Gradient modulus coated silicon-carbon composite material as well as preparation method and application thereof

Abstract

The invention discloses a preparation method of a gradient modulus coated silicon-carbon composite material, which comprises the following steps: firstly, preparing a primary precursor solution containing an organic carbon source and a silicon substrate, and carrying out spray drying to form an amorphous carbon buffer layer with an elastic modulus; then, the metal salt composite carbon source solution serves as a secondary precursor, and a metal doped functional layer is deposited on the surface of the buffer layer through controllable spraying parameters; and finally, carrying out third-stage spraying by adopting the carbon-silicon mixed precursor solution to form a compact silicon carbide shell. And through a gradient heat treatment process, buffer layer carbonization, functional layer metal-carbon bonding and outer layer silicon carbide crystallization are realized in sequence. In the three-dimensional gradient coating structure, the flexible carbon on the inner layer effectively buffers volume deformation, the middle functional layer enhances electron conduction and regulates and controls lithium ion flux through metal doping, and the silicon carbide on the outer layer inhibits particle breakage by means of high strength. Through the synergistic effect of the multi-dimensional coating layers, the structural integrity and the interface stability of the silicon-based material in the charging and discharging process are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon-carbon composite electrodes, and specifically relates to a method for preparing a gradient modulus coated silicon-carbon composite material, also relates to a gradient modulus coated silicon-carbon composite material prepared by the preparation method, and also relates to applications of the composite material. Background Art

[0002] With the widespread application of lithium-ion batteries in electric vehicles, portable electronic devices and large-scale energy storage systems, the demand for negative electrode materials with high energy density and long cycle life is becoming increasingly urgent. Traditional graphite negative electrodes have low theoretical specific capacity (about 372mAh / g), which makes it difficult to meet the requirements of the next generation of high energy density batteries. Silicon-based materials have extremely high theoretical specific capacity (about 4200 mAh / g, corresponding to Li 22 Si5 alloying reaction) has become the most promising alternative material. However, the dramatic volume expansion of silicon during the lithiation / delithiation process (approximately 300%) not only causes particle fracture and active material detachment, but also repeatedly damages the solid electrolyte interface (SEI) layer on the electrode surface, causing continuous electrolyte decomposition, increased interfacial impedance, and rapid capacity decay.

[0003] To alleviate the volume expansion effect, existing technologies have proposed a variety of strategies: (1) Nano-sizing silicon particles to shorten the lithium ion diffusion path and reduce the absolute deformation, but the high surface energy of nanomaterials easily causes agglomeration, and the preparation cost is high; (2) Constructing a porous carbon skeleton to load silicon particles, using the physical confinement effect of the carbon matrix to inhibit expansion, but the interface bonding strength between carbon and silicon is insufficient, and structural peeling is still prone to occur after long-term cycling; (3) Using carbon coating technology to form a conductive protective layer through asphalt or polymer carbonization, but the modulus gradient of a single carbon layer is missing, and it cannot meet the stress buffering and mechanical support requirements at the same time, and the low lithium ion diffusion rate of carbon materials leads to limited rate performance; (4) Introducing a metal or metal oxide coating layer to enhance conductivity, but the modulus mismatch between the rigid coating layer and the silicon matrix will aggravate the interface stress concentration and accelerate particle rupture.

[0004] In addition, the dynamic stability of the SEI layer is a key factor limiting the cycle life of silicon-based negative electrodes. Traditional coating layer designs often only focus on physical confinement or conductivity improvement, while ignoring the active regulation of the chemical composition and structure of the SEI. For example, although the carbon coating can partially block the direct contact between the electrolyte and silicon, an uneven SEI film will still form on its surface, and it is prone to rupture and regeneration under volume deformation, resulting in continuous consumption of active lithium. Although metal doping can improve electron conduction, it lacks directional guidance of lithium ion flux, and local current density unevenness further aggravates the uneven growth of SEI. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a gradient modulus coated silicon-carbon composite material. In the prior art, the volume expansion of silicon-carbon materials causes a problem of structural instability.

[0006] Another object of the present invention is to provide a composite material prepared by the preparation method of the gradient modulus coated silicon-carbon composite material.

