High-metamorphic-degree coal-based hard carbon negative electrode material for sodium-ion battery and preparation method of negative electrode material

Abstract

The invention discloses a high-metamorphic-degree coal-based hard carbon negative electrode material of a sodium ion battery and a preparation method of the negative electrode material. The preparation method comprises the following steps: crushing the high-metamorphic-degree coal to obtain the required pulverized coal granularity; the high-metamorphic-degree pulverized coal is sequentially subjected to ultrasonic pickling deliming, filtering and washing, nitric acid pre-oxidation, filtering and washing, inorganic pore-forming agent activation pore-forming, carbonization and coating. The method can improve the yield, simplify the treatment process and reduce the cost.

Description

Technical Field

[0001] The present invention relates to the field of anode materials for sodium-ion batteries, and particularly to a high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have developed rapidly in the past thirty-odd years and are widely used in various fields of production and life. However, the limited sources of lithium-ion battery materials (especially metal materials such as lithium, cobalt, manganese, and copper) have led to a sharp increase in material prices, restricting the future development of lithium-ion batteries.

[0003] Sodium-ion batteries have the advantages of rich materials, wide sources, low cost, and high safety compared with lithium-ion batteries, and have developed rapidly in recent years. Sodium-ion batteries have a working principle similar to that of lithium-ion batteries. During charging and discharging, sodium ions pass through the electrolyte and diaphragm and repeatedly intercalate and deintercalate between the positive and negative electrodes, while electrons perform external work through the external circuit.

[0004] The anode material of lithium-ion batteries is mainly composed of graphite. Since the radius of sodium ions is larger than that of lithium ions, and sodium ions cannot form a stable intercalation compound with graphite, the reversible capacity of graphite as the anode material of sodium-ion batteries is low (35 mAh / g). Hard carbon materials are the mainstream in the research and industrialization of anode materials for sodium-ion batteries due to their excellent performance and cost advantages.

[0005] The precursors of hard carbon anode materials mainly include biomass, resin, coal, etc. Coal has the advantages of low cost, stable source, and batch supply compared with the other two precursor materials. Coal with a high degree of metamorphism has low impurity content, low volatile matter, and high carbon yield, but also has disadvantages such as low specific capacity and low initial efficiency in terms of electrical properties.

[0006] The present invention provides a high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries and a preparation method thereof, as well as a method for improving its electrical properties. Summary of the Invention

[0007] The purpose of the present invention is to provide a high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries and a preparation method thereof. The present invention uses a coal with a high degree of metamorphism as the precursor of the hard carbon anode for sodium-ion batteries, which can improve the yield, simplify the processing technology, and reduce the cost.

[0008] The technical solution of the present invention is as follows:

[0009] A preparation method of a high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries, the steps include:

[0010] 1) Taking a coal with a high degree of metamorphism as the precursor;

[0011] 2) Crush and pickling the high-rank coal to obtain pulverized coal;

[0012] 3) Carry out low-temperature pre-oxidation on the pulverized coal with a nitric acid solution;

[0013] 4) Mix the ammonia-based inorganic pore-forming agent with the pre-oxidized pulverized coal and activate pore formation at 500 - 1000 °C;

[0014] 5) Carry out high-temperature carbonization on the pore-formed coal-based hard carbon precursor material in the temperature range of 1000 - 1600 °C and in an inert atmosphere to obtain a high-rank coal-based hard carbon negative electrode material for sodium-ion batteries;

[0015] 6) Carry out carbon coating treatment on the carbonized pulverized coal by means of gas-phase coating, and the coating temperature is 500 - 1300 °C.

[0016] In the present invention, in step 1), the precursor is high-rank coal, and the type of the high-rank coal is defined by the maximum reflectance of vitrinite and the volatile matter content. The maximum reflectance of vitrinite is 1.6% - 5.0%, preferably 1.6% - 3.0%; the volatile matter content is 1% - 20%, and the preferred volatile matter content is 1% - 10%. The low volatile matter content of the high-rank coal can improve the yield of hard carbon after carbonization and reduce the usage amount and cost of the precursor coal.

