Preparation method of coal-based hard carbon material and application of coal-based hard carbon material in negative electrode of sodium-ion battery

Abstract

The invention belongs to the technical field of sodium-ion batteries, and particularly relates to a preparation method of a coal-based hard carbon material and application of the coal-based hard carbon material in a sodium-ion battery negative electrode. The preparation method of the hard carbon material comprises the following steps: carrying out acid-base chemical heat treatment on a coal-based material to obtain an ash-removed and dried precursor material, and activating the material by water vapor to obtain a target sample with etched micropore inner gaps, and in the subsequent carbonization process, the carbon layer is twisted to form a closed pore structure to obtain the electrode material. According to the invention, the application of the rich-resource coal precursor and the high carbon conversion rate in the sodium ion battery is realized, and the storage capacity of sodium and the cycling stability of the material are effectively improved. The coal-based hard carbon material can be used for preparing a sodium ion battery and has important application value.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery negative electrode materials, and in particular to a method for preparing a high-performance hard carbon material by acid-base pretreatment of a coal-based precursor combined with water vapor activation, specifically to a method for preparing a coal-based hard carbon material and its application in the negative electrode of a sodium ion battery. Background Art

[0002] Faced with challenges such as global warming, environmental pollution, and fossil fuel shortages, the development and utilization of alternatives to non-renewable energy sources has become increasingly urgent. Utilizing clean, renewable energy sources such as solar, geothermal, tidal, and biomass is a crucial means of addressing climate change and energy shortages. However, these renewable energy sources are difficult to directly utilize in our daily lives. Therefore, developing sustainable, low-cost physical or electrochemical energy storage systems to collect and store renewable energy and ensure sustained power generation is crucial. The development of large-scale energy storage systems is a key cornerstone of the new energy revolution and the development and promotion of new energy sources, and the country has incorporated them into its overall strategic planning. Among various energy storage technologies, lithium-ion batteries (LIBs) stand out from other electrochemical energy storage devices due to their high energy density, fast conversion efficiency, and long cycle life. They are widely used in large-scale energy storage, electric vehicles, portable electronic devices, and other fields. However, due to the limited and uneven distribution of lithium resources, the use of organic electrolytes poses significant safety challenges, hindering their long-term development. Considering that sodium and lithium belong to the same element family and have similar chemical properties, sodium resources are abundant and low-cost, and the energy density and cycle stability of sodium-ion batteries are comparable to those of lithium-ion batteries, researchers have begun to invest in high-performance sodium-ion batteries based on the existing achievements of lithium-ion batteries.

[0003] Benefiting from the rapid development of the new energy industry, graphite materials have been applied to the market as negative electrode materials and have shown excellent performance in the commercial application of lithium-ion batteries. However, because the ionic radius of sodium ions is larger than that of lithium ions, sodium ions cannot smoothly enter the interlayer of traditional graphite, and it is difficult for sodium ions to form stable intercalation compounds with graphite, resulting in a low sodium storage capacity (capacity of only 30mAh g). -1 ), therefore, there is an urgent need to study a negative electrode material that can smoothly deintercalate and deintercalate sodium ions and has high capacity, excellent rate performance and stable cycle performance during the electrochemical process.

[0004] Coal-based materials, used in fields such as sodium-ion battery anode materials, present both advantages and challenges. Their abundant reserves and widespread global distribution provide resource security for large-scale production. However, due to their complex formation process and high impurity content, including metallic elements and harmful substances such as sulfur and nitrogen, impurity removal increases costs and process difficulty. Furthermore, coal's complex structure makes it difficult to precisely control the degree of graphitization. Excessive graphitization reduces the material's specific capacity, while moderate graphitization balances conductivity and sodium storage performance. Developing green and efficient impurity treatment, structure regulation, and activation technologies is key to promoting the development of coal-based materials. Summary of the Invention

[0005] In order to overcome the technical problems mentioned in the above background technology, the first purpose of the present invention is to provide a method for preparing coal-based hard carbon materials, which solves the problems of difficult impurity removal and easy graphitization of existing coal-based negative electrode materials.

[0006] A second object of the present invention is to provide the above-mentioned electrode material.

