Preparation method of sodium storage hard carbon negative electrode material

Abstract

This invention provides a method for preparing a sodium-storage hard carbon anode material, relating to the field of battery electrode material preparation. The method includes: step S1, preparation of a composite solvent system; step S2, cellulose swelling; step S3, cellulose regeneration and phase separation; step S4, filtration and washing; step S5, drying; and step S6, pyrolysis and carbonization. Through a synergistic process involving molecular chain swelling and decrystallization, interface covalent anchoring, precise pore formation through phase separation, gradient pyrolysis and carbonization, and in-situ interface etching, the structural mutual exclusion limitation between the graphite domain conductive network and the closed-pore sodium storage sites is overcome. This achieves comprehensive optimization of the overall performance of the hard carbon anode material, resulting in a hard carbon anode material with excellent electrochemical performance.

Description

[0001] This invention is a divisional application of patent application number 2026104849477, entitled "A method for preparing a cellulose-based hard carbon anode material". Technical Field

[0002] This invention relates to the field of battery electrode material preparation technology, and in particular to a method for preparing a sodium-storage hard carbon anode material. Background Technology

[0003] With the rapid development of large-scale energy storage power stations and low-speed electric vehicles, sodium-ion batteries, thanks to their abundant and widely distributed sodium resources, low manufacturing costs, and excellent safety performance, have become an important alternative to lithium-ion batteries in energy storage and low-to-medium power applications, and their industrialization process continues to accelerate. The anode material is the core component determining the electrochemical performance and manufacturing cost of sodium-ion batteries. Hard carbon materials, due to their disordered layered structure and abundant internal pores, can accommodate the insertion / extraction and storage of sodium ions, making them the only commercially viable anode material for sodium-ion batteries.

[0004] Among the many raw materials for hard carbon production, cellulose-based biomass is widely available, highly renewable, and has a high carbon conversion rate, making it the mainstream precursor for low-cost hard carbon materials. However, natural cellulose has a highly crystalline molecular chain structure, and hard carbon obtained by direct pyrolysis and carbonization suffers from problems such as uneven graphite domain size, low order, and a pore structure dominated by open and macropores, with a low proportion of closed pores and insufficient low-potential plateau capacity, making it difficult to meet the application requirements of high-energy-density batteries.

[0005] To improve the electrochemical performance of cellulose-based hard carbon, existing technologies often employ physical activation, chemical activation, or solution regeneration modification to regulate the pore structure of the precursor. While activation can increase the porosity of the material, it easily leads to an increase in carbon framework defects and excessive enrichment of surface functional groups, resulting in low coulombic efficiency in the first cycle. Furthermore, the activation process easily destroys the closed-pore structure, forming a large number of open pores, making it difficult to achieve effective low-potential sodium storage. Solution regeneration can reduce the crystallinity of cellulose to some extent, but the bidirectional diffusion rate of solvent and non-solvent during conventional phase separation is difficult to control, easily forming a dense surface skin and an internal macroporous structure. Moreover, capillary forces during subsequent drying can easily cause the nanopores to collapse, making it impossible to stably retain the closed-pore structure. More importantly, existing modification methods cannot solve the core problem of shrinkage mismatch between the microcrystalline and amorphous regions during pyrolysis and carbonization: the difference in shrinkage rates between the two phases during carbonization generates internal stress, which causes microcracks in the carbon skeleton and tearing of the closed-pore walls. This not only destroys the sodium storage closed-pore space but also interrupts the graphite domain conductive network. Ultimately, this leads to a "one-for-one" dilemma in the specific capacity, rate performance, and cycle stability of hard carbon materials, making it impossible to achieve synergistic improvement of multiple properties. This severely restricts the large-scale application and performance upgrade of cellulose-based hard carbon materials. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a method for preparing a sodium-storage hard carbon anode material. This method overcomes the structural mutual exclusion limitation between the graphite domain conductive network and the closed-pore sodium storage sites through a synergistic process of molecular chain swelling and decrystallization, interface covalent anchoring, phase separation for precise pore formation, gradient pyrolysis carbonization, and in-situ interface etching, thereby achieving comprehensive optimization of the overall performance of the hard carbon anode material.

[0007] The objective of this invention is achieved through the following technical solution: A method for preparing a sodium-storage hard carbon anode material includes: Step S1, Preparation of the composite solvent system: Anhydrous LiCl is pretreated and added to DMAc (N,N-dimethylacetamide) solvent, and stirred in a constant temperature oil bath to obtain a DMAc / LiCl composite solvent, which is then sealed for later use; Step S2, Cellulose swelling: Cellulose, KH560, and triethylamine are added sequentially to the DMAc / LiCl composite solvent and stirred to form a premixed system; the premixed system is then placed in a constant temperature oil bath and stirred to swell, followed by vacuum degassing to obtain a cellulose composite dispersion; Step S3, Cellulose regeneration phase separation: A pre-prepared coagulation bath is prepared... The vacuum-degassed cellulose composite dispersion was added dropwise at a uniform rate to a coagulation bath. After the addition was completed, the mixture was allowed to stand at a constant temperature to obtain a regenerated phase dispersion. Step S4: Filtration and washing: The regenerated phase dispersion was subjected to solid-liquid separation, primary coordination washing, secondary anchoring washing, and tertiary solvent replacement in sequence to obtain cellulose regenerated wet material. Step S5: Drying: The cellulose regenerated wet material after filtration and washing in step S4 was subjected to supercritical CO2 gradient drying to obtain cellulose regenerated powder. Step S6: Pyrolysis and carbonization: The cellulose regenerated powder was subjected to pyrolysis and carbonization and etching treatment in sequence to obtain a hard carbon anode material for sodium-ion batteries.

[0008] Based on further optimization of the above scheme, in step S1, the anhydrous LiCl pretreatment specifically involves: placing the anhydrous LiCl in a vacuum oven and drying it for 3.5 to 4.5 hours at a temperature of 118–122°C and a vacuum of -0.08 to -0.1 MPa to remove the water of crystallization. After drying, immediately place it in a desiccator to cool to room temperature, avoiding contact with air and moisture absorption throughout the process. The mass ratio of anhydrous LiCl to DMAc solvent is 5–15:100; the stirring is specifically carried out in a constant temperature oil bath at 30–60°C, with a stirring speed of 280–320 rpm for 0.5–2 hours.

[0009] Based on further optimization of the above scheme, in step S2, the mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 35–45:0.42–0.54:0.7–0.9:1000; the premixed system is stirred at a speed of 180–220 rpm for 8–12 minutes. The cellulose has a mesh size of 200–1000 mesh; the cellulose is one or more of coconut shell fiber, bamboo fiber, and straw fiber.

[0010] Based on further optimization of the above scheme, in step S2, the swelling specifically involves: continuously stirring at a speed of 180–220 rpm in a constant-temperature oil bath at 70–75°C; simultaneously applying low-frequency pulsed ultrasound during stirring, with ultrasound parameters of: frequency 28–32 kHz and power density 7.5–8.5 W / cm². 2 The pulse duty cycle is 1:3, and the treatment is continued for 12 to 24 hours. Vacuum degassing is specifically performed as follows: after swelling, the composite dispersion is placed in a vacuum drying oven and degassed for 28 to 32 minutes at 24 to 26°C and -0.08 to -0.1 MPa.

