Preparation method of silicon-carbon composite electrode material

Abstract

The invention relates to the field of electrode materials, in particular to a preparation method of a silicon-carbon composite electrode material. The method comprises the following steps: carrying out crushing pretreatment on the silicon-containing agricultural residues, and mixing the silicon-containing agricultural residues with tetraethyl orthosilicate according to a mass ratio of (3-5): 1; raising the temperature to 200-800 DEG C at the speed of 5 DEG C / min in a nitrogen atmosphere, and performing constant-temperature pyrolysis for 30-60 minutes; in the pyrolysis process, the TEOS is decomposed to generate a SiO2 nano-template to regulate and control a pore structure, part of the SiO2 nano-template is reduced into SiO nano-particles, and a SiO (at) C core-shell structure material with microporous and mesoporous graded pores is formed in situ; the product, conductive carbon and polyvinylidene fluoride are prepared into slurry according to the ratio of (6-7): (1.5-3): (1-1.5), the slurry is coated on activated carbon cloth, and the activated carbon cloth is freeze-dried to prepare the electrode. According to the preparation method, through design of the green activating agent, synchronous silicon-based compounding, pore regulation and control and pyrolysis process optimization, the defects of the traditional technology in the aspects of environmental protection, pore regulation and control accuracy and functional compounding are effectively overcome, and the electrochemical performance of the agricultural residue-based electrode material is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of electrode materials, and more specifically to a method for preparing a silicon-carbon composite electrode material. Background Technology

[0002] Supercapacitors have attracted much attention in the field of energy storage due to their high power density and long cycle life. Among them, biomass porous carbon electrode materials prepared from agricultural residues (such as coffee grounds) have become a research hotspot due to their wide availability, low cost, high sustainability, and the potential of their naturally occurring porous structures. The microporous structure of these materials provides abundant charge storage sites, while the mesoporous and macroporous structures together construct efficient ion transport channels, shortening ion diffusion distances and reducing resistance, which helps to achieve rapid charge adsorption / desorption processes.

[0003] However, existing activation techniques for preparing porous carbon materials from biomass still have significant shortcomings. Traditional physical activation methods (such as those using CO2 or steam), while relatively environmentally friendly, typically require extremely high temperatures (850-1100℃), resulting in high energy consumption. Furthermore, the resulting pore structures are often disordered, making it difficult to precisely control the ratio and distribution of micropores and mesopores. In contrast, the widely used chemical activation method can effectively form high specific surface areas (typically reaching 1000-3000 m²) through chemical etching. 2 While possessing a high specific surface area and abundant porous structure, this process is accompanied by serious environmental and technological problems. Highly corrosive activators (such as KOH) not only corrode equipment but also require extensive acid and alkali cleaning after activation to remove residues, generating large amounts of polluting wastewater. Some activators (such as ZnCl2) are inherently toxic, failing to meet the requirements of green chemistry development. Furthermore, traditional chemical activation methods often struggle to simultaneously optimize high specific surface area and a reasonable micropore / mesopore ratio. Micropores provide the primary ion adsorption sites, while mesopores are crucial for shortening ion transport paths at high current densities. Improper matching between the two leads to significant capacity decay at high current densities.

[0004] To overcome the shortcomings of traditional methods, researchers have explored various unconventional activation pathways, such as self-activation (utilizing elements like K and Na naturally present in biomass), bio-activation (e.g., microbial fermentation), template-assisted activation (using SiO2, ZnO, etc. as templates), and the use of green activators (e.g., K2CO3, Na2S2O3). While these methods have made some progress in terms of environmental friendliness or the construction of specific pore structures, they still have their limitations. For example, self-activation is highly dependent on the inherent components of biomass raw materials, limiting its applicability; bio-activation has low pore formation efficiency and poor controllability; template-based methods require the additional introduction and removal of template agents, increasing process complexity and cost; and some green activators are insufficient in terms of activation efficiency or the ability to finely control pore structure, making it difficult to meet the comprehensive electrochemical performance requirements of high-performance electrodes.

[0005] Of particular note is that certain agricultural residues, such as coffee grounds, are not only rich in carbon sources, but their plant cell walls also naturally contain siliceous structures, making them ideal precursors for the preparation of high-performance silicon-carbon composite electrode materials. However, current research on such raw materials mostly still uses the aforementioned traditional or unconventional activation methods, failing to fully utilize their inherent silicon resources. More importantly, existing technologies struggle to achieve uniform composite and structural control of silicon-based nanoparticles (such as SiO with good conductivity) with the carbon matrix during the activation process, which limits further improvements in the conductivity, cycle stability, and energy density of the resulting materials.

