Silicon-carbon negative electrode material, preparation method thereof and lithium ion battery

Abstract

The embodiment of the invention provides a silicon-carbon negative electrode material, a preparation method thereof and a lithium ion battery, and relates to the field of lithium secondary batteries. The silicon-carbon negative electrode material comprises a porous silicon-containing carbon skeleton, a second silicon material and a carbon coating layer, the porous silicon-containing carbon skeleton comprises a carbon skeleton with a porous structure and a first silicon material located in the carbon skeleton, the second silicon material is attached to the surface of the carbon skeleton and in the porous structure, and the second silicon material and the porous silicon-containing carbon skeleton jointly form a matrix. The surface of the matrix is coated with the carbon coating layer. According to the silicon-carbon negative electrode material, the preparation method thereof and the lithium ion battery material provided by the embodiment of the invention, the silicon coating property is good, the silicon content is high, and the tap density is relatively high; the battery is high in capacity and first efficiency and good in cycle performance.

Description

Technical Field

[0001] This application relates to the field of lithium secondary batteries, and more specifically, to a silicon-carbon anode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Currently, silicon-carbon materials with porous carbon structures internally loaded with silicon particles are a promising silicon-carbon anode material for lithium-ion batteries. These materials are typically prepared by ball milling a carbon-based precursor with silicon particles, or by directly generating the precursor through vapor deposition. However, the silicon coating in these materials is insufficient, resulting in low silicon content and difficulty in achieving optimal density, ultimately affecting battery performance.

[0003] Specifically, while silicon-carbon materials prepared by direct ball milling have a high tap density, their silicon coating is insufficient, resulting in spherical silicon particles on the material surface and a lower specific capacity in electrical performance tests. Silicon-carbon materials prepared by direct vapor deposition have a lower tap density and a larger specific surface area, but the single-layer vapor-phase carbon coating is insufficient for silicon particle coating, and there is also the problem of uneven distribution of silicon particles within the carbon substrate.

[0004] Therefore, how to prepare a silicon-carbon material with good silicon coating properties, high silicon content, and high tap density is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this application is to provide a silicon-carbon anode material, its preparation method, and a lithium-ion battery. The material has good silicon coating properties and high silicon content, and high tap density. The battery has high capacity and initial efficiency, and good cycle performance.

[0006] In a first aspect, embodiments of this application provide a silicon-carbon anode material, comprising: A porous silicon-containing carbon framework, the porous silicon-containing carbon framework comprising a carbon framework having a porous structure and a first silicon material located inside the carbon framework; The second silicon material is attached to the surface and porous structure of the carbon skeleton, and the second silicon material and the porous silicon-containing carbon skeleton together form the matrix; A carbon coating layer, wherein the carbon coating layer is applied to the surface of the substrate.

[0007] In the above technical solution, a portion of the silicon material (the first silicon material) in the silicon-carbon anode material exists inside the carbon skeleton, and another portion of the silicon material (the second silicon material) exists outside the carbon skeleton (specifically, it is attached to the surface of the carbon skeleton and inside the porous structure of the carbon skeleton). Therefore, the silicon content in the material is high. Moreover, the carbon coating layer coats the surface of the substrate containing the above-mentioned silicon material, resulting in good silicon coating properties and a high tap density in the material.

[0008] In one possible implementation, based on the mass of the silicon-carbon anode material, the content of the first silicon material in the porous silicon-carbon framework is 5-10 wt%, and the total content of the first silicon material and the second silicon material is 60-70 wt%.

[0009] In the above technical solutions, the silicon content in the material is high.

[0010] Secondly, embodiments of this application provide a method for preparing a silicon-carbon anode material, which includes the following steps: S1. Mix biomass carbon-based precursor, silicon nanoparticles and pore-forming agent, first activate the biomass in the biomass carbon-based precursor until the biomass softens and separates and the silicon nanoparticles enter the interior of the biomass, and then perform carbonization treatment to obtain a porous silicon-containing carbon framework. S2. The porous silicon-containing carbon framework is subjected to silicon deposition to obtain a substrate, and then the substrate is subjected to carbon coating treatment.

[0011] In the above technical solution, the biomass carbon-based precursor, silicon nanoparticles, and pore-forming agent are first activated and carbonized. The activation treatment softens and separates the lignin, cellulose, and other biomass components in the biomass carbon-based precursor, allowing the silicon nanoparticles to enter the interior of the lignin, cellulose, and other biomass components. The carbonization treatment then pre-places the silicon nanoparticles within the carbon skeleton substrate, forming a porous silicon-containing carbon skeleton by embedding the silicon nanoparticles into the carbon skeleton formed by the carbonization of the lignin, cellulose, and other biomass components. This results in a tighter silicon-carbon contact and improves material stability. Next, the porous silicon-containing carbon skeleton undergoes silicon deposition treatment to deposit silicon material on its exterior (surface and pores). The nitrogen element abundant in the biomass carbon-based precursor forms CN bonds with carbon elements during the carbonization treatment. In the subsequent silicon deposition process, these CN bonds induce the silicon source gas used for silicon deposition to uniformly deposit silicon nanoparticles on the exterior of the porous silicon-containing carbon skeleton, increasing the silicon content in the material and thus improving the capacity and initial efficiency of the battery formed when used as a negative electrode material.