[0007] The technical solution adopted by the present invention is a method for preparing a gradient modulus coated silicon-carbon composite material, which is specifically implemented according to the following steps: Step 1: uniformly dispersing the silicon-carbon composite material in deionized water, performing ultrasonic dispersion, adding a solid carbon source, stirring and mixing, and spray drying to form a precursor material; Step 2, heat-treating the precursor material to carbonize the solid carbon source; Step 3: ultrasonically disperse the heat-treated precursor material in deionized water, spray a secondary precursor solution of a metal salt composite carbon source onto the surface of the buffer layer, and spray dry again to form an intermediate metal-doped functional layer; Step 4, sintering the material dried in step 3; Step 5, mixing the material treated in step 4 with a tertiary precursor solution of a carbon source for a third-stage spraying; Step 6, performing a high-temperature silicon carbide crystallization treatment on the spray-dried material in step 5 under an inert atmosphere; Step 7, grinding and screening the material processed in step 6 to obtain.

[0008] The present invention is also characterized in that: In step 1, the solid carbon source is selected from one or more of phenolic resin, glucose or polyacrylonitrile, and the mass ratio of the silicon-carbon composite material to the solid carbon source is 1:9~9:1; the frequency of ultrasonic dispersion is 20kHz-100kHz, the time is 10min-60min, the inlet temperature of spray drying is 150℃-220℃, and the outlet temperature is 80℃-120℃.

[0009] In step 2, heat treatment is performed under an inert atmosphere at a temperature of 300° C. to 500° C. for 3 h to 5 h.

[0010] In step 3, the metal salt is a transition metal nitrate or acetate, including one or more of Bi(NO3)3, AgNO3, and Ni(NO3)2. The mass ratio of the metal salt to the raw material in the secondary precursor solution is 1:10~1:8. The inlet temperature of the spray drying is 120℃-220℃, and the outlet temperature is 80℃-120℃.

[0011] In step 4, the sintering temperature is 600° C.-800° C., and the sintering time is 2 h-6 h; the thickness of the metal-doped functional layer after sintering is 20 nm-100 nm, and the metal doping amount is 1 wt %-8 wt %.

[0012] In step 5, the tertiary precursor solution contains a silicon source and a carbon source. The silicon source is the base material coated with a metal layer in the previous stage. The carbon source is selected from sucrose and asphalt. The mass ratio of the silicon source to the carbon source is 1:1~1:0.5. The inlet temperature of the spray drying is 200℃-300℃, and the outlet temperature is 120℃-180℃.

[0013] In step 6, the inert atmosphere is argon, the temperature of the high-temperature silicon carbide crystallization treatment is 900° C.-1200° C., and the time is 6 h-8 h.

[0014] In step 7, the grinding is carried out by ball milling process, with a rotation speed of 100 r / min-300 r / min and a time of 10 min-60 min; the particle size of the material after screening is 1 μm-20 μm.

[0015] Another technical solution adopted by the present invention is to prepare a gradient modulus coated silicon-carbon composite material using a preparation method of a gradient modulus coated silicon-carbon composite material.

[0016] The third technical solution adopted by the present invention is the application of gradient modulus coated silicon-carbon composite materials in lithium ion battery negative electrode materials. The beneficial effects of the present invention are: 1) This invention uses a multi-step spray drying method to sequentially construct an amorphous carbon buffer layer, a metal-doped functional layer, and a silicon carbide protective layer. Multidimensional coating is achieved through gradient modulus design. The inner, flexible carbon precursor preferentially penetrates the silicon substrate surface, forming an elastic buffer layer. The outer, silicon carbide precursor, then directionally coats the substrate during the spraying process, ultimately forming a three-dimensional gradient structure with increasing modulus from the inside out. This effectively disperses stress during the lithiation / delithiation process and inhibits particle fracture.

[0017] 2) A gradient heat treatment process (low-temperature carbonization, medium-temperature metal bonding, and high-temperature crystallization) allows for precise control of each coating layer. The amorphous carbon buffer layer retains high elasticity at low temperatures, the metal-doped functional layer forms a stable conductive network through medium-temperature sintering, and the outer silicon carbide layer crystallizes into a high-strength protective shell at high temperatures. These three layers work synergistically to provide stress buffering, electronic conduction, and mechanical support.

