A multi-stage sandwich structure silicon-carbon material based on lignin carbon and a preparation method and application thereof

Abstract

This invention relates to a multi-level sandwich structure silicon-carbon material based on lignin-carbon, its preparation method, and its applications. The invention first involves dissolving ammoniated lignin in water and mixing it with positively charged nano-silicon to achieve electrostatic adsorption, followed by low-temperature calcination for pre-carbonization, yielding a lignin-carbon-coated silicon composite material (Si@C). Subsequently, the Si@C composite material is ball-milled and embedded with expanded graphite to obtain a lignin-carbon-coated silicon-embedded expanded graphite composite material (Si@C-EG). Finally, pitch is ball-milled and mixed with the Si@C-EG composite material, followed by high-temperature calcination to soften the pitch and allow it to penetrate between the expanded graphite layers, forming a closed macroporous sandwich structure that effectively provides space to alleviate volume expansion, thus obtaining the multi-level sandwich structure silicon-carbon material based on lignin-carbon. Verification shows that the silicon-carbon composite material prepared by this invention possesses excellent cycle stability and rate performance, and the constructed multi-level sandwich structure effectively mitigates the volume expansion of silicon. Lithium-ion half-cells assembled from the lignin-carbon-based multi-level sandwich structure silicon-carbon material prepared by this invention exhibit excellent cycle stability and charge transport capability.

Description

Technical Field

[0001] This invention belongs to the field of silicon-carbon anode materials for lithium-ion batteries, specifically relating to a multi-level sandwich structure silicon-carbon material based on lignin carbon, its preparation method, and its application. Background Technology

[0002] Energy is a crucial material foundation for human survival and the progress of social civilization. With the rapid development of the global economy and industrialization, the demand for energy has increased significantly, and energy supply has become more diversified. However, the overexploitation of non-renewable fossil fuels such as oil and coal has led to a year-on-year decline in reserves, resulting in energy shortages. Solar and wind power are affected by region, weather, and season; the clean electricity generated by these sources needs to be stored through advanced energy storage systems to ensure continuous and stable power output. Furthermore, with increasingly higher demands for 3C electronic products and new energy vehicles, and a growing market size, there is an urgent need to develop high-energy-density and high-safety secondary batteries. Currently, the capacity of widely used lithium-ion battery graphite anodes is low (372 mAh / g) and has reached its limit; there is an urgent need to develop next-generation lithium-ion battery anodes with high energy density, long cycle life, and safety and environmental friendliness. Silicon (Si) has a high theoretical capacity (4200 mAh / g) and a low operating voltage (0.4V vs. Li / Li). + Silicon anodes are considered the best choice for improving the energy density of lithium-ion batteries. However, silicon anodes still face problems such as volume expansion, easy pulverization and shedding of materials, and low electronic conductivity, resulting in poor battery stability and rate performance.

[0003] To address the bottlenecks of silicon-based materials, such as large volume expansion, poor conductivity, and unstable SEI films, researchers have developed three major technical pathways through material structure control and composite system design: (1) Nanostructured materials: Low-dimensional structural systems such as nanoparticles (20-200 nm), nanowires (diameter < 100 nm), and nanotubes have been developed through size effect control. Among them, oriented silicon nanowires (such as vertical array structures) can not only reduce the volume expansion rate to below 150%, but also establish continuous electron conduction channels. Studies have shown that three-dimensional porous silicon nanofibers (specific surface area > 200 m²) prepared by electrospinning can achieve significant improvements in volume expansion. 2 / g) After 200 cycles at 0.5C, it still maintains 82% capacity retention. However, the agglomeration problem caused by the high surface energy of nanomaterials still needs to be solved by strategies such as constructing graphene composite carriers or introducing spatial confinement structures. (2) Surface functionalization modification: Carbon coating technology has developed from single coating to inner and outer layer composite coating system. For example, CVD method grows 3-5nm graphene-like carbon layer on silicon surface, and constructs Si@C@TiO2 double shell structure to improve the conductivity and mechanical stability of electrode materials. However, the CVD method for depositing graphene carbon layer is complicated and costly, which is not conducive to promoting the industrialization of silicon-carbon anode. (3) Multi-scale composite system design: ① Introduce a three-dimensional graphene network (content 15-30wt%) to form a stress buffer skeleton through chemical bonding, thereby improving the Young's modulus of the composite material; ② Develop silicon-metal alloys (such as Si-Fe-Si gradient structures) to achieve adaptive strain adjustment by utilizing the ductility of the metallic phase; ③ Construct hollow composite materials, such as egg yolk-shell structures (void space > 40%), to effectively confine volume expansion within the shell. However, these technical approaches are complex, costly, and difficult to mass-produce, necessitating the development of green and low-cost technologies that are easy to mass-produce. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of severe volume expansion, high cost, and difficulty in mass production of silicon-carbon anodes. It provides a multi-level sandwich structure silicon-carbon material based on lignin-carbon, its preparation method, and its applications. This silicon-carbon anode material, based on the inexpensive, environmentally friendly, and mechanically stable properties of lignin, overcomes the problems of pure lignin-carbon being unable to withstand the volume expansion of silicon and being prone to pulverization. The prepared silicon-carbon anode exhibits excellent long-cycle stability and rate performance. This structure effectively alleviates the volume expansion stress of silicon during cycling, providing a new approach for the engineering application of high-silicon-content anodes and contributing to the industrialization of silicon-carbon anodes.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution.

