Silicon-based carbon fiber negative pole piece as well as preparation method and application thereof

Abstract

The invention discloses a silicon-based carbon fiber negative pole piece as well as a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The silicon-based carbon fiber negative pole piece comprises carbon fibers, silicon particles and a non-tube-bundle type single dispersed oxidized single-walled carbon nanotube; the silicon particles are embedded in the carbon fibers; the non-tube-bundle type single dispersed oxidized single-walled carbon nanotubes are uniformly distributed in the carbon fibers and are distributed among the silicon particles in a shuttling manner; the carbon fibers are interwoven to form a self-supporting flexible pole piece. Different from a traditional lithium ion battery negative pole piece, the silicon-based carbon fiber negative pole piece can be directly used as a lithium ion battery negative pole without additionally adding a conductive agent and a binder and using a current collector, and has excellent electrochemical performance.

Description

Technical Field

[0001] The present application relates to a silicon-based carbon fiber negative electrode sheet, a preparation method thereof and an application, belonging to the technical field of lithium-ion batteries. Background Art

[0002] Silicon has a high theoretical capacity (≈4200 mAh / g), abundant crustal reserves, and a suitable lithium intercalation potential (~0.4V vs Li), and is expected to replace graphite negative electrodes as the next-generation negative electrode material for lithium-ion batteries. However, silicon materials have problems of volume expansion (≈400%) and poor conductivity during charge and discharge, which hinder their commercial promotion. Silicon-carbon composite and constructing a carbon coating on the surface of silicon are currently widely recognized technical routes. Electrospinning technology is a common means for preparing silicon-based carbon fibers with a carbon-coated silicon structure. However, when directly electrospinning with silicon, silicon is easily exposed outside the carbon fibers and agglomerates on the outer surface of the carbon fibers, making it difficult to form an effective carbon-coated structure, resulting in silicon being directly exposed to the electrolyte and making it difficult to maintain good electrochemical performance. At the same time, the formed carbon fiber sheet structure is also unstable. Summary of the Invention

[0003] In order to solve the above technical problems existing in the process of preparing a silicon-based carbon fiber negative electrode sheet by electrospinning, and to enable the silicon-based carbon fiber negative electrode sheet prepared by electrospinning to be better applied to lithium-ion batteries and achieve better electrochemical performance, the present application provides a silicon-based carbon fiber negative electrode sheet, a preparation method thereof and an application.

[0004] According to the first aspect of the present application, a silicon-based carbon fiber negative electrode sheet is provided. The silicon-based carbon fiber negative electrode sheet includes carbon fibers, silicon particles, and non-bundled single-dispersed oxidized single-walled carbon nanotubes; the microstructure unit of a single carbon fiber in the silicon-based carbon fiber negative electrode sheet has the following structural general formula: Si / SWCNT@CNF, where Si represents silicon particles, SWCNT represents non-bundled single-dispersed oxidized single-walled carbon nanotubes, and CNF represents carbon fibers; the non-bundled single-dispersed oxidized single-walled carbon nanotubes and silicon particles are both embedded inside the carbon fibers, and the non-bundled single-dispersed oxidized single-walled carbon nanotubes are uniformly distributed inside the carbon fibers and shuttle between the silicon particles. The carbon fibers in the silicon-based carbon fiber negative electrode sheet are intertwined with each other to form a self-supporting and flexible sheet that can be bent and folded.

[0005] A silicon-based carbon fiber negative electrode sheet, the silicon-based carbon fiber negative electrode sheet includes carbon fibers, silicon particles, and non-bundled single-dispersed oxidized single-walled carbon nanotubes;

[0006] The silicon particles are embedded inside the carbon fibers;

[0007] The non-bundled single-dispersed oxidized single-walled carbon nanotubes are uniformly distributed inside the carbon fiber and shuttle between the silicon particles;

[0008] The carbon fibers are intertwined with each other to form a self-supporting flexible electrode sheet.

[0009] The silicon-based carbon fiber negative electrode sheet does not need to add additional conductive agents, binders and current collectors, and can be directly used as the negative electrode of a lithium-ion battery.

[0010] Optionally, the silicon particles are selected from one or both of pure silicon particles and silicon monoxide particles.

[0011] Optionally, the surface of the non-bundled single-dispersed oxidized single-walled carbon nanotubes contains oxygen-containing functional groups.

[0012] Optionally, the oxygen-containing functional groups include hydroxyl groups, carboxyl groups and epoxy groups.