[0006] In summary, existing technologies for preparing porous carbon electrode materials from biomass have significant shortcomings in terms of the environmental friendliness of the activation process, the precise controllability of pore structure (especially the synergistic effect of micropores and mesopores), and the functionalization of specific raw materials (such as silicon-containing agricultural residues) (e.g., uniform silicon doping to form SiO@C structures). Therefore, there is an urgent need to develop a novel, green, environmentally friendly, and efficient activation method that can simultaneously achieve precise pore control and silicon-based functionalization, in order to fully exploit the potential of agricultural residues and enhance their overall performance as electrode materials for supercapacitors. Summary of the Invention

[0007] To address the aforementioned technical problems in the prior art, this invention provides a method for preparing a silicon-carbon composite electrode material, comprising the following steps:

[0008] (1) The silicon-containing agricultural residues are crushed and pretreated to obtain particles with a particle size of less than 2 mm;

[0009] (2) Mix the pretreated agricultural residues and additives evenly at a mass ratio of (3-5):1;

[0010] (3) Under a nitrogen atmosphere, the temperature is increased to 200-800℃ at a heating rate of 5℃ / min, and then pyrolyzed at a constant temperature for 30-60 minutes.

[0011] (4) During the pyrolysis process, tetraethyl orthosilicate decomposes to generate SiO2 nanoparticles as templates to regulate the pore structure. At the same time, some SiO2 is reduced to SiO nanoparticles, which are uniformly embedded in the carbon matrix to form a silicon-carbon core-shell structure material with hierarchical pores.

[0012] (5) The obtained silicon-carbon core-shell structure material, conductive carbon, and polyvinylidene fluoride are prepared into carbon paste in a mass ratio of (6-7):(1.5-3):(1-1.5), coated onto activated carbon cloth, and then freeze-dried to obtain the electrode.

[0013] The mass ratio of agricultural residue to tetraethyl orthosilicate in step (2) is 4:1.

[0014] In step (3), the pyrolysis temperature is 800℃ and the constant temperature time is 45 minutes.

[0015] The hierarchical porous silicon-carbon core-shell structure includes micropores with a diameter of less than 2 nm and mesopores with a diameter of 2-50 nm, and the SiO nanoparticles have a diameter of less than 2 nm.

[0016] In step (5), the mass ratio of active material: conductive carbon: polyvinylidene fluoride in the carbon slurry component is 7:2:1.

[0017] The additive is tetraethyl orthosilicate or a silicon-doped cellulose complex.

[0018] The silicon-doped cellulose composite is composed of tetraethyl orthosilicate, cellulose, and potassium chloride.

[0019] On the other hand, the present invention provides a silicon-carbon composite electrode material, wherein the electrode material comprises a core-shell structure formed by uniformly embedding SiO nanoparticles into a carbon matrix, and has hierarchical pores with micropores and mesopores working together; in a 6 mol / L KOH electrolyte, the specific capacitance is ≥446.3 F / g at a current density of 5 A / g.

[0020] The carbon matrix is ​​derived from at least one silicon-containing agricultural residue, such as coffee grounds, rice husks, or sugarcane bagasse.

[0021] Furthermore, the present invention provides a supercapacitor electrode, which is made of the aforementioned silicon-carbon composite electrode material.

[0022] The present invention has the following advantages over the prior art:

[0023] This invention innovatively employs tetraethyl orthosilicate (TEOS) or a silicon-doped cellulose composite as a multifunctional green activator, achieving simultaneous one-step pyrolysis. TEOS or the silicon-doped cellulose composite decomposes to generate SiO2 nano-hard templates, constructing a hierarchical structure with synergistic micropores during coffee grounds carbonization, solving the industry problem of traditional activators struggling to balance the micro / mesoporous ratio. Part of the SiO2 is reduced by the carbon matrix to ultra-small SiO nanoparticles, uniformly embedded to form a SiO@C core-shell structure, significantly improving electronic conductivity and enhancing interfacial stability. The method described in this invention completely avoids highly corrosive reagents such as KOH; the pyrolysis product is only ethanol vapor, requiring no acid or alkali post-treatment, overcoming the inherent drawback of heavy pollution in chemical activation methods.

[0024] More significantly, during pyrolysis, some SiO2 is reduced to SiO particles, which are uniformly embedded in the carbon framework to form a core-shell structure. Energy dispersive spectroscopy (EDS) analysis confirms a silicon content of 0.70%. This composite structure improves electron transport efficiency by 32% and endows the material with ultra-high specific capacitance and excellent cycle stability. Furthermore, the process is both highly efficient and versatile. The pyrolysis temperature and time are significantly lower than the energy consumption of physical activation. Simultaneously, using the method described in this invention, the raw materials can be extended to silicon-containing agricultural residues such as rice husks, sugarcane bagasse, and coffee grounds, and the activator is compatible with alternative silicon sources such as methyl orthosilicate, providing a green and efficient complete solution for the resource utilization of agricultural waste and the industrialization of high-performance supercapacitors. Attached Figure Description

[0025] Figure 1 This is the cyclic voltammetric characteristic curve of Example 1;

[0026] Figure 2 The cyclic volt-ampere characteristic curve is shown in Comparative Example 1.

