A method for preparing a biomass-based hard carbon negative electrode material based on a pre-carbonization coupled sodium bicarbonate in-situ mild activation technology

Abstract

A method for preparing biomass-based hard carbon anode materials based on a pre-carbonization coupled with in-situ mild activation using sodium bicarbonate is disclosed, belonging to the technical field of sodium-ion battery anode materials. First, the biomass raw material undergoes acid washing and pre-carbonization treatment to transform the structurally unstable natural biomass into a rigid aromatized carbon framework precursor. Then, the precursor is mixed with solid sodium bicarbonate and heat-treated under an inert atmosphere, utilizing in-situ decomposition of NaHCO3 to achieve a three-stage synergistic activation. This invention achieves precise control over the microstructure of hard carbon through a synergistic strategy of pre-carbonization coupled with in-situ mild activation. NaHCO3 is non-corrosive, the process is simple, and it is recyclable. The hard carbon anode material is applied as an anode material in biomass-based sodium-ion batteries, achieving a performance of 30 mA g. ‑1 At current density, its initial discharge specific capacity is 314 mAh g. ‑1 The initial coulombic efficiency was over 87%, demonstrating excellent rate performance and cycle stability, providing a new approach for the large-scale preparation of high-performance sodium-ion battery anode materials.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology and relates to a method for preparing hard carbon anode materials for sodium-ion batteries. Specifically, it relates to a method for preparing biomass-based hard carbon anode materials based on a pre-carbonization coupled sodium bicarbonate in-situ mild activation technology. Background Technology

[0002] With the rapid development of the large-scale energy storage market, sodium-ion batteries have attracted much attention due to their abundant resources and low cost. Among various anode materials, hard carbon, with its disordered carbon structure, large interlayer spacing, high theoretical capacity, low sodium storage potential, and excellent cycle stability, has become the most promising anode material for sodium-ion batteries.

[0003] Biomass materials are widely available, green, and low-carbon, making them ideal precursors for the preparation of hard carbon materials. However, the performance of traditional biomass-based hard carbon materials is highly dependent on the natural structure of the precursor, resulting in poor reproducibility, low plateau capacity, and poor first-pass coulombic efficiency, which limits their commercial application. Therefore, optimizing the structure of biomass and achieving effective editing of the carbon skeleton is crucial. Commonly used optimization methods include gas activation, template methods, and strong base activation methods. Among them, gas activation methods (such as CO2 and water vapor) utilize oxidizing gases to selectively activate amorphous components in the carbon skeleton, effectively introducing porous structures. However, it is difficult to precisely control the degree and uniformity of activation in gas-phase reactions, and over-activation can easily lead to local collapse of the carbon skeleton. Template methods introduce nano-sized templates (such as ZnO and SiO2) to occupy the space within the carbon skeleton. After carbonization, the templates are removed to form pores. Although this method can achieve precise control of pore size, the introduction and removal of templates involve multiple chemical processes, which are cumbersome and carry the risk of template residue, increasing the complexity of large-scale preparation. While strong base activation methods (such as KOH) can create high specific surface areas, the reaction process is violent and will severely damage the integrity of the carbon skeleton, thereby introducing a large number of irreversible defects, resulting in a significant decrease in the initial coulombic efficiency, as well as heavy equipment corrosion and environmental burden.

[0004] Therefore, finding a mild, efficient, and less corrosive technical strategy to effectively control the specific surface area and optimize the closed-pore structure while achieving fine activation of the carbon skeleton, thereby obtaining hard carbon materials with both high reversible capacity and high first-time coulombic efficiency, is a technical challenge that urgently needs to be solved in the field of sodium-ion batteries. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies and achieve the above objectives, this invention aims to provide a low-cost, easily controllable, simple method for preparing high-specific-capacity biomass hard carbon materials with recyclable activators, specifically a method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled with in-situ mild activation using sodium bicarbonate. This invention utilizes CO2 and H2O released from the decomposition of sodium bicarbonate to in-situ activate the aromatized carbon framework carbon precursor obtained after pre-carbonization. The in-situ generated Na2CO3 serves as a template, filling the pores in nanocrystalline form to form a rigid carbon framework, preventing pore collapse. Subsequent water washing removes the template, resulting in secondary pore expansion, thereby achieving the pre-production of a precursor with hierarchical, porous, and rigid framework. Further washing and high-temperature carbonization transform the open-pore structure into a closed-pore structure, producing high-efficiency and high-capacity hard carbon materials.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled with sodium bicarbonate in-situ mild activation technology, the method comprising the following steps: Step (1) involves acid washing the biomass raw material to remove impurities, followed by drying to obtain purified biomass. The purified biomass is then pre-carbonized under an inert atmosphere to obtain a rigid aromatized carbon skeleton precursor, specifically a stable carbon skeleton with a rigid aromatic ring structure. In this step, acid washing removes ash and metallic impurities from the biomass, reducing the interference of inorganic components on the pore formation of the carbon skeleton. Pre-carbonization causes pyrolysis and condensation of components such as cellulose and lignin in the biomass, forming a stable carbon skeleton with a rigid aromatic ring structure while preserving the macroscopic morphology of the precursor. This aromatized carbon skeleton exhibits high thermal stability and mechanical strength, providing a reproducible and homogenized high-quality precursor for subsequent in-situ NaHCO3 activation.

