A lignin-rich nitrogen carbon / silicon oxide composite material with a core-shell structure and preparation and application thereof

Abstract

The application discloses a lignin-rich nitrogen carbon / silicon oxide composite material with a core-shell structure and a preparation and application thereof. Lignin is used as a carbon source, and etherification modification is carried out on the lignin, and then a silicon source is added under alkaline conditions to obtain an etherification-modified lignin / silicon oxide composite precursor, and then carbonization is carried out to obtain a lignin carbon / silicon oxide composite material, and then the lignin carbon / silicon oxide composite material is subjected to hydrothermal treatment together with melamine, an oxalate and a lignin sulfonate to obtain a supramolecular structure, and finally, carbonization is carried out again to obtain the lignin-rich nitrogen carbon / silicon oxide composite material. The method disclosed by the application realizes uniform and stable coating of the lignin-rich nitrogen carbon on the silicon oxide, inhibits volume expansion of the silicon oxide generated in the process of stripping / embedding lithium ions and enhances the conductivity of the silicon oxide. On the other hand, the method regulates the pore structure of the material, reduces consumption of lithium ions in the process of forming a solid electrolyte interface film, and significantly improves the first coulomb efficiency and the cycle stability of a lithium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery anode material technology, specifically relating to a lignin-rich nitrogen-carbon / silicon oxide composite material with a core-shell structure and its preparation and application. Background Technology

[0002] Lithium-ion batteries (LIBs) possess advantages such as high energy density, low self-discharge, long cycle life, and environmental friendliness, and have been widely used in electric vehicles, aerospace, and many other fields in recent years. The anode material is one of the most important components of a lithium-ion battery, playing a crucial role in its performance. Graphite has become the most popular choice due to its high conductivity and good cycle stability. However, its low theoretical capacity (372 mAh / g) makes it difficult for commercial graphite to meet the demands of high energy and power density energy storage applications. Therefore, there is an urgent need to develop high-performance anode materials for next-generation lithium-ion batteries with higher energy and power densities.

[0003] Alloy-based anode materials have received widespread attention in recent years as anode materials for lithium-ion batteries due to their significantly higher theoretical capacity compared to carbon-based materials. Silicon oxide (SiO₂) x Due to its high theoretical specific capacity (1965 mAh / g for SiO2 and 2680 mAh / g for SiO), high mechanical strength, low cost, and wide availability, SiO2 has gradually become one of the most promising anode materials for lithium-ion batteries. However, SiO2... x As a negative electrode material for lithium-ion batteries, there are two problems that need to be solved: (1) there is a severe volume expansion effect (volume change of about 200%) during the lithiation / delithiation process, which leads to a significant capacity decay; (2) SiO x Its low electronic conductivity is detrimental to long-cycle performance and rate performance.

[0004] To address the above issues, researchers have proposed several methods to improve SiO₂ production. x The strategy for improving lithium storage performance is first to utilize SiO₂. x Combined with carbon to prepare SiO x In the / C composite material, highly conductive carbon fibers are linked together to form a continuous conductive network, improving the overall conductivity of the composite material. The carbon skeleton can also effectively accommodate the internal SiO₂. x Repeated volume changes ensure the structural integrity of the electrode material. Furthermore, the continuous carbonaceous network prevents SiO₂ from forming. xDirect contact with the electrolyte facilitates the formation of a stable SEI film during cycling, which enhances the reversibility of lithium-ion insertion / extraction. Yang et al. (Adv. Mater. Interfaces (2019) 6(6): 1801-809.) used tetraethyl orthosilicate as the silicon source and sucrose as the carbon source, and controlled the size and morphology of silicon oxide with hydrofluoric acid to prepare a silica with a specific surface area of ​​208 m². 2 / g of SiO x / C composite material, as a negative electrode material for lithium-ion batteries, exhibits a capacity of 820 mAh / g after 200 cycles at a current density of 100 mA / g. However, a larger specific surface area leads to a larger irreversible capacity, resulting in a lower initial coulombic efficiency for lithium-ion batteries. Luo et al. (ACS Appl. Energy Mater. (2022) 5:8982-8989) first prepared SiO2 using vinyltriethoxysilane as a raw material. x A core-shell structured SiO₂ was prepared by two-step carbonization of a composite material, which was then surface-modified with polydiallyldimethylammonium chloride and coated with polymethyl methacrylate. x / C composite material, which has 22.2m 2 With a low specific surface area of ​​ / g, SiO₂ exhibited an initial coulombic efficiency of 68.5% under charge-discharge cycle testing at a current density of 0.5 A / g, and maintained a high discharge specific capacity of 770 mAh / g after 500 cycles. Therefore, by controlling the structure of the composite material through two-step carbonization, SiO₂ with suitable specific surface area and pore structure can be prepared. x The goal is to address the issues of poor conductivity and low initial coulombic efficiency in carbon / silicon oxide composites. However, current carbon sources for carbon / silicon oxide composites are typically sugars such as sucrose and glucose, polymers such as phenolic resins and epoxy resins, and nanomaterials such as carbon nanotubes and graphene. These carbon sources are costly and complex to prepare, making it difficult to achieve low-cost and large-scale industrial production.

[0005] Lignin is the most abundant aromatic polymer with a three-dimensional network structure in nature and the second largest natural biomass resource. It accounts for up to 30% of plant content, contains up to 50% carbon, is low in cost, renewable, and rich in functional groups such as hydroxyl, benzyl, methoxy, ether, and carboxyl groups, exhibiting excellent structural designability. It is an ideal precursor for preparing carbon-based materials in carbon / silicon oxide composites.

[0006] Currently, there are many reports on research regarding lignin / silica materials and lignin-carbon / silica composites, but reports on nitrogen-doped lignin-carbon / silica composites are relatively few, as detailed below:

[0007] In the research of lignin / silica materials, Zhong Ruisheng et al. (Journal of Chemical Industry and Engineering (2015) 66(8):3255-3261.) used alkali lignin as raw material, obtained phosphorylated alkali lignin through phosphorylation modification, and then prepared lignin / SiO2 composite nanoparticles by acid precipitation co-precipitation method. When added to high-density polyethylene plastic, the tensile strength and tensile strength at break increased by 48.68% and 73.57%, respectively. Liu et al. (Industrial Crops and Products (2022) 189:115842.) modified lignin by sol-gel method, in which tetraethyl orthosilicate precursor was hydrolyzed / condensed to deposit SiO2 on the lignin surface. After ball milling, a lignin / SiO2 composite with uniform particle size was obtained. The high-density polyethylene material with 3.0 wt.% of this composite had good tensile strength and thermal stability. However, these studies mainly address the problems of SiO2 particles' strong surface polarity and hydrophilicity, leading to easy aggregation and poor dispersibility. They cannot achieve uniform coating of SiO2 with lignin. Therefore, if these materials are directly carbonized, the lignin carbon network will collapse and aggregate, which is not conducive to the insertion and extraction of lithium ions. Brovko et al. (Materials Chemistry and Physics (2021) 269:124768.) obtained a novel porous aerogel material using sodium lignin sulfonate as raw material and sol-gel technology and supercritical fluid process. However, this material has a large number of micropores and high ion transport resistance, resulting in poor rate performance as a negative electrode material for lithium-ion batteries. He et al. (Waste Management (2021) 135:381-388.) prepared hybrid nanoparticles with different particle sizes and different lignin coating amounts using a two-step acid precipitation method. By changing the pH value during the silicate polycondensation process, the particle size of the obtained SiO2 can be controlled. The optimized product exhibits excellent reinforcing properties, promoting the application of lignin in the field of rubber reinforcing agents. Xiong et al. (Chemical Engineering Journal (2017) 326: 803-810.) prepared a lignin / SiO2 composite based on sodium metasilicate and quaternized modified alkali lignin by co-precipitation method, further improving the dispersion stability between lignin and SiO2. This material was applied in the field of UV protection. After being blended with polyurethane, the UV transmittance below 315nm and 400nm decreased by 99.3% and 87.0%, respectively. However, due to the high silica content (over 50%) in these materials, lignin only disperses silica without coating it. If it is directly carbonized, it will still cause a large volume expansion effect, resulting in low reversible capacity, poor cycle stability and rate performance as a lithium battery anode material.