[0018] 3) The gradient modulus coating structure of the present invention significantly improves the interfacial stability of the silicon-carbon composite material: the inner carbon layer buffers the volume expansion stress and reduces the direct contact between the silicon substrate and the electrolyte; the middle metal doping layer induces the uniform deposition of lithium ions and stabilizes the SEI film through the ion-electron cooperative transport mechanism; the outer silicon carbide layer suppresses particle pulverization with its high hardness, thereby significantly improving the material's first coulombic efficiency and cycle capacity retention rate.

[0019] 4) The multi-step spray drying combined with gradient heat treatment process proposed in this invention offers strong process controllability and low cost. By adjusting the precursor solution composition and spray parameters, the thickness, composition, and modulus gradient of each coating layer can be precisely controlled, making it suitable for large-scale production and providing a feasible technical path for the practical application of high-silicon-content anodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an XRD characterization diagram of the silicon carbide high modulus protective layer before and after high-temperature crystallization in the present invention. DETAILED DESCRIPTION

[0021] The present invention will be described in detail below with reference to specific embodiments and accompanying drawings.

[0022] The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: uniformly dispersing the silicon-carbon composite material in deionized water, performing ultrasonic dispersion, adding a solid carbon source, stirring and mixing, and performing a first spray drying to form a precursor material; The solid carbon source is selected from one or more of phenolic resin, glucose or polyacrylonitrile, and the mass ratio of the silicon-carbon composite material to the solid carbon source is 1:9 to 9:1; the frequency of ultrasonic dispersion is 20kHz-100kHz, the time is 10min-60min, the inlet temperature of spray drying is 150℃-220℃, and the outlet temperature is 80℃-120℃; Step 2: heat-treating the precursor material to carbonize the solid carbon source, so that the carbon precursor is carbonized to form an amorphous carbon buffer layer with a thickness of 10nm-200nm. The buffer layer has a flexible modulus and can effectively relieve the volume expansion stress of the silicon substrate; Heat treatment is carried out in an inert atmosphere at a temperature of 300°C-500°C for 3h-5h; Step 3: ultrasonically disperse the precursor material after the heat treatment in step 2 in deionized water, spray a secondary precursor solution of a metal salt composite carbon source onto the surface of the buffer layer, and spray dry it again to form an intermediate metal-doped functional layer (secondary material coated with a metal precursor); The metal salt is a transition metal nitrate or acetate, including one or more of Bi(NO3)3, AgNO3, and Ni(NO3)2. The mass ratio of the metal salt to the raw material in the secondary precursor solution is 1:10 to 1:8. The inlet temperature of the spray drying is 120°C to 220°C, and the outlet temperature is 80°C to 120°C. Step 4: heat-treating and sintering the dried material in step 3 to stabilize the structure of the metal layer and form a metal bond. This functional layer has both the functions of enhancing electron conduction and regulating lithium ion flux. The sintering temperature is 600-800°C, and the sintering time is 2-6 hours. The thickness of the metal-doped functional layer after sintering is 20nm-100nm, and the metal doping amount is 1wt%-8wt%. Step 5: The material treated in step 4 is mixed with a tertiary precursor solution of a carbon source for a third-stage spraying to form a silicon carbide precursor coating structure on the outer layer; The tertiary precursor solution contains a silicon source and a carbon source. The silicon source is the base material coated with a metal layer in the previous stage. The carbon source is selected from sucrose and asphalt. The mass ratio of the silicon source to the carbon source is 1:1 to 1:0.5. The inlet temperature of the spray drying is 200°C-300°C, and the outlet temperature is 120°C-180°C. Step 6: subjecting the spray-dried material in step 5 to a high-temperature silicon carbide crystallization treatment under an inert atmosphere to form a gradient coating structure with increasing elastic modulus from the inside to the outside; The inert atmosphere is argon, the temperature of high-temperature silicon carbide crystallization treatment is 900℃-1200℃, and the time is 6h-8h; Step 7, grinding and screening the material processed in step 6 to obtain a gradient modulus coated silicon-carbon composite material with uniform particles; The grinding adopts a ball milling process with a rotation speed of 100 r / min-300 r / min and a time of 10 min-60 min; the particle size of the material after screening is 1 μm-20 μm.