[0013] Optionally, the content of the non-bundled single-dispersed oxidized single-walled carbon nanotubes relative to the silicon particles is 1-5 wt.%.

[0014] Optionally, the content of the non-bundled single-dispersed oxidized single-walled carbon nanotubes relative to the silicon particles is any value among 1 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, 5 wt.% or the range value between any two of them.

[0015] Optionally, the content of the non-bundled single-dispersed oxidized single-walled carbon nanotubes relative to the silicon particles is 1-3 wt.%.

[0016] Optionally, the content of the silicon particles in the silicon-based carbon fiber negative electrode sheet is 30-55 wt.%.

[0017] Optionally, the content of the silicon particles in the silicon-based carbon fiber negative electrode sheet is any value among 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, 55 wt.% or the range value between any two of them.

[0018] Optionally, the content of the silicon particles in the silicon-based carbon fiber negative electrode sheet is 35-50 wt.%.

[0019] According to the second aspect of the present application, a preparation method of the above-mentioned silicon-based carbon fiber negative electrode sheet is provided.

[0020] The preparation method of the above-mentioned silicon-based carbon fiber negative electrode sheet includes the following steps:

[0021] (a1) Uniformly disperse non-bundled single-dispersed oxidized single-walled carbon nanotubes in N,N-dimethylformamide solvent to form a homogeneous and stable dispersion solution, then disperse Si particles in the above dispersion solution to form a stable dispersion, and then add polyacrylonitrile to completely dissolve it in the dispersion to obtain a homogeneous and stable dispersion precursor;

[0022] (b1) Electrospinning the above dispersion precursor to obtain a nanofiber cloth;

[0023] (e1) After pre-oxidation treatment of the above nanofiber cloth in air, then high-temperature annealing and carbonization to obtain the silicon-based carbon fiber negative electrode sheet (Si-SWCNT@CNF).

[0024] Optionally, step (a1) includes: uniformly dispersing SWCNT in N,N-dimethylformamide (DMF) solvent to form a homogeneous and stable dispersion solution (SWCNT / DMF), then dispersing Si particles in the above dispersion solution to form a stable dispersion (Si / SWCNT / DMF), and then adding polyacrylonitrile to completely dissolve it in the dispersion to obtain a homogeneous and stable dispersion precursor.

[0025] Optionally, in step (a1), the content of the non-bundled single-dispersed oxidized single-walled carbon nanotubes relative to the Si particles is 1-5 wt.%.

[0026] Optionally, in step (a1), the concentration of polyacrylonitrile relative to N,N-dimethylformamide is 10-15 wt.%.

[0027] Optionally, in step (a1), the mass ratio of the Si particles to polyacrylonitrile is 0.2-0.8:1.

[0028] Optionally, step (a1) includes: uniformly dispersing non-bundled single-dispersed oxidized single-walled carbon nanotubes in N,N-dimethylformamide (DMF) solvent by ultrasonic means, then dispersing Si particles in the above dispersion solution by ultrasonic means, and then adding polyacrylonitrile, heating and stirring to completely dissolve it in the dispersion to obtain a homogeneous and stable dispersion precursor; the heating and stirring temperature is 50-70 °C; the content of the non-bundled single-dispersed oxidized single-walled carbon nanotubes relative to the Si particles is 1-5 wt.%; the concentration of polyacrylonitrile relative to DMF is 10-15 wt.%; the surface of the non-bundled single-dispersed oxidized single-walled carbon nanotubes contains oxygen-containing functional groups; the oxygen-containing functional groups include hydroxyl, carboxyl and epoxy groups.

[0029] Optionally, in step (a1), the preparation method of the non-bundled single-dispersed oxidized single-walled carbon nanotubes includes:

[0030] First, add potassium permanganate to an excessive amount of concentrated sulfuric acid solution until the reaction is complete. Then, add the original ash of single-walled carbon nanotubes to the above solution. After stirring evenly, the resulting reaction mixture is continuously stirred and reacted at a temperature of 50 - 60 °C for 2 - 3 h. Then, dilute the reacted mixture with water, and add an excessive amount of hydrogen peroxide solution to reduce and remove the manganese dioxide solid generated during the reaction. After filtration and washing with an acid solution, the obtained solid is further washed with water to remove some impurity ions in the solution. When the pH of the aqueous solution of the single-walled carbon nanotube mixture is 4 - 5, centrifuge at 5000 - 14000 revolutions per minute, then take out the upper homogeneous solution, and filter and wash and collect this homogeneous solution to obtain a black solid.

