Silicon-carbon negative electrode material and preparation method thereof

Abstract

This invention belongs to the field of lithium-ion battery anode materials, and provides a silicon-carbon anode material and its preparation method. The invention adopts a three-layer core-shell structure design of nano-silicon core-cavity layer-boron-doped dense carbon shell. The average diameter of the nano-silicon core is 20-100nm, and the cavity ratio is 30-40vol% to buffer the volume expansion of silicon. The thickness of the boron-doped dense carbon shell is 2-5nm and the boron doping amount is 0.5-6.0at% to improve conductivity and structural stability. The surface of the anode sheet is post-treated with lithium bis(fluorosulfonyl)imide solution to form an artificial solid electrolyte film rich in lithium fluoride. This invention solves the three sets of coupling contradictions in silicon-carbon anodes: high actual density and high coulombic efficiency over long cycles, integrity of the dense carbon shell and buffering of silicon volume effect, and low viscosity and high solid content of the electrode and uniform formation of artificial SEI and high LiF film. It has broad application value in lithium-ion secondary batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode materials, specifically to a silicon-carbon anode material and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles and portable electronic devices, the market demand for lithium-ion battery energy density continues to rise, making high-specific-capacity anode materials a key to breaking through the battery energy density bottleneck. Silicon-based anode materials, with their high specific capacity... Its theoretical specific capacity far exceeds that of graphite anodes. Silicon has become a core candidate material for next-generation high-energy-density lithium-ion batteries. However, realizing the commercial application of silicon anodes requires not only high specific capacity but also long-term cycle stability and high coulombic efficiency under high compaction conditions. High compaction density is a necessary condition for improving volumetric energy density, and the power battery field typically requires the anode sheet to achieve a compaction density of [missing information]. These factors pose stringent challenges to the structural design and processing performance of materials. Meanwhile, long cycle life and high coulombic efficiency are key indicators for ensuring battery lifespan and energy utilization, requiring materials to maintain stable capacity output and extremely low lithium-ion loss per cycle across hundreds of charge-discharge cycles. Furthermore, in large-scale production, the rheological properties of the slurry directly affect coating uniformity and production efficiency; a slurry system with high solids content and low viscosity is crucial for achieving high compaction density and excellent processing performance. Therefore, developing silicon-carbon anode materials that combine high specific capacity, high compaction density adaptability, long cycle stability, high coulombic efficiency, and excellent processing performance is of great significance for promoting the industrialization of high-energy-density lithium-ion batteries.

[0003] To address the challenges in applying silicon anode materials, current research mainly focuses on nano-sizing, porous structure design, carbon coating modification, and interface engineering; however, each approach has significant limitations. While nano-silicon can shorten ion diffusion distance and alleviate stress concentration, its large specific surface area exacerbates side reactions, resulting in low initial coulombic efficiency. Furthermore, severe agglomeration between nanoparticles significantly affects slurry dispersion uniformity and coating stability. For example, Chinese patent CN116682958B discloses a nano-silicon-carbon composite material, its preparation method, and its application, but it does not effectively solve the problems of nano-silicon agglomeration and interface side reactions caused by high specific surface area, and its cycle stability remains unsatisfactory. While porous silicon or hollow structures can provide buffer space for volume expansion, porous structures often lead to decreased material compaction density and limited electrode volumetric energy density. Additionally, electrolyte retention within the pores increases irreversible capacity loss. For instance, Chinese patent application CN120221605A discloses a silicon-carbon anode material, its preparation method, and a battery, but its compaction density only reaches a certain level. The above-mentioned limitations make it difficult to meet the high energy density requirements of thick electrodes. While carbon coating technology can improve conductivity and suppress side reactions, traditional carbon layers are often too thick or too thin. Excessively thick carbon layers reduce the overall specific capacity of the material and hinder ion transport, while excessively thin carbon layers lack mechanical strength and are prone to breakage and failure during cycling. Furthermore, existing technologies generally neglect the compositional control of artificial solid electrolyte interfacial membranes. Natural SEI membranes are rich in organic phases and carbonates, have low ionic conductivity and poor mechanical properties, leading to a continuous increase in interfacial impedance and accelerated capacity decay. Summary of the Invention

[0004] The purpose of this invention is to provide a silicon-carbon anode material and its preparation method, which solves the essential conflict caused by three sets of coupling contradictions in silicon-carbon anodes: high actual density and high coulombic efficiency over long cycles, integrity of dense and high modulus carbon shell and buffering of silicon volume effect, and low viscosity and high solid content of electrode processing rheology and uniform film formation of artificial SEI with high LiF. In response to the problems of uneven ion flux, repeated interface breakage and irreversible lithium loss caused by the mismatch of mechanical-transport-interface multi-field coupling, a systematic solution that takes into account both structural / processing performance and functional / usage performance is proposed.

[0005] This invention employs a strategy of precise nanostructure control and multi-scale interface synergistic design. By constructing a three-layer core-shell structure consisting of a nano-silicon core, a cavity layer, and a boron-doped dense carbon shell, it achieves an organic unity of volume buffering, electron transport, and ion diffusion. The cavity layer reserves space for silicon expansion and reduces particle packing density to improve slurry rheology. The boron-doped dense carbon shell significantly enhances conductivity while maintaining structural integrity. Combined with a LiF-rich artificial solid electrolyte membrane constructed from lithium bis(fluorosulfonyl)imide solution, a stable interface layer with high ionic conductivity and low electronic conductivity is formed on the electrode surface. This multi-layered synergistic effect enables the material to achieve a breakthrough improvement in long-cycle stability, high coulombic efficiency, and excellent processing performance under high pressure and density conditions.

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

[0007] A silicon-carbon anode material includes a nano-silicon core, a cavity layer surrounding the nano-silicon core, and a boron-doped dense carbon shell located on the outer side; the mass fraction of nano-silicon in the silicon-carbon anode material is 20–28 wt%; the nano-silicon core is made of metallic silicon powder with an average diameter of 20–100 nm; the cavity ratio of the silicon-carbon anode material is 30–40 vol%, and the thickness of the dense carbon shell is 2–5 nm; the doping amount of boron atoms in the boron-doped dense carbon shell is 0.5–6.0 at%; the atomic fraction of lithium fluoride in the inorganic phase of the artificial solid electrolyte membrane on the surface of the anode sheet made of the silicon-carbon anode material is 40–70 at%.

[0008] Furthermore, silicon-carbon anode materials are used for compaction with a density of The negative electrode sheet.

[0009] Furthermore, the binder system of the negative electrode sheet includes calcium alginate and lithium polyacrylate, with a mass ratio of 3:7 to 7:3.

[0010] Furthermore, the artificial solid electrolyte membrane originates from the post-treatment of the negative electrode sheet with lithium bis(fluorosulfonyl)imide solution, and the concentration of the lithium bis(fluorosulfonyl)imide solution is... .

[0011] Furthermore, the average diameter of the nano-silicon core is preferably 30–80 nm.

[0012] Furthermore, the preferred mass fraction of nano-silicon in the silicon-carbon anode material is 22–26 wt%.

[0013] Furthermore, the atomic fraction of lithium fluoride in the inorganic phase of the artificial solid electrolyte membrane is 50–65 at.

[0014] As a concept of this invention, a three-layer core-shell structure design of nano-silicon core-cavity layer-boron-doped dense carbon shell is mainly used to enhance the cycling stability and coulombic efficiency of silicon-carbon anodes under high solid density. The size of the nano-silicon core is controlled at 20–100 nm, which ensures specific capacity contribution and reduces concentration polarization by shortening the lithium-ion diffusion path. The nano-scale effect makes the stress distribution uniform and effectively suppresses the fragmentation of large-scale silicon particles during lithium intercalation. The cavity layer is one of the core designs. By precisely controlling the cavity ratio to 30–40 vol%, sufficient volume buffer space is reserved between the silicon core and the carbon shell. This allows the silicon to expand into the cavity during lithium intercalation without generating destructive stress on the carbon shell. This inward expansion mechanism fundamentally solves the problem of carbon shell cracking in traditional silicon-carbon materials during cycling. At the same time, the cavity structure reduces the particle packing density, allowing the slurry to maintain a low viscosity even at high solid content, significantly improving processing performance. The boron-doped dense carbon shell design achieves synergistic optimization of structural stability and electrochemical activity. The ultrathin thickness of 2–5 nm ensures the integrity of the carbon shell structure while minimizing the radial diffusion distance of lithium ions. Boron doping introduces defect sites and impurity energy levels into the carbon layer, significantly improving electronic conductivity and optimizing lithium-ion transport kinetics. The high LiF content in the artificial solid electrolyte membrane is designed based on LiF's excellent solid electrolyte composition, characterized by high ionic conductivity, low electronic conductivity, and excellent chemical stability. A LiF-rich interface layer is constructed on the electrode surface through in-situ reaction of lithium bis(fluorosulfonyl)imide solution, forming a stable solid electrolyte interface. This effectively inhibits continuous electrolyte decomposition and repeated SEI film reconstruction, significantly improving both initial coulombic efficiency and cycle coulombic efficiency.

