Flow battery composite diaphragm and preparation method thereof

Abstract

The invention discloses a flow battery composite diaphragm and a preparation method thereof, and belongs to the technical field of flow battery manufacturing. According to the preparation method of the flow battery composite diaphragm provided by the invention, the front and back surfaces of a Nafion membrane are subjected to solvent spraying activation to swell the Nafion membrane, then the swelled surface is coated with a Nafion solution containing a nano material through electrostatic spraying and pre-dried, and an ultrathin blended layer is formed; and then carrying out heat treatment on the composite membrane to obtain the composite diaphragm with a sandwich structure. According to the method, the surface swelling treatment and the electrostatic spraying technology are combined, so that fusion of the coating and the base film is promoted, and layering and nanoparticle agglomeration are prevented; the ultra-thin blending coating with high nano filler content is constructed on the two sides of the Nafion film through electrostatic spraying, the permeability of vanadium ions is reduced, and meanwhile the original conductivity and mechanical performance of a base body are kept.

Description

Technical Field

[0001] This invention relates to the field of flow battery manufacturing technology, specifically to a flow battery composite separator and its preparation method. Background Technology

[0002] Flow batteries, as a long-term energy storage technology, offer advantages such as high safety, long lifespan, independent capacity and power, and easy recyclability, attracting widespread attention and application both domestically and internationally. The separator, a crucial component of flow batteries, functions to conduct ions and separate the active materials of the positive and negative electrodes. Nafion membranes are currently the most widely used separator material for flow batteries, characterized by high proton conductivity and good mechanical and chemical stability. Although vanadium redox flow batteries are considered one of the most mature technologies, the high ion permeability of Nafion ion-conducting membranes leads to vanadium ion cross-contamination, resulting in battery capacity loss and reduced cycle life. Therefore, effectively reducing the permeability of Nafion membranes to active ions while maintaining their high proton conductivity is one of the key technological bottlenecks for improving the overall performance of flow batteries.

[0003] To address the aforementioned issues, existing technologies have undertaken various attempts. For example, patent application CN114243209B discloses a method for preparing composite membranes containing inorganic nanomaterials for alkaline batteries via electrospinning. This method involves adding inorganic materials such as vermiculite and montmorillonite and then using electrospinning to prepare membranes with high porosity and low resistance. However, the high porosity membranes prepared by this method do not meet the proton conductivity requirements in the flow battery field; furthermore, the polyacrylonitrile (PAN) material used in this method does not meet stability requirements in the highly oxidizing environment of flow batteries. Patent application CN120048963A discloses a method for directly adding graphene to a Nafion solution, coating, and drying to prepare a blended composite membrane. While this improves the methanol barrier properties of the membrane, achieving effective modification usually requires the addition of a large amount of nanomaterials. This not only increases costs but may also lead to a significant decrease in the overall proton conductivity of the membrane due to the aggregation or improper distribution of nanomaterials (especially unmodified graphene) in the matrix, thus offsetting some of the benefits of improved barrier performance.

[0004] Therefore, how to achieve both high conductivity and low ion permeability has become a technical challenge that urgently needs to be overcome by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a composite separator for flow batteries and its preparation method, so as to overcome the problem that the existing technology is unable to effectively reduce ion permeability while maintaining high conductivity.

[0006] The present invention solves the above-mentioned technical problems through the following technical solution: This invention provides a method for preparing a composite separator for a flow battery, comprising the following steps: S1. Pre-treat the front and back surfaces of the Nafion film respectively to obtain a surface-swollen Nafion film A; the pre-treatment specifically involves: activating the surface of the Nafion film by solvent spraying to cause the surface of the Nafion film to swell. S2. A Nafion solution containing nanomaterials is coated onto the swollen surface of Nafion membrane A by electrostatic spraying. An ultrathin blend layer is formed on the swollen surface of Nafion membrane A by pre-drying. After heat treatment, a composite separator for flow batteries with a sandwich structure is obtained.

[0007] A further improvement of the present invention is that the solvent in step S1 is N,N-dimethylformamide, and the surface coating amount of the Nafion film is 0.5 to 2 ml / m².

[0008] A further improvement of the present invention is that the swelling of the surface of the Nafion membrane specifically involves forming a 1-2 μm swelling layer on the surface of the Nafion membrane.

