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

Abstract

The invention relates to the technical field of battery silicon negative electrodes, in particular to a silicon-carbon composite material and a preparation method and application thereof. The silicon-carbon composite material comprises a three-dimensional through mesoporous carbon skeleton, nano silicon particles loaded in mesopores and a gradient carbon coating layer, nano silicon and the carbon skeleton are connected through Si-O-C covalent bonds, and the gradient carbon coating layer is composed of a soft carbon inner layer, a graphitized carbon middle layer and a compact hard carbon outer layer. The preparation method comprises three steps of synthesis of a three-dimensional through mesoporous carbon skeleton, confinement silicon loading and gradient carbon coating. The material solves the problems of silicon particle agglomeration, volume effect and interface instability, has excellent electrochemical performance, can be used as a lithium ion battery negative electrode material, is suitable for a next-generation high-energy lithium ion battery, and improves the cycling stability and capacity performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of silicon anode technology for batteries, specifically to a silicon-carbon composite material, its preparation method, and its application. Background Technology

[0002] In the field of lithium-ion batteries, the performance of the anode material directly determines the battery's energy density, cycle life, and rate performance. Traditional graphite anodes have a theoretical specific capacity of only about 372 mAh / g, which is insufficient to meet the high-capacity requirements of next-generation high-energy lithium-ion batteries (such as those used in electric vehicles and energy storage systems). Silicon, with its ultra-high theoretical specific capacity of 4200 mAh / g, has become an ideal alternative to graphite anodes. It is often combined with carbon materials to form silicon-carbon composite materials, achieving a balance between high capacity and structural stability.

[0003] However, the preparation and application of existing silicon-carbon composite materials still face many technical bottlenecks. First, traditional mechanical mixing methods easily lead to uneven distribution of nano-silicon particles in the carbon matrix, causing severe agglomeration. Agglomerated silicon particles not only block active sites and reduce capacity utilization, but also break electron and ion transport channels, resulting in a significant decrease in battery rate performance and failing to meet the requirements of high-current charge and discharge scenarios. Second, the physical mixing of silicon and carbon results in extremely weak interfacial bonding energy and insufficient contact: during long-term charge and discharge, the two are easily decoupled, causing repeated rupture and regeneration of the solid electrolyte interphase (SEI) film, which consumes electrolyte and lithium source, and leads to a sharp increase in battery impedance and a continuous decrease in coulombic efficiency. Third, silicon undergoes a volume expansion of over 300% during lithium intercalation. This drastic volume change creates concentrated stress inside the electrode, causing electrode structure damage and active material shedding. Most existing silicon-carbon composite materials have a capacity retention rate of less than 50% after 100 cycles, making it difficult to meet commercial standards.

[0004] Although the industry has attempted to improve the situation through simple carbon coating and adjusting the silicon-carbon ratio, single-layer coating either lacks the flexibility to buffer expansion or has poor density, making it difficult to prevent electrolyte side reactions. Simply adjusting the ratio also fails to fundamentally solve the interface and dispersion problems, thus failing to break through the bottleneck. These problems collectively restrict the large-scale application of silicon-carbon composite materials, and new preparation technologies are urgently needed to address these issues simultaneously. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a silicon-carbon composite material based on multi-level channel confined assembly and gradient carbon coating, its preparation method and application, which is suitable as a next-generation high-energy lithium-ion battery anode material and has good electrochemical performance.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a silicon-carbon composite material, comprising: a three-dimensional interconnected mesoporous carbon skeleton, nano-silicon particles loaded within the mesopores of the three-dimensional interconnected mesoporous carbon skeleton, and a gradient carbon coating layer covering the surfaces of the carbon skeleton and the nano-silicon particles; the nano-silicon particles and the three-dimensional interconnected mesoporous carbon skeleton are connected by Si-OC covalent bonds; the gradient carbon coating layer comprises a soft carbon inner layer, a graphitized carbon intermediate layer, and a dense hard carbon outer layer.

