Composite ion-conducting membrane, method for preparing the same, and use thereof

Abstract

The application discloses a simple preparation method of a composite ion conductive membrane and application of the composite ion conductive membrane in an alkaline zinc-iron flow battery, and the composite membrane comprises a support layer and a nitrogen-doped carbon nanotube layer attached to or partially embedded in the support layer. The membrane is prepared by uniformly coating inorganic substances such as nitrogen-doped carbon nanotubes on a glass plate, then coating a casting solution containing organic polymer resin on the modified glass plate, and performing phase inversion in a non-solvent. The membrane has high ion selectivity, high chemical stability and high mechanical strength, can realize uniform electric field distribution and induce initial nucleation to realize uniform deposition of zinc, and avoids short circuit of the battery caused by zinc dendrite piercing the diaphragm. The alkaline zinc-iron flow battery assembled by using the prepared composite ion conductive membrane has good battery performance and service life, and has a good application prospect.

Description

Technical Field

[0001] This invention relates to a composite ion-conducting membrane, its preparation method, and its application, particularly to the application of this type of membrane in alkaline flow battery systems. Background Technology

[0002] Net-zero emissions have accelerated the demand for renewable energy sources such as solar and wind power, necessitating economical large-scale energy storage devices. Aqueous flow batteries, which store energy in an externally stored electrolyte, are well-suited for grid storage needs due to their inherent advantages such as high safety, flexible design, ease of expansion, and long lifespan. Vanadium redox flow batteries, as one of the most mature technologies, have made significant progress, with a recent 100MW / 400MWh vanadium redox flow battery energy storage station connected to the grid. However, the sustainable development of flow batteries remains limited by relatively high cost and low energy density.

[0003] In recent years, zinc-based flow batteries have gained increasing attention due to their unique properties, such as high abundance in the Earth's crust, low cost, and the environmental friendliness of zinc-based materials. A major bottleneck in the practical application of zinc-based flow batteries is the uneven electroplating of zinc at the negative electrode, which can induce the formation of zinc dendrites. Uncontrolled zinc dendrites can puncture the separator, reducing battery reliability. These problems are exacerbated at higher current densities or areal capacities, reducing the power or energy density of zinc-based flow batteries. Typically, the electrochemical reaction of the zincate couple occurs at the membrane-electrode interface. Adjusting the interfacial characteristics of the membrane and electrode, such as temperature distribution, active material distribution, and spatial distribution, can effectively achieve a uniform electroplating process for the zincate couple and further prevent zinc dendrite formation. Composite films consisting of functional and support layers are considered an effective strategy for adjusting the membrane-electrode interface because the functional and support layers can be designed and adjusted independently. Currently, functional nanomaterials such as metal-organic frameworks, MXenes, and layered double hydroxides are the most widely used materials for the functional layers of composite membranes. They are typically introduced onto polymer substrates via vacuum-assisted filtration or spraying to form the composite membrane. Although these methods are simple and convenient, the nanomaterials in the functional layer easily detach from the polymer substrate under flowing electrolytes due to poor anchoring strength between the nanomaterials and the polymer substrate, as well as damage caused by external loads applied by zinc. In-situ growth methods can obtain defect-free functional layers on polymer substrates, but they are not universally applicable due to the relatively complex processes and harsh conditions required. Therefore, new methods are needed to prepare composite membranes with excellent properties. Summary of the Invention

[0004] To address the aforementioned technical problems, the purpose of this invention is to develop a low-cost, highly selective, zinc dendrite-resistant composite ion membrane for alkaline zinc-iron flow batteries.

[0005] To achieve the above objectives, the specific technical solution of the present invention is as follows:

[0006] A method for preparing a composite ion-conducting membrane is disclosed, comprising the following steps: An inorganic dispersion is uniformly coated onto the upper surface of a glass plate, covering the entire glass plate. After the solvent has completely evaporated, a casting solution is scraped onto the glass plate and immersed in water. The polymer is easily peeled off from the glass plate, yielding the composite ion-conducting membrane.

