Porous flexible self-supporting carbon nanofiber membrane material as well as preparation method and application thereof

Abstract

The invention discloses a porous flexible self-supporting carbon nanofiber membrane material and a preparation method and application thereof, the preparation method comprises the following steps: dissolving an acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile, a pore forming agent, a pH regulator and a titanium source in an organic solvent, and then carrying out electrostatic spinning to obtain a precursor fiber membrane; carrying out pre-oxidation treatment on the precursor fiber membrane in an air atmosphere to obtain a pre-oxidized fiber membrane; and carrying out carbonization treatment on 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 the invention can be directly used as an independent negative electrode of a sodium-ion battery, does not need a conductive agent and a binder, simplifies an electrode preparation process, has higher sodium storage capacity, good reversibility and excellent structural stability, has the characteristics of light weight and flexibility, and is suitable for large-scale production. And the practical development of the high-performance sodium ion battery is promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon nanofiber membrane materials, and in particular to a porous flexible self-supporting carbon nanofiber membrane material and a preparation method and application thereof. Background Art

[0002] Sodium ion batteries (SIBs) have broad application prospects in large-scale energy storage and other fields due to their abundant sodium resource reserves, low cost, and similar electrochemical properties to lithium. However, the development of sodium ion batteries still faces many technical bottlenecks. + ) The large ionic radius leads to slow diffusion kinetics in the electrode material, which places extremely high demands on the structural design and performance optimization of the electrode material. Currently commonly used negative electrode materials each have their own defects: traditional carbon-based materials, such as hard carbon and soft carbon, have good conductivity and stability, but their theoretical specific capacity is limited, making it difficult to meet the needs of high energy density; titanium-based materials, such as TiO2, have a stable structure and low volume expansion rate, but their intrinsic conductivity is extremely low, which seriously restricts the rate performance and cycle life of the battery. In addition, the flexible self-supporting negative electrode prepared by electrospinning in the existing technology can simplify the process and avoid the use of binders, but there are problems such as weak interface bonding between TiO2 and the carbon matrix and insufficient active sites, resulting in the actual capacity being far lower than the theoretical value.

[0003] In order to improve the performance of negative electrode materials for sodium ion batteries, researchers have carried out a lot of work. For example, by using electrospinning technology combined with doping strategies, phosphorus or nitrogen elements are introduced into carbon-based materials, which improves the conductivity and structural stability of the materials to a certain extent. Chinese patent CN106711410A uses a phosphorus source and a carbon-containing polymer to blend and spin, and then prepares phosphorus-doped nanofibers through vacuum drying and high-temperature treatment, which improves battery performance by increasing the spacing between carbon layers and forming stable bonds; Chinese patent CN114361450A prepares nitrogen-doped porous carbon fibers by electrospinning, and optimizes material performance using porous structures and flexible fiber networks. However, these single element doping methods still have the defects of limited capacity improvement and insufficient long-term cycle stability, and lack the synergistic optimization of composite doping and microstructure, making it difficult to achieve a breakthrough improvement in the performance of negative electrode materials. Summary of the Invention

[0004] In order to solve the problems of insufficient specific capacity, poor conductivity and poor cycle stability of sodium ion battery negative electrode materials in the existing technology, the purpose of the present invention is to provide a porous flexible self-supporting carbon nanofiber membrane material and its preparation method and 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 negative electrode material for assembling sodium ion batteries.

[0005] The above-mentioned object of the present invention is achieved through the following technical solutions: A first aspect of the present invention provides a method for preparing a porous flexible self-supporting carbon nanofiber membrane material, comprising the following steps: (1) Acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile (PAN), a pore-forming agent, a pH regulator, and a titanium source are dissolved in an organic solvent, and the obtained electrospinning solution is electrospun 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; the concentration of ZIF-8 particles in the electrospinning solution is 0.1-1 wt%; (2) pre-oxidizing the precursor fiber membrane at 250-300° C. in an air atmosphere 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.

[0006] The present invention prepares a flexible self-supporting negative electrode material of acidic CDs modified nano-TiO2 particles / carbon matrix through electrospinning and high-temperature carbonization technology. On the basis of single-atom doping (using the nitrogen environment in high-temperature pyrolysis), ZIF-8 is used as the Zn source to introduce Zn atoms into the TiO2 lattice, resulting in a larger interlayer spacing, which is beneficial to Na + Intercalation reaction can be carried out, and the point defects in TiO2 (oxygen vacancies, lattice defects, etc.) can be regulated by changing its content; in addition, CDs are introduced by generating Ti-OC bonds between the functional groups (-COOH, -OH, etc.) of CDs and nano-TiO2 particles, which not only improves the conductivity of carbon nanofiber membrane materials, promotes sodium ion transmission, increases porosity and active sites, but also enhances interfacial bonding and prolongs cycle life.

[0007] Furthermore, in step (1), the concentration of carbon dots in the acidic carbon dot solution is 15-20 wt%.

[0008] Furthermore, in step (1), the size of the carbon dots in the acidic carbon dot solution is 2-5 nm.

