Composite separator, method for manufacturing the same, and secondary battery

Abstract

The application discloses a kind of composite diaphragm and its preparation method and secondary battery, the composite diaphragm includes base film and at least the composite coating of one side surface of base film, composite coating includes PVDF porous framework and the functional group modified MOF particle dispersed in it, the average pore size range of composite coating is 1µm~5µm, average porosity is 45%~60%. MOF particle with functional group is randomly distributed in coating, both be coated by PVDF and be exposed to channel surface, while can maintain coating mechanical structure and give full play to its functional role. The intrinsic microporous of MOF and the macroscopic hierarchical pore formed after phase separation with PVDF cooperatively construct dual ion conduction network. The structure brings high ionic conductivity, excellent wettability and lithium dendrite inhibition effect.

Description

Technical Field

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

[0002] With the rapid development of lithium-ion batteries and new energy storage devices, the demand for high-performance separator materials is increasing. As a key component of batteries, the separator not only isolates the positive and negative electrodes to prevent short circuits, but also plays a crucial role in ion transport, maintaining electrolyte stability, and influencing battery safety. In existing technologies, polyvinylidene fluoride (PVDF) is widely used in the preparation of porous separators for batteries due to its excellent electrochemical stability, mechanical strength, and film-forming properties. A common preparation method involves dissolving PVDF in a polar solvent to form a coating slurry, coating it onto a polyolefin-based membrane, and then immersing it in a coagulation bath to induce pore formation through a non-solvent phase separation (NIPS) process, thereby forming a composite separator with a porous structure.

[0003] Currently, the common industrial approach involves coating a PVDF solution onto a base film and then using a coagulation bath to separate the phases to form a porous structure. While this method offers advantages such as simple process flow and ease of large-scale production, it still faces several key technical bottlenecks in practical applications: First, PVDF itself is a non-polar or weakly polar polymer with low surface energy, resulting in poor wettability to polar electrolytes. This manifests as a slow electrolyte wetting rate, high initial interfacial contact resistance, and insufficient liquid retention capacity during long-term cycling, thus affecting the battery's charge-discharge efficiency and cycle life. Second, the pores constructed from pure PVDF mainly rely on thermodynamically driven phase separation processes, lacking functional active sites or fixed charge centers, thus failing to effectively regulate lithium-ion migration behavior, leading to... + The low migration number limits the improvement of overall ionic conductivity. Secondly, under single coagulation bath conditions, it is difficult to precisely control the film formation kinetics, making it impossible to simultaneously achieve a gradient interconnected structure of "open micropores on the surface to dense nanopores at the bottom." Poor pore size distribution uniformity and poor pore connectivity make it difficult to balance rapid ion transport and electrolyte retention capacity. Finally, traditional PVDF separators lack effective physical or chemical control mechanisms to suppress lithium dendrite growth. They cannot homogenize the current density distribution at the electrode interface or intervene in the lithium-ion solvation structure. Therefore, under high current density or long-term cycling, local electric field concentration easily occurs, inducing lithium dendrite nucleation and penetration, leading to serious safety hazards. These technical bottlenecks severely restrict the development and application of high-performance lithium battery separators.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a composite separator, its preparation method, and a secondary battery to improve the above-mentioned technical problems.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a composite membrane comprising a base membrane and a composite coating located at least on one side surface of the base membrane, the composite coating comprising a PVDF porous framework and MOF particles modified with functional groups dispersed therein, the composite coating having an average pore size ranging from 1µm to 5µm and an average porosity of 45% to 60%.

[0007] In an optional embodiment, a portion of the MOF particles are exposed within the pores of the PVDF porous framework.

[0008] In an optional embodiment, the average pore size of the composite coating ranges from 1.9µm to 2.5µm, and the average porosity ranges from 50% to 55%.

[0009] In an optional embodiment, the functionalized group includes at least one selected from sulfonic acid, carboxyl, amino, hydroxyl, phosphate, carbonyl, and amide groups.

[0010] In an optional embodiment, the functionalized group is one of sulfonic acid group, carboxyl group and amino group.

[0011] In an optional embodiment, the functionalized group is a sulfonic acid group or a carboxyl group.

[0012] In an optional embodiment, the MOF particles have a BET specific surface area of ​​800 m². 2 / g~950m 2 / g, with an average pore size ranging from 1.8nm to 2.5nm, and a micropore ratio greater than 85%.

[0013] In an optional embodiment, the molar ratio of the organic ligand of the MOF particle to the metal center is in the range of (1~3):1, preferably 1:1.

[0014] In an optional embodiment, the mass of the MOF particles is 1% to 50% of the mass of the porous PVDF, preferably 15% to 25%.