[0009] A further improvement of the present invention is that the Nafion solution containing nanomaterials is composed of a solvent, nanomaterials and Nafion polymer, wherein the solvent accounts for 90 to 99 wt% of the mass of the Nafion solution containing nanomaterials; and the amount of nanomaterials added is 5% to 8% of the mass of the Nafion polymer.

[0010] A further improvement of the present invention is that the nanomaterial is one of graphene oxide, surface amino-modified SiO2, or a covalent organic framework material containing sulfonate groups.

[0011] A further improvement of the present invention is that the electrostatic spraying method is specifically as follows: The spraying voltage is 15–20 kV, the spacing between the spraying electrodes is 20 cm, the spraying speed is 1.5–2 ml / h, and the spraying amount is 1–5 ml / m².

[0012] A further improvement of the present invention is that the pre-drying temperature is 50-80°C and the pre-drying time is 3-5 hours.

[0013] A further improvement of the present invention is that the heat treatment temperature is 100-150°C and the heat treatment time is 1-3 hours.

[0014] The present invention also provides a flow battery composite separator, which is prepared by the above-described method for preparing a flow battery composite separator. The flow battery composite separator has a sandwich structure, with the middle layer of the sandwich structure being a Nafion membrane A, and the upper and lower layers of the sandwich structure being ultrathin blended layers.

[0015] A further improvement of the present invention is that the thickness of the ultrathin blend layer is 0.3 to 1 μm.

[0016] Compared with the prior art, the positive and progressive effects of the present invention are as follows: The present invention provides a method for preparing a composite separator for flow batteries. An ultrathin blended layer of nanoparticles is prepared by electrostatic spraying onto both sides of a Nafion membrane. The high barrier properties of this ultrathin nanofiller-containing blended coating reduce the vanadium ion permeability of the flow battery. Since the Nafion membrane itself does not contain nanofillers, its conductivity and mechanical properties remain unchanged, avoiding the decrease in conductivity and mechanical properties caused by nanofiller agglomeration and poor interphase compatibility during direct blending. Solvent spraying allows the Nafion surface to fully swell, enabling the swollen layer and the subsequent electrostatically sprayed layer to fully fuse. The process of preparing the nanomaterial blend layer using electrostatic spraying promotes the bonding between the nanofiller-containing blend coating and the Nafion film substrate, preventing delamination. During this process, nanoparticles are rapidly propelled towards the Nafion base film under electrostatic traction, penetrating the Nafion film's swelling layer and promoting the fusion of the nanofiller blend layer and the Nafion film substrate. Simultaneously, the electrostatic traction prevents agglomeration of the nanoparticles. The swelling layer, lacking fluidity, effectively fixes the nanoparticles, preventing aggregation. Attached Figure Description

[0017] The accompanying drawings are provided to further understand the invention and constitute a part of this invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0018] Figure 1 This is a schematic flowchart of a method for preparing a composite separator for a flow battery according to the present invention. Figure 2 This is a schematic diagram of the structure of a composite separator for a flow battery according to the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0021] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0022] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This is an explanation of the present invention and not a limitation thereof.

[0024] Doping Nafion films with inorganic nanomaterials can effectively reduce the vanadium ion permeability of the films, making it an effective method for Nafion film modification. Traditional blending and doping methods suffer from drawbacks such as nanofiller agglomeration, reduced film conductivity, and decreased mechanical properties. While spin coating and in-situ growth methods can prepare composite films with ultrathin surface layers, they still face certain challenges in continuous large-area industrial production.

[0025] Therefore, see Figure 1 This invention provides a method for preparing a composite separator for a flow battery, comprising the following steps: S1. Pre-treat the front and back surfaces of the Nafion film respectively to obtain a surface-swollen Nafion film A; the pre-treatment specifically involves: activating the surface of the Nafion film by solvent spraying to cause the surface of the Nafion film to swell. S2. A Nafion solution containing nanomaterials is coated onto the swollen surface of Nafion membrane A by electrostatic spraying. An ultrathin blend layer is formed on the swollen surface of Nafion membrane A by pre-drying. After heat treatment, a composite separator for flow batteries with a sandwich structure is obtained.