[0007] Secondly, the present invention provides a method for preparing the aforementioned silicon-carbon composite material, comprising the following steps: Synthesis of three-dimensional interconnected mesoporous carbon skeleton: by mixing carbon source and template agent in solvent, adding supercritical carbon dioxide as foaming agent for foaming treatment, followed by high-temperature carbonization under protective atmosphere, and cooling to obtain the three-dimensional interconnected mesoporous carbon skeleton. Confined silicon loading: The three-dimensional through-porous carbon skeleton is immersed in a solution containing a silicon source and a silane coupling agent, treated under vacuum negative pressure, and then treated with ammonia water vapor to obtain a carbon / silicon loaded sample. Gradient carbon coating: The carbon / silicon loaded sample is subjected to staged high-temperature calcination under a protective atmosphere to form a carbon coating layer consisting of a soft carbon inner layer, a graphitized carbon intermediate layer, and a dense hard carbon outer layer, thereby obtaining the silicon-carbon composite material.

[0008] As a preferred embodiment of the present invention, the carbon source includes at least one of phenolic resin, asphalt, biomass carbon source, polyacrylonitrile, and polyvinyl alcohol; the template agent includes at least one of polystyrene microspheres, silica microspheres, inorganic salts, and metal oxides; and the mass ratio of the carbon source to the template agent is (3-4):(1-2).

[0009] In the above schemes, asphalt (such as petroleum asphalt / coal tar pitch) has extremely low cost, is easy to graphitize, and has a high residual carbon rate; the residual carbon rate of high-quality asphalt can reach over 50%. Biomass carbon sources (such as cellulose, lignin, and sucrose) are renewable, environmentally friendly, widely available, and low-cost, and are inherently porous: some biomass (such as coconut shells) has a hierarchical porous structure and can serve as a self-template. Polyacrylonitrile (PAN): is a commonly used precursor for preparing carbon fibers; it has high strength and good electrical conductivity after carbonization. Polyvinyl alcohol (PVA): has good water solubility and is easy to mix with the template.

[0010] Silica microspheres exhibit high thermal and chemical stability, and also possess the monodispersity of polystyrene microspheres, making their surface easily modifiable. Inorganic salts (such as sodium chloride, NaCl) are extremely low-cost, easy, and safe to remove: they can be removed simply by washing with water, making them environmentally friendly. Metal oxides (such as magnesium oxide, MgO) are also easy to remove: they can be etched with dilute acids (such as hydrochloric acid), making them safer than silica. Their nanoscale size allows them to act as pore-forming agents, creating smaller mesopores or larger specific surface areas.

[0011] As a preferred embodiment of the present invention, the high-temperature carbonization temperature is 800~1000℃, the heating rate is 2~5℃ / min, and the holding time is 1~3h.

[0012] As a preferred embodiment of the present invention, the supercritical carbon dioxide has a foaming pressure of 20-50 MPa, a foaming temperature of 60-100℃, and a foaming time of 1-3 h.

[0013] As a preferred embodiment of the present invention, the silicon source comprises silicon tetrachloride or tetraethyl orthosilicate, and the silane coupling agent comprises at least one of vinyl silane coupling agent, amino silane coupling agent, epoxy silane coupling agent, and methyl silane coupling agent; the molar concentration of the silicon source in the solution is 0.1-1 mol / L, and the volume concentration of the silane coupling agent in the solution is 0.5-2 vol.

[0014] Advantages of tetraethyl orthosilicate: High safety: mild operating conditions, no need for strict waterproofing; Controllable hydrolysis: relatively mild hydrolysis rate, easy to control.

[0015] Aminosilanes (such as 3-aminopropyltriethoxysilane) possess strong reactivity and catalytic activity due to their amino groups, promoting certain condensation reactions and exhibiting good compatibility with many polymers. Epoxysilanes (such as (3-glycidoxypropyl)trimethoxysilane) have highly reactive epoxy groups that can react with various functional groups after ring-opening. Methylsilanes (such as methyltriethoxysilane) are highly hydrophobic, improving the water resistance of materials, and exhibit superior thermal stability compared to amino and epoxy groups.