[0009] Furthermore, in step (1), the carbon dots in the acidic carbon dot solution may be citric acid-based carbon dots.

[0010] Furthermore, in step (1), the carboxyl group density of the carbon dots in the acidic carbon dot solution is 0.5-1.2 mmol / g.

[0011] Preferably, in step (1), the concentration of ZIF-8 particles in the electrospinning solution is 0.5-1 wt%.

[0012] After the ZIF-8 particles are introduced into the present invention, the fibers after high-temperature calcination tend to become brittle and breakage occurs between the carbon fibers. As the doping concentration increases until 0.5 wt%, the breakage between the fibers is significantly reduced, the fiber diameter becomes more uniform, the TiO2 particles and pores are more prominent, and the fibers are more oriented. When the ZIF-8 doping concentration continues to increase, the fibers become tortuous and messy, which is not conducive to Na + When the doping concentration is higher than 1 wt%, the lattice of the nano-TiO2 particles is distorted, resulting in uneven thickness between fibers and uneven internal stress distribution, which easily leads to stress concentration at the defects, resulting in fracture. At the same time, due to the increase of defects in TiO2 caused by appropriate N doping, doping the TiO2 carbon nanofiber membrane with an appropriate concentration of ZIF-8 can produce a higher degree of defects than single atom doping, which is more conducive to Na + De-embedding.

[0013] Furthermore, in step (1), the concentration of PAN in the electrospinning solution is 2-12 wt%.

[0014] Furthermore, in step (1), the pore-forming agent is one or more of styrene acrylonitrile copolymer (SAN), polyvinyl pyrrolidone (PVP), polystyrene (PS), and polymethyl methacrylate (PMMA), preferably SAN and PVP.

[0015] Furthermore, in step (1), when the pore-forming agents are SAN and PVP, the mass ratio of SAN to PVP is (0.5-3):1.

[0016] Furthermore, in step (1), the concentration of the pore-forming agent in the electrospinning solution is 3-15 wt%.

[0017] Furthermore, 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.

[0018] The pH regulator can slow down the hydrolysis rate of the titanium source, avoid the generation of TiO2 agglomerated particles due to rapid hydrolysis, and ensure the uniformity of the spinning solution; it can also prevent TBOT from absorbing moisture. After TBOT absorbs moisture, the solution becomes too viscous, which is not conducive to electrospinning.

[0019] Furthermore, the concentration of the pH regulator in the electrospinning solution is 1-5 wt%.

[0020] Furthermore, in step (1), the titanium source is tetrabutyl titanate (TBOT) or titanium tetrachloride (TiCl4).

[0021] Furthermore, in step (1), the concentration of the titanium source in the electrospinning solution is 3-10 wt%.

[0022] Furthermore, 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.

[0023] Furthermore, in step (1), the electrospinning conditions are: using an 18-22 G needle, a receiving distance of 10-20 cm, a temperature of 25-30 ° C, a humidity of 30%-60%, and an electrospinning liquid flow rate of 0.5-1.5 mL / h.

[0024] Furthermore, in step (1), the process parameters of the 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.

[0025] Furthermore, in step (1), the surface density of the precursor fiber membrane is 1.5-2.5 mg / cm 2 .

[0026] In step (2), the pre-oxidation treatment allows the precursor fiber membrane to form a stable cross-linked structure. The pore-forming agent undergoes thermal decomposition at this stage, such as SAN and PVP, which are thermally decomposed to form micropores and mesopores, respectively. At the same time, the organic ligands in the ZIF-8 particles that have been dissociated by acid are further decomposed, partially forming porous carbon materials with a high specific surface area. This provides more active sites for SIBs, reduces charge transfer resistance, facilitates the penetration of electrolyte ions, and ensures rapid ion transport.

[0027] Furthermore, in step (2), the pre-oxidation treatment is specifically as follows: placing the precursor fiber membrane in a heating device, heating it to 250-300°C at a heating rate of 1-5°C / min and keeping it warm for 1-3 hours.

[0028] In step (3), the carbonization treatment causes the pre-oxidized fiber membrane to form a porous carbon-based skeleton, in which the diameter of the fiber is 200-500 nm and the BET specific surface area is 300-400 m 2 / g, and the fibers form a continuous conductive network through carbonization cross-linking.

[0029] In step (3), the titanium source is carbonized under an inert atmosphere to self-generate nano-TiO2 particles with a particle size of 10-50 nm.

[0030] Furthermore, in step (3), the carbonization treatment is specifically as follows: placing the pre-oxidized fiber membrane in a heating device, heating it to 700-800°C at a heating rate of 1-5°C / min and keeping it warm for 2-3 hours.

[0031] The porous flexible self-supporting carbon nanofiber membrane material prepared by the present invention contains more micropores and mesopores, and has a large specific surface area, which is beneficial to the storage of active sites and reduces Na + The pore structure of the nanostructured nanoparticles is mainly composed of micropores (<2 nm), which account for 30-40% and are formed by the pyrolysis of SAN; mesopores (2-50 nm), which account for 50-60% and are formed by the pyrolysis of PVP; and macropores (>50 nm), which account for 10-20% and are formed by the stacking of electrospun fibers.

