Metal-organic framework diaphragm with high selectivity and rigidity-flexibility as well as preparation method and application of metal-organic framework diaphragm

Abstract

The invention provides a rigid-flexible metal-organic framework diaphragm with high selectivity as well as a preparation method and application of the rigid-flexible metal-organic framework diaphragm. Through a seed secondary growth strategy, the problems that MOF particles are easy to fall off and are incompatible with a substrate film are successfully avoided. On one hand, grain seeds embedded in the polyvinylidene fluoride (PVDF) substrate film in advance provide stable anchoring points for growth of MOF, and MOF particles are not prone to falling off due to the high adhesive force of the grain seeds. The high Young modulus of the MOF crystal also effectively improves the thermal shrinkage resistance of the diaphragm, the excellent flexibility of PVDF is complementary with the diaphragm in mechanical property, and the battery diaphragm shows excellent mechanical property and thermal stability due to the rigid-flexible combination strategy. The invention provides an efficient diaphragm construction strategy for developing a lithium metal battery with long cycle life and high safety, and shows a wide application prospect in a next-generation high-safety battery.

Description

Technical Field

[0001] The present invention belongs to the field of battery separators, and in particular relates to a metal-organic framework separator with high selectivity and both rigidity and flexibility, as well as a preparation method and application thereof. Background Art

[0002] In order to achieve sustainable development of mankind, the concept of green and low carbon has become an irreversible development trend. With the continuous advancement of national policies, energy conservation and emission reduction have become key measures to cope with global climate change and achieve sustainable development. Traditional fuel vehicles are gradually withdrawing from the market due to their carbon emissions of more than 80%. New energy vehicles are rapidly occupying the transportation field with their high energy efficiency and low emissions. At present, the world is competing to develop rechargeable battery systems with lower costs and higher energy density. Lithium-ion batteries have high energy density (≥200 Wh kg -1 ), low self-discharge rate and wide operating temperature range, it has been widely used in fields such as pure electric vehicles and has become an important pillar of the emerging industry. However, due to the high reactivity of lithium metal, it is easy to undergo irreversible reactions with the electrolyte during charging and discharging, resulting in volume expansion, damage to the solid electrolyte interface (SEI), and the formation of lithium dendrites. Lithium dendrites increase internal resistance, consume electrolyte and may puncture the diaphragm, causing short circuits, thermal runaway and even explosions, thereby reducing coulombic efficiency and battery life. To address this series of problems, researchers are committed to developing new lithium battery technologies to improve the safety and cycle performance of lithium metal batteries.

[0003] Metal-organic frameworks (MOFs) are a class of porous crystalline materials formed by the self-assembly of metal ion centers and organic ligands. Their structure and pore size can be adjusted through design. MOFs not only have a high specific surface area and excellent porosity, but their high Young's modulus also significantly improves the material's resistance to shrinkage. In battery separator applications, MOFs can efficiently screen ions and precisely control ion transport processes, thereby optimizing battery performance. Although a variety of functionalized MOF-based separators have been explored, the successful introduction of MOFs into battery separators still faces many challenges. Summary of the Invention

[0004] To address the above-mentioned technical problems, the present invention aims to provide a highly selective and "rigid-flexible" metal-organic framework (MOF) separator, as well as its preparation method and application. This invention utilizes MOF secondary growth to in-situ grow a MOF layer on a base separator, avoiding issues such as uneven MOF dispersion and gaps between MOF layers. Furthermore, its excellent electrolyte adsorption capacity and ability to regulate ion transport demonstrate its great potential for lithium-ion battery applications, providing a novel technical approach for improving lithium-ion battery safety and performance.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A method for preparing a metal-organic framework membrane with high selectivity and both rigidity and flexibility comprises the following steps: (1) Dissolving the metal salt and the organic ligand in an organic solvent, such as dissolving zinc nitrate and 2-methylimidazole in methanol, and then rapidly stirring and mixing to prepare ZIF-8 seeds; (2) Dissolve PVDF in an organic phase, then add the MOF seeds obtained in step (1) and stir until the mixture is uniform; (3) Pour the slurry obtained in step (2) onto a glass plate and apply it with a scraper. The glass plate is immersed in deionized water to obtain a ZIF-8-Seeds@PVDF membrane, which is then placed in deionized water to exchange the residual organic solvent. (4) Dissolving the metal salt and the organic ligand in an organic solvent, such as dissolving zinc nitrate, 2-methylimidazole, and sodium formate in methanol; (5) The solution obtained in step (4) and the ZIF-8-Seeds@PVDF membrane obtained in step (3) were transferred to a reactor and subjected to a hydrothermal reaction at 80°C for 12 hours to obtain a ZIF-8@PVDF membrane.

