Silicon-carbon composite material as well as preparation method and application thereof

Abstract

The invention relates to a silicon-carbon composite material and a preparation method and application thereof.The preparation method comprises the steps that a porous graphite raw material is subjected to acid pickling and smashing to obtain pretreated porous graphite, then the pretreated porous graphite is soaked in a carbon precursor solution in a vacuum mode, carbonization treatment is conducted after centrifugal separation, pores of the porous graphite are filled with carbides, physical activation is conducted, and the silicon-carbon composite material is obtained; the preparation method comprises the following steps: forming pores in a carbide to form a graded pore structure to obtain a porous graphite composite material, then carrying out silicon deposition in a fluidized bed to obtain a uniform nano silicon layer, and then carrying out carbon coating treatment to obtain a silicon-carbon composite material; the graphite with relatively low cost is adopted as a matrix, high capacity and long cycle performance of the graphite-based silicon-carbon negative electrode material are realized through hierarchical pore structure design, carbonization-activation synergistic pore forming and interface deposition process optimization, and the graphite-based silicon-carbon negative electrode material can be applied to the fields of power batteries and high-energy-density energy storage, especially the field of lithium ion batteries and has wide application prospects. The expansion rate of the battery can be reduced, and the specific capacity, cycling stability and safety of the lithium ion battery can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery materials, and particularly to a silicon-carbon composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Among the anode materials of lithium-ion batteries, the traditional graphite anode has a low theoretical specific capacity (372 mAh / g), which is difficult to meet the requirements of high-energy-density batteries. While the theoretical specific capacity of silicon-based anodes is as high as 4200 mAh / g, which can significantly improve the energy density of the battery and meet the demand for long endurance in fields such as new energy vehicles and consumer electronics. However, silicon-based anodes face serious volume expansion during charge and discharge (greater than 300%), resulting in the pulverization of the structure of silicon-based anodes and poor cycle life.

[0003] Currently, the method of silicon-carbon compounding is adopted to improve silicon-based anodes. However, the dispersion of silicon and the interfacial bonding in the silicon-carbon composite material are not good, which is prone to cause electrode failure. The silicon-carbon composite material obtained with porous graphite as the matrix can effectively alleviate the volume expansion of silicon. However, the pore size distribution of conventional porous graphite is uneven, resulting in uneven silicon loading. Moreover, directly depositing silicon on porous graphite, the deposited silicon is easy to block the pores of porous graphite, reducing the ion / electron transport efficiency and affecting the capacity and cycle performance of the material.

[0004] To improve the practical application potential of graphite-based silicon-carbon anode materials, various modification methods have been developed in the industry. For example:

[0005] Chinese invention patent with publication number CN119170752A (publication date: December 20, 2024) discloses a silicon-carbon anode material and a preparation method thereof. First, silicon nanoparticles are embedded in the graphite matrix, and then a carbon layer is coated on the outer layer. The multi-layer core-shell structure buffers the expansion stress. However, for the silicon-carbon anode material prepared by this patent, due to the insufficient binding force of the carbon layer, the physical coating is easy to fall off during mechanical processing (such as airflow pulverization), exposing a new surface, and the process is complex, requiring multiple high-temperature sinterings, with high energy consumption and easy to cause silicon oxidation failure.

[0006] Chinese invention patent with publication number CN118888703A (publication date: November 01, 2024) discloses a graphite anode material, a preparation method thereof, and an application thereof. A silicon precursor and graphite are mixed in a solvent, and a silicon-carbon composite material is formed through solvothermal method and subsequent pyrolysis. This patent has the problem of uneven coating. The volatilization of the solvent leads to local concentration differences, incomplete coverage in the corner areas, poor compactness of the carbon layer, and microcracks in the pyrolytic carbon, accelerating capacity decay during cycling due to the penetration of the electrolyte.

[0007] The Chinese invention patent with the publication number CN112366294A (publication date: February 12, 2021) directly mixes graphite with silicon dioxide, then generates silicon through magnesiothermic reduction, and finally coats a carbon layer on its outer layer. However, in the preparation method provided by this patent, local silicon agglomeration is likely to occur during the reduction reaction, resulting in uneven silicon distribution and stress concentration. Moreover, it is difficult to completely remove the residual impurities such as magnesiothermic reaction by-products (such as MgO), which affects the electrochemical performance.

[0008] Although the above-mentioned Chinese invention patent has improved the electrochemical performance of the silicon-carbon composite material to a certain extent, there are still some technical problems:

[0009] (1) Insufficient control of silicon expansion stress: The improvement of silicon volume expansion is limited, resulting in easy pulverization of the electrode structure, rapid attenuation of the cycle capacity, and poor cycle performance.

[0010] (2) Weak interfacial bonding force: The physical carbon-coated layer is mainly bonded to the graphite / silicon matrix by van der Waals forces and is easily detached by mechanical impact.

[0011] (3) Low first-cycle Coulombic efficiency: There are many side reactions on the silicon surface, and the solid electrolyte interface film (SEI film) repeatedly breaks and regenerates, consuming active lithium.

[0012] (4) Poor process economy: Multi-step high-temperature treatment and high-purity raw materials lead to high costs. Summary of the Invention

[0013] The object of the present invention is to address the deficiencies of the prior art and propose a silicon-carbon composite material, its preparation method and application.

[0014] To achieve the above object, in the first aspect, an embodiment of the present invention provides a silicon-carbon composite material, which includes: a porous graphite composite material, a nano-silicon layer uniformly deposited in the pores and on the surface of the porous graphite composite material, and a carbon coating layer coated on the outermost layer of the porous graphite composite material;

[0015] The porous graphite composite material includes: a porous graphite skeleton, and a carbide with a hierarchical pore structure filled in the pores of the porous graphite skeleton;

[0016] The porous graphite composite material is obtained by vacuum impregnating the pretreated porous graphite after pickling and pulverization in a carbon precursor solution, and then successively through centrifugation, carbonization treatment, and physical activation.

[0017] Preferably, the micropore proportion of the porous graphite composite material is 10% - 60%, and the mesopore proportion is 40% - 90%;

[0018] The specific surface area of the porous graphite composite material is 200m 2 / g to 1500 m 2 / g, with an average pore diameter of 2 nm to 10 nm and a total pore volume of 0.2 cm 3 / g to 0.8 cm 3 / g;

[0019] The thickness of the carbon coating layer is 5 nm to 20 nm;

[0020] The mass percentage of the nano-silicon layer in the total mass of the silicon-carbon composite material is 10% to 65%;

[0021] The mass percentage of the carbon coating layer in the total mass of the silicon-carbon composite material is 2% to 6%.

