A porous flexible self-supporting carbon nanofiber membrane material, its preparation method and application

Abstract

This invention discloses a porous flexible self-supporting carbon nanofiber membrane material, its preparation method, and its application. The preparation method includes the following steps: dissolving an acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile, a pore-forming agent, a pH adjuster, and a titanium source in an organic solvent, followed by electrospinning to obtain a precursor fiber membrane; pre-oxidizing the precursor fiber membrane in an air atmosphere to obtain a pre-oxidized fiber membrane; and carbonizing the pre-oxidized fiber membrane in a nitrogen atmosphere to obtain the porous flexible self-supporting carbon nanofiber membrane material. The porous flexible self-supporting carbon nanofiber membrane material provided by this invention can be directly used as an independent negative electrode for sodium-ion batteries, eliminating the need for conductive agents and binders, thus simplifying the electrode preparation process. It also possesses greater sodium storage capacity, good reversibility, and excellent structural stability, while combining lightweight and flexible characteristics, which is beneficial for promoting the practical development of high-performance sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of carbon nanofiber membrane materials technology, specifically to a porous flexible self-supporting carbon nanofiber membrane material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries (SIBs) have shown great promise for large-scale energy storage due to the abundance and low cost of sodium resources and their similar electrochemical properties to lithium. However, the development of sodium-ion batteries still faces many technical bottlenecks. Sodium ions (Na...) + The large ionic radius of the ions results in slow diffusion kinetics in electrode materials, posing extremely high requirements for the structural design and performance optimization of electrode materials. Currently used anode materials each have their drawbacks: traditional carbon-based materials, such as hard carbon and soft carbon, while possessing good conductivity and stability, have limited theoretical specific capacity, making it difficult to meet the demands of high energy density; titanium-based materials, such as TiO2, although structurally stable and with low volume expansion, have extremely low intrinsic conductivity, severely limiting the rate performance and cycle life of the battery. Furthermore, while existing technologies using flexible self-supporting anodes prepared by electrospinning simplify the process and avoid the use of binders, they suffer from weak interfacial bonding between TiO2 and the carbon matrix and insufficient active sites, resulting in actual capacities far lower than theoretical values.

[0003] To improve the performance of sodium-ion battery anode materials, researchers have conducted extensive work. For example, by combining electrospinning technology with doping strategies, phosphorus or nitrogen elements have been introduced into carbon-based materials, which has improved the conductivity and structural stability of the materials to some extent. Chinese patent CN106711410A uses a phosphorus source to co-spin with a carbon-containing polymer, followed by vacuum drying and high-temperature treatment to prepare phosphorus-doped nanofibers, thereby increasing the carbon interlayer spacing and forming stable bonds to improve battery performance. Chinese patent CN114361450A prepares nitrogen-doped porous carbon fibers through electrospinning, utilizing the porous structure and flexible fiber network to optimize material performance. However, these single-element doping methods still have drawbacks such as limited capacity improvement and insufficient long-term cycle stability, and lack synergistic optimization of composite doping and microstructure, making it difficult to achieve a breakthrough improvement in the performance of anode materials. Summary of the Invention

[0004] To address the problems of insufficient specific capacity, poor conductivity, and unsatisfactory cycle stability in existing sodium-ion battery anode materials, the present invention aims to provide a porous flexible self-supporting carbon nanofiber membrane material, its preparation method, and its application. Based on electrospinning and high-temperature carbonization technology, a synergistic strategy of dual-element (N / Zn) doping and acidic carbon dots (CDs) interface modification is proposed to construct a porous TiO2 / carbon composite flexible self-supporting carbon nanofiber membrane material, which can be used as a sodium-ion battery anode material for assembling sodium-ion batteries.

[0005] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0006] The first aspect of this invention provides a method for preparing a porous flexible self-supporting carbon nanofiber membrane material, comprising the following steps:

[0007] (1) An acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile (PAN), a pore-forming agent, a pH adjuster, and a titanium source are dissolved in an organic solvent. The resulting electrospinning solution is electrospinned at a spinning voltage of 15-25 kV to obtain a precursor fiber membrane. The pH value of the electrospinning solution is 3.5-4.5, and the mass of the acidic carbon dot solution accounts for 15-20% of the mass of the electrospinning solution. The concentration of ZIF-8 particles in the electrospinning solution is 0.1-1 wt%.

[0008] (2) In an air atmosphere, the precursor fiber membrane is pre-oxidized at 250-300 °C to obtain a pre-oxidized fiber membrane;

[0009] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane is carbonized at 700-800 °C to obtain the porous flexible self-supporting carbon nanofiber membrane material.

[0010] This invention prepares a flexible, self-supporting anode material based on acidic CDs modified nano-TiO2 particles / carbon matrix using electrospinning and high-temperature carbonization techniques. Building upon single-atom doping (utilizing the nitrogen environment during high-temperature pyrolysis), Zn atoms are introduced into the TiO2 lattice using ZIF-8 as the Zn source, resulting in increased interlayer spacing, which is beneficial for Na... + Intercalation reactions can be carried out, and the point defects (oxygen vacancies, lattice defects, etc.) in TiO2 can be controlled by changing their content. In addition, CDs are introduced by forming Ti-OC bonds between the functional groups (-COOH, -OH, etc.) on CDs and nano-TiO2 particles. This not only improves the conductivity of carbon nanofiber membrane materials, promotes sodium ion transport, increases porosity and active sites, but also enhances interfacial bonding and extends cycle life.

[0011] Further, in step (1), the concentration of carbon dots in the acidic carbon dot solution is 15-20 wt%.

[0012] Further, in step (1), the size of the carbon dots in the acidic carbon dot solution is 2-5 nm.

[0013] Furthermore, in step (1), the carbon dots in the acidic carbon dot solution can be citric acid-based carbon dots.

[0014] Further, in step (1), the carboxyl group density of the carbon dots in the acidic carbon dot solution is 0.5-1.2 mmol / g.

[0015] Preferably, in step (1), the concentration of ZIF-8 particles in the electrospinning solution is 0.5-1 wt%.

