A method for preparing cellulose-based hard carbon anode material

Abstract

This invention provides a method for preparing a cellulose-based hard carbon anode material, relating to the field of battery electrode material preparation. The method includes: composite solvent system preparation – cellulose swelling – cellulose regeneration phase separation – filtration and washing – drying – pyrolysis and carbonization. The cellulose swelling process involves sequentially adding cellulose, KH560, and triethylamine to a DMAc / LiCl composite solvent, stirring and mixing to form a premixed system, then placing the premixed system in a constant-temperature oil bath for stirring and swelling, followed by vacuum degassing. The cellulose regeneration phase separation process involves: pre-preparing a coagulation bath, uniformly adding the vacuum-degassed cellulose composite dispersion to the coagulation bath, and allowing it to stand at a constant temperature after addition to obtain the regenerated phase dispersion. This method utilizes a synergistic process of cellulose molecular chain swelling and phase separation to reconstruct the crystal structure, thereby preparing a hard carbon anode material with excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of battery electrode material preparation technology, and in particular to a method for preparing a cellulose-based hard carbon anode material. Background Technology

[0002] Compared to lithium-ion batteries, sodium-ion batteries offer advantages such as low cost, abundant resources, and high output power, making them a key pathway to alleviate resource dependence and reduce battery costs. Unlike the graphite materials used in the negative electrode of lithium-ion batteries, sodium-ion batteries use sodium ions as charge carriers. Hard carbon has a typical disordered carbon structure and does not have a fixed theoretical specific capacity; it possesses multiple sodium storage mechanisms, among which plateau region sodium storage is crucial for improving the energy density of sodium-ion batteries. During charging: Na… + Sodium atoms are extracted from the lattice of the positive electrode material, migrate through the electrolyte, and embed themselves in closed pores between hard carbon layers to form sodium clusters. Electrons synchronously flow from the positive electrode to the negative electrode through the external circuit to achieve charge balance. During discharge: Na + Electrons escape from the closed pores through the graphite sheets, return to the positive electrode via the electrolyte, and flow from the negative electrode to the positive electrode through the external circuit, thus outputting electrical energy.

[0003] Hard carbon is currently the only anode material for large-scale commercial application in sodium-ion batteries. The sodium storage capacity of hard carbon comes from two core sites: interlayer embedding sites and closed-pore filling sites. However, interlayer embedding sites depend on the densification and ordering of the carbon skeleton (i.e., high-temperature carbonization is required to drive carbon layer rearrangement, forming larger and more ordered graphite microcrystals to provide stable reversible embedding capacity), while closed-pore filling sites depend on the loose porous structure of the carbon skeleton (i.e., low-temperature carbonization or pore-forming modification is required to retain stable nanopores, providing low-voltage platform filling capacity and fast ion diffusion channels). The two (interlayer embedding sites and closed-pore filling sites) are in fundamental competition in terms of carbon skeleton structure, and their requirements for carbon skeleton structure are completely mutually exclusive. This causes all the core performance of hard carbon anodes to fall into a "seesaw effect," making it impossible to achieve a comprehensive breakthrough in core electrochemical performance, mass production cost, and application scenario adaptability. In other words, it is impossible to achieve a synergistic breakthrough in multi-dimensional performance, which seriously restricts the large-scale production and application of sodium-ion batteries. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a method for preparing cellulose-based hard carbon anode materials. This method utilizes a synergistic process of swelling cellulose molecular chains and phase separation to reconstruct the crystalline structure, thereby efficiently preparing hard carbon anode materials with excellent electrochemical performance and optimizing the conductivity and sodium storage capacity of the hard carbon anode materials.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for preparing a cellulose-based hard carbon anode material, comprising:

[0007] Step S1, Preparation of composite solvent system: After pretreatment of anhydrous LiCl, add it to DMAc (N,N-dimethylacetamide) solvent and stir in a constant temperature oil bath to obtain DMAc / LiCl composite solvent, and seal for later use;

[0008] 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;

[0009] 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.

[0010] 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;

[0011] 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;

[0012] 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.

[0013] 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 to 122°C and a vacuum of -0.08 to -0.1 MPa 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.

[0014] Based on further optimization of the above scheme, in step S1, 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℃, and stirred at a speed of 280-320 rpm for 0.5-2 hours.

[0015] 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 stirring of the premixed system is specifically carried out at a speed of 180-220 rpm for 8-12 minutes.

