Porous carbon material prepared from rice husks and preparation method and application thereof

Abstract

The present application relates to the technical field of porous carbon material, and particularly relates to porous carbon prepared from rice husk, a preparation method and application thereof. The rice husk is sequentially subjected to pretreatment, crosslinking treatment, five-stage carbonization, activation and removal of silicon dioxide, so that the porous carbon material with complex pore system of micropore, mesopore and macropore, a specific surface area of 1800-2400 m2 / g and a total pore volume of 0.8-2.15 cm3 / g is obtained. Through the synergistic effect of component regulation, in-situ interface strengthening and five-stage gradient carbonization, the porous carbon material with excellent performance is successfully prepared. The silicon-carbon material and the battery negative electrode prepared based on the porous carbon material both exhibit excellent electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of porous carbon materials technology, specifically to porous carbon materials prepared using rice husks, their preparation methods, and applications. Background Technology

[0002] Porous carbon materials have the characteristics of large specific surface area, well-developed pore structure, good chemical stability and excellent electrical conductivity. They have shown broad application prospects in fields such as supercapacitors, lithium / sodium ion batteries, carbon dioxide capture, heavy metal ion adsorption and heterogeneous catalyst supports. The development of high-performance, low-cost and environmentally friendly porous carbon materials has become a research hotspot in the field of materials science.

[0003] Currently, the main methods for preparing porous carbon materials include activation methods, template methods, and sol-gel methods. Among them, activation methods (such as KOH activation and steam activation) are relatively simple to operate, but the resulting carbon materials have poor pore uniformity and the pore size distribution is difficult to control precisely. In contrast, template methods, by replicating the pore structure of a template, can achieve precise control over pore size, morphology, and pore structure, thus preparing porous carbon materials with highly ordered pores. Template methods are divided into hard template methods and soft template methods according to the template type. Hard template methods use mesoporous silica (SiO2), zeolites, or metal-organic frameworks (MOFs) as templates. After filling the template pores with carbon precursors, porous carbon is obtained through high-temperature carbonization and template removal steps. This method can synthesize carbon materials with uniform pore size, large specific surface area, and ordered pore structure. However, the hard template method has many inherent drawbacks: template preparation is costly (e.g., ordered mesoporous SiO2 templates can account for more than 60% of the total material cost); the template removal process requires the use of highly corrosive etching agents such as hydrofluoric acid (HF) or concentrated alkali, which is not only cumbersome and time-consuming, but also generates toxic waste liquid, increasing the environmental burden and safety risks; in addition, templates are usually difficult to recycle and reuse, further increasing the raw material and environmental costs of material synthesis. The soft template method utilizes the self-assembly characteristics of surfactants or block copolymers to guide the directional alignment of carbon sources. Although it does not require a template removal step, soft templates have poor stability, and the pore size is easily affected by fluctuations in conditions such as temperature and pH, making it difficult to guarantee batch stability. Moreover, the reagents used are often expensive and have a certain degree of toxicity, which is not conducive to large-scale production and sustainable application.

[0004] Rice husks, a major byproduct of rice processing, are produced in vast quantities globally each year. Rich in natural biomass silica (SiO2 content approximately 15%–20%) and lignocellulose, they form a unique natural silicon / carbon blend system. Currently, conventional methods of disposing of rice husks mainly include direct combustion for heating or landfill, which not only result in low resource utilization but also cause a certain degree of environmental pollution. Researchers have attempted to extract SiO2 from rice husks to prepare high-value-added products such as precipitated silica, or to directly carbonize rice husks to prepare biochar. However, the resulting carbon materials have low specific surface area and underdeveloped pore structure, making it difficult to meet the needs of high-performance energy storage and adsorption applications.

[0005] Developing a method for preparing porous carbon that can fully utilize the natural silicon / carbon blending properties of rice husks is of great practical significance and has broad application prospects for reducing the production cost of porous carbon materials, alleviating the environmental burden, and promoting the high-value utilization of agricultural waste. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a porous carbon material prepared from rice husks, its preparation method, and its applications.

[0007] In a first aspect, the present invention provides a method for preparing porous carbon from rice husks, comprising: Step S1: Rice husks are pretreated with an aqueous solution of ethanol or an aqueous solution of sodium carbonate, dried, and pulverized to obtain rice husk powder. Step S2: The rice husk powder is cross-linked with a cross-linking agent to obtain the rice husk precursor. Step S3: The rice husk precursor undergoes a five-stage heat treatment in an inert atmosphere. The heat treatment temperature for the first stage is 180~220℃; The heat treatment temperature for the second stage is 260~320℃; The heat treatment temperature for the third stage is 380~450℃; The heat treatment temperature for the fourth stage is 600~750℃; The heat treatment temperature for the fifth stage is 800~950℃; After heat treatment, a carbonized matrix is ​​obtained; Step S4: The carbonized matrix and composite activator are activated under an inert atmosphere to obtain the activated product; Step S5: Etch in an alkaline solution to remove SiO2 from the activated product to obtain a porous carbon material.