[0016] After introducing ZIF-8 particles, the fibers became brittle after high-temperature calcination, and fractures occurred between the carbon fibers. As the doping concentration increased to 0.5 wt%, the fiber fractures significantly decreased, the fiber diameter became more uniform, the TiO2 particles and pores became more prominent, and the fibers showed better orientation. However, as the ZIF-8 doping concentration continued to increase, the fibers became tortuous and disordered, which was detrimental to Na… + After diffusion and doping concentrations exceeding 1 wt%, the lattice distortion of nano-TiO2 particles leads to uneven fiber thickness and uneven internal stress distribution, making stress concentration at defects prone to occur, resulting in fracture. Simultaneously, appropriate N doping increases defects in TiO2; therefore, doping TiO2 carbon nanofiber films with an appropriate concentration of ZIF-8 can generate a higher degree of defects than single-atom doping, which is more conducive to Na… + De-embedded.

[0017] Further, in step (1), the concentration of PAN in the electrospinning solution is 2-12 wt%.

[0018] Further, in step (1), the pore-forming agent is one or more of styrene-acrylonitrile copolymer (SAN), polyvinylpyrrolidone (PVP), polystyrene (PS), and polymethyl methacrylate (PMMA), preferably SAN and PVP.

[0019] Further, in step (1), when the pore-forming agent is SAN and PVP, the mass ratio of SAN to PVP is (0.5-3):1.

[0020] Further, in step (1), the concentration of the pore-forming agent in the electrospinning solution is 3-15 wt%.

[0021] Further, in step (1), the pH adjuster is selected from one or more of acetic acid, formic acid, citric acid and dilute nitric acid, preferably glacial acetic acid.

[0022] pH adjusters can slow down the hydrolysis rate of titanium sources, avoid the formation of TiO2 agglomerates due to rapid hydrolysis, and ensure the uniformity of the spinning solution; they can also prevent TBOT from absorbing moisture, as the solution becomes too viscous after TBOT absorbs moisture, which is not conducive to electrospinning.

[0023] Furthermore, the concentration of the pH adjuster in the electrospinning solution is 1-5 wt%.

[0024] Further, in step (1), the titanium source is tetrabutyl titanate (TBOT) or titanium tetrachloride (TiCl4).

[0025] Further, in step (1), the concentration of the titanium source in the electrospinning solution is 3-10 wt%.

[0026] Further, in step (1), the organic solvent is selected from one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) and tetrahydrofuran (THF), preferably DMF.

[0027] Further, in step (1), the conditions for electrospinning are: using an 18-22 G needle, a receiving distance of 10-20 cm, a temperature of 25-30 ℃, a humidity of 30%-60%, and an electrospinning solution flow rate of 0.5-1.5 mL / h.

[0028] Further, in step (1), the process parameters for electrospinning are: the viscosity of the electrospinning solution is 2000-3000 cP, and the rotation speed of the fiber collecting roller is 400-600 rpm.

[0029] Furthermore, in step (1), the areal density of the precursor fiber membrane is 1.5-2.5 mg / cm³. 2 .

[0030] In step (2), the pre-oxidation treatment enables the precursor fiber membrane to form a stable cross-linked structure. During this stage, the pore-forming agent undergoes pyrolysis, such as SAN and PVP, which pyrolyze to generate micropores and mesopores, respectively. At the same time, the organic ligands in the acid-dissociated ZIF-8 particles are further decomposed, partially generating porous carbon materials, forming a high specific surface area, providing more active sites for SIBs, reducing charge transfer resistance, facilitating the penetration of electrolyte ions, and ensuring rapid ion transport.

[0031] Further, in step (2), the pre-oxidation treatment specifically involves placing the precursor fiber membrane in a heating device and heating it to 250-300 ℃ at a heating rate of 1-5 ℃ / min and holding it at that temperature for 1-3 h.

[0032] In step (3), the carbonization treatment causes the pre-oxidized fiber membrane to form a porous carbon-based framework. The diameter of the fibers in the porous carbon-based framework is 200-500 nm, and the BET specific surface area is 300-400 m². 2 / g, the fibers form a continuous conductive network through carbonization cross-linking.

[0033] In step (3), the titanium source is carbonized in an inert atmosphere to generate nano-TiO2 particles with a particle size of 10-50 nm.

[0034] Further, in step (3), the carbonization treatment specifically involves placing the pre-oxidized fiber membrane in a heating device and heating it to 700-800 ℃ at a heating rate of 1-5 ℃ / min and holding it at that temperature for 2-3 h.

[0035] The porous flexible self-supporting carbon nanofiber membrane material prepared by this invention contains numerous micropores and mesopores, resulting in a large specific surface area, which is beneficial for the storage of active sites and reduces Na+ content. + The diffusion distance is increased, improving electrochemical performance. Its pore structure mainly consists of: micropores (<2 nm) accounting for 30-40%, formed by SAN pyrolysis; mesopores (2-50 nm) accounting for 50-60%, formed by PVP pyrolysis; and macropores (>50 nm) accounting for 10-20%, formed by electrospun fiber stacking.

[0036] The second aspect of the present invention provides a porous flexible self-supporting carbon nanofiber membrane material prepared by the preparation method described in the first aspect.

[0037] The porous flexible self-supporting carbon nanofiber membrane material provided by this invention exhibits flexible self-supporting characteristics, with a bending strength >50 MPa and a structural integrity retention rate >95% after 10,000 bending cycles.

[0038] Furthermore, the loading of CDs in the porous flexible self-supporting carbon nanofiber membrane material is 2.5-5 wt%.

[0039] Furthermore, the Zn doping concentration in the porous flexible self-supporting carbon nanofiber membrane material is 0.1-0.5 wt%.

[0040] Furthermore, the doping concentration of nitrogen element in the porous flexible self-supporting carbon nanofiber membrane material is 1-3 wt%.

[0041] The third aspect of this invention provides an application of the porous flexible self-supporting carbon nanofiber membrane material described in the second aspect in the assembly of sodium-ion batteries, which is suitable for high-energy-density sodium-ion battery systems.