[0008] In the research of lignin-carbon / silica composites, Jiang et al. (Composites Science and Technology (2022) 230: 109775.) used lignin as raw material, added formaldehyde and reacted at 90℃ for 3h to obtain hydroxymethyl modified lignin, and then reacted at 60℃ for half an hour to modify it with silane coupling, thereby improving the affinity between lignin and SiO2 sol. The lignin-carbon / SiO2 composite was prepared by carbonization at 800℃. When it was added to styrene-butadiene rubber, the tensile strength and elongation at break of the composite rubber were significantly improved. In the prepared lignin-carbon / SiO2 composite, the lignin carbon did not coat the silica, so lithium ion storage could not be achieved. Zhong et al. (International Journal of Biological Macromolecules (2022) 217: 66-76.) prepared a lignin-carbon / SiO2 composite material by carbonizing silicon and lignin at 900℃ with the addition of tetraethyl silicate for crosslinking with lignin. This material has a high specific surface area and is mainly used in supercapacitors. However, this material has a rich porous structure, mainly micropores around 1 nm in size. As a negative electrode material for lithium-ion batteries, it suffers from slow ion transport rate, low initial coulombic efficiency, and low reversible capacity.

[0009] In the research on lignin-carbon / silica composite materials as lithium-ion battery anode materials, Chinese patent application CN108878813A describes the process of combining industrial lignin with nano-SiO2, obtaining lignin-carbon / SiO2 composite materials after carbonization, and then obtaining porous lignin-carbon / SiO2 composite materials through hydrofluoric acid etching. These composite materials exhibit certain performance when applied to lithium-ion battery anodes. However, this process suffers from problems such as uneven lignin-SiO2 composite formation, high specific surface area, and low initial coulombic efficiency. Huang et al. (Microporous and Mesoporous Materials (2021) 307: 111004-111012.) synthesized honeycomb-shaped lignin-mesoporous carbon / SiO2 composite materials using quaternized alkali lignin as the carbon source, sodium dodecylbenzenesulfonate as a soft template agent and structure directing agent, and nano-SiO2 as a hard template agent, employing a dual-template assisted self-assembly method. The material has a silica content of 21 wt.% and exhibits a reversible capacity of up to 1109 mAh / g after 100 cycles at 0.1 A / g, demonstrating excellent electrochemical performance. However, this process requires prior surface modification, resulting in a complex and costly synthesis route. Furthermore, the process mitigates the volume expansion effect by etching part of the SiO2 with sodium hydroxide solution, leading to an excessively rich pore structure and a high specific surface area, which in turn significantly reduces the initial coulombic efficiency. Simultaneously, the material exhibits poor rate performance, with a capacity of only 372 mAh / g at 1 A / g. Huang Si (Huang Si. Microstructure Modification and Lithium / Sodium Storage Performance of Lignin Mesoporous Carbon [D]. Guangzhou: South China University of Technology, 2022) prepared open-cell hollow spherical lignin mesoporous carbon / SiO2 based on electrostatic interaction and double hydrolysis using sodium metasilicate as the silicon source and quaternized alkali lignin as the carbon source. x Composite material, SiO obtained after optimization x The composite material with a content of 9.0 wt.% exhibits excellent electrochemical performance as a lithium-ion battery anode, providing a high specific capacity of 989 mAh / g after 400 cycles at a current density of 0.1 A / g. However, since most of the silica in this material serves as a hard template, some incompletely coated silica remains, requiring removal through alkaline etching. This does not solve the problem of low initial coulombic efficiency in lignin-carbon materials, and the specific surface area remains as high as 902 m². 2 The initial coulombic efficiency is only 47.6% per g. Furthermore, due to weak electrostatic interaction, the material's structural stability is poor, leading to partial collapse after etching and electrode breakage during cycling. In summary, current technologies and processes for preparing lignin-carbon / silicon oxide composite materials still suffer from poor rate performance and low initial coulombic efficiency when used as anode materials in lithium-ion batteries.

[0010] Additionally, it can be found in SiO xNitrogen doping is performed on C / C composite materials. Nitrogen doping has been shown to provide additional abundant defect sites for effective adsorption of lithium ions, thereby improving the cycle performance and rate performance of lithium-ion batteries at high current densities. Generally, under the condition of fixed porosity and graphitization degree, the higher the nitrogen doping amount, the better the electrochemical performance. Among them, melamine has low cost, high nitrogen content, and the graphitic carbon nitride (g-C3N4) produced by decomposition at high temperature still has abundant nitrogen atoms, and is often used as the nitrogen source for nitrogen doping materials. Kensy et al. (Carbon (2020) 161:190-197.) prepared nitrogen-doped carbon materials using melamine as the nitrogen source, and the nitrogen doping amount was 1.14 wt.% at 700℃. Zhang et al. (Carbon(2021)178:202-210.) prepared nitrogen-doped carbon nanosheets / silica composite materials using melamine through simple ball milling and carbonization. As a negative electrode material for lithium-ion batteries, it exhibited a high specific capacity of 1129 mAh / g after 50 cycles at a current density of 0.5 A / g, while the material without nitrogen doping under the same conditions only had a specific capacity of 688 mAh / g. However, the g-C3N4 produced by the decomposition of melamine has poor thermal stability and begins to decompose and be lost at around 550 °C, resulting in a low nitrogen doping content (generally less than 5.0 at.%) in the carbon material at high temperatures.

[0011] To address the issues of low nitrogen doping levels and uncontrollable doping sites, researchers have discovered that metal cyanamide compounds can delay the decomposition of g-C3N4 generated from the high-temperature decomposition of nitrogen sources such as melamine, thereby enabling the preparation of carbon materials with high nitrogen doping levels. Zhang et al. (Nano Energy (2021) 87:106184.) prepared nitrogen-doped carbon materials by directly carbonizing a mixture of melamine and zinc oxide. Even at a carbonization temperature of 800℃, the nitrogen doping level of this material reached 21.2 at.%, with an edge nitrogen content as high as 18.9 at.%. Further research revealed that the pyrolysis of calcium oxide mixed with melamine can also achieve the preparation of high nitrogen-doped carbon materials. This is because the metal oxide reacts with g-C3N4 to generate thermally stable cyanamide salts (ZnNCN and CaNCN), which act as catalysts, delaying the complete decomposition of nitrogen species. This material exhibits excellent electrochemical performance for potassium / sodium storage. However, this material cannot be used to coat silicon oxide, therefore, its lithium storage capacity is relatively low when used directly as a negative electrode material for lithium-ion batteries.