[0006] The first aspect of this invention provides a method for preparing a multi-level sandwich structure silicon-carbon material based on lignin carbon, comprising the following steps:

[0007] (1) Add nano-silicon and surfactant to water, mix evenly by ultrasonication, wash and dry to obtain nano-silicon with positive charge on the surface (modified nano-silicon);

[0008] (2) Dissolve lignin in water, then add nano-silicon with a positive charge on the surface, let it settle so that the lignin is electrostatically adsorbed on the surface of the nano-silicon, filter and dry, and then calcine in a tube furnace to fix the carbon layer to obtain lignin carbon coated with nano-silicon (Si@C).

[0009] (3) After mixing lignin-coated silicon nanoparticles and expanded graphite, a solvent was added to form a spheroidal graphite to achieve uniform mixing, thus obtaining a lignin-coated silicon-embedded expanded graphite composite material (Si@C-EG).

[0010] (4) Add pitch to the lignin-carbon coated silicon-embedded expanded graphite composite material obtained in step (3) and ball mill again. After drying the solvent, calcine the obtained powder material at high temperature to obtain a multi-level sandwich structure silicon-carbon material (Si@C-EGC composite material) based on lignin-carbon.

[0011] Preferably, the surfactant in step (1) is selected from hexadecyltrimethylammonium bromide (CTAB).

[0012] Preferably, the mass ratio of nano-silicon to surfactant in step (1) is (8-1):1; more preferably, the mass ratio of nano-silicon to surfactant is (2-5):1; most preferably, the mass ratio of nano-silicon to surfactant is 2:1.

[0013] Preferably, the ultrasonic mixing time in step (1) is 10-120 min; more preferably, the ultrasonic mixing time is 20-60 min; most preferably, the ultrasonic mixing time is 30 min.

[0014] Preferably, the particle size of the nano-silicon in step (1) is 10-200 nm; more preferably, the particle size of the nano-silicon is 30-100 nm.

[0015] Preferably, the drying temperature in step (1) is 50-120°C and the drying time is 2-8h; more preferably, the drying temperature is 70-80°C and the drying time is 3-5h.

[0016] Preferably, the lignin in step (2) is selected from one or more of alkali lignin, enzymatically hydrolyzed lignin, and ammonia-oxidized lignin; more preferably, the lignin is selected from ammonia-oxidized lignin.

[0017] Preferably, the mass ratio of lignin to positively charged nano-silicon in step (2) is 1:(1-8); more preferably, the mass ratio of lignin to positively charged nano-silicon is 1:(2-5); most preferably, the mass ratio of lignin to positively charged nano-silicon is 1:4.

[0018] Preferably, the settling time in step (2) is 1-15 hours and the temperature is 10-50°C; more preferably, the settling time is 6-14 hours and the temperature is 20-30°C; most preferably, the settling time is 12 hours and the temperature is 25°C.

[0019] Preferably, the calcination temperature in step (2) is 100-350℃ and the time is 1-6h; more preferably, the calcination temperature is 250-350℃ and the time is 1-3h; most preferably, the calcination temperature is 300℃ and the time is 1h.

