Method for preparing hard carbon material from low-rank coal hot melt

Abstract

The invention relates to a method for preparing a hard carbon material from a low-rank coal hot melt, the hard carbon material obtained by using the method, application of the hard carbon material and a sodium ion battery containing the hard carbon material. The method comprises the steps of raw material pretreatment, degradation solvent extraction, precipitation separation, spray granulation, crosslinking, primary carbonization, secondary carbonization, cooling and the like. According to the method, microcrystalline defects of the coal-based hard carbon material are regulated and controlled through self-foaming of the low-rank coal hot melt, effective regulation and control of the hard carbon microcrystalline structure are achieved, the ultrafine grinding process is omitted, meanwhile, in-situ oxygen element doping is achieved, and the surface activity of the material and the first efficiency of the negative electrode material are effectively improved. Through the method, the low-rank coal can be converted into the sodium battery hard carbon negative electrode material in a high-valued manner, the application scene of the low-rank coal is expanded, and the comprehensive utilization level of coal resources is improved.

Description

Technical Field

[0001] This invention relates to the field of coal-based sodium-ion battery anode materials, and more particularly to a method for preparing hard carbon materials from low-rank coal thermal melts, the hard carbon materials obtained by the method, the uses of the hard carbon materials, and sodium-ion batteries containing the hard carbon materials. Background Technology

[0002] my country has large reserves of low-quality coal (low-rank coal, coal slime, etc.). Using low-quality coal as raw material, hard carbon anode materials can be produced through purification technology, which can replace raw materials such as phenolic resin, asphalt, and coconut shell. This can not only solve the problem of utilizing low-quality coal, but also alleviate the raw material pressure on the sodium battery industry, and has significant economic value and application potential.

[0003] While some progress has been made in the research of sodium-ion battery anode materials, the preparation of hard carbon anodes from coal still faces several shortcomings. Traditional methods for preparing hard carbon from coal are hampered by the complex structure of coal and the difficulty in precisely controlling the pyrolysis process. This leads to challenges in regulating the microcrystalline structure of the hard carbon products, typically requiring physical or chemical activation, which is energy-intensive. Furthermore, directly preparing hard carbon anode materials from raw coal necessitates grinding the coal to achieve a D50 of ~7μm (2200 mesh), requiring specialized equipment and consuming significant energy. Additionally, ordinary coal-based hard carbon has an inert surface and weak ion adsorption capacity, resulting in unsatisfactory reversible specific capacity and initial coulombic efficiency when used as a sodium-ion battery anode material.

[0004] The above problems greatly limit the improvement of the energy density of sodium-ion batteries. Summary of the Invention

[0005] Therefore, the present invention focuses on a method for preparing coal-based hard carbon materials using low-rank coal thermal melts, wherein the microcrystalline defects of the coal-based hard carbon materials are controlled by the self-foaming of low-rank coal thermal melts, thereby achieving effective control of the hard carbon microcrystalline structure, omitting the ultrafine abrasive process, and simultaneously achieving in-situ oxygen doping, which effectively improves the surface activity of the material and the first-stage efficiency of the anode material.

[0006] This invention provides a method for preparing coal-based hard carbon materials from low-rank coal melt, comprising the following steps: (1) Raw material pretreatment: The low-rank coal is crushed to a mesh size of 80 or larger and dried to a moisture content of less than 2 wt%. (2) Degradation solvent extraction (DSE): Pretreated coal was mixed with industrial compound solvent (ICS) at a mass ratio of 1:10-1:15, extracted at 330-360℃, and then hot filtered to obtain the extract and extraction residue. (3) Separation: The extraction solution obtained in step (2) is cooled to room temperature, during which precipitation occurs, and the precipitate and solution are collected separately; (4) Spray granulation The precipitate collected in step (3) is spray granulated at 220-260°C under a nitrogen atmosphere with purity ≥ 99.9% to obtain spherical particles; (5) Cross-linking The spherical particles obtained in step (4) are heated at a temperature increasing rate of 2-4°C / min from room temperature to 220-260°C under an oxygen and nitrogen mixed gas atmosphere with a volume ratio of 1:99, and are kept at the temperature for 5-7h; (6) Primary carbonization The product obtained in step (5) is further heated at a temperature increasing rate of 4-6°C / min to 580-620°C under an argon atmosphere with purity ≥ 99.999%, and is kept at the temperature for 90-150min, thereby performing primary carbonization to form a primary carbide; (7) Secondary carbonization The primary carbide of step (6) is further heated at a temperature increasing rate of 4-6°C / min to 950-1050°C under a nitrogen atmosphere containing 5vol% hydrogen, and is further heated at a temperature increasing rate of 4-6°C / min to 1400°C under an argon atmosphere with purity ≥ 99.999%, and is kept at the temperature for 1-2h; (8) Cooling The product of step (7) is naturally cooled to room temperature under an argon atmosphere with purity ≥ 99.999% to obtain a coal-based hard carbon material.

[0007] The above steps are described in detail as follows.

[0008] (1) Raw material pretreatment In this step, the low-rank coal is crushed to 80 mesh or more, for example, 80-100 mesh, and is dried to a moisture content of 2wt% or less.

[0009] The low-rank coal can be lignite, sub-bituminous coal, or a combination thereof. The lignite can be, for example, Naomaohu (NMH) lignite, lignite in the Zhundong coalfield of Xinjiang, etc. The sub-bituminous coal can be, for example, sub-bituminous coal in the Zhundong coalfield of Xinjiang.

[0010] The low-rank coal as a raw material needs to meet the following requirements: volatile matter 20wt%-40wt%, moisture content ≤ 30wt% (typical value of lignite 15wt%-30wt%), ash content ≤ 40wt%.

[0011] The crushing can be performed using conventional methods, as long as the target particle size is obtained. For example, the crushing and screening can be performed as follows: two-stage crushing using a jaw crusher + hammer crusher, first crushing to 20-50 mm coarse particles, and then screening to 0.15-0.20 mm (80-100 mesh) using a vibrating screen. Within the above particle size range, it is helpful to ensure sufficient contact between the solvent and the coal particles in the subsequent extraction process.