[0022] In a second aspect, an embodiment of the present invention provides a preparation method of the silicon-carbon composite material described in the first aspect. The preparation method includes:

[0023] Step S1: Subject the porous graphite raw material to pickling and crushing in sequence to obtain pretreated porous graphite;

[0024] Step S2: Place the pretreated porous graphite in a carbon precursor solution for vacuum impregnation, so that the carbon precursor solution enters and fills the pores of the pretreated porous graphite, and then perform centrifugal separation treatment to obtain a centrifuged product;

[0025] Step S3: Place the centrifuged product in a high-temperature device for carbonization treatment to obtain a porous graphite skeleton with carbides filled in the pores;

[0026] Step S4: Place the porous graphite skeleton in an activation furnace for physical activation to form a hierarchical pore structure in the carbides in the pores of the porous graphite skeleton, and obtain a porous graphite composite material;

[0027] Step S5: Place the porous graphite composite material in a fluidized bed for silicon deposition to obtain a graphite-based silicon-carbon semi-finished product;

[0028] Step S6: Perform carbon coating treatment on the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

[0029] Preferably, the specific surface area of the porous graphite raw material is 0.1 m 2 / g to 20 m 2 / g, and the average pore diameter is 5 nm to 100 nm;

[0030] The pickling includes: washing the porous graphite raw material with hydrochloric acid having a molar concentration of 1 mol / L to 3 mol / L for 2 hours to 4 hours at a temperature of 60°C to 80°C; the ash content of the pickled porous graphite raw material is less than 0.3%, and the Fe content is lower than 50 ppm;

[0031] The median particle size D of the pretreated porous graphite 50 is 4 μm to 15 μm.

[0032] Preferably, the carbon precursor in the carbon precursor solution includes any one of phenolic resin, coal, pitch, glucose, polyacrylonitrile, and lignin; wherein, when the carbon precursor is a resin, the solvent of the carbon precursor solution is ethanol; when the carbon precursor is coal or pitch, the solvent of the carbon precursor solution is tetrahydrofuran; when the carbon precursor is glucose, the solvent of the carbon precursor solution is deionized water; when the carbon precursor is polyacrylonitrile, the solvent of the carbon precursor solution is N,N-dimethylformamide; when the carbon precursor is lignin, the solvent of the carbon precursor solution is an aqueous NaOH solution; the mass concentration of the carbon precursor in the carbon precursor solution is 10% to 30%;

[0033] The degree of vacuum for the vacuum impregnation is 1×10 -4 to 1×10 -3 Pa, and the time for the vacuum impregnation is 0.5 hour to 1.5 hours.

[0034] Preferably, the carbonization treatment includes: in a protective gas environment, heating the high-temperature equipment at a heating rate of 2 °C / min to 5 °C / min to 600 °C to 800 °C, and holding for 2 hours to 4 hours;

[0035] The high-temperature equipment includes any one of a tube furnace, a box furnace, a rotary furnace, and a pusher furnace;

[0036] The protective gas includes one or more of nitrogen, argon, or helium.

[0037] Preferably, the physical activation includes: introducing carbon dioxide or water vapor with a flow rate of 100 mL / min to 500 mL / min into the activation furnace, performing high-temperature treatment at 800 °C to 1000 °C for 1 hour to 4 hours, and then cooling to room temperature in a protective gas;

[0038] The proportion of micropores in the hierarchical pore structure is 20% to 60%, and the proportion of mesopores is 40% to 90%;

[0039] The specific surface area of the porous graphite composite material is 200 m 2 / g to 1000 m 2 / g, the average pore diameter is 2 nm to 10 nm, and the total pore volume is 0.2 cm 3 / g to 0.8 cm 3 / g.

[0040] Preferably, the silicon deposition includes: heating the temperature of the fluidized bed to 450°C to 900°C at a rate of 2°C / min to 5°C / min in a protective gas environment, introducing a silicon source gas, and maintaining the temperature for 1 hour to 4 hours;

[0041] The protective gas includes one or more of nitrogen, argon, and helium;

[0042] The silicon source gas includes one or more gases of silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, hexachloroethylsilane; the flow rate of the silicon source gas is 1 L / min to 100 L / min;

[0043] The volume ratio of the silicon source gas to the protective gas is 1:1 to 1:3.

[0044] Preferably, the carbon coating treatment is gas-phase carbon coating, including: placing the graphite-based silicon-carbon semi-finished product in a coating furnace, heating it to 500°C to 580°C at a heating rate of 2°C / mi to 5°C / min in a protective atmosphere, introducing a carbon source gas, and maintaining the temperature for 0.5 hour to 2 hours, so that the carbon element decomposed from the carbon source gas is deposited on the surface of the silicon-carbon composite semi-finished product to form a carbon coating layer, and finally obtaining a silicon-carbon composite material;

[0045] Among them, the carbon source gas includes one or more of alkanes, alkenes, and alkynes; the flow rate of the carbon source gas is 5 L / min to 60 L / min.

[0046] In a third aspect, an embodiment of the present invention provides a lithium-ion battery, which includes the silicon-carbon composite material described in the first aspect above, or the silicon-carbon composite material prepared by the preparation method described in the second aspect above.

[0047] A silicon-carbon composite material, a preparation method and an application thereof provided by an embodiment of the present invention have the following

[0048] Beneficial effects:

[0049] (1) In the preparation method of the silicon-carbon composite material provided by the embodiment of the present invention, the porous graphite raw material is pickled and crushed to obtain pretreated porous graphite, and then vacuum-impregnated in a carbon precursor solution. After centrifugal separation, carbonization treatment is carried out to fill the pores of the porous graphite with carbide, and then physical activation is carried out to form hierarchical pores in the carbide to obtain a porous graphite composite material. Then, silicon deposition is carried out in a fluidized bed, and finally carbon coating treatment is carried out to obtain a silicon-carbon composite material.

[0050] By controlling the initial pore size, impurity content and particle size of porous graphite, and optimizing the activation process parameters, the structure of the formed silicon-carbon negative electrode can be controlled; a carbide layer is filled in the pores, and a hierarchical pore structure is formed through physical activation design, and the micro-mesopore distribution can be regulated to avoid the pore structure collapse caused by traditional alkali activation; silicon gas deposition is carried out in a fluidized bed, so that the nano-silicon generated by the decomposition of the silicon source material is uniformly deposited on the surface of the porous graphite composite material, and the gas deposition interface is optimized, and the nano-silicon layer is chemically bonded to the graphite carbon skeleton, thereby inhibiting the volume expansion of silicon; the raw material cost of the present invention is low, the process is simple and controllable, and it is beneficial to large-scale production and popularization and application.

[0051] (2) The silicon-carbon negative electrode material obtained by the preparation method provided by the embodiment of the present invention uses relatively inexpensive graphite as the matrix, and through the design of the hierarchical pore structure, the synergistic pore formation of carbonization-activation and the optimization of the interface deposition process, the high capacity and long cycle performance of the graphite-based silicon-carbon negative electrode material are realized.

