A dual-scale nano-silicon solar cell anode material, its preparation method, and its application.

Abstract

This invention discloses a dual-scale nano-silicon battery anode material, its preparation method, and its application, belonging to the field of lithium battery technology. The preparation method includes the following steps: preparing a biomass porous carbon framework material, preparing a composite material of porous carbon and small-sized silicon particles, and preparing a composite material of carbon nanotubes coated with dual-scale nano-silicon and then co-loaded on the porous carbon framework. The application is as an anode material in lithium batteries. The dual-scale nano-silicon battery anode material prepared by this invention has a dual-scale structure that disperses stress in a hierarchical manner, inhibits crack propagation, improves the mechanical toughness of the material, alleviates the volume expansion problem of silicon during charging and discharging, extends battery cycle life, and has excellent electrochemical performance.

Description

Technical Field

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

[0002] Lithium-ion batteries (LIBs), as the current mainstream electrochemical energy storage technology, play a crucial role in consumer electronics, electric vehicles, and large-scale energy storage. However, the theoretical specific capacity of traditional graphite anodes (372 mAh / g) is insufficient to meet the development requirements of high-energy-density batteries, necessitating the development of novel high-performance anode materials. Silicon (Si)-based anodes, due to their extremely high theoretical specific capacity (pure silicon can reach 4200 mAh / g, approximately 10 times that of graphite) and suitable operating potential (0.4 V vs. Li), offer a promising alternative. + Silicon (Li₂O₃) and its abundant natural reserves are considered among the most promising next-generation anode materials. However, silicon materials exhibit a volume expansion effect of approximately 300% during charge and discharge, leading to problems such as electrode structure pulverization and continuous growth of the solid electrolyte interphase (SEI) film, which severely restricts its cycle stability. In addition, the low intrinsic conductivity of silicon also limits the improvement of rate performance.

[0003] In recent years, researchers have significantly improved the electrochemical performance of silicon-based anodes through strategies such as nanostructure design (e.g., porous silicon, silicon nanowires), composite materials (carbon coating, silicon / metal alloys), and the development of novel binders. With breakthroughs in material preparation processes and pre-lithiation technologies, silicon-based anodes are gradually moving from the laboratory to industrial applications. However, their long-term cycle stability and large-scale production costs remain key scientific issues and technological challenges that urgently need to be addressed.

[0004] Chinese patent application CN117317161A discloses a method for preparing a Chlorella-derived porous carbon / silicon composite material. The method involves first pretreating Chlorella to obtain a Chlorella precursor; then mixing nano-silicon particles, a pore-forming agent, and a hydrophilic solvent to obtain a suspension; adding the Chlorella precursor to the suspension to react and obtain an intermediate product; and finally carbonizing the intermediate product to obtain the Chlorella-derived porous carbon / silicon composite material. While this method uses biomass porous carbon to reduce costs, and the Chlorella-derived porous carbon / silicon composite material improves the conductivity of nano-silicon and provides a buffer space for silicon expansion, thus improving the material's cycle stability, it has at least the following problems: (1) This method uses one-step sintering to prepare disordered pores. In the one-step method, the decomposition of volatiles and pore formation are carried out simultaneously, resulting in pores of different sizes (randomly mixed micropores, mesopores and macropores) and a lack of hierarchical structure, i.e. disordered pores. The "disordered pores" may lead to tortuous lithium-ion transport paths, affecting rate performance. At the same time, when biomass is directly sintered at high temperature, the rapid escape of volatiles will generate internal stress, leading to particle cracking and skeleton collapse. It will also result in low strength of the molded material, making it difficult to withstand the mechanical load in subsequent processing or use.

[0005] (2) Defects in silicon dispersion and loading method: This method uses nano-silicon particles (<50nm) directly mixed with pore-forming agent, which are prone to agglomeration during carbonization; the problem of interfacial bonding between silicon and carbon matrix is ​​not solved, and they are easy to fall off during cycling; although CN117550598A first uses pore-forming agent to prepare porous carbon and then combines it with silicon, the introduction of silicon nanoparticles in this method is carried out by CVD method, which is a high-temperature decomposition of silicon source gas (such as SiH4 → Si + 2H2), which requires continuous energy input and has too high production cost; moreover, when CVD method deposits silicon nanoparticles on substrate, temperature gradient or gas flow unevenness will lead to particle distribution differences.