[0015] In an optional embodiment, the particle size of the MOF particles ranges from 30 nm to 300 nm, preferably from 50 nm to 150 nm.

[0016] In an optional embodiment, the base film is a polyolefin base film.

[0017] In an optional embodiment, the electrolyte wetting rate of the composite diaphragm is 70 mm to 110 mm / min.

[0018] In an optional implementation, the polarization voltage increment of a lithium / lithium symmetric battery assembled according to GB / T 36363-2018 after 800 hours of cycling is less than 40mV.

[0019] In an optional embodiment, the surface of the MOF particles is further grafted with a silane coupling agent, preferably 3-aminopropyltriethoxysilane or polyethylene glycol silane.

[0020] Secondly, the present invention provides a method for preparing a composite membrane as described in any of the foregoing embodiments, comprising: coating a PVDF solution containing the MOF particles onto the base membrane, then transferring it to a coagulation bath for coagulation treatment, and after phase separation, removing the membrane for washing and drying.

[0021] Thirdly, the present invention provides a secondary battery comprising a composite separator as described in any of the foregoing embodiments.

[0022] This invention offers the following advantages: Functionalized metal-organic framework (MOF) particles (e.g., surface-modified with functional groups such as -SO3H, -COOH, or -NH2) are introduced into a PVDF matrix, and these MOF particles are uniformly and randomly distributed within the porous composite coating. On one hand, the MOF particles are effectively coated by PVDF, forming a good interfacial bond and significantly enhancing the mechanical stability and structural integrity of the composite material. On the other hand, some MOF particles are locally exposed on the inner surface of the porous structure, allowing the functional groups loaded on their surfaces to fully contact the electrolyte and exert their chemical activity. Furthermore, the regular microporous structure of the MOF material itself and the macroscopic hierarchical porous structure formed by phase separation in the PVDF matrix are interconnected, synergistically constructing a "dual ion conduction network." This network not only facilitates the rapid transport of lithium ions but also optimizes the wetting behavior of the electrolyte inside the membrane. Therefore, the resulting composite membrane exhibits excellent electrolyte affinity, significantly improving ionic conductivity and lithium-ion transference number. Meanwhile, functionalized MOF components have a directional regulatory effect on lithium-ion deposition behavior, which can effectively inhibit the growth of lithium dendrites, thereby improving the safety performance and cycle stability of the battery. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 The graphs show the lithium / lithium symmetric battery cycle performance of the separator structures in Examples 1-3 and Comparative Examples 1-2 of this invention. Figure 2 The graph shows the polarization voltage increment of the lithium / lithium symmetric battery assembled with the composite separator in Example 1 of the present invention after 800 hours of cycling. Figure 3 This is a SEM image of the membrane before treatment in Example 1 of the present invention; Figure 4 The image obtained by adjusting the image threshold of the surface image using ImageJ is the SEM image of Embodiment 1 of the present invention. Figure 5 This is a 15,000x magnified SEM image of the composite diaphragm of Embodiment 1 of the present invention. Figure 6 This is a 20,000x magnified SEM image of the composite diaphragm of Embodiment 1 of the present invention. Figure 7 This is a 15,000x magnified SEM image of the cross-section of the composite diaphragm in Embodiment 1 of the present invention. Figure 8 This is a 5000x magnified SEM image of the lithium sheet surface after 800 hours of cycling of the composite separator-assembled Li / Li symmetric battery of Example 1 of the present invention. Figure 9 This is a 5000x SEM image of the lithium sheet surface after 500 hours of cycling of the composite separator-assembled Li / Li symmetric battery of Comparative Example 2 of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0026] Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible subranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0027] In the description of this invention, the terms "comprising," "including," etc., mean "including but not limited to." In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c," or "at least one of a, b, and c," can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can each be single or multiple.

[0028] In this invention, the abbreviation MOF corresponds to the Chinese name Metal-Organic Framework and the English name Metal organic Framework; the abbreviation PVDF corresponds to the Chinese name Polyvinylidene Difluoride and the English name Polyvinylidene Difluoride; the abbreviation DMAc corresponds to the Chinese name N,N-Dimethylacetamide and the English name Dimethylacetamide; and the abbreviation DMF corresponds to the Chinese name N,N-Dimethylformamide and the English name Dimethylformamide.

[0029] The following is a detailed description of a composite separator, its preparation method, and a secondary battery provided by the present invention.

[0030] Some embodiments of the present invention provide a composite membrane comprising a base membrane and a composite coating at least on one side of the base membrane. The composite coating comprises a PVDF porous framework and MOF particles modified with functional groups dispersed therein. The average pore size of the composite coating ranges from 1 µm to 5 µm, and the average porosity is 45% to 60%.