[0016] As a preferred embodiment of the present invention, the vacuum negative pressure is -60~-100kPa, the number of cycles is 2~5, and the pressure holding time for each cycle is 20~40min; the temperature of the ammonia steam treatment is 40~60℃, and the treatment time is 0.5~2h.

[0017] As a preferred embodiment of the present invention, the staged high-temperature calcination includes: First calcination stage: Heat to 300-400℃ at 0.5-2℃ / min, hold for 0.5-2h to form a soft carbon inner layer; Second calcination stage: Heat to 600-700℃ at 2-5℃ / min and hold for 0.25-1h to form a graphitized carbon intermediate layer; The third calcination stage: the temperature is increased to 800-1000℃ at a rate of 3-8℃ / min, and held for 1-3 hours to form a dense hard carbon outer layer.

[0018] The above preparation method uses a carbon source-template agent-supercritical foaming agent as the core system, and forms a through-pore structure through multi-action coupling. The mechanism can be divided into three stages: foaming and pore formation, high-temperature carbonization, and template removal. I. Synthesis of a three-dimensional interconnected mesoporous carbon framework: 1. Supercritical CO2 foaming and pore formation: Supercritical CO2 (20-50 MPa, 60-100℃) has both high diffusivity and solubility, rapidly penetrating the phenolic resin (carbon source) and polystyrene microspheres (template agent) system to form uniform bubble nuclei that grow and connect. The microspheres act as rigid spacers to prevent pore collapse. The mass ratio of carbon source to template agent (3-4):(1-2) controls the mesopore density, ultimately forming a precursor with interconnected bubble pores and microsphere gaps.

[0019] 2. High-temperature carbonization and curing: Under nitrogen protection, the temperature is increased to 800-1000℃ at a rate of 2-5℃ / min and held for 1-3 hours. Polystyrene microspheres will undergo thermal decomposition and vaporization at ~400℃. The carbonization temperature of phenolic resin is ~500℃. The high-temperature carbonization and curing process can complete template removal and carbon skeleton curing in one step.

[0020] II. Confined silicon load: 1. Vacuum negative pressure driven silicon source deposition: Under a vacuum of -60~-100kPa, air is extracted from the mesopores, and a solution containing silicon tetrachloride (silicon source) and vinylsilane coupling agent fills the mesopores (cycled 2-5 times to ensure no dead zones). Ammonia steam at 40-60℃ catalyzes the hydrolysis of silicon tetrachloride: SiCl4 + 4NH3·H2O → H4SiO4 + 4NH4Cl. Orthosilicic acid dehydrates and condenses to SiO2, which is subsequently reduced to nano-silicon. The mesopore confinement controls the silicon particle size, preventing agglomeration.

[0021] 2. Si-OC covalent bond formation: The siloxy group at one end of the coupling agent condenses with the hydroxyl group on the silicon (or SiO2) surface to form a Si-O-Si bond, and the vinyl group at the other end adds to the unsaturated carbon sites of the carbon skeleton at a high temperature. Through the bridging effect, the silicon and carbon form a Si-OC covalent bond, which replaces the traditional physical action, strengthens the interfacial bonding, and reduces the impedance.

[0022] III. Gradient carbon coating: 1. Soft carbon inner layer (300-400℃, 0.5-2℃ / min): The carbon source is slightly carbonized at a low heating rate to form a soft carbon layer with high disorder and strong flexibility. It can absorb the silicon expansion stress through elastic deformation, avoid the outer layer from cracking, and does not hinder ion transport.

[0023] 2. Graphitized carbon intermediate layer (600-700℃, 2-5℃ / min): Medium-speed heating causes carbon chain rearrangement to form a partially graphitized structure. High conductivity reduces electronic resistance, and medium modulus connects soft and hard carbon layers, avoiding interface peeling caused by abrupt changes in modulus.

