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

Abstract

The invention provides a silicon-carbon composite material as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The preparation method of the silicon-carbon composite material comprises the following steps: S1, uniformly mixing a nitrogen-containing carbon source precursor, an amphiphilic block copolymer and a solvent to obtain a precursor solution; performing plasma treatment on the precursor solution, and then performing heating treatment to obtain nitrogen-doped carbon spheres; s2, dispersing the nitrogen-doped carbon spheres in a solution containing an organic silicon source, adding a reducing agent, and performing microwave treatment to obtain an intermediate; and S3, mixing the intermediate with graphene oxide, introducing CO2, reacting in a supercritical state, and releasing pressure to obtain the silicon-carbon composite material. The battery assembled by the silicon-carbon composite material prepared by the invention has excellent rate capability and cycle performance.

Description

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

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

[0002] With the increasing demand for energy density in lithium-ion batteries, silicon-based anode materials have become a research hotspot due to their extremely high theoretical specific capacity (approximately 4200 mAh / g, far exceeding the 372 mAh / g of graphite anodes). However, silicon materials experience severe volume expansion (up to 300% or more) during charge and discharge, leading to electrode structure pulverization, active material shedding, and repeated rupture and regeneration of the solid electrolyte interphase (SEI) film. This process not only continuously consumes electrolyte and active lithium but also exacerbates the increase in interfacial impedance, ultimately causing a rapid decline in battery cycle performance.

[0003] To address these issues, existing technologies typically employ strategies such as nanostructuring, core-shell structure design, or carbon coating. For example, silicon nanoparticles are coated within a carbon shell (core-shell structure), or gaps are left (yolk-shell structure). While these methods alleviate volume expansion to some extent, they still have the following drawbacks: First, simple physical mixing or coating cannot achieve a strong bond between silicon and the carbon matrix, and the interface is prone to separation after long-term cycling; Second, a single buffer structure (such as only gaps) cannot withstand the mechanical stress accumulated over long-term cycling, and the carbon shell is prone to cracking; Third, traditional coating methods are difficult to form a dense and uniform protective layer, and the stability of the SEI film remains insufficient.

[0004] Therefore, developing a silicon-carbon composite material with a multi-level mechanical buffering mechanism, strong interfacial bonding, and excellent conductivity is key to realizing the commercialization of silicon anodes. Summary of the Invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a silicon-carbon composite material, its preparation method and application.

[0006] The present invention proposes a silicon-carbon composite material comprising nitrogen-doped carbon spheres, silicon quantum dots, and a graphene layer, wherein the nitrogen-doped carbon spheres serve as the core, the silicon quantum dots are embedded in the pores of the nitrogen-doped carbon spheres, and the graphene layer serves as the outer shell.

[0007] The silicon-carbon composite material proposed in this invention has a three-layer buffer structure from the inside out; the primary buffer structure is a nitrogen-doped carbon sphere that provides internal expansion space; the secondary buffer structure is a silicon quantum dot stress dispersion layer embedded in the wall of the nitrogen-doped carbon sphere by microwave in-situ reduction; and the tertiary buffer structure is a gradient graphene network formed by supercritical fluid technology and coated on the surface of the nitrogen-doped carbon sphere containing silicon quantum dots.

[0008] Preferably, the nitrogen-doped carbon spheres have a particle size of 5–50 nm and an average outer diameter of 80–800 nm.

[0009] Preferably, the silicon quantum dots have a grain size of 5–10 nm.

[0010] Preferably, the number of graphene layers is 3 to 10.

[0011] Having 3 to 10 graphene layers helps to ensure uniform coating and effective lithium ion transport. If the coating layer is too thin, the coating effect is poor; if it is too thick, lithium ions cannot be transported.

[0012] Preferably, the graphene layer is wrapped around the surface of a nitrogen-doped carbon sphere containing silicon quantum dots.

[0013] This invention also proposes a method for preparing silicon-carbon composite materials, comprising the following steps:

[0014] S1. Mix the nitrogen-containing carbon source precursor, amphiphilic block copolymer, and solvent evenly to obtain a precursor solution; subject the precursor solution to plasma treatment and then heat treatment to obtain nitrogen-doped carbon spheres.

[0015] S2. Nitrogen-doped carbon spheres are dispersed in a solution containing an organosilicon source, a reducing agent is added, and the mixture is microwaved to obtain an intermediate.

