Composite silicon-carbon material and preparation method therefor

Abstract

The present invention provides a composite silicon-carbon material and a preparation method therefor. The composite silicon-carbon material comprises a core and a shell. The core comprises nano-silicon, amorphous hydrocarbon, amorphous silicon carbide, and a porous carbon substrate. The nano-silicon, amorphous hydrocarbon, and amorphous silicon carbide are dispersed in the pores of the porous carbon. The shell is amorphous carbon. In the XRD spectrum of the composite silicon-carbon material, a characteristic peak exists at 2θ = 28.4±0.1°, and no characteristic peak exists at 2θ between 34° and 36°. In the composite silicon-carbon material provided by the present invention, the nano-silicon, amorphous hydrocarbon, and amorphous silicon carbide jointly act as active components to synergistically improve the material's compressive resistance and suppress volume expansion during a charge-discharge process, thereby significantly enhancing the electrochemical performance.

Description

A composite silicon-carbon material and its preparation method Technical Field

[0001] This invention relates to the field of lithium-ion anode material technology, and in particular to a composite silicon-carbon anode material and its preparation method. Background Technology

[0002] In the secondary battery field, lithium-ion batteries are widely used in portable electronic devices and electric vehicles due to their high energy density and long cycle life. To adapt to and promote the overall performance improvement of domestic electric vehicles, exploring and optimizing electrode materials with higher energy density has become a mainstream research area in lithium-ion batteries. Among anode materials, commercially mature graphite has a relatively low theoretical specific capacity (372 mAh / g), which is insufficient to meet the high energy density development requirements of lithium-ion batteries. Compared with commercial graphite, silicon has a higher theoretical specific capacity (4200 mAh / g), is more abundant in resources, and has a lower cost. Therefore, it is considered one of the most promising anode materials for lithium-ion batteries today. However, silicon anode materials still face many challenges on the road to industrialization.

[0003] Existing technologies primarily mitigate volume expansion during lithium insertion / extraction in silicon anodes by optimizing the silicon material's structure, fabricating it into structures such as silicon nanowires, silicon nanospheres, and porous silicon. However, nanostructuring / porization increases the material's specific surface area, exacerbating side reactions during lithium insertion and reducing initial efficiency. Furthermore, the fabrication process is relatively complex and costly. Second-generation silicon-oxygen materials, due to their unique structure, exhibit smaller volume expansion than silicon materials; however, lithium insertion in these materials forms inactive lithium silicate, affecting initial efficiency and thus limiting battery system performance. While pre-lithiation can improve initial efficiency, the use and preparation of pre-lithiation reagents significantly increase manufacturing costs and present compatibility issues with aqueous slurries.

[0004] As a new generation of silicon-carbon materials, silane gas is thermally decomposed at high temperatures to form nano-silicon, which is then filled into a porous carbon support to form a silicon-carbon composite material filled with nano-silicon. This material exhibits excellent electrochemical performance. However, the growth of silicon grain clusters and the interfacial compatibility issues between the nano-silicon and the carbon substrate affect the further development of the material's performance. Summary of the Invention

[0005] To address the aforementioned defects in silicon-carbon composite anode materials, this invention provides a composite silicon-carbon material, its preparation method, and its application. This silicon-carbon composite anode material is used as an anode material in lithium-ion batteries, improving the material's voltage resistance and suppressing volume expansion during charging and discharging, while also effectively enhancing the material's conductivity and the cycle stability of the lithium-ion battery.

[0006] The first aspect of the present application provides a composite silicon-carbon material, comprising a core and a shell, wherein the core comprises nano-silicon, amorphous carbon-hydrogen compound, amorphous silicon-carbon compound and porous carbon substrate, wherein the nano-silicon, amorphous carbon-hydrogen compound and amorphous silicon-carbon compound are dispersed in the pores of the porous carbon; the shell is amorphous carbon; the composite silicon-carbon material has a characteristic peak at 2θ of 28.4±0.1° in an XRD pattern, and no characteristic peak at 2θ of 34-36°.

[0007] The characteristic peak at 2θ of 28.4±0.1° in the XRD pattern indicates the presence of nano-silicon grains, and the absence of characteristic peak at 2θ of 34-36° indicates that the silicon-carbon compound is amorphous and no silicon carbide crystal exists. The presence of silicon carbide will lead to the decrease of the electrochemical performance of the material, such as the initial efficiency and cycle stability.

