Method for preparing coal-based carbon material by molten salt electrochemical method and coal-based carbon material

Abstract

The invention relates to the technical field of battery materials, in particular to a method for preparing a coal-based carbon material through a molten salt electrochemical method and the coal-based carbon material. The invention provides a method for preparing a coal-based carbon material by a molten salt electrochemical method, which comprises the following steps: step 1, uniformly mixing pulverized coal and nickel chloride hexahydrate, and tabletting to obtain a mixed sheet body; step 2, wrapping the mixed sheet body in foamed nickel to obtain a cathode material; 3, heating the chlorine metal salt mixture to be molten, and immersing the cathode material and the anode material into the molten chlorine metal salt mixture for preheating; 4, voltage is applied between the anode material and the cathode material for electrolysis, and the coal-based carbon material is obtained on the surface of the foamed nickel. The embodiment of the invention provides a method for preparing a coal-based carbon material by a molten salt electrochemical method and the coal-based carbon material, and can provide the coal-based carbon material for electric energy storage.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and particularly relates to a method for preparing coal-based carbon materials by molten salt electrochemistry and the coal-based carbon materials. Background Art

[0002] As the main application mode of coal resources, coal-fired power generation has long faced severe environmental challenges. The nitrogen oxides (NO x ), sulfur oxides (SO x ), CO2 and volatile organic pollutants (VOCs) released during its combustion process will not only cause air pollution, but also trigger other environmental problems such as acid rain, photochemical smog, global warming and an increase in extreme weather. In addition, some volatile organic compounds are carcinogenic.

[0003] In view of the above deficiencies, there is an urgent need for a new way of coal utilization. Summary of the Invention

[0004] Embodiments of the present invention provide a method for preparing coal-based carbon materials by molten salt electrochemistry and the coal-based carbon materials, which can provide a coal-based carbon material for electric energy storage.

[0005] In a first aspect, the present invention provides a method for preparing coal-based carbon materials by molten salt electrochemistry, and the method includes the following steps:

[0006] Step 1: Mix pulverized coal and nickel chloride hexahydrate, and after mixing evenly, press into tablets to obtain a mixed tablet;

[0007] Step 2: Wrap the mixed tablet in nickel foam to obtain a cathode material;

[0008] Step 3: Heat the chloro-metal salt mixture to melting, and immerse the cathode material and the anode material in the molten chloro-metal salt mixture for preheating;

[0009] Step 4: Apply a voltage between the anode material and the cathode material for electrolysis to obtain coal-based carbon materials on the surface of the nickel foam.

[0010] In a possible design, the chloro-metal salt mixture includes LiCl, KCl and CaCl2.

[0011] In a possible design, the molar ratio of LiCl, KCl and CaCl2 is 0.4207:0.4843:0.0949.

[0012] In a possible design, in Step 1, 0.2 - 1 mmol of nickel chloride hexahydrate is added to every 0.1 g of pulverized coal.

[0013] In a possible design, in step one, 0.25 mmol of nickel chloride hexahydrate is added to every 0.1 g of pulverized coal.

[0014] In a possible design, in step four, the electrolysis temperature is 460 - 650 °C.

[0015] In a possible design, in step four, the electrolysis temperature is 550 °C.

[0016] In a possible design, in step four, the electrolysis voltage applied between the anode material and the cathode material is 2.6 - 3.4 V.

[0017] In a possible design, in step four, the electrolysis time is 1 - 5 h.

[0018] In a second aspect, the embodiments of the present invention further provide a coal-based carbon material prepared according to any one of the above methods.

[0019] The present invention has at least the following beneficial effects compared with the prior art:

[0020] In this embodiment, with molten salt as the reaction system, steam coal is converted into a carbon material by applying an external voltage, and it is applied to the electrode material of a supercapacitor. The possible reaction mechanisms involved in the molten salt electrolysis process are as follows:

[0021] Pyrolysis of molten salt: High temperature causes volatile components in coal (such as H2O, CO, CH4, etc.) to escape, leaving a porous carbon skeleton.

[0022] Template effect of molten salt: Migration and dynamic adsorption of ions (such as Li + , Ca 2+ , K + ) in the molten salt may form a temporary pore template on the carbon surface, and then the metal ions are removed by electrolysis later, retaining the pore structure in the carbon.

[0023] Etching effect of molten salt: Cl- in the molten salt forms micropores and mesoporous structures on the carbon surface through electrochemical etching, significantly increasing the specific surface area and providing relatively abundant ion active sites for the carbon material in energy storage applications.

[0024] Cathode material (including coal): The organic matter in coal undergoes dehydrogenation and deoxidation to finally form elemental carbon. At the same time, Ni 2+ produced by the decomposition of NiCl2·6H2O is reduced to metallic Ni nanoparticles. These Ni particles act as catalysts to promote subsequent carbon atom rearrangement, thereby causing changes in the morphology of the carbon material. Li + , K + , Ca 2+ and other ions are reduced to metallic elements.

[0025] Molten salt electrolysis inhibits the graphitization process through the synergistic effects of electrochemical reduction deconstruction, Ni single - element catalyzed rearrangement, Cl - chemical etching, and high - temperature pyrolysis, promoting carbon to be arranged in a disordered sp 2 / sp 3 hybridization, forming an amorphous structure, thereby converting steam coal into a disordered carbon material with a high specific surface area. This process combines electrochemical, catalytic, and high - temperature pyrolysis mechanisms, providing a new idea for the high - value utilization of coal - based carbon materials. Description of the Drawings

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0027] Figure 1 It is a comparative cyclic voltammogram of an electrode material provided by the present invention in a three - electrode system within the voltage range of - 1 to 0 V at a scanning rate of 20 mV s-1;

[0028] Figure 2a It is a comparative GCD curve of different electrode materials at a current density of 5 A·g within the voltage range of - 1 to 0 V provided by the present invention; -1

[0029] Figure 2b It is a comparative GCD curve of different electrode materials at a current density of 0.5 A·g within the voltage range of - 1 to 0 V provided by the present invention; -1

[0030] Figure 3 It is a relationship diagram of specific capacitance and current density of the present invention in a three - electrode system within the voltage range of - 1 to 0 V;

[0031] Figure 4 It is a Nyquist diagram and an equivalent circuit diagram provided by the present invention, and the inset is a partial enlarged view of the high - frequency region;

[0032] Figure 5 It is a comparative cyclic voltammogram of another electrode material provided by the present invention in a three - electrode system within the voltage range of - 1 to 0 V at a scanning rate of 20 mVs-1;

[0033] Figure 6a It is a comparative GCD curve of different electrode materials at a current density of 1 A·g within the voltage range of - 1 to 0 V provided by the present invention; -1 ​​​

[0034] Figure 6b It is a comparison diagram of GCD curves of different electrode materials at a current density of 0.1 A·g within the voltage range of -1 to 0 V provided by the present invention; -1

[0035] Figure 7 It is another relationship diagram between specific capacitance and current density within the voltage range of -1 to 0 V in a three-electrode system provided by the present invention;

[0036] Figure 8 It is another Nyquist diagram and equivalent circuit diagram provided by the present invention, and the inset is a partial enlarged view of the high-frequency region;

[0037] Figure 9 It is a comparison diagram of cyclic voltammograms of another electrode material within the voltage range of -1 to 0 V in a three-electrode system at a scanning rate of 20 mV s-1;

[0038] Figure 10a It is another comparison diagram of GCD curves of different electrode materials at a current density of 1 A·g within the voltage range of -1 to 0 V provided by the present invention; -1

