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

Abstract

This application belongs to the field of lithium-ion battery material preparation technology, and more specifically, relates to a silicon-carbon anode material for lithium batteries, its preparation method, anode sheet, and battery. This invention employs an improved magnesothermic reduction method, carbon coating, and secondary pore-forming technology to obtain a high specific surface area silicon-carbon anode material with a microporous-mesoporous structure. The prepared silicon-carbon anode material has a three-dimensional hierarchical porous structure, which can provide buffer space for the volume expansion of silicon during cycling. The presence of the porous carbon coating layer improves the electronic conductivity of the material and accelerates the electron and ion transport rates during charging and discharging. When used as the anode active material in the preparation of lithium-ion batteries, it exhibits high reversible capacity, first-cycle efficiency, and cycle stability, at 0.2 A g. ‑1 After 150 cycles at the current density, it maintains 1529.4 mAh g. ‑1 Its capacity can be applied to the manufacture of high-energy-density lithium-ion batteries.

Description

Technical Field

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

[0002] Lithium-ion batteries, as high-performance energy storage devices, are widely used in portable electronic devices and electric vehicles. However, the theoretical capacity of traditional graphite anode materials is limited (only 372 mAh / g), restricting further improvements in the energy density of lithium batteries. Silicon-based materials, due to their high theoretical capacity (4200 mAh / g) and low lithium intercalation potential (approximately 0.2 V vs. Li), offer a more competitive advantage. + Silicon (Li₂O₃) and its abundant reserves make it a strong candidate material for next-generation lithium-ion battery anodes. However, silicon undergoes significant volume changes during charge and discharge (expansion rate exceeding 300%), leading to particle pulverization, active material shedding, and repeated rupture and reconstruction of the solid electrolyte interphase (SEI) film, resulting in rapid capacity decay and low coulombic efficiency. Furthermore, silicon has poor intrinsic conductivity (approximately 10⁻⁶ ppm). - ³ S / cm) and low lithium-ion diffusion coefficient (10 - ¹ 4 -10 - (¹³ cm² / s), which further limits its electrochemical performance.

[0003] To overcome these challenges, existing technologies for preparing high-energy-density silicon-carbon composite materials mainly focus on three aspects: material nanostructuring, porosity, and carbon coating. Nanostructuring strategies reduce stress concentration and alleviate volume effects by preparing nano-silicon particles, nanowires, or nanofilms. For example, Ortaboy et al. reported a manganese oxide-modified carbonized porous silicon nanowire array material (Energy Environ. Sci., 2017, 10, 1505-1516), aiming to mitigate volume changes and improve charge transport using a one-dimensional nanowire structure. However, this process is complex and costly, and the high specific surface area of ​​the nanoparticles exacerbates side reactions.

[0004] Porous structure design provides expansion space by introducing pores into silicon anode materials, thereby improving ion transport efficiency. Traditional methods include chemical etching and the magnesium thermal reduction of silicon dioxide (SiO2). Chemical etching typically uses hydrofluoric acid (HF) to etch single-crystal silicon. HF is highly corrosive and hazardous, and the resulting pollutants are difficult to treat. The traditional SiO2 magnesium thermal reduction method often involves temperatures around 700℃ and is an exothermic reaction; the enormous heat easily causes sintering of the porous silicon product. HF is also needed during the synthesis process to remove excess SiO2. For example, Chen Ke et al. reported the preparation of nanoporous silicon using SiO2 aerogel as a template via magnesium thermal reduction (Template-limited preparation and performance of porous silicon via magnesium thermal reduction, Acta Physico-Chimica Sinica, Vol. 27, No. 11, 2719-2725, November 2011), which is a high-temperature metallothermic reduction process with high energy consumption and high pollution. Furthermore, SiO2 has poor pore uniformity, and high-temperature treatment easily leads to pore collapse.

[0005] Molecular sieves, a type of silicate or aluminosilicate material with abundant pore structures, are considered excellent templates for preparing porous silicon. Zhu et al. reduced MCM-48 molecular sieves to porous silicon using a magnesothermic reduction method (Journal of Materials Science, 46 (2011), 7223-7227). However, the traditional magnesothermic reduction method for preparing porous silicon still has many limitations. During the high-temperature reduction process, the large amount of heat released during the reaction causes the porous silicon to agglomerate and clump, thus affecting the electrochemical performance of the material; moreover, porous silicon has low intrinsic conductivity, which is not conducive to electron transport.

