Preparation and application of nitrogen-doped lignin-based silicon-carbon negative electrode material

Abstract

The application discloses a preparation and application of nitrogen-doped lignin-based silicon-carbon negative electrode material and belongs to the technical field of negative electrode material preparation. The preparation method of the nitrogen-doped lignin-based silicon-carbon negative electrode material comprises the following steps: (1) mixing a mixed solution of lignin and a eutectic solvent containing nitrogen with nano-silicon with positive charges on the surface, and high-temperature calcining to obtain a nitrogen-doped silicon-carbon composite material; (2) embedding the nitrogen-doped silicon-carbon composite material into the surface of graphite to obtain a nitrogen-doped silicon-carbon / graphite composite material; and (3) mixing the nitrogen-doped silicon-carbon / graphite composite material with a mixed solution of lignin and the eutectic solvent containing nitrogen, and high-temperature calcining to obtain the nitrogen-doped lignin-based silicon-carbon negative electrode material. The lithium ion semi-battery assembled from the nitrogen-doped lignin-based silicon-carbon negative electrode material has excellent cycle stability and charge transmission capacity.

Description

Technical Field

[0001] This invention relates to the field of anode material preparation technology, and in particular to the preparation and application of a nitrogen-doped lignin-based silicon-carbon anode material. Background Technology

[0002] With the vigorous development of the new energy industry, the market for electric vehicles (EVs) is gradually expanding. However, the development of electric vehicles is hindered by the low energy density of lithium-ion batteries (LIBs), which have a limited driving range. Silicon (Si), on the other hand, offers advantages due to its high theoretical capacity (4200 mAh / g) and low operating voltage (0.4V vs. Li / Li). + Silicon anodes are considered the best choice for improving the energy density of lithium-ion batteries. However, silicon anodes also face problems such as volume expansion, easy pulverization and shedding of materials, and low electronic conductivity, resulting in poor mechanical and electrochemical properties.

[0003] To overcome these problems, researchers have proposed solutions such as nanostructures, surface modification, and composite material design. 1) Nanostructured silicon materials, such as nanoparticles, nanowires, and nanotubes, can alleviate stress concentration by reducing the unit size of the material while maintaining the high capacity of silicon. However, as the material size decreases, the increased surface energy can cause the nanomaterials to aggregate. 2) Surface modification enhances mechanical stability, improves electronic conductivity, and promotes the formation of a stable SEI layer by carbon coating or forming a core-shell structure. However, a single carbon coating layer cannot limit the volume expansion of silicon in subsequent cycles, leading to a decrease in capacity. 3) Adding carbonaceous materials or metal alloys to silicon-based composite materials can synergistically improve the overall electrochemical performance, but the mechanical properties are not guaranteed.

[0004] The reported Si / C composite designs can not only adapt to volume changes during cycling but also improve overall electronic conductivity, thereby enhancing the cycle performance of lithium-ion batteries. For example, Si / graphite composites prepared by ball milling exhibit a specific capacity of 883 mAh / g after 200 cycles at a current density of 0.2 A / g, demonstrating good cycle stability at low current densities but not suitable for fast charging at high current densities. To address this, researchers have employed various carbonization methods such as chemical vapor deposition (CVD) and high-temperature pyrolysis. For instance, core-shell structured materials were prepared by depositing a controllable carbon layer on Si particles using CVD technology. This material exhibited a reversible capacity of 1211 mAh / g at a current density of 0.2 A / g, with a capacity retention of 64% after 200 cycles. However, the CVD method requires the decomposition of silicon source gases such as silane at high temperatures and the deposition of silicon-carbon composite materials on the surface of porous carbon-based materials. Currently, it still has some drawbacks: 1) Silane gas is flammable and explosive, posing poor safety; 2) It is limited by factors such as the porous carbon structure and uniformity; 3) The deposition process is complex, generally requiring secondary carbon coating after silicon deposition; 4) Deposition equipment has low throughput, making mass production difficult; 5) Production costs are high. Therefore, to meet the market demand for high-energy-density lithium-ion battery anodes, it is urgent to develop a low-energy-consumption, low-cost, simple, and easily commercializable technology. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing and applying nitrogen-doped lignin-based silicon-carbon anode materials, so as to solve the problems existing in the prior art.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of the present invention: a method for preparing a nitrogen-doped lignin-based silicon-carbon anode material, comprising the following steps:

[0008] (1) Add nano-silicon and hexadecyltrimethylammonium bromide (CTAB) to water, mix evenly by ultrasonication, wash and dry to obtain nano-silicon with positive charge on the surface (modified nano-silicon);

[0009] Electrostatic repulsion can effectively reduce the aggregation of nano-silicon.