[0012] The drying can be performed using conventional methods, as long as the target moisture content is achieved. For example, the drying to remove water can be performed as follows: drying the screened coal sample at 150°C to a moisture content of < 2 wt%. Different coal types have different moisture contents, and thus the drying time will also vary based on the initial moisture of the coal. For example, sub-bituminous coal can be dried for 6-8 h, while lignite, due to its high initial moisture, requires an extended drying time of 8-10 h to avoid affecting the solvent extraction efficiency.

[0013] (2) Degraded solvent extraction (DSE): In this step, the pretreated coal is mixed with the industrial complex solvent (ICS) at a mass ratio of 1:10-1:15, for example 1:10, 1:11, 1:12, 1:13, 1:14, or 1:15, extracted at 330-360°C (for example 330, 335, 340, 345, 350, 355, or 360°C, but not limited to), and then hot filtered to obtain an extract and an extraction residue.

[0014] The extraction can be performed using any suitable extraction equipment. For example, a batch high-pressure reaction kettle (for example, volume 0.5-500 L) can be used, and the supporting system includes: A feeding unit: a premixing kettle (with stirring, speed 120-300 rpm) for uniform mixing of coal and solvent; A reaction unit: a high-pressure reaction kettle (material 316L stainless steel, pressure resistance ≥10 MPa, temperature control accuracy ±5°C), equipped with a stirring paddle (speed 120-200 rpm), a thermocouple (insertion depth ≥ 2 / 3 of the material height in the kettle), and a pressure sensor (range 0-15 MPa); A separation unit: an internal 0.5 μm steel filter (kettle bottom outlet), a cold-hot separator (for cooling of the extract); A solvent recovery unit: a vacuum distillation column (vacuum degree -0.09 to -0.095 MPa, temperature 125-150°C), a solvent storage tank (with nitrogen protection).

[0015] The industrial complex solvent (ICS) comprises, based on the total weight of the solvent, (a) 40 to 50 wt%, in particular 45 wt% of 1-methylnaphthalene; (b) 35 to 45 wt%, particularly 40 wt%, of a co-solvent selected from 2-methylnaphthalene, 2,6-dimethylnaphthalene, and 1,6-dimethylnaphthalene; and (c) 10 wt% to 20 wt%, particularly 15 wt%, of a polarity modifier, which is at least one selected from acenaphthene or fluorene.

[0016] In the aforementioned industrial complex solvent (ICS), the main solvent is 1-methylnaphthalene, whose core function is to serve as the basic component of the solvent system. It can maintain thermal stability and not decompose at an extraction temperature of 340-360℃, while effectively dissolving low-polarity aromatic components in coal, thus providing a stable solvent environment for subsequent extraction.

[0017] The main function of the synergistic solvent is to enhance the solvent system's ability to dissolve monocyclic aromatics (such as benzene and toluene derivatives) and bicyclic aromatics in coal, help increase the proportion of target aromatic extracts, better meet the component requirements of hard carbon precursors, and form a synergistic effect with 1-methylnaphthalene to further improve the overall extraction yield.

[0018] In some embodiments, the synergistic solvent is a mixture of 2-methylnaphthalene and 2,6-dimethylnaphthalene, preferably in a mass ratio of 5:3.

[0019] The polarity modifier provides weakly polar sites to the solvent system, interacting with oxygen-containing functional groups such as hydroxyl groups and ether bonds in coal, promoting the breaking of weak covalent bonds (such as ether bonds and long-chain C / C bonds) in coal molecules. It also helps improve the thermal stability of the entire solvent system, preventing component decomposition at extraction temperatures. In some embodiments, acenaphthene is selected as the polarity modifier.

[0020] In some embodiments, the industrial complex solvent (ICS) comprises, based on the total weight of the solvent, 45 wt% of 1-methylnaphthalene, 25 wt% of 2-methylnaphthalene, 15 wt% of 2,6-dimethylnaphthalene, and 15 wt% of acenaphthene. From the perspective of formulation advantages, on the one hand, 2-methylnaphthalene and 2,6-dimethylnaphthalene together account for 40 wt% of the mass ratio. Among them, the solubility of 2,6-dimethylnaphthalene for monocyclic aromatics is 1.2-1.5 times that of 1-methylnaphthalene. Combined with the polarity regulating effect of acenaphthene, the proportion of aromatic extracts can be increased by 12 wt%-15 wt% compared with 1-methylnaphthalene solvent alone, which is fully compatible with the composition requirements of hard carbon precursors. On the other hand, each component has excellent thermal stability. 1-methylnaphthalene has a boiling point of 240℃, 2-methylnaphthalene has a boiling point of 241℃, 2,6-dimethylnaphthalene has a boiling point of 262℃, and acenaphthene has a boiling point of 279℃. They can exist stably in the extraction temperature range of 340-360℃, and will not affect the extraction effect or introduce impurities due to solvent decomposition.

[0021] In some implementations, the extraction process is operated and parameters are controlled as follows.

[0022] (A1) Preparation before reaction Inert atmosphere replacement: High-purity nitrogen gas (purity ≥99.999%) is introduced into the reactor and replaced three times (each time the pressure is increased to 0.5 MPa and then released to 0.1 MPa). Inert atmosphere replacement can remove oxygen from the reactor and prevent the oxidation of organic matter in the coal (such as the decomposition of carboxyl groups to generate CO2, which affects product quality).

[0023] (A2) Temperature rise program setting A staged heating method is adopted, initially heating at 4-6℃ / min to 150℃ (to remove residual moisture and light volatiles), and then heating at 5℃ / min to the target extraction temperature to avoid coal particle agglomeration caused by excessively rapid heating.

[0024] (A3) Core Extraction Parameters Extraction temperature: Controlled between 330-360℃. Different coal types require different temperatures to achieve the best extraction results.

[0025] In some embodiments, the low-rank coal is lignite, and the extraction temperature is controlled at 330-340℃. Lignite is an aliphatic-enriched low-rank coal, rich in aliphatic structures. Controlling the extraction temperature of lignite at 330-340℃ can effectively avoid excessive decomposition of aliphatic components due to excessively high temperatures, thereby ensuring the quality and yield of the extracted aliphatic enrichment, providing a suitable precursor for the subsequent preparation of hard carbon materials.

[0026] In some implementations, the low-rank coal is sub-bituminous coal, and the extraction temperature is controlled at 340-360℃. Sub-bituminous coal is an aromatic-enriched low-rank coal. Controlling the extraction temperature of sub-bituminous coal at 340-360℃ can promote the breaking of weak covalent bonds (ether bonds, CC long chains) in sub-bituminous coal, which helps to increase the proportion of aromatic extracts and meet the requirements of hard carbon precursors for aromatic components.