[0006] Carbon coating technology combines silicon and carbon materials, utilizing the electrical conductivity and mechanical stability of carbon to improve conductivity and suppress expansion. Traditional mechanical ball milling directly mixes silicon powder and carbon materials to obtain silicon-carbon composites (Si / C). However, the resulting Si / C particles are prone to agglomeration, and it is difficult to control the thickness and uniformity of the carbon coating layer, thus affecting conductivity and structural stability, and reducing the battery's initial coulombic efficiency and cycle life. Other methods, such as chemical vapor deposition (CVD), can achieve more uniform carbon coating, but require specialized equipment and are more expensive. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this application is to provide a multi-level porous silicon-carbon composite material with uniform pores, low cost, simple process, and environmental friendliness, as well as its preparation method. When applied to the negative electrode of lithium-ion batteries, it has advantages such as good cycle stability, high specific capacity, and high initial coulombic efficiency.

[0008] To achieve the above objectives, in a first aspect, this application provides a method for preparing a silicon-carbon anode material, comprising the following steps: S1. Mix and ball-mill the all-silica molecular sieve and magnesium silicide to obtain a mixed raw material; S2. The mixed raw materials are subjected to a magnesium thermoelectric reduction reaction under a protective atmosphere. After the reaction is completed, the mixture is cooled, washed, and dried to obtain porous silicon nanoparticles. S3. The porous silicon nanoparticles are mixed and dispersed with a carbon source in an organic solvent. After being fully dispersed, the organic solvent is removed by drying and then heated to carbonize, thereby obtaining a primary porous silicon-carbon composite material. S4. The primary porous silicon-carbon composite material is ball-milled and mixed with a pore-forming agent, and then sintered to create pores again, thereby obtaining a multi-level porous silicon-carbon composite material.

[0009] Preferably, the all-silica molecular sieve is Silicalite-1 or MCM-41; the mass ratio of the all-silica molecular sieve to magnesium silicide is 1:1 to 1:2.

[0010] Preferably, the temperature of the magnesium thermal reduction reaction in step S2 is 650-750 °C, the reaction time is 5-6 h, and the protective atmosphere is N2 or Ar.

[0011] Preferably, the carbon source in step S3 is one or more of asphalt, lignocellulose, chitosan, glucose, sucrose, and citric acid; the organic solvent is tetrahydrofuran and / or toluene; and the mass ratio of the porous silicon nanoparticles to the carbon source is 0.75:1-2:1.

[0012] Preferably, the carbonization in step S3 is carried out at a temperature of 700-800 ℃ and a carbonization time of 2-3 h.

[0013] Preferably, the pore-forming agent in step S4 is one or more of potassium chloride, sodium chloride, and zinc chloride; the mass ratio of the primary porous silicon-carbon composite material to the pore-forming agent is 1:0.5-1:1.

[0014] Preferably, the sintering in step S4 is performed at a sintering temperature of 650-750 ℃ ​​and a sintering time of 2-3 h.

[0015] According to another aspect of the present invention, a silicon-carbon anode material is provided, which is prepared by the preparation method described above.

[0016] According to another aspect of the present invention, a negative electrode sheet is provided, comprising the aforementioned silicon-carbon negative electrode material.

[0017] According to another aspect of the invention, a lithium battery is provided, comprising the aforementioned negative electrode.

[0018] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: (1) This invention employs an improved magnesothermic reduction method, carbon coating, and secondary pore-forming technology to obtain a high specific surface area silicon-carbon anode material with a microporous-mesoporous structure. The prepared silicon-carbon anode material has a three-dimensional hierarchical porous structure, which can provide a buffer space for the volume expansion of silicon during cycling. The presence of the porous carbon coating layer improves the electronic conductivity of the material and accelerates the electron and ion transport rate during charging and discharging. When used as the anode active material to prepare lithium-ion batteries, it exhibits high reversible capacity, first-time efficiency, and cycle stability at 0.2 A g. -1 After 150 cycles at the current density, it maintains a capacity of 1529.4 mAh g. -1 Its capacity can be applied to the manufacture of high-energy-density lithium-ion batteries.