[0007] Furthermore, the preparation method mainly includes the following steps:

[0008] (1) Using organic polymer resin as raw material, the organic polymer resin is dissolved in an organic solvent and stirred at room temperature for 24-48 hours to obtain casting solution, wherein the concentration of the organic polymer resin is 20-40 wt%.

[0009] (2) Disperse the inorganic powder in anhydrous ethanol solvent, stir with a magnetic stirrer for 1 hour, and then sonicate the solution with an ultrasonic cleaner for 2 hours to form a concentration of 0.1-2 mg / mL. -1 A uniformly dispersed liquid;

[0010] (3) The inorganic dispersion is uniformly sprayed onto the designated area of ​​the glass plate, with a spraying amount of 0.01-0.07 mg / cm². -2 Then, the glass plate is heated on a hot plate at 40-60℃ for 0.2-2 hours to remove residual ethanol.

[0011] (4) Then, under the conditions of room temperature and humidity less than 50%, use a doctor blade with a thickness of 180-230μm to evenly coat the organic polymer casting liquid onto the spraying area of ​​the glass plate.

[0012] (5) Finally, immerse the glass plate in water for 5-20 minutes to form a composite ion-conducting membrane. Preferably, the inorganic material is a nitrogen-doped carbon nanotube with an outer diameter of 30-80 nm, an inner diameter of 5-20 nm, a length of 10-30 μm, and a nitrogen doping amount of 2.5-5 wt%; more preferably, the nitrogen-doped carbon nanotube has an outer diameter of 50-60 nm, an inner diameter of 10-15 nm, a length of 15-20 μm, and a nitrogen doping amount of 2.5 wt%.

[0013] Preferably, the dispersion contains 0.5-1 mg / mL of [a certain substance]. -1 An ethanol solution; more preferably, the dispersion concentration is 0.5-0.7 mg / mL. -1 ;

[0014] Preferably, the area of ​​the glass plate is 72-500 cm². 2 ;

[0015] Preferably, the spraying amount of the inorganic dispersion is 0.03-0.07 mg / cm³. -2 More preferably, the spraying amount of the inorganic dispersion is 0.05-0.06 mg / cm³. -2

[0016] Preferably, the organic polymer resin is: polyethersulfone and sulfonated polyetheretherketone; the mass ratio of polyethersulfone to sulfonated polyetheretherketone is 95:5-85:15, more preferably 94:6-92:8; the degree of sulfonation of sulfonated polyetheretherketone is 0.6-0.8.

[0017] Preferably, the organic solvent is N,N-dimethylacetamide;

[0018] Preferably, the solid content of the casting solution is 30-40 wt%; more preferably, the solid content of the casting solution is 35-37 wt%.

[0019] The present invention also provides a composite ion-conducting membrane, characterized in that: the composite membrane includes a support layer and a nitrogen-doped carbon nanotube layer, the thickness of the nitrogen-doped carbon nanotube layer is 1-3 μm (preferably 1.5-2.8 μm), and the thickness of the composite membrane is 90-125 μm (preferably 90-120 μm).

[0020] The composite ion-conducting membrane is used in alkaline zinc-iron flow batteries.

[0021] The beneficial effects of this invention are:

[0022] 1. The nitrogen-doped carbon nanotube (N-CNTs) composite ion-conducting membrane (PECM) prepared in this invention differs from traditional composite membranes. Its highly conductive N-CNTs functional layer can be firmly embedded into the PECM membrane surface through a "partial embedding" strategy and will not detach under flowing electrolyte conditions. This simple and efficient preparation method can be extended to the preparation of other composite membranes with different functional layers and has the advantage of easy scale-up.

[0023] 2. The PECM membrane prepared by this invention has the advantages of high electronic conductivity, high ion selectivity and high ion conductivity, and is expected to endow alkaline zinc-iron flow batteries with high performance;

[0024] 3. The PECM membrane prepared in this invention has a zinc-loving N-CNTs conductive functional layer on its surface, which is then used in an alkaline zinc-iron flow battery with its face facing the negative electrode. By changing the initial nucleation site of the zincate redox couple, metallic zinc is preferentially deposited on the N-CNTs-based composite ion-conducting membrane, and the current density distribution and zincate ion concentration distribution at the membrane-electrode interface are uniformly distributed, thereby achieving uniform deposition of metallic zinc on the electrode.