[0027] In some implementations, the extraction time can be 30-90 minutes. The extraction time can be adjusted as needed. For example, in a laboratory setting, the extraction time can be 30-60 minutes for product structure studies; in an industrial setting, the extraction time can be extended to 60-90 minutes to ensure that the target component (aliphatic, heterocyclic compound) is fully dissolved, avoiding incomplete dissolution due to too short a time (<30 minutes) or reduced production efficiency due to too long a time (>90 minutes).

[0028] In some implementations, stirring is performed simultaneously during extraction at a rate of 120-200 rpm. The stirring rate can be adjusted as needed. For example, in pilot-scale / industrial applications, due to the large material volume, it needs to be increased to 180-200 rpm to ensure uniform mixing of the coal-solvent system and reduce mass transfer resistance. If the stirring rate is too low, for example below 100 rpm, the extract yield will decrease, for example, by 8%-12%.

[0029] In some implementations, the extraction process is controlled at a pressure of 4-7 MPa, typically 5 MPa at 350°C. If the pressure is too high, for example, >8 MPa, it is necessary to release the pressure to prevent the solvent from vaporizing too quickly and causing material overflow; if the pressure is too low, for example, <3 MPa, the vessel seal must be checked and nitrogen gas must be added to a pressure of 4 MPa or higher.

[0030] After extraction, hot filtration is performed to obtain the extract and extraction residue. For example, the material can be forced into a separation tank with a built-in 0.5μm steel filter at 330-360℃ using internal pressure (4-5MPa) to retain insoluble matter (upgraded coal). The main components of upgraded coal are a large molecular carbon skeleton and ash, which can be used to prepare activated carbon or as a gasification feedstock.

[0031] (3) Separation: The filtered extract (containing solvent) was cooled to room temperature (approximately 25°C). During the process, a precipitate was formed. The precipitate and solution were collected separately after filtration.

[0032] The extract contains high molecular weight aromatic concentrates and low molecular weight aliphatic / heterocyclic compounds extracted from low-rank coal. The high molecular weight aromatic concentrates are soluble in the solvent at high temperatures and precipitate out of the solvent at low temperatures, while the low molecular weight aliphatic / heterocyclic compounds remain soluble in the solvent at low temperatures. Therefore, the difference in solubility at high and low temperatures can be used to separate the high molecular weight aromatic concentrates and the low molecular weight aliphatic / heterocyclic compounds.

[0033] The precipitate (high molecular weight aromatic enrichment) collected by filtration has a fixed carbon content of ≥48 wt%. The collected precipitate is used as a hard carbon precursor for subsequent preparation of hard carbon materials.

[0034] When the raw coal is lignite, the extracted precipitate (high molecular weight aromatic concentrate) is sometimes abbreviated as HM-Ex; when the raw coal is bituminous coal, the extracted precipitate (high molecular weight aromatic concentrate) is sometimes abbreviated as CYM-Ex.

[0035] In this invention, the high molecular weight aromatic enrichment used as a hard carbon precursor exhibits thermoplasticity. This is due to its high content of small and medium molecular weight aromatic and aliphatic components with a certain degree of fluidity. Upon heating, the intermolecular forces weaken, resulting in plastic behavior. Therefore, the obtained high molecular weight aromatic enrichment can be subsequently spray-granulated.

[0036] The solution contains solvent and low molecular weight aliphatic / heterocyclic compounds dissolved in the solvent. The solvent is then recovered in the subsequent solvent recovery stage and reused.

[0037] Solvent recovery can be carried out as follows: The collected solution is sent to a vacuum distillation column and distilled at 125℃ and -0.095MPa. The solvent recovery rate is ≥99% (after ICS is recycled 5 times, the extraction efficiency only decreases by 2%). The distillation residue is a low molecular weight aliphatic / heterocyclic compound (H / C ratio ≥1.2).

[0038] (4) Spray granulation In this step, the precipitate collected in step (3) is spray-granulated at 220-260℃, for example 230℃, under a nitrogen atmosphere with a purity ≥99.9%. The precipitate is heated to a liquid state and melted, then sprayed out from a nozzle under a certain pressure, while simultaneously atomizing it into fine particles using air. These particles are naturally cooled by air in a cooling tower to form a semi-finished product. The cooled semi-finished product is then sent from the bottom of the cooling tower to a three-dimensional vibrating screen filter for grading and screening to remove particles that do not meet the particle size requirements, thus obtaining spherical particles that meet the requirements.

[0039] In some embodiments, the precipitate is a high molecular weight aromatic concentrate (HM-Ex) extracted from lignite.

[0040] In some embodiments, the precipitate is a high molecular weight aromatic concentrate (CYM-Ex) extracted from bituminous coal.

[0041] In some embodiments, the precipitate is a mixture of HM-Ex and CYM-Ex. In some embodiments, the mass ratio of HM-Ex to CYM-Ex is 1:1 to 1:3, for example, 1:1.5, 1:2, 1:2.5, etc.

[0042] In some embodiments, the sphericity of the spherical particles is controlled to be 0.8-0.9, and the particle size is controlled to be 0.9-3.88 μm.

[0043] Spray granulation can be carried out using a spray granulator. A spray granulator is equipped with a screw conveyor, feed inlet, high-pressure atomizing nozzles, etc.

[0044] In some embodiments, spray granulation is carried out as follows: the precipitate obtained in step (3) is conveyed at a constant speed through a screw conveyor to the feed inlet of the spray granulator at 220-260°C, for example, 230°C (at which point it is in a plastic molten state). The feed rate is controlled to a stable value (6 kg / h) that matches the atomization rate. The precipitate is then broken into micron-sized droplets / particles by a high-pressure atomizing nozzle (atomization pressure 0.5 MPa). These particles are naturally cooled by air in a cooling tower to form a semi-finished product. The cooled semi-finished product is then fed from the bottom of the cooling tower into a three-dimensional vibrating screen filter for grading and screening to remove particles that do not meet the particle size requirements, thereby obtaining spherical particles that meet the requirements.

[0045] (5) Crosslinking In this step, in a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) are heated from room temperature to 220-260°C, for example 230°C, at a heating rate of 2-4°C / min, for example 3°C / min, and held at that temperature for 5-7 hours, for example 6 hours.

