Optimized preparation process of biomass-based precursor for hard carbon anode of sodium-ion battery by carbonization

Abstract

The application discloses a sodium ion battery hard carbon negative electrode biomass-based precursor carbonization optimization preparation process, comprising the following steps: S1, coconut shell, walnut shell, straw biomass raw materials are sequentially washed, 80-120 DEG C drying, crushing, acid leaching, washing to neutral and 60-80 DEG C vacuum drying, vacuum degree <=-0.08MPa, get purified biomass precursor, the beneficial effects of the application are: through raw material compounding and process synergy, the customization of hard carbon microstructure is realized, the high-strength carbon base is constructed by using the nut shell raw material with high carbon content, the volatile produced in the pyrolysis process of the specific type of straw raw material is combined, the rich initial pore network is constructed in the matrix, the carbon layer spacing is expanded through the synergistic doping of multiple heteroatoms and the liquid-phase carbon source backfilling technology, and part of the open pores is converted into closed nanopores, and the sodium storage active site is increased.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to an optimized carbonization preparation process for biomass-based precursors for hard carbon anodes in sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries, as an important supplement to lithium-ion batteries, have attracted much attention in the field of large-scale energy storage due to their advantages such as low raw material cost and high safety. Hard carbon materials, with their large interlayer spacing and abundant porous structure, are among the most promising anode materials for sodium-ion batteries. Among the preparation methods of hard carbon, biomass carbonization is favored due to its wide availability of raw materials and environmental friendliness. However, in existing biomass-based hard carbon preparation technologies, metallic impurities in the raw materials can easily catalyze the graphitization of the carbon lattice during high-temperature carbonization, leading to a reduction in interlayer spacing and hindering sodium ion insertion. Traditional processes are difficult to control the pore structure, resulting in too many open pores and irreversible consumption of the electrolyte at defect sites, leading to low initial coulombic efficiency. Conventional doping modifications are difficult to simultaneously optimize the bulk and interface structures, and traditional coating techniques are prone to uneven coating layers or weak bonding with the matrix, making them prone to peeling off during cycling and failing to effectively suppress electrode structure disintegration, resulting in poor cycling stability. Summary of the Invention

[0003] The purpose of this invention is to provide an optimized carbonization preparation process for biomass-based precursors of hard carbon anodes in sodium-ion batteries, so as to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution: an optimized carbonization preparation process for biomass-based precursors for sodium-ion battery hard carbon anodes, comprising the following steps: S1. Coconut shells, walnut shells, and straw biomass raw materials are sequentially washed with water to remove impurities, dried at 80-120℃, crushed and sieved, acid-leached to remove ash, washed with water until neutral, and vacuum-dried at 60-80℃ with a vacuum degree ≤-0.08MPa to obtain purified biomass precursors. S2. Under an inert gas atmosphere, the purified biomass precursor is pre-carbonized at 300-500℃, crushed and passed through a 200-mesh sieve to obtain the pre-carbonized precursor. S3. Under inert gas protection, the pre-carbonized precursor is heated to 600-800℃, then switched to a mixed atmosphere with a hydrogen volume ratio of 10%-30% and heated to 1100-1400℃. The inert gas is switched back 30-60 minutes before the end of the holding period. After cooling, a preliminary hard carbon matrix is ​​obtained. S4. Controlling the solid-liquid ratio to 1:5-1:10, the preliminary hard carbon matrix is ​​stirred with an aqueous solution containing at least one dopant source among phosphoric acid, urea and boric acid at 60-80℃ and then dried to constant weight. Subsequently, it is calcined at 700-900℃ to obtain the doped modified hard carbon matrix. S5. Disperse the doped and modified hard carbon matrix in a solvent, add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, reflux and dry to constant weight, and then heat treat at 1000-1200℃ under an inert atmosphere to obtain the hard carbon matrix with reconstructed pores. S6. The hard carbon matrix with reconstructed pores is mixed with an aqueous solution of lithium, sodium and potassium chlorides in a chloride molar ratio of 4:4:2-5:3:2. After stirring at 60-80℃, the mixture is freeze-dried and ground to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. After pre-embedded hard carbon matrix in molten salt at 650-850℃, the matrix is ​​washed with water until the conductivity of the filtrate is less than 10μS / cm, and then dried to obtain surface-coated hard carbon material. S8. The surface-coated hard carbon material is crushed, sieved, and dried to obtain hard carbon anode material.

[0005] Preferably, step S1 specifically includes the following steps: S11. Select coconut shells, walnut shells, and straw biomass raw materials, wash them with water to remove surface dust and soluble impurities, then dry them in an 80-120℃ forced-air drying oven to constant weight to remove free moisture, then pulverize the dried raw materials and pass them through an 80-120 mesh sieve to obtain biomass powder. S12. Immerse the biomass powder in a 0.5-2 mol / L hydrochloric acid or nitric acid aqueous solution and let it stand at room temperature for 6-12 hours to dissolve and remove the metal ash in the raw material, thereby reducing the impact of catalytic graphitization in the subsequent carbonization process. S13. Separate the solid and liquid components of the soaked mixture, remove the solid residue, and wash the solid residue repeatedly with deionized water until the pH value of the filtrate stabilizes in the range of 6-7 to remove residual acid and dissolved metal ions. S14. Place the washed solid residue in a vacuum drying oven and dry it at 60-80℃ and vacuum degree ≤-0.08MPa for 12-24h until constant weight is achieved, removing physically adsorbed water and chemically bound water to obtain purified biomass precursor.

[0006] Preferably, step S2 specifically includes the following steps: S21. Place the purified biomass precursor in the quartz boat of the tubular furnace, push it into the constant temperature zone, seal the furnace tube, and then introduce inert gas with a flow rate of 50-200 mL / min. Continue to ventilate for 20-30 minutes to exhaust the air in the furnace. S22. Under the condition of maintaining an inert gas flow rate of 50-200 mL / min, the temperature is increased from room temperature to 300-500℃ at a heating rate of 1-5℃ / min and kept at this temperature for 1-3 hours to promote thermal dehydration of hemicellulose and cellulose and form a preliminary cross-linked network structure. S23. After the heat preservation is completed, stop heating and keep inert gas in the air supply to allow the material to cool naturally to room temperature with the furnace and eliminate thermal stress. S24. Take out the cooled block product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve to collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor.

