A gradient conductivity electrode, method of making the same, and zinc-based flow battery

Abstract

This application relates to the field of battery materials technology, disclosing a gradient conductivity electrode, its preparation method, and a zinc-based flow battery. The electrode is a self-supporting three-dimensional carbon fiber network structure, and its bulk conductivity exhibits a continuous gradient distribution along the thickness direction perpendicular to the electrode. The electrode is composed of a first component and a second component. Along the thickness direction, the volume percentage of the first component decreases from the membrane side near the electrode to the side away from the membrane; the volume percentage of the second component increases from the membrane side near the electrode to the side away from the membrane. The bulk conductivity of the first component is less than that of the second component. The continuous gradient distribution of bulk conductivity is configured to form an electric field gradient pointing towards the interior of the electrode during charging, guiding zinc ions to diffuse and deposit into the electrode. The technical solution provided by this application can guide the zinc deposition reaction from the membrane interface to the interior of the electrode body, thereby inhibiting zinc dendrite growth and preventing membrane puncture.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and in particular to a gradient conductivity electrode, its preparation method, and a zinc-based flow battery. Background Technology

[0002] Zinc-based flow batteries have attracted much attention in the field of large-scale energy storage due to their high energy density, low cost, and high safety. However, zinc dendrites easily form on the negative electrode during charging, which seriously restricts their long-life cycle performance. Traditional improvement methods, such as using homogeneous three-dimensional carbon felt electrodes or surface zincophilic modification, can provide deposition space or reduce nucleation overpotential, but they fail to fundamentally change the electric field and ion concentration distribution at the electrode / electrolyte interface. In traditional homogeneous electrode systems, the membrane characteristics result in the strongest electric field and the shortest zinc ion transport path on the electrode surface closest to the membrane during the initial stage of charging. This leads to preferential reduction and deposition of zinc ions at the electrode / membrane interface, forming a "near-membrane preferential deposition" mechanism. This causes zinc to accumulate rapidly and locally near the membrane, forming dendrites. Once the dendrites pierce the membrane, it will directly cause a short circuit between the positive and negative electrodes, posing a serious safety hazard.

[0003] Therefore, how to overcome the near-membrane side electric field concentration and ion transport inhomogeneity caused by traditional homogeneous electrodes, guide the zinc deposition reaction from the membrane interface to the inside of the electrode body, thereby inhibiting zinc dendrite growth and preventing membrane puncture, is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] This application provides a gradient conductivity electrode and its preparation method, as well as a zinc-based flow battery. It achieves the technical effect of overcoming the near-membrane side electric field concentration and ion transport unevenness caused by traditional homogeneous electrodes, guiding the zinc deposition reaction from the membrane interface to the interior of the electrode body, thereby inhibiting zinc dendrite growth and preventing membrane puncture.

[0005] To achieve the above objectives, the main technical solutions adopted in this application include: In a first aspect, embodiments of this application provide a gradient conductivity electrode, wherein the electrode is a self-supporting three-dimensional carbon fiber network structure, and the bulk conductivity of the electrode exhibits a continuous gradient distribution in the direction perpendicular to the thickness of the electrode. The electrode is composed of a first component and a second component, wherein... Along the thickness direction, the volume percentage of the first component decreases from the membrane side near the electrode to the side away from the membrane. Along the thickness direction, the volume percentage of the second component increases from the membrane side near the electrode to the side away from the membrane. Wherein, the bulk conductivity of the first component is less than that of the second component; The continuous gradient distribution of the bulk conductivity is configured to form an electric field gradient pointing inwards during charging, so as to guide zinc ions to diffuse and deposit inwards.

[0006] In one embodiment, the first component is polyvinylidene fluoride source carbon fiber, and the second component is polyacrylonitrile source carbon fiber.

[0007] In one embodiment, the specific surface area of ​​the self-supporting three-dimensional carbon fiber network structure is 1.8-5 m². 2 / g.

[0008] In one embodiment, the bulk conductivity of the first component is 0.1-2 S / cm, and the bulk conductivity of the second component is 5-20 S / cm.

[0009] Secondly, embodiments of this application provide a method for preparing a gradient conductivity electrode, the method comprising: Prepare the first and second components; The dual-nozzle electrospinning technology is used to perform co-spinning deposition on the receiving device. During the deposition process, the first liquid supply rate of the nozzle corresponding to the first component is controlled to decrease, while the second liquid supply rate of the nozzle corresponding to the second component is controlled to increase, so as to form a gradient composite fiber membrane. The gradient composite fiber membrane is subjected to pre-oxidation and carbonization treatment to obtain a gradient conductivity electrode with a self-supporting three-dimensional carbon fiber network structure.

