A bamboo-based and lignin-based hard carbon composite material, its preparation method and application

Abstract

This invention belongs to the field of hard carbon technology, specifically relating to a bamboo-based and lignin-based hard carbon composite material, its preparation method, and its applications. The process involves pretreating raw materials to obtain pretreated material; pretreating the pretreated material with an acidic oxidant to obtain acidic oxidant pretreated material; pre-oxidizing the acidic oxidant pretreated material to obtain pre-oxidized material; pre-carbonizing the pre-oxidized material under a protective atmosphere to obtain pre-carbonized material; pulverizing the pre-carbonized material to obtain pulverized material; acid washing followed by water washing to obtain purified material; soaking the purified material in acid, filtering, and drying to obtain soaked material; mixing the soaked material with a modifier, and high-temperature coating and carbonizing under a protective atmosphere to obtain bamboo-based coated hard carbon or lignin-based coated hard carbon. Mixing the two types of hard carbon and carbon coating yields a bamboo-based and lignin-based hard carbon composite material, which is used in negative electrode sheets and batteries. When applied to batteries, the hard carbon composite material of this invention can improve battery energy density.

Description

Technical Field

[0001] This invention belongs to the field of hard carbon technology, specifically relating to a bamboo-based and lignin-based hard carbon composite material, its preparation method, and its application. Background Technology

[0002] Currently, sodium-ion battery technology is undergoing rapid evolution and innovation. Benefiting from its advantages such as low cost, long cycle life, fast charging, environmental friendliness, excellent high and low temperature performance, and safety, sodium-ion batteries are continuously developing towards lower cost, higher energy density, and longer lifespan. Despite these advantages, sodium-ion batteries have a lower energy density than lithium-ion batteries, approximately 160Wh / kg, only two-thirds that of lithium iron phosphate batteries. This means that sodium-ion batteries store less energy per unit weight or volume, thus providing shorter driving ranges for sodium-electric vehicles.

[0003] The main reasons for the low energy density of sodium-ion batteries are: sodium ions have a larger ionic radius, affecting phase stability during the reaction process and resulting in slower ion transport and diffusion; sodium ions have a high mass-to-charge ratio, reducing the theoretical specific capacity of the material; and sodium has a high standard electrode potential. Given these reasons, the energy density of sodium-ion batteries can be improved by optimizing the battery structure and the energy density of the materials. A sodium-ion battery mainly consists of four parts: a positive electrode, a negative electrode, an electrolyte, and a separator. The structure and performance of the positive and negative electrode materials determine the overall sodium storage performance of the battery. The negative electrode material accounts for approximately 16% of the total battery mass and has a significant impact on the battery's energy density, which ultimately manifests as the battery's sodium storage capacity. The higher the compaction density of the negative electrode material, the greater the mass of active material that can be stored per unit volume, resulting in higher battery capacity and energy density.

[0004] Therefore, it is particularly important to improve the capacity of hard carbon anode materials by designing a low-cost, high-pressure anode material. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing bamboo-based coated hard carbon; the second objective is to provide a method for preparing lignin-based coated hard carbon; the third objective is to provide a method for preparing a bamboo-based and lignin-based hard carbon composite material; the fourth objective is to provide a negative electrode sheet; and the fifth objective is to provide a battery. This invention prepares two types of hard carbon: bamboo-based coated hard carbon and lignin-based coated hard carbon. These two types of hard carbon are then mixed and further coated with carbon to prepare a hard carbon composite material. When applied to sodium-ion batteries, this composite material can improve the energy density of the sodium-ion battery.

[0006] To achieve the above and other related objectives, the present invention provides the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing bamboo-based coated hard carbon, comprising the following steps:

[0008] The raw materials are pretreated to obtain pretreated material; the raw materials include bamboo-based biomass raw materials;

[0009] The pretreated material is pretreated with an acidic oxidant to obtain an acidic oxidant pretreated material;

[0010] The acidic oxidant pretreatment material is pre-oxidized to obtain a pre-oxidized material;

[0011] The pre-oxidized material is pre-carbonized under a protective atmosphere to obtain a pre-carbonized material;

[0012] The pre-carbonized material is crushed to obtain crushed material;

[0013] The pulverized material is first acid-washed and then washed with water to obtain purified material;

[0014] The purified material is soaked in acid, filtered, and dried to obtain the soaked material; the acid used for soaking includes phosphoric acid.

[0015] The soaked material is mixed evenly with the modifier, and then high-temperature coating and carbonization is carried out under a protective atmosphere to obtain bamboo-based coated hard carbon.

[0016] Secondly, the present invention provides a method for preparing lignin-coated hard carbon, comprising the following steps:

[0017] The raw materials are pretreated to obtain pretreated material; the raw materials include lignin-based biomass raw materials;

[0018] The pretreated material is pretreated with an acidic oxidant to obtain an acidic oxidant pretreated material;

[0019] The acidic oxidant pretreatment material is pre-oxidized to obtain a pre-oxidized material;

[0020] The pre-oxidized material is pre-carbonized under a protective atmosphere to obtain a pre-carbonized material;

[0021] The pre-carbonized material is crushed to obtain crushed material;

[0022] The pulverized material is first acid-washed and then washed with water to obtain purified material;

[0023] The purified material is soaked in acid, filtered, and dried to obtain the soaked material; the acid used for soaking includes phosphoric acid.

[0024] The soaked material is mixed evenly with the modifier, and then subjected to high-temperature coating carbonization under a protective atmosphere to obtain lignin-based coated hard carbon.

[0025] Thirdly, the present invention provides a method for preparing a bamboo-based and lignin-based hard carbon composite material, comprising the following steps: preparing bamboo-based coated hard carbon and lignin-based coated hard carbon respectively; mixing the bamboo-based coated hard carbon and lignin-based coated hard carbon and then carbon coating to obtain a bamboo-based and lignin-based hard carbon composite material.

[0026] The bamboo-based coated hard carbon is prepared by the aforementioned method for preparing bamboo-based coated hard carbon.

[0027] The lignin-coated hard carbon is prepared by the aforementioned method for preparing lignin-coated hard carbon.

[0028] In one embodiment of the present invention, the raw material comprises at least one of bamboo and wood.

[0029] In one embodiment of the present invention, the pretreatment parameters for preparing bamboo-based coated hard carbon and lignin-based coated hard carbon both include: washing the raw materials with water, and then drying them in a drying oven at a temperature of 60–120°C for 4–24 hours.