[0007] The third object of the present invention is to provide applications of the above-mentioned electrode material.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for preparing a coal-based hard carbon material comprises the following steps:

[0010] Step 1: Dry-mill the coal raw material in a ball mill to obtain fine coal powder with a particle size of less than 70 μm, so as to increase the contact area between the coal and the reagent in subsequent treatment and enhance the reaction effect;

[0011] Step 2: Soak the fine coal powder in an acidic solution at a specific solid-liquid ratio in an 80°C constant-temperature water bath for 2-4 hours. Continuous stirring ensures full contact between the acid and the coal, dissolving impurities such as metal oxides and carbonates in the coal. After the treatment, solid-liquid separation is performed using a vacuum filtration device, and the filter cake is repeatedly washed with deionized water until the pH reaches 7. The filter cake is then placed in a vacuum drying oven to remove any residual moisture.

[0012] Step 3: Add the dried acid-treated coal powder to the prepared alkaline solution and immerse it in a constant temperature water bath at 80°C for 2-4 hours. During the water bath heating process, the alkaline solution reacts with some organic matter in the coal to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, wait for the reactor to cool naturally to room temperature, take out the material for centrifugal separation, and wash it with deionized water several times until the pH value is about 7. Finally, place the material in a freeze dryer and dry it at -50-40°C and a vacuum degree of less than 10Pa;

[0013] Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to the activation temperature at a certain rate, and introduce water vapor for activation treatment. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to inert gas (such as nitrogen or argon) for protection. After the furnace temperature drops to room temperature, remove the material;

[0014] Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Inert gas is introduced into the box furnace as a protective gas for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. The carbonized coal-based hard carbon material is taken out, ground, crushed and sieved, and then can be used for the preparation of sodium ion battery negative electrode materials.

[0015] The above method prepares a coal-based hard carbon electrode material, and the electrode material can be used in the field of sodium ion batteries.

[0016] As a preferred embodiment of the present invention, the rotation speed of the ball mill in step 1 is 300-500 r / min, and the grinding time is 2-4 hours.

[0017] As a preferred embodiment of the present invention, the solid-to-liquid ratio in step 2 is 1:5-1:10.

[0018] As a preferred embodiment of the present invention, the acid solution in step 2 is 3-10 mol / L acetic acid, hydrofluoric acid, sulfuric acid, or a mixed acid solution thereof, and the immersion treatment is performed in a constant temperature water bath at 60-80°C for 2-4 hours.

[0019] As a preferred embodiment of the present invention, the solid-liquid ratio in step three is 1:8-1:15, and the concentration of the alkaline solution is 2-6 mol / L sodium hydroxide or potassium hydroxide solution.

[0020] As a preferred embodiment of the present invention, the heating rate in step 4 is 1-5°C / min, the activation temperature is 400-1000°C, and the water vapor flow rate is 50-100 mL / min to carry out the activation treatment for 2-8 hours.

[0021] As a preferred embodiment of the present invention, the protective gas in step 4 is an inert hydrogen-argon mixture gas (1%:99%).

[0022] As a preferred embodiment of the present invention, the inert gas in step five is hydrogen or nitrogen, the heating rate is 5-10°C / min, the carbonization temperature is 1000-1400°C, and the carbonization treatment time is 2-6 hours.

[0023] The coal-based hard carbon electrode material prepared by the above method can be used in the field of sodium ion batteries.

[0024] The beneficial effect of the present invention is that by innovatively integrating the intrinsic structural advantages of coal-based precursors and a multi-stage modification process, the industry's difficult problem of closed-pore control and synergistic optimization of electrochemical properties of hard carbon materials has been successfully solved.

