A coaxial composite fiber, its preparation method and application

Abstract

This invention relates to a coaxial composite fiber, its preparation method, and its application, belonging to the field of new energy materials technology. This invention introduces polyvinylidene fluoride (PVDF), manganese dioxide, and carbon dots into a spinning system, utilizing microfluidic wet spinning technology and coaxial structure design to achieve controllable and continuous fabrication of a core-shell structured composite fiber electrode. First, an outer spinning solution and an inner spinning solution are prepared, with the ratio of PVDF to carbon dots controlled separately. Coaxial needles are used for microfluidic spinning, followed by coagulation, drawing, winding, and curing to obtain the coaxial composite fiber. This coaxial composite fiber exhibits good structural uniformity and interfacial stability, significantly improving electron conduction and ion diffusion performance, achieving continuous and controllable fabrication, high specific capacitance, and excellent cycle stability, making it suitable for flexible supercapacitors.

Description

Technical Field

[0001] This invention relates to the field of new energy materials technology, specifically to a coaxial composite fiber, its preparation method, and its application. Background Technology

[0002] With the rapid development of wearable electronic devices and flexible energy systems, the demand for high-performance flexible energy storage devices continues to grow. Fibrous supercapacitors, due to their one-dimensional flexible morphology, excellent weavability, and portable integration capabilities, have become an important research direction for next-generation flexible energy storage technologies. Among these, electrode materials, as key components, directly determine device performance and scalability through their flexibility, structural optimization, and continuous fabrication capabilities. Currently, there is an urgent need in this field to develop a flexible electrode material construction strategy that possesses both excellent electrochemical performance and low energy consumption, controllable structure, and continuous molding capability.

[0003] Manganese dioxide (MnO2) is widely used in supercapacitor electrode research due to its advantages such as high theoretical specific capacitance, safety, environmental friendliness, and low cost. However, its poor intrinsic conductivity, limited specific capacitance utilization, and insufficient cycle stability severely restrict its high-performance development. To improve its overall performance, carbon-based materials are often introduced as composite components to construct conductive networks and improve electron / ion transport efficiency. Among them, carbon dots (CDs) are a novel type of nanocarbon material with advantages such as small particle size, large specific surface area, abundant edge active sites, and good dispersibility. They can form a synergistic effect with MnO2, improving structural stability and electrochemical reaction kinetics. However, existing MnO2 / CDs composite materials are mostly prepared by deposition, electroplating, or static doping, which have problems such as uncontrollable structure, weak interfacial bonding, and complex processes, making it difficult to achieve continuous fibrous molding, which is not conducive to the large-scale fabrication and integrated application of flexible electrodes.

[0004] Existing patent CN115020113A discloses an electrode preparation method that modifies carbon cloth with strong acid and grows MnO2 in situ in a high-temperature KMnO4 solution. Although this method improves some electrochemical performance by controlling the valence ratio of Mn, its process steps are complex, requiring long-term acid treatment and high-temperature reaction, resulting in high energy consumption and the inability to achieve continuous production. Furthermore, the electrode obtained by this method consists of MnO2 particles attached to the surface of the carbon cloth, resulting in an uncontrollable structure, poor flexibility, and unsuitability for integration into flexible or fibrous energy storage devices, thus limiting its application scenarios. Patent CN107204242A discloses a manganese dioxide-porous polyaniline composite electrode and its preparation method. This method involves a two-step electrochemical deposition process to first deposit a manganese dioxide layer on the surface of a metal or carbon-based current collector, and then polymerizes it using a kinetic potential to generate a porous polyaniline fiber layer, thereby constructing a composite electrode structure. This technology has achieved certain results in improving electrode conductivity and electrochemical activity, featuring the advantages of no binder, compact structure, and high specific capacitance. However, its structure is a simple layer-by-layer stack, lacking a three-dimensional coaxial configuration, and the material composition cannot be independently controlled, resulting in an uncontrollable internal structure. Patent CN112768258A uses a hydrothermal method and in-situ polymerization process to construct a polyaniline / aluminum-doped MnO2 composite electrode on carbon cloth, improving conductivity and specific capacitance. However, this method is complex, time-consuming, relies on high-temperature static reactions, cannot achieve continuous preparation, and the resulting structure is a non-self-supporting planar electrode, unsuitable for flexible fiber applications.

[0005] Therefore, there is an urgent need to develop a novel coaxial composite fiber electrode fabrication technology with adjustable layered structure, precise component distribution, and continuous molding capability, which has significant research value and application prospects. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a coaxial composite fiber, its preparation method, and its application. This invention provides a continuous preparation method for coaxial composite fiber electrodes based on microfluidic wet spinning technology. Using microfluidic wet spinning technology and polyvinylidene fluoride as a flexible substrate, a core-shell structured coaxial composite fiber is constructed, achieving continuous and controllable preparation of the composite fiber electrode. This method precisely controls the composition and flow rate of the inner and outer spinning solutions to form a coaxial fiber structure with stable interfaces and uniform structure. Furthermore, carbon dots are introduced as a functional component, enhancing the dispersibility of MnO2 in the fiber system and improving overall conductivity and interfacial reactivity. The structural design of this invention not only overcomes the continuous molding obstacles caused by the addition of carbon dots but also provides greater parameter control space, offering an effective solution for high-performance, flexible, and scalable fibrous supercapacitors.