[0011] Based on further optimization of the above scheme, in step S3, the coagulation bath preparation is as follows: deionized water and polyethylene glycol diglycidyl ether (PEGDGE) are added to the coagulation bath and stirred at a low speed of 45-55 rpm until completely dissolved. The volume-to-mass ratio of deionized water to PEGDGE is 9-11 L: 27-33 g. After dissolution, the mixture is placed in a constant temperature water bath to stabilize the system temperature at 20-50℃. In step S3, the cellulose regeneration phase separation is as follows: first, the vacuum-degassed cellulose composite dispersion is slowly added to the coagulation bath at a rate of 1-5 mL / min using a constant flow peristaltic pump. The height of the adding needle from the surface of the coagulation bath is 9.5-10.5 cm. During the adding process, the temperature of the coagulation bath system is kept stable at 20-50℃ to ensure that the droplets fall vertically and evenly into the coagulation bath. After the adding is completed, the mixture is allowed to stand at a constant temperature of 20-50℃ for 28-32 minutes.

[0012] Based on further optimization of the above scheme, in step S4, the solid-liquid separation specifically involves: using a Buchner funnel to vacuum filter the regenerated phase dispersion obtained in step S3 to obtain a cellulose wet filter cake, and filtering until no filtrate drips; the primary coordination washing specifically involves: adding the cellulose wet filter cake to the primary coordination washing solution, with a mass-to-volume ratio of cellulose wet filter cake to primary coordination washing solution of 1:20, stirring at 23-27°C and a speed of 140-160 rpm for 28-32 minutes, followed by vacuum filtration, and repeating the operation 3-4 times; the secondary anchoring washing specifically involves: the primary coordination... The washed filter cake is added to the secondary anchoring washing solution at a mass-to-volume ratio of 1:10, and soaked at 23–27°C for 28–32 min. Then, it undergoes vacuum filtration, deionized water rinsing, and vacuum filtration again. The tertiary solvent replacement involves adding the rinsed, vacuum-filtered filter cake to anhydrous ethanol at a mass-to-volume ratio of 1:15, stirring at 140–160 rpm for 18–22 min at 23–27°C, followed by vacuum filtration. This process is repeated 3–4 times to displace the internal moisture of the particles, obtaining regenerated cellulose wet material. The primary coordination washing solution comprises 12-crown ether-4, triethanolamine, anhydrous ethanol, and deionized water in a mass ratio of 0.9–1.1:1.8–2.2:78.3–95.7:99–121. The secondary anchoring washing solution is a 0.1 wt% deionized cellulose nanocrystal dispersion.

[0013] Based on further optimization of the above scheme, step S5 specifically involves: using supercritical CO2 gradient drying, firstly, evenly spreading the regenerated cellulose wet material in the material basket of the supercritical reactor, with a filling thickness not exceeding 2 cm, and sealing the reactor after filling; then, introducing liquid CO2 into the reactor, slowly raising the temperature to 37–39°C, and simultaneously increasing the pressure to 8.5–9.5 MPa, and statically soaking for 2.8–3.2 h under constant temperature and pressure, allowing the supercritical CO2 to completely penetrate into the nanopores inside the particles and replace the ethanol solvent; then, slowly reducing the pressure at a constant rate of 0.14–0.16 MPa / min until the pressure in the reactor drops to atmospheric pressure, with the temperature remaining stable at 37–39°C throughout the depressurization process; after depressurization, opening the reactor, removing the regenerated cellulose powder, and immediately placing it in a desiccator for sealed storage.

[0014] Based on further optimization of the above scheme, in step S6, the pyrolysis carbonization specifically involves: first, uniformly spreading the cellulose regenerated powder in the corundum boat, with a filling thickness not exceeding 5 mm; placing the boat in the constant temperature zone of a tubular furnace, introducing inert gas as a protective gas, and continuously purging at a rate of 90–110 mL / min for 28–32 min to purge the air from the tubular furnace and prevent oxygen from seeping in; then, rapidly heating to 790–810 °C at a heating rate of 19.5–20.5 °C / min and holding at that temperature for 28–32 min; subsequently, slowly heating to 1300–1500 °C at a heating rate of 1.5–2.5 °C / min and holding at that temperature for 5–7 h; finally, cooling to room temperature at a cooling rate of 4.5–5.5 °C / min, shutting off the inert gas supply, and removing the carbonized hard carbon crude product.

[0015] Based on further optimization of the above scheme, in step S6, the etching process specifically involves: placing the crude hard carbon material into a 9.5–10.5 wt% hydrofluoric acid solution, with a mass-to-volume ratio of crude hard carbon material to hydrofluoric acid solution of 1:10; stirring at 180–220 rpm for 1.8–2.2 h at 25 ± 1 °C to etch away the nano-SiO2 generated by the pyrolysis of KH560, while simultaneously forming additional nanopores at the microcrystalline-amorphous interface; subsequently, repeatedly rinsing the etched hard carbon material with deionized water until the pH of the filtrate is 6–7 to avoid residual F. - Ions affect electrochemical performance; after rinsing, the hard carbon material is dried in a vacuum oven at 115-125℃ and -0.08--0.1MPa for 11-13 hours, and then sieved to obtain the hard carbon anode material for sodium-ion batteries.

[0016] The following are the technical effects of this solution: This invention utilizes Li to prepare a DMAc / LiCl composite solvent. +The coordination of PEGDGE with cellulose hydroxyl groups and the disruption of hydrogen bonds between cellulose molecules lay the foundation for the uniform swelling and modification of cellulose in the subsequent process. During the swelling process, KH560 undergoes a ring-opening addition reaction with cellulose hydroxyl groups. Combined with the catalytic effect of triethylamine and the synergistic effect of temperature and catalyst in a constant-temperature oil bath, covalent bonds are formed at the microcrystalline-amorphous interface to anchor the cellulose, avoiding problems such as local stress concentration leading to pore tearing and microcracks in the carbon skeleton during subsequent pyrolysis and carbonization. During the regeneration phase separation process, a specific coagulation bath is used to utilize the cross-linking effect of PEGDGE and the anchoring effect of KH560 to form a dual structural stability of "surface cross-linking + internal anchoring". This avoids the collapse of the pore structure during subsequent washing and drying processes. At the same time, the cross-linking effect of PEGDGE forms an interface layer on the surface of cellulose droplets that allows unidirectional diffusion of DMAc molecules and blocks the inward diffusion of water molecules, thereby avoiding problems such as the opening of closed pore structures or widening of pore size caused by bidirectional diffusion mismatch. This invention utilizes a swelling step and a regenerated phase separation step to cause the cellulose molecular chains to swell and reconstruct, reducing the crystallinity of cellulose and transforming it from a predominantly crystalline region into a microcrystalline-amorphous coexisting structure. This results in the formation of numerous closed-pore structures during pyrolysis, while the graphite domain structures formed in the microcrystalline regions provide a continuous electronic conductivity network. The closed-pore structure creates excellent sodium storage space, effectively improving the sodium storage performance of the hard carbon anode. Furthermore, this invention employs a washing process involving solid-liquid separation, primary coordination washing, secondary anchoring washing, and tertiary solvent replacement to thoroughly remove the Li groups coordinated to the cellulose hydroxyl groups. + With free Li + To avoid Li during pyrolysis + Excessive growth of catalytic graphite domains leads to uncontrollable domain size and breakage of the continuous conductive network. It also ensures that the washing solution fully penetrates the inner wall of the particles and prevents impurity encapsulation. Supercritical drying, utilizing the absence of gas-liquid phase change in supercritical CO2, effectively avoids the ice crystal growth of freeze-drying and the capillary shrinkage of oven drying that cause closed-pore rupture, preserving the nanoporous structure formed by phase separation while removing residual DMAc and ethanol. Reverse gradient pyrolysis carbonization, in synergy with KH560 interface anchoring, eliminates the internal stress caused by two-phase shrinkage mismatch, achieving the synergistic existence of graphite domains and closed-pore structures, effectively resolving the mutual repulsion between hard carbon graphite domains and closed-pore structures. Etching is used to form additional nanopores at the microcrystalline-amorphous interface, further expanding the sodium storage space.