[0010] In one embodiment, the gradient composite fiber membrane is formed by stacking multiple sub-fiber membranes; During the deposition of each sub-fiber membrane, the first liquid supply rate and the second liquid supply rate are kept constant; When switching to the next sub-fiber membrane, the first liquid supply rate and the second liquid supply rate are adjusted step by step so that the volume percentage of the first component decreases along the thickness direction and the volume percentage of the second component increases along the thickness direction.

[0011] In one embodiment, the first component is a polyvinylidene fluoride solution, the polyvinylidene fluoride solution having a mass concentration of 12-18%, and the solvent being a mixture of N,N-dimethylformamide and acetone; wherein the mass ratio of N,N-dimethylformamide to acetone is 6:4-7:3. The second component is a polyacrylonitrile solution with a mass concentration of 8-12% and N,N-dimethylformamide as the solvent.

[0012] In one embodiment, the receiving voltage during the deposition process is 12-18kV, and the receiving distance is 15-20cm.

[0013] In one embodiment, the pre-oxidation treatment includes: heating to 220-280°C at a heating rate of 1-5°C / min in an air atmosphere and holding at that temperature for 0.5-2 hours; The carbonization process includes: heating to 800-1200°C at a heating rate of 2-10°C / min under a nitrogen or argon atmosphere, holding at that temperature for 1-3 hours, and then cooling to room temperature with the furnace.

[0014] Thirdly, embodiments of this application provide a zinc-based flow battery, including a positive electrode, a negative electrode, a separator, an electrolyte, and a circulation system. The negative electrode is a gradient conductivity electrode as described above or a gradient conductivity electrode prepared by the preparation method described above. The gradient conductivity electrode is disposed with the first component enrichment side facing the membrane.

[0015] The technical solution provided by one or more embodiments of this application involves an electrode composed of a first component with low conductivity and a second component with high conductivity. The volume percentage of the first component decreases from the separator side to the current collector side, while that of the second component increases. Due to the significant difference in conductivity between the two components, this gradient distribution leads to the formation of a non-uniform potential field inside the electrode, generating an electric field gradient from the separator side to the current collector side during charging. This electric field gradient actively drives zinc ions to overcome diffusion limitations, migrate to the low-potential region inside the electrode, and preferentially deposit, thereby transferring the zinc deposition reaction from the traditional separator interface to the depth of the electrode body. Simultaneously, the self-supporting three-dimensional network structure provides a high specific surface area, effectively reducing local current density and providing sufficient nucleation sites. Combined with the directional guidance of the ion transport path by the gradient electric field, uniform lateral spreading and deposition of zinc are ultimately achieved, suppressing longitudinal dendrite growth and achieving the technical effect of dendrite-free deep charging. This fundamentally avoids the risk of separator puncture and significantly extends the battery cycle life. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 A flowchart illustrating a method for preparing a gradient conductivity electrode according to an embodiment of this application; Figure 2 This is a schematic diagram of the electrode material preparation and structure provided in the embodiments of this application; Figure 3 This is a graph showing the test results of the specific surface area and bulk conductivity of the gradient conductivity electrode provided in Embodiment 1 of this application; Figure 4This is a comparison chart of the charge-discharge energy efficiency of the batteries prepared in Example 1 and Comparative Example 1 of this application at different current densities. Figure 5 These are electrode morphology diagrams prepared in Example 1 and Comparative Example 2 of this application. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] Zinc-based flow batteries have attracted much attention in the field of large-scale energy storage due to their high energy density, low cost, and high safety provided by aqueous electrolytes. However, during charging, zinc dendrites are prone to form on the negative electrode (zinc deposition side), a problem that severely restricts the battery's long-life cycle performance.

[0019] Traditional improvement methods mainly focus on optimizing the physical structure of the electrode or modifying its surface chemically. For example, using a homogeneous three-dimensional carbon felt electrode can provide some deposition space, or zinc-loving modification of the electrode surface can reduce the nucleation overpotential. However, these methods fail to fundamentally change the electric field and ion concentration distribution at the electrode / electrolyte interface.

[0020] Specifically, in traditional homogeneous electrode systems, because the separator typically carries a negative charge or exhibits specific ion selectivity, the electric field strength is strongest near the electrode surface closest to the separator during the initial charging phase, while the transport path for zinc ions is shortest. This causes zinc ions to preferentially reduce and deposit at the electrode / separator interface, where the electric field is strongest and ion transport resistance is lowest. This "near-membrane preferential deposition" mechanism directly leads to intense local accumulation of zinc on the near-membrane side, resulting in a dendritic structure with a magnified tip.

