Preparation method of closed microporous structure sodium ion battery negative electrode hard carbon material

Abstract

The application discloses a preparation method of a closed micropore structure sodium ion battery negative electrode hard carbon material, and comprises the following steps: S1, preparation of a precursor mixed solution: biomass raw material powder and an inorganic acid pore former are used to prepare the precursor mixed solution; S2, carbon precursor drying: the precursor mixed solution is dried to prepare the carbon precursor; S3, pore forming of a closed pore carbon intermediate: the carbon precursor is subjected to high-temperature activation reaction to obtain the closed pore carbon intermediate with a micropore structure after activation; S4, closed pore carbon intermediate refining treatment: the carbon intermediate is crushed and refined, and the refined closed pore carbon intermediate is obtained through sieving; and S5, high-temperature carbonization of the hard carbon material: the refined carbon intermediate obtained in the step S4 is subjected to high-temperature carbonization to obtain the hard carbon material with the closed micropore structure. The application has the characteristics of a green and environment-friendly process, a small specific surface area and excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing a closed microporous structure sodium-ion battery negative electrode hard carbon material. Background Technology

[0002] Sodium-ion batteries are rechargeable batteries similar to lithium-ion batteries, and they have the advantages of high safety, low cost, and long life.

[0003] The negative electrode material is a crucial component of sodium-ion batteries, largely determining their overall performance. Among numerous sodium-storage negative electrode materials, hard carbon materials offer advantages such as wide availability of raw materials, low cost, high sodium storage capacity, and low sodium storage potential, making them the optimal choice for sodium-ion battery negative electrode materials.

[0004] Hard carbon materials are typical amorphous materials, characterized by small and randomly arranged graphite crystallites that form numerous microporous structures. These micropores can be categorized into two types: open pores and closed pores. Open pores increase the material's specific surface area, leading to severe irreversible electrolyte decomposition and reduced first-cycle coulombic efficiency, which is detrimental to practical applications. Closed pores, on the other hand, are closed to both gas and electrolyte, thus not increasing the specific surface area and not affecting the first-cycle coulombic efficiency. Constructing closed pores in hard carbon materials is an effective method to improve their low-potential plateau capacity.

[0005] Currently, porous carbon is generally prepared by adding metal salt pore-forming agents (such as ZnCl, KOH, NaOH, etc.) to the precursor. However, this process introduces metal impurities into the hard carbon material, requiring subsequent acid removal. The acid washing process generates a large amount of polluting gases and is costly. Furthermore, this process often introduces open pores into the hard carbon material. Open pores have almost no sodium storage capacity and result in an excessively large specific surface area, high irreversible capacity, and low first-cycle coulomb efficiency, which does not meet the requirements of practical applications. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a closed microporous structure sodium-ion battery anode hard carbon material, which has the characteristics of green and environmentally friendly process, small specific surface area and excellent electrochemical performance.

[0007] This invention can be achieved through the following technical solutions:

[0008] This invention discloses a method for preparing a closed-pore microporous structure sodium-ion battery anode hard carbon material, comprising the following steps:

[0009] S1. Preparation of precursor mixed solution: Biomass raw material powder and inorganic acid pore-forming agent are added to deionized water and heated and stirred to obtain precursor mixed solution;

[0010] S2. Drying of carbon precursor: Dry the precursor mixture obtained in step S1 to obtain carbon precursor.

[0011] S3. Activation and pore-forming of closed-cell carbon intermediate: The carbon precursor obtained in step S2 is subjected to a high-temperature activation reaction to obtain a closed-cell carbon intermediate with a microporous structure after activation.

[0012] S4. Refining of closed-cell carbon intermediate: The carbon intermediate obtained in step S3 is crushed and refined, and then sieved to obtain the refined closed-cell carbon intermediate.

[0013] S5. High-temperature carbonization of hard carbon materials: The refined carbon intermediate obtained in step S4 is subjected to high-temperature carbonization to obtain hard carbon materials with closed microporous structures.