[0039] Figure 10b It is another comparison diagram of GCD curves of different electrode materials at a current density of 0.1 A·g within the voltage range of -1 to 0 V provided by the present invention; -1

[0040] Figure 11 It is another relationship diagram between specific capacitance and current density within the voltage range of -1 to 0 V in a three-electrode system provided by the present invention;

[0041] Figure 12 It is another Nyquist diagram and equivalent circuit diagram provided by the present invention, and the inset is a partial enlarged view of the high-frequency region;

[0042] Figure 13 It is another comparison diagram of cyclic voltammograms of an electrode material within the voltage range of -1 to 0 V in a three-electrode system at a scanning rate of 20 mV s -1 ;

[0043] Figure 14a It is another comparison diagram of GCD curves of different electrode materials at a current density of 1 A·g within the voltage range of -1 to 0 V provided by the present invention; -1

[0044] Figure 14b It is another comparison diagram of GCD curves of different electrode materials at a current density of 0.1 A·g within the voltage range of -1 to 0 V provided by the present invention; -1 ​​​​​

[0045] Figure 15 It is another graph showing the relationship between specific capacitance and current density in the voltage range of -1 to 0V in a three-electrode system provided by the present invention;

[0046] Figure 16 It is another Nyquist plot and equivalent circuit diagram provided by the present invention, and the inset is a partially enlarged view of the high-frequency region;

[0047] Figure 17 It is another comparison graph of cyclic voltammograms of another electrode material provided by the present invention in a three-electrode system in the voltage range of -1 to 0V at a scanning rate of 20mVs -1 ;

[0048] Figure 18a It is another comparison graph of GCD curves of different electrode materials at a current density of 1A·g in the voltage range of -1 to 0V provided by the present invention; -1

[0049] Figure 18b It is another comparison graph of GCD curves of different electrode materials at a current density of 0.1A·g in the voltage range of -1 to 0V provided by the present invention; -1

[0050] Figure 19 It is another graph showing the relationship between specific capacitance and current density in the voltage range of -1 to 0V in a three-electrode system provided by the present invention;

[0051] Figure 20 It is another Nyquist plot and equivalent circuit diagram provided by the present invention, and the inset is a partially enlarged view of the high-frequency region;

[0052] Figure 21 It is another comparison graph of cyclic voltammograms of another electrode material provided by the present invention in a three-electrode system in the voltage range of -1 to 0V at a scanning rate of 20mVs -1 ;

[0053] Figure 22 It is a comparison graph of GCD curves of different electrode materials at a current density of 1A·g in the voltage range of -1 to 0V provided by the present invention; -1

[0054] Figure 23 It is another comparison graph of cyclic voltammograms of another electrode material provided by the present invention in a three-electrode system in the voltage range of -1 to 0V at a scanning rate of 20mVs -1 ;

[0055] Figure 24a It is another comparison graph of GCD curves at a current density of 1A·g in the voltage range of -1 to 0V provided by the present invention; -1Comparison diagram of GCD curves of different electrode materials at a current density;

[0056] Figure 24b Another one provided by the present invention is within the voltage range of -1 to 0V and at a current density of 0.1A·g -1 Comparison diagram of GCD curves of different electrode materials at a current density;

[0057] Figure 25 Another one provided by the present invention is a relationship diagram of specific capacitance and current density within the voltage range of -1 to 0V in a three-electrode system;

[0058] Figure 26 Another one provided by the present invention is a Nyquist diagram and an equivalent circuit diagram, and the inserted figure is a partial enlarged view of the high-frequency region;

[0059] Figure 27 Another one provided by the present invention is a comparison diagram of cyclic voltammograms of an electrode material within the voltage range of -1 to 0V in a three-electrode system at a scanning rate of 20mVs -1 ;

[0060] Figure 28a Another one provided by the present invention is within the voltage range of -1 to 0V and at a current density of 1A·g -1 Comparison diagram of GCD curves of different electrode materials at a current density;

[0061] Figure 28b Another one provided by the present invention is within the voltage range of -1 to 0V and at a current density of 0.1A·g -1 Comparison diagram of GCD curves of different electrode materials at a current density;

[0062] Figure 29 Another one provided by the present invention is a relationship diagram of specific capacitance and current density within the voltage range of -1 to 0V in a three-electrode system;

[0063] Figure 30 Another one provided by the present invention is a Nyquist diagram and an equivalent circuit diagram, and the inserted figure is a partial enlarged view of the high-frequency region. Detailed implementation manners

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts in the embodiments of the present invention belong to the scope of protection of the present invention.

[0065] Please refer to Figure 1To Figure 2, the present invention provides a method for preparing coal-based carbon materials by molten salt electrochemistry, and the method comprises the following steps:

[0066] Step 1: Mix pulverized coal and nickel chloride hexahydrate, and after mixing evenly, press into tablets to obtain a mixed tablet;

[0067] Step 2: Wrap the mixed tablet in nickel foam to obtain a cathode material;

[0068] Step 3: Heat the chloro-metal salt mixture to melting, and immerse the cathode material and the anode material in the molten chloro-metal salt mixture for preheating;

[0069] Step 4: Apply a voltage between the anode material and the cathode material for electrolysis to obtain coal-based carbon materials on the surface of the nickel foam.

[0070] In this embodiment, using a molten salt as a reaction system, steam coal is converted into carbon materials by applying an external voltage, and it is applied to the electrode materials of supercapacitors. The possible reaction mechanisms involved in the molten salt electrolysis process are as follows:

[0071] Pyrolysis of molten salt: High temperature promotes the volatilization of volatile components in coal (such as H2O, CO, CH4, etc.), leaving a porous carbon skeleton.

[0072] Template effect of molten salt: Migration and dynamic adsorption of ions (such as Li + , Ca 2+ , K + ) in the molten salt may form a temporary pore template on the carbon surface, and then the metal ions are removed by electrolysis later, retaining the pore structure in the carbon.

[0073] Etching effect of molten salt: Cl- in the molten salt forms micropores and mesoporous structures on the carbon surface through electrochemical etching, significantly increasing the specific surface area and providing relatively abundant ion active sites for the carbon materials during energy storage applications.

[0074] Cathode material (including coal): The organic matter in coal undergoes dehydrogenation and deoxidation to finally form elemental carbon. At the same time, Ni decomposed from NiCl2·6H2O 2+ is reduced to metallic Ni nanoparticles. These Ni particles act as catalysts to promote the subsequent rearrangement of carbon atoms, thus causing changes in the morphology of the carbon materials. Li + , K + , Ca 2+ and other ions are reduced to metallic elements.

[0075] Molten salt electrolysis inhibits the graphitization process through the synergistic effects of electrochemical reduction deconstruction, Ni single-catalyst rearrangement, Cl - chemical etching, and high-temperature pyrolysis, promoting carbon to be in the sp 2 / sp3 Hybrid disordered arrangement forms an amorphous structure, thus converting steam coal into a disordered carbon material with a high specific surface area. This process combines electrochemical, catalytic, and high-temperature pyrolysis mechanisms, providing new ideas for the high-value utilization of coal-based carbon materials.

[0076] In some embodiments of the present invention, the metal chloride mixture includes LiCl, KCl, and CaCl2.