[0032] 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.

[0033] The porous flexible self-supporting carbon nanofiber membrane material provided by the present invention exhibits flexible self-supporting properties, a bending strength of >50 MPa, and a structural integrity retention rate of >95% after 10,000 bending cycles.

[0034] Furthermore, the loading amount of CDs in the porous flexible self-supporting carbon nanofiber membrane material is 2.5-5 wt%.

[0035] Furthermore, the doping concentration of the Zn element in the porous flexible self-supporting carbon nanofiber membrane material is 0.1-0.5 wt%.

[0036] Furthermore, the doping concentration of the N element in the porous flexible self-supporting carbon nanofiber membrane material is 1-3 wt %.

[0037] The third aspect of the present invention provides an application of the porous flexible self-supporting carbon nanofiber membrane material described in the second aspect in assembling a sodium ion battery, which is suitable for a high energy density sodium ion battery system.

[0038] The porous flexible self-supporting carbon nanofiber membrane material provided by the present invention can be used as the negative electrode of a sodium ion battery. When sodium ions are transmitted, the porous skeleton shortens the diffusion path, the dual-element doping reduces the ion migration barrier, and the carbon point bonding inhibits particle agglomeration, synergistically achieving the high specific capacity, ultra-long cycle stability and excellent rate performance of the sodium ion battery.

[0039] The beneficial effects of the present invention are: 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. The micropores provide sodium storage active sites, the mesopores serve as ion transmission channels, and the macropores alleviate volume expansion. By adopting Ti-OC chemical bonding interface engineering, a strong interaction is formed between carbon dots and TiO2 to inhibit the shedding of active substances. After 10,000 cycles, the fiber morphology integrity is greater than 90%, which significantly improves the cycle stability.

[0040] 2. The porous flexible self-supporting carbon nanofiber membrane material provided by the present invention can be used as the negative electrode of a sodium ion battery, wherein Zn is doped into the TiO2 lattice to introduce additional electrons, which helps to improve the transmission rate of the electrode; N and Zn doping can improve the structural stability of TiO2, reduce the degradation of the electrode and the attenuation of electrochemical performance during the cycle, thereby maintaining a high specific capacity; in addition, diatomic doping in the TiO2 lattice forms oxygen vacancies and heteroatom doping defects, which can adjust the band structure of the material, narrow the band gap of TiO2, reduce the energy barrier in the charge transfer path, make it easier for the charge to move in the electrode, increase the electrochemical active sites, enhance the reversibility of the electrochemical reaction, and help to improve the specific capacity; the surface functional groups of CDs form strong chemical bonds with TiO2, enhance the interfacial bonding force, and synergistically suppress the volume expansion with the gradient pore structure, significantly improving the sodium ion diffusion kinetics, ultimately making the carbon nanofiber membrane material have a greater sodium storage capacity, good reversibility and excellent structural stability.

[0041] 3. The porous flexible self-supporting carbon nanofiber membrane material provided by the present invention can be directly used as an independent negative electrode of sodium ion batteries, without the need for conductive agents and binders, streamlining the electrode preparation process, and its flexible self-supporting structure eliminates the need for traditional metal supports, increasing the surface capacity by 30-50%, and providing a new electrode design for the development of flexible energy storage devices; the porous flexible self-supporting carbon nanofiber membrane material as a negative electrode has a high conductivity at 0.1 A·g -1 At a current density of 1.5 mAh·g, the specific capacity can reach 304.5 mAh·g -1 ; at 2 A·g -1 At a current density of -1 The porous flexible self-supporting carbon nanofiber membrane material as the negative electrode showed an ultra-long cycle life. In 1000 cycles, when the current density was 1 A·g -1 When the battery is charged, it can reach 262.5mAh·g -1 The reversible specific capacity of the -1 After 10,000 cycles, it still showed 120.8 mAh·g -1The reversible specific capacity of the material is as high as 200.9% (based on the first cycle capacity). 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. The carbon nanofiber membrane material has both lightweight and flexible characteristics, which is conducive to promoting the practical development of high-performance sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Scanning electron microscope (SEM) images of the prepared porous flexible self-supporting carbon nanofiber membrane material and the carbon nanofiber membrane materials prepared in Comparative Examples 1-3; wherein, (a) is a SEM image of the PCF-1-2.5 carbon nanofiber membrane (scale is 5 μm), (b) is a SEM image of the PCF-1-2.5 carbon nanofiber membrane (scale is 500 nm), (c) is a SEM image of the CCF-1-2.5 carbon nanofiber membrane (scale is 5 μm), (d) is a SEM image of the CCF-1-2.5 carbon nanofiber membrane (scale is 500 nm), (e) is a SEM image of the CF-1-2.5 carbon nanofiber membrane (scale is 5 μm), (f) is a SEM image of the CF-1-2.5 carbon nanofiber membrane (scale is 500 nm), and (g) is a SEM image of the CF-2-2.5 carbon nanofiber membrane (scale is 5 µm), (h) is the SEM image of CF-2-2.5 carbon nanofiber membrane (scale bar is 500 nm), (i) is the SEM image of F-1-2.5 carbon nanofiber membrane (scale bar is 5 µm), and (j) is the SEM image of F-1-2.5 carbon nanofiber membrane (scale bar is 500 nm).