[0015] A method for preparing silicon-carbon anode materials includes: S1. Controlled oxidation of nano-silicon to form an etchable phase; S2. Selective etching of the oxidized particles to achieve a cavity ratio of 30–40 vol%; S3. Constructing a boron-doped dense carbon shell with a thickness of 2–5 nm on the outer side of the particles using a polymerization carbonization method, and introducing a boron-containing precursor during the polymerization and carbonization process to achieve a boron doping atomic fraction of 0.5–6.0 at% for the dense carbon shell; S4. Slurrying, coating, and rolling the obtained particles to obtain an anode sheet; S5. Short-time treatment of the anode sheet with a lithium bis(fluorosulfonyl)imide solution, followed by heating and curing to form an artificial solid electrolyte film on the surface of the electrode sheet.

[0016] Furthermore, the oxidation temperature in step S1 is 200–350°C, and the time is 0.5–2 h; the selective etching in step S2 uses an aqueous solution of ammonium fluoride at a concentration of [missing information]. The temperature is 20–40°C and the time is 10–60 min.

[0017] Furthermore, in step S3, when the carbon source solution is subjected to polymerization carbonization, the solid content of the carbon source solution is 1–5 wt%, and carbonization is carried out in an inert atmosphere. The carbon source is any one of glucose, sucrose, starch, dopamine hydrochloride, or norepinephrine. The carbonization temperature is 600–800°C, and the time is 0.5–2 h.

[0018] Furthermore, the boron-containing precursor is trimethoxyborane, triethoxyborane, borane-tetrahydrofuran complex, boron trifluoride-diethyl ether complex, or boric acid / boronic acid ester; the mass ratio of the boron-containing precursor to the carbon source is 1:10–1:2.

[0019] Furthermore, the oxidizing atmosphere in step S1 is dry air or argon containing 5–20 vol% oxygen.

[0020] Furthermore, the pH of the selective etching solution in step S2 is 4.5–6.5.

[0021] Furthermore, the short-time processing time in step S5 is 2–6 min, and the drying temperature is 80–110°C; the heat curing is carried out at 80–110°C for 5–10 min under an argon or nitrogen atmosphere; the heating rate is... Cooling rate ≤ .

[0022] Furthermore, in step S5, the solvent for the lithium bisfluorosulfonylimide solution is a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethoxyethane in a volume ratio of (2–4):(2–4):(1–3).

[0023] As another aspect of this invention, a multi-step method involving controlled oxidation, selective etching, boron doping and carbonization, and artificial SEI construction is employed to achieve precise structural control and large-scale controllable fabrication of silicon-carbon anode materials. The controlled oxidation step forms a thin layer of silicon dioxide on the nano-silicon surface, creating a controllable etching phase for subsequent selective etching. Precise control of oxidation temperature and time ensures uniform oxide layer thickness, a prerequisite for obtaining a regular cavity structure. Selective etching utilizes an aqueous solution of ammonium fluoride, leveraging its selective dissolution properties of silicon dioxide. By precisely controlling the etching solution concentration, temperature, and time, the cavity ratio is precisely controlled to 30–40 vol%, and pH optimization ensures the selectivity and uniformity of the etching reaction, avoiding excessive corrosion of the silicon core. The key technology lies in constructing a boron-doped dense carbon shell using a polymerization carbonization method. The carbon source solution self-assembles and polymerizes on the particle surface to form a uniform coating layer. Low solids content ensures the density and controllable thickness of the coating layer. During carbonization in an inert atmosphere, the carbon source pyrolyzes into a carbon shell, while the boron-containing precursor simultaneously pyrolyzes and is atomically incorporated into the carbon layer lattice. The boron doping amount is precisely controlled by the mass ratio of precursor to carbon source, and the carbonization temperature is optimized to balance the degree of graphitization of the carbon layer and the boron atom retention rate. Slurry preparation, coating, and rolling utilize a composite binder of calcium alginate and lithium polyacrylate; their synergistic effect ensures both the mechanical strength of the electrode and good ionic conductivity. The artificial SEI is constructed using a short-time treatment strategy with lithium bis(fluorosulfonyl)imide solution. The lithium bis(fluorosulfonyl)imide in the solution decomposes in situ on the electrode surface, directionally generating an inorganic phase interface layer rich in lithium fluoride. Solvent system optimization ensures uniform wetting of the electrode and density of the reaction layer. The heating and curing process further optimizes the microstructure and interfacial bonding strength of the SEI film by controlling the heating and cooling rates and the heat preservation conditions, ultimately achieving an artificial solid electrolyte interface film with high LiF content, high ionic conductivity, and high stability.

[0024] The application of the silicon-carbon anode material in the anode of lithium-ion secondary batteries further demonstrates that the anode sheet retains ≥85% of its capacity after 100 cycles following the first charge-discharge cycle, and has an average coulombic efficiency ≥99.3% at a 0.5C rate.

[0025] This study delves into the synergistic mechanism of nano-silicon cores, a cavity layer, a boron-doped dense carbon shell, and an artificial solid electrolyte membrane. The nano-silicon core, as the active component for lithium storage, contributes to high specific capacity. The nanoscale shortens the lithium-ion diffusion path, improving rate performance, and the nanoscale effect ensures uniform stress distribution, suppressing particle fragmentation during lithium intercalation. The cavity layer provides buffer space for the volume expansion of the silicon core. A 30–40 vol% cavity ratio allows the silicon to expand into the cavity during lithium intercalation without applying destructive stress to the carbon shell, effectively buffering mechanical strain while reducing particle packing density and improving slurry rheology. The boron-doped dense carbon shell exhibits synergistic structural stability and electrochemical activity. Its ultra-thin 2–5 nm thickness ensures the integrity of the carbon shell and shortens the lithium-ion diffusion distance. Boron doping introduces defect sites and impurity energy levels into the carbon layer, enhancing electronic conductivity through p-type doping and optimizing lithium-ion transport kinetics. The high LiF content in the artificial solid electrolyte membrane enhances interfacial stability. LiF possesses high ionic conductivity and low electronic conductivity, forming a stable passivation layer on the electrode surface and inhibiting continuous electrolyte decomposition. The synergistic effect of the four components is reflected in the precise matching of multi-scale structures. The nano-silicon core provides the capacity basis, the cavity layer realizes mechanical buffering and processability optimization, the boron-doped carbon shell constructs the electron transport network and ion diffusion channel, and the artificial SEI film stabilizes the interface reaction. The multi-level synergistic breakthrough overcomes the contradiction between structural stability, electrochemical performance and processing performance of traditional silicon-carbon materials, and achieves a synergistic improvement in long-cycle stability and high coulombic efficiency under high pressure real density.

[0026] (3) Beneficial technical effects

[0027] 1. Significantly improved cycle stability and coulombic efficiency: Through a three-layer core-shell structure design of nano-silicon core-cavity layer-boron-doped dense carbon shell, the cavity ratio is precisely controlled at 30-40 vol%, reserving sufficient buffer space for silicon expansion. This allows silicon to expand into the cavity during lithium intercalation without causing destructive stress to the carbon shell, fundamentally solving the problems of electrical contact loss and repeated SEI film reconstruction caused by carbon shell cracking during the cycling process of traditional silicon-carbon materials. Combined with the high LiF content design in the artificial solid electrolyte membrane, a stable solid electrolyte interface is formed, effectively suppressing the continuous decomposition of the electrolyte and irreversible lithium loss. This results in the negative electrode plate achieving a capacity retention rate of over 85% after 100 cycles following the first charge-discharge cycle, and an average coulombic efficiency of over 99.3% at 0.5C rate, representing a cycle life improvement of over 50% compared to traditional silicon-carbon materials.

[0028] 2. Achieving synergy between high compaction density and excellent processing performance: The introduction of the cavity structure provides volume buffering while reducing particle packing density. This allows the nano-silicon-carbon material to maintain its high specific surface area while significantly improving the rheological properties of the slurry. Under high solids content conditions, the slurry viscosity is reduced by more than 40%, supporting the negative electrode sheet to achieve a compaction density of [missing value]. The high level of technology has broken through the contradiction between the processing performance and electrochemical performance of traditional nano-silicon-carbon materials, laying the foundation for the large-scale production of silicon-carbon anodes. At the same time, the high density directly improves the volumetric energy density of the battery, meeting the dual requirements of power batteries and high-end consumer electronics for thinner and lighter batteries and higher energy density.

[0029] 3. Boron-doped dense carbon shell achieves dual optimization of conductivity and structural stability: The ultrathin carbon shell thickness of 2–5 nm minimizes the radial diffusion distance of lithium ions while ensuring structural integrity. Boron atoms are introduced into the carbon layer lattice at a doping amount of 0.5–6.0 at%, introducing impurity energy levels near the Fermi level through p-type doping effect, significantly improving the electronic conductivity of the material. The conductivity is 2–3 orders of magnitude higher than that of the undoped carbon shell. At the same time, the introduction of boron atoms optimizes the micro-defect distribution of the carbon layer, constructing an ordered ion diffusion channel while maintaining the density of the carbon shell, realizing the synergistic optimization of electron transport and ion transport, and laying the foundation for high-rate performance.

[0030] 4. Directional Construction of High-Stability Interface with Artificial Solid Electrolyte Membrane: A LiF-rich artificial solid electrolyte membrane is constructed on the surface of the negative electrode through the in-situ reaction of lithium bis(fluorosulfonyl)imide solution. The lithium fluoride atomic fraction is precisely controlled at 40–70 at%. LiF, as an excellent solid electrolyte component, has high ionic conductivity, low electronic conductivity, and excellent chemical stability. It forms a dense and uniform passivation layer on the electrode surface, effectively isolating the direct contact between the electrolyte and silicon-carbon materials. This inhibits the repeated rupture and reconstruction process of the SEI film from the source, improving the initial coulombic efficiency to over 88%, which is 10–15 percentage points higher than that of traditional silicon-carbon negative electrodes. This significantly reduces irreversible lithium loss and improves the energy utilization rate and cycle life of the battery.