[0016] S3. Mix the intermediate with graphene oxide, introduce CO2 and react in a supercritical state, then release the pressure to obtain a silicon-carbon composite material.

[0017] This invention utilizes a nitrogen-containing carbon source precursor and an amphiphilic block copolymer to prepare hollow nitrogen-doped carbon spheres via plasma-induced self-assembly and high-temperature carbonization. The carbon spheres are then dispersed in a nonpolar solvent containing an organosilicon source, and in-situ growth of silicon quantum dots is achieved within the pores under microwave radiation and a reducing agent. Finally, the spheres are mixed with graphene oxide, and gradient coating is achieved through supercritical CO2 treatment and rapid depressurization. This invention effectively solves the pulverization and interface failure problems caused by the volume expansion of silicon anodes through a three-stage mechanical linkage mechanism of "internal depressurization-internal solidification-external beam," significantly improving the cycling stability and rate performance of the material.

[0018] Preferably, in S1, the nitrogen-containing carbon source precursor is selected from one or more of glucosamine hydrochloride, dopamine hydrochloride, chitosan, aniline, pyrrole, and melamine.

[0019] Preferably, in S1, the amphiphilic block copolymer is selected from one or more of triblock copolymers, hexadecyltrimethylammonium bromide (CTAB), and polystyrene-polyethylene oxide (PS-b-PEO).

[0020] More preferably, the triblock copolymer is selected from one or more of F127 and P123.

[0021] Preferably, in S1, the mass ratio of the nitrogen-containing carbon source precursor to the amphiphilic block copolymer is 1:0.1 to 2.

[0022] Within a certain range, the mass ratio of nitrogen-containing carbon source precursor to amphiphilic block copolymer helps to form hollow nitrogen-doped carbon spheres with moderate wall thickness and complete structure. The ratio of nitrogen-containing carbon source precursor to amphiphilic block copolymer is crucial to the performance of the multi-level buffer structure: when the mass ratio is 1:0.3 to 0.8, hollow carbon spheres with moderate wall thickness (5 to 50 nm) and complete structure can be obtained. This wall thickness can ensure mechanical strength to suppress the volume expansion of silicon quantum dots and maintain good ion transport channels. If the ratio is lower than 1:0.1, a solid structure tends to be formed, losing the first-level buffer space; if the ratio is higher than 1:2.0, the carbon wall is too thin and is prone to cracking and pulverization during repeated lithium-ion insertion and extraction.

[0023] Preferably, in S1, the solvent is selected from one or more of ethanol and water.

[0024] More preferably, the volume ratio of ethanol to water is 1:1 to 3.

[0025] Controlling the volume ratio of ethanol to water helps to stabilize the formation of micelle nuclei, prevents the micelle nuclei from sticking together, and ensures that the spheres are of uniform size.

[0026] Preferably, in step S1, the plasma treatment power is 100-300W, the plasma treatment time is 0.5-2h, and the plasma treatment atmosphere is a protective gas; the protective gas is selected from one or more of nitrogen and argon.

[0027] Plasma treatment conditions help to obtain perfectly monodisperse hollow spheres in a short time. When the power is <100W, the yield of carbon spheres is low and they are mostly fragments. When the power is >300W, pitting will occur on the surface of the spheres.

[0028] Preferably, in step S1, the heating treatment includes: heating to 700-1000°C at a heating rate of 1-5°C / min under a protective gas atmosphere, holding at that temperature for 1-4 hours, and then naturally cooling to room temperature; the protective gas is selected from one or more of nitrogen and argon.

[0029] This invention involves high-temperature carbonization of plasma-treated materials through heat treatment to obtain hollow nitrogen-doped carbon spheres.

[0030] Preferably, in step S2, the solution containing the organosilicon source includes an organosilicon source and a solvent; the organosilicon source is selected from one or more of polymethylhydrosiloxane (PMHS), hexamethyldisiloxane, octamethylcyclotetrasiloxane (D4), and tetraethyl orthosilicate (TEOS); the solvent is a nonpolar organic solvent, selected from one or more of n-hexane, n-heptane, cyclohexane, toluene, and xylene; the mass ratio of the nitrogen-doped carbon spheres to the organosilicon source is 1:0.5-3; the mass-volume ratio of the nitrogen-doped carbon spheres to the organic solvent is 1g:50-150mL.

