Similar coconut shell type elastic inner shell carbon-silicon-carbon structure silicon-carbon negative electrode material, preparation and application

Abstract

The invention discloses a coconut shell-like elastic inner shell carbon-silicon-carbon structure silicon-carbon negative electrode material, preparation and application, and belongs to the technical field of electrode materials. The material adopts a three-layer gradient functional coating structure, an inner layer elastic hollow carbon nanocage can realize stress buffering through inward expansion deformation, a middle layer is a silicon nanoparticle active layer, an outer layer is a rigid carbon coating layer, and the whole material is in a coconut shell-like shape. Based on the deformation recovery characteristic of the elastic inner shell and the synergistic effect of the three-layer gradient functional structure, the volume expansion effect of a silicon-based active substance in the lithium embedding / de-embedding process can be effectively relieved, and pulverization and agglomeration of active particles and repeated rupture and regeneration of an SEI film are inhibited; when the material is applied to a lithium ion half battery, the material still has the capacity of 886 mAh g <-1 > after being circulated for 500 circles under the current density of 2.5 A g <-1 >; after the lithium ion battery is assembled with a LiCoO2 positive electrode to form a total battery, the capacity can still reach 131 mAh g <-1 > after 700 circles of circulation under the current density of 1 C; the material can be widely applied to power batteries, lithium ion batteries for energy storage and lithium ion batteries for consumer electronics, and has remarkable industrialization potential and application value.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a coconut shell-like elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material, and discloses its preparation method and related applications. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density and long cycle life, have been widely used in mobile electronic devices, electric vehicles, and other fields. However, the actual usable capacity of the graphite anode commonly used in commercial lithium-ion batteries is approaching 372 mAh g⁻¹. -1 The theoretical limits of the anode materials are insufficient to meet the urgent demand for high energy density in next-generation energy storage devices, making the development of high-performance alternative anode materials a key direction for industry development.

[0003] Silicon-based materials have a capacity of up to 4200 mAh g. -1 With its theoretical specific capacity and low lithiation potential close to that of metallic lithium, silicon-based anodes are widely recognized as one of the most promising next-generation lithium-ion battery anode materials. However, silicon-based anodes suffer from two major defects that restrict their large-scale application: First, silicon has extremely low intrinsic conductivity, resulting in high charge transport resistance and poor rate performance. Second, silicon undergoes a volume expansion of up to 300% during lithium insertion / extraction, which easily leads to silicon particle pulverization and agglomeration. This process also promotes the formation of an unstable solid electrolyte interphase (SEI) film on the electrode surface, causing continuous loss of active materials and excessive consumption of electrolyte, ultimately resulting in rapid capacity decay and extremely poor cycle stability.

[0004] To address the aforementioned issues, silicon-carbon composites have proven to be a simple and effective modification strategy. By coating or doping with carbon materials, the conductivity and structural stability of silicon-based anodes can be improved simultaneously. To date, researchers have developed silicon-carbon composite anode materials with various configurations, such as core-shell, yolk-shell, and shell-shell, but all of these structures have inherent weaknesses that are difficult to avoid. In the core-shell structure, the silicon core is directly encased in a carbon shell. During lithiation, the volume expansion of the silicon core generates enormous radial stress, which can easily cause the outer carbon shell to crack, leading to repeated cracking and regeneration of the SEI film and accelerating battery performance degradation. In the yolk-shell structure, although the cavity between the silicon yolk and the carbon shell alleviates the pressure of volume expansion, the poor electrical contact between the silicon and the carbon shell hinders charge transfer, resulting in low silicon utilization and failing to fully realize its high capacity advantage. Although the shell-shell structure balances stress buffering and charge transfer efficiency to some extent, the silicon shell tends to aggregate inward after multiple charge-discharge cycles, eventually evolving into a yolk-shell structure and losing its original structural advantages.

[0005] Furthermore, in the paper "High-performance silicon-carbon anode enabled by nitrogen-doped porous carbon-silicon-carbon (NPC-Si-C) composite material," researchers proposed a nitrogen-doped porous carbon-silicon-carbon (NPC-Si-C) composite material. This material achieves uniform dispersion of nano-silicon particles through a porous carbon framework and utilizes the channel structure to buffer the volume expansion of silicon, exhibiting a performance of 1725.17 mAh g⁻¹ at 0.5 C rate. -1 The initial discharge specific capacity and initial coulombic efficiency were 91.02%, and the capacity retention rate after 100 cycles was 75.66%. In the full cell system, the capacity retention rate after 150 cycles at 0.5C was 62.25%. This scheme represents a significant improvement over the traditional shell-to-shell structure. However, due to the lack of elastic deformation capability of the porous carbon core, the volume expansion of silicon particles can only be passively buffered by the pores, and there is still an inevitable tendency for outward expansion, which limits the long-term cycle stability and the potential for improving specific capacity.