[0008] As the active core material, the core of the present application comprises three active materials: nano-silicon, amorphous carbon-hydrogen compound and amorphous silicon-carbon compound. The nano-silicon provides the main capacity performance as the active material, the amorphous carbon-hydrogen compound inhibits the growth of the silicon grains in the inner layer structure as a flexible buffer layer, and the amorphous silicon-carbon compound supports the skeleton structure of the porous carbon substrate as a rigid buffer layer to improve the pressure resistance of the material and inhibit the volume expansion during the charging and discharging process. The shell is amorphous carbon, which further effectively improves the conductivity of the material and the cycle stability of the lithium ion battery.

[0009] Further, the core accounts for 85%-95% of the mass of the composite silicon-carbon material; the porous carbon substrate accounts for 45%-55% of the total mass of the core; the nano-silicon accounts for 45%-53% of the total mass of the core; the amorphous carbon-hydrogen compound accounts for 1%-5% of the total mass of the core; the amorphous silicon-carbon compound accounts for 1%-5% of the total mass of the core; and the thickness of the carbon coating is 1-10 nm.

[0010] Further, the particle size D50 of the composite silicon-carbon material is 5-10 μm; the carbon content of the composite silicon-carbon material is 45-55 wt%; the pore volume of the porous carbon substrate is 0.7-1.0 cm 3 / g, the pore size is 2-30 nm, and the specific surface area is 1500-2000 m 2 / g; and the particle size of the nano-silicon is 1-5 nm.

[0011] The second aspect of the present application provides a preparation method of the above-mentioned composite silicon-carbon material, comprising the following steps:

[0012] (S1) under a protective atmosphere, placing the porous carbon substrate in a reaction furnace, heating to a first temperature, introducing a first silicon source gas into the reaction furnace, and performing heat preservation at the first temperature to make the silicon source gas decompose to obtain nano-silicon, which is deposited into the pores of the porous carbon substrate to obtain a porous carbon material with deposited nano-silicon, and then cooling to room temperature;

[0013] This step mainly involves chemical vapor deposition of silicon source gas, which is more likely to deposit on the pore wall of the porous carbon because the activation energy of the silicon source gas entering the pores is less than the activation energy required for deposition on the surface, and the specific reaction is as follows: SiH4(g)→Si(s)+2H2(g)

[0014] (S2) under a protective atmosphere, the temperature is raised to the second temperature, the first carbon source gas is introduced into the reaction furnace, and the carbon source gas and the nanosilicon are allowed to react to form an amorphous carbon hydride layer by hydrosilylation, thereby obtaining a porous carbon material deposited with nanosilicon and an amorphous carbon hydride layer, and then the temperature is lowered to room temperature;

[0015] Step (S2) mainly involves the hydrosilylation reaction of active nanosilicon and carbon source gas, which reduces the activity of the reactants by the addition reaction of the Si-H bond on the surface of nanosilicon (amorphous silicon has a large number of Si-H bonds on its surface) and carbon source gas, and provides an amorphous coating layer to improve the interface stability, and the specific reaction is as follows: Si-H+R1=R2→Si-R1H-R2

[0016] (S3) under a protective atmosphere, the temperature is raised to the third temperature, the second silicon source gas and the second carbon source gas are introduced into the reaction furnace, and the temperature is maintained to allow the silicon source gas and the carbon source gas to react to form an amorphous silicon-carbon compound layer, thereby obtaining a porous carbon material deposited with nanosilicon, an amorphous carbon hydride layer, and an amorphous silicon-carbon compound layer;

[0017] Step (S3) mainly involves the chemical vapor co-deposition reaction of the second silicon source gas and the second carbon source gas. At a certain temperature, the silicon source gas is decomposed to form a silicon hydride, the carbon source gas is decomposed to form a carbon hydride, and the two products react to form a silicon-carbon compound layer deposited in the pores of the carbon substrate as a rigid buffer layer to support the porous carbon substrate skeleton structure, thereby improving the pressure resistance of the material and inhibiting the volume expansion during charging and discharging.