[0031] 5. The preparation method is highly controllable and easy to scale up: The multi-step preparation method of controlled oxidation-selective etching-boron doping carbonization-artificial SEI construction has clear and well-defined parameters for each step. Key parameters such as oxidation temperature, etching solution concentration, carbonization temperature, and boron doping amount can be precisely controlled. The process has good reproducibility. The use of aqueous solution system etching and polymerization carbonization method avoids the use of toxic and harmful solvents, which is environmentally friendly and cost-controllable. The equipment and processes involved in the preparation process are all mature technologies, which are easy to realize industrial scale-up production, providing a reliable technical route for the commercial application of silicon-carbon anode materials. Attached Figure Description

[0032] Figure 1 The effect of boron doping amount on the initial discharge specific capacity and capacity retention rate after 100 cycles is shown.

[0033] Figure 2 The effect of cavity ratio on the volume retention rate and compaction density after 100 cycles.

[0034] Figure 3 The effect of LiF atomic fraction in artificial SEI films on initial coulombic efficiency and cycling capacity retention.

[0035] Figure 4 The image shows the X-ray diffraction pattern of the silicon-carbon anode material of Example 1 of this invention.

[0036] Figure 5 This is a B1s peak fitting diagram of the X-ray photoelectron spectrum of the boron-doped dense carbon shell in Example 1 of the present invention.

[0037] Figure 6 This is a high-resolution F1s X-ray photoelectron spectrum of the silicon-carbon anode material of Example 1 of the present invention.

[0038] Figure 7 This is a morphological diagram of the boron-doped dense carbon shell of Embodiment 1 of the present invention.

[0039] Figure 8 This is a morphology diagram of the boron-doped dense carbon shell after etching silicon in Example 1 of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0041] Example 1

[0042] In this embodiment, the silicon-carbon anode material has a nano-silicon mass fraction of 24 wt%, an average nano-silicon core diameter of 60 nm, a cavity ratio of 35 vol%, a boron-doped dense carbon shell thickness of 3.5 nm, a boron doping amount of 3.0 at, and a lithium fluoride atomic fraction of 58 at in the artificial solid electrolyte membrane. The compaction density of the anode sheet in this embodiment is... The adhesive system uses calcium alginate and lithium polyacrylate in a mass ratio of 5:5.

[0043] Preparation steps:

[0044] S1. Controlled oxidation of nano-silicon

[0045] Weigh 100g of silicon metallic powder (average diameter 60nm, purity ≥99.5%, commercially available), spread it evenly in an alumina crucible (approximately 5mm thick), and place it in a tube furnace. Pour in dry air (relative humidity <5%, flow rate...). ),by The temperature was increased to 280°C and held for 1 hour for controlled oxidation, forming a silicon oxide layer of approximately 2-3 nm on the surface of the silicon particles. After cooling to room temperature, the particles were removed to obtain surface-oxidized silicon nanoparticles.

[0046] S2. Selective etching to construct cavities

[0047] Preparation of ammonium fluoride aqueous solution: Weigh 75.0 g of ammonium fluoride (analytical grade, purity ≥98%, commercially available) and dissolve it in 1000 mL of deionized water to prepare the solution. The solution was adjusted to pH 5.5 with ammonia. 100 g of the silicon oxide nanoparticles obtained in step S1 were dispersed in 800 mL of this solution and stirred (300 rpm) for 30 minutes in a 30°C constant temperature water bath. Ammonium fluoride selectively etched the oxide layer on the silicon surface, forming cavities. After the reaction, the mixture was rapidly washed with deionized water until neutral (pH = 6.5-7.5), vacuum filtered, and vacuum dried at 60°C (<10 Pa) for 12 hours to obtain silicon nanoparticles with a cavity structure. The cavity percentage was 35 vol%, determined by nitrogen adsorption.

[0048] S3. Construction of boron-doped dense carbon shell

[0049] Preparation of carbon source solution: Weigh 30g of glucose (analytical grade, purity ≥99%, commercially available) and dissolve it in 1000mL of deionized water to prepare a 3.0 wt% glucose solution.

[0050] Addition of boron-containing precursor: Weigh 3.0g of boric acid ( (Analytical grade, purity ≥99.5%, commercially available) was dissolved in the above glucose solution and stirred until completely dissolved. The mass ratio of boric acid to glucose was 1:10.

[0051] Polymerization coating: 100g of the hollow silicon nanoparticles obtained in step S2 were dispersed in 1000mL of the above boron-containing glucose solution and ultrasonically dispersed for 30 minutes (power 400W). Then, the mixture was stirred (500rpm) and heated in an 80°C water bath for 12 hours, during which the glucose underwent a polymerization reaction, forming a polymer coating layer on the particle surface. The mixture was then filtered and vacuum dried at 80°C for 8 hours.

[0052] Carbonization: The dried sample is placed in a tube furnace and heated under a nitrogen atmosphere (flow rate...). (Purity ≥ 99.99%, commercially available) The temperature was increased to 700°C and held for 1 hour for carbonization, during which boric acid decomposed and incorporated into the carbon layer. After natural cooling to room temperature, a boron-doped, dense carbon-coated silicon-carbon anode material was obtained. The carbon shell thickness was 3.5 nm (TEM measurement), and the boron doping amount was 3.0 at% (XPS measurement).

[0053] S4. Electrode Preparation

[0054] Slurry preparation: Add 80g of the silicon-carbon anode material obtained in step S3, 10g of conductive carbon black (Super P, commercially available), 5g of calcium alginate (molecular weight 100,000-200,000, commercially available), and 5g of lithium polyacrylate (molecular weight 150,000-250,000, commercially available) to deionized water, adjust the solid content to 48 wt%, and stir in a planetary mixer (600 rpm) for 2 hours to obtain a uniform slurry. The slurry viscosity is 2800 mPa·s (measured using a Brookfield viscometer at 60 rpm and 25°C).

[0055] Coating and Rolling: The slurry is evenly coated onto copper foil (8 μm thick), achieving a coating thickness of approximately 80 μm (wet film), and dried at 80°C for 2 hours. Then, it is rolled using a roller press (linear pressure). Roller speed The compaction density is obtained as follows: The negative electrode sheet has an area density of approximately .

[0056] S5. Construction of Artificial Solid Electrolyte Membranes

[0057] LiFSI solution preparation: Weigh 16.8g of lithium bis(fluorosulfonyl)imide (LiFSI, battery grade, purity ≥99.5%, commercially available) and dissolve it in a mixed solvent. The mixed solvent consists of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethoxyethane (DME) in a volume ratio of 3:3:2, with a total volume of 200mL. LiFSI solution.

[0058] Short-term treatment: Cut the negative electrode sheet obtained in step S4 into an appropriate size (e.g., 5cm × 5cm), immerse it in the above LiFSI solution, and soak it at room temperature (25°C) for 4 minutes. Remove the electrode sheet, quickly rinse the surface with deionized water for 5 seconds (to remove excess solution), and then place it in a nitrogen atmosphere glove box ( , Dry at 80°C for 5 minutes.

[0059] Heat curing: The dried electrode is placed in a tube furnace and cured under a nitrogen atmosphere (flow rate... (with a purity ≥ 99.99%) The heating rate was increased to 95°C, and the temperature was maintained for 8 minutes to allow for the curing reaction. LiFSI decomposed on the surface of the silicon-carbon anode, generating an artificial solid electrolyte membrane rich in lithium fluoride. The material was cooled to room temperature at a specific cooling rate to obtain the final negative electrode product. The atomic fraction of lithium fluoride in the artificial solid electrolyte membrane was 58 at% (XPS depth profile determination).

[0060] Analysis of the characteristics of Example 1:

[0061] This embodiment employs a moderately optimized parameter design. The nano-silicon mass fraction of 24 wt% falls within the preferred range of 22-26 wt%, the cavity ratio of 35 vol% is in the middle of the range of 30-40 vol%, and the boron doping amount of 3.0 at% and carbon shell thickness of 3.5 nm are both at moderate levels. This parameter combination balances high capacity (nano-silicon content) with long-term cycling stability (moderate cavity buffering), processing rheology (spherical hollow structure reduces packing density) with interfacial stability (appropriate boron doping enhances carbon shell toughness). (Packing density...) At an upper-middle level, it is suitable for high-energy-density battery systems. The LiF atomic fraction of 58 at% falls within the preferred range of 50-65 at%, effectively suppressing initial irreversible lithium loss and improving cycle coulombic efficiency. This embodiment is suitable for lithium-ion battery anode applications in consumer electronics, such as high-end smartphones and laptops, where high energy density and cycle life are required.

[0062] Example 2

[0063] In this embodiment, the silicon-carbon anode material has a nano-silicon mass fraction of 26 wt%, an average nano-silicon core diameter of 50 nm, a cavity ratio of 38 vol%, a boron-doped dense carbon shell thickness of 4.0 nm, a boron doping amount of 4.5 at, and a lithium fluoride atomic fraction of 62 at in the artificial solid electrolyte membrane. The compaction density of the anode sheet in this embodiment is... The adhesive system uses calcium alginate and lithium polyacrylate in a mass ratio of 4:6.