[0012] Therefore, the preparation method of this application uses a low-cost and environmentally friendly biomass carbon-based precursor. It achieves two silicon loading processes through silicon nanoparticle pre-placement and silicon deposition, significantly increasing the silicon content in the material. Further carbon coating enhances the silicon coating properties while maintaining a high tap density, thereby effectively improving battery capacity and initial efficiency, as well as cycle performance. This preparation method for silicon-carbon anode materials solves the problems of low silicon content, uneven silicon distribution, incomplete carbon coating, and low tap density in traditional silicon-carbon anode materials, which lead to low specific capacity, low initial efficiency, and poor cycle performance.

[0013] In one possible implementation, in step S1, the biomass carbon-based precursor includes one or at least two of hemp fiber, wood fiber, coconut shell, straw, bamboo, apricot shell, and walnut shell. And / or, the particle size of the biomass carbon-based precursor is 1-20 mm, and the particle size of the silicon nanoparticles is 2-10 nm; And / or, the mass ratio of the biomass carbon-based precursor to the silicon nanoparticles is 1-2:1; And / or, the pore-forming agent includes one or at least two of zinc chloride, magnesium chloride, potassium chloride, calcium chloride, aluminum chloride, ferric chloride, and phosphoric acid; And / or, the concentration of the pore-forming agent is 0.5-3 mol / L.

[0014] In one possible implementation, in step S1, the activation treatment method includes: stirring at 30-60°C for 1-3 hours; And / or, the carbonization treatment method includes: carbonizing at 500-1000°C for 2-5 hours in an inert gas atmosphere; And / or, after carbonization, acid washing, water washing and drying are performed to obtain the silicon-containing carbon skeleton.

[0015] In one possible implementation, in step S2, the silicon deposition process includes: depositing at 450-600°C for 3-7 hours in a mixed atmosphere of inert gas and silicon source gas.

[0016] In one possible implementation, in step S2, the carbon coating process includes: sequentially performing carbon deposition on the substrate and non-gas phase carbon coating.

[0017] In the above technical solution, two carbon coatings are used to improve the coating of carbon on the silicon-containing matrix and reduce the exposure of silicon. At the same time, the two carbon coating layers can improve the electronic conductivity (i.e., electrical conductivity) of the material, alleviate the expansion of the material during charging and discharging, reduce the direct contact between silicon and electrolyte, reduce the occurrence of side reactions, and improve the cycle performance of the material.

[0018] In one possible implementation, the carbon deposition process includes: depositing at 450-650°C for 2-5 hours in a mixed atmosphere of inert gas and carbon source gas.

[0019] In one possible implementation, the non-gas phase carbon coating method includes: fusing the carbon-deposited matrix and the carbon source material, followed by carbonization treatment; wherein the mass ratio of the carbon-deposited matrix to the carbon source material is 2-4:1, and the carbon source material includes one or at least two of asphalt, resin, glucose, starch, polyvinyl alcohol, and polypyrrole; the carbonization treatment method includes: carbonizing at 700-1200℃ for 2-5 hours in an inert atmosphere.

[0020] In the above technical solution, the matrix after carbon deposition treatment (one-time carbon coating) and the carbon source material are fused together and then carbonized to achieve two carbon coatings, which can improve the tap density of the material. Moreover, the fusion treatment during non-gas phase carbon coating can make the silicon nanoparticles and carbon framework substrates more closely connected on the basis of the aforementioned silicon nanoparticle pre-placement, making the silicon nanoparticles more uniformly distributed in the carbon framework substrate, improving the consistency of different material particles, and ultimately improving the batch stability of the material.

[0021] Thirdly, embodiments of this application provide a lithium-ion battery, which includes a positive electrode and a negative electrode, wherein the negative electrode includes the silicon-carbon negative electrode material provided in the first aspect or the silicon-carbon negative electrode material prepared by the preparation method provided in the second aspect.