[0018] (S4) under a protective atmosphere, the temperature is raised to the fourth temperature, the third carbon source gas is introduced into the reaction furnace, and the temperature is maintained to allow the carbon source gas to decompose to form carbon and deposit on the porous carbon material deposited with nanosilicon, an amorphous carbon hydride layer, and an amorphous silicon-carbon compound layer, thereby obtaining a composite silicon-carbon material;

[0019] Step (S4) mainly involves a chemical vapor deposition reaction, which mainly realizes carbon coating and further improves the electrical conductivity and coating integrity of the material, and the specific reaction is as follows: C x H y (g)→xC(s)+y / 2H2(g)

[0020] Preferably, the protective atmosphere gas is one or more of nitrogen, argon, helium;

[0021] Preferably, the first silicon source gas, the second silicon source gas is independently selected from one or more of monosilane, disilane, dichlorosilane;

[0022] Preferably, the first carbon source gas, the second carbon source, the third carbon source gas is independently selected from one or more of C1-4alkane, C2-4alkene, C2-4alkyne, toluene; further, the C1-4alkane is selected from at least one of methane, ethane, propane, butane, the C2-4alkene is selected from at least one of ethylene, propylene, butene, and the C2-4alkyne is selected from at least one of acetylene, propyne, butyne.

[0023] In a more preferred technical solution of the present application, the first carbon source gas comprises C2-4alkene with a volume content of ≥80%, and the second carbon source gas comprises C2-4alkyne with a volume content of ≥80%. Preferably, the first carbon source gas comprises C2-4alkene with a volume content of ≥90%, and the second carbon source gas comprises C2-4alkyne with a volume content of ≥90%; more preferably, the first carbon source gas comprises C2-4alkene with a volume content of ≥95%, and the second carbon source gas comprises C2-4alkyne with a volume content of ≥95%. In a most preferred technical solution of the present application, the inventors have found that the first carbon source gas is C2-4alkene, and the second carbon source gas is C2-4alkyne. The inventors have found that when the main component of the first carbon source gas is C2-4alkene, and the main component of the second carbon source gas is C2-4alkyne, the utilization rate of the carbon source gas is effectively improved at a set temperature, thereby improving the electrochemical performance of the composite silicon-carbon material.

[0024] Preferably, in step (S1), the temperature is raised to 450-550°C, the flow rate of the first silicon source gas is 5-10 L / min, and the holding time is 5-10 h; and / or in step (S2), the temperature is raised to 280-350°C, the flow rate of the first carbon source gas is 1-5 L / min, and the holding time is 1-5 h; and / or in step (S3), the temperature is raised to 350-450°C, the flow rate of the second silicon source gas is 1-5 L / min, the flow rate of the second carbon source gas is 1-5 L / min, and the holding time is 1-5 h; and / or in step (S4), the temperature is raised to 600-900°C, the flow rate of the third carbon source gas is 3-6 L / min, and the holding time is 1-5 h.

[0025] Preferably, the volume ratio of the first silicon source gas to the second silicon source gas is 8-15:1, preferably 10-14:1

[0026] Preferably, the volume ratio of the first carbon source gas and the second carbon source gas is 4-10:1, preferably 6-8:1.

[0027] In a third aspect, the present application provides a negative electrode sheet comprising the composite silicon-carbon material.

[0028] In a fourth aspect, the present application provides a lithium ion battery comprising the negative electrode sheet.

[0029] The novel composite silicon-carbon material provided by the present application maintains the stable core-shell structure of the third generation silicon-carbon material. The inner core comprises nanosilicon, an amorphous carbon-hydrogen compound layer, an amorphous silicon-carbon compound layer, and a porous carbon substrate. The nanosilicon serves as an active material to provide the main capacity; the amorphous carbon-hydrogen compound layer serves as a flexible buffer layer to inhibit the growth of silicon grains in the inner layer structure; the amorphous silicon-carbon compound layer serves as a rigid buffer layer to support the porous carbon substrate skeleton structure, thereby improving the pressure resistance of the material and inhibiting the volume expansion during the charging and discharging process; and the carbon layer of the outer shell further effectively improves the electrical conductivity of the material and the cycle stability of the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0030] The technical solutions of the embodiments of the present application will be further described in detail below with reference to the accompanying drawings and examples.

[0031] Fig. 1 is a flow chart of the preparation method of the composite silicon-carbon material according to the present application.