[0025] Specifically:

[0026] 1. Closed-pore directional construction: Taking full advantage of the aromatic condensed ring structure of coal, under gradient acid-base treatment (HCl-HF composite pickling combined with NaOH hydrothermal reconstruction), the ash removal rate is >98%, and graphite-like microcrystals with short-range ordered characteristics (0.30nm<d 002 <0.42nm), combined with a two-stage steam activation (600℃ pore expansion + 1000℃ closed-pore molding), a bimodal pore structure (0.5-2nm mesopores and 1-3nm closed-pores coordinated distribution) is precisely constructed, with a closed-pore density of 45-55% (SAXS determination), which is more than 20% higher than that of biomass hard carbon;

[0027] 2. Improved electrochemical performance: Thanks to the unique arrangement of graphite microcrystals in coal-based hard carbon, the sodium storage platform voltage is stable in the range of 0.1-0.3V (vs. Na + / Na), while maintaining a high specific capacity (350-380mAh / g at 0.1C), significantly alleviating the risk of sodium dendrite growth in the low-voltage region, and for the first time exceeding 90% Coulombic efficiency (a 10-15% improvement over traditional processes);

[0028] 3. Coal Defect Conversion – For high-sulfur / high-ash coals, a pre-oxidation-pickling synergistic process (pre-oxidation at 200°C converts sulfur into soluble sulfate, with a combined pickling removal rate of >95%), combined with free radical recombination kinetics (pulsating H2O concentration during the steam activation phase to suppress excessive graphitization of the carbon layer), transforms impurity interference into closed-cell nucleation sites. This process combines environmental friendliness (water vapor replaces KOH activation, reducing wastewater emissions by 70%) with cost advantages (raw material costs are 40% lower than those of biomass systems), providing an innovative solution for the industrialization of sodium battery anodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0030] Figure 1 The X-ray diffraction pattern of the electrode material prepared in Example 1;

[0031] Figure 2 This is a scanning electron microscope image of the front electrode material prepared in Example 1;

[0032] Figure 3 This is a transmission electron microscope image of the electrode material prepared in Example 1;

[0033] Figure 4 This is the full rate discharge diagram of the electrode material prepared in Example 1;

[0034] Figure 5 Statistical graph of the charge and discharge capacity of the sodium ion batteries prepared in Examples 1-3 and Comparative Example 1 at a current density of 20 mA / g. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] Example 1:

[0037] See also Figure 1-Figure 5 As shown, a method for preparing a coal-based hard carbon material comprises:

[0038] Step 1: Shanxi anthracite (82% fixed carbon, 5.2% ash, 0.8% sulfur) was ball milled at 400 rpm for 3 h to obtain fine coal powder with a particle size of D50≈65 μm. This increased the contact area between the coal and the reagents in subsequent treatments and enhanced the reaction effect.

[0039] Step 2: Soak the above-mentioned fine coal powder in a mixed acidic solution of 6mol / L HCl+4mol / L HF (volume ratio 3:1) at a solid-liquid ratio of 1:8, and carry out an immersion treatment in a constant temperature water bath at 80°C for 3 hours. During the period, continuous stirring is carried out to ensure full contact between the acid solution and the coal, and to directionally dissolve impurities such as metal oxides and carbonates in the coal. After the treatment, solid-liquid separation is carried out through a vacuum filtration device, and the filter cake is repeatedly washed with deionized water to a pH value of 7. The filter cake is then placed in a vacuum drying oven to remove residual moisture;

[0040] Step 3: Add the dried acid-treated coal powder to a 4mol / L NaOH alkaline solution at a solid-liquid ratio of 1:10, and immerse it in a constant temperature water bath at 80°C for 3 hours. During the water bath heating process, the alkaline solution reacts with part of the organic matter in the coal to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, wait for the reactor to cool naturally to room temperature, take out the material for centrifugal separation, and wash it with deionized water several times until the pH value is about 7. Finally, place the material in a freeze dryer and dry it at -50°C and a vacuum degree of less than 10Pa;

[0041] Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to an activation temperature of 800°C at a rate of 3°C / min, and introduce water vapor for activation treatment for 6 hours. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to pure Ar gas (flow rate 1L / min) for protection. After the furnace temperature drops to room temperature, take out the material;

[0042] Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Introduce nitrogen into the box furnace as a protective gas for carbonization. Raise the temperature to 1200°C at 8°C / min and keep it warm for 4 hours for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it with a planetary ball mill (300 rpm, 1h), and pass it through a 300-mesh sieve to obtain the final coal-based hard carbon material, which can be used for the preparation of sodium ion battery negative electrode materials.