[0008] Secondly, the present invention provides a porous carbon material, wherein the porous carbon material satisfies the following characteristics: (a) Specific surface area of ​​1800-2400 m² / g, total pore volume of 0.8-2.15 cm³ / g; (b) It has a three-level pore structure consisting of micropores, mesopores, and macropores; (c) Based on the pore volume ratio Micropores with a pore size of 0.5-2 nm account for 5%-15%; Mesopores with a pore size concentrated in the range of 3-15 nm account for 65%-85%; Macropores with a pore size concentrated in the 50-90 nm range account for 5%-25%; Furthermore, the sum of the pore volume ratios of micropores, mesopores, and macropores is 100%; (d) It has a continuous, interconnected three-dimensional network of channels.

[0009] The porous carbon material was prepared by the above-described preparation method.

[0010] Thirdly, the present invention provides a silicon-carbon material obtained by depositing silicon in the pores of the aforementioned porous carbon material using chemical vapor deposition.

[0011] Fourthly, the present invention provides a battery negative electrode comprising the aforementioned silicon-carbon material.

[0012] Fifthly, the present invention provides a battery comprising the aforementioned negative electrode.

[0013] The above-described one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: The problem of organic matter coagulation and agglomeration and SiO2 migration during carbonization was suppressed, and the prepared porous carbon had a concentrated pore size distribution.

[0014] Using a segmented carbonization process adapted to porous carbon, the prepared porous carbon can achieve a specific area of ​​1800~2400 m² / g and a pore volume of 0.8~2.15 cm³. 3 / g has a rich pore system and continuous interconnected three-dimensional channels.

[0015] The removal of silica templates requires only a low-concentration alkaline solution, which is lower in cost and more environmentally friendly.

[0016] Porous carbon can be prepared at a lower cost and can be mass-produced without the need for external template agents and exogenous carbon sources. Attached Figure Description

[0017] Figure 1 Fourier transform infrared spectrum of pretreated rice husk powder.

[0018] Figures 2-4 The images show SEM images of the porous carbon material obtained in Example 1 at different magnifications. Detailed Implementation

[0019] In a first aspect, the present invention provides a method for preparing porous carbon from rice husks, comprising: Step S1, Rice husk pretreatment: Rice husks are pretreated with an aqueous solution of ethanol or an aqueous solution of sodium carbonate, dried, and pulverized to obtain rice husk powder. Step S2, cross-linking reaction: The rice husk powder is cross-linked with a cross-linking agent to obtain the rice husk precursor; Step S3, Segmented Carbonization Treatment: The rice husk precursor undergoes five-stage heat treatment in an inert atmosphere: First stage: Increase the temperature to 180-220℃ at a rate of 1-2℃ / min, and hold for 1-2 hours; Second stage: Increase the temperature to 260-320℃ at a rate of 1-3℃ / min, and hold for 2-3 hours; Third stage: Increase the temperature to 380-450℃ at a rate of 2-3℃ / min, and hold for 3-4 hours; Fourth stage: Increase the temperature to 600-750℃ at a rate of 3-5℃ / min, and hold for 2-3 hours; Fifth stage: Increase the temperature to 800-950℃ at a rate of 3-5℃ / min, and hold for 1-2 hours; After heat treatment, a carbonized matrix is ​​obtained; Step S4, Activation and Pore Formation: The carbonized matrix and composite activator are activated under an inert atmosphere to obtain the activated product; Step S5, post-processing: Etching in an alkaline solution removes SiO2 from the activated product to obtain porous carbon material.

[0020] Furthermore, prior to step S1, the process includes washing and drying the rice husks. The rice husks are ultrasonically washed with deionized water to remove impurities such as mud and dust, and then vacuum dried at 60-100℃ for 4-6 hours.

[0021] First, step S1, rice husk pretreatment.

[0022] Rice husks are pretreated with an aqueous solution of ethanol or sodium carbonate to remove small molecule impurities such as fats and proteins that can cause severe brittleness.

[0023] Furthermore, the volume fraction of ethanol in the aqueous solution of ethanol is 10-30%.

[0024] Furthermore, the mass fraction of sodium carbonate in the aqueous solution of sodium carbonate is 0.1~0.5%.

[0025] Furthermore, the solid-liquid ratio of rice husk to an aqueous solution of ethanol or sodium carbonate is 1:10~15.

[0026] Furthermore, the pretreatment temperature is 30~60℃, and the pretreatment time is 1~3h.

[0027] Furthermore, the drying described in step S1 is vacuum drying, with a drying temperature of 60-100℃ and a drying time of 4-6 hours.

[0028] Furthermore, in step S1, the rice husks are crushed to below 100 mesh.

[0029] Second, step S2, cross-linking reaction.

[0030] Furthermore, the crosslinking agent is selected from at least one of boric acid, silane coupling agent, and citric acid.

