A method for preparing silicon-carbon negative electrode material based on organic pyrolysis-modified porous carbon

Abstract

The present invention relates to a method for preparing a silicon-carbon negative electrode material based on porous carbon modified by pyrolysis of organic matter, comprising the following steps: S1: activating a carbon raw material to form pores, and obtaining a porous carbon material; S2: placing the porous carbon material in an inert atmosphere, and performing high-temperature pretreatment; S3: pyrolyzing and modifying the pretreated porous carbon material in the presence of organic small molecule vapor; S4: performing silicon vapor deposition on the modified porous carbon by silane gas, and obtaining a silicon-carbon precursor; S5: using a carbon-containing gas to coat the surface of the silicon-carbon precursor at high temperature, and obtaining a product silicon-carbon negative electrode material. The present invention uses small molecule organic matter pyrolysis to modify porous carbon, fill ultra-micropores and repair defects in the carbon layer of the pore wall, and form a dense carbon layer structure on the surface of the pore wall, thereby improving the conductivity and mechanical strength of the material and reducing irreversible capacity loss.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery materials, and in particular relates to a method for preparing a silicon-carbon negative electrode material based on porous carbon modified by pyrolysis of organic matter. Background Art

[0002] Lithium-ion batteries are widely used in consumer electronics, energy storage systems, and electric vehicles due to their high energy density, long cycle life, and stability. However, the specific capacity limit of traditional graphite anode materials (theoretical specific capacity of 372 mAh / g) can no longer meet the current demand for batteries with higher energy density. Silicon is considered to be the most promising anode material to replace graphite because of its high theoretical specific capacity (about 4200 mAh / g) and abundant reserves. However, the silicon anode will undergo a volume expansion of up to 300% during the charge and discharge process, resulting in structural damage to the material, interface failure, and irreversible capacity loss, which seriously affects the cycle performance and stability of the battery. These problems limit the widespread application of silicon anode materials.

[0003] To solve the above problems, silicon-carbon composite negative electrode materials came into being. By combining silicon and carbon materials, not only can the conductivity and structural stability of carbon be used to buffer the volume expansion of silicon, but also the electrochemical performance of silicon negative electrode can be improved. Among many carbon materials, porous carbon, due to its high specific surface area and rich pore structure, can provide deposition sites and confined space for silicon, inhibiting its volume expansion, and thus has become the research focus of new silicon-carbon negative electrodes.

[0004] Although the existing technology can produce silicon-carbon composite materials with good performance, there are still some technical challenges in practical applications, mainly in terms of the pore structure, surface chemical properties and silicon deposition efficiency of porous carbon as a silicon negative electrode carrier. As a carrier of silicon, porous carbon materials play a vital role in the uniform and efficient deposition of silicon and the electrochemical performance of silicon-carbon negative electrode materials. Specifically, the pore structure and specific surface area of ​​porous carbon directly affect the deposition mode and distribution uniformity of silicon, and thus affect the capacity, cycle stability and charge and discharge efficiency of the battery. The ideal porous carbon material should have pores with moderate pore diameters to provide sufficient space to accommodate silicon, which can effectively alleviate the volume expansion of silicon during the charge and discharge process, thereby reducing the mechanical stress of the material and improving its cycle stability and life. However, there are usually many ultra-micropores in current porous carbon materials, because these ultra-micropores are not only difficult to provide effective deposition space for silicon, but also easy to form irreversible "dead lithium" during the insertion and deinsertion of lithium ions, thereby reducing the first coulombic efficiency and overall cycle performance of the battery. At the same time, some heteroatoms and defects often remain in the graphite layer of porous carbon, which will capture lithium ions, increase irreversible capacity loss, and reduce the conductivity and mechanical strength of carbon materials. Therefore, how to accurately control the pore size distribution, surface chemical properties and structural defects of porous carbon to achieve efficient and uniform deposition of silicon and improve the electrochemical performance of silicon-carbon negative electrode materials is still a key problem facing current technology. Summary of the invention