[0016] Based on further optimization of the above scheme, the mesh size of the cellulose is 200-1000 mesh; the cellulose is one or more of coconut shell fiber, bamboo fiber, and straw fiber.

[0017] 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.

[0018] Based on further optimization of the above scheme, in step S2, vacuum degassing specifically involves: after swelling is completed, 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.

[0019] Based on further optimization of the above scheme, in step S3, the coagulation bath is specifically prepared 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 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℃.

[0020] Based on further optimization of the above scheme, in step S3, the separation of the cellulose regenerated phase is specifically as follows: First, a constant flow peristaltic pump is used to slowly add the vacuum-degassed cellulose composite dispersion to the coagulation bath at a rate of 1-5 mL / min. The height of the dropping needle from the surface of the coagulation bath is 9.5-10.5 cm. During the dropping process, the temperature of the coagulation bath system is kept stable at 20-50℃ to ensure that the droplets fall vertically and uniformly into the coagulation bath. After the dropping is completed, the mixture is allowed to stand at a constant temperature of 20-50℃ for 28-32 min.

[0021] 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 minutes. Then, it undergoes vacuum filtration, deionized water rinsing, and vacuum filtration again. The tertiary solvent replacement involves adding the rinsed and vacuum-filtered filter cake to anhydrous ethanol at a mass-to-volume ratio of 1:15. The mixture is stirred at 23–27°C and 140–160 rpm for 18–22 minutes, followed by vacuum filtration. This process is repeated 3–4 times to remove the water from the particles, yielding regenerated cellulose wet material.

[0022] Based on further optimization of the above scheme, the primary coordination washing solution includes 12-crown ether-4, triethanolamine, anhydrous ethanol and deionized water, with 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%.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The following are the technical effects of this solution:

[0027] This invention utilizes Li to prepare a DMAc / LiCl composite solvent. +The coordination with cellulose hydroxyl groups and the disruption of hydrogen bonds between cellulose molecules lay the foundation for subsequent uniform swelling and modification of cellulose. 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 the constant-temperature oil bath, covalent bonds are formed at the microcrystalline-amorphous interface, preventing problems such as closed-cell tearing and microcracks in the carbon skeleton caused by localized stress concentration during subsequent pyrolysis and carbonization (closed-cell tearing or microcracks destroy the closed-cell space of sodium storage and disrupt the continuous conductive network of graphite domains, thus severely reducing the performance of the hard carbon anode). During the regeneration phase separation process, a specific coagulation bath is used, utilizing PE... The cross-linking effect of GDGE works synergistically with the anchoring effect of KH560 to form a dual structural stability of "surface cross-linking + internal anchoring", which avoids the collapse of the pore structure during subsequent washing and drying processes. At the same time, the cross-linking effect of PEGDGE will form an interface layer on the surface of cellulose droplets that allows DMAc small molecules to diffuse unidirectionally and blocks water molecules from diffusing inward, thereby avoiding problems such as the opening of closed pore structures or widening of pore size caused by bidirectional diffusion mismatch (when cellulose droplets enter the coagulation bath, if the rate of DMAc diffusion outward and the rate of water molecule diffusion inward in the coagulation bath are mismatched, a dense skin layer will first form on the surface of the droplets. The internal pressure generated by the continuous diffusion of the internal solvent will break through the skin layer, eventually forming a large number of open pores or through pores). This invention uses a swelling step and a regeneration 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 a large number of closed-pore structures during pyrolysis, while the graphite domain structure formed in the microcrystalline region provides a continuous electronic conductivity network. The closed-pore structure forms an excellent sodium storage space, effectively improving the sodium storage performance of the hard carbon anode.

[0028] This invention, through a series of washing steps including solid-liquid separation, primary coordination washing, secondary anchoring washing, and tertiary solvent replacement, not only thoroughly removes the Li groups coordinated to the cellulose hydroxyl groups, but also... + With free Li + To avoid Li during pyrolysis + Excessive growth of catalytic graphite domains leads to uncontrollable graphite domain size and breakage of the continuous conductive network (while, residual Li...). +It also competes for sodium storage sites during the first charge and discharge cycle, lowering the initial coulombic efficiency and cycle stability. It also ensures that the washing liquid fully penetrates the inner wall of the particles, preventing impurity encapsulation. Through supercritical drying, utilizing the characteristic of supercritical CO2 without gas-liquid phase change, it effectively avoids the ice crystal growth of freeze-drying and the capillary shrinkage of oven drying that lead to closed-pore rupture, preserving the nanoporous structure formed by phase separation while removing residual DMAc and ethanol. Through reverse gradient pyrolysis carbonization treatment, in synergy with the interface anchoring of KH560, it eliminates the internal stress caused by the shrinkage mismatch between the two phases, achieving the synergistic existence of graphite domains and closed-pore structures, effectively solving the mutual repulsion between hard carbon graphite domains and closed-pore structures. Using etching treatment, additional nanopores are formed at the microcrystalline-amorphous interface, further expanding the sodium storage space.