[0077] In this embodiment, the metal chloride can include a combination of various different salts. For example, it can be CaCl2-NaCl-KCl (0.51503:0.40522:0.07975, mol%), LiCl-KCl-NaCl (0.4207:0.4843:0.0949, mol%), LiCl-KCl-CaCl2 (0.52893:0.42422:0.04685, mol%), LiCl-KCl (0.595:0.405, mol%), and CaCl2-KCl (0.25:0.75, mol%). Different salt compositions result in different electrochemical properties of the coal-based carbon materials obtained by electrolysis. Among them, the coal-based carbon materials prepared from the molten salt system obtained by melting LiCl, KCl, and CaCl2 have the best electrochemical properties.

[0078] In some embodiments of the present invention, in step one, 0.2 - 1 mmol of nickel chloride hexahydrate is added to every 0.1 g of pulverized coal.

[0079] In this embodiment, the electrochemical properties of the coal-based carbon materials obtained by electrolysis of the cathode materials prepared with different ratios of pulverized coal and nickel chloride hexahydrate are different. Among them, the coal-based carbon materials obtained by electrolysis of the cathode material prepared by adding 0.25 mmol of nickel chloride hexahydrate to every 0.1 g of pulverized coal have the best electrochemical properties.

[0080] In some embodiments of the present invention, in step four, the electrolysis temperature is 460 - 650 °C.

[0081] In this embodiment, when the electrolysis temperature is 550 °C, the coal-based carbon materials obtained have the best electrochemical properties.

[0082] In some embodiments of the present invention, in step four, the electrolysis voltage applied between the anode material and the cathode material is 2.6 - 3.4 V.

[0083] In this embodiment, when the voltage is 3.2 V, the coal-based carbon materials obtained have the best electrochemical properties.

[0084] In some embodiments of the present invention, in step four, the electrolysis time is 1 - 5 h.

[0085] In this embodiment, when the electrolysis time is 3 hours, the electrochemical performance of the obtained coal-based carbon material is the best.

[0086] In some embodiments of the present invention, before step 1, the method further includes:

[0087] The coal powder is immersed in water for hydrothermal treatment.

[0088] In a high-temperature and high-pressure water environment, water molecules can penetrate into the microstructure of coal and trigger hydrolysis reactions of oxygen-containing functional groups (such as carboxylic acid groups, phenolic hydroxyl groups, ether bonds, etc.). These functional groups usually need to be decomposed at higher temperatures (above 300°C) in untreated coal, but hydrothermal treatment causes them to break in advance, resulting in a reduction in the energy required for the release of volatiles during subsequent pyrolysis. Therefore, the maximum weight loss temperature is shifted forward. The cross-linked structure in coal (such as the bridge bonds between aromatic rings) may break under hydrothermal conditions, causing the macromolecular network structure of coal to become loose. This structural loosening reduces the activation energy of chemical bond breaking during pyrolysis, making it easier for volatiles to be released at lower temperatures. Hydrothermal treatment can dissolve some minerals or organic matter in coal, increase porosity and specific surface area. This improves the heat and mass transfer efficiency during pyrolysis and reduces the diffusion resistance of volatiles, thereby accelerating the pyrolysis reaction and reducing the maximum weight loss temperature.

[0089] In some embodiments of the present invention, the hydrothermal treatment time is 3 to 30 hours.

[0090] In this embodiment, the hydrothermal treatment time is 18 hours, and the electrochemical performance of the obtained coal-based carbon material is the best.

[0091] In some embodiments of the present invention, the temperature of the hydrothermal treatment is 100-200°C.

[0092] In this embodiment, the temperature of the hydrothermal treatment is 200° C., and the electrochemical performance of the obtained coal-based carbon material is the best.

[0093] An embodiment of the present invention also provides a coal-based carbon material, which is prepared according to any of the above methods.

[0094] In order to more clearly illustrate the technical solution and advantages of the present invention, a method for preparing coal-based carbon materials by molten salt electrochemical method and coal-based carbon materials are described in detail through several embodiments below.

[0095] Embodiment 1:

[0096] Step 1: Hammer the steam coal into small pieces, then put it into a planetary ball mill for grinding into powder. After screening with a 100-mesh sieve, steam coal powder is obtained. Weigh 0.1 g of steam coal powder and mix it with 0.25 mmol of NiCl₂·6H₂O, add an appropriate amount of absolute ethanol for dissolution, ultrasonic for 30 min, then put it into a blast drying oven for drying at 60 °C for 12 h. Grind the dried mixture evenly with a small mortar and press it into tablets under a pressure of 10 MPa to obtain a mixed tablet, and weigh the mass after pressing.

[0097] Step 2: Wrap the pressed mixed tablet with nickel foam and connect it with galvanized iron wire to serve as the cathode material for the molten salt pyrolysis and molten salt electrochemical conversion of coal-based carbon materials.

[0098] Step 3: The mixture of chloro-metal salts is pre-dried in a vacuum drying oven at 120 °C for 12 h to remove moisture. Weigh the salts according to the following molar ratio, LiCl-KCl-CaCl₂ (0.52893:0.42422:0.04685, mol%), and mix the molten salts evenly, then put them into a 50 mL corundum crucible. Place the corundum crucible in the center of a muffle furnace, heat the muffle furnace to a certain temperature and keep it for 1 h, and the mixed molten salts change from a solid state to an ionic liquid state. Immerse the galvanized iron anode material (35×20×0.2 mm) and the nickel foam-wrapped steam coal cathode material (25×20×0.5 mm) into the molten salts for preheating for 30 min.

[0099] Step 4: After the preheating is completed, carry out molten salt electrolysis at 3.2 V and 550 °C for 3 h. After the electrolysis, take out the cathode from the molten salts. After the molten salts on the cathode cool down, separate the carbon product in the nickel foam cathode with hot deionized water. Wash and purify the separated carbon product with 5 M HCl. Finally, repeatedly rinse the carbon product with deionized water and dry it in a vacuum drying oven at 60 °C for 12 h.

[0100] Example 2

[0101] Example 2 is basically the same as Example 1, the difference is that in Step 4, the electrolysis temperature is 650 °C.

[0102] Example 3

[0103] Example 3 is basically the same as Example 2, the difference is that in Step 3, the mixture of chloro-metal salts is CaCl₂-NaCl-KCl (0.51503:0.40522:0.07975, mol%).

[0104] Example 4

[0105] Example 4 is basically the same as Example 2, the difference is that in Step 3, the mixture of chloro-metal salts is LiCl-KCl-NaCl (0.4207:0.4843:0.0949, mol%).

[0106] Example 5

[0107] Example 5 is basically the same as Example 2, except that in step 3, the metal chloride mixture is LiCl-KCl (0.595:0.405, mol%).

[0108] Example 6

[0109] Example 6 is basically the same as Example 2, except that in step 3, the metal chloride mixture is CaCl2-KCl (0.25:0.75, mol%).

[0110] In the preparation of coal-based carbon materials by molten salt electrolysis, different molten salt systems will cause differences in the electrochemical properties of the prepared carbon materials (Examples 2-6). The tests were carried out in 6M KOH electrolyte, the voltage range was -1 to 0V, and the scanning rate was 20mV·s-1. The CV curves are as Figure 1 shown. The CV curves of the carbon materials prepared by the five chloride salt systems show a similar rectangular shape, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes prepared by the five molten salt systems exhibit good electric double layer capacitance characteristics. Among them, the peak current density of the coal-based carbon material prepared by the LiCl-KCl-CaCl2 system is the highest, and the enclosed area of the CV curve is the largest, indicating that the LiCl-KCl-CaCl2 system has the most significant effect on improving the capacitance of the coal-based carbon material, and the LiCl-KCl-NaCl system has the second-best effect on improving the capacitance of the coal-based carbon material. It can be Figure 1 observed that the CV curves of LiCl-KCl-CaCl2 and CaCl2-KCl are the most similar in shape. The addition of LiCl and the change of the molten salt ratio can further improve the capacitance performance of the coal-based carbon material.

