Silicon-carbon composite material, silicon-carbon composite material preparation method and negative electrode sheet

Abstract

The application provides a silicon-carbon composite material, a silicon-carbon composite material preparation method and a negative electrode sheet, and comprises a core and a composite coating layer. The core is a silicon-carbon core, and the composite coating layer coats the core. The composite coating layer comprises, from inside to outside, a first coating layer with a pore structure, a transition layer and a second coating layer with a dense structure. The dielectric constant of the first coating layer is greater than that of the second coating layer, and the dielectric constant of the transition layer is between the dielectric constants of the first coating layer and the second coating layer. The silicon-carbon composite material provided by the application has a gradient distribution of the dielectric constant of the silicon-carbon composite material through the multi-layer structure in the composite coating layer, thereby optimizing the migration path of ions and electrons, reducing the transmission resistance, improving the interface compatibility of the material and the electrolyte, and realizing the comprehensive improvement of the overall electrical performance of the lithium ion battery through the synergistic effect of the multi-layer structure in the composite coating layer.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet. Background Technology

[0002] With the rapid development of modern electronic devices and electric vehicles, the demand for high-performance batteries is increasing. Silicon-carbon anode materials, due to their high theoretical specific capacity (far exceeding that of traditional graphite anodes), have become highly promising next-generation battery anode materials. Silicon-carbon materials prepared by chemical vapor deposition (CVD) have unique advantages in structure and performance, but also have some problems.

[0003] On the one hand, silicon undergoes significant volume changes during charging and discharging, leading to material structure damage and electrode pulverization, which in turn causes a sharp decline in battery cycle performance. On the other hand, the poor interfacial stability between silicon-carbon materials and electrolytes can trigger side reactions, consume active lithium, and reduce the battery's initial coulombic efficiency and cycle life.

[0004] To address these issues, coating modification of CVD silicon-carbon materials is an effective approach. However, traditional uniform coatings have limitations in improving material performance, including poor cycle performance, low initial coulombic efficiency, and poor interfacial stability with the electrolyte. Summary of the Invention

[0005] To address the problems of poor cycle performance, low initial coulombic efficiency, and poor interfacial stability with electrolyte when silicon-carbon materials are applied to lithium-ion batteries in the prior art, a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet are provided.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: On one hand, the present invention provides a silicon-carbon composite material, comprising a core and a composite coating layer, wherein the core is a silicon-carbon core, the composite coating layer covers the core, and the composite coating layer comprises, from the inside to the outside, a first coating layer having a porous structure, a transition layer, and a second coating layer having a dense structure, wherein the dielectric constant of the first coating layer is greater than the dielectric constant of the second coating layer, and the dielectric constant of the transition layer is between the dielectric constant of the first coating layer and the dielectric constant of the second coating layer.

[0007] Optionally, the first coating layer comprises a first dielectric material, which includes one or more of barium titanate, strontium titanate, and lead zirconate titanate; and / or, The transition layer includes a second dielectric material, which is a metal oxide composite system, comprising one or more of aluminum oxide, yttrium oxide, hafnium oxide, and lanthanum oxide; and / or, The second coating layer includes a third dielectric material, which includes one or more of silicon dioxide, aluminum oxide, and silicon nitride.

[0008] Optionally, the pore size of the pore structure in the first coating layer is 2-100 nm, and the porosity of the first coating layer is 30%-70%.

[0009] Optionally, the thickness of the first coating layer is 5-100nm; The thickness of the transition layer is 10-80 nm; The thickness of the second coating layer is 5-80 nm.

[0010] Optionally, the dielectric constant of the first coating layer is >150; The dielectric constant of the transition layer is 10-150; The dielectric constant of the second coating layer is <10.

[0011] Optionally, the silicon-carbon core includes a porous carbon framework and nano-silicon dispersed on the porous carbon framework.

[0012] Optionally, the method for preparing the silicon-carbon composite material includes the following operations: Obtaining silicon-carbon cores; The first dielectric material precursor, solvent, first catalyst and pore-forming agent are mixed and reacted to obtain a gel mixture; The core is immersed in a gel mixture, dried, and calcined to obtain a first precursor with a first coating layer; The second dielectric material precursor is prepared into a solution, the first precursor is placed in the solution for thermal reaction, and then sintered to obtain the second precursor with a transition layer. A third dielectric material is deposited on the surface of the second precursor to obtain a silicon-carbon composite material.

[0013] Optionally, the silicon-carbon core is prepared by the following method: A carbon source and a silicon source are introduced into a reaction chamber, and a silicon-carbon core is grown on a substrate, which is a carbon-based material, by chemical vapor deposition under a second catalyst and high temperature conditions.

[0014] Optionally, the first dielectric material precursor comprises an organic compound and / or salt of one or more elements selected from titanium, barium, strontium, zirconium, and lead; and / or The second dielectric material precursor includes an organic compound and / or salt of one or more elements selected from aluminum, yttrium, hafnium, and lanthanum.