[0052] (3) The silicon-carbon negative electrode material provided by the embodiment of the present invention can be applied to power batteries and high-energy density energy storage fields, especially the lithium-ion battery field, which can reduce the expansion rate of the battery and improve the specific capacity, cycle stability and safety of the lithium-ion battery. Description of the Drawings

[0053] Figure 1 It is a flowchart of the preparation method of the silicon-carbon composite material provided by the embodiment of the present invention.

[0054] Figure 2 It is an X-ray diffraction (XRD) pattern of the porous graphite composite material prepared in Example 1 of the present invention.

[0055] Figure 3 It is a scanning electron microscope (SEM) image of the porous graphite composite material prepared in Example 1 of the present invention.

[0056] Figure 4 It is an SEM image of the graphite-based silicon-carbon semi-finished product prepared in Example 1 of the present invention.

[0057] Figure 5 It is an SEM image of the graphite-based silicon-carbon semi-finished product prepared in Comparative Example 4 of the present invention. Detailed Embodiments

[0058] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0059] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0060] The silicon-carbon composite material prepared by the preparation method provided in the embodiment of the present invention includes: a porous graphite composite material, a nano-silicon layer uniformly deposited in the pores and on the surface of the porous graphite composite material, and a carbon coating layer coated on the outermost layer of the porous graphite composite material.

[0061] Among them, the porous graphite composite material includes: a porous graphite skeleton, and a carbide with a hierarchical pore structure filled in the pores of the porous graphite skeleton; specifically, the porous graphite composite material is obtained by vacuum impregnating the pretreated porous graphite after pickling and crushing into a carbon precursor solution, and then successively undergoing centrifugation, carbonization treatment, and physical activation; the precursor of the carbide includes any one of phenolic resin, coal, asphalt, glucose, polyacrylonitrile, and lignin.

[0062] A hierarchical pore structure is formed in the carbide of the porous graphite composite material, the proportion of micropores is 10% - 60%, the proportion of mesopores is 40% - 90%, and the remaining pores are macropores.

[0063] The proportion of micropores in the porous graphite composite material is 10% - 60%, which can be any value within this range, for example: 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0064] The proportion of mesopores in the porous graphite composite material is 40% - 90%, which can be any value within this range, for example: 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0065] The specific surface area of the porous graphite composite material is 200 m 2 / g - 1500 m 2 / g, which can be any value within this range, for example: 200 m 2 / g, 300 m 2 / g, 400 m 2 / g, 500 m 2 / g, 600 m 2 / g, 700 m 2 / g, 800 m 2 / g, 900 m 2 / g, 1000 m 2 / g, 1100 m 2 / g, 1200 m2 / g, 1300 m 2 / g, 1400 m 2 / g, 1500 m 2 / g, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0066] The average pore size of the porous graphite composite is 2 nm to 10 nm, and can be any value within this range, such as: 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0067] The total pore volume of the porous graphite composite is 0.2 cm 3 / g to 0.8 cm 3 / g, and can be any value within this range, such as: 0.2 cm 3 / g, 0.3 cm 3 / g, 0.4 cm 3 / g, 0.5 cm 3 / g, 0.6 cm 3 / g, 0.7 cm 3 / g, 0.8 cm 3 / g, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0068] The percentage of the mass of the nano-silicon layer in the total mass of the silicon-carbon composite is 10% to 65%, and can be any value within this range, such as: 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0069] The thickness of the carbon coating layer in the silicon-carbon composite is 5 nm to 20 nm, and can be any value within this range, such as: 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0070] The percentage of the mass of the carbon coating layer in the total mass of the silicon-carbon composite is 2% to 6%, and can be any value within this range, such as: 2%, 3%, 4%, 5%, 6%, etc., but not limited to the listed values, and other unlisted values within this range are equally applicable.

[0071] An embodiment of the present invention provides a method for preparing a silicon-carbon composite material, as Figure 1 shown, the preparation method includes:

[0072] Step S1, subjecting the porous graphite raw material to pickling and pulverization in sequence to obtain pretreated porous graphite;

[0073] Among them, the specific surface area of the porous graphite raw material is 0.1 m 2 / g to 20 m 2 / g, and the average pore diameter is 5 nm to 100 nm;

[0074] The pickling includes: at a temperature of 60 °C to 80 °C, washing the porous graphite raw material with hydrochloric acid having a molar concentration of 1 mol / L to 3 mol / L for 2 hours to 4 hours;

[0075] The ash content of the pickled porous graphite raw material is less than 0.3%, and the Fe content is lower than 50 ppm; the present invention controls the ash content and Fe content of the porous graphite raw material to reduce the interference of impurities on subsequent silicon deposition, inhibit the formation of inactive silicides (such as AlSi, CaSi, FeSi2), and improve the silicon deposition efficiency; avoid the occurrence of micro-short circuits and catalytic side reactions of impurities existing in the porous graphite raw material during the battery cycle, and ensure cycle stability;

[0076] The median particle size D 50 of the pretreated porous graphite is 4 μm to 15 μm.

[0077] Step S2, placing the pretreated porous graphite in a carbon precursor solution for vacuum impregnation, enabling the carbon precursor solution to enter and fill the pores of the pretreated porous graphite, and then performing centrifugal separation treatment to obtain a centrifuged product;

[0078] Among them, the carbon precursor in the carbon precursor solution includes: any one of phenolic resin, coal, asphalt, glucose, polyacrylonitrile, and lignin; among them, when the carbon precursor is resin, the solvent of the carbon precursor solution is ethanol; when the carbon precursor is coal or asphalt, the solvent of the carbon precursor solution is tetrahydrofuran; when the carbon precursor is glucose, the solvent of the carbon precursor solution is deionized water; when the carbon precursor is polyacrylonitrile, the solvent of the carbon precursor solution is N,N-dimethylformamide (DMF); when the carbon precursor is lignin, the solvent of the carbon precursor solution is an aqueous NaOH solution; the mass concentration of the carbon precursor in the carbon precursor solution is 10% to 30%;

[0079] The vacuum degree of the vacuum impregnation is 1×10 -4 Pa to 1×10 -3 Pa, and the time of the vacuum impregnation is 0.5 hour to 1.5 hours;

[0080] In the present invention, the carbon precursor solution is fully filled into the pores of the pretreated porous graphite by means of vacuum impregnation. First, the gas and moisture adsorbed inside the pores can be removed, and the resin solution is infiltrated into the pores driven by negative pressure to reduce the local concentration difference, so that it is uniformly dispersed inside the pores. Compared with traditional mechanical mixing, the pore filling rate is high, and the carbon precursor solution will not accumulate on the surface of the pretreated porous graphite, which is beneficial to constructing a uniform and complete carbide filling structure in the pores subsequently.

