High-rate silicon-carbon composite negative electrode material and preparation method thereof

Abstract

The invention discloses a high-rate silicon-carbon composite negative electrode material and a preparation method thereof, and belongs to the technical field of new energy materials and electrochemistry. The preparation method comprises the following steps: performing oxidation layer reconstruction on photovoltaic silicon waste to obtain a two-dimensional Si / SiOx composite material, then mixing the two-dimensional Si / SiOx composite material with metal salt, and performing spray drying granulation and CVD sectional carbon deposition to realize in-situ growth and carbon coating of carbon nanotubes CNTs, thereby obtaining the high-rate silicon-carbon composite negative electrode material. According to the invention, the in-situ grown carbon nanotubes and the silicon-based material form a good conductive network and closer carbon shell coating, Si-C bonds are formed, a multi-dimensional conductive network is constructed, excellent electron / ion conductivity of the composite material is ensured, a reconstructed oxide layer inhibits volume expansion of silicon, pulverization and failure of the material are prevented, and the performance of the composite material is improved. In-situ growth CNTs are interspersed around silicon to buffer the interface stress of the silicon. The prepared composite material has excellent multiplying power and cycle performance, has kilogram-level production capacity, and shows a wide application prospect in the field of electrochemical materials.

Description

Technical Field

[0001] The invention relates to a high-rate silicon-carbon composite negative electrode material and a preparation method thereof, and belongs to the technical field of new energy materials and electrochemistry. Background Art

[0002] With the rapid growth of electronic equipment and new energy vehicle markets, the demand and technical level of lithium-ion batteries continue to increase. In the current market, carbon-based materials such as graphite dominate as negative electrode materials. However, the theoretical specific capacity of commercial graphite negative electrodes is close to the limit (372mAh / g), which greatly limits the improvement of battery energy density. To meet this challenge, the development of new negative electrode materials with higher specific capacity has become an important research direction in the current lithium battery field. Silicon-based negative electrode materials are considered to be one of the ideal negative electrode materials for the next generation of lithium-ion batteries because of their theoretical specific capacity of about 4200mAh / g. However, the volume expansion of silicon-based materials during charging and discharging is as high as 300% or more, resulting in structural rupture and rapid decline in battery performance. In addition, silicon has poor conductivity, which makes its electrochemical performance unable to reach the ideal level at high rates.

[0003] At present, some metals such as nickel, cobalt, iron, vanadium, magnesium, copper, zinc silicon-based particles are coated with carbon nanotubes in the negative electrode active materials. Although the cycle stability of the material is improved, the combination of the directly inserted carbon nanotubes and the silicon-based materials is not tight enough. The added CNTs cannot return to their original position after the electrode cycle, and they degenerate after electrical contact with silicon, resulting in obstruction of electron transmission and capacity loss. At the same time, the polymer amorphous carbon formed at high temperature is not tightly coated enough, resulting in the direct contact between silicon and the electrolyte to trigger side reactions. In addition, the application demand for high-rate silicon-based negative electrode materials prepared by existing technologies is still a bottleneck. The volume expansion of silicon during charging and discharging is the main reason for the failure of traditional silicon-based materials. Therefore, the preparation of a material that can effectively deal with the volume expansion, conductivity and high-rate problems of silicon-based materials while ensuring stable cycle performance is the key to promoting its commercial application. Summary of the invention

[0004] In order to solve these problems, the present invention proposes a method and application of preparing a high-rate silicon-carbon composite negative electrode material using photovoltaic silicon waste. By reconstructing the oxide layer of the pretreated two-dimensional photovoltaic silicon waste, a two-dimensional Si / SiOx composite material with a stable structure is obtained; on this basis, Si / SiOx is mixed with metal salts (such as Cu, Fe, Ni, Co), spray-dried and granulated to prepare a uniformly distributed spherical granular material, and then carbon is deposited in a rotary CVD furnace in stages to achieve in-situ growth and carbon coating of carbon nanotubes (CNTs), thereby obtaining a silicon-carbon composite negative electrode material with a core-shell structure.