[0007] Preferably, step S3 specifically includes the following steps: S31. Place the pre-carbonized precursor in a quartz boat of a tubular furnace, first introduce inert gas to purge the air, then heat it to 600-800℃ at a heating rate of 2-5℃ / min, and hold it at this temperature for 0.5-1h to remove residual volatiles. S32. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, control the hydrogen volume ratio to 10-30%, continue to heat to 1100-1400℃ at a heating rate of 1-3℃ / min, and hold for 2-5 hours to suppress excessive carbon layer growth by using hydrogen etching. S33. 30-60 minutes before the end of the heat treatment at 1100-1400℃, switch back to inert gas and maintain this atmosphere until the reaction is complete. S34. Maintain inert gas protection to allow the material inside the furnace to cool naturally to room temperature, then remove it, crush and grind it to obtain a preliminary hard carbon matrix.

[0008] Preferably, step S4 specifically includes the following steps: S41. Add the preliminary hard carbon matrix to an aqueous solution at a solid-liquid ratio of 1:5-1:10. The aqueous solution contains phosphoric acid, urea and boric acid. The dopant source molecules diffuse into the pores and surface defect sites of the preliminary hard carbon matrix through impregnation. S42. Place the mixed system in a constant temperature water bath at 60-80℃ and stir continuously for 2-4 hours to promote the adsorption or exchange between the doped source ions and the oxygen-containing functional groups of the preliminary hard carbon matrix. S43. Take out the saturated material and dry it at 80-100℃ to constant weight. The difference between two consecutive weighings should be ≤0.5mg. S44. Place the dried product in the reaction chamber, introduce ammonia or nitrogen gas, and calcine at 700-900℃ for 1-3 hours at a rate of 2-5℃ / min to induce doped atoms to embed into the carbon lattice, thereby obtaining a doped modified hard carbon matrix.

[0009] Preferably, step S5 specifically includes the following steps: S51. Disperse the doped and modified hard carbon matrix in an alcohol solvent, and add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, so that the organic carbon source molecules can fully contact and diffuse into the pores of the doped and modified hard carbon matrix. S52. The mixed slurry is heated and refluxed in an oil bath at 70-90℃ for 1-3 hours. The solvent is prevented from evaporating by a condenser. The organic carbon source undergoes prepolymerization in the pores of the doped and modified hard carbon matrix. S53. Filter the refluxed mixture, dry the solid material at 80-100℃ to constant weight, and weigh it twice consecutively. The difference in mass should be ≤0.5mg. S54. Place the dried sample under an inert atmosphere and heat it to 1000-1200℃ at a rate of 3-8℃ / min. Hold the temperature for 0.5-2h. The precursor polymer will be decomposed and carbonized, filling the pore defects and obtaining a hard carbon matrix with reconstructed pores.

[0010] Preferably, step S6 specifically includes the following steps: S61. Dissolve lithium, sodium and potassium chlorides in deionized water in a molar ratio of 4:4:2-5:3:2 to prepare a mixed salt solution; then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chloride to be 1:0.5-1:2. S62. Stir continuously in a constant temperature water bath of 60-80℃ for 2-4 hours to utilize capillary action to fully wet and pre-fill the pores of the hard carbon matrix after pore reconstruction with salt ions. S63. Freeze-dry the stirred slurry, with a cold trap temperature ≤ -50℃, a vacuum degree ≤ -0.08MPa, and a drying time of 24-48h. Avoid salt particle migration and agglomeration through ice crystal sublimation, and fix chloride particles in situ. S64. Take out the freeze-dried product and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded.

[0011] Preferably, step S7 specifically includes the following steps: S71. Place the hard carbon matrix pre-embedded in molten salt carrier in the reaction chamber, and under a protective atmosphere, heat it to 650-850℃ at 3-8℃ / min and hold it for 1-3 hours to melt the pre-embedded chloride and grow a carbonaceous coating layer on the surface of the hard carbon matrix pre-embedded in molten salt carrier. S72. After the heat preservation is completed, stop heating and slowly cool to room temperature at a rate of ≤5℃ / min; S73. Take out the cooled product and wash it repeatedly with deionized water until the conductivity of the filtrate is less than 10 μS / cm to remove residual soluble salts. Then dry it at 80-100℃ and vacuum degree ≤-0.08MPa for 12-24h to obtain the surface-coated hard carbon material.

[0012] Preferably, in the biomass raw materials, the mass ratio of coconut shells and walnut shells is not less than 60%, and the straw raw materials are selected from one or more of corn straw, wheat straw, or cotton straw.

[0013] Preferably, the organic carbon source in step S51 is at least two of glucose, sucrose and citric acid, and the mass ratio of glucose to sucrose is 1:0.5-1:2.

[0014] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: by combining raw materials and synergistic processes, the microstructure of hard carbon is customized. A high-strength carbon substrate is constructed using nut shell raw materials with high carbon content. Combined with the volatiles generated during the pyrolysis of specific types of straw raw materials, a rich initial pore network is constructed inside the substrate. Through the synergistic doping of multi-component heteroatoms and liquid-phase carbon source backfilling technology, the carbon interlayer spacing is increased, and some open pores are transformed into closed nanopores, increasing sodium storage active sites and reducing the irreversible consumption of electrolyte at defect sites. Freeze-drying technology was used to achieve in-situ uniform pre-embedding of molten salt in the pores. A dense carbonaceous coating layer was grown in-situ on the surface of hard carbon particles at high temperature using the molten salt medium. The coating layer has strong adhesion to the matrix, isolates the electrolyte from direct contact with the active material, reduces interfacial impedance, and inhibits the structural disintegration of the electrode material during cycling. In combination with the use of small and large molecule carbon sources, surface defects of the material were further repaired and the pore size distribution was optimized. Attached Figure Description

[0016] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the method steps of the present invention. Detailed Implementation

[0018] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses consistent with some aspects of this disclosure as detailed in the appended claims.