[0010] (2) A mixed solution of lignin and nitrogen-containing eutectic solvent (DES) is mixed with the nano-silicon with positive surface charge. After stirring and mixing evenly, the mixture is calcined at high temperature to obtain nitrogen-doped silicon-carbon composite material (Si@NC composite material).

[0011] During the conversion of lignin into carbon materials, it can form a uniform and ordered pore structure, increasing the specific surface area, thereby improving the storage capacity, charge and discharge efficiency and cycle stability of lithium ions, and further enhancing the energy density of lithium batteries.

[0012] The nitrogen provided by the nitrogen-containing eutectic solvent can be used to simultaneously achieve nitrogen doping (high nitrogen-doped lignin carbon layer coated on nano-silicon surface) during subsequent high-temperature calcination to form lignin-based carbon layer, thereby enhancing the bulk electron cloud density of the composite material, improving the electron transport capability of the composite material, and increasing the overall conductivity of the material.

[0013] Nitrogen doping is an effective method to improve the electrical conductivity of carbon materials. By doping nitrogen atoms into the lattice of lignin carbon layers, the number of free electrons or holes can be increased. Nitrogen atoms exist in carbon materials in different forms (such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen). Furthermore, the introduction of nitrogen atoms can alter the electronic structure of carbon materials. The 2p orbitals of nitrogen atoms hybridize with the 2p orbitals of carbon atoms to form new band structures, thereby reducing the band gap and making it easier for electrons to migrate within the material. Therefore, the electrical conductivity of carbon materials is improved.

[0014] (3) The nitrogen-doped silicon-carbon composite material and graphite are mixed and ball-milled to obtain a nitrogen-doped silicon-carbon / graphite composite material (Si@NC-G composite material);

[0015] During ball milling, the milling beads apply shear force to impact the silicon-carbon composite material and the graphite material. Since both the nitrogen-doped lignin carbon shell layer and the graphite contain benzene ring structures, a π-π stacking effect occurs when they are in close contact, which enhances the interfacial adhesion. This allows the nano-silicon to be firmly embedded in the graphite and not easily detached (solving the problem of weak silicon-graphite interfacial bonding), resulting in a Si@NC-G composite material with higher stability and better conductivity.

[0016] (4) The nitrogen-doped silicon-carbon / graphite composite material is mixed with a mixed solution of lignin and nitrogen-containing eutectic solvent (DES), stirred and mixed evenly, and then calcined at high temperature to obtain the nitrogen-doped lignin-based silicon-carbon anode material (Si@NC-G@NC composite material).

[0017] The high-temperature calcination in step (4) allows the surface of the Si@NC-G composite material to be coated with a nitrogen-doped lignin carbon shell layer, thereby improving the conductivity of the composite material (electrode material) and suppressing silicon volume expansion, preventing the composite material structure from collapsing, and thus improving the battery cycle stability and rate performance.

[0018] This invention utilizes CTAB to modify nano-silicon, giving it a positive charge. The electrostatic repulsion effectively alleviates nano-silicon agglomeration. A nitrogen-containing eutectic solvent effectively dissolves lignin, and high-temperature calcination coats the nano-silicon surface with a highly nitrogen-doped lignin carbon layer. The CN-C bonds formed between nitrogen atoms and lignin carbon atoms have higher conductivity than CC bonds, and the increased electronegativity leads to a redistribution of electron density, thereby improving the overall conductivity of the material. Ball milling embeds the modified nano-silicon into the surface of graphite particles, establishing a stable contact between the lignin carbon layer coating the modified nano-silicon and the graphite, forming a complete conductive pathway and improving the material's conductivity. Simultaneously, the bulk graphite possesses stable mechanical properties, effectively alleviating the expansion stress of silicon and preventing performance degradation caused by silicon volume expansion. DES is then used to dissolve lignin for carbon coating as an outer carbon layer. This sealed outer carbon layer ensures that the nano-silicon is not easily detached during subsequent cycles, enhancing the bulk electron cloud density of the composite material, improving its electron transport capability, stabilizing SEI film growth, and providing superior mechanical properties. The lithium-ion half-cell assembled from the Si@NC-G@NC composite material prepared by the above method exhibits excellent cycle stability and charge conduction capability.