[0025] 3. The PECM membrane prepared by this invention has low cost, simple manufacturing process, and good stability;

[0026] 4. The PECM membrane prepared in this invention broadens the application range of membrane materials for alkaline zinc-iron flow batteries. This type of membrane is prepared by uniformly coating nitrogen-doped carbon nanotubes and other inorganic materials onto a glass plate, followed by coating a casting solution containing dissolved organic polymer resin onto the modified glass plate, and then undergoing a phase inversion in a solvent-free environment. This type of membrane exhibits high ion selectivity, high chemical stability, and high mechanical strength. It can achieve uniform deposition of metallic zinc by uniformly distributing the electric field and inducing initial nucleation, avoiding zinc dendrites piercing the separator and causing short circuits in the battery. Alkaline zinc-iron flow batteries assembled with the prepared composite ion-conducting membrane exhibit good battery performance and lifespan, showing promising application prospects. Attached Figure Description

[0027] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0028] Figure 1 Surface morphology images (a) and (b) magnified images of the nitrogen-doped carbon nanotube composite ion-conducting membrane (PECM) prepared in Example 1, and cross-sectional morphology images (c) and (d) magnified images of the cross-sectional morphology.

[0029] Figure 2 X-ray diffraction pattern of the PECM film prepared in Example 1.

[0030] Figure 3 (a) Force-displacement curves of PM prepared in Comparative Example 1 and PECM prepared in Example 1; alkaline zinc-iron flow battery at 80 mA cm⁻¹ -2 Digital photographs of the SNCM prepared in Comparative Example 2 and the PECM prepared in Example 1 at the end of the 10th discharge under the current density (bc).

[0031] Figure 4 The alkaline zinc-iron flow battery assembled with the PECM membrane prepared in Example 1 was tested at 0.5 mAh cm⁻¹. -2 Area capacity, 80 mA / cm -2 (a)(b) Surface morphology of the PECM film prepared in Example 1 and (c)(d) morphology of the negative electrode carbon felt at current density; Alkaline zinc-iron flow battery assembled with PM film prepared in Comparative Example 1 at 0.5 mAh cm⁻¹ -2 Area capacity, 80mA cm -2 Morphology of the negative electrode carbon felt at current density (e)(f).

[0032] Figure 5 Alkaline zinc-iron flow batteries assembled from the PECM prepared in Example 1 and the PM prepared in Comparative Example 1 (a), (b) 60 mA cm-2 (c), (d) 80mA cm -2 Simulation results of current density distribution in (e) and (f).

[0033] Figure 6 At a current density of 80 mA cm -2 The morphology of the zinc deposited on the negative electrode of the alkaline zinc-iron flow battery assembled from (a) the PM prepared in Comparative Example 1 and its (b) scaled-up version, and (d) the PECM prepared in Example 1 and its (e) scaled-up version, is shown. The alkaline zinc-iron flow battery assembled using (c) the PM prepared in Comparative Example 1 and (d) the PECM prepared in Example 1 is shown at 80 mA cm⁻¹. -2 Ultra-deep microscopic images of zinc morphology at current density.

[0034] Figure 7 An alkaline zinc-iron flow battery using the PECM prepared in Example 1 was tested at 80 mA cm⁻¹. -2 Cyclic performance at current density.

[0035] Figure 8 Alkaline zinc-iron flow batteries using the PECM prepared in Example 1 and the PM prepared in Comparative Example 1 were tested at 80 mA cm⁻¹. -2 Voltage-time curves at current density. Detailed Implementation

[0036] The present invention will be described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0037] The alkaline zinc-iron flow battery performance test conditions used for the ion-conducting membranes in the following examples and comparative examples are as follows:

[0038] The assembly sequence of an alkaline zinc-iron flow battery is as follows: negative electrode plate, graphite current collector, 3 x 3 cm... 2 Carbon felt (0.5cm thick square carbon felt, negative electrode), 5x5cm 2 Ion-conducting membrane, 3 x 3 cm 2 Carbon felt (0.5cm thick square carbon felt, positive electrode), graphite current collector, positive electrode plate.