[0015] Optionally, the drying time is 1-3 hours and the drying temperature is 60-120℃; The calcination time is 1-3 hours, and the calcination temperature is 500-800℃.

[0016] Optionally, the solvent includes one or more of ethanol, methanol, and acetone; The first catalyst includes one or more of hydrochloric acid, nitric acid, and acetic acid; The pore-forming agent includes one or more of polyvinyl alcohol, polyethylene glycol, and starch.

[0017] Optionally, in the thermal reaction for preparing the transition layer, the thermal reaction time is 0.5-2 hours and the thermal reaction temperature is 30-80°C.

[0018] Optionally, during the sintering process, the sintering temperature is 400-700℃ and the sintering time is 1-3 hours.

[0019] On the other hand, the present invention provides a negative electrode sheet, including a negative electrode material layer, wherein the negative electrode material layer includes a binder, a conductive agent and the silicon-carbon composite material, or a silicon-carbon composite material prepared by the method for preparing the silicon-carbon composite material.

[0020] Optionally, the mass ratio of the silicon-carbon composite material, the binder, and the conductive agent is (70-90):(5-15):(5-15).

[0021] The beneficial effects of this application are as follows: The silicon-carbon composite material provided in this application includes a core and a composite coating layer. The composite coating layer comprises multiple first coating layers with different dielectric constants and porous structures, a transition layer, and a second coating layer with a dense structure. The first coating layer, with its high dielectric constant and porous structure, utilizes the compressibility of its pores to provide a buffer space for the volume expansion and contraction of the silicon-carbon core during charging and discharging, reducing the structural damage caused by changes in core volume. The transition layer, with a dielectric constant between the first and second coating layers, enhances the bonding force between the layers of the composite coating layer, ensuring long-term stability under cycling conditions. The structural integrity of the composite coating layer, with its dense second coating layer of low dielectric constant forming a physical barrier, prevents direct contact between the electrolyte and the core, thereby isolating electrolyte penetration and core reaction, reducing active lithium loss, and improving the first coulombic efficiency. The multilayer structure in the composite coating layer enables the dielectric constant of the silicon-carbon composite material to exhibit a gradient distribution, which optimizes the migration paths of ions and electrons, reduces transport resistance, and improves the interfacial compatibility between the material and the electrolyte. Through the synergistic effect of the multilayer structure in the composite coating layer, the overall electrical performance of the battery is comprehensively improved after the application of silicon-carbon composite material in lithium-ion batteries. Detailed Implementation

[0022] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] This invention provides a silicon-carbon composite material, comprising a core and a composite coating layer. The core is a silicon-carbon core, and the composite coating layer covers the core. The composite coating layer comprises, from the inside out, a first coating layer with a porous structure, a transition layer, and a second coating layer with a dense structure. The dielectric constant of the first coating layer is greater than that of the second coating layer, and the dielectric constant of the transition layer is between the dielectric constants of the first and second coating layers.

[0024] Specifically, the silicon-carbon composite material provided in this application includes a core and a composite coating layer. The composite coating layer has multiple first coating layers with porous structures and different dielectric constants, a transition layer, and a second coating layer with a dense structure. The first coating layer, with its high dielectric constant and porous structure, utilizes the compressibility of its pores to provide a buffer space for the volume expansion and contraction of the silicon-carbon core during charging and discharging, reducing the damage to the structure caused by changes in the core volume. The transition layer, with a dielectric constant between the first and second coating layers, enhances the bonding force between the layers of the composite coating layer, ensuring long-term cycling. The structural integrity of the under-ring composite coating layer and the dense second coating layer with low dielectric constant form a physical barrier, blocking direct contact between the electrolyte and the core, thereby isolating electrolyte penetration and core reaction, reducing active lithium loss, and improving the first coulombic efficiency. The multi-layer structure in the composite coating layer enables the dielectric constant of the silicon-carbon composite material to be distributed in a gradient, thereby optimizing the migration path of ions and electrons, reducing transport resistance, and improving the interfacial compatibility between the material and the electrolyte. Through the synergistic effect of the multi-layer structure in the composite coating layer, the overall electrical performance of the battery is comprehensively improved after the silicon-carbon composite material is applied to lithium-ion batteries.

[0025] In some embodiments, the first coating layer comprises a first dielectric material, the first dielectric material comprising one or more of barium titanate, strontium titanate, and lead zirconate titanate; and / or, The transition layer includes a second dielectric material, which is a metal oxide composite system, comprising one or more of aluminum oxide, yttrium oxide, hafnium oxide, and lanthanum oxide; and / or, The second coating layer includes a third dielectric material, which includes one or more of silicon dioxide, aluminum oxide, and silicon nitride.