[0024] 3. Dense hard carbon outer layer (800-1000℃, 3-8℃ / min): The carbon source is deeply carbonized under high-speed heating to form a hard carbon layer with high density and high mechanical strength. This layer blocks the electrolyte from contacting silicon, inhibits repeated growth of the SEI film, and at the same time constrains excessive expansion of silicon to maintain structural stability.

[0025] The above three steps form a structure-interface-performance closed loop: mesopores solve the problems of silicon agglomeration and transport, Si-OC bonds strengthen interfacial bonding, and gradient carbon coating controls volume effects and SEI stability. The three complement each other; for example, mesopores provide a uniform substrate for gradient coating, and covalent bonds avoid stress-induced interfacial separation, ultimately achieving high capacity, high stability, and low expansion rate in the material.

[0026] Thirdly, the present invention provides the use of the silicon-carbon composite material described above as a negative electrode material for lithium-ion batteries.

[0027] Fourthly, the present invention provides a lithium-ion battery comprising a positive electrode, a separator, an electrolyte, and a negative electrode, wherein the negative electrode is an electrode sheet made of a silicon-carbon composite material as described above, mixed with a conductive agent and a binder.

[0028] Compared with the prior art, the beneficial effects of the present invention include: (1) The present invention provides a uniform and independent storage space for nano-silicon by constructing a three-dimensional interconnected mesoporous carbon framework through supercritical foaming and dual template method, which fundamentally solves the problem of silicon particle aggregation and ensures efficient ion / electron transport channels.

[0029] (2) The gradient modulus coating layer constructed in this invention is a soft-medium-hard sandwich carbon layer structure (soft carbon inner layer, graphitized carbon middle layer, and dense hard carbon outer layer), which realizes a continuous transition of modulus from the inside to the outside. The inner soft carbon absorbs the stress generated by silicon expansion, while the outer hard carbon maintains the structural integrity and prevents electrolyte side reactions, thus synergistically solving the problem of uncontrolled silicon volume effect.

[0030] (3) The present invention introduces a vinyl silane coupling agent (VTES) and forms a Si-OC covalent bond under specific pyrolysis conditions, which transforms the fragile physical contact between silicon and carbon into a strong chemical connection, greatly enhancing the interface stability and significantly reducing the interface impedance. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0032] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0033] Example 1 This embodiment provides a method for preparing a silicon-carbon composite material with high cycle stability, including the following steps: Step 1. Synthesis of three-dimensional interconnected mesoporous carbon framework: Using phenolic resin and polystyrene microspheres as carbon source and template, they were mixed in ethanol (solid content 30%) at a mass ratio of 7:3. Supercritical carbon dioxide was added as a foaming agent and foamed at 35 MPa / 80℃ for 2 hours to prepare a precursor. The precursor was placed in a tube furnace and protected with nitrogen. The temperature was increased to 900℃ at 3℃ / min and held for 2 hours for high-temperature carbonization treatment. After cooling to room temperature, a three-dimensional interconnected mesoporous carbon framework was obtained. Step 2. Confined silicon loading: Using SiCl4 as the silicon source, a 0.5 mol / L SiCl4-ethanol solution was prepared, and 1 vol% ethylenetriethoxysilane (VTES) was added as a silane coupling agent. The sample from Step 1 was immersed in this solution and circulated three times under negative pressure of -90 kPa (holding pressure for 30 min each time). Then, it was placed in a 50℃ saturated ammonia vapor atmosphere for 1 h to obtain a carbon / silicon loaded sample.

[0034] Step 3. Gradient carbon coating: The precursor obtained in Step 2 is transferred to a tube furnace and a protective atmosphere (ethylene / argon = 5 / 95) is introduced. The temperature is increased to 350℃ at 1℃ / min and held for 1h to form a soft carbon inner layer. Then, the temperature is increased to 650℃ at 3℃ / min and held for 30min to form a graphitized carbon intermediate layer. Finally, the temperature is increased to 900℃ at 5℃ / min and held for 2h to form a dense hard carbon outer layer, thus obtaining a gradient carbon-coated silicon-carbon composite material.