[0027] Figure 3 This is the cyclic voltammetric characteristic curve of Example 2;

[0028] Figure 4 The cyclic volt-ampere characteristic curve is shown in Comparative Example 2.

[0029] Figure 5 This is the cyclic voltammetric characteristic curve of Example 3;

[0030] Figure 6 The cyclic voltammetric characteristic curves are those of Comparative Example 3.

[0031] Figure 7 This is the constant current charge-discharge curve of Example 1;

[0032] Figure 8 The constant current charge-discharge curves are those of Comparative Example 1.

[0033] Figure 9 This is the constant current charge-discharge curve of Example 2;

[0034] Figure 10 The constant current charge-discharge curves are those of Comparative Example 2.

[0035] Figure 11-16 This is the constant current charge-discharge curve of Example 3;

[0036] Figure 17-22 The constant current charge-discharge curves are for Comparative Example 3.

[0037] Figure 23 These are the AC impedance spectra of Examples 1 to 3 and Comparative Examples 1 to 3;

[0038] Figure 24-27 This is a scanning electron microscope image of Comparative Example 3;

[0039] Figure 28 This is the energy spectrum of the electrode material in Comparative Example 3;

[0040] Figure 29-32 These are scanning electron microscope images of Example 3;

[0041] Figure 33 This is the energy spectrum of the electrode material in Example 3. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0043] Example 1

[0044] 1. Raw material pretreatment

[0045] Coffee grounds generated during coffee production are selected as agricultural residue raw materials (including coffee grounds, coffee shells, etc.). First, the raw materials are washed to remove surface impurities, then dried in an oven at 60-80℃ to constant weight (about 12-24 hours), and then crushed into particles with a particle size of less than 2mm to ensure the uniformity of subsequent reactions.

[0046] 2. Preparation of SiO@C composite materials

[0047] Coffee grounds and tetraethyl orthosilicate were thoroughly mixed at a mass ratio of 4:1 (i.e., 5.0000g coffee grounds : 1.2500g tetraethyl orthosilicate) and placed in a quartz boat. The mixture was then placed in a biomass pyrolysis furnace and pyrolyzed at a constant temperature of 200℃ for 45 minutes (heating rate 5℃ / min, nitrogen atmosphere). During pyrolysis, tetraethyl orthosilicate decomposed to generate SiO2 nanoparticles, which served as templates. Simultaneously, these nanoparticles reacted with the carbon matrix produced from the pyrolysis of coffee grounds, with some SiO2 being reduced to SiO, forming a composite structure in which SiO nanoparticles were uniformly embedded in the carbon matrix (labeled SiO@C-1).

[0048] 3. Electrode preparation

[0049] (1) Carbon slurry preparation: Accurately weigh 0.5000g of active material, 0.1428g of conductive carbon, and 0.0714g of PVDF according to the mass ratio of SiO@C-1:conductive carbon:polyvinylidene fluoride (PVDF) = 7:2:1. Add the active material and conductive carbon to 20.00mL glass bottles respectively, add 6.00mL of N-methylpyrrolidone (NMP) as solvent to each, and sonicate (60℃, 30 minutes); add PVDF to another glass bottle, add 6mL of NMP, and sonicate to dissolve (60℃, 1 hour). Then mix the three components and sonicate for 30 minutes (60℃) to obtain a uniformly dispersed carbon slurry.

[0050] (2) Carbon cloth treatment: Cut 1cm×1cm carbon cloth, place it in 17% dilute nitric acid solution for 1.5 hours to remove surface impurities and enhance hydrophilicity; then rinse 5 times with deionized water, then ultrasonically wash with anhydrous ethanol for 20 minutes, dry in an oven (60℃, 12 hours), and weigh for later use. Coat the activated carbon cloth surface with carbon paste (active material loading of about 0.0450g), freeze dry (-50℃, 24 hours) to fix the carbon paste, and obtain SiO@C-1 working electrode.

[0051] Example 2

[0052] The steps are the same as in Example 1. The difference between Example 2 and Example 1 is that in the preparation process of SiO@C composite material in step 2, the isothermal pyrolysis condition is 300°C for 45 minutes. In Example 2, a composite structure in which SiO nanoparticles are uniformly embedded in the carbon matrix is ​​formed (labeled as SiO@C-3), and finally, the SiO@C-3 working electrode is obtained.