[0042] The porous flexible self-supporting carbon nanofiber membrane material provided by this invention can be used as the negative electrode of sodium-ion batteries. When sodium ions are transported, the porous framework shortens the diffusion path, the dual-element doping reduces the ion migration barrier, and the carbon point bonding inhibits particle aggregation, thus synergistically achieving high specific capacity, ultra-long cycle stability and excellent rate performance of sodium-ion batteries.

[0043] The beneficial effects of this invention are:

[0044] 1. In the porous flexible self-supporting carbon nanofiber membrane material provided by the present invention, the porous carbon matrix provides a three-dimensional conductive network and forms a gradient pore structure through a dual-mode pore-forming system. Micropores provide sodium storage active sites, mesopores serve as ion transport channels, and macropores alleviate volume expansion. By adopting Ti-OC chemical bonding interface engineering, strong interactions are formed between carbon dots and TiO2 to inhibit the shedding of active materials. After 10,000 cycles, the fiber morphology integrity is >90%, which significantly improves the cycling stability.

[0045] 2. The porous flexible self-supporting carbon nanofiber membrane material provided by this invention can be used as the negative electrode of sodium-ion batteries. Zn doping into the TiO2 lattice introduces additional electrons, which helps improve the electrode's charge transport rate. N and Zn doping improves the structural stability of TiO2, reducing electrode degradation and electrochemical performance decay during cycling, thus maintaining high specific capacity. Furthermore, diatomic doping in the TiO2 lattice forms oxygen vacancies and heteroatom doping defects. These defects can adjust the material's band structure, narrow the band gap of TiO2, and lower the energy barrier in the charge transport path, making it easier for charges to move within the electrode. This increases the number of electrochemical active sites, enhances the reversibility of electrochemical reactions, and helps improve specific capacity. Strong chemical bonds are formed between the surface functional groups of CDs and TiO2, enhancing interfacial bonding. The synergistic gradient pore structure suppresses volume expansion, significantly improving sodium ion diffusion kinetics. Ultimately, this results in a carbon nanofiber membrane material with greater sodium storage capacity, good reversibility, and excellent structural stability.

[0046] 3. The porous flexible self-supporting carbon nanofiber membrane material provided by this invention can be directly used as an independent negative electrode in sodium-ion batteries, eliminating the need for conductive agents and binders, thus simplifying the electrode fabrication process. Furthermore, its flexible self-supporting structure eliminates the need for traditional metal supports, increasing the areal capacity by 30-50%, providing a novel electrode design solution for the development of flexible energy storage devices. The porous flexible self-supporting carbon nanofiber membrane material, as a negative electrode, achieves a capacity of 0.1 A·g -1 At current density, the specific capacity can reach 304.5 mAh·g. -1 ; in 2 A·g -1 At the specified current density, the specific capacity can reach 148.8 mAh·g. -1 Porous, flexible, self-supporting carbon nanofiber membrane materials exhibit ultra-long cycle life as a negative electrode. During 1000 long cycles at a current density of 1 A·g-1, the membrane life remains exceptionally long. -1 At that time, it can reach 262.5 mAh·g -1 Reversible specific capacity; at a current density of 5 A·g -1 Even after 10,000 long cycles, it still exhibits 120.8 mAh·g. -1The reversible specific capacity has a capacity retention rate (based on the first cycle capacity) of up to 200.9%; the porous flexible self-supporting carbon nanofiber membrane material, as a flexible self-supporting electrode, still maintains its complete shape after 10,000 long cycles. This carbon nanofiber membrane material has both lightweight and flexible characteristics, which is conducive to promoting the practical development of high-performance sodium-ion batteries. Attached Figure Description

[0047] Figure 1 Scanning electron microscope (SEM) images of the prepared porous flexible self-supporting carbon nanofiber membrane materials and the carbon nanofiber membrane materials prepared in Comparative Examples 1-3 are shown below. (a) is the SEM image of the PCF-1-2.5 carbon nanofiber membrane (scale bar: 5 µm), (b) is the SEM image of the PCF-1-2.5 carbon nanofiber membrane (scale bar: 500 nm), (c) is the SEM image of the CCF-1-2.5 carbon nanofiber membrane (scale bar: 5 µm), (d) is the SEM image of the CCF-1-2.5 carbon nanofiber membrane (scale bar: 500 nm), (e) is the SEM image of the CF-1-2.5 carbon nanofiber membrane (scale bar: 5 µm), (f) is the SEM image of the CF-1-2.5 carbon nanofiber membrane (scale bar: 500 nm), and (g) is the SEM image of the CF-2-2.5 carbon nanofiber membrane (scale bar: 5 µm). (h) is a SEM image of CF-2-2.5 carbon nanofiber membrane (scale bar is 500 nm), (i) is a SEM image of F-1-2.5 carbon nanofiber membrane (scale bar is 5 µm), and (j) is a SEM image of F-1-2.5 carbon nanofiber membrane (scale bar is 500 nm).

[0048] Figure 2 The images shown are transmission electron microscope (TEM) images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; where (a) is the TEM image and (b) is the corresponding high-resolution image.

[0049] Figure 3 The images shown are high-resolution transmission electron microscopy (HRTEM) images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; where (a) is an HRTEM image, (b) is C element, (c) is N element, (d) is O element, (e) is Ti element, and (f) is Zn element.

[0050] Figure 4 The images show the X-ray diffraction (XRD) patterns of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1, and the carbon nanofiber membrane materials prepared in Comparative Examples 3 and 4.

[0051] Figure 5The images show the physical images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; where (a) is the unfolded image, (b) is the folded image, (c) is the entangled image, and (d) is the coiled image.

[0052] Figure 6 For the CF-1-2.5 electrode at a current density of 200 mA·g -1 Constant current charge-discharge curves for cycles 2, 5, 10, 50, 100, and 200.