[0026] The method for preparing a composite separator for flow batteries provided by this invention involves constructing an ultrathin blend layer containing nanofillers only on the front and back surfaces of the Nafion membrane substrate (matrix), while the Nafion membrane substrate itself does not incorporate any nanofillers. This ensures that the original high proton conductivity and excellent mechanical properties of the membrane substrate, which serves as the core channel for ion conduction, are not degraded or altered. The ultrathin blend layer on the surface serves as a high-barrier functional layer, utilizing the physical barrier effect of nanomaterials to effectively reduce the transmembrane permeation of active substances such as vanadium ions. This avoids the inevitable defects caused by directly blending nanomaterials with Nafion solution to form a membrane, namely, the tendency of a large number of nanofillers to aggregate and distribute unevenly in the substrate, which severely hinders the proton transport channel and leads to a significant decrease in the overall proton conductivity of the membrane. Before electrostatically spraying the nanomaterial blend layer, the surface of the Nafion membrane is first activated by solvent spraying, causing its surface layer to swell and loosening the polymer chains on the base film surface, forming a swollen layer of a certain thickness. Subsequently, during the electrostatic spraying process, nanoparticles are driven by electrostatic force to propel the nanoparticles at high speed toward the base film, penetrating and embedding into the swollen layer. After pre-drying, the swollen layer and the sprayed blend layer solution are fully fused and entangled, and finally, through heat treatment, an integrated interface is formed. This effectively prevents the interface delamination problems that may occur with simple coating or lamination, ensuring the structural integrity of the composite membrane under long-term operation and complex conditions of the flow battery. Surface blending layers are prepared using electrostatic spraying technology. Charged nanomaterials / solution droplets are subjected to strong electrostatic attraction in a high-voltage electrostatic field and are deposited on the surface of the base film at high speed and uniformly. This electrostatic dispersion effect can effectively prevent the agglomeration of nanoparticles during the spraying process. More importantly, when nanoparticles are embedded in the aforementioned Nafion film swelling layer, the swelling layer has a certain viscosity and does not have free flow, which can fix the nanoparticles in situ and prevent them from migrating and secondary agglomerating due to Brownian motion or solvent evaporation during the subsequent film formation process. This overcomes the technical bottleneck of easy agglomeration and uneven distribution of nanofillers in solution or solid film in traditional blending methods, so that a small amount of nanofiller can form a barrier network efficiently and uniformly.

[0027] The method of this invention confines nanomaterials within an ultrathin functional layer on the surface, without altering the Nafion membrane matrix. Therefore, the Nafion membrane, as the substrate, retains its inherent excellent chemical stability in the highly oxidizing and acidic electrolyte environment of flow batteries (especially vanadium redox flow batteries), avoiding the chemical degradation risks associated with using other unstable polymer matrices (such as polyacrylonitrile PAN). Furthermore, since the nanomaterials are used only for surface modification, and the electrostatic spraying process has good compatibility with them, various inorganic or organic nanomaterials (such as graphene oxide) can be flexibly selected to achieve specific barrier or functionalization goals without excessive concern about their negative impact on the overall membrane's mechanical strength or conductivity.

[0028] Preferably, the solvent in step S1 is N,N-dimethylformamide, and the surface coating amount of the Nafion film is 0.5–2 ml / m².

[0029] N,N-Dimethylformamide (DMF) was chosen as the activation solvent because it has a moderate swelling capacity for Nafion, effectively relaxing the surface polymer chains to form the desired swollen layer without causing excessive dissolution or deformation of the base film. Precisely controlling the spraying amount within a low range of 0.5–2 ml / m² allows for micro-area, controllable activation of the Nafion film surface, ensuring that the swelling process occurs only on the outermost layer and does not deeply affect the structure and performance of the bulk film.

[0030] Preferably, the swelling of the Nafion membrane surface specifically involves forming a 1-2 μm swelling layer on the surface of the Nafion membrane.

[0031] Limiting the thickness of the swollen layer to 1–2 μm provides a sufficient soft base layer for the subsequent electrostatic spraying of nanoparticles to penetrate and embed, ensuring the robustness of the interface fusion and preventing delamination. On the other hand, it is thin enough to allow the swollen layer to fully fuse with the sprayed Nafion solution during subsequent heat treatment, without forming an independent, weakly conductive layer that may hinder proton transport.

[0032] Preferably, the Nafion solution containing nanomaterials is composed of a solvent, nanomaterials and Nafion polymer, wherein the solvent accounts for 90 to 99 wt% of the mass of the Nafion solution containing nanomaterials; and the amount of nanomaterials added is 5% to 8% of the mass of the Nafion polymer.