[0005] A method for preparing a high-rate silicon-carbon composite negative electrode material comprises the following specific steps:

[0006] (1) drying and crushing photovoltaic silicon waste to obtain coarse silicon powder, placing the coarse silicon powder in a mixed solution of hydrofluoric acid (HF) and alcohol, performing ultrasound and stirring to remove the oxide layer and other impurities on the surface, then performing solid-liquid separation, repeatedly washing the solid with deionized water until the washing liquid is neutral, and obtaining purified silicon powder;

[0007] (2) placing the purified silicon powder in an aqueous solution, uniformly growing an oxide layer under water bath heating conditions, and obtaining a Si / SiOx composite material having a surface oxide layer of different thicknesses by adjusting the reaction conditions. After the reaction is completed, solid-liquid separation is performed to obtain a Si / SiOx composite material;

[0008] (3) mixing the Si / SiOx composite material and the metal salt in an aqueous solution, stirring them thoroughly, and spray drying and granulating the mixture to obtain a spherical Si / SiOx / metal salt composite material;

[0009] (4) placing the spherical Si / SiOx / metal salt composite material obtained in step (3) in a rotary CVD furnace, including three stages S1 to S3, during which the CVD furnace is always rotated to ensure that the reaction proceeds fully;

[0010] S1: placing the spherical Si / SiOx / metal salt composite material in a rotary CVD furnace, gradually increasing the temperature in an Ar-H2 atmosphere for a heat preservation reaction, and reducing the metal salt to metal while maintaining a stable atmosphere to obtain a Si / SiOx / metal composite material;

[0011] S2: On the basis of reducing the metal, a mixed gas of a carbon-containing gas source and Ar is introduced, and the carbon-containing gas source is reacted by pre-treatment with a radio frequency power supply to decompose the carbon-containing gas source into activated carbon and hydrogen atoms. Under the catalytic action of metal particles, the activated carbon in situ grows carbon nanotubes inside the Si / SiOx metal composite material to form a Si / SiOx / metal / CNTs composite material;

[0012] S3: After the growth of carbon nanotubes is completed, the temperature is lowered to the set temperature in an Ar-H2 atmosphere, and a carbon-containing gas source is introduced again. The reaction is kept warm after being treated with a radio frequency power supply. At this time, the growth of carbon nanotubes is stopped for 0.1-10 seconds, and the outside is evenly coated with a carbon layer to obtain the final Si / SiOx / metal / CNTs / C silicon-carbon composite negative electrode material, where x=0-2.

[0013] As an embodiment of the present invention, the photovoltaic silicon waste in the step (1) is diamond wire cutting silicon waste recovered in the photovoltaic industry, the concentration of hydrofluoric acid is 0.1-20 mol / L, the alcohol is at least one of methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, propylene alcohol, and vinyl alcohol, the stirring time is 10-720 min, and the solid-liquid ratio of the coarse silicon powder to the mixed solution is 1:(10-500) g / mL.

[0014] As an embodiment of the present invention, the solid-liquid ratio of the purified silicon powder to water is 1:(10-500) g / mL, the heating temperature is 30-100° C., and the heating time is 10-720 min.

[0015] As an embodiment of the present invention, during the spray drying process of step (2), the solid-liquid ratio of the two-dimensional Si / SiOx composite material to the aqueous solution is 1:(10-500) g / mL, the feed rate is 0.1-200 mL / min, and the spray outlet temperature is 80-200°C.

[0016] As an embodiment of the present invention, the metal salt in step (3) is a copper salt, an iron salt, a nickel salt, a cobalt salt or a silver salt; the copper salt is one of copper sulfate, copper chloride, copper nitrate and copper acetate; the iron salt is one of iron chloride, iron sulfate, iron nitrate, iron chloride and iron sulfate; the nickel salt is one of nickel sulfate, nickel chloride, nickel nitrate and nickel acetate; the cobalt salt is one of cobalt sulfate, cobalt chloride, cobalt nitrate and cobalt acetate; the silver salt is one of silver nitrate, silver chloride and silver acetate, wherein the theoretical mass of the metal accounts for 1% to 50% of the total mass of the spherical Si / SiOx / metal salt composite material.

[0017] As an implementation scheme of the present invention, in S1 and S3, the volume fraction of H2 in the Ar-H2 mixed atmosphere is 1% to 30%, and the balance is Ar; the flow rate of the Ar-H2 mixed gas is 10 to 500 mL / min, the heating rate is 1-30°C / min, the insulation temperature is 800 to 1200°C, and the insulation time is 0.1-10h.

[0018] As an implementation scheme of the present invention, the carbon-containing gas source is at least one of methane, ethylene, acetylene, propylene, propane, carbon tetrafluoride, trifluoromethane, carbon dioxide, carbon monoxide, n-butane, cyclohexane, and naphthalene. The volume fraction of the carbon-containing gas source in the mixed gas is 1% to 50%, the volume remainder is high-purity Ar, and the flow rate of the carbon-containing gas source is 10 to 500 mL / min.

[0019] As an implementation scheme of the present invention, the frequency of the RF power supply in S2 is 10-1000W, the reaction temperature is 800-1200°C, and the reaction time is 10-720min; the frequency of the RF power supply in S3 is 10-1000W, the cooling rate is 1-30°C / min, the insulation temperature is 400-800°C, and the insulation time is 0.1-10h.