[0031] The crosslinking agent undergoes esterification with the ortho-hydroxyl groups on the cellulose / lignin in rice husk, and simultaneously undergoes condensation with the silanol groups on the surface of SiO2. Ultimately, a three-dimensional covalent crosslinking network is formed on the original structure of rice husk, which completely locks the original SiO2 onto the carbon skeleton at the molecular level, solving the problems of SiO2 migration and aggregation and organic matter coagulation and agglomeration during carbonization.

[0032] Furthermore, the amount of crosslinking agent added is 0.5-5% of the mass of rice husk powder.

[0033] Furthermore, the solid-liquid ratio of the crosslinking reaction system is 1:5~10.

[0034] Furthermore, the cross-linking reaction is carried out at a temperature of 90~130℃ for a time of 2~5h.

[0035] Third, step S3, segmented carbonization treatment.

[0036] The rice husk precursor is subjected to segmented carbonization treatment, specifically five-stage heat treatment.

[0037] The first stage is a low-temperature cross-linking and shaping stage, which allows the cross-linked network to further complete bonding, slowly removes residual solvent and bound water, and avoids damage to the original structure caused by rapid heating and high temperature.

[0038] The second stage is the hemicellulose pyrolysis stage, which is matched to the pyrolysis temperature of hemicellulose to allow hemicellulose to slowly and uniformly pyrolyze and volatilize without destroying the rigid skeleton of cellulose-lignin.

[0039] The third stage is the cellulose-lignin pyrolysis stage, which matches the pyrolysis temperatures of cellulose and lignin, allowing them to pyrolyze and carbonize simultaneously to form a continuous rigid carbon skeleton. At the same time, the cross-linked network is strengthened twice to lock in the uniform distribution of native SiO2.

[0040] The fourth stage is the carbon skeleton aromatization stage, which transforms amorphous carbon into graphite-like microcrystals, improving the conductivity and structural stability of the carbon skeleton and providing a foundation for subsequent activation and pore formation.

[0041] The fifth stage is the high-temperature structural shaping stage, which completes the carbonization of the carbon skeleton and preserves the in-situ uniformly distributed SiO2 template.

[0042] Fourth, step S4, activate the pore formation.

[0043] Furthermore, the composite activator is a mixture of potassium hydroxide and potassium carbonate. Preferably, the mass ratio of potassium hydroxide to potassium carbonate in the composite activator is 2-3:1.

[0044] When potassium hydroxide and potassium carbonate are mixed and used as a composite activator, the reaction rates of strong and mild activation are matched, avoiding both pore collapse caused by excessive etching with KOH and incomplete pore formation caused by insufficient activation with K2CO3. In contrast, a single activator can only selectively etch micropores or mesopores, resulting in low activation efficiency and a lack of pore control capabilities.

[0045] Furthermore, the mass ratio of the composite activator to the carbonized matrix is ​​1~2.5:1.

[0046] Furthermore, the activation treatment temperature is 750~850℃, the heating rate is 2~4℃ / min, and the holding time is 1~2h.

[0047] Fifth, step S5, post-processing.

[0048] Further, the alkaline solution is an aqueous solution of sodium hydroxide and / or potassium hydroxide at a concentration of 0.5~1.5 mol / L.

[0049] This invention requires only a low concentration of alkaline solution to complete template removal, without the need for high concentration of strong alkali etching.

[0050] Furthermore, the etching temperature is 50~70℃, and the etching time is 1~3h.

[0051] Furthermore, it also includes: crushing, classifying, and magnetically separating and demagnetizing the porous carbon material obtained in step S5.

[0052] Secondly, the present invention provides a porous carbon material, wherein the porous carbon material satisfies the following characteristics: (a) Specific surface area of ​​1800-2400 m² / g, total pore volume of 0.8-2.15 cm³ / g; (b) It has a three-level pore structure consisting of micropores, mesopores, and macropores; (c) Based on the pore volume ratio Micropores with a pore size of 0.5-2 nm account for 5%-15%; Mesopores with a pore size concentrated in the range of 3-15 nm account for 65%-85%; Macropores with a pore size concentrated in the 50-90 nm range account for 5%-25%; Furthermore, the sum of the pore volume ratios of micropores, mesopores, and macropores is 100%; (d) It has a continuous, interconnected three-dimensional network of channels.

[0053] The porous carbon material was prepared by the above-described preparation method.

[0054] The five-stage carbonization process mainly forms a primary carbon framework, and the activation process etches the carbon framework, thereby obtaining a porous carbon material with a complex pore system of micropores, mesopores, and macropores.

[0055] In porous carbon-carbon materials, micropores provide extremely high specific surface areas, offering numerous nucleation and anchoring sites for silicon nanoparticles deposited in the vapor phase. Mesopores, with their pore size perfectly matched to nano-silicon, serve as the primary deposition site for silicon. When silicon absorbs lithium and expands, the space provided by mesopores effectively buffers this expansion, preventing particles from squeezing against each other; simultaneously, they construct efficient high-speed ion pathways. Macropores act as rapid permeation channels for the electrolyte, reducing concentration polarization; and under high silicon loading, they can accommodate macroscopic volume changes in the electrode, maintaining the overall structural stability of the electrode.