[0017] This solution effectively addresses the problem of the repulsion between interlayer graphite domain embedding sites and closed-pore filling sites in existing hard carbon anode materials. The prepared hard carbon material possesses both continuous interlayer graphite domain embedding sites and high closed-pore ratio pore filling sites, synergistically enhancing sodium storage sites. Simultaneously, the carbon framework is free of microcracks and the pore structure does not collapse, resulting in excellent cycle stability. Furthermore, the abundant nanopores construct efficient ion transport channels, which, combined with the continuous conductive network, result in high capacity retention, making it suitable for the high-rate charge and discharge requirements of sodium-ion battery power and energy storage scenarios. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the molecular chain structure during the cellulose swelling and rephase separation process in an embodiment of the present invention.

[0019] Figure 2 The images shown are high-resolution transmission electron microscope images of the hard carbon anode material in the embodiments of the present invention; wherein, a) is a graphite domain framework structure and closed-pore distribution diagram of the hard carbon material, and b) is a microporous structure and graphite domain detail feature diagram of the hard carbon material. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0021] Example 1: A method for preparing a sodium-storage hard carbon anode material includes: Step S1, Preparation of the composite solvent system: The pretreatment of anhydrous LiCl is as follows: Anhydrous LiCl is placed in a vacuum oven and dried at 118℃ and -0.08MPa for 4.5h to remove the water of crystallization. After drying, it is immediately placed in a desiccator to cool to room temperature, avoiding contact with air and water absorption throughout the process. The pretreated anhydrous LiCl is added to DMAc (N,N-dimethylacetamide, CAS: 127-19-5) solvent and stirred in a constant temperature oil bath. The mass ratio of anhydrous LiCl to DMAc solvent is 5:100. The stirring and mixing are as follows: Stir at 280rpm for 2h in a constant temperature oil bath at 30℃. The DMAc / LiCl composite solvent is obtained and sealed for later use.

[0022] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidyl etheroxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine were sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent was 35:0.42:0.7:1000. The mixture was stirred to form a premixed system, specifically: stirred at 180 rpm for 12 minutes. The cellulose used was 300-mesh coconut shell fiber. The premixed system was then placed in a constant-temperature oil bath for swelling, specifically: continuously stirred at 180 rpm in a 70°C constant-temperature oil bath. Low-frequency pulsed ultrasound was applied simultaneously during stirring, with the following parameters: frequency 28 kHz, power density 7.5 W / cm³. 2 The pulse duty cycle was 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment was continued for 24 hours. After swelling, vacuum degassing was performed. Specifically, after swelling, the composite dispersion was placed in a vacuum drying oven and degassed at 24°C and -0.08MPa for 32 minutes.

[0023] Step S3, Cellulose Regeneration Phase Separation: A pre-prepared coagulation bath is prepared as follows: Deionized water and polyethylene glycol diglycidyl ether (PEGDGE, CAS: 39443-66-8) are added to the coagulation bath and stirred at a low speed of 45 rpm until completely dissolved. The volume-to-mass ratio of deionized water to polyethylene glycol diglycidyl ether is 9 L: 27 g. After dissolution, the mixture is placed in a constant-temperature water bath to stabilize the system temperature at 20℃. The vacuum-degassed cellulose composite dispersion is then added dropwise to the coagulation bath at a uniform rate. After addition, the mixture is kept at a constant temperature and allowed to stand. Specifically, a constant-flow peristaltic pump is used to slowly add the vacuum-degassed cellulose composite dispersion to the coagulation bath at a rate of 1 mL / min. The height of the dropper tip from the surface of the coagulation bath is 9.5 cm. During the addition process, the temperature of the coagulation bath system is kept stable at 20℃ to ensure that the droplets fall vertically and uniformly into the coagulation bath. After addition, the mixture is kept at a constant temperature of 20℃ for 32 min to obtain the regenerated phase dispersion.

[0024] Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation, primary coordination washing, secondary anchoring washing and tertiary solvent replacement in sequence; The solid-liquid separation process involves vacuum filtration of the regenerated phase dispersion obtained in step S3 using a Buchner funnel to obtain a wet cellulose filter cake. Filtration continues until no filtrate drips, avoiding over-filtration which could cause the filter cake to crack and collapse. The primary coordination washing process involves adding the wet cellulose filter cake to the primary coordination washing solution at a mass-to-volume ratio of 1:20. The mixture is stirred at 23°C and 140 rpm for 32 minutes, followed by vacuum filtration. This process is repeated three times to completely remove residual Li+ strongly coordinated with the cellulose hydroxyl groups. +The primary coordination washing solution comprises 12-crown ether-4 (CAS: 294-93-9), triethanolamine (CAS: 102-71-6), anhydrous ethanol, and deionized water in a mass ratio of 0.9:1.8:78.3:99. The secondary anchoring washing process involves adding the filter cake from the primary coordination washing to the secondary anchoring washing solution at a mass-to-volume ratio of 1:10. The mixture is soaked at 23°C for 32 minutes, followed by vacuum filtration, deionized water rinsing, and then vacuum filtration again to remove free Li. + Inclusion complex and residual reagents; wherein, the secondary anchoring washing solution is a deionized water dispersion of 0.1 wt% sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent); the tertiary solvent replacement is specifically as follows: the filter cake after rinsing and vacuum filtration is added to anhydrous ethanol, the mass-to-volume ratio of filter cake to anhydrous ethanol is 1:15, and after stirring at 23℃ and 140 rpm for 22 min, vacuum filtration is performed. The operation is repeated 3 times to replace the water inside the particles and obtain cellulose regenerated wet material.

[0025] Step S5, Drying: The cellulose regenerated wet material after filtration and washing in Step S4 is subjected to supercritical CO2 gradient drying to obtain cellulose regenerated powder. Specifically, the cellulose regenerated wet material is first evenly spread in the material basket of the supercritical reactor, with a filling thickness not exceeding 2 cm to avoid uneven drying caused by particle stacking. After filling, the reactor is sealed. Liquid CO2 is then introduced into the reactor, and the temperature is slowly raised to 37°C while the pressure is simultaneously increased to 8.5 MPa. The material is statically soaked for 3.2 hours under constant temperature and pressure to allow the supercritical CO2 to completely penetrate into the nanopores inside the particles and replace the ethanol solvent. Afterward, the pressure is slowly reduced at a constant rate of 0.14 MPa / min until the pressure in the reactor drops to atmospheric pressure. The temperature is kept stable at 37°C throughout the depressurization process to avoid the CO2 phase change generating capillary force and damaging the closed-pore structure. After the depressurization is completed, the reactor is opened, the cellulose regenerated powder is taken out, and it is immediately placed in a desiccator for sealed storage.

