A hard carbon material, a method for preparing the same, and an application thereof

Abstract

This invention relates to the field of new material preparation, and particularly to a hard carbon material, its preparation method, and its applications. The invention provides a method for preparing a hard carbon material, comprising mixing biomass powder with a biomass salt composed of biomass-derived organic anions and metal cations, followed by sequential heat treatment for self-activation, washing and drying, and high-temperature carbonization. This method utilizes the molecular affinity of the homologous biomass salt to achieve uniform mixing with the biomass powder. During heat treatment, the synergistic effect of organic anion carbon supplementation and metal cation etching pore formation is simultaneously achieved, resulting in a final closed-pore size concentrated within a suitable sodium storage range of 0.9–1.3 nm. The obtained hard carbon material has a specific capacity of over 352 mAh / g, a plateau capacity of over 221 mAh / g, and an initial coulombic efficiency exceeding 86%. Furthermore, the preparation process is simple and low-cost, making it suitable as a negative electrode material for sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of new material preparation, and in particular to a hard carbon material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, due to the abundance and low cost of sodium resources, have shown broad application prospects in the field of large-scale energy storage. The anode material is a key component determining the energy density and cycle life of sodium-ion batteries. Hard carbon materials, due to their disordered carbon structure, abundant closed pores, and microcrystalline characteristics suitable for sodium storage, are considered the most commercially promising anode material for sodium-ion batteries.

[0003] Studies on the sodium storage mechanism of hard carbon have shown that its high plateau capacity mainly originates from the quasi-metallic cluster filling of sodium ions by the closed-pore structure. Therefore, constructing closed pores of suitable size and uniform distribution is the core path to improve the performance of hard carbon anodes. Currently, the construction of closed-pore structures mainly relies on two approaches: one is high-temperature thermally induced pore formation, which involves increasing the carbonization temperature (usually >1400℃) to promote carbon layer rearrangement, converting some open pores into closed pores. However, simply relying on high-temperature control often leads to the shrinkage of the interlayer spacing, making it difficult to balance closed pores and interlayer spacing, and the ability to control the size of closed pores is limited. The second approach is chemical activation-assisted pore formation, which involves introducing activators (such as KOH, ZnCl2, H3PO4, etc.) during the low-temperature pyrolysis stage to form micropores through etching reactions, which are then converted into closed pores by high-temperature carbonization.

[0004] However, the activators used in existing technologies are mostly inorganic salts or small molecule compounds, which differ significantly from biomass precursors in terms of particle size, chemical polarity, and charge properties. During solid- or liquid-phase mixing, these activators are prone to aggregation or uneven distribution, leading to localized excessive or insufficient etching in subsequent heat treatment stages. This results in uneven distribution and significant size variations in the formed micropore structure, making it difficult to control and ultimately leading to poor performance of the prepared sodium-ion batteries. Secondly, existing activators primarily function in etching and pore-forming, without participating in the construction of the carbon framework. The pore-forming process often involves excessive carbon loss, resulting in a decrease in carbon yield.

[0005] Therefore, developing a method for preparing hard carbon materials with suitable and controllable closed-pore size, high uniformity of distribution, and good electrochemical performance is of great significance. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of large differences in closed-pore size, low distribution uniformity, and poor electrochemical performance in existing hard carbon materials, and to provide a hard carbon material, its preparation method, and its application.

[0007] The first aspect of this invention provides a method for preparing a hard carbon material, comprising the following steps: Biomass powder is mixed with biomass salt to obtain a first mixture; wherein the biomass salt is composed of negatively charged biomass source organic anions and metal cations; The first mixture was heat-treated under a protective atmosphere to obtain a self-activated product; The self-activated product is washed and dried to obtain activated carbon powder; Activated carbon powder is carbonized at high temperature under a protective atmosphere to obtain hard carbon material.