[0019] (2) This invention uses all-silicon molecular sieves as the silicon source and magnesium silicide as the reducing agent. During the magnesian reduction process, magnesium vapor can easily enter the inner surface of the molecular sieve and react fully with SiO2. The heat of reaction can be effectively dissipated, thereby preventing excessive sintering of silicon nanocrystals and obtaining porous silicon materials with high specific surface area and pore volume. Moreover, the heat of reaction between Mg2Si and SiO2 is reduced by 62.8% compared with traditional magnesian reduction, which greatly reduces the heat release of the reaction and fundamentally solves the problem of high heat generation, preventing sintering. This invention uses molecular sieves as the silicon source to prepare porous silicon, which has the advantages of preventing sintering, readily available raw materials, and simple process, and does not require the use of corrosive hydrofluoric acid.

[0020] (3) After preparing carbon-coated porous silicon nanomaterials, the present invention introduces a pore-forming agent for secondary pore formation, resulting in a novel hierarchical porous silicon-carbon material with a "porous silicon core-porous carbon layer" structure. The carbon coating and carbon layer pore formation steps are carried out separately. Compared with directly mixing porous nano-silicon, carbon source and pore-forming agent in one step for carbon coating and carbon layer pore formation, the experiment shows that the present invention performs carbon coating and secondary pore formation in steps, which is more conducive to improving the electrochemical performance of the obtained hierarchical porous silicon-carbon composite material. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the preparation of a hierarchical porous silicon-carbon anode material according to an embodiment of the present invention; Figure 2 A scanning electron microscope image of the silicon-carbon anode material prepared in Example 1 is shown; Figure 3 The X-ray diffraction pattern of the hierarchical porous silicon-carbon anode material prepared in Example 1 is shown. Figure 4 The contents of the hierarchical porous silicon-carbon anode material prepared in Example 1 are shown as: a) nitrogen adsorption-desorption curve and b) pore size distribution diagram. Figure 5The charge-discharge cycle performance of the multi-level porous silicon-carbon anode materials prepared in Example 1 and Comparative Example 1 after being assembled into batteries is shown. Figure 6 A scanning electron microscope image of the hierarchical porous silicon-carbon anode material prepared in Example 2 is shown. Figure 7 The following diagrams are shown: a) nitrogen adsorption-desorption curve and b) pore size distribution diagram of the hierarchical porous silicon-carbon anode material prepared in Example 2. Figure 8 The charge-discharge cycle performance of the porous silicon anode materials prepared in Examples 1 and 2 after being assembled into batteries is shown. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0023] This invention provides a method for preparing a silicon-carbon anode material, such as... Figure 1 As shown, it includes the following steps: S1. Mix and ball-mill the all-silica molecular sieve and magnesium silicide to obtain a mixed raw material; S2. The mixed raw materials are subjected to a magnesium thermoelectric reduction reaction under a protective atmosphere. After the reaction is completed, the mixture is cooled, washed, and dried to obtain porous silicon nanoparticles. S3. The porous silicon nanoparticles are mixed and dispersed with a carbon source in an organic solvent. After being fully dispersed, the organic solvent is removed by drying and then heated to carbonize, thereby obtaining a primary porous silicon-carbon composite material. S4. The primary porous silicon-carbon composite material is ball-milled and mixed with a pore-forming agent, and then sintered to create pores again, thereby obtaining a multi-level porous silicon-carbon composite material.

[0024] In a preferred embodiment, the all-silica molecular sieve is Silicalite-1 or MCM-41; the mass ratio of the all-silica molecular sieve to magnesium silicide is 1:1-1:2, more preferably 1:1.2-1:2.

[0025] In some embodiments, during the ball milling process in step S1, the balls are placed in the milling jar at a ball-to-material ratio of 8:1 to 10:1, and the total volume of the all-silica molecular sieve and magnesium silicide does not exceed 2 / 3 of the volume of the milling jar. The mixture is mechanically mixed at a speed of 300-400 r / min for 3-4 h, so that Mg2Si can be well coated on the surface of the molecular sieve.