[0026] Specifically, the first coating layer uses high dielectric materials such as barium titanate, strontium titanate, and lead zirconate titanate. The high dielectric properties of the first coating layer, combined with its porous structure, effectively buffer the volume changes of the silicon-carbon core during charging and discharging, and enhance the storage and conduction capabilities of charge, enabling rapid ion migration. The transition layer uses a composite system of alumina, yttrium oxide, hafnium oxide, and lanthanum oxide as the second dielectric material. This optimizes and enhances the bonding force with adjacent first and second coating layers, reduces the risk of interlayer delamination, and ensures the long-term stability of the coating structure. Stable; the second coating layer uses silicon dioxide, aluminum oxide, and silicon nitride as the third dielectric material with low dielectric constant. Its dense structure can form a stable physical barrier, effectively blocking direct contact between the electrolyte and the core, reducing the loss of active lithium caused by side reactions. At the same time, the low dielectric properties can reduce the accumulation of interface charge, further optimizing the ion transport environment. The rational selection and combination of the three types of dielectric materials enables the precise realization of the gradient dielectric structure of the composite coating layer, ultimately synergistically improving the cycle stability, first coulombic efficiency and overall electrical performance of silicon-carbon composite materials in lithium-ion batteries.

[0027] In some embodiments, the pore size of the pore structure in the first coating layer is 2-100 nm, and the porosity of the first coating layer is 30%-70%.

[0028] Specifically, the pore size of the pore structure in the first coating layer is controlled within the range of 2-100 nm. This pore size range can provide sufficient buffer space for the volume change of the silicon-carbon core during charging and discharging, avoiding insufficient structural support due to excessively large pores or ineffective stress relief due to excessively small pores. The porosity of 30%-70% can ensure the structural stability of the first coating layer itself, while also taking into account the buffering effect on the core volume change and ion transport efficiency. In conjunction with the high dielectric properties of the first dielectric material, it further enhances the cycling stability and electrochemical performance of the material.

[0029] In some embodiments, the thickness of the first coating layer is 5-100 nm; The thickness of the transition layer is 10-80 nm; The thickness of the second coating layer is 5-80 nm.

[0030] Specifically, the thickness of the first coating layer is 5-100 nm. This thickness range ensures that its porous structure provides sufficient buffer space for changes in the volume of the silicon-carbon core, while maintaining a stable bond with the transition layer. This avoids increased ion transport resistance due to excessive thickness or ineffective stress relief due to insufficient thickness. The thickness of the transition layer is 10-80 nm. This thickness allows the transition layer to fully utilize the dielectric constant gradient transition effect, enhancing the interfacial bonding force with the first and second coating layers and ensuring stable interlayer connections. The thickness of the second coating layer is 5-80 nm. Within this thickness range, the dense structure of the second coating layer can effectively block electrolyte penetration and maintain the overall stability of the material while reducing the obstruction to ion transport. This avoids a decrease in electrochemical performance due to excessive thickness. The multi-layer structure of the composite coating layer in the silicon-carbon composite material described in this application works synergistically with its corresponding dielectric materials to further optimize the comprehensive function of the composite coating layer and significantly improve the electrical performance of the silicon-carbon composite material.

[0031] In some embodiments, the dielectric constant of the first coating layer is >150; The dielectric constant of the transition layer is 10-150; The dielectric constant of the second coating layer is <10.

[0032] Specifically, the high dielectric constant of the first coating layer (>150) enhances its ability to adsorb and conduct charges. Combined with its porous structure, it buffers changes in core volume while accelerating ion migration. The dielectric constant of the transition layer is in the range of 10-150, which is beneficial for the transition of dielectric properties between the transition layer and its two adjacent layers (the first coating layer and the second coating layer). This reduces the accumulation of interfacial charges and energy loss, thereby strengthening the bonding force between the composite coating layers and the continuity of charge transport. The low dielectric constant of the second coating layer (<10) reduces the interfacial polarization effect. Combined with the dense structure of the second coating layer, it can effectively block electrolyte penetration and optimize the ion transport environment. That is, the synergistic effect of the differential distribution of the three types of dielectric constants in the composite coating layer further improves the ionic conductivity, structural stability and interfacial compatibility of the material, thereby enhancing the cycle performance and overall electrical performance of silicon-carbon composite materials in lithium-ion batteries.

[0033] In some embodiments, the silicon-carbon core comprises a porous carbon framework and nano-silicon dispersed on the porous carbon framework.

[0034] Specifically, the porous carbon framework in the silicon-carbon core can provide stable support for nano-silicon, and the pore structure of the porous carbon framework provides corresponding space for the volume expansion during the charging and discharging process of lithium-ion batteries. At the same time, the good conductivity of the carbon framework can promote the rapid transport of electrons; while the nano-silicon is dispersed on the porous carbon framework in a dispersed form, which helps to shorten the diffusion path of lithium ions and improve the ion migration rate.