[0006] In summary, existing silicon-carbon composite anode materials have failed to achieve a balance between high capacity and long cycle life due to issues such as insufficient structural stress release, impeded interfacial charge transport, or structural evolution over long cycles. Therefore, developing a novel silicon-carbon composite anode material that combines elastic stress buffering capacity, high charge transport efficiency, and stable structural characteristics remains a key technical challenge that urgently needs to be overcome in the current lithium-ion battery field. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a coconut shell-like elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material, its preparation, and its applications. This anode material, relying on the deformation recovery characteristics of the elastic inner shell and the synergistic effect of the three-layer gradient functional coating structure, can precisely alleviate the volume expansion effect of silicon-based active materials during lithium insertion / extraction, inhibit the pulverization and agglomeration of active particles and repeated rupture of the SEI film, while simultaneously improving the material's conductivity and interfacial compatibility. This material can be widely used in energy storage devices such as lithium-ion batteries and sodium-ion batteries, significantly improving the battery's energy density, cycle life, and rate performance, meeting the high-performance application requirements of multiple fields.

[0008] The technical solution of this invention is as follows: This invention provides a method for preparing a coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material. Through gradient functionalized structural design and precise process control, it solves the core problems of silicon-based anodes such as volume expansion and poor cycle stability. The specific preparation steps are as follows: 1) Elastic hollow carbon nanocage materials were prepared using the in-situ magnesium oxide template method; 2) Silicon-coated hollow carbon nanocage materials were prepared by in-situ deposition of silicon nanoparticles on the surface of elastic hollow carbon nanocage materials using chemical vapor deposition (CVD). 3) By using the asphalt liquid phase coating-high temperature carbonization method, a rigid outer carbon shell is constructed on the surface of the silicon coating layer to obtain a coconut shell-like carbon@silicon@carbon three-layer coated silicon-carbon anode material with an elastic inner shell.

[0009] Further, in step 1), the elastic hollow carbon nanocage material is either an undoped elastic hollow carbon nanocage or a nitrogen-doped elastic hollow carbon nanocage; both are prepared using basic magnesium carbonate as a template agent, and reacted at 750~850℃ for 0.5~2 h in an inert gas atmosphere to obtain the corresponding elastic hollow carbon nanocage; wherein, the carbon source for preparing the undoped elastic hollow carbon nanocage is one or more organic compounds such as benzene, toluene, xylene, sucrose, and glucose, and the nitrogen-doped elastic hollow carbon nanocage is prepared using one or more organic compounds such as pyridine, pyrrole, imidazole, melamine, and aniline as both carbon and nitrogen sources.

[0010] Preferably, the elastic hollow carbon nanocage material is a nitrogen-doped elastic hollow carbon nanocage, prepared using pyridine as the carbon and nitrogen source, and the mass-to-volume ratio of basic magnesium carbonate to pyridine is 8 g: 0.3~1.5 mL; more preferably, the mass-to-volume ratio of basic magnesium carbonate to pyridine is 8 g: 0.5 mL.

[0011] Furthermore, in step 2), the silicon source is silane, and the deposition reaction is carried out at 450~500℃ in an inert gas atmosphere. The volume ratio of silane in the argon / silane mixed gas is 5~10%, the total flow rate is 80~200 sccm, and the deposition time is 15~30 min. These process parameters can achieve uniform loading of silicon nanoparticles on the carbon cage surface and avoid particle agglomeration.

[0012] Further, in step 3), the mass ratio of silicon-coated hollow carbon nanocage material to asphalt is 1.5~3:1. After the two are dispersed in an organic solvent and mixed evenly, the solvent is removed by vacuum evaporation to obtain the asphalt-coated precursor. The precursor is then placed in an inert gas atmosphere and carbonized at 700~900℃ for 2~5 h to obtain the carbon@silicon@carbon three-layer coated silicon-carbon anode material.

[0013] Furthermore, the asphalt is one or more of coal tar pitch, petroleum asphalt, and mesophase asphalt. This type of asphalt has a high carbon yield and good film-forming properties, and is suitable for liquid phase coating and high-temperature carbonization processes, which can ensure the rigidity and density of the outer carbon shell.

[0014] This application also claims protection for a coconut-shell-like elastic inner-shell carbon@silicon@carbon structured silicon-carbon anode material prepared by the above method. This material has a typical three-layer gradient functional coating structure: the inner layer is an elastic hollow carbon nanocage with excellent deformation recovery ability, which can efficiently buffer the volume expansion of the silicon-based active layer; the middle layer is a silicon nanoparticle active layer, which provides a high specific capacity core for the material; and the outer layer is a rigid carbon coating layer, which can not only improve the conductivity of the material, but also form a physical barrier to prevent electrolyte corrosion. The synergistic effect of the three layers makes the material exhibit a coconut-shell-like morphology, balancing high capacity, high stability and high conductivity.

[0015] Furthermore, the silicon mass fraction in the silicon-carbon anode material is 30-40 wt.%, preferably 35 wt.%, which allows for a balance between high energy density and structural stability.