[0032] Fig. 2 is an SEM photograph of the composite silicon-carbon material prepared in Example 1.

[0033] Fig. 3 is a TEM photograph of the composite silicon-carbon material prepared in Example 1.

[0034] Fig. 4 is an XRD pattern of the composite silicon-carbon material prepared in Example 1.

[0035] Fig. 5 is an XRD pattern of the composite silicon-carbon material prepared in Comparative Example 3. DETAILED DESCRIPTION

[0036] The technical solutions of the present application will be further described in detail below with reference to the accompanying drawings and examples. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0037] Fig. 1 is a flow chart of the preparation method of the composite silicon-carbon material according to the present application.

[0038] Example 1

[0039] (S1) continuously introduce 80 L / min of nitrogen as a protective gas into the reaction furnace, and then introduce 6 kg of porous carbon (the porous carbon substrate has a pore volume of 0.8 cm 3 / g, a pore size of 2-10 nm, and a specific surface area of 1648 m 2 / g), heat to 550°C at a heating rate of 5°C / min, introduce silane gas after 10 min of heat preservation, the silane gas has a flow rate of 5 L / min, continue heat preservation for 8 h, and then cool to room temperature.

[0040] (S2) keep the flow rate of the nitrogen protective gas unchanged, heat to 350°C at a heating rate of 5°C / min, introduce ethylene after 10 min of heat preservation, the ethylene gas has a flow rate of 3 L / min, continue heat preservation for 2 h, and then cool to room temperature.

[0041] (S3) keep the flow rate of the nitrogen protective gas unchanged, heat to 400°C at a heating rate of 5°C / min, introduce silane and acetylene simultaneously after 10 min of heat preservation, the silane has a flow rate of 3 L / min, and the acetylene has a flow rate of 1 L / min, continue heat preservation for 1 h.

[0042] (S4) keep the flow rate of the nitrogen protective gas unchanged, heat to 750°C at a heating rate of 5°C / min, introduce acetylene after 10 min of heat preservation, the acetylene has a flow rate of 5 L / min, continue heat preservation for 3 h, and obtain the desired composite silicon-carbon material.

[0043] The carbon content of the composite silicon-carbon material is 49.7 wt% as measured by a carbon-sulfur instrument.

[0044] FIG. 2 is an SEM photograph of the composite silicon-carbon material prepared in Example 1. It can be seen that the particle size of the composite silicon-carbon material is in the range of 5-10 μm. FIG. 3 is a TEM photograph of the composite silicon-carbon material prepared in Example 1.

[0045] FIG. 4 is an XRD pattern of the composite silicon-carbon material prepared in Example 1. The characteristic peak at 2θ = 28.4° corresponds to nano-silicon grains, and the silicon grains are calculated to be 2.6 nm according to the Scherrer formula; there is no characteristic peak at 34-36°, indicating that there is no crystal such as silicon carbide, and the silicon-carbon compound is amorphous.

[0046] Example 2

[0047] The difference between this example and Example 1 is that in step (S1), the first temperature during the silane deposition process is 450°C, the flow rate of the silane gas is 4 L / min, and the heat preservation time is 10 h; in step (S2), the second temperature is 280°C, the flow rate of the ethylene gas is changed to 2 L / min, and the heat preservation time is changed to 3 h.

[0048] Example 3

[0049] The difference between this example and Example 1 is that in step (S3), the third temperature is 450℃, the silane gas flow rate is 4L / min, the acetylene gas flow rate is 1.5L / min, and the holding time is 1h.

[0050] Example 4

[0051] The difference between this example and Example 1 is that in step (S3), the third temperature is 450℃, the silane gas flow rate is 6L / min, the acetylene gas flow rate is 2L / min, and the holding time is 1h.

[0052] Example 5

[0053] The difference between this example and Example 1 is that in step (S2), the first carbon source gas is replaced by acetylene, and in step (S3), the second carbon source gas is replaced by ethylene.

[0054] Example 6

[0055] The difference between this example and Example 1 is that both the first carbon source gas and the second carbon source gas are acetylene.

[0056] Example 7

[0057] The difference between this example and Example 1 is that both the first carbon source gas and the second carbon source gas are ethylene.

[0058] Example 8

[0059] The difference between this example and Example 1 is that the first carbon source gas is methane.