[0017] In the present invention, in step 2), drying treatment needs to be carried out first. The drying temperature is 105 °C - 120 °C, preferably 105 °C - 110 °C. After drying is completed, subsequent treatment is carried out;

[0018] The crushing method includes one or several of crushing methods such as ball milling, jet milling, and stirred milling. The particle size of the pulverized coal obtained after crushing is 0.01 μm - 70 μm, and the preferred particle size range is 1 - 20 μm, where the D50 particle size of the pulverized coal is 0.01 - 15 μm;

[0019] The pickling in step 2) is to remove the minerals in the coal and increase the carbon content. The acid used is a mixed solution of one of hydrochloric acid and sulfuric acid and hydrofluoric acid; the concentration of each acid in the mixed solution is 5 mol / L - 10 mol / L; the ratio of coal to the mixed solution is coal:mixed acid solution = 1 g:2 ml - 10 ml, and the preferred ratio is 1 g:2 ml - 6 ml. Since the particle size of the pulverized coal after crushing is small, to prevent agglomeration, the pickling container is placed in an ultrasonic disperser, the temperature of the ultrasonic disperser is 30 - 70 °C, and the time for ultrasonic and pickling deashing is 0.5 - 24 h, and the preferred deashing time is 12 - 24 h. The pickled coal needs to be filtered and washed with deionized water until the pH value of the filtrate is 7.

[0020] In the present invention, in step 3), pre-oxidation with nitric acid can increase the graphite layer spacing and defect sites, enhance the crosslinking of the precursor, and make the graphite layers tend to be disordered during the carbonization process. The concentration of the nitric acid solution is 0.01 mol / L - 5 mol / L, and the preferred range is 0.01 mol / L - 2 mol / L. The ratio of coal to the nitric acid solution is coal: nitric acid solution = 1 g: 3 ml - 10 ml, and the preferred range is 1 g: 3 ml - 5 ml. The pre-oxidation temperature is 30 - 60 °C, the pre-oxidation time is 0.5 h - 24 h, and the preferred pre-oxidation time is 1 - 6 h. After pre-oxidation, it needs to be filtered, washed with deionized water until the pH value of the filtrate is 7, and dried. The drying temperature is 105 - 120 °C to obtain pre-oxidized coal powder.

[0021] In the present invention, in step 4), an ammonia-based inorganic pore former is mixed with the pre-oxidized coal powder and activated to form pores at 500 - 1000 °C. High-rank coal needs to be porous to improve the capacity of the hard carbon anode material. The ammonia-based inorganic pore former includes ammonia-based inorganic pore formers such as ammonium carbonate / ammonium bicarbonate, with a mass fraction of 1% - 30%, and the preferred ratio is 5% - 10% based on the mass of the coal powder. The coal powder and the ammonia-based inorganic pore former are mixed evenly in a mixer. When the heating rate is 1 °C / min - 10 °C / min, the mixed material is heated programmedly to 500 - 1000 °C and held for 0.5 - 6 h to obtain pore-formed coal powder.

[0022] In the present invention, in step 5), the preferred carbonization temperature range is 1000 - 1400 °C; carbonization treatment is carried out under an inert atmosphere of N2 or Ar, and the carbonization time is 1 - 5 h to obtain the hard carbon anode material for sodium-ion batteries.

[0023] In the present invention, the hard carbon anode material prepared in step 5) has more open pore structures, while the closed pore structures play a role in storing sodium ions. Therefore, in step 6), the hard carbon anode material prepared in step 5) needs to be treated with carbon coating. The carbon source for the carbon coating in step 6) comes from one or more of naphtha, PO / SM tar, propylene, methane, acetylene, etc.; the type of carrier gas during the coating process is N2 or Ar; the coating temperature is 500 - 1300 °C, and the preferred coating temperature is 900 - 1300 °C; the coating time is 0.5 - 12 h, and the preferred coating time is 1 - 5 h;

[0024] The boiling point of the PO / SM tar is 200 - 500 °C;

[0025] The carbon coating method in step 6) is gas-phase coating. First, the carbon source and the carrier gas are preheated, and then the coal-based hard carbon is coated. Finally, the hard carbon anode material for sodium-ion batteries based on high-rank coal is obtained.

[0026] Compared with the prior art, the present invention has the following characteristics:

[0027] (1) High-metamorphism coal has a high yield and few impurities, which can reduce costs and simplify the process;

[0028] (2) The negative electrode electrical performance can be improved through pre-oxidation, pore formation, and coating modification. Specific Embodiments

[0029] The present invention will be further described below in conjunction with embodiments. It should be noted that the embodiments do not constitute a limitation on the scope of protection required by the present invention.