[0016] This invention also protects the application of the above-mentioned coconut shell-type elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material in lithium-ion batteries, wherein the lithium-ion battery includes a lithium-ion half-cell and a lithium-ion full-cell; wherein the lithium-ion half-cell uses pure lithium foil as the counter electrode, and the lithium-ion full-cell uses lithium cobalt oxide (LiCoO2) as the positive electrode, and the silicon-carbon anode material is used as the anode active material, which can be widely used in power batteries, energy storage lithium-ion batteries and consumer electronics lithium-ion batteries, and can significantly improve the cycle life and energy density of the battery.

[0017] The beneficial effects of this invention are as follows: 1. The coconut shell-like elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material disclosed in this application can effectively alleviate the volume expansion problem of silicon-based active materials during charge and discharge (especially lithium insertion / deintercalation) by virtue of the excellent deformation recovery capability of the elastic inner shell. On the one hand, the elastic inner shell effectively releases lithiation-induced stress through inward elastic deformation, avoiding silicon particle breakage and active material shedding, and ensuring the integrity of the material structure. On the other hand, the elastic inner shell and the outer coating structure form a synergistic support, which can significantly suppress the agglomeration, pulverization and electrode film peeling of electrode materials during cycling, providing core structural support for achieving long cycle life. 2. The silicon-carbon anode material disclosed in this application has a three-layer coating structure. The high specific surface area of ​​the inner nitrogen-doped carbon nanocage enables uniform dispersion of silicon nanoparticles and enhances the interfacial bonding force with silicon particles. The middle silicon-based active layer provides high specific capacity and ensures the energy density of the electrode material. The outer carbon layer not only improves the conductivity of the material, but also forms a physical barrier to prevent the electrolyte from eroding the internal structure. At the same time, the rigid outer carbon shell can inhibit the repeated formation / destruction of the SEI film during silicon alloying / dealloying. The three-layer structure design, along with the elastic inner shell and coconut shell-like morphology, works synergistically to enable the material to simultaneously possess high specific capacity, high conductivity, and high stability. 3. After silicon deposition and carbon coating, the number of mesopores in the elastic hollow cavity is reduced, and the pore volume and porosity are reduced simultaneously. This can effectively suppress the excessive wetting of electrolyte in the porous structure and the occurrence of side reactions, reduce electrolyte consumption in the electrochemical process, and thus improve the cycle stability and coulombic efficiency of the battery. 4. The three-layer coated silicon-carbon anode material disclosed in this application exhibits excellent comprehensive electrochemical performance when applied to lithium-ion battery anodes. In the half-cell system, at 2.5 A g... -1 It still has 886 mAh g after 500 cycles at current density. -1 Capacity: In a full-cell system matched with a LiCoO2 cathode, the capacity reaches 131mAh after 700 cycles at a 1C current density. -1 It can meet the core requirements of power batteries and energy storage batteries for high energy density, long cycle life and high rate performance. 5. The preparation method of the negative electrode material described in this application has clear steps, the raw materials used are widely available and the cost is controllable, and each process can be scaled up for industrial production; by adjusting the process parameters, the thickness of the elastic inner shell, the coconut shell-like pore structure and the thickness of the three-layer coating can be precisely controlled, ensuring the consistency and stability of product performance, and providing technical support for the large-scale application of the material. 6. The application scope of the negative electrode material described in this application can be extended to new energy storage systems such as sodium-ion batteries, and it is applicable to multiple fields such as power batteries, energy storage batteries, and consumer electronics batteries. Its excellent comprehensive performance and industrialization potential can help energy storage devices upgrade towards high energy density, long life and high safety, and have significant economic value and social significance. Attached Figure Description

[0018] Figure 1 In the image, a is a high-resolution transmission electron microscope (HRTEM) image of hNCNC-0.3; b is an HRTEM image of hNCNC-0.5; c is an HRTEM image of hNCNC-1.0; and d is an HRTEM image of hNCNC-1.5. Figure 2 In the image, a is a TEM transmission electron microscope (TEM) image of NS-0.5; b is a high-resolution transmission electron microscope (HRTEM) image of NS-0.5; c is a particle size distribution diagram of silicon nanoparticles in NS-0.5. Figure 3In the image, a is a transmission electron microscope (TEM) image of NSC-0.5; b is a high-resolution transmission electron microscope (HRTEM) image of NSC-0.5; c is an X-ray diffraction (XRD) pattern of hNCNC-0.5, NS-0.5, and NSC-0.5; d is a silicon-carbon thermogravimetric map of NSC-0.5; e is a Raman spectrum of hNCNC-0.5, NS-0.5, and NSC-0.5; f is a full X-ray photoelectron spectroscopy (XPS) spectrum of hNCNC-0.5, NS-0.5, and NSC-0.5; g is a silicon 2p orbital X-ray photoelectron spectroscopy (Si 2p XPS) pattern of hNCNC-0.5, NS-0.5, and NSC-0.5. Figure 4 In the diagram, a represents the N2 adsorption-desorption isotherms of hNCNC-0.5, NS-0.5, and NSC-0.5, and b represents the pore size distribution of hNCNC-0.5, NS-0.5, and NSC-0.5. Figure 5 In the image, a is a scanning electron microscope (SEM) image of hNCNC-0.5, b is a SEM image of NS-0.5, and c is a SEM image of NSC-0.5. Figure 6 In the figure, a represents the rate performance curves of half-cells fabricated based on NSC-0.3, NSC-0.5, NSC-1.0, and NSC-1.5 materials; b represents the rate performance curves of the corresponding half-cells at 2.5 Ag. -1 Long-cycle performance graph at current density, where c represents the corresponding half-cell at 0.5 Ag. -1 Long-cycle performance at current density; Figure 7 In the figure, a is the rate performance curve of the NSC-0.5 / / LiCoO2 full cell; b is the long-cycle performance curve of the NSC-0.5 / / LiCoO2 full cell at 1C current density. Figure 8 In the image, a is a transmission electron microscope (TEM) image of the ASC electrode; b is a long-cycle performance graph based on the ASC electrode. Figure 9 In the image, a is a three-dimensional distribution image of the main species in the SEI film on the NSC-0.5 electrode surface; b is a TOF-SIMS sputtering depth profile of the SEI film on the NSC-0.5 electrode surface; c is a three-dimensional distribution image of the main species in the SEI film on the ASC electrode surface; and d is a TOF-SIMS sputtering depth profile of the SEI film on the ASC electrode surface. Detailed Implementation

