Si (at) g-SiC (at) CNF negative electrode material and preparation method and application thereof

Abstract

The invention discloses a Si (at) g-SiC (at) CNF negative electrode material, a silicon nanoparticle (Si) is used as a core, the surface of the silicon nanoparticle is coated with a gradient silicon carbide (g-SiC) interface layer, and the surface of the silicon carbide interface layer is coated with a shell carbon nanofiber (CNF). Accurate regulation and control of a silicon carbide layer are achieved through ultrafast Joule thermal shock, the SiC grain size is regulated and controlled through fast heating and cooling, a continuous gradient structure (Si-SiC-C) from a silicon core to a carbon shell is formed, volume expansion is buffered through gradient difference (the expansion rate after circulation is smaller than 15%), the interface stability is enhanced through chemical bonding, and the preparation process is simple and convenient. The compact outer layer isolates electrolyte permeation, and the loss of active lithium is reduced. The material can be directly cut into pieces to be used for a lithium ion battery negative electrode, the initial capacity of the material is increased to 1200 mAh / g, the capacity retention rate after 200 cycles is larger than 85%, and the rate performance and cycle stability of the battery are remarkably improved.

Description

Technical Field

[0001] This application relates to the field of anode materials for lithium-ion batteries, and particularly to a Si@g-SiC@CNF anode material, its preparation method and application. Background Art

[0002] With the increasing demand for high-performance electronic devices and electric vehicles, the market's demand for lithium-ion batteries with high power density and energy density is becoming increasingly urgent. In terms of anode materials, traditional graphite anode materials have a relatively low theoretical capacity (372 mAh / g), which cannot meet the energy requirements of large-scale power devices. Silicon (Si) has attracted much attention due to its ultra-high theoretical capacity (about 4200 mAh / g) and low de-lithiation potential (<0.5 V vs. Li + / Li). However, the volume change of silicon during charge and discharge reaches 300 - 400%, which will lead to structural collapse, particle fragmentation, and the formation of an unstable solid electrolyte interface (SEI), resulting in rapid attenuation of the electrode capacity and difficulty in stable cycling of the battery; on the other hand, silicon has poor electrical conductivity and slow lithium-ion diffusion, which seriously affects the charge and discharge kinetics of the electrode, exacerbates the electrochemical polarization, and leads to poor rate performance of the battery.

[0003] To solve the problem of performance attenuation caused by volume expansion of silicon anodes, silicon is usually nano-sized or silicon particles are dispersed in graphite carbon (silicon content less than 10%) to relieve the stress on the electrode stability caused by volume change. However, nano-silicon has high chemical activity and is easy to react with the electrolyte and lose its activity, resulting in low charge and discharge efficiency; while the silicon content in graphite carbon is too low to significantly improve the anode capacity. Therefore, reducing its contact with the electrolyte and improving the electrical conductivity through nano-silicon surface coating technology have become research hotspots.

[0004] Electrospinning technology is a feasible nano-particle surface coating technology, which encapsulates nano-silicon in a fiber structure through the curling and winding of polymer macromolecular chains. For example, Patent CN 111900411 A proposes a core-shell structure silicon / carbon fiber composite electrode material, which encapsulates nano-silicon particles in a carbon fiber shell layer through electrospinning. However, the carbon shell and silicon in this structure are only physically coated, lacking a chemically bonded interface layer. During cycling, the volume expansion of silicon will cause the carbon shell and silicon particles to gradually peel off, exposing the fresh silicon surface to react with the electrolyte and form an unstable SEI film. After 30 cycles, its reversible capacity rapidly decays to 487 mAh / g (capacity retention rate < 55%). Therefore, it is urgent to introduce a chemically stable bonding layer (such as a SiC layer) at the silicon-carbon interface to inhibit the peeling problem between silicon and the carbon shell.