[0005] Compared with the prior art, the present invention modifies porous carbon by pyrolysis of small molecule organic matter, such as alcohols (methanol, ethanol, propanol, butanol, etc.), aldehydes (formaldehyde, acetaldehyde, propionaldehyde, etc.) and hydrocarbons (benzene, toluene, cyclohexane, etc.). These organic matter have small kinetic size and can enter the very tiny gap-like pores inside the porous carbon. At high temperature, the organic matter first decomposes into highly active small molecule carbon substances (such as carbon free radicals), which react with some oxygen-containing functional groups to generate water and carbon dioxide, while filling ultra-micropores and repairing defects in the carbon layer of the pore wall, forming a dense carbon layer structure on the surface of the pore wall, improving the conductivity and mechanical strength of the material, and reducing irreversible capacity loss. Subsequently, high-performance silicon-carbon negative electrode materials are obtained through silane deposition and carbon coating processes. This method can effectively reduce the ultramicropores in porous carbon, obtain a more concentrated pore size distribution, promote the efficient and uniform deposition of silicon, alleviate the irreversible capacity and conductivity decline caused by heteroatoms and pore wall defects, and significantly improve the first coulombic efficiency and conductivity of silicon-carbon negative electrode materials, providing an efficient and reliable technical solution for the development of high-performance silicon-carbon negative electrode materials.

[0006] The present invention aims to provide a silicon-carbon negative electrode material based on organic pyrolysis-modified porous carbon and a preparation method thereof, which significantly improves the first coulombic efficiency and cycle stability of the material by filling ultra-micropores, repairing pore wall defects to form a dense carbon layer structure and removing heteroatoms, and meets the practical application requirements of high energy density lithium-ion batteries. Specifically, the present invention provides the following technical solutions to solve the above technical problems:

[0007] A method for preparing a silicon-carbon negative electrode material based on organic pyrolysis-modified porous carbon comprises the following steps:

[0008] S1: activating the carbon raw material to form pores to obtain a porous carbon material;

[0009] S2: placing the porous carbon material in an inert atmosphere for high temperature pretreatment;

[0010] S3: performing pyrolysis modification on the pretreated porous carbon material in the presence of organic small molecule vapor;

[0011] S4: performing silicon vapor deposition on the modified porous carbon by using silane gas to obtain a silicon-carbon precursor;

[0012] S5: Use carbon-containing gas to coat the surface of the silicon-carbon precursor at high temperature to obtain the product silicon-carbon negative electrode material.

[0013] Further, in step S1, the specific surface area of ​​the obtained porous carbon is 1500-3000m² / g and the pore volume is 0.5-2 cm³ / g; preferably, the specific surface area of ​​the obtained porous carbon is 1600-2200m² / g and the pore volume is 0.8-1.2 cm³ / g. The carbon raw material is not particularly limited, and the carbon source commonly used in the art to prepare porous carbon can be used, such as resin-based carbon precursors (including at least one of phenolic resin, urea resin, and amide resin), biomass-based carbon precursors (including at least one of coconut shell carbon, apricot shell carbon, peach shell carbon, walnut shell carbon, and rice husk carbon). The activation pore-forming method is well known in the art, such as alkali activation, carbon dioxide activation, and water vapor activation, as long as a porous carbon composite of the above provisions can be obtained.

[0014] Furthermore, in step S2, the inert atmosphere is at least one of nitrogen, argon and helium, and the high temperature pretreatment is carried out at 800-1000°C for 1-5h; the purpose of the high temperature pretreatment is to remove impurities and gases adsorbed on the surface to ensure a clean surface for subsequent modification processes.

[0015] Furthermore, in step S3, the organic small molecule is selected from at least one of methanol, ethanol, propanol, butanol, formaldehyde, acetaldehyde, propionaldehyde, cyclohexanone, acetone, benzene, toluene, xylene, and cyclohexane; the organic small molecule is introduced into the furnace through a peristaltic pump to complete gasification, and is brought to the surface of the material by an inert gas to perform pyrolysis modification on the porous carbon; the inert gas is at least one of nitrogen, argon, and helium; and the inert gas flow rate is 1-10 L / min.

[0016] Furthermore, in step S3, the ratio of the liquid volume of the organic small molecule to the porous carbon is 100-300 mL: 1 kg, preferably 150-200 mL: 1 kg, more preferably 160-180 mL: 1 kg, such as 166.7 mL: 1 kg. The conditions for pyrolysis modification are heat treatment at 500-800 ° C for 0.5-2 h. The purpose of pyrolysis modification in the presence of organic small molecules is to reduce the presence of ultra-micropores and form a dense carbon layer structure on the surface of the pore wall, effectively improving the stability of the pore structure.

