Coal-based hard carbon negative electrode material and preparation method and application thereof

Abstract

This invention relates to the field of coal-based hard carbon anode materials, and proposes a coal-based hard carbon anode material, its preparation method, and its application. The preparation steps of the coal-based hard carbon anode material include: providing low-rank coal raw materials, crushing and grinding them; bombarding the treated coal powder with plasma under an inert gas or nitrogen-containing atmosphere to obtain an activated precursor; subjecting the activated precursor to pre-oxidative crosslinking treatment; and performing programmed temperature-controlled carbonization treatment on the activated precursor under an inert atmosphere, the programmed temperature-controlled carbonization including a first low-temperature holding stage and a second medium-temperature holding stage; finally obtaining the coal-based hard carbon anode material. The coal-based hard carbon anode material, its preparation method, and its application provided by this invention, through multi-step synergy, improve cycle stability and first-cycle coulombic efficiency, achieving a comprehensive balance between high reversible capacity and excellent cycle and rate performance.

Description

Technical Field

[0001] This invention relates to the field of coal-based hard carbon anode materials technology, specifically to a coal-based hard carbon anode material, its preparation method, and its application. Background Technology

[0002] With the increasing global demand for clean energy and sustainable development, the importance of energy storage technology is becoming increasingly prominent. Lithium-ion batteries, as the most widely used rechargeable batteries, have achieved great success in portable electronic devices, electric vehicles, and other fields. However, the limited and uneven distribution of lithium resources in the Earth's crust leads to the high cost of lithium-ion batteries, making it difficult to meet the growing demand in large-scale energy storage. Against this backdrop, sodium-ion batteries, due to their abundant resources and low cost, have gradually become a research hotspot in the energy storage field and are considered a powerful supplement and potential alternative to lithium-ion batteries.

[0003] Coal, as an abundant fossil energy source, has vast reserves and is widely distributed in my country. Using coal as a raw material to prepare hard carbon anode materials not only effectively utilizes coal resources and reduces dependence on imported lithium resources, but also provides a new approach for the clean and efficient utilization of coal, possessing significant strategic and economic value. Coal-based hard carbon anode materials are produced by converting coal into amorphous hard carbon materials through specific processes. These materials contain numerous micropores and defects, providing abundant active sites for sodium ion storage, thus endowing them with high sodium storage capacity. Simultaneously, coal-based hard carbon materials exhibit good structural stability and conductivity, demonstrating excellent cycle stability and rate performance in sodium-ion batteries.

[0004] Existing coal-based hard carbon anode materials generally have low sodium storage capacity, making it difficult to meet the demands of high-energy-density sodium-ion batteries. This is mainly due to the limited number of sodium storage active sites provided by the chemical composition and structural characteristics of coal-based raw materials. During the charge and discharge process of sodium-ion batteries, the anode material undergoes repeated insertion and extraction of sodium ions, causing volume changes in the material. During cycling, existing coal-based hard carbon anode materials are prone to structural collapse and pulverization due to insufficient internal structure, leading to poor contact between the electrode and the current collector, increased internal resistance, and consequently affecting the cycle stability of the battery. Based on this, this invention proposes a coal-based hard carbon anode material, its preparation method, and its applications. Summary of the Invention

[0005] This invention proposes a coal-based hard carbon anode material, its preparation method, and its application. Through multi-step synergy, it improves sodium storage capacity, structural stability, and conductivity; it also improves cycle stability and first-cycle coulombic efficiency, achieving a comprehensive balance between high reversible capacity and excellent cycle and rate performance.

[0006] The technical solution of the present invention is as follows: In a first aspect, this invention proposes a method for preparing a coal-based hard carbon anode material, comprising the following steps: (a) Raw material preparation steps: Provide low-rank coal raw materials, and crush and grind them; (b) Plasma activation step: Under an inert gas or nitrogen-containing atmosphere, the coal powder treated in step (a) is bombarded with plasma to obtain an activated precursor; (c) Pre-oxidative crosslinking treatment of the activated precursor; (d) Gradient carbonization treatment: Under the protection of an inert atmosphere, the activated precursor is subjected to programmed temperature-controlled carbonization treatment, which includes a first low-temperature heat preservation stage and a second medium-temperature heat preservation stage in sequence; finally, the coal-based hard carbon anode material is obtained.

