Functional composite membrane for zinc-bromine flow battery and preparation and application thereof

Abstract

The application discloses application of a composite membrane with a functional coating in a zinc-bromine flow battery. The membrane is composed of a support layer and a coating layer with zinc dendrite inhibition function and bromine diffusion inhibition function. The support layer can endow the composite membrane with good ion conduction performance and mechanical performance. The functional coating contains a zinc dendrite inhibitor, which can inhibit the generation and growth of zinc dendrites while inhibiting bromine diffusion, thereby solving the self-discharge problem of the zinc-bromine flow battery caused by bromine diffusion and the short circuit problem of the battery caused by the negative zinc dendrite puncturing the separator during the charging and discharging process, and finally improving the performance and cycle stability of the zinc-bromine flow battery.

Description

Technical Field

[0001] This invention provides the preparation and application of a porous polyolefin composite membrane, particularly its application in the field of zinc-bromine flow batteries. Background Technology

[0002] In recent years, there has been a growing call for the use of renewable and clean energy. However, renewable energy power generation, such as wind and solar power, is significantly discontinuous and unstable due to seasonal, meteorological, and geographical conditions. The generated electricity fluctuates greatly and has poor adjustability, potentially causing significant impacts on the power grid. Therefore, with the rapid rise of renewable energy sources like wind and solar power and the smart grid industry, energy storage technology has become a focus of attention. Large-scale energy storage technology is considered a strategic technology supporting the widespread adoption of renewable energy and has received high attention from governments and businesses worldwide.

[0003] Energy storage technologies are broadly categorized into physical energy storage and chemical energy storage. Physical energy storage includes pumped hydro storage, compressed air storage, and flywheel energy storage. Chemical energy storage primarily includes lead-acid batteries, sodium-sulfur batteries, flow batteries, and lithium-ion batteries. However, each energy storage technology has its suitable application areas. Chemical energy storage technologies suitable for large-scale energy storage mainly include flow batteries, sodium-sulfur batteries, lead-acid batteries, and lithium-ion batteries. Considering the advantages and disadvantages of various energy storage technologies, flow battery energy storage technology has received more widespread attention. Among them, zinc-bromine flow batteries have the advantages of high energy density and low cost, making them particularly suitable for distributed energy storage and attracting increasing attention. However, bromine has strong volatility and diffusion properties. During battery operation, bromine generated at the positive electrode diffuses to the negative electrode, reducing the active material content of the positive electrode and affecting the active material of the negative electrode, causing self-discharge, reducing battery efficiency, and severely impacting battery capacity and cycle life. Furthermore, during charging, zinc dendrites at the negative electrode can pierce the separator, causing a short circuit in the battery. Therefore, bromine diffusion and zinc dendrite problems severely affect the performance and lifespan of zinc-bromine flow batteries, thus hindering their further commercialization and industrialization.

[0004] In zinc-bromine flow batteries, the ion-conducting membrane is a crucial component, accounting for a significant proportion of the battery cost. Therefore, developing a low-cost, high-performance, and stable ion-conducting membrane for batteries is one of the important ways to suppress bromine diffusion and zinc dendrite formation, reduce the cost of zinc-bromine flow batteries, and improve battery performance.

[0005] Polyolefin porous membranes possess advantages such as good chemical and mechanical stability, excellent conductivity, and low cost, leading to their widespread application in lead-acid batteries and other fields. However, when applied to flow batteries, polyolefin porous membranes exhibit low selectivity and cannot effectively address the zinc dendrite problem, easily causing short circuits and resulting in decreased battery performance and shortened lifespan. Therefore, functional coatings are applied to the surface of polyolefin porous membranes to improve their selectivity. The coating contains zinc dendrite inhibitors, which can effectively regulate zinc deposition behavior, inhibit the formation and growth of zinc dendrites, and achieve a smooth and uniform zinc deposition morphology on the negative electrode surface, thereby reducing battery polarization and improving battery performance. Furthermore, the coating has a non-porous, dense structure that effectively prevents bromine diffusion and reduces battery self-discharge reactions. In addition, the high stability of the polyolefin porous membrane substrate ensures excellent stability of the composite membrane in flow batteries, guaranteeing long-term stable battery operation. Moreover, there are numerous commercially available polyolefin porous membranes, and the preparation process of the functional coating is simple and easy to implement, with a simple and controllable membrane fabrication process suitable for large-scale production. According to the requirements of zinc-bromine flow batteries, the ability of polyolefin composite films to suppress bromine diffusion and zinc dendrites can be adjusted by regulating the type and content of zinc dendrite inhibitors in the coating and the coating thickness, thereby controlling the performance and lifespan of zinc-bromine flow batteries. Summary of the Invention

[0006] The purpose of this invention is to prepare a polyolefin composite membrane with a functional coating for zinc-bromine flow batteries. By utilizing the zinc dendrite inhibitor in the functional coating, the problem of zinc dendrites caused by uneven zinc deposition during the charging process can be solved, which damages the membrane and ultimately leads to micro-short circuit failure of the battery. The barrier effect of the non-porous dense coating can be used to solve the problem of battery self-discharge caused by bromine diffusion, thereby improving the performance and cycle life of zinc-bromine flow batteries.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A composite membrane includes a polyolefin porous membrane support layer; a functional coating is laminated to one side surface of the support layer;

[0009] The functional coating contains a zinc dendrite inhibitor, which includes at least one or more organic compounds such as polyethylene glycol, polyethyleneimine, polyethylene glycol octylphenyl ether, polyvinyl alcohol, polyethyleneimine, polytetrafluoroethylene, cellulose urea, and aniline.