[0021] Even more seriously, once dendrites pierce the diaphragm, it will directly cause a short circuit between the positive and negative electrodes, posing a serious safety hazard.

[0022] In summary, how to overcome the near-membrane side electric field concentration and ion transport inhomogeneity caused by traditional homogeneous electrodes, and guide the zinc deposition reaction from the membrane interface to the interior of the electrode body, thereby inhibiting zinc dendrite growth and preventing membrane puncture, is a technical problem that urgently needs to be solved.

[0023] To address the aforementioned technical problems, this application provides an embodiment of a gradient conductivity electrode, which is a self-supporting three-dimensional carbon fiber network structure, and the bulk conductivity of the electrode exhibits a continuous gradient distribution in the direction perpendicular to the thickness of the electrode. The electrode is composed of a first component and a second component, wherein, Along the thickness direction, the volume percentage of the first component decreases from the membrane side near the electrode to the side away from the membrane. Along the thickness direction, the volume percentage of the second component increases from the membrane side near the electrode to the side away from the membrane. The bulk conductivity of the first component is less than that of the second component. The continuous gradient distribution of bulk conductivity is configured to form an electric field gradient pointing inwards during charging to guide zinc ions to diffuse and deposit inwards.

[0024] Specifically, the electrode is a self-supporting three-dimensional carbon fiber network structure. This structure not only reduces interfacial impedance but also provides abundant porosity to accommodate the electrolyte. The bulk conductivity of the electrode exhibits a continuous gradient distribution along the thickness direction perpendicular to the electrode (Z-axis direction).

[0025] Along the thickness direction, the side closer to the diaphragm is defined as the "diaphragm side", and the side farther away from the diaphragm is defined as the "current collector side".

[0026] The electrode is composed of a first component and a second component.

[0027] The volume percentage of the first component (low conductivity component) decreases from the membrane side near the electrode to the side far from the membrane.

[0028] The volume percentage of the second component (high conductivity component) increases from the membrane side near the electrode to the side far from the membrane.

[0029] Because the bulk conductivity of the first component is significantly lower than that of the second component, the aforementioned component distribution leads to an uneven potential distribution within the electrode. During charging, this conductivity difference creates an electric field gradient pointing inwards towards the electrode interior (i.e., from the diaphragm side to the current collector side). This electric field gradient can effectively drive zinc ions (Zn... 2+ Overcoming diffusion limitations, it migrates and deposits into the low-potential region inside the electrode, thereby achieving dendrite-free deep charging and protecting the membrane interface from damage.

[0030] Preferably, the first component is polyvinylidene fluoride (PVDF) source carbon fiber. After carbonization, PVDF usually has more defects and functional groups, and its bulk electrical conductivity is relatively low, making it suitable as a "slow" component for gradient regulation.

[0031] The second component is polyacrylonitrile (PAN)-based carbon fiber. PAN-based carbon fiber has a high degree of graphitization and excellent electronic conductivity, serving as a "fast" component to construct an electron transport highway.

[0032] Preferably, the specific surface area of ​​the self-supporting three-dimensional carbon fiber network structure is 1.8-5 m². 2 / g. The high specific surface area greatly reduces the local current density, providing ample nucleation sites for zinc ions.

[0033] Because the bulk conductivity of the first component (preferably 0.1-2 S / cm) is significantly lower than that of the second component (preferably 5-20 S / cm), the gradient distribution of these components leads to an uneven potential distribution inside the electrode. During charging, this difference in conductivity creates an electric field gradient pointing towards the inside of the electrode (i.e., from the diaphragm side to the current collector side).

[0034] This embodiment provides a gradient conductivity electrode, which consists of a first component with low conductivity and a second component with high conductivity. The volume percentage of the first component decreases from the separator side to the current collector side, while that of the second component increases. Due to the significant difference in conductivity between the two components, this gradient distribution results in a non-uniform potential field inside the electrode, generating an electric field gradient from the separator side to the current collector side during charging. This electric field gradient actively drives zinc ions to overcome diffusion limitations, migrate to the low-potential region inside the electrode, and preferentially deposit, thereby transferring the zinc deposition reaction from the traditional separator interface to the depth of the electrode body. Simultaneously, the self-supporting three-dimensional network structure provides a high specific surface area, effectively reducing local current density and providing sufficient nucleation sites. Combined with the directional guidance of the ion transport path by the gradient electric field, this ultimately achieves uniform lateral spreading and deposition of zinc, suppressing longitudinal dendrite growth and achieving the technical effect of dendrite-free deep charging. This fundamentally avoids the risk of separator puncture and significantly extends battery cycle life.