[0043] Figure 2 These are transmission electron microscope (TEM) images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; (a) is the TEM image, and (b) is the corresponding high-resolution image.

[0044] Figure 3 These are high-resolution transmission electron microscopy (HRTEM) images of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; wherein, (a) is the HRTEM image, (b) is the C element, (c) is the N element, (d) is the O element, (e) is the Ti element, and (f) is the Zn element.

[0045] Figure 4 These are 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.

[0046] Figure 5These are actual pictures of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1; wherein, (a) is the unfolded view, (b) is the folded view, (c) is the wound view, and (d) is the curled view.

[0047] Figure 6 The CF-1-2.5 electrode was charged at a current density of 200 mA·g -1 The constant current charge and discharge curves of the 2nd, 5th, 10th, 50th, 100th and 200th cycles.

[0048] Figure 7 The CF-1-2.5 electrode and CF-2-2.5 electrode were 0.1, 0.2, 0.5, 1, and 2 A·g -1 and recovered to 0.1 A·g -1 The rate diagram and the electrochemical impedance spectroscopy (EIS) test results of CF-1-2.5 electrode, CF-2-2.5 electrode and F-1-2.5 electrode; (a) is the CF-1-2.5 electrode at 0.1, 0.2, 0.5, 1, and 2 A·g -1 and recovered to 0.1 A·g -1 (b) shows the rate diagram of CF-2-2.5 electrode at 0.1, 0.2, 0.5, 1, and 2 A·g -1 and recovered to 0.1 A·g -1 (c) is the EIS test result diagram of CF-1-2.5 electrode, CF-2-2.5 electrode and F-1-2.5 electrode, and (d) is a local enlarged diagram of (c).

[0049] Figure 8 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 1 A·g -1 1000-cycle long-cycle stability test results under ; (a) is the CF-1-2.5 electrode, and (b) is the CF-2-2.5 electrode.

[0050] Figure 9 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 10,000 long cycle stability test results under ; (a) is the CF-1-2.5 electrode, and (b) is the CF-2-2.5 electrode.

[0051] 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 under 37°C; (a) is the SEM image of the CF-1-2.5 electrode, (b) is a local enlarged image of (a), (c) is the SEM image of the CF-2-2.5 electrode, and (d) is a local enlarged image of (c). DETAILED DESCRIPTION

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0053] 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 the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0054] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are all commercially available unless otherwise specified.

[0055] Example 1 A method for preparing a porous flexible self-supporting carbon nanofiber membrane material comprises the following steps: (1) 2.5 mL of an acidic carbon dot solution (17.2 wt%) with a pH of 2 was mixed with 9 g of DMF. 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 the solution was vigorously stirred at 40 °C for 12 h. ZIF-8 particles were then added (the concentration of ZIF-8 particles in the electrospinning solution was 0.5 wt%) to form a homogeneous solution. The solution was ultrasonicated for 2 h to obtain an electrospinning solution 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 conditions of spinning voltage of 16 kV, receiving distance of 16 cm, temperature of 25 °C, 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 °C for 3 h to obtain.

[0056] (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 pre-oxidation treatment for 2 h to form a stable cross-linked structure and obtain a pre-oxidized fiber membrane.

[0057] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tubular furnace and heated to 700 °C at a heating rate of 2.5 °C / min for carbonization treatment for 2 h to obtain the porous flexible self-supporting carbon nanofiber membrane material, which was named CF-1-2.5 carbon nanofiber membrane.

[0058] Example 2 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%. The finally obtained porous flexible self-supporting carbon nanofiber membrane material is named CF-1-2.5-0.1 carbon nanofiber membrane.

[0059] Example 3 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.

[0060] Example 4 A method for preparing a porous flexible self-supporting carbon nanofiber membrane material comprises the following steps: (1) 3 mL of acidic carbon dot solution with a pH of 2 (the concentration of carbon dots in the acidic carbon dot solution is 17.2 wt%) was mixed with 10 g of DMF, and 1 g of PAN and 1 g of PVP were added thereto. After stirring at room temperature for 3 h to form a uniform solution, glacial acetic acid was added dropwise to adjust the solution to pH 4. After stirring for 4 h, 0.8 g of TBOT and 1.5 g of SAN were added, and the mixture was vigorously stirred at 40 °C for 12 h. Then, ZIF-8 particles (the concentration of ZIF-8 particles in the electrospinning solution was 0.5 wt%) were added to form a uniform solution. The solution was ultrasonicated for 2 h to obtain the electrospinning solution, which was named F-1-3 solution. The 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 °C, humidity of 30%, electrospinning solution flow rate of 1.5 mL / h, electrospinning solution viscosity of 3000 cP, and speed of fiber collecting roller of 400 rpm. After spinning, the precursor fiber membrane was dried at 60 °C for 3 h to obtain the precursor fiber membrane.