[0010] The thickness of the functional coating is 0.01–60 μm, preferably 0.1–40 μm, and more preferably 1–20 μm.

[0011] The functional coating consists of a zinc dendrite inhibitor and a binder, wherein the binder is a perfluorosulfonic acid polymer, and the mass ratio of the zinc dendrite inhibitor to the binder is 1-60:3-40, preferably 5-40:5-30, and more preferably 5-20:8-20.

[0012] The polyolefin porous membrane is an ion-conducting membrane with a porous structure composed of at least one or more of polyethylene, polypropylene, ultra-high molecular weight polyethylene, and ultra-high density polyethylene.

[0013] The support layer is a commercially available polyolefin membrane with a porous structure, having a thickness of 175-1000 μm, a porosity of 30%-86%, and a pore size distribution range of 0.001-500 nm.

[0014] The method for preparing the composite membrane includes the following steps:

[0015] (1) The functional coating material, namely zinc dendrite inhibitor, binder and solvent, is mixed to obtain a functional coating solution;

[0016] (2) The functional coating solution is applied to one side surface of the support layer by one or more of the following methods: spraying, spin coating, scraping, electrospinning, dip coating or interfacial polymerization, to obtain the composite film.

[0017] The solvent in step (1) is at least one or more of the following: acetic acid, water, ethanol, methanol, isopropanol, N,N-dimethylacetamide, N,N-dimethylformamide, chloroform, 1,2-dichloroethane, benzene, toluene, and xylene.

[0018] The concentration of the functional coating solution is 1wt%-60wt%, and the concentration of the binder is 3-40wt%.

[0019] The composite membrane is used as a separator in zinc-bromine flow batteries to solve the problems of bromine diffusion and zinc dendrite formation. When a functional coating is laminated on one side of the support layer, it is preferable to place the side with the functional coating closer to the negative electrode side.

[0020] Beneficial results of the present invention

[0021] 1. The functional composite membrane for zinc-bromine flow batteries prepared by this invention effectively alleviates the problems of short circuits caused by zinc dendrites piercing the separator and self-discharge caused by bromine diffusion during battery operation.

[0022] 2. The functional composite membrane for zinc-bromine flow batteries prepared by this invention contains zinc dendrite inhibitors in its functional coating material, which have a good ability to inhibit zinc dendrites, and the barrier effect of the non-porous dense coating can effectively prevent the diffusion of bromine.

[0023] 3. The functional composite membrane for zinc-bromine flow batteries prepared by this invention has adjustable types and contents of zinc dendrite inhibitors in the functional coating, and adjustable coating thickness, which enables controllable performance of zinc-bromine flow batteries.

[0024] 4. The functional composite membrane for zinc-bromine flow batteries prepared by this invention broadens the types and application range of membrane materials for zinc-bromine flow batteries.

[0025] Figure 3 Cyclic performance of zinc-bromine flow batteries assembled with composite membrane A1 and Daramic base membrane Attached Figure Description

[0026] Figure 1 Surface morphology of the A1 coating side of the composite film

[0027] Figure 2 (a) Composite membrane A1; (b) Zinc deposition morphology on the negative electrode surface of a zinc-bromine flow battery assembled with Daramic base membrane.

[0028] Figure 3 Cyclic performance of zinc-bromine flow batteries assembled with composite membrane A1 and Daramic base membrane

[0029] Figure 4 Zinc deposition morphology on the negative electrode of the battery assembled with composite film A1 after 120 cycles.

[0030] Figure 5 After more than 20 cycles, the zinc deposition morphology on the negative electrode of the battery assembled with the Daramic base film is shown.

[0031] Figure 6 Surface morphology of composite film a9 coating measured Detailed Implementation

[0032] Zinc-bromine flow battery cycle performance test conditions: end plates are made of stainless steel, bipolar plates are made of graphite, and both positive and negative electrodes use carbon felt as electrodes with an effective electrode area of ​​36 cm². 2 The positive and negative electrode electrolytes are both 2 mol / L zinc bromide + 3 mol / L potassium chloride + 0.8 mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The volume of each electrolyte is 60 mL. The battery uses a constant current charge-discharge mode at 40 mA cm⁻¹. -2 Charged for 60 minutes under the given current density, then under voltage cutoff conditions, at 40 mA cm. -2 Discharged to 0.8V under the current density conditions.

[0033] The following embodiments are further illustrations of the present invention, but not limitation thereof. The support layer is a commercially available Daramic membrane (polyethylene) (thickness: 200 μm, porosity: 60%, pore size distribution range: 0.1-100 nm), the zinc dendrite inhibitor is polyvinyl alcohol (PVA), and the binder is perfluorosulfonic acid resin or perfluorosulfonic acid polymer (PFSA). The self-discharge test is performed by charging the battery for 60 minutes, allowing it to stand for 24 hours, and then discharging it at a cutoff voltage of 0.8V to test the battery's capacity retention.

[0034] Example 1

[0035] First, PVA and PFSA were dissolved in the organic solvent isopropanol (IPA) and stirred thoroughly at 25°C for 48 hours to prepare a homogeneous blend solution; the mass concentration of PVA was 10%, and the mass concentration of PFSA was 10%. Then, the prepared PVA / PFSA blend solution was uniformly coated onto one side surface of a Daramic membrane substrate using a blade coating method. The substrate was then transferred to a 50°C hot plate and heated for 4 hours. After cooling to room temperature, a PVA / PFSA / Daramic composite membrane (defined as composite membrane A1) with a non-porous, dense PVA / PFSA coating was obtained; the thickness of the non-porous, dense PVA / PFSA coating was 5 μm. Figure 1 As shown, the surface morphology of the A1 coating side of the composite film is dense and flat, with no pores present.