[0081] Step S3: Place the centrifuged product in a high-temperature device for carbonization treatment to obtain a porous graphite framework with carbides filled in the pores;

[0082] Among them, the carbonization treatment includes: under the environment of a protective gas, heating the high-temperature device at a heating rate of 2 °C / min to 5 °C / min to 600 °C to 800 °C, and keeping it warm for 2 hours to 4 hours;

[0083] The high-temperature device includes any one of a tube furnace, a box furnace, a rotary furnace, and a pusher furnace;

[0084] The protective gas includes one or more of nitrogen, argon, or helium.

[0085] Step S4: Place the porous graphite framework in an activation furnace for physical activation to form a hierarchical pore structure in the carbides in the pores of the porous graphite framework, and obtain a porous graphite composite material;

[0086] Among them, the physical activation includes: introducing carbon dioxide or water vapor with a flow rate of 100 mL / min to 500 mL / min into the activation furnace, performing high-temperature treatment at 800 °C to 1000 °C for 1 hour to 4 hours, and then cooling to room temperature under the protective gas;

[0087] The proportion of micropores in the hierarchical pore structure is 10% to 60%, the proportion of mesopores is 40% to 90%, and the rest are macropores;

[0088] The specific surface area of the porous graphite composite material is 200 m 2 / g to 1500 m 2 / g, the average pore diameter is 2 nm to 10 nm, and the total pore volume is 0.2 cm 3 / g to 0.8 cm 3 / g;

[0089] In this step, carbides are filled in the pores of the pre-treated porous graphite, and a hierarchical pore structure is physically activated in the carbides. The micropores and mesopores in the carbides account for a relatively high proportion, and thus a porous graphite composite material with a relatively high specific surface area and pore volume is obtained. This is conducive to enhancing the stability of the carbon structure and effectively suppressing the volume expansion of silicon particles during charge and discharge. The porous carbon and graphite form a continuous electron conduction path, reducing the interfacial resistance and improving the conductivity. The hierarchical pore structure shortens the lithium-ion diffusion path and improves the rate performance. In addition, the surface of the micro-mesopores is rich in defects and functional groups (such as -OH, -COOH), which can enhance the chemical adsorption of silicon precursors, realizing high loading and uniform distribution of silicon.

[0090] Step S5: Place the porous graphite composite material in a fluidized bed for silicon deposition to obtain a graphite-based silicon-carbon semi-finished product.

[0091] Among them, the silicon deposition includes: under the environment of a protective gas, heating the temperature of the deposition equipment to 450°C - 900°C at a rate of 2°C / min - 5°C / min, introducing a silicon source gas, and maintaining the temperature for 1 hour - 4 hours. The protective gas includes one or more of nitrogen, argon, and helium.

[0092] The silicon source gas includes one or more of silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, and hexachloroethylsilane; the flow rate of the silicon source gas is 1 L / min - 100 L / min.

[0093] The volume ratio of the silicon source gas to the protective gas is 1:1 - 1:3.

[0094] The silicon deposition process of the present invention is carried out in a fluidized bed, which can enable the porous graphite composite material to be in full contact with the silicon source gas in a fluidized state. The diffusion of the silicon source gas is not restricted, the silicon deposition is more uniform and not easy to agglomerate, the deposition efficiency is higher, and at the same time, the phenomenon that large-sized silicon particles formed by the agglomeration of silicon particles hinder ion diffusion is avoided, effectively alleviating the volume expansion, thereby improving the specific capacity and rate performance.

[0095] Step S6: Perform carbon coating treatment on the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

[0096] Among them, the carbon coating treatment is gas-phase carbon coating, including: placing the graphite-based silicon-carbon semi-finished product in a coating furnace, under a protective atmosphere, heating to 500°C - 580°C at a heating rate of 2°C / min - 5°C / min, introducing a carbon source gas, and maintaining the temperature for 0.5 hour - 2 hours, so that the carbon element decomposed from the carbon source gas is deposited on the surface of the silicon-carbon composite material semi-finished product to form a carbon coating layer, and finally obtaining a silicon-carbon composite material.

[0097] Among them, the carbon source gas includes one or more of alkanes, alkenes, and alkynes; the flow rate of the carbon source gas is 5 L / min - 60 L / min.

[0098] In the present invention, the method for testing ash content is as follows: Put the sample to be tested into a ceramic crucible, and then place it in a muffle furnace. Remove the organic components in the carbon matrix by high-temperature burning, and use an analytical balance to measure the weight of the remaining inorganic substances (such as oxides, salts, etc.), which is the ash content.

[0099] In the present invention, the method for testing the Fe content is as follows: Convert iron into an ionic state by acid digestion, and quantitatively detect the iron concentration by spectroscopic analysis. Among them, the quantitative spectroscopic analysis is carried out using an inductively coupled plasma optical emission spectrometer (ICP-OES).

[0100] In the present invention, the median particle size of the material is measured using a Malvern laser particle size analyzer, and the median particle size of the material is measured based on the principle of laser scattering.

[0101] In the present invention, based on the gas adsorption method, the Micromeritics specific surface area analyzer is used to measure the specific surface area and pore size distribution of the sample by using the adsorption behavior of gas molecules on the solid surface.

[0102] The silicon-carbon composite material prepared by the above preparation method provided in the embodiment of the present invention can be used as a negative electrode active material to prepare a negative electrode sheet, and this negative electrode sheet containing the silicon-carbon composite material is used to assemble a lithium-ion battery. The lithium-ion battery includes a liquid lithium-ion battery, a semi-solid lithium-ion battery or a solid-state lithium-ion battery.

[0103] To better understand the technical solution provided by the present invention, the following uses multiple specific examples to separately illustrate the preparation process and characteristics of the silicon-carbon composite material of the present invention.

[0104] Example 1

[0105] This example provides a preparation process and performance test of a silicon-carbon composite material, which are as follows.

[0106] (1) Take 1 kg of porous graphite raw material with a specific surface area of 3.07 m 2 / g and an average pore diameter of 29.5 nm, pickle it with 2 mol / L hydrochloric acid at 60 °C for 2 hours and then dry it. The ash content of the pickled porous graphite raw material is measured to be 0.07%, and the Fe content is 29 ppm; then the pickled porous graphite raw material is pulverized by air flow to obtain pretreated porous graphite with a median particle size D 50 of 10.9 μm.

[0107] (2) Vacuum impregnate the pretreated porous graphite in a phenolic resin ethanol solution with a mass concentration of 20% under a vacuum of 1×10 -3 Pa for 0.5 hours, and then centrifuge to remove the excess solution to obtain a centrifuged product.

[0108] (3) Place the centrifuged product in a tubular furnace. Under a nitrogen atmosphere, heat it to 750 °C at a heating rate of 4 °C / min and hold for 3 hours for carbonization treatment to obtain a porous graphite skeleton with carbides filled in the pores.