[0022] In the above technical solution, the silicon-carbon anode material used has good silicon coating properties and high silicon content, and high tap density. Therefore, the lithium-ion battery has high capacity and first-time efficiency, and good cycle performance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the preparation process of the silicon-carbon anode material provided in the embodiments of this application; Figure 2 This is an elemental energy dispersive spectroscopy (EDS) scan of the cross-section of the porous silicon-carbon framework in Example 4; Figure 3 SEM images of the silicon carbide anode materials of Examples 1-3; Figure 4 SEM images of the silicon-carbon anode materials in Comparative Examples 1-3; Figure 5 The images show the XRD patterns of the silicon carbide anode material and the silicon material in Examples 1-3. Figure 6The images show the XRD patterns of the carbon-silicon anode materials and silicon materials in Comparative Examples 1-3. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0026] The silicon-carbon anode material, its preparation method, and lithium-ion battery of the present application embodiments are described in detail below.

[0027] This application provides a silicon-carbon anode material, comprising: A porous silicon-containing carbon framework, comprising a carbon framework having a porous structure and a first silicon material located inside the carbon framework; The second silicon material is attached to the surface of the carbon skeleton and the porous structure, and the second silicon material and the porous silicon-containing carbon skeleton together form the matrix. Carbon coating layer, which is coated on the surface of the substrate.

[0028] In some embodiments of this application, the first silicon material inside the carbon skeleton is silicon nanoparticles, i.e., nanoscale silicon particles; the second silicon material outside the silicon skeleton can also be silicon nanoparticles, which are distributed at different positions outside the carbon skeleton: some of the second silicon material is attached to the pores of the porous structure of the carbon skeleton, and some of the second silicon material is attached to the non-pore positions on the surface of the silicon skeleton, forming a regional coating type silicon coating layer or a completely coating type silicon coating layer.

[0029] In some embodiments of this application, based on the mass of the silicon-carbon anode material, the content of the first silicon material in the porous silicon-carbon framework is 5-10 wt%, and the total content of the first silicon material and the second silicon material is 60-70 wt%.

[0030] In some embodiments of this application, the carbon coating layer is mainly located on the non-porous surface of the substrate, making the material granular and without exposed pores on the surface. The carbon coating layer can be divided into two layers: a first carbon coating layer located on the surface of the substrate; and a second carbon coating layer located on the surface of the first carbon coating layer.

[0031] Please refer to Figure 1 This application also provides a method for preparing a silicon-carbon anode material, which includes the following steps: S1. A mixture of biomass carbon-based precursor, silicon nanoparticles, and a pore-forming agent is first activated until the biomass in the biomass carbon-based precursor softens and separates, allowing the silicon nanoparticles to penetrate the biological interior. Then, a carbonization process is performed to obtain a porous silicon-containing carbon framework. Figure 1 As can be seen from the diagram, the porous silicon-containing carbon framework includes a carbon framework with a porous structure and silicon nanoparticles (the first silicon material) located inside the carbon framework.

[0032] In some embodiments of this application, in step S1, the biomass carbon-based precursor includes one or at least two of hemp fiber, wood fiber, coconut shell, straw, bamboo, apricot shell and walnut shell. The biomass carbon-based precursor used contains a high content of lignin, cellulose and other biomass to carbonize and form a carbon skeleton, and is also rich in nitrogen to induce the subsequent uniform deposition of silicon material. And / or, the particle size of the biomass carbon-based precursor is 1-20 mm, and the particle size of the silicon nanoparticles is 2-10 nm; And / or, the mass ratio of biomass carbon-based precursor to silicon nanoparticles is 1-2:1; And / or, the pore-forming agent includes one or at least two of zinc chloride, magnesium chloride, potassium chloride, calcium chloride, aluminum chloride, ferric chloride, and phosphoric acid; And / or, the concentration of the pore-forming agent in the corresponding solution is 0.5-3 mol / L.

[0033] In some embodiments of this application, the activation treatment in step S1 includes stirring at 30-60°C for 1-3 hours; And / or, the carbonization process includes: carbonizing at 500-1000℃ for 2-5 hours in an inert gas atmosphere; And / or, after carbonization, acid washing, water washing and drying are performed to obtain a silicon-containing carbon skeleton (wherein, the acid used for acid washing includes one or at least two of dilute hydrochloric acid, dilute sulfuric acid and dilute nitric acid, and the drying method includes vacuum drying at 50-100℃ or freeze drying).

[0034] S2. The porous silicon-containing carbon framework is subjected to silicon deposition to obtain a matrix (intermediate product). This matrix comprises a porous silicon-containing carbon framework and a second silicon material attached to the surface of the carbon framework and within the porous structure. Figure 1 As can be seen from the diagram, after silicon deposition, the pores of the carbon framework of this intermediate product contain silicon material; then the substrate is carbon-coated, combined with... Figure 1 As can be seen from the illustration, after carbon coating, the material appears as particles with a carbon material surface.

[0035] In some embodiments of this application, in step S2, the silicon deposition process employs chemical vapor deposition (CVD), specifically including: deposition at 450-600°C for 3-7 hours in a mixed atmosphere of inert gas and silicon source gas. The volume ratio (also referring to the flow rate ratio) of the inert gas to the silicon source gas is 2-4:1. The inert gas includes at least one of nitrogen, argon, and helium, and the silicon source gas includes at least one of silane and disilane.