[0036] The composite membrane A1 underwent performance testing and was compared with the performance of the Daramic base membrane. The sheet resistivity of composite membrane A1 is shown in Table 1. Due to the extremely high hydrophilicity of PVA, its introduction promotes ion conduction within the membrane, increasing the ionic conductivity and resulting in a lower sheet resistivity for composite membrane A1 compared to the Daramic base membrane. This demonstrates that composite membrane A1 exhibits higher ionic conductivity. The bromide ion concentration (Br) of composite membrane A1 is also shown in Table 1. - The permeability is shown in Table 1. Because the PVA / PFSA coating is a non-porous, dense coating, it enhances the membrane's barrier effect against bromine, resulting in a lower bromide ion permeability for the composite membrane A1 compared to the Daramic base membrane. This demonstrates that the composite membrane A1 also possesses excellent ion selectivity. Therefore, coating the Daramic membrane with a non-porous, dense PVA / PFSA coating can improve both its conductivity and selectivity.

[0037] The fabricated composite membrane A1 was applied to a zinc-bromine flow battery at 40 mA cm⁻¹. -2Under the operating current density conditions, the battery assembled with the Daramic base film exhibits a coulombic efficiency of 94.11%, a voltage efficiency of 78.52%, and a capacity retention rate of 43% after self-discharge testing. However, for the composite film A1, firstly, the barrier effect of the non-porous, dense coating effectively reduces the bromine diffusion rate of the composite film, decreasing the battery's self-discharge reaction and improving the battery's coulombic efficiency, capacity retention rate after self-discharge testing, and cycle performance. Furthermore, during charging, the highly hydrophilic PVA significantly promotes ion migration, thus replenishing the zinc ions consumed at the negative electrode and ensuring a uniform zinc ion concentration distribution on the negative electrode surface. Secondly, the PVA polymer's insulating properties cause a decrease in the current density at the zinc tip upon contact with the deposited zinc, preventing newly reduced zinc atoms from depositing at the already deposited zinc tip, instead depositing them in other areas not in contact with the coating, thus ensuring uniform and smooth zinc deposition. Therefore, the zinc deposited on the negative electrode of the battery assembled with the composite film A1 is more uniform and smooth. Figure 2 a) Therefore, zinc dendrites will not form and grow on the negative electrode surface, micro-short circuits will not occur in the battery, and zinc will not grow into the pores of the porous membrane, hindering ion migration and leading to increased battery polarization. Therefore, batteries assembled with composite films can have lower battery polarization, higher coulombic efficiency, higher capacity retention, and better cycle performance. Therefore, at 40 mA cm⁻¹ -2 Under the operating current density conditions, the coulombic efficiency of the battery assembled with composite membrane A1 increased to 98.89%, the voltage efficiency increased to 84.63%, and the capacity retention rate after self-discharge testing was as high as 80% (Table 2). Furthermore, the zinc-bromine-zinc-iron flow battery assembled with the composite membrane achieved a high efficiency at 40 mA cm⁻¹. -2 Under operating current density conditions, it can operate continuously and stably for more than 120 cycles, and the battery performance remains stable, demonstrating excellent stability. Figure 3 After 120 cycles, the zinc deposited on the negative electrode of the battery assembled with composite film A1 remained uniform and smooth, without the formation of zinc dendrites. Figure 4 However, when the Daramic film is applied to a zinc-bromine flow battery, the zinc deposited on the negative electrode is very uneven and rough, containing a large number of hexagonal zinc flakes with very sharp edges. Figure 2 a) It can puncture the membrane and come into contact with the positive electrode, causing a micro-short circuit or even a short circuit in the battery, and it can react with the bromine in the positive electrode, leading to self-discharge of the battery. Therefore, zinc-bromine flow batteries assembled with Daramic base membranes have low coulombic efficiency, severe capacity decay, and short cycle life, only about 20 cycles (Table 2 and...). Figure 3 Furthermore, after the cycle is complete, the zinc deposited on the negative electrode is extremely uneven and rough. Figure 5Therefore, coating the surface of the Daramic base film with a functional coating containing zinc dendrite inhibitors can effectively suppress bromine diffusion and zinc dendrites, thereby improving battery efficiency, reducing self-discharge reactions, and increasing battery life (Table 3).

[0038] Example 2

[0039] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the thickness of the PVA / PFSA coating was 0.05 μm (defined as composite membrane A2).

[0040] The performance of composite membrane A2 was tested. The sheet resistivity of composite membrane A2 is shown in Table 1. Due to the decrease in the thickness of the non-porous dense PVA / PFSA coating, the membrane's ion conductivity is enhanced, resulting in a lower sheet resistivity for composite membrane A2 compared to composite membrane A1. This demonstrates that composite membrane A2 has higher ion conductivity. The bromide ion concentration (Br) of composite membrane A2 is also shown in Table 1. - As shown in Table 1, due to the decrease in the thickness of the PVA / PFSA dense coating, the membrane's ability to block bromine decreases, resulting in a higher bromide ion permeability of composite membrane A2 than that of composite membrane A1, but still lower than that of the Daramic base membrane. This indicates that composite membrane A2 still has relatively high bromine selectivity.