[0007] As a further technical solution, in step (a), the low-rank coal is lignite or long-flame coal; the particle size D50 of the coal powder is 5-20 μm.

[0008] As a further technical solution, in step (b), the plasma is a normal pressure nitrogen-hydrogen mixed gas plasma, wherein the volume percentage of hydrogen is 5%-20%; the power of the plasma treatment is 1-5 kW, the treatment temperature is 300-500℃, and the treatment time is 10-60 min.

[0009] As a further technical solution, in step (c), the oxidative crosslinking treatment is carried out in an air atmosphere, with a heating rate of 1-5℃ / min to 300-400℃, and then kept at that temperature for 2-4 hours.

[0010] As a further technical solution, during the pre-oxidation crosslinking treatment, 10% to 20% of a phosphorus-containing compound by mass is incorporated into the coal-based porous carbon material precursor as a crosslinking promoter; the phosphorus-containing compound includes at least one of phosphoric acid, ammonium dihydrogen phosphate, or phytic acid.

[0011] As a further technical solution, in step (d), the temperature of the first low-temperature insulation stage is 600-800℃, and the insulation time is 2-4h; the temperature of the second medium-temperature insulation stage is 800-1000℃, and the insulation time is 1-2h.

[0012] This invention employs a gradient carbonization process, sequentially performing a first low-temperature holding stage and a second medium-temperature holding stage, achieving synergistic optimization of the pore structure and carbon matrix. In the first low-temperature holding stage, the primary function is to stabilize the carbon framework, further consolidating the preliminary structure formed by pre-oxidative crosslinking and preventing structural collapse during subsequent temperature increases. Simultaneously, low-temperature carbonization initiates the formation of microporous structures, providing initial channels for sodium ion insertion. As the temperature rises to the second medium-temperature holding stage, the carbon matrix is ​​further refined, and the pore structure is further adjusted and optimized, resulting in a more balanced pore structure. This gradient carbonization approach, first stabilizing the framework at low temperatures and then refining the structure at medium temperatures, facilitates the formation of a more stable carbon matrix, optimizes sodium ion transport kinetics, and ensures structural integrity during cycling. In contrast, a single-step high-temperature carbonization method may lead to rapid structural stabilization, resulting in defects and poorer cycle stability and rate performance. Therefore, the gradient carbonization steps, through the synergistic control of temperature and time, optimize the pore structure and carbon matrix, improving the overall electrochemical performance of the material.

[0013] As a further technical solution, the heating rate from the first low-temperature insulation stage to the second medium-temperature insulation stage is 2-5℃ / min.

[0014] As a further technical solution, step (d) is followed by step (e): cooling, ball milling and sieving the product after gradient carbonization to obtain the final product.

[0015] Secondly, this invention proposes a coal-based hard carbon anode material prepared using the aforementioned preparation method.

[0016] Thirdly, the coal-based hard carbon anode material of this invention is used in sodium-ion batteries.

[0017] The working principle and beneficial effects of this invention are as follows: This invention activates pulverized coal, optimizing its surface structure through synergistic effects. The coal powder is bombarded with atmospheric pressure nitrogen-hydrogen mixed gas plasma under an inert or nitrogen-containing atmosphere. Hydrogen plasma possesses unique etching and reduction effects. On one hand, it etches the coal powder surface, creating abundant active sites and defect structures. These sites and structures provide more space for sodium ion storage, effectively increasing the material's sodium storage capacity. On the other hand, its reduction effect modulates the surface chemical properties of the coal powder, reducing irreversible decomposition of the electrolyte during the first cycle and promoting the formation of a stable solid electrolyte interphase (SEI) film. Simultaneously, appropriate plasma treatment power (1-5 kW), temperature (300-500 °C), and time (10-60 min) ensure sufficient energy input, allowing for thorough activation and significant alteration of the physicochemical structure of the coal powder. If the power is too low, activation is insufficient, and the precursor cannot form enough defects and pores. If the power is too high, excessive bombardment may damage the carbon precursor structure, generating too many unstable defects and an excessively large specific surface area, exacerbating side reactions in the electrolyte. Therefore, the parameters in the plasma activation step work together to lay the foundation for the preparation of high-performance coal-based hard carbon anode materials.