[0031] In the composite coating structure, MOF particles are randomly distributed: some particles are encased within the PVDF porous framework, effectively ensuring the mechanical strength and structural integrity of the composite coating; some particles are exposed on the pore surface of the PVDF porous framework, allowing the functional groups on their surface to be fully exposed and exert their chemical activity. This structural design achieves multiple optimization effects through synergistic interaction between the physical configuration and chemical function of the material: First, by introducing polar functional groups such as -SO3H, -COOH, and -NH2, the surface energy of the membrane surface is significantly enhanced, thereby greatly improving its wettability to the electrolyte and enhancing the electrolyte's adsorption capacity and liquid holding stability, which is beneficial for strengthening the interfacial electrochemical contact between the electrode and the membrane. Second, relying on the inherent regular microporous structure and high specific surface area of ​​MOF material, multiple lithium-ion preferential conduction pathways are constructed. At the same time, the fixed negative charge or coordination sites introduced by the functional groups have a selective repulsion or confinement effect on anions, which helps to increase the lithium-ion transference number, reduce the impedance in the ion transport process, and improve the overall ionic conductivity. Third, the functional sites exposed on the pore wall surface and the through-type multi-level pore structure synergistically regulate ion transport behavior, which can effectively homogenize the electric field distribution at the solid-liquid interface, reduce the local current density, regulate the lithium-ion solvation / desolvation kinetics, promote a more uniform spatial distribution of lithium metal during deposition, and significantly inhibit the nucleation and penetration growth of lithium dendrites. In summary, through synergistic innovative design in two dimensions—micromorphology control and surface chemical functionalization—a fundamental breakthrough and systematic improvement have been achieved in key performance indicators such as wettability, ion-selective permeability, and dendrite suppression ability of membranes prepared by the traditional PVDF coagulation bath method.

[0032] It should be noted that the average pore size and porosity in this embodiment of the invention are measured using SEM image analysis. The specific detection method is as follows: Using ImageJ software, the image format is adjusted to 8-bit. After adding a scale, the image threshold range of the surface image is adjusted (0≤MinThr≤70, 150≤MaxThr≤255, for example, MinThr is 65 and MaxThr is 220) to make the edges of the pore structures in the image prominent. The software is then used to draw along the contours of the pore structures, specifically along the areas with higher grayscale values, appearing as white or near-white, to obtain the drawn image. The smallest unit in the drawn image is recorded as one pore structure, and even the smallest unit located at the edge of the drawn image that is not fully displayed is also recorded as one pore structure. The area S of the two-dimensional image fitted to the edges of the pore structures in the drawn image is measured using ImageJ software. The average pore size and porosity range can then be calculated using the software.

[0033] In some embodiments, the average pore size of the composite coating ranges from 1.9µm to 2.5µm, and the average porosity ranges from 50% to 55%. The pore size and porosity of this composite coating enable the membrane to have a better hierarchical pore structure, thereby giving it better air permeability and wettability.

[0034] In some embodiments, the mass of MOF particles is 1% to 50% of the mass of PVDF, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%, preferably 15% to 25%. An appropriate proportion of MOF particles can provide sufficient intrinsic micropores to synergistically construct a "dual ion conduction network" with the macroscopic hierarchical pores formed by the separation of the PVDF phase, without being excessive and affecting the overall average pore size and porosity of the porous composite coating, thereby avoiding adverse effects on the air permeability and wettability of the composite membrane.

[0035] Furthermore, in some embodiments, the particle size of the MOF particles ranges from 30 nm to 300 nm, preferably from 50 nm to 150 nm. MOF particles in this particle size range are beneficial for uniform dispersion in the PVDF framework, have moderate porosity, and are suitable for suppressing lithium dendrite penetration.

[0036] In some embodiments, the functionalized group includes, but is not limited to, at least one of sulfonic acid group (-SO3H), carboxyl group (-COOH), amino group (-NH2), hydroxyl group (-OH), phosphoric acid group (-PO3H2), and carbonyl / amide group (-C=O, amide group).

[0037] In some embodiments, the functionalized group is one of sulfonic acid group, carboxyl group, and amino group, preferably sulfonic acid group or carboxyl group. The functionalization treatment of MOF particles, especially the introduction of sulfonic acid group and amino group, greatly improves the affinity of the separator for electrolyte, which is beneficial to the rapid wetting of the battery and interfacial ion transport, indicating that the introduction of MOF helps to build a more efficient lithium ion conduction channel.

[0038] In particular, sulfonic acid functional groups have excellent selective transport capabilities for lithium ions, which helps to suppress concentration polarization and improve battery rate performance and cycle stability.