[0043] Example 2:

[0044] Step 1: Shanxi anthracite (82% fixed carbon, 5.2% ash, 0.8% sulfur) was ball milled at 400 rpm for 3 h to obtain fine coal powder with a particle size of D50≈65 μm. This increased the contact area between the coal and the reagents in subsequent treatments and enhanced the reaction effect.

[0045] Step 2: The fine coal powder is immersed in a mixed acidic solution of 8 mol / L HCl and 6 mol / L HF (3:1 by volume) at a solid-liquid ratio of 1:8 in an 80°C water bath for 3 hours. Continuous stirring is performed to ensure full contact between the acid and the coal. After the treatment, solid-liquid separation is performed using a vacuum filtration device. The filter cake is repeatedly washed with deionized water to a pH of 7, and then placed in a vacuum drying oven to remove any residual moisture.

[0046] Step 3: The dried, acid-treated coal powder is added to a 4 mol / L NaOH alkaline solution at a solid-liquid ratio of 1:10 and immersed in an 80°C constant-temperature water bath for 3 hours to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, the reactor is naturally cooled to room temperature, the material is removed and centrifuged, and washed with deionized water multiple times to a pH of approximately 7. Finally, the material is placed in a freeze dryer and dried at -50°C and a vacuum degree of less than 10Pa.

[0047] Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to an activation temperature of 800°C at a rate of 3°C / min, and introduce water vapor for activation treatment for 6 hours. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to pure Ar gas (flow rate 1L / min) for protection. After the furnace temperature drops to room temperature, take out the material;

[0048] Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Introduce nitrogen into the box furnace as a protective gas for carbonization. Raise the temperature to 1200°C at 8°C / min and keep it warm for 4 hours for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it with a planetary ball mill (300 rpm, 1h), and pass it through a 300-mesh sieve to obtain the final coal-based hard carbon material, which can be used for the preparation of sodium ion battery negative electrode materials.

[0049] Example 3:

[0050] Step 1: Shanxi anthracite (82% fixed carbon, 5.2% ash, 0.8% sulfur) was ball milled at 400 rpm for 3 h to obtain fine coal powder with a particle size of D50≈65 μm. This increased the contact area between the coal and the reagents in subsequent treatments and enhanced the reaction effect.

[0051] Step 2: The fine coal powder is immersed in an 8 mol / L H2SO4 acidic solution at a solid-to-liquid ratio of 1:8 in an 80°C water bath for 3 hours. Continuous stirring is performed to ensure full contact between the acid and the coal. After the treatment, solid-liquid separation is performed using a vacuum filtration device. The filter cake is repeatedly washed with deionized water to a pH of 7 and then placed in a vacuum drying oven to remove any residual moisture.

[0052] Step 3: The dried, acid-treated coal powder is added to a 2 mol / L NaOH alkaline solution at a solid-liquid ratio of 1:10 and immersed in an 80°C constant-temperature water bath for 3 hours to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, the reactor is naturally cooled to room temperature, the material is removed and centrifuged, and washed with deionized water multiple times to a pH of approximately 7. Finally, the material is placed in a freeze dryer and dried at -50°C and a vacuum degree of less than 10 Pa.

[0053] Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to an activation temperature of 800°C at a rate of 3°C / min, and introduce water vapor for activation treatment for 6 hours. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to pure Ar gas (flow rate 1L / min) for protection. After the furnace temperature drops to room temperature, take out the material;

[0054] Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Introduce nitrogen into the box furnace as a protective gas for carbonization. Raise the temperature to 1200°C at 8°C / min and keep it warm for 4 hours for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it with a planetary ball mill (300 rpm, 1h), and pass it through a 300-mesh sieve to obtain the final coal-based hard carbon material, which can be used for the preparation of sodium ion battery negative electrode materials.

[0055] Comparative Example 1: No acid treatment

[0056] Step 1: Shanxi anthracite (82% fixed carbon, 5.2% ash, 0.8% sulfur) was ball milled at 400 rpm for 3 h to obtain fine coal powder with a particle size of D50≈65 μm. This increased the contact area between the coal and the reagents in subsequent treatments and enhanced the reaction effect.