[0030] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the parameters for pretreating the pretreated material with an acidic oxidant include: reacting the pretreated material and the acidic oxidant at 60-100°C for 4-12 hours, wherein the acidic oxidant includes an oxidant and an acid, the oxidant including at least one of hydrogen peroxide, peracetic acid, and ozone, wherein the concentration of the oxidant is 1-5%, the acid including at least one of formic acid and acetic acid, the concentration of the acid is 0.5-2%, and the solid-liquid mass ratio of the pretreated material and the acidic oxidant is 1:1.2-5.

[0031] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the acidic oxidant pretreatment material is pre-oxidized to obtain pre-oxidized material. The parameters of the pre-oxidized material are as follows: temperature is 200-350°C and pre-oxidation time is 3-5 hours.

[0032] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pre-oxidized material is pre-carbonized under a protective atmosphere. The parameters of the pre-carbonized material include: a heating rate of 2-10°C / min, a pre-carbonization temperature of 200-800°C, and a holding time of 1-3 hours. The pre-carbonization is carried out in a rotary kiln, and the protective atmosphere includes at least one of nitrogen, argon, and helium.

[0033] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pre-carbonized material is pulverized to obtain pulverized material with the following parameters: the pulverizing equipment includes at least one of a mechanical pulverizer, a roller mill, and an air jet mill, and the particle size D50 of the pulverized material is controlled between 3 and 10 μm.

[0034] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pulverized material is first acid-washed and then water-washed to obtain purified material. The parameters of the purified material include: the acid used for acid washing includes at least two of hydrochloric acid, nitric acid, hydrofluoric acid, and sulfuric acid; the acid concentration for acid washing is 20-50%; the acid washing reaction temperature is 60-100°C; the solid-liquid mass ratio for acid washing is 1:0.7-5.5; and the acid washing time is 4-24 hours. The water washing process includes: washing with deionized water multiple times until the filtrate is close to neutral; then filtration and drying at a temperature of 60-120°C for 6-24 hours.

[0035] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the purified material is soaked in a phosphoric acid solution. The parameters of the soaked material include: the concentration of phosphoric acid is 3-30%, the soaking time is 3-24 h, and the solid-liquid mass ratio is 1:1.2-3. The material is stirred while soaking, and after soaking, it is filtered and dried at a temperature of 60-120°C for 4-24 h.

[0036] In one embodiment of the present invention, when preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the parameters for uniformly mixing the soaked material with a modifier and then performing high-temperature coating carbonization under a protective atmosphere include: the modifier includes at least one of asphalt and resin, the mass of the modifier accounts for 1 to 10 wt% of the mass of the soaked material, and the mixing time is 0.5 to 3 hours.

[0037] The high-temperature coating carbonization is a high-temperature segmented carbonization process: the first stage is low-temperature carbonization, with a temperature of 800-1000℃, a heating rate of 4-6℃ / min, and a holding time of 1-2h; the second stage is high-temperature carbonization, with a temperature of 1200-1400℃, a heating rate of 1-2℃ / min, and a holding time of 2-4h; the third stage is cooling, with the temperature reduced to 800-1100℃, a cooling rate of 3-5℃ / min, and then naturally cooled to 25-30℃ under a protective atmosphere.

[0038] The protective atmosphere includes at least one of nitrogen, argon, and helium.

[0039] In one embodiment of the present invention, when bamboo-based coated hard carbon and lignin-based coated hard carbon are mixed and carbon coated to obtain a bamboo-based and lignin-based hard carbon composite material, the mixture of bamboo-based coated hard carbon and lignin-based coated hard carbon is added to a CVD carbon coating device, and then a carbon source and a protective gas are introduced, the temperature is raised, the temperature is maintained, and the temperature is cooled to obtain the bamboo-based and lignin-based hard carbon composite material.

[0040] The mass ratio of bamboo-based coated hard carbon to lignin-based coated hard carbon is 1:9 to 9:1, the mixing time is 0.5 to 2 hours, the carbon source includes at least one of acetylene, methane, propane, ethylene, propylene, and benzene, the protective gas includes at least one of argon and nitrogen, the volume fraction of carbon source and protective gas is 4% to 10%: 96% to 90%, the heating rate is 5 to 10 °C / min, the heating temperature is 650 to 1000 °C, and the holding time is 0.5 to 4 hours; the CVD carbon coating equipment includes at least one of a CVD rotary furnace, an intermittent CVD furnace, and a continuous CVD furnace.

[0041] Furthermore, the powder compaction density of bamboo-based and lignin-based hard carbon composite materials is 0.8900–1.2000 g / cm³. 3 .

[0042] Fourthly, the present invention provides a negative electrode sheet, wherein the negative electrode sheet uses a bamboo-based and lignin-based hard carbon composite material as the active material of the battery negative electrode material, and the bamboo-based and lignin-based hard carbon composite material is prepared by the aforementioned method for preparing bamboo-based and lignin-based hard carbon composite materials.

[0043] Fifthly, a battery, which is a sodium-ion battery, comprising a negative electrode, a positive electrode, a separator, and an electrolyte, wherein the negative electrode comprises the aforementioned negative electrode sheet, and a charge-discharge test is performed at a current density of 0.1C, wherein the initial charge specific capacity of the sodium-ion battery is greater than 330 mAh / g, and the initial coulombic efficiency is greater than 90%.

[0044] The aforementioned negative electrode sheet and battery utilize bamboo-based and lignin-based hard carbon composite materials.

[0045] The beneficial effects of this invention are:

[0046] 1. Environmentally Friendly Raw Materials: The bamboo-based and lignin-based biomass raw materials used in this invention are abundant, short-cycle, renewable, and green materials. They possess excellent characteristics such as wide distribution, rapid growth, early maturity, and high strength. They represent the largest biomass resources on Earth, with billions of tons of bamboo-based and lignin-based biomass raw materials produced annually through photosynthesis, ensuring a stable supply of raw materials. Furthermore, as sustainable and environmentally friendly materials, bamboo-based and lignin-based biomass raw materials exhibit low energy consumption and low emissions during their preparation process, aligning with current green development requirements. Therefore, using bamboo-based and lignin-based biomass raw materials to prepare hard carbon anodes not only improves energy efficiency but also helps reduce dependence on fossil fuels, promoting energy transition.