[0041] When the prepared composite membrane A2 was used in a zinc-bromine flow battery, its thin coating resulted in weak bromine diffusion suppression, leading to increased bromine permeability and exacerbated self-discharge. Furthermore, the thin coating implied a lower content of zinc dendrite inhibitors, further reducing its ability to suppress zinc dendrites. Zinc dendrites at the negative electrode easily damage the membrane, leading to decreased battery performance. Therefore, at 40 mA cm⁻¹... -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A2 decreased to 98.23%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane; the voltage efficiency was 85.46%, higher than that of the zinc-bromine flow batteries assembled with composite membrane A1 and Daramic base membrane (Table 2). After the self-discharge test, the battery's capacity retention rate was 75%, lower than that of composite membrane A1, but higher than that of the battery assembled with Daramic base membrane (Table 2). Due to the thinner coating, the battery can operate stably for more than 100 cycles, with cycle performance inferior to composite membrane A1 but superior to Daramic base membrane (Table 3).

[0042] Example 3

[0043] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the thickness of the PVA / PFSA coating was 50 μm (defined as composite membrane A3).

[0044] The performance of composite membrane A3 was tested. The sheet resistivity of composite membrane A3 is shown in Table 1. Due to the increased thickness of the dense PVA / PFSA coating, the membrane's ion conductivity decreased, resulting in a higher sheet resistivity for composite membrane A3 compared to composite membrane A1. However, it remained lower than the sheet resistivity of the Daramic base membrane, indicating that composite membrane A3 still exhibits high ion conductivity. The bromide ion concentration (Br) of composite membrane A3 was also tested. - As shown in Table 1, due to the increased thickness of the PVA / PFSA dense coating, the membrane's ability to block bromine is enhanced, resulting in a lower bromide ion permeability of composite membrane A3 compared to composite membrane A1.

[0045] When composite membrane A3 is used in zinc-bromine flow batteries, its thicker coating enhances its ability to suppress bromine diffusion, reducing bromine permeability and thus weakening self-discharge. The thicker coating also implies a higher content of zinc dendrite inhibitors, further increasing its ability to suppress zinc dendrites. However, the increased coating thickness also means a higher PFSA content. Both the increased thickness and PFSA content lead to a decrease in the membrane's ion transport capacity, preventing the timely transfer of zinc ions from the positive electrode to the negative electrode to compensate for the zinc ions consumed at the negative electrode. This results in a relatively uneven zinc ion concentration distribution on the negative electrode surface, which is detrimental to improving battery performance. Therefore, at 40 mA cm⁻¹... -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A3 was 98.72%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane; the voltage efficiency was 84.22%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane (Table 2). After self-discharge testing, the battery's capacity retention rate was 76%, lower than that of composite membrane A1, but higher than that of the battery assembled with Daramic base membrane (Table 2). However, due to the decreased ion conductivity of composite membrane A3, its cycle performance was inferior to that of the zinc-bromine flow battery assembled with composite membrane A1. The zinc-bromine flow battery assembled with composite membrane A3 could operate stably for more than 100 cycles, and its cycle performance was better than that of the Daramic base membrane (Table 3).

[0046] Example 4

[0047] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PVA was 60% (defined as composite membrane A4).

[0048] The performance of composite membrane A4 was tested. The sheet resistivity of composite membrane A4 is shown in Table 1. Due to the increased content of highly hydrophilic PVA in the dense PVA / PFSA coating, the membrane's ion conductivity is enhanced, resulting in a lower sheet resistivity for composite membrane A4 compared to composite membrane A1. This demonstrates that composite membrane A4 has higher ion conductivity. The bromide ion concentration (Br) of composite membrane A4 is also shown in Table 1. - As shown in Table 1, the high ion conductivity of composite membrane A4 enhances its ability to conduct bromine, while reducing its bromine barrier capacity. This results in a higher bromine ion permeability for composite membrane A4 compared to composite membrane A2. However, the presence of a non-porous, dense coating ensures that the bromine ion permeability of composite membrane A4 is still lower than that of the Daramic base membrane, indicating that composite membrane A4 still exhibits relatively high bromine selectivity.

[0049] When the prepared composite membrane A4 was used in a zinc-bromine flow battery, the increased PVA content in the membrane coating reduced its ability to suppress bromine diffusion, leading to increased bromine permeability and exacerbated battery self-discharge. However, the increased PVA content enhanced the membrane's ability to suppress zinc dendrite formation, resulting in improved battery performance. Therefore, at 40 mA cm⁻¹, [the following is a continuation of the previous sentence, but the context is unclear]. -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A4 increased to 98.81%, slightly lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane; the voltage efficiency was 85.26%, higher than that of the zinc-bromine flow batteries assembled with composite membrane A1 and Daramic base membrane (Table 2). After self-discharge testing, the battery capacity retention rate was 78%, lower than that of composite membrane A1, but higher than that of the battery assembled with Daramic base membrane (Table 2). In addition, the increased content of zinc dendrite inhibitor in the coating improved the membrane's ability to suppress zinc dendrites, making the zinc deposited on the negative electrode surface more uniform and smooth, thereby enhancing battery stability. The zinc-bromine flow battery assembled with composite membrane A4 could operate stably for more than 100 cycles with stable performance, which was better than that of the Daramic base membrane. However, due to the decreased ability of composite membrane A4 to prevent bromine diffusion, its cycle performance was inferior to that of composite membrane A1 but superior to that of the zinc-bromine flow battery assembled with Daramic base membrane (Table 3).

[0050] Example 5

[0051] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PVA was 3% (defined as composite membrane A5).