[0031] Optionally, the non-bundled single-dispersed single-walled carbon nanotubes refer to that the single-walled carbon nanotubes do not exist in the form of bundles, but each single-walled carbon nanotube is independently peeled off from the single-walled carbon nanotube bundle and exists in a single discrete form.

[0032] Optionally, in the original ash of single-walled carbon nanotubes, the purity of the single-walled carbon nanotubes is 60 - 95 wt%.

[0033] Optionally, in the original ash of single-walled carbon nanotubes, the impurities include at least one of crystalline carbon sphere particles, multi-walled carbon nanotubes, graphite flakes, graphene flakes, fullerene spheres, metal nanoparticles, and metal crystalline carbon particles.

[0034] Optionally, the metal crystalline carbon particles are crystalline carbon sphere particles coated with metal nanoparticles.

[0035] Optionally, in the non-bundled single-dispersed oxidized single-walled carbon nanotubes, the purity of the single-walled carbon nanotubes is 99.5 - 100 wt.%.

[0036] Optionally, step (b1) includes: electrospinning the above dispersion precursor to obtain a nanofiber cloth; the electrospinning is single-axis electrospinning, and its injection speed is 1 - 1.5 ml·h -1 , and the applied voltage is 12 - 15 kV.

[0037] Optionally, the pre-oxidation treatment in air in step (c1) is to keep the temperature constant at 260 - 300 °C for 2 - 4 h.

[0038] Optionally, the high-temperature annealing carbonization in step (c1) includes: annealing at 600 - 1000 °C for 0.5 - 12 h in an inert atmosphere or under air isolation.

[0039] Optionally, the inert atmosphere is at least one of nitrogen or noble gases.

[0040] According to the third aspect of the present application, there is provided an application of the above-mentioned silicon-based carbon fiber negative electrode sheet or a silicon-based carbon fiber negative electrode sheet prepared according to the above-mentioned preparation method in an electrode.

[0041] Optionally, the electrode is selected from the electrodes of a primary electrochemical generator, a secondary electrochemical generator, and a high-energy generator.

[0042] According to the fourth aspect of the present application, there is provided a secondary battery, wherein the negative electrode sheet of the secondary battery is the above-mentioned silicon-based carbon fiber negative electrode sheet or a silicon-based carbon fiber negative electrode sheet prepared according to the above-mentioned preparation method;

[0043] Optionally, the secondary battery is a lithium-ion battery.

[0044] Optionally, the secondary battery includes a positive electrode, a negative electrode, and an electrolyte.

[0045] The beneficial effects that can be produced by the present application include:

[0046] (1) In the preparation method of the silicon-based carbon fiber negative electrode sheet provided by the present application, an electrospinning method is used to prepare a self-supporting electrode sheet. Before electrospinning, when preparing the spinning solution precursor, by adding non-bundled single-dispersed oxidized single-walled carbon nanotubes to the spinning solution, strong hydrogen bond interactions are formed between the oxygen-containing groups on the surface of the single-walled carbon nanotubes and the surface of the silicon particles and PAN respectively, which not only promotes the dispersion of Si particles in the spinning solution, but also enables the Si particles to be more effectively coated by PAN. Finally, a silicon-based carbon fiber with a regular core-shell structure is obtained by electrospinning. On the contrary, in the case of not adding SWCNT, for the obtained Si@CNF silicon-based carbon fiber, a lot of silicon particle agglomerations appear on the surface of the carbon fiber, resulting in a large number of nodules, and the carbon fiber structure is irregular and unstable, which is likely to cause the shedding and exposure of silicon particles and direct contact with the electrolyte, affecting the final electrochemical performance.

[0047] (2) The silicon-based carbon fiber negative electrode sheet provided by this application has a regular core-shell fiber structure. Silicon particles and SWCNT are completely coated inside the regular fibers to form a regular core-shell fiber structure; SWCNT not only enhances the flexibility and strength of the fibers, but also maintains the electrical contact between silicon particles, effectively improving the toughness, strength, and electrical conductivity uniformity and effectiveness of the entire electrode sheet; the external carbon fiber shell effectively blocks the direct contact between silicon particles and the electrolyte, reduces the occurrence of side reactions, and ensures the continuous and stable electrode reaction. The interweaving of each fiber not only forms a stable self-supporting electrode sheet, but also ensures the rapid transmission of electrons between the fibers. The multi-level structure composite and synergy effectively promote the mechanical stability of the electrode structure and the effective electrical conductivity connection. Si@CNF-1 and Si / SWCNT@CNF-1 are used as the negative electrode sheets of lithium-ion batteries respectively, and the charge-discharge cycle stability test is carried out. The results show that Si / SWCNT@CNF-1 maintains a specific capacity of 608.4 mAh g -1 after 160 cycles at 0.5 Ag -1 , while Si@CNF-1 only has 299.5 mAh g -1 , indicating that Si / SWCNT@CNF-1 has better electrochemical performance. Due to its continuous and uniform core-shell structure, the addition of single-walled carbon nanotubes can fix Si particles well in the carbon fiber tube, avoiding direct contact with the electrolyte. At the same time, single-walled carbon nanotubes and the core-shell structure can further alleviate the volume expansion of Si.