[0056] Furthermore, the porous carbon material is prepared by the above-described preparation method.

[0057] Thirdly, the present invention provides a silicon-carbon material obtained by depositing silicon in the pores of the aforementioned porous carbon material using chemical vapor deposition.

[0058] Fourthly, the present invention provides a battery negative electrode comprising the aforementioned silicon-carbon material.

[0059] Fifthly, the present invention provides a battery comprising the aforementioned negative electrode.

[0060] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0061] 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.

[0062] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0063] Example 1 (1) Raw material pretreatment: Take rice husk raw material, ultrasonically clean it 3 times with deionized water to remove mud and impurities, and vacuum dry it at 80℃ for 4 hours. Take 100g of dried rice husks, add 1200mL of 20% ethanol-water solution, stir in a water bath at 45℃ for 2h for pretreatment, filter, wash twice with deionized water, vacuum dry at 80℃ for 4h, and pulverize to below 100 mesh to obtain pretreated rice husk powder. Figure 1 The image shows the Fourier transform infrared spectrum of the pretreated rice husk powder. As can be seen from the image, the Si-OC bonds are fully preserved in the pretreated rice husk powder. (2) Crosslinking reaction: Take 100g of pretreated rice husk powder, add 2g of boric acid crosslinking agent and 800mL of deionized water, stir at high speed for 30min to mix evenly, transfer to a hydrothermal reactor, and perform hydrothermal crosslinking reaction at 110℃ for 3h. After filtration and drying, obtain the interface-reinforced rice husk precursor. (3) Segmented carbonization treatment: The interface-enhanced rice husk precursor was placed in a tube furnace, and high-purity argon gas was introduced at a flow rate of 200 mL / min. The following five-stage heating program was then executed: Low-temperature cross-linking and setting section: heat to 200℃ at a rate of 1.5℃ / min and hold for 1.5h; Hemicellulose fractionation pyrolysis section: Heat to 300℃ at 2℃ / min and hold for 2.5h; Cellulose-lignin co-pyrolysis section: Heat to 420℃ at 2.5℃ / min and hold for 3.5h; Carbon skeleton aromatization section: Heat to 700℃ at 4℃ / min and hold for 2.5h; High-temperature structural shaping section: The temperature is increased to 850℃ at 4℃ / min and held for 1.5h; then naturally cooled to room temperature to obtain a carbonized matrix containing an in-situ uniformly distributed SiO2 template; (4) Activation treatment: Take 100g of carbonized matrix and mix it evenly with 150g of composite activator (potassium hydroxide: potassium carbonate = 3:1). Place it in a tube furnace, introduce high-purity argon gas at a flow rate of 200mL / min, heat it to 800℃ at 3℃ / min, hold it at that temperature for 1.5h, and let it cool naturally to room temperature to obtain the pore-forming carbon material. (5) Post-processing: After pore formation, the carbon material was placed in a 1 mol / L sodium hydroxide aqueous solution and etched by stirring at 60°C for 2 hours to complete deep template removal. It was then washed with deionized water until the pH of the filtrate was neutral, dried under vacuum at 105°C for 8 hours, crushed and classified by an air jet mill to a D50 of 10 μm, sieved through a 300-mesh sieve, and demagnetized by 10000Gs to obtain the porous carbon product.

[0064] Figures 2-4The images show SEM images of the porous carbon material obtained in Example 1 at different magnifications. It can be seen that the porous carbon material is blocky with dense pores on its surface. Since the pores are at the nanometer level (<100 nm), the internal connectivity is difficult to observe directly. However, because SiO2 is naturally and uniformly distributed within the rice husk, etching the porous carbon will inevitably form interconnected three-dimensional channels.

[0065] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that the cross-linking reaction in step (2) is omitted. That is, the pretreated rice husk powder obtained in step (1) is directly subjected to the segmented carbonization treatment in step (3), the activation treatment in step (4), and the post-treatment in step (5) to obtain the porous carbon product.

[0066] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that the dried rice husks were not pretreated with an ethanol-water solution in step (1). The dried rice husks were directly subjected to the crosslinking reaction in step (2), the segmented carbonization treatment in step (3), the activation treatment in step (4), and the post-treatment in step (5) to obtain the porous carbon product.

[0067] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that step (3) is not a segmented carbonization process, but only a single-stage carbonization process. Specifically, step (3) of Comparative Example 3 is as follows: the interface-reinforced rice husk precursor is placed in a tube furnace, high-purity argon gas is introduced at a flow rate of 200 mL / min, and the temperature is directly raised to 850°C at a rate of 4°C / min. The temperature is held for 11 hours (which is exactly the same as the total holding time of the five-stage carbonization in Example 1), and then naturally cooled to room temperature to obtain the carbonized matrix.