[0006] (6) The membrane obtained in step (5) was washed with methanol and vacuum dried at 120° C. for 12 hours to obtain a highly selective metal-organic framework membrane.

[0007] Furthermore, in step (1), the metal salt and the organic ligand are subjected to solution reaction at a mass ratio of 1:2.

[0008] Furthermore, in step (2), the organic phase for dissolving PVDF is N,N-dimethylacetamide (DMAc), and the organic phase for dissolving the metal salt and the organic ligand is N,N-dimethylformamide or methanol.

[0009] Furthermore, in step (1), the concentration of zinc nitrate in methanol is 15.0 g / L.

[0010] Furthermore, in step (1), the concentration of 2-methylimidazole in methanol is 30.0 g / L, and the dosage ratio of zinc nitrate to 2-methylimidazole is 1:2.

[0011] Furthermore, in step (2), the MOF monomer is one of ZIF-8, HKUST-1, UIO-66, ZIF-67 and MOF-5.

[0012] Furthermore, in step (3), the mass ratio of PVDF, N,N-dimethylacetamide and MOF seeds is 86:13:1.

[0013] Furthermore, in step (4), the molar ratio of the metal ion to the organic ligand is 2:3.

[0014] The present invention also provides a metal-organic framework membrane with high selectivity and both rigidity and flexibility obtained by the above preparation method.

[0015] The present invention provides a metal-organic framework separator with high selectivity and both rigidity and flexibility for use in lithium-ion batteries.

[0016] This invention successfully avoids the problems of MOF particle shedding and incompatibility with the substrate membrane through a seeded secondary growth strategy. Firstly, the seed crystals pre-embedded in the polyvinylidene fluoride (PVDF) substrate membrane provide a stable anchoring point for MOF growth, and their strong adhesion prevents MOF particles from shedding. The high Young's modulus of the MOF crystals also effectively improves the separator's resistance to thermal shrinkage, while the excellent flexibility of PVDF complements this mechanical performance. This "rigid-flexible" combination strategy results in a battery separator with excellent mechanical properties and thermal stability. The ZIF-8@PVDF composite separator exhibits excellent flexibility and thermal stability. Even after 200 repeated bends, the separator maintains its structural integrity without cracking or breaking, demonstrating excellent mechanical toughness and deformation adaptability, making it suitable for practical applications in complex working environments. Furthermore, the separator remains structurally stable even after 90 days in an unprotected atmosphere at high temperatures (190°C). Furthermore, the introduction of ZIF-8 not only improves the thermodynamic properties of the separator but also further enhances its interfacial stability and ion conductivity. This invention provides an efficient separator construction strategy for the development of lithium metal batteries with long cycle life and high safety, showing broad application prospects in next-generation high-safety batteries.

[0017] The beneficial effects of the present invention are: This study developed a flexible sheet-like MOF-based functional separator for stabilizing lithium metal anodes. By pre-embedding MOF seeds within a PVDF substrate, in-situ vertical growth of the MOF sheet was achieved, eliminating the need for a binder and enhancing the separator's structural stability. The vertically aligned, dense MOF layers form ordered, uniform lithium-ion channels, effectively guiding the uniform deposition of lithium ions while restricting anion migration, achieving a lithium-ion transference number as high as 0.90. This ion screening effect significantly inhibits dendrite growth, stabilizes the lithium-metal interface, and enables stable cycling of over 1200 hours in Li||Li symmetric cells at extremely low overpotentials. In actual battery testing, the ZIF-8@PVDF separator significantly improved the cycling stability of Li-LFP full cells, retaining 90% of the capacity after 1800 cycles. The flexible PVDF effectively alleviates the MOF's brittleness, while the incorporation of the high-modulus MOF enhances the separator's thermal stability and resistance to mechanical deformation. The two synergistically create a composite separator structure that combines flexibility with thermal stability. Compared with PP separators, the electrochemical performance of LiFePO4 / Li-ion batteries based on this functional separator is improved. This work provides a new separator design strategy for achieving high-safety and high-stability lithium metal batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the synthesis of ZIF-8@PVDF; the MOF exhibits a preferential growth plane of 002. The figure shows the different stages of MOF growth. The MOF first grows outward from the seed, forming a dense MOF layer. Flower-like clusters and flake-like MOFs then grow within this layer, ultimately covering the membrane surface with uniform, continuous MOF flakes.