[0019] In step (1), the mass ratio of the nano-silicon to hexadecyltrimethylammonium bromide is 1:(1-8), preferably 1:(2-5), and more preferably 1:4;

[0020] Preferably, the ultrasonic mixing time is 10 min to 2 h, more preferably 30 min;

[0021] Preferably, the particle size of the nano-silicon is 10-200 nm, more preferably 30-100 nm;

[0022] Preferably, the drying temperature is 50–120°C, more preferably 70–80°C.

[0023] In step (2), the stirring time is 1 to 15 hours and the temperature is 10 to 50°C; preferably, the stirring time is 3 to 4 hours and the temperature is 20 to 25°C.

[0024] Preferably, the mass ratio of the positively charged nano-silicon to lignin is (0.5-5):1, more preferably (1-3):1, and even more preferably 1.5:1;

[0025] Preferably, the high-temperature calcination temperature is 200–1000°C and the time is 1–6 hours; more preferably, the high-temperature calcination temperature is 400–700°C and the time is 2–4 hours; even more preferably, the high-temperature calcination temperature is 500°C and the time is 3 hours.

[0026] In step (3), the mass ratio of the nitrogen-doped silicon-carbon composite material to graphite is (1-6):1, preferably (1-3):1, and more preferably 2:1;

[0027] Preferably, the ball mill rotation speed is 400-700 rpm; the ball milling time is 1-10 h, more preferably 2-8 h, and even more preferably 5 h;

[0028] In steps (2) and (4), the lignin is selected from one or more of alkali lignin, sodium lignin sulfonate, and enzymatically hydrolyzed lignin;

[0029] Preferably, the mass ratio of the nitrogen-containing eutectic solvent (DES) to lignin is (1-30):1, more preferably (10-20):1, and even more preferably 15:1;

[0030] Preferably, the nitrogen-containing eutectic solvent is a mixture of choline chloride and urea.

[0031] Preferably, the molar ratio of choline chloride to urea is 1:(1-4), more preferably 1:(1-2).

[0032] In step (4), the high-temperature calcination temperature is 200-1000℃ and the time is 1-10h; preferably, the high-temperature calcination temperature is 400-800℃ and the time is 2-4h; more preferably, the high-temperature calcination temperature is 500℃ and the time is 3h.

[0033] In step (4), the mass ratio of the nitrogen-doped silicon-carbon / graphite composite material to lignin is (1-4):1.

[0034] The second technical solution of the present invention: a nitrogen-doped lignin-based silicon-carbon anode material prepared by the above preparation method.

[0035] The third technical solution of the present invention: the application of the above-mentioned nitrogen-doped lignin-based silicon-carbon anode material in the preparation of battery anode materials.

[0036] The battery anode material includes lithium-ion battery anode materials.

[0037] The present invention discloses the following technical effects:

[0038] (1) The lithium-ion half-cell assembled from the Si@NC-G@NC composite material prepared in this invention has excellent cycle stability and charge transport capability.

[0039] (2) The preparation process of the present invention is simple and efficient. The low eutectic solvent used is green, non-toxic and environmentally friendly. The raw material cost is low. No dangerous acid or special equipment is required. It can be prepared on a large scale and has the prospect of commercial development of silicon-carbon anode. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 Here is a SEM image of the nitrogen-doped Si@C material prepared in Comparative Example 4;

[0042] Figure 2 SEM image of the Si@NC-G composite material prepared in Comparative Example 1;

[0043] Figure 3 SEM image of the Si@NC-G@NC composite material prepared in Example 1;

[0044] Figure 4 The image shows the XRD pattern of the Si@NC-G@NC composite material prepared in Example 1.

[0045] Figure 5 The nitrogen adsorption-desorption curves of the Si@NC-G@NC composite material prepared in Example 1 are shown.