[0111] At a voltage range of -1 to 0V, constant current charge-discharge tests were carried out at current densities of 5A·g -1 and 0.5A·g -1 respectively, as Figure 2a and Figure 2b shown. The charge-discharge duration of the LiCl-KCl-CaCl2 system is the longest. According to calculations, at a current density of 0.5 to 5A·g -1 , the specific capacitance of the coal-based carbon material electrode prepared by the LiCl-KCl-CaCl2 system is 117.5 to 157.3F·g -1 , the specific capacitance of the coal-based carbon material electrode prepared by the LiCl-KCl-NaCl system is 94.5 to 128F·g -1 , and the specific capacitance of the coal-based carbon material electrode prepared by the CaCl2-KCl system is 80 to 121.3F·g -1, the specific capacitance of the coal-based carbon material electrode prepared from the CaCl2-NaCl-KCl system is 66-103.7 F·g -1 , the specific capacitance of the coal-based carbon material electrode prepared from the LiCl-KCl system is 38.5-74.5 F·g -1 , which is consistent with the CV curve results, indicating that the coal-based carbon material electrode prepared from the LiCl-KCl-CaCl2 system has the highest specific capacitance. At a current density of 0.5 A·g -1 , the capacitance of the raw steam coal is 5.8 F·g -1 , the capacitance of the coal-based carbon material electrode prepared from the LiCl-KCl-CaCl2 system is about 27 times higher than that of the raw steam coal. The specific capacitance of the LiCl-KCl-NaCl system is increased by about 22 times, that of the CaCl2-KCl system is increased by about 21 times, that of the CaCl2-NaCl-KCl system is increased by about 18 times, and that of the LiCl-KCl system is increased by about 13 times, further proving that the coal-based carbon material prepared from the LiCl-KCl-CaCl2 system has the best capacitive performance.

[0112] Calculate the specific capacitance of the coal-based carbon material electrodes prepared from the five molten salt systems at different current densities, and plot the relationship diagram between the specific capacitance and the current density, as Figure 3 shown. At a current density of 0.5-10 A·g -1 , the capacitance retention rates of the coal-based carbon materials prepared from the LiCl-KCl-CaCl2, LiCl-KCl-NaCl, CaCl2-KCl, CaCl2-NaCl-KCl and LiCl-KCl molten salt systems are 69%, 66%, 56%, 57% and 42% respectively. The coal-based carbon material electrode prepared from the LiCl-KCl-CaCl2 system has a higher capacitance retention rate. As the current density increases, the specific capacitance of the coal-based carbon material gradually decreases. The specific capacitance of the coal-based carbon material electrode prepared from the LiCl-KCl-CaCl2 system at different current densities is significantly higher than that of the other four molten salt systems, indicating that the preparation of coal-based carbon materials from the LiCl-KCl-CaCl2 system not only has excellent capacitance retention, but also can significantly improve the specific capacitance.

[0113] Under the open circuit voltage, an electrochemical impedance spectroscopy test is carried out to obtain a Nyquist diagram, as Figure 4 shown. Use ZView software for equivalent circuit simulation to fit the measured EIS data. From Figure 4It can be seen that the semicircle diameters of the coal-based carbon material electrodes prepared from the LiCl-KCl, CaCl2-NaCl-KCl, LiCl-KCl-NaCl, LiCl-KCl-CaCl2, and CaCl2-KCl systems decrease in turn. The semicircle diameter reflects the charge transfer resistance of the electrode. The Rct fitting value of LiCl-KCl-CaCl2 is 0.458 Ω. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. The slope of the straight line of LiCl-KCl-CaCl2 is the largest, indicating that it has a relatively small Warburg impedance. Considering the comprehensive performance of capacitance, capacity retention rate, Rct, and Warburg impedance, the coal-based carbon material prepared from the LiCl-KCl-CaCl2 system has the best electrochemical performance.

[0114] Example 7

[0115] Example 7 is basically the same as Example 1, except that in Step 1, NiCl2·6H2O is not added to the cathode material.

[0116] Example 8

[0117] Example 8 is basically the same as Example 1, except that in Step 1, 0.5 mmol of NiCl2·6H2O is added to every 0.1 g of pulverized coal in the cathode material.

[0118] Example 9

[0119] Example 9 is basically the same as Example 1, except that in Step 1, 0.75 mmol of NiCl2·6H2O is added to every 0.1 g of pulverized coal in the cathode material.

[0120] Example 10

[0121] Example 10 is basically the same as Example 1, except that in Step 1, 1 mmol of NiCl2·6H2O is added to every 0.1 g of pulverized coal in the cathode material.

[0122] There are differences in the electrochemical performance of the carbon products (Examples 1, 7-9) electrolyzed from the cathodes prepared by mixing different addition amounts of NiCl2·6H2O with steam coal in the molten salt system. Their CV curves are as Figure 5As shown in the figure. The CV curves of carbon materials prepared with different addition amounts of NiCl2·6H2O show a similar rectangular shape, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes prepared with different addition amounts of NiCl2·6H2O all exhibit good electric double-layer capacitance characteristics. When the addition amount of NiCl2·6H2O is 0.25 mmol, the peak current density of the prepared coal-based carbon material is the highest, and the enclosed area of the CV curve is the largest, indicating that the addition amount of 0.25 mmol of NiCl2·6H2O has the most significant effect on improving the capacitance performance of the coal-based carbon material. This may be attributed to the fact that the coal-based carbon material prepared under this condition is composed of the accumulation of spherical-like structures, forming a three-dimensional interconnected conductive network, shortening the ion transport path inside the material, and the surface of the spherical-like particles is rough, providing more ion adsorption sites. In addition, the coal-based carbon material prepared under this condition has a high degree of disorder, contains more defects and pores, which can enhance the charge storage ability and thus improve the capacitance performance.

[0123] The coal-based carbon materials prepared with different addition amounts of NiCl2·6H2O were subjected to constant current charge-discharge tests at current densities of 1 A·g -1 and 0.1 A·g -1 respectively, as shown in Figure 6a and Figure 6b . When the addition amount of NiCl2·6H2O is 0.25 mmol, the charge-discharge duration is the longest and the specific capacitance is the highest, which is consistent with the CV curve results. According to the calculation, at current densities of 0.1 - 1 A·g -1 , the specific capacitance of the coal-based carbon material electrode prepared with an addition amount of 0.25 mmol of NiCl2·6H2O is 211.3 - 358.2 F·g -1 . At a current density of 1 A·g -1 , the capacitance of the raw steam coal is 3.3 F·g -1 . The capacitance of the coal-based carbon material electrode prepared with an addition amount of 0.25 mmol of NiCl2·6H2O is about 64 times higher than that of the raw steam coal. The specific capacitance is about 30 times higher when no NiCl2·6H2O is added, about 53 times higher when the addition amount is 0.5 mmol, about 48 times higher when the addition amount is 0.75 mmol, and about 31 times higher when the addition amount is 1 mmol, further proving that the coal-based carbon material prepared with an addition amount of 0.25 mmol of NiCl2·6H2O has the best capacitance performance.