[0033] This quantitative range is determined based on the concept of surface modification (rather than overall blending). In ultrathin surface layers, this amount of addition is sufficient to form an effective nano-barrier network, significantly reducing ion penetration. At the same time, due to the small amount of addition and the suppression of agglomeration through electrostatic spraying, the problems of soaring costs, severe agglomeration of nanomaterials, and severe blockage of proton conduction pathways caused by high addition amounts (usually >10% or even higher) are avoided.

[0034] Preferably, the nanomaterial is one of graphene oxide, surface amino-modified SiO2, or a covalent organic framework material containing sulfonate groups.

[0035] The layered structure of graphene oxide provides excellent physical barrier pathways, extending the tortuosity of vanadium ion penetration. Simultaneously, its surface oxygen-containing groups may interact with Nafion, enhancing compatibility. Surface-modified SiO2, with its amino (-NH2) basic groups, can electrostatically interact with sulfonate groups in Nafion materials, forming a denser network and providing additional chemical barrier effects. This synergistic effect with physical barrier further improves selectivity, and the modified SiO2 exhibits better dispersibility. Covalent organic framework materials containing sulfonate groups inherently possess regular channels and abundant sulfonate groups (-SO3H). Their introduction not only provides physical barrier but also allows for proton (H+) absorption. + The transport has a promoting effect (size sieving or electrostatic attraction), while the sulfonate group can form a continuous proton conduction channel with the sulfonate group of Nafion, which is expected to improve or at least not damage the proton conductivity of the composite membrane while blocking vanadium ions.

[0036] Preferably, the electrostatic spraying method is as follows: The spraying voltage is 15–20 kV, the spacing between the spraying electrodes is 20 cm, the spraying speed is 1.5–2 ml / h, and the spraying amount is 1–5 ml / m².

[0037] Preferably, the pre-drying temperature is 50–80°C and the pre-drying time is 3–5 hours.

[0038] The use of relatively mild pre-drying conditions (50~80℃) aims to allow the solvent to evaporate slowly, giving the surface swollen layer and the sprayed blend solution sufficient time for interdiffusion and entanglement of molecular chain segments, promoting initial fusion at the interface and laying a solid foundation for the final heat treatment. On the other hand, it avoids coating cracking, nanoparticle migration and aggregation, or interface stress concentration caused by high-temperature rapid drying. The 3~5 hour duration ensures uniform and thorough pre-curing even for thicker spray coatings.

[0039] Preferably, the heat treatment temperature is 100-150°C and the heat treatment time is 1-3 hours.

[0040] Heat treatment is a key step in forming the final stable sandwich structure. A temperature range of 100–150°C allows residual solvents to evaporate fully, while simultaneously enabling the polymer chains on the surface of the blended layer and at its interface with the base film to move, reconfigure, and tightly bond, achieving complete integration and fusion of the blended layer and the swollen layer of the base film. This eliminates micro-interfaces, enhances interlayer bonding, and improves the mechanical integrity of the composite film. Furthermore, this temperature range is the standard processing temperature for Nafion materials and will not cause decomposition or performance degradation. The 1–3 hour timeframe ensures the adequacy of the heat treatment.

[0041] Based on the same inventive concept, see [link to inventive concept] Figure 2 The present invention also provides a flow battery composite separator, which is prepared by the above-described method for preparing a flow battery composite separator. The flow battery composite separator has a sandwich structure, with the middle layer of the sandwich structure being a Nafion membrane A, and the upper and lower layers of the sandwich structure being ultrathin blended layers.

[0042] The composite separator for flow batteries provided by this invention, by preparing an ultrathin blend layer of nanomaterials on both sides of the Nafion membrane, can effectively reduce the ion permeability of the flow battery while having high conductivity and mechanical strength.

[0043] Preferably, the thickness of the ultrathin blend layer is 0.3–1 μm.