[0046] Figure 6 The rate performance of the Si@NC-G@NC composite material prepared in Example 1 assembled into a coin cell at different current densities is shown in the graph.

[0047] Figure 7 The Si@NC-G@NC composite material prepared in Example 1 was assembled into a coin cell, and in 1Ag -1 Long-cycle performance at current density;

[0048] Figure 8 The images show top views of the materials prepared in Example 1 and Comparative Example 1 before and after 100 cycles as electrode materials. Specifically, a is a top view of the Si@NC-G composite material prepared in Comparative Example 1 before cycling, b is a top view of the Si@NC-G composite material prepared in Comparative Example 1 after 100 cycles as electrode materials, c is a top view of the Si@NC-G@NC composite material prepared in Example 1 before cycling, and d is a top view of the Si@NC-G@NC composite material prepared in Example 1 after 100 cycles as electrode materials.

[0049] Figure 9The images show cross-sectional views of the materials prepared in Example 1 and Comparative Example 1 before and after 100 cycles as electrode materials. Specifically, a is a cross-sectional view of the Si@NC-G composite material prepared in Comparative Example 1 before cycling, b is a cross-sectional view of the Si@NC-G composite material prepared in Comparative Example 1 after 100 cycles as electrode material, c is a cross-sectional view of the Si@NC-G@NC composite material prepared in Example 1 before cycling, and d is a cross-sectional view of the Si@NC-G@NC composite material prepared in Example 1 after 100 cycles as electrode material. Detailed Implementation

[0050] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0051] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0052] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0053] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0054] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0055] Example 1

[0056] A method for preparing nitrogen-doped lignin-based silicon-carbon anode material:

[0057] (1) Add 0.15g of nano-silicon (30nm) and 0.6g of CTAB to 200mL of ultrapure water, mix ultrasonically at room temperature (25℃) for 30min, then wash with water to remove excess CTAB, filter and vacuum dry at 80℃ for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0058] (2) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, and heat and stir in a water bath at 75°C for 5 hours to obtain a eutectic solvent (DES);

[0059] 0.1g of alkali lignin was dissolved in 1g of eutectic solvent and stirred for 1h to obtain an alkali lignin solution;

[0060] Add 0.15g of the modified nano-silicon obtained in step (1) to the alkali lignin solution and stir for 2 hours (at a temperature of 25°C) to uniformly coat the surface of the nano-silicon and obtain a mixture.

[0061] The mixture was placed in a tube furnace, N2 was introduced, and it was calcined at 500℃ for 3 hours to obtain nitrogen-doped silicon-carbon composite material (Si@NC composite material).

[0062] (3) Add 0.2g of Si@NC composite material and 0.1g of graphite into a ball mill jar, add ball milling beads (ball-to-material mass ratio of 20:1), and ball mill at 500rpm for 5h to obtain Si@NC-G composite material.

[0063] (4) Dissolve 0.1g of alkali lignin in 1g of DES (same as step (2)) and stir for 1h. Add 0.2g of Si@NC-G composite material and stir for 0.5h to mix evenly. Transfer the mixture to a tube furnace and calcine at 500℃ for 3h under N2 atmosphere to obtain nitrogen-doped lignin-based silicon-carbon anode material (Si@NC-G@NC).

[0064] Example 2

[0065] Same as Example 1, except that in step (2), the calcination temperature is 700°C.

[0066] Example 3

[0067] Same as Example 1, except that in step (4), the calcination temperature is 700°C.

[0068] Example 4

[0069] Same as Example 1, except that in step (3), the amount of Si@NC composite material used is 0.4g.

[0070] Comparative Example 1

[0071] A method for preparing Si@NC-G composite material:

[0072] Same as Example 1, except that step (4) is omitted.

[0073] Comparative Example 2

[0074] A method for preparing Si@CG composite material:

[0075] Same as Example 1, except that in step (2), the raw materials for preparing the eutectic solvent are choline chloride and ethylene glycol in a mass ratio of 1:2.

[0076] Step (4) is omitted.

[0077] Comparative Example 3

[0078] Preparation method of nitrogen-doped Si@C materials:

[0079] Choline chloride and urea were added to a reagent bottle in a molar ratio of 1:2, and the mixture was heated and stirred in a water bath at 75°C for 5 hours to obtain a eutectic solvent (DES).