[0124] The specific capacitance of the coal-based carbon material electrodes prepared with different addition amounts of NiCl2·6H2O at different current densities was calculated, and the relationship diagram between the specific capacitance and the current density was plotted, as shown in Figure 7 . At 1 - 10 A·g -1At current densities, the capacitance retention rates of the coal-based carbon materials prepared with the addition amounts of NiCl₂·6H₂O being 0, 0.25, 0.5, 0.75, and 1 mmol were 67%, 79%, 78%, 76%, and 59% respectively. As the current density increased, the specific capacitance of the coal-based carbon material electrode prepared with an addition amount of 0.25 mmol of NiCl₂·6H₂O was significantly higher than that of other addition amounts of NiCl₂·6H₂O at different current densities.

[0125] Under open-circuit voltage, an electrochemical impedance spectroscopy test was carried out to obtain a Nyquist plot, as Figure 8 shown. The equivalent circuit simulation was performed using ZView software to fit the measured EIS data. From Figure 8 it can be seen that the semicircle diameters of the coal-based carbon material electrodes prepared with the addition amounts of NiCl₂·6H₂O being 1, 0.25, 0.75, 0.5, and 0 mmol decreased in turn, and the charge transfer resistances of the electrodes decreased in turn. The Rct fitting value for the addition amount of 0.25 mmol of NiCl₂·6H₂O was 0.217 Ω. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. The slopes of the straight lines of the coal-based carbon material electrodes prepared with the addition amounts of NiCl₂·6H₂O being 0.75, 0.5, 0.25, 0, and 1 mmol decreased in turn, and the Warburg impedance increased in turn. Considering the comprehensive performance of capacitance, capacity retention rate, Rct, and Warburg impedance, although the Rct and Warburg impedance of the coal-based carbon material prepared with an addition amount of 0.25 mmol of NiCl₂·6H₂O are not the best, its capacitance and capacity retention rate are the highest. Therefore, considering the above electrochemical properties, the coal-based carbon material prepared with an addition amount of 0.25 mmol of NiCl₂·6H₂O has the best performance.

[0126] Example 11

[0127] Example 11 was basically the same as Example 1, except that in Step 4, the electrolysis temperature was 460 °C.

[0128] Example 12

[0129] Example 12 was basically the same as Example 1, except that in Step 4, the electrolysis temperature was 500 °C.

[0130] Example 13

[0131] Example 13 was basically the same as Example 1, except that in Step 4, the electrolysis temperature was 600 °C.

[0132] The electrochemical properties of the coal-based carbon materials prepared at different temperatures (Examples 1-2 and Examples 11-13) are different due to the differences in their microscopic morphology and disorder. The test was carried out in 6M KOH electrolyte with a voltage range of -1 to 0 V and a scan rate of 20 mV·s -1 , its CV curve is as follows Figure 9 As shown. The CV curves of the carbon materials prepared under different temperature conditions present a rectangular shape, and there is no obvious redox peak in the CV curve. The carbon material electrode exhibits good double-layer capacitance characteristics. Among them, the coal-based carbon material prepared at an experimental temperature of 550°C has the highest peak current density and the largest closed area of the CV curve, indicating that the coal-based carbon material prepared at an experimental temperature of 550°C has the best capacitance performance. This may be because the coal-based carbon material prepared at 550°C is composed of a spherical structure with uniform particle size. The surface of the spherical structure is rough, the carbon material has a high degree of disorder, and there are abundant defects, which is conducive to increasing the ion active sites, thereby improving the capacitance performance of the carbon material.

[0133] Coal-based carbon materials prepared under different temperature conditions at 1A·g -1 and 0.1A·g -1 The constant current charge and discharge tests were carried out at the current density of Figure 10a and Figure 10b When the experimental temperature is 550℃, the charge and discharge duration is the longest and the specific capacitance is the highest, which is consistent with the CV curve results. According to calculations, in the range of 0.1~1A·g -1 At a current density of 2.5 and an experimental temperature of 550°C, the specific capacitance of the prepared coal-based carbon material electrode is 211.3 to 358.2 F·g -1 , at 1A·g -1 At the current density, the capacitance of thermal coal raw material is 3.3F·g -1 The electrode capacitance of the coal-based carbon material prepared at an experimental temperature of 550°C increased by about 64 times compared with that of the thermal coal raw material, the specific capacitance increased by about 32 times at an experimental temperature of 460°C, the specific capacitance increased by about 40 times at an experimental temperature of 500°C, the specific capacitance increased by about 55 times at an experimental temperature of 600°C, and the specific capacitance increased by about 44 times at an experimental temperature of 650°C, further proving that the capacitance performance of the coal-based carbon material prepared at an experimental temperature of 550°C is the best.

[0134] The specific capacitance of the coal-based carbon material electrode prepared under different experimental temperature conditions at different current densities was calculated, and the relationship between specific capacitance and current density was plotted, such as Figure 11 As shown. In 1~10A·g -1At a current density of, the capacitance retention rates of the coal-based carbon materials prepared at experimental temperatures of 460, 500, 550, 600, and 650 °C are 64%, 75%, 79%, 80%, and 76% respectively. With the increase of the current density, the specific capacitance of the coal-based carbon material electrode prepared at an experimental temperature of 550 °C is significantly higher than that of the coal-based carbon materials prepared under other temperature conditions at different current densities.

[0135] Under open circuit voltage, an electrochemical impedance spectroscopy test was carried out to obtain a Nyquist plot, as Figure 12 shown. The equivalent circuit simulation was carried out using ZView software to fit the measured EIS data. From Figure 12 it can be seen that the semicircle diameters of the coal-based carbon material electrodes prepared at experimental temperatures of 650, 500, 600, 550, and 460 °C decrease in turn, and the charge transfer resistances of the electrodes decrease in turn. Among them, the Rct fitting value of the coal-based carbon material prepared at an experimental temperature of 550 °C is 0.217 Ω. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. The coal-based carbon material prepared at an experimental temperature of 550 °C has the largest slope of the straight line in the low-frequency region and the smallest Warburg impedance. Considering the capacitance, capacitance retention rate, Rct, and Warburg impedance performance comprehensively, although the Rct resistance of the coal-based carbon material prepared at an experimental temperature of 550 °C is not the best, its capacitance, capacitance retention rate, and Warburg impedance performance are the highest. Therefore, considering the above electrochemical performance, the coal-based carbon material prepared at an experimental temperature of 550 °C has the best performance.

[0136] Example 14

[0137] Example 14 is basically the same as Example 1, except that in Step 4, the electrolysis voltage is 2.6 V.

[0138] Example 15

[0139] Example 15 is basically the same as Example 1, except that in Step 4, the electrolysis voltage is 2.8 V.

[0140] Example 16

[0141] Example 16 is basically the same as Example 1, except that in Step 4, the electrolysis voltage is 3.0 V.

[0142] Example 17

[0143] Example 17 is basically the same as Example 1, except that in Step 4, the electrolysis voltage is 3.4 V.