[0012] In the research of nitrogen-doped lignin-carbon materials, Fan et al. (Journal of Energy Storage (2023) 63:106974.) used sodium lignin sulfonate as a carbon source to synthesize a sodium lignin sulfonate / ZnC2O4 composite precursor through solvent-induced self-assembly. This precursor was then blended with melamine and carbonized at 800℃ to obtain nitrogen-doped lignin-carbon materials. These materials exhibit low specific surface area and high nitrogen content, resulting in excellent volumetric specific capacitance for supercapacitors. However, if this process is used to prepare nitrogen-rich lignin-carbon / silica composite materials for lithium-ion batteries, the problem is that the nitrogen-rich carbon layer generated in the one-step process cannot further coat the silica. Furthermore, the lignin used in this process is unmodified, resulting in weak interaction with silica and an inability to suppress silica expansion, leading to low initial coulombic efficiency and poor cycle stability. Huang et al. (Adv. Funct. Mater. (2020) 32: 2203279.) used an in-situ polycondensation method to crosslink phenolic hydroxyl groups in lignin with formaldehyde and urea to form lignin-amine urea-formaldehyde resin. Then, through electrostatic interaction, they achieved efficient dispersion and uniform coating of SiO2. Finally, the prepared SiO2 / lignin-amine urea-formaldehyde resin precursor was carbonized at 600℃ and then etched with an alkaline etchant to obtain nitrogen-doped lignin mesoporous carbon with high reversible capacity as a sodium-ion battery anode material. However, in this process, SiO2 is only used as a hard template, and the specific surface area of ​​the material is as high as 690 m². 2 / g increases the irreversible reaction of the electrode, which is not conducive to improving the initial coulombic efficiency and reversible specific capacity of the material.

[0013] In summary, current technologies and processes for preparing nitrogen-doped lignin-carbon and lignin-carbon / silicon oxide composites still suffer from poor rate performance and low initial coulombic efficiency when used as anode materials in lithium-ion batteries. Furthermore, because suitable modification methods for lignin have not been found, only modifications to SiO₂ can be achieved. x The efficient dispersion of SiO2 cannot be achieved. x Uniform and stable coating is required. However, most of the silica in these materials only serves as a hard template and needs to be removed by etching, resulting in an excessively large specific surface area and low initial coulombic efficiency. Furthermore, to obtain more lithium-ion adsorption active sites, lignin is directly blended with a nitrogen source for nitrogen atom doping, but this also suffers from low nitrogen doping efficiency and uncontrollable nitrogen doping sites. Therefore, it is impossible to obtain a nitrogen-doped lignin-carbon / silica composite material with good lithium storage performance. Summary of the Invention

[0014] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing a lignin-rich nitrogen-carbon / silica composite material with a core-shell structure.

[0015] The method of this invention uses lignin as a carbon source, which is etherified to increase the hydroxyl groups and molecular weight of the lignin. Then, a silicon source is added under alkaline conditions to obtain an etherified lignin / silica composite precursor. Then, carbonization is performed to obtain a lignin-carbon / silica composite material. In this step, based on hydrogen bonding, the etherified lignin can be efficiently dispersed and uniformly coated with the silica produced by hydrolysis. Further hydrothermal treatment with melamine, oxalate, and lignin sulfonate is performed to obtain a supramolecular structure, which is beneficial to improving the structural stability of the composite material and the efficient coating of silica. Finally, a second carbonization is performed to obtain a nitrogen-rich lignin-carbon / silica composite material. The efficient nitrogen doping of the lignin-carbon / silica composite material is achieved by the cyanamide salt generated by melamine and metal oxide during the second carbonization process.

[0016] The method described in this invention achieves uniform and stable coating of silicon oxide with nitrogen-rich lignin, suppressing the volume expansion of silicon oxide during lithium ion extraction / intercalation and enhancing its conductivity. On the other hand, by performing secondary carbonization on the composite material, the pore structure of the material is further controlled, the structural stability of the carbon layer is maintained, the specific surface area is reduced, and the consumption of lithium ions during the formation of the solid electrolyte interface film is reduced, thus significantly improving the initial coulombic efficiency and cycle stability of the lithium-ion battery.

[0017] Another object of the present invention is to provide a lignin-rich nitrogen-carbon / silica composite material with a core-shell structure prepared by the above method.

[0018] The lignin-carbon is uniformly doped with nitrogen, and the efficient nitrogen retention effect of the cyanamide salt intermediate solves the problems of low nitrogen doping efficiency and uncontrollable nitrogen doping sites in nitrogen-doped lignin-carbon materials. Furthermore, thanks to the stable core-shell structure of the material after secondary carbonization, the nitrogen-rich carbon layer in the shell can further coat silicon oxide, effectively suppressing its expansion. This eliminates the need to etch silicon oxide to mitigate the volume expansion effect, thus allowing for more silicon oxide as an active material. The lignin-nitrogen-rich carbon / silicon oxide composite material has a high nitrogen doping content and a high silicon oxide content, further increasing the number of lithium storage active sites, thereby improving the specific capacity, initial coulombic efficiency, and rate performance of lithium-ion batteries.

[0019] In this invention, the specific surface area of ​​the lignin-rich nitrogen-carbon / silica composite material is less than 100 m². 2 / g, the mass content of silicon dioxide is not less than 20%, and the surface content of nitrogen in the composite material is not less than 20%.

[0020] Another object of the present invention is to provide the above-mentioned lignin-rich nitrogen-carbon / silica composite material with a core-shell structure as a negative electrode material in the fields of lithium-ion batteries, supercapacitors and photoelectrocatalysis.

[0021] The objective of this invention is achieved through the following technical solution:

[0022] A method for preparing a lignin-rich nitrogen-carbon / silica composite material with a core-shell structure includes the following steps:

[0023] (1) Add glycidyl ether compounds to a lignin solution with a pH of 11-13 and react at 70-90℃ for 3-6 hours to obtain an etherified lignin solution;

[0024] (2) After adding soluble silicate to the etherified lignin solution in step (1) and mixing evenly, add soluble ammonium salt to maintain the pH of the solution at 10-11. After reacting for 3-4 hours, add acid regulator to adjust the pH of the reaction system to 7-9. Let it stand at 50-80℃ for 1-3 hours, centrifuge, and dry to obtain etherified lignin / silica complex.

[0025] (3) Carbonize the etherified lignin / silica composite from step (2), wash and dry to obtain lignin carbon / silica composite.

[0026] (4) After mixing the lignin carbon / silica complex, oxalate, melamine, lignin sulfonate and water in step (3), the mixture was hydrothermally reacted at 120-160℃ for 6-12 hours and dried to obtain lignin carbon / silica / nitrogen-containing complex.

[0027] (5) Carbonize, wash and dry the lignin carbon / silica / nitrogen-containing composite material obtained in step (4) to obtain lignin nitrogen-rich carbon / silica composite material.

[0028] Preferably, the mass ratio of lignin, glycidyl ether compounds in step (1) and soluble silicate and soluble ammonium salt in step (2) is 10:1 to 10:5 to 15:5 to 15; more preferably, it is 10:1 to 4:5 to 10:5 to 10.

[0029] Preferably, the lignin solution with a pH of 11 to 13 in step (1) is obtained by the following method: preparing lignin into an aqueous solution with a mass concentration of 10 to 30%, and then adding an alkaline solution to adjust the pH to 11 to 13.

[0030] More preferably, the alkali in the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, and ammonia water; the mass concentration of the alkaline solution is 10-20%.

[0031] Preferably, the lignin in step (1) is at least one of the following: enzymatically hydrolyzed lignin extracted from biorefining residue, organic solvent lignin obtained from solvent pulping, and alkali lignin extracted from black liquor of alkali pulping.

[0032] More preferably, the lignin is obtained by purifying at least one of the following: enzymatically hydrolyzed lignin extracted from the above-mentioned biorefining residue, organic solvent lignin obtained from solvent pulping, and alkali lignin extracted from black liquor from alkaline pulping. The purification can be carried out by conventional purification methods in the art, which can be achieved by the following method: dissolving lignin in alkaline solution, heating and stirring to dissolve, filtering, adding acid to the filtrate to fully aggregate and precipitate the lignin, separating, washing, and drying to obtain purified lignin.

[0033] Preferably, the glycidyl ether compound in step (1) is at least one of o-toluene glycidyl ether, polyoxyethylene glycidyl ether, and ethylene glycol diglycidyl ether.