[0019] To better understand the content of this invention patent, the technical solution of this invention is further illustrated below through specific embodiments and accompanying drawings. However, these examples do not limit the invention. Modifications and substitutions made to the methods, steps, or conditions of this invention without departing from the essence of this invention are all within the scope of this invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0020] Example 1

[0021] 1) Preparation of hNCNC-0.5

[0022] Hollow carbon nanocage material hNCNC-0.5 (0.5 represents the volume of pyridine introduced, unit: mL) was prepared using the in-situ magnesium oxide template method with pyridine as the carbon and nitrogen source. The specific steps are as follows:

[0023] Weigh 8 g of basic magnesium carbonate (4MgCO3·Mg(OH)2·5H2O) and place it in the quartz tube of a vertical tube furnace. Algon gas is alternately introduced and a vacuum is drawn using a mechanical pump. This operation is repeated 3 times to completely remove the air in the tube. Then, the argon atmosphere is maintained. The temperature is increased to 800℃ at a heating rate of 10℃ / min. 0.5 mL of pyridine is injected into the quartz tube using a microsyringe, and the reaction is maintained at this temperature for 1 h. After the reaction is completed, the furnace tube is allowed to cool naturally to room temperature under argon protection. The black powder in the quartz tube is collected and washed repeatedly with 3 mol / L hydrochloric acid aqueous solution and deionized water until neutral. The washed product is freeze-dried for 24 h to obtain hollow carbon nanocages, denoted as hNCNC-0.5.

[0024] 2) Synthesis of NS-0.5

[0025] Silicon nanoparticles (Si NPs) were deposited on the surface of hNCNC-0.5 using chemical vapor deposition (CVD) with silane (SiH4) as the silicon source to prepare silicon-coated hollow carbon nanocage material (denoted as NS-0.5). The specific steps are as follows: 100 mg of hNCNC-0.5 was weighed and placed in the quartz tube of a vertical tube furnace. Argon gas was alternately introduced and the tube was evacuated three times to completely remove the air inside the tube. Then, the argon atmosphere was maintained. The temperature was increased to 475℃ at a heating rate of 5℃ / min. After the temperature stabilized, the gas was switched to an argon / silane mixed gas (argon volume percentage 95% and silane volume percentage 5%). The total gas flow rate was controlled at 100 sccm. The deposition was maintained under these conditions for 20 min. After the deposition was completed, the silane was stopped, the argon gas was continued to be introduced, and the material was naturally cooled to room temperature. The product was collected to obtain silicon-coated hollow carbon nanocage NS-0.5.

[0026] 3) Synthesis of NSC-0.5

[0027] A carbon@silicon@carbon three-layer core-shell composite material (denoted as NSC-0.5) was prepared by constructing an outer carbon shell on the surface of NS-0.5 using a coal tar pitch liquid-phase coating-high-temperature carbonization method. The specific steps are as follows: NS-0.5 and coal tar pitch were weighed at a mass ratio of 2:1 and added together to an appropriate amount of tetrahydrofuran (THF) solvent. The mixture was magnetically stirred at room temperature for 24 h to fully dissolve the coal tar pitch and uniformly coat the surface of NS-0.5. The mixture was then transferred to a rotary evaporator and evaporated at 40 °C under reduced pressure to remove the THF solvent, resulting in a dry coal tar pitch-coated precursor solid. The precursor was placed in a quartz reaction tube of a vertical tube furnace and heated to 800 °C at a heating rate of 5 °C / min under an argon atmosphere. The temperature was maintained for 3 h for carbonization. After carbonization, the mixture was naturally cooled to room temperature, and the product was collected to obtain the carbon@silicon@carbon three-layer composite material NSC-0.5.