[0018] In this invention, the pre-oxidative crosslinking step plays a crucial synergistic role in constructing a stable hard carbon framework. Under air atmosphere, the temperature is raised to 300-400℃ at a rate of 1-5℃ / min and held for 2-4 hours to induce a pre-oxidative crosslinking reaction in the activated precursor. During this process, coal molecules form crosslinking bonds through oxidation, constructing a stable three-dimensional crosslinked network. Based on this, a phosphorus-containing compound, accounting for 10%-20% of the activated precursor mass, is incorporated as a crosslinking promoter, further enhancing the crosslinking effect. The phosphorus-containing compound, with its multifunctional molecules, can bridge coal molecules, making the crosslinked network more robust. This stable hard carbon framework maintains structural integrity during subsequent carbonization, reduces structural disorder, improves conductivity, and suppresses side reactions. If the pre-oxidative crosslinking step is omitted, the carbon precursor cannot form a stable three-dimensional crosslinked network at high temperatures, leading to poor structural disorder, deteriorated conductivity, and severe side reactions, significantly affecting the electrochemical performance of the material. Therefore, the pre-oxidative crosslinking step and the crosslinking promoter work synergistically to construct a stable hard carbon framework, which is crucial for improving material performance.

[0019] This invention achieves the preparation of high-performance coal-based hard carbon anode materials through the synergistic interaction of three core steps: plasma activation, pre-oxidative crosslinking, and gradient carbonization. Plasma activation creates abundant active sites and defect structures in the material, enhancing sodium storage capacity and promoting the formation of a stable SEI film. Pre-oxidative crosslinking constructs a stable hard carbon framework, improving the material's structural stability and conductivity while reducing side reactions. Gradient carbonization optimizes the pore structure and carbon matrix, improving sodium ion transport kinetics and cycling stability. All three steps are indispensable. Through synergistic effects, the prepared coal-based hard carbon material achieves a comprehensive balance of high reversible capacity, high first-cycle coulombic efficiency, and excellent cycling and rate performance, validating the effectiveness of the preparation method of this invention. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 This embodiment provides a method for preparing a coal-based hard carbon anode material, the steps of which include: (1) Raw material preparation: Lignite from Inner Mongolia was selected as raw material and was initially crushed by a jaw crusher, and then transferred to a planetary ball mill for fine grinding; the grinding process was monitored by a laser particle size analyzer to ensure that the final coal powder particle size D50 was 10μm; (2) Plasma activation: The above-mentioned coal powder is evenly spread in an alumina crucible and placed in an atmospheric pressure plasma reactor; after sealing the reaction chamber, a mixed gas consisting of 15% hydrogen and 85% nitrogen by volume is introduced as the plasma working gas, and the gas flow rate is controlled at 1.0 L / min; the plasma generator is started, the radio frequency power is set to 3 kW, the temperature of the reaction area is controlled to be stable at 400℃, and the coal powder is bombarded for 30 minutes. After the treatment, it is cooled to room temperature under atmosphere protection to obtain the activated precursor. (3) Pre-oxidative crosslinking treatment: Take 100 g of the above activated precursor and mix it thoroughly with 15 g of crosslinking promoter phytic acid (accounting for 15% of the precursor mass) in an ethanol solution. Then dry it at 80 °C to remove the solvent. Transfer the uniformly mixed material to a muffle furnace and heat it from room temperature to 350 °C at a heating rate of 2 °C / min under an air atmosphere. Then keep it at 350 °C for 3 hours to complete the pre-oxidative crosslinking process and obtain the crosslinking product. (4) Gradient carbonization treatment: The pre-oxidized crosslinked product is transferred to a tube furnace, and high-purity argon gas (flow rate of 200 mL / min) is continuously introduced into the furnace as a protective gas; temperature-controlled carbonization is carried out according to the following procedure: the temperature is increased from room temperature to the target temperature of 700℃ in the first low-temperature holding stage at a rate of 3℃ / min, and held at 700℃ for 3h; then, the temperature is increased to the target temperature of 900℃ in the second medium-temperature holding stage at a rate of 3℃ / min, and held at 900℃ for 1.5h; after the carbonization procedure is completed, heating is stopped, and the product is naturally cooled to room temperature in an argon gas flow. (5) Post-processing: Take out the cooled carbonized product and put it into the planetary ball mill again for light ball milling to deagglomerate and soften the agglomerates; finally, use a 200-mesh standard sieve to sieve and collect the powder that passes through the sieve to obtain the final coal-based hard carbon anode material product of this embodiment.