[0052] The performance of composite membrane A5 was tested. The sheet resistivity of composite membrane A5 is shown in Table 1. Due to the decrease in PVA content in the dense PVA / PFSA coating, the membrane's ion conductivity decreases, resulting in a higher sheet resistivity for composite membrane A5 compared to composite membrane A1, but lower than that of the Daramic base membrane. This demonstrates that composite membrane A5 still possesses high ion conductivity. The bromide ion concentration (Br) of composite membrane A5... - As shown in Table 1, due to the decrease in PVA content in the PVA / PFSA dense coating, the membrane's ability to block bromine increases, resulting in a lower bromide ion permeability of composite membrane A5 compared to composite membrane A1. This indicates that composite membrane A5 has high bromine selectivity.

[0053] When the prepared composite membrane A5 was used in a zinc-bromine flow battery, the decreased PVA content in the membrane coating reduced its ability to suppress zinc dendrites, leading to a decrease in the smoothness of the zinc deposition morphology on the negative electrode and consequently, a reduction in battery performance. However, its ability to suppress bromine diffusion increased, thus suppressing battery self-discharge. Therefore, at 40 mA cm⁻¹, -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A5 increased to 98.95%, which is higher than that of the zinc-bromine flow battery assembled with composite membrane A1; the voltage efficiency was 84.30%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane (Table 2). After the self-discharge test, the battery's capacity retention rate was 81%, higher than that of the battery assembled with composite membrane A1 (Table 2). However, due to the decreased ability of composite membrane A5 to suppress zinc dendrites, the zinc-bromine flow battery assembled with composite membrane A5 could operate stably for more than 100 cycles, with cycle performance inferior to that of composite membrane A1 but superior to that of Daramic base membrane (Table 3).

[0054] Example 6

[0055] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PFSA was 4% (defined as composite membrane A6).

[0056] The performance of composite membrane A6 was tested. The sheet resistivity of composite membrane A6 is shown in Table 1. Since PFSA is not conducive to ion conduction, the decrease in PFSA content in the dense PVA / PFSA coating increases the membrane's ion conductivity, resulting in a lower sheet resistivity for composite membrane A6 compared to composite membrane A1. This indicates that composite membrane A6 has higher ion conductivity. The bromide ion concentration (Br) of composite membrane A6 is also shown in Table 1. -The permeability is shown in Table 1. Due to the decrease in PFSA content in the PVA / PFSA dense coating, the membrane's ability to block bromine decreases, resulting in a higher bromide ion permeability of composite membrane A6 than composite membrane A1, but lower than that of the Daramic base membrane. This indicates that composite membrane A6 has relatively high bromine selectivity (Table 3).

[0057] When the prepared composite membrane A6 was used in a zinc-bromine flow battery, its ion conductivity increased due to the decrease in PFSA content in the membrane coating, and the smoothness of the zinc deposition morphology on the negative electrode improved, which is beneficial to the battery performance. However, its ability to suppress bromine diffusion also decreased, leading to increased self-discharge of the battery. Therefore, at 40 mA cm⁻¹, -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A6 decreased to 98.63%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane; the voltage efficiency was 84.97%, higher than that of the zinc-bromine flow battery assembled with composite membrane A1 (Table 2). After the self-discharge test, the battery's capacity retention rate was 75%, lower than that of composite membrane A1, but higher than that of the battery assembled with Daramic base membrane (Table 2). However, due to the reduced ability of composite membrane A6 to suppress bromine diffusion, the zinc-bromine flow battery assembled with composite membrane A6 could operate stably for more than 100 cycles, with cycle performance inferior to composite membrane A1 but superior to Daramic base membrane (Table 3).

[0058] Example 7

[0059] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PFSA was 35% (defined as composite membrane A7).

[0060] The performance of composite membrane A7 was tested. The sheet resistivity of composite membrane A7 is shown in Table 1. Since PFSA is not conducive to ion conduction, the increased PFSA content in the PVA / PFSA dense coating reduces the membrane's ion conductivity, resulting in a higher sheet resistivity for composite membrane A7 than composite membrane A1, but lower than that of the Daramic base membrane. This indicates that composite membrane A7 still possesses high ion conductivity. The bromide ion concentration (Br) of composite membrane A7 is also shown in Table 1. - As shown in Table 1, due to the increase in PFSA content in the PVA / PFSA dense coating, the membrane's ability to block bromine increases, resulting in a lower bromide ion permeability of composite membrane A7 compared to composite membrane A1. This indicates that composite membrane A7 has higher bromine selectivity.

[0061] When the prepared composite membrane A7 was used in a zinc-bromine flow battery, the increased PFSA content in the membrane coating reduced its ion conductivity and decreased the smoothness of the zinc deposition morphology on the negative electrode, which was detrimental to improving battery performance. However, its ability to suppress bromine diffusion increased, thus suppressing battery self-discharge. Therefore, at 40 mA cm⁻¹, -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane A7 increased to 99.06%, which is higher than that of the zinc-bromine flow battery assembled with composite membrane A1; the voltage efficiency was 84.19%, lower than that of composite membrane A1, but higher than that of the zinc-bromine flow battery assembled with Daramic base membrane (Table 2). After the self-discharge test, the battery's capacity retention rate was 82%, higher than that of the battery assembled with composite membrane A1 (Table 2). However, due to the decreased ion conductivity of composite membrane A7, the zinc-bromine flow battery assembled with composite membrane A7 can operate stably for more than 100 cycles, with cycle performance inferior to that of composite membrane A1 but superior to that of Daramic base membrane (Table 3).