[0064] Preparation steps:

[0065] S1. Controlled oxidation of nano-silicon

[0066] Weigh 100g of silicon metallic powder (average diameter 50nm, purity ≥99.5%, commercially available), spread it evenly in an alumina crucible, and place it in a tube furnace. Introduce a mixture of argon gas containing 10 vol% oxygen (flow rate...). ),by The temperature was increased to 300°C and held for 0.8 hours for controlled oxidation. After cooling to room temperature, the particles were removed to obtain surface-oxidized silicon nanoparticles (oxide layer thickness approximately 2.5 nm).

[0067] S2. Selective etching to construct cavities

[0068] Preparation of ammonium fluoride aqueous solution: Weigh 100.0 g of ammonium fluoride and dissolve it in 1000 mL of deionized water to prepare... The solution was adjusted to pH 5.8 with ammonia. 100 g of the silicon dioxide nanoparticles obtained in step S1 were dispersed in 800 mL of this solution and stirred (350 rpm) in a 35°C water bath for 40 minutes. After the reaction, the solution was quickly washed with deionized water until neutral, vacuum filtered, and dried under vacuum at 60°C for 12 hours. At this point, the cavity percentage was 38 vol%.

[0069] S3. Construction of boron-doped dense carbon shell

[0070] Weigh 40g of glucose and dissolve it in 1000mL of deionized water to prepare a 4.0wt% glucose solution. Weigh 5.0g of trimethoxyborane ( (Trimethoxyborane, purity ≥ 98%, commercially available) is slowly added dropwise to the glucose solution (note that trimethoxyborane is easily hydrolyzed), and stirred until completely mixed. The mass ratio of trimethoxyborane to glucose is 1:8.

[0071] 100g of the hollow silicon nanoparticles obtained in step S2 were dispersed in 1000mL of the above boron-containing glucose solution and ultrasonically dispersed for 30 minutes. Then, the mixture was stirred (500rpm) and heated in an 85°C water bath for 10 hours, during which glucose polymerized and cross-linked with hydrolyzed trimethoxyborane. The mixture was then filtered and vacuum dried at 80°C for 8 hours.

[0072] The dried sample was placed in a tube furnace and heated under an argon atmosphere (flow rate...). (with a purity ≥ 99.99%) The temperature was increased to 750°C and held for 1.2 hours for carbonization. After natural cooling to room temperature, a silicon-carbon anode material with a boron-doped dense carbon shell was obtained. The carbon shell thickness was 4.0 nm, and the boron doping amount was 4.5 at%.

[0073] S4. Electrode Preparation

[0074] Add 80g of the silicon-carbon anode material obtained in step S3, 10g of conductive carbon black, 4g of calcium alginate, and 6g of lithium polyacrylate to deionized water, adjusting the solid content to 46wt%. Stir in a planetary mixer for 2 hours to obtain a uniform slurry (viscosity 3200 mPa·s). Coat the slurry onto copper foil and dry at 80°C for 2 hours. Roll press (linear pressure) The compaction density is obtained as follows: The negative electrode sheet.

[0075] S5. Construction of Artificial Solid Electrolyte Membranes

[0076] Weigh 22.4 g of lithium difluorosulfonyl imide and dissolve it in a mixed solvent (EC:EMC:DME = 3:3:2, volume ratio, total volume 200 mL) to prepare the following solution: The negative electrode was immersed in the LiFSI solution at room temperature for 5 minutes. The electrode was then removed, quickly rinsed with deionized water, and dried in a nitrogen glove box at 80°C for 6 minutes. The solution was then applied under a nitrogen atmosphere. Raise the temperature to 100°C and hold for 7 minutes to cure. Cool to room temperature. The atomic fraction of lithium fluoride in the artificial solid electrolyte membrane is 62 at%.

[0077] Analysis of the characteristics of Example 2:

[0078] This embodiment employs a parameter combination biased towards high capacity and high performance. The mass fraction of nano-silicon is increased to 26 wt%, while the cavity ratio is increased to 38 vol% to adequately buffer the volume expansion of silicon. The nano-silicon diameter is reduced to 50 nm to provide a larger specific surface area and a shorter lithium-ion diffusion path. Increasing the carbon shell thickness to 4.0 nm and the boron doping amount to 4.5 at% further enhances the mechanical strength and conductivity of the carbon shell, suppressing carbon shell cracking during cycling. Increasing the LiF atomic fraction to 62 at% (the upper limit of the preferred range) constructs a more stable artificial SEI film, effectively suppressing the continuous decomposition of the electrolyte and irreversible lithium-ion consumption. This parameter combination prioritizes high specific capacity and long cycle life, making it suitable for power battery applications with extremely high energy density requirements and strict cycle life requirements, such as high-end electric vehicles and drones. A relatively low compaction density is also achieved. Good processing performance and electrode integrity were maintained even under high silicon content conditions.

[0079] Example 3

[0080] In this embodiment, the silicon-carbon anode material has a nano-silicon mass fraction of 22 wt%, an average nano-silicon core diameter of 70 nm, a cavity ratio of 32 vol%, a boron-doped dense carbon shell thickness of 3.0 nm, a boron doping amount of 2.0 at, and a lithium fluoride atomic fraction of 53 at in the artificial solid electrolyte membrane. The compaction density of the anode sheet in this embodiment is... The adhesive system uses calcium alginate and lithium polyacrylate in a mass ratio of 6:4.

[0081] Preparation steps:

[0082] S1. Controlled oxidation of nano-silicon

[0083] Weigh 100g of silicon metallic powder (average diameter 70nm, purity ≥99.5%, commercially available), spread it evenly in an alumina crucible, and place it in a tube furnace. Pour in dry air (flow rate...). ),by The temperature was increased to 260°C and held for 1.5 hours for controlled oxidation. After cooling to room temperature, the particles were removed to obtain surface-oxidized silicon nanoparticles (oxide layer thickness approximately 3.5 nm).

[0084] S2. Selective etching to construct cavities

[0085] Preparation of ammonium fluoride aqueous solution: Weigh 60.0 g of ammonium fluoride and dissolve it in 1000 mL of deionized water to prepare... The solution was adjusted to pH 5.2 with ammonia. 100 g of the silicon dioxide nanoparticles obtained in step S1 were dispersed in 800 mL of this solution and stirred (280 rpm) for 25 minutes in a 25°C constant temperature water bath. After the reaction, the mixture was quickly washed with deionized water until neutral, vacuum filtered, and dried under vacuum at 60°C for 12 hours. At this point, the cavity percentage was 32 vol%.

[0086] S3. Construction of boron-doped dense carbon shell

[0087] Weigh 20g of sucrose (analytical grade, purity ≥99.5%, commercially available) and dissolve it in 1000mL of deionized water to prepare a 2.0wt% sucrose solution. Weigh 2.0g of boric acid and dissolve it in the above sucrose solution, with a boric acid to sucrose mass ratio of 1:10.

[0088] 100g of the hollow silicon nanoparticles obtained in step S2 were dispersed in 1000mL of the boron-containing sucrose solution and ultrasonically dispersed for 30 minutes. Then, the mixture was stirred (500rpm) and heated in a 75°C water bath for 14 hours, during which the sucrose underwent hydrolysis and polymerization. The mixture was then filtered and vacuum dried at 80°C for 8 hours.

[0089] The dried sample was placed in a tube furnace and heated under a nitrogen atmosphere (flow rate...). ),by The temperature was increased to 650°C and held for 1.5 hours for carbonization. After natural cooling to room temperature, a silicon-carbon anode material with a boron-doped dense carbon shell was obtained. The carbon shell thickness was 3.0 nm, and the boron doping amount was 2.0 at%.

[0090] S4. Electrode Preparation

[0091] Add 80g of the silicon-carbon anode material obtained in step S3, 10g of conductive carbon black, 6g of calcium alginate, and 4g of lithium polyacrylate to deionized water, adjusting the solid content to 50wt%. Stir in a planetary mixer for 2 hours to obtain a uniform slurry (viscosity 2400 mPa·s). Coat the slurry onto copper foil and dry at 80°C for 2 hours. Roll press (linear pressure) The compaction density is obtained as follows: The negative electrode sheet.

[0092] S5. Construction of Artificial Solid Electrolyte Membranes

[0093] Weigh 11.2 g of lithium bis(fluorosulfonyl)imide and dissolve it in a mixed solvent (EC:EMC:DME = 2.5:2.5:2, volume ratio, total volume 200 mL) to prepare the following solution: The negative electrode was immersed in the LiFSI solution for 3 minutes at room temperature. The electrode was then removed, quickly rinsed with deionized water, and dried in a nitrogen glove box at 80°C for 4 minutes. The solution was then applied under a nitrogen atmosphere. Raise the temperature to 90°C and hold for 9 minutes to cure. Cool to room temperature. The atomic fraction of lithium fluoride in the artificial solid electrolyte membrane is 53 at%.

[0094] Analysis of the characteristics of Example 3:

[0095] This embodiment employs a parameter combination prioritizing stability and high compaction density. The mass fraction of nano-silicon is reduced to 22 wt%, which, by decreasing the active silicon content, helps mitigate volume effects and side reactions, improving cycle stability. Increasing the nano-silicon diameter to 70 nm reduces particle surface area, decreasing the contact area with the electrolyte, while larger particles also offer better mechanical stability. A void fraction of 32 vol% is near the lower limit of the range, ensuring sufficient buffering capacity while increasing the material's compaction density and compaction density potential. A carbon shell thickness of 3.0 nm and a boron doping amount of 2.0 at% are at a moderately low level; a relatively thin carbon shell facilitates rapid lithium-ion transport. A LiF atomic fraction of 53 at% is at a moderate level within the preferred range, balancing the ion conductivity and protection of the SEI film. This parameter combination achieves... The high density significantly improves the volumetric energy density of the electrode, making it ideal for applications with stringent requirements for battery volumetric energy density, such as energy storage systems and power tools. The proportion of calcium alginate in the binder is increased to 60%, further enhancing the mechanical strength and anti-expansion capabilities of the electrode.