[0048] (3) The silicon-based carbon fiber negative electrode sheet of this application has self-supporting characteristics and is a flexible electrode sheet that can be bent and folded. This silicon-based carbon fiber negative electrode sheet can be directly used as the negative electrode of a lithium-ion battery without the need to additionally add a conductive agent, binder, and current collector. Description of the Drawings

[0049] Figure 1 is the structural schematic diagram of a single Si / SWCNT@CNF fiber.

[0050] Figure 2 is the optical photograph of the silicon-based carbon fiber negative electrode sheet prepared in Example 1. Figure (a) is when a ruler is placed, and Figure (b) is when picked up by tweezers.

[0051] Figure 3 In, Figures a), b), and c) are the SEM images of Si@CNF-1, Si@CNF-2, and Si@CNF-3 of Comparative Example 1 respectively.

[0052] Figure 4Among them, the SEM images of Si / SWCNT@CNF-1 of Example 1, Si / SWCNT@CNF-2 of Example 2, and Si / SWCNT@CNF-3 of Example 3 are shown in Figures a), b), and c) respectively.

[0053] Figure 5 The cycling performance of the Si@CNF-1 of Comparative Example 1 and the Si / SWCNT@CNF-1 negative electrode sheet of Example 1 at 0.5 A g -1 is shown below.

[0054] Figure 6 The transmission electron microscope photograph of the original bundled single-walled carbon nanotubes described in Comparative Example 2 is shown.

[0055] Figure 7 The transmission electron microscope photograph of the non-bundled single-root dispersed oxidized single-walled carbon nanotubes in the examples is shown.

[0056] Figure 8 The transmission electron microscope photograph of the original ash of the original single-walled carbon nanotubes in the examples is shown. Detailed implementation manners

[0057] The present application will be described in detail below in conjunction with the examples, but the present application is not limited to these examples.

[0058] Unless otherwise specified, the raw materials in the examples of the present application are all purchased through commercial channels. Among them, the non-bundled single-root dispersed oxidized single-walled carbon nanotube dispersion aqueous solution described in this example is prepared according to the steps (1) of Example 1 in the Chinese invention patent (application number: 2024102122774) previously applied by the inventor. Specifically as follows:

[0059] 0.4 g of potassium permanganate solid was added to a beaker containing 15 mL of concentrated sulfuric acid (98 wt%). After stirring until the reaction was complete, 0.2 g of ashed single-walled carbon nanotubes was poured into the above solution. After stirring well, it was placed in a constant-temperature oil bath at 50 °C and stirred for 2 hours. During this period, a glass cover was covered to prevent the concentrated sulfuric acid from absorbing water. After cooling to room temperature, it was diluted with about 100 mL of deionized water and continuously stirred. Then about 10 mL of 30% hydrogen peroxide solution was added. After stirring and reacting for 15 minutes, it was filtered through a microporous filter membrane. The obtained black solid was directly poured into 100 mL of 4% dilute hydrochloric acid, stirred and washed, then filtered through a microporous filter membrane and washed with deionized water by filtration. The obtained black solid was then poured into 200 mL of deionized water, stirred well, and centrifuged at 14,000 rpm for 30 minutes. After stratification, the upper clear liquid was poured off, and 200 mL of deionized water was continuously added to the centrifuge tube, stirred well, washed, and then centrifuged again. After stratification, the upper clear liquid was poured off. This centrifugation and washing process was repeated 3 times until the pH value of the mixture was 4 when deionized water was added to the centrifuge tube. Then it was centrifuged at 10,000 rpm for 30 minutes, the upper clear liquid was extracted, and the collected dispersion solution was filtered and washed through a microporous filter membrane to remove residual impurity ions. The collected black solid was dissolved in deionized water to obtain a dispersion aqueous solution of non-bundled single-dispersed oxidized single-walled carbon nanotubes.