[0026] Step S6, pyrolysis and carbonization: The cellulose regenerated powder is subjected to pyrolysis and carbonization and etching treatment in sequence to obtain the hard carbon anode material for sodium-ion batteries; The pyrolysis carbonization process specifically involves: first, uniformly spreading the regenerated cellulose powder in a corundum boat, with a filling thickness not exceeding 5 mm to avoid uneven carbonization due to stacking; placing the boat in the constant-temperature zone of a tube furnace and introducing an inert gas (such as argon with a purity ≥99%) as a protective gas, continuously purging at a rate of 90 mL / min for 32 min to purge the air from the tube furnace and prevent oxygen infiltration; then, rapidly heating to 790℃ at a rate of 19.5℃ / min and holding at that temperature for 32 min to allow the amorphous cellulose region to complete the main dehydroxylation, volatile matter removal, and volume shrinkage and shaping reactions. Simultaneously, KH560 undergoes preliminary decomposition at this stage, generating nano-SiO2 which anchors at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases; subsequently, heating at a rate of 1.5℃ / min... The temperature was slowly increased to 1300℃ and held for 7 hours to allow the cellulose microcrystalline region to slowly graphitize, forming a uniform and continuous graphite domain conductive network. Finally, the temperature was lowered to room temperature at a rate of 4.5℃ / min, the inert gas supply was shut off, and the carbonized hard carbon crude material was removed. The etching process involved placing the hard carbon crude material in a 9.5wt% hydrofluoric acid solution (mass-volume ratio of hard carbon crude material to hydrofluoric acid solution: 1:10) and stirring at 180 rpm for 2.2 hours at 25±1℃ to etch away the nano-SiO2 generated by the pyrolysis of KH560, while simultaneously forming additional nanopores at the microcrystalline-amorphous interface. Afterward, the etched hard carbon material was repeatedly rinsed with deionized water until the pH of the filtrate was 6 to avoid residual F. - Ions affect electrochemical performance; after rinsing, the hard carbon material is dried in a vacuum oven at 115℃ and -0.08MPa for 13 hours, and then passed through a 300-mesh sieve to obtain the hard carbon anode material for sodium-ion batteries.

[0027] Example 2: A method for preparing a sodium-storage hard carbon anode material includes: Step S1, Preparation of the composite solvent system: The pretreatment of anhydrous LiCl is as follows: Anhydrous LiCl is placed in a vacuum oven and dried at 120℃ and -0.09MPa for 4 hours to remove the water of crystallization. After drying, it is immediately placed in a desiccator to cool to room temperature, avoiding contact with air and water absorption throughout the process. The pretreated anhydrous LiCl is added to DMAc (N,N-dimethylacetamide, CAS: 127-19-5) solvent and stirred in a constant temperature oil bath. The mass ratio of anhydrous LiCl to DMAc solvent is 10:100. The stirring and mixing are as follows: Stir at 300rpm for 1 hour in a constant temperature oil bath at 45℃. The DMAc / LiCl composite solvent is obtained and sealed for later use.

[0028] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidyl etheroxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine were sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent was 40:0.48:0.8:1000. The mixture was stirred to form a premixed system, specifically: stirring at 200 rpm for 10 min; the cellulose used was 400-mesh bamboo fiber. The premixed system was then placed in a constant-temperature oil bath for swelling, specifically: stirring continuously at 200 rpm in a constant-temperature oil bath at 72.5℃; low-frequency pulsed ultrasound was applied simultaneously during the stirring process, with the ultrasound parameters being: frequency 30 kHz, power density 8 W / cm². 2 The pulse duty cycle was 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment was continued for 18 hours. After swelling, vacuum degassing was performed. Specifically, after swelling, the composite dispersion was placed in a vacuum drying oven and degassed at 25°C and -0.09 MPa for 30 minutes.

[0029] Step S3, Cellulose Regeneration Phase Separation: A pre-prepared coagulation bath is prepared as follows: Deionized water and polyethylene glycol diglycidyl ether (PEGDGE, CAS: 39443-66-8) are added to the coagulation bath and stirred at a low speed of 50 rpm until completely dissolved. The volume-to-mass ratio of deionized water to polyethylene glycol diglycidyl ether is 10 L: 30 g. After dissolution, the mixture is placed in a constant-temperature water bath to stabilize the system temperature at 35°C. The vacuum-degassed cellulose composite dispersion is then added dropwise to the coagulation bath at a uniform rate. After addition, the mixture is kept at a constant temperature and allowed to stand. Specifically, a constant-flow peristaltic pump is used to slowly add the vacuum-degassed cellulose composite dispersion to the coagulation bath at a rate of 3 mL / min. The distance between the dropper tip and the surface of the coagulation bath is 10 cm. During the addition process, the temperature of the coagulation bath system is kept stable at 35°C to ensure that the droplets fall vertically and evenly into the coagulation bath. After addition, the mixture is kept at a constant temperature of 35°C for 30 min to obtain the regenerated phase dispersion.

[0030] Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation, primary coordination washing, secondary anchoring washing and tertiary solvent replacement in sequence; The solid-liquid separation process involves vacuum filtration of the regenerated phase dispersion obtained in step S3 using a Buchner funnel to obtain a wet cellulose filter cake. Filtration continues until no filtrate drips, avoiding over-filtration which could cause the filter cake to crack and collapse. The primary coordination washing process involves adding the wet cellulose filter cake to the primary coordination washing solution at a mass-to-volume ratio of 1:20. The mixture is stirred at 150 rpm for 30 minutes at 25°C, followed by vacuum filtration. This process is repeated three times to completely remove residual Li+ strongly coordinated with the cellulose hydroxyl groups. +The primary coordination washing solution comprises 12-crown ether-4 (CAS: 294-93-9), triethanolamine (CAS: 102-71-6), anhydrous ethanol, and deionized water in a mass ratio of 1:2:87:110. The secondary anchoring washing process involves adding the filter cake from the primary coordination washing to the secondary anchoring washing solution at a mass-to-volume ratio of 1:10. The mixture is soaked at 25°C for 30 minutes, followed by vacuum filtration, deionized water rinsing, and then vacuum filtration again to remove free Li. + Inclusion complex and residual reagents; wherein, the secondary anchoring washing solution is a deionized water dispersion of 0.1wt% sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent); the tertiary solvent replacement is as follows: the filter cake after rinsing and vacuum filtration is added to anhydrous ethanol, the mass-volume ratio of filter cake to anhydrous ethanol is 1:15, and after stirring at 25℃ and 150rpm for 20min, vacuum filtration is performed. The operation is repeated 3 times to replace the water inside the particles and obtain cellulose regenerated wet material.

