Zinc-repellent porous composite membrane for implementing crystal face manipulation, preparation method, and use of zinc-repellent porous composite membrane in zinc-based flow battery

Abstract

The present invention relates to the technical field of flow batteries, and provides a zinc-repellent porous composite membrane for implementing crystal face manipulation, a preparation method, and a use of a zinc-repellent porous composite membrane in a zinc-based flow battery. Carboxymethylcellulose (CMC) is introduced to perform customized optimization on the charge properties of the surface of the membrane, and an electrostatic repulsion force-based zinc-repellent interface project is constructed, so that zincate ions are uniformly distributed along a membrane-electrode interface. The zinc-repellent interface project successfully implements crystal face manipulation, so that the exposure of a preferentially oriented crystal face ((002) crystal face) of zinc deposition is increased, thereby obtaining smoother zinc deposition, and prolonging the service life of the zinc-based flow battery. In addition, the good wettability of hydrophilic CMC to an electrolyte and the positive effect of the rich -OH functional groups of hydrophilic CMC on the transport kinetics of charge carriers (OH-) synergistically ensure that the prepared membrane has high ionic conductivity. The present research provides a new crystal face manipulation idea for solving the zinc dendrite problem, thereby promoting the commercial process of the zinc-based flow battery for long-time energy storage.

Description

A zinc-phobic porous composite membrane for realizing crystal plane manipulation, preparation method and application thereof in zinc-based liquid flow battery Technical Field

[0001] The present invention relates to a zinc-phobic porous composite membrane for realizing crystal plane manipulation, a preparation method and application thereof in a zinc-based liquid flow battery, and belongs to the technical field of liquid flow batteries. Background Art

[0002] The depletion of fossil fuels and the intensification of the greenhouse effect have made the transition to an efficient, renewable, and low-carbon energy system imperative. Building a new power system that integrates renewable clean energy with energy storage technologies is one of the effective pathways to achieving a clean, low-carbon energy transition. Among numerous energy storage technologies, flow battery energy storage, with its advantages of safety, reliability, cost-effectiveness, environmental friendliness, and long cycle life, has become a leading technology for large-scale energy storage. In particular, zinc-iron flow batteries, which use zinc and iron as their primary raw materials, two abundant and inexpensive elements, are expected to break through in the long-duration energy storage market and usher in a surge in production capacity after overcoming technical bottlenecks such as the zinc dendrite problem. Battery separators, a key component of zinc-iron flow batteries, are easily punctured by zinc dendrites during charging, and their quality directly impacts battery efficiency and stability. However, traditional organic porous membranes have low mechanical strength, poor dendrite resistance, and a mismatch between ion selectivity and ion conductivity, severely hindering the strategic deployment of zinc-iron flow batteries in the future energy storage sector.

[0003] Because the electrochemical reaction of metal pairs tends to occur on membrane-electrode interface, therefore by regulating the properties (such as hydrophilicity, roughness, rigidity etc.) of surface and interface, it is possible to effectively achieve uniform deposition process, and further avoid the formation of dendrite. Such as Xu et al. will have high mechanical strength and hydrophobic zeolite molecular sieve nanosheet (ns-MFI) load on porous substrate surface, its special fish scale arrangement can be used as buffer layer to enhance anti-dendritic ability, while hydrophobicity ns-MFI reduces the adsorption energy of membrane surface and zincate ion, effectively regulates the distribution of zincate ion at membrane-electrode interface, thus achieving uniform zinc deposition (non-patent literature 1). However, there is an impact on the impact of membrane surface charge on deposition process, and if the diaphragm surface has certain adsorption for zincate ion, it will cause the increase of zinc deposition, and then affect battery operation performance.

[0004] Non-patent literature 1: Hou, Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the formation of zinc dendrites in the zinc-based flow battery during the charge and discharge process leads to membrane damage, battery performance degradation and shortened cycle life; the electrically neutral porous membrane transmits OH - The rate is slightly lower.

[0006] The technical concept adopted by the present invention is to perform functional modification on the surface of the porous composite membrane by spin coating carboxymethyl cellulose (CMC), thereby further improving the surface charge properties of the membrane and obtaining a uniform Zn(OH)4 2- Distribution, avoiding the phenomenon of excessive local zincate ion concentration caused by the "tip effect", thereby inducing zinc to expose more preferred crystal plane orientation (parallel (002) crystal plane) during deposition. In addition, the good wettability of hydrophilic carboxymethyl cellulose to electrolyte and its rich -OH functional groups to OH - The positive influence of transport kinetics on the formation of charge-carrying ions (OH - ) through a rapid transfer channel, exhibiting higher ionic conductivity. This invention provides a new solution for dendrite-free alkaline zinc-based flow batteries, namely, by customizing and optimizing the surface charge properties of the composite membrane to regulate the microenvironmental charge characteristics at the membrane electrode interface to achieve crystal plane manipulation.