[0039] The PECM film (nitrogen-doped carbon nanotube composite ion-conducting film) prepared in this invention has a zinc-loving N-CNTs conductive functional layer on its surface, and is used in alkaline zinc-iron flow batteries with its face facing the negative electrode.

[0040] Performance testing conditions for alkaline zinc-iron flow batteries: The composition of the negative electrode electrolyte and the positive electrode electrolyte are 40 mL and 0.4 mol / L, respectively. -1 Zn(OH)4 2- +3mol L -1 NaOH and 40 mL 0.8 mol L-1 Fe(CN)6 4- +3mol L -1 KOH. The assembled single cell is then circulated through the positive and negative electrodes one-to-one by a magnetic circulation pump (MP-10RN) to ensure a flow rate of 60 mL / min. -1 The battery underwent constant current charge-discharge testing on an ArbinBT 2000, using a time-cutoff method, at a current density of 80 mA cm⁻¹. -2 Charge for 50 minutes and then discharge to 0.1V (each cycle is about 100 minutes, and one cycle refers to charging and then discharging).

[0041] The three key indicators for evaluating battery performance are coulombic efficiency, voltage efficiency, and energy efficiency. Coulombic efficiency (CE) is the ratio of discharged capacity to charged capacity during a single charge-discharge cycle; energy efficiency (EE) is the ratio of discharged energy to charged energy; and voltage efficiency (VE) is approximately equal to the ratio of the integral average of the discharge voltage and the charging voltage over time.

[0042] Example 1

[0043] Weigh 50 mg of nitrogen-doped carbon nanotubes (conductive inorganic material) purchased from XFNANO, with an outer diameter of 50-60 nm, an inner diameter of 10-15 nm, a length of 15-20 μm, and a nitrogen doping amount of 2.5 wt%. Disperse them uniformly in 100 mL of ethanol, stir for 30 min, and then sonicate for 1 h to obtain a nitrogen-doped carbon nanotube dispersion of 0.5 mg / mL.

[0044] Polyethersulfone and sulfonated polyether ether ketone (sulfonation degree 0.7) were dissolved in N,N-dimethylacetamide (DMAC) solvent, mechanically stirred for 12 h and allowed to stand for more than 24 h to remove bubbles, resulting in a casting solution with a solid content (concentration of organic polymer resin, the organic polymer resin being polyethersulfone and sulfonated polyether ether ketone) of 35%, wherein the mass ratio of polyethersulfone to sulfonated polyether ether ketone was 94:6.

[0045] The prepared nitrogen-doped carbon nanotube (N-CNT) dispersion was drawn up with a syringe and uniformly sprayed onto a 72 cm² area using a spray gun. 2 The dispersion was applied to the entire surface of a clean, horizontally placed glass plate at a spraying rate of 0.05 mg / cm². -2The glass plate was then placed horizontally on a hot plate and baked at 50°C for 1 hour to remove the dispersant ethanol. N-CNTs were uniformly dispersed on the glass plate without significant agglomeration, and the conductive inorganic layer had a uniform thickness. At room temperature and 25% relative humidity, a 200 μm thick (coating thickness 200 μm) doctor blade was used to uniformly coat the casting solution onto the horizontally placed glass plate after N-CNTs coating. The coated glass plate was then immersed in water for 10 minutes at room temperature to undergo a solvent-free phase inversion. After the solvent DMAC was completely dissolved, the N-CNTs composite ion-conducting membrane (PECM) was obtained. Figure 1 As can be seen, the PECM membrane surface has an N-CNT network structure, the conductive inorganic layer thickness is 2.0 μm, and the composite membrane thickness is 112.7 μm. The N-CNTs are partially embedded in the polymer surface and partially exposed outside the surface, which endows the PECM with high conductivity, high ion selectivity and high ion conductivity. Figure 2 The XRD pattern showed that, in addition to the polymer peaks, the (002) characteristic diffraction peaks of N-CNTs were also clearly observed, proving that N-CNTs were successfully introduced onto the polymer substrate. An alkaline zinc-iron flow battery was assembled using the PECM membrane, with the N-CNT layer facing the negative electrode. The battery was subjected to a 720mA constant current charge-discharge test on an ArbinBT 2000 under the conditions described above. The performance test results of the alkaline zinc-iron flow battery assembled with the ion-conducting membrane are shown in Table 2. In Table 2, CE, VE, and EE represent the performance of the battery during the second charge-discharge cycle.