[0006] Therefore, it is urgent to improve upon the shortcomings of the aforementioned existing technologies. Summary of the Invention

[0007] One of the objectives of this invention is to provide a method for preparing a dual-scale nano-silicon battery anode material to solve the above-mentioned problems.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a dual-scale nano-silicon battery anode material includes the following steps: Step 1: Preparation of biomass porous carbon framework material (abbreviated as: BC): First, clean the biomass carbon source material, for example, by repeatedly washing it with deionized water. After drying, sinter it in a tube furnace filled with argon at 100-500°C for 1-10 hours. After cooling to room temperature, disperse it in an aqueous solution of 0.01-2M pore-forming agent and mix it continuously for 4-24 hours. Then heat and dry it to form a solid mixture. In the above steps, the first step is low-temperature pre-calcination, which has the following effects: (1) Deep removal of volatiles: Cellulose and hemicellulose in biomass decompose in large quantities at 200-350℃, while lignin degrades gradually at 250-500℃. Low-temperature pre-calcination can basically complete the cracking of these components, removing more than 90% of the volatiles (such as H2O, CO, CO2, CH4 and tar precursors), significantly reducing the pollution risk of subsequent high-temperature treatment; (2) Formation of preliminary carbon skeleton: Biomass begins to carbonize, generating a solid carbon matrix rich in aromatic ring structure (biochar prototype), but has not yet reached a high degree of graphitization. This structure has high thermal stability and chemical inertness; (3) Development of pore structure: After the volatiles escape, micropores and mesopores (pore size 2-50 nm) are formed inside the material, and the specific surface area increases significantly, laying the foundation for subsequent activation or catalytic applications; (4) Reconstruction of surface functional groups. Oxygen-containing functional groups (such as carboxyl and hydroxyl groups) partially decompose, and the material surface changes from hydrophilic to hydrophobic, while retaining some active sites (such as phenolic hydroxyl groups), which affects subsequent chemical modification or adsorption performance. Then, the solid mixture is sintered in a high-temperature furnace filled with argon at 500-1500°C for 1-6 hours, and after repeated washing and drying, it is modified for 1-6 hours to functionalize the porous carbon. Finally, the material was washed with deionized water and dried to obtain a biomass porous carbon framework material. Step 2: Preparation of porous carbon / small-sized silicon particle composite material (abbreviated as: BC / Si): First, 20–200 mg of the biomass porous carbon framework material (BC) obtained in step one is added to 1–20 mL of water, then 5–100 mL of ethanol is added, and the mixture is ultrasonically dispersed for 5–120 min. Then, 2–50 mL of ammonia solution is added, followed by 0.5–20 mL of silicate ester. After stirring for 1–24 h, the mixture is filtered and dried to obtain the BC / SiO2 intermediate. Then, the BC / SiO2 intermediate, NaCl and Mg are mixed in a mass ratio of (0.5-3):(5-50):(1-10) and sintered in a high-temperature furnace filled with argon at 500-1200℃ for 1-24 hours; after cooling to room temperature, impurities are removed. Finally, after washing and drying, a porous carbon / small-sized silicon particle composite material (BC / Si composite material) is obtained. Step 3: Prepare a composite material (BC / Si@Si@CNTs) in which carbon nanotubes are coated with dual-scale nano-silicon and then co-loaded onto a porous carbon framework. First, add 5-150 mg of porous carbon / small-sized silicon particle composite material (BC / Si) obtained in step two to 5-50 ml of water and ultrasonically disperse for 5-120 min; Then, 5–150 mg of large-size silicon nanoparticles were added and ultrasonically dispersed for 5–120 min; subsequently, 5–1000 mg of carbon nanotubes were added and continuously mixed for 1–24 h, followed by repeated washing and drying to obtain a composite material (BC / Si@Si@CNTs) in which carbon nanotubes are coated with dual-scale nano-silicon and co-loaded on a porous carbon framework.

[0009] To address the problem of disordered porosity in existing technologies, this invention involves pre-firing biomass materials at low temperatures in an inert atmosphere. This process partially carbonizes or decomposes the organic components, leaving behind an inorganic framework while preserving the orderliness of the precursor. The pre-firing results in a rigid but not fully dense matrix where pores are not yet fully formed, but rather an ordered "potential pore template" is retained.

