] Furthermore, in step S2, the vacuum condition is 0.01-0.5 MPa, the carbonization temperature is 700-1000°C, and the carbonization time is 1-5 hours.

[0031] Furthermore, in step S3, the high temperature sintering conditions are: a heating rate of 0.5-5°C / min, a carbonization temperature of 1000-1600°C, and a carbonization time of 2-10 h.

[0032] Another aspect of the present invention is to protect the hard carbon negative electrode material of the sodium ion battery, which is prepared by the above-mentioned preparation method.

[0033] Another aspect of the present invention is to protect sodium ion batteries. Specifically, the sodium ion battery uses the hard carbon material prepared by the above preparation method as the negative electrode material. Example 1

[0034] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0035] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0036] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0037] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0038] In this embodiment, in step S1, the pretreatment conditions are: a heating rate of 20°C / min, a treatment temperature of 350°C, and an activation time of 2 hours; the protective gas is nitrogen and / or argon. The organic precursors are sawdust powder and walnut shell powder.

[0039] In this embodiment, in step S2, the vacuum condition is 0.5 MPa, the carbonization temperature is 850° C., and the carbonization time is 1 hour.

[0040] In this embodiment, in step S3, the high-temperature sintering conditions are: a heating rate of 5°C / min, a carbonization temperature of 1300°C, and a carbonization time of 2 h. Example 2

[0041] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0042] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0043] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0044] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0045] In this embodiment, in step S1, the pretreatment conditions are: a heating rate of 15°C / min, a treatment temperature of 100°C, and an activation time of 5 hours; the protective gas is nitrogen. The organic precursors are coffee hull powder, nut shell powder, and wheat straw powder.

[0046] In this embodiment, in step S2, the vacuum condition is 0.25 MPa, the carbonization temperature is 700° C., and the carbonization time is 5 h.

[0047] In this embodiment, in step S3, the high-temperature sintering conditions are: a heating rate of 3°C / min, a carbonization temperature of 1000°C, and a carbonization time of 10 h. Example 3

[0048] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0049] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0050] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0051] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0052] In this embodiment, in step S1, the pretreatment conditions are: a heating rate of 10°C / min, a treatment temperature of 600°C, and an activation time of 3.5 hours; the protective gas is argon. The organic precursors are sawdust powder, walnut shell powder, phenolic resin, epoxy resin, furfural resin, and glucose.

[0053] In this embodiment, in step S2, the vacuum condition is 0.01 MPa, the carbonization temperature is 1000° C., and the carbonization time is 3 h.

[0054] In this embodiment, in step S3, the high-temperature sintering conditions are: a heating rate of 0.5°C / min, a carbonization temperature of 1600°C, and a carbonization time of 6 h. Example 4

[0055] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0056] S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor;

[0057] S2, vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon;

[0058] S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

[0059] In this embodiment, in step S1, the pretreatment conditions are: a heating rate of 15°C / min, a treatment temperature of 400°C, and an activation time of 4 hours; the protective gases are nitrogen and argon. The organic precursors are epoxy resin, furfural resin, glucose, sucrose, and starch.

[0060] In this embodiment, in step S2, the vacuum condition is 0.3 MPa, the carbonization temperature is 850° C., and the carbonization time is 3 h.

[0061] In this embodiment, in step S3, the high-temperature sintering conditions are: a heating rate of 3°C / min, a carbonization temperature of 1300°C, and a carbonization time of 6 h.

[0062] Application Example 1

[0063] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0064] S1. Pretreatment: Place the walnut shell powder in a low-temperature furnace, heat it to 400°C at a heating rate of 15°C / min in a nitrogen atmosphere, and keep it at this temperature for 2 hours to obtain a carbon precursor.

[0065] S2. Vacuum carbonization: The carbon precursor prepared in step S1 is placed in a vacuum furnace. The vacuum furnace is heated to 800° C. at a vacuum degree of 0.3 MPa and kept at this temperature for 2 h to obtain vacuum carbonized carbon.