[0026] In some embodiments, the temperature of the magnesium thermal reduction reaction in step S2 is 650-750 °C, the reaction time is 5-6 h, and the protective atmosphere is N2 or Ar.

[0027] In some embodiments, step S2 involves uniformly spreading the mixed raw materials on the bottom of a magnetic boat, placing it in a tube furnace, and heating it to 650-750 °C under a protective gas atmosphere to carry out a magnesothermic reduction reaction. After maintaining this temperature for 5-6 hours, the mixture is allowed to cool naturally. The reduced product is then immersed in a dilute hydrochloric acid solution of a certain concentration (e.g., 1-10 wt%) for 6-8 hours to completely remove MgO impurities. After centrifugation, the mixture is washed 2-4 times with water and ethanol respectively, and then dried overnight in a vacuum drying oven at 60-90 °C to obtain porous silicon nanoparticles.

[0028] Preferably, the silicon source in step S1 is an all-silica molecular sieve, Silicalite-1 or MCM-41. All-silica molecular sieves have a high specific surface area and also possess mesopores and micropores, making them an excellent silicon source and hierarchical pore template. During the magnesothermic reduction process, magnesium vapor can easily enter the inner surface of the molecular sieve and react fully with SiO2; moreover, the heat of reaction can be effectively dissipated, thereby preventing excessive sintering of silicon nanocrystals and obtaining porous silicon materials with high specific surface area and pore volume.

[0029] The reducing agent in step S1 is Mg₂Si. The reaction formula between Mg₂Si and SiO₂ is as follows: Thermodynamic calculations yielded a reaction exothermic of 214 KJ; while the reaction equation for the traditional reducing agent magnesium powder with SiO2 is... Thermodynamic calculations yielded a reaction heat release of 576 KJ. Comparing the reaction heat releases of the two methods, it can be seen that the reaction heat of Mg2Si with SiO2 is reduced by 62.8% compared to the traditional magnesium thermal reduction, significantly reducing the reaction heat release and fundamentally solving the problem of high heat generation, thus preventing sintering.

[0030] In some embodiments, the carbon source in step S3 is one or more selected from asphalt, lignocellulose, chitosan, glucose, sucrose, and citric acid; the organic solvent is tetrahydrofuran and / or toluene; and the mass ratio of the porous silicon nanoparticles to the carbon source is 0.75:1-2:1. The carbonization in step S3 is carried out at a temperature of 700-800 °C for 2-3 hours.

[0031] In some embodiments, porous silicon nanoparticles and a carbon source are mixed and added to an organic solvent such as a tetrahydrofuran solution. The mixture is then placed in a water bath at 70-80 °C and stirred at 600-800 r / min for 2-3 h. The sample is then placed in a vacuum drying oven at 60-90 °C and dried overnight to completely remove the tetrahydrofuran solvent. Finally, the sample is heated to 700-800 °C and held for 2-3 h under a protective gas atmosphere to carbonize, thereby obtaining a primary porous silicon-carbon composite material.

[0032] The pore-forming agent described in step S4 of this invention is used to create secondary pores in the primary porous silicon-carbon composite material. In some embodiments, the pore-forming agent in step S4 is one or more of potassium chloride, sodium chloride, and zinc chloride; the mass ratio of the primary porous silicon-carbon composite material to the pore-forming agent is in the range of 1:0.5-1:1. The sintering in step S4 is performed at a temperature of 650-750℃ for 2-3 hours.

[0033] In some embodiments of the present invention, the protective gas atmosphere or protective atmosphere refers to an argon or nitrogen atmosphere.

[0034] The present invention also provides a silicon-carbon anode material prepared according to the above method, an anode sheet containing the silicon-carbon anode material, and a lithium battery containing the anode sheet.

[0035] This invention provides a hierarchical porous silicon-carbon anode material prepared according to the above method. The hierarchical porous silicon-carbon anode material prepared by this invention has a three-dimensional porous structure with numerous mesopores and micropores on its surface and interior, resulting in a high specific surface area. Furthermore, silicon nanoparticles are uniformly embedded within the pores, providing expansion space for the silicon. Moreover, the hierarchical porous silicon-carbon anode material can undergo an alloying reaction with lithium, exhibiting good cycle stability, high initial coulombic efficiency, and high specific capacity when applied to lithium-ion batteries.