[0035] Based on the above structural design, this application also provides an embodiment of a method for preparing a gradient conductivity electrode. Figure 1 A flowchart illustrating a method for fabricating a gradient conductivity electrode according to an embodiment of this application is provided. The method includes: Step S1: Prepare the first component and the second component.

[0036] Specifically, the first component (low conductivity precursor) is a polyvinylidene fluoride (PVDF) solution. The mass concentration of this solution should be controlled between 12% and 18%. To obtain good spinning performance and solubility, a mixed solvent of N,N-dimethylformamide (DMF) and acetone is used. The mass ratio of DMF to acetone must be strictly controlled within the range of 6:4 to 7:3 to adjust the evaporation rate and viscosity of the solution.

[0037] The second component (high conductivity precursor) is a polyacrylonitrile (PAN) solution. Its mass concentration is preferably 8%–12%. The solvent for this component is exclusively N,N-dimethylformamide (DMF).

[0038] PVDF typically exhibits a low degree of graphitization after carbonization, resulting in relatively poor electrical conductivity; while PAN is a classic precursor for preparing highly conductive carbon fibers. The concentration ratio of the two is designed to ensure the rheological properties of the two jets are matched during spinning, avoiding spinning failure due to excessive viscosity differences.

[0039] Step S3: Using dual-nozzle electrospinning technology, co-spinning deposition is performed on the receiving device. During the deposition process, the first liquid supply rate of the nozzle corresponding to the first component is controlled to decrease, while the second liquid supply rate of the nozzle corresponding to the second component is controlled to increase, so as to form a gradient composite fiber membrane.

[0040] Specifically, please refer to Figure 2 This is a schematic diagram illustrating the preparation and structure of the electrode material provided in this application embodiment. The two prepared solutions are respectively loaded into two independent syringes and connected to a dual-nozzle electrospinning device. Co-spinning deposition is performed on a receiving device (such as aluminum foil or a roller). The receiving device is a roller receiver, with its surface covered by aluminum foil. The key lies in controlling the liquid supply system: The liquid supply rate of the nozzle corresponding to the first component (PVDF) is controlled to decrease.

[0041] The liquid supply rate of the nozzle corresponding to the second component (PAN) is controlled to be incrementally distributed.

[0042] During the spinning process, dual-nozzle spinning is initiated. A computer program controls the two syringes in tandem: the PVDF solution propulsion rate decreases linearly or non-linearly from an initial value (preferably 1-2 mL / h) to zero over time, while the PAN solution propulsion rate increases synchronously from zero to a final value (preferably 1-2 mL / h). Simultaneously, the syringes are moved horizontally parallel to the receiver direction, ensuring uniform coverage of the receiver plane and guaranteeing homogeneous composition of the film on the same plane. In other words, in the initial stage of spinning, almost all deposited fibers originate from the PVDF nozzle; as spinning progresses, the proportion of PAN fibers gradually increases, while the proportion of PVDF fibers gradually decreases; in the final stage of spinning, almost all deposited fibers originate from the PAN nozzle. Therefore, a continuous gradient composite structure, from a pure PVDF fiber layer to a pure PAN fiber layer, is formed along the thickness direction on the finally collected fiber membrane.

[0043] The electrostatic field parameters during the deposition process are crucial. The receiving voltage is preferably 12-18kV to ensure the stretching and refinement of the jet; the receiving distance (distance from the nozzle to the receiving plate) is preferably 15-20cm to ensure that the solvent fully evaporates before reaching the receiving plate, forming solid fibers.

[0044] By reversing the liquid supply rate as described above, the fiber membrane deposited at the bottom is rich in PVDF (low conductivity component), while the fiber membrane deposited at the top is rich in PAN (high conductivity component), and a smooth transition is formed in the middle layer.

[0045] To achieve a more precise gradient distribution, a "stepped" deposition strategy can be used: Gradient composite fiber membranes are considered to be composed of multiple layers of sub-fiber membranes stacked together.

[0046] During the deposition of each sub-fiber membrane, the liquid supply rate of the two nozzles is kept constant (i.e., the composition within the layer is uniform).

[0047] When switching to the next sub-fiber membrane, the liquid supply rate is adjusted step by step: the first liquid supply rate is reduced while the second liquid supply rate is increased.