[0046] During this process, weak intermolecular bonds (such as intermolecular connections caused by van der Waals forces) break, resulting in oxidative cross-linking, which fixes the molecular structure and prevents excessive melting and disordered structural collapse during carbonization. At the same time, polar functional groups participate in the reaction, further affecting the carbon layer arrangement.

[0047] When high molecular weight aromatic concentrates of different low-rank coals are mixed in proportion (e.g., a mixture of HM-Ex and CYM-Ex) as a hard carbon precursor, the volatile component content of the high molecular weight aromatic concentrates of different low-rank coals varies. During the crosslinking process, the volatilization of volatile components leads to in-situ foaming, which in turn constructs a complex pore structure network after solidification. This provides favorable conditions for regulating the microcrystalline structure of hard carbon. Moreover, this process does not require the use of traditional physical or chemical activators, consumes less energy, and can achieve in-situ regulation of the microcrystalline structure of hard carbon.

[0048] (6) Initial carbonization In this step, under an argon atmosphere with a purity of ≥99.999%, the product obtained in step (5) is heated to 580-620°C, for example 600°C, at a rate of 4-6°C / min, for example 5°C / min, and held for 90-150 min, for example 120 min, thereby undergoing primary carbonization to form primary carbides.

[0049] During the heating process, the cross-linked spherical particles undergo dehydrogenation and condensation, and volatiles (such as CO2, CH4, H2O, etc.) further volatilize, resulting in the initial formation of graphite microcrystals and the construction of the initial interlayer structure of the hard carbon matrix.

[0050] During the heat preservation process, carbon microcrystals continue to grow, the pores further shrink and become denser, and the carbon structure of the hard carbon matrix is ​​initially formed.

[0051] (7) Secondary carbonization In this step, the primary carbide from step (6) is heated to 950-1050°C, for example, 1000°C, at a rate of 4-6°C / min, or 5°C / min, under a nitrogen atmosphere containing 5% hydrogen by volume. Then, under an argon atmosphere with a purity ≥99.999%, the temperature is further increased to 1400°C at a rate of 4-6°C / min, or 5°C / min, and held for 1-2 hours, or for example, 90 minutes.

[0052] During this process, the carbon microcrystalline structure is further improved, and the hard carbon matrix carbon structure is fully formed.

[0053] (8) Cooling In this step, the product of step (7) is naturally cooled to room temperature (around 25°C) under an argon atmosphere with a purity of ≥99.999% to obtain coal-based hard carbon material.

[0054] During this process, by continuously introducing argon gas, the oxidation of the hard carbon material at high temperatures caused by air entering the reaction device can be prevented, thus ensuring the integrity of the carbon skeleton structure.

[0055] In some embodiments, the method for preparing coal-based hard carbon materials of the present invention includes the following steps: (1) Raw material pretreatment: crush lignite and bituminous coal to above 80 mesh and dry to a moisture content of below 2wt%; (2) Mix the pretreated lignite with industrial compound solvent at a mass ratio of 1:10-1:15, heat to 330-340℃, keep warm for 30-90 min, and filter hot to obtain lignite extraction residue and extract. The pretreated bituminous coal was mixed with an industrial compound solvent at a mass ratio of 1:10-1:15, heated to 340-360℃, kept at the temperature for 30-90 min, and hot filtered to obtain the bituminous coal extraction residue and extract. The industrial compound solvent comprises, based on the total weight of the solvent, 45 wt% of 1-methylnaphthalene, 25 wt% of 2-methylnaphthalene, 15 wt% of 2,6-dimethylnaphthalene and 15 wt% of acenaphthene; (3) After cooling the lignite extract and the bituminous coal extract obtained in step (2) to room temperature, filter them and collect the precipitate and solution respectively; wherein, the precipitate obtained from the lignite extract is denoted as lignite extract (HM-Ex) and the precipitate obtained from the bituminous coal extract is denoted as bituminous coal extract (CYM-Ex). (4) In a nitrogen atmosphere with a purity of ≥99.9%, at 220-260℃, for example 230℃, the mixture of lignite extract (HM-Ex) and bituminous coal extract (CYM-Ex) obtained in step (3) at a mass ratio of 1:1 to 1:3 is spray-granulated to obtain spherical particles, wherein the sphericity of the spherical particles is controlled to be 0.8-0.9 and the particle size is controlled to be 0.9-3.88μm; (5) In a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) are heated from room temperature to 220-260°C, for example 230°C, at a heating rate of 2-4°C / min, for example 3°C / min and held at that temperature for 5-7h, for example 6h, to obtain the crosslinked product. (6) Under an argon atmosphere with a purity of ≥99.999%, the crosslinked product obtained in step (5) is heated to 580-620°C, for example 600°C, at a rate of 4-6°C / min, for example 5°C / min, and held at that temperature for 90-150 min, for example 120 min, thereby performing the initial carbonization to form the initial carbide. (7) In a nitrogen atmosphere containing 5 vol% hydrogen, the primary carbide from step (6) is heated to 950-1050°C, for example 1000°C, at a rate of 4-6°C / min, for example 5°C / min, and then heated to 1400°C in an argon atmosphere with a purity ≥99.999%, at a rate of 4-6°C / min, for example 5°C / min, and held for 1-2 hours, for example 90 minutes. (8) Under an argon atmosphere with a purity of ≥99.999%, the product of step (7) is naturally cooled to room temperature (around 25°C) to obtain coal-based hard carbon material.

[0056] In another aspect, the present invention provides a coal-based hard carbon material, which is prepared using the method described above.

[0057] The hard carbon material prepared according to the method of the present invention has a carbon interlayer spacing of 0.34-0.40 nm, an average particle size distribution of 0.9-3.88 μm, and an initial coulombic efficiency of ≥90.1%.

[0058] The hard carbon material according to the present invention can be used as a negative electrode active material in sodium-ion batteries. Therefore, in another aspect, the present invention provides the use of the coal-based hard carbon material according to the present invention in the preparation of a negative electrode active material for sodium-ion batteries.

[0059] In another aspect, the present invention provides a sodium-ion battery comprising the coal-based hard carbon material of the present invention as the negative electrode active material.