[0047] 2. The process of this invention is reasonable, including pretreatment, acidic oxidant pretreatment, pre-oxidation, pre-carbonization, pulverization, acid washing followed by water washing, acid soaking followed by filtration and drying, modifier modification, high-temperature coating carbonization (high-temperature segmented carbonization), and CVD carbon coating, as detailed below:

[0048] (1) Due to the complex and dense cell wall structure of lignocellulose in lignin-based biomass raw materials, it is difficult to dissolve and utilize. Therefore, the lignocellulose of this invention adopts a pretreatment method combining oxidant and acid. The green strong oxidant can oxidize and degrade lignocellulose into aldehydes, phenols, organic acids, and small molecule compounds, thereby achieving the separation of carbohydrates, cellulose, hemicellulose, and lignin in lignocellulose. The acid can promote the decomposition of lignocellulose under mild reaction conditions without high temperature or high pressure, which reduces energy consumption, reduces the risk of side reactions, and improves the usability of lignocellulose. The synergistic effect of the oxidant and acid breaks down the connection between cellulose, lignin, and hemicellulose, achieving the separation of lignocellulose components. This is beneficial for reducing crystallinity and removing impurities by acid washing, increasing porosity, hydrolysis efficiency, and raw material utilization, thereby increasing the active sites of hard carbon composite materials, increasing interlayer spacing, and greatly improving the reversible specific capacity of the battery, ultimately improving energy density and specific capacity. This invention does not use a single pretreatment technology, but overcomes the shortcomings of a single method by combining the pretreatment of oxidant and acid, enhancing the pretreatment technology, and thus effectively improving the resource utilization efficiency of biomass raw materials.

[0049] (2) The biomass raw materials used in this invention are bamboo-based and lignin-based, with lignocellulose as the main component. Lignocellulose is mainly composed of cellulose, hemicellulose, and lignin, with cellulose accounting for approximately 30-50%, lignin and hemicellulose accounting for 10-20% and 15-35%, respectively. Cellulose in lignocellulose forms microfibers, constituting a three-dimensional network framework of fiber cell walls, while hemicellulose and lignin fill the spaces between fibers and microfibers in the form of covalent bonds. Their unique structure, pore channels, and abundant aromatic carbon rings give them a significant advantage as precursors for hard carbon materials in sodium-ion batteries. However, lignocellulose hard carbon materials obtained through direct carbonization cannot exhibit good electrochemical performance due to their relatively low conductivity, cross-linking degree, and limited sodium ion storage sites. Therefore, this invention introduces oxygen-containing, hydroxyl, and carbonyl functional groups and enhances the cross-linking degree between lignocellulose molecules through ester groups by adding a low-temperature pre-oxidation process before pre-carbonization, generating a large number of disordered structures, expanding the carbon interlayer spacing, and promoting Na+ ion storage. + The transmission of gases during heating and the release of small molecule gases during heating induce the formation of nanopores within the amorphous carbon, significantly improving the electrochemical performance of hard carbon composite materials.

[0050] (3) The present invention uses phosphoric acid soaking, which can not only further dissolve and decrystalline cellulose, hemicellulose and lignin, but also introduce P element doping modification, thereby enhancing the reactivity and adding additional Na + The increased number of storage sites improves conductivity and wettability, thereby increasing the specific capacity of hard carbon anode materials.

[0051] (4) This invention does not employ the method of mixing first and then coating with a modifier for carbonization. Instead, it employs the method of coating and carbonizing the two materials separately with a modifier first, and then mixing the two carbonized materials. This is because different hard carbon precursors have different internal microstructures, pore sizes, pore structures, surface areas, and surface functional groups, resulting in different optimal coating ratios for each. The composite material obtained by coating and carbonizing the precursors separately and then mixing the carbonized precursors allows the synergistic effect of the two to fully utilize their respective advantages. This results in a more uniform and denser coating of the hard carbon precursor, while maintaining the high capacity of bamboo-based biomass and the high compaction of lignin-based biomass.

[0052] (5) The reason for the high-temperature segmented carbonization of the present invention: During the first stage of low-temperature carbonization, pyrolysis occurs, which rapidly releases a large number of organic molecules, which is conducive to the formation of a rich porous structure; During the second stage of high-temperature carbonization, the degree of graphitization can be improved and a large interlayer spacing can be retained, which is conducive to the insertion and extraction of sodium ions in the material, thereby improving the initial coulombic efficiency and reversible capacity. However, the heating rate in the high-temperature stage should not be too fast, otherwise it will easily cause insufficient volatilization of small molecules and increase the specific surface area; During the third stage of cooling, the temperature should be kept to decrease slowly, otherwise internal cracks will appear and the performance will be reduced.

[0053] (6) Compared with solid phase coating, the present invention uses CVD to perform carbon coating. High-temperature pyrolysis causes carbon source gas to be deposited on the surface and pores of bamboo-based and lignin-based hard carbon composite materials, resulting in uniform carbon material composition and dense structure. The coating layer can not only increase the conductivity of composite materials, but also reduce the decomposition of electrolyte on the electrode surface, and has good cycle characteristics.

[0054] 3. The powder compaction density of the bamboo-based and lignin-based hard carbon composite material of the present invention is 0.8900~1.2000 g / cm³. 3 When applied to negative electrode sheets and sodium-ion batteries, it can improve the energy density of sodium-ion batteries. Under charge and discharge tests at a current density of 0.1C, the initial charge specific capacity of sodium-ion batteries is greater than 330mAh / g, and the initial coulombic efficiency is greater than 90%. Attached Figure Description

[0055] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0056] Figure 1 These are XRD comparison images of the hard carbon composite materials of Example 1 and Comparative Example 1 of the present invention;

[0057] Figure 2 This is a comparison graph of the charge-discharge curves of the hard carbon composite materials of Example 1 and Comparative Example 7 of the present invention under 0.1C conditions. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] Unless otherwise specified, the equipment and materials used in the embodiments can be readily obtained from commercial companies.

[0060] The CVD rotary kiln model is: KY-RJ(Q) series, Keyuan.

[0061] The intermittent chemical vapor deposition (CVD) furnace model is: KY-R-JQC series, Keyuan.

[0062] The continuous chemical vapor deposition (CVD) furnace model is: KY-R-LQC series, Keyuan.

[0063] Phenolic resin, CAS 9003-35-4, Aladdin reagent;

[0064] CMC (sodium carboxymethyl cellulose), CAS 9004-32-4, C104985, Aladdin reagent;

[0065] Conductive carbon black (SP) and SBR (styrene-butadiene rubber) are both general-purpose reagents suitable for batteries.