[0035] In some embodiments, the method for preparing the silicon-carbon composite material includes the following operations: Obtaining silicon-carbon cores; The first dielectric material precursor, solvent, first catalyst and pore-forming agent are mixed and reacted to obtain a gel mixture; The core is immersed in a gel mixture, dried, and calcined to obtain a first precursor with a first coating layer; The second dielectric material precursor is prepared into a solution, the first precursor is placed in the solution for thermal reaction, and then sintered to obtain the second precursor with a transition layer. A third dielectric material is deposited on the surface of the second precursor to obtain a silicon-carbon composite material.

[0036] The preparation method of the silicon-carbon composite material includes the following specific operations: Carbon and silicon sources are introduced into the reaction chamber, and under the conditions of a second catalyst and high temperature, they are deposited on the substrate by chemical vapor deposition. After the reaction is completed, the silicon-carbon core is obtained by cleaning and drying. The second catalyst can be one or more of metals such as iron, cobalt, and nickel and their oxides. The high-temperature conditions of the reaction are 800-1200℃ and the reaction time is 1-5h.

[0037] The sol-gel method is used to mix and react the second dielectric material precursor, solvent, catalyst and pore-forming agent to obtain a gel mixture. The core is immersed in the gel mixture, and after drying and calcination, a first precursor with a first coating layer is obtained. The second dielectric material precursor is prepared into a solution, and the first precursor is placed in the solution and immersed for 0.5-2 hours at an immersion temperature of 30-80°C to carry out a thermal reaction. After being removed, it is dried (60-120°C, 1-2 hours) to remove the solvent, and then sintered to decompose the first precursor and transform it into a metal oxide composite layer, thus obtaining the second precursor with a transition layer. Using physical vapor deposition, a third dielectric material target is placed in a vacuum chamber. Under the bombardment of high-energy particles, the target atoms or molecules are sputtered out and deposited on the surface of a second precursor with a transition layer, forming a dense second coating layer, thus obtaining a silicon-carbon composite material.

[0038] In this operation, the vacuum level is 10. -3 -10 -5 Pa, sputtering power of 100-500W, deposition time of 1-3h.

[0039] The preparation method first uses a silicon-carbon core as a substrate, and forms a gel by mixing and reacting a first dielectric material precursor with a pore-forming agent. After impregnation, drying and calcination, the pore-forming agent decomposes to leave a porous structure, giving the first coating layer both high dielectric properties and buffer space. Then, through the thermal reaction of a second dielectric material precursor solution, a transition layer is uniformly generated on the surface of the first coating layer. The interfacial bonding during the reaction process ensures its tight connection with the adjacent layers. Finally, a dense second coating layer is formed by depositing a third dielectric material, which effectively protects the core and ensures the synergistic effect of the gradient dielectric properties and functions of the composite coating layer.

[0040] In some embodiments, the silicon-carbon core is prepared by the following method: A carbon source and a silicon source are introduced into a reaction chamber, and a silicon-carbon core is grown on a substrate, which is a carbon-based material, by chemical vapor deposition under a second catalyst and high temperature conditions.

[0041] Specifically, the silicon-carbon core is prepared by chemical vapor deposition. Using a carbon-based material as a substrate, carbon and silicon source gases are introduced under a second catalyst and high temperature conditions. Carbon and silicon atoms are deposited and grown on the substrate surface by gas-phase chemical reaction. The carbon-based substrate has good compatibility with the carbon source, which can provide a stable initial framework for the growth of the silicon-carbon core. Moreover, its carbon components can be integrated into the porous carbon skeleton structure of the core. The high temperature environment promotes the decomposition and reaction of the carbon and silicon sources. Combined with the catalytic effect of the catalyst, the dispersion morphology and size of nano-silicon in the carbon skeleton can be controlled to ensure uniform silicon distribution, thus preparing a silicon-carbon core with both high specific capacity and good conductivity.

[0042] Specifically, the carbon source gas can be selected from one or more of methane, acetylene, and propane, and the silicon source gas can be selected from one or more of silane, dichlorosilane, and silicon tetrachloride.

[0043] In some embodiments, the first dielectric material precursor comprises an organic compound and / or salt of one or more elements selected from titanium, barium, strontium, zirconium, and lead; and / or The second dielectric material precursor includes an organic compound and / or salt of one or more elements selected from aluminum, yttrium, hafnium, and lanthanum.

[0044] Specifically, the first dielectric material precursor is selected from organic compounds and / or salts of elements such as titanium, barium, strontium, zirconium, and lead. After high-temperature calcination, these precursors can generate high dielectric constant materials such as barium titanate (BaTiO3), strontium titanate (SrTiO3), and lead zirconate titanate (PZT). Their organic groups or salt structures have good dispersibility and reactivity in solution. They can be uniformly coated on the surface of silicon-carbon core through sol-gel method, and form a porous structure through the synergistic effect of pore-forming agents, which not only retains the high dielectric properties but also provides volume buffer space. The second dielectric material precursor uses organic compounds and / or salts of elements such as aluminum, yttrium, hafnium, and lanthanum. During the thermal reaction, it can generate in situ a composite system of metal oxides such as aluminum oxide (Al2O3), yttrium oxide (Y2O3), hafnium oxide (HfO2), and lanthanum oxide (La2O3). The molecular structure of its organic precursor can form chemical bonds with the surface of the first coating layer, enhancing the interlayer bonding force. At the same time, by adjusting the element ratio, the dielectric constant of the transition layer can be precisely controlled within the range of 10-150, achieving matching with the dielectric properties of the adjacent layers.