[0029] This invention effectively solves 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 simultaneously possesses continuous interlayer graphite domain embedding sites and high closed-pore filling sites, synergistically enhancing sodium storage sites (the continuous graphite domain conductive network ensures rapid electron transport, and the uniform nanopores provide Na+). + It provides a fast diffusion channel, while the carbon skeleton is free of microcracks and the pore structure does not collapse, resulting in excellent cycle stability. In addition, the abundant nanopores construct efficient ion transport channels, which, together with the continuous conductive network, result in high capacity retention and are suitable for the high-rate charge and discharge requirements of sodium-ion batteries in power and energy storage scenarios. Attached Figure Description

[0030] 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.

[0031] 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

[0032] 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.

[0033] Example 1:

[0034] A method for preparing a cellulose-based hard carbon anode material, comprising:

[0035] Step S1, configuration of composite solvent system: The pretreatment of anhydrous LiCl is as follows: Anhydrous LiCl is placed in a vacuum oven and dried at a temperature of 118℃ and a vacuum degree of -0.08MPa for 4.5h to remove crystal water. After drying, it is immediately placed in a desiccator to cool to room temperature, and contact with air and water absorption are avoided throughout the process.

[0036] Pretreated anhydrous LiCl was 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 was 5:100. The stirring and mixing were carried out in a constant temperature oil bath at 30℃ and a speed of 280 rpm for 2 hours. The DMAc / LiCl composite solvent was obtained and sealed for later use.

[0037] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine are sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 35:0.42:0.7:1000. The mixture is stirred to form a premixed system, specifically by stirring at 180 rpm for 12 minutes. The cellulose used is 300-mesh coconut shell fiber.

[0038] The premixed system was then placed in a constant-temperature oil bath and stirred to swell. Specifically, the system was continuously stirred at 180 rpm in a 70°C constant-temperature oil bath. Simultaneously, low-frequency pulsed ultrasound was applied with the following parameters: frequency 28 kHz, power density 7.5 W / cm³. 2 The pulse duty cycle is 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment continues for 24 hours.

[0039] After swelling, vacuum degassing is performed. Specifically, after swelling is completed, the composite dispersion is placed in a vacuum drying oven and degassed at 24℃ and -0.08MPa for 32 minutes.

[0040] Step S3, Cellulose Regeneration Phase Separation: Pre-prepared coagulation bath, specifically: 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℃.

[0041] The vacuum-degassed cellulose composite dispersion was added dropwise to the coagulation bath at a uniform rate. After the addition was completed, the mixture was allowed to stand at a constant temperature. Specifically, the vacuum-degassed cellulose composite dispersion was slowly added to the coagulation bath at a rate of 1 mL / min using a constant flow peristaltic pump. The height of the dropper tip from the surface of the coagulation bath was 9.5 cm. During the addition, the temperature of the coagulation bath system was kept stable at 20℃ to ensure that the droplets fell vertically and uniformly into the coagulation bath. After the addition was completed, the mixture was allowed to stand at a constant temperature of 20℃ for 32 min to obtain the regenerated phase dispersion.

[0042] 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;

[0043] 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.

[0044] 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 140 rpm for 32 minutes at 23°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 0.9:1.8:78.3:99.

[0045] 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 then soaked at 23°C for 32 minutes, followed by vacuum filtration, deionized water rinsing, and another vacuum filtration to remove free Li. + Inclusion complexes and residual reagents; wherein, the secondary anchoring washing solution is a deionized aqueous dispersion of sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent) with a concentration of 0.1 wt%;

[0046] The three-stage solvent replacement process involves adding the rinsed and vacuum-filtered filter cake to anhydrous ethanol at a mass-to-volume ratio of 1:15. After stirring at 23°C and 140 rpm for 22 minutes, vacuum filtration is performed. This process is repeated three times to replace the moisture inside the particles and obtain regenerated cellulose wet material.