[0031] The mass ratio of nitrogen-doped carbon spheres to organosilicon source is 1:0.5–3. This is because when the amount of organosilicon source is too low (<0.5), the specific capacity improvement of the composite material is limited; while when the amount of organosilicon source is too high (>3.0), the excess organosilicon source cannot completely enter the mesoporous channels of the carbon sphere wall, tending to undergo microwave reduction on the outer surface of the carbon sphere, forming large-sized silicon aggregates, thus defeating the original design intent of the secondary buffer structure. Only under specific ratios, combined with a diluted non-polar organic solvent environment, can the capillary effect of the carbon sphere channels be utilized to achieve confined growth of silicon quantum dots within the carbon wall. Furthermore, this invention preferably uses non-polar solvents (such as n-hexane, toluene, etc.) because their small microwave absorption cross-section ensures that microwave energy is mainly concentrated at the interface between the carbon sphere and the silicon precursor, achieving localized high-temperature-induced in-situ rapid nucleation of quantum dots, avoiding boiling over or runaway reactions caused by overall solvent overheating.

[0032] Preferably, in S2, the reducing agent is selected from one or more of sodium borohydride, potassium borohydride, lithium aluminum hydride, and magnesium powder.

[0033] Preferably, in S2, the molar ratio of the reducing agent to the silicon element in the organosilicon source is 2 to 10:1.

[0034] The molar ratio of reducing agent to silicon in the organosilicon source needs to be controlled within a certain range because if the amount of reducing agent is insufficient (<2:1), the organosilicon source can only undergo partial reduction or only surface cross-linking, resulting in a large amount of amorphous silicon oxide (SiO₂) in the product. x This oxide is non-conductive and inert during charge and discharge, which severely reduces the battery's first-cycle coulombic efficiency (ICE). If too much reducing agent is used (>10:1), the excess strong reducing agent will trigger a violent hydrogen evolution side reaction (reacting with trace amounts of water or hydroxyl groups in the solvent) under microwave heating. The resulting bubbles will generate huge local pressures, which may shatter the thin-walled structure of the hollow carbon spheres and damage the primary buffer chamber. In addition, raw materials will be wasted and post-processing will be difficult.

[0035] Preferably, in S2, the microwave processing parameters include: radiation power of 300-1000W, radiation mode of pulsed or continuous, microwave processing time of 5-40min, and microwave processing temperature of 60-150℃.

[0036] The role of microwave treatment lies in utilizing the excellent dielectric loss characteristics of nitrogen-doped carbon spheres. The nitrogen-doped carbon spheres act as a microscopic heat source, absorbing microwave energy, with the internal channel temperature significantly higher than the external solvent temperature. This "internal heat, external cold" temperature gradient allows siloxanes adsorbed within the channels to be preferentially reduced to silicon quantum dots by reducing agents (such as LiAlH4 or NaBH4), while precursors in the external solvent do not readily nucleate. This enables precise, localized growth of silicon quantum dots within the carbon wall, avoiding silicon particle agglomeration and surface enrichment caused by traditional heating methods.

[0037] Preferably, in S2, the intermediate is a nitrogen-doped carbon sphere containing silicon quantum dots; that is, the intermediate is a composite material in which silicon quantum dots are embedded in the pores of nitrogen-doped carbon spheres.

[0038] Preferably, in S3, the mass ratio of the intermediate to graphene oxide is 1:0.05 to 0.2.

[0039] The mass ratio of intermediate to graphene oxide is controlled within a certain range so that the thickness of the graphene layer is 1 to 5 nm.

[0040] Preferably, in S3, the parameters of the supercritical state include: temperature 40-60℃ and air pressure 10-20MPa.

[0041] The advantage of treatment under supercritical CO2 conditions lies in the fact that supercritical CO2 has extremely low surface tension and a high diffusion coefficient, enabling it to carry GO sheets deep into the micropores and crevices of the carbon sphere surface, achieving "no dead angle" coating. During rapid depressurization, the supercritical CO2 molecules that have penetrated between the GO layers instantly vaporize and expand, generating microscopic peeling forces that effectively overcome the van der Waals forces between the GO sheets, preventing them from recombining into a dense graphite block. This expansion effect from the inside out results in a loose and porous structure in the outermost graphene network, which is beneficial for electrolyte wetting; while the inner graphene layers close to the carbon spheres are relatively dense due to the constraint of the substrate, providing good conductive contact. This gradient structure of "loose on the outside and dense on the inside" cannot be achieved by conventional mechanical mixing.