[0036] This invention also provides a lithium-ion battery comprising the aforementioned hierarchical porous silicon-carbon anode material. The hierarchical porous silicon-carbon anode material prepared by this invention possesses abundant pore size distribution and high specific surface area, significantly improving battery performance when applied to lithium-ion batteries. Specifically, the high specific surface area ensures sufficient active sites, while the three-dimensional interconnected microporous-mesoporous network enables rapid electrolyte wetting, thereby significantly improving the utilization rate of active materials and the first coulombic efficiency. The hierarchical channels within the material effectively buffer the volume expansion of silicon during lithium insertion / extraction, reducing mechanical stress on the electrode material and thus enhancing cycle stability. The open channel system provides a rapid transport path for ions and electrons, ensuring the material's rapid charge / discharge capability. Furthermore, after long-term cycling, the material maintains its original pore structure and mechanical integrity, preventing particle pulverization or active material shedding, ensuring the battery's long-lasting performance.

[0037] The embodiments of the present invention are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally in accordance with conventional conditions.

[0038] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0039] Example 1 S1. Weigh 1 g of pure silicon Silicalite-1 molecular sieve and 1.28 g of magnesium silicide, and place them in a ball mill jar with grinding balls at a ball-to-material ratio of 8:1. Mechanically mix at 300 r / min for 3 h. Spread the ball-milled mixture evenly in a ceramic boat, transfer it to a tube furnace, purge the protective gas in the furnace with Ar gas, raise the temperature to 650℃ at a heating rate of 5℃ / min, maintain for 6 h, and then allow it to cool naturally to room temperature. Soak the obtained product in a 1% (w / w) dilute hydrochloric acid solution for 8 h, then centrifuge, wash, and dry three times with anhydrous ethanol and deionized water. Place the product in a vacuum drying oven at 60℃ overnight to obtain porous silicon nanoparticles.

[0040] S2. Porous silica and pitch were added to a tetrahydrofuran solution at a mass ratio of 2:1 and placed in a 70°C water bath. The mixture was stirred at 600 rpm for 2 h. The sample was then placed in a 60°C vacuum drying oven overnight. The dried powder was then transferred to a tube furnace, and the protective gas inside the furnace was replaced with Ar gas. The temperature was increased to 700°C at a rate of 5°C / min and held for 2 h. The mixture was then allowed to cool naturally to room temperature.

[0041] S3. The dried powder and potassium chloride are ball-milled and mixed at a mass ratio of 1.5:1. The mixture is then transferred to a tube furnace, and Ar gas is introduced to replace the protective gas in the furnace. The temperature is raised to 650°C at a rate of 5°C / min and held for 2 hours. The mixture is then allowed to cool naturally to room temperature. The potassium chloride is removed by washing with deionized water. After drying, a multi-level porous silicon-carbon composite material is obtained.

[0042] S4. Then, using the obtained silicon-carbon composite material as the negative electrode material, the negative electrode material, conductive additive carbon black, and binder polyacrylic acid were weighed according to a mass ratio of 8:1:1, and a slurry was prepared at room temperature using a mixer. The prepared slurry was evenly coated onto copper foil and dried in a forced-air drying oven at 55°C for 2 hours. It was then cut into circular electrode sheets with a diameter of 14 mm and vacuum-dried in a vacuum drying oven at 100°C for 8 hours to obtain silicon-carbon negative electrode sheets. The negative electrode sheets were removed, and a coin cell was assembled using lithium metal as the counter electrode, a glass fiber membrane as the separator, and 1M lithium hexafluorophosphate (LiPF6) + ethyl methyl carbonate (EMC) / ethylene carbonate (EC) / dimethyl carbonate (DMC) (1 / 1 / 1, v / v / v) as the electrolyte. The entire battery assembly process was completed in a glove box.

[0043] Comparative Example 1 The preparation method for porous silicon nanoparticles in step S1 is the same as in Example 1.