[0109] (4) Place the porous graphite skeleton in an activation furnace, introduce CO2 gas with a flow rate of 150 mL / min, heat it to 850 °C, and perform physical activation for 2 hours to form a hierarchical pore structure in the carbides in the pores of the porous graphite skeleton. After cooling, a porous graphite composite material is obtained; among them, the specific surface area of the porous graphite composite material is 852 m 2 / g, the average pore diameter is 2.32 nm, the pore volume is 0.55 cm 3 / g, the proportion of micropores is 22.5%, and the proportion of mesopores is 54.7%.

[0110] The XRD pattern of the porous graphite composite material prepared in this step is as Figure 2 shown. It can be seen from the figure that XRD shows obvious graphite diffraction peaks.

[0111] The SEM image of the porous graphite composite material prepared in this step is as Figure 3 shown. The morphology of the porous graphite composite material presents an irregular shape.

[0112] (5) Place the porous graphite composite material in a fluidized bed, introduce nitrogen with a flow rate of 10 L / min for protection, and heat the fluidized bed to 600 °C at a heating rate of 4 °C / min. Then, introduce a mixed gas of silane and nitrogen with a flow rate of 20 L / min at a volume ratio of 1:1 and hold for 4 hours for silicon deposition, so that the silicon element decomposed from silane is deposited in the pores of the carbides of the porous graphite composite material to obtain a graphite-based silicon-carbon semi-finished product.

[0113] (6) Place the graphite-based silicon-carbon semi-finished product in a coating furnace. Under a nitrogen atmosphere, heat the coating furnace to 550 °C at a heating rate of 5 °C / min, introduce a mixed gas of nitrogen and acetylene with a flow rate of 20 L / min at a volume ratio of 1:1, and hold for 1 hour for carbon coating treatment to form a carbon coating layer on the surface of the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

[0114] The SEM image of the graphite-based silicon-carbon semi-finished product prepared in this example is as Figure 4 shown. It can be seen from the figure that there is no obvious change in the surface morphology after silicon deposition, and there is no agglomeration of silicon particles.

[0115] Use the silicon-carbon composite material prepared in this example to prepare an electrode sheet and assemble it into a coin-type half-cell for testing, as follows.

[0116] Preparation of the electrode sheet: The prepared silicon-carbon composite material, conductive agent Super P, and binder sodium carboxymethyl cellulose were mixed in a mass ratio of 8:1:1, and then an appropriate amount of deionized water was added to form a uniform slurry in a pulper. The slurry was coated on a copper foil current collector and then dried in a vacuum oven at 80 °C for 12 hours. The dried electrode sheet was cut into circular pieces with a diameter of 14 mm as the electrode sheet of the coin-type half-cell.

[0117] Assembly of the coin-type half-cell: The above electrode sheet was assembled into a coin-type half-cell in an argon-filled glove box by a conventional method. The electrolyte used was lithium hexafluorophosphate with a molar concentration of 1 mol / L, and the solvent of the electrolyte was ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and the volume ratio of EC, DMC, and DEC was 1:1:1. The counter electrode used was a lithium sheet.

[0118] The coin-type half-cell was subjected to electrochemical performance testing and electrode sheet swelling rate testing. The test data are shown in Table 1.

[0119] The process of electrochemical performance testing was as follows: The testing was carried out on a BlueTEC battery testing system to evaluate the electrochemical performance of the assembled coin-type half-cell. The testing conditions were: temperature 25 °C, voltage window 0.005 V to 2 V. The charge-discharge conventional test was carried out at a current density of C / 10 to test the initial charge specific capacity; the discharge rate test was carried out at current densities of 1C / 3C, and the cycle capacity retention rates after 500 cycles at a current density of 1C and a current density of 3C were respectively tested.

[0120] The process of electrode sheet swelling rate testing was as follows: Using an optical profiler, the thickness of a single electrode sheet was measured at 10 points before one cycle and the average value D1 was taken. After that, the thickness of the electrode sheet disassembled after one cycle was measured at 10 points and the average value D2 was taken. The electrode sheet swelling rate was calculated by the formula (D2 - D1) / D1×100%.

[0121] Example 2

[0122] This example provides a preparation process and performance testing of a silicon-carbon composite material, which are as follows.

[0123] (1) 1 kg of porous graphite raw material with a specific surface area of 1.60 m 2 / g and an average pore diameter of 43.1 nm was pickled with 1.5 mol / L hydrochloric acid at 70 °C for 2 hours and then dried. The ash content of the pickled porous graphite raw material was measured to be 0.11%, and the Fe content was 24 ppm; then the pickled porous graphite raw material was subjected to air flow pulverization to obtain pretreated porous graphite with a median particle size D 50 of 9.5 μm.

[0124] (2) The pretreated porous graphite was placed at 1×10 -3The product was centrifuged to remove the excess solution after vacuum impregnation in a phenol-formaldehyde resin ethanol solution with a mass concentration of 15% under a Pa vacuum for 0.5 hours, obtaining a centrifuged product.

[0125] (3) The centrifuged product was placed in a tubular furnace. Under a nitrogen atmosphere, it was heated to 700 °C at a heating rate of 3 °C / min and held for 2 hours for carbonization treatment, obtaining a porous graphite skeleton with carbides filled in the pores.

[0126] (4) The porous graphite skeleton was placed in an activation furnace. CO2 gas with a flow rate of 120 mL / min was introduced, and the temperature was raised to 875 °C for physical activation for 2.5 hours. A hierarchical pore structure was formed in the carbides in the pores of the porous graphite skeleton. After cooling, a porous graphite composite material was obtained; among them, the specific surface area of the porous graphite composite material was 614 m 2 / g, the average pore diameter was 2.56 nm, the pore volume was 0.42 cm 3 / g, the proportion of micropores was 18.4%, and the proportion of mesopores was 62.5%.

[0127] (5) The porous graphite composite material was placed in a fluidized bed. Nitrogen with a flow rate of 20 L / min was introduced for protection, and the fluidized bed was heated to 550 °C at a heating rate of 3 °C / min. Then, a mixed gas of silane and nitrogen with a flow rate of 30 L / min was introduced at a volume ratio of 1:2, and held for 3.5 hours for silicon deposition, so that the silicon element decomposed from silane was deposited in the pores of the carbides of the porous graphite composite material, obtaining a graphite-based silicon-carbon semi-finished product.

[0128] (6) The graphite-based silicon-carbon semi-finished product was placed in a coating furnace. Under a nitrogen atmosphere, the coating furnace was heated to 560 °C at a heating rate of 5 °C / min. A mixed gas of nitrogen and acetylene with a flow rate of 20 L / min was introduced into the coating furnace at a volume ratio of 1:1, and held for 0.6 hours for carbon coating treatment to form a carbon coating layer on the surface of the graphite-based silicon-carbon semi-finished product, obtaining a silicon-carbon composite material.