[0023] The gradient modulus coated silicon-carbon composite material prepared by the preparation method of the gradient modulus coated silicon-carbon composite material of the present invention has a structure including an amorphous carbon buffer layer on the inner side; a metal-doped functional layer in the middle with a metal doping amount of 1 wt%-8wt%; and a silicon carbide protective layer on the outer layer.

[0024] The gradient modulus coated silicon-carbon composite material of the present invention buffers volume expansion stress through an inner layer of flexible material, a middle layer of functional layer regulates lithium ion distribution, and an outer layer of high-strength material inhibits particle breakage; constructs a chemically stable coating interface to reduce electrolyte side reactions and induces the formation of a dense and uniform SEI film; and realizes low-cost and controllable preparation of a multi-level coating structure.

[0025] Example 1 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Mix 10 g of the silicon-carbon composite with 2 g of glucose (a solid carbon source) in a 5:1 mass ratio. Add the mixture to 200 mL of deionized water and disperse the mixture using ultrasound at 40 kHz for 30 minutes. Then, mechanically stir for 2 hours until uniformly dispersed. The mixed solution is spray-dried for the first time at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a primary material coated with a glucose precursor. Step 2: The primary material was placed in a tube furnace, nitrogen was introduced (flow rate 100 mL / min), the temperature was increased to 400°C at 5°C / min, and the temperature was kept at this temperature for 4 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.5 g of Bi(NO3)3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 700°C at a rate of 10°C / min, and sinter it for 4 hours. After cooling, a metal-doped functional layer with a thickness of about 60 nm is obtained. The metal doping amount is 5 wt%, and its electronic conductivity is increased to 10 -2 S / cm; Step 5: The metal-doped material was mixed with a tertiary precursor solution of 4 g sucrose (silicon to carbon mass ratio of 1:0.8), ultrasonically dispersed for 30 minutes, and spray-dried for the third time at an inlet temperature of 250°C and an outlet temperature of 150°C to form an outer layer of silicon carbide precursor coating structure; Step 6: Place the above materials in an argon atmosphere, heat to 1100°C at 5°C / min, and keep at this temperature for 7 hours to perform a carbothermal reduction reaction; Step 7: Place the crystallized composite material in a ball mill, use zirconia balls as grinding balls (ball-to-material ratio 10:1), set the speed to 200r / min, ball mill for 30min, and obtain a gradient modulus coated silicon-carbon composite material with a particle size of 5-15μm after sieving through 200 mesh. Figure 1 This is the XRD characterization diagram before and after high-temperature crystallization of the silicon carbide high modulus protective layer.