[0020] Preferably, the solvent in step (3) is selected from ethanol, NMP (N-methylpyrrolidone), and ethylene glycol; more preferably, the solvent is selected from NMP.

[0021] Preferably, the ball milling speed in step (3) is 400-700 rpm and the time is 1-10 h; more preferably, the ball milling speed is 450-650 rpm and the time is 2-8 h; most preferably, the ball milling speed is 600 rpm and the time is 3 h.

[0022] Preferably, the mass ratio of lignin-carbon-coated nano-silicon to expanded graphite in step (3) is (1-6):1; more preferably, the mass ratio of lignin-carbon-coated nano-silicon to expanded graphite is (1-3):1; most preferably, the mass ratio of lignin-carbon-coated nano-silicon to expanded graphite is 2:1.

[0023] Preferably, the mass ratio of the lignin-carbon-coated silicon-embedded expanded graphite composite material in step (3) to the asphalt in step (4) is (1-6):1; more preferably, the mass ratio of the lignin-carbon-coated silicon-embedded expanded graphite composite material to the asphalt is (2-5):1; most preferably, the mass ratio of the lignin-carbon-coated silicon-embedded expanded graphite composite material to the asphalt is 4:1.

[0024] Preferably, the ball milling speed in step (4) is 400-700 rpm and the time is 1-10 h; more preferably, the ball milling speed is 450-650 rpm and the time is 2-8 h; most preferably, the ball milling speed is 600 rpm and the time is 3 h.

[0025] Preferably, the high-temperature calcination in step (4) is carried out at a temperature of 100-800℃ for 1-10 hours; more preferably, the high-temperature calcination is carried out at a temperature of 400-780℃ for 2-4 hours; and most preferably, the high-temperature calcination is carried out at a temperature of 700℃ for 3 hours.

[0026] The second aspect of the present invention provides a multi-level sandwich structure silicon-carbon material based on lignin carbon prepared according to the above preparation method.

[0027] The third aspect of this invention provides the application of the above-mentioned lignin-carbon-based multi-level sandwich structure silicon-carbon material in the preparation of battery anode materials.

[0028] Preferably, the battery is selected from one or more of lithium-ion batteries and sodium-ion batteries.

[0029] This invention utilizes CTAB to modify nano-silicon, giving it a positive charge. Electrostatic repulsion effectively reduces the aggregation of the nano-silicon. The ammonia-oxidized lignin used carries a negative charge and has good water solubility. Through electrostatic adsorption, it can effectively coat the surface of the nano-silicon to form a thin carbon layer, increasing the specific surface area during the conversion to a carbon coating. Furthermore, the coated lignin carbon layer facilitates better embedding into expanded graphite during subsequent ball milling, enhancing the intermolecular bonding between the carbon-coated silicon and the expanded graphite. By ball milling, Si@C composite materials are embedded in expanded graphite layers. During ball milling, the milling beads apply shear force to impact the silicon and expanded graphite materials. The nano-silicon coated with lignin carbon layers and the expanded graphite, both having benzene ring structures, generate strong π-π stacking effects, enhancing interfacial adhesion. This allows the nano-silicon to be embedded in the graphite and is not easily detached (solving the problem of weak silicon-graphite interfacial bonding). This enables the lignin carbon layer on the modified nano-silicon surface to establish a stable contact surface with the expanded graphite, forming a complete conductive path and improving the conductivity of the material. At the same time, the sandwich structure formed by the expanded graphite has stable mechanical properties, which can effectively alleviate the expansion stress of silicon and suppress the problem of electrode material collapse and performance degradation caused by silicon volume expansion. Thus, a Si@C-EG composite material with higher stability and better conductivity is obtained. In addition, this invention creatively utilizes the characteristic of asphalt softening at low temperatures. During ball milling and calcination, the asphalt softens, allowing it to penetrate the spaces between the expanded graphite layers, effectively filling the gaps and alleviating the stress caused by silicon volume expansion. The prepared material possesses a sandwich structure, ensuring that the nano-silicon is not easily detached during subsequent cycles, improving the electron transport capability of the composite material, stabilizing the growth of the SEI film, and providing superior mechanical properties. High-temperature calcination further fixes the carbon layer, thus improving the conductivity of the composite material (electrode material) and suppressing silicon volume expansion, preventing the composite material structure from collapsing, and thereby improving the battery's cycle stability and rate performance. The lithium-ion half-cell assembled from the Si@C-EGC composite material prepared by the above method exhibits excellent cycle stability and charge transport capability.