[0060] Beneficial effects I. Raw material advantages and in-situ control of defects in carbon microcrystals Low-rank coal extracts have low ash, low sulfur and are rich in oxygen-active functional groups. During the pre-oxidation and molding stage, the small molecule organic matter escapes through pyrolysis, thereby effectively regulating the defect structure of coal-based hard carbon microcrystals and improving the electrochemical performance of sodium-ion battery anodes.

[0061] II. Synergistic effects of extract components In the initial carbonization stage, oxygen-containing functional groups (such as carboxyl and hydroxyl groups) in low-rank coal extracts release small molecule gases through dehydrogenation and decarboxylation reactions, which help build pores. On the other hand, some residual oxygen heteroatoms can be doped into the carbon skeleton, regulate the electronic structure of hard carbon, enhance the adsorption capacity of sodium ions, and improve the reversible capacity.

[0062] III. Precursor Coal Particle Size Control Using raw coal directly as the hard carbon anode material requires grinding the raw coal to a finer particle size (D50) of ~7μm (2200 mesh), which necessitates specialized equipment and consumes a significant amount of energy. This invention utilizes a precursor extract, requiring only the grinding of low-rank coal to (80-100) mesh, thus requiring less sophisticated equipment and consuming less energy.

[0063] IV. Simplified Process and Cost Advantages From the overall process perspective: By utilizing natural components extracted from low-rank coal, complex chemical modification steps are eliminated, and the raw materials are readily available and easy to process. Gradient pre-carbonization and microcrystal regulation work together to reduce high-energy-consuming and high-cost processes (such as the addition of additional pore regulators or high-energy-consuming activation processes). Low-rank coal is a coal resource with abundant reserves but low utilization rate. The method of this invention can transform low-rank coal into high-value hard carbon anode material for sodium batteries, expand the application scenarios of low-rank coal, and improve the comprehensive utilization level of coal resources. Attached Figure Description

[0064] Figure 1 XRD patterns of hard carbon material samples prepared in Examples 1-3 and Comparative Example 1; Figure 2 High-magnification SEM image of the hard carbon material sample prepared in Example 1; Figure 3 The first charge-discharge curve of the hard carbon material sample prepared in Example 1. Detailed Implementation

[0065] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0066] Raw materials and reagents The bituminous coal was purchased from the Huaidong Coalfield in Xinjiang, with volatile matter (Vdaf) of 20 wt%-30 wt% and carbon content (Cdaf) of 70 wt%-80 wt%.

[0067] The lignite was purchased from the Huaidong coalfield in Xinjiang, with volatile matter (Vdaf) of 37 wt%-40 wt% and carbon content (Cdaf) of 60 wt%-80 wt%.

[0068] 1-Methylnaphthalene, purchased from Maclean's, purity ≥98 wt.

[0069] 2-Methylnaphthalene, purchased from Aladdin, purity ≥98 wt.

[0070] 2,6-Dimethylnaphthalene, purchased from Aladdin, purity ≥98 wt.

[0071] Acenathois, purchased from 999 Chemical City, with a purity of ≥97 wt.

[0072] The industrial complex solvent (ICS) consists of 45% by mass of 1-methylnaphthalene, 25% by mass of 2-methylnaphthalene, 15% by mass of 2,6-dimethylnaphthalene and 15% by mass of acenaphthene.

[0073] SEM image The samples were tested using a field emission SU-70 scanning electron microscope (Hitachi) to obtain scanning electron microscope (SEM) images for observing the microscopic morphological characteristics of the materials.

[0074] X-ray diffraction pattern Carbonaceous materials were characterized using a Bruker D8 Advance X-ray diffractometer (Germany). Measurement conditions were set as follows: Cu target Kα rays (λ = 0.15406 nm), tube voltage 40 kV, tube current 40 mA, scanning range (2θ) 5°–80°, scanning rate 8° / min, step size 0.02°, and a nickel filter to eliminate Kβ ray interference. Testing was conducted at room temperature. By analyzing the position, intensity, and full width at half maximum (FWHM) of characteristic peaks in the diffraction patterns, the crystal structure (e.g., degree of graphitization, interlayer spacing), phase composition (e.g., presence of impurity phases), and grain size of carbonaceous materials can be determined, providing crucial information for evaluating the performance of hard carbon materials. The interlayer spacing of hard carbon was analyzed using JADE, with the analysis repeated three times and the average value taken. JADE is a classic XRD analysis software commonly used for phase identification and calculation of crystal structure parameters (e.g., interplanar spacing d002, grain size Lc (normal grain size), La (tangential grain size)).

[0075] Elemental analysis The German Elementar vario EL cube elemental analyzer was used. It employs the dynamic flash-chromatographic separation-thermal conductivity detection principle, which can quickly and accurately determine the carbon, hydrogen, oxygen, nitrogen and sulfur content in the sample simultaneously.

[0076] Example 1 (1) Raw material pretreatment: Two-stage crushing is carried out using a jaw crusher and a hammer crusher. First, the particles are crushed to coarse particles of 20-50mm, and then screened by vibrating screens of 80 and 100 mesh.

[0077] The lignite and sub-bituminous coal were crushed to about 100 mesh and then dried at 150℃ until the moisture content was ≤2 wt%.

[0078] (2) Degradation solvent extraction (DSE) Extraction is performed using a batch high-pressure reactor, and the supporting system includes: Feeding unit: Premixing kettle (with stirring, speed 120-300 rpm), used for uniform mixing of coal and solvent; Reaction unit: High-pressure reactor (material 316L stainless steel, pressure resistance ≥10MPa, temperature control accuracy ±5℃), equipped with a stirring paddle (speed 120-200rpm), thermocouple (insertion depth ≥2 / 3 of the material height in the reactor), and pressure sensor (range 0-15MPa). Separation unit: Built-in 0.5μm steel filter (bottom outlet) and hot / cold separator (for cooling the extract); Solvent recovery unit: vacuum distillation column (vacuum degree -0.09 to -0.095 MPa, temperature 125-150℃), solvent storage tank (with nitrogen protection).

[0079] The extraction process and parameter control are as follows: (1) Preparation before reaction Inert atmosphere replacement: High-purity nitrogen (purity ≥99.999%) is introduced into the reactor and replaced 3 times (each time the pressure is released to 0.1 MPa after each purging to 0.5 MPa) to remove oxygen and prevent the oxidation of organic matter in the coal (such as the decomposition of carboxyl groups to generate CO2, which affects the quality of the product).