[0066] Example 1

[0067] A method for preparing a bamboo-based and lignin-based hard carbon composite material, comprising the following steps:

[0068] Note: Since the preparation methods for bamboo-based coated hard carbon and lignin-based coated hard carbon are the same in this embodiment (except for the different raw materials), they are described uniformly in S1 to S7 below. S8 and S9 involve further processing using the bamboo-based coated hard carbon and lignin-based coated hard carbon obtained respectively, specifically as follows:

[0069] S1. Rinse the bamboo or wood with water first, then put it into a drying oven and dry it at 80℃ for 24 hours to obtain the pretreated material;

[0070] S2. The pretreated material is added to a mixed solution of 1% formic acid and 3% H2O2 (hydrogen peroxide) and reacted at 80°C for 6 hours, wherein the solid-liquid mass ratio is 1:2, to obtain the acidic oxidant pretreated material.

[0071] S3. Place the acidic oxidant pretreated material in a rotary kiln, first introduce oxygen, then heat it to 300°C at a heating rate of 3°C / min, pre-oxidize for 3 hours to obtain pre-oxidized material, then introduce nitrogen, and under nitrogen atmosphere, heat it to 700°C at a heating rate of 5°C / min, hold for 2 hours and then cool to room temperature to obtain pre-carbonized material.

[0072] S4. Use an air jet mill to pulverize the pre-carbonized material. After pulverization, the particle size D50 of the particles is controlled at 5μm to obtain pulverized material.

[0073] S5. Place the pulverized material in a 30% acid solution (HCl (hydrochloric acid) and HF (hydrofluoric acid) and react at 80°C for 12 hours. The solid-liquid mass ratio is 1:3. After purification, wash the material several times with deionized water until the filtrate is nearly neutral. Then filter the filtrate and place it in a drying oven at 80°C for 24 hours to obtain the purified material. It should be noted that a 30% concentration of hydrochloric acid and hydrofluoric acid is sufficient. The individual concentrations of hydrochloric acid and hydrofluoric acid are not key characteristics or affect the experimental results, and therefore will not be described in detail.

[0074] S6. Add 10% phosphoric acid to the purified material and soak it at room temperature while stirring. After soaking for 6 hours, filter it first and then place it in a drying oven at 80°C for 24 hours. The solid-liquid mass ratio is 1:2 to obtain the soaked material.

[0075] S7. The soaked material and phenolic resin are mixed evenly at room temperature using a VC machine, wherein the mass of phenolic resin is 5 wt% of the total mass of the soaked material. After mixing for 1 hour, the mixture is placed in a pusher furnace for high-temperature segmented carbonization. Under a nitrogen atmosphere, the temperature is first raised to 800℃ at a heating rate of 5℃ / min and held for 1 hour; then raised to 1300℃ at a heating rate of 2℃ / min and held for 3 hours; finally, the temperature is lowered to 800℃ at a cooling rate of 4℃ / min and then naturally cooled to 30℃ to obtain bamboo-based or lignin-based coated hard carbon.

[0076] S8. Mix bamboo-based coated hard carbon and lignin-based coated hard carbon at a mass ratio of 3:7 at room temperature using a VC mixer. After mixing for 30 minutes, remove iron and pass through a 325-mesh sieve to obtain a mixture of bamboo-based and lignin-based hard carbon.

[0077] S9. Add the bamboo-based and lignin-based hard carbon mixture into a continuous vapor-phase deposition (CVD) furnace, then introduce a mixture of acetylene and argon gas with a volume ratio of 4%:96%, and raise the temperature to 800℃ at a heating rate of 10℃ / min. After holding at 800℃ for 1 hour, cool to room temperature to obtain the bamboo-based and lignin-based hard carbon composite material.

[0078] Application: The bamboo-based and lignin-based hard carbon composite materials prepared above are used as active materials for the negative electrode of sodium-ion batteries.

[0079] Weigh out 0.2g of CMC (sodium carboxymethyl cellulose), 0.4g of SP (conductive carbon black), 9.2g of bamboo-based and lignin-based hard carbon composite material, and 0.5g of SBR (styrene-butadiene rubber, solid content 40%) according to a mass ratio of 2.0%:4.0%:92.0%:2.0%. Add an appropriate amount of deionized water and stir for 8 hours until a uniform slurry is formed. Use a four-sided coating apparatus to evenly coat the slurry onto the surface of copper foil. Dry in a vacuum drying oven at 80℃ for 8 hours. Cut the Cu foil with active material into circular negative electrode sheets and transfer them to a glove box for later use.

[0080] The coin cell assembly was carried out in an Ar atmosphere glove box. The prepared bamboo-based and lignin-based hard carbon composite material was used as the negative electrode, and a 1.0 mol / L commercial electrolyte was used. The commercial electrolyte was prepared by adding NaPF6 to a volume ratio of EC:DMC = 1:1. Na metal sheets or sodium metal blocks were used as the counter electrode. The CR2032 coin cell was assembled, and then charge-discharge tests were carried out at a current density of 0.1C.

[0081] It should be noted that using Na metal sheets or sodium metal blocks as counter electrodes does not affect the final experimental results in this invention.

[0082] Example 2

[0083] In this embodiment, S2 differs from Embodiment 1 as follows:

[0084] S2. The pretreated material is added to a mixed solution of 2% formic acid and 5% H2O2 and reacted at 80°C for 6 hours, with a solid-liquid mass ratio of 1:2, to obtain an acidic oxidant pretreated material.

[0085] The remaining steps are the same as in Example 1.

[0086] Example 3

[0087] In this embodiment, S2 differs from Embodiment 1 as follows:

[0088] S2. The pretreated material is added to a mixed solution of 0.5% formic acid and 1% H2O2 and reacted at 80°C for 6 hours, with a solid-liquid mass ratio of 1:2, to obtain an acidic oxidant pretreated material.

[0089] The remaining steps are the same as in Example 1.

[0090] Example 4

[0091] In this embodiment, S3 differs from Embodiment 1 as follows:

[0092] S3. Place the acidic oxidant pretreated material in a rotary kiln, first introduce oxygen, then heat it to 200℃ at a heating rate of 3℃ / min, and pre-oxidize it for 5 hours to obtain the pre-oxidized material. Then introduce nitrogen gas, and under the nitrogen atmosphere, heat it to 700℃ at a heating rate of 5℃ / min. After holding it at this temperature for 2 hours, cool it to room temperature to obtain the pre-carbonized material.