[0045] In some embodiments, the drying time is 1-3 hours and the drying temperature is 60-120°C; The calcination time is 1-3 hours, and the calcination temperature is 500-800℃.

[0046] Specifically, the above-mentioned drying conditions (drying time 1-3h, temperature 60-120℃) can both promote the full evaporation of the solvent in the gel mixture, avoiding the residual solvent from affecting the formation of the pore structure of the first coating layer during subsequent calcination (avoiding cracks in the pore structure), and prevent the gel mixture from hardening prematurely due to excessive drying time or excessive temperature, thereby affecting the uniform coating of the core. Setting the calcination time to 1-3 hours and the temperature to 500-800℃ ensures that the first dielectric material precursor is fully decomposed and crystallized to form a first dielectric material with stable high dielectric properties. At the same time, it allows the pore-forming agent to be completely decomposed and form a porous structure. This temperature range avoids the damage to the silicon-carbon core structure caused by excessively high temperatures or the inability of excessively low temperatures to affect the complete reaction of the material.

[0047] In some embodiments, the solvent includes one or more of ethanol, methanol, and acetone; The first catalyst includes one or more of hydrochloric acid, nitric acid, and acetic acid; The pore-forming agent includes one or more of polyvinyl alcohol, polyethylene glycol, and starch.

[0048] In some embodiments, the thermal reaction time for preparing the transition layer is 0.5-2 hours, and the thermal reaction temperature is 30-80°C.

[0049] Specifically, the aforementioned relatively mild temperature range can prevent high temperatures from damaging the already formed pore structure of the first coating layer or the silicon-carbon core, while ensuring that the second dielectric material precursor (such as organic compounds or salts of elements such as aluminum and yttrium) is hydrolyzed and condensed in the solution, thereby ensuring that the second dielectric material precursor is uniformly deposited on the surface of the first coating layer. A reaction time of 0.5-2 hours ensures that the precursor reacts fully and forms a continuous and dense transition layer. This avoids both incomplete coverage and insufficient bonding caused by a short reaction time, and excessive thickness of the transition layer that affects ion transport. The transition layer, with its moderate dielectric constant, achieves performance matching with the adjacent layers (first coating layer and second coating layer), and enhances the overall structural stability of the composite coating layer through stable interfacial bonding.

[0050] In some embodiments, during the sintering process for preparing the second precursor with a transition layer, the sintering temperature is 400-700°C and the sintering time is 1-3 hours.

[0051] In another embodiment of the present invention, a negative electrode sheet is provided, comprising a negative electrode material layer, wherein the negative electrode material layer comprises a binder, a conductive agent, and the aforementioned silicon-carbon composite material, or a silicon-carbon composite material prepared by the aforementioned method for preparing silicon-carbon composite material.

[0052] Specifically, the binder may be selected from one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR); the conductive agent may be selected from one or more of acetylene black, Super P, and carbon nanotubes; and the solvent may be selected from one or more of N-methylpyrrolidone (NMP) and deionized water.

[0053] Silicon-carbon composite materials possess excellent structural stability, high specific capacity, and good ion / electron transport properties. When combined with binders, they ensure a stable bond between material particles and with the current collector, reducing shedding during charge-discharge cycles. Conductive agents further optimize the conductive network inside the electrode, reducing electron transport resistance and compensating for the poor conductivity of silicon materials. The synergistic effect of these three components allows the negative electrode to fully leverage the high capacity advantage of silicon-carbon composite materials while improving cycle life through their structural stability. At the same time, by improving the overall conductivity and interfacial contact of the electrode, they help lithium-ion batteries achieve a comprehensive improvement in high energy density, long cycle life, and good rate performance.

[0054] In some embodiments, the mass ratio of the silicon-carbon composite material, the binder, and the conductive agent is (70-90):(5-15):(5-15).

[0055] Specifically, the silicon-carbon composite material, which accounts for a relatively high proportion, serves as the core active material and can fully leverage its high specific capacity to provide sufficient lithium storage sites for the negative electrode. 5-15% binder can effectively bind silicon-carbon composite material particles tightly with conductive agents and current collectors, resist volume change stress during charging and discharging, and reduce the pulverization and detachment of electrode sheets. Adding 5-15% conductive agent can construct a continuous conductive network, which can compensate for the poor conductivity of silicon materials, reduce electron transport resistance, and improve the overall conductivity of the electrode. The optimized balance of the proportions of the silicon-carbon composite material, the binder, and the conductive agent ensures a high proportion of active material to achieve high energy density, while the combination of the binder and the conductive agent enhances the stability of the electrode structure and the electronic conduction efficiency, giving the negative electrode a combination of high capacity, long cycle life, and good electrochemical performance.