[0047] 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.

[0048] 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;

[0049] 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 caused by 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 at a rate of 90 mL / min for 32 minutes to purge the air from the tube furnace and prevent oxygen from seeping in; then, rapidly heating to 790℃ at a rate of 19.5℃ / min and holding at that temperature for 32 minutes to allow the amorphous regions of the cellulose to complete carbonization. The main dehydroxylation and devolatile matter reactions, along with volume shrinkage and shaping, occur simultaneously. KH560 undergoes initial decomposition during this stage, generating nano-SiO2 which anchors at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases. Subsequently, the temperature is slowly increased to 1300℃ at a rate of 1.5℃ / min 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 is lowered to room temperature at a rate of 4.5℃ / min, the inert gas supply is shut off, and the carbonized hard carbon crude product is removed.

[0050] The etching process is as follows: The crude hard carbon material is placed in a 9.5 wt% hydrofluoric acid solution (mass-volume ratio of crude hard carbon to hydrofluoric acid solution is 1:10). The mixture is stirred at 180 rpm for 2.2 hours 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. Afterward, the etched hard carbon material is repeatedly rinsed with deionized water until the pH of the filtrate is 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.

[0051] Example 2:

[0052] A method for preparing a cellulose-based hard carbon anode material, comprising:

[0053] Step S1, configuration of 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 crystal water. After drying, it is immediately placed in a desiccator to cool to room temperature, and contact with air and water absorption are avoided throughout the process.

[0054] Pretreated anhydrous LiCl was 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 was 10:100. The stirring and mixing were carried out in a constant temperature oil bath at 45℃ and a speed of 300 rpm for 1 hour. The DMAc / LiCl composite solvent was obtained and sealed for later use.

[0055] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine are sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 40:0.48:0.8:1000. The mixture is stirred to form a premixed system, specifically by stirring at 200 rpm for 10 minutes. The cellulose used is 400-mesh bamboo fiber.

[0056] The premixed system was then placed in a constant-temperature oil bath and stirred to swell. Specifically, the system was stirred continuously at 200 rpm in a constant-temperature oil bath at 72.5℃. During the stirring process, low-frequency pulsed ultrasound was applied simultaneously with the following parameters: frequency 30 kHz, power density 8 W / cm³. 2 The pulse duty cycle is 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment is continued for 18 hours.

[0057] After swelling, vacuum degassing is performed. Specifically, after swelling is completed, the composite dispersion is placed in a vacuum drying oven and degassed at 25℃ and -0.09MPa for 30 minutes.

[0058] Step S3, Cellulose Regeneration Phase Separation: Pre-prepared coagulation bath, specifically: 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.

[0059] The vacuum-degassed cellulose composite dispersion was added dropwise to the coagulation bath at a constant rate. After the addition was completed, the mixture was allowed to stand at a constant temperature. Specifically, the vacuum-degassed cellulose composite dispersion was slowly added to the coagulation bath at a rate of 3 mL / min using a constant flow peristaltic pump. The height of the drop needle from the surface of the coagulation bath was 10 cm. During the addition, the temperature of the coagulation bath system was kept stable at 35℃ to ensure that the droplets fell vertically and evenly into the coagulation bath. After the addition was completed, the mixture was allowed to stand at a constant temperature of 35℃ for 30 min to obtain the regenerated phase dispersion.

[0060] 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;

[0061] 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.

[0062] 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 includes 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.

[0063] 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 then soaked at 25°C for 30 minutes, followed by vacuum filtration, deionized water rinsing, and another vacuum filtration to remove free Li. + Inclusion complexes and residual reagents; wherein, the secondary anchoring washing solution is a deionized aqueous dispersion of sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent) with a concentration of 0.1 wt%;

[0064] The three-stage solvent replacement process involves adding the rinsed and vacuum-filtered filter cake to anhydrous ethanol at a mass-to-volume ratio of 1:15. After stirring at 25°C and 150 rpm for 20 minutes, vacuum filtration is performed. This process is repeated three times to replace the moisture inside the particles and obtain regenerated cellulose wet material.

[0065] 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.