[0030] The main raw materials involved in the present invention are as follows:

[0031] High-metamorphism coal - the maximum reflectance of vitrinite is 3.2%, and the volatile matter content is 6.1%.

[0032] Performance test method for the coal-based hard carbon negative electrode material: Using the high-metamorphism hard carbon material obtained by high-temperature carbonization as the negative electrode, metallic sodium as the positive electrode, sodium perchlorate as the solute of the electrolyte, carbonate organic solvent as the solvent, and the electrolyte concentration is 1M; after assembling into a button cell, the capacity and first efficiency of the hard carbon negative electrode are tested at a current density of 20 mA / g.

[0033] Example 1

[0034] Crushing and acid washing and deashing operations: The particle size of the coal powder obtained by ball milling 40 g of coal is D50: 5.43 μm, D90: 13.25 μm, D100: 20 μm. Place it in a polytetrafluoroethylene container, and add a mixed acid washing solution with the concentrations of hydrochloric acid and hydrofluoric acid both being 5 mol / L, where the ratio of coal to the acid washing solution is 1 g of coal: 10 ml. Place the polytetrafluoroethylene container in an ultrasonic disperser for dispersion and deashing. The temperature of the ultrasonic disperser is 60 °C, and ultrasonic treatment and acid washing are carried out for 24 h. Then filter, wash, and dry to obtain the acid-washed coal powder.

[0035] Pre-oxidation operation: Weigh 10 g of the acid-washed coal powder, place it in 50 mL of 2 mol / L nitric acid solution for pre-oxidation treatment. The pre-oxidation temperature is 50 °C, and pre-oxidation is carried out for 2 h. After pre-oxidation is completed, filter it, wash it with deionized water until the pH value of the filtrate is 7, and carry out drying treatment at 120 °C to obtain the pre-oxidized coal powder.

[0036] Activation and pore formation operation: Mix 10% ammonium carbonate with the pre-oxidized coal powder evenly in a mixer. Place the mixed material in a high-temperature furnace, and when the N2 flow rate is 100 ml / min, heat it up to 600 °C at a heating rate of 2 °C / min, keep it warm for 3 h, and then cool it down to room temperature to obtain the coal powder with pores formed.

[0037] Carbonization operation: After the pore formation of pulverized coal, it is placed in a high-temperature furnace and carbonized at 1300 °C for 2 h with an Ar flow rate of 100 ml / min to obtain a coal-based hard carbon anode material.

[0038] Carbon coating operation: 10 g of the hard carbon anode material prepared by the carbonization operation is placed in a fixed-bed reactor and heated to 800 °C under a nitrogen atmosphere. At the same time, the preheaters of PO / SM tar and carrier gas N2 are heated to 550 °C. After reaching the set temperature, the flow rate of PO / SM tar is set to 0.1 g / min and the nitrogen flow rate is 200 ml / min. After both components are preheated to 550 °C, they are introduced into the fixed-bed reactor. The carbon coating time is 3 h. After the coating is completed, it is cooled to room temperature under a nitrogen atmosphere of 200 ml / min to finally obtain a high-rank coal-based hard carbon anode material for sodium-ion batteries. 2 Atmosphere and cooled to room temperature to finally obtain a high-rank coal-based hard carbon anode material for sodium-ion batteries.

[0039] After the carbon coating is completed, the high-rank coal-based hard carbon material is used as the anode material of the sodium-ion battery to carry out performance tests. The reversible capacity of the obtained coal-based hard carbon anode material is 270 mAh / g, and the first efficiency is 85.2%.

[0040] Example 2

[0041] Crushing and acid washing and deashing operation: The crushing process is the same as that in Example 1; the acid washing solution is a mixed acid washing solution of sulfuric acid and hydrofluoric acid with a concentration of 10 mol / L each, and the ratio of coal to the acid washing solution is 1 g of coal: 3 ml. It is placed in an ultrasonic disperser for dispersion and deashing at a temperature of 50 °C for 12 h of ultrasonic treatment and acid washing, and then filtered, washed, and dried to obtain the acid-washed pulverized coal.