[0010] To address the problems of silicon particle agglomeration and poor bonding between silicon particles and carbon matrix in existing technologies, this invention first generates SiO2 particles inside and on the surface of porous carbon. Due to the properties of porous carbon itself and the functionalization of mixed acids, SiO2 has good bonding with porous carbon. Subsequently, SiO2 is reduced to silicon nanoparticles, thus introducing small-sized silicon nanoparticles in situ, which are also less prone to agglomeration.

[0011] As a preferred technical solution In step one, the biomass carbon source material is selected from at least one of wood and its by-products, natural fibers, starches, sugars and oils, natural resins and gums, other plant materials, proteins, animal leather, gelatin, chitin and chitosan, and other animal derivatives. The pore-forming agent is selected from at least one of alkaline activators, acidic activators, salt activators, oxidizing activators, and novel activators; The high-temperature furnace is selected from at least one of the following: resistance furnace, induction furnace, microwave sintering furnace, gas furnace, special furnace, electric plasma sintering furnace, or ultra-high temperature electric arc furnace. The drying process is selected from any one of the following: hot air drying, conduction drying, radiation drying, adsorption drying, freeze drying, supercritical drying, and permeation drying.

[0012] As a further preferred technical solution The wood and by-products are selected from at least one of pine, oak, fir, birch, sawdust, wood chips, bark, and shavings; the crop residues are selected from at least one of rice straw, wheat straw, corn stalks, sorghum stalks, sugarcane bagasse, rice husks, coconut shells, and peanut shells; the natural fibers are selected from at least one of cotton, kapok, coconut fiber, flax, ramie, jute, hemp, kenaf, sisal, and abaca; the starches are selected from at least one of corn starch, potato starch, cassava starch, and wheat starch; the sugars and oils are selected from at least one of sugarcane juice, beet syrup, soybean oil, palm oil, and castor oil; the natural resins and gums are selected from at least one of rosin and turpentine; the other plant materials are selected from at least one of bamboo, cork, and algae; the proteins are selected from at least one of silk, wool, feathers, animal leather, gelatin, chitin, and chitosan; and the other animal derivatives are selected from at least one of beeswax, shellac, and animal fat. The alkaline activator is selected from at least one of potassium hydroxide, sodium hydroxide, potassium carbonate, and potassium ferrate; the acidic activator is selected from at least one of phosphoric acid and sulfuric acid; the salt activator is selected from zinc chloride and ferric chloride; the oxidizing activator is selected from at least one of nitric acid and ammonium persulfate; and the novel activator is selected from at least one of urea and metal-organic frameworks. The resistance furnace is selected from at least one of box-type, tube-type, and vacuum resistance furnaces; the gas furnace is selected from at least one of shuttle kiln and tunnel kiln; and the special furnace is a hot-pressing sintering furnace.

[0013] As a preferred technical solution In step two, the impurity removal includes: removing possible MgO, Mg2Si byproducts and excess Mg with HCl solution, and removing excess SiO2 with HF solution; Wash repeatedly with ethanol and deionized water.

[0014] As a preferred technical solution In step three, the carbon nanotubes include multi-walled carbon nanotubes and single-walled carbon nanotubes; The mixing method is any one or more of stirring, ultrasonication, ball milling or sand milling; Wash repeatedly with ethanol and deionized water during washing.

[0015] As a further preferred technical solution The carbon nanotubes have a diameter ≤100nm and a length ≤300μm. For example, the carbon nanotubes are sourced from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences.

[0016] The second objective of this invention is to provide a dual-scale nano-silicon battery anode material prepared by the above-described preparation method.

[0017] The third objective of this invention is to provide the application of the dual-scale nano-silicon battery anode material prepared by the above-mentioned method in the preparation of lithium batteries.

[0018] Specifically, for example, the above-mentioned negative electrode material, conductive agent and binder are mixed into a slurry in a mass ratio of 7:1.5:1.5 and then uniformly coated on copper foil. After the electrode is vacuum dried at 60-120°C for 8-24 hours, it is cut into circular electrode sheets with a diameter of 12 mm, and a button cell is assembled using a lithium sheet as the counter electrode.