[0044] In a specific embodiment of the present invention, a method for preparing a composite separator for a flow battery includes the following steps: 1) Pre-treat the Nafion membrane by fixing it onto the electrode of an electrostatic spraying machine, then spraying N,N-dimethylformamide solvent onto the surface of the Nafion membrane to treat the surface and cause it to swell. 2) The nanoparticles were dispersed in Nafion's N,N-dimethylformamide solution using ultrasonic dispersion; 3) The solution prepared in 2) is sprayed onto the surface of the Nafion film obtained in 2) by electrostatic spraying. The nanomaterials are fixed on the Nafion surface by electrostatic traction to form an ultrathin blend layer. 4) Pre-dry the membrane prepared in step 3) at 50-80°C for 3-5 hours; 5) Repeat steps 1), 2), 3), and 4) to prepare an ultrathin blend layer containing nanomaterials on the other side of the Nafion membrane; 6) Heat treatment at 100-150°C for 1-3 hours to finally obtain a flow battery separator with a Nafion membrane in the middle and ultra-thin blended layers of nanomaterials on both sides.

[0045] The thickness of the swelling layer on the Nafion membrane surface is controlled at 1–2 μm; the amount of nanomaterials added accounts for 5–8 wt% of the Nafion polymer, and the solvent accounts for 90–99 wt% of the total solution weight; the electrostatic spraying voltage is 10–25 kV, the spraying speed is 1.5–2 ml / h, and the spraying amount is 1–5 ml / m. 2 After drying, the thickness of the nanoparticle blend layer on the surface is 0.3~1 μm.

[0046] The nanoparticle content in the blended layer of this invention can reach 5-8 wt%, which is much higher than that of general blended membranes. Furthermore, the electrostatic spraying method for preparing flow battery composite separators with ultra-thin blended layers on both sides is applicable to various nanomaterials and allows for continuous preparation, making it a simple and effective method for preparing multilayer composite membranes.

[0047] Example 1 Graphene oxide (GO) was used as the nanofiller. 0.03 g of GO and 0.4 g of Nafion polymer were added to 6 g of N,N-dimethylformamide (GO nanoparticles accounted for 7.5% of the Nafion polymer mass, and the solvent content was 93.3%), and the mixture was ultrasonically dispersed for 20 min, then allowed to stand for 24 h to homogenize. At room temperature, 0.5 ml of N,N-dimethylformamide solvent was uniformly sprayed onto a 0.3 m² surface. 2 The Nafion substrate was coated with solvent at a rate of 1.67 ml / m², and allowed to stand for 3 minutes to allow for full swelling. The Nafion substrate was then fixed onto the electrodes of an electrostatic spraying machine, and 1.2 ml of a GO-containing solution was sprayed onto it using electrostatic spraying (a rate of 4 ml / m²). The electrostatic spraying voltage was 20 kV, the electrode spacing was 20 cm, and the spraying speed was 2 ml / h. The resulting membrane was dried at 60°C for 3 hours. A nanomaterial blend layer was then prepared on the other side of the Nafion membrane using the same method. Finally, the membrane was heat-treated at 120°C for 1 hour. The resulting membrane had a vanadium ion permeability of 2.8 × 10⁻⁶. -8 s -1 This is lower than that of the commercial Nafion film (3.90×10). -8 cm 2 .s -1 Meanwhile, the sheet resistivity of the film is only 0.49 Ω·cm. 2 It is only slightly higher than that of the Nafion membrane (0.43 Ω·cm). 2 ), Example 2 Amino-modified SiO2 nanoparticles were used as nanofillers. 2g of the nanoparticles were dispersed ultrasonically in a mixed solvent of 10g water and 40g ethanol, with 0.2g of γ-aminopropyltriethoxysilane added. The mixture was ultrasonicated at 40°C for 1 hour, filtered, and washed three times. The resulting amino-modified SiO2 nanoparticles were then placed in a vacuum drying oven at 90°C for 2 hours. The composite membrane was prepared using the same method as in Example 1. Due to the strong cross-interaction between the amino group and the sulfonate group on the Nafion nanoparticles, the membrane structure was more compact, resulting in a lower vanadium ion permeability. The vanadium ion permeability of the prepared membrane was 2.5 × 10⁻⁶. -8 cm 2 .s -1 The resistivity is lower than that of the composite film prepared in Example 1; at the same time, the sheet resistivity of the prepared film is only 0.52 Ω·cm. 2 The film content is slightly higher than that of the film prepared in Example 1.