[0028] Example 2

[0029] The only difference between this embodiment and Example 1 is that in step 1), the amount of pyridine added is adjusted to 0.3 mL during the preparation of hollow carbon nanocages; all other operating steps and reaction conditions are consistent with those in Example 1. The hollow carbon nanocage hNCNC-0.3 prepared under these conditions was further processed using the same subsequent process to obtain intermediate NS-0.3 and the final product NSC-0.3.

[0030] Example 3

[0031] The only difference between this embodiment and Example 1 is that in step 1), the amount of pyridine added is adjusted to 1.0 mL during the preparation of hollow carbon nanocages; all other operating steps and reaction conditions are consistent with those in Example 1. The hollow carbon nanocage hNCNC-1.0 prepared under these conditions was further processed using the same subsequent process to obtain intermediate NS-1.0 and the final product NSC-1.0.

[0032] Example 4

[0033] The only difference between this embodiment and Example 1 is that in step 1), the amount of pyridine added is adjusted to 1.5 mL during the preparation of hollow carbon nanocages; all other operating steps and reaction conditions are consistent with those in Example 1. Based on these conditions, the hollow carbon nanocage hNCNC-1.5 was further prepared using the same subsequent process to obtain intermediate NS-1.5 and the final product NSC-1.5.

[0034] Related performance tests

[0035] 1. Morphological characterization and mechanical property testing of hNCNC-x (x represents the volume of pyridine introduced, unit: mL) series materials

[0036] 1.1 High-resolution transmission electron microscopy (HRTEM) characterization results showed that the wall thicknesses of hNCNC-0.3, hNCNC-0.5, hNCNC-1, and hNCNC-1.5 were 1.77 nm, 1.93 nm, 2.41 nm, and 2.76 nm, respectively. Figure 1 ).

[0037] 1.2 The mechanical property characterization results of the hNCNC-x series materials show that the deformation resistance of the materials increases with the thickness of the shell; among them, the hNCNC-0.3 sample is relatively soft, while the hNCNC-1.0 sample is hard and has poor stress buffering capacity. The fitted Young's modulus (YM) data show that the YM values ​​of hNCNC-0.3 and hNCNC-1.0 are 7.702 GPa and 10.048 GPa, respectively, indicating relatively poor elasticity; while the YM value of hNCNC-0.5 is only 5.492 GPa, confirming that this sample has the best elastic properties.

[0038] 2. Morphological characterization of NS-0.5

[0039] From NS-0.5 HRTEM ( Figure 2 From the characterization results in (b), no silicon lattice fringes were observed. Combined with the deposition process analysis, this phenomenon is attributed to the low deposition temperature causing the silicon nanoparticles to exist in an amorphous structure; particle size statistics ( Figure 2 c) shows that the average particle size of silicon nanoparticles in NS-0.5 is 6.2±1.4nm, indicating that silicon particles have achieved small size and good uniformity loading on the carbon matrix surface.

[0040] 3. Morphological characterization of NSC-0.5

[0041] 3.1 Unlike the amorphous structure of silicon in NS-0.5, the combined HRTEM and XRD characterization results of NSC-0.5 show that the silicon nanoparticles in NSC-0.5 have a crystalline structure. Figure 3 (b, c) Thermogravimetric analysis (TGA) results show that the silicon content in NSC-0.5 is approximately 35 wt.%. Figure 3 (d) Raman spectroscopy characterization confirmed the presence of silicon phase characteristic signals in both NS-0.5 and NSC-0.5; however, the silicon characteristic peak intensity of NSC-0.5 decreased, a phenomenon attributed to the shielding effect of the carbon layer coating the silicon nanoparticles on the silicon characteristic signals. Figure 3 e)

[0042] 3.2 X-ray photoelectron spectroscopy (XPS) analysis results show that both NS-0.5 and NSC-0.5 materials are composed of four elements: silicon (Si), carbon (C), nitrogen (N), and oxygen (O). Figure 3 (f) Among them, the nitrogen content of the hNCNC matrix is ​​as high as 8.8 at.%, and the introduction of nitrogen can construct defect sites in the carbon matrix, which can not only improve the ion permeability of the material, but also effectively alleviate the stress caused by the volume expansion of silicon nanoparticles during lithiation. The Si 2p XPS spectra of NS-0.5 and NSC-0.5 are similar, which indicates that no chemical reaction occurred between the silicon nanoparticles and the carbon coating layer during the high-temperature carbonization process. Figure 3 (g in the text). The two characteristic peaks with binding energies at 99.5 eV and 103.2 eV can be attributed to the chemical states corresponding to Si-Si bonds and O-Si-O bonds, respectively.