[0008] This invention provides a method for preparing hard carbon materials, which involves mixing biomass powder with a biomass salt composed of negatively charged organic anions and metal cations from the biomass source, followed by sequential heat treatment for self-activation, washing and drying, and high-temperature carbonization. In this technical solution, the biomass salt, due to its homologous chemical framework and polarity with the biomass powder, achieves molecular-level uniform dispersion with the main carbon source during mixing through intermolecular forces, fundamentally avoiding the agglomeration and uneven distribution caused by particle size and polarity differences in traditional inorganic salt additives. During the heat treatment for self-activation, the metal cations of the biomass salt gently etch the carbon framework in situ, forming uniformly distributed micropores. Simultaneously, the organic anions are carbonized and added to the carbon framework, achieving a synergistic effect of carbon replenishment, etching, and pore formation. In the subsequent high-temperature carbonization process, the uniformly distributed micropores are further transformed into closed-pore structures, with the closed-pore size controllable within a suitable sodium storage range of 0.9-1.3 nm. By controlling the microstructure, the hard carbon material prepared by the method of this invention maintains a high initial coulombic efficiency (≥86%) while significantly improving the plateau capacity (≥221 mAh / g) and specific capacity (≥352 mAh / g). Furthermore, the process is simple and the raw material cost is low, showing good prospects for commercial application.

[0009] Furthermore, the biomass powder includes one or more of lignin, cellulose, and starch.

[0010] Furthermore, the particle size distribution of the biomass powder is 20-70 μm.

[0011] Furthermore, the biomass salt includes one or more of lignin sulfonate, carboxymethyl cellulose salt, carboxymethyl starch salt, and starch phosphate. Even further, the biomass salt is one or more of sodium lignin sulfonate, potassium lignin sulfonate, calcium lignin sulfonate, sodium carboxymethyl cellulose, sodium carboxymethyl starch, and sodium starch phosphate.

[0012] Furthermore, the mass ratio of the biomass powder to the biomass salt is 1:0.01-0.08. Preferably, the mass ratio of the biomass powder to the biomass salt is 1:0.03-0.06.

[0013] Furthermore, the biomass powder and biomass salt are mixed in a solid-phase mixture or a liquid-phase mixture.

[0014] Furthermore, the solid-phase mixing includes ball milling the biomass powder with biomass salt. Furthermore, the ball milling speed is 300-600 rpm, and the ball milling time is 1-8 h.

[0015] Furthermore, the grinding jar and grinding beads are made of agate or stainless steel, the mass ratio of material to grinding balls in the grinding jar is controlled at 1:10-1:30, and the diameter of the grinding beads is controlled between 5-150 mm.

[0016] Furthermore, the liquid phase mixing includes dispersing biomass powder in a solution containing biomass salt, stirring and mixing, and then drying and pulverizing.

[0017] Furthermore, the solvent for the biomass salt solution is water, ethanol, or N,N-dimethylformamide. Furthermore, in the liquid-phase mixing, the ratio of the total mass of the raw material to the mass of the solvent is 1:3 to 1:250.

[0018] Furthermore, the heat treatment temperature is 400℃-900℃, and the heat treatment time is 1h-4h. Furthermore, the heating rate during heat treatment is 3-20℃ / min. Preferably, the heat treatment temperature is 500-700℃.

[0019] Furthermore, the high-temperature carbonization temperature is 1200℃-1500℃, and the high-temperature carbonization time is 1-8 hours. Furthermore, the heating rate during high-temperature carbonization is 1-5 °C / min. Preferably, the high-temperature carbonization temperature is 1300℃-1400℃.

[0020] A second aspect of the present invention provides a hard carbon material prepared by the above-described method for preparing hard carbon material.

[0021] Furthermore, the closed-pore size in the hard carbon material is 0.9-1.3 nm.

[0022] Furthermore, the particle size distribution range of the hard carbon material is 3-20 μm.