[0031] Step S5, Drying: The cellulose regenerated wet material after filtration and washing in Step S4 is subjected to supercritical CO2 gradient drying to obtain cellulose regenerated powder. Specifically, the cellulose regenerated wet material is first evenly spread in the material basket of the supercritical reactor, with a filling thickness not exceeding 2 cm to avoid uneven drying caused by particle stacking. After filling, the reactor is sealed. Liquid CO2 is then introduced into the reactor, and the temperature is slowly raised to 38°C while the pressure is simultaneously increased to 9 MPa. The material is statically soaked for 3 hours under constant temperature and pressure to allow the supercritical CO2 to completely penetrate into the nanopores inside the particles and replace the ethanol solvent. Afterward, the pressure is slowly reduced at a constant rate of 0.15 MPa / min until the pressure in the reactor drops to atmospheric pressure. The temperature is kept stable at 38°C throughout the depressurization process to avoid the CO2 phase change generating capillary force and damaging the closed-pore structure. After the depressurization is completed, the reactor is opened, the cellulose regenerated powder is taken out, and it is immediately placed in a desiccator for sealed storage.

[0032] Step S6, pyrolysis and carbonization: The cellulose regenerated powder is subjected to pyrolysis and carbonization and etching treatment in sequence to obtain the hard carbon anode material for sodium-ion batteries; The pyrolysis carbonization process specifically involves: first, uniformly spreading cellulose regenerated powder in a corundum boat, with a filling thickness not exceeding 5 mm to avoid uneven carbonization due to stacking; placing the boat in the constant-temperature zone of a tube furnace, and introducing an inert gas (such as argon with a purity ≥99%) as a protective gas, continuously purging at a rate of 100 mL / min for 30 min to purge the air from the tube furnace and prevent oxygen infiltration; then, rapidly heating to 800℃ at a rate of 20℃ / min and holding at that temperature for 30 min, allowing the amorphous cellulose region to complete the main dehydroxylation, volatile matter removal, and volume shrinkage and shaping reactions, while KH560 initially decomposes at this stage, generating nano-SiO2 and anchoring it at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases; subsequently, heating at a rate of 2℃ / min... The temperature was slowly increased to 1400℃ and held for 6 hours to allow the cellulose microcrystalline region to slowly graphitize, forming a uniform and continuous conductive network of graphite domains. Finally, the temperature was lowered to room temperature at a rate of 5℃ / min, the inert gas supply was shut off, and the carbonized hard carbon crude material was removed. The etching process involved placing the hard carbon crude material in a 10wt% hydrofluoric acid solution (mass-volume ratio of hard carbon crude material to hydrofluoric acid solution: 1:10) and stirring at 200 rpm for 2 hours at 25±1℃ to etch away the nano-SiO2 generated by the pyrolysis of KH560, while simultaneously forming additional nanopores at the microcrystalline-amorphous interface. Afterward, the etched hard carbon material was repeatedly rinsed with deionized water until the pH of the filtrate reached 6.5 to avoid residual F. - Ions affect electrochemical performance; after rinsing, the hard carbon material is dried in a vacuum oven at 120℃ and -0.09MPa for 12 hours, and then passed through a 400-mesh sieve to obtain the hard carbon anode material for sodium-ion batteries.

[0033] Example 3: A method for preparing a sodium-storage hard carbon anode material includes: Step S1, Preparation of the composite solvent system: The pretreatment of anhydrous LiCl is as follows: Anhydrous LiCl is placed in a vacuum oven and dried at 122℃ and -0.1MPa for 3.5h to remove the water of crystallization. After drying, it is immediately placed in a desiccator to cool to room temperature, avoiding contact with air and water absorption throughout the process. The pretreated anhydrous LiCl is added to DMAc (N,N-dimethylacetamide, CAS: 127-19-5) solvent and stirred in a constant temperature oil bath. The mass ratio of anhydrous LiCl to DMAc solvent is 15:100. The stirring and mixing are as follows: Stir at 320rpm for 0.5h in a constant temperature oil bath at 60℃. The DMAc / LiCl composite solvent is obtained and sealed for later use.

[0034] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine were sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent was 45:0.54:0.9:1000. The mixture was stirred to form a premixed system, specifically by stirring at 220 rpm for 8 minutes. The cellulose used was 500-mesh straw fiber. The premixed system was then placed in a constant-temperature oil bath for swelling, specifically by continuously stirring at 220 rpm in a 75°C constant-temperature oil bath. Low-frequency pulsed ultrasound was applied simultaneously during the stirring process, with the ultrasound parameters being: frequency 32 kHz and power density 8.5 W / cm³. 2 The pulse duty cycle was 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment was continued for 12 hours. After swelling, vacuum degassing was performed. Specifically, after swelling, the composite dispersion was placed in a vacuum drying oven and degassed at 26°C and -0.1 MPa for 28 minutes.

[0035] Step S3, Cellulose Regeneration Phase Separation: A pre-prepared coagulation bath is prepared as follows: Deionized water and polyethylene glycol diglycidyl ether (PEGDGE, CAS: 39443-66-8) are added to the coagulation bath and stirred at a low speed of 55 rpm until completely dissolved. The volume-to-mass ratio of deionized water to polyethylene glycol diglycidyl ether is 11 L: 33 g. After dissolution, the mixture is placed in a constant-temperature water bath to stabilize the system temperature at 50℃. The vacuum-degassed cellulose composite dispersion is then added dropwise to the coagulation bath at a uniform rate. After addition, the mixture is kept at a constant temperature and allowed to stand. Specifically, a constant-flow peristaltic pump is used to slowly add the vacuum-degassed cellulose composite dispersion to the coagulation bath at a rate of 5 mL / min. The height of the dropper tip from the surface of the coagulation bath is 10.5 cm. During the addition, the temperature of the coagulation bath system is kept stable at 50℃ to ensure that the droplets fall vertically and evenly into the coagulation bath. After addition, the mixture is kept at a constant temperature of 50℃ for 28 min to obtain the regenerated phase dispersion.

[0036] Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation, primary coordination washing, secondary anchoring washing and tertiary solvent replacement in sequence; The solid-liquid separation process involves vacuum filtration of the regenerated phase dispersion obtained in step S3 using a Buchner funnel to obtain a wet cellulose filter cake. Filtration continues until no filtrate drips, avoiding over-filtration which could cause the filter cake to crack and collapse. The primary coordination washing process involves adding the wet cellulose filter cake to the primary coordination washing solution at a mass-to-volume ratio of 1:20. The mixture is stirred at 27°C and 160 rpm for 28 minutes, followed by vacuum filtration. This process is repeated four times to completely remove residual Li+ strongly coordinated with the cellulose hydroxyl groups. +The primary coordination washing solution comprises 12-crown ether-4 (CAS: 294-93-9), triethanolamine (CAS: 102-71-6), anhydrous ethanol, and deionized water in a mass ratio of 1.1:2.2:95.7:121. The secondary anchoring washing process involves adding the filter cake from the primary coordination washing to the secondary anchoring washing solution at a mass-to-volume ratio of 1:10. The mixture is soaked at 27°C for 28 minutes, followed by vacuum filtration, deionized water rinsing, and then vacuum filtration again to remove free Li. + Inclusion complex and residual reagents; wherein, the secondary anchoring washing solution is a deionized water dispersion of 0.1 wt% sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent); the tertiary solvent replacement is specifically as follows: the filter cake after rinsing and vacuum filtration is added to anhydrous ethanol, the mass-volume ratio of filter cake to anhydrous ethanol is 1:15, stirred at 27℃ and 160 rpm for 18 min and then vacuum filtered, the operation is repeated 4 times to replace the water inside the particles and obtain cellulose regenerated wet material.

