Preparation Method and Application of a Lignin Carbon-Coated Si@C-rGO Composite Material

Abstract

The present invention relates to a preparation method and application of a lignin carbon-coated Si@C-rGO composite material. In the present invention, a layer of polydopamine is uniformly coated on the surface of nano-silicon, which can inhibit the aggregation and volume expansion of nano-silicon. Through electrostatic adsorption, the modified nano-silicon is anchored in the layered structure of graphene oxide, effectively avoiding the collapse of the electrode material and improving the mechanical stability of the electrode material. At the same time, reduced graphene oxide can provide a strong electron pathway to enhance the conductivity of the material. The lignin carbon outer shell layer is uniformly wrapped on the surface of the Si@C-rGO material, which can prevent the material from directly contacting the electrolyte and cause the continuous formation of the solid electrolyte interface (SEI). At the same time, the lignin carbon outer shell layer can improve the conductivity of the electrode material and inhibit the silicon volume expansion, avoiding the collapse of the electrode material structure. Moreover, the present invention has no complex preparation process, can complete electrostatic adsorption only in water, does not need to use any dangerous acids or special equipment, and can be prepared on a large scale.

Description

Technical Field

[0001] The present invention belongs to the technical field of anode materials for lithium-ion batteries, and particularly relates to a preparation method and application of a lignin carbon-coated Si@C-rGO composite material, and more particularly to the application of a lignin carbon-coated Si@C-rGO composite material in anode materials for lithium-ion batteries. Background Art

[0002] In commercial lithium-ion batteries, graphite is used as the anode due to its good electrical conductivity and excellent cycle stability, and its theoretical capacity is 372 mAh / g. However, when the graphite anode is charged at high rates, the insertion and extraction rates of lithium ions between the graphite layers are limited, resulting in a decline in battery performance and even safety problems. In contrast, silicon-based anodes have advantages such as a high theoretical specific capacity (4200 mAh / g), non-toxicity, abundant reserves, and a low working voltage (0.4 V vs. Li / Li + ) and are considered the best choice for the next-generation anode materials for lithium-ion batteries. However, the huge volume change (370%) causes the silicon anode to pulverize during the lithiation-dellithiation process and is easily peeled off from the current collector, resulting in a rapid capacity decay. Another problem is that silicon has poor electrical conductivity, which is not conducive to electron transport and lithium-ion diffusion, thus weakening the rate performance and cycle stability of the material.

[0003] To address the above two problems, researchers have proposed many methods for preparing silicon-carbon composite materials. There are mainly the following three solutions: 1) Carbon coating. This method first disperses biomass carbon or organic carbon in a solvent, coats it on the surface of nano-silicon, and then calcines it at high temperature to form an amorphous carbon layer. This structure can improve the electrical conductivity of the electrode material and use the carbon shell to inhibit the volume expansion of silicon. However, a single carbon shell cannot inhibit the severe volume expansion of silicon in the later stage of battery cycling, resulting in the fragmentation of the carbon shell during cycling. 2) Ball milling method. The original bulk material is crushed and ground into micro / nano particles by high-speed rotating ball milling beads to composite two different materials. However, the repeated impact of the ball milling beads will damage the crystallinity of the Si material, leading to adverse electrochemical side reactions during charge and discharge. 3) Chemical vapor deposition (CVD). CVD is a method that uses gases or vapors to react on the gas phase or gas-solid interface to produce high-purity solid deposits. Its advantage compared to other methods is that it can control the thickness of the deposited carbon layer, but its equipment cost and operating cost are high, the operation is complex, and it is difficult to achieve large-scale industrial production.

[0004] Most of the reported preparation processes of silicon-carbon composite materials are relatively complex. For example, a layer of TiO2 is in-situ coated on the surface of silicon nanoparticles for surface modification to make it positively charged, and it combines with the negatively charged carboxyl group of graphene oxide through electrostatic adsorption. Then, the adhesion between materials is further enhanced through a hydrothermal reaction. Subsequently, a thin outer carbon layer is covered on the precursor material through chemical vapor deposition. Finally, the silicon-carbon composite material is obtained by calcination in an inert gas atmosphere in a tube furnace. The initial discharge and charge capacities of this material are 3017.6 and 2715.8 mAh g -1 -1 respectively at a current density of 1 A g -1 -1. After 200 cycles, the capacity retention rate is 68%. However, the preparation technology has strict conditions. Especially during electrostatic adsorption, an additional hydrothermal method is required to stabilize the bonding force between materials, and it involves CVD method, which has certain production cost constraints and is difficult to realize industrial application. Therefore, it is necessary to explore a simple, efficient and easy-to-implement method for preparing silicon-carbon composite materials. To improve the poor conductivity of silicon-based materials, some researchers have doped elements such as nitrogen and sulfur to increase the conductivity of materials. However, the additives used face problems such as high cost and difficulty in purifying them in subsequent reactions, which may cause unnecessary side reactions. Therefore, most of the current preparation processes of silicon-carbon materials require the use of synthetic chemicals as carbon precursors, with high costs and complex preparation processes, making it difficult to achieve industrialization.

[0005] Lignin is the most abundant aromatic compound in biomass, with low price and high carbon content (55% - 65%). Due to its highly cross-linked amorphous aromatic ring structure, lignin can be carbonized at high temperature to produce hard carbon materials. Its large interlayer spacing allows lithium ions to achieve rapid insertion and extraction during charge and discharge. Moreover, lignin contains rich active groups such as phenolic hydroxyl groups, carboxyl groups, and carbon-carbon double bonds, and lignin can be modified through redox, polycondensation, or graft copolymerization reactions. Research shows that directly using lignin-coated nanosilicon as the anode material exhibits poor specific capacity, initial coulombic efficiency, and cycle stability. The reason is the weak bonding force between lignin and silicon. The silicon-carbon material with lignin as a single carbon shell cannot withstand the volume expansion of silicon and is extremely easy to crush and collapse.