[0019] Figure 2 SEM images of ZIF-8@PVDF membrane; (a) top view; (b) cross-sectional view.

[0020] Figure 3 This is the EDX element distribution map of Zn, N, F, and C elements in ZIF-8@PVDF membrane.

[0021] Figure 4 This is a physical picture of the ZIF-8@PVDF membrane and SEM images of the membrane bending, folding and recovery.

[0022] Figure 5 (a) shows the thermal stability results of ZIF-8@PVDF membrane, (b) PP membrane, (c) PVDF membrane and (d) ZIF-8@PVDF membrane at different temperatures for half an hour and the corresponding infrared thermal imaging images.

[0023] Figure 6The chronoamperometric curve of the lithium-lithium symmetric battery based on ZIF-8@PVDF separator at a polarization of 10 mV (inset: Nyquist plot before and after polarization).

[0024] Figure 7 For the lithium-lithium symmetric battery equipped with ZIF-8@PVDF membrane and PP membrane: (a) 1 mA / cm 2 , 1 mAh / cm 2 and (b) 2 mA / cm 2 , 2 mAh / cm 2 .

[0025] Figure 8 0.5 mA / cm for Li||Cu batteries based on ZIF-8@PVDF membrane and PP membrane 2 , 0.5 mAh / cm 2 and Coulomb efficiency.

[0026] Figure 9 Long-term cycling performance of LFP / separator / Li half-cells with ZIF-8@PVDF separator, PVDF separator, and PP separator at 1 C rate.

[0027] Figure 10 Rate performance of LFP / membrane / Li half-cells with ZIF-8@PVDF separator, PVDF separator, and PP separator.

[0028] Figure 11 The surface morphology of the block ZIF-8@PVDF membrane.

[0029] Figure 12 The surface morphology of HKUST-1@PVDF membrane.

[0030] Figure 13 The surface morphology of UIO-66-NH2@PVDF membrane.

[0031] Figure 14 The surface morphology of UIO-66@PVDF membrane. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The exemplary implementation methods of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0033] The molecular weight of the PVDF used in the present invention is greater than 300,000.

[0034] Preparation of ZIF-8 seeds: Dissolve zinc nitrate in methanol (the concentration of zinc nitrate is 15.0 g / L) and 2-methylimidazole in methanol (the concentration of 2-methylimidazole is 30.0 g / L). Then, quickly stir the two solutions at equal volumes to prepare MOF seeds.

[0035] Preparation of sheet-like ZIF-8@PVDF membrane First, 0.1 g of ZIF-8 seeds were dispersed in a DMAc solution containing 13% PVDF (the mass ratio of DMAc, PVDF, and ZIF-8 seeds was 86:13:1). This slurry was then doctor-bladed onto a glass plate, which was then placed in water to produce a ZIF-8-Seeds@PVDF membrane via phase inversion. A 60 mL methanol solution containing 1 g of zinc nitrate hexahydrate, 0.4 g of 2-methylimidazole, and 0.23 g of sodium formate was prepared. This solution and the ZIF-8-Seeds@PVDF membrane were placed in a reactor and hydrothermally reacted at 80°C for 12 hours to produce a sheet-like ZIF-8@PVDF membrane. This membrane was then soaked in methanol, with fresh methanol replaced every 12 hours, for six cycles. Finally, the membrane was cut into the desired size and dried under vacuum at 120°C for 4 hours before testing.