[0048] Although this method forms a "quasi-gradient" or "step gradient", it can still effectively construct a volume ratio change from the diaphragm side (low conductivity) to the current collector side (high conductivity) on a macroscopic level.

[0049] Step S5 involves pre-oxidizing and carbonizing the gradient composite fiber membrane to obtain a gradient conductivity electrode with a self-supporting three-dimensional carbon fiber network structure.

[0050] Specifically, the obtained gradient composite fiber membrane is heat-treated to transform it into a carbon fiber structure.

[0051] Pre-oxidation treatment: The gradient composite fiber membrane is placed in an air atmosphere and heated to 220-280℃ at a heating rate of 1-5℃ / min, and kept at this temperature for 0.5-2h to cause PAN to cyclize and PVDF to dehydrofluorinate, forming a thermally stable gradient structure.

[0052] Carbonization: The pre-oxidized sample is transferred to a tube furnace and heated to 800-1200℃ at a rate of 2-10℃ / min under an inert atmosphere such as nitrogen or argon. After holding at this temperature for 1-3 hours, the sample is cooled to room temperature with the furnace to remove non-carbon elements, forming a carbon structure rich in graphite microcrystals. Since the conductivity of carbonized PAN is much higher than that of PVDF, this process successfully transforms the gradient of chemical composition into a continuous gradient distribution of bulk conductivity, resulting in a gradient conductivity electrode with a self-supporting three-dimensional carbon fiber network structure.

[0053] This embodiment provides a method for preparing a gradient conductivity electrode. By preparing low-conductivity and high-conductivity solutions, it ensures that the two jets have matched viscosity and volatility characteristics during electrospinning, laying the foundation for subsequent co-spinning deposition. Subsequently, a dual-nozzle electrospinning technique is used to perform co-spinning deposition on a receiving device. The liquid supply rate of the two syringes is controlled by a computer program: the propulsion rate of the first component decreases to zero over time, while the propulsion rate of the second component increases synchronously from zero. This ensures that the fibers initially consist almost entirely of the first component and later almost entirely of the second component, forming a continuous gradient composite structure from pure first component to pure second component in the fiber film thickness direction. Finally, the gradient composite fiber membrane was pre-oxidized and carbonized to successfully transform the gradient of chemical components into a continuous gradient distribution of bulk conductivity, resulting in a gradient conductivity electrode with a self-supporting three-dimensional carbon fiber network structure. After pre-oxidation and carbonization, the conductivity gradient is transformed, causing an electric field gradient pointing towards the current collector side to be formed inside the electrode during charging. This actively guides zinc ions to migrate and deposit in the low-potential region inside the electrode, thereby transferring the zinc deposition reaction from the membrane interface to the depth of the electrode body. At the same time, the self-supporting three-dimensional carbon fiber network structure provides abundant pore structure and high specific surface area, effectively reducing interfacial impedance and local current density, providing sufficient nucleation sites for zinc ions, and ultimately achieving uniform zinc deposition, inhibiting dendrite growth, protecting the membrane interface, and extending battery cycle life.

[0054] This application also provides an embodiment of a zinc-based flow battery, including a positive electrode, a negative electrode, a separator, an electrolyte, and a circulation system. The negative electrode is a gradient conductivity electrode as described above or a gradient conductivity electrode prepared by the above preparation method. The gradient conductivity electrode is disposed with the first component enrichment side facing the separator.

[0055] Specifically, the electrode has a continuous gradient structure along the thickness direction, from the PVDF-enriched layer to the PAN-enriched layer. During battery assembly, the first component enriched side (i.e., the PVDF-enriched side) of the gradient conductivity electrode is positioned facing the separator.

[0056] The reason for this configuration is that PVDF has excellent chemical stability and corrosion resistance. Positioning its enriched side towards the separator effectively blocks the electrolyte from eroding the electrode substrate. Simultaneously, the hydrophobic properties of PVDF help reduce the deposition of zinc dendrites on the separator side, thereby improving the battery's cycle stability. Meanwhile, the PAN enriched side has higher conductivity and zinc affinity, which is beneficial for the uniform deposition and stripping of zinc ions, reducing polarization voltage.