[0022] Example 2 This embodiment provides a method for preparing a coal-based hard carbon anode material, the steps of which include: (1) Raw material preparation: Long-flame coal from Xinjiang Uygur Autonomous Region was selected as raw material. It was initially crushed by a jaw crusher and then transferred to a planetary ball mill for fine grinding. The grinding process was monitored by a laser particle size analyzer to ensure that the final coal powder particle size D50 was 5μm. (2) Plasma activation: The above-mentioned coal powder is evenly spread in an alumina crucible and placed in an atmospheric pressure plasma reactor; after sealing the reaction chamber, a mixed gas consisting of 5% hydrogen and 95% nitrogen by volume is introduced as the plasma working gas, and the gas flow rate is controlled at 1.0 L / min; the plasma generator is started, the radio frequency power is set to 1 kW, and the temperature of the reaction area is controlled to be stable at 300℃, and the coal powder is bombarded for 10 minutes; after the treatment, it is cooled to room temperature under atmosphere protection to obtain the activated precursor; (3) Pre-oxidative crosslinking treatment: Take 100g of the above activated precursor and mix it thoroughly with 10g of crosslinking accelerator ammonium dihydrogen phosphate (accounting for 10% of the precursor mass) in an ethanol solution. Then dry it at 80℃ to remove the solvent. Transfer the uniformly mixed material to a muffle furnace and heat it from room temperature to 300℃ at a heating rate of 1℃ / min under an air atmosphere. Then keep it at 300℃ for 2h to complete the pre-oxidative crosslinking process and obtain the crosslinking product. (4) Gradient carbonization treatment: The pre-oxidized crosslinked product is transferred to a tube furnace; high-purity argon gas (flow rate of 200 mL / min) is continuously introduced into the furnace as a protective gas; temperature-controlled carbonization is carried out according to the following procedure: the temperature is increased from room temperature to the target temperature of 600℃ in the first low-temperature holding stage at a rate of 2℃ / min, and held at 600℃ for 2 hours; then, the temperature is increased to the target temperature of 800℃ in the second medium-temperature holding stage at a rate of 2℃ / min, and held at 800℃ for 1 hour; after the carbonization procedure is completed, heating is stopped, and the product is naturally cooled to room temperature in an argon gas flow. (5) Post-processing: Take out the cooled carbonized product and put it into the planetary ball mill again for light ball milling to deagglomerate and soften the agglomerates; finally, use a 200-mesh standard sieve to sieve and collect the powder that passes through the sieve to obtain the final coal-based hard carbon anode material product of this embodiment.