[0007] Step (2) involves thoroughly mixing the aromatized carbon skeleton precursor with solid sodium bicarbonate and heat-treating it under an inert atmosphere to obtain an activated intermediate. In this step, the CO2 and H2O gases released from the decomposition of NaHCO3 partially activate the carbon skeleton, creating pores. The in-situ generated Na2CO3 acts as an in-situ template, filling the pores in the form of nanocrystals to provide rigid support, forming a rigid carbon skeleton and preventing pore collapse. The synergistic effect of gas activation and in-situ Na2CO3 template filling achieves a balance between the degree of activation and the integrity of the skeleton.

[0008] Step (3) involves washing the activated intermediate to remove the in-situ Na2CO3 template, collecting the washed solid and filtrate separately; drying the collected solid to obtain a porous carbon material; and then subjecting the porous carbon material to high-temperature carbonization under an inert atmosphere to obtain a high-performance hard carbon anode material. Simultaneously, the Na2CO3-containing solution obtained after washing is collected and retained. In this step, after washing away the in-situ Na2CO3 template in the activated intermediate, the space it occupies is transformed into a porous structure, thereby achieving secondary pore expansion and obtaining a porous carbon material with abundant open-pore structures. Subsequently, high-temperature carbonization is performed at a higher temperature, causing carbon layer rearrangement, with some open-pore structures closing and transforming into closed pores, while the disordered carbon structure is further ordered. The closed-pore structure provides more storage sites for sodium ions, which helps to improve the low-potential plateau capacity and the first coulombic efficiency.

[0009] Step (4) collects and purifies the CO2 gas generated from the decomposition of NaHCO3 in step (2) and the CO2 gas generated during the high-temperature carbonization process in step (3); the Na2CO3-containing solution collected after washing in step (3) is placed in an ice-water bath for cooling, and then the collected CO2 gas is continuously introduced until saturation. CO2 reacts with Na2CO3 to generate NaHCO3, and the resulting NaHCO3-containing solution is cooled and crystallized, filtered under reduced pressure, and dried at low temperature to obtain sodium bicarbonate solid, which is reused in step (2).

[0010] Furthermore, the biomass raw material mentioned in step (1) is a natural biomass rich in cellulose, preferably a processing by-product, including but not limited to one or more of cinnamon residue, apricot shells, and rice husks; the acid used in the acid washing treatment is hydrochloric acid or sulfuric acid, used to remove ash and inorganic impurities.

[0011] Furthermore, the temperature of the pre-carbonization treatment in step (1) is 200-600 ℃, and the holding time is 1-3 hours.

[0012] Further, in step (2), the mass ratio of the carbon skeleton precursor to sodium bicarbonate solid is 1:0.5 to 1:4; the heat treatment temperature is 600-1000 ℃, and the holding time is 1-3 hours.

[0013] Furthermore, the washing in step (3) is water washing to completely remove the Na2CO3 in-situ template; the temperature of the high-temperature carbonization treatment is 1200-1600 ℃, and the holding time is 1-3 hours.

[0014] Further, in step (4), the rate of CO2 gas introduced is 50-200 mL / min; the cooling crystallization temperature is 0-10 ℃, and the reaction time is 1-3 hours; the low-temperature drying temperature is 20-50 ℃, and the drying time is 8-24 hours.