[0066] S3. High-temperature carbonization: The vacuum carbonized carbon obtained in step S2 is placed in a high-temperature carbonization furnace, and the temperature is increased to 1300°C at a heating rate of 2°C / min in a nitrogen atmosphere, and kept at this temperature for 3 h to obtain a hard carbon material.

[0067] The electrochemical performance of the resulting material was tested as follows: Hard carbon material, Super P, CMC, and SBR were mixed into a slurry at a mass ratio of 94:1.5:2:2.5. The black slurry was then coated onto copper foil using a 120 μm four-sided film prep machine. The film was then dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6 mm using a sheet puncher. CR2016 coin cells were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator. These cells were subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 300 mAh / g) over a voltage range of 2–0.005 V.

[0068] Application Example 2

[0069] This embodiment discloses a low specific surface area hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0070] S1. Pretreatment: Place the phenolic resin in a low-temperature furnace, heat it to 500°C at a heating rate of 15°C / min in a nitrogen atmosphere, and keep it at this temperature for 2 hours to obtain a carbon precursor.

[0071] S2. Vacuum carbonization: The carbon precursor prepared in step S1 is placed in a vacuum furnace. The vacuum furnace is heated to 700° C. at a vacuum degree of 0.3 MPa and kept at this temperature for 2 h to obtain vacuum carbonized carbon.

[0072] S3. High-temperature carbonization: The vacuum carbonized carbon obtained in step S2 is placed in a high-temperature carbonization furnace, and the temperature is increased to 1400°C at a heating rate of 1°C / min in a nitrogen atmosphere, and kept at this temperature for 3 hours to obtain a hard carbon material.

[0073] The electrochemical performance of the resulting material was tested as follows: Hard carbon material, Super P, CMC, and SBR were mixed into a slurry at a mass ratio of 94:1.5:2:2.5. The black slurry was then coated onto copper foil using a 120 μm four-sided film prep machine. The film was then dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6 mm using a sheet puncher. CR2016 coin cells were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator. These cells were subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 300 mAh / g) over a voltage range of 2–0.005 V. Comparative Example 1

[0074] This embodiment discloses a hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0075] S1. Pretreatment: Place the walnut shell powder in a low-temperature furnace, heat it to 400°C at a heating rate of 15°C / min in a nitrogen atmosphere, and keep it at this temperature for 2 hours to obtain a carbon precursor.

[0076] S2. High-temperature carbonization: The carbon precursor obtained in step S1 is placed in a high-temperature carbonization furnace, heated to 1300° C. at a heating rate of 2° C. / min in a nitrogen atmosphere, and kept at this temperature for 3 h to obtain a hard carbon material.

[0077] The electrochemical performance of the resulting material was tested as follows: Hard carbon material, Super P, CMC, and SBR were mixed into a slurry at a mass ratio of 94:1.5:2:2.5. The black slurry was then coated onto copper foil using a 120 μm four-sided film prep machine. The film was then dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6 mm using a sheet puncher. CR2016 coin cells were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator. These cells were subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 300 mAh / g) over a voltage range of 2–0.005 V. Comparative Example 2

[0078] This embodiment discloses a hard carbon negative electrode material for sodium ion batteries and a preparation method thereof, wherein the preparation method comprises the following steps:

[0079] S1. Pretreatment: Place the phenolic resin in a low-temperature furnace, heat it to 500°C at a heating rate of 15°C / min in a nitrogen atmosphere, and keep it at this temperature for 2 hours to obtain a carbon precursor.

[0080] S2. High-temperature carbonization: The carbon precursor obtained in step S12 is placed in a high-temperature carbonization furnace, and the temperature is increased to 1400° C. at a heating rate of 1° C. / min in a nitrogen atmosphere, and kept at this temperature for 3 h to obtain a hard carbon material.

[0081] The electrochemical performance of the resulting material was tested as follows: Hard carbon material, Super P, CMC, and SBR were mixed into a slurry at a mass ratio of 94:1.5:2:2.5. The black slurry was then coated onto copper foil using a 120 μm four-sided film prep machine. The film was then dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6 mm using a sheet puncher. CR2016 coin cells were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator. These cells were subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 300 mAh / g) over a voltage range of 2–0.005 V.