[0062] Example 8

[0063] First, PVA and PFSA resins were dissolved in the organic solvent IPA and stirred thoroughly at 25°C for 48 hours to prepare a homogeneous blend solution; the mass concentration of PVA was 10%. Then, the prepared PVA / PFSA blend solution was spin-coated onto a Daramic membrane substrate, and then transferred to a 50°C hot plate for heating for 3-12 hours. After cooling to room temperature, a PVA / PFSA / Daramic composite membrane was obtained; the thickness of the PVA / PFSA coating was 5 μm. This membrane was then used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery's coulombic efficiency is 98.39%, voltage efficiency is 85.41%, and capacity retention after self-discharge testing is 79%. The zinc deposition on the negative electrode surface is uniform and smooth, and the battery can operate continuously and stably for more than 100 cycles with stable performance, which is superior to the performance of the Daramic base film (Table 3).

[0064] Example 9

[0065] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the zinc dendrite inhibitor was polyethyleneimine (PEI). It was then used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery has a coulombic efficiency of 98.42%, a voltage efficiency of 85.46%, and a capacity retention rate of 79% after the self-discharge test. The zinc deposition on the negative electrode surface is uniform and smooth. The battery can operate stably for more than 100 cycles with stable performance, which is superior to the performance of the Daramic base film (Table 3).

[0066] Example 10

[0067] The composite membrane was prepared according to the method described in Example 1, except that the base membrane was an ultra-high density polyethylene (UPE) porous membrane (300 μm thick, 89% porosity, pore size distribution range of 1-500 nm). It was used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery has a coulombic efficiency of 98.53%, a voltage efficiency of 85.40%, and a capacity retention rate of 79% after the self-discharge test. The zinc deposition on the negative electrode surface is uniform and flat. The battery can operate stably for more than 100 cycles with stable performance, which is superior to the performance of the Daramic base film (Table 3).

[0068] Comparative Example 1

[0069] First, PFSA resin was dissolved in the organic solvent IPA and stirred thoroughly at 25°C for 48 hours to prepare a homogeneous blend solution; the mass concentration of PFSA was 10%; then, the prepared PFSA blend solution was coated onto the Daramic membrane substrate by a blade coating method, and then transferred to a 50°C hot stage for heating for 3-12 hours. After cooling to room temperature, a PFSA / Daramic composite membrane (defined as composite membrane a1) was obtained; the thickness of the PFSA coating was 5 μm.

[0070] The composite membrane a1 underwent performance testing. The sheet resistivity of composite membrane a1 is shown in Table 1. Due to the absence of PVA in the coating, the membrane's ion conductivity decreases, resulting in a higher sheet resistivity for composite membrane a1 compared to composite membrane A1 and the Daramic base membrane. The bromide ion concentration (Br) of composite membrane a1... - The permeability is shown in Table 1. Since the coating does not contain PVA, the membrane's ability to block bromine is enhanced, making the bromide ion permeability of composite membrane a1 lower than that of composite membrane A1.

[0071] The composite membrane a1 was used in a zinc-bromine flow battery at 40 mA cm⁻¹ -2 Under certain operating current density conditions, the battery cannot operate normally because the membrane impedance is too high (Table 3).

[0072] Comparative Example 2

[0073] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the thickness of the PVA / PFSA coating was 100 μm (defined as composite membrane a2).

[0074] The performance of composite membrane a2 was tested. The sheet resistivity of composite membrane a2 is shown in Table 1. Due to the excessive coating thickness and binder content, the membrane's ion conductivity decreased, and the increased PVA content failed to promote ion transport, resulting in a higher sheet resistivity for composite membrane a2 compared to composite membrane A1 and the Daramic base membrane. The bromide ion concentration (Br) of composite membrane a2... - The permeability is shown in Table 1. Due to the increased thickness of the non-porous dense coating, the membrane's ability to block bromine is enhanced, resulting in the lower bromide ion permeability of composite membrane a1 compared to composite membrane A1.

[0075] Composite membrane a2 was used in a zinc-bromine flow battery at 40 mA cm⁻¹ -2 Under certain operating current density conditions, the battery cannot operate normally because the membrane impedance is too high (Table 3).

[0076] Comparative Example 3

[0077] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the thickness of the PVA / PFSA coating was 0.001 μm (defined as composite membrane a3).

[0078] The performance of composite membrane a3 was tested. The sheet resistivity of composite membrane a3 is shown in Table 1. Due to the low thickness of the dense PVA / PFSA coating, the membrane's ion conductivity does not change significantly, making the sheet resistivity of composite membrane a3 similar to that of the Daramic base membrane. The bromide ion concentration (Br) of composite membrane a3... - The permeability is shown in Table 1. Due to the low thickness of the PVA / PFSA dense coating, the membrane's ability to block bromine does not change much, making the bromide ion permeability of the composite membrane a3 similar to that of the Daramic base membrane.

[0079] The prepared composite membrane a3 was used in a zinc-bromine flow battery. However, due to the thin coating, its ability to suppress bromine diffusion and zinc dendrite formation was weak, resulting in minimal change in the battery performance. Therefore, at 40 mA cm⁻¹, the performance was not optimal. -2 Under the operating current density conditions, the coulombic efficiency of the zinc-bromine flow battery assembled with composite membrane a3 was 94.09%, similar to that of the zinc-bromine flow battery assembled with Daramic base membrane; the voltage efficiency was 78.54%, similar to that of the zinc-bromine flow battery assembled with Daramic base membrane (Table 2). After self-discharge testing, the battery's capacity retention rate was 42%, similar to that of the battery assembled with Daramic base membrane (Tables 2 and 3).

[0080] Comparative Example 4

[0081] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PVA was 0.001% (defined as composite membrane a4).