[0023] A third aspect of the present invention provides a hard carbon material prepared by the above-described preparation method, or the application of the hard carbon material as described above in the preparation of a negative electrode active material for sodium-ion batteries.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for preparing hard carbon materials, which involves mixing biomass powder with a biomass salt composed of negatively charged organic anions and metal cations from the biomass source, followed by sequential heat treatment for self-activation, washing and drying, and high-temperature carbonization. In this technical solution, the biomass salt, due to its homologous chemical framework and polarity with the biomass powder, achieves molecular-level uniform dispersion with the main carbon source during mixing through intermolecular forces, fundamentally avoiding the agglomeration and uneven distribution caused by particle size and polarity differences in traditional inorganic salt additives. During the heat treatment for self-activation, the metal cations of the biomass salt gently etch the carbon framework in situ to form uniformly distributed micropores, while the organic anions are simultaneously carbonized and added to the carbon framework, achieving a synergistic effect of carbon replenishment, etching, and pore formation. In the subsequent high-temperature carbonization process, the uniformly distributed micropores are further transformed into closed-pore structures, and the closed-pore size can be controlled within a suitable sodium storage range of 0.9-1.3 nm. By controlling the microstructure, the hard carbon material prepared by the method of this invention maintains a high initial coulombic efficiency (≥86%) while significantly improving the plateau capacity (≥221 mAh / g) and specific capacity (≥352 mAh / g). Furthermore, the process is simple and the raw material cost is low, showing good prospects for commercial application.

[0025] 2. The hard carbon material provided by this invention has a concentrated and controllable closed-pore size of 0.9-1.3 nm, which is highly matched with the excellent sodium storage size of sodium ions. Thus, while maintaining a high initial coulombic efficiency, it significantly improves the plateau capacity and specific capacity, overcoming the technical problem that traditional hard carbon materials cannot balance plateau capacity and initial efficiency. Moreover, the material has a uniform structure and good batch stability, and can be directly used as a negative electrode material for sodium-ion batteries, showing excellent electrochemical performance and practical application prospects. Attached Figure Description

[0026] Figure 1 The image shows the HRTEM image of the material prepared in Example 1.

[0027] Figure 2 The image shows the HRTEM image of the material prepared in Example 4-1.

[0028] Figure 3 The image shows the first charge-discharge cycle of a sodium-ion battery assembled with the hard carbon anode material prepared in Example 1. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0030] Existing technologies utilize activators that are mostly inorganic salts or small-molecule compounds, which differ significantly from biomass precursors in particle size, chemical polarity, and charge properties. During solid- or liquid-phase mixing, these activators are prone to aggregation or uneven distribution, leading to localized over- or under-etching during subsequent heat treatment. This results in uneven distribution and significant size variations in the formed micropore structure, making it difficult to control and ultimately leading to poor performance of the prepared sodium-ion batteries. Furthermore, existing activators primarily function in etching and pore-forming, without participating in the construction of the carbon framework. The pore-forming process often involves excessive carbon loss, resulting in decreased carbon yield.

[0031] The first aspect of this embodiment provides a method for preparing hard carbon material, including the following steps: Biomass powder is mixed with biomass salt to obtain a first mixture; wherein the biomass salt is composed of negatively charged biomass source organic anions and metal cations; The first mixture was heat-treated under a protective atmosphere to obtain a self-activated product; The self-activated product is washed and dried to obtain activated carbon powder; Activated carbon powder is carbonized at high temperature under a protective atmosphere to obtain hard carbon material.

[0032] Due to its homologous chemical framework and polarity with biomass powder, biomass salt achieves molecular-level uniform dispersion with the main carbon source during mixing through intermolecular forces. This fundamentally avoids the agglomeration and uneven distribution caused by particle size and polarity differences in traditional inorganic salt additives. During the self-activation stage of heat treatment, the metal cations of biomass salt gently etch the carbon framework in situ, forming uniformly distributed micropores. Simultaneously, its organic anions are carbonized and added to the carbon framework, achieving a synergistic effect of carbon replenishment, etching, and pore formation. In the subsequent high-temperature carbonization process, the uniformly distributed micropores are further transformed into closed-pore structures, and the closed-pore size can be controlled within a suitable sodium storage range of 0.9-1.3 nm. Through microstructure control, the hard carbon material prepared by the method of this invention maintains a high initial coulombic efficiency (≥86%) while significantly improving the plateau capacity (≥221 mAh / g) and specific capacity (≥352 mAh / g). Furthermore, the process is simple and the raw material cost is low, showing good prospects for commercial application.

[0033] In some embodiments, the biomass powder includes one or more of lignin, cellulose, and starch.

[0034] In some embodiments, the particle size distribution of the biomass powder is 20-70 μm.