[0019] The main inventive points of this invention include: (1) Design of dual-scale silicon nanoparticles: Large-sized silicon nanoparticles (>100nm) provide structural support and reduce agglomeration caused by volume expansion; small-sized silicon nanoparticles (<100nm) shorten the lithium-ion diffusion path and improve rate performance; the dual-scale structure disperses stress in a hierarchical manner, inhibits crack propagation, and improves the mechanical toughness of the material, which can alleviate the volume expansion problem of silicon during charging and discharging and extend the battery cycle life. (2) The "anchoring" effect of carbon nanotubes on silicon nanoparticles and their role as conductive bridges: The flexibility of carbon nanotubes allows them to tightly encapsulate silicon particles, maintaining electrical contact even as silicon expands, thus preventing the active material from detaching and failing. Simultaneously, the high aspect ratio and excellent conductivity of carbon nanotubes can form a continuous conductive network between silicon particles and between silicon particles and the porous carbon framework, significantly reducing electrode resistance and improving electron transport efficiency. In addition, carbon nanotubes provide physical insulation: carbon nanotubes partially cover the silicon surface, reducing direct contact between silicon and electrolyte and inhibiting side reactions (such as LiPF6 decomposition producing HF that corrodes silicon). (3) Biomass has a wide range of carbon sources, is green and environmentally friendly, and has excellent performance: The inherent structure of biomass can serve as a natural template to form hierarchical pores (micropores <2 nm, mesopores 2–50 nm, macropores >50 nm), eliminating the need for complex pore-forming processes. The specific surface area can be controlled through activation. The proteins and amino acids contained in biomass can retain heteroatoms after carbonization (e.g., nitrogen content can reach 2–8 at%), significantly improving the conductivity and catalytic activity of carbon materials. Some biomass carbons (such as bamboo charcoal and coconut shell carbon) have a high degree of graphitization and a compressive strength >50 MPa, making them suitable for the recycling of electrode materials. They are stable in acidic / alkaline environments and have a thermal stability >500℃ (inert atmosphere), making them suitable for harsh conditions.

[0020] Compared with the prior art, the advantages of the present invention are as follows: (1) The present invention adopts a two-step sintering method, namely, low-temperature pre-firing followed by activation to create pores. Low-temperature pre-firing first removes volatiles (cellulose, hemicellulose, etc.) from biomass to form a preliminary carbon skeleton and pore structure. The subsequent activation further expands and creates pores on the stable carbon skeleton to generate hierarchical pores, which makes the pore size distribution more uniform and the specific surface area higher. The invention discovered through numerous experiments that if there is no low-temperature pre-firing, directly reacting the activator with the undecomposed organic matter in the biomass can easily lead to excessive etching or pore collapse, forming messy channels. (2) The biomass of the present invention has a natural hierarchical porous structure. After carbonization, the porosity can be retained or further expanded to form a structure in which micropores (<2 nm), mesopores (2-50 nm) and macropores (>50 nm) coexist. The high specific surface area provides a large number of active sites, which is beneficial to electrolyte wetting and ion adsorption, improves electrode reaction kinetics, and is suitable for high-rate charge and discharge. The graphite microcrystalline structure formed after carbonization provides good electronic conductivity, the carbon skeleton structure is stable, and the volume change is small during cycling, making it suitable for long-term use. Biomass naturally contains elements such as O and N, which can form defect sites and active groups (such as carboxyl groups and pyridine nitrogen) after carbonization, thereby improving the pseudocapacitive contribution and interfacial wettability of electrode materials.

[0021] Compared to traditional fossil-based carbon materials (such as pitch coke), biomass carbon production processes are less polluting and align with the trend of green chemistry. Besides being used as battery anodes, it can also be applied to supercapacitors (high power), catalyst supports (high specific surface area), and adsorbent materials (such as wastewater treatment), among other applications, demonstrating its wide range of potential uses. (3) This invention effectively solves the key problems of traditional silicon-based anodes through the synergistic effect of multi-level size of nano-silicon: small-sized silicon nanoparticles (<100 nm) can alleviate mechanical stress during lithiation and reduce particle breakage; large-sized silicon nanoparticles (>100 nm) serve as a structural framework to inhibit the aggregation of nanoparticles and maintain the overall integrity of the electrode. Small-sized silicon nanoparticles (<100 nm) shorten the lithium-ion diffusion path and improve reaction kinetics; large-sized silicon nanoparticles (>100 nm) reduce excessive interfacial contact resistance in the electrode. The introduction of large-size silicon nanoparticles (>100 nm) can reduce the proportion of high specific surface area nano-silicon (<100 nm) and reduce costs; (4) The hollow structure and surface defects of carbon nanotubes can provide additional Li⁺ transport channels, improving the reaction kinetics of silicon-based anodes; Carbon nanotubes can be wound around the surface of silicon nanoparticles or interwoven between silicon and porous carbon frameworks, forming a "reinforced concrete" network that inhibits silicon particle expansion and pulverization during cycling. Simultaneously, the three-dimensional network formed by interwoven carbon nanotubes provides abundant porosity, promoting electrolyte penetration and enhancing Li... + Transmission efficiency; Coating with carbon nanotubes can reduce side reactions on the surface of silicon particles, preventing the continuous growth of the SEI film and electrolyte decomposition. Attached Figure Description