[0023] Example 3 This embodiment provides a method for preparing a coal-based hard carbon anode material, the steps of which include: (1) Raw material preparation: Shanxi lignite was selected as raw material, and a jaw crusher was used for preliminary crushing, followed by fine grinding in a planetary ball mill; the grinding process was monitored by a laser particle size analyzer to ensure that the final coal powder particle size D50 was 20μm; (2) Plasma activation: The above-mentioned coal powder is evenly spread in an alumina crucible and placed in an atmospheric pressure plasma reactor; after sealing the reaction chamber, a mixed gas consisting of 20% hydrogen and 80% nitrogen by volume is introduced as the plasma working gas, and the gas flow rate is controlled at 1.0 L / min; the plasma generator is started, the radio frequency power is set to 5kW, and the temperature of the reaction area is controlled to be stable at 500℃, and the coal powder is bombarded continuously for 60min; after the treatment, it is cooled to room temperature under atmosphere protection to obtain the activated precursor; (3) Pre-oxidative crosslinking treatment: Take 100 g of the above activated precursor and mix it thoroughly with 20 g of crosslinking promoter phosphoric acid (accounting for 20% of the precursor mass) in an ethanol solution. Then dry it at 80°C to remove the solvent. Transfer the uniformly mixed material to a muffle furnace and heat it from room temperature to 400°C at a heating rate of 5°C / min under an air atmosphere. Then keep it at 400°C for 4 hours to complete the pre-oxidative crosslinking process and obtain the crosslinking product. (4) Gradient carbonization treatment: The pre-oxidized crosslinked product is transferred to a tube furnace; high-purity argon gas (flow rate of 200 mL / min) is continuously introduced into the furnace as a protective gas; temperature-controlled carbonization is carried out according to the following procedure: the temperature is increased from room temperature to the target temperature of 800℃ in the first low-temperature holding stage at a rate of 5℃ / min, and held at 800℃ for 4 hours; then, the temperature is increased to the target temperature of 1000℃ in the second medium-temperature holding stage at a rate of 5℃ / min, and held at 1000℃ for 2 hours; after the carbonization procedure is completed, heating is stopped, and the product is naturally cooled to room temperature in an argon gas flow. (5) Post-processing: Take out the cooled carbonized product and put it into the planetary ball mill again for light ball milling to deagglomerate and soften the agglomerates; finally, use a 200-mesh standard sieve to sieve and collect the powder that passes through the sieve to obtain the final coal-based hard carbon anode material product of this embodiment.

[0024] Comparative Example 1 In Comparative Example 1, step (2) plasma activation treatment was omitted, and the coal powder was directly pre-oxidized and cross-linked; the rest was the same as in Example 1.

[0025] Comparative Example 2 In Comparative Example 2, step (2) was performed using pure nitrogen plasma without adding hydrogen, and the rest was the same as in Example 1.

[0026] Comparative Example 3 In Comparative Example 3, step (2) was performed with a plasma processing power of 0.5 kW; the rest was the same as in Example 1.

[0027] Comparative Example 4 In Comparative Example 4, step (2) was set to 7 kW for plasma processing power, and the rest was the same as in Example 1.

[0028] Comparative Example 5 In Comparative Example 5, step (3) pre-oxidation crosslinking treatment was omitted, and the plasma-activated precursor was directly carbonized. The rest was the same as in Example 1.

[0029] Comparative Example 6 In Comparative Example 6, step (3) was performed without adding any crosslinking promoters, and the rest was the same as in Example 1.

[0030] Comparative Example 7 In Comparative Example 7, step (4) was canceled, and the temperature was directly increased to 900℃ at 5℃ / min and kept at that temperature for 4.5h. The rest was the same as in Example 1.

[0031] Experimental Example 1: The coal-based hard carbon anode materials prepared in Examples 1-3 and Comparative Examples 1-7 were tested as follows: The coal-based hard carbon anode material, conductive agent acetylene black, and binder (sodium carboxymethyl cellulose CMC and styrene-butadiene rubber SBR, mass ratio 1:1) prepared in the embodiments and comparative examples of this invention were mixed in a mass ratio of 8:1:1, using deionized water as solvent, and stirred in a vacuum mixer for more than 4 hours to form a uniform slurry. The slurry was then uniformly coated onto copper foil using a coater and placed in a 100°C forced-air drying oven for initial drying for 2 hours. Subsequently, it was transferred to a 120°C vacuum oven for drying for 12 hours to completely remove moisture. The electrode sheets were then punched into round sheets with a diameter of 12 mm using a punching machine, and the mass of each electrode was accurately weighed. The active material loading was 1.5 mg / cm³. 2 In an argon-filled glove box, a button cell was assembled using a sodium metal sheet as the counter electrode and reference electrode, a Whatman glass fiber membrane as the separator, a NaPF6 solution with a NaPF6 concentration of 1 mol / L, and diethylene glycol dimethyl ether as the solvent.