[0093] The remaining steps are the same as in Example 1.

[0094] Example 5

[0095] In this embodiment, S4 differs from that in Embodiment 1 as follows:

[0096] S4. Use an air jet mill to pulverize the pre-carbonized material. After pulverization, the particle size D50 of the particles is controlled at 7μm to obtain pulverized material.

[0097] The remaining steps are the same as in Example 1.

[0098] Example 6

[0099] In this embodiment, S6 differs from Embodiment 1 as follows:

[0100] S6. Add 30% phosphoric acid to the purified material and soak it at room temperature while stirring. After soaking for 6 hours, filter it first and then place it in a drying oven at 80℃ for 24 hours. The solid-liquid mass ratio is 1:2 to obtain the soaked material.

[0101] The remaining steps are the same as in Example 1.

[0102] Example 7

[0103] In this embodiment, S7 differs from Embodiment 1 as follows:

[0104] S7. The obtained soaked material is mixed evenly with phenolic resin at room temperature using a VC machine. The mass of phenolic resin is 3 wt% of the total mass of the bamboo-based soaked material or 5 wt% of the total mass of the lignin-based soaked material. After mixing for 1 hour, the mixture is placed in a pusher furnace for high-temperature segmented carbonization. Under a nitrogen atmosphere, the temperature is first raised to 800℃ at a heating rate of 5℃ / min and held for 1 hour. Then, the temperature is raised to 1300℃ at a heating rate of 2℃ / min and held for 3 hours. Finally, the temperature is lowered to 800℃ at a cooling rate of 4℃ / min and then naturally cooled to 30℃ to obtain bamboo-based or lignin-based coated hard carbon.

[0105] The remaining steps are the same as in Example 1.

[0106] Note: To avoid ambiguity, when the mass of phenolic resin is 3 wt% of the total mass of the raw material after soaking bamboo-based materials, bamboo-based coated hard carbon is obtained; when the mass of phenolic resin is 5 wt% of the total mass of the raw material after soaking lignin-based materials, lignin-based coated hard carbon is obtained.

[0107] Example 8

[0108] In this embodiment, S7 differs from Embodiment 7 as follows:

[0109] S7. The obtained soaked material and phenolic resin are mixed evenly at room temperature using a VC machine. The mass of phenolic resin is 5 wt% of the total mass of the bamboo-based soaked material or 3 wt% of the total mass of the lignin-based soaked material. After mixing for 1 hour, the mixture is placed in a pusher furnace for high-temperature segmented carbonization. Under a nitrogen atmosphere, the temperature is first raised to 800℃ at a heating rate of 5℃ / min and held for 1 hour. Then, the temperature is raised to 1300℃ at a heating rate of 2℃ / min and held for 3 hours. Finally, the temperature is lowered to 800℃ at a cooling rate of 4℃ / min and then naturally cooled to 30℃ to obtain bamboo-based or lignin-based coated hard carbon.

[0110] The remaining steps are the same as in Example 1.

[0111] Note: To avoid ambiguity, when the mass of phenolic resin is 5 wt% of the total mass of the raw material after soaking bamboo-based materials, bamboo-based coated hard carbon is obtained; when the mass of phenolic resin is 3 wt% of the total mass of the raw material after soaking lignin-based materials, lignin-based coated hard carbon is obtained.

[0112] Example 9

[0113] In this embodiment, S8 differs from Embodiment 1 as follows:

[0114] S8. Mix bamboo-based coated hard carbon and lignin-based coated hard carbon at a mass ratio of 7:3 at room temperature using a VC mixer. After mixing for 30 minutes, remove iron and pass through a 325-mesh sieve to obtain a mixture of bamboo-based and lignin-based hard carbon.

[0115] The remaining steps are the same as in Example 1.

[0116] Example 10

[0117] In this embodiment, S8 differs from Embodiment 1 as follows:

[0118] S8. Mix bamboo-based coated hard carbon and lignin-based coated hard carbon at a mass ratio of 5:5 using a VC mixer at room temperature. After mixing for 30 minutes, remove iron and pass through a 325-mesh sieve to obtain a mixture of bamboo-based and lignin-based hard carbon.

[0119] The remaining steps are the same as in Example 1.

[0120] Example 11

[0121] In this embodiment, S8 differs from Embodiment 1 as follows:

[0122] S8. Mix bamboo-based coated hard carbon and lignin-based coated hard carbon at a mass ratio of 4:6 at room temperature using a VC mixer. After mixing for 30 minutes, remove iron and pass through a 325-mesh sieve to obtain a mixture of bamboo-based and lignin-based hard carbon.

[0123] The remaining steps are the same as in Example 1.

[0124] Example 12

[0125] In this embodiment, S8 differs from Embodiment 1 as follows:

[0126] S8. Mix bamboo-based coated hard carbon and lignin-based coated hard carbon at a mass ratio of 6:4 at room temperature using a VC mixer. After mixing for 30 minutes, remove iron and pass through a 325-mesh sieve to obtain a mixture of bamboo-based and lignin-based hard carbon.

[0127] The remaining steps are the same as in Example 1.

[0128] Comparative Example 1

[0129] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that: the acidic oxidant pretreatment process in step 2 is omitted, i.e. the pretreated material obtained in step S1 is directly processed in step S3.

[0130] The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1.

[0131] Comparative Example 2

[0132] A method for preparing a bamboo-based and lignin-based hard carbon composite material, compared with Example 1, differs in that: the pre-oxidation process in step 3 is omitted, and direct pre-carbonization is performed.

[0133] S3. Place the acidic oxidant pretreated material in a rotary kiln, introduce nitrogen gas, and raise the temperature to 700°C at a rate of 5°C / min under nitrogen atmosphere. Hold the temperature for 2 hours and then cool to room temperature to obtain the pre-carbonized material.

[0134] The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1.

[0135] Comparative Example 3

[0136] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that the phosphoric acid soaking process in step 6 is omitted, and the purified material obtained in step S5 is directly processed in step S7.

[0137] The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1.

[0138] Comparative Example 4

[0139] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that: in step S7, the obtained soaked material is mixed with phenolic resin at room temperature using a VC mixer, wherein the mass of phenolic resin is 5 wt% of the total mass of the soaked material. After mixing for 1 hour, the mixture is placed in a pusher furnace and heated from room temperature to 1300°C at a heating rate of 2°C / min under a nitrogen atmosphere and held for 3 hours. Then, it is naturally cooled to room temperature. The rest of the steps are the same as those in Example 1, and the obtained battery material is tested using the same method as in Example 1.