[0144] The coal-based carbon materials (Example 1 and Examples 14-17) prepared at different voltages were tested in 6M KOH electrolyte with a voltage range of -1 to 0 V and a scan rate of 20 mV·s due to differences in their micromorphology and disorder, which would further affect their electrochemical properties. -1 , its CV curve is as follows Figure 13 As shown. The CV curves of the carbon materials prepared under different voltage conditions all show a rectangular shape, and there is no obvious redox peak in the CV curve, indicating that the carbon material electrodes prepared under different voltages show good double-layer capacitance characteristics. Among them, the peak current density of the coal-based carbon material prepared at an electrolysis voltage of 3.2V is the highest, and the closed area of the CV curve is the largest, indicating that the coal-based carbon material prepared at a voltage of 3.2V has the best capacitance performance. This may be because the carbon material prepared under this condition is composed of spherical particles with uniform particle size. Compared with the lamellar structure generated under low voltage, the accumulation of spherical particles forms a three-dimensional cross-linked network, shortening the transmission path of ions. The lamellar structure may cause pore blockage due to too tight stacking, reducing the utilization of carbon materials and causing the capacitance performance to decay.

[0145] Coal-based carbon materials prepared under different voltage conditions at 1A·g -1 and 0.1A·g -1 The constant current charge and discharge tests were carried out at the current density of Figure 14a and 14b When the electrolysis voltage is 3.2V, the charge and discharge duration is the longest and the specific capacitance is the highest, which is consistent with the CV curve results. According to calculations, in the range of 0.1 to 1A·g -1 At a current density of 2.50 W and an electrolysis voltage of 3.2 V, the specific capacitance of the prepared coal-based carbon material electrode is 211.3 to 358.2 F·g -1 , at 1A·g -1 At the current density, the capacitance of thermal coal raw material is 3.3F·g -1 The electrode capacitance of the coal-based carbon material obtained at an electrolysis voltage of 3.2V is about 64 times higher than that of the thermal coal raw material, the specific capacitance is about 41 times higher when the electrolysis voltage is 2.6V, the specific capacitance is about 48 times higher when the electrolysis voltage is 2.8V, the specific capacitance is about 50 times higher when the electrolysis voltage is 3.0V, and the specific capacitance is about 62 times higher when the electrolysis voltage is 3.4V, which further proves that the coal-based carbon material obtained at an electrolysis voltage of 3.2V has the best capacitance performance.

[0146] The specific capacitances of the coal-based carbon material electrodes prepared under different voltage conditions were calculated at different current densities, and the relationship diagram between the specific capacitance and the current density was plotted, as shown in Figure 14. As the current density increases, the specific capacitance of the coal-based carbon material electrodes prepared at an electrolysis voltage of 3.2 V is significantly higher than that of the coal-based carbon materials prepared under other electrolysis voltage conditions at different current densities. At a current density of 1-10 A·g -1 −1, the capacitance retention rates of the coal-based carbon materials prepared at electrolysis voltages of 2.6, 2.8, 3.0, 3.2, and 3.4 V are 60%, 78.4%, 78.3%, 79%, and 72%, respectively. The three-dimensional interconnected network structure formed by the spherical-like structure reduces the electron transport path, can adapt to the rapid charge and discharge requirements at high current densities, and is beneficial to improving the capacity retention rate.

[0147] Under the open circuit voltage, the electrochemical impedance spectroscopy test was carried out to obtain the Nyquist diagram, as Figure 15 shown. The equivalent circuit simulation was carried out using ZView software to fit the measured EIS data. It can be Figure 15 seen that the semicircle diameters of the coal-based carbon material electrodes prepared at electrolysis voltages of 3.4, 2.6, 3.2, 2.8, and 3.0 V decrease in turn, and the charge transfer resistances of the electrodes decrease in turn. Among them, the Rct fitting value of the coal-based carbon material prepared at an electrolysis voltage of 3.2 V is 0.217 Ω. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. The slope of the straight line of the coal-based carbon material prepared at an electrolysis voltage of 3.2 V in the low-frequency region is the largest, and the Warburg impedance is the smallest. Considering the comprehensive performance of capacitance, capacity retention rate, Rct, and Warburg impedance, although the Rct resistance of the coal-based carbon material prepared at an electrolysis voltage of 3.2 V is not the best, its capacitance, capacity retention rate, and Warburg impedance performance are the highest. Therefore, considering the above electrochemical performance, the coal-based carbon material prepared at an electrolysis voltage of 3.2 V has the best performance.

[0148] Example 18

[0149] Example 18 is basically the same as Example 1, except that in Step 4, the electrolysis time is 1 h.

[0150] Example 19

[0151] Example 19 is basically the same as Example 1, except that in Step 4, the electrolysis time is 2 h.

[0152] Example 20

[0153] Example 20 is basically the same as Example 1, except that in Step 4, the electrolysis time is 4 h.

[0154] Example 21

[0155] Example 21 is basically the same as Example 1, except that in Step 4, the electrolysis time is 5 h.

[0156] In the coal-based carbon materials prepared with different electrolysis times (Example 1 and Examples 18 - 21), due to the differences in their microscopic morphology and degree of disorder, their electrochemical performance will be further affected. They were tested in 6 M KOH electrolyte with a voltage range of -1 to 0 V and a scanning rate of 20 mV·s -1 , and their CV curves are as Figure 17 shown. The CV curves of the carbon materials prepared under different electrolysis times all show a shape similar to a rectangle, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes prepared at different times exhibit good electric double-layer capacitance characteristics. Among them, when the electrolysis time is 3 h, the peak current density of the coal-based carbon material prepared is the highest, and the enclosed area of the CV curve is the largest, indicating that the coal-based carbon material prepared by electrolysis for 3 h has the best capacitance performance. This may be because the carbon material prepared under this condition is composed of randomly stacked spherical-like particles with uniform particle size, forming a three-dimensional cross-linked network structure through point contact, which is conducive to shortening the ion transport path and improving the capacitance performance of the carbon material. In addition, the carbon material generated by electrolysis for 4 h has a larger spherical-like structure than that for 3 h, and the stacking mode of the spherical-like structure is more loose, and the degree of disorder is lower than that of the carbon material for 3 h. During the CV curve test, the coal-based carbon material prepared by electrolysis for 4 h peeled off seriously in the KOH electrolyte, which may directly lead to a sharp attenuation of its capacitance.

[0157] The coal-based carbon materials prepared under different electrolysis times were subjected to constant current charge-discharge tests at current densities of 1 A·g -1 and 0.1 A·g -1 respectively, as Figure 18a and 18b shown. When the electrolysis time is 3 h, the charge-discharge duration is the longest and the specific capacitance is the highest, which is consistent with the results of the CV curve. According to the calculation, at a current density of 0.1 - 1 A·g -1 , when the electrolysis time is 3 h, the specific capacitance of the coal-based carbon material electrode is 211.3 - 358.2 F·g -1 . At a current density of 1 A·g -1 , the capacitance of the raw steam coal is 3.3 F·g -1 . The capacitance of the coal-based carbon material electrode prepared by electrolysis for 3 h is about 64 times higher than that of the raw steam coal. The specific capacitance is increased by about 38 times for electrolysis for 1 h, about 45 times for electrolysis for 2 h, about 35 times for electrolysis for 4 h, and about 49 times for electrolysis for 5 h, further proving that the coal-based carbon material prepared by electrolysis for 3 h has the best capacitance performance.

[0158] The specific capacitances of the coal-based carbon material electrodes prepared under different electrolysis time conditions at different current densities were calculated, and a graph of the relationship between the specific capacitance and the current density was plotted, as Figure 19 shown. At a current density of 1-10 A·g -1 , the capacitance retention rates of the coal-based carbon materials prepared by electrolysis for 1, 2, 3, 4, and 5 h were 79.8%, 76.8%, 79.0%, 77.4%, and 78.1%, respectively. As the current density increased, the specific capacitance of the coal-based carbon material electrode prepared by electrolysis for 3 h at different current densities was significantly higher than that of the coal-based carbon materials prepared by other electrolysis times.