[0030] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0031] (1) The half-cell assembled from the Si@C-EGC composite material prepared in this invention has excellent cycle stability and charge transport capability.

[0032] (2) The preparation process of the present invention is simple and efficient, the solvent is green, non-toxic and environmentally friendly, the raw material cost is low, no dangerous acid or special equipment is required, it can be prepared on a large scale, and has the prospect of commercial development of silicon-carbon anode. Attached Figure Description

[0033] Figure 1 The image shows the SEM image of the Si@C material prepared in Comparative Example 2.

[0034] Figure 2 The image shows a SEM image (scale bar is 500 nm) of the Si@C-EGC composite material prepared in Example 1.

[0035] Figure 3 The image shows a SEM image (scale bar is 1 μm) of the Si@C-EGC composite material prepared in Example 1.

[0036] Figure 4 The image shows the XRD pattern of the Si@C-EGC composite material prepared in Example 1.

[0037] Figure 5 The rate performance of the Si@C-EGC composite material assembled into a coin cell in Example 1 at different current densities is shown in the figure.

[0038] Figure 6 The Si@C-EGC composite material prepared in Example 1 was assembled into a coin cell at a current density of 1 Ag. -1 The long-cycle performance graph. Detailed Implementation

[0039] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0040] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention are commercially available or can be prepared by existing methods.

[0041] Example 1

[0042] A multi-level sandwich structure silicon-carbon material based on lignin carbon, the preparation method of which includes the following steps:

[0043] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon (modified nano-silicon) with positive charge on the surface; the aggregation of nano-silicon is effectively reduced by electrostatic repulsion.

[0044] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C); this step can uniformly coat ammonia-oxidized lignin onto the surface of nano-silicon through electrostatic adsorption, and the low-temperature calcination process can fix the carbon layer to obtain the lignin carbon outer shell layer.

[0045] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0046] (4) Add 0.05g of pitch and continue ball milling at 600rpm for 3h. After drying the solvent, place the resulting powder material in a tube furnace under a nitrogen atmosphere and calcine at 700℃ for 3h to obtain a multi-level sandwich structure silicon-carbon material based on lignin carbon (Si@C-EGC composite material). In this step, the ball milling beads apply shear force to impact silicon and expanded graphite. The nano-silicon coated with the lignin carbon layer and the expanded graphite both have benzene ring structures, resulting in a strong π-π stacking effect, which enhances the interfacial adhesion. Therefore, ball milling allows the nano-silicon to be stably embedded in the expanded graphite. Then, taking advantage of the fact that pitch softens easily at low temperatures, the pitch penetrates into the interlayer of the expanded graphite during calcination to fill the gaps. The closed sandwich structure ensures that the nano-silicon is not easily detached during subsequent cycles, resulting in a Si@C-EGC composite material with higher stability and better conductivity.

[0047] Example 2

[0048] A multi-level sandwich structure silicon-carbon material based on lignin carbon, the preparation method of which includes the following steps:

[0049] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0050] (2) Dissolve 0.05g of ammoniated lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow the ammoniated lignin to be electrostatically adsorbed on the surface of the nano-silicon, filter and dry, and then calcine and carbonize in a tube furnace under nitrogen atmosphere at 400°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0051] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0052] (4) Add 0.05g of asphalt and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and calcination at 700℃ for 3h to obtain a multi-level sandwich structure silicon carbide material (Si@C-EGC composite material) based on lignin carbon.

[0053] Example 3

[0054] A multi-level sandwich structure silicon-carbon material based on lignin carbon, the preparation method of which includes the following steps:

[0055] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0056] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0057] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0058] (4) Add 0.05g of asphalt and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and calcination at 800℃ for 3h to obtain a multi-level sandwich structure silicon carbide material (Si@C-EGC composite material) based on lignin carbon.