[0017] Furthermore, in step S3, the organic small molecules are alcohols (at least one of methanol, ethanol, propanol, butanol) and cyclohexane in a liquid volume ratio of 1-2: 1-2. The inventor unexpectedly found that alcohols and cyclohexane synergistically optimized the pore structure and carbon layer properties through different mechanisms of action during the modification of porous carbon materials. Alcohols, such as ethanol molecules and their decomposition products, are more likely to penetrate into fine pores and fill ultrafine pores through carbon deposition during pyrolysis. At the same time, under high temperature conditions, ethanol decomposition produces reducing gases (such as water vapor, acetaldehyde, etc.), which react in the pores, remove oxygen-containing functional groups in porous carbon, and promote graphitization. The carbon atoms and carbon clusters generated by the cracking of cyclohexane are more deposited on the pore walls of porous carbon, which promotes the densification of the pore wall carbon layer. During the pyrolysis process, the olefins and aromatic hydrocarbon intermediates generated by the decomposition of cyclohexane will further polymerize and accumulate on the pore wall surface, enhancing the continuity and stability of the carbon layer. As the temperature rises, these carbon clusters are gradually transformed into graphitized structures, especially under high temperature conditions, carbon atoms tend to be orderly stacked through dynamic driving, thereby forming a more stable and highly conductive graphite layer. In this process, the orderly arrangement of carbon atoms and the strengthening of carbon-carbon bonds help to improve the compactness, mechanical strength and electrical conductivity of the carbon layer. Through synergistic effects, alcohol small molecules effectively remove oxygen-containing functional groups, fill ultrafine pores and optimize the pore structure, while cyclohexane improves the stability of carbon materials by enhancing the compactness and graphitization modification of the carbon layer. The two work together to make the porous carbon material not only have a lower ultrafine porosity, a more concentrated pore size distribution, but also form a denser pore wall structure, significantly improving the first coulomb efficiency and cycle stability of the silicon-carbon composite negative electrode material. Methanol can also theoretically complete the present invention, but considering toxicity, ethanol is preferably used, but the technical solution in which the organic small molecule is methanol is not excluded from the scope of protection of the present invention.

[0018] Further, in step S4, the silane gas is selected from at least one of monosilane, disilane, dichlorosilane, and chlorosilane, the silane gas flow rate is 0.3-5L / min, the inert gas flow rate is 3-20L / min, and the pyrolysis reaction is carried out at 400-600°C for 5-20h; the silane gas flow rate and pyrolysis time are controlled so that the silicon content of the silicon-carbon negative electrode material is 45-50wt%. Alternatively, the silane gas flow rate and pyrolysis time are such that the ratio of the total silane gas volume to the porous carbon mass is 700-1000L:1kg. In step S4, the nano-silicon produced by the pyrolysis of the silane gas is uniformly deposited in the pores of the porous carbon in an amorphous state.

[0019] Further, in step S5, the carbon-containing gas is selected from at least one of methane, ethane, propane, butane, ethylene, propylene, butene, butadiene, acetylene, and propyne, the carbon-containing gas flow rate is 0.5-10L / min, preferably 1-5L / min, and the coating treatment is carried out at 400-600°C for 1-12h; the carbon-containing gas flow rate and coating time are controlled so that the carbon content increment of the silicon-carbon negative electrode material obtained in step S5 relative to the silicon-carbon precursor in step S4 is 1-3wt%; or the total carbon-containing gas volume and silicon-carbon precursor mass ratio is 300-500L:1kg. After carbon coating, a uniform and dense carbon layer is formed on the surface of the material, which improves the conductivity and stability of the silicon-carbon negative electrode material.

[0020] Furthermore, in steps S3, S4 and S5, an inert gas is used as a carrier gas, and the carrier gas flow rate is 1-20 L / min. The inert gas is selected from at least one of nitrogen and argon.

[0021] Compared with the existing technology, the beneficial effects of this technical solution are as follows:

[0022] 1. The organic small molecules of the present invention include alcohols, aldehydes, ketones and hydrocarbons, which have a small kinetic size and can enter the tiny slit-like pores inside the porous carbon. At the same time, they can be pyrolyzed at high temperatures to produce carbon free radicals that can effectively repair the porous carbon and prevent organic matter from depositing on the surface of the porous carbon and blocking the pores. This is also a significant improvement compared to the prior art.