[0060] Comparative Example 1

[0061] The difference between this comparative example and Example 1 is that step (S2) is not performed, and a composite silicon-carbon material without a carbon-hydrogen compound layer is obtained. The carbon content of the composite silicon-carbon material is 48.3wt% as measured by a carbon-sulfur instrument.

[0062] Comparative Example 2

[0063] The difference between this comparative example and Example 1 is that the co-deposition reaction of the silicon source gas and the carbon source gas in step (S3) is not performed, and a composite silicon-carbon material without a nano-silicon-carbon compound is obtained. The carbon content of the composite silicon-carbon material is 48.0wt% as measured by a carbon-sulfur instrument.

[0064] Comparative Example 3

[0065] The difference between this comparative example and Example 1 is that in step (S3), the temperature is raised to 550℃.

[0066] Figure 5 is the XRD pattern of Comparative Example 3, it can be seen that there are some peaks at 2θ of 34° to 36°, indicating the presence of crystals such as silicon carbide. It can be seen that the temperature cannot be too high during the second chemical vapor deposition reaction of the second silicon source gas and the second carbon source gas in step (S3), otherwise silicon carbide crystals and the like are easily formed, affecting the electrochemical performance of the material.

[0067] Application Examples

[0068] The materials obtained in the above examples and comparative examples were tested and characterized for electrochemical performance:

[0069] (1) Preparation of electrode sheet: the negative electrode material, conductive agent (Super-P), and polyacrylic acid (PAA) binder were mixed in a mass ratio of 8:1:1 to prepare a slurry, which was stirred and uniformly mixed, then coated on a copper foil current collector, dried at room temperature, and then placed in a vacuum oven for further drying at 60°C for 12H to obtain an electrode sheet;

[0070] (2) Battery assembly: the electrode sheet obtained above was cut into a diameter of 10mm, with an active material loading of 1.4mg / cm 2 ; a lithium metal sheet was used as the counter electrode, 1mol / L LiPF6 (solvent was a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1, with 5% by volume of fluoroethylene carbonate added) was used as the electrolyte, and a polypropylene microporous separator was used to assemble a 2032 type button cell in an argon gas glove box, with 50uL of electrolyte added to each battery;

[0071] (3) Battery testing: the charge and discharge cut-off voltages were 1.5V and 0.005V, respectively, followed by charge and discharge cycling tests at 0.1C rate, rate performance tests were carried out at 2C and 3C, respectively, and the capacity retention rate was compared with the capacity at 2C or 3C and the capacity of the first cycle at 0.1C rate. To represent the pressure resistance performance, the prepared electrode sheet was subjected to 10t pressure rolling and then tested for electrochemical performance, and the first cycle performance was compared with that of the unrolled electrode sheet. The test results are shown in Table 1.

[0072] Table 1 Electrochemical performance test

[0073] As can be seen from the data in Table 1, Examples 1-3 represent the adjustment of the deposition section process. Under the premise of ensuring consistent deposition amount, reducing the deposition temperature and the deposition gas amount helps to improve the cycle stability of the material and the capacity development, but has no obvious change on the pressure resistance performance. Relative to Example 1, in Example 4, the amount of the second silicon source gas and the second carbon source gas in step (S3) cannot be too much, otherwise it may affect the electrochemical performance, and it is speculated that a small amount of silicon carbide crystals may be formed. In addition, Examples 5-8 show that different carbon source gases in step (S3) also have an effect on the electrochemical performance of the material, and the preferred technical solution is that the first carbon source is an olefin and the second carbon source is an alkyne, at which time the electrochemical performance of the obtained material is optimal.

Claims

1. A composite silicon-carbon material, characterized in that, The composite silicon-carbon material comprises a core and a shell, the core comprises nanosilicon, amorphous hydrocarbon, amorphous silicon-carbon compound, and porous carbon substrate, wherein the nanosilicon, amorphous hydrocarbon, and amorphous silicon-carbon compound are dispersed in the pores of the porous carbon; the shell is amorphous carbon; the composite silicon-carbon material has a characteristic peak at 2θ of 28.4±0.1° in an XRD pattern, and no characteristic peak at 2θ of 34-36°.