[0020] The high-rate silicon-carbon composite negative electrode material of the present invention is a core-shell composite material composed of two-dimensional Si / SiOx, metal particles and carbon nanotubes CNTs interspersed therein, and coated with a carbon shell on the outside.

[0021] The high-rate silicon-carbon composite negative electrode material of the present invention has excellent rate and cycle performance, has kilogram-level production capacity, and shows broad application prospects in the field of electrochemical materials.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention combines spray drying and segmented carbon deposition methods to achieve the in-situ growth of carbon nanotubes catalyzed by metal copper, iron, nickel, cobalt, and silver. The carbon nanotubes (CNTs) grown in-situ in a rotary CVD furnace are uniformly distributed inside or on the surface of the silicon material, forming a good conductive network and a tighter carbon shell coating with the silicon-based material, and forming Si-C bonds, thereby reducing the specific surface area of ​​the composite material, thereby maintaining good electrical contact with silicon during silicon expansion and improving the conductivity of electrons and ions. At the same time, the in-situ grown carbon nanotubes (CNTs) relieve the interfacial stress of the silicon material during the lithiation process, and the multi-dimensional conductive network constructed in collaboration with the metal particles further enhances the conductivity of the material, so that the composite material has excellent rate and cycle performance.

[0024] (2) The present invention develops a pre-treatment oxide layer reconstruction strategy based on the two-dimensional structure of photovoltaic silicon waste, which can controllably and uniformly generate an oxide layer on the silicon surface to obtain a two-dimensional Si / SiOx composite material. The reconstructed oxide layer effectively buffers the volume expansion problem of silicon during the charging and discharging process, significantly improves the structural stability of the material, and reduces the risk of silicon pulverization and failure. At the same time, the silicon of this two-dimensional structure has a large specific surface area and can form a richer interface contact with carbon nanotubes, thereby realizing the preparation of high-rate silicon-carbon negative electrodes.

[0025] (3) The present invention adopts processes such as spray drying granulation and segmented CVD carbon deposition to optimize the material preparation process. The preparation method of the present invention not only ensures the production of high-performance silicon-based negative electrode materials, but also has kilogram-level large-scale production capacity. Compared with the traditional complex preparation process, this solution significantly reduces the manufacturing cost, realizes the value-added utilization of waste resources, and greatly improves the commercial application potential of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the SEM image of the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 1.

[0027] Figure 2 This is the XRD pattern of the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 2.

[0028] Figure 3 This is a SEM image of the Si / SiOx / Fe / CNTs / C silicon-carbon composite negative electrode material prepared in Example 3, and Figure (b) is a partial enlarged image of Figure (a).

[0029] Figure 4 This is the rate performance diagram of the Si / SiOx / Co / CNTs / C silicon-carbon composite negative electrode material prepared in Example 4.

[0030] Figure 5 This is a comparison chart of the half-cell cycle performance of the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 1 and the Si / SiOx / Cu / CNTs silicon-carbon composite material prepared in Comparative Example 1.

[0031] Figure 6 This is a comparison chart of the cycle performance of a full battery composed of the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 1 and the Si / SiOx / Cu / CNTs silicon-carbon composite material prepared in Comparative Example 1 and lithium iron phosphate. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below in conjunction with specific implementation modes, but the protection scope of the present invention is not limited to the described contents.

[0033] Example 1

[0034] A method for preparing a high-rate Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material, the specific steps are as follows:

[0035] (1) Cut silicon waste generated in the photovoltaic industry is used as a raw material, dried and crushed to obtain coarse silicon powder, and the coarse silicon powder is placed in a mixed solution of HF-ethanol, the concentration of HF in the HF-ethanol solution is 1 mol / L, and the solid-liquid ratio of the coarse silicon powder to the mixed solution is 1 g:100 mL, and ultrasonic stirring is performed for 80 minutes. After the stirring is completed, the solid and liquid are separated, and the powder is washed with deionized water until the washing liquid is neutral. After vacuum drying at 80°C for 5 hours, purified silicon powder is obtained.

[0036] (2) Purified silicon powder is placed in an aqueous solution and heated at 80° C. to uniformly grow an oxide layer for 3 hours. After the reaction is completed, solid-liquid separation is performed and the mixture is dried at 80° C. for 5 hours to obtain a Si / SiOx composite material.