[0015] A biomass-based hard carbon anode material is prepared using a pre-carbonization coupled sodium bicarbonate in-situ mild activation technique. The prepared material exhibits a typical non-graphitized carbon structure with interlaced, bent carbon layers forming a disordered carbon structure. X-ray diffraction analysis revealed a carbon layer spacing of 0.38-0.42 nm. The nitrogen adsorption-desorption isotherm is a type IV curve with an H3-type hysteresis loop, and the pore size is concentrated in the 2-5 nm range. The closed-pore size is mainly concentrated in the 1-3 nm range. This biomass-based hard carbon anode material prepared using the pre-carbonization coupled sodium bicarbonate in-situ mild activation technique is applied as the active material in biomass-based sodium-ion batteries, achieving a performance of 30 mA g / g. -1 At the specified current density, the initial discharge specific capacity of the carbon material used as the negative electrode in this biomass sodium-ion battery is 314 mAh g. -1 The initial coulombic efficiency is over 87%, demonstrating excellent sodium storage performance.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention first pre-carbonizes waste biomass, then activates the resulting biochar with NaHCO3 activator at a certain temperature and ratio, and then performs high-temperature carbonization under an inert atmosphere to obtain a biomass-based sodium-ion battery anode carbon material with good performance. Pre-carbonization transforms the structurally unstable natural biomass into a rigid aromatized carbon framework, providing a controllable and high-quality precursor for subsequent activation. During the activation process, NaHCO3 decomposes and releases CO2 and H2O, which preferentially activate the defect sites at the edge of the carbon layer to form initial nanopores. The in-situ generated Na2CO3 fills the pores in the form of nanocrystals, which not only prevents pore collapse but also serves as a pore-forming template. After the template is removed by washing with water, the space occupied by the original template is transformed into a pore structure, thereby achieving secondary pore expansion. At subsequent higher temperatures, the pore structure gradually shrinks, providing filling space for sodium ions, significantly improving the plateau capacity at low potentials (<0.1V) and enhancing the sodium storage performance of the material.

[0017] (2) The process of this invention is simple, environmentally friendly, and low-cost. This invention only involves simple steps such as mixing, heat treatment, and water washing, without the need for complex template synthesis and removal processes. The NaHCO3 used is inexpensive, has a much lower corrosiveness than strong alkalis such as KOH, does not corrode equipment, is environmentally friendly, and is easy to scale up for production. At the same time, by using biomass processing byproducts such as cinnamon residue as raw materials, high-value utilization of waste biomass resources is realized.

[0018] (3) This invention enables the recovery and recycling of the activator, making it environmentally friendly. This invention uses NaHCO3 as the activator, which decomposes in situ during heat treatment to generate Na2CO3 nanotemplates and CO2 / H2O activation gas. After activation, the Na2CO3 template is washed with water and introduced into the filtrate. The CO2 and other gases generated during carbonization are then introduced into the solution. Through cooling crystallization, filtration, and low-temperature drying, it can be converted back into solid NaHCO3, thus completing the recovery and recycling of the activator, significantly reducing raw material costs and waste emissions, and achieving green chemistry.

[0019] In summary, this invention achieves the recovery and recycling of activators through pre-carbonization coupled with in-situ mild activation of sodium bicarbonate. While preparing biomass-based hard carbon anode materials with high platform capacity and high first coulombic efficiency, it significantly reduces raw material costs and environmental burden, and has broad prospects for industrial application. Attached Figure Description

[0020] Figure 1 This is a SEM image of the hard carbon material obtained in Example 1 of this invention; Figure 1 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 1 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 2 This is a SEM image of the hard carbon material obtained in Example 2 of this invention; Figure 2 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 2 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 3 This is a SEM image of the hard carbon material obtained in Example 3 of this invention; Figure 3 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 3 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 4 This is a SEM image of the hard carbon material obtained in Example 4 of this invention; Figure 4 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 4 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 5 This is a SEM image of the hard carbon material obtained in Example 5 of this invention; Figure 5 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 5(b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 6 This is a SEM image of the hard carbon material obtained in Comparative Example 1 of this invention; Figure 6 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 6 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 7 This is a SEM image of the hard carbon material obtained in Comparative Example 2 of this invention; Figure 7 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 7 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 8 This is a SEM image of the hard carbon material obtained in Comparative Example 3 of this invention; Figure 8 (a) in the image is a SEM image magnified 2500 times (scale bar 20 μm); Figure 8 (b) in the image is a SEM image magnified 10,000 times (scale bar 5 μm). Figure 9 The half-cell assembled from the sodium-ion battery negative electrode carbon material prepared in Examples 1-5 of this invention has a performance of 30 mAg. -1 First-cycle charge-discharge curves at current density; Figure 10 The half-cell assembled from the sodium-ion battery negative electrode carbon material prepared in Examples 6-8 of this invention has a performance of 30 mAg. -1 First-cycle charge-discharge curves at current density; Figure 11 The half-cells assembled from the sodium-ion battery negative electrode carbon materials prepared in Comparative Examples 1-3 of this invention achieve a 30 mAg performance. -1 The first charge-discharge curve at current density. Detailed Implementation

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. However, these embodiments are only for illustrating the present invention and are not intended to limit the scope of the present invention. Any non-substantial improvements or substitutions made by those skilled in the art based on the present invention are within the protection scope of the present invention.