[0042] Pre-oxidation operation: Weigh 10 g of the acid-washed pulverized coal and place it in 100 ml of 0.1 mol / L nitric acid solution for pre-oxidation treatment at a pre-oxidation temperature of 50 °C for 5 h. Then it is filtered, washed, and dried.

[0043] Activation and pore formation operation: 20 wt% of ammonium carbonate, ammonium bicarbonate and the pre-oxidized pulverized coal are mixed evenly in a mixer, and the proportions of ammonium carbonate and ammonium bicarbonate are both 10 wt%. The mixed material is heated to 1000 °C at a heating rate of 10 °C / min under a nitrogen atmosphere of 100 ml / min and held for 5 h, and then cooled to room temperature to obtain the pore-formed pulverized coal.

[0044] Carbonization operation: The high-temperature carbonization operation is the same as that in Example 1.

[0045] Carbon Coating Operation: 10 g of the hard carbon anode material prepared by the carbonization operation was placed in a fixed-bed reactor and heated to 900 °C under a nitrogen atmosphere. At the same time, the preheaters for propylene and carrier gas N2 were heated to 300 °C. After reaching the set temperature, the flow rate of propylene was set to 50 ml / min and the nitrogen flow rate was 200 ml / min. After both components were preheated to 300 °C, they were introduced into the fixed-bed reactor. The carbon coating time was 3 h. After the coating was completed, the temperature was lowered to room temperature under a nitrogen atmosphere of 200 ml / minN 2 Atmosphere, and finally a high-rank coal-based hard carbon anode material for sodium-ion batteries was obtained.

[0046] The high-rank coal-based hard carbon material after the coating was completed was used as the anode material of the sodium-ion battery to carry out performance tests. The reversible capacity of the obtained coal-based hard carbon anode material was 285.3 mAh / g, and the initial efficiency was 86.7%.

[0047] Example 3

[0048] Crushing and Acid Washing and Ash Removal Operation: After the high-rank coal was crushed by a ball mill, the particle size of the pulverized coal was D50: 8.2 μm and D100: 30 μm. After ball milling, it was placed in a polytetrafluoroethylene container for acid washing and ash removal operation. The concentrations of hydrochloric acid and hydrofluoric acid in the acid washing solution were both 5 mol / L, and the ratio of coal to the acid washing solution was 1 g of coal: 10 ml. Then the polytetrafluoroethylene container was placed in an ultrasonic disperser for dispersion and acid washing and ash removal. The temperature during the acid washing process was 60 °C, and the acid washing time was 24 h. After that, filtration, washing, and drying were carried out to obtain the acid-washed coal.

[0049] Pre-oxidation Operation: Weigh 40 g of the acid-washed pulverized coal and carry out pre-oxidation in 200 ml of 5 mol / L nitric acid solution at a temperature of 60 °C for 2 h. After pre-oxidation, filtration, washing, and drying were carried out to obtain the pre-oxidized pulverized coal.

[0050] Activation and Pore Formation Operation: 5% of ammonium carbonate and ammonium bicarbonate were mixed evenly with the pre-oxidized pulverized coal in a mixer, where the proportion of ammonium carbonate was 1% and the proportion of ammonium bicarbonate was 4%. The mixed material was placed in a high-temperature furnace and heated to 500 °C at a heating rate of 10 °C / min with a N2 flow rate of 100 ml / min, and held for 2 h. Then the temperature was lowered to room temperature to obtain the pulverized coal with pores formed.

[0051] Carbonization Operation: The pre-oxidized pulverized coal was placed in a tube furnace and heated to the set temperature at a heating rate of 5 °C / min under a N2 atmosphere, and carbonized at 1400 °C for 2 h.

[0052] Carbon coating operation: 10 g of the hard carbon anode material after carbonization was placed in a fixed-bed reactor and heated to 800 °C under a nitrogen atmosphere. At the same time, the preheater for naphtha and carrier gas N2 was heated to 300 °C. The pipeline before entering the coating reactor needs to be insulated, and the temperature of the pipeline also needs to be maintained at 300 °C to ensure no condensation occurs. After reaching the set temperature, the flow rate of naphtha was set to 0.1 g / min, the nitrogen flow rate was 200 ml / min, and the carbon coating time was 5 h. After coating, it was cooled to room temperature under a N2 atmosphere of 200 ml / min, and finally, a high-rank coal-based hard carbon anode material for sodium-ion batteries was obtained.