[0007] A zinc-based flow battery separator comprises a porous base membrane. The surface of the porous base membrane is covered with a modification layer. The modification layer contains a modification material and a binder. The modification material contains -COOH and -OH.

[0008] The charge properties of one side of the porous base membrane are adjusted by introducing molecules with -COOH and -OH to construct a functional layer, so that the surface of the porous composite membrane has a strong negative charge. The functional layer is evenly distributed on the surface of the porous base membrane and interacts with the zincate ions (Zn(OH)4 2- ) Based on the mutual repulsion of the Donnan effect, a zinc-phobic barrier is constructed for the porous membrane.

[0009] The base film is a porous base film with a thickness of 20 to 500 μm; the modified layer has a thickness of 0.1 to 10 μm.

[0010] The Zeta potential value of the surface of the zinc-based flow battery separator is less than -10 mV, preferably less than -20 mV.

[0011] The material of the porous base membrane is selected from one or more of polysulfones, polyketones, polyimide, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, polybenzimidazole, polyvinyl pyridine, polyethylene, polypropylene, chitosan, and cellulose acetate; the polymer is preferably a mixture of one or two of polyethersulfone and sulfonated polyetheretherketone.

[0012] The modifying material is selected from one or a mixture of carboxymethyl cellulose, carboxyethyl cellulose, carboxypropyl cellulose, carboxylated carbon nanotubes or carboxylated chitosan; the binder is selected from one or more of Nafion, sulfonated polyetheretherketone, polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene and polyimide.

[0013] The above-mentioned method for preparing the zinc-based flow battery separator comprises the following steps:

[0014] Step 1, obtaining a porous basement membrane;

[0015] Step 2: Disperse the modifying material and the binder in a solvent, apply the resultant to the surface of the porous base film, and obtain the zinc-based flow battery separator after drying.

[0016] In the step 1, the base film is prepared by the following method: dispersing the polymer in an organic solvent as a casting liquid; coating the casting liquid on the surface of the substrate, and performing a film-forming treatment by a phase inversion method to obtain a porous base film; the organic solvent is selected from one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, chloroform or dimethyl sulfoxide.

[0017] In the phase inversion method, a poor solvent for the polymer is used as a coagulation bath, and the immersion time in the coagulation bath is 1-600 seconds. In the phase inversion method, after the membrane is treated in the coagulation bath, it is treated with an extraction solvent at a set temperature for a certain time; the temperature is 20-200°C, and the treatment time is 0.5-24 hours.

[0018] In step 2, the coating method is selected from flat blade coating, spray coating or spin coating; the mass ratio of the modifying material to the binder is 1:(3-8), and the binder accounts for 1-20% of the weight of the solvent.

[0019] Application of battery separators in zinc-based flow batteries.

[0020] In the application described above, the modified layer faces the negative electrode of the battery.

[0021] The application also includes a method for predicting battery VE performance, comprising the following steps:

[0022] Determination of Fe(CN)6 in battery separators obtained under different preparation conditions 4- The permeability or surface resistance of the battery separator is selected, and the battery separator with the highest permeability or lowest surface resistance is regarded as the separator with the highest VE performance prediction.

[0023] The present invention also provides a method for regulating the crystal orientation of a zinc deposit layer (002) on a diaphragm surface during operation of a zinc-based liquid flow battery, comprising the following steps:

[0024] Step 1, obtaining a porous basement membrane;

[0025] Step 2: dispersing the modifying material and the binder in a solvent, coating the mixture on the surface of the porous base film, and drying the resulting zinc-based flow battery separator.