[0044] S2. Porous silica, pitch, and potassium chloride were added to a tetrahydrofuran solution in a mass ratio of 2:1:2. The solution was placed in a 70°C water bath and stirred at 600 r / min for 2 h. The sample was then placed in a 60°C vacuum drying oven overnight. Finally, the dried powder was transferred to a tube furnace, and the protective gas inside the furnace was replaced with Ar gas. The temperature was increased to 700°C at a rate of 5°C / min and held for 2 h. The mixture was then allowed to cool naturally to room temperature. Potassium chloride was removed by washing with deionized water to obtain a hierarchical porous silica-carbon composite material.

[0045] Step S3 is omitted.

[0046] Step S4, the preparation steps for the silicon-carbon negative electrode, are the same as in Example 1.

[0047] Figure 2 The image shown is a scanning electron microscope image of the hierarchical porous silicon-carbon anode material obtained in Example 1. The results show that the hierarchical porous silicon-carbon anode material retains the three-dimensional structure of Silicalite-1 molecular sieve, with dense micropores and mesopores distributed on the surface and inside.

[0048] Figure 3 The X-ray diffraction pattern of the multi-porous silicon-carbon anode material obtained in Example 1 shows that its diffraction peaks correspond to the characteristic diffraction peaks of crystalline silicon (PDF#77-2108), indicating good crystallinity.

[0049] Figure 4 The nitrogen adsorption-desorption curve (content a) and pore size distribution diagram (content b) of the hierarchical porous silicon-carbon anode material obtained in Example 1 have a specific surface area and pore volume of 209 m³. 2 / g and 0.27 cm 3 / g.

[0050] Comparative Example 1 is the same as Example 1 except that after preparing carbon-coated porous silicon nanomaterials in Example 1, a pore-forming agent is introduced for secondary pore formation, resulting in a multi-level porous silicon-carbon material with a "porous silicon core-porous carbon layer" structure; while in Comparative Example 1, porous nano-silicon, carbon source and pore-forming agent are mixed in one step for carbon coating and carbon layer pore formation. Figure 5 The graphs show the charge-discharge cycle performance of the multi-porous silicon-carbon anode materials obtained in Example 1 and Comparative Example 1 after assembly into batteries at a current density of 0.2 A / g. The results show that the first-cycle coulombic efficiency of the electrode in Comparative Example 1 (pSi@C) is 84%, and from the second week onwards, the electrode begins stable reversible cycling, with a capacity of 1104.9 mAh / g after 150 cycles. In contrast, the first-cycle coulombic efficiency of the electrode in Example 1 (pSi@C) is 87%, and from the second week onwards, the electrode begins stable reversible cycling, with a capacity of 1529.4 mAh / g after 150 cycles. This demonstrates that Example 1, by performing carbon-coated porous silicon and pore re-creation in two steps, yields a silicon-carbon anode material with significantly improved cycle performance compared to the silicon-carbon anode material obtained in the single-step pore-creation method of Comparative Example 1. The capacity after 150 cycles is increased by nearly 40%, leading the cycle capacity performance of similar materials in the art.

[0051] Example 2 S1. Weigh 1 g of pure silicon MCM-41 molecular sieve and 1.28 g of magnesium silicide, and place them in a ball mill jar with grinding balls at a ball-to-material ratio of 8:1. Mechanically mix at 300 r / min for 3 h. Spread the ball-milled mixture evenly in a ceramic boat, transfer it to a tube furnace, purge the protective gas in the furnace with Ar gas, raise the temperature to 650℃ at a heating rate of 5℃ / min, hold for 6 h, and then allow it to cool naturally to room temperature. Soak the obtained product in a 1% (w / w) dilute hydrochloric acid solution for 8 h, then centrifuge, wash, and dry three times with anhydrous ethanol and deionized water. Place the product in a vacuum drying oven at 60℃ overnight to obtain porous silicon nanoparticles.

[0052] S2. Porous silica and pitch were added to a tetrahydrofuran solution at a mass ratio of 2:1 and placed in a 70°C water bath. The mixture was stirred at 600 rpm for 2 h. The sample was then placed in a 60°C vacuum drying oven overnight. The dried powder was then transferred to a tube furnace, and the protective gas inside the furnace was replaced with Ar gas. The temperature was increased to 700°C at a rate of 5°C / min and held for 2 h. The mixture was then allowed to cool naturally to room temperature.