[0080] Heating program settings: Segmented heating is adopted. Initially, the temperature is increased at 4-6℃ / min to 150℃ (to remove residual moisture and light volatiles), and then increased at 5℃ / min to the target extraction temperature to avoid coal particle agglomeration caused by excessive heating.

[0081] (2) Core extraction parameters Extraction temperature: The extraction temperature for lignite is controlled at 330-340℃, and the extraction temperature for sub-bituminous coal is set at 340-360℃; Incubation time: 60 minutes; Stirring speed: 120-200 rpm; Pressure control: The pressure inside the vessel is controlled at 4-7 MPa during the extraction process.

[0082] The lignite powder prepared in step (1) was mixed with ICS solvent at a mass ratio of 1:12, kept at 340℃ for 60 min, and then hot filtered to obtain lignite extraction residue and extract.

[0083] The bituminous coal powder prepared in step (1) was mixed with ICS solvent at a mass ratio of 1:12, kept at 340℃ for 60 min, and then hot filtered to obtain bituminous coal extraction residue and extract.

[0084] (3) Separation The lignite extract and the bituminous coal extract obtained in step (2) were cooled to 25°C and filtered, and the precipitates were collected to obtain high molecular weight aromatic concentrates. The high molecular weight aromatic concentrate obtained from lignite is hereinafter referred to as lignite extract (HM-Ex), with a volatile matter content of 53.9 wt%. The high molecular weight aromatic concentrate obtained from bituminous coal is hereinafter referred to as bituminous coal extract (CYM-Ex), with a volatile matter content of 48.3 wt%. The volatile matter content was determined according to the "Industrial Analysis Methods for Coal" (GB / T 212-2021).

[0085] Elemental analysis of HM-Ex and CYM-Ex was performed, and the results are shown in Table 1.

[0086] Table 1: Elemental Analysis Table

[0087] The analysis results show that the extracts CYM-Ex and HM-Ex are rich in C and O substances, while the ash content of CYM-Ex is reduced to below 1 compared with HM-Ex.

[0088] HM-Ex and CYM-Ex were blended at a mass ratio of 1:2 as a hard carbon precursor for subsequent steps.

[0089] (4) Spray granulation The HM-Ex and CYM-Ex blend from step (3) with a mass ratio of 1:2 is heated to 230°C and molten. It is then conveyed at a constant speed via a screw conveyor to the feed inlet of a spray granulator, with a feed rate controlled at 6 kg / h. After entering the granulator, the material is broken into micron-sized droplets / particles by a high-pressure atomizing nozzle under an atomization pressure of 0.5 MPa. The heated liquid molten extract is sprayed from the nozzle under a certain pressure, simultaneously atomized into fine particles using air. These particles are naturally cooled by air in a cooling tower, forming a semi-finished product. The cooled semi-finished product is then fed from the bottom of the cooling tower into a three-dimensional vibrating screen filter for grading and screening particles with a diameter greater than 0.7 μm and less than 4.0 μm. Particles that do not meet the size requirements are removed, resulting in particles that initially meet the size requirements. The particles were detected by an image analyzer and found to have a sphericity of 0.8-0.9. Laser particle size analyzer showed that the particle size distribution was concentrated in the range of 0.9-3.88 μm, with an average particle size of 2.17 μm.

[0090] (5) Crosslinking In a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) were heated from room temperature to 230°C at a rate of 3°C / min and kept at that temperature for 6 hours to obtain the crosslinked product.

[0091] (6) Initial carbonization Under an argon atmosphere (purity ≥ 99.999%), the crosslinked product obtained in step (5) is heated to 600°C at 5°C / min to carry out the initial carbonization and form the initial carbide.

[0092] (7) Secondary carbonization First, the primary carbide from step (6) is heated from 600°C to 1000°C at 5°C / min in a nitrogen atmosphere containing 5% hydrogen by volume. Then, in an argon atmosphere with a purity ≥99.999%, the temperature is increased from 1000°C to 1400°C at 5°C / min, and then held at that temperature for 90 min.

[0093] (8) Natural cooling and inert atmosphere protection: After the heat preservation is completed, the argon atmosphere is kept constant, heating is stopped, and the reaction system is allowed to cool naturally to room temperature (around 25°C) to obtain coal-based hard carbon materials.

[0094] Example 2 Using only bituminous coal as raw material, and employing only CYM-Ex as a hard carbon precursor in step (4), coal-based hard carbon materials were prepared according to the method of Example 1.

[0095] Example 3 Using only lignite as raw material, and in step (4) using only HM-Ex as a hard carbon precursor, coal-based hard carbon materials were prepared according to the method of Example 1.

[0096] Comparative Example 1: Using sub-bituminous coal as a precursor Using bituminous coal (CYM) as the sole hard carbon precursor, without low-rank coal pretreatment and extraction, the carbonization heating process is the same as in Example 1, but without crosslinking and spray granulation steps. The specific process is as follows.

[0097] (1) Precursor preparation: The raw coal is made directly from bituminous coal, which is then finely crushed to 2200 mesh and dried for later use. The moisture content is tested to be ≤2wt%.

[0098] (2) Initial carbonization Under an argon atmosphere (purity ≥ 99.999%), the sub-bituminous coal powder obtained in step (1) is heated to 600℃ at 5℃ / min to complete the primary carbonization and form primary carbides; (3) Secondary carbonization The primary carbide obtained in step (2) was directly transferred into a nitrogen atmosphere containing 5 vol% hydrogen and heated to 1000℃ at 5℃ / min; then switched to an argon atmosphere with a purity ≥99.999% and heated to 1400℃ at 5℃ / min, and held for 90min. The subsequent natural cooling and inert atmosphere protection were the same as in Example 1, resulting in a sub-bituminous coal hard carbon material.

[0099] Comparative Example 2: Using lignite as a precursor Except for using lignite (HM) raw coal as the sole hard carbon precursor, lignite hard carbon materials were prepared using the same steps as in Comparative Example 1.

[0100] Experimental Example 1: XRD Detection The carbonaceous structure of the sub-bituminous coal hard carbon materials obtained in Examples 1-3 and Comparative Example 1 was observed using XRD. The results are shown in the figure. Figure 1 .