[0032] The assembled batteries were left to stand for 8 hours before testing, and then constant current charge-discharge tests were conducted to evaluate the material's reversible specific capacity, first-cycle coulombic efficiency, cycle stability, and rate performance.

[0033] First week of charge and discharge: Charge and discharge tests were conducted at 30mA / g within a voltage window of 0.01-2.5V, and the charging capacity and discharging capacity were recorded; Calculate the first-week coulombic efficiency: CE = (first-week charging capacity / first-week discharging capacity) × 100%; Cyclic performance testing: Under the same voltage window, perform continuous charge-discharge cycle testing at 100mA / g (e.g., 100 cycles) and record the capacity retention rate each week.

[0034] Rate performance test: On the same battery, charge and discharge tests are performed sequentially from low rate to high rate (e.g., 50, 100, 200, 500, 1000 mA / g), with 5-10 cycles at each rate. Finally, the battery is returned to the initial low rate to examine the capacity recovery ability and evaluate its high rate charge and discharge performance.

[0035] The results are shown in Table 1 below: Table 1

[0036] As discussed above, omitting the plasma activation step in Comparative Example 1 resulted in a significant decrease in the material's first-cycle discharge capacity and coulombic efficiency. This indicates that plasma activation is crucial for creating abundant active sites and defect structures on the coal powder surface, which can effectively enhance the material's sodium storage capacity and promote the formation of a stable SEI film. Unactivated coal powder exhibits low reactivity, leading to a comprehensive deterioration in electrochemical performance.

[0037] Compared to Example 1, the material prepared using pure nitrogen plasma in Comparative Example 2 showed a significant decrease in coulombic efficiency during the first cycle. This demonstrates the crucial role of hydrogen in plasma: hydrogen plasma possesses etching and reduction effects, enabling more effective generation of suitable microporous structures and modulation of surface chemistry, thereby reducing irreversible electrolyte decomposition during the first cycle and improving coulombic efficiency.

[0038] The excessively low plasma power in Comparative Example 3 resulted in insufficient activation, and the precursor failed to form a sufficient number of defects and pores. This manifested as lower capacity and coulombic efficiency compared to Example 1. This demonstrates that sufficient energy input is a necessary condition for effective activation; insufficient energy cannot adequately alter the physicochemical structure of the pulverized coal.

[0039] While the excessively high plasma power in Comparative Example 4 resulted in a higher first-cycle discharge capacity, it also led to the lowest coulombic efficiency and poor cycle stability. This indicates that excessive bombardment may damage the structure of the carbon precursor, generating too many unstable defects and an excessively large specific surface area, exacerbating electrolyte side reactions, and impairing the electrochemical compatibility of the material.

[0040] Comparative Example 5, which omitted the pre-oxidative crosslinking step, resulted in the worst overall performance, with significant decreases in capacity, efficiency, and cycle stability. This strongly demonstrates that pre-oxidative crosslinking is indispensable for constructing a stable and robust hard carbon framework during carbonization. Without this step, the carbon precursor cannot form a stable three-dimensional crosslinked network at high temperatures, leading to poor structural disorder, deteriorated conductivity, and severe side reactions.

[0041] In Comparative Example 6, without the addition of phytic acid crosslinking promoter, the first-week coulombic efficiency and cycling stability of the prepared material both decreased. This indicates that the crosslinking promoter can bridge coal molecules through its multifunctional molecular structure, strengthen the crosslinking network, thereby contributing to the formation of a more stable carbon structure, reducing irreversible capacity loss, and improving structural stability during long-term cycling.