[0053] Example 3

[0054] 1.2500 g of tetraethyl orthosilicate and 2.2500 g of cellulose were dissolved sequentially in a beaker containing 20 mL of deionized water. 2.0000 g of potassium chloride was added, and the mixture was stirred at 70 °C for 5 hours. Then, the treated coffee grounds were added at a mass ratio of 4:1 (i.e., 5.0000 g coffee grounds : 1.2500 g tetraethyl orthosilicate). The mixture was stirred at 70 °C for another 5 hours until the solids were evenly dispersed to form a sol. Stirring was then stopped, and the mixture was cooled to room temperature and allowed to gel for 12 hours. The resulting gel was then placed in a freeze-drying apparatus and dried at -20 °C for 10 hours to obtain a fully aged gel. 5.0000 g of pretreated gel was accurately weighed into a quartz boat and placed in a biomass pyrolysis furnace. The mixture was pyrolyzed at 800℃ for 45 minutes (heating rate 5℃ / min, under nitrogen atmosphere protection). After natural cooling to room temperature, the gel was removed and ground into a uniform powder using an agate mortar (passing through a 200-mesh sieve), yielding a composite structure of SiO nanoparticles uniformly embedded in a carbon matrix (labeled SiO@C-13). During pyrolysis, tetraethyl orthosilicate decomposed to generate SiO2 nanoparticles, which served as a template. Simultaneously, these nanoparticles reacted with the carbon matrix generated from the pyrolysis of coffee grounds, reducing some of the SiO2 to SiO.

[0055] This invention employs a silicon-doped cellulose composite to replace traditional highly corrosive activators (such as KOH / H3PO4), achieving pollution-free activation via a sol-gel method. TEOS hydrolyzes and condenses to form SiO nanoparticles during pyrolysis, following the reaction pathway:

[0056] Hydrolysis: Si(OEt)₄ + 4×Cellose → Si(Cellose)₃(OEt) + 3EtOH (acid / base catalysis)

[0057] Polycondensation: 2Si(Cellose)3(OEt)→(Cellose)3-Si-O-Si-(Cellose)3+EtOEt

[0058] Pyrolysis: (Cellose)3-Si-O-Si-(Cellose)3→-Si-O-Si-O-

[0059] This technology innovatively employs tetraethyl orthosilicate (TEOS) or silicon-doped cellulose composites as green activators, simultaneously achieving multiple functions during nitrogen atmosphere pyrolysis: The SiO2 nanoparticles generated from TEOS decomposition serve as hard templates, in-situ constructing a hierarchical pore structure in a carbon matrix, synergistically combining micropores (<2nm) and mesopores (2-50nm). The micropores provide high-density ion adsorption sites, while the mesopores shorten ion diffusion paths, solving the capacity decay problem at high current densities caused by the imbalance of the micro / mesopore ratio in traditional activation methods; simultaneously, the carbon matrix reduces some SiO2 to ultra-small SiO nanoparticles (<2nm in diameter), uniformly embedding them... The process forms a SiO@C core-shell structure, which enhances electronic conductivity (32% increase in electron transport efficiency when silicon content reaches 0.70%) and structural stability through the interface interaction between silicon and carbon. The entire process requires no highly corrosive reagents, and the pyrolysis product is only ethanol vapor, thus avoiding acid and alkali pollution and equipment corrosion problems associated with chemical activation methods. Furthermore, the pyrolysis temperature (200-800℃) is significantly lower than that of physical activation (850-1100℃), resulting in a substantial reduction in energy consumption. Ultimately, through the synergistic effect of pore control, silicon-based composites, and green processes, the electrode material achieves a specific capacitance of ≥446.3 F / g at a current density of 5 A / g, with excellent cycle stability. In addition, the method described in this invention involves no strong acids / alkalis, avoiding the acid and alkali cleaning steps required in traditional activation methods and completely eliminating pollution from salt-containing / heavy metal wastewater.

[0060] Example 4

[0061] The steps are the same as in Example 1, but the mass ratio of coffee grounds to TEOS is adjusted to 3:1 (5.000g:1.667g) and 5:1 (5.000g:1.000g). Increasing the silicon source ratio (e.g., 3:1) can increase the loading of SiO nanoparticles and enhance conductivity; decreasing the ratio (e.g., 5:1) can reduce silicon source consumption and lower costs.

[0062] Electrochemical tests showed that the specific capacitance was highest at a mass ratio of 4:1 (446.3 F / g), conductivity improved but porosity decreased at a mass ratio of 3:1 (421.5 F / g), and silicon doping was insufficient at a mass ratio of 5:1 (402.8 F / g).

[0063] Example 5

[0064] The steps are the same as in Example 1, but the pyrolysis temperature is adjusted to 700℃ and 900℃.

[0065] Results: The material formed the optimal hierarchical pore structure at 800℃ (microporosity 32% + mesoporosity 58%), and the specific capacitance reached its peak value; carbonization was incomplete at 700℃ (specific capacitance 382.4 F / g); and pore collapse occurred at 900℃ (specific capacitance 415.6 F / g).

[0066] Example 6

[0067] Replace coffee grounds with rice husks or sugarcane bagasse (containing natural silicon sources), and the rest is the same as in Example 1.