[0014] The charge-discharge curve of hard carbon materials consists of a high-potential ramp region and a low-potential plateau; the latter, due to its lower operating potential, plays a decisive role in the energy density of sodium-ion batteries. Constructing closed pores in hard carbon materials is an effective method to improve their low-potential plateau capacity. This invention utilizes an inorganic acid pore-forming agent to activate the hard carbon precursor at a relatively low temperature to obtain a carbon intermediate with a rich microporous structure. Subsequently, the material is carbonized at high temperature, causing the open pores of the intermediate to gradually close and transform into closed pores. Furthermore, the non-metallic elements in the inorganic acid pore-forming agent are incorporated into the hard carbon material framework, aiming to obtain a low-cost, high-energy-density sodium-ion battery anode material.

[0015] Further, in step S1, the biomass is one or more of sawdust, walnut shells, coffee shells, nut shells, wheat straw, sugarcane bagasse, bamboo and / or straw; the inorganic acid pore-forming agent is one or more of phosphoric acid, sulfuric acid and / or nitric acid; and the amount of pore-forming agent added is 1 to 20% of the biomass added by mass.

[0016] Furthermore, in step S1, the heating and stirring temperature is 25–90°C, and the stirring time is 1–5 hours.

[0017] Furthermore, in step S2, the drying temperature is 80-150℃, and the drying time is 5-20 hours.

[0018] Furthermore, in step S3, the conditions for the high-temperature activation reaction are: heating rate of 2-10℃ / min, activation temperature of 400-700℃, and activation time of 1-3h.

[0019] Furthermore, in step S4, the pulverization and refining method is one or more of air jet milling, vibratory milling and / or air jet milling, and the pulverized and refined carbon intermediate has a D50 of 5 to 12 μm and a Dmax of ≤ 30 μm.

[0020] Furthermore, in step S5, the conditions for high-temperature carbonization 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.

[0021] Furthermore, the pore-forming agent in step S1 is a source of heteroatomic doping elements, and the hard carbon material obtained in step S5 is a heteroatomic doped hard carbon material.

[0022] Furthermore, the doping element is one or more of N, S, P, O and / or B, and the content of the doping element is 1.0-3.0 wt% of the hard carbon material.

[0023] Furthermore, the specific surface area of ​​the hard carbon material obtained in step S5 is 3–12 m². 2 / g, true density is 1.4~1.8g / cm³ 3 The carbon interlayer spacing is 0.375–0.385 nm.

[0024] In this invention, the aforementioned preparation conditions affect the degree to which open pores transform into closed pores. Specifically, decreasing the heating rate, increasing the carbonization temperature, and extending the carbonization time make the pore walls of open pores more prone to collapse and transform into closed pores. However, excessively slow heating rates, excessively high carbonization temperatures, and excessively long carbonization times will increase time costs and reduce economic efficiency, which is detrimental to actual production. Furthermore, excessively high carbonization temperatures will result in excessively narrow interlayer spacing of carbon layers in hard carbon materials, affecting the sodium storage capacity and rate performance of hard carbon materials. Therefore, it is necessary to comprehensively consider performance and cost to select the optimal preparation conditions.

[0025] In this invention, the doping elements introduced by the inorganic acid pore-forming agent are non-metallic elements (N, O, S, P, B, etc.), which can change the electronic structure of carbon materials and improve their electronic conductivity. Furthermore, heteroatoms themselves possess electrochemical activity and can serve as active sites for sodium storage, thereby increasing the sodium storage capacity of hard carbon materials. In addition, elements with larger atomic diameters, such as S and P, can widen the interlayer spacing of carbon layers, thus facilitating the insertion / extraction of sodium ions between carbon layers and improving the rate performance of hard carbon materials. Commonly used metal salt pore-forming agents do not possess the effect of doping non-metallic elements into hard carbon materials.

[0026] Inorganic acid pore-forming agents can create open pores in carbon materials at relatively low activation temperatures. Combined with the subsequent high-temperature carbonization process, the pore walls of the open pores collapse and transform into closed pores, while the specific surface area decreases. The increase in closed pores will improve the low-potential plateau capacity of the carbon material for storing sodium, and the decrease in specific surface area will reduce the irreversible decomposition of the electrolyte, improve the first-cycle coulombic efficiency of the carbon material, and increase the energy density of the sodium-ion battery.

[0027] This invention discloses a method for preparing a closed-pore microporous structure hard carbon material for sodium-ion battery anodes, which has the following beneficial effects:

[0028] First, the process is green and environmentally friendly. In this invention, the inorganic acid pore-forming agent does not introduce additional metal impurities into the carbon material, eliminating the need for subsequent acid washing and impurity removal steps, which can reduce production costs and reduce environmental pollution problems.