[0037] Step S5, Drying: The cellulose regenerated wet material after filtration and washing in Step S4 is subjected to supercritical CO2 gradient drying to obtain cellulose regenerated powder. Specifically, the cellulose regenerated wet material is first evenly spread in the material basket of the supercritical reactor, with a filling thickness not exceeding 2 cm to avoid uneven drying caused by particle stacking. After filling, the reactor is sealed. Liquid CO2 is then introduced into the reactor, and the temperature is slowly raised to 39°C while the pressure is simultaneously increased to 9.5 MPa. The material is statically soaked for 2.8 hours under constant temperature and pressure to allow the supercritical CO2 to completely penetrate into the nanopores inside the particles and replace the ethanol solvent. Afterward, the pressure is slowly reduced at a constant rate of 0.16 MPa / min until the pressure in the reactor drops to atmospheric pressure. The temperature is kept stable at 39°C throughout the depressurization process to avoid the CO2 phase change generating capillary force and damaging the closed-pore structure. After the depressurization is completed, the reactor is opened, the cellulose regenerated powder is taken out, and it is immediately placed in a desiccator for sealed storage.

[0038] Step S6, pyrolysis and carbonization: The cellulose regenerated powder is subjected to pyrolysis and carbonization and etching treatment in sequence to obtain the hard carbon anode material for sodium-ion batteries; The pyrolysis carbonization process specifically involves: first, uniformly spreading the regenerated cellulose powder in a corundum boat, with a filling thickness not exceeding 5 mm to avoid uneven carbonization due to stacking; placing the boat in the constant-temperature zone of a tube furnace and introducing an inert gas (such as argon with a purity ≥99%) as a protective gas, continuously purging at a rate of 110 mL / min for 28 min to purge the air from the tube furnace and prevent oxygen infiltration; then, rapidly heating to 810℃ at a rate of 20.5℃ / min and holding at that temperature for 28 min to allow the amorphous cellulose region to complete the main dehydroxylation, volatile matter removal, and volume shrinkage and shaping reactions. Simultaneously, KH560 undergoes preliminary decomposition at this stage, generating nano-SiO2 which anchors at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases; subsequently, heating at a rate of 2.5℃ / min... The temperature was slowly increased to 1500℃ and held for 5 hours to allow the cellulose microcrystalline region to slowly graphitize, forming a uniform and continuous graphite domain conductive network. Finally, the temperature was lowered to room temperature at a rate of 5.5℃ / min, the inert gas supply was shut off, and the carbonized hard carbon crude material was removed. The etching process involved placing the hard carbon crude material in a 10.5wt% hydrofluoric acid solution (mass-volume ratio of hard carbon crude material to hydrofluoric acid solution: 1:10) at 25±1℃ and stirring at 220 rpm for 1.8 hours to etch away the nano-SiO2 generated by the pyrolysis of KH560, while simultaneously forming additional nanopores at the microcrystalline-amorphous interface. Afterward, the etched hard carbon material was repeatedly rinsed with deionized water until the pH of the filtrate was 7 to avoid residual F. - Ions affect electrochemical performance; after rinsing, the hard carbon material is dried in a vacuum oven at 125℃ and -0.1MPa for 11 hours, and then passed through a 500-mesh sieve to obtain the hard carbon anode material for sodium-ion batteries.

[0039] Comparative Example 1: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0040] Step S2, Cellulose Swelling: Cellulose and triethylamine are added sequentially to the DMAc / LiCl composite solvent. The mass ratio of cellulose, triethylamine, and DMAc / LiCl composite solvent is 40:0.8:1000. The mixture is stirred to form a premixed system. Specifically, the mixture is stirred at 200 rpm for 10 minutes. The cellulose used is 400-mesh bamboo fiber. The premixed system is then placed in a constant temperature oil bath for stirring and swelling, followed by vacuum degassing. The stirring and swelling and vacuum degassing processes are consistent with step S2 in Example 2.

[0041] Step S3, Cellulose regeneration phase separation: Same as step S3 in Example 2. Step S4, Filtration and washing: Same as step S4 in Example 2. Step S5, Drying: Same as step S5 in Example 2. Step S6, Pyrolysis and carbonization: Same as step S6 in Example 2.

[0042] Comparative Example 2: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2. Step S2, Swelling of cellulose: Same as step S2 in Example 2.

[0043] Step S3, Cellulose Regeneration Phase Separation: A pre-prepared coagulation bath is prepared as follows: Deionized water and anhydrous ethanol are added to the coagulation bath and stirred at a low speed of 50 rpm until completely dissolved. The volume-to-mass ratio of deionized water to anhydrous ethanol is 10 L: 30 g. After dissolution, the mixture is placed in a constant-temperature water bath to stabilize the system temperature at 35°C. The vacuum-degassed cellulose composite dispersion is then added dropwise to the coagulation bath at a uniform rate. After the addition is complete, the mixture is allowed to stand at a constant temperature. The specific steps are consistent with step S3 in Example 2.

[0044] Step S4, Filtration and washing: Same as step S4 in Example 2. Step S5, Drying: Same as step S5 in Example 2. Step S6, Pyrolysis and carbonization: Same as step S6 in Example 2.

[0045] Comparative Example 3: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2. Step S2, Swelling of cellulose: Same as step S2 in Example 2. Step S3, Separation of the cellulose regenerated phase: Same as step S3 in Example 2.

[0046] Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation and washing in sequence; The solid-liquid separation process is as follows: the regenerated phase dispersion obtained in step S3 is vacuum filtered using a Buchner funnel to obtain a cellulose wet filter cake. The filter cake is then filtered until no filtrate drips, avoiding excessive filtration that could cause the filter cake to crack and collapse. The washing process is as follows: the wet filter cake after solid-liquid separation is added to anhydrous ethanol at a mass-to-volume ratio of 1:15. The mixture is stirred at 25°C and 150 rpm for 20 minutes, followed by vacuum filtration. This process is repeated three times to remove the water inside the particles and obtain the regenerated cellulose wet material.

[0047] Step S5, Drying: Same as step S5 in Example 2. Step S6, Pyrolysis and Carbonization: Same as step S6 in Example 2.

[0048] Comparative Example 4: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2. Step S2, Swelling of cellulose: Same as step S2 in Example 2. Step S3, Separation of the cellulose regenerated phase: Same as step S3 in Example 2. Step S4, Filtration and washing: Same as step S4 in Example 2.

[0049] Step S5, Drying: The cellulose regenerated wet material after filtration and washing in step S4 is dried to obtain cellulose regenerated powder. Freeze drying is adopted, specifically: the cellulose regenerated wet material is first pre-frozen at -30℃ for 3 hours, then the pre-frozen wet material is quickly immersed in liquid nitrogen at -196℃ for 8 minutes, and then transferred to a freeze dryer. Under the conditions of cold trap temperature of -80℃ and vacuum degree of 5Pa, it is first sublimated at -10℃ for 24 hours, and then dried at 25℃ for 8 hours. The cellulose regenerated powder is then taken out and immediately placed in a desiccator for sealed storage.