[0026] Step 3: increasing the loading amount of the modification material on the surface of the porous base film, thereby increasing the content of zinc with a (002) crystal plane in the zinc deposition layer. Beneficial effects

[0027] According to the Donnan effect, by adjusting the microenvironmental charge characteristics of the membrane-electrode interface, the membrane surface and zincate ions are repelled from each other, which can effectively prevent their accumulation at certain tips, thereby achieving crystal surface manipulation. The present invention selects carboxymethyl cellulose (CMC) with rich functional groups (-COOH, -OH), uses a small amount of binder, and evenly distributes it on the surface of the porous base membrane by spin coating. The -COOH functional group gives the composite membrane surface a strong negative charge. The membrane and Zn(OH)4 2- The electrostatic repulsion between the zincate ions makes them follow the Donnan mechanism and move away from the membrane surface, which in turn weakens the "tip effect" during zinc deposition and is conducive to more exposure of the preferred orientation crystal plane ((002) crystal plane) of zinc deposition. At the same time, the good electrolyte wettability of hydrophilic CMC and its rich -OH functional groups are conducive to OH - The synergistic effect of the positive influence of transport kinetics on the ionic conductivity of the composite membrane is enhanced. Ultimately, the battery equipped with the composite membrane exhibits excellent battery performance and long-term cycling stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG1 is a schematic diagram of a porous composite membrane having a zinc-repellent functional layer constructed by a spin coating method.

[0029] Figure 2 SEM and EDX characterization of porous composite membranes: a), b) P / S, c), d) P / S-Nf-CMC-400.

[0030] FIG3 shows a) porosity and b) FTIR of the porous composite membrane.

[0031] Figure 4 shows a) water absorption and swelling rate, b) tensile strength and c) nanoindentation of the porous composite membrane.

[0032] Figure 5 is the Fe(CN)6 porous composite membrane 4- Permeability test: a) test apparatus, b) permeability curve.

[0033] Figure 6 is the OH of the porous composite membrane - Permeability curve.

[0034] FIG7 is an electrochemical impedance spectroscopy test of the porous composite membrane: a) conductivity, b) membrane surface resistance.

[0035] FIG8 is the electrochemical impedance spectroscopy test of Comparative Examples 1 and 2.

[0036] Figure 9 shows the porous composite membrane at 80 mA cm -2 Battery performance under .

[0037] Figure 10 shows the porous composite membrane at 40-240 mA cm -2 Battery testing. a) Schematic diagram of membrane orientation, b) CE, c) VE, d) EE.

[0038] Figure 11 shows the battery cycling stability test of the porous composite membrane. a) Charge-discharge voltage curves of the battery loaded with P / S and P / S-Nf-CMC-400, b) enlarged view of the charge-discharge voltage curves, and c) battery discharge capacity.

[0039] Figure 12 shows the digital photographs, SEM images, and EDX spectra of the porous composite membrane surfaces after the battery cycling stability test: a) P / S, b) P / S-Nf-CMC-400.

[0040] Figure 13 shows the performance of cells equipped with different membranes at 80 mA cm -2 Long cycle test at different current densities.

[0041] Figure 14 is an SEM image of zinc deposition on carbon felt after cycling of batteries equipped with different membranes for a period of time: a) P / S, b) P / S-Nf-CMC-400, c) P / S-Nf-CMC-500, d) P / S-Nf-CMC-800, e) P / S-Nf-CMC-1000.

[0042] Figure 15 is a Wiener scan of zinc deposition on carbon felt after cycling of cells loaded with different membranes for a period of time: a) P / S, b) P / S-Nf-CMC-400, c) P / S-Nf-CMC-500, d) P / S-Nf-CMC-800, e) P / S-Nf-CMC-1000.

[0043] Figure 16 shows the zinc deposition orientation on the carbon felt of cells equipped with different membranes at the end of cycling. a) Schematic diagram of zinc deposition orientation, b) XRD test results, c) Ratio of XRD diffraction peak intensity of (002) crystal plane to (100) crystal plane.

[0044] FIG17 is a Zeta potential test of the porous composite membrane surface.

[0045] FIG18 shows the quartz crystal microbalance (QCM) test results of different film layers.

[0046] FIG19 is a potential energy scan between different molecules and zincate ions.

[0047] FIG20 is a schematic diagram of the mechanism by which CMC regulates zinc deposition behavior. DETAILED DESCRIPTION

[0048] The present invention selects carboxymethyl cellulose (CMC) with rich functional groups (-COOH, -OH), uses a small amount of binder, and evenly distributes it on the surface of the porous base membrane by spin coating (Figure 1). The presence of a large number of -COOH functional groups effectively regulates the surface charge properties of the composite membrane, making it have a strong negative charge. The zincate ions follow the Donnan mechanism and repel each other with the negatively charged membrane surface, staying away from the membrane surface. This in turn weakens the "tip effect" during zinc deposition, which is conducive to more exposure of the preferred orientation crystal plane ((002) crystal plane) of zinc deposition. At the same time, the good electrolyte wettability of hydrophilic CMC and its rich -OH functional groups have a great effect on OH - The positive effects of transport kinetics synergistically enhance the ionic conductivity of the composite membrane. Ultimately, batteries equipped with this composite membrane exhibit excellent battery performance and long-term cycling stability.