[0006] To develop a cheap, environmentally friendly and high-performance silicon-carbon anode material based on lignin, problems such as the weak interfacial interaction force between lignin and silicon itself after dissolution and the easy crushing and collapse of lignin carbon due to the inability to withstand the volume expansion of silicon need to be solved urgently. Exploring the strong bonding force between graphene oxide and nanosilicon, and exploring the use of lignin as an outer carbon layer coating to further improve the mechanical properties to avoid electrode crushing and collapse, and developing a material with high mechanical stability, thus effectively suppressing the volume expansion of silicon in the anode of lithium-ion batteries is of great significance. Summary of the Invention

[0007] The purpose of the present invention is to solve the problems of poor structural stability of silicon-carbon negative electrode materials in the prior art, thereby providing a lignin carbon-coated Si@C-rGO composite material and a preparation method thereof. The Si@C-rGO composite material can solve the problems of easy agglomeration of nano-silicon and weak bonding force between silicon and graphene oxide, and realize that nano-silicon is firmly anchored between graphene oxide layers. Then, by coating a nitrogen-doped lignin carbon outer shell layer, the conductivity and mechanical properties of the material are effectively improved to enhance the rate performance of the electrode material and prevent the collapse of the electrode material, and the coated lignin carbon outer shell layer can also prevent the continuous formation of a solid electrolyte interface (SEI) and reduce the consumption of the electrolyte.

[0008] In order to solve the above technical problems, the present invention is implemented through the following technical solutions.

[0009] The first aspect of the present invention provides a method for preparing a lignin carbon-coated Si@C-rGO composite material, comprising the following steps:

[0010] (1) adding silicon and dopamine hydrochloride to a buffer solution and stirring them thoroughly to polymerize the dopamine hydrochloride, followed by filtering and drying to obtain modified silicon with amine groups on the surface;

[0011] (2) dispersing the modified silicon and graphene oxide in water respectively, mixing by ultrasonication and then drying; then calcining at high temperature in an inert gas containing a reducing gas to perform a carbonization reduction reaction to obtain a Si@C-rGO composite material;

[0012] (3) adding the Si@C-rGO composite material obtained in step (2) to the lignin solution and stirring sufficiently to allow the lignin to be evenly coated on the surface of the Si@C-rGO composite material to obtain a mixed solution;

[0013] (4) The mixed solution is calcined at high temperature in an inert gas atmosphere for carbonization to obtain a lignin carbon-coated Si@C-rGO composite material, denoted as Si@C-rGO@C.

[0014] Preferably, in step (1), the mass ratio of silicon to dopamine hydrochloride is 1:(0.1-4); more preferably, the mass ratio of silicon to dopamine hydrochloride is 1:(0.5-1.5); most preferably, the mass ratio of silicon to dopamine hydrochloride is 1:1.

[0015] Preferably, the buffer solution in step (1) is a Tris buffer solution (Tris); more preferably, the buffer solution is a Tris buffer solution with a pH value of 6-9; most preferably, the buffer solution is a Tris buffer solution with a pH value of 8.5.

[0016] Preferably, the silicon in step (1) is nano-silicon with a particle size of 10 - 200 nm, and more preferably, the silicon is nano-silicon with a particle size of 20 - 100 nm.

[0017] Preferably, the stirring time in step (1) is 10 - 40 h, and the temperature is 10 - 40 °C; more preferably, the stirring time is 20 - 30 h, and the temperature is 20 - 25 °C.

[0018] Preferably, the mass ratio of the modified silicon to graphene oxide in step (2) is (0.5 - 5):1; more preferably, the mass ratio of the modified silicon to graphene oxide is (1 - 3):1; most preferably, the mass ratio of the modified silicon to graphene oxide is 2:1.

[0019] Preferably, the ultrasonic mixing time in step (2) is 10 - 60 min; more preferably, the ultrasonic mixing time is 20 - 40 min.

[0020] Preferably, the drying temperature in step (2) is 60 - 100 °C; more preferably, the drying temperature is 70 - 90 °C.

[0021] Preferably, the high-temperature calcination temperature in step (2) is 400 - 1000 °C, and the time is 1 - 5 h; more preferably, the high-temperature calcination temperature is 600 - 900 °C, and the time is 2 - 4 h; most preferably, the high-temperature calcination temperature is 700 °C, and the time is 2 h.

[0022] Preferably, the inert gas in step (2) is selected from one or more of nitrogen, argon, helium, and carbon dioxide; the reducing gas is selected from one or more of hydrogen and carbon monoxide; more preferably, the inert gas containing the reducing gas is an argon / hydrogen mixed gas.

[0023] Preferably, the mass ratio of the Si@C-rGO composite material to lignin in the lignin solution in step (3) is (0.5 - 5):1; more preferably, the mass ratio of the Si@C-rGO composite material to lignin in the lignin solution is (1 - 3):1; most preferably, the mass ratio of the Si@C-rGO composite material to lignin in the lignin solution is 2:1.

[0024] Preferably, the lignin solution in step (3) is prepared by the following method: dissolving lignin in a solvent and performing a stirring treatment.

[0025] Preferably, the lignin is selected from one or more of alkali lignin, sodium lignosulfonate, and enzymatically hydrolyzed lignin.

[0026] Preferably, the solvent is selected from deep eutectic solvents.