[0022] Example 1 Step (1) Take 50 g of cinnamon residue powder, add 600 mL of 2 mol / L hydrochloric acid solution for acid washing and deashing, stir for 12 hours, filter and wash until neutral, dry the filter residue at 80 ℃ to obtain purified biomass powder. Place the purified biomass powder in a tube furnace, heat to 500 ℃ at a heating rate of 10 ℃ / min under argon atmosphere, and hold for 2 hours for pre-carbonization; after natural cooling, a stable aromatized carbon skeleton precursor is obtained.

[0023] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and 5 g of NaHCO3 solid (mass ratio 1:1) and mix thoroughly; place the mixture in a corundum boat, put it into a tube furnace, and heat it to 800°C at a heating rate of 10 °C / min under an argon atmosphere, and hold it at that temperature for 2 hours; after natural cooling, obtain the activated intermediate.

[0024] Step (3) The activated intermediate is repeatedly washed with deionized water until neutral to completely remove the Na2CO3 template. After drying at 80 °C, a porous carbon material is obtained. This porous carbon material is placed in a graphite crucible and put into a tube furnace. Under an argon atmosphere, the temperature is first increased to 1400 °C at 10 °C / min, and then increased to 1500 °C at 5 °C / min. The temperature is held for 2 hours. After natural cooling, a high-performance hard carbon anode material is obtained.

[0025] The morphology of the sodium-ion battery hard carbon anode material prepared in Example 1 is as follows: Figure 1 As shown in (a) and (b), the material surface is smooth with pores at the edges; this material was assembled into a sodium-ion half-cell for testing at 30 mA g. -1 At current density, its initial charge specific capacity is 362 mAh g. -1 The initial Coulomb efficiency reached 92%.

[0026] Example 2 Step (1) Take 50 g of cinnamon residue powder, add 600 mL of 2 mol / L hydrochloric acid solution for acid washing and deashing, stir for 12 hours, filter and wash until neutral, and dry the filter residue at 80 ℃ to obtain purified biomass powder. Place the purified biomass powder in a tube furnace, and under an argon atmosphere, heat it to 200 ℃ at a heating rate of 10 ℃ / min, and hold it at that temperature for 1 hour for pre-carbonization; after natural cooling, a stable aromatized carbon skeleton precursor is obtained.

[0027] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and mix thoroughly with 2.5 g of NaHCO3 solid (mass ratio 1:0.5); heat the mixture to 1000 °C at a heating rate of 10 °C / min under argon atmosphere and hold for 1 hour; obtain the activated intermediate after natural cooling.

[0028] Step (3) The activated intermediate is repeatedly washed with deionized water until neutral, and dried at 80 °C to obtain porous carbon material. The porous carbon material is placed in a graphite crucible and placed in a tube furnace. Under an argon atmosphere, the temperature is first increased to 1400 °C at 10 °C / min, and then increased to 1500 °C at 5 °C / min. The temperature is held for 2 hours. After natural cooling, hard carbon material is obtained.

[0029] SEM images of the sodium-ion battery hard carbon anode material prepared in Example 2 are shown below. Figure 2 As shown in (a) and (b), a small number of dispersed nanopores appear on the surface of the material; when this material is assembled into a sodium-ion half-cell for testing, at 30 mA g -1 At current density, its initial charge specific capacity is 351 mAh g. -1 The initial Coulomb efficiency was 90%.

[0030] Example 3 Step (1) Take 50 g of cinnamon residue powder, add 600 mL of 2 mol / L hydrochloric acid solution for acid washing and deashing, stir for 12 hours, filter and wash until neutral, and dry the filter residue at 80 ℃ to obtain purified biomass powder. Place the purified biomass powder in a tube furnace, and under an argon atmosphere, heat it to 600 ℃ at a heating rate of 10 ℃ / min, and hold for 3 hours for pre-carbonization; after natural cooling, a stable aromatized carbon skeleton precursor is obtained.