[0005] In the prior art, a SiC layer is usually formed at the silicon-carbon interface by a conventional heating method, as described in Patent CN113097469B. This method involves multi-step high-temperature sintering (1500 - 2000 °C, holding for 2 - 20 h), which is energy-consuming and time-consuming. Due to the long-time high-temperature treatment, the grains of the SiC layer are coarsened and the interfacial stress is concentrated, resulting in poor uniformity of the obtained silicon carbide layer (thickness deviation > 30%), a capacity retention rate < 80% after 200 cycles, and a volume expansion rate > 25%. Summary of the Invention

[0006] Aiming at the technical problems existing in the prior art that "during the charge and discharge process, the diffusion and migration path of lithium ions is long, resulting in a decline in battery performance; the shedding of silicon nanoparticles will cause the loss of electrode active materials, damage the electrode structure, and further cause a rapid decay of the battery's reversible capacity; in addition, the shed Si nanoparticles exposed in the electrolyte will continuously consume active lithium ions, leading to the continuous fragmentation and formation of the SEI film, affecting the Coulomb efficiency and cycle life of the battery", the purpose of the present invention is to provide a Si@g-SiC@CNF negative electrode material, its preparation method and application. By regulating the Joule heat parameters, a gradient silicon carbide layer is in-situ formed at the Si-C interface to form a mechanical buffer layer to inhibit the volume expansion of silicon.

[0007] To achieve the above purpose, the present application provides the following technical solutions:

[0008] The present invention provides a Si@g-SiC@CNF negative electrode material, with silicon nanoparticles (Si) as the core, a gradient silicon carbide (g-SiC) interface layer coated on the surface of the silicon nanoparticles, and a carbon nanofiber (CNF) shell coated on the surface of the silicon carbide interface layer, forming a continuous gradient structure from the silicon core through the gradient silicon carbide interface layer to the carbon nanofiber shell.

[0009] As a further improvement of the above solution, the size of the silicon nanoparticles is 30 - 200 nm; preferably 50 - 100 nm.

[0010] As a further improvement of the above solution, the thickness of the silicon carbide interface layer is 5 - 30 nm.

[0011] As a further improvement of the above solution, the diameter of the carbon nanofiber is 100 - 500 nm.

[0012] As a further improvement of the above solution, in the Si@g-SiC@CNF negative electrode material, the mass percentage of silicon is 45% - 50%, the mass percentage of silicon carbide is 5% - 15%, and the mass percentage of carbon nanofiber is 40% - 45%, and the sum of the mass percentages of each component is 100%.

[0013] The present invention also provides a method for preparing a Si@g-SiC@CNF anode material, comprising the following steps:

[0014] (1) Dissolve a silicon source and a polymer in an organic solvent, ultrasonically disperse, and then electrospin to obtain a fibrous membrane precursor;

[0015] (2) Pre-oxidize the fibrous membrane precursor obtained in step (1) in air and perform high-temperature carbonization in an inert atmosphere to obtain a self-supporting silicon-carbon nanofiber membrane;

[0016] (3) Place the self-supporting silicon-carbon nanofiber membrane obtained in step (2) in a Joule heat reaction device, regulate the pulsed voltage, current, and reaction time, induce the atomic diffusion at the Si-C interface by ultrafast Joule heat shock, in-situ generate a SiC interface layer, and form a continuous gradient structure from the silicon (Si) core through the gradient silicon carbide (g-SiC) interface layer to the carbon nanofiber (CNF) shell. After the reaction, cool to room temperature and take out to obtain a nanofiber membrane, that is, the Si@g-SiC@CNF anode material.

[0017] In a specific embodiment, in step (1), the addition amount of the silicon source is 30% to 50% of the mass of the polymer; the polymer is at least one of polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), and polyimide (PI), with a weight-average molecular weight of 100,000 to 500,000, preferably polyacrylonitrile and polyvinylpyrrolidone.

[0018] In a specific embodiment, in step (1), the organic solvent is a polar solvent such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), etc., preferably N,N-dimethylformamide (DMF).

[0019] In a specific embodiment, in step (1), the electrospinning parameters include: a liquid supply speed of 0.5 to 5 mm / h, a receiver rotation speed of 400 to 1000 r / min, a voltage of 10 to 30 kV, and a needle size of 8 to 30 G; the preferred electrospinning parameters are: a liquid supply speed of 1 - 3 mm / h, a rotation speed of 800 to 1000 r / min, a voltage of 20 to 25 kV, and a needle size of 15 to 19 G.