[0046] Examples 2-8

[0047] The preparation process of the ion-conducting membrane, the assembly process of the battery, and the performance testing process are the same as those in Example 1. The difference is that one or more of the following are used: the type of conductive inorganic material, the type and proportion of organic polymer resin, the concentration of organic polymer resin, and the amount of inorganic material sprayed. The specific relevant materials and data are shown in Table 1. The thickness of the conductive inorganic layer is uniform. The performance test results of the alkaline zinc-iron flow battery assembled with the ion-conducting membrane are shown in Table 2.

[0048] Comparative Example 1

[0049] Polyethersulfone and sulfonated polyetheretherketone (sulfonation degree 0.7) were dissolved in DMAC solvent, mechanically stirred for 12 h and allowed to stand for more than 24 h to remove air bubbles, resulting in a casting solution with a solid content of 35%, wherein the mass ratio of polyethersulfone to sulfonated polyetheretherketone was 94:6. Under room temperature and 25% relative humidity, the casting solution was uniformly coated onto a sprayed glass plate using a 200 μm thick doctor blade. The glass plate was then immersed in water for 10 min at room temperature to perform a non-solvent phase inversion. After the solvent was completely dissolved, a porous base membrane (PM) was obtained. The PM was applied to an alkaline zinc-iron flow battery, and the application method (battery assembly process and performance testing process) was the same as in Example 1, the only difference being that the separator was replaced with PM instead of PECM. The performance of the assembled alkaline zinc-iron flow battery was tested under the same battery testing conditions as in Example 1, and the results are shown in Table 2.

[0050] Comparative Example 2

[0051] Take 0.5 mg mL of the solution prepared in Example 1 -1 An N-CNTs dispersion was prepared, and then a 1% Nafion solution (by mass concentration of binder) was added. The mixture was stirred for 30 min and sonicated for 1 h. The mass ratio of N-CNTs to Nafion was 8:2. 4 mL of the prepared N-CNTs dispersion was drawn using a syringe and sprayed evenly onto one side of the PM prepared in Comparative Example 1, resulting in a composite ion-conducting membrane (SNCM) prepared by spraying. The composite membrane was applied to an alkaline zinc-iron flow battery under the same testing conditions as in Example 1 (the N-CNTs side facing the negative electrode in the alkaline zinc-iron flow battery). The results are shown in Table 2.

[0052] Comparative Example 3-13

[0053] The preparation process of the ion-conducting membrane, the assembly process of the battery, and the performance testing process are the same as those in Example 1. The difference is that one or more of the following are used: the type of conductive inorganic material, the type and proportion of organic polymer resin, the concentration of organic polymer resin, and the amount of inorganic material sprayed. The specific relevant substances and data are shown in Table 1. The performance of the assembled alkaline zinc-iron flow battery was tested under the same battery testing conditions as in Example 1. The performance test results of the alkaline zinc-iron flow battery assembled with the ion-conducting membrane are shown in Table 2.

[0054] Comparative Example 14

[0055] The same preparation process and conditions as in Example 1 were used for the ion-conducting membrane, except that the dispersant ethanol in the N-CNTs dispersion was replaced with ultrapure water. The N-CNTs were sprayed onto a glass plate using ultrapure water as the dispersant and then heated on a hot plate. After drying, the N-CNTs could not be uniformly distributed on the glass plate; instead, they agglomerated to form random black spots, resulting in an uneven surface and preventing the formation of a uniform conductive inorganic layer.