[0031] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and mix thoroughly with 20 g of NaHCO3 solid (mass ratio 1:4); place the mixture in a corundum boat, put it into a tube furnace, and heat it to 600°C at a heating rate of 10 °C / min under an argon atmosphere, and hold it for 3 hours; after natural cooling, obtain the activated intermediate.

[0032] Step (3) The activated intermediate is repeatedly washed with deionized water until neutral to completely remove the Na2CO3 template. After drying at 80 °C, a porous carbon material is obtained. This porous carbon material is placed in a graphite crucible and then placed in a tube furnace. Under an argon atmosphere, the temperature is first increased to 1400 °C at 10 °C / min, and then increased to 1500 °C at 5 °C / min. The temperature is held for 2 hours. After natural cooling, a hard carbon material is obtained.

[0033] SEM images of the sodium-ion battery hard carbon anode material prepared in Example 3 are shown below. Figure 3 As shown in (a) and (b), the material surface exhibits an increased number of pores and fragmented particle edges, indicating overactivation. When this material was assembled into a sodium-ion half-cell for testing, at 30 mA g... -1 At current density, its initial charge specific capacity is 334 mAh g. -1 The initial Coulomb efficiency was 90%.

[0034] Example 4 Step (1) Take 50 g of apricot shell powder, add 600 mL of 2 mol / L sulfuric acid solution for acid washing and deashing, stir for 12 hours, filter and wash until neutral, and dry the filter residue at 80 ℃ to obtain purified biomass powder. Place the purified biomass powder in a tube furnace and heat it to 400 ℃ at a heating rate of 10 ℃ / min under an argon atmosphere, and hold for 2 hours for pre-carbonization; after natural cooling, a stable aromatized carbon skeleton precursor is obtained.

[0035] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and mix thoroughly with 10 g of NaHCO3 solid (mass ratio 1:2); place the mixture in a corundum boat, put it into a tube furnace, and heat it to 800°C at a heating rate of 10 °C / min under an argon atmosphere, and hold it for 2 hours; after natural cooling, obtain the activated intermediate.

[0036] Step (3) The activated intermediate is repeatedly washed with deionized water until neutral to completely remove the Na2CO3 template. After drying at 80 °C, a porous carbon material is obtained. This porous carbon material is placed in a graphite crucible and put into a tube furnace. Under an argon atmosphere, the temperature is increased to 1200 °C at 10 °C / min and held for 3 hours. After natural cooling, a hard carbon material is obtained.

[0037] SEM images of the sodium-ion battery hard carbon anode material prepared in Example 4 are shown below. Figure 4 As shown in (a) and (b), the number of pores on the material surface is reduced, and the structure is more loose. When this material is assembled into a sodium-ion half-cell for testing, it achieves a yield of 30 mA g... -1 At current density, the initial charge specific capacity is 348 mAh g. -1 The initial Coulomb efficiency was 89%.

[0038] Example 5 Step (1) Take 50 g of rice husk powder, add 600 mL of 2 mol / L sulfuric acid solution for acid washing and deashing, stir for 12 hours, filter and wash until neutral, and dry the filter residue at 80 ℃ to obtain purified biomass powder. Place the purified biomass powder in a tube furnace and heat it to 600 ℃ at a heating rate of 10 ℃ / min under an argon atmosphere, and hold for 1 hour for pre-carbonization; after natural cooling, a stable aromatized carbon skeleton precursor is obtained.

[0039] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and 5 g of NaHCO3 solid (mass ratio 1:1) and mix thoroughly; place the mixture in a corundum boat, put it into a tube furnace, and heat it to 900℃ at a heating rate of 10 ℃ / min under an argon atmosphere, and hold it for 1 hour; after natural cooling, obtain the activated intermediate.

[0040] Step (3) The activated intermediate is repeatedly washed with deionized water until neutral to completely remove the Na2CO3 template. After drying at 80 °C, a porous carbon material is obtained. This porous carbon material is placed in a graphite crucible and put into a tube furnace. Under an argon atmosphere, the temperature is increased to 1600 °C at 10 °C / min and held for 1 hour. After natural cooling, a hard carbon material is obtained.