[0053] Figure 7 For CF-1-2.5 and CF-2-2.5 electrodes at 0.1, 0.2, 0.5, 1, and 2 A·g -1 and restored to 0.1 A·g -1 The magnification plots and electrochemical impedance spectroscopy (EIS) results of the CF-1-2.5, CF-2-2.5, and F-1-2.5 electrodes are shown; among them, (a) shows the magnification plots of the CF-1-2.5 electrode at 0.1, 0.2, 0.5, 1, and 2 A·g. -1 and restored to 0.1 A·g -1 The magnification diagram, (b) shows the CF-2-2.5 electrode at 0.1, 0.2, 0.5, 1, and 2 A·g. -1 and restored to 0.1 A·g -1 The magnification diagrams are shown in (c), which is the EIS test result diagram of the CF-1-2.5 electrode, the CF-2-2.5 electrode and the F-1-2.5 electrode, and (d) is a partial magnified view of (c).

[0054] Figure 8 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 1 A·g -1 Figure 1 shows the results of a long-cycle stability test of 1000 cycles; where (a) is the CF-1-2.5 electrode and (b) is the CF-2-2.5 electrode.

[0055] Figure 9 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 Figure 1 shows the results of a long-cycle stability test of 10,000 cycles; where (a) is the CF-1-2.5 electrode and (b) is the CF-2-2.5 electrode.

[0056] Figure 10 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1SEM images after 10,000 long-cycle stability tests are shown below; (a) is the SEM image of the CF-1-2.5 electrode, (b) is a magnified view of (a), (c) is the SEM image of the CF-2-2.5 electrode, and (d) is a magnified view of (c). Detailed Implementation

[0057] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0058] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0059] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0060] Example 1

[0061] A method for preparing a porous flexible self-supporting carbon nanofiber membrane material includes the following steps:

[0062] (1) 2.5 mL of acidic carbon dot solution with pH=2 (the concentration of carbon dots in the acidic carbon dot solution is 17.2 wt%) was stirred with 9 g of DMF until homogeneous. 0.5 g of PAN and 0.5 g of PVP were added to the solution. After stirring at room temperature for 3 h to form a homogeneous solution, 0.34 mL of glacial acetic acid was added dropwise to adjust the pH of the solution to 4 (the carbon dots in the solution at this time were named CDs-1). After stirring for 4 h, 0.5 g of TBOT and 0.25 g of SAN were added and stirred vigorously at 40 ℃ for 12 h. Then ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution is 0.5 wt%) were added to form a homogeneous solution. The solution was sonicated for 2 h to obtain the electrospinning solution, which was named F-1-2.5 solution (2.5 refers to the amount of acidic carbon dot solution added). The F-1-2.5 solution was injected into an 18 G needle, and electrospinning was carried out under the following conditions: spinning voltage of 16 kV, receiving distance of 16 cm, temperature of 25 ℃, humidity of 50%, electrospinning solution flow rate of 1 mL / h, electrospinning solution viscosity of 2500 cP, and fiber collecting roller speed of 500 rpm. After spinning, the precursor fiber membrane was dried at 60 ℃ for 3 h to obtain the precursor fiber membrane.

[0063] (2) In an air atmosphere, the precursor fiber membrane is placed in a muffle furnace and heated to 250 °C at a heating rate of 2 °C / min for 2 h for pre-oxidation treatment to form a stable cross-linked structure and obtain the pre-oxidized fiber membrane.

[0064] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane is placed in a tube furnace and heated to 700 °C at a heating rate of 2.5 °C / min for 2 h to obtain the porous flexible self-supporting carbon nanofiber membrane material, named CF-1-2.5 carbon nanofiber membrane.

[0065] Example 2

[0066] A method for preparing a porous flexible self-supporting carbon nanofiber membrane material is basically the same as that in Example 1, except that: in step (1), the concentration of ZIF-8 particles in the electrospinning solution is 0.1 wt%, and the porous flexible self-supporting carbon nanofiber membrane material is named CF-1-2.5-0.1 carbon nanofiber membrane.

[0067] Example 3

[0068] A method for preparing a porous flexible self-supporting carbon nanofiber membrane material is basically the same as that in Example 1, except that: in step (1), the concentration of ZIF-8 particles in the electrospinning solution is 1 wt%, and the carbon nanofiber membrane material finally obtained is named CF-1-2.5-1 carbon nanofiber membrane.

[0069] Example 4

[0070] A method for preparing a porous flexible self-supporting carbon nanofiber membrane material includes the following steps:

[0071] (1) 3 mL of acidic carbon dot solution with pH=2 (the concentration of carbon dots in the acidic carbon dot solution is 17.2 wt%) was stirred with 10 g DMF. 1 g PAN and 1 g PVP were added to the solution. After stirring at room temperature for 3 h to form a homogeneous solution, glacial acetic acid was added dropwise to adjust the pH of the solution to 4. After stirring for 4 h, 0.8 g TBOT and 1.5 g SAN were added. The solution was stirred vigorously at 40 ℃ for 12 h. Then ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution is 0.5 wt%) were added to form a homogeneous solution. The solution was sonicated for 2 h to obtain the electrospinning solution, which was named F-1-3 solution. Electrospinning solution was injected into a 22 G needle, and electrospinning was carried out under the following conditions: spinning voltage of 25 kV, receiving distance of 10 cm, temperature of 30 ℃, humidity of 30%, electrospinning solution flow rate of 1.5 mL / h, electrospinning solution viscosity of 3000 cP, and fiber collection roller speed of 400 rpm. After spinning, the precursor fiber membrane was dried at 60 ℃ for 3 h to obtain the precursor fiber membrane.

[0072] (2) In an air atmosphere, the precursor fiber membrane is placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min for 3 h for pre-oxidation treatment to form a stable cross-linked structure and obtain the pre-oxidized fiber membrane.

[0073] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane is placed in a tube furnace and heated to 800 °C at a heating rate of 4 °C / min for 3 h to obtain the porous flexible self-supporting carbon nanofiber membrane material, named CF-1-3 carbon nanofiber membrane.