[0049] Example 1 Preparation of sulfonated polyetherketone

[0050] Sulfonated polyetherketone (SPEEK) is prepared by direct sulfonation of PEEK with sulfuric acid (98%) at 50°C for 5 hours (Non-Patent Document 2). 1 H NMR measurement showed that the degree of sulfonation (DS) of the prepared SPEEK was about 67%.

[0051] Non-patent document 2: J.Xi, Z.Li, L.Yu, B.Yin, L.Wang, L.Liu, X.Qiu, L.Chen Effect of degree of sulfonation and casting solvent on sulfonated poly(ether ether ketone)membrane for vanadium redox flow battery J.Power Sources,285(2015),pp.195-204,10.1016 / j.jpowsour.2015.03.104.

[0052] Example 2 Preparation of porous base membrane

[0053] A porous base membrane (P / S) was prepared using a phase inversion method. Polyethersulfone (PES) and sulfonated polyetherketone (SPEEK) were added to the solvent DMAc. The mixture was then ball-milled and ultrasonicated, and then allowed to stand to defoam, forming a uniform casting solution. The total polymer concentration in the casting solution was 35 wt%, with SPEEK comprising 20 wt% and PES as the remainder.

[0054] The casting solution was poured onto a flat glass plate and a membrane was prepared at room temperature using a flat film scraper. The glass plate was then immersed in water until the phase inversion was complete, and the membrane peeled off automatically. The resulting membrane was soaked in isopropyl alcohol for 30 minutes and then evaporated at room temperature for 2 hours to ensure complete evaporation of the isopropyl alcohol. Finally, the membrane was stored in deionized water until ready for use. The resulting porous base membrane had a thickness of 70 ± 5 μm.

[0055] Example 3 Construction of zinc-phobic functional layer

[0056] CMC suspension (denoted as Nf-CMC) was prepared by adding CMC-V (DS = 0.7MW = 90000) powder to 5wt% Nafion solution (CMC / Nafion mass ratio was 1 / 5) and then ultrasonically treating for 30 minutes, wherein the Nafion solution was used as a binder. The prepared P / S base membrane (3.5cm 2 ) was coated with Nf-CMC. A series of porous composite membranes with different functional layer thicknesses were prepared by varying the spin-coating amount of Nf-CMC (400, 500, 800, and 1000 μL). They were then dried in an oven at 60°C for 6 hours and stored in deionized water before use.

[0057] Comparative Example 1 Preparation of PES / SPEEK membrane

[0058] The difference from Example 3 is that there is no zinc-phobic functional layer, that is, the original P / S film.

[0059] Comparative Example 2: Porous composite membrane with pure Nafion functional layer

[0060] The difference from Example 3 is that no CMC is added to the spin coating solution, and a 5 wt % Nafion solution is directly used. The preparation method is the same as that of Example 3.

[0061] Comparative Example 3: Using high-substitution CMC to construct a porous composite membrane with a zinc-repellent functional layer

[0062] The difference from Example 3 is that CMC-V is replaced by highly substituted CMC-I (DS = 0.9MW = 700000) and CMC-III (DS = 0.9MW = 250000), the number of -COOH functional groups is increased and the number of -OH functional groups is reduced. The obtained porous composite membranes are respectively recorded as P / S-Nf-CMC (H1) and P / S-Nf-CMC (H2).

[0063] Film morphology characterization

[0064] Digital photos show that the morphology of the membrane is not changed after spin coating of CMC (Figure 2a and Figure 2c). The prepared P / S-Nf-CMC membrane is translucent and uniform (spin coating volume 400μL), with no obvious defects. The surface and cross-sectional morphology of the membrane were characterized using field emission scanning electron microscopy (SEM), and the elemental composition of the membrane was detected using energy dispersive X-ray spectrometry. The cross-sectional SEM and EDX of the membrane (Figure 2d) proved the successful spin coating of the CMC functional layer, and there was no obvious seepage of the spin coating liquid. The overall cross-section still retained a sponge-like porous structure consistent with the original P / S membrane (Figure 2b), and the porosity did not change significantly compared with the P / S membrane (Figure 3a). The surface SEM of the membrane (Figure 2c) showed that its surface was not significantly different from the original P / S membrane (Figure 2a), which indicates that the Nf-CMC mixture was evenly spin-coated on the membrane surface and did not change the structure of the original P / S membrane. The Fourier transform infrared spectroscopy (FTIR) test results showed that at 1720cm -1 The typical C=O characteristic peak of carboxyl group (-COOH) appeared (Fig. 3b), which also proved that CMC was successfully loaded on the surface of P / S membrane.