[0056] The present invention will be further illustrated by the following examples.

[0057] Example 1 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention, including the following operational steps: Methane and silane gases were introduced into the reaction chamber at a volume ratio of 3:1, with iron as a catalyst, and reacted at 1000°C for 3 hours. The silicon-carbon core was then grown on the carbon substrate by chemical vapor deposition. The first dielectric material precursor (including tetrabutyl titanate containing titanium and barium acetate containing barium, 1:1), ethanol, hydrochloric acid and polyvinyl alcohol were mixed in a mass ratio of 10:20:0.5:1 and stirred evenly to obtain a gel mixture. The silicon-carbon core was immersed in a gel mixture, dried at 80°C for 2 h, and then calcined at 600°C for 2 h to obtain a first precursor with a first coating layer, wherein the pore size of the pore structure in the first coating layer is 10 nm and the porosity of the first coating layer is 50%. A solution of a second dielectric precursor (aluminum isopropoxide containing aluminum and yttrium acetate containing yttrium, 7:3) was prepared. The first precursor was placed in the solution and immersed at 50°C for 1 h to carry out a thermal reaction. After being removed, it was dried (60-120°C, 1-2 h) to remove the solvent. Then, it was sintered at 400°C for 1 h to obtain a second precursor with a transition layer. The third dielectric material, silicon dioxide target, is placed in a vacuum of 10. -4 In a vacuum chamber of Pa, a third dielectric material was deposited on the surface of the second precursor at a sputtering power of 300W for 2 hours to obtain a silicon-carbon composite material. In the silicon-carbon composite material, the thickness of the first coating layer is 40 nm, the thickness of the transition layer is 30 nm, and the thickness of the second coating layer is 40 nm.

[0058] Silicon-carbon composite material, polyvinylidene fluoride (PVDF), and acetylene black are mixed in a mass ratio of 80:10:10, and an appropriate amount of N-methylpyrrolidone (NMP) solvent is added and stirred evenly to form a negative electrode slurry. The negative electrode slurry is coated onto a copper foil current collector, dried at 120°C for 8 hours, and then rolled to form a negative electrode sheet.

[0059] Example 2 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the preparation of the silicon-carbon core, the carbon source is acetylene gas and the silicon source is dichlorosilane gas, with a volume ratio of acetylene gas to dichlorosilane gas of 4:1.

[0060] Example 3 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The pore size of the pore structure in the first coating layer is 100 nm, and the porosity of the first coating layer is 70%.

[0061] Example 4 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The pore size of the pore structure in the first coating layer is 2 nm, and the porosity of the first coating layer is 30%.

[0062] Example 5 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the silicon-carbon composite material, the thickness of the first coating layer is 100 nm, the thickness of the transition layer is 10 nm, and the thickness of the second coating layer is 80 nm.

[0063] Example 6 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the silicon-carbon composite material, the thickness of the first coating layer is 5 nm, the thickness of the transition layer is 10 nm, and the thickness of the second coating layer is 5 nm.

[0064] Example 7 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The first dielectric material precursor is formed by the reaction of titanium and strontium (tetrabutyl titanate and strontium acetate) to form strontium titanate.

[0065] Example 8 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The first dielectric material precursor is formed by the reaction of titanium, zirconium, and lead to form lead zirconate titanate.

[0066] Example 9 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The second dielectric material precursor is hafnium oxide-lanthanum oxide generated from hafnium and lanthanum elements, wherein the mass ratio of hafnium oxide to lanthanum oxide is 7:3.

[0067] Example 10 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The first dielectric material precursor (including tetrabutyl titanate containing titanium and barium acetate containing barium, in a 1:1 ratio), ethanol, hydrochloric acid and polyvinyl alcohol are mixed in a mass ratio of 12:25:0.8:1.5 and stirred evenly to obtain a gel mixture.

[0068] Example 11 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the preparation of the first precursor, the silicon-carbon core is immersed in a gel mixture, dried at 90°C for 1.5 h, and then calcined at 700°C for 1.5 h.

[0069] Example 12 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: The third dielectric material, aluminum oxide target, is placed in a vacuum of 10. -3In a vacuum chamber of Pa, a third dielectric material was deposited on the surface of the second precursor at a sputtering power of 400W for 1.5 hours.

[0070] Example 13 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the preparation of the negative electrode slurry, silicon-carbon composite material, polyvinylidene fluoride (PVDF), and acetylene black are mixed in a mass ratio of 70:15:15.