[0068] Rice husk-based SiO@C has a specific capacitance of 431.2 F / g (5 A / g); sugarcane bagasse-based SiO@C has a specific capacitance of 428.7 F / g (5 A / g), confirming the universality of the method described in this invention.

[0069] Example 7

[0070] The carbon paste ratio is adjusted as follows:

[0071] When SiO@C-13:Super P:PVDF = 6:3:1, conductivity is improved, and rate performance is improved by 12%.

[0072] When SiO@C-13:Super P:PVDF = 7:1.5:1.5, adhesion is enhanced, and cycle life is increased to 98.2%.

[0073] Comparative Example 1

[0074] In Comparative Example 1, step 2 involved accurately weighing 5.0000g of pretreated coffee grounds into a quartz boat, placing it in a biomass pyrolysis furnace, and pyrolyzing it at a constant temperature of 200℃ for 45 minutes (heating rate 5℃ / min, under nitrogen atmosphere protection). After naturally cooling to room temperature, the ground coffee grounds were removed and ground into a uniform powder using an agate mortar and pestle (passing through a 200-mesh sieve) to obtain the blank control material (labeled BlankControl-2). The remaining steps were the same as in Example 1 to obtain the Blank Control-2 working electrode.

[0075] Comparative Example 2

[0076] In Comparative Example 1, step 2 involved accurately weighing 5.0000g of pretreated coffee grounds into a quartz boat, placing it in a biomass pyrolysis furnace, and pyrolyzing it at a constant temperature of 300℃ for 45 minutes (heating rate 5℃ / min, under nitrogen atmosphere protection). After naturally cooling to room temperature, the ground coffee grounds were removed and ground into a uniform powder using an agate mortar and pestle (passing through a 200-mesh sieve) to obtain the blank control material (labeled BlankControl-4). The remaining steps were the same as in Example 1 to obtain the Blank Control-4 working electrode.

[0077] Comparative Example 3

[0078] In Comparative Example 1, step 2 was as follows: 5.0000g of pretreated coffee grounds were accurately weighed into a quartz boat, placed in a biomass pyrolysis furnace, and pyrolyzed at 800℃ for 45 minutes (heating rate 5℃ / min, under nitrogen atmosphere protection). After naturally cooling to room temperature, the ground grounds were removed and ground into a uniform powder using an agate mortar and pestle (passing through a 200-mesh sieve) to obtain the blank control material (labeled BlankControl-14). The remaining steps were the same as in Example 1 to obtain the Blank Control-14 working electrode.

[0079] During pyrolysis, the silicon-doped cellulose composite decomposes to generate SiO nanoparticles. These nanoparticles act as hard templates, forming mesopores (2-50 nm) within the carbon matrix. Some SiO reacts with carbon to form SiO nanoparticles (<2 nm), which are uniformly embedded in the carbon matrix to form a SiO@C core-shell structure. This improves the material's conductivity (SiO has better electron transport capabilities than pure carbon) and enhances cycle stability through the interfacial interaction between the nanoparticles and carbon. Furthermore, the SiO nanoparticles (10-50 nm) generated from the decomposition of the silicon-doped cellulose composite act as hard templates, forming interconnected mesoporous channels (2-50 nm in diameter) within the carbon matrix, shortening the ion diffusion path. Finally, the volatiles (H₂O / CO₂) released during the pyrolysis of coffee grounds etch the carbon layer, forming abundant micropores (<2 nm), providing high-density ion adsorption sites.

[0080] By comparing pyrolysis conditions of 200℃ and 800℃, this invention proposes that at 800℃, the organic matter in coffee grounds is fully carbonized, the silicon-doped cellulose composite is completely decomposed, and the SiO@C material forms a hierarchical pore structure with abundant micropores (<2nm, providing ion adsorption sites) and mesopores (shortening ion transport paths). At the same time, the SiO nanoparticles are tightly bonded to the carbon matrix, and finally the specific capacitance reaches 446.3F / g at a current density of 5A / g (superior to 395.6F / g of the blank control material), and the cycle stability is excellent.

[0081] Combined with appendix Figure 1-33 The beneficial effects of the present invention will be further explained. Figure 1 (1) is the cyclic voltammetric characteristic curve of Example 1 (scan rate 0.005V / s to 0.04V / s); Figure 1 (2) is the cyclic voltammetric characteristic curve of Example 1 (scan rate 0.06V / s to 0.10V / s). Figure 1 (1, 2) are the CV curves of 0.0050 g of silicon-added SiO@C-1 material with 6MKOH as electrolyte solution, in a voltage range of -0.3 to 0.45 V, at different scan rates of 0.005 V / s, 0.01 V / s, 0.02 V / s, 0.04 V / s, 0.06 V / s, 0.08 V / s, and 0.1 V / s, and on carbon cloth.

[0082] At scan rates of 0.005–0.1 V / s, the CV curves exhibited symmetrical rectangular shapes with no obvious redox peaks, verifying the typical characteristics of double-layer capacitance. Compared to Comparative Example 1, the curve enclosed a larger area, indicating that the SiO@C core-shell structure enhances charge storage capacity and solves the problem of low capacitance values ​​in traditional biomass carbon materials.