[0080] 0.1g of alkali lignin was dissolved in 1g of eutectic solvent and stirred for 1h to obtain an alkali lignin solution;

[0081] 0.15g of nano-silicon (30nm) was added to an alkali lignin solution and stirred for 2 hours (at 25℃) to uniformly coat the surface of the nano-silicon with alkali lignin, resulting in a mixed solution.

[0082] The mixture was placed in a tube furnace, N2 was introduced, and it was calcined at 500℃ for 3 hours to obtain nitrogen-doped Si@C material.

[0083] Comparative Example 4

[0084] Preparation method of nitrogen-doped Si@C materials:

[0085] (1) Add 0.15g of nano-silicon (30nm) and 0.6g of CTAB to 200mL of ultrapure water, mix ultrasonically at room temperature (25℃) for 30min, then wash with water to remove excess CTAB, filter and vacuum dry at 80℃ for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0086] (2) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, and heat and stir in a water bath at 75°C for 5 hours to obtain a eutectic solvent (DES);

[0087] 0.1g of alkali lignin was dissolved in 1g of eutectic solvent and stirred for 1h to obtain an alkali lignin solution;

[0088] Add 0.15g of the modified nano-silicon obtained in step (1) to the alkali lignin solution and stir for 2 hours (at a temperature of 25°C) to uniformly coat the surface of the nano-silicon and obtain a mixture.

[0089] The mixture was placed in a tube furnace, N2 was introduced, and it was calcined at 500℃ for 3 hours to obtain nitrogen-doped Si@C material.

[0090] Comparative Example 5

[0091] Preparation method of Si-G@NC:

[0092] (1) Add 0.2g of nano-silicon (30nm) and 0.8g of CTAB to 200mL of ultrapure water, sonicate at room temperature (25℃) for 30min, then wash away excess CTAB with water, filter and vacuum dry at 80℃ for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0093] (2) Add 0.2g of modified nano-silicon and 0.1g of graphite to a ball mill jar, add ball milling beads (ball-to-material mass ratio of 20:1), and ball mill at 500rpm for 5h to obtain Si-G composite material.

[0094] (3) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, and heat and stir in a water bath at 75°C for 5 hours to obtain a eutectic solvent (DES);

[0095] 0.1g of alkali lignin was dissolved in 1g of eutectic solvent and stirred for 1h to obtain an alkali lignin solution;

[0096] 0.2g of Si-G composite material was added to an alkali lignin solution and stirred for 0.5h (at 25℃) until homogeneous. The mixture was then transferred to a tube furnace and calcined at 500℃ for 3h under a N2 atmosphere to obtain Si-G@NC.

[0097] Comparative Example 6

[0098] Preparation method of Si@NC@NC:

[0099] (1) Add 0.15g of nano-silicon (30nm) and 0.6g of CTAB to 200mL of ultrapure water, mix ultrasonically at room temperature (25℃) for 30min, then wash with water to remove excess CTAB, filter and vacuum dry at 80℃ for 4h to obtain nano-silicon with positive charge on the surface (modified nano-silicon).

[0100] (2) Add choline chloride and urea to a reagent bottle in a molar ratio of 1:2, and heat and stir in a water bath at 75°C for 5 hours to obtain a eutectic solvent (DES);

[0101] 0.1g of alkali lignin was dissolved in 1g of eutectic solvent and stirred for 1h to obtain an alkali lignin solution;

[0102] Add 0.15g of the modified nano-silicon obtained in step (1) to the alkali lignin solution and stir for 2 hours (at a temperature of 25°C) to uniformly coat the surface of the nano-silicon and obtain a mixture.

[0103] The mixture was placed in a tube furnace, N2 was introduced, and it was calcined at 500℃ for 3 hours to obtain nitrogen-doped silicon-carbon composite material (Si@NC composite material).

[0104] (3) Dissolve 0.1g of alkali lignin in 1g of DES (same as step (2)) and stir for 1h. Add 0.2g of Si@NC composite material and stir for 0.5h to mix evenly. Transfer the mixture to a tube furnace and calcine at 500℃ for 3h under N2 atmosphere to obtain Si@NC@NC.

[0105] Comparative Example 7

[0106] Same as Example 1, except that the eutectic solvent in steps (2) and (4) is replaced with an equal mass of dimethyl sulfoxide (DMSO).