[0140] Comparative Example 5

[0141] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that:

[0142] Step S1 involves rinsing the bamboo with water and then drying it in a drying oven at 80℃ for 24 hours to obtain a pretreated material. Step S7 involves mixing the bamboo-based soaked material with phenolic resin at room temperature using a VC mixer. The mass of the phenolic resin is 5 wt% of the total mass of the bamboo-based soaked material. After mixing for 1 hour, the mixture is placed in a pusher furnace and heated to 1300℃ at a rate of 2℃ / min under a nitrogen atmosphere for high-temperature coating carbonization. The temperature is maintained for 3 hours, and the mixture is cooled to room temperature to obtain a bamboo-based coated hard carbon material. Step S8 is omitted. Step S9 involves adding the bamboo-based coated hard carbon material to a continuous vapor-phase deposition (CVD) furnace and then introducing a mixture of acetylene and argon gas with a volume ratio of 4%:96%. The temperature is raised to 800℃ at a rate of 10℃ / min and maintained at 800℃ for 1 hour before cooling to room temperature to obtain a bamboo-based hard carbon composite material. The rest of the steps are the same as in Example 1, and the obtained battery material is tested using the same method as in Example 1 (when preparing the test, the bamboo-based hard carbon composite material is replaced with the bamboo-based and lignin-based hard carbon composite materials).

[0143] It should be noted that, in this embodiment, lignin-coated hard carbon is not prepared.

[0144] Comparative Example 6

[0145] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that:

[0146] Step S1 involves rinsing the wood with water and then drying it in a drying oven at 80°C for 24 hours to obtain a pretreated material. Step S7 involves mixing the obtained lignin-based impregnated material with phenolic resin at room temperature using a VC mixer, wherein the mass of the phenolic resin is 5 wt% of the total mass of the lignin-based impregnated material. After mixing for 1 hour, the mixture is placed in a pusher furnace and heated to 1300°C at a rate of 2°C / min under a nitrogen atmosphere for high-temperature coating carbonization. The temperature is held for 3 hours, and after cooling to room temperature, a lignin-based coated hard carbon material is obtained. Step S8 is omitted. Step S9 involves adding the lignin-based coated hard carbon material to a continuous vapor-phase deposition (CVD) furnace and then introducing a mixture of acetylene and argon gas with a volume ratio of 4%:96%. The temperature is raised to 800°C at a rate of 10°C / min and held at 800°C for 1 hour before cooling to room temperature to obtain a lignin-based hard carbon composite material. The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1 (when preparing the test, the lignin-based hard carbon composite material is replaced with the bamboo-based and lignin-based hard carbon composite materials).

[0147] It should be noted that, in this embodiment, bamboo-based coated hard carbon is not prepared.

[0148] Comparative Example 7

[0149] A method for preparing a bamboo-based and lignin-based hard carbon composite material differs from Example 1 in that: step S7 involves mixing bamboo-based impregnation material and lignin-based impregnation material at a mass ratio of 3:7 with phenolic resin at room temperature using a VC mixer, wherein the mass of phenolic resin is 5 wt% of the total mass of the bamboo-based and lignin-based impregnation materials. After mixing for 30 minutes, the mixture is placed in a pusher furnace for high-temperature segmented carbonization. Under a nitrogen atmosphere, the temperature is first raised to 800°C at a heating rate of 5°C / min and held for 1 hour; then raised to 1300°C at a heating rate of 2°C / min and held for 3 hours; finally, the temperature is lowered to 800°C at a cooling rate of 4°C / min and then naturally cooled to 30°C to obtain the bamboo-based and lignin-based hard carbon composite material. Step S8 is omitted, and the bamboo-based and lignin-based hard carbon composite material is then prepared in step S9.

[0150] The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1.

[0151] It should be noted that in this embodiment, in step S7, bamboo-based coated hard carbon and lignin-based coated hard carbon are not prepared separately. Instead, bamboo-based impregnating material and lignin-based impregnating material are mixed and used as raw materials to prepare bamboo-based and lignin-based hard carbon composite materials.

[0152] Comparative Example 8

[0153] A method for preparing a bamboo-based and lignin-based hard carbon composite material, which differs from Example 1 in that step S9 is omitted, and the bamboo-based and lignin-based hard carbon mixture obtained in step S8 is used to prepare a sodium-ion battery.

[0154] The rest of the steps are the same as in Example 1, and the obtained battery materials are tested using the same method as in Example 1.

[0155] Comparative Example 9

[0156] A method for preparing a bamboo-based and lignin-based hard carbon composite material, which differs from Example 1 in that: in step S9, carbon coating is performed using VC mixed coating, while the rest of the steps are the same as in Example 1, and the obtained battery material is tested using the same method as in Example 1.

[0157] The 21 bamboo-based and lignin-based hard carbon composite materials prepared in Examples 1-12 and Comparative Examples 1-9 were subjected to powder compaction density testing and electrochemical performance testing after being assembled into sodium-ion coin cells. The test results are shown in Table 1 and 2012. Figures 1-2 As shown.

[0158] Specific testing requirements:

[0159] Powder compaction density test: 5T powder compaction was performed using a PCD2000 powder compaction density meter and in accordance with the national standard GB / T24533-2019 for powder compaction density.

[0160] Electrochemical performance testing: The test was conducted using the Xinwei testing system; Test conditions: Charge and discharge at a rate of 0.1C within the voltage range of 0 to 2V.

[0161] Table 1 Electrochemical performance of sodium-ion batteries

[0162]

[0163]

[0164] According to the test results of Examples 1-12 in Table 1, the present invention uses bamboo-based or lignin as biomass raw materials, which are pretreated with acidic oxidant, pre-oxidized and pre-carbonized, acid-washed and soaked, and then coated with high-temperature segmented carbonization. Finally, the bamboo-based and lignin-based coated carbonized materials are mixed to prepare a bamboo-based and lignin-based hard carbon composite material with high sodium storage capacity and compaction density.

[0165] Compared with Example 1, Example 3 has a lower concentration of formic acid and H2O2, while Example 2 has a higher concentration of formic acid and H2O2. The higher concentration of formic acid and H2O2 makes the connection between cellulose, lignin and hemicellulose more thoroughly broken, and the separation of each component more complete. This is beneficial to improving the carbon yield after subsequent acid washing and carbonization, thereby greatly improving the reversible specific capacity and compaction density, and ultimately improving the initial coulombic efficiency, specific capacity and energy density.