[0061] (2) In an air atmosphere, the precursor fiber membrane was placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min for pre-oxidation treatment for 3 h to form a stable cross-linked structure and obtain a pre-oxidized fiber membrane.

[0062] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tubular furnace and heated to 800 °C at a heating rate of 4 °C / min for carbonization treatment for 3 h to obtain the porous flexible self-supporting carbon nanofiber membrane material, which was named CF-1-3 carbon nanofiber membrane.

[0063] Comparative Example 1 A method for preparing a carbon nanofiber membrane material comprises the following steps: (1) 0.5 g PAN and 0.5 g PVP were dissolved in 9 g DMF and stirred at room temperature for 3 h to form a homogeneous solution. 0.34 mL of glacial acetic acid was then added dropwise. After stirring for 4 h, 0.5 g of TBOT and 0.25 g of SAN were added and vigorously stirred at 40 °C for 12 h. ZIF-8 particles were then added (the concentration of ZIF-8 particles in the electrospinning solution was 0.5 wt%) to form a homogeneous solution. Ultrasonic treatment was performed for 2 h to obtain an electrospinning solution. The electrospinning solution was injected into an 18 G needle and electrospun under the conditions of a spinning voltage of 16 kV, a receiving distance of 16 cm, a temperature of 25 °C, a humidity of 50%, an electrospinning solution flow rate of 1 mL / h, an electrospinning solution viscosity of 2500 cP, and a fiber collection roller speed of 500 rpm. After spinning, the solution was dried at 60 °C for 3 h to obtain a precursor fiber membrane.

[0064] (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 pre-oxidation treatment for 2 h. After forming a stable cross-linked structure, a pre-oxidized fiber membrane was obtained. The pre-oxidized fiber membrane was infiltrated with an acidic carbon dot solution with a pH of 4 (the concentration of carbon dots in the acidic carbon dot solution was 17.2 wt%, the original acidic carbon dot solution had a pH of 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 a pre-oxidized membrane treated with CDs.

[0065] (3) In a nitrogen atmosphere, the pre-oxidized film treated with CDs was placed in a tubular furnace and heated to 700 °C at a heating rate of 2.5 °C / min for carbonization treatment for 2 h to obtain a carbon nanofiber membrane material, which was named PCF-1-2.5 carbon nanofiber membrane.

[0066] Comparative Example 2 A method for preparing a carbon nanofiber membrane material comprises the following steps: (1) 0.5 g PAN and 0.5 g PVP were dissolved in 9 g DMF and stirred at room temperature for 3 h to form a homogeneous solution. 0.34 mL of glacial acetic acid was then added dropwise. After stirring for 4 h, 0.5 g of TBOT and 0.25 g of SAN were added and vigorously stirred at 40 °C for 12 h. ZIF-8 particles were then added (the concentration of ZIF-8 particles in the electrospinning solution was 0.5 wt%) to form a homogeneous solution. Ultrasonic treatment was performed for 2 h to obtain an electrospinning solution. The electrospinning solution was injected into an 18 G needle and electrospun under the conditions of a spinning voltage of 16 kV, a receiving distance of 16 cm, a temperature of 25 °C, a humidity of 50%, an electrospinning solution flow rate of 1 mL / h, an electrospinning solution viscosity of 2500 cP, and a fiber collection roller speed of 500 rpm. After spinning, the solution was dried at 60 °C for 3 h to obtain a precursor fiber membrane.

[0067] (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 pre-oxidation treatment for 2 h. After forming a stable cross-linked structure, a pre-oxidized fiber membrane was obtained.

[0068] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tubular furnace and heated to 700 °C at a heating rate of 2.5 °C / min for carbonization treatment for 2 h. The obtained fiber membrane was infiltrated with an acidic carbon dot solution with a pH of 4 (the concentration of carbon dots in the acidic carbon dot solution was 17.2 wt%, the original acidic carbon dot solution had a pH of 2, and glacial acetic acid was added to adjust the pH to 4). The fiber membrane was then dried in a vacuum oven at 60 °C for 3 h to obtain a carbon nanofiber membrane material, which was named CCF-1-2.5 carbon nanofiber membrane.

[0069] Comparative Example 3 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 solution pH 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.

[0070] Comparative Example 4 A method for preparing a carbon nanofiber membrane material comprises the following steps: (1) 0.5 g PAN and 0.5 g PVP were dissolved in 9 g DMF and stirred at room temperature for 3 h to form a homogeneous solution. 0.34 mL of glacial acetic acid was then added dropwise. After stirring for 4 h, 0.5 g of TBOT and 0.25 g of SAN were added and vigorously stirred at 40 °C for 12 h. ZIF-8 particles were then added (the concentration of ZIF-8 particles in the electrospinning solution was 0.5 wt%) to form a homogeneous solution. Ultrasonic treatment was performed for 2 h to obtain an electrospinning solution. The electrospinning solution was injected into an 18 G needle and electrospun under the conditions of a spinning voltage of 16 kV, a receiving distance of 16 cm, a temperature of 25 °C, a humidity of 50%, an electrospinning solution flow rate of 1 mL / h, an electrospinning solution viscosity of 2500 cP, and a fiber collection roller speed of 500 rpm. After spinning, the solution was dried at 60 °C for 3 h to obtain a precursor fiber membrane.