[0083] Figure 2 (1) is the cyclic voltammetric characteristic curve of Comparative Example 1 (scan rate from 0.005 V / s to 0.04 V / s); Figure 2 (2) Cyclic voltammetric characteristic curves of Comparative Example 1 (scan rate from 0.06 V / s to 0.10 V / s)

[0084] Figure 2 (1, 2) are the CV curves of Blank Control-2 material without silicon dopant, with 6MKOH as electrolyte solution, in a voltage range of -0.3 to -0.45V, at different scan rates of 0.005V / s, 0.01V / s, 0.02V / s, 0.04V / s, 0.06V / s, 0.08V / s, and 0.1V / s, and with an active material attachment amount of about 0.0050g on the carbon cloth.

[0085] Figure 2 (1, 2) The curves have poor symmetry and small enclosing area, indicating that when the pure carbon matrix lacks silicon-based composite structure, the reversibility of ion adsorption and desorption is insufficient, which highlights the effect of SiO nanoparticle embedding on electrochemical stability in Example 1.

[0086] Figure 3 The CV curves are obtained using 6M KOH as the electrolyte solution, within a voltage range of -0.3 to 0.45V, at different scan rates of 0.005V / s, 0.01V / s, 0.02V / s, 0.04V / s, 0.06V / s, 0.08V / s, and 0.1V / s, with 0.0050g of silicon-added SiO@C-3 material attached to carbon cloth. Figure 3 The results show that under pyrolysis conditions of 300℃, the CV curve is nearly rectangular in shape but has a smaller area than that of Example 1 (800℃), indicating the influence of pyrolysis temperature on pore structure and silicon reduction efficiency. This verifies that 800℃ is the optimal pyrolysis temperature, enabling complete carbonization of organic matter and complete decomposition of SiO2, thus resolving the performance defects caused by incomplete carbonization at low temperatures.

[0087] Figure 4The CV curves are for Blank Control-4 material without silicon dopant, with 6M KOH as electrolyte solution, in a voltage range of -0.3 to 0.45V, at different scan rates of 0.005V / s, 0.01V / s, 0.02V / s, 0.04V / s, 0.06V / s, 0.08V / s, and 0.1V / s, and with an active material attachment amount of approximately 0.0050g on the carbon cloth. Figure 4 The curve distortion is obvious, further confirming that without the addition of a silicon source, the electrochemical performance of carbon materials improves only slightly with increasing temperature, highlighting the key role of the SiO@C composite structure in performance optimization under high-temperature pyrolysis conditions.

[0088] Figure 5 The CV curves are for SiO@C-13 material with silicon added, using 6MKOH as the electrolyte solution, in a voltage range of -0.3 to 0.45V, at different scan rates of 0.005V / s, 0.01V / s, 0.02V / s, 0.04V / s, 0.06V / s, 0.08V / s, and 0.1V / s, with an active material attachment amount of approximately 0.0450g on the carbon cloth. Figure 5 The results show that the curves exhibit high rectangularity and the largest area at scan rates of 0.005-0.1 V / s, indicating that the silicon-doped cellulose composite, when used as an activator, results in a more complete microporous-mesoporous hierarchical structure and lower ion diffusion resistance. This verifies that the sol-gel method can precisely control the pore ratio, solving the micro / mesoporous imbalance problem of traditional activation methods.

[0089] Figure 6 The CV curves are obtained using 6MKOH as the electrolyte solution, within a voltage range of -0.3 to 0.45V, at different scan rates of 0.005V / s, 0.01V / s, 0.02V / s, 0.04V / s, 0.06V / s, 0.08V / s, and 0.1V / s, with approximately 0.0450g of silicon-free Blank Control-14 material attached to the carbon cloth as the active material.

[0090] Figure 6 The curve shows severe distortion as the scan rate increases, indicating that without silicon doping, the ion transport path is long and the resistance is high at high current density, which contrasts with the rate performance improved by the synergistic effect of SiO nanoparticles and mesoporous channels in Example 3.

[0091] Figure 7 , 8 The GCD curves of silicon-added SiO@C-1 and non-silicon-added Blank Control-2 materials are shown, using 6MKOH as the electrolyte solution, in a voltage range of -0.3 to -0.45V, at the same current density of 1A / g, with an active material attachment amount of about 0.0050g on the carbon cloth.

[0092] Figure 7The curves in the middle form a symmetrical triangle, the charge-discharge time is longer than that of Comparative Example 1, and the voltage drop (IR drop) is smaller. This indicates that the SiO@C structure reduces the internal resistance and improves the electronic conductivity (transmission efficiency is improved by 32%), thus solving the problem of insufficient conductivity of pure carbon materials.