[0034] Preferably, the soluble silicate in step (2) is at least one of potassium silicate and sodium silicate, and its cation must be the same as the cation of the alkali in the lignin solution in step (1).

[0035] Preferably, the soluble ammonium salt in step (2) is selected from at least one of ammonium carbonate, ammonium bicarbonate, ammonium chloride, ammonium sulfate, and ammonium nitrate.

[0036] Preferably, the acid regulator in step (2) is an acid solution with a mass concentration of 10-20%; the acid in the acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid, and its anion must be the same as the anion in the soluble ammonium salt in step (2).

[0037] Preferably, the centrifugation speed in step (2) is 5000-20000 rpm; the centrifugation time is 10-30 min.

[0038] Preferably, the drying in step (2) is at least one of blower drying, vacuum drying and infrared drying; more preferably, it is carried out at 40-60°C.

[0039] Preferably, the carbonization in step (3) is carried out in an inert gas atmosphere, wherein the inert gas is at least one of nitrogen, argon and helium.

[0040] Preferably, the carbonization conditions in step (3) are: carbonization at 150-350℃ for 10-60 min followed by carbonization at 500-700℃ for 1-5 h; more preferably, carbonization at 200-300℃ for 30-60 min followed by carbonization at 500-600℃ for 1-3 h.

[0041] Preferably, the carbonization process in step (3) is as follows: heating to 150-350°C at 5-15°C / min and holding for 10-60 min; then heating to 500-700°C at 5-15°C / min and holding for 1-5 h, and then cooling to room temperature; more preferably, heating to 200-300°C at 10°C / min and holding for 30-60 min; then heating to 500-600°C at 10°C / min and holding for 1-3 h, and then cooling to room temperature.

[0042] Preferably, the washing in step (3) refers to immersing the carbonized product in water to wash away any residual pyrolysis products; the drying is carried out at 60-100°C.

[0043] Preferably, the carbonization process in step (3) is carried out in a tubular furnace.

[0044] Preferably, the mass ratio of the lignin carbon / silica composite, oxalate, melamine and lignin sulfonate in step (4) is 1:1 to 10:1 to 10:1 to 10; more preferably, it is 1:1 to 4:1 to 4:1 to 4.

[0045] Preferably, the oxalate in step (4) is at least one of zinc oxalate, calcium oxalate and magnesium oxalate.

[0046] Preferably, the lignin sulfonate in step (4) is one of calcium lignin sulfonate, ammonium lignin sulfonate, or sodium lignin sulfonate.

[0047] Preferably, the lignin carbon / silica composite, oxalate, melamine, lignin sulfonate and water in step (4) are mixed to prepare a mixture with a mass concentration of 5-20%.

[0048] Preferably, the temperature of the hydrothermal reaction in step (4) is 140-160°C and the time is 8-12 hours.

[0049] Preferably, the drying in step (4) is at least one of blower drying, vacuum drying and infrared drying; more preferably, it is carried out at 40-60°C.

[0050] Preferably, the carbonization in step (5) is carried out in an inert gas atmosphere, wherein the inert gas is at least one of nitrogen, argon and helium.

[0051] Preferably, the carbonization conditions in step (5) are: carbonization at 150-350℃ for 10-60 min followed by carbonization at 800-1000℃ for 1-5 h; more preferably, carbonization at 200-300℃ for 30-60 min followed by carbonization at 800-900℃ for 1-3 h.

[0052] Preferably, the carbonization process in step (5) is as follows: heating to 150-350°C at 5-15°C / min and holding for 10-60 min; then heating to 800-1000°C at 5-15°C / min and holding for 1-5 h, and then cooling to room temperature; more preferably, heating to 200-300°C at 10°C / min and holding for 30-60 min; then heating to 800-900°C at 10°C / min and holding for 1-3 h, and then cooling to room temperature.

[0053] Preferably, the washing in step (5) refers to immersing the carbonized product in a dilute acid solution to wash away any residual pyrolysis products; the mass concentration of the dilute acid solution is 5-20%; the acid in the dilute acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid, and its anion must be the same as the anion in the soluble ammonium salt in step (2).

[0054] Preferably, the carbonization process in step (5) is carried out in a tubular furnace.

[0055] Preferably, the drying in step (5) is at least one of blower drying, vacuum drying and infrared drying; more preferably, it is carried out at 40-60°C.

[0056] The above method yields a lignin-rich nitrogen-carbon / silica composite material with a core-shell structure.

[0057] The above-mentioned lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure is used in lithium-ion batteries, supercapacitors, and photoelectrocatalysis.

[0058] Preferably, the application of lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure as a negative electrode material in lithium-ion batteries.

[0059] The present invention patent will now be described in more detail.

[0060] (1) Prepare a lignin solution with a mass concentration of 10-30%, add an alkaline solution to adjust the pH to 11-13; add glycidyl ether compounds to the lignin solution and react at 70-90℃ for 3-6 hours to obtain an etherified lignin solution;

[0061] This step is to etherify lignin to increase its molecular weight and obtain more oxygen-containing functional groups such as hydroxyl groups. This not only enhances the hydrogen bonding between lignin and silicon dioxide and improves the stability of lignin's coating on silicon dioxide, but also improves the thermal stability of lignin due to its high molecular weight, which is beneficial to improving the structural stability of the composite material after carbonization.

[0062] In this step, glycidyl ether compounds are grafted onto the active hydrogen atoms of the phenolic hydroxyl groups in lignin through an etherification reaction, thereby obtaining etherified lignin with good solubility. This solves the problem of severe aggregation of lignin in aqueous solution and is beneficial to the uniform dispersion of lignin and silica.

[0063] The addition of an alkaline solution and the control of the pH at 11-13 or higher in this step are to ensure that the lignin in the solution is fully dissolved. At the same time, in order to avoid introducing other impurities, the cations of the alkali added in step (1) and the silicate added in step (2) must be the same.

[0064] In this step, the reaction temperature is controlled between 70 and 90°C. Too low a temperature is not conducive to the formation of etherified lignin, resulting in a slow reaction rate, low efficiency, and incomplete reaction. Too high a temperature will cause the etherification reaction to be more intense, and byproducts will be generated, which will also increase energy consumption and costs.

[0065] (2) Add soluble silicate to the etherified lignin solution in step (1), add soluble ammonium salt to maintain the pH of the solution at 10-11, react for 3-4 hours, add acid regulator to adjust the pH of the reaction system to 7-9, let it stand at 50-80℃ for 1-3 hours, then separate by centrifugation, and dry the precipitate to obtain etherified lignin / silica complex.

[0066] This step is to allow strong hydrogen bonds to form between the etherified lignin and the silica generated by hydrolysis, which is beneficial for the uniform dispersion and stable coating of lignin on silica.

[0067] In this step, a soluble ammonium salt is added to maintain the solution pH at 10–11. The weakly acidic ammonium salt reacts with SiO3 in the silicate. 2- Hydrolysis occurs between the silica and lignin, gradually generating silica. Simultaneously, hydrogen bonding between silica and etherified lignin allows them to grow in situ into a loosely structured complex. This step requires precise pH control. If the pH is too low, lignin will prematurely aggregate and precipitate, hindering the adequate coating of silica by the lignin. If the pH is too high, incomplete silicate hydrolysis will result in less silica formation and an unsatisfactory outcome.

[0068] In this step, an acid regulator is added to adjust the pH of the reaction system to 7-9, which is beneficial to the π-π and hydrophobic interactions between etherified lignin molecules, causing the loose complex to gradually shrink and form a dense lignin / silica complex.

[0069] In this step, it is necessary to control the amount of soluble ammonium salt and acid regulator. Too little amount will result in a slow reaction rate, which is not conducive to pH adjustment; too much amount will cause lignin to directly aggregate and precipitate, which is not conducive to the uniform coating of silica by lignin. In order to avoid introducing other impurities, the ammonium salt and the acid solution should have the same anion in step (2).