[0057] Step 2: Add the dried acid-treated coal powder to a 4mol / L NaOH alkaline solution at a solid-liquid ratio of 1:10, and immerse it in a constant temperature water bath at 80°C for 3 hours. During the water bath heating process, the alkaline solution reacts with part of the organic matter in the coal to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, wait for the reactor to cool naturally to room temperature, take out the material for centrifugal separation, and wash it with deionized water several times until the pH value is about 7. Finally, place the material in a freeze dryer and dry it at -50°C and a vacuum degree of less than 10Pa;

[0058] Step 3: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to an activation temperature of 800°C at a rate of 3°C / min, and introduce water vapor for activation treatment for 6 hours. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to pure Ar gas (flow rate 1L / min) for protection. After the furnace temperature drops to room temperature, remove the material;

[0059] Step 4: Transfer the activated material to a box furnace and place it in an inert atmosphere. Introduce nitrogen into the box furnace as a protective gas for carbonization. Raise the temperature to 1200°C at 8°C / min and keep it warm for 4 hours for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it with a planetary ball mill (300rpm, 1h), and pass it through a 300-mesh sieve to obtain the final coal-based hard carbon material, which can be used for the preparation of sodium ion battery negative electrode materials.

[0060] Comparative Example 2: CO2 instead of water vapor

[0061] Step 1: Shanxi anthracite (82% fixed carbon, 5.2% ash, 0.8% sulfur) was ball milled at 400 rpm for 3 h to obtain fine coal powder with a particle size of D50≈65 μm. This increased the contact area between the coal and the reagents in subsequent treatments and enhanced the reaction effect.

[0062] Step 2: Soak the above-mentioned fine coal powder in a mixed acidic solution of 6mol / L HCl+4mol / L HF (volume ratio 3:1) at a solid-liquid ratio of 1:8, and carry out an immersion treatment in a constant temperature water bath at 80°C for 3 hours. During the period, continuous stirring is carried out to ensure full contact between the acid solution and the coal, and to directionally dissolve impurities such as metal oxides and carbonates in the coal. After the treatment, solid-liquid separation is carried out through a vacuum filtration device, and the filter cake is repeatedly washed with deionized water to a pH value of 7. The filter cake is then placed in a vacuum drying oven to remove residual moisture;

[0063] Step 3: Add the dried acid-treated coal powder to a 4mol / L NaOH alkaline solution at a solid-liquid ratio of 1:10, and immerse it in a constant temperature water bath at 80°C for 3 hours. During the water bath heating process, the alkaline solution reacts with part of the organic matter in the coal to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, wait for the reactor to cool naturally to room temperature, take out the material for centrifugal separation, and wash it with deionized water several times until the pH value is about 7. Finally, place the material in a freeze dryer and dry it at -50°C and a vacuum degree of less than 10Pa;

[0064] Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to an activation temperature of 800°C at a rate of 3°C / min, and introduce CO2 for activation treatment for 6 hours. Water vapor undergoes redox reaction with the carbon material at high temperature to form a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After activation, stop introducing water vapor and switch to pure Ar gas (flow rate 1L / min) for protection. After the furnace temperature drops to room temperature, take out the material;

[0065] Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Introduce nitrogen into the box furnace as a protective gas for carbonization. Raise the temperature to 1200°C at 8°C / min and keep it warm for 4 hours for carbonization. After the carbonization is completed, the furnace body naturally cools to room temperature, and the coal-based material is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it with a planetary ball mill (300 rpm, 1h), and pass it through a 300-mesh sieve to obtain the final coal-based hard carbon material, which can be used for the preparation of sodium ion battery negative electrode materials.

[0066] Comparative Example 3:

[0067] Reduce the carbonization temperature in step 5 to 800°C

[0068] The correlation between the process parameters and performance of Examples 1-3 and Comparative Examples 1-3 is shown in the following table:

[0069] Group Key process differences Corresponding performance impact Example 1 Composite pickling + steam activation High closed cell ratio increases platform capacity Example 2 Pre-oxidation + enhanced pickling Sulfur residue, improved cycle stability Example 3 <![CDATA[H2SO4 substitution for HF, lye degradation]]> Cost reduction and decreased closed-cell rate Comparative Example 1 Acid-free treatment The ash residue increases and the specific capacity decreases Comparative Example 2 <![CDATA[Pore coarsening caused by CO2 activation]]> The proportion of mesopores increases but the closed-pore ratio decreases Comparative Example 3 Low-temperature carbonization causes interlayer spacing to shrink Sodium ion diffusion is hindered

[0070] Referring to the data in the table above, and comparing Examples 1-3 with Comparative Examples 1-3, we can see that gradient desulfurization / ash pretreatment can adapt to high-sulfur, high-ash coals, providing a new path for the clean utilization of coal resources. Steam activation replaces chemical etching processes, reducing pollution emissions and further lowering process costs. Acid and alkali cleaning and steam activation achieve synergistic optimization of high closed pores and interlayer spacing.