[0020] In a specific embodiment, in step (2), the pre-oxidation temperature is 200 to 400 °C, and the time is 1 to 2 h; the carbonization temperature is 800 to 1300 °C, and the time is 1 to 3 h.

[0021] In a specific embodiment, in step (3), the thickness of the silicon carbide interface layer satisfies the following relationship:

[0022] The thickness h of the silicon carbide layer = k (heating rate × reaction time) 0.5

[0023] where k is a material constant, and its value range is 0.1 - 0.3 nm·s -0.5 ·°C -0.5 .

[0024] In a specific embodiment, in step (3), the output voltage is 20 - 70 V, the current is 0 - 100 A, the instantaneous heating rate is 1000 - 2000 °C / s, and the reaction time is 1 - 150 s; the preferred Joule heat reaction parameters are: the output voltage is 50 - 60 V, the current is 40 - 80 A, the instantaneous heating rate is 1000 - 1600 °C / s, and the reaction time is 1 - 5 s.

[0025] The present invention also provides the application of the Si@g - SiC@CNF negative electrode material in a lithium - ion battery.

[0026] The preparation process of the lithium - ion battery is as follows: Cut the obtained nanofiber membrane into electrode sheets of 10 * 10 mm, dry, weigh, evacuate, and then put them into a glove box. Using a lithium sheet as the counter - electrode material, a polypropylene (PP) film as the separator, adding a mixed electrolyte with a solute of 1 M LiPF6 and solvents of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1, assemble a CR2025 type button battery in the glove box. The process is: sequentially place a negative electrode case, a shrapnel, a steel sheet, an electrolyte, a lithium sheet, an electrolyte, a separator, an electrolyte, a negative electrode sheet, an electrolyte, and a positive electrode case from bottom to top, gently press and hold them in advance, and then put them into a button battery encapsulation machine to close and press tightly with a pressure of 40 - 60 MPa, and let it stand for 10 - 14 h for use.

[0027] The present invention realizes the precise regulation of the silicon carbide layer through ultrafast Joule heat shock. This technology regulates the SiC grain size through rapid heating and cooling, forming a continuous gradient structure from the silicon core to the carbon shell (Si→SiC→C). It not only buffers volume expansion through gradient differences (the expansion rate after cycling < 15%), but also enhances interface stability through chemical bonding. Its dense outer layer isolates electrolyte penetration and reduces the loss of active lithium. This material can be directly cut into pieces for use as the negative electrode of a lithium - ion battery. Its initial capacity is increased to 1200 mAh / g, and the capacity retention rate is > 85% after 200 cycles, significantly improving the rate performance and cycle stability of the battery.

[0028] The technical solution of the present invention has the following advantages compared with the prior art:

[0029] (1) Gradient silicon carbide layer design: By regulating the Joule heat parameters (voltage, current, reaction time), a gradient silicon carbide layer is in-situ generated at the Si-C interface. The thickness of the silicon carbide layer is adjustable, forming a mechanical buffer layer to inhibit the volume expansion of silicon.

[0030] (2) Instantaneous reaction technology: Compared with traditional high-temperature sintering, the extremely short reaction time (1 - 150 s) of Joule heat shock avoids the destruction of the material structure and improves the preparation efficiency.

[0031] (3) Self-supporting flexible structure: Without adding binders, it can be directly cut and used as the negative electrode, improving the energy density of the battery. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the embodiments. Obviously, the drawings described below are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the provided drawings.

[0033] Figure 1 It is a process flow chart of the prepared silicon-carbon negative electrode material for lithium-ion batteries.

[0034] Figure 2 It is a TEM image of the prepared Si@g-SiC@CNF negative electrode material.

[0035] Figure 3 It is an XRD pattern of the negative electrode materials obtained by different heat treatment methods.

[0036] Figure 4 It is a CV curve graph of the prepared Si@g-SiC@CNF negative electrode material.

[0037] Figure 5 It is a rate performance graph of the negative electrode materials with two different heat treatment methods.