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Example 1:

[0020] See Figure 1 As shown, the optimized carbonization preparation process of the biomass-based precursor for the hard carbon anode of sodium-ion batteries according to an embodiment of the present invention includes the following steps: S1. Coconut shells, walnut shells, and straw biomass raw materials are sequentially washed with water to remove impurities, dried at 80-120℃, crushed and sieved, acid-leached to remove ash, washed with water until neutral, and vacuum-dried at 60-80℃ with a vacuum degree ≤-0.08MPa to obtain purified biomass precursors. S2. Under an inert gas atmosphere, the purified biomass precursor is pre-carbonized at 300-500℃, crushed and passed through a 200-mesh sieve to obtain the pre-carbonized precursor. S3. Under inert gas protection, the pre-carbonized precursor is heated to 600-800℃, then switched to a mixed atmosphere with a hydrogen volume ratio of 10%-30% and heated to 1100-1400℃. The inert gas is switched back 30-60 minutes before the end of the holding period. After cooling, a preliminary hard carbon matrix is ​​obtained. S4. Controlling the solid-liquid ratio to 1:5-1:10, the preliminary hard carbon matrix is ​​stirred with an aqueous solution containing at least one dopant source among phosphoric acid, urea and boric acid at 60-80℃ and then dried to constant weight. Subsequently, it is calcined at 700-900℃ to obtain the doped modified hard carbon matrix. S5. Disperse the doped and modified hard carbon matrix in a solvent, add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, reflux and dry to constant weight, and then heat treat at 1000-1200℃ under an inert atmosphere to obtain the hard carbon matrix with reconstructed pores. S6. The hard carbon matrix with reconstructed pores is mixed with an aqueous solution of lithium, sodium and potassium chlorides in a chloride molar ratio of 4:4:2-5:3:2. After stirring at 60-80℃, the mixture is freeze-dried and ground to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. After pre-embedded hard carbon matrix in molten salt at 650-850℃, the matrix is ​​washed with water until the conductivity of the filtrate is less than 10μS / cm, and then dried to obtain surface-coated hard carbon material. S8. The surface-coated hard carbon material is crushed, sieved, and dried to obtain hard carbon anode material; S5 employs liquid-phase reflux and organic carbon source backfilling technology, utilizing glucose and sucrose carbon sources to fill microcracks after high-temperature pyrolysis and transform some open pores into closed nanopores. This pore reconstruction strategy significantly increases sodium storage active sites while reducing irreversible sodium storage sites, resulting in an initial coulombic efficiency of over 88%. S6's innovative freeze-drying pre-embedding technology utilizes the principle of ice crystal sublimation to avoid salt particle migration and agglomeration, ensuring uniform distribution of molten salt within the pores. The subsequent S7 high-temperature molten salt treatment grows a dense carbonaceous coating layer in situ on the hard carbon surface, effectively isolating the electrolyte from direct contact with the substrate, reducing interfacial impedance, and significantly improving the battery's cycle life. Example 2:

[0021] Step S1 specifically includes the following steps: S11. Select coconut shells, walnut shells, and straw biomass raw materials, wash them with water to remove surface dust and soluble impurities, then dry them in an 80-120℃ forced-air drying oven to constant weight to remove free moisture, then pulverize the dried raw materials and pass them through an 80-120 mesh sieve to obtain biomass powder. S12. Immerse the biomass powder in a 0.5-2 mol / L hydrochloric acid or nitric acid aqueous solution and let it stand at room temperature for 6-12 hours to dissolve and remove the metal ash in the raw material, thereby reducing the impact of catalytic graphitization in the subsequent carbonization process. S13. Separate the solid and liquid components of the soaked mixture, remove the solid residue, and wash the solid residue repeatedly with deionized water until the pH value of the filtrate stabilizes in the range of 6-7 to remove residual acid and dissolved metal ions. S14. Place the washed solid residue in a vacuum drying oven and dry it for 12-24 hours at 60-80℃ and vacuum degree ≤-0.08MPa until constant weight is achieved, removing physically adsorbed water and chemically bound water to obtain purified biomass precursor. Acid leaching effectively removes metallic impurities, preventing the catalytic graphitization of carbon lattices during high-temperature carbonization and preserving the necessary disordered layer structure of hard carbon. Strict pH and vacuum control ensure the purity and chemical stability of the precursor, laying a high-quality foundation for subsequent processes.

[0022] Step S2 specifically includes the following steps: S21. Place the purified biomass precursor in the quartz boat of the tubular furnace, push it into the constant temperature zone, seal the furnace tube, and then introduce inert gas with a flow rate of 50-200 mL / min. Continue to ventilate for 20-30 minutes to exhaust the air in the furnace. S22. Under the condition of maintaining an inert gas flow rate of 50-200 mL / min, the temperature is increased from room temperature to 300-500℃ at a heating rate of 1-5℃ / min and kept at this temperature for 1-3 hours to promote thermal dehydration of hemicellulose and cellulose and form a preliminary cross-linked network structure. S23. After the heat preservation is completed, stop heating and keep inert gas in the air supply to allow the material to cool naturally to room temperature with the furnace and eliminate thermal stress. S24. Take out the cooled block product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve. Collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor. The low-temperature pre-carbonization stage promotes the dehydration and cross-linking of hemicellulose and cellulose, forming a stable preliminary skeleton and preventing structural collapse during subsequent high-temperature treatment; inert gas protection and furnace cooling effectively eliminate thermal stress and ensure the mechanical strength and particle size uniformity of the precursor particles.

[0023] Step S3 specifically includes the following steps: S31. Place the pre-carbonized precursor in a quartz boat of a tubular furnace, first introduce inert gas to purge the air, then heat it to 600-800℃ at a heating rate of 2-5℃ / min, and hold it at this temperature for 0.5-1h to remove residual volatiles. S32. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, control the hydrogen volume ratio to 10-30%, continue to heat to 1100-1400℃ at a heating rate of 1-3℃ / min, and hold for 2-5 hours to suppress excessive carbon layer growth by using hydrogen etching. S33. 30-60 minutes before the end of the heat treatment at 1100-1400℃, switch back to inert gas and maintain this atmosphere until the reaction is complete. S34. Maintain inert gas protection to allow the material inside the furnace to cool naturally to room temperature, then remove it, crush and grind it to obtain a preliminary hard carbon matrix; A variable-temperature, variable-atmosphere carbonization strategy was adopted, which involved inert atmosphere, hydrogen, and inert atmosphere. In the early stage, the inert atmosphere was used to remove volatiles, in the middle stage, hydrogen etching was used to inhibit excessive carbon layer growth and enrich the pores, and in the later stage, the inert atmosphere was switched back to prevent hydrogen embrittlement and side reactions. The interlayer spacing and porosity of hard carbon were controlled, which improved the conductivity and structural stability of the material.