[0023] 2. The small molecule organic matter used in the present invention can be pyrolyzed and repaired under the catalytic effect of the pore wall to form a dense graphite carbon layer on the pore wall, reduce the retention of lithium ions on the carbon surface and the formation of dead lithium, improve its mechanical strength and conductivity, and thus improve the overall performance of the silicon-carbon composite material. This is also a significant improvement compared to the prior art.

[0024] 3. The porous carbon carrier modified by pyrolysis of small molecule organic matter in the present invention has some ultra-micropores effectively filled, which is conducive to the effective deposition and uniform distribution of silane, reduces the irreversible capacity, and thus significantly improves the first coulombic efficiency of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a SEM image of modified porous carbon prepared in Example 4;

[0026] Figure 2 is the nitrogen adsorption isotherm curve of the modified porous carbon prepared in Example 4;

[0027] Figure 3 This is the first charge and discharge curve of a battery assembled with silicon-carbon negative electrode material prepared using modified porous carbon obtained in Example 4. DETAILED DESCRIPTION

[0028] Example 1

[0029] (S1) Using the once activated coconut shell carbon as raw material and water vapor as activating agent, a secondary activation was carried out at 900°C, with a water vapor flow rate of 3 kg / h, a nitrogen flow rate of 2 L / min, and an activation time of 6 h, to obtain a porous carbon with a specific surface area of ​​about 1900 m² / g and a pore volume of about 0.91 cm³ / g.

[0030] (S2) Porous carbon pretreatment step: 3 kg of the porous carbon obtained in step (S1) was placed in a rotary kiln and heated to 900° C. under a nitrogen atmosphere at a gas flow rate of 5 L / min and a holding time of 2 h.

[0031] (S3) Step of pyrolysis modification of porous carbon by organic matter. The porous carbon pretreated in step (S2) is placed in a rotary kiln and nitrogen is introduced. Ethanol enters the rotary kiln through a peristaltic pump for gasification. Nitrogen is used as a carrier gas to contact the porous carbon. The amount of liquid ethanol is 500 mL, the nitrogen gas flow rate is 5 L / min, and the mixture is heated to 600°C for 1.0 h.

[0032] (S4) Chemical vapor deposition step of silicon: 1 kg of the modified porous carbon prepared in step (S3) is placed in a fluidized bed, and monosilane gas and nitrogen gas are introduced. The silane gas flow rate is adjusted to 1 L / min and the nitrogen gas flow rate is adjusted to 9 L / min. The carbon is pyrolyzed at 540°C for 15 h. Nano-silicon is uniformly deposited in the micropores of various porous carbon materials in an amorphous form to obtain a silicon-carbon precursor.

[0033] (S5) Carbon layer coating step: Place 1 kg of the silicon-carbon precursor obtained in step (S4) in a rotary furnace, introduce acetylene gas and nitrogen gas, adjust the acetylene gas flow rate to 1 L / min, and the nitrogen gas flow rate to 5 L / min, and carry out coating treatment at 540°C for 5 hours. Test the carbon content. Compared with the silicon-carbon precursor, the carbon content increases by 2.3 wt%, and a silicon-carbon negative electrode material is obtained.

[0034] Example 2

[0035] The process is similar to that of Example 1, except that in step (S3), 500 mL of ethanol is replaced by 600 mL of methanol, and the pyrolysis temperature is 650°C.

[0036] Example 3

[0037] The process is similar to that of Example 1, except that in step (S3), 500 mL of ethanol is used to replace 300 mL of liquid cyclohexane, the pyrolysis temperature is 750° C., and the holding time is 2 h.

[0038] Example 4

[0039] The process is similar to that of Example 1, except that the 500 mL of ethanol in step (S3) is replaced by a mixture of 250 mL of ethanol and 250 mL of cyclohexane.

[0040] Example 4 Preparation of modified porous carbon SEM image Figure 1 Example 4 Preparation of modified porous carbon nitrogen adsorption isotherm curve see Figure 2 Example 4: The first charge and discharge curve of the silicon-carbon negative electrode material assembled battery obtained by modified porous carbon Figure 3 .

[0041] Example 5

[0042] The process is similar to that of Example 1, except that in step (S3), 500 mL of ethanol is replaced by a mixture of 250 mL of ethanol and 250 mL of toluene.