[0035] In some embodiments, the biomass salt includes one or more of lignin sulfonate, carboxymethyl cellulose salt, carboxymethyl starch salt, and starch phosphate. In further embodiments, the biomass salt is one or more of sodium lignin sulfonate, potassium lignin sulfonate, calcium lignin sulfonate, sodium carboxymethyl cellulose, sodium carboxymethyl starch, and sodium starch phosphate.

[0036] In some embodiments, the mass ratio of the biomass powder to the biomass salt is 1:0.01-0.08.

[0037] Research has found that the mass ratio of biomass powder to biomass salt is a key factor affecting the microstructure and electrochemical performance of hard carbon materials. Controlling the mass ratio within a reasonable range helps ensure sufficient etching and pore formation and carbon replenishment, avoids pore structure deterioration caused by over-activation, and achieves a synergistic improvement in closed-pore size distribution (0.9-1.3 nm), plateau capacity, and first coulombic efficiency.

[0038] The possible reasons are as follows: When the mass ratio is too low, the biomass salt content is insufficient, resulting in weak etching by metal cations and an inability to form a sufficient number of micropores. This leads to a small number of closed pores and limited improvement in plateau capacity after subsequent high-temperature carbonization. At the same time, the carbon supplementation effect of organic anions is not significant, and the improvement in carbon yield is not obvious. When the mass ratio is too high, the excessive amount of biomass salt causes the local etching reaction to be too intense, and the micropores are excessively enlarged to form mesopores or macropores. During subsequent high-temperature carbonization, these macropores are difficult to close effectively, or the size of the closed pores after conversion is too large, deviating from the optimal sodium storage size for sodium ions. This, in turn, leads to a decrease in plateau capacity and a deterioration in rate performance.

[0039] For example, the mass ratio of the biomass powder to the biomass salt is 1:0.01; 1:0.02; 1:0.03; 1:0.04; 1:0.05; 1:0.06; 1:0.07 or 1:0.08. Preferably, the mass ratio of the biomass powder to the biomass salt is 1:0.03-0.06.

[0040] In some embodiments, the biomass powder and biomass salt are mixed as a solid-phase mixture or a liquid-phase mixture. More preferably, the mass ratio of the biomass powder to the biomass salt is 1:0.04-0.05.

[0041] In some embodiments, the solid-phase mixing includes ball milling the biomass powder with biomass salt. In some embodiments, the ball milling speed is 300-600 rpm and the ball milling time is 1-8 h.

[0042] In some embodiments, the grinding jar and grinding beads are made of agate or stainless steel, the mass ratio of material to grinding balls in the grinding jar is controlled at 1:10-1:30, and the diameter of the grinding beads is controlled between 5-150 mm.

[0043] In some embodiments, the liquid phase mixing includes dispersing biomass powder in a solution containing biomass salt, stirring and mixing, and then drying and pulverizing.

[0044] In some embodiments, the solvent for the biomass salt solution is water, ethanol, or N,N-dimethylformamide. In some embodiments, the ratio of the total mass of the raw material to the mass of the solvent in the liquid phase mixing is 1:3 to 1:250.

[0045] In some embodiments, the heat treatment temperature is 400°C-900°C. Studies have found that, preferably, the heat treatment temperature is 500-700°C.

[0046] Research has found that the heat treatment temperature is a key factor affecting the effective synergistic effect of carbon supplementation and etching in biomass salts. Controlling the heat treatment temperature within a reasonable range enables gentle and uniform etching of biomass salts and sufficient carbonization of organic anions, forming microporous precursors with concentrated and uniform size distribution. This lays the foundation for obtaining suitable closed-pore structures of 0.9-1.3 nm, thereby achieving a synergistic improvement in plateau capacity and initial coulombic efficiency.

[0047] It is possible that when the heat treatment temperature is too low, the organic anions of biomass salt are not completely carbonized, which cannot effectively replenish the carbon source. At the same time, the etching reaction rate of metal cations is too low, making it difficult to form a sufficient number of micropores, resulting in insufficient number of closed pores and limited improvement in plateau capacity. When the heat treatment temperature is too high, the etching reaction of biomass salt is too violent, and the micropores are excessively expanded to form mesopores or macropores. In the subsequent high-temperature carbonization, these macropores are difficult to effectively convert into closed pores or their size is too large after conversion, deviating from the optimal sodium storage size of sodium ions, which leads to a decrease in plateau capacity.