[0037] (3) Si / SiOx and Cu(NO3)2 are mixed in an aqueous solution and stirred thoroughly, wherein the theoretical mass of Cu added accounts for 10% of the total mass of the composite material. The mixed solution is spray-dried and granulated to obtain spherical Si / SiOx / Cu(NO3)2 particles.

[0038] (4) placing the spherical Si / SiOx / Cu(NO3)2 composite particles obtained in step (3) in a rotary CVD furnace, and performing carbon deposition in the CVD furnace in three stages, during which the CVD furnace is always kept rotating to ensure that the reaction proceeds fully; Stage 1: placing the spherical Si / SiOx / Cu(NO3)2 composite material in a rotary CVD furnace, in an Ar-H2 atmosphere with a H2 volume fraction of 10% and a gas flow rate of 200 mL / min, gradually heating to 1000°C, keeping the atmosphere stable, so that Cu(NO3)2 is reduced to metallic Cu, thereby obtaining a Si / SiOx / Cu composite material; Stage 2: stopping the introduction of Ar-H2, and introducing a mixed gas of CH4 and Ar with a CH4 volume fraction of 10%, performing RF power pretreatment, and decomposing the carbon-containing gas source into Activated carbon and hydrogen atoms, the radio frequency power is 200W, under the catalytic action of metal particles, the activated carbon grows carbon nanotubes in situ inside the Si / SiOx / Cu composite material to form a Si / SiOx / Cu / CNTs composite material; Stage three: After the growth of carbon nanotubes is completed, the temperature is cooled to 700°C in an Ar-H2 atmosphere at a cooling rate of 10°C / min, and a mixed gas of CH4 and Ar is introduced again, with a CH4 volume fraction of 10%. The carbon-containing gas source is decomposed into activated carbon and hydrogen atoms through radio frequency power pretreatment, and the radio frequency power is 200W. At this time, the growth of carbon nanotubes stops, and the outside is evenly coated with a carbon layer to obtain the final Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material. The single output during the preparation process is 1-20kg, and the product has good uniformity.

[0039] The Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 1 was analyzed by scanning electron microscopy. Figure 1 It can be seen that the in-situ grown carbon nanotubes can be evenly distributed inside or on the surface of the silicon material to form a good conductive network. The segmented carbon deposition in the rotary CVD furnace and the activated carbon layer decomposed by the carbon-containing gas source are evenly coated on the outside of Si / SiOx / Cu / CNTs, achieving a more uniform distribution of carbon nanotubes in the composite material, which is more conducive to improving the electrochemical properties of silicon-based materials.

[0040] Example 2

[0041] A method for preparing a high-rate Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material, the specific steps are as follows:

[0042] (1) Cut silicon waste generated in the photovoltaic industry is used as a raw material, which is dried and crushed to obtain coarse silicon powder, and the coarse silicon powder is placed in an HF-methanol solution and ultrasonically stirred for 120 minutes. The concentration of HF in the HF-methanol solution is 2 mol / L, and the solid-liquid ratio of the coarse silicon powder to the mixed solution is 1 g:200 mL. After the stirring is completed, the solid and liquid are separated, and the powder is washed with deionized water until the washing liquid is neutral. After vacuum drying at 100° C. for 2 hours, purified silicon powder is obtained.

[0043] (2) Purified silicon powder is placed in an aqueous solution and heated at 60° C. to uniformly grow an oxide layer for 5 hours. After the reaction is completed, solid-liquid separation is performed and the mixture is dried at 80° C. for 2 hours to obtain a Si / SiOx composite material.

[0044] (3) Si / SiOx and CuSO4 are mixed and stirred in an aqueous solution, and the theoretical mass of the added Cu accounts for 20% of the total mass of the composite material. The mixed solution is spray-dried and granulated to obtain spherical Si / SiOx / CuSO4 composite particles.

[0045] (4) placing the spherical Si / SiOx / CuSO4 composite particles obtained in step (3) in a rotary CVD furnace for three-stage CVD furnace carbon deposition, wherein the CVD furnace is always kept rotating in the three stages to ensure that the reaction proceeds fully; Stage 1: placing the spherical Si / SiOx / CuSO4 composite particles in a rotary CVD furnace, in an Ar-H2 atmosphere, with a H2 volume fraction of 5%, a gas flow rate of 500 mL / min, gradually heating to 900°C, keeping the atmosphere stable, so that CuSO4 is reduced to metallic Cu, and a Si / SiOx / Cu composite material is obtained; Stage 2: stopping the introduction of Ar-H2, introducing a C2H2 and Ar mixed gas, with a C2H2 volume fraction of 20%, and pre-treating with a radio frequency power supply to decompose the carbon-containing gas source into activated carbon and hydrogen atoms , the RF power is 100W. Under the catalytic action of the metal particles, the activated carbon in situ grows carbon nanotubes inside the Si / SiOx / Cu composite material to form a Si / SiOx / Cu / CNTs composite material; Stage three: After the growth of the carbon nanotubes is completed, the temperature is cooled to 600°C in an Ar-H2 atmosphere at a cooling rate of 5°C / min, and a mixed gas of C2H2 and Ar is introduced again, with a C2H2 volume fraction of 20%. The carbon-containing gas source is decomposed into activated carbon and hydrogen atoms through RF power pretreatment. The RF power is 100W. At this time, the growth of the carbon nanotubes stops, and the outside is evenly coated with a carbon layer to obtain the final Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material. The single output during the preparation process is 1-20kg, and the product has good uniformity.