[0068] Comparative Example 4 The only difference between Comparative Example 4 and Example 1 is that step (4) is not a five-stage carbonization process, but a two-stage carbonization process. Step (3) of Comparative Example 4 is as follows: the interface-reinforced rice husk precursor is placed in a tube furnace, high-purity argon gas is introduced at a flow rate of 200 mL / min, the temperature is raised to 400°C at a rate of 2°C / min, and held for 5.5 h (pre-carbonization stage, consistent with the total holding time of the first two stages in Example 1); then the temperature is raised to 850°C at a rate of 4°C / min, and held for 5.5 h (high-temperature carbonization stage, consistent with the total holding time of the last three stages in Example 1), and then naturally cooled to room temperature to obtain the carbonized matrix.

[0069] Comparative Example 5 The only difference between Comparative Example 5 and Example 1 is that step (4) is not a five-stage carbonization process, but a three-stage carbonization process. Step (3) of Comparative Example 5 is as follows: the interface-reinforced rice husk precursor is placed in a tube furnace, high-purity argon gas is introduced at a flow rate of 200 mL / min, and the temperature is raised to 250°C at a rate of 2°C / min and held for 1.5 h (low temperature setting stage); then the temperature is raised to 420°C at a rate of 3°C / min and held for 6 h (medium temperature pyrolysis stage, combining the hemicellulose pyrolysis and synergistic pyrolysis stages of Example 1); then the temperature is raised to 850°C at a rate of 4°C / min and held for 3.5 h (high temperature carbonization stage, combining the aromatization and high temperature setting stages of Example 1), and then naturally cooled to room temperature to obtain the carbonized matrix.

[0070] Comparative Example 6 The only difference between Comparative Example 6 and Example 1 is that step (4) is not a five-stage carbonization process, but a four-stage carbonization process. Step (3) of Comparative Example 6 is as follows: the interface-strengthened rice husk precursor is placed in a tube furnace, high-purity argon gas is introduced at a flow rate of 200 mL / min, and the following four-stage heating program is executed: 1. Low-temperature cross-linking and setting section: Heat to 200℃ at a rate of 1.5℃ / min and hold for 1.5 hours; 2. Cellulose-lignin co-pyrolysis section: The temperature was increased to 420℃ at 2.5℃ / min and held for 6 hours (combining the holding time of the hemicellulose pyrolysis section in Example 1). 3. Carbon skeleton aromatization section: Heat to 700℃ at a rate of 4℃ / min and hold for 2.5h; 4. High-temperature structural shaping section: Heat to 850℃ at 4℃ / min and hold for 1.5h; cool naturally to room temperature to obtain carbonized matrix.

[0071] Comparative Example 7 The only difference between Comparative Example 7 and Example 1 is that step (5) is omitted, and step (4) yields the porous carbon product.

[0072] Example 2 (1) Raw material pretreatment: Take rice husk raw material, ultrasonically clean it twice with deionized water to remove mud and impurities, and vacuum dry it at 60℃ for 6 hours. Take 100g of dried rice husks, add 1000mL of 10% ethanol-water solution, stir in a water bath at 60℃ for 2h, filter, wash twice with deionized water, vacuum dry at 80℃ for 4h, and pulverize to below 100 mesh to obtain pretreated rice husk powder. (2) Crosslinking reaction: Take 100g of pretreated rice husk powder, add 0.5g of boric acid crosslinking agent and 500mL of deionized water, stir at high speed for 30min to mix evenly, transfer to a hydrothermal reactor, and perform hydrothermal crosslinking reaction at 130℃ for 2h. After filtration and drying, obtain the interface-reinforced rice husk precursor. (3) Segmented carbonization treatment: The interface-enhanced rice husk precursor was placed in a tube furnace, and high-purity argon gas was introduced at a flow rate of 200 mL / min. The following five-stage heating program was then executed: Low-temperature cross-linking and shaping section: Heat to 180℃ at a rate of 1℃ / min and hold for 2 hours; Hemicellulose fractionation pyrolysis section: Heat to 320℃ at 1.5℃ / min and hold for 2 hours; Cellulose-lignin co-pyrolysis section: Heat to 380℃ at 2℃ / min and hold for 4 hours; Carbon skeleton aromatization section: Heat to 600℃ at 3℃ / min and hold for 3h; High-temperature structural shaping section: The temperature is increased to 800℃ at 3℃ / min and held for 2 hours; then naturally cooled to room temperature to obtain a carbonized matrix containing an in-situ uniformly distributed SiO2 template; (4) Activation treatment: Take 100g of carbonized matrix and mix it evenly with 100g of composite activator (potassium hydroxide: potassium carbonate = 2:1). Place it in a tube furnace, introduce high-purity argon gas at a flow rate of 200mL / min, heat it to 750℃ at 2℃ / min, keep it at the temperature for 2h, and cool it naturally to room temperature to obtain the pore-forming carbon material. (5) Post-processing: After pore formation, the carbon material was placed in a 0.5 mol / L sodium hydroxide aqueous solution and etched by stirring at 50°C for 3 hours to complete deep template removal. It was then washed with deionized water until the pH of the filtrate was neutral, dried under vacuum at 105°C for 8 hours, crushed and classified by an air jet mill to a D50 of 10 μm, sieved through a 300-mesh sieve, and demagnetized by 10000Gs to obtain the porous carbon product.