[0026] Example 2 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Take 10 g of silicon-carbon composite material and 2 g of phenolic resin (solid carbon source), mix them in a mass ratio of 5:1, add them to 200 mL of deionized water, use ultrasonic dispersion at a frequency of 40 kHz for 30 minutes, then mechanically stir for 2 hours until uniformly dispersed, and perform the first spray drying of the mixed solution with the inlet temperature set at 220°C and the outlet temperature set at 120°C to obtain the primary material of the surface-coated phenolic resin precursor; Step 2: The primary material was placed in a tube furnace, introduced with argon (flow rate 100 mL / min), heated to 400°C at 5°C / min, and kept at this temperature for 4 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.5 g of Bi(NO3)3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 700°C at a rate of 10°C / min, and sinter it for 5 hours. After cooling, a metal-doped functional layer with a thickness of about 60 nm is obtained. The metal doping amount is 5 wt%, and its electronic conductivity is increased to 10 -2 S / cm; Step 5: The metal-doped material was mixed with the tertiary precursor solution of 4 g sucrose (mass ratio of silicon to carbon 1:0.8), ultrasonically dispersed for 30 min, and spray-dried for the third time at an inlet temperature of 250 °C and an outlet temperature of 150 °C to form an outer layer of silicon carbide precursor coating structure; Step 6: Place the above materials in an argon atmosphere, heat to 1100°C at 5°C / min, and keep at this temperature for 7 hours to perform a carbothermal reduction reaction; Step 7: The crystallized composite material was placed in a ball mill, using zirconia balls as grinding balls (ball-to-material ratio of 10:1), set the speed to 300 r / min, and ball milled for 30 minutes. After sieving through 200 mesh, a gradient modulus coated silicon-carbon composite material with a particle size of 5-15 μm was obtained; Example 3 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Take 10 g of silicon-carbon composite material and 2 g of phenolic resin (solid carbon source), mix them in a mass ratio of 2:1, add them to 200 mL of deionized water, use ultrasonic dispersion at a frequency of 40 kHz for 30 minutes, then mechanically stir for 2 hours until uniformly dispersed, and perform the first spray drying of the mixed solution with the inlet temperature set at 220°C and the outlet temperature set at 120°C to obtain the primary material of the surface-coated phenolic resin precursor; Step 2: The primary material was placed in a tube furnace, introduced with argon (flow rate 100 mL / min), heated to 300°C at 5°C / min, and kept at this temperature for 5 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.8 g of AgNO3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 700°C at a rate of 5°C / min, hold it for 5 hours, and sinter it. After cooling, a metal-doped functional layer with a thickness of approximately 60 nm is obtained. Step 5: Mix the metal-doped material 6 g with the three-stage precursor solution of asphalt (mass ratio of silicon to carbon is 1:0.8), ultrasonically disperse for 30 minutes, and perform a third spray drying at an inlet temperature of 250°C and an outlet temperature of 150°C to form an outer layer of silicon carbide precursor coating structure. Step 6: Place the above materials in an argon atmosphere, heat to 1000°C at 5°C / min, and keep at this temperature for 8 hours to perform a carbothermal reduction reaction. Step 7: Place the crystallized composite material in a ball mill, use zirconia balls as grinding balls (ball-to-material ratio 10:1), set the speed to 300 r / min, ball mill for 40 minutes, and obtain a gradient modulus coated silicon-carbon composite material with a particle size of 5-15 μm after sieving through 200 mesh.

[0027] Example 4 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Take 10 g of silicon-carbon composite material and 3 g of PVP k80 (solid carbon source), mix them in a mass ratio of 2:1, add them to 200 mL of deionized water, use ultrasonic dispersion at a frequency of 40 kHz for 30 minutes, then mechanically stir for 2 hours until uniformly dispersed, and perform the first spray drying of the mixed solution with the inlet temperature set at 200 ° C and the outlet temperature set at 100 ° C to obtain the primary material of the surface-coated phenolic resin precursor; Step 2: The primary material was placed in a tube furnace, introduced with argon (flow rate 100 mL / min), heated to 400°C at 5°C / min, and kept at this temperature for 5 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.8 g of AgNO3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 700°C at a rate of 5°C / min, hold it for 5 hours, and sinter it. After cooling, a metal-doped functional layer with a thickness of approximately 60 nm is obtained. Step 5: Mix the metal-doped material with 4 g of the tertiary precursor solution of asphalt (silicon to carbon mass ratio of 1:0.8), ultrasonically disperse for 30 minutes, and perform a third spray drying at an inlet temperature of 250°C and an outlet temperature of 150°C to form an outer layer of silicon carbide precursor coating structure; Step 6: Place the above materials in an argon atmosphere, heat to 900°C at 5°C / min, and keep at this temperature for 8 hours to perform a carbothermal reduction reaction; Step 7: Place the crystallized composite material in a ball mill, use zirconia balls as grinding balls (ball-to-material ratio 10:1), set the speed to 300 r / min, ball mill for 40 minutes, and obtain a gradient modulus coated silicon-carbon composite material with a particle size of 5-15 μm after sieving through 200 mesh.