[0007] This invention is achieved through the following technical solution:

[0008] The purpose of this invention is to provide a method for preparing coaxial composite fibers, comprising the following steps:

[0009] S1. Polyvinylidene fluoride, MnO2 and carbon dots are mixed and the mixture is heated and stirred, ultrasonically dispersed and magnetically stirred to obtain the outer spinning solution; the mass ratio of carbon dots to polyvinylidene fluoride is 5:100-15:100; preferably 8:100-12:100.

[0010] S2. Polyvinylidene fluoride, manganese dioxide and carbon dots are mixed, heated and stirred, ultrasonically dispersed and magnetically stirred to obtain the inner layer spinning solution; the mass ratio of carbon dots to polyvinylidene fluoride is 1:1-15:1; specifically, it is one of 1:1, 5:1 and 15:1; by adjusting the ratio of polyvinylidene fluoride to carbon dots, the optimal balance between the flexibility and conductivity of the inner layer structure is achieved.

[0011] S3. The obtained inner spinning solution and outer spinning solution are spun and solidified using microfluidic wet spinning technology to obtain composite fibers.

[0012] S4. The obtained composite fibers are stretched, wound and cured to obtain coaxial composite fibers.

[0013] Further, in step S1, the mass fraction of polyvinylidene fluoride in the outer spinning solution is 10%-25%; preferably 13%-17%.

[0014] Further, in step S1, the mass ratio of manganese dioxide to polyvinylidene fluoride is 3:100-11:100; preferably 3:100-7:100.

[0015] Polyvinylidene fluoride provides a flexible fiber matrix, manganese dioxide provides high specific capacitance, and carbon dots, as a conductive component, promote electron transport and improve structural stability. The resulting spinning solution has excellent stability, fluidity, and processing performance suitable for coaxial spinning.

[0016] Further, in step S2, the mass fraction of polyvinylidene fluoride in the inner spinning solution is 1%-8%; specifically 8%, 6%, 5%, 3% or 1%.

[0017] Further, in step S2, the mass ratio of manganese dioxide to polyvinylidene fluoride is 3:100-11:100; preferably 3:100-7:100.

[0018] The proportion of manganese dioxide is maintained at the same level as the outer spinning solution to ensure the overall functional consistency of the fiber. The inner spinning solution has suitable viscosity and good stability, allowing for the stable formation of the fiber core layer structure via a microfluidic coaxial nozzle. The electronic conductivity and ion transport performance of the composite fiber electrode are optimized to ensure that the inner layer possesses appropriate conductivity and ion transport capabilities.

[0019] Furthermore, in steps S1 and S2, the ultrasonic dispersion time is 2 h-4 h; the magnetic stirring time is 0.5 h-1 h. The purpose of ultrasonic dispersion and magnetic stirring is to enhance the dispersibility of nanoparticles. Through ultrasonic treatment and magnetic stirring, the spinning solution achieves a state with suitable rheological properties, uniform dispersion, and stable composition, ensuring that the requirements for shell spinning solution formation and interface quality in the subsequent microfluidic wet spinning process are met.

[0020] Further, step S3 specifically involves: loading the obtained outer and inner spinning solutions into syringes, which are then connected to the outer and inner propulsion pumps of the microfluidic wet spinning device, respectively. Both are connected to the outer and central channels of the coaxial spinning needle via flexible tubes, achieving synchronous and stable propulsion of the two liquids. During spinning, the coaxial needle is placed above a deionized water coagulation bath. Under the influence of gravity and traction, the spinning solution forms a coating structure at the needle and then enters the coagulation bath. The polyvinylidene fluoride system undergoes phase separation and coagulation reactions under aqueous phase induction, rapidly solidifying to form continuous fibers with a core-shell structure.

[0021] Further, in step S3, the flow rate of the outer spinning solution in the microfluidic wet spinning is 0.1 mL / min-0.5 mL / min; preferably 0.3 mL / min; and the flow rate of the inner spinning solution is 0.1 mL / min-0.3 mL / min; preferably 0.2 mL / min.

[0022] Furthermore, in step S4, the stretching speed during the stretching process is 1.5 m / min-3.0 m / min.

[0023] The flow velocity ratio and stretching rate of the inner and outer layers directly affect the core diameter and shell thickness. Typical core diameters are 40-60 μm, and shell thicknesses are 20-40 μm, which can be optimized according to different energy storage requirements. Precisely controlling the stability and morphological uniformity of the fluid flow through a microfluidic system to achieve spatial distribution regulation of inner and outer layer components and complete interface fusion is a key step in ensuring the continuous and structured fabrication of core-shell fiber electrodes.

[0024] After the composite fiber formed by coaxial spinning enters the deionized water coagulation bath, the outer polyvinylidene fluoride body first undergoes rapid phase separation to form a preliminary solid structure, while the coated manganese dioxide and carbon dots are stably embedded in it; the inner spinning solution solidifies simultaneously, ultimately constructing a core-shell composite fiber with a clear interface and complete structure.

[0025] In the coagulation bath, the fibers undergo tension control and directional stretching via a stretching device, which enhances the molecular chain orientation and structural density, further improving the fiber's mechanical properties and the effectiveness of electron / ion channels. Subsequently, a winding device enables continuous winding and finishing, allowing the fiber electrodes to be stably collected in a continuous filament state, facilitating subsequent processing and integrated applications.