[0036] In some embodiments of this application, in step S2, the carbon coating process includes two carbon coatings, specifically by sequentially performing carbon deposition treatment (first carbon coating) and non-gas phase carbon coating (second carbon coating) on ​​the substrate.

[0037] In some embodiments of this application, the carbon deposition process employs a vapor phase deposition method, specifically including: deposition at 450-650°C for 2-5 hours in a mixed atmosphere of inert gas and carbon source gas. The volume ratio of inert gas to carbon source gas is 4-6:1, and the inert gas includes at least one of nitrogen, argon, and helium, while the carbon source gas includes at least one of methane, acetylene, ethylene, and benzene.

[0038] In some embodiments of this application, the non-gas phase carbon coating adopts a fusion method, specifically including: fusing the carbon-deposited matrix and the carbon source material, followed by carbonization treatment; wherein, the mass ratio of the carbon-deposited matrix to the carbon source material is 2-4:1, and the carbon source material includes one or at least two of asphalt, resin, glucose, starch, polyvinyl alcohol, and polypyrrole; the fusion is performed by ball milling or fusion machine; the carbonization treatment method includes: carbonizing at 700-1200℃ for 2-5 hours in an inert atmosphere.

[0039] In some embodiments of this application, both silicon deposition and carbon deposition (single carbon coating) are performed using vapor deposition, and can be carried out sequentially using the same equipment.

[0040] This application also provides a lithium-ion battery, which includes a positive electrode and a negative electrode. The negative electrode includes the silicon-carbon negative electrode material of the foregoing embodiments or the silicon-carbon negative electrode material prepared by the preparation method of the foregoing embodiments.

[0041] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0042] Example 1 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take materials with a mass ratio of silicon nanoparticles: wood fiber = 1:1.5. The particle size of silicon nanoparticles is 2-5nm and the particle size of wood fiber is 1-5mm. Add silicon nanoparticles and wood fiber to 1mol / L ZnCl2 solution, stir at 40℃ for 2h, and filter to obtain filter residue. Carbonize the filter residue at 850℃ for 3h in Ar atmosphere, and then perform acid washing with dilute hydrochloric acid, water washing, and freeze drying to obtain a porous silicon-containing carbon skeleton.

[0043] (2) The porous silicon-carbon skeleton is transferred to a tube furnace and a gas with a flow rate ratio of N2:SiH4=2.5:1 is introduced. Silicon deposition is carried out at 550℃ for 5 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of N2:C2H2=5:1 is introduced and carbon deposition is carried out at 600℃ for 3 hours in a mixed atmosphere to obtain a primary carbon-coated substrate.

[0044] (3) The primary carbon-coated matrix and asphalt are mixed at a mass ratio of 2:1 and ball-milled for 0.5 h, and then carbonized at 1000 °C for 3 h in an Ar atmosphere to obtain silicon-carbon anode material.

[0045] Example 2 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles and straw in a mass ratio of 1:2. The particle size of silicon nanoparticles is 5-8 nm and the particle size of straw is 5-20 mm. Add silicon nanoparticles and straw to 2 mol / L KCl solution, stir at 60℃ for 1 h, and filter to obtain filter residue. Carbonize the filter residue at 1000℃ for 2 h in N2 atmosphere, and then wash with dilute sulfuric acid, wash with water, and dry under vacuum at 80℃ to obtain a porous silicon-containing carbon skeleton.

[0046] (2) The porous silicon-carbon skeleton is transferred to a fluidized bed and a gas with a flow rate ratio of N2:SiH4=3:1 is introduced. Silicon deposition is carried out at 500℃ for 6 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of N2:SiH4=4:1 is introduced and carbon deposition is carried out at 650℃ for 5 hours in a mixed atmosphere to obtain a primary carbon-coated matrix.

[0047] (3) The carbon-coated matrix and polyvinyl alcohol are mixed at a mass ratio of 3:1 and ball-milled for 1 hour. Then, the mixture is carbonized again at 1200°C in N2 atmosphere for 2 hours to obtain silicon-carbon anode material.

[0048] Example 3 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles and hemp fibers in a mass ratio of 1:1. The particle size of silicon nanoparticles is 5-8 nm and the particle size of hemp fibers is 5-10 mm. Add silicon nanoparticles and hemp fibers to a 0.5 mol / L CaCl2 solution and stir at 50 °C for 1.5 h. Filter to obtain filter residue. Carbonize the filter residue at 700 °C for 4 h in an Ar atmosphere. After acid washing with dilute nitric acid, water washing, and vacuum drying at 50 °C, a porous silicon-carbon skeleton is obtained.