[0038] Figure 6 It is a cycle performance graph of the negative electrode materials with two different heat treatment methods during charge and discharge. Detailed Embodiments

[0039] The following will clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0040] The following will further elaborate on the present application in detail with reference to the drawings and specific embodiments:

[0041] Example 1:

[0042] 1. Prepare the spinning solution: Weigh 0.2 g of Si nanoparticles (50 - 100 nm) into 5 g of DMF, ultrasonically treat for 40 min, then add 0.4 g of PAN, and stir for 12 h for standby.

[0043] 2. Prepare the silicon-based spinning film precursor by electrospinning: Use a 10 ml syringe to suck up the spinning solution, set the electrospinning parameters: 25 °C, 19G needle, voltage 20 kV, rotation speed 600 r / min, flow rate 3 mm / h, and the distance between the needle and the receiving roller is 15 cm. Collect the spinning film precursor.

[0044] 3. Prepare the silicon carbide nanofiber film by pre-oxidation and carbonization treatment: Dry the nanofiber film at 60 °C for 6 h, cut it and press it with graphite sheets, then keep it at 250 °C for 1 h in a muffle furnace (air), and then carry out carbonization treatment at 800 °C for 3 h in a tube furnace (argon), with a heating rate of 3 °C / min. After cooling, the silicon carbide nanofiber film is obtained.

[0045] 4. Rapidly prepare the Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber film prepared in step 3 into a 3×5 cm square, sandwich it in a 3×6 cm carbon paper, and fix it in a heating reactor.

[0046] Set the Joule heat reactor: Voltage 30 V, current 40 A, heating rate 1000 °C / s, evacuate,

[0047] The target temperature is 1500 °C, and the reaction time is 1 s. After the reaction is completed, cool it down and take out the anode material.

[0048] Example 2:

[0049] The preparation methods of the first three steps of this example are the same as those of Example 1 to obtain the silicon carbide nanofiber film.

[0050] Rapidly prepare the Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber film prepared in step 3 into a 3×5 cm square, sandwich it in a 3×6 cm carbon paper, and fix it in a heating reactor. Set the Joule heat reactor: Voltage 30 V, current 70 A, heating rate 1500 °C / s, evacuate, the target temperature is 1500 °C, and the reaction time is 1 s. After the reaction is completed, cool it down and take out the anode material.

[0051] Example 3:

[0052] The preparation methods of the first three steps of this example are the same as those of Example 1 to obtain the silicon carbide nanofiber film.

[0053] Rapid preparation of Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber membrane prepared in step 3 into a square of 3×5 cm, sandwich it in a carbon paper of 3×6 cm, and fix it in a heating reactor. Set the Joule heat reactor: voltage 30 V, current 40 A, heating rate 1000 °C / s, evacuate, target temperature 1500 °C, hold for 3 s. After the reaction is completed, cool down and take out the anode material.

[0054] Example 4:

[0055] The preparation method of the first three steps of this example is the same as that of Example 1 to obtain a silicon carbide nanofiber membrane.

[0056] Rapid preparation of Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber membrane prepared in step 3 into a square of 3×5 cm, sandwich it in a carbon paper of 3×6 cm, and fix it in a heating reactor. Set the Joule heat reactor: voltage 30 V, current 70 A, heating rate 1500 °C / s, evacuate, target temperature 1500 °C, hold for 3 s. After the reaction is completed, cool down and take out the anode material.

[0057] Example 5:

[0058] The preparation method of the first three steps of this example is the same as that of Example 1 to obtain a silicon carbide nanofiber membrane.

[0059] Rapid preparation of Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber membrane prepared in step 3 into a square of 3×5 cm, sandwich it in a carbon paper of 3×6 cm, and fix it in a heating reactor. Set the Joule heat reactor: voltage 30 V, current 40 A, heating rate 1000 °C / s, evacuate, target temperature 1500 °C, hold for 5 s. After the reaction is completed, cool down and take out the anode material.

[0060] Example 6:

[0061] The preparation method of the first three steps of this example is the same as that of Example 1 to obtain a silicon carbide nanofiber membrane.