[0027] Preferably, the mass ratio of the lignin to the deep eutectic solvent is 1:(1 - 20); more preferably, the mass ratio of the lignin to the deep eutectic solvent is 1:(10 - 20); most preferably, the mass ratio of the lignin to the deep eutectic solvent is 1:15.

[0028] Preferably, the deep eutectic solvent is selected from one or more of choline chloride - urea, choline chloride - lactic acid, and choline chloride - ethylene glycol; more preferably, the deep eutectic solvent is choline chloride - urea, where the molar ratio of choline chloride to urea is 1:(1 - 4); more preferably, the molar ratio of choline chloride to urea is 1:(1 - 2).

[0029] Preferably, the temperature of the stirring treatment is 20 - 90°C and the time is 1 - 8 h; most preferably, the temperature of the stirring treatment is 60 - 80°C and the time is 2 - 5 h.

[0030] Preferably, in step (4), the temperature of the high-temperature calcination is 400 - 1000°C and the time is 1 - 10 h; more preferably, the temperature of the high-temperature calcination is 500 - 800°C and the time is 2 - 4 h; most preferably, the temperature of the high-temperature calcination is 600°C and the time is 2 h.

[0031] Preferably, in step (4), the inert gas is selected from one or more of nitrogen, argon, and helium; more preferably, the inert gas is nitrogen.

[0032] In the second aspect of the present invention, there is provided a lignin carbon-coated Si@C-rGO composite material prepared according to the above preparation method.

[0033] In the third aspect of the present invention, there is provided the application of the lignin carbon-coated Si@C-rGO composite material prepared by the above preparation method in the preparation of a negative electrode material for a battery.

[0034] Preferably, the negative electrode material for the battery is selected from negative electrode materials for lithium-ion batteries.

[0035] First, dopamine hydrochloride is polymerized on the surface of nano-silicon in an alkaline environment, introducing secondary amine groups to make the modified nano-silicon carry a positive charge. The repulsive force of the charge effectively alleviates the agglomeration problem of nano-silicon. Furthermore, various oxygen-containing functional groups (such as carboxyl groups and hydroxyl groups) contained in the structure of graphene oxide provide negative charges. The amine-group modified nano-silicon is adsorbed on the surface of graphene oxide through electrostatic adsorption, and the nano-silicon is anchored in the layered structure of reduced graphene oxide by calcination in a mixed atmosphere of H2 / Ar, obtaining the Si@C-rGO composite material. Then, the deep eutectic solvent synthesized from choline chloride and urea is used to dissolve and disperse lignin, and the lignin is uniformly wrapped on the surface of the Si@C-rGO composite material. The lignin is calcined and carbonized again to make it uniformly wrap on the surface of the Si@C-rGO composite material as an outer carbon layer, further enhancing the mechanical stability of the material. In addition, the deep eutectic solvent synthesized from choline chloride and urea contains nitrogen elements, which can be calcined at high temperature to form lignin carbon and simultaneously realize nitrogen doping, enhancing the electron cloud density of the composite material bulk phase, improving the electron transport ability of the electrode material, and increasing the overall conductivity of the material.

[0036] The present invention has the following technical effects compared with the prior art:

[0037] (1) A layer of polydopamine is uniformly coated on the surface of nano-silicon in the present invention. The shell layer formed by polydopamine not only effectively inhibits the agglomeration of nano-silicon, but also effectively inhibits the volume expansion of nano-silicon.

[0038] (2) In the present invention, the modified nano-silicon is anchored in the layered structure of graphene oxide through electrostatic adsorption, and reduced graphene oxide is obtained by calcination in an inert gas containing a reducing gas. The electrode material has a large specific surface area, and its sheet structure has a supporting effect on the electrode material, effectively avoiding the collapse of the electrode material, thus improving the mechanical stability of the electrode material. At the same time, reduced graphene oxide can provide a strong electron pathway to enhance the conductivity of the material.

[0039] (3) The lignin carbon outer shell layer is uniformly wrapped on the surface of the Si@C-rGO material, which can prevent the material from directly contacting the electrolyte and causing the continuous formation of the solid electrolyte interface (SEI). At the same time, the lignin carbon outer shell layer can improve the conductivity of the electrode material and inhibit the silicon volume expansion, avoiding the collapse of the electrode material structure. The prepared choline chloride-urea deep eutectic solvent can achieve high nitrogen doping of the lignin carbon outer shell layer during the high-temperature calcination stage, enhancing the electron cloud density of the composite material bulk phase and improving the electron transport ability of the electrode material. And the present invention has no complex preparation process, can complete electrostatic adsorption only in water, does not need to use any dangerous acids or special equipment, and can be prepared on a large scale. Description of the Drawings

[0040] Figure 1 SEM image of the Si@C material prepared in Comparative Example 1.

[0041] Figure 2 SEM image of the Si@C-rGO composite material prepared in Comparative Example 2.

[0042] Figure 3 SEM image of the Si@C-rGO@C composite material prepared in Example 1.

[0043] Figure 4 XRD pattern of the Si@C-rGO@C composite material prepared in Example 1.

[0044] Figure 5 Rate performance graph of the Si@C-rGO@C composite material assembled into a coin cell at different current densities, prepared in Example 1.

[0045] Figure 6 For the Si@C-rGO@C composite material prepared in Example 1 assembled into a coin cell at a current density of 1 A g -1 Long cycle performance graph.

[0046] Figure 7 SEM image of the modified nano-silicon prepared in Comparative Example 5.