[0159] Under the open circuit voltage, an electrochemical impedance spectroscopy test was carried out to obtain a Nyquist plot, as Figure 20 shown. The equivalent circuit simulation was carried out using ZView software to fit the measured EIS data. From Figure 20 , it can be seen that the fitted values of Rct of the coal-based carbon materials prepared by electrolysis for 1, 2, 3, 4, and 5 h were 0.203, 0.233, 0.217, 0.213, and 0.164 Ω, respectively. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. Among them, the slope of the straight line of the carbon material electrode prepared by electrolysis for 4 h in the low-frequency region was the largest, and the Warburg impedance was the smallest. The carbon material electrode prepared by electrolysis for 3 h was the second. This may be because the loose packing of the spherical-like structures in the carbon material prepared by electrolysis for 4 h formed more mesopores or macropores, shortening the ion transport path and reducing the diffusion resistance. Therefore, the Warburg impedance was smaller.

[0160] In summary, the sample SC-550℃-3.2V-3h prepared in the optimal condition example 1 was subjected to 10,000 charge-discharge cycles at a large current density of 5 A·g -1 in the voltage range of -1 to 0 V. After 10,000 cycles, the capacitance of the optimal sample hardly decreased, and the Coulomb efficiency fluctuated but almost remained at 100%, indicating that the sample had good cycle stability in the KOH electrolyte. The specific capacitance of the SC-550℃-3.2V-3h sample under different current density conditions was higher than that of the carbon materials prepared under other conditions. At the same time, it had good cycle stability. Therefore, combined with the electrochemical evaluation, it can be seen that the SC-550℃-3.2V-3h electrode material had excellent electrochemical performance, which may be attributed to its spherical-like structure packing with uniform particle size and high degree of disorder. Spherical carbon is the most typical and common three-dimensional material type due to its isotropic and compressible characteristics. The pores between spherical particles and the internal pores constitute a hierarchical pore structure, which can provide a buffer space for the electrolyte and reduce the ion transport path.

[0161] Example 22

[0162] Example 22 is basically the same as Example 2, except that in step 4, the electrolysis voltage is 2.8 V and the electrolysis time is 2 h.

[0163] Example 23

[0164] Example 23 is basically the same as Example 22, except that before step 1, the carbon powder is hydrothermally treated, including: weighing 0.1 g of coal powder and pouring it into a 50 mL inner liner, adding 25 mL of deionized water and 5 mL of absolute ethanol thereto for hydrothermal treatment, the hydrothermal temperature is 200 °C, and the hydrothermal time is 18 h.

[0165] The carbon materials obtained in Example 22 and Example 23 were respectively tested in 6 M KOH electrolyte, the voltage range was -1 to 0 V, and the scanning rate was 20 mV·s -1 , and its CV curve is as Figure 21 shown. The CV curves all show a shape similar to a rectangle, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes exhibit good electric double-layer capacitance characteristics. Among them, the coal-based carbon material prepared by molten salt electrolysis after hydrothermal pretreatment of coal powder has the highest peak current density and the largest enclosed area of the CV curve, indicating that the coal-based carbon material prepared after hydrothermal pretreatment has the best capacitance performance.

[0166] The coal-based carbon material was tested by constant current charge and discharge at a current density of 1 A·g -1 , as Figure 22 shown. Among them, the coal-based carbon material prepared by molten salt electrolysis after hydrothermal pretreatment of steam coal has the longest charge and discharge duration and the highest specific capacitance, which is consistent with the CV curve results. According to calculations, at a current density of 1 A·g -1 , the specific capacitance of the coal-based carbon material electrode prepared by molten salt electrolysis after hydrothermal pretreatment of steam coal is 138.3 F·g -1 , and at a current density of 1 A·g -1 , the specific capacitance of the coal-based carbon material prepared without hydrothermal pretreatment is 120.3 F·g -1 , and the specific capacitance of the coal-based carbon material electrode prepared by molten salt electrolysis after hydrothermal pretreatment of steam coal is 15% higher than that of the carbon material electrode prepared without hydrothermal treatment.

[0167] Example 24

[0168] Example 24 is basically the same as Example 23, except that the hydrothermal treatment time is 6 h.

[0169] Example 25

[0170] Example 25 is basically the same as Example 23, except that the hydrothermal treatment time is 12 h.

[0171] Example 26

[0172] Example 26 is basically the same as Example 23, except that the hydrothermal treatment time is 24 h.

[0173] Example 27

[0174] Example 27 is basically the same as Example 23, except that the hydrothermal treatment time is 30 h.

[0175] Among the coal-based carbon materials prepared with different hydrothermal times (Examples 23 and Examples 24 - 27), due to the differences in their microscopic morphology and degree of disorder, their electrochemical performance will be further affected. The test was carried out in 6 M KOH electrolyte, the voltage range was -1 to 0 V, and the scanning rate was 20 mV·s -1 , and its CV curve is as Figure 23 shown. The CV curves of the carbon materials prepared under different hydrothermal time conditions all show a similar rectangular shape, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes prepared under different hydrothermal times exhibit good electric double-layer capacitance characteristics. Among them, when the hydrothermal time is 18 h, the peak current density of the coal-based carbon material is the highest, and the enclosed area of the CV curve is the largest, indicating that the coal-based carbon material prepared by hydrothermal treatment for 18 h has the best capacitance performance. This may be because the coal-based carbon material prepared by hydrothermal treatment for 18 h has a loose and porous surface, and the degree of disorder of the carbon material is relatively high, which increases the ion adsorption sites and thus improves the specific capacitance of the carbon material.

[0176] The coal-based carbon materials prepared under different hydrothermal time conditions were tested by constant current charge and discharge at the current densities of 1 A·g -1 and 0.1 A·g -1 , as Figure 24a and 24b shown. When the hydrothermal time is 18 h, the charge and discharge duration is the longest and the specific capacitance is the highest, which is consistent with the results of the CV curve. According to the calculation, at the current density of 0.1 - 1 A·g -1 , when the hydrothermal time is 18 h, the specific capacitance of the coal-based carbon material electrode is 212.4 - 332.2 F·g -1 . When the coal-based carbon material electrode prepared with a hydrothermal time of 30 h was tested at a current density of 0.1 A·g -1 , the GCD curve could not be measured.

[0177] The specific capacitances of the coal-based carbon material electrodes prepared under different hydrothermal time conditions at different current densities were calculated, and the relationship diagram between the specific capacitance and the current density was plotted, as Figure 25 shown. At 1 - 10 A·g -1At the current density, the capacitance retention rates of the coal-based carbon materials prepared by hydrothermal pretreatment for 6, 12, 18, 24, and 30 h are 62%, 76%, 81%, 73%, and 73%, respectively. As the current density increases, the specific capacitance of the coal-based carbon material electrode prepared by hydrothermal treatment for 18 h at 1-10 A·g -1 is significantly higher than that of the coal-based carbon materials prepared by other hydrothermal times.