[0026] Furthermore, in step S3, deionized water is used for coagulation, and the temperature is maintained at room temperature (20℃-25℃). The fiber stays in the bath for a period of 10 s-30 s to ensure full curing and formation of a complete structure.

[0027] Furthermore, in step S4, the winding rate and the stretching speed are matched and set at 1.5 m / min-3.0 m / min to ensure uniform fiber tension and avoid breakage or deformation.

[0028] The present invention also provides coaxial composite fibers obtained by the preparation method, wherein the coaxial composite fibers have a core-shell structure.

[0029] The present invention also provides the application of the coaxial composite fiber in the fabrication of flexible energy storage devices or fibrous supercapacitors.

[0030] The technical solution of the present invention has the following advantages compared with the prior art:

[0031] This invention provides a coaxial composite fiber, its preparation method, and its applications. The invention utilizes microfluidic coaxial wet spinning technology to construct a core-shell structured coaxial composite fiber, achieving spatial separation and synergistic optimization of the inner and outer functional materials. The inner layer forms a flexible supporting framework and conductive channels, while the outer layer is enriched with manganese dioxide and doped with carbon dots, significantly improving the overall reactive surface area and electron / ion transport efficiency. The core-shell structure allows electrons to conduct at high speed along the core, while the shell provides ample reaction sites, forming a synergistic conductive-energy storage system. This effectively reduces interfacial resistance, inhibits structural aggregation and volume expansion, thereby improving rate performance and cycle stability.

[0032] The composite fiber electrode of this invention exhibits excellent mechanical properties, with a tensile strength of 4.5 MPa-5.4 MPa and a tensile strain retention rate of 43%-52%. The coaxial composite fiber has a specific capacitance of 540 F / g-775 F / g at a current density of 1 A / g. After 5000 constant current charge-discharge cycles at a current density of 10 A / g, its specific capacitance still retains more than 95% of its initial value, indicating excellent specific capacitance and cycle stability.

[0033] This invention achieves a high balance between conductivity, flexibility, specific capacitance, and cycling performance in coaxial composite fibers produced by microfluidic wet spinning, demonstrating outstanding comprehensive performance. This verifies the significant synergistic effect of the core-shell composite structure design and carbon doping strategy described in this invention. The appropriate doping of carbon dots in the shell does not affect spinning stability; the entire system can still be stably and continuously spun at a drawing speed of 1.5–3.0 m / min, effectively avoiding carbon dot agglomeration and rheological anomalies, ensuring the structural consistency and batch controllability of the electrode material. This low-energy, low-cost, and structurally simple coaxial configuration provides a reliable path for improving the performance of manganese dioxide-based flexible electrodes and their large-scale continuous fabrication.

[0034] 2. This invention introduces carbon dots to regulate intermolecular interactions and electron migration pathways, significantly enhancing the conductivity and structural coupling of fiber electrodes: This invention introduces carbon dots with abundant surface functional groups and excellent conductivity as synergistic functional components. By regulating their distribution in the polyvinylidene fluoride / MnO2 matrix, multi-scale synergistic optimization of structure and performance is achieved. Specifically, active groups such as carboxyl and hydroxyl groups on the surface of CDs can form hydrogen bonds with the –CF2– groups in PVDF, promoting the orderly arrangement of PVDF molecular chains, thereby enhancing the fiber's density and mechanical strength; simultaneously, CDs can also form C–O–Mn chemical bonds with the MnO2 surface, enhancing interfacial bonding and constructing stable electron transport pathways.

[0035] During coaxial wet spinning, the reduction in the inner layer polyvinylidene fluoride (PVDF) content, in synergy with carbon dots, resulted in greater orientational stretching of PVDF segments per unit volume, significantly increasing the β-phase content in the crystals. Furthermore, the behavior was primarily capacitively controlled under different voltage and scan rate conditions, fully demonstrating the crucial role of carbon dots in improving electron / ion migration efficiency and electrochemical performance.

[0036] 3. The preparation of traditional MnO2 electrode materials typically relies on processes such as high-temperature calcination, electrochemical deposition, or chemical oxidation. These processes not only suffer from high energy consumption, intermittent operation, and uncontrollable structure, but also hinder the large-scale manufacturing of flexible electronic devices. This invention innovatively employs microfluidic wet spinning technology to achieve the controllable and continuous construction of coaxial composite fibers based on microfluidic wet spinning at room temperature without the need for high-temperature heat treatment or corrosive reagents. This process combines the precise transport and regulation capabilities of microfluidics with the characteristics of wet-induced solidification, achieving a highly consistent core-shell structure configuration for the composite fibers by controlling the component ratio, flow rate, and drawing speed of the inner and outer spinning solutions.

[0037] This method not only boasts significant advantages such as low energy consumption, simple process, and environmental friendliness, but also exhibits excellent scalability and process stability. In continuous laboratory preparation, it has already achieved the output of high-performance composite fibers exceeding 100 meters in length, facilitating the industrial integration of subsequent flexible electrode sheet fabrication, winding, and electrode integration processes. This technology provides a stable and efficient solution for the mass production of MnO2-based flexible electrode materials and the widespread application of flexible energy storage devices.