[0041] SEM images of the sodium-ion battery hard carbon anode material prepared in Example 5 are shown below. Figure 5 As shown in (a) and (b), the number of pores on the material surface decreases and tends to become denser, indicating that excessively high temperatures cause partial collapse of the pore structure. The material was assembled into a sodium-ion half-cell for testing at 30 mA g. -1 At current density, the initial charge specific capacity is 355 mAh g. -1 The initial Coulomb efficiency was 88%.

[0042] Example 6 Step (1) Collect the CO2 gas generated during the heat treatment in steps (2) and (3) of Example 1, and introduce it into the reaction flask containing the Na2CO3 solution obtained from washing in step (3) of Example 1 through a conduit at a rate of 100 mL / min. Stir the reaction at 0°C for 2 hours.

[0043] After the reaction in step (2) is completed, the reaction solution is cooled and crystallized at 0 °C. The resulting white solid is filtered and dried at 30 °C for 12 hours to obtain the recovered sodium bicarbonate solid. The recovery rate of sodium bicarbonate is 84.5% according to the weighing calculation. XRD analysis shows that there are no impurity peaks, and the purity meets the requirements for reuse.

[0044] Step (3) replace commercial sodium bicarbonate with recycled sodium bicarbonate solid and re-prepare the hard carbon material according to steps (2) and (3) of Example 1.

[0045] The electrochemical performance of the sodium-ion battery anode material prepared in Example 6 at 30 mA g -1 At current density, its initial charge specific capacity can reach 351 mAh g. -1 The initial coulombic efficiency was 89%, and the performance retention rate was 96.7% compared with Example 1, confirming that NaHCO3 can be effectively recycled.

[0046] Example 7 Step (1) Collect the CO2 gas generated during the heat treatment in steps (2) and (3) of Example 2, and pass it through a conduit into a reaction flask containing the Na2CO3 solution obtained from washing in step (3) of Example 2 at a rate of 200 mL / min. Stir the reaction at 10°C for 3 hours.

[0047] After the reaction in step (2) is completed, the reaction solution is cooled and crystallized at 10 °C. The resulting white solid is filtered and dried at 50 °C for 8 hours to obtain the recovered sodium bicarbonate solid. The recovery rate of sodium bicarbonate is 78.6% according to the weighing calculation. XRD analysis shows no impurity peaks, and the purity meets the requirements for reuse.

[0048] Step (3) replace commercial sodium bicarbonate with recycled sodium bicarbonate solid and re-prepare the hard carbon material according to steps (2) and (3) of Example 2.

[0049] The electrochemical performance of the sodium-ion battery anode material prepared in Example 7 at 30 mA g -1 At current density, its initial charge specific capacity can reach 330 mAh g. -1 The initial coulombic efficiency was 89%, and the performance retention rate was 93.5% compared with Example 2.

[0050] Example 8 Step (1) Collect the CO2 gas generated during the heat treatment in steps (2) and (3) of Example 3, and introduce it into the reaction flask containing the Na2CO3 solution obtained from washing in step (3) of Example 3 through a conduit at a rate of 50 mL / min. Stir the reaction at 5 °C for 1 hour.

[0051] After the reaction in step (2) is completed, the reaction solution is cooled and crystallized at 5 °C. The resulting white solid is filtered and dried at 20 °C for 24 hours to obtain the recovered sodium bicarbonate solid. The recovery rate of sodium bicarbonate is 82.1% according to the weighing calculation. XRD analysis shows no impurity peaks, and the purity meets the requirements for reuse.

[0052] Step (3) replace commercial sodium bicarbonate with recycled sodium bicarbonate solid and re-prepare the hard carbon material according to steps (2) and (3) of Example 3.

[0053] The electrochemical performance of the sodium-ion battery anode material prepared in Example 8 at 30 mA g -1 At current density, its initial charge specific capacity can reach 314 mAh g. -1 The initial coulomb efficiency was 87%, and the performance retention rate was 94.1% compared with Example 3.

[0054] Comparative Example 1 This comparative example aims to illustrate the impact of omitting the NaHCO3 activation step and only subjecting the acid-washed biomass feedstock to high-temperature carbonization on the final hard carbon material structure and properties.

[0055] Step (1) is the same as step (1) in Example 1, to obtain the aromatized carbon skeleton precursor.

[0056] Step (2) The aromatized carbon skeleton precursor is placed directly in a graphite crucible and the comparative hard carbon material is obtained by the same high-temperature carbonization process as step (3) of Example 1.