[0071] Example 14 This embodiment illustrates a silicon-carbon composite material, a method for preparing the silicon-carbon composite material, and a negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but differs in that: In the preparation of the negative electrode slurry, silicon-carbon composite material, polyvinylidene fluoride (PVDF), and acetylene black are mixed in a mass ratio of 90:5:5.

[0072] Comparative Example 1 This comparative example is used to illustrate the silicon-carbon composite material, the method for preparing the silicon-carbon composite material, and the negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but the difference is: The prepared silicon-carbon composite material has no composite coating layer.

[0073] Comparative Example 2 This comparative example is used to illustrate the silicon-carbon composite material, the method for preparing the silicon-carbon composite material, and the negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but the difference is: The prepared silicon-carbon composite material does not have a first coating layer in its composite coating layer.

[0074] Comparative Example 3 This comparative example is used to illustrate the silicon-carbon composite material, the method for preparing the silicon-carbon composite material, and the negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but the difference is: The prepared silicon-carbon composite material has no transition layer in its composite coating layer.

[0075] Comparative Example 4 This comparative example is used to illustrate the silicon-carbon composite material, the method for preparing the silicon-carbon composite material, and the negative electrode sheet disclosed in this invention. It includes most of the operations in Example 1, but the difference is: The prepared silicon-carbon composite material has no second coating layer in its composite coating layer.

[0076] Performance testing The following performance tests were performed on Examples 1-14 and Comparative Examples 1-4 prepared above: The negative electrode sheets prepared in Examples 1-14 and Comparative Examples 1-4 were assembled into lithium-ion half-cells and subjected to charge-discharge tests at a current density of 0.1C at room temperature for 100 cycles. The discharge capacity of each cycle was recorded and the capacity retention rate was calculated.

[0077] The test results are entered into Table 1.

[0078] Table 1 As can be seen from the test results in Table 1, the initial charge-discharge efficiency of Examples 1-14 is between 81% and 87%, and the capacity retention rate after 100 cycles is between 80% and 88%. In contrast, the initial charge-discharge efficiency of Comparative Examples 1-4 is only 65%-72%, and the capacity retention rate after 100 cycles is 52%-72%. That is, the silicon-carbon composite material with a gradient dielectric material coating layer provided by the present invention is significantly better than the comparative examples without the silicon-carbon composite material described in this application in terms of initial charge-discharge efficiency and cycle stability, which verifies the effectiveness of the synergistic effect of the multilayer structure in the composite coating layer in improving the battery's electrical performance. In Example 1, the initial efficiency was 87% and the capacity retention was 88%, which is the best performance compared to other examples. It is speculated that the reason is that in Example 1, multiple parameters such as the pore size, porosity and thickness of each layer in the composite coating layer fully exerted a synergistic effect, further optimizing the comprehensive function of the composite coating layer and promoting a significant improvement in the electrical performance of the silicon-carbon composite material. The performance of Examples 3 and 4 is slightly lower than that of Example 1, indicating that there is an optimal value when the pore size of the first coating layer is within the range of 2-100 nm and the porosity is within the range of 30%-70%. Too large or too small a value will have a certain impact on its buffering effect and ion transport efficiency. The capacity retention rates of Examples 5 (first coating layer 100nm, transition layer 10nm, second coating layer 80nm) and 6 (first coating layer 5nm, transition layer 10nm, second coating layer 5nm) are slightly lower than those of Example 1, indicating that the thickness of each coating layer needs to be matched (for example, if the transition layer is too thin, it may weaken the interlayer bonding force) in order to achieve the best synergistic effect. As can be seen from the test results of Example 9, its initial efficiency is 81% and its capacity retention is 81%, which is slightly worse than the overall performance of other examples. The mass ratio of the hafnium oxide-lanthanum oxide composite system in its transition layer is 7:3, which is relatively low. It is speculated that the reason for the poor test results is that the dielectric matching between the composite system and the adjacent layers (first coating layer and second coating layer) is slightly poor, the interlayer bonding force is weakened, which leads to a decrease in structural stability during cycling and thus affects the performance. In Example 11, the drying and calcination conditions for the preparation of the first precursor were changed (drying at 90°C for 1.5 h and calcining at 700°C for 1.5 h). The initial efficiency was 84% ​​and the capacity retention rate was 83%, which was slightly lower than that in Example 1. This may be because the increased calcination temperature caused some pore structures to shrink, resulting in a slight decrease in buffering capacity, which in turn affected the initial efficiency and capacity retention rate. Comparative Example 1, without a composite coating layer, had an initial charge-discharge efficiency of only 65% ​​and a capacity retention rate of 52% after 100 cycles, resulting in the worst test results. This was due to the lack of protection from the composite coating layer. The silicon-carbon core underwent volume changes during charge and discharge, leading to structural damage. Furthermore, it reacted violently with the electrolyte interface, consuming a large amount of active lithium and exhibiting extremely poor cycle stability. Comparative Example 2 lacks a first coating layer, with an initial efficiency of 70% and a capacity retention of 72%. Due to the lack of a first coating layer with a high dielectric constant and a porous structure, it cannot effectively buffer changes in core volume and also weakens charge conduction ability, resulting in performance degradation. Comparative Example 3 lacks a transition layer, with an initial efficiency of 72% and a capacity retention of 62%. The absence of the transition layer weakens the bonding force between the first and second coating layers, making the layers easy to peel off, which damages the integrity of the coating structure and results in significant performance degradation during cycling. Comparative Example 4 lacks a second coating layer, with an initial efficiency of 66% and a capacity retention of 70%. The lack of a dense second coating layer makes it easy for the electrolyte to penetrate and react with the core, increasing the loss of active lithium, reducing the initial efficiency, and the insufficient structural stability affects the cycle performance. Based on the above analysis, the multi-layer structure of the composite coating layer in the silicon-carbon composite material described in this application works synergistically with its corresponding dielectric materials to further optimize the comprehensive function of the composite coating layer and significantly improve the electrical properties of the silicon-carbon composite material.