[0107] Comparative Example 8

[0108] Same as Example 1, except that the alkali lignin in step (4) is replaced with an equal mass of polyvinylpyrrolidone (PVP).

[0109] Example 1

[0110] The materials prepared in Example 1, Comparative Example 1, and Comparative Example 4 were used to test their morphology and size using a scanning electron microscope (SEM, Hitach SU8220). The results are shown in the figure. Figures 1-3 .

[0111] in, Figure 1 The image shows the SEM image of the nitrogen-doped Si@C material prepared in Comparative Example 4. As can be seen from the image, the surface of the nano-silicon is coated with a thin carbon layer, the silicon particles are clearly dispersed and there is no obvious agglomeration. This indicates that CTAB can modify the nano-silicon to carry a positive charge to avoid the agglomeration effect and successfully coat lignin carbon.

[0112] Figure 2 The image shows a SEM image of the Si@NC-G composite material prepared in Comparative Example 1.

[0113] Figure 3 The image shows a SEM image of the Si@NC-G@NC composite material prepared in Example 1, and... Figure 2In contrast, the Si@NC-G@NC composite material prepared in Example 1 is coated with a lignin outer carbon layer. The alkali lignin carbon outer shell can further suppress the collapse of the electrode material caused by silicon volume expansion. Nano-silicon can be uniformly distributed in the carbon layer. The smooth and flat outer carbon layer is conducive to the formation of a stable SEI film and reduces the consumption of electrolyte.

[0114] Figure 4 The XRD pattern of the Si@NC-G@NC composite material prepared in Example 1 shows that the material has five crystalline peaks belonging to silicon, indicating that the silicon crystal phase was not altered during the preparation process. The peak observed near 26.5° corresponds to the characteristic peak of graphite, and the amorphous carbon peak at 23° proves that the Si@NC-G@NC composite material was successfully synthesized.

[0115] Example 2

[0116] The nitrogen adsorption-desorption curve of the Si@NC-G@NC composite material prepared in Example 1 is shown in the figure. Figure 5 .

[0117] from Figure 5 As can be seen, the Si@NC-G@NC composite material is mainly composed of mesoporous and macroporous structures, with a specific surface area of ​​109.56 m². 2 / g.

[0118] Example 3

[0119] Lithium-ion half-cells were assembled using the materials finally prepared in Examples 1-4 and Comparative Examples 1-8, and their electrochemical performance was tested. The battery model was CR2032; the negative electrode material consisted of 70 wt% of the materials finally prepared in the examples or comparative examples, 20 wt% carbon black, and 10 wt% CMC; a lithium sheet was used as the counter electrode; the electrolyte was prepared using 1 mol / L LiPF6 as the solute and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 as the solvent. The entire assembly process of the lithium-ion half-cells was completed in an argon-protected glove box. The Neware battery performance testing system was used to test the performance at 0.1 Ag in the voltage range of 0.01-3.0V. -1 and 1Ag -1 The battery's constant current charge / discharge performance was tested at a current density of 0.1Ag, while the rate performance was tested at 0.1Ag. -1 0.2Ag -1 0.5Ag -1 1Ag -1 2Ag -1 and 5Ag -1 The experiment was conducted at current density. Results are shown in Table 1 and... Figures 6-7 As shown.

[0120] Figure 6 The graph shows the rate performance of the Si@NC-G@NC composite material prepared in Example 1 assembled into a coin cell at different current densities.

[0121] from Figure 6 As can be seen, the specific capacity of the composite material remains stable during five cycles at different current densities, and increases from 0.1 Ag. -1 up to 5Ag -1 Back to 0.1Ag -1 It can still stabilize quickly and maintain a high specific capacity, especially in 5Ag. -1 It can still maintain 808.13mAh g even under high current density. -1 The discharge specific capacity demonstrates that the prepared electrode material has excellent rate performance and can be adapted to different charge and discharge rates.

[0122] Figure 7 The Si@NC-G@NC composite material prepared in Example 1 was assembled into a coin cell, and in 1Ag -1 Long-cycle performance at current density.

[0123] from Figure 7 As can be seen, the discharge specific capacity is still as high as 986.31 mAh g after 200 cycles. -1 It is significantly superior to similar materials.