[0057] This embodiment provides a zinc-based flow battery that, by aligning the first component enrichment side (i.e., the PVDF enrichment side) of the gradient conductivity electrode towards the separator, fully leverages the excellent chemical stability and corrosion resistance of the PVDF carbide layer, effectively blocking the electrolyte from eroding the electrode substrate. Simultaneously, its hydrophobic properties reduce zinc dendrite deposition on the separator side. Meanwhile, the PAN enrichment side faces the current collector, utilizing its high conductivity and zinc affinity to provide a low-polarization, highly uniform deposition and stripping environment for zinc ions. Thus, the continuous gradient distribution of conductivity within the electrode forms an electric field gradient pointing inwards during charging, actively driving zinc ions to migrate deeper into the electrode body and preferentially deposit. This shifts the zinc deposition reaction from the traditional separator interface to the electrode interior, fundamentally suppressing longitudinal dendrite growth and the risk of separator puncture, achieving dendrite-free deep charging, and significantly improving the battery's cycle stability and lifespan.

[0058] To better explain and facilitate understanding of this application, a detailed description of its specific embodiments is provided below. Unless otherwise specified in the embodiments, all raw materials used in the embodiments of this application are purchased commercially.

[0059] Example 1 Fabrication of gradient conductivity electrodes: Prepare a first component with a mass concentration of 16% using a mixed solvent of N,N-dimethylformamide and acetone; wherein the mass ratio of N,N-dimethylformamide to acetone is 7:3. Prepare a second component with a mass concentration of 10%, using N,N-dimethylformamide as the solvent.

[0060] A dual-nozzle electrostatic spinning device was used, with the following spinning parameters: receiving voltage 15kV, receiving distance 18cm. The receiving device was a flat plate covered with aluminum foil, set to horizontal uniform speed translation, with a total stroke of 20cm and a total time of 4 hours.

[0061] The gradient program was designed as follows: At the start of the translation, the first liquid supply rate of the first component was 1.0 mL / h, and the first liquid supply rate of the second component was 0 mL / h. Then, every 15 minutes, the first liquid supply rate decreased by 0.0625 mL / h, and the second liquid supply rate increased by 0.0625 mL / h, until the spinning ended and the first liquid supply rate dropped to 0 while the second liquid supply rate increased to 1.0 mL / h. A gradient composite fiber membrane with a thickness of approximately 200 μm was finally obtained.

[0062] The gradient composite fiber membrane was pre-oxidized in air at 2℃ / min to 250℃ for 1h; then carbonized in an argon atmosphere at 5℃ / min to 1000℃ for 2h to obtain a gradient conductivity electrode.

[0063] Battery assembly and testing: The aforementioned gradient conductivity electrode was used as the negative electrode, with its low conductivity side (first component) facing the Nafion 212 membrane during assembly. The positive electrode was made of carbon felt. The positive electrode electrolyte was a 0.8 mol / L sodium ferrocyanide solution, and the negative electrode electrolyte was a 0.4 mol / L zinc oxide solution, assembled into a single cell.

[0064] The gradient conductivity electrode prepared in Example 1 was uniformly cut into 5 layers along the thickness direction using a cryostat. The specific surface area of ​​each layer was measured using the nitrogen adsorption-desorption method, and the bulk conductivity of each layer was measured using a four-probe resistivity meter.

[0065] Test results are as follows Figure 3 As shown, Figure 3 The graph shows the test results of the specific surface area and bulk conductivity of the gradient conductivity electrode provided in Embodiment 1 of this application. Along the thickness direction from the first component (PVDF enriched side) to the second component (PAN enriched side), the specific surface area of ​​the gradient conductivity electrode increases from approximately 4.6 m² / s. 2 / g gradually decreased to 1.1m 2 / g, showing a decreasing gradient; while the bulk conductivity increases significantly from close to 0 S / cm to over 16 S / cm, showing an increasing gradient.

[0066] The results show that the gradient conductivity electrode prepared in Example 1 successfully constructed an opposite gradient distribution of conductivity and specific surface area in the thickness direction. This unique structure is beneficial for balancing electron transport and ion diffusion.

[0067] Example 2 Fabrication of gradient conductivity electrodes: Prepare a first component with a mass concentration of 12% using a mixed solvent of N,N-dimethylformamide and acetone; wherein the mass ratio of N,N-dimethylformamide to acetone is 6:4. Prepare a second component with a mass concentration of 8%, using N,N-dimethylformamide as the solvent.

[0068] A dual-nozzle electrostatic spinning device was used, with the following spinning parameters: receiving voltage 12kV, receiving distance 15cm. The receiving device was a flat plate covered with aluminum foil, set to move horizontally at a uniform speed, with a total stroke of 20cm and a total time of 4 hours.