[0035] Example 2 Example 2 is basically the same as Example 1, except that in step 1, the supercritical carbon dioxide foaming pressure is 25 MPa.

[0036] Example 3 Example 3 is basically the same as Example 1, except that in step 1, the supercritical carbon dioxide foaming pressure is 45 MPa.

[0037] Example 4 Example 4 is basically the same as Example 1, except that in step 3, the temperature is increased to 350°C at 1°C / min and held for 30 min to form a soft carbon inner layer; then the temperature is increased to 650°C at 3°C / min and held for 15 min to form a graphitized carbon intermediate layer; finally, the temperature is increased to 900°C at 5°C / min and held for 1 h to form a dense hard carbon outer layer.

[0038] Example 5 Example 5 is basically the same as Example 1, except that in step 3, the temperature is increased to 350°C at 1°C / min and held for 90 min to form a soft carbon inner layer; then the temperature is increased to 650°C at 3°C / min and held for 60 min to form a graphitized carbon intermediate layer; finally, the temperature is increased to 900°C at 5°C / min and held for 3 h to form a dense hard carbon outer layer.

[0039] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that in step 1, supercritical carbon dioxide is not added for foaming treatment.

[0040] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that VTES silane coupling agent is not added in step 2.

[0041] Comparative Example 3 Comparative Example 3 is basically the same as Example 1, except that in step 3, the high-temperature calcination is directly heated to 900°C at 5°C / min and held for 2 hours to form a single layer of hard carbon coating.

[0042] Comparative Example 4 Comparative Example 4 is basically the same as Example 1, except that in step 3, the low-temperature calcination is directly heated to 400°C at 2°C / min and held for 2 hours to form a single layer of soft carbon coating.

[0043] Comparative Example 5 Comparative Example 4 is basically the same as Example 1, except that in step 2, the negative pressure of -30 kPa is cyclically applied three times (each time for 30 min).

[0044] Comparative Example 6 Comparative Example 4 is basically the same as Example 1, except that in step 2, the negative pressure is -90 kPa, the pressure is maintained for 30 minutes, and there is no circulation.

[0045] Comparative Example 7 This comparative example provides a common silicon-carbon anode material, such as: mixing nano-silicon powder (average particle size 100 nm) and conductive carbon black at a mass ratio of 3:7 and placing it in a planetary ball mill jar. Stainless steel grinding balls (ball-to-material ratio 15:1) and an appropriate amount of anhydrous ethanol are added, and the mixture is ball-milled at 350 rpm for 12 hours. The slurry is then removed, filtered to separate the grinding balls, vacuum-dried at 80°C for 24 hours, and passed through a 400-mesh sieve to obtain the comparative sample.

[0046] Application examples The materials prepared in Examples 1-5 and Comparative Examples 1-3 were assembled into lithium-ion coin cells for electrochemical performance testing. The specific methods are as follows: Using 1-methyl-2-pyrrolidone (NMP) as a solvent, conductive agent carbon black (SuperP), binder polyvinylidene fluoride (PVDF), and silicon-carbon composite material were mixed in a mass ratio of 1:1:8 to form a slurry. This slurry was then coated onto copper foil as a current collector and dried under vacuum at 80°C for 6 hours. After cooling, the slurry was cut into electrode sheets. In a glove box, the positive electrode shell, electrode sheets, separator, lithium sheet, nickel foam, and negative electrode shell were stacked sequentially, and an appropriate electrolyte was added before encapsulation. The battery shell used was a CR2016 type, the separator was Celgard2400, and the electrolyte was a mixed electrolyte containing 1M LiPF6 of ethylene carbonate (EC) and diethyl carbonate (DEC) (the volume ratio of EC to DEC in the mixed electrolyte was 1:1). The prepared batteries were subjected to cycle testing, and the electrochemical performance tests were performed on a Blue Electric CT2001A battery testing system. After 100 cycles at 0.5C, the electrode was fully charged and disassembled. The thickness of the negative electrode was measured with a micrometer and the expansion rate was calculated by comparing it with the uncycled electrode.