[0022] Figure 1 This is a nitrogen adsorption / desorption curve of the porous carbon prepared in step one of Example 1 of the present invention; Figure 2 This is a transmission electron microscope (TEM) image of BC / Si@Si@CNTs prepared in Example 1 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments. Example 1

[0024] A dual-scale nano-silicon solar cell anode material, the preparation method of which includes the following steps: Step 1: Preparation of porous carbon framework material (BC): The tea stems were repeatedly washed with deionized water to remove impurities and dust. After drying, the biomass material was sintered in a tube furnace filled with argon at 200°C for 2 hours. After cooling to room temperature, it was dispersed in a 0.12M K₂FeO₄ aqueous solution and stirred continuously for 12 hours. Then, it was heated and dried in a water bath at 80°C to form a solid mixture. The solid mixture was sintered in a high-temperature tube furnace filled with argon at 800°C for 2 hours. After repeated washing and drying, it was dispersed in a mixture of nitric acid and sulfuric acid for 2 hours to modify the porous carbon. Finally, it was washed with deionized water and dried in a vacuum oven at 80°C for 12 hours. Figure 1 The results show that the nitrogen adsorption-desorption curve of the obtained porous carbon framework material has a typical type I pattern, indicating that the material has a large specific surface area. Step 2: Prepare a composite material of porous carbon and small-sized silicon particles (BC / Si): First, 120 mg of BC was added to 5 mL of water, followed by 40 mL of ethanol. The mixture was ultrasonically dispersed for 30 min, then 10 mL of ammonia solution was added, followed by 2.5 mL of tetraethyl orthosilicate. After stirring for 6 h, the mixture was filtered and dried to obtain a BC / SiO2 intermediate. The BC / SiO2 intermediate, NaCl, and Mg were mixed at a mass ratio of 1:10:2 and sintered in a tube furnace filled with argon at 650 °C for 6 h. After cooling to room temperature, any MgO, Mg2Si byproducts, and excess Mg were removed with HCl solution, and excess SiO2 was removed with HF solution. Finally, the mixture was repeatedly washed with ethanol and deionized water and dried to obtain the BC / Si composite material. Step 3: Prepare a composite material (BC / Si@Si@CNTs) in which carbon nanotubes are coated with dual-scale nano-silicon and then co-loaded on a porous carbon framework: First, 80 mg of BC / Si was added to 20 ml of water and ultrasonically dispersed for 15 min. Then, 40 mg of commercial silicon nanoparticles (Guangzhou Hongwu Materials Technology Co., Ltd., ultrafine silicon powder, 100-200 nm, 99.9%) were added and ultrasonically dispersed for 30 min. Afterward, 37.5 mg of carbon nanotubes were added and stirred continuously for 12 h. The mixture was then repeatedly washed with ethanol and deionized water to obtain the BC / Si@Si@CNTs composite material. The transmission electron microscope (TEM) image of the prepared BC / Si@Si@CNTs composite material is shown below. Figure 2 As shown, Figure 2 In the middle: the large circle represents large-size nano-silicon, and the small circle represents small-size nano-silicon. Figure 2 It can be seen that the negative electrode material obtained by the present invention has large-sized silicon particles with a particle size >100nm and small-sized silicon particles with a particle size <100nm, and the silicon particles are almost not agglomerated and are orderly dispersed. Example 2

[0025] Compared with Example 1, this embodiment is the same as Example 1 except that in step one, the biomass material is placed in a tube furnace filled with argon at 300°C and sintered for 2 hours. Example 3