[0070] (3) Carbonize the etherified lignin / silica composite from step (2), wash and dry to obtain lignin carbon / silica composite.

[0071] The carbonization atmosphere in this step does not necessarily need to be nitrogen; it can be replaced with other inert gases such as argon. The carbonization temperature should be in the range of 500–600℃, and the time should be between 1 and 5 hours. If the temperature is too low or the time is too short, incomplete carbonization will result in poor material conductivity; if the temperature is too high or the time is too long, the material structure will become unstable, leading to partial collapse, while also increasing energy consumption and production costs.

[0072] (4) Oxalate, melamine and lignin sulfonate are added to the lignin carbon / silica composite in step (3), and a certain amount of water is added to prepare a mixture with a mass concentration of 5-20%. The mixture is then subjected to hydrothermal reaction at 120-160℃ for 6-12 hours and dried to obtain lignin carbon / silica / nitrogen-containing composite.

[0073] This step is to further coat the silica to alleviate the volume expansion effect of silica; at the same time, it regulates the pore structure of the lignin carbon / silica composite material to reduce the specific surface area of ​​the material; in addition, nitrogen doping of the lignin carbon / silica composite material is beneficial to the formation of a nitrogen-rich carbon layer in the subsequent secondary carbonization process.

[0074] In this step, the hydrothermal reaction allows melamine to undergo a condensation reaction with lignin sulfonate, further coating the surface of the synthesized continuous nitrogen-rich lignin molecular network onto the lignin-carbon / silica composite material. The nitrogen-rich carbon layer formed after carbonization acts as a support for the material, enhancing its structural stability and preventing collapse during secondary carbonization. Furthermore, the nitrogen-rich carbon layer increases the nitrogen content in the composite material, thereby improving its lithium storage performance. It is crucial to control the hydrothermal reaction time and temperature. If the reaction time is too long or the temperature is too high, melamine monomers may aggregate, preventing melamine grafting into the lignin and resulting in unsatisfactory subsequent nitrogen doping. Conversely, if the reaction time is too short or the temperature is too low, the reaction will be incomplete, affecting the formation of the nitrogen-rich carbon layer and hindering the structural control and nitrogen doping of the composite material.

[0075] In this step, nitrogen doping of the composite material requires the addition of oxalate. The lignin-carbon / silica composite and melamine cannot be directly mixed and carbonized because melamine has poor thermal stability and is prone to decomposition during carbonization, resulting in extremely low nitrogen doping levels and unsatisfactory doping effects. The metal oxides produced by oxalate decomposition can react with melamine to form cyanamide salts, achieving high-temperature nitrogen retention, reducing nitrogen loss, improving nitrogen doping efficiency, and providing additional lithium storage sites. Simultaneously, the metal ions also stabilize the carbon framework. It is important to note that the temperature at which the metal oxides are produced by oxalate decomposition must be higher than the carbonization temperature of lignin (around 350°C). Otherwise, the metal ions will react with sulfur in the lignin to form metal sulfides instead of cyanamide salts, failing to achieve efficient nitrogen retention.

[0076] (5) Carbonize, wash and dry the lignin carbon / silica / nitrogen-containing composite material from step (4) to obtain lignin nitrogen-rich carbon / silica composite material.

[0077] This step is to carbonize and form a stable nitrogen-rich carbon layer, which is then used to further coat the lignin carbon / silica composite material, thus preparing a lignin nitrogen-rich carbon / silica composite material with a core-shell structure. This is used to reduce the specific surface area, alleviate the volume expansion of silica, increase additional lithium storage active sites, and thereby improve the lithium storage performance of the composite material.

[0078] The carbonization atmosphere in this step does not necessarily need to be nitrogen; it can be replaced with other inert gases such as argon. The carbonization temperature should be in the range of 800–900℃, and the time should be between 1 and 5 hours. If the temperature is too low or the time is too short, incomplete carbonization will result in poor material conductivity. If the temperature is too high or the time is too long, the material structure will become unstable, leading to partial collapse and nitrogen loss, resulting in low nitrogen content in the material. In addition, it will increase energy consumption and production costs.

[0079] In this invention, the specific surface area of ​​the lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure is less than 100 m². 2 / g, with a silicon dioxide content of not less than 20% by mass and a nitrogen surface element content of not less than 20%. It can be applied in the fields of lithium-ion battery anode materials, supercapacitors, and photoelectrocatalysis (as a photoelectrocatalyst).

[0080] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0081] (1) The lignin-rich nitrogen-carbon / silica composite material prepared in this invention has a low specific surface area and a high nitrogen doping content. Silica, which acts as the main active material, is uniformly distributed in the lignin-rich nitrogen-carbon-based material. The continuous lignin-rich nitrogen-carbon layer not only improves the overall conductivity of the composite material and reduces the specific surface area, but also effectively alleviates the volume expansion of silica, thereby achieving a higher initial coulombic efficiency, better rate performance, and cycle performance. In addition, thanks to the efficient nitrogen retention of cyanamide salt, the composite material has a high nitrogen content, which further enhances the lithium storage performance of the material. As a lithium-ion battery anode material, compared with pure silica, it has a higher initial coulombic efficiency, cycle performance, and rate performance, showing good application prospects.

[0082] (2) The preparation method of the lignin-rich nitrogen-carbon / silica composite material of the present invention uses lignin as the carbon source, melamine as the nitrogen source, and silicate as the silicon source, achieving uniform dispersion and stable coating of lignin-rich nitrogen-carbon onto silica. The raw materials of the present invention are abundant, low-cost renewable resources, and the preparation process is safe, green, and environmentally friendly, enabling high-value utilization of papermaking black liquor or biorefining waste, thus saving resources and protecting the environment. Attached Figure Description

[0083] Figure 1 The constant current charge-discharge spectrum of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of the present invention at a current density of 100 mA / g is shown.

[0084] Figure 2 The constant current charge-discharge spectrum of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of the present invention is shown at a current density of 200 mA / g.

[0085] Figure 3 This is a rate performance spectrum of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of the present invention.

[0086] Figure 4 This is the SEM image of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of the present invention.

[0087] Figure 5 The TEM image and elemental mapping spectrum of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of this invention are shown.

[0088] Figure 6 The attached image shows the nitrogen adsorption-desorption spectrum of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of this invention. Detailed Implementation

[0089] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0090] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0091] Example 1

[0092] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0093] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0094] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0095] Weigh 1g of lignin carbon / silica composite, add 4g of sodium lignin sulfonate, 4g of zinc oxalate, and 4g of melamine, add 130ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0096] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0097] Example 2

[0098] Take 10g of alkali lignin powder and add it to 100ml of water to prepare a 10% solution. Add 20% potassium hydroxide solution to adjust the pH to 11. Add 1g of ethylene glycol diglycidyl ether and react at 70℃ for 3h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0099] Weigh 5g of potassium silicate and add it to the above etherified lignin solution. Add 5g of ammonium sulfate and maintain the pH value at 10. After reacting for 3 hours, add 20% sulfuric acid solution to adjust the pH value of the reaction system to 7. Let it stand at 50℃ for 1 hour and then centrifuge at 5000rpm for 10 minutes. Transfer the centrifuged precipitate to an infrared oven at 40℃ and dry it for 24 hours to obtain the etherified lignin / silica composite.