[0071] (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 pre-oxidation treatment for 2 h to form a stable cross-linked structure and obtain a pre-oxidized fiber membrane.

[0072] (3) In a nitrogen atmosphere, the pre-oxidized fiber membrane was placed in a tubular furnace and heated to 700 °C at a heating rate of 2.5 °C / min for carbonization treatment for 2 h to obtain a carbon nanofiber membrane material, which was named F-1-2.5 carbon nanofiber membrane.

[0073] Comparative Example 5 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 by MOF-5, and the carbon nanofiber membrane material finally obtained is named CF-1-2.5-MOF carbon nanofiber membrane.

[0074] Comparative Example 6 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.

[0075] Test Example 1 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 in terms of morphology. Figure 1The following are SEM 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. Acidic CDs can serve as a self-template for generating pores. They are rich in functional groups such as carboxyl (-COOH) and hydroxyl (-OH). Some functional groups decompose under high temperature to generate gas, causing the formation of pore structures. Figure 1 As shown in (a) and (b), the PCF-1-2.5 carbon nanofiber membrane still maintains a clear fiber structure, with more holes on the fiber surface, but no obvious nano-TiO2 particles; Figure 1 In (c) and (d), the surface of CCF-1-2.5 carbon nanofiber membrane has more pores and the fiber surface is rougher. Figure 1 As can be seen from (e) and (f), the CF-1-2.5 carbon nanofiber membrane becomes finer, with more pores, an increased specific surface area, and an improved fiber surface roughness, indicating that the addition of CDs-1 to the precursor solution can regulate the thickness of the fiber and also generate pores and a rough fiber surface. Figure 1 As can be seen in (g) and (h), the morphology of the CF-2-2.5 carbon nanofiber membrane has changed to form "pod-like" spherical protrusions, and the fiber surface is still rough and uneven. This is because the addition of CDs with different acidity will change the acidic environment of the solution. Under strong acid spinning conditions, it will destroy the stability of the material, causing cross-linking of polymer molecular chains, affecting the viscosity of the polymer solution, and making the fibers too thick; 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, the addition of CDs of varying acidity to the precursor solution can significantly alter the fiber morphology, increasing surface roughness and porosity, and increasing active sites, thereby affecting electrode performance.

[0076] Figure 2 This is a TEM image of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. Figure 2 In (b), distinct lattice fringes can be seen. 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. A disordered transition region appears at the interface between the two, suggesting that new chemical bonds may have been formed between them, resulting in a two-phase product.

[0077] Figure 3 This is the HRTEM image of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. Figure 3 As can be seen from (a), the CF-1-2.5 nanofibers are uniform in thickness and have a rough surface. Figure 3It can be seen from the element distribution diagrams in (b)-(f) that C, N, O, Ti and Zn elements are evenly distributed in the CF-1-2.5 nanofibers, which shows that the carbonized CF-1-2.5 nanofiber membrane not only generates TiO2, but also successfully doped with Zn and N.

[0078] Figure 4 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 are shown. Figure 4 As can be seen in the figure, after doping with CDs, the peak height of the amorphous carbon middle layer (002) of the fiber membrane decreases, while the peak height belonging to anatase TiO2 gradually becomes flat, which indirectly indicates that CDs and TiO2 form new chemical bonds, thereby affecting the broadening of the characteristic peak of TiO2 and forming more defects. The CF-2-2.5 shows a more obvious performance than CF-1-2.5, with a flatter peak height, which 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.

[0079] The X-ray photoelectron spectroscopy (XPS) 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 showed that both the CF-1-2.5 carbon nanofiber membrane and the CF-2-2.5 carbon nanofiber membrane contained Zn, C, N, O and Ti elements. The functional groups carried by different acidic CDs affected the electron cloud density of carbon atoms, resulting in a slight shift in the binding energy. The characteristic peak with a binding energy of 458.3 eV was attributed to the Ti-OC bond, which also proved that the chemical bond generated between the introduction of acidic CDs and the nano-TiO2 particles was a Ti-OC bond, which was consistent with the HRTEM and XRD test results. The appearance of the Ti-OC bond would strengthen the bonding between the nano-TiO2 particles and the carbon matrix, and improve the structural stability. The characteristic peaks of the CF-1-2.5 carbon nanofiber membrane were more prominent and neat than those of the CF-2-2.5 carbon nanofiber membrane. This was because the -COOH in the weakly acidic CDs could inhibit the Zn 2+ The oxidation state of Zn fluctuates, reducing its oxidation tendency, thereby making Zn 2+ Easier to dope with nano-TiO2 particles.