[0048] In some embodiments, the heat treatment time is 1 hour to 4 hours. For example, the heat treatment time is 1 hour, 2 hours, 3 hours, or 4 hours.

[0049] In some embodiments, the heating rate during heat treatment is 3-20 °C / min. For example, the heating rate during heat treatment is 3 °C / min, 5 °C / min, 10 °C / min, 15 °C / min, or 20 °C / min.

[0050] In some embodiments, the high-temperature carbonization temperature is 1200℃-1500℃.

[0051] Research has found that the high-temperature carbonization temperature is a key factor affecting the conversion efficiency from open to closed pores and the final closed-pore size. Controlling the high-temperature carbonization temperature within the range of 1200-1500℃, especially the optimal range of 1300-1400℃, ensures efficient conversion from open to closed pores while avoiding pore collapse and excessive graphitization of the carbon layer. This results in the final closed-pore size being concentrated within a suitable sodium storage range of 0.9-1.3 nm, achieving synergistic optimization of high plateau capacity and high initial coulombic efficiency.

[0052] When the high-temperature carbonization temperature is too low, the migration ability of carbon atoms is insufficient, and the pore walls of open pores cannot fully melt, shrink, and cross-link, resulting in incomplete conversion of closed pores. A large number of open pore structures remain in the material, which form a large number of solid electrolyte interface films during charging and discharging due to excessive contact with the electrolyte, causing a significant decrease in the initial coulombic efficiency and limiting the improvement of plateau capacity. When the high-temperature carbonization temperature is too high, carbon microcrystals grow excessively and tend to be arranged in an orderly manner. The already formed closed pores are squeezed and collapsed or the carbon interlayer spacing shrinks excessively, resulting in a reduction in sodium storage sites. The specific capacity and plateau capacity decrease instead, and energy consumption and cost increase.

[0053] Preferably, the high-temperature carbonization temperature is 1300℃-1400℃.

[0054] In some embodiments, the high-temperature carbonization time is 1-8 hours. For example, the high-temperature carbonization time is 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours.

[0055] In some embodiments, the heating rate in high-temperature carbonization is 1-5 °C / min. For example, the heating rate in high-temperature carbonization is 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min, or 5 °C / min. The second aspect of this embodiment provides a hard carbon material prepared by the above-described method for preparing hard carbon materials.

[0056] In some embodiments, the closed-pore size in the hard carbon material is 0.9-1.3 nm.

[0057] In some embodiments, the particle size distribution of the hard carbon material ranges from 3 to 20 μm.

[0058] The third aspect of this embodiment provides a hard carbon material prepared by the above-described preparation method, or the application of the hard carbon material as described above in the preparation of a negative electrode active material for sodium-ion batteries.

[0059] To better understand the technical solutions of the above embodiments, the following more detailed examples are provided for further explanation.

[0060] Example 1 The lignin was crushed and graded to obtain lignin powder; the mass ratio of lignin powder to biomass salt was controlled at 1:0.05.

[0061] 2 g of lignin powder was added to a solution prepared by 0.1 g of sodium lignin sulfonate and 250 mL of deionized water to obtain a mixed solution. After stirring and mixing for 12 h, the mixture was evaporated to dryness. The dried product was then heated at 150 °C for 2 h.

[0062] The dried sample was ground until homogeneous and placed in a high-temperature tube furnace. It was then heated to 800 ℃ in a high-purity argon atmosphere at a heating rate of 5 ℃ / min and held for 1 h for one-step carbonization activation. After activation, the material was ground and dissolved in deionized water, neutralized with hydrochloric acid to pH 5, and the temperature was controlled at 25 ℃. The mixture was stirred for 10 h, washed until neutral, and then dried in an oven at 110 ℃ for 8 h.

[0063] The dried material was placed in a high-temperature tube furnace and heated from room temperature to 1400 ℃ in a high-purity argon atmosphere at a heating rate of 3 ℃ / min, and held for 3 h to obtain hard carbon material.

[0064] Example 2 Cellulose is crushed and graded to obtain cellulose powder; the mass ratio of cellulose powder to biomass salt is controlled at 1:0.01.