[0100] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to an argon atmosphere, heated to 150°C at a heating rate of 5°C / min, held for 10 min, then heated to 500°C at a heating rate of 5°C / min, held for 1 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0101] Weigh 1g of lignin carbon / silica composite, add 1g of calcium lignin sulfonate, 1g of calcium oxalate, and 1g of melamine, add 80ml of water to prepare a 5% mass concentration mixture, heat in a hydrothermal autoclave at 120℃ for 6 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0102] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in an argon atmosphere, heated to 150°C at a rate of 5°C / min, held for 10 min, then heated to 800°C at a rate of 5°C / min, held for 1 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% sulfuric acid solution and washed with water. The washed product was then placed in an infrared oven at 60°C and dried for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0103] Example 3

[0104] Take 10g of enzymatically hydrolyzed lignin powder and add it to 67ml of water to prepare a solution with a mass concentration of 15%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 2g of o-toluene glycidyl ether and react at 75℃ for 4h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0105] Weigh 7g of sodium silicate and add it to the above etherified lignin solution. Add 7g of ammonium nitrate and maintain the pH value at 11. After reacting for 4 hours, add 20% nitric acid solution to adjust the pH value of the reaction system to 8. Let it stand at 55℃ for 2 hours and then centrifuge at 10000rpm for 20 minutes. Transfer the centrifuged precipitate to a 50℃ forced-air oven and dry for 24 hours to obtain the etherified lignin / silica composite.

[0106] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 200°C at a heating rate of 10°C / min, held for 20 min, then heated to 550°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0107] Weigh 1g of lignin carbon / silica composite, add 2g of ammonium lignin sulfonate, 2g of magnesium oxalate, and 2g of melamine, add 88ml of water to prepare a mixture with a mass concentration of 8%, heat it in a hydrothermal autoclave at 130℃ for 7h, let it cool to room temperature, and dry it to obtain lignin carbon / silica / nitrogen-containing composite.

[0108] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 200°C at a rate of 10°C / min, held for 20 min, then heated to 850°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% nitric acid solution and washed with water. The washed product was then placed in a 50°C forced-air oven and dried for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0109] Example 4

[0110] Take 10g of enzymatically hydrolyzed lignin powder and add it to 42ml of water to prepare a solution with a mass concentration of 24%. Add a 20% potassium hydroxide solution to adjust the pH to 13. Add 6g of o-toluene glycidyl ether and react at 85℃ for 6h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0111] Weigh 12g of potassium silicate and add it to the above etherified lignin solution. Add 12g of ammonium chloride and maintain the pH value at 11. After reacting for 4 hours, add 20% hydrochloric acid solution to adjust the pH value of the reaction system to 8. Let it stand at 65℃ for 3 hours and then centrifuge at 15000rpm for 20 minutes. Transfer the centrifuged precipitate to a 40℃ forced-air oven and dry for 24 hours to obtain the etherified lignin / silica composite.

[0112] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to an argon atmosphere, heated to 300°C at a heating rate of 15°C / min, held for 40 min, then heated to 650°C at a heating rate of 15°C / min, held for 3 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0113] Weigh 1g of lignin carbon / silica composite, add 6g of sodium lignin sulfonate, 6g of zinc oxalate, 6g of melamine, and add 146ml of water to prepare a 13% mass concentration mixture. Heat the mixture in a hydrothermal autoclave at 150℃ for 9 hours, and then let it cool to room temperature and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0114] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in an argon atmosphere, heated to 300℃ at a heating rate of 15℃ / min, held for 40 min, then heated to 950℃ at a heating rate of 15℃ / min, held for 3 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and the washed product was placed in a 90℃ forced-air oven and dried for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0115] Example 5

[0116] Take 10g of organic solvent lignin powder and add it to 37ml of water to prepare a solution with a mass concentration of 27%. Add a 20% sodium hydroxide solution to adjust the pH to 11. Add 8g of polyoxyethylene glycidyl ether and react at 90℃ for 3h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0117] 14g of sodium silicate was weighed and added to the above etherified lignin solution. 14g of ammonium sulfate was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% sulfuric acid solution was added to adjust the pH value of the reaction system to 9. The mixture was allowed to stand and age at 70℃ for 2 hours. Then, it was centrifuged at 20,000 rpm for 30 minutes. The centrifuged precipitate was transferred to a vacuum oven at 50℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0118] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 350°C at a heating rate of 5°C / min, held for 50 min, then heated to 700°C at a heating rate of 5°C / min, held for 4 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0119] Weigh 1g of lignin carbon / silica composite, add 8g of calcium lignin sulfonate, 8g of calcium oxalate, and 8g of melamine, and add 139ml of water to prepare a mixture with a mass concentration of 18%. Heat the mixture in a hydrothermal autoclave at 160℃ for 10 hours, and then let it cool to room temperature and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0120] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 350°C at a rate of 5°C / min, held for 50 min, then heated to 1000°C at a rate of 5°C / min, held for 4 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% sulfuric acid solution and washed with water. The washed product was then placed in a vacuum oven at 100°C and dried for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0121] Example 6

[0122] Take 10g of organic solvent lignin powder and add it to 33ml of water to prepare a solution with a mass concentration of 30%. Add a 20% potassium hydroxide solution to adjust the pH to 13. Add 10g of polyoxyethylene glycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0123] Weigh 15g of potassium silicate and add it to the above etherified lignin solution. Add 15g of ammonium nitrate and maintain the pH value at 11. After reacting for 4 hours, add 20% nitric acid solution to adjust the pH value of the reaction system to 9. Let it stand at 80℃ for 3 hours and then centrifuge at 5000rpm for 30 minutes. Transfer the centrifuged precipitate to a vacuum oven at 60℃ and dry it for 24 hours to obtain the etherified lignin / silica composite.

[0124] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to an argon atmosphere, heated to 250°C at a heating rate of 15°C / min, held for 60 min, then heated to 600°C at a heating rate of 15°C / min, held for 5 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0125] Weigh 1g of lignin carbon / silica composite, add 10g of ammonium lignin sulfonate, 10g of magnesium oxalate, and 10g of melamine, add 155ml of water to prepare a 20% mass concentration mixture, heat it in a hydrothermal autoclave at 140℃ for 12h, and then let it cool to room temperature and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0126] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in an argon atmosphere, heated to 250°C at a rate of 15°C / min, held for 60 min, then heated to 900°C at a rate of 15°C / min, held for 5 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% nitric acid solution, washed with water, and dried in a vacuum oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0127] Comparative Example 1 (pure silicon dioxide)

[0128] Weigh 10g of sodium silicate and add it to 50ml of water. Add 20% sodium hydroxide solution to adjust the pH to 12. Add 10g of ammonium chloride and maintain the pH at 10. React for 3 hours. Then centrifuge at 10000rpm for 10 minutes. Transfer the centrifuged precipitate to an infrared oven at 60℃ and dry for 24 hours to obtain silicon dioxide.

[0129] The silicon dioxide obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the carbonized silicon dioxide material.

[0130] The carbonized silicon oxide material was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the silicon oxide material after two carbonizations.

[0131] Comparative Example 2 (using lignin directly without etherification modification)

[0132] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a 20% solution. Add a 20% sodium hydroxide solution to adjust the pH to 12.

[0133] 10g of sodium silicate was added to the above alkali lignin solution, and 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the alkali lignin / silica composite.

[0134] The alkali lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0135] Weigh 1g of lignin carbon / silica composite, add 4g of sodium lignin sulfonate, 4g of zinc oxalate, and 4g of melamine, add 130ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0136] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0137] Comparative Example 3 (Carbonization in one step only)

[0138] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0139] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0140] Weigh 1g of etherified lignin / silica complex, add 4g of sodium lignin sulfonate, 4g of zinc oxalate, and 4g of melamine, add 130ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing complex.

[0141] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0142] Comparative Example 4 (without melamine or nitrogen doping)

[0143] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0144] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0145] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0146] Weigh 1g of lignin carbon / silica composite, add 4g of sodium lignin sulfonate and 4g of zinc oxalate, add 90ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-free composite.

[0147] The lignin carbon / silica / nitrogen-free composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain a secondary carbonized lignin carbon / silica composite material.