[0057] SEM images of the sodium-ion battery hard carbon anode material prepared in Comparative Example 1 are shown below. Figure 6 As shown in (a) and (b), the material surface is dense and smooth, without any porous structure; when this material is assembled into a sodium-ion half-cell for testing, it achieves a yield of 30 mA g. -1 At current density, the initial charge specific capacity is 246 mAh g. -1 The initial coulombic efficiency was 77%. Compared with Example 1, pre-carbonization and high-temperature carbonization can only form limited natural pores and cannot produce abundant closed-pore structures for sodium ion storage, resulting in a significantly lower plateau capacity.

[0058] Comparative Example 2 This comparative example aims to illustrate the effect of omitting the pre-carbonization step and directly activating the acid-washed purified biomass powder with NaHCO3 on the structure and properties of the final hard carbon material.

[0059] Step (1) Example 1 After the acid washing treatment in step (1) of the purified biomass powder, it directly enters the activation process without pre-carbonization.

[0060] Step (2) Weigh 5 g of purified biomass powder and 5 g of NaHCO3 solid (mass ratio 1:1) and mix them thoroughly. The activated intermediate is obtained by the same heat treatment process as in step (2) of Example 1.

[0061] Step (3) The activated intermediate is placed directly in a graphite crucible and the comparative sample hard carbon material is obtained by the same high-temperature carbonization process as in step (3) of Example 1.

[0062] SEM images of the sodium-ion battery hard carbon anode material prepared in Comparative Example 2 are shown below. Figure 7 As shown in (a) and (b), the material exhibits a bulk, dense structure with no obvious pores, and some areas show remnants of incompletely pyrolyzed original biomass morphology. When this material was assembled into a sodium-ion half-cell for testing, it achieved a yield of 30 mA g... -1 At current density, the initial charge specific capacity is 267 mAh g. -1 The initial coulombic efficiency was 71%. Compared with Example 1, when the pre-carbonization step was omitted, the cellulose, lignin and other components in the original biomass underwent violent pyrolysis during the heating process, the carbon skeleton collapsed severely, and NaHCO3 could not be effectively embedded and formed pores, resulting in extremely poor activation effect, with both capacity and initial efficiency being far lower than in Example 1.

[0063] Comparative Example 3 This comparative example aims to illustrate the effect of using Na2CO3 directly as an activator on the structure and properties of the final hard carbon material.

[0064] Step (1) is the same as in Example 1, to obtain the aromatized carbon skeleton precursor.

[0065] Step (2) Weigh 5 g of aromatized carbon skeleton precursor and 5 g of Na2CO3 solid (mass ratio 1:1) and mix thoroughly. The activated intermediate is obtained by the same heat treatment process as in step (2) of Example 1.

[0066] Step (3) The activated intermediate is washed and subjected to high-temperature carbonization in the same manner as in step (3) of Example 1 to obtain the comparative sample hard carbon material.

[0067] SEM images of the sodium-ion battery hard carbon anode material prepared in Comparative Example 3 are shown below. Figure 8 As shown in (a) and (b), the material surface is smooth and dense, with almost no pores and regular particle edges; when this material is assembled into a sodium-ion half-cell for testing, it achieves a performance of 30 mA g. -1 At current density, the initial charge specific capacity is 215 mAh g. -1 The initial coulombic efficiency was 76%. Compared with Example 1, Na2CO3 cannot decompose and release CO2 and H2O gases during heat treatment, lacking the synergistic effect of gas activation. Relying solely on the template effect of Na2CO3 molten salt is insufficient for effective pore formation; Na2CO3 directly replacing NaHCO3 cannot achieve excellent activation effects.

[0068] This invention successfully prepared a hard carbon anode material for sodium-ion batteries with high plateau capacity and high initial coulombic efficiency through a synergistic process of pre-carbonization, in-situ mild activation with NaHCO3, and high-temperature carbonization. The sodium-ion battery anode materials prepared in Examples 1-5 showed significantly higher initial charge specific capacity and initial coulombic efficiency than those in Comparative Examples 1-3, indicating that the pre-carbonization coupled with in-situ mild activation with sodium bicarbonate is effective for preparing biomass-based hard carbon anode materials.

[0069] Examples 1-5 illustrate the effects of activator ratio and carbonization temperature on performance. Both excessively low or high activator ratios and excessively low or high carbonization temperatures lead to a decrease in capacity. Regarding the NaHCO3 dosage, a mass ratio of 1:1 is optimal. As for carbonization temperature, a pre-carbonization temperature of 500 °C and a high-temperature carbonization temperature of 1500 °C are optimal.