[0066] 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;

[0067] The pyrolysis carbonization process specifically involves: first, uniformly spreading the regenerated cellulose powder in a corundum boat, with a filling thickness not exceeding 5mm to avoid uneven carbonization caused by 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 at a rate of 100mL / min for 30 minutes to purge the air from the tube furnace and prevent oxygen from seeping in; then, rapidly heating to 800℃ at a rate of 20℃ / min and holding at that temperature for 30 minutes to allow the amorphous regions of the cellulose to fully develop. The main processes include dehydroxylation, devolatile matter removal, and volume shrinkage for stabilization. Simultaneously, KH560 undergoes initial decomposition at this stage, generating nano-SiO2 which anchors at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases. Subsequently, the temperature is slowly increased to 1400℃ at a rate of 2℃ / min and held for 6 hours to allow the cellulose microcrystalline region to slowly graphitize, forming a uniform and continuous graphite domain conductive network. Finally, the temperature is lowered to room temperature at a rate of 5℃ / min, the inert gas supply is shut off, and the carbonized hard carbon crude product is removed.

[0068] The etching process is as follows: The crude hard carbon material is placed in a 10wt% hydrofluoric acid solution (mass-volume ratio of crude hard carbon to hydrofluoric acid solution is 1:10). The mixture is stirred 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 is repeatedly rinsed with deionized water until the pH of the filtrate is 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.

[0069] Example 3:

[0070] A method for preparing a cellulose-based hard carbon anode material, comprising:

[0071] Step S1, configuration of composite solvent system: The pretreatment of anhydrous LiCl is as follows: Anhydrous LiCl is placed in a vacuum oven and dried at a temperature of 122℃ and a vacuum degree of -0.1MPa for 3.5h to remove crystal water. After drying, it is immediately placed in a desiccator to cool to room temperature, and contact with air and water absorption are avoided throughout the process.

[0072] Pretreated anhydrous LiCl was 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 was 15:100. The stirring and mixing were carried out in a constant temperature oil bath at 60℃ and a speed of 320 rpm for 0.5 h. The DMAc / LiCl composite solvent was obtained and sealed for later use.

[0073] Step S2, Cellulose Swelling: Cellulose, KH560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8), and triethylamine are sequentially added to a DMAc / LiCl composite solvent. The mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 45:0.54:0.9:1000. The mixture is stirred to form a premixed system, specifically by stirring at 220 rpm for 8 minutes. The cellulose used is 500-mesh straw fiber.

[0074] The premixed system was then placed in a constant-temperature oil bath and stirred to swell. Specifically, the system was continuously stirred at 220 rpm in a 75°C constant-temperature oil bath. Simultaneously, low-frequency pulsed ultrasound was applied with the following parameters: frequency 32 kHz, power density 8.5 W / cm³. 2 The pulse duty cycle is 1:3 (i.e., 1 second of ultrasound followed by 3 seconds of pause), and the treatment is continued for 12 hours.

[0075] After swelling, vacuum degassing is performed. Specifically, after swelling is completed, the composite dispersion is placed in a vacuum drying oven and degassed at 26℃ and -0.1MPa for 28 minutes.

[0076] Step S3, Cellulose Regeneration Phase Separation: Pre-prepared coagulation bath, specifically: 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℃.

[0077] The vacuum-degassed cellulose composite dispersion was added dropwise to the coagulation bath at a uniform rate. After the addition was completed, the mixture was allowed to stand at a constant temperature. Specifically, the vacuum-degassed cellulose composite dispersion was slowly added to the coagulation bath at a rate of 5 mL / min using a constant flow peristaltic pump. The height of the dropper tip from the surface of the coagulation bath was 10.5 cm. During the addition, the temperature of the coagulation bath system was kept stable at 50°C to ensure that the droplets fell vertically and uniformly into the coagulation bath. After the addition was completed, the mixture was allowed to stand at a constant temperature of 50°C for 28 min to obtain the regenerated phase dispersion.

[0078] 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;

[0079] 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.

[0080] 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 160 rpm for 28 minutes at 27°C, 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.

[0081] 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 then soaked at 27°C for 28 minutes, followed by vacuum filtration, deionized water rinsing, and another vacuum filtration to remove free Li. +Inclusion complexes and residual reagents; wherein, the secondary anchoring washing solution is a deionized aqueous dispersion of sulfonated cellulose nanocrystals (SCNC, purchased from Aladdin Reagent or Maclean Reagent) with a concentration of 0.1 wt%;

[0082] The three-stage solvent replacement process involves adding the rinsed and vacuum-filtered filter cake to anhydrous ethanol at a mass-to-volume ratio of 1:15. After stirring at 160 rpm for 18 minutes at 27°C, the mixture is vacuum-filtered. This process is repeated four times to replace the moisture inside the particles and obtain regenerated cellulose wet material.