[0082] The performance of composite membrane a4 was tested. The sheet resistivity of composite membrane a4 is shown in Table 1. Due to the low PVA content in the dense PVA / PFSA coating, the properties of the PVA / PFSA coating are similar to those of the pure PFSA coating. Therefore, the membrane's ion conductivity decreases. The sheet resistivity of composite membrane a4 is higher than that of composite membrane A1 and the Daramic base membrane, indicating that composite membrane a4 has very poor ion conductivity. The bromide ion concentration (Br) of composite membrane a4 is... - The permeability is shown in Table 1. Due to the low PVA content in the PVA / PFSA dense coating, the membrane's ability to block bromine is increased, resulting in the lower bromide ion permeability of composite membrane a4 compared to composite membrane A1.

[0083] When the prepared composite membrane a4 was used in a zinc-bromine flow battery, the battery could not operate normally because the membrane impedance was too high (Table 3).

[0084] Comparative Example 5

[0085] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PVA was 70% (defined as composite membrane a5).

[0086] The performance of composite membrane a5 was tested. The sheet resistivity of composite membrane a5 is shown in Table 1. Because the concentration of zinc dendrite inhibitor is too high, the binder cannot fix it completely to the membrane surface, and zinc dendrite inhibitor is usually a water-soluble polymer, excess zinc dendrite inhibitor will dissolve in the aqueous electrolyte of zinc-bromine flow battery. Therefore, the sheet resistivity of composite membrane a5 is similar to that of composite membrane A4, and the bromide ion permeability of composite membrane a5 is also similar to that of composite membrane A4.

[0087] The prepared composite membrane a5 was used in a zinc-bromine flow battery, and the results were obtained at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery's coulombic efficiency is 98.80%, and its voltage efficiency is 85.24%. After the self-discharge test, the battery's capacity retention rate is 78%, and its performance is similar to that of the zinc-bromine flow battery assembled with composite membrane A4 (Table 2). The zinc-bromine flow battery assembled with composite membrane a5 can operate stably for more than 100 cycles with stable performance, and its cycle performance is similar to that of the zinc-bromine flow battery assembled with composite membrane A4 (Table 3).

[0088] Comparative Example 6

[0089] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PFSA was 0.001% (defined as composite membrane a6).

[0090] The performance of composite membrane a6 was tested. The sheet resistivity of composite membrane a6 is shown in Table 1. When the binder concentration is too low, the binder cannot fix the PVA to the membrane surface, causing the zinc dendrite inhibitor to dissolve in the zinc-bromine flow battery electrolyte. Therefore, the sheet resistivity and bromide ion permeability of composite membrane a6 are similar to those of the Daramic base membrane.

[0091] The prepared composite membrane a6 was used in a zinc-bromine flow battery, and the results were obtained at 40 mA cm⁻¹. -2 Under the operating current density conditions, the battery has a coulombic efficiency of 94.15% and a voltage efficiency of 75.44%. After the self-discharge test, the battery retains 43% of its capacity, and its performance is similar to that of the Daramic base film (Tables 2 and 3).

[0092] Comparative Example 7

[0093] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the mass concentration of PFSA was 40% (defined as composite membrane a7).

[0094] The performance of composite membrane a7 was tested. The sheet resistivity of composite membrane a7 is shown in Table 1. Due to the excessively high PFSA content in the dense PVA / PFSA coating, the membrane's ion conductivity is significantly reduced, resulting in a higher sheet resistivity for composite membrane a7 compared to composite membrane A1 and the Daramic base membrane. This indicates that composite membrane a7 has lower ion conductivity. The bromide ion concentration (Br) of composite membrane a7 is also shown in Table 1. - As shown in Table 1, due to the increase in PFSA content in the PVA / PFSA dense coating, the membrane's ability to block bromine increases, resulting in a lower bromide ion permeability of composite membrane a7 compared to composite membrane A1. This indicates that composite membrane A7 has higher bromine selectivity.

[0095] The composite membrane a7 was used in a zinc-bromine flow battery at 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate normally because the membrane impedance is too high (Table 3).

[0096] Comparative Example 8

[0097] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the binder was polyvinylidene fluoride (PVDF) (defined as composite membrane a8).

[0098] The composite membrane a8 was subjected to performance testing. Due to the strong hydrophobicity and crystallinity of PVDF, ion conduction is hindered, resulting in a significant decrease in the membrane's ion conductivity. When used in a zinc-bromine flow battery, it achieved performance at 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate normally due to excessive membrane impedance (Table 3).

[0099] Comparative Example 9

[0100] First, PVA and PFSA were dissolved in the organic solvent isopropanol (IPA) and stirred thoroughly at 25°C for 48 hours to prepare a homogeneous blend solution; the mass concentration of PVA was 10%, and the mass concentration of PFSA was 10%. Then, the prepared PVA / PFSA blend solution was uniformly coated onto one side surface of a Daramic membrane substrate using a blade coating method. The substrate was then placed in water for phase inversion to obtain a PVA / PFSA / Daramic composite membrane (defined as composite membrane a9) with a porous PVA / PFSA coating; the thickness of the porous PVA / PFSA coating was 5 μm. Figure 6 As shown, the surface morphology of the composite membrane a9 is porous (porosity of 59%, pore size distribution range of 1-100 nm).

[0101] The composite membrane a9 was subjected to performance testing. Due to the water solubility of PVA, it directly dissolves in water during phase inversion, resulting in a PVA / PFSA coating that is essentially a porous PFSA coating, leading to a significant decrease in the membrane's ion conductivity. When used in a zinc-bromine flow battery, it performed well at 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate normally due to excessive membrane impedance (Table 3).