[0074] Comparative Example 1

[0075] A method for preparing a carbon nanofiber membrane material includes the following steps:

[0076] (1) Dissolve 0.5 g PAN and 0.5 g PVP in 9 g DMF, stir at room temperature for 3 h to form a homogeneous solution, then add 0.34 mL glacial acetic acid dropwise, stir for 4 h, then add 0.5 g TBOT and 0.25 g SAN, and stir vigorously at 40 ℃ for 12 h; then add ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution is 0.5 wt%) to form a homogeneous solution, and sonicate for 2 h to obtain the electrospinning solution. Inject the electrospinning solution into an 18 G needle, and perform electrospinning under the following conditions: spinning voltage of 16 kV, receiving distance of 16 cm, temperature of 25 ℃, humidity of 50%, electrospinning solution flow rate of 1 mL / h, electrospinning solution viscosity of 2500 cP, and fiber collecting roller speed of 500 rpm. After spinning, dry at 60 ℃ for 3 h to obtain the precursor fiber membrane.

[0077] (2) In an air atmosphere, the precursor fiber membrane was placed in a muffle furnace and heated to 250 °C at a heating rate of 2 °C / min for 2 h for pre-oxidation treatment to form a stable cross-linked structure, thus obtaining the pre-oxidized fiber membrane. The pre-oxidized fiber membrane was then soaked in an acidic carbon dot solution with pH=4 (the concentration of carbon dots in the acidic carbon dot solution was 17.2 wt%, the original pH of the acidic carbon dot solution was 2, and glacial acetic acid was added to adjust the pH to 4) and dried in a vacuum oven at 60 °C for 3 h to obtain the CDs-treated pre-oxidized membrane.

[0078] (3) In a nitrogen atmosphere, the pre-oxidized membrane treated with CDs was placed in a tube furnace and heated to 700 °C at a heating rate of 2.5 °C / min for 2 h to obtain carbon nanofiber membrane material, named PCF-1-2.5 carbon nanofiber membrane.

[0079] Comparative Example 2

[0080] A method for preparing a carbon nanofiber membrane material includes the following steps:

[0081] (1) Dissolve 0.5 g PAN and 0.5 g PVP in 9 g DMF, stir at room temperature for 3 h to form a homogeneous solution, then add 0.34 mL glacial acetic acid dropwise, stir for 4 h, then add 0.5 g TBOT and 0.25 g SAN, and stir vigorously at 40 ℃ for 12 h; then add ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution is 0.5 wt%) to form a homogeneous solution, and sonicate for 2 h to obtain the electrospinning solution. Inject the electrospinning solution into an 18 G needle, and perform electrospinning under the following conditions: spinning voltage of 16 kV, receiving distance of 16 cm, temperature of 25 ℃, humidity of 50%, electrospinning solution flow rate of 1 mL / h, electrospinning solution viscosity of 2500 cP, and fiber collecting roller speed of 500 rpm. After spinning, dry at 60 ℃ for 3 h to obtain the precursor fiber membrane.

[0082] (2) In an air atmosphere, the precursor fiber membrane is placed in a muffle furnace and heated to 250 °C at a heating rate of 2 °C / min for 2 h for pre-oxidation treatment. After forming a stable cross-linked structure, the pre-oxidized fiber membrane is obtained.

[0083] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tube furnace and heated to 700 °C at a heating rate of 2.5 °C / min for 2 h for carbonization treatment. The obtained fiber membrane was soaked in an acidic carbon dot solution with pH=4 (the concentration of carbon dots in the acidic carbon dot solution was 17.2 wt%, the original pH of the acidic carbon dot solution was 2, and glacial acetic acid was added to adjust the pH to 4) and dried in a vacuum oven at 60 °C for 3 h to obtain carbon nanofiber membrane material, named CCF-1-2.5 carbon nanofiber membrane.

[0084] Comparative Example 3

[0085] A method for preparing a carbon nanofiber membrane material is basically the same as that in Example 1, except that: in step (1), glacial acetic acid is added dropwise to adjust the pH of the solution to 3 (the carbon dots in the solution at this time are named CDs-2), and the carbon nanofiber membrane material finally obtained is named CF-2-2.5 carbon nanofiber membrane.

[0086] Comparative Example 4

[0087] A method for preparing a carbon nanofiber membrane material includes the following steps:

[0088] (1) Dissolve 0.5 g PAN and 0.5 g PVP in 9 g DMF, stir at room temperature for 3 h to form a homogeneous solution, then add 0.34 mL glacial acetic acid dropwise, stir for 4 h, then add 0.5 g TBOT and 0.25 g SAN, and stir vigorously at 40 ℃ for 12 h; then add ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution is 0.5 wt%) to form a homogeneous solution, and sonicate for 2 h to obtain the electrospinning solution. Inject the electrospinning solution into an 18 G needle, and perform electrospinning under the following conditions: spinning voltage of 16 kV, receiving distance of 16 cm, temperature of 25 ℃, humidity of 50%, electrospinning solution flow rate of 1 mL / h, electrospinning solution viscosity of 2500 cP, and fiber collecting roller speed of 500 rpm. After spinning, dry at 60 ℃ for 3 h to obtain the precursor fiber membrane.

[0089] (2) In an air atmosphere, the precursor fiber membrane is placed in a muffle furnace and heated to 250 °C at a heating rate of 2 °C / min for 2 h for pre-oxidation treatment to form a stable cross-linked structure and obtain the pre-oxidized fiber membrane.

[0090] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tube furnace and heated to 700 °C at a heating rate of 2.5 °C / min for 2 h to obtain carbon nanofiber membrane material, named F-1-2.5 carbon nanofiber membrane.

[0091] Comparative Example 5

[0092] A method for preparing a carbon nanofiber membrane material is basically the same as that in Example 1, except that in step (1), ZIF-8 particles are replaced with MOF-5, and the carbon nanofiber membrane material is finally named CF-1-2.5-MOF carbon nanofiber membrane.