[0024] Step S4 specifically includes the following steps: S41. Add the preliminary hard carbon matrix to an aqueous solution at a solid-liquid ratio of 1:5-1:10. The aqueous solution contains phosphoric acid, urea and boric acid. The dopant source molecules diffuse into the pores and surface defect sites of the preliminary hard carbon matrix through impregnation. S42. Place the mixed system in a constant temperature water bath at 60-80℃ and stir continuously for 2-4 hours to promote the adsorption or exchange between the doped source ions and the oxygen-containing functional groups of the preliminary hard carbon matrix. S43. Take out the saturated material and dry it at 80-100℃ to constant weight. The difference between two consecutive weighings should be ≤0.5mg. S44. Place the dried product in the reaction chamber, introduce ammonia gas or nitrogen gas, and calcine at 700-900℃ for 1-3 hours at a rate of 2-5℃ / min to promote the insertion of doped atoms into the carbon lattice and obtain a doped modified hard carbon matrix. By synergistic doping of phosphorus, nitrogen, and boron heteroatoms, heteroatoms were successfully embedded into the carbon lattice, increasing the carbon interlayer spacing and introducing abundant active defect sites. The constant weight drying standard avoided oxidation caused by moisture, while high-temperature calcination ensured the stable embedding of doped atoms, improving the sodium storage activity and rate performance of the material.

[0025] Step S5 specifically includes the following steps: S51. Disperse the doped and modified hard carbon matrix in an alcohol solvent, and add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, so that the organic carbon source molecules can fully contact and diffuse into the pores of the doped and modified hard carbon matrix. S52. The mixed slurry is heated and refluxed in an oil bath at 70-90℃ for 1-3 hours. The solvent is prevented from evaporating by a condenser. The organic carbon source undergoes prepolymerization in the pores of the doped and modified hard carbon matrix. S53. Filter the refluxed mixture, dry the solid material at 80-100℃ to constant weight, and weigh it twice consecutively. The difference in mass should be ≤0.5mg. S54. Place the dried sample under an inert atmosphere and heat it to 1000-1200℃ at a rate of 3-8℃ / min. Hold the temperature for 0.5-2h. The precursor polymer is decomposed and carbonized, filling the pore defects and obtaining a hard carbon matrix with reconstructed pores. By utilizing liquid-phase reflux and organic carbon source backfilling technology, in-situ repair of microcracks and open pores was achieved; high-temperature heat treatment caused deep pyrolysis and carbonization of the precursor polymer, transforming some open pores into closed nanopores, increasing sodium storage active sites, and optimizing pore size distribution, thereby improving the material's reversible capacity and low-pressure plateau performance.

[0026] Step S6 specifically includes the following steps: S61. Dissolve lithium, sodium and potassium chlorides in deionized water in a molar ratio of 4:4:2-5:3:2 to prepare a mixed salt solution; then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chloride to be 1:0.5-1:2. S62. Stir continuously in a constant temperature water bath of 60-80℃ for 2-4 hours to utilize capillary action to fully wet and pre-fill the pores of the hard carbon matrix after pore reconstruction with salt ions. S63. Freeze-dry the stirred slurry, with a cold trap temperature ≤ -50℃, a vacuum degree ≤ -0.08MPa, and a drying time of 24-48h. Avoid salt particle migration and agglomeration through ice crystal sublimation, and fix chloride particles in situ. S64. Take out the freeze-dried product, and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded. By using freeze-drying technology and the principle of ice crystal sublimation, the migration and agglomeration of salt particles caused by liquid bridge tension in traditional hot drying are avoided, and the in-situ uniform fixation of chloride salt particles inside the pores is achieved. The specific molar ratio design of ternary chloride optimizes the eutectic point and surface tension of the molten salt system, creating conditions for subsequent uniform coating.

[0027] Step S7 specifically includes the following steps: S71. Place the hard carbon matrix pre-embedded in molten salt carrier in the reaction chamber, and under a protective atmosphere, heat it to 650-850℃ at 3-8℃ / min and hold it for 1-3 hours to melt the pre-embedded chloride and grow a carbonaceous coating layer on the surface of the hard carbon matrix pre-embedded in molten salt carrier. S72. After the heat preservation is completed, stop heating and slowly cool to room temperature at a rate of ≤5℃ / min; S73. Take out the cooled product and wash it repeatedly with deionized water until the conductivity of the filtrate is less than 10 μS / cm to remove residual soluble salts. Then dry it at 80-100℃ and vacuum degree ≤-0.08MPa for 12-24h to obtain the surface-coated hard carbon material. Molten salt grows a dense carbonaceous coating layer in situ at high temperature, which isolates the electrolyte from direct contact with the hard carbon matrix and reduces interfacial impedance. Slow cooling prevents the coating layer from peeling off due to thermal stress, and strict conductivity washing standards ensure that residual salt is completely removed, improving the cycle life and safety of the battery.

[0028] Among the biomass raw materials, coconut shells and walnut shells account for no less than 60% of the mass, and the straw raw materials are selected from one or more of corn straw, wheat straw or cotton straw. By limiting the use of high-carbon nut shell materials (coconut shells, walnut shells) as the dominant raw material, the carbon yield and mechanical strength of the substrate after carbonization are guaranteed. By combining specific types of straw raw materials and utilizing the specific volatiles generated by pyrolysis, it is helpful to build a rich initial pore structure in the hard carbon matrix, while reducing the cost of raw materials.