[0073] Example 3 (1) Raw material pretreatment: Take rice husk raw material, ultrasonically clean it 3 times with deionized water to remove mud and impurities, and vacuum dry it at 100℃ for 4 hours. Take 100g of dried rice husks, add 1500mL of 30% ethanol-water solution, stir in a water bath at 30℃ for 3h for pretreatment, filter, wash twice with deionized water, vacuum dry at 80℃ for 4h, and pulverize to below 100 mesh to obtain pretreated rice husk powder. (2) Crosslinking reaction: Take 100g of pretreated rice husk powder, add 3g of silane coupling agent and 1000mL of deionized water, stir at high speed for 30min to mix evenly, transfer to a hydrothermal reactor, and perform hydrothermal crosslinking reaction at 90℃ for 4h. After filtration and drying, obtain the interface-reinforced rice husk precursor. (3) Segmented carbonization treatment: The interface-enhanced rice husk precursor was placed in a tube furnace, and high-purity argon gas was introduced at a flow rate of 200 mL / min. The following five-stage heating program was then executed: Low-temperature cross-linking and shaping section: heat up to 220℃ at a rate of 2℃ / min and hold for 1 hour; Hemicellulose fractionation pyrolysis section: Heat to 260℃ at 1℃ / min and hold for 3 hours; Cellulose-lignin synergistic pyrolysis section: Heat to 450℃ at 3℃ / min and hold for 3h; Carbon skeleton aromatization section: Heat to 750℃ at 3℃ / min and hold for 2h; High-temperature structural shaping section: The temperature is increased to 900℃ at 4℃ / min and held for 1 hour; then naturally cooled to room temperature to obtain a carbonized matrix containing an in-situ uniformly distributed SiO2 template; (4) Activation treatment: Take 100g of carbonized matrix and mix it evenly with 250g of composite activator (potassium hydroxide: potassium carbonate = 2:1). Place it in a tube furnace, introduce high-purity argon gas at a flow rate of 200mL / min, heat it to 850℃ at 4℃ / min, hold it at that temperature for 1h, and let it cool naturally to room temperature to obtain the pore-forming carbon material. (5) Post-processing: After pore formation, the carbon material was placed in a 1 mol / L potassium hydroxide aqueous solution and etched by stirring at 60°C for 1 hour to complete deep template removal. It was then washed with deionized water until the pH of the filtrate was neutral, dried under vacuum at 105°C for 8 hours, crushed and classified by an air jet mill to a D50 of 10 μm, sieved through a 300-mesh sieve, and demagnetized by 10000Gs to obtain the porous carbon product.

[0074] Example 4 (1) Raw material pretreatment: Take rice husk raw material, ultrasonically clean it 3 times with deionized water to remove mud and impurities, and vacuum dry it at 90℃ for 5 hours. Take 100g of dried rice husks, add 1000mL of sodium carbonate solution with a mass fraction of 0.5%, stir in a water bath at 50℃ for 1h for pretreatment, filter, wash twice with deionized water, vacuum dry at 80℃ for 4h, and pulverize to below 100 mesh to obtain pretreated rice husk powder. (2) Crosslinking reaction: Take 100g of pretreated rice husk powder, add 5g of citric acid and 800mL of deionized water, stir at high speed for 30min to mix evenly, transfer to a hydrothermal reactor, perform hydrothermal crosslinking reaction at 100℃ for 5h, filter and dry to obtain interface-reinforced rice husk precursor. (3) Segmented carbonization treatment: The interface-enhanced rice husk precursor was placed in a tube furnace, and high-purity argon gas was introduced at a flow rate of 200 mL / min. The following five-stage heating program was then executed: Low-temperature cross-linking and shaping section: Heat to 200℃ at a rate of 1℃ / min and hold for 2 hours; Hemicellulose fractionation pyrolysis section: Heat to 320℃ at 3℃ / min and hold for 2 hours; Cellulose-lignin synergistic pyrolysis section: Heat to 400℃ at 2℃ / min and hold for 4h; Carbon skeleton aromatization section: heat up to 750℃ at 5℃ / min and hold for 2.5h; High-temperature structural shaping section: The temperature is increased to 950℃ at 5℃ / min and held for 1 hour; then naturally cooled to room temperature to obtain a carbonized matrix containing an in-situ uniformly distributed SiO2 template; (4) Activation treatment: Take 100g of carbonized matrix and mix it evenly with 200g of composite activator (potassium hydroxide: potassium carbonate = 3:1). Place it in a tube furnace, introduce high-purity argon gas at a flow rate of 200mL / min, heat it to 850℃ at 3℃ / min, keep it at that temperature for 1h, and let it cool naturally to room temperature to obtain the pore-forming carbon material. (5) Post-processing: After pore formation, the carbon material was placed in a 1.5 mol / L sodium hydroxide aqueous solution and etched by stirring at 70°C for 1 hour to complete deep template removal. It was then washed with deionized water until the pH of the filtrate was neutral, dried under vacuum at 105°C for 8 hours, crushed and classified by an air jet mill to a D50 of 10 μm, sieved through a 300-mesh sieve, and demagnetized by 10000 Gs to obtain the porous carbon product.