[0047] Figure 8 SEM image of the Si@C-rGO composite material prepared by ball milling in Comparative Example 7. Detailed implementation manners

[0048] To make the objectives, technical solutions and effects of the present invention clearer and more definite, the following further describes the present invention in detail with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0049] Example 1

[0050] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0051] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of hydrochloric acid dopamine to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize the hydrochloric acid dopamine, then filter by suction and vacuum dry for 8 h to obtain modified nano-silicon with amine groups on the surface. Hydrochloric acid dopamine can effectively polymerize on the surface of nano-silicon under alkaline conditions and make it carry secondary amine groups with positive charges, and effectively reduce the aggregation of nano-silicon by means of electrostatic repulsion.

[0052] (2)Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven to dry until it becomes powder. Subsequently, place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine at 700 °C for 2 h for carbonization reduction reaction to obtain the Si@C-rGO composite material. In this step, the modified nano-silicon and graphene oxide are pre-dispersed by ultrasonic waves. After mixing, the secondary amine groups on the modified nano-silicon are connected to the carbonyl groups on the graphene oxide to form hydrogen bonds, firmly anchoring the nano-silicon on the graphene oxide sheets. Then, under a high-temperature calcination in a H2 / Ar mixed atmosphere, the graphene oxide is reduced to prepare a Si@C-rGO composite material with higher stability and better conductivity.

[0053] (3)Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir in a water bath at 75 °C for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution. The deep eutectic solvent synthesized from choline chloride and urea contains nitrogen elements, which can achieve nitrogen doping synchronously when forming lignin carbon during subsequent high-temperature calcination, enhance the electron cloud density of the composite material bulk phase, improve the electron transport ability of the electrode material, and increase the overall conductivity of the material. Add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the alkali lignin on the surface of the Si@C-rGO composite material to obtain a mixed solution. This step can uniformly coat the alkali lignin on the surface of the Si@C-rGO composite material, and can achieve the purpose of nitrogen doping through the pyrolysis of urea during subsequent calcination to prepare a nitrogen-doped lignin carbon outer shell layer.

[0054] (4)Transfer the mixed solution to a tube furnace and calcine at 600 °C for 2 h in a N2 atmosphere for carbonization to obtain the lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C. Through high-temperature calcination, a nitrogen-doped lignin carbon outer shell layer is coated on the surface of the Si@C-rGO composite material prepared in step (2), thereby improving the conductivity of the electrode material and inhibiting the volume expansion of silicon, avoiding the collapse of the electrode material structure, and further improving the battery cycle stability and rate performance.

[0055] Example 2

[0056] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0057] (1)Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, and then filter and vacuum dry for 8 h to obtain modified nano-silicon with amine groups on the surface.

[0058] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder. Subsequently, place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 900 °C for 2 h to carry out a carbonization reduction reaction to obtain a Si@C-rGO composite material.

[0059] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-rGO composite material with alkali lignin to obtain a mixed solution.

[0060] (4) Transfer the mixed solution to a tube furnace and calcine it at 600 °C for 2 h in a N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0061] Example 3

[0062] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0063] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, and then carry out suction filtration and vacuum drying for 8 h to obtain modified nano-silicon with amino groups on the surface.

[0064] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder. Subsequently, place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 700 °C for 2 h to carry out a carbonization reduction reaction to obtain a Si@C-rGO composite material.

[0065] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-rGO composite material with alkali lignin to obtain a mixed solution.

[0066] (4) Transfer the mixed solution to a tubular furnace and calcine it at 800 °C for 2 h under a N2 atmosphere for carbonization to obtain the lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0067] Example 4

[0068] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0069] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and vacuum dry for 8 h to obtain modified nano-silicon with amine groups on the surface.

[0070] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, disperse 0.1 g of graphene oxide in 20 mL of deionized water, ultrasonically mix for 30 min and then place it in a blast drying oven to dry to powder; then place the obtained powder in a tubular furnace, introduce a H2 / Ar mixed gas, and calcine at 700 °C for 2 h for a carbonization reduction reaction to obtain the Si@C-rGO composite material.

[0071] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-rGO composite material with alkali lignin to obtain a mixed solution.

[0072] (4) Transfer the mixed solution to a tubular furnace and calcine it at 600 °C for 2 h under a H2 / Ar mixed gas atmosphere for carbonization to obtain the lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0073] Example 5

[0074] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0075] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and vacuum dry for 8 h to obtain modified nano-silicon with amine groups on the surface.

[0076] (2) Disperse 0.4 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder. Subsequently, place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and carry out a carbonization reduction reaction at 700 °C for 2 h to obtain a Si@C-rGO composite material.

[0077] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent. Dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue stirring for 3 h to prepare an alkali lignin solution. Add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-rGO composite material with alkali lignin to obtain a mixed solution.

[0078] (4) Transfer the mixed solution to a tube furnace and carry out carbonization at 600 °C for 2 h under a N2 atmosphere to obtain a lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0079] Comparative Example 1

[0080] A Si@C material, and its preparation method includes the following steps:

[0081] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then carry out suction filtration and vacuum drying for 8 h to obtain modified nano-silicon with amino groups on the surface.

[0082] (2) Place the modified nano-silicon in a tube furnace, introduce N2, and carry out carbonization at 600 °C for 2 h to obtain a polydopamine-coated nano-silicon material Si@C.

[0083] Comparative Example 2

[0084] A Si@C-rGO composite material, and its preparation method includes the following steps:

[0085] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then carry out suction filtration and vacuum drying for 8 h to obtain modified nano-silicon with amino groups on the surface.

[0086] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder; then place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 700 °C for 2 h for carbonization reduction reaction to obtain the Si@C-rGO composite material.

[0087] Comparative Example 3

[0088] A Si-rGO@C composite material, and its preparation method includes the following steps:

[0089] (1) Ultrasonically disperse 0.2 g of nano-silicon (30 nm) in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder; then place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 700 °C for 2 h to obtain the Si-rGO composite material.