[0042] Preferably, in step S3, the reaction time is 1 to 4 hours.

[0043] Preferably, in step S3, after depressurization, heat treatment is further included, wherein the heat treatment includes heat treatment at 650-700°C for 1-3 hours in a reducing gas atmosphere; the reducing gas atmosphere includes a reducing gas and a protective gas, wherein the reducing gas is hydrogen and the protective gas is selected from one or more of nitrogen and argon; the volume ratio of the reducing gas to the protective gas is 5-10:90-95.

[0044] The purpose of heat treatment is to reduce graphene oxide to graphene.

[0045] Application of the above-described silicon-carbon composite material or the silicon-carbon composite material prepared by the above-described preparation method in lithium-ion batteries.

[0046] This invention effectively solves the problems of pulverization and interface failure caused by the volume expansion of silicon anodes through a three-level mechanical linkage mechanism of "internal leakage-middle consolidation-external binding", and significantly improves the cycle stability and rate performance of the material.

[0047] The beneficial effects of this invention are as follows:

[0048] The purpose of this invention is to provide a high-performance silicon-carbon composite material with a multi-level buffer structure and its preparation method. The aim is to solve the problems of pulverization and interface failure caused by the volume expansion of silicon anode by constructing a three-level buffer system of "internal leakage-middle solid-external binding", and significantly improve the cycle stability and rate performance of the material.

[0049] The silicon-carbon composite material disclosed in this invention possesses a three-tiered buffer structure, which can progressively dissipate the enormous volumetric expansion stress. The first-tier buffer structure's internal cavity acts as a "volume throughput chamber," allowing silicon to expand freely inward during lithium intercalation, effectively eliminating radial expansion stress and preventing overall particle breakage. The second-tier buffer structure utilizes microwave in-situ reduction technology to firmly embed silicon quantum dots (<10nm) within the carbon wall pores. The nanoscale effect prevents silicon pulverization, while the rigid constraint of the carbon wall restricts silicon agglomeration, achieving "point-wall" synergistic stress resistance. The third-tier buffer structure utilizes a gradient graphene network constructed using supercritical fluid technology. The dense inner layer provides strong binding force and high conductivity, while the loose outer layer facilitates electrolyte wetting. This network not only enhances overall mechanical strength but also stabilizes the SEI film, preventing repeated breakage. Furthermore, compared to traditional hydrothermal methods, plasma-induced self-assembly allows plasma treatment to rapidly initiate precursor cross-linking at low temperatures, forming a robust pre-carbonized shell, ensuring the integrity and uniformity of the hollow structure after high-temperature carbonization. Microwave in-situ reduction utilizes the excellent microwave absorption properties of carbon spheres to create microscopic localized high temperatures, enabling the localized growth of silicon quantum dots within carbon channels and avoiding ineffective nucleation of silicon sources in solvents. Supercritical CO2 gradient coating utilizes the high permeability of supercritical fluids and the instantaneous expansion force during rapid depressurization to achieve the layer-by-layer exfoliation and recombination of graphene oxide, constructing a gradient coating structure that cannot be achieved through conventional physical mixing. Detailed Implementation

[0050] The technical solution of the present invention will be described in detail through specific embodiments.

[0051] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0052] Example 1

[0053] A method for preparing a silicon-carbon composite material includes the following steps:

[0054] S1. Preparation of hollow nitrogen-doped carbon spheres: Dissolve 1.0 g glucosamine hydrochloride and 0.5 g F127 in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stir until transparent; place the solution in a plasma reactor and treat it for 1 h under an argon atmosphere and 200 W power to induce micelle self-assembly and cross-linking; then carbonize it at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and obtain hollow nitrogen-doped carbon spheres after natural cooling.

[0055] S2. In-situ embedding of silicon quantum dots: Take 0.5g of the above hollow nitrogen-doped carbon spheres and disperse them in 50mL of n-hexane; add 0.5g of polymethylhydrosiloxane (PMHS) and ultrasonically disperse for 30min; then add 0.5g of sodium borohydride powder, seal and place in a microwave reactor, and react at 500W power and 80℃ for 15min; after the reaction, centrifuge and wash three times with ethanol to obtain the intermediate, namely hollow nitrogen-doped carbon spheres embedded with silicon quantum dots.