[0096] Example 4

[0097] In this embodiment, the silicon-carbon anode material has a nano-silicon mass fraction of 28 wt%, an average nano-silicon core diameter of 30 nm, a cavity ratio of 40 vol%, a boron-doped dense carbon shell thickness of 2.0 nm, a boron doping amount of 6.0 at, and a lithium fluoride atomic fraction of 70 at in the artificial solid electrolyte membrane. The compaction density of the anode sheet in this embodiment is... The adhesive system uses calcium alginate and lithium polyacrylate in a mass ratio of 3:7.

[0098] Preparation steps:

[0099] S1. Controlled oxidation of nano-silicon

[0100] Weigh 100g of silicon metal powder (average diameter 30nm, purity ≥99.5%, commercially available), spread it evenly in an alumina crucible, and place it in a tube furnace. Introduce a mixture of argon gas containing 15 vol% oxygen (flow rate...). ),by The temperature was increased to 320°C and held for 0.6 hours for controlled oxidation. Due to the small diameter (30 nm) of the silicon nanoparticles, the oxidation rate was relatively fast, so a relatively short oxidation time was used to avoid over-oxidation. After cooling to room temperature, the nanoparticles were removed, resulting in surface-oxidized silicon nanoparticles (oxide layer thickness approximately 2 nm).

[0101] S2. Selective etching to construct cavities

[0102] Preparation of ammonium fluoride aqueous solution: Weigh 125.0 g of ammonium fluoride and dissolve it in 1000 mL of deionized water to prepare... The solution was adjusted to pH 6.2 with ammonia. 100 g of the silicon dioxide nanoparticles obtained in step S1 were dispersed in 800 mL of this solution and stirred (400 rpm) in a 40°C constant temperature water bath for 50 minutes. Higher etching concentration, temperature, and time ensured thorough etching and the formation of a larger cavity ratio. After the reaction, the mixture was quickly washed with deionized water until neutral, vacuum filtered, and vacuum dried at 60°C for 12 hours. At this point, the cavity ratio was 40 vol%.

[0103] S3. Construction of boron-doped dense carbon shell

[0104] Weigh 50g of glucose and dissolve it in 1000mL of deionized water to prepare a 5.0wt% glucose solution. Weigh 10.0g of borane-tetrahydrofuran complex ( Slowly add 1M tetrahydrofuran solution (commercially available) to the glucose solution (operate in a fume hood, as borane is volatile and toxic), and stir for 30 minutes to ensure thorough mixing. The mass ratio of the borane complex (calculated as boron) to glucose is approximately 1:5.

[0105] 100g of the hollow silicon nanoparticles obtained in step S2 were dispersed in 1000mL of the above boron-containing glucose solution and ultrasonically dispersed for 40 minutes (due to the small particle size, a longer ultrasonic time is required to ensure dispersion). Then, the mixture was stirred (600rpm) and heated in a 90°C water bath for 8 hours. After filtration, the mixture was vacuum dried at 80°C for 8 hours.

[0106] The dried sample was placed in a tube furnace and heated under a nitrogen atmosphere (flow rate...). ),by The heating rate was increased to 800°C, and carbonization was carried out by holding at that temperature for 0.5 hours. A higher carbonization temperature is beneficial for sufficient boron doping, but a relatively short carbonization time is necessary to avoid excessive carbon shell growth. After natural cooling to room temperature, a silicon-carbon anode material with a dense boron-doped carbon shell was obtained. The carbon shell thickness was 2.0 nm, and the boron doping amount was 6.0 at%.

[0107] S4. Electrode Preparation

[0108] Add 80g of the silicon-carbon anode material obtained in step S3, 10g of conductive carbon black, 3g of calcium alginate, and 7g of lithium polyacrylate to deionized water, adjusting the solid content to 44wt%. Stir in a planetary mixer for 2.5 hours to obtain a uniform slurry (viscosity 3600 mPa·s). Due to the high content (28wt%) and small particle size (30nm) of nano-silicon, the slurry viscosity is high, requiring an extended stirring time and a reduced solid content. Coat the slurry onto copper foil and dry at 80°C for 2 hours. Roll press (linear pressure) (To avoid damaging the ultrathin carbon shell with relatively low linear pressure), a compaction density of [value missing] was obtained. The negative electrode sheet.

[0109] S5. Construction of Artificial Solid Electrolyte Membranes

[0110] Weigh 28.0 g of lithium difluorosulfonyl imide and dissolve it in a mixed solvent (EC:EMC:DME = 4:3:2, volume ratio, total volume 200 mL) to prepare the following solution: The negative electrode was immersed in the LiFSI solution at room temperature for 6 minutes. The electrode was then removed, quickly rinsed with deionized water, and dried in a nitrogen glove box at 85°C for 7 minutes. The solution was then applied under a nitrogen atmosphere. Raise the temperature to 110°C and hold for 5 minutes to cure. Cool to room temperature. Higher LiFSI concentrations, processing times, and curing temperatures can produce thicker artificial SEI films with higher LiF content. The atomic fraction of lithium fluoride in the artificial solid electrolyte membrane is 70 at%.

[0111] Analysis of the characteristics of Example 4:

[0112] This embodiment employs a boundary value verification strategy for multiple parameters to demonstrate the feasibility and technical rationality of the parameter ranges claimed in the claims. A nano-silicon mass fraction of 28 wt% achieves the highest theoretical specific capacity, while a cavity ratio of 40 vol% adequately buffers the significant volume expansion under the limiting silicon content (the theoretical volume expansion of silicon is approximately 300%). The nano-silicon diameter is reduced to the lower limit of 30 nm; the ultra-small particle size significantly shortens the lithium-ion solid-phase diffusion path, which is beneficial for improving rate performance, but also increases the risk of surface side reactions. The carbon shell thickness is only 2.0 nm; the ultra-thin carbon shell maximizes lithium-ion transport efficiency, but places extremely high demands on the integrity and uniformity of the carbon shell. Therefore, the boron doping amount is increased to the upper limit of 6.0 at% to enhance the mechanical toughness and conductivity of the carbon shell, compensating for the insufficient thickness. A LiF atomic fraction as high as 70 at% constructs an ultra-thick artificial SEI film, maximally suppressing electrolyte decomposition and irreversible lithium-ion consumption, which is particularly necessary for ultra-small nano-silicon particles with highly active surfaces. This parameter combination represents a design direction that pursues the ultimate specific capacity and rate performance, but due to the boundary values ​​of multiple parameters, process control is difficult and the compaction density is relatively low. The proportion of lithium polyacrylate in the binder is increased to 70% to enhance bonding strength. This embodiment is suitable for special application scenarios with extreme requirements for comparative capacity and fast charging performance and insensitivity to cost, such as high-performance racing drones and military portable power supplies. The successful preparation and testing of this embodiment demonstrates the feasibility and technical feasibility of the parameter range defined in the claims, providing crucial support for the legality of the patent protection scope.

[0113] Comparative Example 1

[0114] It is basically the same as Example 1, except that the mass fraction of nano-silicon is 15wt%, while the amounts of other components and preparation conditions remain unchanged.

[0115] Comparative Example 2

[0116] It is basically the same as Example 1, except that the mass fraction of nano-silicon is 32wt%, while the amounts of other components and preparation conditions remain unchanged.

[0117] Comparative Example 3

[0118] The preparation method is basically the same as in Example 1, except that the average diameter of the nano-silicon core is 15 nm. In the specific preparation, metallic silicon powder with an average diameter of 15 nm is used as the raw material, while other preparation conditions remain unchanged.

[0119] Comparative Example 4

[0120] The preparation method is basically the same as in Example 1, except that the average diameter of the nano-silicon core is 120 nm. In the specific preparation, metallic silicon powder with an average diameter of 120 nm is selected as the raw material, and other preparation conditions remain unchanged.

[0121] Comparative Example 5

[0122] Essentially the same as Example 1, except that the cavity ratio is 22 vol%. This is achieved by adjusting the etching conditions in step S2: the concentration of the ammoniacal ammonium fluoride solution is reduced to... The etching time was shortened to 15 minutes, the etching temperature was reduced to 25°C, and other preparation conditions remained unchanged.

[0123] Comparative Example 6

[0124] This is essentially the same as Example 1, except that the cavity ratio is 48 vol%. This is achieved by adjusting the etching conditions in step S2: the concentration of the ammoniacal ammonium fluoride solution is increased to... The etching time was extended to 70 minutes, the etching temperature was increased to 45°C, and other preparation conditions remained unchanged.

[0125] Comparative Example 7

[0126] The process is essentially the same as in Example 1, except that the thickness of the boron-doped dense carbon shell is 1.0 nm. By adjusting the solid content of the carbon source solution in step S3 to 0.5 wt%, the carbonization time is shortened to 0.3 hours, while other preparation conditions remain unchanged.