[0080] 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 BET ) and pore size distribution data are shown in Table 1: Table 1 Specific surface area and pore size distribution data of CF-1-2.5 carbon nanofiber membrane and F-1-2.5 carbon nanofiber membrane

[0081] The BET specific surface area of ​​CF-1-2.5 carbon nanofiber membrane is 404.49 m 2 / g, which is 18% higher than that of F-1-2.5 carbon nanofiber membrane, and the average pore size is 5.66 nm, which shows that the specific surface area and pore size of CF-1-2.5 carbon nanofiber membrane increase due to the addition of CDs, which is beneficial to shorten the Na + The diffusion path of the electrolyte is improved, thereby promoting electrolyte infiltration and improving the electrochemical performance.

[0082] Figure 5 This is a physical picture of the porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1. Figure 5 As can be seen in the figure, the CF-1-2.5 carbon nanofiber membrane has a smooth surface, no obvious wrinkles, and no cracks, making it suitable for use as a self-supporting flexible anode. Using tweezers to grip the fiber membrane and change its shape, such as folding, winding, and curling, after multiple tests, the CF-1-2.5 carbon nanofiber membrane still maintains its original shape and shows no signs of cracking, demonstrating excellent mechanical stability.

[0083] Test Example 2 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 punched into circular sheets (D = 16 mm) and used as the negative electrode of a sodium ion half-cell. The half-cell was assembled in a nitrogen-filled glove box (H2O < 0.5 ppm, O2 < 0.5 ppm). The battery case was a CR2032 type. The sodium sheet served as the counter electrode, a glass fiber membrane (Whatman, GF / D grade) served as the separator, and the electrolyte consisted of 1 mol / L NaClO4 dissolved in a 1:1:1 volume ratio of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC), with 5% fluoroethylene carbonate (FEC) added.

[0084] The electrochemical performance of sodium ion half-cells assembled with 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 was tested in accordance with GB / T 44265-2024. The test conditions were: 0.01-3 V voltage window, 0.2 A·g -1 The current density was tested at 0.2 A·g -1 The discharge capacity after 200 cycles and the discharge capacity after 0.1 A·g -1 Discharge specific capacity.

[0085] The test results are shown in Table 2: Table 2

[0086] As can be seen from Table 2, doping CDs-1 in the precursor solution can significantly improve the cycle and rate performance of the electrode. The porous flexible self-supporting carbon nanofiber membrane material prepared in Example 1 was used as the negative electrode at a current density of 0.2 A·g -1 After 200 cycles, the discharge capacity can reach 200.6 mAh·g -1 ; After charge and discharge at a rate of 0.1 A·g -1 The discharge capacity reaches 304.5 mAh g -1 In comparative example 5, after replacing ZIF-8 with MOF-5, the electrode performance deteriorated significantly (the cycle capacity after 200 cycles was only 130 mAh·g -1 The reason is that the thermal decomposition temperature of MOF-5 (about 400°C) is much lower than the carbonization temperature (700-800°C). The skeleton collapses during the pre-oxidation stage (250-300°C), making it unable to provide an effective Zn doping source.

[0087] Test Example 3 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 a CR2032 button half-cell and test its electrochemical performance. The assembly and testing method is as follows: the carbon nanofiber membrane is punched into a circular self-supporting electrode with a diameter of 16 mm (without a binder / conductive agent), and is 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 with a volume ratio of 1:1:1 + 5% FEC) in a nitrogen glove box (H2O / O2<0.5 ppm). The test voltage range is 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 and discharge curves of the 2nd, 5th, 10th, 50th, 100th and 200th cycles are as follows: Figure 6 The CF-1-2.5 electrode showed 143.1 mAh·g at the 10th cycle. -1 The discharge capacity of the battery increased slightly from the 50th cycle to 200.6 mAh·g -1On the one hand, this is because the electrolyte gradually penetrates into the interior of the nanofibers and deep pores during cycling, 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, CDs are compounded with nano-TiO2 particles, inhibiting the agglomeration of nano-TiO2 particles, maintaining a high ion transmission rate and improving the electrochemical performance.

[0088] Figure 7 The CF-1-2.5 electrode and CF-2-2.5 electrode were 0.1, 0.2, 0.5, 1, and 2 A·g -1 and recovered to 0.1 A·g -1 The rate diagram and the electrochemical impedance spectroscopy (EIS) test results of CF-1-2.5 electrode, CF-2-2.5 electrode and F-1-2.5 electrode, from 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 specific capacity of the CF-1-2.5 electrode is higher at current densities of 0.1, 0.2, 0.5, 1, and 2 A·g -1 When the discharge capacity is 304.5, 264.4, 219.7, 184.3 and 148.8 mAh·g, respectively. -1 , which shows that the electrode has excellent rate performance. After high rate cycling, the current density returned to 0.1 A·g -1 The specific capacity reaches 265.3 mAh·g -1 This indicates that the CF-1-2.5 electrode has excellent reversibility for sodium ion diffusion. However, as a comparison electrode, the reversible specific capacity of CF-2-2.5 only reached 191.7, 178.7, 165.1, 149.6, and 128.3 mAh·g under the same test conditions. -1 When the current density returns to 0.1 A·g -1 When the specific capacity is restored to 186.4 mAh·g -1 This difference is attributed to the carboxyl groups of weakly acidic CDs bonding with nano-TiO2 particles to form a continuous conductive network; while the strong electron-withdrawing effect of the sulfonic acid groups of strongly acidic CDs leads to uneven surface charge distribution and inhibits sodium ion adsorption, thus reducing its specific capacity. Figure 7 It can be seen from (c) and (d) that after adding CDs, the polarization resistance (R p ) increases slightly, which is due to the uneven dispersion of some CDs in the carbon matrix, resulting in the formation of local insulating regions, which hinder electron transmission and cause R p Although CDs doping leads to the overall R p However, in the CF-1-2.5 electrode, the Ti-OC bonding effectively reduces the Rp , although the overall R p The interface contact resistance decreased significantly.