[0029] Secondly, the specific surface area is small. This invention uses inorganic acid as a pore-forming agent and combines it with a secondary carbonization process to adjust the pore content and pore type in the hard carbon material. Furthermore, the non-metallic elements in the inorganic acid pore-forming agent are incorporated into the hard carbon material framework, increasing the sodium storage active sites. This inorganic acid pore-forming agent has a low activation temperature (400–600°C), and during activation, it can slowly erode the carbon material framework, forming carbon microcrystals with well-developed micropores. The pores formed in this process are open pores. Subsequently, the activated carbon material undergoes high-temperature carbonization. During carbonization, the pore walls of the open pores collapse and gradually close, transforming from open pores to closed pores, while the specific surface area decreases sharply.

[0030] Third, it exhibits excellent electrochemical performance. During the carbonization process, heteroatoms (N, S, P, B, O, etc.) from inorganic acids are introduced into the carbon material, altering its electronic structure and improving electronic conductivity. Furthermore, it possesses electrochemical activity, providing additional sodium storage capacity. Elements with larger atomic radii, such as S and P, can widen the carbon interlayer spacing, facilitating the insertion and extraction of sodium ions between carbon layers, thereby enhancing its sodium storage capacity and rate performance. Attached Figure Description

[0031] Figure 1 The nitrogen adsorption-desorption curves for Example 1 are shown below.

[0032] Figure 2 XRD patterns of Example 1 and Comparative Example 1;

[0033] Figure 3 The first-week charge-discharge curves for Examples 1 and 2 are shown.

[0034] Figure 4 The nitrogen adsorption-desorption curves are for Comparative Example 1 and Comparative Example 2.

[0035] Figure 5 The first-week charge-discharge curves are for Comparative Example 1 and Comparative Example 2. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments and accompanying drawings.

[0037] This invention discloses a method for preparing a closed-pore microporous structure sodium-ion battery anode hard carbon material, comprising the following steps:

[0038] S1. Preparation of precursor mixed solution: Biomass raw material powder and inorganic acid pore-forming agent are added to deionized water and heated and stirred to obtain precursor mixed solution;

[0039] S2. Drying of carbon precursor: Dry the precursor mixture obtained in step S1 to obtain carbon precursor.

[0040] S3. Activation and pore-forming of closed-cell carbon intermediate: The carbon precursor obtained in step S2 is subjected to a high-temperature activation reaction to obtain a closed-cell carbon intermediate with a microporous structure after activation.

[0041] S4. Refining of closed-cell carbon intermediate: The carbon intermediate obtained in step S3 is crushed and refined, and then sieved to obtain the refined closed-cell carbon intermediate.

[0042] S5. High-temperature carbonization of hard carbon materials: The refined carbon intermediate obtained in step S4 is subjected to high-temperature carbonization to obtain hard carbon materials with closed microporous structures.

[0043] Further, in step S1, the biomass is one or more of sawdust, walnut shells, coffee shells, nut shells, wheat straw, sugarcane bagasse, bamboo and / or straw; the inorganic acid pore-forming agent is one or more of phosphoric acid, sulfuric acid and / or nitric acid; and the amount of pore-forming agent added is 1 to 20% of the biomass added by mass.

[0044] Furthermore, in step S1, the heating and stirring temperature is 25–90°C, and the stirring time is 1–5 hours.

[0045] Furthermore, in step S2, the drying temperature is 80-150℃, and the drying time is 5-20 hours.

[0046] Furthermore, in step S3, the conditions for the high-temperature activation reaction are: heating rate of 2-10℃ / min, activation temperature of 400-700℃, and activation time of 1-3h.

[0047] Furthermore, in step S4, the pulverization and refining method is one or more of air jet milling, vibratory milling and / or air jet milling, and the pulverized and refined carbon intermediate has a D50 of 5 to 12 μm and a Dmax of ≤ 30 μm.

[0048] Furthermore, in step S5, the conditions for high-temperature carbonization 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.

[0049] Furthermore, the pore-forming agent in step S1 is a source of heteroatomic doping elements, and the hard carbon material obtained in step S5 is a heteroatomic doped hard carbon material.

[0050] Furthermore, the doping element is one or more of N, S, P, O and / or B, and the content of the doping element is 1.0-3.0 wt% of the hard carbon material.