[0127] Comparative Example 8

[0128] The process was essentially the same as in Example 1, except that the thickness of the boron-doped dense carbon shell was 7 nm. The solid content of the carbon source solution in step S3 was adjusted to 8 wt%, and the carbonization time was extended to 3 hours, while other preparation conditions remained unchanged.

[0129] Comparative Example 9

[0130] The process is essentially the same as in Example 1, except that the amount of boron atoms in the boron-doped dense carbon shell is 0.2 at%. By adjusting the mass ratio of the boron-containing precursor (boric acid) to the carbon source (glucose) in step S3 to 1:30, other preparation conditions remain unchanged.

[0131] Comparative Example 10

[0132] The process is essentially the same as in Example 1, except that the boron doping concentration in the boron-doped dense carbon shell is 8.0 at%. The mass ratio of the boron-containing precursor (boric acid) to the carbon source (glucose) in step S3 is adjusted to 1:1.5, while other preparation conditions remain unchanged.

[0133] Comparative Example 11

[0134] The process is essentially the same as in Example 1, except that the atomic fraction of lithium fluoride in the artificial solid electrolyte membrane is 30 at%. This is achieved by adjusting the concentration of the lithium difluorosulfonyl imide solution in step S5 to... The short-time processing time was shortened to 1 minute, the heating curing temperature was reduced to 70°C and the time was shortened to 3 minutes, while other preparation conditions remained unchanged.

[0135] Comparative Example 12

[0136] The process is essentially the same as in Example 1, except that the atomic fraction of lithium fluoride in the artificial solid electrolyte membrane is 80 at%. This is achieved by adjusting the concentration of the lithium bis(fluorosulfonyl)imide solution in step S5 to... The short-time processing time was extended to 10 minutes, the heating curing temperature was increased to 130°C and the time was extended to 15 minutes, while other preparation conditions remained unchanged.

[0137] Comparative Example 13

[0138] The process is basically the same as in Example 1, except that in step S3, no boron-containing precursor is added. Instead, pure glucose is used as the carbon source for polymerization, coating, and carbonization to obtain an undoped ordinary carbon shell (0 at%). The thickness of the carbon shell is still 3.5 nm, and other preparation conditions remain unchanged.

[0139] Comparative Example 14

[0140] The process is basically the same as in Example 1, except that step S2 is skipped without selective etching, resulting in a silicon-carbon anode material with no cavity structure (cavity rate 0 vol%). Other preparation conditions remain unchanged.

[0141] Performance testing:

[0142] The electrochemical cycling performance test aims to evaluate the cycling stability and capacity retention of silicon-carbon anode materials, directly verifying the effectiveness in addressing the "short cycle life" problem. The test principle is based on a constant current charge-discharge cycle process. By examining the reversible capacity, cycle life, and coulombic efficiency of the active material, it reflects the ability of the boron-doped carbon shell and cavity layer to synergistically buffer silicon volume expansion, thereby suppressing electrode pulverization and repeated SEI film rupture and improving cycling stability. The specific experimental method involves mixing silicon-carbon anode material, conductive carbon black, and binder in a mass ratio of 8:1:1 to prepare a compacted material with a density of [missing information]. The negative electrode uses a lithium metal sheet as the counter electrode. CR2032 coin cells were assembled using a 1:1 volume ratio electrolyte. Charge-discharge cycle tests were conducted at room temperature on a Neware battery testing system at a 0.2C rate within a voltage range of 0.01-1.5V, with initial activation at 0.1C. The discharge specific capacity, coulombic efficiency, and cycle number-capacity retention curves were recorded for each cycle. Testing was performed according to GB / T 18287-2013 "General Specification for Lithium-ion Batteries and Battery Packs for Mobile Phones" or similar lithium battery testing standards. Key parameters were controlled as follows: test temperature 25±2℃, current density 0.2C (approximately 420mA / g), cutoff voltage 0.01-1.5V, and cycle number ≥100 cycles. During data processing, cycle number-discharge specific capacity curves and cycle number-coulombic efficiency curves were plotted. The capacity retention rate after 100 cycles (100th cycle / 2nd cycle × 100%) and the average coulombic efficiency (average of cycles 2-100) were calculated to comprehensively evaluate the material's cycle performance.

[0143] The initial coulombic efficiency and rate performance test aimed to evaluate the irreversible capacity loss during the first charge-discharge cycle and the discharge capacity at different rates, verifying the effect of the artificial SEI film on improving interface stability. The test principle is that the initial coulombic efficiency reflects the degree of irreversible lithium-ion consumption during the first charge-discharge cycle, while the LiF enrichment layer in the artificial SEI film can reduce electrolyte decomposition and lithium loss. Rate performance reflects the material's fast-charging capability and ion diffusion kinetics. The experimental method employed the same battery assembly method as the electrochemical cycling performance test. The initial coulombic efficiency (initial discharge capacity / initial charge capacity × 100%) was measured at a 0.1C rate. Subsequently, cycles were performed at 0.2C, 0.5C, 1C, 2C, and 5C rates, each for 5 cycles, followed by a return to 0.2C for 10 cycles. The average discharge specific capacity and capacity recovery rate at each rate were recorded. The tests were conducted according to GB / T 18287-2013 or IEC 61960 standards. Key parameters were controlled as follows: test temperature 25±2℃, voltage range 0.01-1.5V, rate gradient from 0.1C to 5C, and each rate was repeated 5 times with the average value taken to ensure data reliability. Data processing included calculating the initial coulombic efficiency, average discharge specific capacity at each rate, 5C rate capacity retention rate (5C capacity / 0.2C capacity × 100%), and capacity recovery rate after returning to 0.2C. These indicators were used to comprehensively evaluate the initial efficiency and fast-charging performance of the material.

[0144] The compaction density and electrode performance testing experiment aims to determine the compaction density of the negative electrode and verify the material's properties. The test assesses the processability and electrode integrity within a specific range. The principle is that compaction density reflects the degree of material packing and directly affects the electrode's volumetric energy density. The cavity structure provides buffer space during compaction, while the boron-doped carbon shell enhances mechanical strength, ensuring the carbon shell does not crack under high compaction density. The experimental method involves preparing silicon-carbon anode material according to the formulation in Example 1, coating it onto copper foil, drying it, and then rolling it under different linear pressures (10-20 kN / cm). Standard-sized electrodes (e.g., 2cm × 2cm) are cut, their thickness is measured with a micrometer, and they are weighed using an electronic balance. The compaction density is calculated using the formula ρ = m / (S × d). Simultaneously, the cross-sectional morphology of the electrode is observed using a scanning electron microscope to check the integrity of the carbon shell. The test is conducted according to GB / T 24533-2009 "Graphite Anode Materials for Lithium-ion Batteries" or enterprise standards. Key parameters are controlled as follows: adjustable rolling linear pressure (10-20 kN / cm), electrode size accuracy to ±0.01 mm, and repeated testing at each compaction density point five times with the average value taken to ensure data accuracy. During data processing, plotting the linear pressure-compacted density curve determines the achievement of [the desired result]. The process window is used to record the surface flatness of the electrode and to count the carbon shell breakage rate using SEM, thereby comprehensively evaluating the compaction performance and structural stability of the material.

[0145] The transmission electron microscopy (TEM) structural characterization experiment aims to visually observe the core-shell-cavity structure of silicon-carbon anode materials, determine the diameter of the nano-silicon core, the thickness of the cavity layer, and the thickness of the carbon shell, and verify the realization of the structural design. The testing principle is that TEM utilizes a high-energy electron beam to penetrate ultrathin samples, and the interaction between electrons and the sample forms a contrasting image that can resolve nanoscale structures. Under TEM, the core-shell-cavity structure appears as a three-layered concentric circle structure: a dark central silicon core, a bright central cavity, and an outer gray carbon shell. The experimental method involves dispersing silicon-carbon anode material powder in anhydrous ethanol, sonicating it for 10 minutes, then dropping it onto a copper grid and allowing it to dry naturally. The powder is observed under a TEM, such as a FEI Tecnai G2 F20 with an accelerating voltage of 200 kV. High-resolution images of 30-50 randomly selected particles are captured, and the pixel size is calibrated using ImageJ software. The diameter of the nano-silicon core, the thickness of the cavity layer, and the thickness of the carbon shell are then measured, and the particle size distribution is statistically analyzed. The tests were conducted according to GB / T 19077-2016 "Particle Size Analysis - Laser Diffraction Method". Key parameters were controlled as follows: accelerating voltage 200kV, magnification 50,000-200,000x, statistical sample size ≥30 particles, and measurement accuracy ±0.5nm to ensure the accuracy of structural parameters. Data processing included plotting a histogram of the nano-silicon core particle size distribution, calculating the average thickness of the cavity layer, the average thickness of the carbon shell, and the standard deviation, and outputting representative TEM images with labeled dimensional data, thus providing intuitive evidence for the accurate characterization of the core-shell-cavity structure.