[0079] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, It includes a core and a composite coating layer. The core is a silicon-carbon core, and the composite coating layer covers the core. The composite coating layer includes, from the inside to the outside, a first coating layer with a porous structure, a transition layer, and a second coating layer with a dense structure. The dielectric constant of the first coating layer is greater than that of the second coating layer, and the dielectric constant of the transition layer is between that of the first coating layer and the second coating layer. The porosity of the first coating layer is 30%-70%; The dielectric constant of the first coating layer is >150; The dielectric constant of the transition layer is 10-150; The dielectric constant of the second coating layer is <10.

2. The silicon-carbon composite material according to claim 1, characterized in that, The first coating layer includes a first dielectric material, which includes one or more of barium titanate, strontium titanate, and lead zirconate titanate; and / or, The transition layer includes a second dielectric material, which is a metal oxide composite system, comprising one or more of aluminum oxide, yttrium oxide, hafnium oxide, and lanthanum oxide; and / or, The second coating layer includes a third dielectric material, which includes one or more of silicon dioxide, aluminum oxide, and silicon nitride.

3. The silicon-carbon composite material according to claim 1, characterized in that, The pore size of the porous structure in the first coating layer is 2-100 nm.

4. The silicon-carbon composite material according to claim 1, characterized in that, The thickness of the first coating layer is 5-100 nm; The thickness of the transition layer is 10-80 nm; The thickness of the second coating layer is 5-80 nm.

5. The silicon-carbon composite material according to claim 1, characterized in that, The silicon-carbon core comprises a porous carbon framework and nano-silicon dispersed on the porous carbon framework.

6. The method for preparing the silicon-carbon composite material according to any one of claims 1-5, characterized in that, Includes the following operations: Obtaining silicon-carbon cores; The first dielectric material precursor, solvent, first catalyst and pore-forming agent are mixed and reacted to obtain a gel mixture; The core is immersed in a gel mixture, dried, and calcined to obtain a first precursor with a first coating layer; The second dielectric material precursor is prepared into a solution, the first precursor is placed in the solution for thermal reaction, and then sintered to obtain the second precursor with a transition layer. A third dielectric material is deposited on the surface of the second precursor to obtain a silicon-carbon composite material.

7. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, The silicon-carbon core is prepared by the following method: A carbon source and a silicon source are introduced into a reaction chamber, and a silicon-carbon core is grown on a substrate, which is a carbon-based material, by chemical vapor deposition under a second catalyst and high temperature conditions.

8. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, The first dielectric material precursor includes an organic compound and / or salt of one or more elements selected from titanium, barium, strontium, zirconium, and lead; and / or The second dielectric material precursor includes an organic compound and / or salt of one or more elements selected from aluminum, yttrium, hafnium, and lanthanum.

9. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, The drying time is 1-3 hours, and the drying temperature is 60-120℃; The calcination time is 1-3 hours, and the calcination temperature is 500-800℃.

10. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, The solvent includes one or more of ethanol, methanol, and acetone; The first catalyst includes one or more of hydrochloric acid, nitric acid, and acetic acid; The pore-forming agent includes one or more of polyvinyl alcohol, polyethylene glycol, and starch.

11. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, In the thermal reaction for preparing the transition layer, the thermal reaction time is 0.5-2 hours and the thermal reaction temperature is 30-80°C.

12. The method for preparing the silicon-carbon composite material according to claim 6, characterized in that, During the sintering process, the sintering temperature is 400-700℃ and the sintering time is 1-3 hours.

13. A negative electrode sheet, characterized in that, The material includes a negative electrode material layer, which comprises a binder, a conductive agent, and a silicon-carbon composite material as described in any one of claims 1-5, or a silicon-carbon composite material prepared by the method described in any one of claims 6-12.

14. The negative electrode sheet according to claim 13, characterized in that, The mass ratio of the silicon-carbon composite material, the binder, and the conductive agent is (70-90):(5-15):(5-15).

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