[0029] In step S51, the organic carbon source is at least two of glucose, sucrose and citric acid, and the mass ratio of glucose to sucrose is 1:0.5-1:2. By combining carbon sources of different molecular weights, the excellent permeability of small-molecule glucose is used to fill micropores, while the high residual carbon rate of large-molecule sucrose is used to repair surface defects. The abundant carboxyl groups provided by citric acid promote cross-linking. By limiting the specific mass ratio of glucose to sucrose, the cross-linking density of pyrolysis products is controlled, the pore size distribution is optimized, the proportion of closed pores in the final product is increased, and the sodium storage capacity and first coulombic efficiency of the material are improved. Example 3:

[0030] S1. Select corn stalks (coconut shells and walnut shells accounting for less than 60% of the total weight), wash with water to remove surface dust and soluble impurities, then dry in an 80℃ forced-air drying oven to constant weight to remove free water. Crush the dried raw material and pass it through an 80-mesh sieve to obtain biomass powder. Immerse the biomass powder in a 0.5mol / L hydrochloric acid aqueous solution and let it stand at room temperature for 6 hours to dissolve and remove metallic ash from the raw material. Perform solid-liquid separation on the soaked mixture, remove the solid residue, and repeatedly wash the solid residue with deionized water until the pH of the filtrate stabilizes at 6.0 to remove residual acid. Place the washed solid residue in a vacuum drying oven and dry at 60℃ and a vacuum degree ≤-0.08MPa for 24 hours until constant weight to obtain purified biomass precursor. S2. Place the purified biomass precursor in a quartz boat of a tubular furnace, push it into the constant temperature zone, seal the furnace tube, and introduce inert gas at a flow rate of 50 mL / min for 20 minutes to exhaust the air in the furnace. While maintaining the inert gas flow rate at 50 mL / min, raise the temperature from room temperature to 300℃ at a rate of 1℃ / min and hold for 1 hour. After the holding period, stop heating, keep the inert gas in the furnace, and allow the material to cool naturally to room temperature. Take out the cooled blocky product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve. Collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor. S3. Place the pre-carbonized precursor in a quartz boat in a tubular furnace. First, introduce inert gas to purge the air, and heat to 600°C at a heating rate of 2°C / min, and hold at this temperature for 0.5 hours. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, controlling the hydrogen volume ratio to 10%, and continue heating to 1100°C at a heating rate of 1°C / min, and hold for 2 hours. 60 minutes before the end of the 1100°C holding period, switch back to inert gas and maintain this atmosphere until the reaction is complete. Maintain inert gas protection and allow the material in the furnace to cool naturally to room temperature with the furnace. Then, remove the material, crush and grind it to obtain a preliminary hard carbon matrix. S4. Add the preliminary hard carbon matrix to an aqueous solution at a solid-liquid ratio of 1:5. The aqueous solution contains only phosphoric acid (excluding urea and boric acid). Impregnation allows the dopant source molecules to diffuse into the pores of the preliminary hard carbon matrix. Place the mixture in a 60°C constant temperature water bath and stir continuously for 4 hours. Take out the saturated material and dry it at 80°C to constant weight. The mass difference between two consecutive weighings is ≤0.5mg. Place the dried product in a reaction chamber, introduce ammonia gas, and calcine it at 700°C at a rate of 2°C / min for 3 hours to obtain the doped modified hard carbon matrix. S5. Disperse the doped and modified hard carbon matrix in ethanol, add glucose (single carbon source) at 5% by mass of the doped and modified hard carbon matrix to ensure that the organic carbon source molecules fully contact and diffuse into the pores; heat the mixed slurry in a 70℃ oil bath under reflux for 3 hours, and prevent solvent evaporation by using a condenser; filter the refluxed mixture, dry the solid material at 80℃ to constant weight, and weigh it twice consecutively with a mass difference ≤0.5mg; place the dried sample under an inert atmosphere, heat it to 1000℃ at 3℃ / min and hold it for 2 hours to obtain the hard carbon matrix with reconstructed pores; S6. Dissolve lithium, sodium, and potassium chlorides in deionized water at a molar ratio of 4:4:2 to prepare a mixed salt solution. Then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chloride to be 1:0.5. Stir continuously in a 60℃ constant temperature water bath for 4 hours to allow the salt ions to fully wet and pre-fill the pores using capillary action. Freeze-dry the stirred mixed slurry at a cold trap temperature ≤-50℃, a vacuum degree ≤-0.08MPa, and a drying time of 48 hours. Take out the freeze-dried product and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. Place the hard carbon matrix pre-embedded with molten salt carrier in the reaction chamber, and heat it to 650℃ at 3℃ / min and hold it for 3h under a protective atmosphere. After holding, stop heating and slowly cool it to room temperature at a rate of 5℃ / min. Take out the cooled product and wash it repeatedly with deionized water until the conductivity of the filtrate is less than 10μS / cm to remove residual soluble salts. Then dry it at 80℃ and vacuum degree ≤-0.08MPa for 24h to obtain the surface-coated hard carbon material A. Example 4:

[0031] S1. Select coconut shells and cotton stalks (mass ratio 1:1), wash with water to remove surface dust and soluble impurities, then dry in a 100℃ forced-air drying oven to constant weight to remove free water. Then crush the dried raw materials and pass them through a 100-mesh sieve to obtain biomass powder. Immerse the biomass powder in a 1mol / L hydrochloric acid aqueous solution and let it stand at room temperature for 9 hours to dissolve and remove the metallic ash from the raw materials. Perform solid-liquid separation on the soaked mixture, take out the solid residue, and wash the solid residue repeatedly with deionized water until the pH value of the filtrate stabilizes at 6.5 to remove residual acid. Place the washed solid residue in a vacuum drying oven and dry it at 70℃ and a vacuum degree ≤-0.08MPa for 18 hours until constant weight to obtain purified biomass precursor. S2. Place the purified biomass precursor in a quartz boat of a tubular furnace, push it into the constant temperature zone, seal the furnace tube, and introduce inert gas at a flow rate of 125 mL / min for 25 min to exhaust the air in the furnace. While maintaining the inert gas flow rate at 125 mL / min, raise the temperature from room temperature to 400℃ at a rate of 3℃ / min and hold for 2 h. After the holding period, stop heating, keep the inert gas in the furnace, and allow the material to cool naturally to room temperature. Take out the cooled block product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve. Collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor. S3. Place the pre-carbonized precursor in a quartz boat in a tubular furnace. First, introduce inert gas to purge the air, and heat to 700℃ at a heating rate of 3.5℃ / min, and hold at this temperature for 0.75h. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, controlling the hydrogen volume ratio to 20%, and continue heating to 1250℃ at a heating rate of 2℃ / min, and hold at this temperature for 3.5h. 45min before the end of the 1250℃ holding period, switch back to inert gas and maintain this atmosphere until the reaction is complete. Maintain inert gas protection and allow the material in the furnace to cool naturally to room temperature with the furnace. Then, remove the material, crush and grind it to obtain a preliminary hard carbon matrix. S4. The preliminary hard carbon matrix is ​​added to an aqueous solution containing phosphoric acid and urea (but no boric acid) at a solid-liquid ratio of 1:7.5. The dopant source molecules diffuse into the pores of the preliminary hard carbon matrix through impregnation. The mixture is placed in a 70°C constant temperature water bath and stirred continuously for 3 hours. The saturated material is taken out and dried at 90°C to constant weight. The mass difference between two consecutive weighings is ≤0.5 mg. The dried product is placed in a reaction chamber, nitrogen gas is introduced, and the temperature is raised to 800°C at 3.5°C / min for 2 hours to obtain the doped modified hard carbon matrix. S5. The doped and modified hard carbon matrix is ​​dispersed in ethylene glycol, and sucrose at a mass of 12.5% ​​of the doped and modified hard carbon matrix is ​​added to ensure that the organic carbon source molecules fully contact and diffuse into the pores. The mixed slurry is heated and refluxed in an oil bath at 80°C for 2 hours, and a condenser is used to prevent solvent evaporation. The refluxed mixture is filtered, and the solid material is dried at 90°C to constant weight. The mass difference between two consecutive weighings is ≤0.5mg. The dried sample is placed in an inert atmosphere and heated to 1100°C at a rate of 5.5°C / min and held for 1.25 hours to obtain the hard carbon matrix with reconstructed pores. S6. Dissolve lithium, sodium, and potassium chlorides in deionized water at a molar ratio of 4.5:3.5:2.5 to prepare a mixed salt solution. Then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chlorides to be 1:0.75. Stir continuously in a 70℃ constant temperature water bath for 3 hours to allow the salt ions to fully wet and pre-fill the pores using capillary action. Freeze-dry the stirred mixed slurry at a cold trap temperature ≤-50℃, a vacuum degree ≤-0.08MPa, and a drying time of 36 hours. Take out the freeze-dried product and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. The hard carbon matrix pre-embedded with molten salt carrier is placed in the reaction chamber and heated to 750℃ at 5.5℃ / min under a protective atmosphere and held for 2 hours. After the holding period, heating is stopped and the material is slowly cooled to room temperature at a rate of 2.5℃ / min. The cooled product is taken out and repeatedly washed with deionized water until the conductivity of the filtrate is less than 10μS / cm to remove residual soluble salts. Then it is dried at 90℃ and vacuum degree ≤-0.08MPa for 18 hours to obtain the surface-coated hard carbon material B. Example 5

[0032] S1. Select coconut shells and walnut shells (mass ratio 1:1, no straw), wash with water to remove surface dust and soluble impurities, then dry in a 120℃ forced-air drying oven to constant weight to remove free water. Then crush the dried raw material and pass it through a 120-mesh sieve to obtain biomass powder. Immerse the biomass powder in a 2mol / L nitric acid aqueous solution and let it stand at room temperature for 12 hours to dissolve and remove the metallic ash from the raw material. Perform solid-liquid separation on the soaked mixture, take out the solid residue, and wash the solid residue repeatedly with deionized water until the pH value of the filtrate stabilizes at 7.0 to remove residual acid. Place the washed solid residue in a vacuum drying oven and dry it at 80℃ and a vacuum degree ≤-0.08MPa for 12 hours until constant weight to obtain purified biomass precursor. S2. Place the purified biomass precursor in a quartz boat of a tubular furnace, push it into the constant temperature zone, seal the furnace tube, and introduce inert gas at a flow rate of 200 mL / min for 30 minutes to purge the air from the furnace. While maintaining the inert gas flow rate at 200 mL / min, raise the temperature from room temperature to 500℃ at a rate of 5℃ / min and hold for 3 hours. After holding, stop heating, keep the inert gas in the furnace, and allow the material to cool naturally to room temperature. Take out the cooled blocky product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve. Collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor. S3. Place the pre-carbonized precursor in a quartz boat in a tubular furnace. First, introduce inert gas to purge the air, and heat to 800°C at a heating rate of 5°C / min, and hold at this temperature for 1 hour. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, controlling the hydrogen volume ratio to 30%, and continue heating to 1400°C at a heating rate of 3°C / min, and hold at this temperature for 5 hours. 30 minutes before the end of the 1400°C holding period, switch back to inert gas and maintain this atmosphere until the reaction is complete. Maintain inert gas protection and allow the material in the furnace to cool naturally to room temperature with the furnace. Then, remove the material, crush and grind it to obtain a preliminary hard carbon matrix. S4. The preliminary hard carbon matrix is ​​added to an aqueous solution containing phosphoric acid, urea, and boric acid at a solid-liquid ratio of 1:10. The dopant source molecules diffuse into the pores of the preliminary hard carbon matrix through impregnation. The mixture is placed in an 80°C constant temperature water bath and stirred continuously for 2 hours. The saturated material is taken out and dried at 100°C to constant weight. The mass difference between two consecutive weighings is ≤0.5 mg. The dried product is placed in a reaction chamber, ammonia gas is introduced, and the temperature is increased to 900°C at 5°C / min for 1 hour to obtain the doped modified hard carbon matrix. S5. Disperse the doped and modified hard carbon matrix in glycerol, add citric acid at 20% by mass of the doped and modified hard carbon matrix to ensure that the organic carbon source molecules fully contact and diffuse into the pores; heat the mixed slurry in a 90℃ oil bath and reflux for 1 hour, using a condenser to prevent solvent evaporation; filter the refluxed mixture, dry the solid material at 100℃ to constant weight, and weigh it twice consecutively with a mass difference ≤0.5mg; place the dried sample in an inert atmosphere and heat it to 1200℃ at 8℃ / min and hold for 0.5 hours to obtain the hard carbon matrix with reconstructed pores. S6. Dissolve lithium, sodium, and potassium chlorides in deionized water at a molar ratio of 5:3:2 to prepare a mixed salt solution. Then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chloride to be 1:2. Stir continuously in an 80℃ constant temperature water bath for 2 hours to allow the salt ions to fully wet and pre-fill the pores using capillary action. Freeze-dry the stirred mixed slurry at a cold trap temperature ≤-50℃, a vacuum degree ≤-0.08MPa, and a drying time of 24 hours. Take out the freeze-dried product and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. Place the hard carbon matrix pre-embedded with molten salt carrier in the reaction chamber, and heat it to 850℃ at 8℃ / min and hold it for 1h under a protective atmosphere. After holding, stop heating and slowly cool it to room temperature at a rate of 1℃ / min. Take out the cooled product and wash it repeatedly with deionized water until the conductivity of the filtrate is less than 10μS / cm to remove residual soluble salts. Then dry it at 100℃ and vacuum degree ≤-0.08MPa for 12h to obtain the surface-coated hard carbon material C. Example 6:

[0033] Comparative experiment: Hard carbon materials A, B, C, and comparative example G were used as negative electrode active materials, and were mixed with conductive carbon black, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 96:2:1:1. Deionized water was added to form a slurry, which was then coated onto copper foil. The electrodes were then dried, rolled, and stamped to form electrodes. A sodium metal sheet was used as the counter electrode, and a 1M NaPF6 EC / DEC (volume ratio 1:1) solution was used as the electrolyte. CR2032 coin cells were assembled in an argon glove box. Constant current charge-discharge tests were performed on the Blue Electric testing system with a voltage window of 0.01-2.5V (vs. Na+ / Na), an initial current density of 30mA / g, and a cycle test current density of 100mA / g. Test data and results:

[0034] Results analysis and discussion: The material of this invention has superior performance compared to existing commercial materials: Data shows that the hard carbon materials prepared in Examples 3, 4, and 5 of this invention are superior to Comparative Example G (existing commercial materials) in terms of initial discharge capacity, initial coulombic efficiency, and cycle stability. In particular, Example 4 achieved an initial discharge capacity of 335.8 mAh / g, which is about 8.3% higher than that of commercial materials; and an initial coulombic efficiency of 88.7%, which is about 4.2 percentage points higher than that of commercial materials. This demonstrates that this invention improves the sodium storage kinetics and structural stability of hard carbon materials through synergistic processes such as temperature-coupled carbonization, multi-element heteroatom doping, and molten salt coating. Example 4 has the best overall performance: Of the three embodiments, the hard carbon material B prepared in Example 4 performed the best; the balanced process conditions (such as calcination at 700-750℃ and a salt-to-material ratio of 1:0.75) and the scientific raw material ratio (coconut shell to cotton stalk mass ratio of 1:1) provided a high carbon yield substrate, while the cotton stalk provided abundant pore templates during pyrolysis. The combination of the two with the optimized calcination regime constructed an ideal closed-cell structure. The influence of process conditions on performance: Comparing Examples 3 and 5, it can be seen that the process conditions have a significant impact on performance. The lower temperature (1000℃) used in Example 3 resulted in insufficient pore development. In contrast, the higher temperature (1200℃) and lack of straw raw materials used in Example 5 may lead to micropore sintering or excessive carbon layer rearrangement, which in turn reduces the effective sodium storage sites. This proves that the selection of process conditions is the key to achieving high performance, and higher temperatures are not necessarily better. The hard carbon anode preparation process provided by this invention, through optimization of raw material ratio and process parameters, successfully prepared hard carbon anode materials with performance superior to existing commercial products; among them, Example 4 (coconut shell and cotton stalk composite matrix, calcined at 700-750℃) showed the best comprehensive performance and has high industrial application value.

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0036] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope of this disclosure is indicated by the following claims.

Claims

1. An optimized preparation process for carbonization of biomass-based precursors for hard carbon anodes in sodium-ion batteries, characterized in that... Includes the following steps: S1. Coconut shells, walnut shells, and straw biomass raw materials are sequentially washed with water to remove impurities, dried at 80-120℃, crushed and sieved, acid-leached to remove ash, washed with water until neutral, and vacuum-dried at 60-80℃ with a vacuum degree ≤-0.08MPa to obtain purified biomass precursors. S2. Under an inert gas atmosphere, the purified biomass precursor is pre-carbonized at 300-500℃, crushed and passed through a 200-mesh sieve to obtain the pre-carbonized precursor. S3. Under inert gas protection, the pre-carbonized precursor is heated to 600-800℃, then switched to a mixed atmosphere with a hydrogen volume ratio of 10%-30% and heated to 1100-1400℃. The inert gas is switched back 30-60 minutes before the end of the holding period. After cooling, a preliminary hard carbon matrix is ​​obtained. S4. Controlling the solid-liquid ratio to 1:5-1:10, the preliminary hard carbon matrix is ​​stirred with an aqueous solution containing at least one dopant source among phosphoric acid, urea and boric acid at 60-80℃ and then dried to constant weight. Subsequently, it is calcined at 700-900℃ to obtain the doped modified hard carbon matrix. S5. Disperse the doped and modified hard carbon matrix in a solvent, add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, reflux and dry to constant weight, and then heat treat at 1000-1200℃ under an inert atmosphere to obtain the hard carbon matrix with reconstructed pores. S6. The hard carbon matrix with reconstructed pores is mixed with an aqueous solution of lithium, sodium and potassium chlorides in a chloride molar ratio of 4:4:2-5:3:

2. After stirring at 60-80℃, the mixture is freeze-dried and ground to obtain a hard carbon matrix with molten salt carrier pre-embedded in it. S7. After pre-embedded hard carbon matrix in molten salt at 650-850℃, the matrix is ​​washed with water until the conductivity of the filtrate is less than 10μS / cm, and then dried to obtain surface-coated hard carbon material. S8. The surface-coated hard carbon material is crushed, sieved, and dried to obtain hard carbon anode material.

2. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 1, characterized in that, Step S1 specifically includes the following steps: S11. Select coconut shells, walnut shells, and straw biomass raw materials, wash them with water to remove surface dust and soluble impurities, then dry them in an 80-120℃ forced-air drying oven to constant weight to remove free moisture, then pulverize the dried raw materials and pass them through an 80-120 mesh sieve to obtain biomass powder. S12. Immerse the biomass powder in a 0.5-2 mol / L hydrochloric acid or nitric acid aqueous solution and let it stand at room temperature for 6-12 hours to dissolve and remove the metal ash in the raw material, thereby reducing the impact of catalytic graphitization in the subsequent carbonization process. S13. Separate the solid and liquid components of the soaked mixture, remove the solid residue, and wash the solid residue repeatedly with deionized water until the pH value of the filtrate stabilizes in the range of 6-7 to remove residual acid and dissolved metal ions. S14. Place the washed solid residue in a vacuum drying oven and dry it at 60-80℃ and vacuum degree ≤-0.08MPa for 12-24h until constant weight is achieved, removing physically adsorbed water and chemically bound water to obtain purified biomass precursor.

3. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 2, characterized in that, Step S2 specifically includes the following steps: S21. Place the purified biomass precursor in the quartz boat of the tubular furnace, push it into the constant temperature zone, seal the furnace tube, and then introduce inert gas with a flow rate of 50-200 mL / min. Continue to ventilate for 20-30 minutes to exhaust the air in the furnace. S22. Under the condition of maintaining an inert gas flow rate of 50-200 mL / min, the temperature is increased from room temperature to 300-500℃ at a heating rate of 1-5℃ / min and kept at this temperature for 1-3 hours to promote thermal dehydration of hemicellulose and cellulose and form a preliminary cross-linked network structure. S23. After the heat preservation is completed, stop heating and keep inert gas in the air supply to allow the material to cool naturally to room temperature with the furnace and eliminate thermal stress. S24. Take out the cooled block product, crush it mechanically, grind it, and then pass it through a 200-mesh sieve to collect the fine powder that passes through the sieve to obtain the pre-carbonized precursor.

4. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 3, characterized in that, Step S3 specifically includes the following steps: S31. Place the pre-carbonized precursor in a quartz boat of a tubular furnace, first introduce inert gas to purge the air, then heat it to 600-800℃ at a heating rate of 2-5℃ / min, and hold it at this temperature for 0.5-1h to remove residual volatiles. S32. While maintaining the inert gas supply, switch to a mixed atmosphere of hydrogen and inert gas, control the hydrogen volume ratio to 10-30%, continue to heat to 1100-1400℃ at a heating rate of 1-3℃ / min, and hold for 2-5 hours to suppress excessive carbon layer growth by using hydrogen etching. S33. 30-60 minutes before the end of the heat treatment at 1100-1400℃, switch back to inert gas and maintain this atmosphere until the reaction is complete. S34. Maintain inert gas protection to allow the material inside the furnace to cool naturally to room temperature, then remove it, crush and grind it to obtain a preliminary hard carbon matrix.

5. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 4, characterized in that, Step S4 specifically includes the following steps: S41. Add the preliminary hard carbon matrix to an aqueous solution at a solid-liquid ratio of 1:5-1:

10. The aqueous solution contains phosphoric acid, urea and boric acid. The dopant source molecules diffuse into the pores and surface defect sites of the preliminary hard carbon matrix through impregnation. S42. Place the mixed system in a constant temperature water bath at 60-80℃ and stir continuously for 2-4 hours to promote the adsorption or exchange between the doped source ions and the oxygen-containing functional groups of the preliminary hard carbon matrix. S43. Take out the saturated material and dry it at 80-100℃ to constant weight. The difference between two consecutive weighings should be ≤0.5mg. S44. Place the dried product in the reaction chamber, introduce ammonia or nitrogen gas, and calcine at 700-900℃ for 1-3 hours at a rate of 2-5℃ / min to induce doped atoms to embed into the carbon lattice, thereby obtaining a doped modified hard carbon matrix.

6. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 5, characterized in that, Step S5 specifically includes the following steps: S51. Disperse the doped and modified hard carbon matrix in an alcohol solvent, and add an organic carbon source with a mass of 5%-20% of the doped and modified hard carbon matrix, so that the organic carbon source molecules can fully contact and diffuse into the pores of the doped and modified hard carbon matrix. S52. The mixed slurry is heated and refluxed in an oil bath at 70-90℃ for 1-3 hours. The solvent is prevented from evaporating by a condenser. The organic carbon source undergoes prepolymerization in the pores of the doped and modified hard carbon matrix. S53. Filter the refluxed mixture, dry the solid material at 80-100℃ to constant weight, and weigh it twice consecutively. The difference in mass should be ≤0.5mg. S54. Place the dried sample under an inert atmosphere and heat it to 1000-1200℃ at a rate of 3-8℃ / min. Hold the temperature for 0.5-2h. The precursor polymer will be decomposed and carbonized, filling the pore defects and obtaining a hard carbon matrix with reconstructed pores.

7. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 6, characterized in that, Step S6 specifically includes the following steps: S61. Dissolve lithium, sodium and potassium chlorides in deionized water in a molar ratio of 4:4:2-5:3:2 to prepare a mixed salt solution; then add the pore-reconstructed hard carbon matrix, controlling the mass ratio of the pore-reconstructed hard carbon matrix to the chloride to be 1:0.5-1:

2. S62. Stir continuously in a constant temperature water bath of 60-80℃ for 2-4 hours to utilize capillary action to fully wet and pre-fill the pores of the hard carbon matrix after pore reconstruction with salt ions. S63. Freeze-dry the stirred slurry, with a cold trap temperature ≤ -50℃, a vacuum degree ≤ -0.08MPa, and a drying time of 24-48h. Avoid salt particle migration and agglomeration through ice crystal sublimation, and fix chloride particles in situ. S64. Take out the freeze-dried product and disperse it evenly by mechanical grinding to obtain a hard carbon matrix with molten salt carrier pre-embedded.

8. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 7, characterized in that, Step S7 specifically includes the following steps: S71. Place the hard carbon matrix pre-embedded in molten salt carrier in the reaction chamber, and under a protective atmosphere, heat it to 650-850℃ at 3-8℃ / min and hold it for 1-3 hours to melt the pre-embedded chloride and grow a carbonaceous coating layer on the surface of the hard carbon matrix pre-embedded in molten salt carrier. S72. After the heat preservation is completed, stop heating and slowly cool to room temperature at a rate of ≤5℃ / min; S73. Take out the cooled product and wash it repeatedly with deionized water until the conductivity of the filtrate is less than 10 μS / cm to remove residual soluble salts. Then dry it at 80-100℃ and vacuum degree ≤-0.08MPa for 12-24h to obtain the surface-coated hard carbon material.

9. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 8, characterized in that, In the biomass raw materials, coconut shells and walnut shells account for no less than 60% of the total mass, and the straw raw materials are selected from one or more of corn straw, wheat straw, or cotton straw.

10. The optimized preparation process of carbonization of biomass-based precursor for sodium-ion battery hard carbon anode according to claim 9, characterized in that, The organic carbon source in step S51 is at least two of glucose, sucrose and citric acid, and the mass ratio of glucose to sucrose is 1:0.5-1:2.

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