[0038] In summary, this invention enables the controllable and continuous fabrication of flexible electrode materials using coaxial composite fibers under ambient temperature and low energy consumption conditions. The designed synergistic system of inner and outer layer components fully leverages the flexible support of polyvinylidene fluoride (PVDF), the high specific capacitance of MnO2, and the conductivity regulation function of carbon dots at the molecular level. This significantly improves the mechanical and electrochemical properties of the fiber electrode while maintaining structural stability and engineering processability. It can be further integrated into novel energy systems such as wearable electronics, smart textiles, flexible energy modules, and linear supercapacitors, providing key electrode support for next-generation micro-flexible energy storage devices. Attached Figure Description

[0039] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0040] Figure 1 This is a flowchart of the continuous preparation process of the PVDF / MnO2 / CDs coaxial composite fiber electrode of the present invention; wherein, 1: heating and stirring device; 2: ultrasonic dispersion device; 3: magnetic stirring device; 4: inner spinning solution channel; 5: outer spinning solution channel; 6: deionized water coagulation bath; 7: winding device;

[0041] Figure 2 SEM images of the coaxial composite fiber prepared in Example 1 of the present invention: (a) is a planar view; (b) is a cross-sectional view;

[0042] Figure 3 A comparison of stress-strain curves of coaxial composite fibers and uniaxial fibers prepared in Example 1 of the present invention;

[0043] Figure 4 This is a mass specific capacitance diagram of the coaxial composite fiber in Embodiment 1 of the present invention;

[0044] Figure 5 This is a CV diagram of the coaxial composite fiber in Example 1 of the present invention. Detailed Implementation

[0045] 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.

[0046] The coaxial composite fibers prepared by the microfluidic wet coaxial spinning technology significantly improve the mechanical properties, electrochemical properties and structural stability of flexible electrodes, making them suitable for the continuous and large-scale fabrication of flexible energy storage devices.

[0047] Figure 1 This is a flowchart illustrating the continuous fabrication process of the PVDF / MnO2 / CDs coaxial composite fiber electrode of the present invention. Specifically: 1 is a heating and stirring device: the mixture is heated and stirred in a water bath to promote complete dissolution of PVDF; 2 is an ultrasonic dispersion device: the solution is ultrasonically treated to break up nanoparticle agglomerates and improve the dispersibility of MnO2 and CDs; 3 is a magnetic stirring device: secondary homogenization and stirring are performed after ultrasonic treatment to improve the stability and spinning adaptability of the system; 4 is an inner spinning solution channel, used to transport the PVDF / CDs spinning solution with controlled proportions to the inner layer of the coaxial needle; 5 is an outer spinning solution channel, used to transport the outer spinning solution containing PVDF, MnO2, and CDs to the outer layer of the coaxial needle; 6 is a deionized water coagulation bath, used for rapid solidification and molding of the spinning solution under aqueous phase induction; 7 is a winding device, used to stretch and collect the continuously formed coaxial composite fibers to ensure continuous fiber forming and subsequent processing.

[0048] This invention aims to construct a core-shell coaxial composite fiber that can be continuously prepared in batches, has excellent performance, and a controllable structure by introducing coaxial structure design, carbon dot synergistic functional materials, and microfluidic wet spinning technology. This will enable the synergistic transport of electrons and ions and flexible integration of electrode structures, thereby improving the overall performance of supercapacitors.

[0049] The coaxial composite fiber of this invention features a typical core-shell coaxial structure. During microfluidic coaxial wet spinning, the inner spinning solution is injected through the central channel of the coaxial needle, forming the fiber core structure. The outer spinning solution coats the outer side through the outer channel and simultaneously solidifies in a coagulation bath, forming a core-shell composite fiber structure with continuous interfaces, clear layering, and complementary components. The outer layer (shell) provides the main electrochemical energy storage function and mechanical support; the inner layer (core) is a proportionally controlled system of polyvinylidene fluoride and carbon dots, providing enhanced conductivity and flexible support. The core and shell dimensions of this invention can be precisely controlled by adjusting the flow rate ratio, component concentration, and drawing speed of the inner and outer spinning solutions. This structural design allows electrons to conduct at high speed along the core layer, and ions to participate in energy storage reactions through the high specific surface area of ​​the shell layer. The synergistic effect of the inner and outer layers improves the overall performance of the electrode, while also enhancing fiber stability and weavability, making it suitable for constructing high-performance flexible supercapacitor systems.

[0050] The method described in this invention does not require high-temperature calcination or highly corrosive chemical reactions. It can achieve continuous and low-energy preparation of composite fibers under normal temperature conditions through controllable flow rate adjustment and precise component design, and has good green manufacturing characteristics and industrial scale-up application prospects.

[0051] In this invention, unless otherwise stated, all materials used in the following embodiments are commercially available.

[0052] Polyvinylidene fluoride (PVDF) was purchased from Shanghai Magnesium Plastics Co., Ltd., specification Kynar 705;

[0053] N,N-Dimethylformamide (DMF), purchased from Beijing Bailingwei Technology Co., Ltd., product number 983353;

[0054] Manganese dioxide (MnO2) was purchased from Shanghai Mairui Biochemical Technology Co., Ltd., with a purity of 99%.

[0055] Deionized water (DI) was prepared using the Milli-Q Plus water purification system;

[0056] Carbon dots (CDs) are prepared by electrolysis of graphite rods.