[0036] Preparation of bulk ZIF-8@PVDF membrane First, 0.1 g of ZIF-8 seeds were dispersed in a DMAc solution containing 13% PVDF (the mass ratio of DMAc, PVDF, and ZIF-8 seeds was 86:13:1). This slurry was then doctor-bladed onto a glass plate, which was then placed in water to produce a ZIF-8-Seeds@PVDF membrane via phase inversion. A 60 mL methanol solution containing 1 g of zinc nitrate hexahydrate, greater than 0.4 g of 2-methylimidazole, and 0.23 g of sodium formate (the molar ratio of zinc nitrate to 2-methylimidazole ranged from 2:3 to 2:10) was prepared. This solution and the ZIF-8-Seeds@PVDF membrane were placed in a reactor and hydrothermally reacted at 80°C for 12 hours to produce a bulk ZIF-8@PVDF membrane. This membrane was then soaked in methanol, with fresh methanol replaced every 12 hours, for six cycles. Finally, the membrane was cut into the desired size and dried under vacuum at 120°C for 4 hours before testing.

[0037] Preparation of HKUST-1@PVDF membrane First, 0.1 g of HKUST-1 seeds were dispersed in a DMAc solution containing 13% PVDF (the mass ratio of DMAc, PVDF, and HKUST-1 seeds was 86:13:1). This slurry was then doctor-bladed onto a glass plate, which was then placed in water to produce a HKUST-1-Seeds@PVDF membrane via phase inversion. A 100 mL mixed solution (C2H5OH:H2O = 1:1 vol%) containing 5 mM copper nitrate and 5 mM trimesic acid (the molar ratio of copper nitrate:trimesic acid ranged from 1:1 to 1:8) was prepared. This solution and the HKUST-1-Seeds@PVDF membrane were placed in a reactor and hydrothermally reacted at 80°C for 12 hours to produce the HKUST-1@PVDF membrane. This membrane was then soaked in ethanol, with fresh ethanol replaced every 12 hours for six times. Finally, the membrane was cut into the desired size and dried under vacuum at 120°C for 4 hours before testing.

[0038] Preparation of UIO-66-NH2@PVDF membrane First, 0.1 g of UIO-66-NH2 seeds were dispersed in a DMAc solution containing 13% PVDF (the mass ratio of DMAc, PVDF, and UIO-66-NH2 seeds was 86:13:1). This slurry was then doctor-bladed onto a glass plate, which was then placed in water to produce a UIO-66-NH2-Seeds@PVDF membrane via phase inversion. A 50 mL solution of 2 mM zirconium chloride and 15 mM 2-aminoterephthalic acid (the molar ratio of zirconium chloride to 2-aminoterephthalic acid ranged from 2:15 to 2:40) in N,N-dimethylformamide (DMF) was prepared. The above solution and UIO-66-NH2-Seeds@PVDF membrane were placed in a reactor and hydrothermally reacted at 60°C for 6 hours to obtain UIO-66-NH2@PVDF membrane. The membrane was then soaked in methanol, and fresh methanol was replaced every 12 hours. This was repeated 6 times. Finally, the membrane was cut into the required size and dried under vacuum at 120°C for 4 hours before testing.

[0039] Preparation of UIO-66@PVDF membrane First, 0.1 g of UIO-66 seeds were dispersed in a DMAc solution containing 13% PVDF (the mass ratio of DMAc, PVDF, and UIO-66 seeds was 86:13:1). This slurry was then doctor-bladed onto a glass plate, which was then placed in water to produce a UIO-66-Seeds@PVDF membrane via phase inversion. A 50 mL solution of 5 mM zirconium chloride and 5 mM terephthalic acid (the molar ratio of zirconium chloride to terephthalic acid ranged from 2:15 to 2:40) in N,N-dimethylformamide (DMF) was prepared. This solution and the UIO-66-Seeds@PVDF membrane were placed in a reactor and hydrothermally reacted at 60°C for 6 hours to produce the UIO-66@PVDF membrane. The membrane was then soaked in methanol, with fresh methanol replaced every 12 hours, for six cycles. Finally, the membrane was cut into the desired size and dried under vacuum at 120°C for 4 hours before testing.