[0056] S3. Gradient graphene coating: 1.0 g of the above intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an autoclave, CO2 was introduced, the temperature was raised to 50 °C, the pressure was increased to 15 MPa, and the supercritical state was maintained for 2 h; then the pressure relief valve was opened within 30 seconds to quickly release the pressure to atmospheric pressure; finally, GO was reduced by heat treatment at 700 °C for 2 h under an argon-hydrogen mixed gas with an argon-hydrogen volume ratio of 95:5 to obtain the final product.

[0057] Example 2

[0058] A method for preparing a silicon-carbon composite material includes the following steps:

[0059] S1. Preparation of hollow nitrogen-doped carbon spheres: 1.0 g of dopamine hydrochloride and 0.8 g of P123 were dissolved in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stirred until transparent; the solution was placed in a plasma reactor and treated for 1 h under an argon atmosphere and a power of 200 W to induce micelle self-assembly and cross-linking; then carbonized at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and obtained hollow nitrogen-doped carbon spheres after natural cooling.

[0060] S2. In-situ embedding of silicon quantum dots: Take 0.5g of the above hollow nitrogen-doped carbon spheres and disperse them in 50mL of n-hexane; add 0.5g of polymethylhydrosiloxane (PMHS) and ultrasonically disperse for 30min; then add 0.5g of sodium borohydride powder, seal and place in a microwave reactor, and react at 500W power and 80℃ for 15min; after the reaction, centrifuge and wash three times with ethanol to obtain the intermediate, namely hollow nitrogen-doped carbon spheres embedded with silicon quantum dots.

[0061] S3. Gradient graphene coating: 1.0 g of the above intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an autoclave, CO2 was introduced, the temperature was raised to 50 °C, the pressure was increased to 15 MPa, and the supercritical state was maintained for 2 h; then the pressure relief valve was opened within 30 seconds to quickly release the pressure to atmospheric pressure; finally, GO was reduced by heat treatment at 700 °C for 2 h under an argon-hydrogen mixed gas with an argon-hydrogen volume ratio of 95:5 to obtain the final product.

[0062] Example 3

[0063] A method for preparing a silicon-carbon composite material includes the following steps:

[0064] S1. Preparation of hollow nitrogen-doped carbon spheres: Dissolve 1.0 g glucosamine hydrochloride and 0.5 g F127 in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stir until transparent; place the solution in a plasma reactor and treat it for 1 h under an argon atmosphere and 200 W power to induce micelle self-assembly and cross-linking; then carbonize it at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and obtain hollow nitrogen-doped carbon spheres after natural cooling.

[0065] S2. In-situ embedding of silicon quantum dots: Take 0.5g of the above hollow nitrogen-doped carbon spheres and disperse them in 50mL of n-hexane; add 0.5g of tetraethyl orthosilicate (TEOS) and sonicate for 30min; then add 0.5g of LiAlH4 powder, seal and place in a microwave reactor, and react at 500W power and 80℃ for 15min; after the reaction, centrifuge and wash three times with ethanol to obtain the intermediate, namely hollow nitrogen-doped carbon spheres embedded with silicon quantum dots.

[0066] S3. Gradient graphene coating: 1.0 g of the above intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an autoclave, CO2 was introduced, the temperature was raised to 50 °C, the pressure was increased to 15 MPa, and the supercritical state was maintained for 2 h; then the pressure relief valve was opened within 30 seconds to quickly release the pressure to atmospheric pressure; finally, GO was reduced by heat treatment at 700 °C for 2 h under an argon-hydrogen mixed gas with an argon-hydrogen volume ratio of 95:5 to obtain the final product.

[0067] Comparative Example 1

[0068] A method for preparing a silicon-carbon composite material includes the following steps:

[0069] S1. Preparation of nitrogen-doped carbon spheres: 1.0 g glucosamine hydrochloride and 0.05 g F127 were dissolved in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stirred until transparent; the solution was placed in a plasma reactor and treated for 1 h under an argon atmosphere and a power of 200 W to induce micelle self-assembly and cross-linking; then carbonized at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and naturally cooled to obtain nitrogen-doped carbon spheres; the nitrogen-doped carbon spheres were solid nitrogen-doped carbon spheres.