[0050] Step S6, pyrolysis and carbonization: consistent with step S6 in Example 2.

[0051] Comparative Example 5: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2. Step S2, Swelling of cellulose: Same as step S2 in Example 2. Step S3, Separation of the cellulose regenerated phase: Same as step S3 in Example 2. Step S4, Filtration and washing: Same as step S4 in Example 2. Step S5, Drying: Same as step S5 in Example 2.

[0052] Step S6, Pyrolysis Carbonization: The cellulose regenerated powder is subjected to pyrolysis carbonization and etching treatment in sequence to obtain the hard carbon anode material for sodium-ion batteries. Specifically, the pyrolysis carbonization is performed as follows: the cellulose regenerated powder is placed in a tube furnace, and an inert gas (such as argon with a purity ≥99%) is introduced as a protective gas. The gas is continuously introduced at a rate of 100 mL / min for 30 min to purge the air in the tube furnace and prevent oxygen from penetrating. Then, the temperature is increased to 1400℃ at a heating rate of 8℃ / min and held for 6 h. Finally, the temperature is reduced to room temperature at a cooling rate of 5℃ / min, the inert gas is turned off, and the carbonized hard carbon crude product is taken out. The etching treatment is consistent with step S6 in Example 2.

[0053] Comparative Example 6: A method for preparing a hard carbon anode, comprising: Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2. Step S2, Swelling of cellulose: Same as step S2 in Example 2. Step S3, Separation of the cellulose regenerated phase: Same as step S3 in Example 2. Step S4, Filtration and washing: Same as step S4 in Example 2. Step S5, Drying: Same as step S5 in Example 2.

[0054] Step S6, pyrolysis carbonization: The cellulose regenerated powder is pyrolyzed and carbonized sequentially. The pyrolysis carbonization steps are consistent with step S6 in Example 2 to obtain a hard carbon anode material for sodium-ion batteries.

[0055] Example 4: A method for assembling a sodium-ion battery includes: first, adding sodium carboxymethyl cellulose (CMC, thickener) to deionized water and stirring at 2000 rpm for 2 hours to obtain a 2 wt% CMC transparent adhesive solution; then adding superconducting carbon black (conductive agent) to the transparent adhesive solution and stirring under vacuum at 3000 rpm for 30 minutes; subsequently adding hard carbon material (any hard carbon material prepared in Examples 1 to 3) in three batches, stirring under vacuum at 3000 rpm for 20 minutes after each addition, and continuing stirring for 1 hour after all additions are completed; finally, adding styrene-butadiene rubber (adhesive) and stirring at 1500 rpm. The anode slurry was obtained by vacuum stirring at low speed for 15 minutes and then allowing it to stand for 10 minutes. The mass ratio of hard carbon material, superconducting carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber was 90%, 5%, 2%, and 3%, respectively. The anode slurry was then uniformly coated onto battery-grade copper foil (8–10 μm thick, single-sided smooth). The coated electrode was then air-dried for 10 minutes and subsequently dried in a vacuum oven at 120°C and -0.1 MPa for 12 hours. The dried electrode was then punched into circular sheets using a punching machine and rolled using a roller press to control the compaction density to 1.1–1.3 g / cm³. 3 After rolling, the electrode sheet showed no powder shedding, cracking, or obvious warping. It was then dried again in a vacuum oven at 80℃ and -0.1MPa for 6 hours. Finally, a sodium metal sheet was placed in the center of the negative electrode shell (CR2032), and electrolyte (20-25 μL) was added. A diaphragm was then placed, and electrolyte (20-25 μL) was added again. The negative electrode sheet, gasket, and spring were then placed in sequence, and the positive electrode shell was installed. The entire mold was then placed in a sealing machine, with a pressure of 500-600 kg / cm². 2 Hold the pressure for 3-5 seconds to complete the encapsulation of the sodium-ion battery.

[0056] The hard carbon materials prepared in Examples 1-3 and Comparative Examples 1-6 were used as the negative electrode active materials for sodium-ion batteries and assembled into half-cells as described in Example 4. Constant current charge-discharge tests at different current densities and charge-discharge cycle tests at 5 A / g were performed on the samples using the Xinwei BTS series battery testing system (BTS8.0 / BTS9.0 software). The charge-discharge test voltages ranged from 0.01 to 3 V. The test results are shown in Table 1 below. Table 1. Charge and discharge test results

[0057] As shown in the table above: Comparative Example 1, compared to Example 2, removed the anchoring agent KH560. During pyrolysis, the internal stress generated by the difference in shrinkage rates between the microcrystalline and amorphous regions could not be released, leading to tearing of the closed-cell walls, significant collapse of closed cells, microcracks in the carbon skeleton, uneven graphite domain size, and breakage of the conductive network, thus causing a decrease in sodium storage capacity, rate performance, and cycle stability. Comparative Example 2, compared to Example 2, had a changed coagulation bath system. During phase separation, there was no interfacial cross-linking reaction, preventing the formation of a semi-permeable interfacial layer. Solvent-nonsolvent bidirectional diffusion mismatch resulted in the formation of numerous open or through-pores in the cellulose particles, leading to a decrease in sodium storage capacity, rate performance, and cycle stability. Comparative Example 3, compared to Example 2, involved simple washing with anhydrous ethanol, which removed the Li groups strongly coordinated with hydroxyl groups in the cellulose. + Unable to be effectively removed; during the first sodium embedding, a large amount of Na... + SEI side reaction with Li + Site consumption results in the highest total sodium storage capacity in the first cycle, but extremely low reversible sodium removal capacity; during the cycling process, Li + The continuous occurrence of side reactions led to a rapid drop in capacity and extremely poor cycling stability. In Comparative Example 4, the drying method was changed compared to Example 2, causing ice crystal growth, which in turn resulted in the formation of nanopores through compression and tearing phase separation. Some of these nanopores became open pores, significantly reducing sodium storage sites and thus lowering performance. In Comparative Example 5, the pyrolysis method was changed compared to Example 2, completely eliminating the core role of stepwise carbonization. Simultaneous carbonization of the amorphous and microcrystalline regions prevented the release of internal stress caused by shrinkage mismatch, leading to the collapse of the nanopores. Simultaneously, the graphitization degree in the microcrystalline region was severely uneven, failing to form a continuous conductive network, resulting in a significant drop in sodium storage capacity, rate performance, and cycling stability. In Comparative Example 6, compared to Example 2, no etching was performed after pyrolysis. The SiO2 generated by the pyrolysis of KH560 could not form additional nanopores, reducing sodium storage sites and thus lowering performance.