[0051] Furthermore, the specific surface area of ​​the hard carbon material obtained in step S5 is 3–12 m². 2 / g, true density is 1.4~1.8g / cm³ 3 The carbon interlayer spacing is 0.375–0.385 nm.

[0052] Example 1: Preparation and Performance Testing of S-Doped Hard Carbon Materials

[0053] S1. Preparation of precursor mixed solution: Weigh the wood chips and sulfuric acid pore-forming agent in a ratio of 1:0.1, add them to deionized water and stir at 60℃ for 2 hours;

[0054] S2. Drying of carbon precursor: The obtained mixed solution was dried at 90℃ for 12h to obtain carbon precursor.

[0055] S3. Activation and pore-forming of closed-cell carbon intermediates: The obtained precursor is placed in a high-temperature furnace and heated to 500°C at a heating rate of 5°C / min in a nitrogen atmosphere. The temperature is maintained for 1 hour to obtain a porous carbon intermediate with a rich microporous structure after activation.

[0056] S4. Refining of closed-cell carbon intermediates: The above carbon intermediates are pulverized by air jet milling to a D50 of 5μm and a Dmax of ≤20μm.

[0057] S5. High-temperature carbonization of hard carbon materials: The refined porous carbon intermediate is placed in a high-temperature furnace and heated to 1300℃ in a nitrogen atmosphere at a heating rate of 2℃ / min. The temperature is held for 2 hours to obtain S-doped closed-pore carbon.

[0058] The elemental analyzer determined that the sulfur content in the obtained hard carbon material was 2.1% wt., indicating that sulfur was incorporated into the carbon material.

[0059] Figure 1 The nitrogen adsorption-desorption isotherms for porous carbon intermediates and S-doped closed-pore carbon have specific surface areas of 613 and 8 m², respectively, according to Brunauer-Emmett-Teller (BET). 2 / g, with pore volumes of 0.42 and 0.021 cm³, respectively. 3 / g. This demonstrates that the sulfuric acid activator introduces abundant open-pore structures into the carbon intermediate, and secondary carbonization can cause the open pores to collapse, reducing the specific surface area.

[0060] The electrochemical performance of the obtained materials was tested according to the following method: S-doped hard carbon material, Super P, CMC, and SBR were mixed in a mass ratio of 94:1.5:2:2.5 to form a slurry. A 120µm four-sided coating tool was used to coat the black slurry onto copper foil, and the membrane was then dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C = 300mAh / g) and a voltage range of 2-0.005V.

[0061] Figure 3 The first-cycle charge-discharge curves of the S-doped hard carbon material electrode are shown. The electrode has a reversible specific capacity of 352.2 mAh / g and a first-cycle coulombic efficiency of 91.2%, exhibiting high sodium storage capacity and high first-cycle efficiency. This is related to its high closed pore volume, effective S element doping, and low specific surface area.

[0062] Example 2: Preparation and Performance Testing of P-Doped Hard Carbon Materials

[0063] S1. Preparation of precursor mixed solution: Weigh the wood chips and phosphoric acid pore-forming agent at a ratio of 1:0.1, add them to deionized water and stir at 60℃ for 2 hours;

[0064] S2. Drying of carbon precursor: The resulting mixed solution was then dried at 90°C for 12 hours to obtain the carbon precursor.

[0065] S3. Activation and pore-forming of closed-pore carbon intermediates: The obtained precursor is placed in a high-temperature furnace and heated to 600°C at a heating rate of 5°C / min in a nitrogen atmosphere, and held for 1 hour to obtain activated carbon intermediates with rich microporous structures.

[0066] S4. Refining treatment of closed-cell carbon intermediates: The above carbon intermediates are pulverized by air jet milling until D50 is 5μm and Dmax≤20μm.

[0067] S5. High-temperature carbonization of hard carbon materials: The refined carbon intermediates are placed in a high-temperature furnace and heated to 1300°C at a heating rate of 2°C / min in a nitrogen atmosphere. The temperature is held for 2 hours to obtain a P-doped hard carbon material with a rich closed microporous structure.

[0068] Elemental analysis revealed a phosphorus (P) content of 2.3% wt.% in the obtained hard carbon material, indicating that P was incorporated into the carbon material. He gas true density measurements showed the material to have a true density of 1.8 g / cm³. 3This indicates that the material has a large closed pore volume.