[0039] In some embodiments, the electrolyte wetting rate of the composite diaphragm is 70 mm to 110 mm / min. This wetting rate is determined by cutting the diaphragm sample into a strip with dimensions of 200 × 5 mm, adding 25 μL of electrolyte to it using a pipette, and measuring the diffusion distance of the electrolyte after 60 seconds using a ruler.

[0040] In some implementations, sulfonic acid and carboxyl-functionalized MOF particles can effectively suppress polarization intensification during cycling, improving the long-term cycle stability of the battery. In particular, sulfonic acid-functionalized MOF particles maintain extremely low polarization growth even with extended cycle times, demonstrating their superior performance in interface stability. In contrast, amino-functionalized MOF particles have limited effectiveness in suppressing dendrites.

[0041] Specifically, in some implementations, the polarization voltage increment of a lithium / lithium symmetric battery assembled according to GB / T 36363-2018 after 800 hours of cycling is less than 40mV.

[0042] In some embodiments, the surface of the MOF particles is also grafted with a silane coupling agent, for example, APTES silane coupling agent (3-aminopropyltriethoxysilane) or PEG-silane (polyethylene glycol silane).

[0043] MOF particles suffer from problems such as strong surface polarity, high tendency to aggregate, and weak interfacial bonding with the PVDF matrix. Silane coupling agents modify the MOF surface through a "molecular bridging" effect. The silane end of the silane coupling agent (Si... OR reacts with the hydroxyl groups on the MOF surface to form covalent bonds for anchoring. The organic functional end (Y) undergoes a chemical reaction or physical interaction with the polymer matrix, and the alkyl chain ( (CH2) n The silane coupling agent acts as a flexible spacer arm, relieving interfacial stress between the MOF and the polymer PVDF. Furthermore, after grafting with the silane coupling agent, an organic thin film forms on the surface of the MOF particles, significantly enhancing the steric hindrance effect and preventing particle aggregation through hydrogen bonds and coordination bonds, thus achieving uniform dispersion in the polymer matrix.

[0044] In some implementations, the MOF particles include, but are not limited to, one or more of the following: Zr-based MOFs (UIO series, NU-1000, MOF-808), ZIF series (ZIF-8, ZIF-67, ZIF-90), MIL-based MOFs (MIL-101, MIL-53), Cu-based MOFs (HKUST-1), and Fe-based MOFs.

[0045] In some implementations, the PVDF model includes, but is not limited to, one or more of Arkema Kynar® HSV 900, Kynar® HSV1810, Kynar Flex® LBG series, Solvay Solef® 5130 and Ruyuan Dongyangguang 702, preferably Arkema Kynar® HSV 900.

[0046] In some embodiments, the base membrane is a polyolefin-based base membrane. Exemplarily, the base membrane may be selected as a polypropylene or polyethylene membrane. The base membrane has sponge-like nanopores, which, together with the micropores of the porous composite coating, form a hierarchical porous structure.

[0047] Some embodiments of the present invention also provide a method for preparing a composite membrane as described in any of the foregoing embodiments, comprising: coating a PVDF solution containing MOF particles onto a base membrane, then transferring it to a coagulation bath for coagulation treatment, and after phase separation, removing the membrane for washing and drying.

[0048] Specifically, in some embodiments, the preparation method of the above-mentioned composite separator includes the following steps: S1. Preparation of MOF particles.

[0049] The MOF organic ligand with functionalized groups and the metal salt are dissolved in an organic solvent and subjected to solvothermal treatment. The molar ratio of the MOF organic ligand to the metal salt is in the range of (1~3):1, preferably 1:1.

[0050] In some embodiments, the temperature of the solvothermal reaction is 100℃~150℃, such as 100℃, 110℃, 120℃, 130℃, 140℃ or 150℃, and the reaction time is 12~24h, such as 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h or 24h.

[0051] In some embodiments, the surface of MOF particles is modified with a silane coupling agent, for example, by immersing the MOF particles in an alcohol solution containing 1-2% by mass of the silane coupling agent to enhance their interfacial compatibility with the polymer matrix.

[0052] S2. Prepare PVDF / MOF composite slurry.

[0053] Specifically, in some embodiments, MOF particles are added to a PVDF DMAc solution, a dispersing agent is introduced, followed by dispersion treatment, and finally vacuum degassing. The dispersion treatment includes ultrasonic dispersion or high-shear dispersion followed by stirring. The dispersing agent may be polyvinylpyrrolidone (e.g., PVP K30).

[0054] In some embodiments, the primary particle size of PVDF ranges from 100 nm to 500 nm, preferably from 200 nm to 400 nm.