[0060] The purity of the non-bundled single-dispersed oxidized single-walled carbon nanotubes described above reaches over 99.5 wt%. The elemental analysis results of the X-ray energy spectrometer show that no residual metal impurities are detected. No other impurity components such as graphene, multi-walled carbon nanotubes, carbon particles, or metal particles are detected by transmission electron microscopy and scanning electron microscopy.

[0061] The tube diameter of the non-bundled single-dispersed oxidized single-walled carbon nanotubes described above is significantly smaller than that of the original single-walled carbon nanotubes, and it is a non-bundled single-dispersed single-walled carbon nanotube ( Figure 7 ).

[0062] The surface of the ultra-high purity single-walled carbon nanotubes described above is attached with oxygen-containing groups, and the oxygen-containing groups include hydroxyl groups, epoxy groups, carboxyl groups, and carbonyl groups.

[0063] The purity of the ashed single-walled carbon nanotubes is 75 wt%; the impurities in the ashed single-walled carbon nanotubes include crystalline carbon sphere particles, graphene sheets, fullerene spheres, metal crystalline carbon particles, and multi-walled carbon nanotubes, and the content of metal impurities is 8 wt%. The particle size of the metal particles in the metal crystalline carbon particles is 1 - 50 nm; the particle size of the metal particles in the metal crystalline carbon particles is 2 - 10 nm. The single-walled carbon nanotubes in the ashed single-walled carbon nanotubes exist in bundles ([[]] Figure 8) The tube bundles are interlaced with each other and intertwined irregularly. The metal crystal carbon particles are embedded between the interlaced grids of the single-walled carbon nanotubes. Some are adhered to the surface of the single-walled carbon nanotubes, and some are bonded to the surface of the single-walled carbon nanotubes.

[0064] Multi-walled carbon nanotubes were monitored to be oxidized and peeled into long graphene sheets in the centrifugal sediment, indicating that multi-walled carbon nanotubes can also be removed by this method.

[0065] Unless otherwise specified, conventional methods are used for the testing methods, and the instrument settings are all the settings recommended by the manufacturers. Among them, X-ray photoelectron spectroscopy analysis is carried out using an X-ray photoelectron spectrometer (XPS, Thermo Fisher, ESCALAB 250Xi); cyclic voltammetry testing and electrochemical impedance testing are carried out using an electrochemical workstation (Shanghai Chenhua, CHI660D); scanning electron microscopy analysis is carried out using a field emission scanning electron microscope (FESEM, SU-8010, Hitachi, Japan); the conductivity of the electrode sheet is tested using a four-probe conductivity meter; thermogravimetric analysis is carried out using a synchronous thermal analyzer (TG / DTA, STA449F3, Netzsch); battery performance testing is carried out using a battery testing system (Wuhan Blue Electric, CT3001A).

[0066] Example 1

[0067] (1) Preparation of Si / SWCNT@CNF

[0068] First, non-bundled single-dispersed oxidized single-walled carbon nanotubes (SWCNT) were uniformly dispersed in 3.2 mL of N,N-dimethylformamide (DMF) solvent and ultrasonically dispersed evenly; then silicon particles were added, and ultrasonic treatment was continued for 1 h. Then, 460 mg of polyacrylonitrile (PAN, Mw = 150000, relative to the DMF concentration of 12.5 wt.%) was added under stirring, and stirring was continued at 60 °C for 3 h. Subsequently, electrospinning was carried out under specific conditions (a voltage of 13.5 kV and a flow rate of 1 mL / h). The collected fiber cloth was named Si / SWCNT@PAN. Then, Si / SWCNT@PAN was placed in a muffle furnace and heated to 260 °C at a rate of 2 °C / min, and pre-oxidized at this temperature for 3 h, and then cut into electrode sheets. Finally, the cut electrode sheets were heated to 800 °C at a rate of 5 °C / min in an argon atmosphere and carbonized at this temperature for 3 hours. The obtained sample was named Si / SWCNT@CNF-1, and the silicon content in the electrode sheet was 51 wt.%. In this example, the silicon dosage was 260 mg, and the SWCNT dosage was 5.2 mg.