[0129] The silicon-carbon composite material prepared in this example was used to prepare an electrode sheet and assembled into a button half-cell for testing. The assembly and testing processes of the button half-cell were the same as those in Example 1. The test data are shown in Table 1.

[0130] Example 3

[0131] This example provides a preparation process and performance test of a silicon-carbon composite material, which are specifically as follows.

[0132] (1) With a specific surface area of 4.05 m 21 kg of porous graphite raw material with an average pore diameter of 20.8 nm was pickled with 3 mol / L hydrochloric acid at 65 °C for 2 hours and then dried. The ash content of the pickled porous graphite raw material was measured to be 0.04%, and the Fe content was 18 ppm. Then, the pickled porous graphite raw material was pulverized by air flow to obtain pretreated porous graphite with a median particle size D 50 of 8.6 μm.

[0133] (2) The pretreated porous graphite was vacuum impregnated in a 20% by mass asphalt tetrahydrofuran solution under a vacuum of 1×10 -3 Pa for 0.5 hour, and then the excess solution was removed by centrifugation to obtain a centrifuged product.

[0134] (3) The centrifuged product was placed in a tubular furnace and heated to 600 °C at a heating rate of 5 °C / min under a nitrogen atmosphere, and carbonized for 3 hours to obtain a porous graphite skeleton with carbides filled in the pores.

[0135] (4) The porous graphite skeleton was placed in an activation furnace, and CO2 gas with a flow rate of 180 mL / min was introduced. It was heated to 875 °C and physically activated for 3 hours to form a hierarchical pore structure in the carbides in the pores of the porous graphite skeleton. After cooling, a porous graphite composite material was obtained. Among them, the specific surface area of the porous graphite composite material was 1074 m 2 / g, the average pore diameter was 2.84 nm, the pore volume was 0.76 cm 3 / g, the proportion of micropores was 37.1%, and the proportion of mesopores was 44.5%.

[0136] (5) The porous graphite composite material was placed in a fluidized bed, and nitrogen with a flow rate of 30 L / min was introduced for protection. The fluidized bed was heated to 600 °C at a heating rate of 5 °C / min, and then a mixed gas of silane and nitrogen with a flow rate of 40 L / min in a volume ratio of 1:3 was introduced and held for 3 hours for silicon deposition, so that the silicon element decomposed from silane was deposited in the pores of the carbides of the porous graphite composite material to obtain a graphite-based silicon-carbon semi-finished product.

[0137] (6) The graphite-based silicon-carbon semi-finished product was placed in a coating furnace, and the coating furnace was heated to 500 °C at a heating rate of 5 °C / min under a nitrogen atmosphere. A mixed gas of nitrogen and acetylene with a flow rate of 25 L / min in a volume ratio of 1:1 was introduced into the coating furnace and held for 1 hour for carbon coating treatment to form a carbon coating layer on the surface of the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

[0138] The silicon-carbon composite material prepared in this example was used to prepare a pole piece and assembled into a button half-cell for testing. The assembly and testing process of the button half-cell were the same as those in Example 1. The test data are shown in Table 1.

[0139] Example 4

[0140] This embodiment provides a preparation process and performance test of a silicon-carbon composite material, which are as follows.

[0141] (1) 1 kg of porous graphite raw material with a specific surface area of 9.66 m 2 / g and an average pore diameter of 26.3 nm was pickled with 2 mol / L hydrochloric acid at 70 °C for 3 hours and then dried. The ash content of the pickled porous graphite raw material was measured to be 0.03%, and the Fe content was 15 ppm; then the pickled porous graphite raw material was pulverized by air flow to obtain pretreated porous graphite with a median particle size D 50 of 11 μm.

[0142] (2) The pretreated porous graphite was vacuum impregnated in a phenolic resin ethanol solution with a mass concentration of 25% under a vacuum of 1×10 -3 Pa for 0.5 hour, and then the excess solution was removed by centrifugation to obtain a centrifuged product.

[0143] (3) The centrifuged product was placed in a tubular furnace and heated to 650 °C at a heating rate of 3 °C / min under a nitrogen atmosphere and kept for 2 hours for carbonization treatment to obtain a porous graphite skeleton with carbides filled in the pores.

[0144] (4) The porous graphite skeleton was placed in an activation furnace, and CO2 gas with a flow rate of 200 mL / min was introduced, heated to 875 °C, and physically activated for 4 hours to form a hierarchical pore structure in the carbides in the pores of the porous graphite skeleton. After cooling, a porous graphite composite material was obtained; among them, the specific surface area of the porous graphite composite material was 756.42 m 2 / g, the average pore diameter was 3.98 nm, the pore volume was 0.75 cm 3 / g, the proportion of micropores was 21.5%, and the proportion of mesopores was 62.7%.

[0145] (5) The porous graphite composite material was placed in a fluidized bed, and nitrogen with a flow rate of 24 L / min was introduced for protection, and the fluidized bed was heated to 570 °C at a heating rate of 5 °C / min. Then, a mixed gas of silane and nitrogen with a flow rate of 32 L / min was introduced at a volume ratio of 1:3, and kept for 2.5 hours for silicon deposition, so that the silicon element decomposed from silane was deposited in the pores of the carbides of the porous graphite composite material to obtain a graphite-based silicon-carbon semi-finished product.

[0146] (6) The graphite-based silicon-carbon semi-finished product was placed in a coating furnace, and the coating furnace was heated to 480 °C at a heating rate of 5 °C / min under a nitrogen atmosphere, and a mixed gas of nitrogen and ethylene with a flow rate of 30 L / min was introduced into the coating furnace at a volume ratio of 1:1, and kept for 1 hour for carbon coating treatment to form a carbon coating layer on the surface of the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

[0147] The silicon-carbon composite material prepared in this example was used to prepare an electrode sheet and assembled into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell were the same as those in Example 1. The test data are shown in Table 1 in detail.

[0148] Example 5

[0149] This example provides a preparation process and performance test of a silicon-carbon composite material, which are as follows.

[0150] (1) 1 kg of porous graphite raw material with a specific surface area of 5.31 m 2 / g and an average pore diameter of 18.8 nm was pickled with 2 mol / L hydrochloric acid at 60 °C for 2 hours and then dried. The ash content of the pickled porous graphite raw material was measured to be 0.08%, and the Fe content was 25 ppm; then the pickled porous graphite raw material was pulverized by air flow to obtain pretreated porous graphite with a median particle size D 50 of 8.3 μm.

[0151] (2) The pretreated porous graphite was vacuum impregnated in a 20% glucose aqueous solution under a vacuum of 1×10 -3 Pa for 0.5 hours, and then the excess solution was removed by centrifugation to obtain a centrifuged product.

[0152] (3) The centrifuged product was placed in a tube furnace, and under a nitrogen atmosphere, it was heated to 800 °C at a heating rate of 4 °C / min and held for 2 hours for carbonization treatment to obtain a porous graphite skeleton with carbides filled in the pores.