[0043] 3.3 After silicon deposition and carbon coating, the pore structure and specific surface area (SSA) of the material undergo significant changes: this can be seen from the N2 adsorption-desorption isotherm of the material. Figure 4 In the a), hNCNC-0.5 exhibits a typical type IV isotherm and an H4 type hysteresis loop, corresponding to a rich mesoporous structure, with a specific surface area as high as 886 m². 2 / g; NS-0.5 obtained by silicon deposition has a specific surface area that drops sharply to 191 m². 2 / g; after further carbon coating, NSC-0.5 has a specific surface area of ​​only 86.2 m². 2 / g. The changes in adsorption capacity on the isotherm directly reflect the decreasing trend of pore volume: hNCNC-0.5 has a significantly higher adsorption capacity in the high relative pressure region, indicating the largest pore volume; while the adsorption capacity of NS-0.5 and NSC-0.5 decreases sequentially, confirming the filling effect of silicon deposition and carbon coating on the pores. Combined with the pore size distribution diagram (dV / dD), ( Figure 4 In Figure b), hNCNC-0.5 exhibits multiple distinct pore volume differential peaks in the 2–100 nm range, indicating well-developed mesopores with a wide pore size distribution. After silicon deposition and carbon coating, the intensity of the pore volume differential peaks in NS-0.5 and NSC-0.5 significantly decreases, and the peak shape narrows, indicating a reduction in the number of mesopores, a more concentrated pore size distribution, and a simultaneous decrease in pore volume and porosity. Furthermore, SEM images of hNCNC-0.5, NS-0.5, and NSC-0.5 further confirm from a microscopic morphology perspective that the pore volume of the materials decreases progressively with the gradual modification of silicon deposition and carbon coating (Figure 5).

[0044] The above results show that NSC-0.5 has a lower specific surface area and porosity. This characteristic can effectively suppress the excessive wetting of electrolyte in the porous structure and side reactions, thereby reducing electrolyte consumption in the electrochemical process and helping to improve the cycle stability and coulombic efficiency of the battery.

[0045] 4. Electrochemical performance testing of NSC-x series materials

[0046] Tests revealed that the bulk conductivity of the three materials, hNCNC-0.5, NS-0.5, and NSC-0.5, was 116.4 Sm. -1 5.5 S m -1 and 46.3 S m -1 This demonstrates that the unique three-layer coating structure of NSC-0.5 not only effectively enhances the electron transport capability of silicon-coated hNCNC (i.e., NS-0.5), but also helps to improve the electrochemical cycling stability of the material.

[0047] The morphology and composition of the three materials NSC-0.3, NSC-1.0 and NSC-1.5 are similar to those of NSC-0.5. The main differences are in the thickness and elastic properties of the inner shell. Based on the NSC-x series carbon@silicon@carbon three-layer composite material, silicon-carbon anodes were further prepared, and the electrochemical performance of the corresponding electrode materials was systematically tested.

[0048] The preparation process of the working electrode is as follows: NSC-x active material, carbon black conductive agent, and sodium carboxymethyl cellulose (CMC) binder are dispersed in deionized water at a mass ratio of 8:1:1, and stirred thoroughly to form a homogeneous slurry. The resulting slurry is then uniformly coated onto the surface of a copper foil current collector and dried in a vacuum oven at 80°C for 12 h. The surface loading of active material on all electrodes is controlled to be approximately 1.27 mg·cm³. -2 A CR2032 coin cell half-cell was assembled using pure lithium foil as the counter electrode; a full cell was assembled using lithium cobalt oxide (LiCoO2) as the positive electrode. The electrolyte was a 1 mol / L lithium hexafluorophosphate (LiPF6) solution, with the solvent system consisting of 89% by volume of a ethylene carbonate / diethyl carbonate (EC / DEC, volume ratio 1:1) mixture, 10% by volume of vinyl fluorocarbonate (FEC), and 1% by volume of vinylene carbonate (VC). A Celgard 2500 membrane was used. After assembly, the cells were allowed to stand for 12 hours to ensure sufficient electrolyte wetting of the electrodes and membrane before electrochemical performance testing. Constant current charge-discharge tests were performed using a NEWARE CT4008T multichannel battery testing system, with the half-cell test potential range being 0.01–1.00 V (relative to Li / Li). + (Electrodes), the test voltage range for the full cell is 2.7~4.2 V.

[0049] For rate capability and cycle performance of half-cells, please refer to [link / reference]. Figure 6 In stepped current density tests, NSC-0.5 exhibited optimal rate adaptability: even at 12.5 Ag... -1 It can still output stable capacity even at ultra-high current densities; when the current density drops back to 0.25 Ag... -1When the specific capacity is restored, it can quickly recover to near its initial level, demonstrating excellent electron transport and structural stability. Figure 6 a) in 2.5 Ag -1 After 500 cycles at high current density, the NSC-0.5 still maintains 886 mAh g⁻¹. -1 High specific capacity ( Figure 6 (b) in 0.5 Ag -1 After 200 cycles at the current density, the specific capacity remained stable at 1293 mAh g⁻¹. -1 ( Figure 6 (c) Compared with other components (NSC-0.3, NSC-1.0, NSC-1.5), NSC-0.5 has better capacity retention and coulombic efficiency stability, further confirming the advantages of its three-layer coating structure in mitigating silicon volume expansion and improving cycle life.