[0075] The specific surface area, pore volume, mesopore ratio, and pore size of the porous carbon products obtained in Examples 1-4 and Comparative Examples 1-7 were tested using the following methods.

[0076] Reference standard: GB / T 19587-2017 Determination of specific surface area of ​​solid substances by gas adsorption BET method.

[0077] Testing instrument: BSD-PS type BSD specific surface area tester.

[0078] Specific steps: Sample pretreatment: Pass the porous carbon powder through a 200-mesh sieve, weigh about 100-200 mg of sample and place it in a sample tube, and degas it under vacuum at 300℃ for 6-12 h to remove moisture and impurity gases adsorbed on the sample surface.

[0079] Test Procedure: The treated sample tubes were installed in the analysis station, and the nitrogen adsorption-desorption isotherm was measured at liquid nitrogen temperature (77K). The relative pressure (P / P0) was measured in the range of 0.005 to 0.995. The specific surface area was calculated using the multi-point BET method, and adsorption data points with relative pressures in the range of 0.05 to 0.30 were selected for linear fitting. The BJH model was used to analyze the pore size distribution and pore volume of mesopores (3-15 nm). The BJH extended model was used to analyze the pore size distribution and pore volume of macropores (50-90 nm). The DFT model was used to analyze the pore size distribution and pore volume of micropores (<2 nm). The total pore volume was calculated from the adsorption amount at a relative pressure of P / P0≈0.99.

[0080] Key testing parameters: specific surface area (S_BET), total pore volume (V_total), micropore volume (V_micro), mesopore volume (V_meso), and average pore size (D_average). The proportion of mesopore volume to total pore volume is calculated as V_meso / V_total × 100%.

[0081] The test results are shown in Table 1.

[0082] Table 1 The porous carbon products obtained in Examples 1, 1-7, and 2-4 were deposited with silicon in the following manner to obtain silicon-carbon materials: 1. Loading and Atmosphere Replacement: 10g of porous carbon product was evenly spread in a quartz boat and pushed into the constant temperature zone of a tube furnace; after sealing the furnace, a vacuum was evacuated to below 10Pa, and high-purity argon gas was introduced to atmospheric pressure. This process was repeated 3 times to purge the air, with the argon gas flow rate maintained at 50mL / min throughout. 2. Heating Pre-stabilization: The temperature was increased to 520℃ at a rate of 5℃ / min and held at this temperature for 1 hour to ensure uniform temperature within the furnace and remove residual adsorbed water from the porous carbon surface. 3. Isothermal Silicon Deposition: While maintaining the temperature and argon gas flow rate, a 10% silane-argon mixture was introduced, with the total flow rate of the mixture controlled at 100mL / min. The mixture was deposited at this temperature for 6 hours. After deposition, the silane mixture was immediately shut off, and pure argon gas purging was resumed. 4. Annealing and cooling discharge: Maintain pure argon purging, continue annealing at 520℃ for 1 hour, then naturally cool to room temperature at a rate of 5℃ / min. After discharge, pass through a 200-mesh sieve to obtain silicon-carbon material.

[0083] The silicon-carbon material was used to prepare the negative electrode sheet: N-methylpyrrolidone (NMP) was added according to the mass ratio of silicon-carbon material: conductive agent SP: binder PVDF = 92:3:5, and the mixture was stirred at high speed to make a slurry. The slurry was coated on copper foil, dried under vacuum at 80°C for 4 hours, and then rolled (compacted density 1.5 g / cm³) and cut into sheets (diameter 12 mm) to obtain the negative electrode sheet.

[0084] A CR2032 coin cell was assembled in an argon glove box using a lithium metal sheet as the counter electrode, Celgard 2400 as the separator, and 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio 1:1:1) as the electrolyte.

[0085] Electrochemical performance testing was performed on the battery. The specific testing method was as follows: A battery testing system was used, with a test voltage range of 0.01~2.0V. First, constant current charge-discharge cycles were performed at a 0.1C rate, and the initial lithium insertion specific capacity (i.e., initial discharge specific capacity) and initial coulombic efficiency were recorded. Subsequently, 400 constant current charge-discharge cycles were performed at a 1C rate, and the discharge capacity retention rate at the 400th cycle was calculated (retention rate = discharge capacity at the 400th cycle / discharge capacity at the first cycle × 100%). All tests were performed with at least three parallel samples, and the average value was taken.