[0055] S3. Preparation of composite coating.

[0056] Specifically, a PVDF solution containing MOF particles is coated onto a base membrane using a blade coating method, with the wet membrane thickness controlled at 2~20µm. The membrane is then transferred to a coagulation bath for coagulation treatment. After phase separation, the membrane is removed for washing and drying.

[0057] In some embodiments, the coagulation bath treatment involves sequentially applying a 55-65 wt% DMAc aqueous solution, a 25-35 wt% DMAc aqueous solution, 100 wt% water, and 100 wt% water, with each concentration solution undergoing coagulation bath treatment for 1.5-2.5 minutes. The water used is deionized water. The coagulation bath treatment temperature is 20°C-30°C. Through precise process control, the porous structure described in these embodiments is formed.

[0058] In some embodiments, drying is performed under vacuum at a temperature of 70°C to 90°C, for example, 80°C, for a drying time of 5 hours to 7 hours, for example, 6 hours.

[0059] Furthermore, some embodiments of the present invention also provide a secondary battery containing a composite separator as described in any of the foregoing embodiments.

[0060] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0061] The PVDF model used in the examples and comparative examples is Arkema Kynar® HSV 900.

[0062] Example 1 This embodiment provides a method for preparing a composite membrane with a sulfonic acid-functionalized MOF / PVDF composite coating, which includes the following steps: (1) Zirconium tetrachloride (ZrCl4) and 2-sulfonic terephthalic acid were dissolved in N,N-dimethylformamide (DMF) at a molar ratio of 1:1 and subjected to a solvothermal reaction at 150°C for 24 h to obtain the UiO-66-SO3H precursor. Subsequently, the product was centrifuged and washed, and neutralized with 0.5M lithium hydroxide (LiOH) aqueous solution to convert the sulfonic acid group (-SO3H) into lithium sulfonate (-SO3Li). After washing with deionized water and ethanol alternately, it was vacuum dried at 80°C for 12 h to obtain UiO-66-SO3Li nanoparticles. The particle size distribution was 50~120 nm.

[0063] (2) Surface modification of UiO-66-SO3Li nanoparticles: treatment with 1wt% 3-aminopropyltriethoxysilane (APTES) ethanol solution for 2h to enhance their interfacial compatibility with the polymer matrix.

[0064] (3) A PVDF solution with a solid content of 8 wt% was prepared using N,N-dimethylacetamide (DMAc) as a solvent. UiO-66-SO3Li nanoparticles were added to the PVDF solution at a ratio of 20 wt% of the total PVDF mass, and 0.5 wt% polyvinylpyrrolidone (PVP K30) was added as a dispersing agent. The MOF was first uniformly dispersed by ultrasonic treatment for 15 min, then planetary stirring was performed for 2 h to form a homogeneous slurry, and finally vacuum degassing was performed.

[0065] (4) The above slurry was coated onto the substrate by a scraping method, and the wet film thickness was controlled at 12µm. It was then immediately transferred to a coagulation bath, which was a series of mixed solutions of deionized water and DMAc. The coated membrane was treated sequentially with coagulation baths of ratios of 60wt%DMAc / H2O-30wt%DMAc / H2O-100wt%H2O-100wt%H2O, with the temperature maintained at 20~30°C, and the treatment time for each concentration was 2min. After phase separation was completed, the membrane was removed, and residual solvent was removed by gradual water washing. Finally, it was vacuum dried at 80°C for 6h to obtain the composite membrane coating.

[0066] Example 2 This embodiment provides a method for preparing a composite membrane with a carboxyl-functionalized MOF / PVDF composite coating, which includes the following steps: (1) 2,5-dicarboxyterephthalic acid or 2-carboxyterephthalic acid was selected as an organic ligand and reacted with zirconium tetrachloride (ZrCl4) in N,N-dimethylformamide (DMF) at a molar ratio of 1:1 for 24 h at 150°C. After centrifugation, washing and vacuum drying at 80°C for 12 h, UiO-66-(COOH) nanoparticles were obtained. The particle size range of the obtained MOF particles was 50~150 nm.

[0067] (2) Surface modification of UiO-66-(COOH) nanoparticles: treatment with 1wt% 3-aminopropyltriethoxysilane (APTES) ethanol solution for 2h to enhance their interfacial compatibility with the polymer matrix.

[0068] (3) The preparation, coating and phase separation of the carboxyl functionalized MOF / PVDF composite slurry are the same as those in Example 1. The only difference is that the MOF used has carboxyl groups.