[0053] S3. The dried powder and potassium chloride are ball-milled and mixed at a mass ratio of 1.5:1. The mixture is then transferred to a tube furnace, and Ar gas is introduced to replace the protective gas in the furnace. The temperature is raised to 650°C at a rate of 5°C / min and held for 2 hours. The mixture is then allowed to cool naturally to room temperature. The potassium chloride is removed by washing with deionized water. After drying, a multi-level porous silicon-carbon composite material is obtained.

[0054] S4. Then, using the obtained silicon-carbon composite material as the negative electrode material, the negative electrode material, conductive additive carbon black, and binder polyacrylic acid were weighed according to a mass ratio of 8:1:1, and a slurry was prepared at room temperature using a mixer. The prepared slurry was evenly coated onto copper foil and dried in a forced-air drying oven at 55°C for 2 hours. It was then cut into circular electrode sheets with a diameter of 14 mm and vacuum-dried in a vacuum drying oven at 100°C for 8 hours to obtain silicon-carbon negative electrode sheets. The negative electrode sheets were removed, and a coin cell was assembled using lithium metal as the counter electrode, a glass fiber membrane as the separator, and 1M lithium hexafluorophosphate (LiPF6) + ethyl methyl carbonate (EMC) / ethylene carbonate (EC) / dimethyl carbonate (DMC) (1 / 1 / 1, v / v / v) as the electrolyte. The entire battery assembly process was completed in a glove box.

[0055] Figure 6 The scanning electron microscope image of the hierarchical porous silicon-carbon anode material obtained in Example 2 shows that the size of the hierarchical porous silicon-carbon anode material is 500-1000 nm, retains the three-dimensional framework structure of MCM-41 molecular sieve, and has dense micropores and mesopores distributed on the surface and inside.

[0056] Figure 7 The nitrogen adsorption-desorption curve (content a) and pore size distribution diagram (content b) of the hierarchical porous silicon-carbon anode material obtained in Example 2 show that its specific surface area and pore volume are 161.8 m². 2 / g and 0.29 cm 3 / g.

[0057] Example 2 is the same as Example 1 except that the all-silica molecular sieve used in Example 1 is Silicalite-1, while the all-silica molecular sieve used in Example 2 is MCM-41. Figure 8 The graphs show the charge-discharge cycle performance of the silicon-carbon anode materials pSi@C (Example 2 and Example 1) after assembly into batteries at a current density of 0.2 A / g. The long-cycle performance test results show that pSi@C (Example 2) exhibits a higher initial discharge specific capacity, which may be related to the additional lithium storage sites provided by its higher micropore ratio. However, the relatively higher micropore ratio in its structure may lead to more severe electrode / electrolyte side reactions and a more unstable solid-state electrolyte interface (SEI), resulting in a capacity retention of only about 74% after 150 cycles. In contrast, although pSi@C (Example 1) has a lower initial capacity, its excellent cycle stability is likely due to the interconnected mesopore-dominated framework in its pore structure providing an effective buffer space for silicon volume expansion, mitigating structural pulverization, and promoting ion transport kinetics and stable SEI formation. The capacity retention after 150 cycles is approximately 92%.

[0058] Table 1 shows the BET nitrogen physical adsorption-desorption test data for four materials: Silicalite-1, MCM-41, pSi@C-Example 1, and pSi@C-Example 2. It can be seen that the initial molecular sieve MCM-41 has a much larger specific surface area and mesoporous distribution ratio than Silicalite-1. Interestingly, after magnesian reduction and pore structure reforming according to the embodiments of this invention, pSi@C-Example 1 exhibits a higher specific surface area and mesoporous distribution ratio than pSi@C-Example 2. It is speculated that the rich and interconnected mesoporous network is likely beneficial to lithium-ion diffusion kinetics, thus enabling pSi@C-Example 1 to exhibit superior cycling performance and rate performance.

[0059] Table 1. Changes in pore structure of silicon-carbon anode materials prepared using silicon sources and examples.