[0049] (2) The porous silicon-carbon skeleton is transferred to an atmosphere furnace and a gas with a flow rate ratio of Ar:Si2H6=4:1 is introduced. Silicon deposition is carried out at 600℃ for 3 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of Ar:C2H4=6:1 is introduced and carbon deposition is carried out at 650℃ for 2 hours in a mixed atmosphere to obtain a primary carbon-coated substrate.

[0050] (3) The primary carbon-coated matrix and resin are mixed at a mass ratio of 3:1 and ball-milled for 3 hours, and then carbonized at 900°C for 4 hours in an Ar atmosphere to obtain silicon-carbon anode material.

[0051] Example 4 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles: bamboo = 1:2 by mass ratio. The particle size of silicon nanoparticles is 2-5 nm and the particle size of bamboo fragments is 1-5 mm. Add silicon nanoparticles and bamboo fragments to 1.5 mol / L MgCl2 solution, stir at 30℃ for 3 h, and filter to obtain filter residue. Carbonize the filter residue at 900℃ for 3 h in He atmosphere, and then wash with dilute hydrochloric acid, wash with water, and dry in vacuum at 100℃ to obtain a porous silicon-carbon skeleton.

[0052] (2) The porous silicon-carbon skeleton is transferred to a fluidized bed and a gas with a flow rate ratio of He:SiH4=2:1 is introduced. Silicon deposition is carried out at 450℃ for 7h in a mixed atmosphere. Then, a gas with a flow rate ratio of He:C2H4=5.5:1 is introduced and carbon deposition is carried out at 550℃ for 4h in a mixed atmosphere to obtain a primary carbon-coated matrix.

[0053] (3) The carbon-coated matrix and starch are mixed and fused at a mass ratio of 1:1 for 2 hours, and then carbonized again at 700°C for 5 hours in a He atmosphere to obtain silicon-carbon anode material.

[0054] Example 5 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles and coconut shells in a mass ratio of 1:1.5. The particle size of silicon nanoparticles is 5-8 nm and the particle size of coconut shell fragments is 2-10 mm. Add silicon nanoparticles and coconut shell fragments to 3 mol / L H3PO4 solution and stir at 45℃ for 2.5 h. Filter to obtain filter residue. Carbonize the filter residue at 500℃ for 5 h in N2 atmosphere. After acid washing with dilute nitric acid, water washing and vacuum drying at 60℃, a porous silicon-carbon skeleton is obtained.

[0055] (2) The porous silicon-carbon skeleton is transferred to a tube furnace and a gas with a flow rate ratio of Ar:SiH4=3.5:1 is introduced. Silicon deposition is carried out at 500℃ for 4 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of Ar:C6H6=4.5:1 is introduced and carbon deposition is carried out at 500℃ for 5 hours in a mixed atmosphere to obtain a primary carbon-coated substrate.

[0056] (3) The carbon-coated matrix and polypyrrole are mixed at a mass ratio of 2.5:1 and ball-milled for 2.5 hours. Then, the mixture is carbonized again at 1100°C for 3 hours in a N2 atmosphere to obtain silicon-carbon anode material.

[0057] Example 6 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles and walnut shells in a mass ratio of 1:1.5. The particle size of silicon nanoparticles is 5-8 nm and the particle size of walnut shells is 5-10 mm. Add silicon nanoparticles and walnut shells to 1 mol / L AlCl3 solution, stir at 55℃ for 3 h, and filter to obtain filter residue. Carbonize the filter residue at 800℃ for 4 h in Ar atmosphere, and then wash with dilute sulfuric acid, wash with water, and dry under vacuum at 90℃ to obtain a porous silicon-carbon skeleton.

[0058] (2) The porous silicon-carbon skeleton is transferred to an atmosphere furnace and a gas with a flow rate ratio of N2:Si2H6=4:1 is introduced. Silicon deposition is carried out at 450°C for 6 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of N2:C2H2=4:1 is introduced and carbon deposition is carried out at 450°C for 5 hours in a mixed atmosphere to obtain a primary carbon-coated substrate.

[0059] (3) The carbon-coated matrix and glucose were mixed and fused at a mass ratio of 1.5:1 for 1 hour, and then carbonized again at 800°C for 2 hours in a N2 atmosphere to obtain silicon-carbon anode material.

[0060] Example 7 This embodiment provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take silicon nanoparticles and apricot shells in a mass ratio of 1:2. The particle size of silicon nanoparticles is 5-8 nm and the particle size of apricot shells is 2-10 mm. Add silicon nanoparticles and apricot shells to a 1 mol / L FeCl3 solution and stir at 35 °C for 1 h. Filter to obtain filter residue. Carbonize the filter residue at 600 °C for 3 h in N2 atmosphere. After acid washing with dilute hydrochloric acid, water washing, and vacuum drying at 70 °C, a porous silicon-carbon skeleton is obtained.