[0148] Comparative Example 5 (without oxalate added before secondary carbonization)

[0149] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0150] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0151] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0152] Weigh 1g of lignin carbon / silica composite, add 4g of sodium lignin sulfonate and 4g of melamine, add 90ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0153] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0154] Comparative Example 6 (without sodium lignosulfonate added before secondary carbonization)

[0155] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0156] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0157] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0158] Weigh 1g of lignin carbon / silica composite, add 4g of zinc oxalate and 4g of melamine, add 90ml of water to prepare a 10% mass concentration mixture, heat in a hydrothermal autoclave at 140℃ for 8 hours, let it cool to room temperature, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0159] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0160] Comparative Example 7 (no hydrothermal treatment before secondary carbonization)

[0161] Take 10g of alkali lignin powder and add it to 50ml of water to prepare a solution with a mass concentration of 20%. Add a 20% sodium hydroxide solution to adjust the pH to 12. Add 4g of ethylene glycol diglycidyl ether and react at 80℃ for 5h. After cooling the reaction system to room temperature, the etherified lignin solution can be obtained.

[0162] 10g of sodium silicate was weighed and added to the above etherified lignin solution. 10g of ammonium chloride was added to maintain and adjust the pH value to 10. After reacting for 3 hours, 20% hydrochloric acid solution was added to adjust the pH value of the reaction system to 7. The mixture was allowed to stand and age at 60℃ for 1 hour, and then centrifuged at 10000rpm for 10 minutes. The centrifuged precipitate was transferred to an infrared oven at 60℃ and dried for 24 hours to obtain the etherified lignin / silica composite.

[0163] The etherified lignin / silica composite obtained above was ground to the micron level, transferred to a nitrogen atmosphere, heated to 250°C at a heating rate of 10°C / min, held for 30 min, then heated to 600°C at a heating rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was washed with water and dried to obtain the lignin carbon / silica composite.

[0164] Weigh 1g of lignin carbon / silica composite, add 4g of sodium lignin sulfonate, 4g of zinc oxalate, and 4g of melamine, add 130ml of water to prepare a 10% mass concentration mixture, evaporate the water at 140℃, and dry to obtain lignin carbon / silica / nitrogen-containing composite.

[0165] The lignin-carbon / silica / nitrogen-containing composite obtained above was ground to the micron level, placed in a nitrogen atmosphere, heated to 250°C at a rate of 10°C / min, held for 30 min, then heated to 900°C at a rate of 10°C / min, held for 2 h, and allowed to cool to room temperature. The carbonized product was then immersed in a 10% hydrochloric acid solution, washed with water, and dried in an infrared oven at 80°C for 24 h to obtain the lignin-nitrogen-rich carbon / silica composite material.

[0166] The morphology and size of the samples of this invention were determined by field emission scanning electron microscopy (SEM, Hitachi S-550).

[0167] The nitrogen surface element content of the samples of this invention was determined by X-ray photoelectron spectroscopy (Thermo Scientific).

[0168] The silica mass content of the samples of this invention was determined by a comprehensive thermal analyzer (STA499C).

[0169] The specific surface area of ​​the samples of this invention was measured using a fully automated specific surface area and pore size analyzer (ASAP2020).

[0170] The battery assembly uses a half-cell assembly, model CR2032. The positive electrode material consists of 80 wt.% active material, 10 wt.% carbon black, and 10 wt.% polyvinylidene fluoride (PVDF), coated using N-methyl-2-pyrrolidone (NMP) as a solvent. The active material is the lignin-rich nitrogen-rich carbon / silica composite material prepared above. A lithium sheet serves as the counter electrode. The electrolyte is prepared by dissolving 1 mol / L LiPF6 in a 1:1 volume ratio mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), with the addition of 5 wt.% fluoroethylene carbonate (FEC). The entire assembly process of the lithium-ion half-cell is completed in an argon-protected glove box. The constant current charge / discharge performance of the battery was tested using the Neware battery performance testing system at a current density of 200 mA / g within a voltage range of 0.01V to 3.0V. The rate performance test was performed at current densities of 50 mA / g, 100 mA / g, 250 mA / g, 500 mA / g, and 1000 mA / g.

[0171] The lignin-rich nitrogen-enriched carbon / silica composite material prepared in Example 1 was applied to the anode material of a lithium-ion battery, and electrochemical tests and material characterization were performed. The results are shown in Tables 1 and 2. Figures 1-5 .

[0172] Table 1 compares the cycling performance of the lignin-rich nitrogen-enriched carbon / silica composite material prepared in the above examples with that of the sample prepared in the above comparative examples.

[0173] Table 1 Cyclic performance of lignin-rich nitrogen-rich carbon / silica composites and comparative examples 1–7

[0174]

[0175] Table 1 Explanation:

[0176] At a current density of 200 mA / g, the initial coulombic efficiency of the lignin-nitrogen-rich carbon / silica composites in Examples 1–6 was greater than 63%, and the discharge specific capacity after 100 cycles was greater than 800 mAh / g, which was superior to other comparative samples. The sample prepared in Example 1 achieved an initial coulombic efficiency of 64.5%, and the discharge specific capacity after 100 cycles was 822 mAh / g, with good cycle stability, which was significantly better than similar materials. This is mainly due to the small specific surface area of ​​the lignin-nitrogen-rich carbon / silica composite, the core-shell structure with a nitrogen-rich carbon layer, and the appropriate carbon / nitrogen / silica ratio in the composite, which can give full play to the role of the three in the composite.

[0177] Table 1 shows the cycling performance data of the comparative samples. After 100 cycles at 200 mA / g, the discharge capacity of Comparative Example 1 (pure silica) was only 11 mAh / g due to severe volume expansion during charging and discharging. Comparative Example 2, lacking lignin modification, had poor water solubility and weak interaction with silica, making it difficult to achieve uniform dispersion and stable coating. The composite material still contained some uncoated silica after carbonization, resulting in a discharge capacity of only 478 mAh / g. Comparative Example 3, undergoing only one carbonization step, failed to further coat the lignin-carbon / silica material with a nitrogen-rich carbon layer, preventing the formation of a stable core-shell structure, thus its discharge capacity was only 605 mAh / g. Comparative Example 4, without melamine for nitrogen doping, experienced lignin condensation during carbonization, preventing the formation of a continuous and stable core-shell structure on the silica surface. Comparative Example 5, with its nitrogen-rich carbon layer network structure and fewer lithium storage active sites, has a discharge specific capacity of 523 mAh / g. Comparative Example 6, lacking sodium lignin sulfonate, cannot form a carbon layer during secondary carbonization to achieve efficient nitrogen retention. Furthermore, melamine's poor thermal stability leads to easy decomposition and loss during carbonization, resulting in a low nitrogen content in the lignin carbon / silica composite material, with a discharge specific capacity of 516 mAh / g. Comparative Example 7, without hydrothermal treatment, only mixes and disperses the raw materials, failing to form a nitrogen-rich lignin network. This results in a large amount of fragmented lignin carbon after carbonization, leading to poor lithium storage performance and a discharge specific capacity of 553 mAh / g.

[0178] Figure 1 , Figure 2 The images show the constant current charge-discharge spectra of the lignin-nitrogen-rich carbon / silica composite material prepared in Example 1 of this invention. At a current density of 100 mA / g, the initial charge-discharge specific capacities of the lignin-nitrogen-rich carbon / silica composite material are 1103 mAh / g and 1659 mAh / g, respectively, with an initial coulombic efficiency of 66.5%. After 100 cycles, the reversible capacity is 1286 mAh / g. At a current density of 200 mA / g, the initial charge-discharge specific capacities are 815 mAh / g and 1264 mAh / g, respectively, with an initial coulombic efficiency of 64.5%. After 100 cycles, the reversible capacity is 822 mAh / g. This is mainly due to the small specific surface area and high nitrogen doping content of the composite material.