[0046] The Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in this example was characterized by an X-ray diffraction instrument. Figure 2 It can be seen that compared with the silicon raw material, the XRD spectrum of the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material shows diffraction peaks of copper and carbon, confirming the introduction of metallic copper and the in-situ generation of crystalline CNTs.

[0047] Example 3

[0048] A method for preparing a high-rate Si / SiOx / Fe / CNTs / C silicon-carbon composite negative electrode material, the specific steps are as follows:

[0049] (1) Cut silicon waste generated in the photovoltaic industry is used as a raw material, which is dried and crushed to obtain coarse silicon powder, and the coarse silicon powder is placed in an HF-methanol solution and ultrasonically stirred for 20 minutes. The HF concentration in the HF-methanol solution is 5 mol / L, and the solid-liquid ratio of the coarse silicon powder to the HF-ethanol solution is 1 g:500 mL. After stirring, the solid and liquid are separated, and the powder is washed with deionized water until the washing liquid is neutral. After vacuum drying at 50° C. for 8 hours, purified silicon powder is obtained.

[0050] (2) Purified silicon powder is placed in an aqueous solution and heated at 50° C. to uniformly grow an oxide layer for 10 hours. After the reaction is completed, solid-liquid separation is performed and the mixture is dried at 80° C. for 2 hours to obtain a Si / SiOx composite material.

[0051] (3) Si / SiOx and Fe(NO3)3 are mixed in an aqueous solution and stirred thoroughly, wherein the theoretical mass of Fe added accounts for 30% of the total mass of the composite material. The mixed solution is spray-dried and granulated to obtain spherical Si / SiOx / Fe(NO3)3 particles.

[0052] (4) placing the spherical Si / SiOx / Fe(NO3)3 composite particles obtained in step (3) in a rotary CVD furnace, and performing three-stage CVD furnace carbon deposition, wherein the CVD furnace is always kept rotating in the three stages to ensure that the reaction is fully carried out; Stage 1: placing the spherical Si / SiOx / Fe(NO3)3 composite particles in a rotary CVD furnace, in an Ar-H2 atmosphere, with a H2 volume fraction of 25%, a gas flow rate of 50 mL / min, and gradually heating to 900°C, keeping the atmosphere stable, so that Fe(NO3)3 is reduced to metallic Fe, and a Si / SiOx / Fe composite material is obtained; Stage 2: stopping the introduction of Ar-H2, and introducing a mixed gas of C3H8 and Ar, with a C3H8 volume fraction of 35%, by radio frequency Power pretreatment, decomposition of carbon-containing gas source into activated carbon and hydrogen atoms, the radio frequency power is 350W, the activated carbon under the catalytic action of metal particles, in situ growth of carbon nanotubes inside the Si / SiOx / Fe composite material, forming a Si / SiOx / Fe / CNTs composite material; Stage three: After the growth of carbon nanotubes, the temperature is cooled to 500°C in an Ar / H2 atmosphere at a cooling rate of 25°C / min, and a mixed gas of C3H8 and Ar is introduced again, with a C3H8 volume fraction of 10%. The carbon-containing gas source is decomposed into activated carbon and hydrogen atoms through radio frequency power pretreatment, and the radio frequency power is 350W. At this time, the growth of carbon nanotubes stops, and the outside is evenly coated with a carbon layer to obtain the final Si / SiOx / Fe / CNTs / C silicon-carbon composite material.

[0053] The Si / SiOx / Fe / CNTs / C silicon-carbon composite negative electrode material prepared in this example was analyzed by scanning electron microscopy. Figure 3 It can be seen that in the process of segmented carbon deposition, Fe successfully catalyzed the in-situ generation of CNTs. The formed CNTs were dense and uniform, constructing a conductive network and improving the electrochemical properties of the composite material.