[0065] Mechanical properties test of membrane

[0066] The water absorption and swelling test results of the membrane (Figure 4a) show that the introduction of a thin and dense Nf-CMC functional layer on the surface of the porous membrane effectively inhibits the swelling of the membrane, allowing the membrane to maintain a stable morphology in practical applications. However, as the amount of spin-coated Nf-CMC mixture increases, the hydrophilic CMC causes the membrane to absorb more water, resulting in a slight increase in the swelling rate of the membrane. At the same time, the introduction of CMC can effectively improve the tensile strength of the membrane, and the transverse mechanical tensile strength of the membrane is significantly improved (Figure 4b). In addition, in order to further verify the longitudinal impact resistance of the membrane, secondary pressurized nanoindentation characterization was carried out. As shown in Figure 4c, with the thickening of the functional layer on the surface of the porous membrane, the hardness of the prepared membrane continues to increase, and is much higher than the original membrane. The significantly improved film hardness will ultimately improve the membrane's resistance to dendrite puncture and ensure the safe and stable operation of the battery.

[0067] Membrane selective transport capacity test

[0068] The Nf-CMC functional layer facilitates selective ion transport. On the one hand, the presence of this additional, dense functional layer significantly enhances the barrier capacity of the P / S-Nf-CMC membrane for active ions compared to the P / S-based membrane. On the other hand, the hydrophilic CMC and its abundant -OH functional groups facilitate the rapid transport of OH- via standard diffusion (onboard mechanism) and proton hopping (Grotthuss mechanism).

[0069] The diffusion method ion permeability test process used is as follows:

[0070] Fe(CN)6 4- The permeability of ions through the membrane is determined using a pair of diffusion cells separated by a membrane. The left cell is filled with 0.4 mol L -1 3 mol L of K4Fe(CN)6 -1 NaOH solution (volume: 50 mL), while the right half of the cell was filled with 0.4 mol L -1 3 mol L of K2SO4 -1 NaOH solution (volume: 50 mL) was added to balance the ionic strength on both sides. During the experiment, the solutions in both half cells were vigorously stirred with a magnetic stirrer to avoid the influence of concentration polarization. 3 mL of sample solution was collected from the right half cell regularly, and then 3 mL of fresh K2SO4 solution was added to the right half cell to keep the solution volume stable. The concentration of K4Fe(CN)6 in the sample solution was detected using a UV-visible spectrophotometer. Fe(CN)6 4- The permeability of the membrane was calculated using Fick's diffusion law. The permeability of hydroxide ions through the membrane was also determined in a similar manner. The left half of the cell was filled with 50 mL of 3 mol L -1 NaOH solution, the right half of the cell is filled with deionized water. The hydroxide ion concentration in the right half of the cell with different diffusion times is measured by a pH meter. Diffusion method for Fe(CN)64- and OH - The devices with different membrane permeabilities are shown in FIG5 a.

[0071] Fe(CN)6 measured by diffusion method (Fig. 5a) 4- The permeability curves (Fig. 5b) fully demonstrate that the P / S-Nf-CMC membrane has an improved barrier capacity for active ions. In addition, all P / S-Nf-CMC membranes exhibit faster OH removal than P / S membranes. - transfer rate (Figure 6). Electrochemical impedance spectroscopy (EIS) experiments (Figures 7a and 7b) further revealed that the P / S-Nf-CMC film has higher ionic conductivity (Figure 7a) and lower surface resistance (Figure 7b) than the P / S-based membrane. It is worth noting that there is an optimal spin coating amount of Nf-CMC, which is about 400μL. This is because, as the spin coating amount increases, the thickness of the functional layer increases, which prolongs the OH - transmission path, reducing its transmission capacity. In addition, the presence of more hydrophilic CMC will enhance the membrane's wettability to the electrolyte, thereby increasing the chance of active ions passing through the membrane, and the ion selectivity of the prepared membrane will also decrease slightly. Based on the above tests, the present invention also provides a method for predicting battery VE performance, comprising the following steps: determining the Fe(CN)6 4- The permeability or surface resistance of the battery separator is selected, and the battery separator with the highest permeability or lowest surface resistance is regarded as the separator with the highest VE performance prediction.