[0093] Figure 8 The results indicate that the charge-discharge time is short and the voltage drop is large, verifying that the carbon matrix has poor conductivity and high charge transfer resistance without the addition of TEOS, further highlighting the conductivity-enhancing effect of SiO nanoparticles.

[0094] Figure 9 , 10 The GCD curves of silicon-added SiO@C-3 and non-silicon-added Blank Control-4 materials are shown, using 6MKOH as the electrolyte solution, in a voltage range of -0.3 to -0.45V, at the same current density of 1A / g, with an active material attachment amount of about 0.0050g on the carbon cloth.

[0095] Figure 9 The results show that the GCD curve symmetry is better than that of Comparative Example 2 at 300℃ pyrolysis, but the discharge time is shorter than that of Example 1, indicating that silicon reduction is incomplete under medium temperature conditions. This verifies the importance of 800℃ pyrolysis for the generation of SiO nanoparticles and carbonization of carbon matrix.

[0096] Figure 10 The nonlinear characteristics of the curve are obvious, confirming that the electrochemical performance of carbon materials at intermediate temperatures is worse without a silicon source, which contrasts with the improved adaptability of the SiO@C composite structure to pyrolysis temperature.

[0097] Figure 11 Figures 12, 13, and 14 show the GCD curves of SiO@C-13 electrode material with an attachment amount of 0.0450 g, using 6MKOH as the electrolyte solution, at a voltage range of -0.3 to 0.45 V, a constant temperature of 800 °C, and different current densities (0.5 A / g, 1.0 A / g, 2.0 A / g, and 3.0 A / g).

[0098] Figure 15 and 16 The GCD curves of SiO@C-13 electrode material with an active material attachment amount of 0.0450g are obtained using 6M KOH as electrolyte solution, under voltage range of -0.3-0.45V, the same temperature of 800℃, and different current densities (4.0A / g, 5.0A / g).

[0099] Figure 11-16 The results show that the curves are symmetrical across the entire current density range, the discharge time remains relatively long at 5 A / g, and the specific capacitance reaches 446.3 F / g. This verifies the efficient ion storage capability of the hierarchical porous structure (microporous adsorption + mesoporous transport) under high current, solving the problem of poor rate performance of traditional materials.

[0100] Figure 17 and 18 The GCD curves of Blank Control-14 electrode material with 6M KOH as electrolyte solution, in a voltage range of -0.30-0.45V, at the same temperature of 800℃, with different current densities (0.5A / g, 1A / g), and with an active material attachment amount of 0.0450g.

[0101] Figure 19 GCD curves of Blank Control-14 electrode materials with 6M KOH as electrolyte solution, in a voltage range of -0.30-0.45V, at the same temperature of 800℃, with different current densities (2.0A / g, 3.0A / g, 4.0A / g, 5.0A / g), and with an active material attachment amount of 0.0450g.

[0102] Figure 17-22 The results show that the curve deviates significantly from the triangle at high current density (≥3A / g), with a specific capacitance of only 395.6F / g. This indicates that without silicon doping, the pore structure is disordered and ion diffusion is hindered, highlighting the role of the SiO@C structure in improving cycle stability in Example 3.

[0103] Figure 23 The EIS curves are for electrode materials with 6M KOH as electrolyte solution, input signal frequency range of 1-100kHz, amplitude of 0.01V, temperature of 200, 300, and 800℃, and active material attachment amounts of 0.0050g for SiO@C-1, SiO@C-3, Blank Control-2, and Blank Control-4, and 0.0450g for SiO@C-13 and Blank Control-14.

[0104] Figure 23 The results show that the Nyquist plot of the sample material has a smaller high-frequency semicircle diameter (charge transfer resistance reduced by 48%) and a low-frequency straight line slope close to 90° (ion diffusion close to ideal capacitance). This verifies that the SiO@C core-shell structure and hierarchical porosity synergistically optimize the interface properties, solving the problem of high interface impedance in traditional activation methods.

[0105] Figure 24 and 25 These are electron microscope images of coffee grounds biochar. Figure 26 and 27 These are electron microscope images of coffee grounds biochar. Figure 24-27 The structure exhibits a disordered, blocky appearance with uneven pore distribution and no nanoparticle dispersion. This confirms that without the addition of an activator, the carbon material has a coarse microstructure and cannot form effective ion transport channels, highlighting the structural regulation role of TEOS as a template agent. Figure 28 This is an energy dispersive spectroscopy (EDS) image of coffee grounds biomass, showing a silicon content of 0.17%, a carbon content of 91.01%, and an oxygen content of 8.01%, revealing its main components and distribution characteristics. The low silicon content of only 0.17% and the high carbon content of 91.01% indicate low silicon source utilization of the raw material itself. This verifies that effective silicon doping cannot be achieved without the addition of TEOS, highlighting the necessity of adding a silicon source to increase silicon content.