[0146] The X-ray photoelectron spectroscopy (XPS) experiment on boron doping and surface chemical state analysis aims to quantitatively analyze the atomic fraction and chemical valence state of boron in boron-doped dense carbon shells, determine the atomic fraction of lithium fluoride in artificial SEI films, and verify whether the content of key components meets the design requirements. The testing principle is that XPS excites inner-shell electrons from the sample surface atoms using X-rays, and the element type and chemical state are determined based on the electron binding energy. The B 1s peak of boron can distinguish between BC bonds (approximately 188 eV) and BO bonds (192 eV). The F 1s peak of LiF in the SEI film (685 eV) can distinguish it from other fluorine-containing compounds. The experimental method involves taking silicon-carbon anode material powder or anode sheet samples and testing them under Al Kα radiation using an XPS instrument such as a Thermo Fisher ESCALAB 250Xi. For boron doping determination, the B 1s peak of C, B, and O elements was scanned and fitted to calculate the boron atomic percentage. For the LiF content in the SEI film, the anode sheet underwent deep profiling with argon ion etching from 0-20 nm. The F 1s peak of Li, F, C, and O elements was fitted to differentiate LiF (approximately 685 eV), LixPFy (approximately 687 eV), and Li₂CO₃ (approximately 532 eV), and the atomic fraction of LiF in F was calculated. Tests were performed according to the ISO 18115-1 surface chemical analysis standard or the NIST XPS Database, with key parameters controlled to ensure vacuum level ≤ The method employed an energy resolution ≤0.5 eV, a depth profiling etching rate of approximately 0.5 nm / min, and repeated scanning of each layer three times with averaging to ensure quantitative analysis accuracy. Data processing included outputting an elemental atomic percentage table, high-resolution B 1s and F 1s spectra and fitting results, and calculating boron doping amount (at%) and LiF atomic fraction (at%), providing a quantitative basis for the composition optimization of artificial SEI films.

[0147] The nitrogen adsorption method for determining cavity ratio and pore structure aims to measure the specific surface area, pore volume, and cavity ratio of silicon-carbon anode materials, verifying whether the cavity layer formation effect after selective etching achieves the design target. The testing principle is based on the nitrogen adsorption-desorption method, using the physical adsorption principle. The specific surface area is calculated through a BET model, and the pore size distribution is analyzed through a BJH or DFT model. The cavity ratio can be calculated from the total pore volume and material density using the formula: Cavity Ratio = Total Pore Volume / (Total Pore Volume + 1 / Framework Density) × 100%. The experimental method involves taking approximately 100 mg of silicon-carbon anode material powder and first degassing it under vacuum at 200℃ for 4 hours on a surface area analyzer such as Micromeritics ASAP 2020 to remove adsorbed water and gas. Then, a nitrogen adsorption-desorption isotherm test is performed at 77K liquid nitrogen temperature, with a relative pressure range... Adsorption equilibrium time was ≥200 seconds / point to ensure sufficient equilibrium. Key parameters were controlled as follows: degassing temperature 200℃, degassing time 4 hours, test temperature 77K, relative pressure step size 0.05, and BET linear fitting correlation coefficient R²>0.999 to ensure data reliability. Data processing included outputting BET specific surface area (m² / g), total pore volume (cm³ / g), average pore size (nm), and pore size distribution curves. The cavity ratio was calculated based on the weighted average of silicon framework density (2.33 g / cm³) and carbon framework density (2.0 g / cm³) to quantitatively evaluate the formation effect of the cavity structure and its buffering capacity against volume expansion.

[0148] The rheological properties test of the slurry aims to determine the viscosity, thixotropy, and upper limit of solid content of the negative electrode slurry, and to verify whether the improvement effect of the spherical hollow structure on the processing rheology meets the requirements of industrial coating. The test principle is that the rheology of the slurry is affected by the particle morphology, particle size distribution, and surface properties. Spherical particles reduce interparticle friction, and the cavity structure reduces the bulk density, thus maintaining a lower viscosity at high solid content, which is beneficial for coating processing. The experimental method is to prepare negative electrode slurries with different solid contents (40-55wt%) according to the formulation of Example 1. The viscosity-shear rate curve (10-100rpm) is measured at 25°C using a rotational viscometer such as Brookfield DV-III. The apparent viscosity at 60rpm is recorded to evaluate thixotropy, i.e., the hysteresis loop area of ​​the acceleration and deceleration curves. The sedimentation stability of the slurry is observed, i.e., the height of the supernatant / total height after standing for 24 hours, and the coating uniformity is observed, i.e., visual inspection and thickness gauge detection after coating on copper foil. The test was conducted according to GB / T 22314-2008 "Determination of Viscosity of Epoxy Resins in Plastics" or similar rheological testing methods. Key parameters were controlled as follows: test temperature 25±0.5℃, shear rate 10-100 rpm, three repetitions per solid content point, wet film thickness 100 μm, and measurement of the standard deviation of thickness at five points after drying to ensure data accuracy. Data processing included plotting viscosity-shear rate curves and solid content-viscosity curves to determine the upper limit of processable solid content (viscosity < 5000 mPa·s), calculating the thixotropic index and coating thickness uniformity RSD < 5%, thereby comprehensively evaluating the processing adaptability of the slurry and the stability of coating quality.

[0149] Figure 1-3 Each experiment corresponds to one of three single-factor variable experiments. Each experiment is based on the preparation conditions of Example 1, changing only one key parameter while keeping other parameters unchanged. Figure 1 To investigate the effect of boron doping on the initial discharge specific capacity and capacity retention after 100 cycles, this experiment maintained the following parameters: 24 wt% silicon nanoparticles, 60 nm silicon core diameter, 35 vol% void fraction, 3.5 nm carbon shell thickness, 58 at% LiF atomic fraction, and compaction density. The boron doping level remained unchanged, but was varied within the range of 0-8.0 at% by adjusting the mass ratio of boric acid to glucose (1:50 to 1:4.5). Figure 1 It can be seen that the cycle capacity retention rate reaches the optimal level (85-88%) when the boron doping amount is 2.5-4.0 at%. When the boron doping amount is too low (<0.5 at%) or too high (>6.0 at%), the cycle stability decreases significantly, indicating that appropriate boron doping plays an important role in enhancing the toughness and cycle performance of the carbon shell. Figure 2 To investigate the effect of cavity ratio on capacity retention and compaction density after 100 cycles, this experiment kept the following parameters constant: nano-silicon mass fraction of 24 wt%, nano-silicon core diameter of 60 nm, carbon shell thickness of 3.5 nm, boron doping of 3.0 at%, and LiF atomic fraction of 58 at%. The concentration of ammonium fluoride solution was adjusted... Etching temperature (25-45°C) and time (15-70 min) cause the cavity ratio to vary within the range of 0-48 vol%. Figure 2 It can be seen that when the cavity ratio is 32-38 vol%, the cycle capacity retention rate is the highest (85-88%) and the compaction density is maintained within a reasonable range of 1.36-1.46 g / cm³. When the cavity ratio is too small (<30 vol%), insufficient volume buffering leads to cycle decay, and when it is too large (>40 vol%), the compaction density decreases significantly. This indicates that optimizing the cavity ratio is crucial for balancing cycle performance and energy density. Figure 3 To investigate the effect of LiF atomic fraction in artificial SEI films on initial coulombic efficiency and cycling capacity retention, this experiment maintained the following parameters: 24 wt% silicon nanoparticles, 60 nm silicon core diameter, 35 vol% void fraction, 3.5 nm carbon shell thickness, 3.0 at% boron doping, and compaction density. The concentration of the LiFSI solution remains unchanged, but is adjusted by adjusting the concentration of the LiFSI solution. The processing time (1-10 min) and curing temperature (70-130°C) cause the LiF atomic fraction to vary within the range of 0-80 at%. Figure 3 It can be seen that the initial coulombic efficiency and cycle stability are optimal when the LiF atomic fraction is 50-65 at%. When the LiF content is too low (<40 at%), insufficient interfacial protection leads to continuous electrolyte decomposition, while too high (>70 at%) may increase interfacial impedance and affect ion transport. This indicates that precise control of the LiF content in the artificial SEI membrane is of great significance for improving electrochemical performance. The above conclusions are jointly proved by the consistency between the spectrum and the process parameters: Example 1 was analyzed by XRD ( Figure 4 Characteristic diffraction peaks appear at 2θ≈28.4°, 47.3°, and 56.1°, corresponding to the (111), (220), and (311) crystal planes of metallic silicon, respectively. A broad amorphous carbon peak is present at 2θ≈26.5°, which is consistent with the characteristics of silicon-carbon composite materials with silicon core and dense carbon shell. Figure 5The statement regarding the B 1s peak fitting diagram of the X-ray photoelectron spectroscopy of the boron-doped dense carbon shell in Example 1 of this invention is correct because: the sample is a boron-doped dense carbon shell, and selecting the B 1s energy level of elemental boron for XPS testing best reflects the chemical state of boron in the carbon shell; the obtained spectrum shows multiple resolvable components in the range of approximately 188.5–192.5 eV, corresponding to typical chemical shifts such as BC3, BC2O, BCO2, and B–O bonds, respectively. The peak positions are consistent with the boron-carbon and boron-oxygen coordination energy ranges reported in the literature. The peak width and relative intensity, after peaking using a pseudo-Voigt function, can be linearly superimposed back to the original experimental spectrum with a residual close to zero, indicating that the fitting has physical rationality and statistical self-consistency; at the same time, the background adopts a slowly varying background model consistent with XPS, with the count value as the ordinate, eliminating the distortion caused by arbitrary unit normalization, thus accurately restoring the qualitative and semi-quantitative information of each valence state; in summary, Figure 5 The B 1s peak fitting diagram is indeed the one for this sample, and it can effectively characterize the chemical environment of boron in the dense carbon shell. For silicon-carbon anode materials, X-ray photoelectron spectroscopy full spectrum testing shows main peaks such as C 1s≈284.8 eV, O 1s≈531 eV, Si 2p≈99–104 eV, Li 1s≈55 eV, and F 1s≈685 eV, with the F 1s peak being particularly strong, consistent with the formation of a fluorine-containing inorganic phase (artificial solid electrolyte film) on the surface after treatment with lithium bis(fluorosulfonyl)imide solution. In the high-resolution F 1s region (Figure 6, energy range 675–695 eV), the main peak is located at ≈685.0 eV, which is consistent with the LiF binding energy recorded in the literature and also matches the LiF atomic fraction of 58 at% in the quantitative results of X-ray photoelectron spectroscopy. This proves that Figure 6 is the high-resolution F 1s X-ray photoelectron spectrum of silicon-carbon anode material.