[0054] Example 4

[0055] A method for preparing a high-rate Si / SiOx / Co / CNTs / C silicon-carbon composite negative electrode material, the specific steps are as follows:

[0056] (1) Cut silicon waste generated in the photovoltaic industry is used as a raw material, which is dried and crushed to obtain coarse silicon powder, and the coarse silicon powder is placed in an HF-propanol solution and ultrasonically stirred for 120 minutes. The HF concentration in the HF-propanol solution is 2 mol / L, and the solid-liquid ratio of the coarse silicon powder to the HF-ethanol solution is 1 g:400 mL. After the stirring is completed, the solid-liquid separation is performed, and the powder is washed with deionized water until the washing liquid is neutral. After vacuum drying at 120° C. for 2 hours, purified silicon powder is obtained.

[0057] (2) Purified silicon powder is placed in an aqueous solution and heated at 100° C. to uniformly grow an oxide layer for 1 hour. After the reaction is completed, solid-liquid separation is performed and the mixture is dried at 100° C. for 2 hours to obtain a Si / SiOx composite material.

[0058] (3) Si / SiOx and Fe(NO3)3 are mixed in an aqueous solution and stirred thoroughly, wherein the theoretical mass of Fe added accounts for 30% of the total mass of the composite material. The mixed solution is spray-dried and granulated to obtain spherical Si / SiOx / Co(NO3)2 particles.

[0059] (4) placing the spherical Si / SiOx / Co(NO3)2 composite particles obtained in step (3) in a rotary CVD furnace for three-stage CVD furnace carbon deposition, wherein the CVD furnace is always kept rotating in the three stages to ensure that the reaction proceeds fully; stage one: placing the spherical Si / SiOx / Co(NO3)2 composite particles in a rotary CVD furnace, in an Ar-H2 atmosphere, with a H2 volume fraction of 25%, a gas flow rate of 50 mL / min, gradually heating to 1000°C, and keeping the atmosphere stable, so that Co(NO3)2 is reduced to metallic Co, thereby obtaining a Si / SiOx / Co composite material; stage two: stopping the introduction of Ar-H2 and introducing C6H 12 and Ar mixed gas, C3H8 volume fraction is 35%, through radio frequency power pretreatment, decompose carbon-containing gas source into activated carbon and hydrogen atoms, radio frequency power is 550W, activated carbon under the catalytic action of metal particles, in situ grow carbon nanotubes inside Si / SiOx / Co composite material, forming Si / SiOx / Co / CNTs composite material; stage three: after the growth of carbon nanotubes, cool down to 550℃ in Ar / H2 atmosphere, cooling rate is 25℃ / min, and C6H 12 and Ar mixed gas, C6H 12 The volume fraction is 10%. The carbon-containing gas source is decomposed into activated carbon and hydrogen atoms through RF power pretreatment. The RF power is 550W. At this time, the growth of carbon nanotubes stops and the outside is evenly coated with a carbon layer to obtain the final Si / SiOx / Co / CNTs / C silicon-carbon composite material.

[0060] The Si / SiOx / Co / CNTs / C silicon-carbon composite negative electrode material prepared in this embodiment was subjected to rate performance testing. Figure 4 It can be seen that the prepared high-rate silicon-carbon negative electrode exhibits higher conductivity and better structural stability, improves the electrochemical properties of the overall composite material, and can be rapidly charged and discharged at a large current of 20A / g, meeting the application requirements of high rate.

[0061] Comparative Example 1

[0062] A method for preparing a high-rate Si / SiOx / Cu / CNTs silicon-carbon composite negative electrode material, the specific steps are as follows:

[0063] (1) Cut silicon waste generated in the photovoltaic industry is used as a raw material, which is dried and crushed to obtain coarse silicon powder, and the coarse silicon powder is placed in an HF-ethanol solution and ultrasonically stirred for 80 minutes. The HF concentration in the HF-ethanol solution is 1 mol / L, and the solid-liquid ratio of the coarse silicon powder to the HF-ethanol solution is 1 g:100 mL. After the stirring is completed, the solid and liquid are separated, and the powder is washed with deionized water until the washing liquid is neutral. After vacuum drying at 80° C. for 5 hours, purified silicon powder is obtained.

[0064] (2) Purified silicon powder is placed in an aqueous solution and heated at 80° C. to uniformly grow an oxide layer for 3 hours. After the reaction is completed, solid-liquid separation is performed and the mixture is dried at 80° C. for 5 hours to obtain a Si / SiOx composite material.

[0065] (3) Si / SiOx and Cu(NO3)2 are mixed in an aqueous solution and stirred thoroughly, wherein the theoretical mass of Cu added accounts for 10% of the total mass of the composite material. The mixed solution is spray-dried and granulated to obtain spherical Si / SiOx / Cu(NO3)2 particles.