[0124] Table 1 Comparison of the performance of different lithium-ion battery anodes prepared in the examples and comparative examples.

[0125]

[0126] Table 1 shows that, in Example 1, 1Ag -1 The initial charge / discharge specific capacity is 1255.23 / 1287.56 mAh g. -1 After 200 cycles, the discharge specific capacity is 986.31 mAh g. -1 As can be seen, the material prepared in Example 1 can maintain a high specific capacity and has good cycle stability during the initial cycle, which is superior to similar materials.

[0127] Comparative Examples 1 and 2 compared the effects of the outer lignin carbon layer on the overall material. In contrast, using nitrogen-doped lignin carbon as the outer carbon layer can enhance the bulk electron cloud density of the composite material, improve the electron transport capability of the electrode material, stabilize the growth of the SEI film, and provide better mechanical properties.

[0128] Comparative Example 3 shows that the nitrogen-doped Si@C material exhibits a mAh / g ratio of 1422.09 / 1463.28. -1The high initial capacity is due to the fact that the material is mostly composed of nano-silicon and a small amount of nitrogen-doped lignin carbon. However, the capacity decays rapidly after 200 cycles because the lack of CTAB activation causes the nano-silicon to agglomerate, resulting in high stress at the silicon-to-silicon interface and ultimately structural collapse.

[0129] Comparative Example 4 shows that coating with a lignin-based outer carbon layer provides a significantly higher initial specific capacity of 1456.45 / 1493.39 mAh g. -1 However, the single core-shell structure cannot limit the volume expansion of silicon during subsequent cycles. Therefore, after 200 cycles, the carbon shell breaks, silicon detaches from the current collector, and the electrode capacity rapidly decays. Comparison of Examples 3 and 4 shows that introducing CTAB to activate silicon can solve the problem of easy agglomeration of nano-silicon, thereby improving the specific capacity and cycle stability of the battery.

[0130] Comparative Example 5, lacking the lignin-carbon layer, involved direct ball milling with graphite. Due to the weak interfacial bonding between silicon and graphite, silicon alone could not be firmly embedded on the graphite surface through ball milling. Therefore, the role of graphite in limiting silicon volume expansion could not be effectively utilized, and silicon significantly detached from the graphite during cycling. The resulting performance was 1153.27 / 1179.35 mAh g. -1 The initial specific capacity was reduced to only 421.94 mAh g after 200 cycles. -1 .

[0131] Comparative Example 6 involved coating two layers of lignin-carbon onto the surface of nano-silicon. On the one hand, as the thickness of the carbon layers increased, the lithiation of silicon within the core was insufficient, thus failing to contribute to the capacity and resulting in a decrease in the initial capacity of the electrode material. On the other hand, the dual-carbon structure constructed from lignin-carbon did not possess the strong mechanical stability of graphite to limit the volume expansion of silicon. The stress generated by the expansion concentrated between the carbon layers, leading to the breakage of the carbon shell.

[0132] The solvent used in Comparative Example 7 was DMSO, a universal solvent that can effectively dissolve lignin. The prepared material exhibited relatively good electrical properties, with a value of 1018.65 / 1059.89 mAh g. -1 And after 200 cycles, it has 528.31 mAh g. -1 While DMSO retains its capacity, it is highly toxic and poses significant risks to the environment and human health. In contrast, the formulated choline chloride-urea DES is a green and non-toxic solvent with strong dissolving power, low cost, and nitrogen content that allows for nitrogen doping of the lignin carbon layer. This increases the electronegativity of the carbon layer and improves the overall conductivity of the material, making it significantly superior to DMSO from a solvent perspective.

[0133] Comparative Example 8 uses a carbon layer coated with PVP to replace lignin. PVP is a polymer with good film-forming and adhesive properties, which causes the intermediate Si@NC-G to agglomerate during the carbon coating process in step (4). The particle size after carbon coating is significantly larger than that in Example 1. Therefore, the stress of silicon expansion is concentrated at the bonding points between graphite particles, resulting in the material's cycle stability being inferior to that of Example 1.

[0134] As can be seen from the above results, the cycling performance of the materials prepared in the embodiments of the present invention is better than that of other comparative samples. This is mainly because graphite can provide stable and tough mechanical properties, effectively alleviating the volume expansion of nano-silicon; nitrogen-doped lignin carbon layer enhances the bulk electron cloud density of composite material, improves the electron transport capability of electrode material, and increases the overall conductivity of material.