[0059] Example 4

[0060] A multi-level sandwich structure silicon-carbon material based on lignin carbon, the preparation method of which includes the following steps:

[0061] (1) Add 0.3g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0062] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.3g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0063] (3) Add 0.3g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and then mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0064] (4) Add 0.05g of asphalt and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and calcination at 700℃ for 3h to obtain a multi-level sandwich structure silicon carbide material (Si@C-EGC composite material) based on lignin carbon.

[0065] Comparative Example 1

[0066] A lignin-carbon-coated silicon-embedded expanded graphite Si@C-EG composite material, the preparation method of which includes the following steps:

[0067] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0068] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0069] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0070] (4) The Si@C-EG composite material was placed in a tube furnace under a nitrogen atmosphere and calcined at 700°C for 3 hours to obtain lignin carbon coated silicon inlaid expanded graphite composite material (Si@C-EG composite material).

[0071] Comparative Example 2

[0072] A lignin-carbon-coated silicon (Si@C) composite material is prepared by the following steps:

[0073] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0074] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0075] Comparative Example 3

[0076] A multi-level sandwich structure silicon-carbon material based on alkali lignin carbon, the preparation method of which includes the following steps:

[0077] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0078] (2) Dissolve 0.05g of alkali lignin (AL) in water, then add 0.2g of nano-silicon with positive charge on the surface prepared in step (1), let it stand at 25°C for 12h to allow alkali lignin to be electrostatically adsorbed on the surface of nano-silicon, filter and dry, and then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0079] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0080] (4) Add 0.05g of pitch and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and calcination at 700℃ for 3h to obtain a multi-level sandwich structure composite silicon-carbon material (Si@C-EGC composite material) based on alkali lignin carbon and pitch carbon.

[0081] Comparative Example 4

[0082] A Si@C-GC composite material with lignin-carbon coated silicon-embedded graphite and coated with an outer carbon layer of pitch is prepared by the following steps:

[0083] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0084] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0085] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@CG composite material.

[0086] (4) Add 0.05g of pitch and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and calcination at 700℃ for 3h to obtain a composite material (Si@C-GC composite material) with lignin carbon coated with silicon inlaid graphite and coated with pitch outer carbon layer.

[0087] Comparative Example 5

[0088] A multi-level sandwich structure silicon-carbon material based on lignin carbon, the preparation method of which includes the following steps:

[0089] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0090] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0091] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of ethylene glycol, and then mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0092] (4) Add 0.05g of asphalt and continue ball milling at 600rpm for 3h. After drying the solvent, place the resulting powder material in a tube furnace under nitrogen atmosphere and high temperature of 700℃ for 3h to obtain a multi-level sandwich structure silicon-carbon material (Si@C-EGC composite material) based on lignin carbon with ethylene glycol as the ball milling solvent.

[0093] Comparative Example 6

[0094] A multi-level sandwich structure Si@C-EGC composite material based on lignin carbon and polyvinylpyrrolidone carbon, the preparation method of which includes the following steps:

[0095] (1) Add 0.2g of nano-silicon (particle size of 30nm) and 0.1g of CTAB to 100mL of ultrapure water, mix ultrasonically at 25℃ for 30min, then wash away excess CTAB with water, filter and vacuum dry for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0096] (2) Dissolve 0.05g of ammonia-oxidized lignin (AOL) in water, then add 0.2g of nano-silicon with positive surface charge prepared in step (1), let it stand at 25°C for 12h to allow ammonia-oxidized lignin to electrostatically adsorb onto the surface of nano-silicon, filter and dry, then calcine and carbonize in a tube furnace under nitrogen atmosphere at 300°C for 1h to fix the carbon layer, and obtain lignin carbon coated nano-silicon (Si@C).

[0097] (3) Add 0.2g of the Si@C composite material prepared in step (2) and 0.1g of expanded graphite into a ball mill jar, add grinding beads and an appropriate amount of NMP, and mix and ball mill at 600rpm for 3h to obtain the Si@C-EG composite material.

[0098] (4) Add 0.05g of polyvinylpyrrolidone (PVP) and continue ball milling at 600rpm for 3h. After drying the solvent, place the obtained powder material in a tube furnace under nitrogen atmosphere and high temperature calcination at 700℃ for 3h to obtain a multi-level sandwich structure composite silicon-carbon material (Si@C-EGC composite material) based on lignin carbon and polyvinylpyrrolidone carbon.