[0089] Figure 8 The CF-1-2.5 and CF-2-2.5 electrodes were -1 1000 times long cycle stability test results. Figure 8 As shown, first, at a current density of 200 mA·g -1 The CF-1-2.5 and CF-2-2.5 electrodes were pre-sodiumized under the conditions of 400 nm, and then the current density was gradually increased to 1 A·g -1 After 1000 cycles, the CF-1-2.5 electrode showed a capacity of 262.5 mAh·g -1 The reversible specific capacity of the CF-2-2.5 electrode is 103.7 mAh·g -1 , its capacity retention rate remained at 116.9%.

[0090] Figure 9 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 10000 times long cycle stability test results. At a current density of 200 mA·g -1 The CF-1-2.5 and CF-2-2.5 electrodes were pre-sodiumized at a constant current to eliminate the irreversible capacity loss caused by the first cycle. -1 When the CF-1-2.5 electrode is used, it still shows a high capacity of 120.8 mAh·g after 10,000 cycles. -1 The reversible specific capacity of the CF-2-2.5 electrode was 74.3 mAh·g under the same conditions. -1 reversible specific capacity. The results show that the long-cycle stability of the CF-1-2.5 electrode is significantly higher than that of the CF-2-2.5 electrode. This is because the weakly acidic CDs anchor the nano-TiO2 particles through the Ti-OC bond, preventing the agglomeration or shedding of the nano-TiO2 particles, enhancing the interfacial binding force, and thus improving the utilization rate of the active material. The phenomenon that the specific capacity of the electrode with the addition of CDs continues to increase at high current density is attributed to the continuous enhancement of the redox reaction on the surface of CDs and the increase in the contribution of pseudocapacitance, and the substantial increase in its 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 has outstanding long-cycle stability and can be used as a negative electrode material for sodium-ion batteries with high structural durability.

[0091] Figure 10 For CF-1-2.5 and CF-2-2.5 electrodes at a current density of 5 A·g -1 SEM image after 10,000 long cycle stability tests. Figure 10 As can be seen from the graph, the CF-1-2.5 electrode exhibits minimal morphological changes, maintaining an intact carbon conductive network and maintaining minimal fiber thickness. However, the CF-2-2.5 electrode exhibits fiber diameter thickening and localized adhesion, which is attributed to volume expansion caused by high current and repeated intercalation and deintercalation during cycling. This suggests that the weakly acidic CDs, combined with the carbon matrix and TiO2 particles, enhance interfacial bonding and prevent fiber breakage caused by volume expansion. Therefore, the CF-1-2.5 electrode can serve as an excellent anode material for sodium-ion batteries, demonstrating outstanding cycling stability.

[0092] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications may be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall 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: The following steps are involved: (1) Acidic carbon dot solution, ZIF-8 particles, polyacrylonitrile, pore-forming agent, pH regulator and titanium source are dissolved in an organic solvent, and the obtained electrospinning solution is electrospun 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; the concentration of ZIF-8 particles in the electrospinning solution is 0.1-1 wt%; (2) pre-oxidizing the precursor fiber membrane at 250-300° C. in an air atmosphere 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, wherein In step (1), the concentration of carbon dots in the acidic carbon dot solution is 15-20 wt%.

3. The preparation method according to claim 1, wherein In step (1), the concentration of polyacrylonitrile in the electrospinning solution is 2-12 wt%; the pore-forming agent is one or more of styrene acrylonitrile copolymer, polyvinyl pyrrolidone, polystyrene, and polymethyl methacrylate; and the concentration of the pore-forming agent in the electrospinning solution is 3-15 wt%.

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

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

6. The preparation method according to claim 1, wherein In step (1), the electrospinning conditions are: using an 18-22 G needle, a receiving distance of 10-20 cm, a temperature of 25-30 ° C, a humidity of 30%-60%, and an electrospinning liquid flow rate of 0.5-1.5 mL / h.

7. The preparation method according to claim 1, wherein In step (2), the pre-oxidation treatment is specifically as follows: placing the precursor fiber membrane in a heating device, heating it to 250-300°C at a heating rate of 1-5°C / min and keeping it warm for 1-3 hours.

8. The preparation method according to claim 1, wherein In step (3), the carbonization treatment is specifically as follows: placing the pre-oxidized fiber membrane in a heating device, heating it to 700-800°C at a heating rate of 1-5°C / min and keeping it warm for 2-3 hours.

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

10. Use of the porous flexible self-supporting carbon nanofiber membrane material according to claim 9 in assembling sodium ion batteries.

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