[0056] Comparative Example 15

[0057] N-CNTs were added to the casting solution prepared in Example 1 (where the mass content of N-CNTs was 15 wt%). The N-CNTs blend casting solution was then mechanically stirred for 12 h at room temperature and allowed to stand for 24 h until all bubbles disappeared. Subsequently, at room temperature and 25% relative humidity, the N-CNTs blend casting solution was uniformly coated onto the surface of a horizontally placed glass plate using a 5 μm thick doctor blade. The glass plate was then placed horizontally on a hot plate and baked at 50°C for 1 h. At room temperature and 25% relative humidity, the casting solution prepared in Example 1 was uniformly coated onto the surface of the sprayed horizontally placed glass plate using a 200 μm thick doctor blade. The horizontally placed glass plate was then immersed in water for 10 min at room temperature to undergo a non-solvent phase inversion. After the solvent was completely dissolved, a composite ion-conducting membrane with a blended N-CNTs layer was obtained. The composite membrane was applied to an alkaline zinc-iron flow battery, and the battery testing conditions were the same as in Example 1. The results are shown in Table 2.

[0058] Table 1. Preparation process parameters of ion-conducting membranes in Examples 2-7 and Comparative Examples 2-12

[0059]

[0060]

[0061] Table 2 Battery performance test results corresponding to composite membranes

[0062]

[0063]

[0064]

[0065] In Table 2, CE, VE, and EE represent the battery's performance during the second charge-discharge cycle.

[0066] As can be seen from the implementation data of Examples 1-8 above, the composite ion-conducting membrane prepared by the present invention has a conductive functional layer and a porous support layer. By controlling the type and proportion of organic polymer resin, the prepared ion-conducting membrane achieves a tight bond between the conductive functional layer and the sponge-like porous layer, thus improving the stability of the composite membrane. The N-CNTs layer of the PECM membrane imparts higher mechanical strength to the membrane. Figure 3a). Furthermore, strong hydrogen bonds exist between N-CNTs and PES and SPEEK polymers, which enhances the anchoring strength between N-CNTs. After the battery has been operating stably, the battery was disassembled for observation of the membrane. The embedded N-CNT layer is firmly bonded to the porous support and does not detach from the support layer in the flowing electrolyte environment. Figure 3 de).

[0067] By adjusting multiple technical parameters such as the concentration of organic polymer resin and the amount of inorganic coating, the thickness of the conductive functional layer is optimized, thereby improving the ion selectivity and conductivity of the membrane. When applied to alkaline zinc-iron flow batteries, this results in high ion selectivity (high coulombic efficiency) and high ion conductivity (high voltage efficiency), demonstrating excellent performance in the battery. A small amount of SPEEK can adjust the microstructure of the porous PES support, thus achieving high battery voltage efficiency, while the N-CNTs functional layer ensures the battery's coulombic efficiency.

[0068] By controlling the types of conductive inorganic materials, the initial nucleation sites of the zincate redox couple can be altered, guiding zinc metal to preferentially deposit on the PECM film surface and uniformly distributing the current density and zincate ion concentration at the film-electrode interface. This results in uniform zinc deposition on the electrode, suppressing zinc dendrite formation and improving the battery's cycle stability. SEM observations of the alkaline zinc-iron flow battery assembled with PECM at 0.5 mAh cm⁻¹... -2 The morphological changes of the negative electrode zinc metal and N-CNTs functional layer surface under low areal capacity. For example Figure 4 As shown in ab, when the deposition surface capacity is 0.5 mAh cm⁻¹ -2 At that time, uniform spherical deposits appeared on the surface of the PECM film. In contrast, almost no metallic zinc (Zn) was found on the carbon felt electrode. Figure 4 The results (cd) indicate that the N-CNTs functional layer can induce the initial nucleation and growth of metallic zinc on the surface of the PECM film. Simulations of the zincate ion concentration and current density distribution at the electrode-film interface revealed that the N-CNTs functional layer can uniformly distribute the zincate ion concentration and current density at the film-electrode interface. Figure 5 This effectively suppresses the negative effects of the "sharp edge effect" and achieves uniform deposition of zinc metal in the negative electrode.