[0043] Example 6

[0044] The process is similar to that of Example 1, except that in step (S3), 500 mL of ethanol is replaced by a mixture of 250 mL of acetone and 250 mL of cyclohexane.

[0045] Example 7

[0046] (S1) Carbon raw material activation step. Resin carbon is used as a raw material, potassium hydroxide is used as an activator, potassium hydroxide and carbon are mixed in a mass ratio of 1:1, and activated at 800°C for 2.0h under a nitrogen atmosphere with a nitrogen flow rate of 3L / min. After acid washing, water washing, and drying, a porous carbon with a specific surface area of ​​about 2200m² / g and a pore volume of about 0.95cm³ / g is obtained.

[0047] (S2) Porous carbon pretreatment step: 3 kg of the porous carbon obtained in step (S1) was placed in a rotary kiln and heated to 1000° C. in a nitrogen atmosphere at a gas flow rate of 5 L / min for 3 h.

[0048] (S3) Step of modifying porous carbon by pyrolysis of organic matter. The porous carbon pretreated in step (S2) is placed in a rotary kiln and nitrogen is introduced. A mixture of 250 mL of ethanol and 250 mL of cyclohexane enters the rotary kiln for gasification through a peristaltic pump. Nitrogen is used as a carrier gas to contact the porous carbon. The nitrogen gas flow rate is 5 L / min. The porous carbon is heated to 600°C and the holding time is 1.0 h.

[0049] (S4) Chemical vapor deposition step of silicon: 1 kg of porous carbon material is placed in a fluidized bed, and monosilane gas and nitrogen gas are introduced. The silane gas flow rate is adjusted to 1 L / min and the inert gas flow rate is adjusted to 9 L / min. Pyrolysis is carried out at 540°C for 15 hours. Nano-silicon is uniformly deposited in the micropores of various porous carbon materials in an amorphous form to obtain a silicon-carbon precursor.

[0050] (S5) Carbon layer coating step: Place 1 kg of the silicon-carbon precursor obtained in step (S4) in a rotary furnace, introduce acetylene gas and nitrogen gas, adjust the acetylene gas flow rate to 1 L / min, and the nitrogen gas flow rate to 5 L / min, and carry out coating treatment at 540°C for 5 hours, maintaining the carbon content increased by 2.4 wt% to obtain a silicon-carbon negative electrode material.

[0051] Comparative Example 1

[0052] The remaining processes are the same as those in Example 1, except that there is no step of pyrolysis modification of organic matter (S3).

[0053] Comparative Example 2

[0054] The rest of the process is the same as that of Example 5, except that there is no step of pyrolysis modification of organic matter (S3).

[0055] Application Example 1

[0056] The pyrolysis-modified porous carbon sample obtained in step S3 of the above embodiment and the porous carbon sample obtained in step S2 of the comparative example were tested using a specific surface area and pore size analyzer (BET) combined with an argon adsorption-desorption method to determine the specific surface area, pore size distribution and pore volume of the samples. The test results are shown in Table 1.

[0057] Table 1 BET test data of porous carbon

[0058] .

[0059] According to the results in Table 1, ethanol has the best effect in filling ultramicropores, and it can significantly reduce the proportion of ultramicropores; while cyclohexane has a significant effect on pore wall repair, and the average pore size is reduced after treatment. The combination of ethanol and cyclohexane can synergistically optimize the pore size distribution and the carbon layer structure of the pore wall, thereby improving the overall performance of porous carbon materials.

[0060] Application Example 2

[0061] To test the electrochemical properties of silicon-carbon negative electrode materials, the silicon-carbon negative electrode materials, polyacrylic acid (PAA) and Super P prepared in the examples and comparative examples were mixed in a ratio of 8:1:1, and deionized water was used to make an electrode slurry. The slurry was coated on a copper foil, vacuum dried, and cut into 12 mm electrode sheets. In a glove box, a 16 mm lithium sheet was used as the counter electrode, a Celgard 2600 diaphragm, an electrolyte of 1M LiPF6 and EC:EDC (1:1:1 volume ratio), and 5% FEC was added. Assembled into CR2032 button cells, electrochemical tests were performed using a Blue Electric CT2001A system with a voltage range of 0.01-1.5 V and a current density of 200 mA / g. The test results are shown in Table 2.

[0062] Table 2 Electrochemical performance test

[0063] .