[0061] (2) The porous silicon-carbon skeleton is transferred to a tube furnace and a gas with a flow rate ratio of N2:SiH4=3:1 is introduced. Silicon deposition is carried out at 480℃ for 7 hours in a mixed atmosphere. Then, a gas with a flow rate ratio of N2:CH4=5:1 is introduced and carbon deposition is carried out at 550℃ for 4 hours in a mixed atmosphere to obtain a primary carbon-coated matrix.

[0062] (3) The carbon-coated matrix and asphalt are mixed at a mass ratio of 2:1 and ball-milled for 0.5h. Then, the mixture is carbonized again at 700℃ for 2h in N2 atmosphere to obtain silicon-carbon anode material.

[0063] Example 8 The silicon-carbon anode material provided in this embodiment differs from that in Example 1 in that: (2) The porous silicon-carbon skeleton is transferred to a tube furnace, and a gas with a flow rate ratio of N2:SiH4=2.5:1 is introduced. Silicon deposition is carried out at 550℃ for 5h in a mixed atmosphere. Then, a gas with a flow rate ratio of N2:C2H2=5:1 is introduced, and carbon deposition is carried out at 600℃ for 4h in a mixed atmosphere to obtain silicon-carbon anode material.

[0064] Example 9 The silicon-carbon anode material provided in this embodiment differs from that in Example 1 in that: (2) The porous silicon-carbon skeleton is transferred to a tube furnace and a gas with a flow rate ratio of N2:SiH4=2.5:1 is introduced. Silicon deposition is carried out at 550℃ for 5 hours in a mixed atmosphere to obtain the substrate.

[0065] (3) The matrix and asphalt are mixed at a mass ratio of 2:1 and ball-milled for 0.5h, and then carbonized at 1000℃ for 3h in an Ar atmosphere to obtain silicon-carbon anode material.

[0066] Comparative Example 1 This comparative example provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take samples at a mass ratio of silicon nanoparticles: wood-based biochar material = 1:2. The particle size of silicon nanoparticles is 2-5 nm, and the particle size of wood-based biochar material (porous carbon particles formed by the activation and carbonization of wood fibers, which do not contain silicon material) is 3-8 μm. Ball mill the silicon nanoparticles and wood-based biochar material for 2 h, and then add asphalt of equal mass to the mixture and continue ball milling for 2 h to obtain the intermediate product.

[0067] (2) The intermediate product is placed in a tube furnace and carbonized at 1000°C for 3 hours in an Ar atmosphere to obtain silicon-carbon anode material.

[0068] Comparative Example 2 This comparative example provides a silicon-carbon anode material, the preparation method of which is as follows: (1) Take materials according to the mass ratio of silicon nanoparticles: wood fiber = 1:1.5 (same as in Example 1), add silicon nanoparticles and wood fiber to 1 mol / L ZnCl2 solution, stir at 40℃ for 2 h, filter to obtain filter residue; carbonize the filter residue at 850℃ for 3 h in Ar atmosphere, and after acid washing with dilute hydrochloric acid, water washing, and freeze drying, a porous silicon-carbon skeleton is obtained, namely silicon-carbon anode material.

[0069] Comparative Example 3 This comparative example provides a silicon-carbon anode material, the preparation method of which is as follows: (1) The mass ratio of silicon nanoparticles to wood fiber is 1:1.5 (same as in Example 1). The silicon nanoparticles and wood fiber are added to a 1 mol / L ZnCl2 solution and stirred at 40°C for 2 h. The residue is filtered to obtain filter residue. The filter residue is carbonized at 850°C for 3 h in an Ar atmosphere. After acid washing with dilute hydrochloric acid, water washing, and freeze drying, a porous silicon-containing carbon skeleton is obtained.

[0070] (2) The porous silicon-carbon skeleton and asphalt were mixed at a mass ratio of 2:1 and ball-milled for 0.5h. Then, the mixture was carbonized again at 1000℃ for 3h in an Ar atmosphere to obtain silicon-carbon anode material.

[0071] Comparative Example 4 This comparative example provides a silicon-carbon anode material, the preparation method of which is as follows: (1) The silicon nanoparticles and asphalt were mixed in a mass ratio of 1:2 and ball-milled for 2 hours. Then, the mixture was carbonized at 700°C for 2 hours in a N2 atmosphere to obtain silicon-carbon anode material.

[0072] Comparative Example 5 This comparative example provides a silicon-carbon anode material. The difference between its preparation method and that of Example 1 is that no activation treatment is performed. Instead, silicon nanoparticles, wood fibers and ZnCl2 are mixed and then directly subjected to carbonization, acid washing, water washing and drying in step (2) to obtain a porous carbon skeleton, i.e., silicon-carbon anode material.