[0101] like Figure 1 As shown, the hard carbon materials in Examples 1-3 and Comparative Example 1 exhibit d(002) crystal plane diffraction peaks at around 2θ = 25°. The peaks are relatively wide and weak, exhibiting a "bun peak" pattern. There are no obvious diffraction peaks. This is mainly because the carbon foam has not been graphitized and is therefore mostly amorphous carbon, exhibiting a disordered amorphous state, which is consistent with the carbonaceous structure of hard carbon.

[0102] JADE analysis was performed on the XRD spectra of the hard carbon materials in Examples 1-3. The average interlayer spacing of the hard carbon material in Example 1 was calculated to be approximately 0.374 nm; the average interlayer spacing of the hard carbon material in Example 2 was approximately 0.355 nm (between 0.34 and 0.4 nm); and the average interlayer spacing of the hard carbon material in Example 3 was approximately 0.4 nm, which was higher than that in Example 1.

[0103] Experimental Example 2: SEM Characterization The hard carbon material in Example 1 was characterized by SEM, and the results are shown in [Figure 1]. Figure 2 .

[0104] like Figure 2 As shown, the prepared samples are spherical with a particle size distribution of 0.9-3.88 μm and an average particle size of 2.17 μm.

[0105] For improving battery energy density, the packing density of materials is one of the key factors. Hard carbon particles with a diameter of approximately 2 μm can achieve good packing performance while ensuring a certain specific surface area. Compared with larger particles (such as those larger than 20 μm), 2 μm particles can be packed more tightly during electrode fabrication, reducing voids and thus increasing the overall packing density of the electrode, thereby improving the battery's energy density.

[0106] Experiment Example 3: Electrochemical Testing Using the spherical hard carbon materials (HC) obtained in Examples 1-3 and Comparative Examples 1-2 as anode materials, CR2032 batteries were assembled and electrochemical tests were performed.

[0107] 3.1 Assembly of CR2032 batteries The battery assembly was assembled in an argon-filled glove box (MBRAUN MB Labstar 1500 / 780) with both water and oxygen content less than 0.1 ppm.

[0108] The CR2032 battery casing was purchased from Dongguan Rongfeng Technology Co., Ltd.; the battery separator is a GF / D separator (Shenzhen Boseng Technology).

[0109] Super P is a conductive carbon black (model: Super P Li, average particle size 15 nm, specific surface area 62 m²) purchased from Timcal, Switzerland. 2 / g).

[0110] Dongguan Shengli New Materials Co., Ltd.: Its styrene-butadiene rubber latex is model SL, with a solid content of 45±1wt%.

[0111] CMC was sodium carboxymethyl cellulose (model: CMC-Na, viscosity 500-800 mPa·s, degree of substitution 0.7-0.9, used as electrode dispersant and thickener) purchased from Anhui Sorbitol Co., Ltd. in China, and prepared as an aqueous solution with a solid content of 1.5 wt%.

[0112] The electrode is a sodium metal sheet purchased from Guangdong Daxiao Chemical Co., Ltd., with a diameter of 12mm, a thickness of 0.45-0.5mm, a purity of 99.7 wt% or higher, 400 sheets / can, net weight 50g.

[0113] The coin cell test used Zhuhai Saiwei SWMHS-NAK01 electrolyte. The raw materials were prepared into a slurry according to a solid mass ratio of HC:Super P:SBR:CMC = 93:2:3:2, with a total solid content of 35wt%. The slurry was then coated onto the electrode sheets with a loading of approximately 6.0 mg / cm³. 2 After rolling, the density is 0.8-1.0 g / cm³. 3 Electrochemical tests were performed using the NEWARE-BTS-4008 system.

[0114] Example 1: The initial charge-discharge curve of hard carbon material is as follows Figure 3 As shown, the initial coulombic efficiency was 90.1%, and the reversible specific capacity was 316 mAh / g, significantly higher than that of Examples 2 and 3. These results indicate that further microcrystalline structure regulation can be achieved through blending CYM-Ex and HM-Ex, significantly optimizing the electrochemical performance of coal-based hard carbon. This facilitates sodium ion intercalation and provides ample space for electrolyte wetting, enabling the material to maintain a high reversible capacity (>316 mAh / g) even at a current of 100 mA / g.

[0115] The electrochemical test results of the hard carbon materials in Examples 1-3 and Comparative Examples 1-2 are summarized in Table 2.

[0116] Table 2

[0117] This invention addresses the pain points of traditional preparation of sodium-ion battery hard carbon anode materials by innovatively employing a low-rank coal "enrichment-based coal blending technology." This enables precise control of the microcrystalline structure of the coal-based hard carbon anode, allowing for precise control of the interlayer distance of the hard carbon to the optimal sodium storage range of 0.34-0.40 nm. In particular, the interlayer distance reaches 0.38 nm when blended at a 1:2 ratio, achieving a high reversible specific capacity of 316 mAh / g and a high first coulombic efficiency of 90.1%, significantly superior to hard carbon prepared by a single enrichment. This technology not only overcomes the bottleneck of interlayer distance control and improves material performance but also meets the economic needs of industrial mass production, providing a key solution for the commercialization of sodium-ion battery hard carbon anodes and the high-value utilization of low-rank coal.

[0118] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing coal-based hard carbon materials from low-rank coal melt, characterized in that, The method includes the following steps: (1) Raw material pretreatment: The low-rank coal is crushed to a mesh size of 80 or larger and dried to a moisture content of less than 2 wt%. (2) Degradation solvent extraction: Pretreated coal is mixed with industrial compound solvent at a mass ratio of 1:10-1:15, extracted at 330-360℃, and then hot filtered to obtain extract and extraction residue. The industrial compound solvent comprises, based on the total weight of the solvent, (a) 40 to 50 wt% of 1-methylnaphthalene; (b) 35 to 45 wt% of a co-solvent selected from 2-methylnaphthalene, 2,6-dimethylnaphthalene and 1,6-dimethylnaphthalene; and (c) 10 wt% to 20 wt% of a polarity modifier selected from at least one of acenaphthene or fluorene; (3) Separation: The extract obtained in step (2) was cooled to room temperature. During the process, a precipitate was formed. The precipitate and solution were collected separately by filtration. (4) Spray granulation The precipitate collected in step (3) was spray-granulated at 220-260℃ under a nitrogen atmosphere with a purity of ≥99.9% to obtain spherical particles; (5) Crosslinking In a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) are heated from room temperature to 220-260℃ at a heating rate of 2-4℃ / min and held at that temperature for 5-7h. (6) Initial carbonization Under an argon atmosphere with a purity of ≥99.999%, the product obtained in step (5) is heated to 580-620℃ at a rate of 4-6℃ / min and held for 90-150min to carry out the initial carbonization and form the initial carbide. (7) Secondary carbonization Under a nitrogen atmosphere containing 5 vol% hydrogen, the primary carbide from step (6) is heated to 950-1050℃ at 4-6℃ / min, and then heated to 1400℃ at 4-6℃ / min under an argon atmosphere with a purity ≥99.999%, and held for 1-2 hours. (8) Cooling Under an argon atmosphere with a purity of ≥99.999%, the product from step (7) was naturally cooled to room temperature to obtain coal-based hard carbon material.