[0069] The non-bundled single-dispersed oxidized single-walled carbon nanotubes (SWCNTs) described in this embodiment were prepared according to the steps of Example 1 in the Chinese invention patent (Application No.: 2024102122774) previously applied by the inventor. The specific usage method is as follows: The solid obtained by filtering 10.4 mL of an aqueous solution of non-bundled single-dispersed oxidized single-walled carbon nanotubes (SWCNTs) with a concentration of 0.5 mg / mL is directly added to DMF without drying to obtain a uniform dispersion solution for further use.

[0070] (2) Lithium-ion battery assembly and performance testing

[0071] The above-mentioned electrode sheets were cut into circular electrode sheets with a diameter of 10 mm and weighed to obtain the mass of each electrode sheet. Finally, the electrode sheets were quickly transferred to a glove box for assembling coin cells. The coin cells were assembled in a glove box filled with argon. For the assembly of lithium-ion battery (LIB) half-cells, the counter electrode and separator were lithium foil and Celgard 2500 membrane respectively, and the electrolyte was 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (volume ratio 1:1) containing 10 wt% fluoroethylene carbonate (FEC). The constant current charge-discharge test mainly examined the charge-discharge specific capacity and cycling performance of the lithium-ion half-cells at different current densities.

[0072] Example 2

[0073] The difference between this example and Example 1 is that the dosage of silicon is 200 mg and the dosage of SWCNT is 2 mg. The finally obtained silicon-based carbon fiber negative electrode sheet is denoted as Si-SWCNT@CNF-2, and the content of silicon particles is 44 wt.%.

[0074] Example 3

[0075] The difference between this example and Example 1 is that the dosage of silicon is 140 mg and the dosage of SWCNT is 7 mg; the concentration of polyacrylonitrile relative to DMF is 10 wt.%; the high-temperature annealing and carbonization conditions are annealing at 600 °C for 12 h in a nitrogen atmosphere; the finally obtained silicon-based carbon fiber negative electrode sheet is denoted as Si-SWCNT@CNF-3, and the content of silicon particles is 30 wt.%.

[0076] Example 4

[0077] The difference between this embodiment and embodiment 1 is that the dosage of silicon is 320 mg, the dosage of SWCNT is 3.2 mg. The concentration of polyacrylonitrile relative to DMF is 15 wt.%; the high temperature annealing carbonization condition is annealing at 1000 °C for 0.5 h in a nitrogen atmosphere; the silicon-based carbon fiber negative electrode sheet finally obtained is recorded as Si-SWCNT@CNF-4, and its silicon particle content is 55 wt.%. A small number of silicon particles appear in Si-SWCNT@CNF-4 agglomerated in the outer shell of the fiber.

[0078] Comparative Example 1

[0079] The difference between this comparative example 1 and the example is that SWCNT is not used. The dosage of silicon is 260 mg, 200 mg, and 140 mg, respectively, and three samples are obtained, which are recorded as Si@CNF-1, Si@CNF-2, and Si@CNF-3. Without using SWCNT, the dispersion uniformity of the prepared electrospinning precursor is obviously not as good as that of the corresponding examples 1-3.

[0080] Comparative Example 2

[0081] The difference between this embodiment and embodiment 1 is that the non-bundled single dispersed oxidized single-walled carbon nanotubes in step (1) are replaced with original bundled single-walled carbon nanotubes. The single-walled carbon nanotubes in the electrospinning precursor solution prepared in step (1) are obviously agglomerated and fail to form a uniform precursor solution, so the expected electrospinning cannot be carried out smoothly. The purity of the original bundled single-walled carbon nanotubes is greater than 98wt.%. Figure 6 The transmission electron microscope photograph of the original bundled single-walled carbon nanotubes is shown. The single-walled carbon nanotube bundles can be seen in the figure. The cross-sectional diameter of the bundles is concentrated in the range of 10-100nm, and the bundles are entangled with each other.

[0082] Characterization tests were performed using Example 1 and Comparative Example 1 as typical examples:

[0083] Figure 1It is a schematic diagram of the microstructure of a single carbon fiber in the Si / SWCNT@CNF silicon-based carbon fiber negative electrode sheet. From the figure, we can see that the microstructure unit of a single carbon fiber in the silicon-based carbon fiber negative electrode sheet has the following structural general formula: Si / SWCNT@CNF; where CNF represents carbon fiber, SWCNT represents single dispersed oxidized single-walled carbon nanotubes without bundles, and Si represents silicon particles. The single dispersed oxidized single-walled carbon nanotubes without bundles and silicon particles are both embedded inside the carbon fiber, and the single dispersed oxidized single-walled carbon nanotubes without bundles are evenly distributed inside the carbon fiber and shuttle between the silicon particles. The surface groups of the single-walled carbon nanotubes have hydrogen bond interactions with the inner surface of the carbon fiber and the surface of the silicon particles respectively. The single dispersed oxidized single-walled carbon nanotubes without bundles can effectively alleviate the aggregation of silicon particles. The flexibility of the single-walled carbon nanotubes can dissipate the huge stress generated by the volume expansion of silicon and can construct a more effective conductive network, which is beneficial to improving the performance of the silicon-based negative electrode.