[0099] Comparative Example 7

[0100] Prepare 50 mL of an aqueous solution containing 0.5 g of CTAB, then add 0.2 g of nano-silicon powder (30 nm), sonicate for 30 min, centrifuge and dry to obtain positively charged nano-silicon powder. Dissolve 0.5 g of lignin in 0.8 g of a eutectic solvent (choline chloride: ethylene glycol = 1:2), stir for 3 h until completely dissolved to obtain a lignin-eutectic solvent mixture. Add 0.2 g of positively charged nano-silicon powder (30 nm) to the prepared lignin-eutectic solvent mixture, stir for 3 h to obtain a homogeneous lignin / nano-silicon mixture. Place the obtained lignin / nano-silicon mixture in a tube furnace, purge with nitrogen, and calcine at 500 °C for 3 h to carbonize, obtaining a lignin-based silicon-carbon composite material. Ball mill 0.1 g of the obtained lignin-based silicon-carbon composite material with 0.05 g of graphite for 5 h, then calcine at 650 °C for 3 h to obtain a graphite-layered lignin-carbon / nano-silicon synergistic composite material.

[0101] Verification Example 1

[0102] The materials prepared in Example 1 and Comparative Example 2 were used to test their morphology and size using a scanning electron microscope (SEM, Hitach SU8220). The results are shown in the figure. Figure 1-3 .

[0103] in, Figure 1 The image shows the SEM image of the nitrogen-doped Si@C material prepared in Comparative Example 2. As can be seen from the image, the surface of the nano-silicon is coated with a thin carbon layer, the silicon particles are clearly dispersed and there is no obvious agglomeration, indicating that the lignin carbon was successfully coated by modification and electrostatic adsorption.

[0104] Figure 2 (Scale bar is 500nm) Figure 3 (Scale bar is 1 μm) is a SEM image of the Si@C-EGC composite material prepared in Example 1. It can be seen that the Si@C-EGC composite material successfully prepared a sandwich structure. Nano-silicon can be observed between the sheet-like expanded graphite layers. The stacked sandwich structure can effectively suppress the collapse of the electrode material caused by the volume expansion of silicon. Nano-silicon can be observed to be uniformly distributed in the carbon layer, and the pitch permeated between the layers can effectively prevent silicon from falling off during subsequent cycles.

[0105] Figure 4The XRD pattern of the Si@C-EGC composite material prepared in Example 1 shows that the material has five crystalline peaks belonging to silicon, indicating that the silicon crystalline phase of the composite material is intact. The peaks observed near 26.5° and 54° correspond to the carbon peaks of expanded graphite, and the peaks observed at and near 29° correspond to the carbon peaks of pitch material, proving that the Si@C-EGC composite material was successfully synthesized.

[0106] Verification Example 2

[0107] Lithium-ion half-cells were assembled using the materials finally prepared in Examples 1-4 and Comparative Examples 1-6, respectively, and their electrochemical performance was tested. The battery model was CR2032; the negative electrode material consisted of 70 wt% of the material finally prepared in the examples or comparative examples, 20 wt% carbon black, and 10 wt% PAA; a lithium sheet was used as the counter electrode; the electrolyte was prepared using 1 mol / L LiPF6 as the solute and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 as solvents. The entire assembly process of the lithium-ion half-cells was completed in an argon-protected glove box. The Neware battery performance testing system was used to test the performance at 0.1 Ag in the voltage range of 0.01-3.0V. -1 and 1Ag -1 The battery's constant current charge / discharge performance was tested at a current density of 0.1Ag, while the rate performance was tested at 0.1Ag. -1 0.2Ag -1 0.5Ag -1 1Ag -1 and 5Ag -1 The experiment was conducted at current density. Results are shown in Table 1 and... Figure 5-6 As shown.

[0108] Figure 5 The graph shows the rate performance of the Si@C-EGC composite material prepared in Example 1 assembled into a coin cell at different current densities. Figure 5 It can be seen that the specific capacity of the composite material remains stable during five cycles at different current densities, and increases from 0.1 Ag. -1 up to 5Ag -1 Back to 0.1Ag -1 It can still stabilize quickly and maintain a high specific capacity, especially in 5Ag. -1 It can still maintain 1137.28mAh g even under high current density. -1 The discharge specific capacity demonstrates that the prepared electrode material has excellent rate performance and can be adapted to different charge and discharge rates.