[0072] In order to further verify the benefits of hydrophilic CMC and its abundant -OH functional groups in detail, Nf-CMC solutions (denoted as Nf-CMC(H1) and Nf-CMC(H2)) prepared from pure Nafion solution (denoted as Nf) and two CMCs with higher substitution degree (the number of -OH functional groups is less than that of the CMC selected in Example 3) were used to construct functional layers. According to the EIS test in Figure 8, the ionic conductivity of the composite membranes (denoted as P / S-Nf-CMC(H1)-500 and P / S-Nf-CMC(H2)-500) with Nf-CMC(H1) and Nf-CMC(H2) functional layers showed that the gradual reduction in the number of -OH functional groups did have a significant effect on OH. - The transport kinetics via the Grotthuss mechanism was slightly adversely affected. In addition, the ionic conductivity of the composite membrane with the Nf functional layer (denoted as P / S-Nf-500) was slightly higher than that of the P / S membrane, but much lower than that of the P / S-Nf-CMC-500 membrane. This result fully confirms that hydrophilicity is essential for the rapid OH reaction following the on-board mechanism. - The importance of transport (Figure 8). After three-way comparison, it was found that the hydrophilicity of CMC is important for achieving OH - The rapid transportation plays a greater role.

[0073] Evaluation of battery efficiency and zinc dendrite-free performance

[0074] In order to verify the applicability of the prepared P / S-Nf-CMC membrane in alkaline zinc-iron flow battery (AZIFB), battery performance tests were carried out.

[0075] The process of battery assembly and performance testing is as follows:

[0076] A diaphragm to be tested is sandwiched between two carbon felt electrodes, which are then clamped between two graphite plates. All of these components are then fixed between two acrylic plates to form a battery assembly. The effective area of ​​the electrodes is 3×3 cm 2 10mL 0.6M K4Fe(CN)6+5M NaOH and 10mL 0.3M Zn(OH)4 2- A solution consisting of +5M NaOH was used as the positive and negative electrolytes. During battery operation, a peristaltic pump was used to force the positive and negative electrolytes to circulate in their respective reaction chambers to participate in the electrochemical reaction. Charge and discharge cycle tests were performed using an ArbinBT 2000: the charging process was controlled by a fixed charging time to maintain a constant charge capacity, while the discharge process was terminated by setting a cutoff voltage of 0.1V. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the battery were calculated using the following equations:

[0077] Among them C d and C c are the discharge capacity and charge capacity respectively, E d and E C are the discharge energy and the charge energy respectively.

[0078] First, at 80 mA cm -2 The cells equipped with different membranes were tested under 400 μL spin coating volume. Figure 9 shows that all the cells have relatively high CE, which proves that these membranes have good ion selectivity. The VE of the cell reaches a peak at a spin coating volume of 400 μL, which is consistent with the OH - The permeability and EIS test results are consistent, which further confirms the correctness of the above prediction method. Through this prediction method, simple test conditions can be used to predict the battery performance of battery separators obtained under different conditions when applied to liquid flow batteries, thereby improving the efficiency of experimental optimization.

[0079] In addition to confirming that the prepared porous composite membrane has good battery efficiency, solving the zinc dendrite problem is a top priority to ensure the efficient and stable operation of ZFBs, especially considering that the operating conditions of high current density will aggravate the growth of zinc dendrites. In order to further verify the rationality and superiority of the P / S-Nf-CMC membrane in alleviating the zinc dendrite problem, the operational stability of AZIFBs equipped with different membranes was studied. Since the Nf-CMC functional layer is only loaded on one side surface of the porous base membrane, this side surface is facing the negative electrode (the side where zinc plating / stripping occurs) during battery operation to maximize the benefit (Figure 10a). As shown in Figure 10bd, all batteries equipped with P / S-Nf-CMC membranes have a higher current density (from 40mA cm -2 to 240mA cm -2 ) can maintain stable operation even when the current density is higher than 200mA cm -2 Even more encouragingly, when the current density was reduced to 80 mA cm -2 The above results prove that even at high current density, where zinc dendrites are more likely to grow, the battery using the P / S-Nf-CMC membrane can still maintain a stable operating state without the problem of dendrites piercing the membrane. In contrast, when the original P / S membrane is used, the battery performance can still be restored to the previous level when the current density reaches 200mA cm -2 Even worse, when the current density dropped again to 80 mA cm -2 When the P / S membrane structure is completely destroyed, the battery performance will be irreversibly reduced.