[0153] (4) The porous graphite skeleton was placed in an activation furnace, and CO2 gas with a flow rate of 150 mL / min was introduced. It was heated to 950 °C and physically activated for 4 hours to form a hierarchical pore structure in the carbides in the pores of the porous graphite skeleton. After cooling, a porous graphite composite material was obtained; among them, the specific surface area of the porous graphite composite material was 630.37 m 2 / g, the average pore diameter was 5.89 nm, the pore volume was 0.93 cm 3 / g, the micropore proportion was 14.3%, and the mesopore proportion was 79.8%.

[0154] (5) The porous graphite composite material was placed in a fluidized bed, and nitrogen with a flow rate of 27 L / min was introduced for protection. The fluidized bed was heated to 550 °C at a heating rate of 5 °C / min, and then a mixed gas of silane and nitrogen with a flow rate of 36 L / min was introduced at a volume ratio of 1:3. It was held for 3.5 hours for silicon deposition, so that the silicon element decomposed from silane was deposited in the pores of the carbides of the porous graphite composite material to obtain a graphite-based silicon-carbon semi-finished product.

[0155] (6) Place the graphite-based silicon-carbon semi-finished product in a coating furnace. Under a nitrogen atmosphere, heat the coating furnace to 500 °C at a heating rate of 5 °C / min. Then, introduce a mixed gas of nitrogen and methane with a flow rate of 15 L / min into the coating furnace at a volume ratio of 1:1, and keep it warm for 2 hours for carbon coating treatment to form a carbon coating layer on the surface of the graphite-based silicon-carbon semi-finished product, thus obtaining a silicon-carbon composite material.

[0156] Use the silicon-carbon composite material prepared in this example to prepare a pole piece and assemble it into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0157] To better illustrate the effects of the embodiments of the present invention, a comparative example is compared with the above embodiments.

[0158] Comparative Example 1

[0159] This comparative example provides a preparation process and performance test of a silicon-carbon composite material. Different from Example 1, in step (1), the porous graphite raw material is not pickled, and the porous graphite raw material is directly crushed by air flow. The ash content of the porous graphite raw material is 2.1%, and the Fe content is 315 ppm. The preparation processes of steps (2) to (6) are the same as those in Example 1. Among them, the specific surface area of the porous graphite composite material obtained in step (4) is 846 m 2 / g, the average pore diameter is 2.33 nm, the pore volume is 0.57 cm 3 / g, the micropore proportion is 22.8%, and the mesopore proportion is 54.5%.

[0160] Use the silicon-carbon composite material prepared in this comparative example to prepare a pole piece and assemble it into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0161] Comparative Example 2

[0162] This comparative example provides a preparation process and performance test of a silicon-carbon composite material. Different from Example 1, the preparation processes of steps (2) to (4) are not carried out, that is, it is not vacuum-impregnated in a phenolic resin ethanol solution, nor is it physically activated. Silicon is directly deposited in the pores and on the surface of the pretreated porous graphite after pickling and air flow crushing, and then carbon coating treatment is carried out to obtain a silicon-carbon composite material. The preparation processes of steps (1), (5), and (6) are the same as those in Example 1.

[0163] Use the silicon-carbon composite material prepared in this comparative example to prepare a pole piece and assemble it into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0164] Comparative Example 3

[0165] This comparative example provides a preparation process and performance test of a silicon-carbon composite material. The difference from Example 1 lies in that in step (2), the vacuum impregnation method is not adopted. Instead, the pretreated porous graphite is mechanically mixed and stirred with a phenolic resin ethanol solution with a mass concentration of 20% for 2 hours, and then the excess solution is removed by centrifugation to obtain a centrifuged product; the processes of other steps are the same as those in Example 1.

[0166] The silicon-carbon composite material prepared in this comparative example is used to prepare an electrode sheet and assembled into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0167] Comparative Example 4

[0168] This comparative example provides a preparation process and performance test of a silicon-carbon composite material. The difference from Example 1 lies in that the deposition equipment used in step (5) is an ordinary chemical vapor deposition furnace; the processes of other steps are the same as those in Example 1.

[0169] The SEM image of the graphite-based silicon-carbon semi-finished product prepared in this comparative example is as Figure 5 shown. It can be seen that there is obvious silicon particle agglomeration on the graphite-based silicon-carbon semi-finished product in this comparative example.

[0170] The silicon-carbon composite material prepared in this comparative example is used to prepare an electrode sheet and assembled into a button-type half-cell for testing. The assembly and testing processes of the button-type half-cell are the same as those in Example 1. The test data are shown in Table 1 for details.

[0171] Table 1 summarizes the test data of the button-type half-cells assembled in Examples 1-5 and Comparative Examples 1-4:

[0172]

[0173]

[0174] Table 1

[0175] From the comparison of the test data between Example 1 and Comparative Example 1, it can be seen that the ash content and metal impurity content of the porous graphite raw material in Comparative Example 1 without acid washing pretreatment are relatively high, which will cause micro short circuits and catalytic side reactions, significantly reducing the cycle life. Therefore, the capacity retention rate of the button-type half-cell in Comparative Example 1 is much lower than that in Example 1 after 500 cycles.

[0176] From the comparison of the test data between Example 1 and Comparative Example 2, it can be seen that there is no composite carbide in the pores of the porous graphite raw material of Comparative Example 2, nor is there a gradient pore structure created in the carbide. Instead, silicon is directly deposited on the pretreated porous graphite raw material. Due to the lack of carbide and gradient pore structure in the porous graphite, the degree of buffering silicon expansion is limited. There are too many mesopores and macropores on the graphite surface, resulting in uneven silicon deposition and a small loading amount, leading to a relatively lower capacity of the coin-type half-cell in Comparative Example 2 compared to Example 1 and poorer cycle stability.

[0177] From the comparison of the test data between Example 1 and Comparative Example 3, it can be seen that Comparative Example 3 did not use vacuum impregnation but adopted the method of ordinary mixing, mechanically mixing and stirring the pretreated porous graphite with a phenolic resin ethanol solution with a mass concentration of 20%. Affected by gravity and surface tension, the phenolic resin ethanol solution is prone to accumulate on the surface of the pretreated porous graphite, and the internal pore filling rate is low. In contrast, the vacuum impregnation method used in Example 1 can first remove the adsorbed gas and moisture inside the pores, and the negative pressure drives the resin solution to penetrate into the pores, reducing the local concentration difference and making it evenly dispersed inside the pores, and the required time is shorter. The material obtained after physical activation has a higher specific surface area and pore volume. After silicon deposition, the electrochemical performance is significantly improved, and the expansion of the electrode sheet is also improved.