[0069] Therefore, the battery assembled using the PECM membrane operates at 80 mA cm -2 The deposited zinc metal at the operating current density has a uniform and dense morphology. Figure 6 (df), and is flatter, with a height difference of only 33.0μm ( Figure 6 c) This uniform and smooth zinc deposition morphology is beneficial to improving battery stability. The alkaline zinc-iron flow battery assembled using a PECM film exhibits performance at 80 mA / cm². -2It can operate stably for 350 hours at its operating current density, with a coulombic efficiency of 98.5% and an energy efficiency of 88.6%. Figure 7-8 ).

[0070] In Comparative Examples 2 and 8-9, the battery performance was good, but the cycle performance was poor; the voltage efficiency gradually decreased with charging and discharging. This is attributed to the external load imposed by the electrolyte flow and the high-modulus zinc deposition on the negative electrode, leading to the shedding of the conductive functional layer, increasing the interfacial resistance, weakening the ability to improve current density distribution, and causing zinc dendrites to penetrate the film, forming dead zinc, thus continuously increasing the film resistance and continuously decreasing the voltage efficiency. Figure 3 bc It can be observed that, due to the external load applied by the flow of electrolyte and the deposition of high-modulus zinc metal on the negative electrode, the functional layer on the surface of the SNCM in Comparative Example 2 has been largely detached.

[0071] In Comparative Examples 1 and 3-5, the cycling performance decreased. This is mainly due to the lack of an N-CNT functional layer, which prevents the control of zinc deposition morphology. For example, in Comparative Example 1, at 80 mA cm⁻¹... -2 Current density and 66mAh cm -2 Under the given areal capacity, the negative electrode zinc exhibits anisotropic moss-like morphology with an undulation height as high as 128.15 μm. Figure 6 Therefore, during charging, zinc dendrites penetrate the membrane, causing a micro-short circuit in the battery and degrading battery performance. Figure 8 ).

[0072] In Comparative Examples 6 and 10, both voltage efficiency and energy efficiency were lower than in Example 1. This was mainly because the changes in experimental parameters hindered the smooth passage of charge carriers through the membrane. In Comparative Examples 7 and 11-13, both coulombic efficiency and energy efficiency were lower than in Example 1. This was mainly because the changes in experimental parameters affected the thickness of the inorganic layer and the structure of the membrane, causing cross-contamination of active materials, resulting in poor ion selectivity and lower coulombic efficiency of the battery.

[0073] In Comparative Example 15, the excessive interfacial resistance between the blend layer and the support layer led to a decrease in the ion conductivity of the composite ion-conducting membrane, an increase in the internal resistance of the battery, and a significant decrease in voltage efficiency and energy efficiency.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a composite ion-conducting membrane, characterized in that: A nitrogen-doped carbon nanotube dispersion is coated onto the upper surface of a horizontally placed flat substrate, forming a coating area on the upper surface of the substrate. After the solvent in the dispersion in the coating area is completely evaporated, a nitrogen-doped carbon nanotube layer is formed. A casting solution containing an organic polymer resin is scraped onto the nitrogen-doped carbon nanotube layer in the coating area, and then the coating area of ​​the substrate is immersed in water. Finally, the organic polymer resin and the nitrogen-doped carbon nanotube layer are peeled off from the substrate to obtain the composite ion-conducting membrane. The casting solution is prepared by dissolving organic polymer resin in an organic solvent. The organic polymer resin is polyethersulfone and sulfonated polyether ether ketone; the mass ratio of polyethersulfone to sulfonated polyether ether ketone is 95:5-85:

15.

2. The preparation method according to claim 1, characterized in that: The organic polymer resin is polyethersulfone and sulfonated polyether ether ketone; the mass ratio of polyethersulfone to sulfonated polyether ether ketone is 95:5-90:

10.

3. The preparation method according to claim 1, characterized in that: The organic polymer resin is polyethersulfone and sulfonated polyether ether ketone; the mass ratio of polyethersulfone to sulfonated polyether ether ketone is 94:6-92:

8.

4. The preparation method according to claim 1, characterized in that: Nitrogen-doped carbon nanotubes have an outer diameter of 30-80 nm, an inner diameter of 5-20 nm, a length of 10-30 μm, and a nitrogen doping concentration of 2.5-5 wt%. The dispersion contains a concentration of 0.1-2 mg / mL. -1 An ethanol solution of nitrogen-doped carbon nanotubes; the amount of the nitrogen-doped carbon nanotube dispersion sprayed onto the upper surface of the substrate is 0.01-0.07 mg / cm³. -2 .