[0057] Example 1

[0058] This embodiment provides a method for preparing coaxial composite fibers, specifically including the following steps:

[0059] 1. Preparation of outer spinning solution: Weigh 15 g PVDF, 0.45 g MnO2 (3% of PVDF mass), and 1.5 g CDs (10% of PVDF mass), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then, transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain a uniform outer spinning solution.

[0060] 2. Preparation of inner layer spinning solution: Weigh 0.03 g MnO2, 1 g PVDF, and 5 g CDs (PVDF to CDs mass ratio of 1:5), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain an inner layer spinning solution with good rheological properties.

[0061] 3. Coaxial wet spinning: The inner and outer spinning solutions are each loaded into a 10 mL syringe and connected to the inner and outer layer propulsion pumps of the microfluidic coaxial wet spinning device via tubing. The outer layer flow rate is set to 0.3 mL / min and the inner layer flow rate to 0.2 mL / min. After the fibers are ejected from the coaxial needle, they directly enter the room temperature deionized water coagulation bath to complete coagulation.

[0062] 4. Fiber drawing and collection: After the fibers are solidified in the coagulation bath, they are drawn by a drawing device and continuously collected by a winding device. The drawing speed is 1.68 m / min. The resulting PVDF / MnO2 / CDs coaxial composite fiber has a clear structure, exhibiting a typical core-shell coaxial structure.

[0063] The SEM image of the coaxial composite fiber obtained in this embodiment is as follows: Figure 2 As shown, the composite fiber obtained by the present invention has a typical coaxial structure, wherein the core diameter is about 50 μm and the shell thickness is about 30 μm, and the overall structure is compact and uniform.

[0064] Example 2

[0065] This embodiment provides a method for preparing coaxial composite fibers, specifically including the following steps:

[0066] 1. Preparation of outer spinning solution: Weigh 15 g PVDF, 0.45 g MnO2 (3% of PVDF mass), and 1.2 g CDs (8% of PVDF mass), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain a uniform outer spinning solution.

[0067] 2. Preparation of inner layer spinning solution: Weigh 0.25 g MnO2, 5 g PVDF, and 5 g CDs (PVDF to CDs mass ratio 1:1), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain an inner layer spinning solution with good rheological properties.

[0068] 3. Coaxial wet spinning: The inner and outer spinning solutions are each loaded into a 10 mL syringe and connected to the inner and outer layer propulsion pumps of the microfluidic coaxial wet spinning device via tubing. The outer layer flow rate is set to 0.4 mL / min and the inner layer flow rate to 0.3 mL / min. After the fibers are ejected from the coaxial needle, they directly enter the room temperature deionized water coagulation bath to complete coagulation.

[0069] 4. Fiber drawing and collection: After the fibers are solidified in the coagulation bath, they are drawn by a drawing device and continuously collected by a winding device. The drawing speed is 2 m / min. The resulting PVDF / MnO2 / CDs coaxial composite fiber has a clear structure, exhibiting a typical core-shell coaxial structure.

[0070] Example 3

[0071] This embodiment provides a method for preparing coaxial composite fibers, specifically including the following steps:

[0072] 1. Preparation of outer spinning solution: Weigh 15 g PVDF, 0.45 g MnO2 (3% of PVDF mass), and 1.8 g CDs (12% of PVDF mass), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain a uniform outer spinning solution.

[0073] 2. Preparation of inner layer spinning solution: Weigh 0.07 g MnO2, 1 g PVDF, and 15 g CDs (PVDF to CDs mass ratio of 1:15), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain an inner layer spinning solution with good rheological properties.

[0074] 3. Coaxial wet spinning: The inner and outer spinning solutions are each loaded into a 10 mL syringe and connected to the inner and outer layer propulsion pumps of the microfluidic coaxial wet spinning device via tubing. The outer layer flow rate is set to 0.5 mL / min and the inner layer flow rate to 0.3 mL / min. After the fibers are ejected from the coaxial needle, they directly enter the room temperature deionized water coagulation bath to complete coagulation.

[0075] 4. Fiber drawing and collection: After the fibers are solidified in the coagulation bath, they are drawn by a drawing device and continuously collected by a winding device. The drawing speed is 2.52 m / min. The resulting PVDF / MnO2 / CDs coaxial composite fiber has a clear structure, exhibiting a typical core-shell coaxial structure.

[0076] Example 4

[0077] This embodiment provides a method for preparing coaxial composite fibers, specifically including the following steps:

[0078] 1. Preparation of outer spinning solution: Weigh 15 g PVDF, 0.45 g MnO2 (3% of PVDF mass), and 1.5 g CDs (10% of PVDF mass), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then, transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain a uniform outer spinning solution.

[0079] 2. Preparation of inner layer spinning solution: Weigh 0.18 g MnO2, 6 g PVDF, and 6 g CDs (PVDF to CDs mass ratio 1:1), and adjust the total liquid mass to 100 g with an appropriate amount of DMF. Place the mixture in an 80℃ water bath and stir for 4 hours to fully dissolve the PVDF. Then transfer it to an ultrasonic disperser and sonicate for 3 hours to enhance the dispersion uniformity of CDs and MnO2. Finally, place the resulting solution on a magnetic stirrer and stir for 30 minutes to obtain an inner layer spinning solution with good rheological properties.

