Preparation method and device of fluorine-doped porous carbon-silicon-carbon composite negative electrode material

Abstract

The application belongs to the technical field of electrochemical material preparation, and particularly relates to a fluorine-doped porous carbon-silicon-carbon composite negative electrode material preparation method and device. The multi-channel thermal chemical vapor deposition device is high in safety and accurate in control, and effectively solves the problems such as pollution emission in the prior art. The fluorine-doped porous carbon-silicon-carbon composite negative electrode material preparation method forms C-F bond structure by pyrolysis of PVDF to induce SEI rich in LiF and improve ICE; nano Si-O-C layer is deposited by OMCTS thermal CVD to replace high-risk SiH4 and realize uniform silicon distribution; the CH4-CVD carbon coating layer improves the conductivity and structural integrity; the comprehensive performance is significantly improved (ICE is increased by about 14%, the cycle retention rate is increased by 20%, and the impedance is reduced by 40%); the process is safe, the raw materials are cheap, and the process is suitable for amplification.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical material preparation technology, specifically relating to a method and apparatus for preparing fluorine-doped porous carbon-silicon-carbon composite anode material. Background Technology

[0002] Currently, with the increasing demand for energy density in lithium-ion batteries, traditional graphite anodes can no longer meet the requirements for high specific capacity. While silicon-based materials possess a high theoretical specific capacity (approximately 4200 mAh / g), they suffer from severe volume expansion and poor cycle stability. Constructing a carbon coating layer on the silicon surface and doping with fluorine can effectively suppress volume expansion and interfacial side reactions. However, traditional silicon-carbon composite materials are often prepared using SiH4, propane, or other gas sources in CVD equipment. These gases pose flammable and explosive risks, challenging experimental safety. Furthermore, existing CVD equipment often only supports a single gas source input, making atmosphere ratio adjustment difficult, and the exhaust gas is not adequately purified, resulting in the emission of organic matter and fluorine-containing gases. Summary of the Invention

[0003] The purpose of this invention is to provide a method and apparatus for preparing fluorine-doped porous carbon-silicon-carbon composite anode materials.

[0004] The first aspect of this application provides a multi-channel thermochemical vapor deposition apparatus, comprising: The system includes a three-way independent gas supply system, a liquid constant temperature vaporization system, a gas mixing and preheating system, a three-temperature zone microfluidic tubular reactor, a tail gas purification and recovery system, and a vacuum pump. The three independent gas supply systems are used to supply inert gas, reducing gas, and carbon source gas to the gas mixing and preheating system; The gas mixing and preheating system is used to mix and preheat the gas and discharge it into a three-temperature zone microfluidic tubular reactor. The liquid isothermal vaporization system is used for independent evaporation and proportional adjustment of liquid organosilicon source, and is integrated into a three-temperature zone microfluidic tubular reactor. The exhaust gas from the three-temperature-zone microfluidic tubular reactor is treated by the exhaust gas purification and recovery system and then discharged through a vacuum pump.

[0005] In one embodiment of this application, the gas mixing and preheating system includes a gas mixer and a preheating pipe section; The three independent gas supply system includes an inert gas branch, a reducing gas branch, and a carbon source gas branch; Each branch includes a gas cylinder, a mass flow controller, a filter, and several valves connected in sequence; Each branch line is connected to the corresponding interface of the gas mixer at its end.

[0006] In one embodiment of this application, the exhaust gas purification and recovery system includes: a cold trap, an alkaline scrubbing tower, and an activated carbon tower connected in sequence; An outlet cooling zone is provided between the cold trap and the three-temperature zone microfluidic tubular reactor.

[0007] In one embodiment of this application, the multi-channel thermochemical vapor deposition apparatus further includes: a PLC safety interlock control system for acquiring flow rate, temperature, pressure, and HF concentration signals, as well as automatic cut-off and flushing interlock control.

[0008] The multi-channel thermochemical vapor deposition device of the present invention has high safety, precise control, and effectively solves the pollution emission problems of existing technologies.