Claims

1. A method for preparing a sodium-storing hard carbon anode material, characterized in that: include: Step S1, Preparation of composite solvent system: After pretreatment of anhydrous LiCl, add it to DMAc solvent and stir in a constant temperature oil bath to obtain DMAc / LiCl composite solvent, and seal for later use; Step S2, Cellulose swelling: Cellulose, KH560, and triethylamine are added sequentially to the DMAc / LiCl composite solvent and stirred to form a premixed system; the premixed system is then placed in a constant temperature oil bath and stirred to swell, and after swelling, vacuum degassing is performed to obtain a cellulose composite dispersion; The vacuum degassing process is as follows: after swelling, the composite dispersion is placed in a vacuum drying oven and degassed at 24-26℃ and -0.08--0.1MPa for 28-32 minutes. Step S3, Cellulose Regeneration Phase Separation: A pre-prepared coagulation bath is used to uniformly add the vacuum-degassed cellulose composite dispersion to the coagulation bath. After the addition is completed, the mixture is kept at a constant temperature and allowed to stand to obtain the regenerated phase dispersion. Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation, primary coordination washing, secondary anchoring washing and tertiary solvent replacement in sequence to obtain cellulose regenerated wet material; Step S5, Drying: The cellulose regenerated wet material after filtration and washing in step S4 is subjected to supercritical CO2 gradient drying to obtain cellulose regenerated powder; Step S6, Pyrolysis and Carbonization: The cellulose regenerated powder is subjected to pyrolysis and carbonization and etching treatment in sequence to obtain the hard carbon anode material for sodium-ion batteries.

2. The method for preparing a sodium-storage hard carbon anode material according to claim 1, characterized in that: In step S1, the anhydrous LiCl pretreatment specifically involves: placing the anhydrous LiCl in a vacuum oven and drying it for 3.5 to 4.5 hours at a temperature of 118–122°C and a vacuum of -0.08 to -0.1 MPa; immediately after drying, placing it in a desiccator to cool to room temperature; the mass ratio of anhydrous LiCl to DMAc solvent is 5–15:100; the stirring specifically involves: stirring in a constant temperature oil bath at 30–60°C at a speed of 280–320 rpm for 0.5–2 hours.

3. The method for preparing a sodium-storage hard carbon anode material according to claim 1 or 2, characterized in that: In step S2, the mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 35-45:0.42-0.54:0.7-0.9:1000; the premixed system is stirred at 180-220 rpm for 8-12 minutes; the cellulose has a mesh size of 200-1000; and the cellulose is one or more of coconut fiber, bamboo fiber, and straw fiber.

4. The method for preparing a sodium-storage hard carbon anode material according to claim 2 or 3, characterized in that: In step S2, the swelling specifically involves: continuously stirring at a speed of 180–220 rpm in a constant-temperature oil bath at 70–75°C; simultaneously applying low-frequency pulsed ultrasound during stirring, with ultrasound parameters of: frequency 28–32 kHz and power density 7.5–8.5 W / cm². 2 The pulse duty cycle is 1:3, and the treatment is continued for 12 to 24 hours.

5. A method for preparing a sodium-storage hard carbon anode material according to claim 2 or 4, characterized in that: In step S3, the coagulation bath is specifically prepared as follows: deionized water and polyethylene glycol diglycidyl ether are added to the coagulation bath tank and stirred at a low speed of 45-55 rpm until completely dissolved. The volume-to-mass ratio of deionized water to polyethylene glycol diglycidyl ether is 9-11 L: 27-33 g. After dissolution, the mixture is placed in a constant temperature water bath to stabilize the system temperature at 20-50℃.

6. The method for preparing a sodium-storage hard carbon anode material according to claim 5, characterized in that: In step S3, the separation of the cellulose regenerated phase is specifically as follows: First, the vacuum-degassed cellulose composite dispersion is slowly added to the coagulation bath at a rate of 1-5 mL / min using a constant flow peristaltic pump. The height of the adding needle from the surface of the coagulation bath is 9.5-10.5 cm. During the adding process, the temperature of the coagulation bath system is kept stable at 20-50℃ to ensure that the droplets fall vertically and evenly into the coagulation bath. After the adding is completed, the mixture is allowed to stand at a constant temperature of 20-50℃ for 28-32 min.

7. The method for preparing a sodium-storage hard carbon anode material according to claim 6, characterized in that: In step S4, the solid-liquid separation specifically involves: vacuum filtration of the regenerated phase dispersion obtained in step S3 using a Buchner funnel to obtain a wet cellulose filter cake; filtration continues until no filtrate drips. The primary coordination washing specifically involves: adding the wet cellulose filter cake to the primary coordination washing solution, with a mass-to-volume ratio of 1:20; stirring at 23–27°C and 140–160 rpm for 28–32 minutes; followed by vacuum filtration; repeating this process 3–4 times. The secondary anchoring washing specifically involves: [The text abruptly ends here, so the translation stops as well.] The washed filter cake is added to the secondary anchoring washing solution at a mass-to-volume ratio of 1:10 and soaked at 23–27°C for 28–32 min. Then, it is subjected to vacuum filtration, deionized water rinsing, and vacuum filtration. The tertiary solvent replacement is as follows: the rinsed and vacuum-filtered filter cake is added to anhydrous ethanol at a mass-to-volume ratio of 1:15 and stirred at 23–27°C and 140–160 rpm for 18–22 min. Then, it is subjected to vacuum filtration. This operation is repeated 3–4 times to obtain cellulose regenerated wet material.

8. The method for preparing a sodium-storage hard carbon anode material according to claim 7, characterized in that: The primary coordination washing solution comprises 12-crown ether-4, triethanolamine, anhydrous ethanol, and deionized water in a mass ratio of 0.9–1.1:1.8–2.2:78.3–95.7:99–121; the secondary anchoring washing solution is a deionized water dispersion of sulfonated cellulose nanocrystals with a concentration of 0.1 wt%.

9. The method for preparing a sodium-storage hard carbon anode material according to claim 8, characterized in that: Step S5 specifically involves: using supercritical CO2 gradient drying, firstly, evenly spreading the wet cellulose regenerated material in the material basket of the supercritical reactor, with a filling thickness not exceeding 2 cm, and sealing the reactor after filling; then, introducing liquid CO2 into the reactor, slowly raising the temperature to 37–39°C, and simultaneously increasing the pressure to 8.5–9.5 MPa, and statically soaking for 2.8–3.2 h under constant temperature and pressure; then, slowly reducing the pressure at a constant rate of 0.14–0.16 MPa / min until the pressure in the reactor drops to atmospheric pressure, with the temperature remaining stable at 37–39°C throughout the depressurization process; after depressurization, opening the reactor, removing the cellulose regenerated powder, and immediately placing it in a desiccator for sealed storage.

10. The method for preparing a sodium-storing hard carbon anode material according to claim 9, characterized in that: In step S6, the pyrolysis carbonization specifically involves: first, uniformly spreading the cellulose regenerated powder in a corundum boat with a filling thickness not exceeding 5 mm; placing the boat in the constant temperature zone of a tubular furnace, introducing inert gas as a protective gas, and continuously purging at a rate of 90–110 mL / min for 28–32 min; then, rapidly heating to 790–810 °C at a heating rate of 19.5–20.5 °C / min, and holding at that temperature for 28–32 min; subsequently, slowly heating to 1300–1500 °C at a heating rate of 1.5–2.5 °C / min, and holding at that temperature for 5–7 h; finally, cooling to room temperature at a cooling rate of 4.5–5.5 °C / min, shutting off the inert gas supply, and removing the carbonized hard carbon crude product.

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