[0090] (2) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; add 0.1 g of alkali lignin and dissolve it in 1.5 g of the deep eutectic solvent and continue stirring for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si-rGO composite material obtained in step (1) to the alkali lignin solution and stir overnight to obtain a mixed solution.

[0091] (3) Transfer the mixed solution to a tube furnace, and calcine it at 600 °C for 2 h in a N2 atmosphere for carbonization to obtain the lignin carbon-coated Si-rGO composite material Si-rGO@C.

[0092] Comparative Example 4

[0093] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0094] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and vacuum dry for 8 h to obtain modified nano-silicon with amine groups on the surface.

[0095] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder; then place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 700 °C for 2 h for carbonization reduction reaction to obtain the Si@C-rGO composite material.

[0096] (3) Dissolve 0.1 g of alkali lignin in dimethylformamide (DMF) and stir for 3 h to obtain an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-rGO composite material with alkali lignin, obtaining a mixed solution.

[0097] (4) Transfer the mixed solution to a tube furnace and calcine it at 600 °C for 2 h under a N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0098] Comparative Example 5

[0099] A lignin carbon-coated Si-rGO composite material, and its preparation method includes the following steps:

[0100] (1) Prepare an aqueous solution containing 0.5 g of cetyltrimethylammonium bromide (CTAB) in 50 ml, then add 0.2 g of nano-silicon powder (30 nm), stir and sonicate for 30 min, filter and dry to obtain a positively charged modified nano-silicon powder;

[0101] (2) Disperse 0.2 g of the modified nano-silicon in 10 mL of deionized water, disperse 0.1 g of graphene oxide in 20 mL of deionized water, ultrasonically mix for 30 min, and then place it in a blast drying oven to dry to powder; subsequently, place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine it at 700 °C for 2 h for a carbonization reduction reaction to obtain a Si-rGO composite material.

[0102] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to obtain a deep eutectic solvent; add 0.1 g of alkali lignin and dissolve it in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to obtain an alkali lignin solution; add 0.2 g of the Si-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si-rGO composite material with alkali lignin, obtaining a mixed solution.

[0103] (4) Transfer the mixed solution to a tube furnace and calcine it at 600 °C for 2 h under a N2 atmosphere for carbonization to obtain a lignin carbon-coated Si-rGO composite material Si-rGO@C.

[0104] Comparative Example 6

[0105] A lignin carbon-coated Si@C-G composite material, and its preparation method includes the following steps:

[0106] (1) Add 0.2 g of nanosilicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and dry in vacuum for 8 h to obtain modified nanosilicon with amino groups on the surface.

[0107] (2) Disperse 0.2 g of modified nanosilicon in 10 mL of deionized water, disperse 0.1 g of graphite (G) in 20 mL of deionized water, ultrasonically mix for 30 min and then dry in a blast drying oven until it becomes a powder; then place the obtained powder in a tube furnace, introduce N2, and calcine at 700 °C for 2 h for carbonization reduction reaction to obtain Si@C-G composite material.

[0108] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir in a water bath at 75 °C for 5 h to prepare a deep eutectic solvent; add 0.1 g of alkali lignin and dissolve it in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-G composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the surface of the Si@C-G composite material with alkali lignin to obtain a mixed solution.

[0109] (4) Transfer the mixed solution to a tube furnace and calcine at 600 °C for 2 h in an N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-G composite material Si@C-G@C.

[0110] Comparative Example 7

[0111] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0112] (1) Add 0.2 g of nanosilicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and dry in vacuum for 8 h to obtain modified nanosilicon with amino groups on the surface.

[0113] (2) Add 0.2 g of modified nanosilicon and 0.1 g of graphene oxide to a ball milling jar, with a ball-to-material ratio of 10:1, ball mill for 5 h to obtain a powder material; then place the obtained powder in a tube furnace, introduce a H2 / Ar mixed gas, and calcine at 700 °C for 2 h for carbonization reduction reaction to obtain a Si@C-rGO composite material.

[0114] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; add 0.1 g of alkali lignin and dissolve it in 1.5 g of the deep eutectic solvent, and continue stirring for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the alkali lignin on the surface of the Si@C-rGO composite material to obtain a mixed solution.

[0115] (4) Transfer the mixed solution to a tube furnace, calcine at 600 °C for 2 h in a N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C.

[0116] Comparative Example 8

[0117] A lignin carbon-coated Si@C-GO composite material, and its preparation method includes the following steps:

[0118] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and dry in vacuum for 8 h to obtain modified nano-silicon with amino groups on the surface.

[0119] (2) Disperse 0.2 g of the modified nano-silicon in 10 mL of deionized water, disperse 0.1 g of graphene oxide in 20 mL of deionized water, ultrasonically mix for 30 min, and then place it in a blast drying oven to dry to powder; then place the obtained powder in a tube furnace, introduce N2, and calcine at 700 °C for 2 h for a carbonization reduction reaction to obtain a Si@C-GO composite material.

[0120] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue stirring for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-GO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the alkali lignin on the surface of the Si@C-GO composite material to obtain a mixed solution.

[0121] (4) Transfer the mixed solution to a tube furnace, calcine at 600 °C for 2 h in a N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-GO composite material Si@C-GO@C.

[0122] Comparative Example 9

[0123] A lignin carbon-coated Si@C-rGO composite material, and its preparation method includes the following steps:

[0124] (1) Add 0.2 g of nano-silicon (30 nm) and 0.2 g of dopamine hydrochloride to 100 mL of Tris buffer solution (pH 8.5), stir at room temperature (20 °C) for 24 h to polymerize dopamine hydrochloride, then filter by suction and dry in vacuum for 8 h to obtain modified nano-silicon with amino groups on the surface.