[0009] The second aspect of this application provides a method for preparing a fluorine-doped porous carbon-silicon-carbon composite anode material, comprising: (A) Porous carbon pretreatment; (B) Fluorine doping to obtain surface-fluorinated porous carbon; (C) Si was deposited by OMCTS thermal CVD on the surface of fluorinated porous carbon; (D) Medium-temperature reduction / residue removal; (E) -CVD carbon coating; (F) Cool and remove, characterize and polarize.

[0010] In one embodiment of this application, the temperature of OMCTS thermal CVD deposition of Si in step (C) is 800-1000℃, and the holding time is 2-4 h.

[0011] In one embodiment of this application, the CH4-CVD carbon coating in step (E) is carried out at 900-1100°C, the volume fraction of CH4 is 5%-20%, and the carrier gas is Ar or N2.

[0012] In one embodiment of this application, the method for pretreating porous carbon includes: drying the porous carbon under vacuum at 100-140°C for 2-12 h; The fluorination doping method includes: dissolving PVDF in NMP to prepare a 5-15 wt% solution, impregnating porous carbon for 5-30 min, drying at 80-120℃ for 1-4 h, and then heat-treating at 300-450℃ for 0.5-2 h under an inert atmosphere to obtain surface-fluorinated porous carbon.

[0013] In one embodiment of this application, the method for OMCTS thermal CVD deposition of Si on surface-fluorinated porous carbon includes: The surface-fluorinated porous carbon sample from step (B) is placed in a three-temperature-zone microfluidic tubular reactor and heated to 800-1100℃; preferably 1000℃, with a heating rate of 5-15℃·min. -1 ; At this temperature, OMCTS vapor and a H2 / Ar mixture are introduced, with an OMCTS equivalent of 5-50 mL / min. -1 The H2 flow rate is set according to the OMCTS ratio, and deposition takes place at this temperature for 1-6 hours. Optionally, the OMCTS steam can be obtained by heating the OMCTS thermostatic vaporizer to 120-150°C; preferably 130°C. Preferably, the ratio of OMCTS steam, H2 flow rate to Ar flow rate is 1:4:5, and the deposition time is 4 hours.

[0014] In one embodiment of this application, the intermediate-temperature reduction / residue removal method includes annealing at 300-600°C for 0.5-2 h in an Ar / H2 mixed gas atmosphere. Preferably, the Ar / H2 flow ratio is 95:5.

[0015] In one embodiment of this application, the CH4-CVD carbon coating method includes: introducing CH4 / H2 / Ar at 700-1050°C with a flow rate ratio of 1:3:10, maintaining this temperature for 5-30 minutes to form an outer carbon coating layer. Preferably, the temperature is 950°C and the reaction time is 15 minutes.

[0016] In one embodiment of this application, the method for preparing the fluorine-doped porous carbon-silicon-carbon composite anode material employs the multi-channel thermochemical vapor deposition apparatus described above.

[0017] The method for preparing fluorine-doped porous carbon-silicon-carbon composite anode material of the present invention involves fluorine doping via PVDF pyrolysis to form a CF bond structure and induce SEI to be rich in LiF, thereby improving ICE; OMCTS thermal CVD deposition of a nano-Si-OC layer to replace high-risk SiH4 and achieve uniform silicon distribution; and CH4-CVD carbon coating to improve conductivity and structural integrity. The overall performance is significantly improved (ICE is increased by about 14%, cycle retention is increased by 20%, and impedance is reduced by 40%). The process is safe, the raw materials are inexpensive, and it is suitable for scale-up.

[0018] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained through the structures particularly pointed out in the description and the drawings.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a system block diagram of a multi-channel thermochemical vapor deposition apparatus according to a preferred embodiment of the present invention; Figure 2 This is a schematic diagram of a multi-channel thermochemical vapor deposition apparatus according to a preferred embodiment of the present invention.