[0069] The gradient program was designed as follows: At the start of the translation, the first liquid supply rate of the first component was 1.5 mL / h, and the first liquid supply rate of the second component was 0 mL / h. Then, every 15 minutes, the first liquid supply rate decreased by 0.0625 mL / h, and the second liquid supply rate increased by 0.0625 mL / h, until the spinning ended and the first liquid supply rate dropped to 0 while the second liquid supply rate increased to 1.5 mL / h. A gradient composite fiber membrane with a thickness of approximately 200 μm was finally obtained.

[0070] The gradient composite fiber membrane was pre-oxidized for 2 hours by heating it to 280°C in air at a rate of 1°C / min; then it was carbonized for 3 hours by heating it to 1200°C in an argon atmosphere at a rate of 2°C / min to obtain a gradient conductivity electrode.

[0071] Battery assembly and testing: The aforementioned gradient conductivity electrode was used as the negative electrode, with its low conductivity side (first component) facing the Nafion 212 separator. The positive electrode was made of carbon felt. The positive electrode electrolyte was a 0.8 mol / L sodium ferrocyanide solution, and the negative electrode electrolyte was a 0.4 mol / L zinc oxide solution. Electrochemical performance was tested after assembling the cells into single cells.

[0072] Example 3 Fabrication of gradient conductivity electrodes: Prepare a first component with a mass concentration of 18% using a mixed solvent of N,N-dimethylformamide and acetone; wherein the mass ratio of N,N-dimethylformamide to acetone is 7:4. Prepare a second component with a mass concentration of 12% using N,N-dimethylformamide as the solvent.

[0073] A dual-nozzle electrostatic spinning device was used, with the following spinning parameters: receiving voltage 18kV, receiving distance 20cm. The receiving device was a flat plate covered with aluminum foil, set to move horizontally at a uniform speed, with a total stroke of 20cm and a total time of 4 hours.

[0074] The gradient program was designed as follows: At the start of the translation, the first liquid supply rate of the first component was 2.0 mL / h, and the first liquid supply rate of the second component was 0 mL / h. Then, every 15 minutes, the first liquid supply rate decreased by 0.0625 mL / h, and the second liquid supply rate increased by 0.0625 mL / h, until the spinning ended and the first liquid supply rate dropped to 0 while the second liquid supply rate increased to 2.0 mL / h. A gradient composite fiber membrane with a thickness of approximately 200 μm was finally obtained.

[0075] The gradient composite fiber membrane was pre-oxidized in air at 5℃ / min to 280℃ for 0.5h; then carbonized in an argon atmosphere at 10℃ / min to 800℃ for 1h to obtain a gradient conductivity electrode.

[0076] Battery assembly and testing: The aforementioned gradient conductivity electrode was used as the negative electrode, with its low conductivity side (first component) facing the Nafion 212 separator. The positive electrode was made of carbon felt. The positive electrode electrolyte was a 0.8 mol / L sodium ferrocyanide solution, and the negative electrode electrolyte was a 0.4 mol / L zinc oxide solution. Electrochemical performance was tested after assembling the cells into single cells.

[0077] Comparative Example 1 Similar to Example 1, except that only the second component (PAN solution) was used for spinning deposition, the liquid supply rate was 1.0 mL / h, the receiving plate was stationary, and a homogeneous PAN carbon fiber electrode was obtained.

[0078] The homogeneous PAN carbon fiber electrode was assembled into a battery in the same manner as in Example 1.

[0079] Please see Figure 4 This is a comparison chart of the charge and discharge energy efficiencies of the batteries prepared in Example 1 and Comparative Example 1 of this application under different current densities.

[0080] As shown in the figure, from 80 mA / cm 2 Gradually increase to 320 mA / cm 2 During the process, the energy efficiency of the battery in Example 1 was consistently better than that in Comparative Example 1.

[0081] This indicates that the gradient conductivity electrode of Example 1 exhibits superior electrochemical kinetic performance. On one hand, the high conductivity of the first component (PAN-enriched side) significantly reduces the ohmic polarization of the electrode, ensuring efficient electron transport; on the other hand, the high specific surface area and suitable pore structure of the second component (PVDF-enriched side) promote rapid ion diffusion, alleviating concentration polarization at high current densities. Furthermore, when the current density recovers to 80 mA / cm², [the electrode exhibits better electrochemical kinetic performance]. 2 When the battery in Example 1 was in operation, its efficiency recovered rapidly and remained high, demonstrating that the gradient conductivity electrode could effectively suppress the growth of zinc dendrites, prevent micro-short circuits and structural damage inside the battery, and exhibit excellent cycle stability.