[0109] Figure 6 The Si@C-EGC composite material prepared in Example 1 was assembled into a coin cell, and in 1Ag -1Long-cycle performance at current density. From Figure 6 It can be seen that the discharge specific capacity is still as high as 877.44 mAh g after 300 cycles. -1 The capacity retention rate is 82.83%, which is significantly better than similar materials.

[0110] Table 1 Comparison of the performance of different lithium-ion battery anodes prepared in the examples and comparative examples.

[0111]

[0112] Table 1 shows that, in Example 1, 1Ag -1 The initial charge / discharge specific capacity is 1022.25 / 1059.28 mAh g. -1 After 200 cycles, the discharge specific capacity is 967.54 mAh g. -1 After 300 cycles, the discharge specific capacity is 877.44 mAh g. -1 As can be seen, the material prepared in Example 1 maintains a high specific capacity and excellent cycle stability during the initial cycling process, which is superior to similar materials. Examples 2 and 3 investigated the effects of the two calcination temperatures on the carbon layer of the Si@C-EGC composite material. It can be seen that increasing the first calcination temperature affects the mechanical properties of the ammonia-oxidized lignin carbon layer, causing some carbon-coated silicon composite material to fall off during ball milling, resulting in a lower initial capacity and poor cycle stability. Increasing the second calcination temperature affects the density of the pitch carbon layer. As the temperature rises, its structure tends to become denser, leading to an increase in lithium-ion transport resistance. Furthermore, excessively high temperatures can cause uneven thermal stress inside the material, reducing the structural integrity of the material. During subsequent cycles, cracking occurs as silicon stress increases, and the capacity decreases rapidly. Example 4 investigated the optimal silicon content of the Si@C-EGC composite material. It can be seen that although increasing the silicon content can improve the capacity, the cycle stability decreases significantly. Therefore, the Si@C-EGC composite material prepared in Example 1 has the best cycle stability and charge-discharge specific capacity, far exceeding similar materials.

[0113] Comparing Examples 1 and 6, the importance of bitumen in Si@C-EGC composites was demonstrated. It is evident that without bitumen to infiltrate and fill the gaps in the expanded graphite, the severe expansion stress of silicon would damage the interlayer structure of the expanded graphite, leading to a rapid decrease in capacity. Furthermore, replacing it with PVP could not achieve the same effect as bitumen because PVP, being a polymer, aggregates on the surface of the Si@C-EG composite during calcination, leaving the spaces in the expanded graphite intact and resulting in low material density. Therefore, it is proven that using bitumen as a soft carbon infiltrated gap in the expanded graphite significantly improves stability.

[0114] Comparative Example 2 shows that coating a layer of ammonia-oxidized lignin with an outer carbon layer can achieve a carbon content of 1 Ag. -1It provides a significantly high initial specific capacity of 1463.68 / 1493.47 mAh g at current density. -1 However, a single core-shell structure cannot limit the volume expansion of silicon during subsequent cycling. Therefore, the carbon shell breaks during cycling, silicon detaches from the current collector, and the electrode capacity decays rapidly.

[0115] Comparative Example 3 shows that replacing alkali lignin cannot achieve the same effect as in Example 1. This is because alkali lignin has poor water solubility, making it difficult to uniformly coat the surface of nano-silicon at the molecular level. Therefore, even though the aqueous solution of alkali lignin carries a negative charge and can generate electrostatic adsorption, the resulting carbon layer is uneven and hydrophobic, leading to a carbon layer stability far inferior to that of ammonia-oxidized lignin. Consequently, during ball milling, external forces can cause some carbon layers to detach, hindering the embedding of silicon onto expanded graphite.

[0116] In Comparative Example 4, because graphite has a blocky microstructure, the Si@C composite material can only be embedded on its surface through ball milling, failing to fully utilize the internal structure of graphite. Furthermore, the blocky graphite cannot achieve the sandwich structure concept of this invention; the subsequently added pitch only serves as an outer carbon coating and cannot penetrate into the spaces between the Si@CG composite materials. Therefore, the result is 1056.13 / 1093.75 mAh g. -1 The initial specific capacity was reduced to only 543.67 mAh g after 300 cycles. -1 .