[0102] Comparative Example 10

[0103] A PVA / PFSA / Daramic composite membrane was prepared according to the method described in Example 1, except that the base membrane was a non-porous and dense polyethylene membrane (thickness: 200 μm, porosity: 0, pore size distribution range: 0) (defined as composite membrane a10).

[0104] The composite membrane a10 was subjected to performance testing. Because the polyethylene-based membrane lacks pores and therefore has no ion-conducting capacity, its ion conductivity is significantly reduced, resulting in extremely high sheet resistance. When used in a zinc-bromine flow battery, it achieved a resistance of 40 mA cm⁻¹. -2 Under certain operating current density conditions, the battery cannot operate normally due to excessive membrane impedance (Table 3).

[0105] Table 1 Comparison of properties of composite membrane and membrane substrate (zinc ion inhibitor is PVA, binder is PFSA) Comparative Example 11

[0106] The composite membrane was prepared according to the method described in Example 1, except that the coating did not contain zinc dendrite inhibitors, but contained poly(p-hydroxystyrene) containing zinc dendrite inhibitor groups - hydroxyl groups but without zinc dendrite inhibitor function. The PHS / PFSA / Daramic composite membrane (defined as composite membrane a11) was prepared according to the method of Example 1.

[0107] The composite membrane a11 was subjected to performance testing. Because this composite membrane lacks zinc dendrite suppression capabilities, the zinc deposited on the negative electrode is highly uneven and irregular, with numerous hexagonal zinc flakes present on the negative electrode surface. The presence of zinc dendrites can puncture the membrane and contact the positive electrode, leading to micro-short circuits or even short circuits in the battery. Furthermore, they can react with the bromine at the positive electrode, causing self-discharge. Therefore, the zinc-bromine flow battery assembled with the composite membrane a11 exhibits low coulombic efficiency, severe capacity decay, and a short cycle life of only about 30 cycles (Table 2). Moreover, after cycling, the zinc deposited on the negative electrode remains extremely uneven and irregular.

[0108] Table 1 Comparison of properties of composite membrane and membrane substrate (zinc ion inhibitor is PVA, binder is PFSA)

[0109]

[0110]

[0111] Table 2. Performance comparison of zinc-bromine flow batteries assembled with composite membranes and membrane substrates (zinc ion inhibitor: PVA; binder: PFSA).

[0112]

[0113]

[0114] Table 3 Summary of the conclusions of the examples and comparative examples

[0115]

[0116]

[0117]

[0118]

Claims

1. A composite membrane, characterized in that: It includes a polyolefin porous membrane support layer; a functional coating is laminated to one side surface of the support layer; The functional coating contains a zinc dendrite inhibitor, which includes at least one or more of polyethylene glycol, polyethyleneimine, polyethylene glycol octylphenyl ether, polyvinyl alcohol, polyethyleneimine, polytetrafluoroethylene, cellulose urea, and aniline organic compounds.

2. The composite membrane according to claim 1, characterized in that, The functional coating is a non-porous, dense layer with a thickness of 0.01~60 μm.

3. The composite membrane according to claim 1, characterized in that, The functional coating is a non-porous, dense layer with a thickness of 0.1-40 μm.

4. The composite membrane according to claim 1, characterized in that, The functional coating is a non-porous, dense layer with a thickness of 1-20 μm.

5. The composite membrane according to any one of claims 1-4, characterized in that: The functional coating consists of a zinc dendrite inhibitor and a binder, wherein the binder is a perfluorosulfonic acid polymer and the mass ratio of the zinc dendrite inhibitor to the binder is 1-60:3-40.

6. The composite membrane according to claim 5, characterized in that: The mass ratio of zinc dendrite inhibitor to binder in the functional coating is 5-40:5-30.

7. The composite membrane according to claim 5, characterized in that: The mass ratio of zinc dendrite inhibitor to binder in the functional coating is 5-20:8-20.

8. The composite membrane according to claim 1, characterized in that: The aforementioned polyolefin porous membrane support layer is a polyolefin membrane with a porous structure, having a thickness of 175-1000 μm, a porosity of 30%-86%, and a pore size distribution range of 0.001-500 nm. The polyolefin membrane is an ion-conducting membrane with a porous structure, composed of at least one or two of polyethylene and polypropylene.

9. A method for preparing the composite membrane according to any one of claims 1-8, characterized in that, Includes the following steps: (1) The functional coating material, namely zinc dendrite inhibitor, binder and solvent, are mixed to obtain a functional coating solution; (2) The functional coating solution is applied to one side surface of the support layer by one or more of the following methods: spraying, spin coating, scraping, electrospinning, dip coating or interfacial polymerization to obtain the composite film.

10. The preparation method according to claim 9, characterized in that, The solvent in step (1) is at least one or more of the following: acetic acid, water, ethanol, methanol, isopropanol, N,N-dimethylacetamide, N,N-dimethylformamide, chloroform, 1,2-dichloroethane, benzene, toluene, and xylene.

11. The preparation method according to claim 9 or 10, characterized in that, The concentration of the zinc dendrite inhibitor in the functional coating solution is 1wt%-60wt%.

12. An application of the composite membrane according to any one of claims 1-8, characterized in that, The composite membrane is used as a separator in zinc-bromine flow batteries.

13. The application of the composite membrane as described in claim 12, characterized in that, When a functional coating is laminated on one side of the support layer, the side with the functional coating should be placed closer to the negative electrode side.

Images