[0022] In the picture: 01-Ar gas cylinder; 02-H2 gas cylinder; 03-CH4 gas cylinder; 04A, 04B, 04C, 04D-Mass flow controllers; 05A, 05B, 05C-Filters; 06-OMCTS constant temperature vaporizer; 07-Gas mixer; 08-Preheating pipe section; 09-Three-temperature zone microfluidic tubular reactor; 11-Outlet cooling zone; 12-Cold trap; 13-Alkali washing tower; 14-Activated carbon tower; 15-Tail gas detector; 16-Vacuum pump; 17-Safety valve assembly; 18-Pressure gauge; 20, 21, 22, 23, 24, 25, 26-Valve assemblies; v20, v21, v22, v23-Manual valves. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] See Figure 1 and Figure 2 In one embodiment, the multi-channel thermochemical vapor deposition apparatus includes: a three-way independent gas supply system, a liquid isothermal vaporization system, a gas mixing and preheating system, a three-temperature zone microfluidic tubular reactor, a tail gas purification and recovery system, and a vacuum pump. The three independent gas supply systems are used to supply inert gas, reducing gas, and carbon source gas to the gas mixing and preheating system; The gas mixing and preheating system is used to mix and preheat the gas and discharge it into a three-temperature zone microfluidic tubular reactor. The liquid isothermal vaporization system is used for independent evaporation and proportional adjustment of liquid organosilicon source, and is integrated into a three-temperature zone microfluidic tubular reactor. The exhaust gas from the three-temperature-zone microfluidic tubular reactor is treated by the exhaust gas purification and recovery system and then discharged through a vacuum pump.

[0025] Optionally, the liquid isothermal vaporization system includes an OMCTS (octamethylcyclotetrasiloxane) isothermal vaporizer 06, connected to a three-temperature zone microfluidic tubular reactor 09, with a vaporization temperature range of 100-180℃.

[0026] Optionally, the three-temperature zone microfluidic tubular reactor 09 can be a commercially available model. It may include a microfluidization section in its center, with a gas velocity of 20-100 mL / min, achieving uniform particle agitation through a porous distribution plate.

[0027] Optionally, the gas mixing and preheating system includes a gas mixer 07 and a preheating pipe section 08; the gas mixer 07 may be a three-way mixer.

[0028] Optionally, the three independent gas supply system includes an inert gas branch, a reducing gas branch, and a carbon source gas branch; each branch includes gas cylinders (e.g., Ar cylinder 01, H2 cylinder 02, CH4 cylinder 03), a mass flow controller 04, a filter 05, and several valves connected in sequence; the end of each branch is connected to the corresponding interface of the gas mixer.

[0029] Optionally, the exhaust gas purification and recovery system includes: a cold trap 12, an alkaline scrubbing tower 13, and an activated carbon tower 14 connected in sequence; an outlet cooling zone 11 is provided between the cold trap 12 and the three-temperature zone microfluidic tubular reactor 09.

[0030] Optionally, the multi-channel thermochemical vapor deposition apparatus further includes: a PLC safety interlock control system for acquiring flow rate, temperature, pressure and HF concentration signals, as well as automatic cut-off and flushing interlock control.

[0031] In one embodiment of this application, the preparation method of fluorine-doped porous carbon-silicon-carbon composite anode material may include: (A) Pretreatment of porous carbon: Dry the porous carbon under vacuum at 100-140 ℃ for 2-12 h; (B) Fluoride doping to obtain surface fluorinated porous carbon: Dissolve PVDF in NMP to prepare a 5-15 wt% solution, impregnate the porous carbon for 5-30 min, dry at 80-120 ℃ for 1-4 h, and then heat treat at 300-450 ℃ for 0.5-2 h under an inert atmosphere to obtain surface fluorinated porous carbon; (C) OMCTS thermal CVD deposition of Si on surface-fluorinated porous carbon: The surface-fluorinated porous carbon sample from step (B) is placed in a three-temperature zone microfluidic tubular reactor and heated to 800-1100 ℃ (preferably 1000 ℃, with a heating rate of 5-15 ℃). At this temperature, OMCTS vapor (obtained by a thermostatic vaporizer heated to 120-150 °C, preferably 130 °C) and an H2 / Ar mixture are introduced, with an OMCTS equivalent of 5-50 mL / min. -1 The H2 and Ar flow rates are set according to the OMCTS ratio (preferably, the ratio of OMCTS steam, H2 flow rate and Ar flow rate is 1:4:5), and deposition is carried out at this temperature for 1-6 h (preferably 4 h). (D) Medium-temperature reduction / residue removal: Annealing at 300-600 ℃ for 0.5-2 h in an Ar / H2 (95:5) mixed gas atmosphere; (E) CH4-CVD carbon coating: The CH4-CVD carbon coating method includes: introducing CH4 / H2 / Ar at 700-1050 °C with a flow rate ratio of 1:3:10. Maintaining this temperature for 5-30 min forms an outer carbon coating layer (preferably 950 °C, reaction time 15 min); (F) Cool and remove, characterize and polarize.