[0080] Afterwards, at 80 mA cm -2A 100-hour cycling stability test was conducted under 40°C (100°F) conditions to further investigate the P / S-Nf-CMC membrane's ability to regulate zinc deposition behavior and ensure stable battery operation. The charge-discharge curves (Figure 11a) show that the battery using the P / S-Nf-CMC membrane maintains a relatively low charge voltage throughout the measurement. In contrast, the battery using the P / S membrane exhibits a continuous increase in charge voltage after nearly 50 hours of stable operation, indicating a breakdown in the battery's stable operation and a severe imbalance in the charge-discharge process. After the test, the membranes were removed from the battery device to investigate their morphology. Significant zinc residues were observed on the P / S membrane surface (Figure 12a). These zinc residues originate from zinc dendrites. These zinc dendrites continue to grow, eventually penetrating the membrane and becoming lodged in the pores, contributing to the increased charge voltage (Figure 11b). They hinder the transport of charge-carrying ions and increase the membrane's resistance. Worse still, they become "dead zinc," unable to participate in subsequent cycles, leading to discharge capacity decay (Figure 11c). However, the morphology of the P / S-Nf-CMC film remains intact without Zn residue (Figure 12b), indicating that the Nf-CMC functional layer successfully achieves Zn dendrite / accumulation-free performance with stable discharge capacity.

[0081] Finally, the cell equipped with P / S-Nf-CMC membrane was -2 The film maintains an exceptionally long operating life (Figure 13), capable of over 3,000 stable cycles (>1,000 hours), far exceeding the performance of the original P / S membrane (~500 cycles) and the P / S-Nf-500 membrane (~900 cycles). Furthermore, the average battery performance remains stable at CE ~98.5%, EE ~79.9%, and VE ~81.1%.

[0082] Regulatory effect of Nf-CMC functional layer on zinc deposition behavior

[0083] To investigate the effect of the Nf-CMC functional layer on zinc deposition behavior, we first conducted an in-depth study of the zinc deposit morphology on carbon felt after cycling batteries equipped with different membranes. Field emission scanning electron microscopy (FE-SEM) was used to characterize the morphology of the zinc coating. SEM images revealed that zinc deposits on carbon felt in batteries equipped with P / S membranes exhibited a large number of typical needle-like morphologies (Figure 14a). In contrast, no sharp zinc deposits were evident on carbon felt in batteries equipped with P / S-Nf-CMC membranes (Figure 14be). Notably, as the amount of Nf-CMC spin-coated increased, the zinc deposits gradually developed a flat, masonry structure. Furthermore, the surface roughness of the zinc deposits on carbon felt was analyzed using a micro-nano scanning analyzer, confirming that metallic zinc deposits on carbon felt in batteries equipped with P / S-Nf-CMC membranes were smoother than those on P / S-based membranes (Figure 15).

[0084] Zinc metal is a typical hexagonal structure, and its main orientations are (002), (100) and (101) crystal planes (Figure 16a), among which the (002) crystal plane is the preferred orientation for forming relatively flat zinc deposition. On this basis, X-ray diffractometer (XRD) was introduced to study the crystal plane orientation of zinc deposition on carbon felt. As shown in Figure 16b, for zinc deposition on battery carbon felt using P / S membrane, the diffraction peak intensity corresponding to the (100) and (101) crystal planes is relatively high. However, for zinc deposition on battery carbon felt using P / S-Nf-CMC membrane, the diffraction peak intensity corresponding to the (002) crystal plane is relatively high. The ratio of the peak intensity corresponding to the (002) crystal plane to the (100) crystal plane (I (002) / I (100) ) as shown in Figure 16(c). The results indicate that increasing the amount of Nf-CMC spin-coated increases the exposure of the preferred orientation ((002) plane) for uniform zinc deposition. Compared to the P / S-based film, the cells using the P / S-Nf-CMC-1000 film exhibit a 50% increase in exposure of the preferred orientation during zinc deposition, consistent with the SEM images.

[0085] The above characterizations all confirm the excellent effect of the Nf-CMC functional layer in regulating the zinc deposition behavior, among which the strong electrostatic repulsion between the -COO functional group of CMC and the zincate ions is the direct reason for inducing uniform zinc deposition. Therefore, the surface charge of different films was first measured by a solid surface ZETA potentiometer. The test results showed that compared with the P / S-based film, as the amount of Nf-CMC spin-coated on the P / S-Nf-CMC film increased, its negative charge was significantly enhanced (Figure 17). According to the Donnan effect, this strong negative charge on the surface of the P / S-Nf-CMC film significantly repels the zincate ions with the same negative charge. A quartz crystal microbalance (QCM) was used to capture the effects of different film layers on Zn(OH)4 2- The slight change in the adsorption amount shows that the -COOH functional group of CMC and Zn(OH)4 2- There is a strong electrostatic repulsion between them, and the amount of zincate ions adsorbed by the film layer decreases with the increase of CMC content in the film layer (Figure 18).