[0086] The test results are shown in Table 2.

[0087] Table 2 As can be seen from the data in Table 2, pretreatment of rice husks to remove small molecule impurities such as fat and protein, as well as reaction of rice husks with crosslinking agents, can significantly improve the electrochemical performance of the resulting silicon-carbon materials.

[0088] Furthermore, this invention employs a five-stage gradient carbonization process to obtain porous carbon materials with richer pore structures and superior overall performance, thereby further improving the electrochemical performance of silicon-carbon materials.

[0089] In summary, this invention successfully prepared high-performance porous carbon materials through the synergistic effects of component regulation, in-situ interface enhancement, and five-stage gradient carbonization. The silicon-carbon materials and battery anodes prepared based on these porous carbon materials both exhibit excellent electrochemical performance.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing porous carbon from rice husks, characterized in that, include: Step S1: Rice husks are pretreated with an aqueous solution of ethanol or an aqueous solution of sodium carbonate, dried, and pulverized to obtain rice husk powder. Step S2: The rice husk powder is cross-linked with a cross-linking agent to obtain the rice husk precursor. Step S3: The rice husk precursor undergoes a five-stage heat treatment in an inert atmosphere. The heat treatment temperature for the first stage is 180~220℃; The heat treatment temperature for the second stage is 260~320℃; The heat treatment temperature for the third stage is 380~450℃; The heat treatment temperature for the fourth stage is 600~750℃; The heat treatment temperature for the fifth stage is 800~950℃; After heat treatment, a carbonized matrix is ​​obtained; Step S4, Activation and Pore Formation: The carbonized matrix and composite activator are activated under an inert atmosphere to obtain the activated product; Step S5, post-processing: Etching in an alkaline solution removes SiO2 from the activated product to obtain porous carbon material.

2. The method for preparing porous carbon from rice husks according to claim 1, characterized in that, Includes at least one of the following features (1) to (5): (1) The volume fraction of ethanol in the aqueous solution of ethanol is 10-30%; (2) The mass fraction of sodium carbonate in the aqueous solution of sodium carbonate is 0.1~0.5%; (3) The solid-liquid ratio of rice husk to aqueous solution of ethanol or aqueous solution of sodium carbonate is 1g: (10~15)mL; (4) Crush the rice husks to below 100 mesh; (5) The temperature of the pretreatment is 30~60℃; the time of the pretreatment is 1~3h.

3. The method for preparing porous carbon from rice husks according to claim 1, characterized in that, Includes at least one of the following features (1) to (5): (1) The crosslinking agent is at least one of boric acid, silane coupling agent, and citric acid; (2) The amount of the crosslinking agent added is 0.5-5% of the mass of the rice husk powder; (3) The temperature of the cross-linking reaction is 90~130℃; (4) The cross-linking reaction takes 2 to 5 hours.

4. The method for preparing porous carbon from rice husks according to claim 1, characterized in that, Includes at least one of the following features (1) to (4): (1) The composite activator is a mixture of potassium hydroxide and potassium carbonate; (2) The mass ratio of potassium hydroxide to potassium carbonate in the composite activator is 2~3:1; (3) The mass ratio of the composite activator to the carbonized matrix is ​​1~2.5:1; (4) The activation treatment temperature is 750~850℃, the heating rate is 2~4℃ / min, and the holding time is 1~2h.

5. The method for preparing porous carbon from rice husks according to claim 1, characterized in that, The alkaline solution is a 0.5~1.5mol / L aqueous solution of sodium hydroxide or potassium hydroxide; the etching temperature is 50~70℃ and the etching time is 1~3h.

6. A porous carbon material, characterized in that, The porous carbon material satisfies the following characteristics: (a) Specific surface area of ​​1800-2400 m² / g, total pore volume of 0.8-2.15 cm³ / g; (b) It has a three-level pore structure consisting of micropores, mesopores, and macropores; (c) Based on the pore volume ratio Micropores with a pore size of 0.5-2 nm account for 5%-15%; Mesopores with a pore size concentrated in the range of 3-15 nm account for 65%-85%; Macropores with a pore size concentrated in the 50-90 nm range account for 5%-25%; Furthermore, the sum of the pore volume ratios of micropores, mesopores, and macropores is 100%; (d) It has a continuous, interconnected three-dimensional network of channels; (e) Prepared by the method described in any one of claims 1-5.

7. A silicon-carbon material, characterized in that, Silicon is obtained by depositing silicon in the pores of the porous carbon material as described in claim 6 using chemical vapor deposition.

8. A battery negative electrode, characterized in that, Includes the silicon-carbon material as described in claim 7.

9. A battery, characterized in that, Includes the negative electrode as described in claim 8.

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