[0060] Example 3 S1. Weigh 1 g of pure silicon Silicalite-1 molecular sieve and 1.92 g of magnesium silicide, and place them in a ball mill jar with grinding balls at a ball-to-material ratio of 10:1. Mechanically mix at 400 r / min for 4 h. Spread the ball-milled mixture evenly in a ceramic boat, transfer it to a tube furnace, purge the protective gas in the furnace with N2 gas, raise the temperature to 750℃ at a rate of 10℃ / min, maintain for 5 h, and then allow it to cool naturally to room temperature. Soak the obtained product in a 10% (w / w) dilute hydrochloric acid solution for 6 h, then centrifuge, wash, and dry three times with anhydrous ethanol and deionized water. Place the product in a 90℃ vacuum drying oven overnight to obtain porous silicon nanoparticles.

[0061] S2. Porous silica and pitch were added to a toluene solution at a mass ratio of 1:1 and placed in a 70°C water bath. The mixture was stirred at 600 r / min for 2 h. The sample was then placed in a 60°C vacuum drying oven overnight. The dried powder was then transferred to a tube furnace, and the protective gas inside the furnace was replaced with Ar gas. The temperature was increased to 700°C at a rate of 5°C / min and held for 2 h. The mixture was then allowed to cool naturally to room temperature.

[0062] S3. The dried powder and zinc chloride are ball-milled and mixed at a mass ratio of 1.5:1. The mixture is then transferred to a tube furnace, and Ar gas is introduced to replace the protective gas in the furnace. The temperature is raised to 750°C at a heating rate of 5°C / min and held for 3 hours. The mixture is then allowed to cool naturally to room temperature. The zinc chloride is removed by washing with deionized water. After drying, a multi-level porous silicon-carbon composite material is obtained.

[0063] S4. Then, it is prepared as a negative electrode, with lithium metal as the counter electrode, glass fiber membrane as the separator, and 1M lithium hexafluorophosphate (LiPF6) + ethyl methyl carbonate (EMC) / ethylene carbonate (EC) / dimethyl carbonate (DMC) (1 / 1 / 1, v / v / v) as the electrolyte, and assembled into a button cell. The entire battery assembly process is completed in a glove box.

[0064] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing a silicon-carbon anode material, characterized in that, Includes the following steps: S1. Mix and ball-mill the all-silica molecular sieve and magnesium silicide to obtain a mixed raw material; S2. The mixed raw materials are subjected to a magnesium thermal reduction reaction under a protective atmosphere. After the reaction is completed, the mixture is cooled, washed, and dried to obtain porous silicon nanoparticles. S3. The porous silicon nanoparticles are mixed and dispersed with a carbon source in an organic solvent. After being fully dispersed, the organic solvent is removed by drying and then heated to carbonize, thereby obtaining a primary porous silicon-carbon composite material. S4. The primary porous silicon-carbon composite material is ball-milled and mixed with a pore-forming agent, and then sintered to create pores again, thereby obtaining a multi-level porous silicon-carbon composite material.

2. The preparation method according to claim 1, characterized in that, The all-silica molecular sieve is Silicalite-1 or MCM-41; the mass ratio of the all-silica molecular sieve to magnesium silicide is 1:1 to 1:

2.

3. The preparation method according to claim 1, characterized in that, The temperature of the magnesium thermal reduction reaction in step S2 is 650-750 °C, the reaction time is 5-6 h, and the protective atmosphere is N2 or Ar.

4. The preparation method according to claim 1, characterized in that, The carbon source in step S3 is one or more of asphalt, lignocellulose, chitosan, glucose, sucrose and citric acid; the organic solvent is tetrahydrofuran and / or toluene; and the mass ratio of the porous silicon nanoparticles to the carbon source is 0.75:1-2:

1.

5. The preparation method according to claim 1, characterized in that, The carbonization process described in step S3 involves a carbonization temperature of 700-800 ℃ and a carbonization time of 2-3 h.

6. The preparation method according to claim 1, characterized in that, The pore-forming agent in step S4 is one or more of potassium chloride, sodium chloride, and zinc chloride; The mass ratio of the primary porous silicon-carbon composite material to the pore-forming agent is 1:0.5-1:

1.

7. The preparation method according to claim 1, characterized in that, The sintering process described in step S4 involves a sintering temperature of 650-750℃ and a sintering time of 2-3 hours.

8. A silicon-carbon anode material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. A negative electrode sheet, characterized in that, It contains the silicon-carbon anode material as described in claim 8.

10. A lithium battery, characterized in that, It contains the negative electrode as described in claim 9.

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