[0073] Test case I. Physical and chemical characterization tests: SEM and XRD tests were performed on the silicon-carbon anode materials of Examples 1-3 and Comparative Examples 1-3. Additionally, cross-sectional EDS scanning was performed on the porous silicon-carbon framework obtained in step (1) of Example 4. The cross-sectional EDS image of the porous silicon-carbon framework in Example 4 is shown below. Figure 2 As shown; SEM images of the silicon-carbon anode materials of Examples 1-3 and Comparative Examples 1-3 are shown below. Figure 3 and Figure 4 As shown, the XRD patterns of the silicon-carbon anode materials and silicon materials of Examples 1-3 and Comparative Examples 1-3 are as follows. Figure 5 and Figure 6 As shown.

[0074] The silicon content of the carbon skeletons in Examples 1-9 and Comparative Examples 1-5 was tested using a carbon-sulfur analyzer and a thermogravimetric analyzer, respectively (Examples 1-9 and Comparative Examples 2-3 were tested for the porous silicon-containing carbon skeletons obtained in step (1), Comparative Example 5 was tested for the porous carbon skeleton, and Comparative Examples 1 and 4 did not form a carbon skeleton and therefore did not need to be tested). The silicon content was calculated based on the final silicon-carbon anode material. In addition, the tap density and total silicon content of each silicon-carbon anode material in Examples 1-9 and Comparative Examples 1-5 were tested using a tap density meter, a carbon-sulfur analyzer, and a thermogravimetric analyzer. The results are shown in Table 1.

[0075] Table 1. Test results of tap density and silicon content

[0076] according to Figure 2 The cross-sectional EDS image of the porous silicon-containing carbon framework in Example 4 shows that silicon is present in the carbon framework, proving that during the activation stage of the biomass carbon-based precursor, silicon nanoparticles successfully entered the interior of biomass such as lignin and cellulose, and thus existed inside the carbon framework.

[0077] according to Figure 3-4 The SEM and XRD patterns of the silicon-carbon anode materials in Examples 1-3 and Comparative Examples 1-3 show that: the SEM images of the silicon-carbon anode materials in Examples 1-3 show smooth surfaces without obvious spherical silicon particles; and the XRD patterns of the silicon-carbon anode materials in Examples 1-3 show no silicon peaks, further demonstrating that the carbon completely coats the silicon in the silicon-carbon anode materials of this application, and there is no silicon exposure on the material surface. The SEM images of the silicon-carbon anode materials in Comparative Examples 1-3 show no obvious abnormalities, but the XRD patterns of these three materials show obvious silicon peaks, proving that silicon is exposed on the surface of these silicon-carbon anode materials.

[0078] Combining the tap density and silicon content test results in Table 1, it can be seen that the silicon-carbon anode material prepared by the method of this application has higher tap density and total silicon content, while still maintaining the absence of exposed silicon material. Furthermore, based on the test results of Comparative Example 1 (no silicon nanoparticles loaded in the carbon skeleton), Comparative Example 3 (no silicon deposition on the exterior of the carbon skeleton), and Comparative Example 5 (no activation treatment), it is evident that the two silicon loading steps (silicon pre-loading + silicon deposition) in this preparation method are necessary and irreplaceable for improving the tap density and silicon content. Based on the test results of Comparative Example 2 (no silicon deposition and carbon coating on the exterior of the carbon skeleton) and Comparative Example 4 (direct ball milling preparation), the overall necessity of this preparation method is demonstrated.

[0079] II. Electrochemical performance testing: The silicon-carbon anode materials of Examples 1-9 and Comparative Examples 1-5 were assembled into coin cells using the same process, and charge-discharge tests were conducted at 0.1C using an electrochemical workstation. The test results are shown in Table 2.

[0080] Negative electrode preparation process: The above silicon-carbon negative electrode material, carboxymethyl cellulose, and SuperP were weighed in a mass ratio of 90:5:5. After adding an appropriate amount of deionized water, the mixture was ball-milled for 1 hour. The slurry was coated onto the current collector and dried under vacuum at 80°C for 12 hours before being cut into negative electrode sheets with a diameter of 1.2 cm. Lithium metal sheets were used as positive electrode sheets. The electrolyte was LiPF6 with a concentration of 1 mol / L. The solvent was a mixed solution of diethyl carbonate and ethylene carbonate with a volume ratio of 1:1. The coin cells were assembled in a glove box filled with Ar.