[0178] At the open circuit voltage, an electrochemical impedance spectroscopy test was carried out to obtain a Nyquist plot, as Figure 26 shown. The equivalent circuit simulation was carried out using ZView software to fit the measured EIS data. From Figure 26 it can be seen that the semicircle diameters of the coal-based carbon material electrodes prepared with hydrothermal times of 24 h, 18 h, 6 h, 30 h, and 12 h decrease in turn, and the charge transfer resistance of the electrodes decreases in turn. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. Among them, the slope of the straight line of the carbon material electrode prepared with a hydrothermal time of 12 h in the low-frequency region is the largest, and the Warburg impedance is the smallest. The Warburg impedances of hydrothermal 6 h, 24 h, 30 h, and 18 h increase in turn. This may be because the fiber network structure and spherical-like structure in the carbon material prepared by hydrothermal treatment for 6 h are disorderly intercalated, shortening the ion transport path and reducing the diffusion resistance, so the Warburg impedance is smaller.

[0179] Example 28

[0180] Example 28 is basically the same as Example 23, except that the hydrothermal temperature is 100 °C.

[0181] Example 29

[0182] Example 29 is basically the same as Example 23, except that the hydrothermal temperature is 125 °C.

[0183] Example 30

[0184] Example 30 is basically the same as Example 23, except that the hydrothermal temperature is 150 °C.

[0185] Example 31

[0186] Example 31 is basically the same as Example 23, except that the hydrothermal temperature is 175 °C.

[0187] Among the coal-based carbon materials prepared at different hydrothermal temperatures (Examples 23 and 28-31), due to the differences in their microscopic morphology and disorder degree, their electrochemical performance will be further affected. The test was carried out in 6 M KOH electrolyte, the voltage range was -1 to 0 V, and the scanning rate was 20 mV·s -1 , and its CV curve is as Figure 27As shown. The CV curves of the carbon materials prepared under different hydrothermal temperature conditions all show a similar rectangular shape, and there are no obvious oxidation-reduction peaks in the CV curves, indicating that the carbon material electrodes prepared under different hydrothermal temperatures exhibit good double-layer capacitance characteristics. Among them, when the hydrothermal temperature is 200 °C, the peak current density of the coal-based carbon material is the highest, and the enclosed area of the CV curve is the largest, indicating that the coal-based carbon material prepared at 200 °C has the best capacitance performance. This may be because the coal-based carbon material prepared at 200 °C is composed of randomly stacked granular structures with smaller particle sizes and rough surfaces, and the degree of disorder of the carbon material is relatively high, which increases the ion adsorption sites and thus improves the specific capacitance of the carbon material.

[0188] The coal-based carbon materials prepared under different hydrothermal temperature conditions were subjected to constant current charge-discharge tests at current densities of 1 A·g -1 and 0.1 A·g -1 , as shown in Figure 28a and 28b . When the hydrothermal temperature is 200 °C, the charge-discharge duration is the longest and the specific capacitance is the highest, which is consistent with the results of the CV curve. According to the calculation, at a current density of 0.1 - 1 A·g -1 , the specific capacitance of the coal-based carbon material electrode prepared at a hydrothermal temperature of 200 °C is 237.1 - 409.5 F·g -1 . Compared with the coal-based carbon material prepared without hydrothermal pretreatment in the first chapter at a current density of 0.1 A·g -1 , the specific capacitance is increased by 51.3 F·g -1 .

[0189] The specific capacitances of the coal-based carbon material electrodes prepared under different hydrothermal temperature conditions at different current densities were calculated, and a relationship diagram between the specific capacitance and the current density was plotted, as shown in Figure 29 . At a current density of 1 - 10 A·g -1 , the capacitance retention rates of the coal-based carbon materials prepared by hydrothermal pretreatment at 100, 125, 150, 175, and 200 °C are 75.9%, 78%, 77%, 76.4%, and 72%, respectively. As the current density increases, the specific capacitance of the coal-based carbon material electrode prepared at 200 °C is significantly higher than that of the coal-based carbon materials prepared at other hydrothermal temperatures at a current density of 1 - 10 A·g -1 .

[0190] At the open circuit voltage, an electrochemical impedance spectroscopy test was carried out to obtain a Nyquist diagram, as shown in Figure 30 . The equivalent circuit simulation was carried out using ZView software to fit the measured EIS data. From Figure 30It can be seen that the semicircle diameters of the coal-based carbon material electrodes prepared at hydrothermal times of 200, 150, 125, 175, 100, and 200 °C decrease in turn, and the charge transfer resistances of the electrodes decrease in turn. The slope of the straight line in the low-frequency region is related to the ion diffusion rate. Among them, the carbon material electrode prepared at a hydrothermal temperature of 150 °C has the largest slope of the straight line in the low-frequency region and the smallest Warburg impedance. The Warburg impedance of the coal-based carbon material prepared at a hydrothermal temperature of 200 °C is the second. This may be because the carbon material prepared at 150 °C of hydrothermal treatment is formed by the accumulation of rough-surfaced and small-particle-size particles and short rod-like structures to form a loose and porous structure, shortening the ion transport path and reducing the diffusion resistance. Therefore, the Warburg impedance is smaller.

[0191] The sample prepared under the optimal conditions (Example 23, hydrothermal treatment at 200 °C for 18 h) was subjected to 10,000 charge-discharge cycles at a large current density of 5 A·g -1 After 10,000 cycles, the capacitance of the optimal sample fluctuated, but remained above 92%. The Coulomb efficiency was relatively high, almost maintaining at 100%, indicating that the sample had good cycle stability in the KOH electrolyte. After pretreatment at 200 °C for 18 h by hydrothermal treatment and then molten salt electrolysis, the specific capacitance of the obtained sample at different current densities was higher than that of the carbon materials prepared under other conditions. At the same time, it had good cycle stability. Therefore, combined with the electrochemical evaluation, it can be seen that the electrode material prepared by pretreatment at 200 °C for 18 h by hydrothermal treatment and then molten salt electrolysis has excellent electrochemical performance, which may be attributed to its randomly stacked granular structure with small particle size and rough surface, and the obtained carbon material has a high degree of disorder.

[0192] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. However, such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing coal-based carbon materials by molten salt electrochemistry, characterized in that, The method includes the following steps: Step 1, mix pulverized coal and nickel chloride hexahydrate, and press the mixture into tablets after uniform mixing to obtain a mixed tablet; Step 2, wrap the mixed tablet in nickel foam to obtain a cathode material; Step 3, heat the chloro-metal salt mixture to melting, and immerse the cathode material and the anode material in the molten chloro-metal salt mixture for preheating; Step 4, apply a voltage between the anode material and the cathode material for electrolysis to obtain a coal-based carbon material on the surface of the nickel foam.

2. The method according to claim 1, wherein The chloro-metal salt mixture includes LiCl, KCl, and CaCl2.

3. The method according to claim 2, wherein The molar ratio of LiCl, KCl, and CaCl2 is 0.4207:0.4843:0.0949.

4. The method according to claim 1, wherein In Step 1, 0.2 - 1 mmol of nickel chloride hexahydrate is added to every 0.1 g of pulverized coal.

5. The method according to claim 4, characterized in that In Step 1, 0.25 mmol of nickel chloride hexahydrate is added to every 0.1 g of pulverized coal.

6. The method according to claim 1, wherein In Step 4, the electrolysis temperature is 460 - 650 °C.

7. The method according to claim 6, characterized in that, In Step 4, the electrolysis temperature is 550 °C.

8. The method according to claim 1, characterized in that In Step 4, the electrolysis voltage applied between the anode material and the cathode material is 2.6 - 3.4 V.

9. The method according to claim 1, wherein In Step 4, the electrolysis time is 1 - 5 h.

10. A coal-based carbon material, characterized in that, Prepared according to any one of claims 1 - 9.

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