[0083] 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.

[0084] 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;

[0085] The pyrolysis carbonization process is as follows: First, the regenerated cellulose powder is evenly spread in a corundum boat, with a filling thickness not exceeding 5 mm to avoid uneven carbonization caused by stacking. The boat is then placed in the constant temperature zone of a tube furnace, and an inert gas (such as argon with a purity ≥99%) is introduced as a protective gas at a rate of 110 mL / min for 28 minutes to purge the air from the tube furnace and prevent oxygen from seeping in. Then, the temperature is rapidly increased to 810℃ at a rate of 20.5℃ / min and held for 28 minutes to allow the amorphous region of the cellulose to complete carbonization. The main dehydroxylation and devolatile matter reactions, along with volume shrinkage and shaping, occur simultaneously. KH560 undergoes initial decomposition during this stage, generating nano-SiO2 which anchors at the microcrystalline-amorphous interface, further restricting the relative displacement of the two phases. Subsequently, the temperature is slowly increased to 1500℃ at a rate of 2.5℃ / min 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 is lowered to room temperature at a rate of 5.5℃ / min, the inert gas supply is shut off, and the carbonized hard carbon crude product is removed.

[0086] The etching process is as follows: The crude hard carbon material is placed in a 10.5 wt% hydrofluoric acid solution (mass-volume ratio of crude hard carbon to hydrofluoric acid solution is 1:10). The mixture is stirred at 220 rpm for 1.8 hours 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. Afterward, the etched hard carbon material is repeatedly rinsed with deionized water until the pH of the filtrate is 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.

[0087] Comparative Example 1:

[0088] A method for preparing a hard carbon anode, comprising:

[0089] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0090] 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 min. The cellulose used is 400 mesh bamboo fiber.

[0091] The premixed system is then placed in a constant temperature oil bath and stirred to swell and then degassed under vacuum. The stirring, swelling and vacuum degassing processes are consistent with step S2 in Example 2.

[0092] Step S3, Cellulose Regeneration Phase Separation: Consistent with Step S3 in Example 2.

[0093] Step S4, filtration and washing: consistent with step S4 in Example 2.

[0094] Step S5, Drying: Same as step S5 in Example 2.

[0095] Step S6, pyrolysis and carbonization: consistent with step S6 in Example 2.

[0096] Comparative Example 2:

[0097] A method for preparing a hard carbon anode, comprising:

[0098] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0099] Step S2, cellulose swelling: consistent with step S2 in Example 2.

[0100] 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.

[0101] Step S4, filtration and washing: consistent with step S4 in Example 2.

[0102] Step S5, Drying: Same as step S5 in Example 2.

[0103] Step S6, pyrolysis and carbonization: consistent with step S6 in Example 2.

[0104] Comparative Example 3:

[0105] A method for preparing a hard carbon anode, comprising:

[0106] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0107] Step S2, cellulose swelling: consistent with step S2 in Example 2.

[0108] Step S3, Cellulose Regeneration Phase Separation: Consistent with Step S3 in Example 2.

[0109] Step S4, Filtration and Washing: The regenerated phase dispersion is subjected to solid-liquid separation and washing in sequence;

[0110] 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.

[0111] The washing process involves adding the wet filter cake after solid-liquid separation 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 moisture inside the particles and obtain regenerated cellulose wet material.

[0112] Step S5, Drying: Same as step S5 in Example 2.

[0113] Step S6, pyrolysis and carbonization: consistent with step S6 in Example 2.

[0114] Comparative Example 4:

[0115] A method for preparing a hard carbon anode, comprising:

[0116] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0117] Step S2, cellulose swelling: consistent with step S2 in Example 2.

[0118] Step S3, Cellulose Regeneration Phase Separation: Consistent with Step S3 in Example 2.

[0119] Step S4, filtration and washing: consistent with step S4 in Example 2.

[0120] 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.

[0121] Step S6, pyrolysis and carbonization: consistent with step S6 in Example 2.

[0122] Comparative Example 5:

[0123] A method for preparing a hard carbon anode, comprising:

[0124] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0125] Step S2, cellulose swelling: consistent with step S2 in Example 2.

[0126] Step S3, Cellulose Regeneration Phase Separation: Consistent with Step S3 in Example 2.