[0086] In order to further provide theoretical support for the mechanism of uniform zinc deposition in Nf-CMC functional layer, density functional theory (DFT) was used to investigate the interaction between different molecules and Zn(OH)4 2- The potential energy scan was performed between (Figure 19). The theoretical calculation results show that CMC and Zn(OH)4 2- The Gibbs free energy value between them is a positive number far greater than 0, indicating that spontaneous reaction is impossible under the action of strong electrostatic repulsion. Therefore, the successful construction of the zinc-phobic functional layer based on electrostatic repulsion is confirmed, which alleviates the "tip effect" and realizes the Zn(OH)4 2-The Zn was well distributed throughout the membrane-electrode interface, thereby inducing uniform Zn deposition ( FIG20 ).

Claims

1. A zinc-based flow battery separator, comprising a porous base membrane, characterized in that: The surface of the porous base film is also covered with a modification layer, the material of the modification layer contains modification materials and a binder, and the modification material contains -COOH and -OH.

2. The zinc-based flow battery separator according to claim 1, characterized in that: The base film is a porous base film with a thickness of 20 to 500 μm; the modified layer has a thickness of 0.1 to 10 μm; 3. The zinc-based flow battery separator according to claim 1, characterized in that: The Zeta potential value of the surface of the zinc-based liquid flow battery separator is less than -10 mV, preferably less than -20 mV.

4. The zinc-based flow battery separator according to claim 1, characterized in that: The material of the porous base membrane is selected from one or more of polysulfones, polyketones, polyimides, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, polybenzimidazole, polyvinyl pyridine, polyethylene, polypropylene, chitosan, and cellulose acetate; the polymer is preferably a mixture of one or two of polyethersulfone and sulfonated polyetheretherketone; the modifying material is selected from one or more of carboxymethyl cellulose, carboxyethyl cellulose, carboxypropyl cellulose, carboxylated carbon nanotubes or carboxylated chitosan; the binder is selected from one or more of Nafion, sulfonated polyetheretherketone, polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, and polyimide.

5. The method for preparing a zinc-based flow battery separator according to any one of claims 1 to 4, characterized in that: The steps include: Step 1, obtaining a porous base membrane; Step 2: Disperse the modifying material and the binder in a solvent, and apply them on the surface of the porous base film, and obtain the zinc-based liquid flow battery separator after drying.

6. The preparation method according to claim 5, characterized in that: In the step 1, the base film is prepared by the following method: dispersing the polymer in an organic solvent as a casting liquid; coating the casting liquid on the surface of the substrate, and performing a film-forming treatment by a phase inversion method to obtain a porous base film; the organic solvent is selected from one or more of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, chloroform or dimethyl sulfoxide.

7. The preparation method according to claim 5, characterized in that: In the phase inversion method, a poor solvent for the polymer is used as a coagulation bath, and the immersion time in the coagulation bath is 1-600s; in the phase inversion method, after the membrane is treated in the coagulation bath, it is treated with an extraction solvent at a set temperature for a certain time; the temperature is 20-200°C, and the treatment time is 0.5-24h; in step 2, the coating method is selected from flat plate scraping, spraying or spin coating; the mass ratio of the modifying material to the binder is 1:(3-8), and the binder accounts for 1-20% of the solvent weight.

8. Use of the battery separator according to any one of claims 1 to 4 in a zinc-based liquid flow battery.

9. The use according to claim, characterized in that: In the application described, the modified layer faces the negative electrode of the battery; The application also includes a method for predicting battery VE performance, including the following steps: Determination of Fe(CN)6 in battery separators obtained under different preparation conditions 4- The permeability or surface resistance of the battery separator is determined, and the battery separator with the highest permeability or lowest surface resistance is taken as the separator with the highest VE performance prediction.

10. A method for regulating the crystal orientation of the zinc deposit layer (002) on the surface of the diaphragm during the operation of a zinc-based liquid flow battery, characterized in that: The steps include: Step 1, obtaining a porous base membrane; Step 2, dispersing the modifying material and the binder in a solvent, and coating the resultant on the surface of the porous base film, and obtaining a zinc-based flow battery separator after drying; Step 3, by increasing the loading amount of the modified material on the surface of the porous base film, the content of zinc with (002) crystal plane in the zinc deposition layer is increased.

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