[0032] The preparation method in this embodiment can be achieved using the multi-channel thermochemical vapor deposition apparatus described above.

[0033] The following are specific preparation examples and comparative examples. Example 1

[0034] Step 1: Preparation of starting materials Weigh 1.000 g of porous carbon, place it in a vacuum oven, dry it at 120℃ under vacuum for 6 h, and store it in a glove box. The sample is recorded as S0.

[0035] Step 2: PVDF solution preparation and impregnation (fluorine doping) Preparation of PVDF solution: Weigh 0.100 g of PVDF and add 0.900 mL of NMP. Stir magnetically and sonicate for 10 min to obtain a 10 wt% solution.

[0036] Add S0 to the PVDF solution, sonicate for 5 min, and impregnate for 15 min (to ensure the slurry is fully impregnated).

[0037] The sample was dried in an 80 °C ventilation oven for 2 h to remove the solvent, and then further removed the residual solvent in a vacuum at 120 °C for 2 h to obtain S1.

[0038] Step 3: PVDF heat treatment (surface fluorination pretreatment) S1 was placed in a quartz boat and then placed in a three-temperature-zone microfluidic tubular reactor. Ar was introduced at a rate of 200 mL / min. -1 Replacement for 20 minutes.

[0039] Temperature program: Room temperature → 100℃ (3℃·min) -1 → 250℃ (3℃·min) -1 → 400℃ (3℃·min) -1 The sample was kept at 400℃ for 60 min, and then purged with Ar and cooled to room temperature to obtain fluorine-doped sample S2.

[0040] QC: Take a small amount of S2 and perform XPS (F1s) to confirm the presence of the CF peak. The target fluorine doping amount is 0.5-3 at.

[0041] Step 4: OMCTS thermal CVD deposition of nano-Si Place S2 in the center of the quartz boat and insert it into a three-zone microfluidic tubular reactor. Begin with Ar at 200 mL / min. -1 Purge for 20 minutes.

[0042] Heating rate 10℃·min -1 The temperature is raised to 1000°C (center temperature). At the same time, the OMCTS are loaded into a heating bubbler and heated to 130°C to generate stable steam.

[0043] When the furnace temperature approaches 1000℃ (or stabilizes), use 20 mL / min of OMCTS steam. -1 (Equivalent meter) Input, simultaneously introduce 80 mL·min H2 -1 (H2 / OMCTS = 4 / 1) with Ar protective gas. Maintain at 1000 ℃ for 4 h (can be adjusted to 1-6 h depending on the required Si content).

[0044] After deposition, the OMCTS supply and H2 supply are stopped, and Ar is used to purge the sample and allow it to cool naturally to room temperature. The sample is then removed and labeled as S3 (at this point, a nanoscale Si or Si-OC mixed phase layer is formed on the sample surface, with a thickness of 10-50 nm, depending on the deposition time).

[0045] Step 5: Medium-temperature residue removal / partial reduction To remove organic residues or partially deoxygenate, S3 is flowed at 200 mL / min in Ar / H2 (95 / 5) atmosphere. -1 The heating rate is 5℃·min -1 Heat to 500℃, hold for 60 minutes, cool and remove S4.

[0046] Step 6: CH4-CVD outer carbon coating S4 was placed in a CH4-CVD quartz boat, Ar was introduced at 300 mL·min⁻¹, and the temperature was raised to 950℃ (10℃·min⁻¹). -1 (To reduce the risk of Si agglomeration, 800-900℃ can be used).