[0028] Example 5 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Take 10 g of silicon-carbon composite material and 5 g of polyacrylonitrile (solid carbon source), mix them in a mass ratio of 2:1, add them to 200 mL of deionized water, use ultrasonic dispersion at a frequency of 40 kHz for 30 minutes, then mechanically stir for 2 hours until uniformly dispersed, and perform the first spray drying of the mixed solution with the inlet temperature set at 200 ° C and the outlet temperature set at 100 ° C to obtain the primary material of the surface-coated phenolic resin precursor; Step 2: The primary material was placed in a tube furnace, introduced with argon (flow rate 100 mL / min), heated to 400°C at 5°C / min, and kept at this temperature for 5 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.7 g of AgNO3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 600°C at a rate of 5°C / min, hold it for 5 hours, and sinter it. After cooling, a metal-doped functional layer with a thickness of approximately 60 nm is obtained. Step 5: Mix the metal-doped material with 4 g of the tertiary precursor solution of asphalt (silicon to carbon mass ratio of 1:0.8), ultrasonically disperse for 40 minutes, and perform a third spray drying at an inlet temperature of 220°C and an outlet temperature of 100°C to form an outer layer of silicon carbide precursor coating structure; Step 6: Place the above materials in an argon atmosphere, heat to 900°C at 5°C / min, and keep at this temperature for 8 hours to perform a carbothermal reduction reaction; Step 7: Place the crystallized composite material in a ball mill, use zirconia balls as grinding balls (ball-to-material ratio 10:1), set the speed to 350 r / min, ball mill for 30 minutes, and after 200 mesh sieving, obtain a gradient modulus coated silicon-carbon composite material with a particle size of 5-15 μm.

[0029] Example 6 The preparation method of the gradient modulus coated silicon-carbon composite material of the present invention is specifically implemented according to the following steps: Step 1: Take 10 g of silicon-carbon composite material and 5 g of PVP (solid carbon source), mix them in a mass ratio of 2:1, add them to 200 mL of deionized water, use ultrasonic dispersion at a frequency of 40 kHz for 30 minutes, then mechanically stir for 2 hours until uniformly dispersed, and perform the first spray drying of the mixed solution with the inlet temperature set at 200 ° C and the outlet temperature set at 100 ° C to obtain the primary material of the surface-coated phenolic resin precursor; Step 2: The primary material was placed in a tube furnace, introduced with argon (flow rate 100 mL / min), heated to 300°C at 5°C / min, and kept at this temperature for 5 hours for carbonization treatment. After cooling, an amorphous carbon buffer layer with a thickness of about 50 nm was obtained; Step 3: The carbon buffer layer-coated silicon-carbon composite material was redispersed in 150 mL of deionized water, and a solution containing 0.43 g of Bi(NO3)3 (the total mass ratio of metal salt to raw material was 1:8) was added. After ultrasonic dispersion for 20 minutes, a second spray drying was performed at an inlet temperature of 180°C and an outlet temperature of 100°C to obtain a secondary material coated with a metal precursor; Step 4: Place the secondary material in an argon atmosphere, heat it to 500°C at a rate of 5°C / min, and sinter it for 5 hours. After cooling, a metal-doped functional layer with a thickness of approximately 60 nm is obtained. Step 5: Mix the metal-doped material (6 g) with the three-stage precursor solution of asphalt (silicon to carbon mass ratio of 1:0.8), ultrasonically disperse for 30 minutes, and spray dry for the third time at an inlet temperature of 220°C and an outlet temperature of 100°C to form an outer layer of silicon carbide precursor coating structure. Step 6: Place the above materials in an argon atmosphere, heat to 1200°C at 5°C / min, and keep at this temperature for 8 hours to perform a carbothermal reduction reaction; Step 7: Place the crystallized composite material in a ball mill, use zirconia balls as grinding balls (ball-to-material ratio 10:1), set the speed to 200 r / min, ball mill for 40 minutes, and obtain a gradient modulus coated silicon-carbon composite material with a particle size of 5-15 μm after sieving through 200 mesh.