[0080] 3. Coaxial wet spinning: The inner and outer spinning solutions are each loaded into a 10 mL syringe and connected to the inner and outer layer propulsion pumps of the microfluidic coaxial wet spinning device via tubing. The outer layer flow rate is set to 0.2 mL / min and the inner layer flow rate to 0.3 mL / min. After the fibers are ejected from the coaxial needle, they directly enter the room temperature deionized water coagulation bath to complete coagulation.

[0081] 4. Fiber drawing and collection: After the fibers are solidified in the coagulation bath, they are drawn by a drawing device and continuously collected by a winding device. The drawing speed is 1.68 m / min. The resulting PVDF / MnO2 / CDs coaxial composite fiber has a clear structure, exhibiting a typical core-shell coaxial structure.

[0082] Comparative Example 1

[0083] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that: no inner spinning solution is used, and only the outer spinning solution is used for uniaxial wet spinning; the other steps are the same as in Example 1.

[0084] The uniaxial wet spinning method is as follows: the spinning solution is loaded into a 10 mL syringe and connected to the propulsion pump of the microfluidic uniaxial wet spinning device through a tubing. The flow rate is set to 0.3 mL / min, and the fibers are ejected from the uniaxial needle and directly enter a room temperature deionized water coagulation bath to complete coagulation.

[0085] This comparative example yielded PVDF / MnO2 / CDs uniaxial composite fibers.

[0086] Comparative Example 2

[0087] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that CDs are not added to either the inner or outer spinning solution, while the other steps are the same as in Example 1.

[0088] Comparative Example 3

[0089] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that: the inner spinning solution is not used, and only the outer spinning solution is used for electrospinning.

[0090] The electrospinning method is as follows: After the mixture is allowed to stand for 12 hours to remove bubbles, a 10 mL syringe is injected and attached to an electrospinning machine. A 21G flat-tipped needle is used, with the voltage set at 15 kV, the receiving distance at 15 cm, and the feed rate at 0.3 mL / h. Fibers are collected on an aluminum foil receiver. After fiber deposition, the fibers are dried in a 60℃ forced-air drying oven for 12 hours. This yields a randomly stacked fiber membrane with a diameter distribution of 20-80 μm and a smooth surface. The membrane is then cut to form a fiber film.

[0091] Comparative Example 4

[0092] This comparative example provides a method for preparing composite fibers, the resulting fibers being post-treated loaded structures, specifically including the following steps:

[0093] 1. Preparation of spinning solution: Dissolve 15 g PVDF in 85 g DMF and stir magnetically at 80°C (800 rpm) for 4 hours to form a homogeneous solution.

[0094] 2. Wet spinning and coagulation: The spinning solution is injected into a syringe and extruded into a deionized water coagulation bath at a flow rate of 0.3 mL / min. After the fiber solidifies, it is wound and collected.

[0095] 3. Drying treatment: The fiber is dried in a 60℃ forced-air drying oven for 12 hours to obtain pure PVDF base fiber (diameter ≈80 μm, smooth surface).

[0096] 4. Fiber pretreatment: Cut PVDF fibers into 10 cm segments, immerse them in anhydrous ethanol and ultrasonically clean for 10 minutes to remove surface impurities.

[0097] 5. MnO2 Deposition and Post-treatment: The fibers were immersed in 200 mL of KMnO4 solution and reacted in an 80℃ water bath. During the reaction, the solution gradually changed from purple to brownish-black. The fibers were then removed, rinsed three times with deionized water, and dried at 60℃ for 6 hours to obtain MnO2 / PVDF fibers (surface covered with MnO2 nanoparticles).

[0098] 6. CDs loading and post-treatment: The CDs dispersion was uniformly sprayed onto the surface of MnO2 / PVDF fibers, three times (with a 10-minute interval) to ensure surface coverage. The coating was then dried at 60℃ for 2 hours to obtain the final electrode material (labeled as MnO2 / PVDF / CDs-coated).

[0099] Comparative Example 5

[0100] This comparative example provides a method for preparing composite fibers, similar to Example 1, except that the amount of PVDF added to the outer spinning solution is 7 g PVDF. All other steps are the same as in Example 1.

[0101] Comparative Example 6

[0102] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that the stretching speed during fiber stretching and collection is 4 m / min, and the other steps are the same as in Example 1.

[0103] Comparative Example 7

[0104] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that the amount of PVDF added in the inner spinning solution is 15 g of PVDF, and the other steps are the same as in Example 1.

[0105] Comparative Example 8

[0106] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that the amount of CDs added in the inner spinning solution is 17 g CDs, and the other steps are the same as in Example 1.

[0107] Comparative Example 9

[0108] This comparative example provides a method for preparing composite fibers, which is similar to Example 1, except that the outer layer flow rate is set to 0.7 mL / min and the inner layer flow rate is set to 0.5 mL / min, while the other steps are the same as in Example 1.

[0109] The composite fibers prepared in Examples 1-4 and Comparative Examples 1-8 were used as electrode materials for performance tests including tensile strength, specific capacity, cycle stability, and charge transfer resistance. The tensile strength test followed GB / T10654-2001; the specific capacity test followed SJ / T 11793-2022; the cycle stability test followed SJ / T 11793-2022; and the charge transfer resistance was obtained through electrochemical impedance spectroscopy. The test results are shown in Table 1.