[0125] (2) Disperse 0.2 g of modified nano-silicon in 10 mL of deionized water, and disperse 0.1 g of graphene oxide in 20 mL of deionized water. After ultrasonic mixing for 30 min, place it in a blast drying oven and dry it to powder; then place the obtained powder in a tubular furnace, introduce a mixed gas of H2 / Ar, and calcine at 700 °C for 2 h for carbonization reduction reaction to obtain Si@C-rGO composite material.

[0126] (3) Add choline chloride and ethylene glycol to the reagent bottle according to a molar ratio of 1:2, heat and stir at 75 °C in a water bath for 5 h to prepare a deep eutectic solvent; dissolve 0.1 g of alkali lignin in 1.5 g of the deep eutectic solvent and continue to stir for 3 h to prepare an alkali lignin solution; add 0.2 g of the Si@C-rGO composite material obtained in step (2) to the alkali lignin solution and stir overnight to uniformly coat the alkali lignin on the surface of the Si@C-rGO composite material to obtain a mixed solution.

[0127] (4) Transfer the mixed solution to a tubular furnace, calcine at 600 °C for 2 h in an N2 atmosphere for carbonization to obtain a lignin carbon-coated Si@C-rGO composite material Si@C-rGO@C without nitrogen doping.

[0128] Verification Example 1

[0129] Respectively take the materials prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 5 and Comparative Example 7, and use a scanning electron microscope (SEM, Hitach SU8220) to test their morphology and size. The results are as Figure 1-3 shown in Figures 7-8. Among them Figure 1 is the SEM image of the poly-dopamine-coated nano-silicon material Si@C prepared in Comparative Example 1. As can be seen from the figure, a layer of poly-dopamine is uniformly coated on the surface of the nano-silicon. The shell layer formed by poly-dopamine can not only effectively inhibit the agglomeration of nano-silicon, but also effectively inhibit the volume expansion of nano-silicon. Figure 3 is the SEM image of the Si@C-rGO@C composite material prepared in Example 1. Compared with Figure 2 , the prepared Si@C-rGO composite electrode material is wrapped by an alkali lignin carbon outer shell layer. Therefore, the alkali lignin carbon outer shell layer can further inhibit the collapse of the electrode material caused by silicon volume expansion and can further improve the conductivity of the electrode material. Subsequently, the Si@C-rGO@C composite material prepared in Example 1 was analyzed by a scanning electron microscope. The results are as Figure 4The results show that the Si@C-rGO@C composite material prepared in Example 1 is mainly composed of amorphous carbon and silicon, without other impurity peaks. Figure 2 This is the SEM image of the Si@C-rGO composite material prepared in Comparative Example 2. It can be seen that the modified nano-silicon can be anchored on the surface and sheets of graphene oxide by electrostatic adsorption.

[0130] Subsequently, the materials prepared in Examples 1-5 and Comparative Examples 1-9 were used for the preparation of negative electrode materials for lithium-ion batteries and the electrochemical performance tests were carried out. The battery assembly adopts half-cell assembly, and the model is CR2032. The composition of the negative electrode material is 70wt% of active material, 20wt% of carbon black, and 10wt% of CMC. Water is used as the solvent for coating, and the active material is the different types of silicon-carbon composite materials prepared in the above-mentioned embodiments and comparative examples. The lithium sheet is used as the counter electrode, and the electrolyte is prepared with 1mol / L LiPF6 as the solute and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 as the solvent. The entire installation process of the lithium-ion half-cell is completed in an argon-protected glove box. The Neware battery performance test system is used to test the negative electrode performance at 0.1A g in the voltage range of 0.01-3.0V. -1 and 1Ag -1 The constant current charge / discharge performance test of the battery was carried out at a current density of 0.1Ag, while the rate performance test was carried out at a current density of 0.1Ag. -1 , 0.2A g -1 , 0.5Ag -1 , 1Ag -1 and 5Ag -1 The test results are shown in Table 1 and Figure 5-6 shown.

[0131] in, Figure 5 The rate performance diagram of button-type batteries assembled with Si@C-rGO@C composite materials prepared in Example 1 at different current densities. At different current densities, the specific capacity of the composite material can reach a stable state after one cycle, and the rate performance of the button-type batteries assembled with Si@C-rGO@C composite materials prepared in Example 1 at different current densities can reach a stable state after one cycle, and the rate performance of the button-type batteries assembled with Si@C-rGO@C composite materials ... -1 To 5Ag -1 Back to 0.1Ag -1 It can still quickly stabilize and maintain a high specific capacity, especially at 5Ag -1 It can still maintain 1099.87mAh g -1 The discharge specific capacity proves that the prepared electrode material has excellent rate performance and can operate stably under different working environments. Figure 6 The Si@C-rGO@C composite material prepared in Example 1 was assembled into a button cell. -1Long cycle performance graph at a current density. The discharge specific capacity is still as high as 632.59 mAh g after 600 cycles -1 , significantly better than similar materials.