[0053] The high-rank coal-based hard carbon material after coating was used as the anode material of a sodium-ion battery to conduct performance tests. The reversible capacity of the obtained coal-based hard carbon anode material was 290 mAh / g, and the initial efficiency was 89.1%.

[0054] Example 4

[0055] Crushing, acid washing and deashing, and pre-oxidation operations: The crushing, acid washing and deashing, and pre-oxidation operations were the same as in Example 1.

[0056] Activation and pore-forming operation: 10% ammonium bicarbonate was mixed evenly with the pulverized coal after crushing and acid washing and deashing in a mixer. The mixed material was placed in a high-temperature furnace and heated to 600 °C at a heating rate of 5 °C / min with a N2 flow rate of 200 ml / min, held for 3 h, and then cooled to room temperature to obtain the pore-formed pulverized coal.

[0057] Carbonization operation: The high-temperature carbonization operation was the same as in Example 1.

[0058] Carbon coating operation: 10 g of the hard carbon anode material after carbonization was placed in a fixed-bed reactor and heated to 900 °C under a nitrogen atmosphere. After reaching the set temperature, the flow rate of acetylene gas was set to 5 ml / min, the nitrogen flow rate was 495 ml / min, and the carbon coating time was 12 h. After coating, it was cooled to room temperature under a N2 atmosphere of 200 ml / min, and finally, a high-rank coal-based hard carbon anode material for sodium-ion batteries was obtained.

[0059] The high-rank coal-based hard carbon material after coating was used as the anode material of a sodium-ion battery to conduct performance tests. The reversible capacity of the obtained coal-based hard carbon anode material was 310 mAh / g, and the initial efficiency was 91.0%.

[0060] Example 5

[0061] Crushing and acid pickling deashing operation: After 40 g of coal is ball-milled, the particle size of the pulverized coal is D50: 14 μm, D100: 35.01 μm. After crushing, it is placed in a polytetrafluoroethylene container and subjected to acid pickling deashing in an acid pickling solution with both hydrochloric acid and hydrofluoric acid concentrations of 7 mol / L, where the ratio of coal to the acid pickling solution is 1 g of coal: 5 ml. The polytetrafluoroethylene container is placed in an ultrasonic disperser for dispersion and deashing at a temperature of 50 °C. After ultrasonic treatment and acid pickling for 12 h, filtration, washing, and drying are carried out to obtain the acid-pickled pulverized coal.

[0062] Pre-oxidation operation: Weigh 10 g of the acid-pickled pulverized coal and place it in 50 mL of 0.5 mol / L nitric acid solution for pre-oxidation treatment. The pre-oxidation temperature is 50 °C, and the pre-oxidation time is 10 h. After that, filtration, washing, and drying at 110 °C are carried out.

[0063] Activation and pore-forming operation: Ammonium carbonate and ammonium bicarbonate are used as a mixed pore-forming agent for activation and pore-forming, and their proportions are both 10 wt%. After the coal, ammonium carbonate, and ammonium bicarbonate are mixed evenly in a mixer, they are placed in a high-temperature furnace and kept at 1000 °C for 2 h to obtain a high-rank coal-based precursor material after activation and pore-forming.

[0064] Carbonization operation: After the pore-forming of the pulverized coal is completed, it is placed in a high-temperature furnace and carbonized at 1200 °C for 1 h with a nitrogen flow rate of 200 ml / min to obtain a high-rank coal-based hard carbon anode material.

[0065] Carbon coating operation: Place 10 g of the hard carbon anode material prepared by the carbonization operation in a fixed-bed reactor, heat it to 1000 °C under a nitrogen atmosphere, and at the same time heat the preheaters of methane and carrier gas N2 to 300 °C. After reaching the set temperature, set the flow rate of methane to 60 ml / min and the nitrogen flow rate to 90 ml / min. After preheating both components to 300 °C, they are introduced into the fixed-bed reactor. The carbon coating time is 5 h. After the coating is completed, it is cooled to room temperature under a nitrogen atmosphere of 200 ml / min 2 to finally obtain a high-rank coal-based hard carbon anode material for sodium-ion batteries.