[0150] Figure 7 and Figure 8 The comparative morphological analysis fully demonstrates the integrity and structural stability of the boron-doped dense carbon shell. Figure 7 The results show that a boron-doped dense carbon shell coats the surface of the nano-silicon core, forming a uniform and continuous spherical shell structure with uniform shell thickness and no obvious defects or cracks. Figure 8 The hollow carbon nanospheres obtained after completely removing the internal silicon nanonuclei through alkaline etching still retain their shape. Figure 7 The identical spherical morphology of the inner and outer shells, along with the smooth and intact surface of the carbon shell without collapse or breakage, provides direct evidence that the boron-doped carbon shell maintains structural integrity before and after etching. This demonstrates that the carbon shell itself possesses sufficient mechanical strength to independently support the spherical structure. Figure 8The existence of medium- and nano-sized hollow carbon spheres also verifies that selective alkaline etching can completely remove the silicon core without damaging the carbon shell, thus forming a cavity layer between the silicon core and the carbon shell. The high consistency of the carbon shell morphology before and after etching not only proves that boron doping enhances the mechanical toughness of the carbon shell so that it does not collapse after losing the core support, but also provides direct morphological evidence for the successful construction of the core-shell-cavity three-layer structure. This further verifies the correctness of the technical route of this invention, which achieves both mechanical strength and structural stability through boron doping and dense carbon shell design.

[0151] The performance of the examples and comparative examples is summarized in Tables 1-2. The experimental results of the comparative examples show that deviations from key material parameters significantly impact electrochemical and processing performance. When the nano-silicon content is too low, such as 15 wt% in Comparative Example 1, the specific capacity of the material is significantly insufficient, only 1520 mAh / g, far below the 2035-2235 mAh / g range of the examples. Conversely, when the silicon content is too high, such as 32 wt% in Comparative Example 2, the volume effect is difficult to control, leading to a significant drop in the initial coulombic efficiency to 75.2% and a cycle capacity retention rate of only 62.3%. The choice of nano-silicon particle size is also crucial. In Comparative Example 3, when the particle size is too small (15 nm), although the specific surface area increases, the side reactions increase significantly, causing the initial coulombic efficiency to drop to 79.8%. In Comparative Example 4, when the particle size is too large (120 nm), the lithium-ion diffusion path is too long, resulting in a significant decrease in rate performance to 1280 mAh / g. Precise control of the cavity ratio is crucial for balancing cycle stability and compaction density. In Comparative Example 5, when the cavity ratio is too small (to 22 vol%), insufficient volume expansion buffering leads to easy material pulverization, resulting in a cycle retention rate of only 76.8% and a slurry viscosity as high as 4200 mPa·s, making processing difficult. In Comparative Example 6, when the cavity ratio is too large (to 48 vol%), although the buffering effect is sufficient, the compaction density is significantly reduced to 1.28 g / cm³, making it prone to problems during compaction, and the cycle retention rate is only 72.5%. The carbon shell thickness plays a dual role in protecting the nano-silicon core and maintaining ion transport channels. In Comparative Example 7, when the carbon shell is too thin (to 1.0 nm), the protective layer is prone to cracking during cycling, leading to a rapid capacity decay to 65.2%. In Comparative Example 8, when the carbon shell is too thick (to 7.0 nm), it significantly hinders lithium-ion transport, causing the rate capacity to drop to 1380 mAh / g and the cycle retention rate to only 78.5%. Optimizing the boron doping concentration is crucial for balancing the mechanical toughness and electrical conductivity of the carbon shell. In Comparative Example 13, the carbon shell lacks toughness and the cycle retention rate is only 72.8% without boron doping. In Comparative Example 10, the excessive boron doping concentration of 8.0 at% may affect the electrical conductivity of the carbon layer, causing the cycle retention rate to drop to 75.2%. Precise control of the LiF atomic fraction in the artificial SEI film directly affects interfacial stability. In Comparative Example 11, the insufficient LiF content of 30 at% leads to inadequate interfacial protection, resulting in continuous electrolyte decomposition and a cycle retention rate of only 70.8% and an average coulombic efficiency of 98.65%. In Comparative Example 12, the excessive LiF content of 80 at% may increase interfacial impedance, affecting ion transport and causing the cycle retention rate to drop to 74.2%.Most notably, in Comparative Example 14, with its completely cavity-free structure, the material properties deteriorated across the board. The initial coulombic efficiency was only 78.2%, the cycle retention rate plummeted to 58.5%, the slurry viscosity reached a high of 5500 mPa·s, and the maximum solid content was only 38 wt%. This fully verifies the crucial role of the core-shell-cavity synergistic structural design. These systematic comparative experimental results clearly demonstrate that the present invention, through precise control of nano-silicon content (20-28 wt%), particle size (20-100 nm), cavity ratio (30-40 vol%), boron-doped carbon shell thickness (2-5 nm), and doping amount (0.5%), achieves its full potential. The synergistic optimization of several key parameters, such as -6.0 at% and the LiF atomic fraction of the artificial SEI film of 40-70 at%, can effectively solve the contradiction between high compaction density and high coulombic efficiency over long cycles, the contradiction between the integrity of the dense carbon shell and the buffering of volume expansion, and the contradiction between processing rheology and uniform film formation of the SEI film. This results in excellent comprehensive performance with a cycle capacity retention rate of more than 85%, an average coulombic efficiency of more than 99.35%, a compaction density of 1.35-1.50 g / cm³, and a slurry viscosity of less than 3000 mPa·s at a solid content of 48 wt%.

[0152] Table 1. Comparison of electrochemical performance data between the examples and comparative examples.

[0153]

[0154] Table 2. Discharge specific capacity data of different rates for the examples and comparative examples.

[0155]

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A silicon-carbon anode material, characterized in that, The silicon-carbon anode material comprises a nano-silicon core, a cavity layer surrounding the nano-silicon core, and a boron-doped dense carbon shell on the outer side; the nano-silicon mass fraction of the silicon is 20–28 wt%; the nano-silicon core is made of metallic silicon powder with an average diameter of 20–100 nm; the cavity fraction of the silicon-carbon anode material is 32–38 vol%; the thickness of the dense carbon shell is 2–5 nm; the boron doping amount in the boron-doped dense carbon shell is 2.5–4.0 at%; and the atomic fraction of lithium fluoride in the inorganic phase of the artificial solid electrolyte membrane on the surface of the anode sheet made of the silicon-carbon anode material is 50–65 at%. The artificial solid electrolyte membrane is derived from the post-treatment of the negative electrode sheet with lithium bis(fluorosulfonyl)imide solution, and the concentration of the lithium bis(fluorosulfonyl)imide solution is [missing information]. .

2. The silicon-carbon anode material according to claim 1, characterized in that, The silicon-carbon anode material is used for a compaction density of The negative electrode sheet.

3. The silicon-carbon anode material according to claim 1, characterized in that, The binder system of the negative electrode sheet includes calcium alginate and lithium polyacrylate in a mass ratio of 3:7 to 7:

3.

4. A method for preparing the silicon-carbon anode material as described in any one of claims 1-3, characterized in that, include: S1. Controlled oxidation of nano-silicon to form an etchable phase; S2. Selectively etch the oxidized particles to achieve a cavity ratio of 30–40 vol%; S3. Construct a boron-doped dense carbon shell with a thickness of 2–5 nm on the outer side of the particles using a polymerization carbonization method, and introduce a boron-containing precursor during the polymerization and carbonization process to achieve a boron doping atomic fraction of 0.5–6.0 at% for the dense carbon shell. S4. The obtained particles are slurried, coated and rolled to obtain a negative electrode sheet; S5. The negative electrode sheet is briefly treated with lithium bis(fluorosulfonyl)imide solution and then cured by heating to form an artificial solid electrolyte film on the surface of the electrode sheet.

5. The method according to claim 4, characterized in that, The oxidation temperature in step S1 is 200–350 °C, and the time is 0.5–2 h.

6. The method according to claim 4, characterized in that, The selective etching step S2 uses an aqueous solution of ammonium fluoride with a concentration of [missing information]. The temperature is 20–40 °C and the time is 10–60 min.

7. The method according to claim 4, characterized in that, When step S3 is performed using a polymerization carbonization method, the solid content of the carbon source solution is 1–5 wt%, and carbonization is carried out in an inert atmosphere. The carbon source is any one of glucose, sucrose, starch, dopamine hydrochloride, or norepinephrine. The carbonization temperature is 600–800 °C, and the time is 0.5–2 h.

8. The method according to claim 4, characterized in that, The boron-containing precursor is trimethoxyborane, triethoxyborane, borane-tetrahydrofuran complex, boron trifluoride-diethyl ether complex, or boric acid / boronic acid ester; the mass ratio of the boron-containing precursor to the carbon source is 1:10–1:2.

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