[0062] Rapid preparation of Si@g-SiC@CNF anode material by high-temperature thermal shock: Cut the silicon carbide nanofiber membrane prepared in step 3 into a square of 3×5 cm, sandwich it in a carbon paper of 3×6 cm, and fix it in a heating reactor. Set the Joule heat reactor: voltage 30 V, current 70 A, heating rate 1500 °C / s, evacuate, target temperature 1500 °C, hold for 5 s. After the reaction is completed, cool down and take out the anode material.

[0063] Comparative Example 1

[0064] 1. Preparation of spinning solution: Weigh 0.2 g of Si nanoparticles into 5 g of DMF, ultrasonically treat for 40 min, then add 0.4 g of PAN, and stir for 12 h for standby.

[0065] 2. Preparation of silicon-based spinning film precursor by electrospinning: Use a 10 ml syringe to suck up the spinning solution, and set the electrospinning parameters: 25 °C, 19G needle, voltage 20 kV, rotation speed 600 r / min, flow rate 3 mm / h, and the distance between the needle and the receiving roller is 15 cm. Collect the spinning film precursor.

[0066] 3. Preparation of silicon-carbon nanofiber membrane by pre-oxidation and carbonization treatment: Dry the nanofiber membrane at 60 °C for 6 h, cut it and press it with graphite sheets, then keep it at 250 °C in a muffle furnace for 1 h

[0067] (in air), and then carry out carbonization treatment at 1500 °C for 3 h (in argon) in a tube furnace with a heating rate of 3 °C / min. After cooling, the Si@SiC@CNF anode material is obtained.

[0068] Example 7: Preparation of electrode sheet and assembly of half-cell

[0069] Directly cut the materials obtained in Examples 1-6 and Comparative Example 1 into electrode sheets of 10*10 mm with a slicing machine. After drying, weighing, and evacuating, put them into a glove box. Using a lithium sheet as the counter electrode material, a PP film as the separator, add a mixed electrolyte with a solute of 1 M LiPF6 and a solvent of EC:EMC:DEC (1:1:1), and assemble a CR2025 type button battery in the glove box. The process is as follows: sequentially place (drop) the negative electrode case, shrapnel, steel sheet, electrolyte, lithium sheet, electrolyte, separator, electrolyte, negative electrode sheet, electrolyte, and positive electrode case from bottom to top, gently press and hold them in advance, and then put them into a button battery encapsulation machine and close and press them with a force of 50 MPa, and let it stand for 12 h to obtain a lithium-ion battery.

[0070] Table 1 Test results

[0071]

[0072] Method for measuring the k value: Measure the thickness of the silicon carbide layer in Examples 1-6 by TEM, and fit to obtain k = 0.18 nm·s -0.5 °C -0.5 , and the actual value of the silicon carbide thickness is close to the theoretical value.

[0073] As can be seen from the data in Table 1, the thickness of the silicon carbide (SiC) layer can be precisely controlled by regulating the heating rate and reaction time: at the same reaction time, increasing the heating rate (e.g., from 1000 to 1500 °C / s) can accelerate the atomic diffusion at the Si-C interface and significantly promote the growth of the SiC layer; while at a fixed heating rate, prolonging the reaction time (from 1 to 5 s) can make the thickness of the SiC layer increase linearly, verifying that the reaction time is the key process parameter determining the thickness. Further, the increase in the thickness of the SiC layer is significantly negatively correlated with the volume expansion rate of the silicon anode (when the thickness increases from 6.1 nm to 25.1 nm, the expansion rate decreases from 20.1% to 8.7%). However, from the relationship diagram of the SiC thickness and the volume expansion rate, as the thickness increases, the expansion rate decreases, but the decrease slows down after exceeding 15 nm. The internal mechanism is that the thick SiC layer effectively buffers the volume deformation of silicon through the mechanical constraint effect, thereby inhibiting the structural collapse and the stripping of active materials. In summary, by synergistically optimizing the heating rate and reaction time, the thickness of the SiC layer can be directionally regulated, providing an important basis for balancing the structural stability and process cost of electrode materials.