[0117] While ethylene glycol used in Comparative Example 5 effectively disperses graphite and silicon-carbon composite materials, NMP, a strongly polar aprotic solvent, exhibits superior wettability on the surface of silicon-carbon materials, effectively reducing particle agglomeration and forming a uniform slurry system. Ethylene glycol, on the other hand, has lower polarity, which may lead to uneven particle dispersion and affect the uniformity of electrode coating.

[0118] Comparative Example 6 uses a PVP outer carbon layer instead of asphalt. PVP is a polymer with good film-forming and adhesive properties, which causes the intermediate Si@C-EG to clump together during ball milling, making it difficult to insert into the gaps of expanded graphite. Furthermore, the expansion stress concentration of nano-silicon leads to silicon shedding, and the cycle stability of the material is not as good as in Example 1.

[0119] Comparative Example 7 used block graphite for ball milling. Due to the large volume of block graphite, the effective space of the graphite could not be better utilized. However, Example 1 of the present invention uses sheet-like expanded graphite, which can make full use of its sheet structure and effectively alleviate the volume expansion of silicon. As a result, the capacity retention rate of Example 1 is better than that of Comparative Example 7 after 200 cycles after 300 cycles.

[0120] As can be seen from the above results, the cycling performance of the materials prepared in the embodiments of the present invention is better than that of other comparative samples. This is mainly because the constructed sandwich structure fully utilizes the characteristics of expanded graphite to provide stable and tough mechanical properties, effectively alleviating the volume expansion of nano-silicon; the pitch successfully penetrates between the expanded graphite layers as soft carbon, limiting the problem of nano-silicon falling off in subsequent cycling processes.

[0121] The above detailed embodiments provide a specific description of the analytical methods involved in this invention. It should be noted that the above description is only intended to help those skilled in the art better understand the methods and ideas of this invention, and is not intended to limit the scope of the invention. Without departing from the principles of this invention, those skilled in the art can make appropriate adjustments or modifications to this invention, and such adjustments and modifications should also fall within the protection scope of this invention.

Claims

1. A method for preparing a multi-level sandwich structure silicon-carbon material based on lignin carbon, characterized in that, Includes the following steps: (1) Add nano-silicon and surfactant to water, mix evenly by ultrasonication, wash and dry to obtain nano-silicon with positive charge on the surface; the surfactant is selected from hexadecyltrimethylammonium bromide; (2) Dissolve lignin in water, then add nano-silicon with a positive charge on the surface, let it stand and settle so that the lignin is electrostatically adsorbed on the surface of the nano-silicon, filter and dry, and then calcine in a tube furnace to fix the carbon layer to obtain lignin carbon coated with nano-silicon; the lignin is selected from ammonia-oxidized lignin. (3) After mixing lignin-coated nano-silicon and expanded graphite, NMP solvent was added and ball milled to achieve uniform mixing, so as to obtain lignin-coated silicon-embedded expanded graphite composite material. (4) Add pitch to the lignin-carbon coated silicon-embedded expanded graphite composite material obtained in step (3) and ball mill again. After drying the solvent, calcine the resulting powder material at high temperature to obtain the final product.

2. The preparation method according to claim 1, characterized in that, The mass ratio of nano-silicon to surfactant in step (1) is (8-1):

1.

3. The preparation method according to claim 1, characterized in that, The particle size of the nano-silicon in step (1) is 10-200 nm.

4. The preparation method according to claim 1, characterized in that, The mass ratio of lignin to nano-silicon with positive surface charge in step (2) is 1:(1-8).

5. The preparation method according to claim 1, characterized in that, The mass ratio of lignin-carbon-coated nano-silicon to expanded graphite in step (3) is (1-6):

1.

6. The preparation method according to claim 1, characterized in that, The high-temperature calcination in step (4) is carried out at a temperature of 100-800℃ for 1-10 hours.

7. The lignin-carbon-based multi-level sandwich structure silicon-carbon material prepared by the preparation method according to any one of claims 1-6.

8. The application of the lignin-carbon-based multi-level sandwich structure silicon-carbon material prepared by the preparation method according to any one of claims 1-6 in the preparation of battery anode materials.

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