[0135] Example of effect 4

[0136] Top views of the Si@NC-G@NC composite material prepared in Example 1 and the Si@NC-G composite material prepared in Comparative Example 1 before and after 100 cycles as electrode materials are shown below. Figure 8 . Figure 8 In Figure 1, a is a top view of the Si@NC-G composite material prepared in Comparative Example 1 before cycling, b is a top view of the Si@NC-G composite material prepared in Comparative Example 1 after 100 cycles, c is a top view of the Si@NC-G@NC composite material prepared in Example 1 before cycling, and d is a top view of the Si@NC-G@NC composite material prepared in Example 1 after 100 cycles.

[0137] Cross-sectional views of the Si@NC-G@NC composite material prepared in Example 1 and the Si@NC-G composite material prepared in Comparative Example 1 before and after 100 cycles as electrode materials are shown below. Figure 9 , Figure 9 In Figure 1, a is a cross-sectional view of the Si@NC-G composite material prepared in Comparative Example 1 before cycling, b is a cross-sectional view of the Si@NC-G composite material prepared in Comparative Example 1 after 100 cycles, c is a cross-sectional view of the Si@NC-G@NC composite material prepared in Example 1 before cycling, and d is a cross-sectional view of the Si@NC-G@NC composite material prepared in Example 1 after 100 cycles.

[0138] from Figure 8 As can be seen, the surfaces of each electrode were relatively smooth before cycling, but after 100 cycles, obvious defects and cracks appeared on the surface of the Si@NC-G electrode. In contrast, the Si@NC-G@NC electrode maintained an intact surface.

[0139] pass Figure 9The expansion of the Si@NC-G and Si@NC-G@NC electrode sheets was calculated, revealing that the expansion rate of the Si@NC-G@NC electrode sheet was 35.8%, while that of the Si@NC-G electrode sheet was as high as 62.2%. This demonstrates that the constructed lignin-carbon outer carbon layer can effectively buffer the stress on silicon during cycling, better limiting the volume expansion of silicon.

[0140] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a nitrogen-doped lignin-based silicon-carbon anode material, characterized in that, Includes the following steps: (1) A mixture of lignin and nitrogen-containing eutectic solvent was mixed with nano-silicon with positive surface charge and calcined at high temperature to obtain nitrogen-doped silicon-carbon composite material. (2) The nitrogen-doped silicon-carbon composite material is embedded in the graphite surface to obtain a nitrogen-doped silicon-carbon / graphite composite material. (3) The nitrogen-doped silicon-carbon / graphite composite material is mixed with a mixed solution of lignin and a nitrogen-containing eutectic solvent and calcined at high temperature to obtain the nitrogen-doped lignin-based silicon-carbon anode material. The nitrogen-containing eutectic solvent is a mixture of choline chloride and urea. The molar ratio of choline chloride to urea is 1:(1-4).

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of the positively charged nano-silicon to lignin is (0.5-5):

1.

3. The preparation method according to claim 1, characterized in that, In step (1), the high-temperature calcination temperature is 200-1000℃ and the time is 1-6h.

4. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of the nitrogen-doped silicon-carbon composite material to graphite is (1-6):

1.

5. The preparation method according to claim 1, characterized in that, Embedding the nitrogen-doped silicon-carbon composite material into the graphite surface includes: mixing the nitrogen-doped silicon-carbon composite material and graphite and ball milling them; And / or, the ball mill rotates at a speed of 400–700 rpm for a time of 1–10 h.

6. The preparation method according to claim 1, characterized in that, In step (3), the high-temperature calcination temperature is 200-1000℃ and the time is 1-10h.

7. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of the nitrogen-doped silicon-carbon / graphite composite material to lignin is (1-4):

1.

8. The preparation method according to claim 1, characterized in that, In steps (1) and (3), the mass ratio of the nitrogen-containing eutectic solvent to lignin is (1-30):

1.

9. A nitrogen-doped lignin-based silicon-carbon anode material prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the nitrogen-doped lignin-based silicon-carbon anode material according to claim 9 in the preparation of battery anode materials.

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