[0026] Compared with Example 1, this embodiment is the same as Example 1 except that in step one, the biomass material is placed in a tube furnace filled with argon at 400°C for 2 hours for sintering. Example 4

[0027] Compared with Example 1, this embodiment is the same as Example 1 except that in step one, the biomass material is placed in a tube furnace filled with argon at 600°C for 2 hours for sintering. Example 5

[0028] Compared with Example 1, this embodiment is the same as Example 1 except that in step one, the biomass material is placed in a tube furnace filled with argon at 600°C for 2 hours for sintering. Example 6

[0029] A dual-scale nano-silicon solar cell anode material, the preparation method of which includes: Step 1: Preparation of porous carbon framework material (BC): The tea stems were repeatedly washed with deionized water to remove impurities and dust. After drying, the biomass material was sintered in a tube furnace filled with argon at 400℃ for 2 hours. After cooling to room temperature, it was dispersed in a 0.12M K2FeO4 aqueous solution and stirred continuously for 12 hours. Then, it was heated and dried in a water bath at 80℃ to form a solid mixture. The solid mixture was sintered in a high-temperature tube furnace filled with argon at 800℃ for 2 hours. After repeated washing and drying, it was dispersed in a mixture of nitric acid and sulfuric acid for 2 hours to modify the porous carbon. Finally, it was washed with deionized water and dried in a vacuum oven at 80℃ for 12 hours. Step 2: Prepare a composite material of porous carbon and small-sized silicon particles (BC / Si): First, 120 mg of BC was added to 5 mL of water, followed by 40 mL of ethanol. The mixture was ultrasonically dispersed for 30 min, then 10 mL of ammonia solution was added, followed by 0.5 mL of tetraethyl orthosilicate. After stirring for 6 h, the mixture was filtered and dried to obtain a BC / SiO2 intermediate. The BC / SiO2 intermediate, NaCl, and Mg were mixed at a mass ratio of 1:10:2 and sintered in a tube furnace filled with argon at 650 °C for 6 h. After cooling to room temperature, any MgO, Mg2Si byproducts, and excess Mg were removed with HCl solution, and excess SiO2 was removed with HF solution. Finally, the mixture was repeatedly washed with ethanol and deionized water and dried to obtain the BC / Si composite material. Step 3: Prepare a composite material (BC / Si@Si@CNTs) in which carbon nanotubes are coated with dual-scale nano-silicon and then co-loaded on a porous carbon framework: First, 80 mg of BC / Si was added to 20 ml of water and ultrasonically dispersed for 15 min. Then, 40 mg of commercial silicon nanoparticles (Guangzhou Hongwu Materials Technology Co., Ltd., ultrafine silicon powder, 100-200 nm, 99.9%) were added and ultrasonically dispersed for 30 min. After that, 37.5 mg of carbon nanotubes were added and stirred continuously for 12 h. The mixture was then repeatedly washed with ethanol and deionized water to obtain the BC / Si@Si@CNTs composite material. Example 5

[0030] Compared with Example 6, this embodiment is the same as Example 6 except that in step two, 4.5 mL of tetraethyl orthosilicate is added to the above solution. Example 6

[0031] Compared with Example 6, this embodiment is the same as Example 6 except that in step two, 6.5 mL of tetraethyl orthosilicate is added to the above solution.