[0178] From the comparison of the test data between Example 1 and Comparative Example 4, it can be seen that Comparative Example 4 uses ordinary chemical vapor deposition. Due to the static stacking of the porous graphite matrix, the diffusion of silane gas is limited, resulting in silicon agglomeration. The large-sized silicon particles hinder ion diffusion, intensify side reactions, consume more active lithium, and the deposited silicon layer is unevenly distributed and has a weak binding force, leading to a significant decrease in the first-cycle charge specific capacity and rate performance of Comparative Example 4 compared to Example 1.

[0179] The specific embodiments described above have further elaborated on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, The silicon-carbon composite material includes: a porous graphite composite material, a nano-silicon layer uniformly deposited in the pores and on the surface of the porous graphite composite material, and a carbon coating layer covering the outermost layer of the porous graphite composite material; The porous graphite composite material includes: a porous graphite skeleton, and a carbide with a hierarchical pore structure filled in the pores of the porous graphite skeleton; The porous graphite composite material is obtained by vacuum impregnating the pretreated porous graphite after pickling and pulverization in a carbon precursor solution, and then successively through centrifugation, carbonization treatment, and physical activation.

2. The silicon-carbon composite material according to claim 1, wherein The proportion of micropores in the porous graphite composite material is 10% - 60%, and the proportion of mesopores is 40% - 90%; The specific surface area of the porous graphite composite is 200 m 2 / g to 1500 m 2 / g, the average pore diameter is 2 nm to 10 nm, and the total pore volume is 0.2 cm 3 / g to 0.8 cm 3 / g; The thickness of the carbon coating layer is 5nm - 20nm; The percentage of the mass of the nano-silicon layer in the total mass of the silicon-carbon composite material is 10% - 65%; The percentage of the mass of the carbon coating layer in the total mass of the silicon-carbon composite material is 2% - 6%.

3. A method for preparing the silicon-carbon composite material according to any one of claims 1-2, characterized in that The preparation method includes: Step S1, successively pickling and pulverizing the porous graphite raw material to obtain pretreated porous graphite; Step S2, placing the pretreated porous graphite in a carbon precursor solution for vacuum impregnation, so that the carbon precursor solution enters and fills the pores of the pretreated porous graphite, and then through centrifugal separation treatment to obtain a centrifuged product; Step S3, placing the centrifuged product in a high-temperature device for carbonization treatment to obtain a porous graphite skeleton with carbide filled in the pores; Step S4, placing the porous graphite skeleton in an activation furnace for physical activation to form a hierarchical pore structure in the carbide in the pores of the porous graphite skeleton to obtain a porous graphite composite material; Step S5, placing the porous graphite composite material in a fluidized bed for silicon deposition to obtain a graphite-based silicon-carbon semi-finished product; Step S6, performing carbon coating treatment on the graphite-based silicon-carbon semi-finished product to obtain a silicon-carbon composite material.

4. The preparation method according to claim 3, characterized in that, The specific surface area of the porous graphite raw material is 0.1 m 2 / g to 20 m 2 / g, and the average pore diameter is 5 nm to 100 nm; The pickling includes: at a temperature of 60°C - 80°C, washing the porous graphite raw material with hydrochloric acid with a molar concentration of 1mol / L - 3mol / L for 2 hours - 4 hours; the ash content of the pickled porous graphite raw material is less than 0.3%, and the Fe content is lower than 50ppm; The median particle size D of the pretreated porous graphite 50 is 4 μm to 15 μm.

5. The preparation method according to claim 3, characterized in that, The carbon precursor in the carbon precursor solution includes any one of phenolic resin, coal, asphalt, glucose, polyacrylonitrile, and lignin; wherein, when the carbon precursor is a resin, the solvent of the carbon precursor solution is ethanol; when the carbon precursor is coal or asphalt, the solvent of the carbon precursor solution is tetrahydrofuran; when the carbon precursor is glucose, the solvent of the carbon precursor solution is deionized water; when the carbon precursor is polyacrylonitrile, the solvent of the carbon precursor solution is N,N-dimethylformamide; when the carbon precursor is lignin, the solvent of the carbon precursor solution is an aqueous NaOH solution; the mass concentration of the carbon precursor in the carbon precursor solution is 10% - 30%; The degree of vacuum for the vacuum impregnation is 1×10 -4 ~1×10 -3 Pa, and the time for the vacuum impregnation is 0.5 to 1.5 hours.

6. The preparation method according to claim 3, wherein The carbonization treatment includes: under a protective gas environment, heating the high-temperature equipment to 600°C - 800°C at a heating rate of 2°C / min - 5°C / min, and holding for 2 hours - 4 hours; The high-temperature equipment includes any one of a tube furnace, a box furnace, a rotary furnace, and a pusher furnace; The protective gas includes one or more of nitrogen, argon, or helium.

7. The preparation method according to claim 3, wherein The physical activation includes: introducing carbon dioxide or water vapor with a flow rate of 100 mL / min - 500 mL / min into the activation furnace, performing high-temperature treatment at 800°C - 1000°C for 1 hour - 4 hours, and then cooling to room temperature under a protective gas; In the hierarchical pore structure, the proportion of micropores is 20% - 60%, and the proportion of mesopores is 40% - 90%; The specific surface area of the porous graphite composite is 200 m 2 / g to 1000 m 2 / g, the average pore diameter is 2 nm to 10 nm, and the total pore volume is 0.2 cm 3 / g to 0.8 cm 3 / g.

8. The preparation method according to claim 3, wherein The silicon deposition includes: under a protective gas environment, heating the temperature of the fluidized bed to 450°C - 900°C at a rate of 2°C / min - 5°C / min, introducing a silicon source gas, and holding for 1 hour - 4 hours; The protective gas includes one or more of nitrogen, argon, or helium; The silicon source gas includes one or more gases of silane, disilane, dichlorosilane, trichlorosilane, tetrachlorosilane, hexachloroethylsilane; the flow rate of the silicon source gas is 1 L / min - 100 L / min; The volume ratio of the silicon source gas to the protective gas is 1:1 - 1:

3.

9. The preparation method according to claim 3, characterized in that, The carbon coating treatment is gas-phase carbon coating, which includes: placing the graphite-based silicon-carbon semi-finished product in a coating furnace, under a protective atmosphere, heating to 500°C - 580°C at a heating rate of 2°C / min - 5°C / min, introducing a carbon source gas, and holding for 0.5 hour - 2 hours, so that the carbon element decomposed from the carbon source gas is deposited on the surface of the silicon-carbon composite semi-finished product to form a carbon coating layer, and finally obtaining a silicon-carbon composite material; Among them, the carbon source gas includes one or more of alkanes, alkenes, and alkynes; the flow rate of the carbon source gas is 5 L / min - 60 L / min.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the silicon-carbon composite material according to any one of claims 1 - 2 above, or the silicon-carbon composite material prepared by the preparation method according to any one of claims 3 - 9 above.

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