[0065] 2 g of cellulose powder was added to a solution prepared by 0.02 g of sodium carboxymethyl cellulose and 250 mL of deionized water to obtain a mixed solution. After stirring and mixing for 12 h, the mixture was evaporated to dryness. The dried product was then heated at 150 °C for 2 h.

[0066] The dried sample was ground until homogeneous and placed in a high-temperature tube furnace. It was then heated to 400 °C at a rate of 3 °C / min and held for 4 hours in a high-purity argon atmosphere for one-step carbonization activation. After activation, the material was ground and dissolved in deionized water. It was then neutralized with hydrochloric acid to pH 5 and the temperature was controlled at 25 °C. The mixture was stirred for 10 hours, washed until neutral, and then dried in an oven at 110 °C for 8 hours.

[0067] The dried material was placed in a high-temperature tube furnace and heated from room temperature to 1200 °C at a heating rate of 1 °C / min in a high-purity argon atmosphere for 8 hours to obtain hard carbon material.

[0068] Example 3 Obtain starch powder; control the mass ratio of starch powder to sodium carboxymethyl starch (carboxymethyl starch) to be 1:0.08.

[0069] Starch powder and sodium carboxymethyl starch were ball-milled at 500 rpm for 5 hours.

[0070] The grinding jar and grinding balls are made of stainless steel. The mass ratio of the grinding balls in the grinding jar is controlled at 1:20, and the diameter of the grinding balls is 100 mm.

[0071] The ball-milled material was placed in a high-temperature tube furnace and heated to 900 °C at a heating rate of 20 °C / min in a high-purity argon atmosphere for one-step carbonization activation. After activation, the material was ground and dissolved in deionized water, neutralized with hydrochloric acid to pH 5, and the temperature was controlled at 25 °C. The mixture was stirred for 10 h, washed until neutral, and then dried in an oven at 110 °C for 8 h.

[0072] The dried material was placed in a high-temperature tube furnace and heated from room temperature to 1500 °C in a high-purity argon atmosphere at a heating rate of 5 °C / min, and held for 1 h to obtain hard carbon material.

[0073] Example 4 Compared with Example 1, Example 4 only adjusted the mass ratio of biomass powder to biomass salt. The specific raw material types, preparation process and process parameters were completely consistent with Example 1.

[0074] In Example 4-1, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0, wherein 2g of lignin powder was used.

[0075] In Example 4-2, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.005, wherein 2g of lignin powder and 0.01g of sodium lignin sulfonate were used.

[0076] In Examples 4-3, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.01, wherein 2g of lignin powder and 0.02g of sodium lignin sulfonate were used.

[0077] In Examples 4-4, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.03, wherein 2g of lignin powder and 0.06g of sodium lignin sulfonate were used.

[0078] In Examples 4-5, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.06, wherein 2g of lignin powder and 0.12g of sodium lignin sulfonate were used.

[0079] In Examples 4-6, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.08, wherein 2g of lignin powder and 0.16g of sodium lignin sulfonate were used.

[0080] In Examples 4-7, the mass ratio of lignin powder to sodium lignin sulfonate was controlled at 1:0.1, wherein 2g of lignin powder and 0.2g of sodium lignin sulfonate were used.

[0081] Example 5 Compared with Example 1, Example 5 only adjusted the heat treatment temperature. The specific raw material types, the mass ratio of biomass powder to biomass salt, the preparation process, and other process parameters were completely consistent with Example 1.

[0082] Specifically, In Example 5-1, the heat treatment temperature was 300°C.

[0083] In Example 5-2, the heat treatment temperature was 400°C.

[0084] In Examples 5-3, the heat treatment temperature was 500°C.

[0085] In Examples 5-4, the heat treatment temperature was 600°C.

[0086] In Examples 5-5, the heat treatment temperature was 700°C.

[0087] In Examples 5-6, the heat treatment temperature was 900°C.

[0088] In Examples 5-7, the heat treatment temperature was 1000℃.

[0089] Example 6 Compared with Example 1, Example 6 only adjusted the high-temperature carbonization temperature. The specific raw material types, the mass ratio of biomass powder to biomass salt, the preparation process, and other process parameters were completely consistent with Example 1.