[0030] The present invention achieves the coordinated optimization of volume expansion suppression, interface stability improvement and charge transfer dynamics of silicon-based materials by constructing a gradient modulus coating structure of "soft inside, conductive in the middle and rigid outside". The inner amorphous carbon buffer layer forms a flexible stress dissipation network with its low modulus characteristics, and accommodates the expansion and contraction of silicon particles through the elastic deformation of the microporous structure, effectively alleviating the local stress concentration caused by volume mutation during the lithiation process; the intermediate metal-doped functional layer forms a three-dimensional conductive framework through chemical bonding between transition metals and carbon matrix. The metal nanoparticles distributed on its surface not only enhance the electron transport capacity, but also guide the uniform diffusion of lithium ions along the three-dimensional channel through the directional distribution of interface charges, thereby inhibiting the rupture and regeneration of SEI film caused by local overpotential; the outer silicon carbide protective layer constructs a mechanical barrier with a high-hardness crystallized structure, and limits the internal penetration of electrolyte side reactions through a covalently bonded Si-C network. The chemical inertness of its surface significantly reduces the decomposition activity of the electrolyte, and the nano-grain boundary provides a rapid migration path for lithium ions. This gradient structure stabilizes the electrode interface through the synergistic effect of physical isolation and chemical regulation: the inner carbon buffer reduces direct contact between silicon and the electrolyte, inhibiting the uncontrolled growth of the native SEI; the Lewis acidic sites in the metal-doped layer preferentially adsorb electrolyte components, inducing the formation of a dense, inorganic-enriched SEI film; and the wide bandgap characteristics of the outer silicon carbide layer optimize charge transfer dynamics through interfacial band matching, reducing interfacial impedance. This cross-scale, multi-dimensional collaborative design breaks through the single-function limitations of traditional coating layers, simultaneously improving the cycling stability and kinetic performance of silicon-based anodes from three dimensions: stress dissipation, electron / ion transport, and interfacial chemistry.

Claims

1. A method for preparing a gradient modulus coated silicon-carbon composite material, characterized in that: Please follow the steps below to implement it: Step 1: uniformly dispersing the silicon-carbon composite material in deionized water, performing ultrasonic dispersion, adding a solid carbon source, stirring and mixing, and spray drying to form a precursor material; Step 2, heat-treating the precursor material to carbonize the solid carbon source; Step 3: ultrasonically disperse the heat-treated precursor material in deionized water, spray a secondary precursor solution of a metal salt composite carbon source onto the surface of the buffer layer, and spray dry again to form an intermediate metal-doped functional layer; Step 4, sintering the material dried in step 3; Step 5, mixing the material treated in step 4 with a tertiary precursor solution of a carbon source for a third-stage spraying; Step 6, performing a high-temperature silicon carbide crystallization treatment on the spray-dried material in step 5 under an inert atmosphere; Step 7, grinding and screening the material processed in step 6 to obtain.

2. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In step 1, the solid carbon source is selected from one or more of phenolic resin, glucose or polyacrylonitrile, and the mass ratio of the silicon-carbon composite material to the solid carbon source is 1:9 to 9:1; the frequency of ultrasonic dispersion is 20kHz-100kHz, the time is 10min-60min, the inlet temperature of spray drying is 150℃-220℃, and the outlet temperature is 80℃-120℃.

3. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In the step 2, the heat treatment is performed under an inert atmosphere at a temperature of 300° C. to 500° C. for 3 h to 5 h.

4. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In step 3, the metal salt is a transition metal nitrate or acetate, including one or more of Bi(NO3)3, AgNO3, and Ni(NO3)2. The mass ratio of the metal salt to the raw material in the secondary precursor solution is 1:10~1:

8. The inlet temperature of the spray drying is 120℃-220℃, and the outlet temperature is 80℃-120℃.

5. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In step 4, the sintering temperature is 600° C.-800° C., and the sintering time is 2 h-6 h. The thickness of the metal-doped functional layer after sintering is 20 nm-100 nm, and the metal doping amount is 1 wt %-8 wt %.

6. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In step 5, the tertiary precursor solution contains a silicon source and a carbon source, the silicon source is the base material coated with a metal layer in the previous stage, the carbon source is selected from sucrose and asphalt, the mass ratio of the silicon source to the carbon source is 1:1~1:0.5, the inlet temperature of the spray drying is 200℃-300℃, and the outlet temperature is 120℃-180℃.

7. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In step 6, the inert atmosphere is argon, the temperature of the high-temperature silicon carbide crystallization treatment is 900° C.-1200° C., and the time is 6 h-8 h.

8. The method for preparing the gradient modulus coated silicon-carbon composite material according to claim 1, wherein: In the step 7, the grinding is carried out by ball milling, with a rotation speed of 100 r / min-300 r / min and a time of 10 min-60 min; the particle size of the material after screening is 1 μm-20 μm.

9. A gradient modulus coated silicon-carbon composite material prepared according to the method for preparing a gradient modulus coated silicon-carbon composite material according to any one of claims 1 to 8.

10. Use of the gradient modulus coated silicon-carbon composite material according to claim 9 in a negative electrode material for lithium-ion batteries.

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