[0084] Figure 2 It is an optical photograph of the silicon-based carbon fiber negative electrode sheet prepared in Example 1. Figure (a) is when a ruler is placed, and Figure (b) is when picked up with tweezers. As a representative optical photograph of the silicon-based carbon fiber negative electrode sheet, this electrode sheet has good flexibility and good self-supporting characteristics and can be directly used as a battery electrode sheet.

[0085] Figure 3 Among them, Figures (a), (b), and (c) are SEM images of Si@CNF-1, Si@CNF-2, and Si@CNF-3 of Comparative Example 1 respectively. For Si@CNF samples with different Si contents, it can be seen that the surface structure of the prepared carbon fiber is irregular, with irregular nodules in the middle. This is mainly caused by particle aggregation, resulting in discontinuous fiber growth, and the severity of aggregation becomes more and more obvious with the increase of Si content, indicating that pure Si is difficult to match with the spinning solution, and the fiber structure and core-shell structure are greatly restricted. This structure is very likely to cause the silicon particles to fall off from the carbon fiber network during charge and discharge, affecting the performance of the electrochemistry.

[0086] In Figure 4In a), b), and c), the morphologies of the Si / SWCNT@CNF samples obtained with different Si contents are observed. It can be seen that after adding SWCNT, no obvious aggregation of silicon particles is observed. Even for the Si / SWCNT@CNF-1 sample with a silicon content much higher than that of Si@CNF-1, it still maintains a regular fiber structure and core-shell structure. This indicates that after adding SWCNT, the oxygen-containing groups on the surface of single-walled carbon nanotubes can form strong hydrogen bond interactions with silicon particles, PAN, and DMF in the precursor solution, thus contributing to the dispersion of silicon particles in the precursor solution. The precursor solution with good dispersibility finally obtains a fiber core-shell structure with a regular structure. This structure can not only prevent the aggregation of silicon particles but also prevent the direct contact between silicon particles and the electrolyte. At the same time, SWCNT shuttling inside the carbon fiber makes the carbon fiber electrode structure stronger, more stable, and the conductivity more uniform. Therefore, it exhibits better electrochemical performance as a negative electrode.

[0087] Figure 5 The cycling stabilities of Si@CNF-1 and Si / SWCNT@CNF-1 were compared. Si / SWCNT@CNF-1 maintained a specific capacity of 608.4 mAh g -1 after 160 cycles at 0.5 A g -1 , while Si@CNF-1 only had 299.5 mAh g -1 . This shows that Si / SWCNT@CNF-1 has better electrochemical performance, attributed to its continuous and uniform core-shell structure. The addition of single-walled carbon nanotubes can well fix the Si particles in the carbon fiber tube, avoiding direct contact with the electrolyte. At the same time, the single-walled carbon nanotubes and the core-shell structure can further alleviate the volume expansion of Si.

[0088] We randomly selected five identical electrodes from each of the two samples, Si@CNF-1 and Si / SWCNT@CNF-1, and tested their conductivity with a four-probe conductivity meter. It was found that the conductivity of the five electrodes of Si@CNF-1 fluctuated greatly, while that of Si / SWCNT@CNF-1 had no fluctuation at all (Table 1), indicating that adding SWCNT helps to form a uniform conductive network for the electrode.

[0089] Table 1 Comparison of conductivity test results of self-supporting electrode sheets (unit: S cm -1 )

[0090] Pole piece 1 2 3 4 5 Average value Si / SWCNT@CNF-1 0.6667 0.6667 0.6667 0.6667 0.6667 0.6667 Si@CNF-1 0.667 0.6250 0.5882 0.6667 0.6250 0.6344

[0091] As described above, these are only several embodiments of the present application and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art, without departing from the scope of the technical solution of the present application, making some changes or modifications using the disclosed technical content above is equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

Claims

1. A silicon-based carbon fiber negative electrode sheet, characterized in that: The silicon-based carbon fiber negative electrode sheet comprises carbon fiber, silicon particles and a single dispersed oxidized single-walled carbon nanotube of non-tube bundle type; The silicon particles are embedded in the carbon fibers; The non-bundled single dispersed oxidized single-walled carbon nanotubes are uniformly distributed inside the carbon fibers and shuttle between the silicon particles; The carbon fibers are interwoven with each other to form a self-supporting flexible pole piece.