[0064] The electrochemical test results show that the silicon-carbon negative electrode treated with ethanol and cyclohexane composite pyrolysis modification exhibits significantly improved first coulombic efficiency and excellent capacity retention.

[0065] This technical solution provides a silicon-carbon negative electrode material based on organic pyrolysis modification, whose ultra-micropores are effectively filled, and the pores effectively improve the effective deposition and distribution of silicon in the porous carbon pores; at the same time, the pore wall defects of the porous carbon are repaired, and a dense graphite carbon layer is formed on the pore wall, which reduces the retention of lithium ions on the carbon surface and the formation of dead lithium, improves its mechanical strength and conductivity, and thus improves the overall performance of the silicon-carbon composite material. Therefore, this technical solution has broad application prospects in the field of battery technology and huge market demand.

Claims

1. A method for preparing a silicon-carbon negative electrode material based on organic pyrolysis-modified porous carbon, characterized in that: The following steps are involved: S1: activating the carbon raw material to form pores to obtain a porous carbon material; S2: placing the porous carbon material in an inert atmosphere for high temperature pretreatment; S3: pyrolyzing and modifying the pretreated porous carbon material in the presence of organic small molecule vapor; the organic small molecule is a compound of alcohol and cyclohexane in a liquid volume ratio of 1-2:1-2; the alcohol is selected from at least one of methanol, ethanol, propanol, and butanol; S4: performing silicon vapor deposition on the modified porous carbon by using silane gas to obtain a silicon-carbon precursor; S5: Use carbon-containing gas to coat the surface of the silicon-carbon precursor at high temperature to obtain the product silicon-carbon negative electrode material.

2. The preparation method according to claim 1, characterized in that: In step S1, the specific surface area of ​​the obtained porous carbon is 1500-3000m² / g and the pore volume is 0.5-2cm³ / g.

3. The preparation method according to claim 1, characterized in that: In step S1, the specific surface area of ​​the obtained porous carbon is 1600-2200 m² / g and the pore volume is 0.8-1.2 cm³ / g.

4. The preparation method according to claim 1, characterized in that: In step S2, the inert atmosphere is at least one of nitrogen, argon and helium, and the high temperature pretreatment is performed at 800-1000°C for 1-5h.

5. The preparation method according to claim 1, characterized in that: In step S3, the organic small molecule is selected from at least one of methanol, ethanol, propanol, butanol, formaldehyde, acetaldehyde, propionaldehyde, cyclohexanone, acetone, benzene, toluene, xylene, and cyclohexane; the ratio of the liquid volume of the organic small molecule to the porous carbon is 100-300mL:1kg; the condition of pyrolysis modification is heat treatment at 500-800℃ for 0.5-2h.

6. The preparation method according to claim 1, characterized in that: In step S3, the ratio of the volume of the organic small molecule liquid to the porous carbon is 150-200 mL: 1 kg.

7. The preparation method according to claim 1, characterized in that: In step S4, the silane gas is selected from at least one of monosilane, disilane, dichlorosilane, and chlorosilane, the silane gas flow rate is 0.3-5L / min, the inert gas flow rate is 3-20L / min, and the pyrolysis reaction is carried out at 400-600°C for 5-20h; the silane gas flow rate and pyrolysis time are controlled so that the silicon content of the silicon-carbon negative electrode material is 45-50wt%; or, the silane gas flow rate and pyrolysis time are controlled so that the ratio of the total silane gas volume to the porous carbon mass is 700-1000L:1kg.

8. The preparation method according to claim 1, characterized in that: In step S5, the carbon-containing gas is selected from at least one of methane, ethane, propane, butane, ethylene, propylene, butene, butadiene, acetylene, and propyne, the flow rate of the carbon-containing gas is 0.5-10L / min, and the coating treatment is carried out at 400-600°C for 1-12h; the carbon-containing gas flow rate and the coating time are controlled so that the carbon content increment of the silicon-carbon negative electrode material obtained in step S5 relative to the silicon-carbon precursor in step S4 is 1-3wt%; or the ratio of the total carbon-containing gas volume to the silicon-carbon precursor mass is 300-500L:1kg.

9. The preparation method according to claim 1, characterized in that: In steps S3, S4 and S5, an inert gas is used as a carrier gas, and the carrier gas flow rate is 1-20 L / min. The inert gas is selected from at least one of nitrogen and argon.

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