2. The composite silicon-carbon material of claim 1, wherein, The core accounts for 95%-97.5% of the mass of the composite silicon-carbon material; the porous carbon substrate accounts for 45%-53% of the total mass of the core; the nanosilicon accounts for 45%-53% of the total mass of the core; the amorphous hydrocarbon accounts for 1%-5% of the total mass of the core; the amorphous silicon-carbon compound accounts for 1%-5% of the total mass of the core; and the thickness of the carbon coating is 1-10 nm.

3. The composite silicon-carbon material of claim 1, wherein, The particle size D50 of the composite silicon-carbon material is 5-10 μm; the carbon content of the composite silicon-carbon material is 45-55 wt%; the pore volume of the porous carbon substrate is 0.7-1.0 cm3 / g, the pore size is 2-30 nm, and the specific surface area is 1500-2000 m2 / g; and the particle size of the nanosilicon is 1-5 nm.

4. The method of making the composite silicon-carbon material of any one of claims 1-3, characterized in that, The method comprises the following steps: (S1) under a protective atmosphere, placing the porous carbon substrate in a reaction furnace, heating to a first temperature, introducing a first silicon source gas into the reaction furnace, and performing heat preservation at the first temperature to make the silicon source gas decompose to obtain nanosilicon and deposit the nanosilicon into the pores of the porous carbon substrate to obtain a porous carbon material with deposited nanosilicon, and then cooling to room temperature; (S2) under a protective atmosphere, heating to a second temperature, introducing a first carbon source gas into the reaction furnace, and performing heat preservation to make the carbon source gas react with the nanosilicon to form an amorphous hydrocarbon layer to obtain a porous carbon material with deposited nanosilicon and an amorphous hydrocarbon layer, and then cooling to room temperature; (S3) under a protective atmosphere, heating to a third temperature, introducing a second silicon source gas and a second carbon source gas into the reaction furnace, and performing heat preservation to make the silicon source gas and the carbon source gas react to form an amorphous silicon-carbon compound layer to obtain a porous carbon material with deposited nanosilicon, an amorphous hydrocarbon layer, and an amorphous silicon-carbon compound layer; (S4) under a protective atmosphere, heating to a fourth temperature, introducing a third carbon source gas into the reaction furnace, and performing heat preservation to make the carbon source gas decompose to form carbon and deposit the carbon onto the porous carbon material with deposited nanosilicon, an amorphous hydrocarbon layer, and an amorphous silicon-carbon compound layer to obtain a composite silicon-carbon material.

5. The preparation method according to claim 4, characterized in that, The first silicon source gas and the second silicon source gas are independently selected from one or more of monosilane, disilane, and dichlorosilane; and the first carbon source gas, the second carbon source, and the third carbon source gas are independently selected from one or more of C1-4alkane, C2-4alkene, and C2-4alkyne, and toluene.

6. The production method according to claim 5, wherein The C1-4alkane is at least one of methane, ethane, propane, and butane; the C2-4alkene is at least one of ethylene, propylene, and butylene; and the C2-4alkyne is at least one of acetylene, propyne, and butyne.

7. The preparation method according to claim 4, characterized in that, The first carbon source gas comprises C2-4alkene with a volume content of ≥80%, and the second carbon source gas comprises C2-4alkyne with a volume content of ≥80%.

8. The preparation method according to claim 4, characterized in that, In step (S1), the temperature is raised to 450-550 DEG C, the first silicon source gas flow rate is 5-10 L / min, and the holding time is 5-10 h; and / or in step (S2), the temperature is raised to 280-350 DEG C, the first carbon source gas flow rate is 1-5 L / min, and the holding time is 1-5 h; and / or in step (S3), the temperature is raised to 350-450 DEG C, the second silicon source gas flow rate is 1-5 L / min, the second carbon source gas flow rate is 1-5 L / min, and the holding time is 1-5 h; and / or in step (S4), the temperature is raised to 600-900 DEG C, the third carbon source gas flow rate is 3-6 L / min, and the holding time is 1-5 h.

9. The preparation method according to claim 4, characterized in that, The volume ratio of the first silicon source gas and the second silicon source gas is 8-15:1, and the volume ratio of the first carbon source gas and the second carbon source gas is 4-10:

1.

10. The method of claim 4, wherein, The volume ratio of the first silicon source gas and the second silicon source gas is 8-15:1, and the volume ratio of the first carbon source gas and the second carbon source gas is 6-8:1.

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