[0110] Table 1

[0111]

[0112] The PVDF / MnO2 / CDs coaxial composite fiber electrodes prepared in Examples 1-4 exhibit significant differences in mechanical properties, electrochemical properties, and structural stability, mainly due to different CDs contents and the controlled ratio of inner and outer layers. Example 1, with its optimized core-shell structure and an inner PVDF to CDs mass ratio of 1:5, achieved structural stability and uniform CDs distribution. This not only enabled continuous fiber preparation (>100 m, drawing speed 1.68 m / min) but also yielded excellent comprehensive properties: tensile stress up to 5.4 MPa, strain retention of 52%, specific capacitance of 775 F / g, capacity retention of 98% after 5000 cycles, and charge transfer resistance as low as 2.5 Ω, demonstrating excellent mechanical support and efficient charge transport performance. In comparison, both examples 2 and 3, with lower or higher CDs ratios, formed typical core-shell structures, but with slightly lower mechanical and electrochemical properties. The tensile strengths were 5.0 MPa and 4.9 MPa, respectively, and the capacities were 680 F / g and 800 F / g, respectively, with slight fluctuations in Rct (2.7 Ω and 2.3 Ω). The PVDF / MnO2 / CDs coaxial composite fiber of example 4, due to its higher polymer concentration, exhibited performance different from example 1. The tensile strength was 4.5 MPa, with a tensile strain retention of 43%, and the capacitance was 540 F / g, with a capacitance retention of 95%. The charge transfer resistance was 4 Ω. Although the electrochemical performance was slightly lower than example 1, its mechanical properties and production capacity remained superior; optimizing the polymer concentration could further improve performance. Overall, the core-shell structure design and synergistic regulation of CDs have significant advantages in improving the specific capacity, flexibility, conductivity, and cycle stability of the fiber electrode. The specific capacitance graph is shown below. Figure 4 As shown, the CV diagram is as follows Figure 5 As shown.

[0113] The performance results shown in Comparative Examples 1-8 are significantly lower than those in Examples 1-4, which fully demonstrates the significant advantages of the core-shell structure electrode constructed by microfluidic coaxial wet spinning in this invention in terms of structural control and comprehensive performance.

[0114] Comparative Example 1 employed a traditional uniaxial wet spinning process. Although the raw material ratio was the same as in Example 1, the lack of inner and outer layer partitioning in the fiber structure resulted in poor material uniformity, leading to a tensile strength of only 2.4 MPa and a tensile strain retention rate of 22%, significantly lower than the 5.4 MPa and 52% of Example 1. Furthermore, due to insufficient structural density and poor electron / ion transport pathways, its specific capacitance was only 200 F / g, its charge transfer resistance was as high as 7.3 Ω, and its cycle stability was only 85%, exhibiting significant electrochemical performance disadvantages. Moreover, the continuous production length was only 25 m, significantly inferior to Example 1. A comparison of stress-strain curves between coaxial composite fibers and uniaxial fibers (Comparative Example 1) is provided. Figure 3 As shown.

[0115] Although Comparative Example 2 employed a coaxial structure, it lacked the introduction of CDs and thus lacked the interfacial modulation between CDs and PVDF or MnO2. Consequently, its mechanical and electrochemical properties were mediocre: tensile strength 2.7 MPa, strain retention 27%, specific capacitance only 250 F / g, and Rct 8.2 Ω. In contrast, in Example 1, CDs formed hydrogen bonds with PVDF and C–O–Mn bonds with MnO2, significantly enhancing electron mobility and structural stability. The continuous length was only 35 m.

[0116] Comparative Example 3, prepared using electrospinning, can form a certain fiber structure, but the fiber distribution is uneven, the pore structure is uncontrollable, and the production method is intermittent and discontinuous, making it difficult to meet the needs of large-scale fabrication of flexible devices. Its performance is the worst: tensile strength is only 1.2 MPa, strain retention rate is 13%, capacitance is only 190 F / g, and Rct is as high as 8.9 Ω, indicating a loose structure and severe blockage of conductive paths. Furthermore, continuous fabrication is not possible.

[0117] Comparative Example 4 is a post-processed loaded structure. Although the surface is coated with MnO2 and CDs, the material adhesion is poor, the interfacial bonding is weak, the CDs coating is uneven, and the processing is complex and discontinuous. Final properties: tensile strength 2.2 MPa, strain retention 17%, specific capacitance 240 F / g, Rct 8.5 Ω, and cycle stability only 83%, significantly inferior to the examples. Continuous preparation is also not feasible.

[0118] Comparative Example 5 employed a microfluidic coaxial wet spinning process during preparation, but its performance was significantly inferior to that of Example 1. The main problem stemmed from the substantial decrease in the PVDF concentration in the outer spinning solution (from a typical concentration of 15 g to 7 g). The lower PVDF content directly resulted in weakened film-forming ability of the outer layer, insufficient fiber wall thickness, and consequently, incomplete core-shell structure coating, leading to decreased electrode structure stability. Especially during fiber formation, the reduced viscosity of the outer fluid made it difficult to effectively "wrap" the inner layer, easily causing localized shell fractures or collapses, affecting the consistency and integrity of the entire fiber. In terms of performance, it exhibits significant disadvantages: the tensile strength is only 3.5 MPa, indicating weakened mechanical properties due to insufficient outer layer support; the strain retention rate is 36%, and the flexible response capability is lower than that of the example; the specific capacitance is only 370 F / g, and the charge transfer resistance is as high as 6.2 Ω, indicating a serious internal resistance bottleneck in the electrode structure and obstruction of the electron transport channel; the cycle stability is 91%, indicating significant structural degradation under long-term charge and discharge; at the same time, its continuous production length is only 50 m, reflecting the limited fiber formation efficiency and fiber yield under low-concentration PVDF conditions, which cannot support efficient large-scale preparation.