[0132] Table 1 Comparison of the performance of different lithium-ion battery anodes prepared in the examples and comparative examples

[0133]

[0134] The results in Table 1 show that in Example 1, at 1 A g -1 , the initial charge-discharge specific capacity is 1567.88 / 1602.76 mAh g -1 , and the discharge specific capacity after 200 cycles is 978.37 mAh g -1 It can be seen that a high specific capacity can be maintained during the initial cycling process, and it has good cycling stability, which is better than similar materials. In Examples 2 and 3, the calcination temperature of the Si@C-rGO precursor and the carbonization temperature of alkali lignin are increased. The initial charge-discharge specific capacity and cycling stability of the electrode material are both lower than those in Example 1. The reason is that too high a calcination temperature will cause the carbon layer formed by polydopamine on the surface of nano-silicon to have a poor interaction with silicon and graphene sheets or the carbon layer to break, resulting in partial silicon shedding and thus a decrease in battery capacity. In Examples 4 and 5, the influence of the calcination atmosphere on the performance of the electrode material is explored. Example 4 shows that the carbonization atmosphere of alkali lignin has little effect on the Si@C-rGO@C electrode material, proving that the key influencing factor leading to the performance difference is the calcination atmosphere of the Si@C-rGO precursor. Example 5 to improve Si@C-rGO The content of silicon in the precursor shows 1503.89 / 1546.22 mAh g -1 It has a high specific capacity, but the discharge specific capacity decreases significantly after 200 cycles, to 701.11 mAh·g -1 , indicating that if the nano-silicon content is too high, some nano-silicon cannot be effectively adsorbed between the graphene oxide sheets, lacking a supporting substrate, resulting in capacity decay and decreased stability of the electrode material.

[0135] From Comparative Example 1, it can be known that the Si@C material formed by simply carbonizing polydopamine has a discharge specific capacity of 510.99 mAh g after cycling 200 times at 1 A g -1 , indicating that the carbon layer formed only by polydopamine is difficult to effectively inhibit the volume expansion of silicon, resulting in the carbon layer breaking during long-cycle charge-discharge processes and rapid decay of the electrode capacity. -1

[0136] From Comparative Example 2, it can be known that the Si@C-rGO composite material has 591.74 mAh g after cycling 200 times at 1 A g -1 , -1The discharge specific capacity shows that introducing graphene oxide and electrostatically adsorbing modified nano-silicon on the graphene surface by using opposite-charge groups between the two can effectively improve the cycle stability of the electrode material. However, the capacity retention rate is still not high. The reason is that the prepared polydopamine carbon shell layer is thin and tightly wraps the nano-silicon. During the charge-discharge process, with the repeated volume expansion and contraction of the nano-silicon, the carbon shell layer formed by polydopamine is extremely likely to rupture, losing the inhibitory effect on the volume expansion of silicon, resulting in the collapse of the electrode material structure and the decline of cycle stability. In contrast, the alkali lignin-coated Si@C-rGO electrode material prepared in Example 1 has excellent cycle stability. This is because the alkali lignin carbon layer is coated on the surface of the polydopamine carbon / silicon core-shell nanospheres / reduced graphene oxide, buffering the volume expansion of the nano-silicon, improving the structural stability of the electrode material, and promoting the stable formation of the SEI film.

[0137] Comparative Example 3 lacks the polydopamine carbon layer and has a higher silicon content than Example 1, so it shows a high initial charge-discharge specific capacity of 1559.12 / 1596.37 mAh g -1 However, the discharge specific capacity after 200 cycles is 469.91 mAh g -1 , because unmodified nano-silicon is difficult to anchor on the graphene oxide surface, and graphene oxide is difficult to play a supporting role during the charge-discharge process, resulting in poor stability and fast capacity decay of the electrode material. Through comparison, it can be found that polydopamine-modified nano-silicon plays an important role in improving the stability of the electrode material.

[0138] From Comparative Example 4, it can be seen that after dissolving alkali lignin with the organic solvent DMF and coating the Si@C-rGO precursor, the prepared electrode material has an initial charge-discharge specific capacity of 923.88 / 1015.31 mAh g -1 at 1 A g -1 , which is significantly lower than the performance of the electrode material prepared by dissolving alkali lignin with the deep eutectic solvent. The reason is that the uniformity of carbon coating on the Si@C-rGO precursor is poor and nitrogen doping cannot be achieved, resulting in poor cycle stability and specific capacity of the battery. In contrast, in Example 1, choline chloride-urea deep eutectic solvent can dissolve alkali lignin well, and high nitrogen doping of the lignin carbon outer shell layer can be achieved during the high-temperature calcination stage, enhancing the bulk electron cloud density of the composite material and improving the electron transport ability of the electrode material, thereby achieving an initial charge-discharge specific capacity as high as 1567.88 / 1602.76 mAh g -1 at 1 A g -1 and excellent long-cycle stability.

[0139] In Comparative Example 5, nano-silicon was modified by CTAB. From Figure 1 and Figure 7By comparison, the effect of modifying nano-silicon with CTAB is worse than that of dopamine. There is still partial silicon agglomeration in CTAB-modified nano-silicon, and it fails to bond well to the layered structure of graphene oxide. With the progress of cycling, some silicon will fall off, resulting in a discharge specific capacity of 498.47 mAh g after 200 cycles. -1 . In contrast, dopamine hydrochloride-modified nano-silicon introduces secondary amine groups, which electrostatically adsorb groups such as carbonyl and carboxyl groups on graphene oxide to form hydrogen bonds, thus better anchoring the modified nano-silicon in the layered structure of graphene oxide.

[0140] In Comparative Example 6, by replacing graphene oxide with graphite, its poor performance is attributed to the fact that the bulk graphite material cannot relieve the volume expansion of silicon, and on the other hand, graphite is an electrically neutral material and fails to bond the positively charged modified nano-silicon through electrostatic adsorption. Therefore, there is no strong binding force between silicon and graphite. During the charge-discharge process, with the repeated volume expansion and contraction of nano-silicon, nano-silicon detaches from graphite, the structure of the electrode material collapses, and the cycle stability decreases. The discharge specific capacity after 200 cycles is 418.77 mAh g -1 . In contrast, groups such as carbonyl and carboxyl groups on graphene oxide can form hydrogen bonds with the modified nano-silicon, thus anchoring the modified nano-silicon in the layered structure of graphene oxide.