[0090] Specifically, In Example 6-1, the high-temperature carbonization temperature was 1000℃.

[0091] In Example 6-2, the high-temperature carbonization temperature was 1200℃.

[0092] In Examples 6-3, the high-temperature carbonization temperature was 1300℃.

[0093] In Examples 6-4, the high-temperature carbonization temperature was 1500℃.

[0094] In Examples 6-5, the heat treatment temperature was 1600℃.

[0095] Comparative Example 1 Compared to Example 1, Comparative Example 1 replaced sodium lignosulfonate with the same mass of zinc acetate, and the preparation process and other process parameters were completely consistent with Example 1.

[0096] Comparative Example 2 Compared to Example 1, Comparative Example 2 replaced sodium lignosulfonate with the same mass of sodium benzenesulfonate, while the preparation process and other process parameters were completely consistent with Example 1.

[0097] Comparative Example 3 Compared to Example 1, Comparative Example 3 replaced sodium lignosulfonate with the same mass of sodium carboxymethyl starch, and the preparation process and other process parameters were completely consistent with Example 1.

[0098] test The materials prepared in Examples 1-6 and Comparative Examples 1-3 were subjected to performance tests. The test procedures are as follows, and the test results are shown in Table 1. Figure 1 The image shows the HRTEM image of the material prepared in Example 1. Figure 2 The image shows the HRTEM image of the material prepared in Example 4-1. Figure 3 The image shows the first charge-discharge cycle of a sodium-ion battery assembled with the hard carbon anode material prepared in Example 1.

[0099] like Figure 1 and Figure 2 As shown in the TEM image, a large number of obvious closed-pore structures appeared after the addition of the homologous modifier. Figure 1 Meanwhile, the closed-pore cavity size in the figure is concentrated in the suitable sodium storage range of 0.9-1.3 nm, while its layer structure without the addition of homologous modifiers ( Figure 2 The structure is rather chaotic, lacking a large number of obvious closed-cell structures.

[0100] Specific testing process: 1. The materials prepared in the embodiments and comparative examples of this invention are used as active materials and accurately weighed with conductive agent (Super P carbon black) and binder (polyvinylidene fluoride, PVDF) at a mass ratio of 80:10:10. The above materials are ground and mixed evenly in an agate mortar, and then an appropriate amount of N-methylpyrrolidone (NMP) solvent is added. Grinding and mixing continue to prepare a slurry with moderate viscosity and uniform dispersion. The resulting slurry is uniformly coated onto a copper foil current collector and then transferred to a vacuum drying oven at 110°C for 12 hours to completely remove the solvent. After drying, the electrode sheet is compacted by roller pressing and then punched into a circular working electrode sheet with a diameter of 12 mm for later use.

[0101] 2. Half-cell tests were conducted using CR2032 coin cells. Battery assembly was performed in a glove box filled with high-purity argon (water and oxygen content <0.1 ppm). The prepared hard carbon electrode was used as the working electrode, and a sodium metal sheet as the counter / reference electrode. A glass fiber diaphragm (Whatman GF / D) was used as the separator. The electrolyte was 1 mol / L NaPF6 dissolved in a 1:1 volume ratio mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The battery was assembled in the following order: negative electrode shell, spring plate, gasket, sodium sheet, electrolyte, separator, hard carbon electrode, and positive electrode shell. After sealing, the battery was allowed to stand for 12 hours before electrochemical testing.

[0102] 3. Constant current charge-discharge tests were performed using the LAND CT2001A battery testing system. The test voltage window was 0.01-2.8 V (vs. Na). + / Na), the test was conducted under constant temperature conditions of 25°C.

[0103] Initial charge-discharge test: Constant current discharge (sodium insertion) was performed at a current density of 0.1C (estimated based on the specific capacity of the hard carbon material, approximately 30 mA / g) to 0.01 V, followed by constant current charging (sodium removal) to 2.8 V. The initial discharge specific capacity and initial charge specific capacity were recorded, and the initial coulombic efficiency was calculated.

[0104] Plateau capacity calculation: The voltage plateau region is defined as a low potential range of 0.01-0.1 V. The plateau capacity is obtained by integrating the charge specific capacity within this voltage range.