[0070] S2. In-situ embedding of silicon quantum dots: Take 0.5g of the above nitrogen-doped carbon spheres and disperse them in 50mL of n-hexane; add 0.5g of polymethylhydrosiloxane (PMHS) and ultrasonically disperse for 30min; then add 0.5g of sodium borohydride powder, seal and place in a microwave reactor, and react at 500W power and 80℃ for 15min; after the reaction, centrifuge and wash three times with ethanol to obtain the intermediate, i.e., nitrogen-doped carbon spheres embedded with silicon quantum dots.

[0071] S3. Gradient graphene coating: 1.0 g of the above intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an autoclave, CO2 was introduced, the temperature was raised to 50 °C, the pressure was increased to 15 MPa, and the supercritical state was maintained for 2 h; then the pressure relief valve was opened within 30 seconds to quickly release the pressure to atmospheric pressure; finally, GO was reduced by heat treatment at 700 °C for 2 h under an argon-hydrogen mixed gas with an argon-hydrogen volume ratio of 95:5 to obtain the final product.

[0072] Comparative Example 2

[0073] A method for preparing a silicon-carbon composite material includes the following steps:

[0074] S1. Preparation of hollow nitrogen-doped carbon spheres: Dissolve 1.0 g glucosamine hydrochloride and 0.5 g F127 in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stir until transparent; place the solution in a plasma reactor and treat it for 1 h under an argon atmosphere and 200 W power to induce micelle self-assembly and cross-linking; then carbonize it at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and obtain hollow nitrogen-doped carbon spheres after natural cooling.

[0075] S2. Take 0.5g of the above hollow nitrogen-doped carbon spheres and mix them evenly with 0.5g of nano-silicon (size 50nm) to obtain a composite intermediate.

[0076] S3. Gradient graphene coating: 1.0 g of the above composite intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an autoclave, CO2 was introduced, the temperature was raised to 50 °C, the pressure was increased to 15 MPa, and the supercritical state was maintained for 2 h; then the pressure relief valve was opened within 30 seconds to quickly release the pressure to atmospheric pressure; finally, GO was reduced by heat treatment at 700 °C for 2 h under an argon-hydrogen mixed gas with an argon-hydrogen volume ratio of 95:5 to obtain the final product.

[0077] Comparative Example 3

[0078] A method for preparing a silicon-carbon composite material includes the following steps:

[0079] S1. Preparation of hollow nitrogen-doped carbon spheres: Dissolve 1.0 g glucosamine hydrochloride and 0.5 g F127 in 60 mL of ethanol / water (volume ratio 1:2) mixed solution and stir until transparent; place the solution in a plasma reactor and treat it for 1 h under an argon atmosphere and 200 W power to induce micelle self-assembly and cross-linking; then carbonize it at 900 °C for 2 h in a tube furnace under argon protection with a heating rate of 5 °C / min, and obtain hollow nitrogen-doped carbon spheres after natural cooling.

[0080] S2. In-situ embedding of silicon quantum dots: Take 0.5g of the above hollow nitrogen-doped carbon spheres and disperse them in 50mL of n-hexane; add 0.5g of polymethylhydrosiloxane (PMHS) and ultrasonically disperse for 30min; then add 0.5g of sodium borohydride powder, seal and place in a microwave reactor, and react at 500W power and 80℃ for 15min; after the reaction, centrifuge and wash three times with ethanol to obtain the intermediate, namely hollow nitrogen-doped carbon spheres embedded with silicon quantum dots.

[0081] S3. Graphene coating: 1.0 g of the above intermediate was dispersed in 50 mL of graphene oxide (GO) ethanol dispersion (concentration 2 mg / mL) and stirred evenly to obtain a slurry; the slurry was placed in an oven to dry and then heat-treated at 700 °C for 2 h under an argon-hydrogen mixture to reduce GO, with an argon-hydrogen volume ratio of 95:5, to obtain the final product.

[0082] The aforementioned silicon-carbon composite material was used as the negative electrode active material, and was mixed with a conductive agent (Super P) and a binder (CMC / SBR) at a mass ratio of 8:1:1 to form a slurry. This slurry was then coated onto copper foil and assembled into CR2032 coin cell half-cells. Its cycle performance and rate performance were tested, and the test results are shown in Table 1.

[0083] Cyclic performance: Constant current charge and discharge test at a rate of 0.5C within a voltage range of 0.01 to 1.5V.

[0084] Rate performance: Constant current charge and discharge tests were conducted at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 3C within a voltage range of 0.01 to 1.5V.