[0141] In Comparative Example 7, the modified nano-silicon was embedded on graphene oxide by the additional external force of ball milling. Figure 8 It shows that only part of the silicon can be embedded on graphene oxide. The reason is that during the ball milling process, the uncontrollable force given by the ball milling beads will knock off some nano-silicon on graphene oxide, resulting in uneven silicon loading on the graphene oxide sheets. The specific capacity is shown as 1022.76 / 1053.89 mAh g -1 , and the discharge specific capacity after 200 cycles is 402.18 mAh g -1 .

[0142] In Comparative Example 8, reduced graphene oxide cannot be generated by calcination in a nitrogen atmosphere, and the obtained precursor is Si@C-GO. Since the conductivity of graphene oxide is poorer than that of reduced graphene oxide, the specific capacity shown is lower than that of Example 1, and the charge-discharge specific capacity is 1168.38 / 1211.22 mAh g -1 .

[0143] The choline chloride-ethylene glycol DES prepared in Comparative Example 9 failed to achieve high nitrogen doping of the lignin carbon outer layer, and the electron transport ability of the electrode material decreased. Therefore, the discharge specific capacity after 200 cycles is 560.75 mAh g -1 .

[0144] As can be seen from the above results, the cycling performance of all the samples of the present invention is superior to that of the other comparative example samples. This is mainly due to the fact that graphene oxide can effectively inhibit the volume expansion of silicon and anchor silicon through electrostatic adsorption during the cyclic charge and discharge process to prevent its peeling, thereby improving the stability and conductivity of the electrode material. By optimizing the two carbonization temperatures, carbonization atmosphere and silicon content in each example, Example 1 is obtained as the optimal embodiment. For the Si@C-rGO@C composite material prepared in Example 1, secondary amine groups are introduced by modifying nano-silicon with dopamine hydrochloride, and hydrogen bonds are formed by electrostatically adsorbing groups such as carbonyl and carboxyl groups on graphene oxide, thereby anchoring the modified nano-silicon in the layered structure of graphene oxide. Reduced graphene oxide is obtained by calcination in a reducing atmosphere, so that the electrode material has a large specific surface area, and its sheet structure plays a supporting role for the electrode material, effectively avoiding the collapse of the electrode material, thus improving the mechanical stability of the electrode material. At the same time, reduced graphene oxide can provide a strong electron pathway to enhance the conductivity of the material. Furthermore, a deep eutectic solvent synthesized from choline chloride and urea is used to dissolve and disperse alkali lignin to obtain an alkali lignin solution. The deep eutectic solvent synthesized from choline chloride and urea contains nitrogen element, and nitrogen doping is simultaneously realized during the subsequent high-temperature calcination to form alkali lignin carbon. The nitrogen-doped alkali lignin carbon is coated on the surface of the Si@C-rGO precursor and calcined at high temperature to form an outer carbon layer. On the one hand, it can prevent the continuous formation of the solid electrolyte interface (SEI). On the other hand, it can improve the conductivity of the electrode material and inhibit the volume expansion of silicon, avoid the collapse of the electrode material structure, and further improve the cycle stability and rate performance of the battery. The nitrogen-doped lignin carbon has higher conductivity, making the electrode material have excellent rate performance.

[0145] The above specific embodiments part specifically introduces the analysis method involved in the present invention. It should be noted that the above introduction is only to help those skilled in the art better understand the method and idea of the present invention, rather than a limitation on the relevant content. Without departing from the principle of the present invention, those skilled in the art can also make appropriate adjustments or modifications to the present invention, and the above adjustments and modifications should also fall within the protection scope of the present invention.

Claims

1. A method for preparing a lignin carbon-coated Si@C-rGO composite material, characterized in that: The following steps are involved: (1) Adding silicon and dopamine hydrochloride into a buffer solution and stirring them thoroughly to polymerize the dopamine hydrochloride, followed by filtering and drying to obtain modified silicon with amino groups on the surface; (2) Dispersing the modified silicon and graphene oxide in water respectively, mixing by ultrasonication and then drying; then calcining at high temperature in an inert gas containing a reducing gas to perform a carbonization reduction reaction to obtain a Si@C-rGO composite material; (3) adding the Si@C-rGO composite material obtained in step (2) to the lignin solution and stirring thoroughly, so that the lignin is evenly coated on the surface of the Si@C-rGO composite material, to obtain a mixed solution; the lignin solution is prepared by the following method: dissolving lignin in a low eutectic solvent and stirring the mixture; the low eutectic solvent is selected from choline chloride-urea; (4) The mixed liquid is calcined at high temperature in an inert gas atmosphere to carbonize the obtained product.

2. The preparation method according to claim 1, characterized in that: In step (1), the mass ratio of silicon to dopamine hydrochloride is 1:(0.1-4).

3. The preparation method according to claim 1, characterized in that: The mass ratio of modified silicon to graphene oxide in step (2) is (0.5-5):

1.

4. The preparation method according to claim 1, characterized in that The high temperature calcination in step (2) is carried out at a temperature of 400-1000°C and a time of 1-5 h.

5. The preparation method according to claim 1, characterized in that: In step (2), the inert gas is selected from one or more of nitrogen, argon, helium, and carbon dioxide; and the reducing gas is selected from one or more of hydrogen and carbon monoxide.

6. The preparation method according to claim 1, characterized in that The high temperature calcination in step (4) is carried out at a temperature of 400-1000°C and a time of 1-10 h.

7. The preparation method according to claim 1, characterized in that: The inert gas in step (4) is selected from one or more of nitrogen, argon and helium.

8. The lignin carbon-coated Si@C-rGO composite material prepared according to the preparation method according to any one of claims 1 to 7.

Images