[0066] The high-rank coal-based hard carbon material after coating is used as the anode material of a sodium-ion battery to carry out performance tests. The reversible capacity of the obtained coal-based hard carbon anode material is 294.6 mAh / g, and the initial efficiency is 88.9%.

[0067] Comparative Example 1

[0068] Compared with Example 1, the crushing, acid washing and ash removal operations and carbonization operation of this high-degree metamorphic coal are the same as those in Example 1, but without pre-oxidation operation, activation pore-forming operation and carbon coating operation. The reversible capacity of the obtained hard carbon anode material based on coal is 201.78 mAh / g, and the initial efficiency is 74.01%. Compared with the reversible capacity of 270 mAh / g and the initial efficiency of 85.2% in Example 1, there is an obvious decrease.

[0069] Comparative Example 2

[0070] Compared with Example 1, the crushing, acid washing and ash removal operations, pre-oxidation operation and carbonization operation of this high-degree metamorphic coal are the same as those in Example 1, but without activation pore-forming operation and carbon coating operation. The reversible capacity of the obtained hard carbon anode material based on coal is 230.2 mAh / g, and the initial efficiency is 76.01%.

[0071] Comparative Example 3

[0072] Compared with Example 1, the crushing, acid washing and ash removal operations, pre-oxidation operation, activation operation and carbonization operation of this high-degree metamorphic coal are the same as those in Example 1, but without carbon coating operation. The reversible capacity of the obtained hard carbon anode material based on coal is 261.5 mAh / g, and the initial efficiency is 78.01%.

Claims

1. A preparation method of a high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries, the steps include: 1) Taking high-degree metamorphic coal as a precursor; 2) Crushing and pickling the high-degree metamorphic coal to obtain coal powder; 3) Performing low-temperature pre-oxidation on the coal powder with a nitric acid solution; 4) Mixing an inorganic pore-forming agent with the pre-oxidized coal powder and activating to form pores; 5) Performing high-temperature carbonization on the pore-formed coal-based hard carbon precursor material under high temperature and an inert atmosphere to obtain a carbonized high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries; 6) Performing carbon coating treatment on the carbonized coal powder by gas-phase coating to obtain the finally coated high-degree metamorphic coal-based hard carbon anode material for sodium-ion batteries.

2. The method according to claim 1, wherein, in step 2), a drying treatment needs to be carried out first, the drying temperature is 105°C - 120°C, and subsequent treatment is carried out after drying; the particle size of the coal powder obtained after crushing is 0.01μm - 70μm, and the D50 coal powder particle size is 0.01 - 15μm.

3. The method according to claim 1, wherein, the acid used for pickling in step 2) is a mixed solution of one of hydrochloric acid and sulfuric acid and hydrofluoric acid; the concentration of each acid in the mixed solution is 5mol / L - 10mol / L; the ratio of coal to the mixed solution is coal:mixed acid solution = 1g:2ml - 10ml; the temperature is 30 - 70°C, and the time is 0.5 - 24h.

4. The method according to claim 1, wherein, in step 3), the concentration of the nitric acid solution is 0.01mol / L - 5mol / L, and the ratio of coal to the nitric acid solution is coal:nitric acid solution = 1g:3ml - 10ml.

5. The method according to claim 1 or 4, wherein, the pre-oxidation temperature is 30 - 60°C, and the pre-oxidation time is 0.5h - 24h.

6. The method according to claim 1, wherein, the inorganic pore-forming agent described in step 4) is selected from ammonium carbonate / ammonium bicarbonate, and the dosage is 1% - 30%, based on the mass of the coal powder.

7. The method according to claim 1 or 6, wherein, activating to form pores at 500 - 1000°C.

8. The method according to claim 1, wherein, in step 5), the carbonization temperature range is 1000 - 1600°C; carbonization treatment is carried out under an inert atmosphere of N2 or Ar atmosphere, and the carbonization time is 1 - 5h.

9. The method according to claim 1, wherein, the carbon source for the carbon coating described in step 6) comes from one or more of naphtha, PO / SM tar, propylene, methane, and acetylene.

10. The method according to claim 1 or 9, wherein, the carrier gas for the coating process is N2 or Ar; the coating temperature is 500 - 1300°C, and the coating time is 0.5 - 12h.