[0119] Comparative Example 6 had a similar raw material mass ratio to Example 1, and the inner layer CDs addition ratio was also maintained within a reasonable range of 1:5. However, this sample showed significant deviations in spinning process parameters, particularly the faster stretching speed, which shortened the core-shell structure formation time, resulting in a blurred core-shell interface and reduced density. This phenomenon led to a decrease in tensile strength to 3.2 MPa, a strain retention rate of 30%, a specific capacitance of 240 F / g, and a charge transfer resistance of 8.2 Ω. Nevertheless, the optimized inner and outer layer flow rates and higher fiber drafting speed maintained its cycle stability at 87%, and the continuous production length was 35 m.

[0120] The excessive addition of PVDF (15 g) to the inner layer of Comparative Example 7 resulted in excessively high viscosity of the inner layer solution, affecting the flow stability during microfluidic spinning. This led to unevenness at the core-shell interface and an increase in pore defects, ultimately resulting in: a tensile strength of only 2.2 MPa, a strain retention rate of 20%, a specific capacitance of only 180 F / g, a charge transfer resistance of 10.5 Ω, a cycle stability decrease to 80%, and a continuous production length of only 22 m. This reflects the dual deficiencies in its structural density and continuous fiber forming ability.

[0121] In Comparative Example 8, the CDs doping ratio was extremely high (PVDF:CDs=1:17), exceeding the optimal CDs distribution concentration range. Excessive CDs easily lead to agglomeration within the fiber, causing structural embrittlement and interruption of conductive pathways. Performance also showed a significant decline: tensile strength was only 1.5 MPa, strain retention was 12%, specific capacitance was only 120 F / g, charge transfer resistance was 14.0 Ω, and cycle stability was 75%. Furthermore, the continuous production length was less than 10 meters, indicating that this ratio severely affected structural integrity and fabrication controllability.

[0122] Compared to Example 1, Comparative Example 9 exhibited insufficient fiber structural stability due to excessively high flow velocities in both the inner and outer layers. This resulted in uneven distribution of core-shell structures (CDs) within the fibers, affecting the quality of core-shell structure formation. Consequently, it exhibited lower tensile stress (2.0 MPa) and strain retention (17%), leading to a significant decrease in mechanical properties. Furthermore, its specific capacitance was only 220 F / g, cycle stability dropped to 81%, and charge transfer resistance increased to 9.4 Ω, indicating that its electrochemical performance was also significantly weakened due to structural disorder.

[0123] In summary, the core-shell structure design, CDs synergistic doping control, and microfluidic continuous processing path embodied in the embodiments, especially Example 1, not only achieve high mechanical strength and excellent flexibility, but also possess excellent electrochemical performance (such as 775 F / g capacitance, 2.5 Ω Rct, and 98% cycle stability), which is far superior to the four comparative examples, verifying the technical advantages and innovative value of the present invention.

[0124] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing coaxial composite fibers, characterized in that, Includes the following steps: S1. Polyvinylidene fluoride, manganese dioxide and carbon dots are mixed and the mixture is heated and stirred, ultrasonically dispersed and magnetically stirred to obtain the outer spinning solution; the mass ratio of carbon dots to polyvinylidene fluoride is 5:100-15:

100. S2. Polyvinylidene fluoride, manganese dioxide and carbon dots are mixed, heated and stirred, ultrasonically dispersed and magnetically stirred to obtain the inner layer spinning solution; the mass ratio of carbon dots to polyvinylidene fluoride is 1:1-15:

1. S3. The obtained inner spinning solution and outer spinning solution are spun and solidified using microfluidic wet spinning technology to obtain composite fibers. S4. The obtained composite fiber is obtained by stretching, winding and curing to obtain coaxial composite fiber; In step S1, the mass fraction of polyvinylidene fluoride in the outer spinning solution is 10%-25%; In step S2, the mass fraction of polyvinylidene fluoride in the inner spinning solution is 1%-8%; In step S3, the flow rate of the outer spinning solution in the microfluidic wet spinning process is 0.1 mL / min-0.5 mL / min; In step S3, the flow rate of the inner spinning solution is 0.1 mL / min-0.3 mL / min; In step S4, the stretching speed during the stretching process is 1.5 m / min to 3.0 m / min.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of manganese dioxide to polyvinylidene fluoride is 3:100-11:

100.

3. The preparation method according to claim 1, characterized in that, In step S2, the mass ratio of manganese dioxide to polyvinylidene fluoride is 3:100-11:

100.

4. The preparation method according to claim 1, characterized in that, In step S3, deionized water is used for coagulation; the coagulation conditions are: 20℃-25℃ for 10 s-30 s.

5. The preparation method according to claim 1, characterized in that, In step S4, the core diameter of the coaxial composite fiber is 40 μm-60 μm, and the shell thickness is 20 μm-40 μm.

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