[0069] Example 3 This embodiment provides a method for preparing a composite membrane with an amino-functionalized MOF / PVDF composite coating, which includes the following steps: (1) 2-Aminoterephthalic acid was selected as the organic ligand and reacted with zirconium salt (zirconium tetrachloride) at a molar ratio of 1:1 in N,N-dimethylformamide (DMF) at 150°C for 24 hours. After centrifugation, washing and vacuum drying at 80°C for 12 hours, UiO-66-NH2 nanoparticles were obtained. The particle size range of the obtained MOF particles was 40–120 nm.

[0070] (2) Surface modification of UiO-66-NH2 nanoparticles: treatment with 1wt% 3-aminopropyltriethoxysilane (APTES) ethanol solution for 2h to enhance their interfacial compatibility with the polymer matrix.

[0071] (3) The preparation, coating and phase separation of the amino-functionalized MOF / PVDF composite slurry are the same as those in Example 1. The only difference is that the MOF used has amino groups.

[0072] Comparative Example 1 This comparative example provides a method for preparing a composite membrane with an unfunctionalized MOF / PVDF composite coating, which includes the following steps: (1) Using terephthalic acid as an organic ligand, zirconium tetrachloride (ZrCl4) was reacted with it in N,N-dimethylformamide (DMF) at a molar ratio of 1:1 for 24 hours at 150°C. After centrifugation, washing, and vacuum drying at 80°C for 12 hours, unfunctionalized UiO-66 nanoparticles were obtained. The particle size of the obtained MOF particles was 50~120 nm.

[0073] (2) Surface modification of UiO-66 nanoparticles: treatment with 1 wt% 3-aminopropyltriethoxysilane (APTES) ethanol solution for 2 h to enhance their interfacial compatibility with the polymer matrix.

[0074] (3) The preparation, coating and phase separation of the unfunctionalized MOF / PVDF composite slurry were the same as those in Example 1. The only difference was that the MOF used did not have functionalized groups.

[0075] Comparative Example 2 This comparative example provides a method for preparing a composite membrane with a PVDF coating, which includes the following steps: (1) A PVDF solution with a solid content of 8 wt% was prepared using N,N-dimethylacetamide (DMAc) as a solvent. No MOF functional filler was added to the slurry, but 0.5 wt% polyvinylpyrrolidone (PVP K30) was added as a dispersant. After magnetic stirring for 1 hour to form a homogeneous slurry, vacuum degassing was performed. The PVDF particle size range used was 200-400 nm.

[0076] (2) The steps of coating the PVDF slurry with the diaphragm and separating the phase to form the film are the same as in Example 1. The only difference is that MOF particles are not added to the PVDF slurry used.

[0077] The performance of the diaphragms in different embodiments and comparative examples was tested, and the specific testing instruments and methods are shown in Table 1.

[0078] Table 1

[0079] The test results are shown in Table 2.

[0080] Table 2. Performance test data of membranes prepared in different embodiments and comparative examples.

[0081] As shown in Table 2, the performance test results indicate that the functionalized MOF-modified membranes (Examples 1-3) provided by this invention exhibit significant improvements in several key performance aspects compared to unfunctionalized MOF (Comparative Example 1) and pure PVDF membranes (Comparative Example 2). Specifically: (1) Wetting performance analysis The electrolyte wetting rates of Examples 1-3 reached 101 mm / min, 73 mm / min, and 102.3 mm / min, respectively, which were significantly higher than those of Comparative Example 1 (42 mm / min) and Comparative Example 2 (30 mm / min). This indicates that the functionalization of the MOF material, especially the introduction of lithium sulfonate groups (Example 1) and amino groups (Example 3), greatly enhances the affinity of the separator for the electrolyte, which is beneficial for rapid wetting of the battery and interfacial ion transport.

[0082] (2) Improved electrochemical performance Regarding ionic conductivity, Example 1 achieved 0.78 mS·cm. -1 Both Example 2 and Comparative Example 1 have a concentration of 0.707 mS·cm. -1 Both are significantly better than the 0.533 mS·cm of Comparative Example 2. -1 This indicates that the introduction of MOFs helps to construct more efficient lithium-ion conduction channels.

[0083] In particular, the lithium-ion transference number of Example 1 was 0.74, which was significantly higher than that of other examples and comparative examples, indicating that the lithium sulfonate functional group has excellent selective transport capability for lithium ions, which helps to suppress concentration polarization and improve battery rate performance and cycle stability.

[0084] (3) Balanced air permeability The initial permeability values ​​of each embodiment and the comparative example are similar (approximately 175~190 s / 100cc), but the permeability increment of Example 1 is 68.9 s / 100cc, which is between Example 2 (55.7) and Comparative Example 2 (72), indicating that the lithium sulfonate functionalized MOF maintains good gas diffusion performance without significantly sacrificing the pore structure and gas permeability of the membrane.