[0047] Introduce CH430 sccm, H290 sccm and Ar300 sccm at 950 ℃, maintain the pressure at approximately 0.5-2 Torr (within the equipment's allowable range), and hold for 15 min.

[0048] After completion, CH4 was turned off, and Ar was used for purging. The sample was then cooled to <200℃ and removed. The sample was designated as the final product S_final_A. This sample should exhibit a thin nano-Si layer on a porous carbon matrix, coated with a continuous graphitized carbon layer, with residual F detectable on the surface.

[0049] Step 7: Electrode preparation and electrochemical testing Active ingredient (S_final_A): Super P : Na-CMC = 80 : 10 : 10 (mass ratio), dispersed in deionized water to prepare a slurry, coated on copper foil, with a dry film active ingredient loading of 1.0 mg·cm³. -2 .

[0050] Assemble CR2032 half-cells (Li metal counter electrode) in an Ar glove box. The electrolyte is 1 M LiPF6 in EC:DEC (1:1) + 10 wt% FEC, and the separator is Celgard 2400.

[0051] Formation: 0.05C × 2 times; Normal cycle: 0.2C; EIS test frequency range: 100 kHz - 0.01 Hz.

[0052] Expected performance: ICE ≈ 80-90%; capacity retention ≥ 80% after 100 cycles.

[0053] Example 2 (Unfluorinated control) The process was followed as in Example 1, but the PVDF impregnation and heat treatment steps were omitted (i.e., directly from step 1 → OMCTSCVD → CH4CVD). The sample was designated B_final. Comparative tests showed that the ICE (70-78%) was significantly lower than that of Example 1, and the cycle stability and interfacial impedance were poorer.

[0054] Example 3 (Uncoated Control) The procedure of Example 1 was followed, but the CH4-CVD carbon coating step was omitted (i.e., annealing and polarization were performed directly after OMCTS deposition), and the sample was designated C_final. Comparative tests showed high initial capacity but rapid cycle decay (low retention rate after 50 cycles), indicating that the outer carbon coating is crucial for cycle stability.

[0055] Performance comparison is as follows Example 1

[0056] The electrochemical performance is shown in the table below: Rate performance: The discharge capacities at 0.1C, 0.5C, 1C and 2C are 956, 910, 865 and 812 mAh·g-1, respectively, and the capacity recovery rate is 98.3% when restored to 0.1C.

[0057] The EIS fitting results are shown in the table below: After 100 cycles, Rct only increased slightly by 8%, indicating that the interface is stable.

[0058] Example 2 (Unfluorinated control) XPS shows no F1s peak; ICE is 74.5%, retention rate after 100 cycles is 71.4%, and EIS Rct = 78.6 Ω. In SEI, FEC decomposition produces less LiF, leading to interface instability and significant impedance increase.

[0059] Example 3 (Uncarbon-coated control) The initial discharge capacity was 1110 mAh·g⁻¹, with an ICE of only 72.3%, and the capacity retention dropped to 62% after 50 cycles. SEM showed cracking of the silicon layer, and the EIS Rct increased by 180%.

[0060] In summary, the preparation method of the fluorine-doped porous carbon-silicon-carbon composite anode material of the present invention involves fluorine doping through PVDF pyrolysis to form a CF bond structure and induce SEI to be rich in LiF, thereby improving ICE; OMCTS thermal CVD deposition of a nano-Si-OC layer to replace the high-risk SiH4 and achieve uniform silicon distribution; and CH4-CVD carbon coating to improve conductivity and structural integrity. The overall performance is significantly improved (ICE is increased by about 14%, cycle retention is increased by 20%, and impedance is reduced by 40%). The process is safe, the raw materials are inexpensive, and it is suitable for scale-up.