[0085] Table 1

[0086]

[0087] As can be seen from the data in Table 1, the battery assembled from the silicon-carbon composite material prepared in this invention has excellent rate performance and cycle performance.

[0088] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, It includes nitrogen-doped carbon spheres, silicon quantum dots, and a graphene layer, with the nitrogen-doped carbon spheres serving as the core, the silicon quantum dots embedded in the pores of the nitrogen-doped carbon spheres, and the graphene layer serving as the outer shell.

2. A method for preparing the silicon-carbon composite material according to claim 1, characterized in that, Includes the following steps: S1. Mix the nitrogen-containing carbon source precursor, the amphiphilic block copolymer, and the solvent evenly to obtain a precursor solution; The precursor solution was subjected to plasma treatment, followed by heating treatment to obtain nitrogen-doped carbon spheres. S2. Disperse nitrogen-doped carbon spheres in a solution containing an organosilicon source, add a reducing agent, and microwave treat to obtain an intermediate; S3. Mix the intermediate with graphene oxide, introduce CO2 to react in a supercritical state, release the pressure, and obtain a silicon-carbon composite material.

3. The preparation method according to claim 2, characterized in that, In S1, the nitrogen-containing carbon source precursor is selected from one or more of glucosamine hydrochloride, dopamine hydrochloride, chitosan, aniline, pyrrole, and melamine; the amphiphilic block copolymer is selected from one or more of triblock copolymer, hexadecyltrimethylammonium bromide, and polystyrene-polyethylene oxide; the triblock copolymer is selected from one or more of F127 and P123; the mass ratio of the nitrogen-containing carbon source precursor to the amphiphilic block copolymer is 1:0.1 to 2.

4. The preparation method according to claim 2, characterized in that, In S1, the solvent is selected from one or more of ethanol and water; the volume ratio of ethanol to water is 1:1 to 3; the plasma treatment power is 100 to 300 W, the plasma treatment time is 0.5 to 2 h, and the plasma treatment atmosphere is a protective gas; the protective gas is selected from one or more of nitrogen and argon. The heating treatment includes: heating to 700-1000°C at a heating rate of 1-5°C / min under a protective gas atmosphere, holding at the temperature for 1-4 hours, and then naturally cooling to room temperature; the protective gas is selected from one or more of nitrogen and argon.

5. The preparation method according to claim 2, characterized in that, In S2, the solution containing the organosilicon source includes an organosilicon source and a solvent; the organosilicon source is selected from one or more of polymethylhydrosiloxane, hexamethyldisiloxane, octamethylcyclotetrasiloxane, and tetraethyl orthosilicate; the solvent is a nonpolar organic solvent, selected from one or more of n-hexane, n-heptane, cyclohexane, toluene, and xylene; the mass ratio of the nitrogen-doped carbon spheres to the organosilicon source is 1:0.5-3; the mass-volume ratio of the nitrogen-doped carbon spheres to the organic solvent is 1g:50-150mL.

6. The preparation method according to claim 2, characterized in that, In S2, the reducing agent is selected from one or more of sodium borohydride, potassium borohydride, lithium aluminum hydride, and magnesium powder; the molar ratio of the reducing agent to silicon in the organosilicon source is 2 to 10:

1.

7. The preparation method according to claim 2, characterized in that, In S2, the microwave processing parameters include: radiation power of 300-1000W, radiation mode of pulsed or continuous, microwave processing time of 5-40min, and microwave processing temperature of 60-150℃.

8. The preparation method according to claim 2, characterized in that, In S3, the mass ratio of the intermediate to graphene oxide is 1:0.05 to 0.2; the parameters of the supercritical state include: temperature 40 to 60°C, gas pressure 10 to 20 MPa; and reaction time 1 to 4 h.

9. The preparation method according to claim 2, characterized in that, In step S3, after depressurization, heat treatment is further included. The heat treatment includes heat treatment at 650-700°C for 1-3 hours in a reducing gas atmosphere. The reducing gas atmosphere includes a reducing gas and a protective gas. The reducing gas is hydrogen, and the protective gas is selected from one or more of nitrogen and argon. The volume ratio of the reducing gas to the protective gas is 5-10:90-95.

10. The application of the silicon-carbon composite material according to claim 1 or the silicon-carbon composite material prepared by any one of claims 2 to 9 in a lithium-ion battery.

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