2. The method according to claim 1, characterized in that, In step (1), the low-rank coal is lignite, sub-bituminous coal, or a combination thereof; and / or In step (2), the industrial compound solvent comprises, based on the total weight of the solvent, (a) 45 wt% of 1-methylnaphthalene; (b) 40 wt% of a co-solvent selected from 2-methylnaphthalene, 2,6-dimethylnaphthalene and 1,6-dimethylnaphthalene; and (c) 15 wt% of a polarity modifier selected from at least one of acenaphthene or fluorene; Specifically, the co-solvent is a mixture of 2-methylnaphthalene and 2,6-dimethylnaphthalene; more specifically, the co-solvent is a mixture of 2-methylnaphthalene and 2,6-dimethylnaphthalene in a mass ratio of 5:

3. Specifically, the industrial compound solvent comprises, based on the total weight of the solvent, 45 wt% of 1-methylnaphthalene, 25 wt% of 2-methylnaphthalene, 15 wt% of 2,6-dimethylnaphthalene, and 15 wt% of acenaphthene.

3. The method according to claim 1, characterized in that, The low-rank coal is lignite, and the extraction temperature is controlled at 330-340℃; or the low-rank coal is sub-bituminous coal, and the extraction temperature is controlled at 340-360℃; and / or In step (2), the extraction time is 30-90 min; and / or the pressure is controlled at 4-7 MPa during the extraction process.

4. The method according to claim 1, characterized in that, In step (4), The precipitate is a high molecular weight aromatic concentrate extracted from lignite; or a high molecular weight aromatic concentrate extracted from bituminous coal; or a mixture of both. Specifically, the precipitate is a mixture of high molecular weight aromatic concentrates extracted from lignite and high molecular weight aromatic concentrates extracted from bituminous coal, with a mass ratio of 1:1 to 1:

3.

5. The method according to claim 1, characterized in that, In step (4), the sphericity of the spherical particles is controlled at 0.8-0.9, and the particle size is controlled at 0.9-3.88μm.

6. The method according to claim 1, characterized in that, In step (5), in a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) are heated from room temperature to 230°C at a heating rate of 3°C / min and kept at that temperature for 6 hours. In step (6), under an argon atmosphere with a purity ≥99.999%, the product obtained in step (5) is heated to 600°C at 5°C / min and held for 120 min to carry out the initial carbonization and form the initial carbide. In step (7), the initial carbide from step (6) is heated to 1000°C at 5°C / min under a nitrogen atmosphere containing 5 vol% hydrogen, and then heated to 1400°C at 5°C / min under an argon atmosphere with a purity ≥99.999%, and held for 90 min.

7. The method according to claim 1, characterized in that, The method includes the following steps: (1) Raw material pretreatment: crush lignite and bituminous coal to above 80 mesh and dry to a moisture content of below 2wt%; (2) Mix the pretreated lignite with industrial compound solvent at a mass ratio of 1:10-1:15, heat to 330-340℃, keep warm for 30-90 min, and filter hot to obtain lignite extraction residue and extract. The pretreated bituminous coal was mixed with an industrial compound solvent at a mass ratio of 1:10-1:15, heated to 340-360℃, kept at the temperature for 30-90 min, and hot filtered to obtain the bituminous coal extraction residue and extract. The industrial compound solvent comprises, based on the total weight of the solvent, 45 wt% of 1-methylnaphthalene, 25 wt% of 2-methylnaphthalene, 15 wt% of 2,6-dimethylnaphthalene and 15 wt% of acenaphthene; (3) Cool the lignite extract and the bituminous coal extract obtained in step (2) to room temperature and filter them respectively, and collect the precipitate and solution respectively; wherein, the precipitate obtained from the lignite extract is called lignite extract and the precipitate obtained from the bituminous coal extract is called bituminous coal extract. (4) In a nitrogen atmosphere with a purity of ≥99.9%, the mixture of lignite extract and bituminous coal extract obtained in step (3) at a mass ratio of 1:1 to 1:3 is spray-granulated at 220-260℃ to obtain spherical particles, wherein the sphericity of the spherical particles is controlled to be 0.8-0.9 and the particle size is controlled to be 0.9-3.88μm; (5) In a mixed gas atmosphere of oxygen and nitrogen with a volume ratio of 1:99, the spherical particles obtained in step (4) are heated from room temperature to 220-260℃ at a heating rate of 2-4℃ / min and kept at that temperature for 5-7h to obtain the crosslinked product. (6) Under an argon atmosphere with a purity of ≥99.999%, the crosslinked product obtained in step (5) is heated to 580-620℃ at 4-6℃ / min and held for 90-150min to carry out the initial carbonization and form the initial carbide. (7) In a nitrogen atmosphere containing 5 vol% hydrogen, the primary carbide from step (6) is heated to 950-1050℃ at 4-6℃ / min, and then in an argon atmosphere with a purity ≥99.999%, the temperature is raised to 1400℃ at 4-6℃ / min and held for 1-2 hours. (8) Under an argon atmosphere with a purity of ≥99.999%, the product of step (7) is naturally cooled to room temperature to obtain coal-based hard carbon material.

8. A coal-based hard carbon material, characterized in that, It is prepared using the method according to any one of claims 1-7.

9. The use of the coal-based hard carbon material according to claim 8 in the preparation of anode active material for sodium-ion batteries.

10. A sodium-ion battery, characterized in that, It includes the coal-based hard carbon material as described in claim 8 as the negative electrode active material.

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