[0050] Further characterization of the electrode thickness before and after cycling revealed that the initial electrode thicknesses of NSC-0.3, NSC-0.5, NSC-1.0, and NSC-1.5 were 53 μm, 54 μm, 43 μm, and 40 μm, respectively. After 500 cycles, the corresponding electrode thicknesses increased to 66 μm, 61 μm, 54 μm, and 61 μm, respectively. Among them, NSC-0.5 had the lowest volume expansion rate at only 13.0%, while NSC-1.5 had the highest volume expansion rate at 52.5%. This result reveals the crucial impact of inner shell thickness on structural stability: when the inner shell is too thick, the material becomes too rigid and cannot buffer lithiation stress through inward deformation; the stress can only be released outward, leading to cracking of the outer carbon shell and agglomeration of silicon particles, ultimately causing rapid capacity decay. When the inner shell is too thin, the material is too soft and difficult to recover after deformation, easily causing the collapse of the three-shell structure, which also leads to performance degradation.

[0051] The above results fully demonstrate that an innovative structural design for silicon-carbon anodes can be achieved by constructing a coconut shell-like three-layer shell structure with an elastic inner shell, fundamentally alleviating the volume expansion problem of silicon-based anodes during charge and discharge. Specifically, the elastic inner shell can undergo controllable inward deformation during silicon volume expansion, effectively buffering lithiation stress; while the optimized three-layer shell structure maintains the overall stability of the electrode, reduces direct contact between silicon particles and the electrolyte, thereby suppressing side reactions and repeated rupture of the SEI film, ultimately significantly improving the electrochemical performance of the silicon-carbon anode.

[0052] The rate capability and long-cycle performance of the full cell prepared based on NSC-0.5 are as follows: Figure 7 As shown, in the stepped current density test, the NSC-0.5 / / LiCoO2 full cell exhibits good rate adaptability. Even as the current density increases to 5 C, the cell can still stably output approximately 90 mAh g⁻¹.-1 The reversible capacity; when the current density drops back to 0.1 C, the capacity can quickly recover to near the initial level, demonstrating the stability of the electrode structure at different rates. Figure 7 (a) After 700 cycles at a current density of 1 C, the discharge specific capacity of the NSC-0.5 / / LiCoO2 full cell remained at 131 mAh g⁻¹. -1 It exhibits excellent cycle stability and charge reversibility, further verifying the reliability of the three-layer coating structure in the full-cell system. Figure 7 (b) in the middle.

[0053] Comparative Example

[0054] To further elucidate the crucial role of the hollow elastic cavity structure in the overall performance of the carbon@silicon@carbon trilayer composite material, this comparative example uses solid core acetylene black (AB) instead of the hollow carbon nanocage (hNCNC) in Example 1 as the core. Through completely identical silicon coating and outer carbon shell preparation processes, an acetylene black@silicon@carbon trilayer composite material (denoted as ASC) was obtained.

[0055] Transmission electron microscopy (TEM) images from ASC, such as Figure 8 As shown in 'a', its long-cycle performance curve ( Figure 8 As shown in b), the electrochemical performance of the ASC anode is significantly inferior to that of the NSC-0.5:2.5 Ag. -1 At current density, its initial specific capacity is only about 100 mAh g. -1 The highest specific capacity is less than 250 mAh g. -1 During cycling, the capacity continuously decayed, and after 500 cycles, only about 100 mAh g⁻¹ remained. Although the coulombic efficiency could be maintained above 98%, the capacity retention was extremely poor.

[0056] This phenomenon stems from the fact that the solid core structure of acetylene black cannot provide a buffer for the volume expansion of silicon: during lithiation, when silicon nanoparticles undergo significant outward volume expansion, the lack of a hollow cavity for buffering directly leads to the rupture of the outer carbon shell and the collapse of the electrode structure, ultimately causing rapid capacity decay. This result further confirms the core role of the hNCNC hollow structure in mitigating silicon volume expansion and maintaining the integrity of the electrode structure.

[0057] To verify the effect of the elastic hollow structure on the stability of the solid electrolyte interphase (SEI) film, time-of-flight secondary ion mass spectrometry (TOF-SIMS) combined with argon ion sputtering was used to systematically characterize the composition and depth distribution of the SEI film on the NSC-0.5 and ASC electrode surfaces, with a focus on monitoring silicon species (Si). - SiO3Li - ), organic components (C2HO)- C4H - ) and inorganic components (LiF2) - PO3 - ) characteristic signal ( Figure 9 ).

[0058] For NSC-0.5 electrode: 3D distribution diagram ( Figure 9 As shown in a), the species distribution on the NSC-0.5 surface is uniform; from the sputtering depth profile ( Figure 9 As shown in b), with increasing sputtering time (i.e., increased detection depth), the organic component C2HO... - C4H - With inorganic component LiF2 - PO3 - The signal strength remained stable throughout, while the silicon species Si - and SiO3Li - The signal gradually increased from a low level to a stable higher value within the initial 150 s. This result indicates that the SEI film formed on the NSC-0.5 electrode surface is mainly composed of conventional organic and inorganic components, and the silicon species content in the outer SEI is extremely low, reflecting that its outer carbon shell structure is intact and no outward expansion and cracking of silicon has occurred, thus ensuring the stability of the SEI film.