[0066] (4) placing the spherical Si / SiOx / Cu(NO3)2 composite particles obtained in step (3) in a rotary CVD furnace for three-stage CVD furnace carbon deposition, wherein the CVD furnace is kept rotating during the three stages to ensure that the reaction proceeds fully; Stage 1: placing the spherical Si / SiOx / Cu(NO3)2 composite particles in a rotary CVD furnace, in an Ar-H2 atmosphere with a H2 volume fraction of 10% and a gas flow rate of 200 mL / min, gradually heating to 1000°C, keeping the atmosphere stable, so that Cu(NO3)2 is reduced to metallic Cu to obtain a Si / SiOx / Cu composite material; Stage 2: stopping the introduction of Ar-H2 and introducing a mixed gas of CH4 and Ar with a CH4 volume fraction of 10%, decomposing the carbon-containing gas source into activated carbon and hydrogen atoms through radio frequency power pretreatment, the radio frequency power being 200 W, and under the catalytic action of the metal particles, the activated carbon in situ grows carbon nanotubes inside the Si / SiOx / Cu composite material to form a Si / SiOx / Cu / CNTs silicon-carbon composite material.

[0067] In order to further verify the feasibility of the silicon-carbon composite negative electrode material prepared by the method of the present invention in practical application, the Si / SiOx / Cu / CNTs / C silicon-based composite negative electrode material prepared in Example 1 and the Si / SiOx / Cu / CNTs composite material prepared in Comparative Example 1 were subjected to a half-cell cycle performance comparison test. Figure 5 It can be seen that the silicon-carbon composite negative electrode material prepared in Example 1 has better cycle stability.

[0068] In addition, the Si / SiOx / Cu / CNTs / C silicon-carbon composite negative electrode material prepared in Example 1 and the Si / SiOx / Cu / CNTs silicon-carbon composite material prepared in the comparative example were respectively used as negative electrodes, commercial lithium iron phosphate was used as the positive electrode, and LiPF6 (EC:EMC:DMC=1:1:1) was used as the electrolyte to assemble a full battery, and the charge and discharge cycle performance test was carried out at a current density of 0.5C. Figure 6As shown, the material prepared in Example 1 has a first discharge specific capacity of 158 mAh / g, and the capacity retention rates after 100, 200 and 300 cycles are 92%, 81% and 76%, respectively. The material prepared in the comparative example has a first discharge specific capacity of 139 mAh / g, and the capacity retention rates after 100, 200 and 300 cycles are 70%, 54% and 35%, respectively. The reason is that when the third stage of CVD deposition is missing, that is, no segmented carbon deposition is performed, the prepared Si / SiOx / Cu / CNTs silicon-carbon composite material has no complete carbon layer coating, and the volume expansion cannot be effectively suppressed, which will also lead to an increase in specific surface area and an increase in side reactions, thereby reducing the electrochemical performance of the composite material. The cycle performance of the Si / SiOx / Cu / CNTs silicon-carbon composite material prepared in the comparative example confirms the importance of segmented carbon deposition. The carbon nanotubes grown in situ by CVD and a tighter carbon layer coating can reduce the specific surface area of ​​the composite material, thereby increasing the cycle stability.

[0069] In summary, the present invention combines spray drying and segmented carbon deposition methods to achieve the in-situ growth of metal Cu, Fe, Ni, Co, and Ag-catalyzed carbon nanotubes. The in-situ grown carbon nanotubes (CNTs) are uniformly distributed inside or on the surface of the silicon material, reducing the specific surface area of ​​the composite material, thereby increasing the cycle performance of the material; secondly, the in-situ grown carbon nanotubes (CNTs) form a good conductive network and a tighter carbon shell coating with the silicon-based material to form Si-C bonds and construct a multidimensional conductive network, thereby ensuring the excellent electronic / ionic conductivity of the composite material. At the same time, the in-situ grown carbon nanotubes (CNTs) effectively relieve the interfacial stress of the silicon material during the lithiation process, and the multidimensional conductive network constructed in collaboration with the metal particles further enhances the conductivity of the material. In addition, the reconstructed oxide layer effectively buffers the volume expansion of silicon during the charge and discharge process, prevents the material from pulverizing and failing, and enables the composite material to have an excellent rate.