[0082] Comparative Example 2 Same as Example 1, except that the mass concentration of the first component is 20% and the mass concentration of the second component is 15%.

[0083] Please see Figure 5 These are electrode morphology images obtained from Examples 1 and 2 of this application. Based on the morphology characterization of Examples 1 and 2, it can be seen that Example 1 successfully achieved the extraction of electrodes from smooth PVDF fibers (…) by adjusting the flow rate of the dual nozzles. Figure 5 (a) to coarse PAN carbon fiber ( Figure 5 The continuous transition in (b) of the microstructure directly corresponds to the gradient change in macroscopic conductivity. Comparative Example 2 ( Figure 5 As shown in (c), strict control of precursor concentration is crucial. In Comparative Example 2, excessively high concentration resulted in severe bead-like defects and pore blockage, preventing the formation of effective ion / electron transport channels.

[0084] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0085] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

[0086] Although embodiments of this application have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of this application, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A gradient conductivity electrode, characterized in that, The electrode is a self-supporting three-dimensional carbon fiber network structure, and the bulk conductivity of the electrode exhibits a continuous gradient distribution in the direction perpendicular to the thickness of the electrode. The electrode is composed of a first component and a second component, wherein... Along the thickness direction, the volume percentage of the first component decreases from the membrane side near the electrode to the side away from the membrane. Along the thickness direction, the volume percentage of the second component increases from the membrane side near the electrode to the side away from the membrane. Wherein, the bulk conductivity of the first component is less than that of the second component; The continuous gradient distribution of the bulk conductivity is configured to form an electric field gradient pointing inwards during charging, so as to guide zinc ions to diffuse and deposit inwards.

2. The electrode according to claim 1, characterized in that, The first component is polyvinylidene fluoride source carbon fiber, and the second component is polyacrylonitrile source carbon fiber.

3. The electrode according to claim 1, characterized in that, The specific surface area of ​​the self-supporting three-dimensional carbon fiber network structure is 1.8-5m². 2 / g.

4. The electrode according to claim 1, characterized in that, The bulk conductivity of the first component is 0.1-2 S / cm, and the bulk conductivity of the second component is 5-20 S / cm.

5. A method for preparing a gradient conductivity electrode, characterized in that, The preparation method includes: Prepare the first and second components; The dual-nozzle electrospinning technology is used to perform co-spinning deposition on the receiving device. During the deposition process, the first liquid supply rate of the nozzle corresponding to the first component is controlled to decrease, while the second liquid supply rate of the nozzle corresponding to the second component is controlled to increase, so as to form a gradient composite fiber membrane. The gradient composite fiber membrane is subjected to pre-oxidation and carbonization treatment to obtain a gradient conductivity electrode with a self-supporting three-dimensional carbon fiber network structure.

6. The preparation method according to claim 5, characterized in that, The gradient composite fiber membrane is composed of multiple layers of sub-fiber membranes stacked together. During the deposition of each sub-fiber membrane, the first liquid supply rate and the second liquid supply rate are kept constant; When switching to the next sub-fiber membrane, the first liquid supply rate and the second liquid supply rate are adjusted step by step so that the volume percentage of the first component decreases along the thickness direction and the volume percentage of the second component increases along the thickness direction.

7. The preparation method according to claim 5, characterized in that, The first component is a polyvinylidene fluoride solution with a mass concentration of 12-18% and a solvent of N,N-dimethylformamide and acetone; wherein the mass ratio of N,N-dimethylformamide to acetone is 6:4-7:

3. The second component is a polyacrylonitrile solution with a mass concentration of 8-12% and N,N-dimethylformamide as the solvent.

8. The preparation method according to claim 5, characterized in that, The receiving voltage during the deposition process is 12-18kV, and the receiving distance is 15-20cm.

9. The preparation method according to claim 5, characterized in that, The pre-oxidation treatment includes: heating to 220-280°C at a heating rate of 1-5°C / min in an air atmosphere, and holding at that temperature for 0.5-2 hours; The carbonization process includes: heating to 800-1200°C at a heating rate of 2-10°C / min under a nitrogen or argon atmosphere, holding at that temperature for 1-3 hours, and then cooling to room temperature with the furnace.

10. A zinc-based flow battery, comprising a positive electrode, a negative electrode, a separator, an electrolyte, and a circulation system, characterized in that, The negative electrode is a gradient conductivity electrode as described in any one of claims 1 to 4 or a gradient conductivity electrode prepared by the preparation method described in any one of claims 5 to 9; The gradient conductivity electrode is disposed with the first component enrichment side facing the membrane.

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