[0059] For ASC electrodes: 3D distribution diagram ( Figure 9 c) shows that the distribution of silicon species on the ASC surface is significantly uneven; its depth profile spectrum ( Figure 9 In d), Si - With C4H - The signal rapidly increases in the initial stage of sputtering (first 20 s), while LiF2 - The signal of the other species (C2HO) showed a significant attenuation trend throughout the 400 s sputtering process. - SiO3Li - PO3 - The signal also exhibited unstable fluctuations. This phenomenon indicates that a thick SEI film rich in silicon species has formed on the ASC surface, directly confirming that its carbon coating layer has ruptured due to the outward volume expansion of silicon, leading to repeated formation and damage of the SEI film caused by the exposed silicon on the electrode surface, ultimately resulting in deterioration of cycle performance.

[0060] The above TOF-SIMS results further confirm from the perspective of interface chemistry that the elastic hollow inner shell of hNCNC-0.5 can buffer the volume expansion of silicon by bending inward, protect the integrity of the outer carbon shell, thereby inhibiting the outward diffusion of silicon species and stabilizing the SEI film. This is one of the core reasons why NSC-0.5 has excellent cycling stability.

[0061] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material, characterized in that, Includes the following steps: 1) Elastic hollow carbon nanocage materials were prepared using the in-situ magnesium oxide template method; 2) Silicon-coated hollow carbon nanocage materials were prepared by in-situ deposition of silicon nanoparticles on the surface of elastic hollow carbon nanocage materials using chemical vapor deposition. 3) By using the asphalt liquid phase coating-high temperature carbonization method, a rigid outer carbon shell is constructed on the surface of the silicon coating layer to obtain a coconut shell-like carbon@silicon@carbon three-layer coated silicon-carbon anode material with an elastic inner shell.

2. The preparation method of the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 1, characterized in that, In step 1), the elastic hollow carbon nanocage material is either an undoped elastic hollow carbon nanocage or a nitrogen-doped elastic hollow carbon nanocage. During preparation, basic magnesium carbonate is used as a template agent, and the reaction is carried out at 750-850℃ for 0.5-2 h in an inert gas atmosphere to obtain the corresponding elastic hollow carbon nanocage. The carbon source for preparing the undoped elastic hollow carbon nanocage is one or more of benzene, toluene, xylene, sucrose, and glucose. When preparing the nitrogen-doped elastic hollow carbon nanocage, one or more of pyridine, pyrrole, imidazole, melamine, and aniline are used simultaneously as both the carbon and nitrogen source.

3. The preparation method of the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 1, characterized in that, In step 2), the silicon source is silane, and the deposition reaction is carried out at 450~500℃ in an inert gas atmosphere. The volume percentage of silane in the argon / silane mixed gas is 5~10%, the total flow rate is 80~200 sccm, and the deposition time is 15~30 min.

4. The preparation method of the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 1, characterized in that, In step 3), the mass ratio of silicon-coated hollow carbon nanocage material to asphalt is 1.5~3:

1. The two are dispersed in an organic solvent and mixed evenly. The solvent is removed by vacuum evaporation to obtain the asphalt-coated precursor. The precursor is then placed in an inert gas atmosphere and carbonized at 700~900℃ for 2~5 h to obtain the carbon@silicon@carbon three-layer coated silicon-carbon anode material.

5. The preparation method of the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 1, characterized in that, The asphalt is one or more of coal tar pitch, petroleum asphalt, and mesophase asphalt.

6. The method for preparing the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 2, characterized in that, In step 1), the elastic hollow carbon nanocage material is a nitrogen-doped elastic hollow carbon nanocage, which is prepared using pyridine as the carbon and nitrogen source, and the mass-volume ratio of basic magnesium carbonate to pyridine is 8 g: 0.3~1.5 mL.

7. The preparation method of the coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material as described in claim 6, characterized in that, In step 1), the mass-to-volume ratio of basic magnesium carbonate to pyridine is 8 g: 0.5 mL.

8. A coconut shell-like elastic inner shell carbon@silicon@carbon structured silicon-carbon anode material, characterized in that, It is prepared based on the preparation method of any one of claims 1-7. The silicon-carbon anode material has a three-layer coating structure, with the inner layer being an elastic hollow carbon nanocage with deformation recovery capability, the middle layer being a silicon nanoparticle active layer, and the outer layer being a rigid carbon coating layer. The overall shape is similar to a coconut shell.

9. The coconut shell-like elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material as described in claim 8, characterized in that, The silicon mass fraction in the silicon-carbon anode material is 30~40 wt.%.

10. The application of the coconut shell-like elastic inner shell carbon@silicon@carbon structure silicon-carbon anode material as described in claim 8 in lithium-ion batteries, characterized in that... The lithium-ion battery includes lithium-ion half-cells and lithium-ion full-cells; the silicon-carbon anode material can be used as an anode active material and applied to power batteries, energy storage lithium-ion batteries or consumer electronics lithium-ion batteries.

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