[0042] Compared to Example 1, which employed gradient carbonization, Comparative Example 7 showed poorer cycling stability and rate performance in the material obtained through a one-step carbonization method. This indicates that the gradient carbonization strategy, which first stabilizes the framework at low temperatures and then refines the structure at medium temperatures, is beneficial for forming a more balanced pore structure and a more stable carbon matrix, thereby optimizing sodium ion transport kinetics and ensuring structural integrity during cycling. One-step high-temperature carbonization, on the other hand, may lead to rapid structural stabilization and defects.

[0043] Based on the test results of the above embodiments and comparative examples, the three core steps involved in this invention—plasma activation, pre-oxidative crosslinking, especially the use of crosslinking promoters and gradient carbonization—are indispensable and synergistic for the preparation of high-performance coal-based hard carbon anode materials. The prepared coal-based hard carbon material achieves a comprehensive balance of high reversible capacity, high first-cycle coulombic efficiency, and excellent cycling and rate performance, verifying the effectiveness of the preparation method of this invention.

[0044] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a coal-based hard carbon negative electrode material, characterized by the steps of include: (a) Raw material preparation steps: Provide low-rank coal raw materials, and crush and grind them; (b) Plasma activation step: Under an inert gas or nitrogen-containing atmosphere, the coal powder treated in step (a) is bombarded with plasma to obtain an activated precursor; (c) Pre-oxidative crosslinking treatment of the activated precursor; (d) Gradient carbonization treatment: Under the protection of an inert atmosphere, the activated precursor is subjected to programmed temperature-controlled carbonization treatment, which includes a first low-temperature heat preservation stage and a second medium-temperature heat preservation stage in sequence; finally, the coal-based hard carbon anode material is obtained.

2. The method according to claim 1, characterized in that, In step (a), the low-rank coal is lignite or long-flame coal; the particle size D50 of the coal powder is 5-20 μm.

3. The method for preparing a coal-based hard carbon anode material according to claim 1, characterized in that, In step (b), the plasma is a nitrogen-hydrogen mixed gas plasma under normal pressure, wherein the hydrogen volume ratio is 5%-20%; the plasma treatment power is 1-5 kW, the treatment temperature is 300-500℃, and the treatment time is 10-60 min.

4. The method for preparing a coal-based hard carbon anode material according to claim 1, characterized in that, In step (c), the oxidative crosslinking treatment is carried out in an air atmosphere, with a heating rate of 1-5℃ / min to 300-400℃, and then held at that temperature for 2-4 hours.

5. The method for preparing a coal-based hard carbon anode material according to claim 4, characterized in that, During the pre-oxidative crosslinking treatment, 10% to 20% by mass of a phosphorus-containing compound is incorporated into the coal-based porous carbon material precursor as a crosslinking promoter; the phosphorus-containing compound includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and phytic acid.

6. The method for preparing a coal-based hard carbon anode material according to claim 1, characterized in that, In step (d), the temperature of the first low-temperature insulation stage is 600-800℃ and the insulation time is 2-4h; the temperature of the second medium-temperature insulation stage is 800-1000℃ and the insulation time is 1-2h.

7. The method for preparing a coal-based hard carbon anode material according to claim 1, characterized in that, The heating rate from the first low-temperature insulation stage to the second medium-temperature insulation stage is 2-5℃ / min.

8. The method for preparing a coal-based hard carbon anode material according to claim 1, characterized in that, Step (d) is followed by step (e): cooling, ball milling and sieving the product after gradient carbonization to obtain the final product.

9. A coal-based hard carbon anode material, prepared by the preparation method of a coal-based hard carbon anode material according to any one of claims 1-8.

10. The preparation method of a coal-based hard carbon anode material according to any one of claims 1-8 or the application of a coal-based hard carbon anode material according to claim 9 in a sodium-ion battery.

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