[0061] It should be noted that all the devices (parts whose specific structures are not specified) selected in this application are general standard parts or parts known to those skilled in the art, and their structures and principles can be known to those skilled in the art through technical manuals or conventional experimental methods.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A multi-channel thermochemical vapor deposition apparatus, characterized in that, include: The system includes a three-way independent gas supply system, a liquid constant temperature vaporization system, a gas mixing and preheating system, a three-temperature zone microfluidic tubular reactor, a tail gas purification and recovery system, and a vacuum pump. The three independent gas supply systems are used to supply inert gas, reducing gas, and / or carbon source gas to the gas mixing and preheating system; The gas mixing and preheating system is used to mix and preheat the gas and discharge it into a three-temperature zone microfluidic tubular reactor. The liquid isothermal vaporization system is used for independent evaporation and proportional adjustment of the liquid organosilicon source, and inputs it into a three-temperature-zone microfluidic tubular reactor. The exhaust gas from the three-temperature-zone microfluidic tubular reactor is treated by the exhaust gas purification and recovery system and then discharged through a vacuum pump.

2. The multi-channel thermochemical vapor deposition apparatus according to claim 1, characterized in that, The gas mixing and preheating system includes a gas mixer and a preheating pipe section; The three independent gas supply system includes an inert gas branch, a reducing gas branch, and a carbon source gas branch; Each branch includes a gas cylinder, a mass flow controller, a filter, and several valves connected in sequence; Each branch line is connected to the corresponding interface of the gas mixer at its end.

3. The multi-channel thermochemical vapor deposition apparatus according to claim 1, characterized in that, The exhaust gas purification and recovery system includes: a cold trap, an alkaline scrubbing tower, and an activated carbon tower connected in sequence; An outlet cooling zone is provided between the cold trap and the three-temperature zone microfluidic tubular reactor.

4. The multi-channel thermochemical vapor deposition apparatus according to claim 3, characterized in that, Also includes: The PLC safety interlock control system is used to collect signals of flow rate, temperature, pressure and HF concentration, as well as automatic shut-off and flushing interlock control.

5. A method for preparing a fluorine-doped porous carbon-silicon-carbon composite anode material, characterized in that, include: (A) Porous carbon pretreatment; (B) Fluorine doping to obtain surface-fluorinated porous carbon; (C) Si was deposited by OMCTS thermal CVD on the surface of fluorinated porous carbon; (D) Medium-temperature reduction / residue removal; (E)CH4-CVD carbon coating; (F) High-temperature graphitization treatment to obtain fluorine-doped porous carbon-silicon-carbon composite anode material; in In step (C), the temperature for OMCTS thermal CVD deposition of Si is 800-1000℃, and the holding time is 2-4 h; and In step (E), CH4-CVD carbon coating is carried out at 900-1100℃, with a CH4 volume fraction of 5%-20%, and the carrier gas is Ar or N2.

6. The preparation method according to claim 5, characterized in that, The method for pretreating porous carbon includes drying the porous carbon under vacuum at 100-140 ℃ for 2-12 h; The fluorination doping method includes: dissolving PVDF in NMP to prepare a 5-15 wt% solution, impregnating porous carbon for 5-30 min, drying at 80-120 ℃ for 1-4 h, and then heat-treating at 300-450 ℃ for 0.5-2 h under an inert atmosphere to obtain surface-fluorinated porous carbon.

7. The preparation method according to claim 5, characterized in that, The method for OMCTS thermal CVD deposition of Si on surface-fluorinated porous carbon includes: The surface-fluorinated porous carbon sample from step (B) was placed in a three-temperature-zone microfluidic tubular reactor and heated to 800-1100℃. At this temperature, OMCTS vapor and a H2 / Ar mixed gas were introduced, with an OMCTS equivalent of 5-50 mL / min. -1 The H2 flow rate is set according to the OMCTS ratio, and deposition is carried out at this temperature for 1-6 hours.

8. The preparation method according to claim 5, characterized in that, The intermediate-temperature reduction / residue removal method includes annealing at 300-600 °C for 0.5-2 h in an Ar / H2 mixed gas atmosphere.

9. The preparation method according to claim 5, characterized in that, The CH4-CVD carbon coating method includes: introducing CH4 / H2 / Ar at 700-1050 °C and maintaining the temperature for 5-30 min to form an outer carbon coating layer.

10. The preparation method according to claim 5, characterized in that, The multi-channel thermochemical vapor deposition apparatus as described in any one of claims 1-4 is used.

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