[0047] Table 1 below shows the button cell test data prepared in Examples 1-5 and Comparative Examples 1-3.

[0048] Table 1 As shown in Table 1, the silicon-carbon composite material prepared in this invention exhibits excellent cycling stability. Example 1 shows an initial coulombic efficiency of 92.1%, indicating that the dense gradient coating reduces irreversible lithium consumption in the SEI during the first cycle. The 100-cycle retention rate is 89.3%, and the negative electrode expansion rate is 28%, indicating that silicon volume expansion is effectively suppressed, the interface is stable, and the cycling performance is stable. Regarding the differences between examples, the supercritical carbon dioxide foaming pressure and gradient calcination holding time are key factors in performance regulation: Example 1 (foaming pressure 35 MPa) shows better initial efficiency (92.1%), 0.5C specific capacity (1523 mAh / g), and 100-cycle retention rate (89.3%) than Examples 2 (25 MPa) and 3 (45 MPa), and its negative electrode expansion rate (28%) is significantly lower than both (45% and 56%), indicating that a foaming pressure of around 35 MPa can construct a pore structure with uniform size and good permeability. The carbon framework provides ample independent space for nano-silicon and ensures ion / electron transport efficiency. The cycle retention rate and expansion rate of Example 1 (soft carbon for 1 h, graphitized carbon for 30 min, hard carbon for 2 h) are better than those of Example 4 (85.2%, 31%) with shorter holding times and Example 5 (82.3%, 35%) with longer holding times. This indicates that the holding time of the staged calcination needs to match the requirements for the formation of the gradient coating layer. Too long or too short a holding time will lead to insufficient density or increased brittleness of the coating layer, weakening the stress buffering and SEI protection effect.

[0049] Comparing the examples and comparative examples: Comparative Example 1 did not use supercritical foaming, and its first-cycle efficiency (76.2%), specific capacity (1025 mAh / g), and 100-cycle retention rate (42.7%) dropped sharply, while its negative electrode expansion rate (120%) soared, proving that the three-dimensional through-porous carbon framework is the basis for solving silicon agglomeration and suppressing volume expansion; Comparative Example 2 did not add VTES coupling agent, and although its specific capacity (1452 mAh / g) was close to that of Example 1, its retention rate (79.5%) and expansion rate (47%) deteriorated, indicating that Si-OC covalent bonds can strengthen the silicon-carbon interface bonding and reduce interface peeling during charging and discharging; Comparative Example 3 used a single-layer hard carbon coating, and its retention rate (71.2%) and expansion rate (63%) were much worse than those of Example 1, highlighting the synergistic advantages of soft carbon absorbing stress and hard carbon inhibiting side reactions brought about by gradient coating. Single-layer coating cannot take into account both buffering and protection functions. Furthermore, compared to single-layer soft carbon coating (Comparative Example 4), the gradient coating of this invention significantly improves cycle stability and volume control through the synergistic effect of soft carbon absorbing stress, graphitized carbon enhancing conductivity, and hard carbon inhibiting side reactions. This is key to achieving low expansion rate (28%) and high retention rate (89.3%) in Example 1. The vacuum negative pressure (-60~-100kPa) and the number of cycles (2~5 times) directly affect the uniformity of silicon source loading; insufficient pressure (Comparative Example 5) or lack of cycles (Comparative Example 6) leads to performance degradation. Performance verification in Comparative Example 7 shows that traditional mechanical mixing methods cannot solve the three major bottlenecks of silicon particle agglomeration, interface instability, and volume expansion. This invention, through the synergistic effect of multiple technologies (mesoporosis confinement, covalent bond enhancement, and gradient coating), achieves a comprehensive breakthrough in specific capacity, cycle stability, and volume control.