[0040] The synthesis process of ZIF-8@PVDF obtained by this method is shown in Figure 1 Tiny seeds are distributed on the ZIF-8-Seeds@PVDF. After a hydrothermal reaction in a reactor for 3-6 hours, flake- and flower-like MOFs grow on the PVDF surface. Extending the reaction time to 12 hours reveals a vertically aligned flake-like MOF covering the PVDF surface.

[0041] The microstructure of the ZIF-8@PVDF membrane was observed by scanning electron microscopy (SEM). Figure 2 As shown in Figures a and b, the surface of ZIF-8@PVDF is covered with evenly distributed, interconnected flakes of MOF. A cross-section of the ZIF-8@PVDF clearly reveals the membrane's bilayer structure of MOF and PVDF. The MOF layer is 9.0 μm thick, and the PVDF is 15 μm thick. PVDF has a porous structure with pores decreasing in size from top to bottom, and MOF grows on the side with larger pores. Because the solvent exchange rate is faster on the side closer to the glass plate during phase inversion of the ZIF-8-Seeds@PVDF, the resulting pores are larger, which facilitates the growth of MOF outward from the larger pores. Figure 3 The element distribution of Zn, N, C, and F on the surface of ZIF-8@PVDF is shown. The elements are evenly distributed and the surface MOF grows uniformly on the substrate.

[0042] Figure 4 The ZIF-8@PVDF separator demonstrates its flexibility. After bending and folding it in half with tweezers, only creases were observed, maintaining its intact shape. This has a positive impact on battery safety during operation. The high surface area of ​​MOF and the CF polar functional groups of PVDF facilitate the adsorption of large amounts of electrolyte, while also providing excellent electrolyte retention and providing good ionic conductivity for the battery.

[0043] The thermal stability of the separator is closely related to the overall safety of the battery. The thermal stability of PP, PVDF and ZIF-8@PVDF separators at different temperatures. Figure 5 As shown, the PP membrane has a thermal shrinkage rate of up to 88.2% at 190°C. The shrinkage rate of PVDF at 190°C reaches 70%, indicating that PVDF does not significantly improve the thermal stability of the membrane. The ZIF-8@PVDF membrane is completely different. In the temperature range of 110°C to 190°C, there is no obvious change in color. Even after heat treatment at 190°C, it only shows a thermal shrinkage of 3.4%. This is because MOF has a low heat transfer rate and acts as an insulating layer. In addition, it has a higher Young's modulus, which increases the membrane's anti-shrinkage ability and improves its resistance to thermal shrinkage. Infrared thermal imaging images show that the ZIF-8@PVDF membrane has significantly faster heat transfer and more uniform heat distribution, indicating that the ZIF-8@PVDF membrane has more excellent thermal stability and can better ensure the safety performance of the battery.

[0044] In order to study the ion transport performance of ZIF-8@PVDF membrane, the classic Bruce-Vincent method was used to measure t Li+ The value is used to evaluate the lithium ion migration number. Figure 6 As shown in Figure 2, a high t of 0.90 was exhibited in an ether electrolyte (1M LiTFSI in DOL:DME with 2wt% LiNO3 (V:V = 1:1)). Li+ Thanks to the strong polar CF bond on PVDF and Li + The interaction between MOF and the screening effect of MOF ion channels on anions can significantly accelerate the Li + The rapid transfer of lithium ions leads to high lithium ion selectivity.

[0045] Application Example 1 In order to study the effect of different separators on lithium deposition, a Li||Li symmetric cell was assembled to compare the reversibility of lithium ion deposition / stripping on lithium metal. Figure 7 As shown in a, when the current density is 1 mA / cm 2 , lithium deposition capacity is 1mAh / cm 2 When the Li||Li symmetric battery equipped with ZIF-8@PVDF separator has a more stable lithium plating and stripping process, the polarization potential is stable at 10 mV after more than 1200 hours, while the polarization potential of the Li||Li symmetric battery with PP separator increases rapidly after more than 300 hours, and the Li||Li battery short-circuits before 400 hours. Figure 7 As shown in b, when the current density is 2mA / cm 2 , lithium deposition capacity is 2 mAh / cm2 When the ZIF-8@PVDF separator is used, the Li||Li symmetric cell with ZIF-8@PVDF separator undergoes stable electroplating and stripping at an overpotential of 50 mV for 1000 h, while the cell with PP separator short-circuit occurs after 100 h.