[0085] (4) Lithium dendrite suppression effect After 800 hours of cycling, Example 1 showed a polarization voltage increment of only 21 mV, and Example 2 showed 35 mV, both significantly lower than Comparative Example 1 (761 mV) and Comparative Example 2 (850 mV) after 500 hours of cycling. This indicates that lithium sulfonate combined with carboxyl-functionalized MOF can effectively suppress polarization intensification during cycling and improve the long-term cycling stability of the battery. Amino-functionalized MOF has limited effect on dendrite suppression. In particular, Example 1 maintained extremely low polarization growth even with extended cycling periods, demonstrating its superior performance in interface stability.

[0086] (5) Comparison of overall performance In summary, the separator constructed with lithium sulfonate functionalized MOF (Example 1) exhibited the best performance in terms of wettability, ionic conductivity, and lithium-ion transference number, demonstrating its significant advantages in improving the overall performance of lithium-ion batteries. Amino-functionalized MOF (Example 3) showed advantages in wettability, while carboxyl-functionalized MOF (Example 2) also outperformed the unfunctionalized sample and the pure polymer matrix in terms of electrochemical performance.

[0087] The cycle performance diagrams of lithium / lithium symmetric batteries assembled with composite separators in Examples 1-3 and Comparative Examples 1-2 are shown below. Figure 1 As shown. The polarization voltage increment of the lithium / lithium symmetric battery assembled with the composite separator in Example 1 after 800 hours of cycling is as follows. Figure 2 As shown in Table 3, the BET specific surface area and pore size of the lithium sulfonate functionalized MOF prepared in Example 1 are shown in Table 3.

[0088] Table 3

[0089] The average pore size and average porosity of the membrane coatings in the examples and comparative examples were measured. The method was as follows: ImageJ software was used to adjust the image format to 8-bit. After adding a scale, the image threshold range of the surface image was adjusted (0≤MinThr≤70, 150≤MaxThr≤255, for example, MinThr is 65 and MaxThr is 220) to make the edges of the pore structures in the image prominent. The software was then used to draw along the contours of the pore structures, specifically along the areas with higher grayscale values, appearing as white or near-white. The smallest unit in the drawn image was recorded as one pore structure, and even the smallest unit located at the edge of the drawn image that was not fully displayed was also recorded as one pore structure. The area S of the two-dimensional image fitted to the edges of the pore structures in the drawn image was measured using ImageJ software. The average pore size and porosity range could then be calculated using the software.

[0090] The test results are shown in Table 4.

[0091] Table 4

[0092] As shown in Table 4, the average pore size of the examples is 1.9~2.5µm and the average porosity is 50~55%.

[0093] The original image before processing in Example 1 is as follows: Figure 3 As shown, the image obtained after adjusting the image threshold of the surface image using ImageJ is as follows. Figure 4 As shown.

[0094] The SEM image of the composite diaphragm of Example 1, magnified 15000x, is shown below. Figure 5 As shown, by Figure 5 As can be seen, MOF particles are randomly distributed on the polymer matrix, and are coated on the surface or inside of PVDF. SEM images magnified 20,000 times are shown below. Figure 6 As shown, by Figure 6 The densely distributed porous morphology on the surface of the PVDF polymer matrix is ​​visible. The interface of the composite membrane in Example 1 was observed using a scanning electron microscope (SEM) at 15000x magnification; the SEM image is shown below. Figure 7 As shown, the micropore morphology formed by phase separation after PVDF and MOF coating and the nanopore morphology of PE membrane are shown.

[0095] The SEM image of the lithium sheet surface after 800 hours of cycling of the composite separator-assembled Li / Li symmetric battery of Example 1 is shown below. Figure 8 As shown, by Figure 8 As can be seen, the surface of the lithium sheet after cycling is relatively smooth, with very little lithium dendrite formation. The SEM image of the lithium sheet surface after 500 hours of cycling in the composite separator-assembled Li / Li symmetric battery of Comparative Example 2 is shown below. Figure 9As shown, by Figure 9 As can be seen, the lithium sheet after cycling exhibits moss-like lithium dendrite growth, which is due to the uneven deposition of lithium. This indicates that the composite separator of Example 1 has a better lithium dendrite suppression effect than that of Comparative Example 2.