5. The preparation method according to claim 4, characterized in that: Nitrogen-doped carbon nanotubes have an outer diameter of 40-60 nm, an inner diameter of 5-15 nm, a length of 10-20 μm, and a nitrogen doping concentration of 2.5-4 wt%. The dispersion contains a concentration of 0.5-1 mg / mL. -1 An ethanol solution of nitrogen-doped carbon nanotubes; the amount of the nitrogen-doped carbon nanotube dispersion sprayed onto the upper surface of the substrate is 0.03-0.07 mg / cm³. -2 .

6. The preparation method according to claim 4, characterized in that: Nitrogen-doped carbon nanotubes have an outer diameter of 50-60 nm, an inner diameter of 10-15 nm, a length of 15-20 μm, and a nitrogen doping concentration of 2.5-3 wt%. The dispersion contains 0.5-0.7 mg / mL of [a solution / concentration]. -1 An ethanol solution of nitrogen-doped carbon nanotubes; the amount of the nitrogen-doped carbon nanotube dispersion sprayed onto the upper surface of the substrate is 0.05-0.06 mg / cm³. -2 .

7. The preparation method according to claim 1, characterized in that: The flat substrate is a glass plate; The coating process of the dispersion is spraying and / or scraping. The solvent in the dispersion in the coating area evaporates by drying at 40-60 ℃ for 0.2-2 h.

8. The preparation method according to claim 1, characterized in that: The organic solvent is N,N-dimethylacetamide (DMAC); The solid content of the casting solution is 20-40 wt%; the degree of sulfonation of the sulfonated polyether ether ketone is 0.6-0.

8.

9. The preparation method according to claim 1, characterized in that: The solid content of the casting solution is 30-40 wt%; the degree of sulfonation of the sulfonated polyether ether ketone is 0.65-0.

75.

10. The preparation method according to claim 1, characterized in that: The solid content of the casting solution is 35-37 wt%; the degree of sulfonation of the sulfonated polyether ether ketone is 0.68-0.

72.

11. The preparation method according to claim 1 or 8, characterized in that: After applying the casting solution containing organic polymer resin to the horizontally placed substrate coating area using a scraper with a coating thickness of 180-230 μm, the horizontally placed substrate coating area is then immersed in water for 5-20 min to perform a non-solvent phase inversion, allowing the solvent DMAC to be completely dissolved. Finally, the organic polymer resin layer and the nitrogen-doped carbon nanotube layer are peeled off from the substrate.

12. The preparation method according to claim 1, 2, or 8, characterized in that: The thickness of the nitrogen-doped carbon nanotube layer after the solvent in the dispersion in the coating area has completely evaporated is 1-3 μm; The thickness of the composite ion-conducting membrane is 80-125 μm.

13. The preparation method according to claim 12, characterized in that: The thickness of the nitrogen-doped carbon nanotube layer after the solvent in the dispersion in the coating area has completely evaporated is 1.5-2.8 μm; The thickness of the composite ion-conducting membrane is 90-120 μm.

14. A composite ion-conducting membrane prepared by any one of the preparation methods described in claims 1-10.

15. The composite ion-conducting membrane according to claim 14, characterized in that: The composite ion-conducting membrane includes a support layer and a nitrogen-doped carbon nanotube layer attached to or partially embedded in the support layer. The thickness of the nitrogen-doped carbon nanotube layer is 1-3 μm, and the thickness of the composite ion-conducting membrane is 90-125 μm.

16. The composite ion-conducting membrane according to claim 15, characterized in that: The thickness of the nitrogen-doped carbon nanotube layer is 1.5-2.8 μm, and the thickness of the composite ion-conducting membrane is 90-120 μm.

17. The application of the composite ion-conducting membrane of claim 14 or 15 as a separator in an alkaline zinc-iron flow battery system.

18. The application according to claim 17, characterized in that: Nitrogen-doped carbon nanotubes with the negative electrode side facing are used in alkaline zinc-iron flow batteries.

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