2. The silicon-based carbon fiber negative electrode sheet according to claim 1, characterized in that: The silicon particles are selected from one or both of pure silicon particles and silicon monoxide particles.

3. The silicon-based carbon fiber negative electrode sheet according to claim 1, characterized in that: The surface of the non-bundled single dispersed oxidized single-walled carbon nanotube contains oxygen-containing functional groups; The oxygen-containing functional groups include hydroxyl, carboxyl and epoxy groups.

4. The silicon-based carbon fiber negative electrode sheet according to claim 1, characterized in that: The content of the non-bundled single dispersed oxidized single-walled carbon nanotubes relative to the silicon particles is 1 to 5 wt.%; Preferably, in the silicon-based carbon fiber negative electrode sheet, the content of the silicon particles is 30 to 55 wt.%.

5. The method for preparing the silicon-based carbon fiber negative electrode sheet according to any one of claims 1 to 4, characterized in that: The steps include: (a1) uniformly dispersing non-bundled single dispersed oxidized single-walled carbon nanotubes in N,N-dimethylformamide solvent to form a uniform and stable dispersion solution, then dispersing Si particles in the above dispersion solution to form a stable dispersion solution, and then adding polyacrylonitrile to completely dissolve it in the dispersion solution to obtain a uniform and stable dispersion precursor; (b1) electrospinning the above dispersion precursor to obtain a nanofiber cloth; (c1) The nanofiber cloth is pre-oxidized in air and then annealed and carbonized at high temperature to obtain the silicon-based carbon fiber negative electrode sheet.

6. The preparation method according to claim 5, characterized in that: In the step (a1), the method for preparing the non-bundled single dispersed oxidized single-walled carbon nanotubes comprises: Potassium permanganate is first added to an excess of concentrated sulfuric acid solution for complete reaction, and then the original single-walled carbon nanotube ash is added to the above solution, and after stirring evenly, the obtained reaction mixture is stirred for reaction at a temperature of 50-60°C for 2-3 hours, and then the reaction mixture is diluted with water, and then an excess of hydrogen peroxide solution is added to reduce and remove the manganese dioxide solid generated in the reaction process, and after filtering and washing with an acid solution, the obtained solid is washed with water to remove some impurity ions in the solution, and washed with water until the pH of the single-walled carbon nanotube mixed aqueous solution is 4-5, centrifuged at 5000-14000 rpm, and then the upper uniform solution is taken out, and then the uniform solution is filtered, washed with water and collected to obtain a black solid.

7. The preparation method according to claim 5, characterized in that: In the step (a1), the content of the non-bundled single dispersed oxidized single-walled carbon nanotubes relative to the silicon particles is 1 to 5 wt.%; Preferably, in the step (a1), the concentration of polyacrylonitrile relative to N,N-dimethylformamide is 10 to 15 wt.%; Preferably, in the step (a1), the mass ratio of the silicon particles to polyacrylonitrile is 0.2-0.8:

1.

8. The preparation method according to claim 5, characterized in that: In the step (b1), the electrospinning is uniaxial electrospinning, and the injection speed is 1 to 1.5 mL·h -1 , the applied voltage is 12-15 kV; Preferably, in the step (c1), the pre-oxidation treatment is carried out at a constant temperature of 260 to 300° C. for 2 to 4 hours; Preferably, in the step (c1), the high temperature annealing and carbonization comprises: annealing at 600-1000° C. for 0.5-12 h in an inert atmosphere or in an airtight state.

9. Application of the silicon-based carbon fiber negative electrode sheet according to any one of claims 1 to 4 or the silicon-based carbon fiber negative electrode sheet prepared by the preparation method according to any one of claims 5 to 8 in an electrode; Preferably, the electrode is selected from the group consisting of a primary electrochemical generator, a secondary electrochemical generator, and a high-energy generator.

10. A secondary battery, characterized in that: The negative electrode sheet of the secondary battery comprises the silicon-based carbon fiber negative electrode sheet according to any one of claims 1 to 4 or the silicon-based carbon fiber negative electrode sheet prepared by the preparation method according to any one of claims 5 to 8; Preferably, the secondary battery is a lithium-ion battery.

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