[0082] Table 1 Performance test results

[0083]

[0084] As shown in Table 1, the yields of carbon materials in Application Example 1 and Comparative Example 1 were 25% and 19%, respectively. The specific surface areas of Application Example 1 and Comparative Example 1 measured by gas adsorption and desorption analyzer were 3.2 and 9.3 m 2 / g. The reversible specific capacity of the electrode in Example 1 was 312 mAh / g, and the first-cycle coulombic efficiency was 92%; the reversible specific capacity of the electrode in Comparative Example 1 was only 302 mAh / g, and the first-cycle coulombic efficiency was only 87%. The essential reason for the improved electrochemical performance of Example 1 is that the tar generated during the production process is deposited on the surface of the hard carbon material. The deposited carbon layer acts as a surface coating, reducing the specific surface area of ​​the hard carbon material and increasing its yield.

[0085] As shown in Table 1, the yields of carbon materials in Application Example 2 and Comparative Example 2 were 63% and 46%, respectively. The specific surface areas of Application Example 2 and Comparative Example 2 measured by the gas adsorption and desorption analyzer were 2.5 and 10.4 m2 / g, respectively. The reversible specific capacity of the electrode in Example 2 was 325 mAh / g, and the coulombic efficiency in the first week was 93%; the reversible specific capacity of the electrode in Comparative Example 2 was only 317 mAh / g, and the coulombic efficiency in the first week was only 90%. The essential reason for the improvement in the electrochemical performance of Application Example 2 is that the tar generated in the production process is deposited on the surface of the hard carbon material. The deposited carbon layer acts as a surface coating layer, which reduces the specific surface area of ​​the hard carbon material and improves its yield.

[0086] The above embodiments are merely specific embodiments of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the scope of the present invention, and such obvious alternatives are all within the scope of protection of the present invention.

Claims

1. A method for preparing a low specific surface area hard carbon negative electrode material for sodium ion batteries, characterized in that The following steps are involved: S1. Pretreatment: placing the organic precursor in a low-temperature carbonization furnace and performing low-temperature sintering pretreatment in a fluid protective gas atmosphere to obtain a carbon precursor; S2. Vacuum carbonization: placing the carbon precursor of step S1 in a vacuum atmosphere furnace and performing vacuum sintering under vacuum conditions to obtain vacuum carbonized carbon. The vacuum conditions are 0.3-0.5 MPa, the carbonization temperature is 700-1000°C, and the carbonization time is 1-5 hours. S3, high-temperature carbonization: placing the vacuum carbonized carbon obtained in step S2 in a high-temperature carbonization furnace and sintering it at high temperature in a fluid protective gas atmosphere to obtain a sodium ion battery negative electrode hard carbon material.

2. The method for preparing a low specific surface area hard carbon negative electrode material for sodium ion batteries according to claim 1, wherein: In step S1, the pretreatment conditions are: heating rate of 10-20°C / min, treatment temperature of 100-600°C, activation time of 2-5 hours; the protective gas is nitrogen and / or argon.

3. The method for preparing a hard carbon negative electrode material for sodium ion batteries with a low specific surface area according to claim 1, wherein: In step S1 , the organic precursor is one or more of sawdust powder, walnut shell powder, coffee shell powder, nut shell powder, wheat straw powder, phenolic resin, epoxy resin, furfural resin, glucose, sucrose, and starch.

4. The method for preparing a low specific surface area hard carbon negative electrode material for sodium ion batteries according to claim 3, wherein: In step S3, the high-temperature sintering conditions are: a heating rate of 0.5-5°C / min, a carbonization temperature of 1000-1600°C, and a carbonization time of 2-10 h.

5. A hard carbon negative electrode material for a sodium ion battery, characterized in that: The invention is prepared by the preparation method according to any one of claims 1 to 4.

6. A sodium ion battery, characterized in that: The hard carbon material is prepared as the negative electrode material by the preparation method according to any one of claims 1 to 4.

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