[0166] Compared with Example 1, Example 4 had a lower pre-oxidation temperature, which prevented the introduced oxygen-containing, hydroxyl, and carbonyl functional groups from forming good cross-linking effects with lignocellulose molecules. As a result, a large number of disordered structures were not generated, and the number of nanopores was small, leading to lower capacity and compaction density of the hard carbon composite material.

[0167] Compared with Example 1, Example 5 shows that the particle size of the pulverized material is larger, the flowability and compressibility of the powder particles are worse, the filling between powder particles is not tight, and the porosity between powder particles is large, which reduces the compaction density and ultimately leads to a lower capacity of the hard carbon material.

[0168] Compared to Example 1, Example 6 shows an increased concentration of phosphoric acid, which not only leads to greater dissolution and decrystalline formation of cellulose, hemicellulose, and lignin, but also introduces more doped phosphorus, resulting in enhanced reactivity and additional Na+. + The increased number of storage sites improves conductivity and wettability, thereby increasing the specific capacity of hard carbon anode materials.

[0169] Compared with Example 1, Examples 7 and 8 use different amounts of modifiers to coat the bamboo-based and lignin-based materials after soaking. Compared with Example 1, Examples 9, 10, 11, and 12 use different composite ratios. Due to the different hard carbon precursors, their internal microstructure, pore size, pore structure, surface area, and surface functional groups are different, resulting in inconsistent optimal ratios for high-temperature coating carbonization and composite after coating carbonization. Coating and carbonizing the precursors separately can make the coating more uniform and dense, and the composite hard carbon material after coating carbonization can produce a synergistic effect to give full play to their respective advantages, maintaining both the high capacity of the bamboo-based material and the high compaction of the lignin-based material.

[0170] According to Table 1, compared with Examples 1-12, the bamboo-based and lignin-based hard carbon composite materials in Comparative Example 1 were not pretreated with acidic oxidants. The bonds between cellulose, lignin and hemicellulose in the biomass raw materials were mostly not destroyed, and the separation of each component was not well achieved. This was not conducive to subsequent acid washing and impurity removal and the utilization rate of raw materials. As a result, the active sites of the hard carbon composite materials were reduced, the interlayer spacing was reduced, and the initial coulombic efficiency, specific capacity and compaction density were ultimately reduced.

[0171] According to Table 1, compared with Examples 1-12, the bamboo-based and lignin-based hard carbon composite materials in Comparative Example 2 were not pre-oxidized. The lignocellulose hard carbon materials obtained by direct carbonization exhibited poor electrochemical performance due to their relatively low conductivity and crosslinking degree and fewer sodium ion storage sites.

[0172] According to Table 1, compared with Examples 1-12, the bamboo-based and lignin-based hard carbon composite materials in Comparative Example 3 did not undergo a phosphoric acid soaking process, and there was no further dissolution and decrystalline cellulose, hemicellulose and lignin, nor was P element doping introduced. This resulted in reduced reactivity, fewer storage sites, and reduced conductivity and wettability, thereby reducing the specific capacity of the hard carbon composite material.

[0173] According to Table 1, compared with Examples 1-12, the bamboo-based and lignin-based hard carbon composite materials in Comparative Example 4 changed the high-temperature segmented carbonization to one-step high-temperature carbonization. The release of organic molecules was insufficient, which was not conducive to the formation of a rich porous structure. It also could not improve the degree of graphitization and expand the interlayer spacing, which was not conducive to the insertion and extraction of sodium ions in the material, thus resulting in a decrease in capacity, initial coulombic efficiency and compaction density.

[0174] According to Table 1, compared with Examples 1-12, Comparative Examples 5 and 6 did not combine bamboo-based and lignin-based hard carbon materials, which prevented them from having a synergistic effect to fully utilize their respective advantages and prevented them from maintaining both the high capacity of bamboo-based materials and the high compaction of lignin-based materials.

[0175] According to Table 1 and Figure 2 Compared with Examples 1-12, the bamboo-based and lignin-based hard carbon composite materials in Comparative Example 7 were mixed first and then coated and carbonized. Because the hard carbon precursors are different, their internal microstructure, pore size, pore structure, surface area and surface functional groups are different, which makes the optimal coating ratio inconsistent. This results in uneven and non-dense coating of the mixed material, which cannot give full play to their respective advantages and make it possible to maintain both the high capacity of bamboo-based materials and the high compaction of lignin-based materials.

[0176] According to Table 1, compared with Examples 1-12, Comparative Example 8 did not have CVD coating, and Comparative Example 9 used VC mixed coating. Due to the uneven and non-dense coating, the electrical conductivity of its composite material was poor, resulting in low capacity and compaction.

[0177] according to Figure 1 The composite material exhibits two distinct characteristic peaks in the ranges of 25°–30° and 45°–50°, corresponding to the (002) crystal plane and the (101) crystal plane, respectively, indicating that the composite material of Comparative Example 1 and Example 1 is a bamboo-based and lignin-based hard carbon composite material.

[0178] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a bamboo-based and lignin-based hard carbon composite material, characterized in that, The process includes the following steps to prepare bamboo-based coated hard carbon and lignin-based coated hard carbon, respectively. A method for preparing bamboo-based coated hard carbon includes the following steps: The raw materials are pretreated to obtain pretreated material; The raw materials include bamboo-based biomass raw materials; The pretreated material is pretreated with an acidic oxidant to obtain an acidic oxidant pretreated material; The acidic oxidant pretreatment material is pre-oxidized to obtain a pre-oxidized material; The pre-oxidized material is pre-carbonized under a protective atmosphere to obtain a pre-carbonized material; The pre-carbonized material is crushed to obtain crushed material; The pulverized material is first acid-washed and then washed with water to obtain purified material; The purified material is soaked in acid, filtered, and dried to obtain the soaked material; the acid used for soaking includes phosphoric acid. The soaked material is mixed evenly with the modifier, and then high-temperature coating and carbonization is carried out under a protective atmosphere to obtain bamboo-based coated hard carbon. A method for preparing lignin-coated hard carbon includes the following steps: The raw materials are pretreated to obtain pretreated material; The raw materials include lignin-based biomass raw materials; The pretreated material is pretreated with an acidic oxidant to obtain an acidic oxidant pretreated material; The acidic oxidant pretreatment material is pre-oxidized to obtain a pre-oxidized material; The pre-oxidized material is pre-carbonized under a protective atmosphere to obtain a pre-carbonized material; The pre-carbonized material is crushed to obtain crushed material; The pulverized material is first acid-washed and then washed with water to obtain purified material; The purified material is soaked in acid, filtered, and dried to obtain the soaked material; the acid used for soaking includes phosphoric acid. The soaked material is mixed evenly with the modifier, and then high-temperature coating carbonization is carried out under a protective atmosphere to obtain lignin-based coated hard carbon. Bamboo-based coated hard carbon and lignin-based coated hard carbon are mixed and then carbon coated to obtain a bamboo-based and lignin-based hard carbon composite material.