[0032] Comparative Example 1: Step 1: Preparation of porous carbon framework material (BC): Tea stems were repeatedly washed with deionized water to remove impurities and dust. After drying, they were dispersed in a 0.12M K₂FeO₄ aqueous solution and stirred continuously for 12 hours. Then, they were heated and dried in a water bath at 80°C to form a solid mixture. The solid mixture was sintered in an argon-filled high-temperature tube furnace at 800°C for 2 hours. After repeated washing and drying, it was dispersed in a mixture of nitric acid and sulfuric acid for 2 hours to modify the porous carbon. Finally, it was washed with deionized water and dried in a vacuum oven at 80°C for 12 hours. Step 2: Prepare a composite material of porous carbon and small-sized silicon particles (BC / Si): First, 120 mg of BC was added to 5 mL of water, followed by 40 mL of ethanol. The mixture was ultrasonically dispersed for 30 min, then 10 mL of ammonia solution was added, followed by 2.5 mL of tetraethyl orthosilicate. After stirring for 6 h, the mixture was filtered and dried to obtain a BC / SiO2 intermediate. The BC / SiO2 intermediate, NaCl, and Mg were mixed at a mass ratio of 1:10:2 and sintered in a tube furnace filled with argon at 650 °C for 6 h. After cooling to room temperature, any MgO, Mg2Si byproducts, and excess Mg were removed with HCl solution, and excess SiO2 was removed with HF solution. Finally, the mixture was repeatedly washed with ethanol and deionized water and dried to obtain the BC / Si composite material. Step 3: Prepare a composite material (BC / Si@Si@CNTs) in which carbon nanotubes are coated with dual-scale nano-silicon and then co-loaded on a porous carbon framework: First, 80 mg of BC / Si was added to 20 ml of water and ultrasonically dispersed for 15 min. Then, 40 mg of commercial silicon nanoparticles (Guangzhou Hongwu Materials Technology Co., Ltd., ultrafine silicon powder, 100-200 nm, 99.9%) were added and ultrasonically dispersed for 30 min. After that, 37.5 mg of carbon nanotubes were added and stirred continuously for 12 h. The mixture was then repeatedly washed with ethanol and deionized water to obtain the BC / Si@Si@CNTs composite material.

[0033] Comparative Example 2: Compared with Example 1, this comparative example is the same as Example 1 except that 40 mg of small-sized nano-silicon is added in step three.

[0034] This comparative example is intended to demonstrate that designs using only single-scale nano-silicon, rather than dual-scale nano-silicon, have poor performance.

[0035] The composite materials obtained in Examples 1-5 and Comparative Examples 1-3 were mixed with conductive agent and binder at a mass ratio of 7:1.5:1.5 to form a slurry, which was then uniformly coated onto copper foil. The electrode sheets were vacuum dried at 105°C for 12 hours and then cut into circular electrode sheets with a diameter of 12 mm. Button cells were assembled using lithium foil as the counter electrode. After the assembled cells were allowed to stand for 8 hours, a multi-channel battery testing system (Neware battery testing station) was used to test them at 0.01–3V (vs. Li). + All batteries were tested within the voltage range of / Li. The test results are shown in Table 1.

[0036] Table 1. Battery performance of different embodiments and comparative examples

[0037] Table 1 shows that Examples 1-5 involved changing the pre-calcination temperature to 200, 300, 400, 500, and 600°C, while keeping other conditions constant. The results indicate that as the pre-calcination temperature increased, the pore volume also increased with rising temperature. However, higher pre-calcination temperatures led to a decrease in initial coulombic efficiency and capacity retention. Examples 6-8 involved changing the amount of silicate ester to 0.5, 4.5, and 6.5 mL, while maintaining a pre-calcination temperature of 400°C. The results show that as the amount of silicate ester increased, the yield of small-sized nano-silicon increased, and the initial discharge capacity also increased. Meanwhile, Comparative Example 1 shows that the electrochemical data of the sample without pre-calcination was significantly lower than that of the other examples; Comparative Example 2 shows that the electrochemical data of the single-sized nano-silicon sample was also significantly lower than that of the other examples.