[0070] The embodiments described above are only preferred specific implementation methods of the present invention, which are used to illustrate the technical solution of the present invention rather than to limit it. However, the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, can make equivalent replacements or changes based on the technical solution and inventive concept of the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a high-rate silicon-carbon composite negative electrode material, characterized in that: The specific steps are as follows: (1) drying and crushing photovoltaic silicon waste to obtain coarse silicon powder, placing the coarse silicon powder in a mixed solution of hydrofluoric acid and alcohol, ultrasonicating, stirring, separating, washing the solid until the washing solution is neutral, and obtaining purified silicon powder; (2) placing the purified silicon powder in water, stirring under water bath heating to uniformly grow an oxide layer, obtaining a Si / SiOx composite material with different oxide layer thicknesses, and performing solid-liquid separation to obtain a two-dimensional Si / SiOx composite material; (3) mixing the two-dimensional Si / SiOx composite material with a metal salt in an aqueous solution, stirring the mixture evenly, and spray drying and granulating the mixture to obtain a spherical Si / SiOx / metal salt composite material; (4) placing the spherical Si / SiOx / metal salt composite material obtained in step (3) in a rotary CVD furnace for segmented carbon deposition; The segmented carbon deposition comprises the following steps: S1: placing the spherical Si / SiOx / metal salt composite material in a rotary CVD furnace, heating it in an Ar-H2 atmosphere for heat preservation reaction, and obtaining a Si / SiOx / metal composite material; S2: Then, in a mixed gas containing a carbon source gas and Ar, a reaction is performed by radio frequency power pretreatment to form a Si / SiOx / metal / CNTs composite material; S3: Finally, in the Ar-H2 atmosphere, a carbon-containing gas source is introduced again, the temperature is cooled to the set temperature, and the temperature is kept warm by radio frequency power treatment to obtain the final Si / SiOx / metal / CNTs / C silicon-carbon composite negative electrode material, where x=0-2.

2. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: The photovoltaic silicon waste is diamond wire cutting silicon waste recovered in the photovoltaic industry, the concentration of hydrofluoric acid is 0.1-20 mol / L, the alcohol is at least one of methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol, propylene alcohol, and vinyl alcohol, and the solid-liquid ratio of coarse silicon powder to the mixed solution is 1: (10-500) g / mL.

3. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: The solid-liquid ratio of the purified silicon powder to water is 1:(10-500) g / mL; the heating temperature is 30-100° C., and the heating time is 10-720 min.

4. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: During the spray drying process of step (2), the solid-liquid ratio of the two-dimensional Si / SiOx composite material to the aqueous solution is 1:(10-500) g / mL, the feed rate is 0.1-200 mL / min, and the spray outlet temperature is 80-200°C.

5. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: In the step (3), the metal salt is a copper salt, an iron salt, a nickel salt, a cobalt salt or a silver salt; the copper salt is one of copper sulfate, copper chloride, copper nitrate and copper acetate; the iron salt is one of iron chloride, iron sulfate, iron nitrate, iron chloride and iron sulfate; the nickel salt is one of nickel sulfate, nickel chloride, nickel nitrate and nickel acetate; the cobalt salt is one of cobalt sulfate, cobalt chloride, cobalt nitrate and cobalt acetate; the silver salt is one of silver nitrate, silver chloride and silver acetate; wherein the theoretical mass of the metal accounts for 1% to 50% of the total mass of the spherical Si / SiOx / metal salt composite material.

6. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: In S1 and S3, the volume fraction of H2 in the Ar-H2 atmosphere is 1% to 30%, and the balance is Ar; the gas flow rate of the Ar-H2 is 10 to 500 mL / min, the heating rate is 1-30°C / min, the insulation temperature is 800 to 1200°C, and the insulation time is 0.1-10h.

7. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: The carbon-containing gas source in S2 and S3 is at least one of methane, ethylene, acetylene, propylene, propane, carbon tetrafluoride, trifluoromethane, carbon dioxide, carbon monoxide, n-butane, cyclohexane, and naphthalene. The volume fraction of the carbon-containing gas source in the mixed gas is 1% to 50%, the volume remainder is Ar, and the flow rate of the carbon-containing gas source is 10 to 500 mL / min.

8. The method for preparing a high-rate silicon-carbon composite negative electrode material according to claim 1, characterized in that: The frequency of the RF power supply in S2 is 10-1000W, the reaction temperature is 800-1200°C, and the reaction time is 10-720min; the frequency of the RF power supply in S3 is 10-1000W, the cooling rate is 1-30°C / min, the insulation temperature is 400-800°C, and the insulation time is 0.1-10h.

9. The high-rate silicon-carbon composite negative electrode material obtained by the preparation method according to any one of claims 1 to 8, characterized in that: A core-shell composite material consisting of a two-dimensional Si / SiOx, metal particles and in-situ grown carbon nanotubes CNTs as a core and a shell covered with a carbon shell.

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