[0046] Application Example 2 In addition, to evaluate the coulombic efficiency (CE) of ZIF-8@PVDF for lithium deposition and stripping in ether-based electrolytes, Li||Cu asymmetric cells with PP and ZIF-8@PVDF separators were assembled, respectively. Figure 8 As shown in Figure 3, the battery using ZIF-8@PVDF separator achieved a coulombic efficiency of 99.7%, which is better than the Li||Cu battery using PP separator (97.8%).

[0047] Application Example 3 In order to study the cycle stability of the separator in the battery, the ZIF-8@PVDF separator was further assembled into a LiFePO4 battery in the order of negative electrode, separator, and positive electrode for cycle performance testing. Figure 9 As shown. When charging and discharging were carried out in the voltage range of 2.5-3.8V, the initial discharge capacities of the batteries using ZIF-8@PVDF membrane, PVDF membrane and PP membrane at 1 C were 147.7, 142.3 and 138.7 mAh / g, respectively. After 600 cycles, the capacity retention rate of the battery using PP membrane was 80.5%. The capacity retention rate of the battery using PVDF membrane was 83.7% after 600 cycles. The capacity retention rate of the battery using ZIF-8@PVDF membrane was 95.8% after 600 cycles. The use of ZIF-8@PVDF membrane significantly improved the cycle stability of the battery. The rate performance of the three LiFePO4 batteries is shown in Figure 2. Figure 10 As shown in Figure 2, the rate performance of the battery equipped with ZIF-8@PVDF separator is significantly better than that of the battery equipped with PP and PVDF separators. At a high rate of 5 C, the battery equipped with ZIF-8@PVDF separator significantly improved the rate performance by 98.1 mA h g compared with the battery equipped with PP separator. -1 Specific capacity.

[0048] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a highly selective and "rigid-flexible" metal-organic framework membrane, characterized by: The steps include: (1) Dissolve zinc nitrate and 2-methylimidazole in methanol and stir them rapidly to prepare MOF seeds; (2) Dissolve PVDF in an organic phase, then add the MOF seeds obtained in step (1) and stir until the mixture is uniform; (3) Pour the slurry obtained in step (2) onto a glass plate, apply it with a scraper, and immerse the glass plate in deionized water to obtain a ZIF-8-Seeds@PVDF membrane; (4) Dissolve zinc nitrate, 2-methylimidazole and sodium formate in methanol; (5) The membrane obtained in step (3) and the solution obtained in step (4) were placed in a reactor and subjected to hydrothermal reaction at 80°C for 12 hours to obtain a ZIF-8@PVDF membrane; (6) The membrane obtained in step (5) is washed with methanol and vacuum-dried at 120° C. for 4-12 h to obtain the metal-organic framework membrane.

2. The preparation method according to claim 1, wherein: The mass ratio of zinc nitrate to 2-methylimidazole in step (1) is 1:

2.

3. The preparation method according to claim 1, wherein: The organic phase in step (2) is N,N-dimethylacetamide.

4. The preparation method according to claim 1, wherein: The mass ratio of the organic phase, PVDF and MOF seeds in step (2) is 86:13:

1.

5. The preparation method according to claim 1, wherein: In step (1), the concentration of zinc nitrate dissolved in methanol is 15.0 g / L, and the concentration of 2-methylimidazole dissolved in methanol is 30.0 g / L.

6. The preparation method according to claim 1, wherein: In step (2), the MOF seeds were replaced with HKUST-1, UIO-66, ZIF-67, and MOF-5.

7. The preparation method according to claim 1, wherein: The molar ratio of zinc nitrate to 2-methylimidazole in step (4) is 2:

3.

8. The metal-organic framework membrane obtained by the preparation method according to any one of claims 1 to 7.

9. Use of the metal-organic framework separator as claimed in claim 8 in a lithium ion battery.

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