[0093] Comparative Example 6

[0094] A method for preparing a carbon nanofiber membrane material is basically the same as that in Example 1, except that: in step (1), the concentration of ZIF-8 particles in the electrospinning solution is 2 wt%, and the carbon nanofiber membrane material finally obtained is named CF-1-2.5-2 carbon nanofiber membrane.

[0095] Test Example 1

[0096] The morphology of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane materials prepared in Comparative Examples 1-4 were characterized. Figure 1SEM images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane materials prepared in Comparative Examples 1-4 are shown. Acidic carbon nanofibers (CDs) can serve as self-templates for pore formation. They are rich in functional groups such as carboxyl groups (-COOH) and hydroxyl groups (-OH). Some of these functional groups decompose under high temperature, generating gas and leading to the formation of porous structures. Figure 1 As shown in (a) and (b), the PCF-1-2.5 carbon nanofiber membrane still maintains a distinct fibrous structure, with increased pores on the fiber surface, but no obvious nano-TiO2 particles; Figure 1 In (c) and (d), the CCF-1-2.5 carbon nanofiber membrane has more pores on its surface, and the fiber surface is rougher; from Figure 1 As can be seen in (e) and (f), the CF-1-2.5 carbon nanofiber membrane becomes finer, exhibits more pores, increases specific surface area, and improves fiber surface roughness. This indicates that adding CDs-1 to the precursor solution can control the fiber thickness, and similarly generates pores and a rough fiber surface. Figure 1 As can be seen from (g) and (h), the morphology of the CF-2-2.5 carbon nanofiber membrane undergoes certain changes, forming "pod-like" spherical protrusions, while the fiber surface remains rough and uneven. This is because the addition of CDs with different acidities alters the acidic environment of the solution. Under strongly acidic spinning conditions, this disrupts the material's stability, leading to cross-linking of the polymer molecular chains and affecting the viscosity of the polymer solution, thus resulting in excessively thick fibers. Figure 1 As can be seen in (i) and (j), the F-1-2.5 carbon nanofiber membrane without the addition of acidic CDs exhibits a continuous and uniform three-dimensional network structure. Therefore, adding CDs of different acidities to the precursor solution can significantly alter the morphology and structure of the fibers, increase surface roughness and porosity, and increase active sites, thereby affecting the performance of the electrode.

[0097] Figure 2 This is a TEM image of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. From... Figure 2 In (b), distinct lattice fringes are visible. The lattice fringes on the left are 0.34 nm, corresponding to the (002) crystal plane of CDs; the lattice fringes on the right are 0.35 nm, corresponding to the (101) crystal plane of anatase TiO2. At the same time, a disordered transition region appears at the interface between the two, which indicates that new chemical bonds may be formed between them, resulting in a two-phase product.

[0098] Figure 3 This is an HRTEM image of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. From... Figure 3 As can be seen in (a), the CF-1-2.5 nanofibers are of uniform thickness and have a rough surface; from Figure 3As can be seen from the elemental distribution diagrams (b)-(f), C, N, O, Ti and Zn are uniformly distributed in the CF-1-2.5 nanofibers. This indicates that the carbonized CF-1-2.5 nanofiber membrane not only generated TiO2, but also successfully doped with Zn and N.

[0099] Figure 4 The images show the XRD patterns of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1, and the carbon nanofiber membrane materials prepared in Comparative Examples 3 and 4. Figure 4 As can be seen, after the doping of CDs, the peak height of the amorphous carbon intermediate layer (002) of the fiber film decreased, while the peak height of the anatase TiO2 gradually became flatter. This indicates that CDs and TiO2 form new chemical bonds, thereby affecting the broadening of the characteristic peaks of TiO2 and forming more defects. The effect of CF-2-2.5 on the graphite is more pronounced than that on CF-1-2.5, with a flatter peak height. This indicates that the addition of strongly acidic CDs excessively reduces the degree of graphitization, which may lead to a decrease in the conductivity of the electrode.

[0100] The X-ray photoelectron spectroscopy (XPS) spectra of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane material prepared in Comparative Example 3 were tested. Both CF-1-2.5 and CF-2-2.5 carbon nanofiber membranes contain Zn, C, N, O, and Ti elements. The functional groups of different acidic CDs affect the electron cloud density of carbon atoms, resulting in a slight shift in binding energy. The characteristic peak with a binding energy of 458.3 eV is attributed to Ti-OC bonds, which proves that the chemical bond formed between the acidic CDs and the nano-TiO2 particles is a Ti-OC bond. This is consistent with the results of HRTEM and XRD tests. The presence of Ti-OC bonds strengthens the bond between the nano-TiO2 particles and the carbon matrix, improving structural stability. The characteristic peaks of the CF-1-2.5 carbon nanofiber membrane are more prominent and orderly than those of the CF-2-2.5 carbon nanofiber membrane. This is because the -COOH in the weakly acidic CDs can inhibit Zn. 2+ The oxidation state fluctuations of Zn reduce its oxidation tendency, thereby making Zn... 2+ It is easier to incorporate nano-TiO2 particles.

[0101] The specific surface area (S) of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane material prepared in Comparative Example 4 is... BET The pore size distribution data are shown in Table 1:

[0102] Table 1. Specific surface area and pore size distribution data of CF-1-2.5 carbon nanofiber membranes and F-1-2.5 carbon nanofiber membranes.

[0103]

[0104] The BET specific surface area of ​​the CF-1-2.5 carbon nanofiber membrane is 404.49 m². 2 The surface area of ​​the CF-1-2.5 carbon nanofiber membrane increased by 18% compared to that of the F-1-2.5 carbon nanofiber membrane, with an average pore size of 5.66 nm. This indicates that the addition of CDs to the CF-1-2.5 carbon nanofiber membrane increases both the specific surface area and pore size, which is beneficial for shortening the Na+ filtration efficiency. + The diffusion pathway promotes electrolyte wetting, thereby improving electrochemical performance.