[0106] Figure 29 , 30 Images 31 and 32 are electron microscope (EM) images of Si@coffee grounds biochar. Technical effects: The silicon content is increased to 0.70%, and it is uniformly distributed with carbon and oxygen. This confirms that silicon-doped cellulose composites can achieve efficient silicon introduction, forming a stable SiO@C core-shell structure, thus solving the problem of insufficient conductivity caused by uneven silicon-based composites in traditional methods. Figure 33 This is an energy dispersive spectroscopy (EDS) image of Si@coffee grounds biomass, showing that the silicon content is 0.70%, the carbon content is 73.79%, and the oxygen content is 24.16%, revealing its main components and distribution characteristics.

[0107] This invention innovatively employs tetraethyl orthosilicate (TEOS) or silicon-doped cellulose composites as green activators, simultaneously achieving pore control and silicon-based composites through a pyrolysis process. This systematically solves the shortcomings of traditional biomass carbon material preparation, such as significant environmental pollution, unbalanced pore structure, and uneven silicon-based composites. Compared to traditional physical or chemical activation, TEOS pyrolysis only generates inert SiO2 nanoparticles, eliminating acid and alkali pollution at the source and reducing equipment wear by over 90%. Simultaneously, SiO2 acts as a hard template to precisely construct a microporous-mesoporous hierarchical structure within the coffee grounds carbon matrix: micropores provide high-density ion adsorption sites, while mesopores shorten ion diffusion paths, resulting in a 48% reduction in charge transfer resistance and ion diffusion behavior approaching that of an ideal capacitor in the electrode material prepared by this invention.

[0108] This invention effectively overcomes the shortcomings of traditional technologies in terms of environmental protection, precise pore control, and functional composites by designing green activators, simultaneously integrating silicon-based composites and optimizing pore control and pyrolysis processes, and significantly improves the electrochemical performance of agricultural residue-based electrode materials.

Claims

1. A method for preparing a silicon-carbon composite electrode material, characterized in that, Includes the following steps: (1) The silicon-containing agricultural residues are crushed and pretreated to obtain particles with a particle size of less than 2 mm; (2) Mix the pretreated agricultural residues and additives evenly at a mass ratio of (3-5):1; (3) Under a nitrogen atmosphere, the temperature is increased to 200-800℃ at a heating rate of 5℃ / min, and then pyrolyzed at a constant temperature for 30-60 minutes. (4) During the pyrolysis process, tetraethyl orthosilicate decomposes to generate SiO2 nanoparticles as templates to regulate the pore structure. At the same time, some SiO2 is reduced to SiO nanoparticles, which are uniformly embedded in the carbon matrix to form a silicon-carbon core-shell structure material with hierarchical pores. (5) The obtained silicon-carbon core-shell structure material, conductive carbon, and polyvinylidene fluoride are prepared into carbon paste in a mass ratio of (6-7):(1.5-3):(1-1.5), coated onto activated carbon cloth, and then freeze-dried to obtain the electrode.

2. The method for preparing silicon-carbon composite electrode material according to claim 1, characterized in that: The mass ratio of agricultural residue to tetraethyl orthosilicate in step (2) is 4:

1.

3. The method for preparing silicon-carbon composite electrode material according to claim 1, characterized in that: In step (3), the pyrolysis temperature is 800℃ and the constant temperature time is 45 minutes.

4. The method for preparing silicon-carbon composite electrode material according to claim 1, characterized in that: The hierarchical porous silicon-carbon core-shell structure includes micropores with a diameter of less than 2 nm and mesopores with a diameter of 2-50 nm, and the SiO nanoparticles have a diameter of less than 2 nm.

5. The method for preparing silicon-carbon composite electrode material according to claim 1, characterized in that: In step (5), the mass ratio of active material: conductive carbon: polyvinylidene fluoride in the carbon slurry component is 7:2:

1.

6. The method for preparing silicon-carbon composite electrode material according to claim 1, characterized in that: The additive is tetraethyl orthosilicate or a silicon-doped cellulose complex.

7. The method for preparing silicon-carbon composite electrode material according to claim 6, characterized in that: The silicon-doped cellulose composite is composed of tetraethyl orthosilicate, cellulose, and potassium chloride.

8. A silicon-carbon composite electrode material prepared by any one of claims 1-7, characterized in that: The electrode material comprises a core-shell structure formed by uniformly embedding SiO nanoparticles into a carbon matrix, and has hierarchical pores with micropores and mesopores working together; in a 6 mol / L KOH electrolyte, the specific capacitance is ≥446.3 F / g at a current density of 5 A / g.

9. The silicon-carbon composite electrode material according to claim 8, characterized in that: The carbon matrix is ​​derived from at least one silicon-containing agricultural residue, such as coffee grounds, rice husks, or sugarcane bagasse.

10. A supercapacitor electrode, characterized in that: It is made using the silicon-carbon composite electrode material as described in claim 8 or 9.

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