[0127] Step S4, filtration and washing: consistent with step S4 in Example 2.

[0128] Step S5, Drying: Same as step S5 in Example 2.

[0129] 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;

[0130] Specifically, the pyrolysis carbonization process involves placing the regenerated cellulose powder in a tube furnace and introducing an inert gas (such as argon with a purity ≥99%) as a protective gas. The gas is continuously introduced at a rate of 100 mL / min for 30 min to purge the air from the tube furnace and prevent oxygen from seeping in. Then, the temperature is increased to 1400℃ at a rate of 8℃ / min and held for 6 h. Finally, the temperature is reduced to room temperature at a rate of 5℃ / min, the inert gas is turned off, and the carbonized hard carbon crude product is removed.

[0131] The etching process is consistent with step S6 in Example 2.

[0132] Comparative Example 6:

[0133] A method for preparing a hard carbon anode, comprising:

[0134] Step S1, Preparation of the composite solvent system: Same as step S1 in Example 2.

[0135] Step S2, cellulose swelling: consistent with step S2 in Example 2.

[0136] Step S3, Cellulose Regeneration Phase Separation: Consistent with Step S3 in Example 2.

[0137] Step S4, filtration and washing: consistent with step S4 in Example 2.

[0138] Step S5, Drying: Same as step S5 in Example 2.

[0139] 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.

[0140] Example 4:

[0141] 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 under low-speed vacuum at 1500 rpm for 15 minutes, and allowing it to stand for 10 minutes after stirring to obtain a negative electrode slurry; the mass percentages of hard carbon material, superconducting carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are 90%, 5%, 2%, and 3%, respectively.

[0142] Next, the negative electrode slurry is uniformly coated onto battery-grade copper foil (8–10 μm thick, single-sided smooth). The coated electrode is then air-dried for 10 minutes, followed by drying in a vacuum oven at 120°C and -0.1 MPa for 12 hours. The dried electrode is 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; after rolling, it was dried again in a vacuum oven at 80℃ and -0.1MPa for 6 hours.

[0143] Finally, place the sodium metal sheet in the center of the negative electrode shell (CR2032), add electrolyte (20-25 μL), then place the diaphragm, add electrolyte (20-25 μL), and then place the negative electrode sheet, gasket, and spring in sequence. Install the positive electrode shell, and place the entire mold into a sealing machine, setting the pressure to 500-600 kg / cm². 2 Hold the pressure for 3-5 seconds to complete the encapsulation of the sodium-ion battery.

[0144] 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.

[0145] Table 1. Charge and discharge test results

[0146]

[0147] 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 cellulose-based 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. After swelling, vacuum degassing is performed to obtain a cellulose composite dispersion; the mass ratio of cellulose, KH560, triethylamine, and DMAc / LiCl composite solvent is 35-45:0.42-0.54:0.7-0.9:1000; 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 to obtain the regenerated phase dispersion. Specifically, 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 addition process, the temperature of the coagulation bath system is kept stable at 20–50 °C to ensure that the droplets fall vertically and uniformly into the coagulation bath. After the addition is completed, the mixture is kept at a constant temperature of 20–50 °C for 28–32 min. The coagulation bath is prepared as follows: Deionized water and polyethylene glycol diglycidyl ether 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 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℃. 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 cellulose-based 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 to 122°C and a vacuum of -0.08 to -0.1 MPa. After drying, the LiCl is immediately placed in a desiccator to cool to room temperature.

3. A method for preparing a cellulose-based hard carbon anode material according to claim 1 or 2, characterized in that: In step S1, 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℃, and stirred at a speed of 280-320 rpm for 0.5-2 hours.

4. The method for preparing a cellulose-based hard carbon anode material according to claim 3, characterized in that: In step S2, the stirring of the premixed system is specifically carried out by stirring at a speed of 180-220 rpm for 8-12 minutes.

5. The method for preparing a cellulose-based hard carbon anode material according to claim 4, characterized in that: The cellulose has a mesh size of 200 to 1000 mesh; the cellulose is made from one or more of coconut shell fiber, bamboo fiber, and straw fiber.

6. The method for preparing a cellulose-based hard carbon anode material according to claim 5, 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.

7. The method for preparing a cellulose-based 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 cellulose-based hard carbon anode material according to claim 7, 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.

9. The method for preparing a cellulose-based hard carbon anode material according to claim 8, 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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