[0050] In summary, this invention achieves breakthroughs in the electrochemical performance and structural stability of silicon-carbon composite materials through key parameter optimization and multi-technology synergy, providing a feasible solution for high-energy lithium-ion battery anode materials.

[0051] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, include: The three-dimensional interconnected mesoporous carbon framework, nano-silicon particles loaded within the mesopores of the three-dimensional interconnected mesoporous carbon framework, and a gradient carbon coating layer covering the surfaces of the carbon framework and the nano-silicon particles; the nano-silicon particles and the three-dimensional interconnected mesoporous carbon framework are connected by Si-OC covalent bonds; the gradient carbon coating layer includes a soft carbon inner layer, a graphitized carbon intermediate layer, and a dense hard carbon outer layer.

2. A method for preparing the silicon-carbon composite material according to claim 1, characterized in that, Includes the following steps: Synthesis of three-dimensional interconnected mesoporous carbon skeleton: by mixing carbon source and template agent in solvent, adding supercritical carbon dioxide as foaming agent for foaming treatment, followed by high-temperature carbonization under protective atmosphere, and cooling to obtain the three-dimensional interconnected mesoporous carbon skeleton. Confined silicon loading: The three-dimensional through-porous carbon skeleton is immersed in a solution containing a silicon source and a silane coupling agent, treated under vacuum negative pressure, and then treated with ammonia water vapor to obtain a carbon / silicon loaded sample. Gradient carbon coating: The carbon / silicon loaded sample is subjected to staged high-temperature calcination under a protective atmosphere to form a carbon coating layer consisting of a soft carbon inner layer, a graphitized carbon intermediate layer, and a dense hard carbon outer layer, thereby obtaining the silicon-carbon composite material.

3. The method according to claim 2, characterized in that, The carbon source includes at least one of phenolic resin, asphalt, biomass carbon source, polyacrylonitrile, and polyvinyl alcohol; the template agent includes at least one of polystyrene microspheres, silica microspheres, inorganic salts, and metal oxides; the mass ratio of the carbon source to the template agent is (3-4):(1-2).

4. The method according to claim 2, characterized in that, The high-temperature carbonization temperature is 800~1000℃, the heating rate is 2~5℃ / min, and the holding time is 1~3h.

5. The method according to claim 2, characterized in that, The supercritical carbon dioxide has a foaming pressure of 20-50 MPa, a foaming temperature of 60-100℃, and a foaming time of 1-3 h.

6. The method according to claim 2, characterized in that, The silicon source includes silicon tetrachloride or tetraethyl orthosilicate; the silane coupling agent includes at least one of vinyl silane coupling agent, amino silane coupling agent, epoxy silane coupling agent, and methyl silane coupling agent; the molar concentration of the silicon source in the solution is 0.1-1 mol / L, and the volume concentration of the silane coupling agent in the solution is 0.5-2 vol.

7. The method according to claim 2, characterized in that, The vacuum negative pressure is -60~-100kPa, the number of cycles is 2~5, and the pressure holding time for each cycle is 20~40min; the temperature of the ammonia water vapor treatment is 40~60℃, and the treatment time is 0.5~2h.

8. The method according to claim 2, characterized in that, The staged high-temperature calcination includes: First calcination stage: Heat to 300-400℃ at 0.5-2℃ / min, hold for 0.5-2h to form a soft carbon inner layer; Second calcination stage: Heat to 600-700℃ at 2-5℃ / min and hold for 0.25-1h to form a graphitized carbon intermediate layer; The third calcination stage: the temperature is increased to 800-1000℃ at a rate of 3-8℃ / min, and held for 1-3 hours to form a dense hard carbon outer layer.

9. The use of the silicon-carbon composite material according to claim 1 as a negative electrode material for lithium-ion batteries.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and a negative electrode, wherein the negative electrode is an electrode sheet made by mixing the silicon-carbon composite material of claim 1 with a conductive agent and a binder.

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