[0096] This invention provides a coated separator based on a composite of functionalized metal-organic framework (MOF) particles and polyvinylidene fluoride (PVDF) and its preparation method. The method involves blending functionalized MOF particles containing polar functional groups such as carboxyl (-COOH), sulfonic acid (-SO3H), or amino (-NH2) groups with PVDF, and then preparing a lithium-ion battery separator with a functional coating using a solidification bath phase inversion method. During film formation, the MOF particles are randomly distributed within the coating: on the one hand, they are effectively encapsulated by the PVDF matrix, forming a stable interfacial bonding structure, significantly improving the mechanical integrity and adhesion strength of the coating; on the other hand, some MOF particles are exposed on the pore surface, allowing their surface functional groups to be fully exposed and participate in interfacial interactions, thereby achieving multiple synergistic effects.

[0097] This design achieves systematic optimization at the material structure and chemical function levels, specifically in the following three aspects: First, strongly polar functional groups such as -SO3H, -COOH, and -NH2 significantly increase the surface energy of the membrane surface, effectively improving the wetting performance of polar electrolytes and enhancing the adsorption capacity and liquid retention stability of the electrolyte, thereby optimizing the interfacial contact between the electrode and the membrane and improving the electrochemical reaction efficiency; Second, the MOF material itself possesses a highly ordered microporous structure and a high specific surface area, which is conducive to lithium-ion (Li... + It provides multiple efficient conduction pathways; at the same time, its fixed negative charge or coordination active sites can target anions (such as PF6). - TFSI - (etc.) generate electrostatic repulsion or confinement effects, inhibiting disordered migration, thereby improving Li + The migration number reduces ion transport impedance and improves the overall ionic conductivity of the membrane. Furthermore, the functional sites exposed on the pore walls and the interconnected hierarchical pore structure synergistically regulate interfacial ion transport behavior, helping to homogenize the local electric field distribution, reduce current density fluctuations, and regulate Li... + The solvation shell structure and its desolvation kinetics promote the uniform deposition of lithium ions on the negative electrode surface, effectively suppress the nucleation and growth of lithium dendrites, and significantly improve the battery cycle safety.

[0098] In summary, this invention, through a synergistic strategy of physical morphology control and chemical function introduction, achieves a fundamental improvement over existing technologies in key indicators such as electrolyte wettability, ion selective transport capability, and lithium dendrite suppression performance without altering the traditional PVDF-based coagulation bath film formation process. It has promising prospects for industrial application and significant value for technology promotion.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite diaphragm, characterized in that, It includes a base film and a composite coating located at least on one side of the surface of the base film. The composite coating includes a PVDF porous framework and MOF particles modified with functional groups dispersed therein. The average pore size of the composite coating ranges from 1µm to 5µm, and the average porosity is 45% to 60%.

2. The composite diaphragm according to claim 1, characterized in that, Some of the MOF particles are exposed in the pores of the PVDF porous framework.

3. The composite diaphragm according to claim 1, characterized in that, The average pore size of the composite coating ranges from 1.9µm to 2.5µm, and the average porosity ranges from 50% to 55%.

4. The composite diaphragm according to claim 1, characterized in that, The functionalized groups include at least one selected from sulfonic acid, carboxyl, amino, hydroxyl, phosphate, carbonyl, and amide groups; Preferably, the functionalized group is one of sulfonic acid group, carboxyl group and amino group; More preferably, the functionalized group is a sulfonic acid group or a carboxyl group.

5. The composite diaphragm according to claim 1, characterized in that, The MOF particles have a BET specific surface area of ​​800 m². 2 / g~950m 2 / g, with an average pore size ranging from 1.8nm to 2.5nm, and a micropore ratio greater than 85%; And / or, the particle size range of the MOF particles is 30nm~300nm, preferably 50nm~150nm; And / or, the molar ratio of the organic ligand to the metal center of the MOF particle is in the range of (1~3):1, preferably 1:

1.

6. The composite diaphragm according to claim 1, characterized in that, The mass of the MOF particles is 1% to 50% of the mass of the PVDF, preferably 15% to 25%; And / or, the base film is a polyolefin base film.

7. The composite diaphragm according to any one of claims 1 to 5, characterized in that, The electrolyte wetting rate of the composite diaphragm is 70 mm to 110 mm / min; And / or, the polarization voltage increment of a lithium / lithium symmetric battery assembled according to GB / T 36363-2018 after 800 hours of cycling is less than 40mV.

8. The composite diaphragm according to claim 1, characterized in that, The surface of the MOF particles is also grafted with a silane coupling agent, preferably 3-aminopropyltriethoxysilane or polyethylene glycol silane.

9. A method for preparing a composite separator as described in any one of claims 1 to 6, characterized in that, It includes: The PVDF solution containing the MOF particles is coated onto the base membrane, and then transferred to a coagulation bath for coagulation treatment. After phase separation, the membrane is removed for washing and drying.

10. A secondary battery, characterized in that, It contains the composite membrane as described in any one of claims 1 to 8.

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