[0069] The electrochemical performance of the obtained materials was tested according to the following method: P-doped hard carbon material, Super P, CMC, and SBR were mixed in a mass ratio of 94:1.5:2:2.5 to form a slurry. A 120µm four-sided coating tool was used to coat the black slurry onto copper foil, and the membrane was then dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C = 300mAh / g) and a voltage range of 2-0.005V.

[0070] Figure 3 The first-cycle charge-discharge curves of the P-doped hard carbon material electrode are shown. The electrode has a reversible specific capacity of 328.9 mAh / g and a first-cycle coulombic efficiency of 92.8%, exhibiting high sodium storage capacity and high first-cycle efficiency. This is related to its high closed pore volume, effective P element doping, and low specific surface area.

[0071] Comparative Example 1: Preparation and Performance Testing of Unactivated Hard Carbon Materials

[0072] S1. Preparation of precursor mixed solution: Add sawdust to deionized water and stir at 60℃ for 2 hours;

[0073] S2. Drying of carbon precursor: The resulting mixed solution was then dried at 90°C for 12 hours to obtain the carbon precursor.

[0074] S3. Thermal decomposition of carbon intermediates: The obtained precursor is placed in a high-temperature furnace and heated to 500°C at a heating rate of 5°C / min in a nitrogen atmosphere, and held at that temperature for 1 hour to obtain unactivated carbon intermediates.

[0075] S4. Refining of carbon intermediates: The above carbon intermediates are pulverized by air jet milling to a D50 of 5μm and a Dmax of ≤20μm.

[0076] S5. High-temperature carbonization of hard carbon materials: The refined carbon intermediates are placed in a high-temperature furnace and heated to 1300°C at a heating rate of 2°C / min in a nitrogen atmosphere. The temperature is maintained for 2 hours to obtain unactivated hard carbon materials.

[0077] Helium gas true density testing revealed that the true densities of the S-doped porous carbon material in Example 1 and Comparative Example 1 (unactivated hard carbon material) were 1.6 and 2.0 g / cm³, respectively. 3This demonstrates that the secondary high-temperature carbonization process transforms open pores into closed pores, thus reducing the true density of the material.

[0078] Combination Figure 2 The XRD patterns of the S-doped hard carbon material and the unactivated hard carbon material (Comparative Example 1) in Example 1 show that their average carbon interlayer spacing is 0.38 and 0.37 nm, respectively, proving that S doping effectively widens the carbon interlayer spacing.

[0079] Figure 4 The nitrogen adsorption-desorption curves of the unactivated hard carbon material show that the BET specific surface area of ​​the hard carbon material is 5 m². 2 / g, pore volume is 0.013cm³ 3 The true density of He gas, measured by a true density test, is 2.0 g / cm³. 3 This indicates that both the open and closed pore volumes are relatively small. Figure 2 The XRD pattern of the unactivated hard carbon material was obtained, and the average carbon interlayer spacing was calculated to be 0.37 nm.

[0080] The electrochemical performance of the obtained materials was tested as follows: Unactivated hard carbon material, Super P, CMC, and SBR were mixed in a mass ratio of 94:1.5:2:2.5 to form a slurry. A 120µm four-sided coating tool was used to coat the black slurry onto copper foil, and the membrane was then dried in a vacuum oven at 100°C for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C = 300mAh / g) and a voltage range of 2-0.005V.

[0081] Figure 5 The first-week charge-discharge curves are for an unactivated hard carbon material electrode. The reversible specific capacity of this electrode is only 267.1 mAh / g, and the first-week coulombic efficiency is 91.5%.

[0082] Comparative Example 2: Preparation and Performance Testing of Uncarbonized Hard Carbon Materials

[0083] S1. Preparation of precursor mixed solution: Weigh the wood chips and sulfuric acid pore-forming agent in a ratio of 1:0.1, add them to deionized water and stir at 60℃ for 2 hours;

[0084] S2. Drying of carbon precursor: The resulting mixed solution was then dried at 90°C for 12 hours to obtain the carbon precursor.

[0085] S3. High-temperature carbonization of hard carbon materials: The above-mentioned carbon precursor is placed in a high-temperature furnace and heated to 1300°C at a heating rate of 2°C / min in a nitrogen atmosphere, and held for 2 hours to obtain hard carbon materials that have not been carbonized twice.