[0179] Figure 3This is a rate performance spectrum of the lignin-nitrogen-rich carbon / silica composite material prepared in Example 1 of the present invention. Under different current intensities, the specific capacity of the lignin-nitrogen-rich carbon / silica composite material can reach a stable state after several cycles, and it can still stabilize rapidly when changing from 1000mA / g to 50mA / g. This indicates that the lignin-nitrogen-rich carbon / silica composite material has excellent rate performance and cycle stability, and can be used normally under different working environments.

[0180] Figure 4 This is a SEM image of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of this invention. The image shows that the particle size of the lignin-rich nitrogen-carbon / silica composite material is approximately 100–200 nm, and the silica particles are stably coated by a continuous lignin-rich nitrogen-carbon layer.

[0181] Figure 5 This is a TEM image and elemental mapping spectrum of the lignin-nitrogen-rich carbon / silica composite material prepared in Example 1 of this invention. The image shows that C, N, O, and Si elements are uniformly distributed, exhibiting a stable core-shell structure, indicating that silica is uniformly coated within the lignin-nitrogen-rich carbon.

[0182] Figure 6 This is a nitrogen adsorption / desorption isotherm diagram of the lignin-rich nitrogen-carbon / silica composite material prepared in Example 1 of this invention. The diagram shows that the nitrogen adsorption capacity of the composite material is relatively small, with a specific surface area of ​​only 76.72 m². 2 / g, the presence of hysteresis loops in the high-pressure zone of the composite material indicates the presence of mesopores in the structure.

[0183] Table 2 compares the surface element content and silica mass content of the lignin-rich nitrogen-rich carbon / silica composite material prepared in the above embodiments with the sample prepared in the above comparative examples.

[0184] Table 2. Surface element content and silica mass content of lignin nitrogen-rich carbon / silica composite materials and comparative examples 1-7

[0185]

[0186] Table 2 Explanation:

[0187] As shown in Table 2, the nitrogen content in the lignin-rich nitrogen-carbon / silica composites prepared in Examples 1-6 is all higher than 20 at.%. High nitrogen doping can increase the conductivity of the material and add additional lithium storage active sites. The low nitrogen content in the samples of Comparative Examples 4-5 demonstrates the necessity of the nitrogen doping treatment step in this scheme. In the lignin-rich nitrogen-carbon / silica composites prepared in Examples 1-6, the surface silicon content is all lower than 1 at.%, while the mass content of silicon oxide is all higher than 20 wt.%, indicating that this scheme achieves uniform and stable coating of silicon oxide. The silicon oxide coated within the nitrogen-rich carbon layer can effectively alleviate its volume expansion effect and improve the lithium storage performance of the material. Furthermore, the higher surface silicon content in the comparative examples indicates the presence of a large amount of unstable silicon oxide, further proving the necessity of the coating treatment step in this scheme.

[0188] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure, characterized in that, Includes the following steps: (1) Add glycidyl ether compounds to a lignin solution with a pH of 11-13 and react at 70-90℃ for 3-6 hours to obtain an etherified lignin solution; (2) After adding soluble silicate to the etherified lignin solution in step (1) and mixing evenly, add soluble ammonium salt to maintain the pH of the solution at 10-11. After reacting for 3-4 hours, add acid regulator to adjust the pH of the reaction system to 7-9. Let it stand at 50-80℃ for 1-3 hours, centrifuge, and dry to obtain etherified lignin / silica complex. (3) Carbonize the etherified lignin / silica composite from step (2), wash and dry to obtain lignin carbon / silica composite. (4) After mixing the lignin carbon / silica complex, oxalate, melamine, lignin sulfonate and water in step (3), the mixture was hydrothermally reacted at 120-160℃ for 6-12 hours and dried to obtain lignin carbon / silica / nitrogen-containing complex. (5) Carbonize, wash and dry the lignin carbon / silica / nitrogen-containing composite material obtained in step (4) to obtain lignin nitrogen-rich carbon / silica composite material.

2. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The mass ratio of lignin, glycidyl ether compounds in step (1) to soluble silicates and soluble ammonium salts in step (2) is 10:1 to 10:5 to 15:5 to 15. The mass ratio of the lignin carbon / silica composite, oxalate, melamine and lignin sulfonate in step (4) is 1:1 to 10:1 to 10:1 to 10.

3. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The carbonization conditions described in step (3) are: first carbonize at 150-350℃ for 10-60 min, then carbonize at 500-700℃ for 1-5 h; The carbonization conditions described in step (5) are: first carbonize at 150-350℃ for 10-60 min, then carbonize at 800-1000℃ for 1-5 h.

4. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, In step (1), the lignin is at least one of the following: enzymatic hydrolyzed lignin extracted from biorefining residue, organic solvent lignin obtained from solvent pulping, and alkali lignin extracted from black liquor from alkaline pulping. The glycidyl ether compound mentioned in step (1) is at least one of o-toluene glycidyl ether, polyoxyethylene glycidyl ether, and ethylene glycol diglycidyl ether; The soluble silicate in step (2) is at least one of potassium silicate and sodium silicate, and its cation must be the same as the cation of the alkali in the lignin solution in step (1). The soluble ammonium salt mentioned in step (2) is at least one of ammonium carbonate, ammonium bicarbonate, ammonium chloride, ammonium sulfate, and ammonium nitrate; The oxalate in step (4) is at least one of zinc oxalate, calcium oxalate and magnesium oxalate.

5. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The lignin solution with a pH of 11-13 in step (1) is obtained by the following method: lignin is prepared into an aqueous solution with a mass concentration of 10-30%, and then an alkaline solution is added to adjust the pH to 11-13; the alkaline solution is at least one of sodium hydroxide, potassium hydroxide and ammonia water; the mass concentration of the alkaline solution is 10-20%. In step (4), the lignin carbon / silica composite, oxalate, melamine, lignin sulfonate, and water are mixed to prepare a mixture with a mass concentration of 5-20%.

6. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The acid regulator in step (2) is an acid solution with a mass concentration of 10-20%; the acid in the acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid, and its anion must be the same as the anion in the soluble ammonium salt in step (2); The carbonization described in steps (3) and (5) is carried out in an inert gas atmosphere, wherein the inert gas is at least one of nitrogen, argon and helium; The heating rate for carbonization in steps (3) and (5) is 5 to 15 °C / min.

7. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The mass ratio of lignin, glycidyl ether compounds in step (1) to soluble silicates and soluble ammonium salts in step (2) is 10:1~4:5~10:5~10; The mass ratio of the lignin carbon / silica composite, oxalate, melamine and lignin sulfonate in step (4) is 1:1 to 4:1 to 4:1 to 4; The hydrothermal reaction in step (4) is carried out at a temperature of 140–160°C for 8–12 hours. The carbonization conditions described in step (3) are: first carbonize at 200-300℃ for 30-60 min, then carbonize at 500-600℃ for 1-3 h; The carbonization conditions described in step (5) are: first carbonize at 200-300℃ for 30-60 minutes, then carbonize at 800-900℃ for 1-3 hours.

8. The method for preparing a lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure according to claim 1, characterized in that, The washing step (5) refers to immersing the carbonized product in a dilute acid solution to wash away any residual pyrolysis products; the mass concentration of the dilute acid solution is 5-20%; the acid in the dilute acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid, and its anion must be the same as the anion in the soluble ammonium salt in step (2).

9. A lignin-rich nitrogen-carbon / silica composite material with a core-shell structure prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the lignin-rich nitrogen-rich carbon / silica composite material with a core-shell structure as described in claim 9 in the fields of lithium-ion batteries, supercapacitors, and photoelectrocatalysis.

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