[0074] Effect evaluation

[0075] Figure 1 : Process flow chart of the prepared flexible silicon-carbon nanofiber membrane for lithium-ion batteries. The spinning solution is loaded into a syringe for electrospinning to obtain a nanofiber membrane, and then the nanofiber membrane is subjected to pre-oxidation and carbonization treatments to obtain a carbon nanofiber membrane. It is sandwiched between two pieces of carbon paper for ultrafast Joule heat shock to obtain a Si@g-SiC@CNF anode material with a thickness gradient distribution. This material can be used as an independent electrode, and no current collector and additional binder are required in the battery assembly, greatly increasing the loading amount of active materials and thus improving the energy density of the battery.

[0076] Figure 2 : TEM image of the nanofiber membrane obtained in Example 4. From Figures (a, b), it can be seen that the obtained fibers are nanoscale and silicon is uniformly distributed inside the carbon nanofibers. From Figure (c), the lattice fringes of Si (0.31 nm) and SiC (0.25 nm) can be seen, indicating the successful formation of SiC, and the thickness of the generated SiC interface layer is about 15 nm, and the diameter of the carbon nanofibers is about 200 nm. In the Si@g-SiC@CNF anode material, the mass percentage of silicon is 45% - 50%, the mass percentage of silicon carbide is 5% - 15%, and the mass percentage of carbon nanofibers is 40% - 45%. The sum of the mass percentages of each component is 100%.

[0077] Figure 3: XRD patterns of anode materials obtained by different heat treatment methods, comparing the effects of traditional high-temperature sintering and Joule heat treatment on the formation of SiC. It can be observed from the figure that for Si@g-SiC@CNF treated by ultrafast Joule heat shock compared with Si@SiC@CNF, the peak value and peak area of SiC in the latter are higher than those in the former, indicating that long-term high-temperature treatment leads to coarsening of SiC grains, interfacial stress concentration, and poor thickness uniformity. Ultrafast Joule heat shock provides high temperature instantaneously, which can accurately control the volume diffusion and grain boundary migration of SiC.

[0078] Figure 4 : CV curve of the Si@g-SiC@CNF anode material in Example 4, showing the electrochemical performance of the material at different cycle numbers. Among them, 1st, 2nd, 3rd, 4th, 5th represent the first to fifth charge-discharge cycles. There is an obvious reduction peak near 0.79V in the first cycle, which can be attributed to the decomposition of the electrolyte and the formation of the SEI film. During the subsequent cycling process, a new reduction peak can be clearly observed at 0.21V, and two oxidation peaks appear at 0.32V and 0.50V, corresponding to the formation of Li x Si phase during the alloying transformation process and the decomposition stage of LixSi phase during the dealloying process to form amorphous silicon, indicating that the formation of SiC does not participate in the redox reaction and does not provide capacity.

[0079] Figure 5 : Rate performance diagrams of anode materials with two different heat treatment methods. It can be seen from the figure that in Example 4 at a current density of 2A g -1 , the discharge capacity still remains at 800mAh g -1 , indicating that the SiC formed in the anode material treated by ultrafast Joule heat treatment is denser, the interfacial stress is more concentrated, and it can better cope with the volume expansion of Si.

[0080] Figure 6 : Charge-discharge cycle performance diagrams of anode materials with two different heat treatment methods. The anode material of Comparative Example 1 treated by traditional high-temperature sintering consumes too much SiC due to the long sintering time. Therefore, the first discharge capacity is only 915mAh g -1 , while the first discharge capacity of the material in Example 4 under ultrafast Joule heat shock is 1209mAh g -1 , and at the same time, Joule heat treatment improves the cycle stability to more than 85%, indicating that the in-situ formation of the Si-C interface gradient silicon carbide layer can maintain better cycle stability.

[0081] Through the ultrafast Joule heat shock technology, the present invention realizes the in-situ generation of a Si-C interface gradient silicon carbide layer in an extremely short time, solving the defects of high energy consumption and insufficient structural control in traditional methods. The prepared silicon-carbon nanofiber membrane has excellent cycle stability (capacity retention rate ≥ 85%) and mechanical properties (volume expansion rate ≤ 15%), and has significant industrial application value in the field of lithium-ion batteries.