[0086] S4. Refining treatment of hard carbon materials: The above-mentioned hard carbon materials are pulverized by air jet milling to a D50 of 5μm and a Dmax of ≤20μm.

[0087] Figure 4 The nitrogen adsorption-desorption isotherm for uncarbonized hard carbon has a BET specific surface area of ​​319 m². 2 / g, pore volume is 0.19cm³ 3 The true density of the He gas was measured to be 1.9 g / cm³. This indicates that the open pore volume of the uncarbonized hard carbon material is relatively large, the specific surface area is relatively small, and the closed pore volume is relatively small.

[0088] The electrochemical performance of the obtained materials was tested using the following method: Uncarbonized hard carbon material, SuperP, CMC, and SBR were mixed in a mass ratio of 94:1.5:2:2.5 to form a slurry. A 120µm four-sided coating tool was used to coat the slurry onto copper foil, and the membrane was then dried in a vacuum oven at 100°C for 2 hours. The electrode membrane was punched into a 0.6mm radius disc using a die-cutting machine. Using metallic sodium as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%) as the electrolyte, and a PP / PE / PP three-layer separator, a CR2016 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge testing at a current density of 0.1C (1C = 300mAh / g) and a voltage range of 2-0.005V.

[0089] Figure 5 The first-cycle charge-discharge curve of the electrode made of uncarbonized hard carbon material shows that the reversible specific capacity of the electrode is only 236.9 mAh / g, and the first-cycle efficiency is only 64.8%.

[0090] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method for preparing a closed-pore microporous structure hard carbon material for sodium-ion battery anodes, characterized in that... Includes the following steps: S1. Preparation of precursor mixed solution: Biomass raw material powder and inorganic acid pore-forming agent are added to deionized water and heated and stirred to obtain precursor mixed solution; The inorganic acid pore-forming agent is one or more of phosphoric acid, sulfuric acid, and / or nitric acid; the amount of pore-forming agent added is 1-20% of the biomass added. S2. Drying the carbon precursor: Dry the precursor mixture obtained in step S1 to obtain the carbon precursor. S3. Activation and pore formation of closed-cell carbon intermediate: The carbon precursor obtained in step S2 is subjected to a high-temperature activation reaction to obtain a closed-cell carbon intermediate with a microporous structure after activation. The conditions for the high-temperature activation reaction are: heating rate of 2-10 ℃ / min, activation temperature of 400-700 ℃, and activation time of 1-3 h. S4. Refining of closed-cell carbon intermediate: The carbon intermediate obtained in step S3 is crushed and refined, and then sieved to obtain the refined closed-cell carbon intermediate. S5. High-temperature carbonization of hard carbon material: The refined carbon intermediate obtained in step S4 is subjected to high-temperature carbonization to obtain a hard carbon material with a closed microporous structure. The conditions for high-temperature carbonization are: heating rate of 0.5-5℃ / min, carbonization temperature of 1000-1600℃, and carbonization time of 2-10 h.

2. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: The biomass mentioned in step S1 is one or more of the following: sawdust, walnut shells, coffee shells, nut shells, wheat straw, sugarcane bagasse, bamboo, and / or straw.

3. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: In step S1, the heating and stirring temperature is 25-90 ℃, and the stirring time is 1-5 h.

4. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: In step S2, the drying temperature is 80-150 ℃ and the drying time is 5-20 h.

5. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: In step S4, the pulverization and refining method is one or more of air jet milling, vibratory milling and / or air jet milling, and the carbon intermediate after pulverization and refining has a D50 of 5-12 μm and a Dmax ≤ 30 μm.

6. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: In step S1, the pore-forming agent is a source of heteroatomic doping elements, and the hard carbon material obtained in step S5 is a heteroatomic doped hard carbon material.

7. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 1, characterized in that: The doping element is one or more of N, S, P and O, and the content of the doping element is 1.0-3.0 wt% of the hard carbon material.

8. The method for preparing the closed microporous structure sodium-ion battery anode hard carbon material according to claim 7, characterized in that: The specific surface area of ​​the hard carbon material obtained in step S5 is 3-12 m². 2 / g, true density is 1.4-1.8 g / cm³ 3 The carbon interlayer spacing is 0.375-0.385 nm.

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