[0038] 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 method for preparing a dual-scale nano-silicon battery anode material, characterized in that, Includes the following steps: Step 1: Preparation of porous carbon framework materials from biomass: First, the biomass carbon source material is cleaned and dried. Then, it is sintered in a tube furnace filled with argon at 200-600℃ for 1-10 hours. After cooling to room temperature, it is dispersed in an aqueous solution of 0.01-2M pore-forming agent and continuously mixed for 4-24 hours. Then, it is heated and dried to form a solid mixture. Then, the solid mixture is sintered in a high-temperature furnace filled with argon at 500-1500°C for 1-6 hours, and after repeated washing and drying, it is dispersed in a mixture of nitric acid and sulfuric acid for 1-6 hours to modify the porous carbon. Finally, the material was washed with deionized water and dried to obtain a biomass porous carbon framework material. Step 2: Preparation of a composite material of porous carbon and small-sized silicon particles: First, add 20-200 mg of the biomass porous carbon framework material obtained in step one to 1-20 mL of water, then add 5-100 mL of ethanol, and ultrasonically disperse for 5-120 min. Then add 2-50 mL of ammonia solution, and then add 0.5-6.5 mL of silicate ester to the above solution. After stirring for 1-24 h, filter and dry to obtain the biomass porous carbon framework material / SiO2 intermediate. Then, the biomass porous carbon framework material / SiO2 intermediate, NaCl and Mg are mixed in a mass ratio of (0.5-3):(5-50):(1-10) and sintered in a high-temperature furnace filled with argon at 500-1200℃ for 1-24 hours; after cooling to room temperature, impurities are removed. Finally, after washing and drying, a porous carbon / small-sized silicon particle composite material is obtained; the particle size of the small-sized silicon particles is <100nm. Step 3: Prepare a composite material in which carbon nanotubes are coated with dual-scale nano-silicon and then co-loaded onto a porous carbon framework. First, add 5-150 mg of the porous carbon / small-sized silicon particle composite material obtained in step two to 5-50 ml of water and ultrasonically disperse for 5-120 min; Then, 5–150 mg of large-size silicon nanoparticles with a particle size >100 nm were added and ultrasonically dispersed for 5–120 min. After that, 5–1000 mg of carbon nanotubes were added and continuously mixed for 1–24 h. After repeated washing and drying, a composite material was obtained in which carbon nanotubes were coated with dual-scale nano-silicon and co-loaded on a porous carbon framework.

2. The method according to claim 1, characterized in that, In step one, the biomass carbon source material is selected from at least one of wood and its by-products, natural fibers, starches, sugars and oils, natural resins and gums, other plant materials, proteins, chitin and chitosan, and other animal derivatives; The wood and by-products are selected from at least one of pine, oak, fir, birch, sawdust, wood chips, bark, and shavings; the natural fibers are selected from at least one of cotton, kapok, coconut fiber, flax, ramie, jute, hemp, kenaf, sisal, and abaca; the starches are selected from at least one of corn starch, potato starch, cassava starch, and wheat starch; the sugars and oils are selected from at least one of sugarcane juice, beet syrup, soybean oil, palm oil, and castor oil; the natural resins and gums are selected from at least one of rosin and turpentine; the other plant materials are selected from at least one of bamboo, cork, and algae; the proteins are selected from at least one of silk, wool, feathers, animal leather, and gelatin; and the other animal derivatives are selected from at least one of beeswax, shellac, and animal fat. The pore-forming agent is selected from at least one of alkaline activators, acidic activators, salt activators, oxidizing activators, and novel activators; The alkaline activator is selected from at least one of potassium hydroxide, sodium hydroxide, potassium carbonate, and potassium ferrate; the acidic activator is selected from at least one of phosphoric acid and sulfuric acid; the salt activator is selected from zinc chloride and ferric chloride; the oxidizing activator is selected from at least one of nitric acid and ammonium persulfate; and the novel activator is selected from at least one of urea and metal-organic frameworks. The high-temperature furnace is selected from at least one of the following: resistance furnace, induction furnace, microwave sintering furnace, gas furnace, special furnace, electric plasma sintering furnace, or ultra-high temperature electric arc furnace. The drying process is selected from any one of the following: hot air drying, conduction drying, radiation drying, adsorption drying, freeze drying, supercritical drying, and permeation drying.

3. The method according to claim 2, characterized in that, The resistance furnace is selected from at least one of box-type, tube-type, and vacuum resistance furnaces; the gas furnace is selected from at least one of shuttle kiln and tunnel kiln; and the special furnace is a hot-pressing sintering furnace.

4. The method according to claim 1, characterized in that, In step two, the impurity removal includes: removing possible MgO, Mg2Si byproducts and excess Mg with HCl solution, and removing excess SiO2 with HF solution; Wash repeatedly with ethanol and deionized water.

5. The method according to claim 1, characterized in that, In step three, the carbon nanotubes include multi-walled carbon nanotubes and single-walled carbon nanotubes; The mixing method is any one or more of stirring, ultrasonication, ball milling or sand milling; Wash repeatedly with ethanol and deionized water during washing.

6. The method according to claim 5, characterized in that, The carbon nanotubes have a diameter ≤100nm and a length ≤300μm.

7. The dual-scale nano-silicon battery anode material prepared by the preparation method of any one of claims 1-6.

8. The application of the dual-scale nano-silicon battery anode material prepared by the preparation method of any one of claims 1-6 in the preparation of lithium batteries.

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