2. The method for preparing bamboo-based and lignin-based hard carbon composite materials according to claim 1, characterized in that, The raw materials include at least one of bamboo and wood; When preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pretreatment parameters include: washing the raw materials with water, then drying them in a drying oven at a temperature of 60–120°C for 4–24 hours.

3. The method for preparing bamboo-based and lignin-based hard carbon composite materials according to claim 1, characterized in that, When preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the parameters for pretreatment of the pretreated material with an acidic oxidant include: reacting the pretreated material and the acidic oxidant at 60-100°C for 4-12 hours, wherein the acidic oxidant includes an oxidant and an acid, wherein the oxidant includes at least one of hydrogen peroxide, peracetic acid, and ozone, and the concentration of the oxidant is 1-5%, the acid includes at least one of formic acid and acetic acid, and the concentration of the acid is 0.5-2%, and the solid-liquid mass ratio of the pretreated material and the acidic oxidant is 1:1.2-5; When preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the acidic oxidant pretreatment material is pre-oxidized to obtain pre-oxidized material. The parameters for the pre-oxidized material are as follows: temperature 200-350℃ and pre-oxidation time 3-5h.

4. The method for preparing the bamboo-based and lignin-based hard carbon composite material according to claim 1, characterized in that, When preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pre-oxidized material is pre-carbonized under a protective atmosphere. The parameters of the pre-carbonized material include: heating rate of 2-10℃ / min, pre-carbonization temperature of 200-800℃, and holding time of 1-3h. The pre-carbonization is carried out in a rotary kiln. The protective atmosphere includes at least one of nitrogen, argon, and helium. When preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pre-carbonized material is pulverized to obtain pulverized material with the following parameters: the pulverizing equipment includes at least one of mechanical pulverizer, roller mill, and air jet mill, and the particle size D50 of the pulverized material is controlled between 3 and 10 μm.

5. The method for preparing the bamboo-based and lignin-based hard carbon composite material according to claim 1, characterized in that, In preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the pulverized material is first acid-washed and then water-washed to obtain purified material. The parameters of the purified material include: the acid used for acid washing includes at least two of hydrochloric acid, nitric acid, hydrofluoric acid, and sulfuric acid; the acid concentration for acid washing is 20-50%; the acid washing reaction temperature is 60-100℃; the solid-liquid mass ratio for acid washing is 1:0.7-5.5; and the acid washing time is 4-24 hours. The water washing process includes: washing with deionized water multiple times until the filtrate is nearly neutral, then filtration and drying at a temperature of 60-120℃ for 6-24 hours. In preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the purified material is soaked in a phosphoric acid solution. The parameters of the soaked material include: the concentration of phosphoric acid is 3-30%, the soaking time is 3-24 h, and the solid-liquid mass ratio is 1:1.2-3. Stirring is performed during soaking, and after soaking, the material is filtered and dried at a temperature of 60-120℃ for 4-24 h. In preparing bamboo-based coated hard carbon and lignin-based coated hard carbon, the soaked material is mixed evenly with a modifier, and then subjected to high-temperature coating and carbonization under a protective atmosphere. The parameters include: the modifier includes at least one of asphalt and resin, the mass of the modifier is 1-10 wt% of the mass of the soaked material, and the mixing time is 0.5-3 h. The high-temperature coating carbonization is a high-temperature segmented carbonization process: the first stage is low-temperature carbonization at 800–1000℃, with a heating rate of 4–6℃ / min and a holding time of 1–2h; the second stage is high-temperature carbonization at 1200–1400℃, with a heating rate of 1–2℃ / min and a holding time of 2–4h; the third stage is cooling down to 800–1100℃ at a cooling rate of 3–5℃ / min, followed by natural cooling under a protective atmosphere to 25–30℃. The protective atmosphere includes at least one of nitrogen, argon, and helium.

6. The method for preparing the bamboo-based and lignin-based hard carbon composite material according to claim 1, characterized in that, When bamboo-based coated hard carbon and lignin-based coated hard carbon are mixed and carbon coated to obtain bamboo-based and lignin-based hard carbon composite materials, the mixture is added to a CVD carbon coating device, and then a carbon source and protective gas are introduced, followed by heating, heat preservation, and cooling to obtain bamboo-based and lignin-based hard carbon composite materials. The mass ratio of bamboo-based coated hard carbon to lignin-based coated hard carbon is 1:9 to 9:1, the mixing time is 0.5 to 2 hours, the carbon source includes at least one of acetylene, methane, propane, ethylene, propylene, and benzene, the protective gas includes at least one of argon and nitrogen, the volume fraction of carbon source and protective gas is 4% to 10%: 96% to 90%, the heating rate is 5 to 10 °C / min, the heating temperature is 650 to 1000 °C, and the holding time is 0.5 to 4 hours; the CVD carbon coating equipment includes at least one of a CVD rotary furnace, an intermittent CVD furnace, and a continuous CVD furnace.

7. A negative electrode sheet, characterized in that, The negative electrode sheet uses a bamboo-based and lignin-based hard carbon composite material as the active material of the battery negative electrode material, and the bamboo-based and lignin-based hard carbon composite material is prepared by the preparation method of the bamboo-based and lignin-based hard carbon composite material according to any one of claims 1 to 6.

8. A battery, characterized in that, This battery is a sodium-ion battery, which includes a negative electrode, a positive electrode, a separator, and an electrolyte. The negative electrode includes the negative electrode sheet as described in claim 7. When a charge-discharge test is performed at a current density of 0.1C, the initial charge specific capacity of the sodium-ion battery is greater than 330 mAh / g, and the initial coulombic efficiency is greater than 90%.

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