[0082] The above content is only a specific implementation case of this application, rather than all application cases of this application. All solutions based on the technical idea of this application and modified on the technical idea of this application are within the protection scope of the claims of this application.

Claims

1. A Si@g-SiC@CNF negative electrode material, characterized in that: The silicon nanoparticles (Si) are used as the core, the surface of the silicon nanoparticles is coated with a gradient silicon carbide (g-SiC) interface layer, and the surface of the silicon carbide interface layer is coated with a carbon nanofiber (CNF) shell, forming a continuous gradient structure from the silicon core through the gradient silicon carbide interface layer to the carbon nanofiber shell.

2. The Si@g-SiC@CNF negative electrode material according to claim 1, characterized in that: The size of silicon nanoparticles is 30 to 200 nm; The thickness of the silicon carbide interface layer is 5 to 30 nm; The diameter of carbon nanofibers is 100 to 500 nm.

3. The Si@g-SiC@CNF negative electrode material according to claim 1, characterized in that: The mass percentage of silicon is 45% to 50%, the mass percentage of silicon carbide is 5% to 15%, the mass percentage of carbon nanofiber is 40% to 45%, and the sum of the mass percentages of each component is 100%.

4. A method for preparing a Si@g-SiC@CNF negative electrode material, characterized in that: The following steps are involved: (1) dissolving a silicon source and a high molecular polymer in an organic solvent, performing ultrasonic dispersion and electrostatic spinning to obtain a fiber membrane precursor; (2) pre-oxidizing the fiber membrane precursor obtained in step (1) in air and carbonizing it at high temperature in an inert atmosphere to obtain a self-supporting silicon-carbon nanofiber membrane; (3) placing the self-supporting silicon-carbon nanofiber membrane obtained in step (2) in a Joule thermal reaction device, and inducing Si-C interface atomic diffusion by regulating the pulse voltage, current and reaction time, using ultrafast Joule thermal shock to generate a SiC interface layer in situ, thereby forming a continuous gradient structure from the silicon (Si) core through the gradient silicon carbide (g-SiC) interface layer to the carbon nanofiber (CNF) shell. After the reaction is completed, the membrane is cooled to room temperature and taken out to obtain a nanofiber membrane, i.e., a Si@g-SiC@CNF negative electrode material.

5. The method for preparing a Si@g-SiC@CNF negative electrode material according to claim 4, characterized in that: In step (1), the amount of silicon source added is 30% to 50% of the mass of the high molecular weight polymer; the high molecular weight polymer is at least one of polyacrylonitrile (PAN), polyvinyl pyrrolidone (PVP), and polyimide (PI), and the weight average molecular weight is 100,000 to 500,000.

6. The method for preparing a Si@g-SiC@CNF negative electrode material according to claim 4, characterized in that: In step (1), the organic solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

7. The method for preparing a Si@g-SiC@CNF negative electrode material according to claim 4, characterized in that: In step (1), the electrospinning parameters include: liquid supply speed 0.5-5 mm / h, receiver speed 400-1000 r / min, voltage 10-30 kV, and needle size 8-30 G.

8. The method for preparing a Si@g-SiC@CNF negative electrode material according to claim 4, characterized in that: In step (2), the pre-oxidation temperature is 200-400° C., and the time is 1-2 h; the carbonization temperature is 800-1300° C., and the time is 1-3 h.

9. The method for preparing a Si@g-SiC@CNF negative electrode material according to claim 4, characterized in that: In step (3), the thickness of the silicon carbide interface layer satisfies the following relationship: Silicon carbide layer thickness h = k (heating rate × reaction time) 0.5 Where k is a material constant, ranging from 0.1 to 0.3 nm·s -0.5 ℃ -0.5 ; The Joule heat reaction parameters are: output voltage of 20 to 70 V, current of 0 to 100 A, instantaneous heating rate of 1000 to 2000° C. / s, and reaction time of 1 to 150 s.

10. Use of the Si@g-SiC@CNF negative electrode material according to any one of claims 1 to 9 in a lithium ion battery.

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