[0081] Table 2 Electrochemical performance test results

[0082] Combining the results in Tables 1 and 2, it can be seen that compared to Comparative Examples 1-3 and 5, the silicon-carbon anode materials in Examples 1-9 have a higher silicon content, resulting in higher capacity and first-efficiency of the assembled coin cells. Compared to Comparative Examples 1-5, the silicon-carbon anode materials in Examples 1-9 do not have exposed silicon particles on their surface, leading to better cycle performance of the assembled coin cells. In contrast, the silicon-carbon anode materials in Comparative Examples 1-5 have low silicon content or low tap density, and exposed silicon particles on their surface, resulting in lower capacity and first-efficiency of the assembled coin cells, and worse cycle performance.

[0083] In summary, the silicon-carbon anode material and its preparation method, as well as the lithium-ion battery of the embodiments of this application, have good silicon coating properties and high silicon content, resulting in high tap density; the battery has high capacity and initial efficiency, and good cycle performance.

[0084] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A silicon-carbon negative electrode material, characterized by, It comprises: a porous silicon-containing carbon skeleton, which comprises a carbon skeleton with a porous structure, and a first silicon material inside the carbon skeleton; a second silicon material, which is attached to the surface and the porous structure of the carbon skeleton, and which, together with the porous silicon-containing carbon skeleton, constitutes a matrix; a carbon coating layer covering the surface of the matrix.

2. The silicon-carbon negative electrode material of claim 1, wherein, The content of the first silicon material in the porous silicon-containing carbon skeleton accounts for 5-10 wt% of the mass of the silicon-carbon negative electrode material, and the total content of the first silicon material and the second silicon material accounts for 60-70 wt%.

3. A method for preparing a silicon-carbon negative electrode material, characterized by, It comprises the following steps: S1, mixing biomass carbon-based precursor, silicon nanoparticles and pore-forming agent, first performing activation treatment to separate the biomass in the biomass carbon-based precursor and make the silicon nanoparticles enter the inside of the biomass, and then performing carbonization treatment to obtain a porous silicon-containing carbon skeleton; S2, performing silicon deposition treatment on the porous silicon-containing carbon skeleton to obtain a matrix, and then performing carbon coating treatment on the matrix.

4. The method of claim 3, wherein the silicon-carbon negative electrode material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon negative electrode material. In step S1, the biomass carbon-based precursor comprises one or at least two of hemp fibers, wood fibers, coconut shells, straw, bamboo, apricot shells and walnut shells; And / or, the particle size of the biomass carbon-based precursor is 1-20 mm, and the particle size of the silicon nanoparticles is 2-10 nm; And / or, the mass ratio of the biomass carbon-based precursor to the silicon nanoparticles is 1-2:1; And / or, the pore-forming agent comprises one or at least two of zinc chloride, magnesium chloride, potassium chloride, calcium chloride, aluminum chloride, iron chloride and phosphoric acid; And / or, the concentration of the pore-forming agent is 0.5-3 mol / L.

5. The method of claim 3, wherein the silicon-carbon negative electrode material is prepared by the steps of: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon negative electrode material. In step S1, the method of activation treatment comprises stirring at 30-60°C for 1-3h; And / or, the method of carbonization treatment comprises carbonization in an inert gas atmosphere at 500-1000°C for 2-5h; And / or, after the carbonization treatment, acid washing, water washing and drying treatment are performed to obtain the silicon-containing carbon skeleton.

6. The method of producing a silicon-carbon negative electrode material according to claim 3, characterized by, In step S2, the method of silicon deposition treatment comprises deposition in a mixed atmosphere of inert gas and silicon source gas at 450-600°C for 3-7h.

7. The method of producing a silicon-carbon negative electrode material according to claim 3, characterized by, In step S2, the carbon coating treatment comprises carbon deposition treatment and non-gas-phase carbon coating of the matrix in sequence.

8. The method of claim 7, wherein the silicon-carbon negative electrode material is prepared by a process comprising: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon negative electrode material. The method of carbon deposition treatment comprises deposition in a mixed atmosphere of inert gas and carbon source gas at 450-650°C for 2-5h.

9. The method of claim 7, wherein the silicon-carbon negative electrode material is prepared by a process comprising: mixing a silicon source and a carbon source to form a mixture; and heating the mixture to form the silicon-carbon negative electrode material. The method of non-gas-phase carbon coating comprises fusing the matrix after carbon deposition treatment and a carbon source material, and then performing carbonization treatment; wherein the mass ratio of the matrix after carbon deposition treatment to the carbon source material is 2-4:1, and the carbon source material comprises one or at least two of pitch, resin, glucose, starch, polyvinyl alcohol and polypyrrole; the method of carbonization treatment comprises carbonization in an inert atmosphere at 700-1200°C for 2-5h.

10. A lithium-ion battery, characterized by, It comprises a positive electrode sheet and a negative electrode sheet, the negative electrode sheet comprising a silicon-carbon negative electrode material as claimed in claim 1 or 2 or a silicon-carbon negative electrode material produced by the production method as claimed in any one of claims 3 to 9.

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