[0071] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0072] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in similar ways. As long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a coal-based hard carbon material, characterized in that: The following steps are involved: Step 1: dry-milling the coal raw material in a ball mill to obtain coal fine powder with a particle size of less than 70 μm; Step 2: The fine coal powder is immersed in an acidic solution at a certain solid-liquid ratio and immersed in a constant temperature water bath at 80°C for 2-4 hours. During the immersion process, the acid solution is continuously stirred to ensure full contact with the coal and to dissolve the metal oxides and carbonate impurities in the coal. After the treatment, the solid-liquid separation is carried out using a vacuum filtration device, and the filter cake is repeatedly washed with deionized water to a pH of 7. The filter cake is then placed in a vacuum drying oven to remove residual moisture. Step 3: Add the dried acid-treated coal powder to the prepared alkaline solution and immerse it in a constant temperature water bath at 80°C for 2-4 hours. During the water bath heating process, the alkaline solution reacts with some organic matter in the coal to further remove impurities and adjust the microstructure of the coal-based material. After the reaction is completed, the reactor is naturally cooled to room temperature, the material is taken out for centrifugal separation, and washed with deionized water several times to a pH value of about 7. Finally, the material is placed in a freeze dryer and dried at -50-40°C and a vacuum degree of less than 10Pa; Step 4: Place the coal-based material after acid and alkali impregnation treatment in a reactor, heat it to the activation temperature at a certain rate, and introduce water vapor for activation treatment. The water vapor undergoes an oxidation-reduction reaction with the carbon material at high temperature, forming a rich microporous and mesoporous structure, thereby increasing the specific surface area and porosity of the material. After the activation is completed, stop introducing water vapor and switch to inert gas protection. After the furnace temperature drops to room temperature, remove the material. Step 5: Transfer the activated material to a box furnace and place it in an inert atmosphere. Inert gas is introduced into the box furnace as a protective gas for carbonization. After carbonization is completed, the furnace body naturally cools to room temperature to obtain a coal-based material, which is further carbonized into a hard carbon material with a stable structure. Take out the carbonized coal-based hard carbon material, grind it, and sieve it, and then use it for the preparation of sodium ion battery negative electrode materials.

2. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The ball mill in step 1 has a rotation speed of 300-500 r / min and a grinding time of 2-4 hours.

3. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The solid-to-liquid ratio in step 2 is 1:5-1:

10.

4. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The acid solution in step 2 is 3-10 mol / L acetic acid, hydrofluoric acid, sulfuric acid or a mixed acid solution thereof, and the immersion treatment is carried out in a constant temperature water bath at 60-80° C. for 2-4 hours.

5. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The solid-liquid ratio in step 3 is 1:8-1:15, and the concentration of the alkaline solution is 2-6 mol / L sodium hydroxide or potassium hydroxide solution.

6. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The heating rate in step 4 is 1-5°C / min, the activation temperature is 400-1000°C, and the water vapor flow rate is 50-100 mL / min for activation treatment for 2-8 hours.

7. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: The protective gas in step 4 is an inert hydrogen-argon mixture with a ratio of 1%:99%.

8. The method for preparing a coal-based hard carbon material according to claim 1, characterized in that: In step 5, the inert gas is hydrogen or nitrogen, the heating rate is 5-10°C / min, the carbonization temperature is 1000-1400°C, and the carbonization treatment time is 2-6 hours.

9. A coal-based hard carbon material, characterized in that: The compound is obtained by the preparation method according to any one of claims 1 to 8.

10. The use of a coal-based hard carbon material according to claim 9, characterized in that: Application as negative electrode in sodium ion batteries.

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