[0105] Figure 5 Here is a physical image of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. Figure 5 As can be seen, the CF-1-2.5 carbon nanofiber membrane has a smooth surface without obvious wrinkles or cracks, making it suitable as a self-supporting flexible negative electrode. Using tweezers to hold the fiber membrane and alter its shape by folding, winding, and curling it, the CF-1-2.5 carbon nanofiber membrane maintained its original shape without any signs of breakage after multiple tests, demonstrating excellent mechanical stability.

[0106] Test Example 2

[0107] The porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane materials prepared in Comparative Examples 1-3 were directly perforated into circular sheets (D=16 mm) and used as the negative electrode of the sodium-ion half-cell. The half-cell was assembled in a nitrogen-filled glove box (H2O<0.5 ppm, O2<0.5 ppm), and the battery casing was of type CR2032. The sodium sheet was used as the counter electrode, and the glass fiber membrane (Whatman, GF / D grade) was used as the separator. The electrolyte was a 1 mol / L NaClO4 solution dissolved in a mixed solution of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1, with 5% fluoroethylene carbonate (FEC) added.

[0108] Electrochemical performance tests were conducted on sodium-ion half-cells assembled from the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 and the carbon nanofiber membrane materials prepared in Comparative Examples 1-3. The standard followed was GB / T 44265-2024, and the test conditions were: within a voltage window of 0.01-3 V and 0.2 A·g -1 At a current density of 0.2 A·g, the test was conducted. -1 Discharge specific capacity after 200 cycles and 0.1 A·g after rate charge-discharge. -1 Discharge specific capacity.

[0109] The test results are shown in Table 2:

[0110] Table 2

[0111]

[0112] As shown in Table 2, doping CDs-1 into the precursor solution significantly improves the cycling and rate performance of the electrode. The porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1, used as the negative electrode, operates at a current density of 0.2 A·g⁻¹. -1 At that time, the discharge specific capacity can reach 200.6 mAh·g after 200 cycles. -1 ; after rate charge and discharge, 0.1 A·g -1 The discharge specific capacity reaches 304.5 mAh·g -1 In Comparative Example 5, replacing ZIF-8 with MOF-5 significantly degraded electrode performance (200-cycle capacity of only 130 mAh·g). -1 The reason is that the thermal decomposition temperature of MOF-5 (about 400 °C) is much lower than its carbonization temperature (700-800 °C). Framework collapse occurs during the pre-oxidation stage (250-300 °C), making it impossible to provide an effective Zn doping source.

[0113] Test Example 3

[0114] The porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 was used as an electrode (CF-1-2.5 electrode), the porous flexible self-supporting carbon nanofiber membrane material prepared in Comparative Example 3 was used as an electrode (CF-2-2.5 electrode), and the porous flexible self-supporting carbon nanofiber membrane material prepared in Comparative Example 4 was used as an electrode (F-1-2.5 electrode) to assemble CR2032 button half-cells and test their electrochemical performance. The assembly and testing method was as follows: the carbon nanofiber membrane was stamped into a circular self-supporting electrode with a diameter of 16 mm (no binder / conductive agent required), and encapsulated with a sodium metal counter electrode, a glass fiber separator (Whatman GF / D grade), and an electrolyte (1 mol / L NaClO4 dissolved in DMC / EC / EMC volume ratio 1:1:1 + 5% FEC) in a nitrogen glove box (H2O / O2 < 0.5 ppm). The test voltage range was 0.001-3 V (vs. Na + / Na), the CF-1-2.5 electrode was tested at a current density of 200 mA·g. -1 The constant current charge-discharge curves for cycles 2, 5, 10, 50, 100, and 200 are shown below. Figure 6 As shown, the CF-1-2.5 electrode exhibited 143.1 mAh·g at the 10th cycle. -1 The discharge specific capacity did not increase significantly at first, but gradually increased from the 50th cycle, reaching 200.6 mAh·g at the 200th cycle.-1 On the one hand, as the electrolyte is cycled, it gradually penetrates into the interior of the nanofibers and deep pores, allowing more and more active sites to react; on the other hand, it is due to the high electronic conductivity caused by Zn and N doping; in addition, the combination of CDs with nano-TiO2 particles inhibits the aggregation of nano-TiO2 particles, maintains high ion transport rate, and improves electrochemical performance.

[0115] Figure 7 For CF-1-2.5 and CF-2-2.5 electrodes at 0.1, 0.2, 0.5, 1, and 2 A·g -1 and restored to 0.1 A·g -1 The magnification plot and the electrochemical impedance spectroscopy (EIS) test results of the CF-1-2.5 electrode, CF-2-2.5 electrode, and F-1-2.5 electrode are shown. Figure 7 As can be seen from (a) and (b), at the same current density, the specific capacity of the CF-1-2.5 electrode is higher than that of the CF-2-2.5 electrode. The CF-1-2.5 electrode exhibits higher specific capacity at current densities of 0.1, 0.2, 0.5, 1, and 2 A·g -1 At that time, it can provide excellent discharge specific capacity, which are 304.5, 264.4, 219.7, 184.3 and 148.8 mAh·g, respectively. -1 This demonstrates that the electrode exhibits excellent rate performance. After high-rate cycling, the current density recovered to 0.1 A·g. -1 At that time, the specific capacity reached 265.3 mAh·g -1 This demonstrates that the CF-1-2.5 electrode exhibits excellent reversibility for sodium ion diffusion. In contrast, the CF-2-2.5 electrode, used as a control, showed reversible specific capacities of only 191.7, 178.7, 165.1, 149.6, and 128.3 mAh·g under the same testing conditions. -1 When the current density returns to 0.1 A·g -1 At that time, the specific capacity recovered to 186.4 mAh·g -1 This difference is attributed to the bonding of the carboxyl groups of weakly acidic CDs with nano-TiO2 particles, forming a continuous conductive network; while the strong electron-withdrawing effect of the sulfonic acid groups in strongly acidic CDs leads to uneven surface charge distribution, inhibiting sodium ion adsorption, and thus reducing their specific capacity. Figure 7 As can be seen from (c) and (d), after adding CDs, the polarization resistance (R) of the electrode increases. p The increase is due to the uneven dispersion of some CDs in the carbon matrix, resulting in the formation of local insulating regions that hinder electron transport, leading to an increase in R. p It increases. Although CDs doping leads to an increase in overall R... pThe value increases, but in the CF-1-2.5 electrode, the effective reduction of R due to Ti-OC bonding... p Although the overall R p It increases slightly, but the interfacial contact resistance decreases significantly.