[0070] Examples 6-8 verified the feasibility of activator recovery and recycling, and the performance retention rate of hard carbon materials prepared using recycled NaHCO3 reached 93.5%-96.7%. In terms of CO2 gas introduction rate, 100 mL / min was optimal; in terms of cooling crystallization temperature, 0 °C was optimal.

[0071] Comparative Examples 1-3 respectively confirmed the necessity of the activation step, the pre-carbonization step, and the irreplaceable role of in-situ activation by NaHCO3.

[0072] The embodiments described above are merely preferred embodiments of the present invention, and not an exhaustive list of all possible implementations of the present invention. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principles and spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled with sodium bicarbonate in-situ mild activation technology, characterized in that, The method includes the following steps: Step (1) The biomass raw material is acid washed to remove impurities and then dried to obtain purified biomass; the purified biomass is pre-carbonized under an inert atmosphere to obtain a rigid aromatized carbon skeleton precursor, which is a stable carbon skeleton with a rigid aromatic ring structure. Step (2) The aromatized carbon skeleton precursor is thoroughly mixed with sodium bicarbonate solid and heat-treated under an inert atmosphere to obtain an activated intermediate. During the heat treatment, NaHCO3 decomposes to release CO2 and H2O gas. The in-situ generated Na2CO3 serves as an in-situ template to fill the pores in the form of nanocrystals to form a rigid carbon skeleton. Step (3) Wash the activated intermediate to remove the in-situ template of Na2CO3, and collect the washed solid and the filtrate containing Na2CO3 respectively; dry the collected solid to obtain porous carbon material; perform high-temperature carbonization treatment on the porous carbon material under an inert atmosphere to obtain high-performance hard carbon anode material. Step (4) collect and purify the CO2 gas generated by the decomposition of NaHCO3 in step (2) and the CO2 gas generated during the high-temperature carbonization process in step (3); place the filtrate collected in step (3) in an ice-water bath to cool and crystallize, then continuously pass the collected CO2 gas through it until saturation, and dry it at low temperature to obtain sodium bicarbonate solid, which is then reused in step (2).

2. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (1): The biomass raw material is natural biomass rich in cellulose; The acid used in the pickling process is hydrochloric acid or sulfuric acid; The pre-carbonization treatment is carried out at a temperature of 200-600 ℃ and for a holding time of 1-3 hours.

3. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (1), the biomass raw material is a processing by-product, including but not limited to one or more of cinnamon residue, apricot shells, and rice husks.

4. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (2): The mass ratio of the carbon skeleton precursor to sodium bicarbonate solid is 1:0.5 to 1:4; The heat treatment temperature is 600-1000 ℃, and the holding time is 1-3 hours.

5. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (3): The washing process is water washing, used to thoroughly remove the Na2CO3 in-situ template; The high-temperature carbonization treatment is carried out at a temperature of 1200-1600 ℃ and a holding time of 1-3 hours.

6. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (4), the rate of CO2 gas introduced is 50-200 mL / min; the cooling crystallization temperature is 0-10 ℃; and the reaction time is 1-3 hours.

7. The method for preparing biomass-based hard carbon anode materials based on pre-carbonization coupled sodium bicarbonate in-situ mild activation technology according to claim 1, characterized in that, In step (4), the low-temperature drying temperature is 20-50 ℃ and the drying time is 8-24 hours.

8. A biomass-based hard carbon anode material prepared based on a pre-carbonization coupled sodium bicarbonate in-situ mild activation technique, characterized in that, It is prepared by any one of the preparation methods described in claims 1-7.

9. The biomass-based hard carbon anode material prepared according to claim 8 using a pre-carbonization coupled sodium bicarbonate in-situ mild activation technique, characterized in that, The biomass-based hard carbon anode material has a typical non-graphitized carbon structure, with the carbon layers bent and interlaced to form a disordered carbon structure; the carbon layer spacing is 0.38-0.42 nm; and the pore size is concentrated in the range of 2-5 nm.

10. An application of the biomass-based hard carbon anode material prepared by the pre-carbonization coupled sodium bicarbonate in-situ mild activation technology as described in claim 8 or 9, characterized in that, It was used as an active material in the negative electrode of a biomass-based sodium-ion battery, at 30 mA g. -1 At current density, the initial discharge specific capacity of the carbon material anode in a biomass sodium-ion battery is 314 mAh g. -1 The initial coulombic efficiency is over 87%, demonstrating excellent sodium storage performance.

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