[0048] Example 3 A composite membrane was prepared using COF containing sulfonate groups (TpPa-SO3H) as the nanofiller, employing the same composite membrane preparation method as in Example 1. Because TpPa-SO3H itself contains a large number of sulfonate groups, which can promote proton conduction, and its regular pore structure can effectively prevent vanadium ion permeation, the membrane prepared in this case exhibits better conductivity and lower vanadium ion permeability compared to the membranes prepared in Examples 1 and 2. The vanadium ion permeability of the prepared membrane is 2.0 × 10⁻⁶. -8 cm 2 .s -1 The resistivity is lower than that of the composite film prepared in Example 1; at the same time, the sheet resistivity of the prepared film is only 0.44 Ω·cm. 2 The concentration is much lower than that of the films prepared in Examples 1 and 2, and comparable to that of the Nafion film.

[0049] Finally, it should be noted that the embodiments listed above are merely one or more specific manifestations of the technical solution of this invention. Their purpose is to clearly illustrate the concept, principle, and application of this invention through specific examples, and is by no means intended to limit the scope of protection of this invention to these specific embodiments. In fact, the true value of this invention lies in its proposed technical ideas and innovations, rather than its manifestations or implementation methods.

[0050] For those skilled in the art, after thoroughly reading and understanding the technical solution of this invention, they are fully capable of making various changes, modifications, or equivalent substitutions to the specific implementation of the invention based on their own professional knowledge and skills. These changes may include, but are not limited to: adjusting the range of technical parameters, optimizing the algorithm flow to improve efficiency, and replacing some technical components to achieve better compatibility or reduce costs. As long as these modified technical solutions substantially retain the technical features claimed by the original invention, that is, they can still achieve the core functions and effects of this invention, then these changes should be considered to fall within the scope of protection of the pending claims of this invention.

[0051] Furthermore, with the continuous progress and development of technology, new technical means and methods are constantly emerging, which provides ample space for further improvement and perfection of this invention. Therefore, the scope of protection of this invention should also include reasonable and foresightful improvements and extensions based on existing technology. As long as these improvements and extensions do not depart from the basic principles and core concepts of this invention, they should be considered equivalents of this invention and are equally protected by patent rights.

Claims

1. A method for preparing a composite separator for a flow battery, characterized in that, Includes the following steps: S1. Pre-treat the front and back surfaces of the Nafion film respectively to obtain a surface-swollen Nafion film A; the pre-treatment specifically involves: activating the surface of the Nafion film by solvent spraying to cause the surface of the Nafion film to swell. S2. A Nafion solution containing nanomaterials is coated onto the swollen surface of Nafion membrane A by electrostatic spraying. An ultrathin blend layer is formed on the swollen surface of Nafion membrane A by pre-drying. After heat treatment, a composite separator for flow batteries with a sandwich structure is obtained.

2. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The solvent in step S1 is N,N-dimethylformamide, and the surface coating amount of the Nafion film is 0.5–2 ml / m².

3. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The process of swelling the surface of the Nafion membrane specifically involves forming a 1-2 μm swelling layer on the surface of the Nafion membrane.

4. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The Nafion solution containing nanomaterials is composed of a solvent, nanomaterials, and Nafion polymer. The solvent accounts for 90–99 wt% of the mass of the Nafion solution containing nanomaterials, and the amount of nanomaterials added is 5%–8% of the mass of the Nafion polymer.

5. The method for preparing a composite separator for a flow battery according to claim 4, characterized in that, The nanomaterial is one of graphene oxide, surface amino-modified SiO2, or a covalent organic framework material containing sulfonate groups.

6. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The electrostatic spraying method is specifically as follows: The spraying voltage is 15–20 kV, the spacing between the spraying electrodes is 20 cm, the spraying speed is 1.5–2 ml / h, and the spraying amount is 1–5 ml / m².

7. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The pre-drying temperature is 50-80℃, and the pre-drying time is 3-5 hours.

8. The method for preparing a composite separator for a flow battery according to claim 1, characterized in that, The heat treatment temperature is 100–150℃, and the heat treatment time is 1–3 hours.

9. A composite separator for a flow battery, characterized in that, The composite separator for a flow battery is prepared by any one of the preparation methods described in claims 1 to 8. The composite separator for the flow battery has a sandwich structure, with the middle layer of the sandwich structure being a Nafion membrane A, and the upper and lower layers of the sandwich structure being ultrathin blended layers.

10. A composite separator for a flow battery according to claim 9, characterized in that, The thickness of the ultrathin blend layer is 0.3–1 μm.

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