[0116] Figure 8 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 1 A·g -1 The following is a graph showing the results of a 1000-cycle long-term stability test. Figure 8 As shown, firstly, at a current density of 200 mA·g -1 The CF-1-2.5 and CF-2-2.5 electrodes were pre-sodiumed respectively, and then the current density was gradually increased to 1 A·g. -1 The electrode underwent long-cycle testing. After 1000 cycles, the CF-1-2.5 electrode exhibited 262.5 mAh·g. -1 The reversible specific capacity of the CF-2-2.5 electrode remained at 127.7%, while the reversible specific capacity of the CF-2-2.5 electrode was 103.7 mAh·g. -1 Its capacity retention rate remained at 116.9%.

[0117] Figure 9 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 The results of a 10,000-cycle long-term stability test are shown in the figure. The current density is 200 mA·g. -1 Preliminary constant-current pre-sodiuming was performed on the CF-1-2.5 and CF-2-2.5 electrodes to eliminate irreversible capacity loss during the first cycle. When the current density increased to 5 A·g... -1 At that time, the CF-1-2.5 electrode still exhibited 120.8 mAh·g after 10,000 long cycles. -1 The reversible specific capacity, with a capacity retention rate as high as 200.9%, indicates a continuous activation effect during cycling; while the CF-2-2.5 electrode, under the same conditions, exhibits 74.3 mAh·g. -1The reversible specific capacity was measured. The results showed that the long-cycle stability of the CF-1-2.5 electrode was significantly higher than that of the CF-2-2.5 electrode. This is because the weakly acidic CDs anchor the nano-TiO2 particles through Ti-OC bonds, preventing their aggregation or detachment, enhancing interfacial bonding, and thus improving the utilization rate of the active material. The phenomenon that the specific capacity of the electrode with added CDs continued to increase at high current densities is attributed to the continuously enhanced redox reaction on the CDs surface, increasing the pseudocapacitive contribution. The significant increase in specific capacity is also due to defects such as oxygen vacancies and the introduction of additional sodium ion adsorption sites by the porous structure. Therefore, the CF-1-2.5 electrode exhibits outstanding long-cycle stability and can be used as a high-structural-durability anode material for sodium-ion batteries.

[0118] Figure 10 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 SEM images after 10,000 long-cycle stability tests. From Figure 10 As can be seen, the morphology of the CF-1-2.5 electrode did not change significantly, maintaining a complete carbon conductive network, and the fiber thickness remained largely unchanged. The CF-2-2.5 electrode showed thicker fiber diameters and localized adhesion, which is attributed to volume expansion caused by high current and repeated insertion / extraction during cycling. This indicates that the weakly acidic carbon dioxide (CDs) combined with the carbon matrix and TiO2 particles enhanced the interfacial bonding, preventing fiber breakage due to volume expansion. Therefore, the CF-1-2.5 electrode can serve as an excellent anode material in sodium-ion batteries, exhibiting outstanding cycle stability.

[0119] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art should understand that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a porous flexible self-supporting carbon nanofiber membrane material, characterized in that, Includes the following steps: (1) An acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile, pore-forming agent, pH adjuster and titanium source are dissolved in an organic solvent. The resulting electrospinning solution is electrospinned at a spinning voltage of 15-25 kV to obtain a precursor fiber membrane. The pH value of the electrospinning solution is 3.5-4.

5. The mass of the acidic carbon dot solution accounts for 15-20% of the mass of the electrospinning solution, and the concentration of carbon dots in the acidic carbon dot solution is 15-20 wt%. The concentration of ZIF-8 particles in the electrospinning solution is 0.1-1 wt%. The pore-forming agent is styrene-acrylonitrile copolymer and polyvinylpyrrolidone. The titanium source is tetrabutyl titanate. (2) In an air atmosphere, the precursor fiber membrane is pre-oxidized at 250-300 °C to obtain a pre-oxidized fiber membrane; (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane is carbonized at 700-800 °C to obtain the porous flexible self-supporting carbon nanofiber membrane material.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of polyacrylonitrile in the electrospinning solution is 2-12 wt%; the concentration of pore-forming agent in the electrospinning solution is 3-15 wt%.

3. The preparation method according to claim 1, characterized in that, In step (1), the concentration of the titanium source in the electrospinning solution is 3-10 wt%; the organic solvent is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide and tetrahydrofuran.

4. The preparation method according to claim 1, characterized in that, In step (1), the pH adjuster is selected from one or more of glacial acetic acid, formic acid, citric acid and dilute nitric acid; the concentration of the pH adjuster in the electrospinning solution is 1-5 wt%.

5. The preparation method according to claim 1, characterized in that, In step (1), the conditions for electrospinning are as follows: using an 18-22 G needle, a receiving distance of 10-20 cm, a temperature of 25-30 ℃, a humidity of 30%-60%, and an electrospinning solution flow rate of 0.5-1.5 mL / h.

6. The preparation method according to claim 1, characterized in that, In step (2), the pre-oxidation treatment specifically involves placing the precursor fiber membrane in a heating device and heating it to 250-300 ℃ at a heating rate of 1-5 ℃ / min and holding it at that temperature for 1-3 h.

7. The preparation method according to claim 1, characterized in that, In step (3), the carbonization process specifically involves placing the pre-oxidized fiber membrane in a heating device and heating it to 700-800 ℃ at a heating rate of 1-5 ℃ / min and holding it at that temperature for 2-3 h.

8. A porous flexible self-supporting carbon nanofiber membrane material prepared by the preparation method according to any one of claims 1-7.

9. The application of the porous flexible self-supporting carbon nanofiber membrane material according to claim 8 in the assembly of sodium-ion batteries.

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