[0105] 4. Perform charge and discharge tests at different current densities in sequence: perform 5 cycles at current densities of 0.1C, 0.2C, 0.5C, 1.0C and 2.0C respectively, record the reversible specific capacity at each current density, and calculate the rate performance (the ratio of the specific capacity measured at 1C to the specific capacity measured at 0.1C).

[0106] Three batteries were prepared in parallel for each set of examples and comparative examples, and the average value of the results was taken to ensure the reliability of the data.

[0107] Table 1 Material Performance Testing

[0108] The data in Table 1 show that this invention achieves synergistic optimization of the electrochemical performance of hard carbon materials by controlling the mass ratio of biomass powder to homologous biomass salt, the heat treatment temperature, and the high-temperature carbonization temperature. When the mass ratio is controlled within the range of 1:0.03-0.06 (Examples 4-4, 1, 4-5), the material exhibits optimal comprehensive performance, with a 0.1C specific capacity of 348-355 mAh / g, an initial coulombic efficiency of 86.3%-86.6%, and a plateau capacity of 218-224 mAh / g. The heat treatment temperature is crucial for micropore formation, with 500-700℃ (Examples 5-3 to 5-5) being the optimal window, at which the specific capacity reaches 358-365 mAh / g and the plateau capacity 228-235 mAh / g. When the high-temperature carbonization temperature is 1300-1400℃ (Examples 6-3, 1), the conversion from open-pore to closed-pore structures is highly efficient, with a specific capacity of 352-359 mAh / g and a plateau capacity of 221-229 mAh / g. Comparative Examples 1-3 further confirm that when using non-biomass salts (zinc acetate, sodium benzenesulfonate) or different-source biomass salts (sodium carboxymethyl starch), the specific capacity (289-295 mAh / g), initial efficiency (78.8%-80.6%), and plateau capacity (159-167 mAh / g) are all much lower than those in the embodiments of this invention because uniform mixing and synergistic carbon etching cannot be achieved.

[0109] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements 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 hard carbon material, characterized in that, Includes the following steps: Biomass powder is mixed with biomass salt to obtain a first mixture; wherein the biomass salt is composed of negatively charged biomass source organic anions and metal cations; The first mixture was heat-treated under a protective atmosphere to obtain a self-activated product; The self-activated product is washed and dried to obtain activated carbon powder; Activated carbon powder is carbonized at high temperature under a protective atmosphere to obtain hard carbon material.

2. The method for preparing hard carbon material according to claim 1, characterized in that, The biomass powder includes one or more of lignin, cellulose, and starch; and / or, the biomass salt includes one or more of lignin sulfonate, carboxymethyl cellulose salt, carboxymethyl starch salt, and starch phosphate.

3. The method for preparing hard carbon material according to claim 1, characterized in that, The mass ratio of the biomass powder to the biomass salt is 1:0.01-0.

08.

4. The method for preparing hard carbon material according to claim 3, characterized in that, The mass ratio of the biomass powder to the biomass salt is 1:0.03-0.

06.

5. The method for preparing hard carbon material according to claim 1, characterized in that, The biomass powder and biomass salt are mixed as a solid-phase mixture or a liquid-phase mixture; and / or, the solid-phase mixture includes ball milling the biomass powder and biomass salt; and / or, the liquid-phase mixture includes dispersing the biomass powder in a solution containing biomass salt, stirring and mixing, and then drying and pulverizing.

6. The method for preparing hard carbon material according to any one of claims 1-5, characterized in that, The heat treatment temperature is 400℃-900℃, and the heat treatment time is 1h-4h.

7. The method for preparing hard carbon material according to any one of claims 1-5, characterized in that, The temperature for high-temperature carbonization is 1200℃-1500℃, and the carbonization time is 1-8 hours.

8. The hard carbon material prepared by the method for preparing hard carbon material according to any one of claims 1-7.

9. The hard carbon material according to claim 8, characterized in that, The closed-pore size in the hard carbon material is 0.9-1.3 nm.

10. The hard carbon material prepared by the preparation method according to any one of claims 1-7, or the application of the hard carbon material according to claim 8 or 9 in the preparation of anode active materials for sodium-ion batteries.

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