Composite separator and electrochemical device

Abstract

This invention discloses a composite separator and an electrochemical device. The composite separator comprises a porous polyimide nanofiber matrix and a multi-metal-organic framework layer grown in situ on its surface, which is firmly bonded to the matrix through an interfacial cross-linking layer. Its preparation method employs a step-by-step controllable strategy: first, a metal precursor is introduced into the matrix through coaxial electrospinning, precursor complexation, or surface functionalization; then, a multi-metal MOF layer is formed in situ through a "seed-selective growth" step; finally, interfacial cross-linking treatment is performed. The separator of this invention can efficiently and selectively capture transition metal ions such as manganese that precipitate during battery cycling, while possessing excellent ionic conductivity, thermal stability, and mechanical strength, significantly improving the cycle life and safety of the battery, and is particularly suitable for manganese-based or high-nickel cathode systems.

Description

Technical Field

[0001] This invention relates to a composite separator and an electrochemical device, belonging to the technical field of lithium-ion battery separator materials. Background Technology

[0002] As a crucial energy storage device, the performance and safety of lithium-ion batteries highly depend on the coordinated operation of their internal components. The separator, a key internal component located between the positive and negative electrodes, plays a vital role in preventing physical short circuits while ensuring efficient ion conduction. However, under long-term cycling, especially under high voltage or high temperature conditions, the separator faces multiple challenges: insufficient electrolyte wettability and retention may lead to localized ion transport obstruction; poor thermal stability can cause thermal shrinkage or even melting, posing a short-circuit risk; furthermore, the positive electrode active material (such as lithium manganese oxide, high-nickel materials, etc.) may dissolve during cycling, releasing Mn. 2+ Transition metal ions, etc. These ions migrate to the negative electrode surface and cause side reactions such as catalytic decomposition of the electrolyte and destruction of the solid electrolyte interphase (SEI), leading to accelerated capacity decay, increased impedance, and shortened cycle life.

[0003] Currently, commercially available separators are mainly polyolefin microporous membranes (such as polyethylene PE and polypropylene PP), which have the advantages of low cost, moderate mechanical strength, and electrochemical inertness. However, they have significant inherent defects: First, they have poor heat resistance (melting point of about 130-165℃), and are prone to shrinkage and deformation at high temperatures, posing a risk of thermal runaway; second, they have poor wettability to polar electrolytes, affecting the rate performance of the battery; most importantly, they do not have chemical adsorption capabilities and cannot effectively capture dissolved transition metal ions in the electrolyte.

[0004] To address the aforementioned issues, existing technologies primarily modify the membrane in two ways: first, by coating a polyolefin matrix with a heat-resistant inorganic ceramic (such as Al2O3 or SiO2) coating to improve thermal stability and electrolyte affinity; however, such coatings lack selective adsorption capacity for metal ions. Second, by introducing functional materials onto the membrane surface to capture metal ions; among these, metal-organic framework materials have attracted attention due to their high specific surface area, tunable pore structure, and abundant coordination unsaturated metal sites.

[0005] Existing patents (such as CN110295498B) disclose the technology of using MOF materials in battery separators, which usually involves dispersing pre-synthesized single metal MOF (such as ZIF-8) powder in a binder and then coating it onto the surface of the separator. However, this method has obvious limitations: (1) MOF exists in the form of physically mixed powders, which is prone to agglomeration and uneven distribution in the coating, which may block ion transport channels; (2) MOF particles are mainly connected to the polymer matrix through physical adsorption or weak binders, resulting in weak interfacial bonding. They are prone to falling off under long-term electrolyte rinsing and volume changes, resulting in poor functional durability; (3) The MOFs used are mostly single metal centers, which have limited ion capture capacity, selectivity and stability; (4) The process involves the pre-synthesis of MOFs and subsequent coating, which is complicated and difficult to achieve uniform and firm functionalization inside the fiber pores.

[0006] Therefore, there is an urgent need in this field for a new technical solution that can directly construct a uniform, stable, and firmly bonded multi-metal MOF functional coating on the separator substrate. Ideally, the method should enable in-situ, controllable growth of MOFs on the fiber surface and within the pores, forming a strong interfacial bond, and utilizing the synergistic effect of multiple metals to enhance the efficient capture capability of target metal ions, thereby significantly extending the cycle life of the battery while ensuring ionic conductivity, mechanical and thermal stability. This constitutes the core technical problem that this invention aims to solve. Summary of the Invention

[0007] The purpose of this invention is to provide a polyimide composite separator and an electrochemical device. The polyimide composite separator is a functionalized coating separator based on a multi-metal organic framework (MOF) material, which can effectively capture transition metal ions such as manganese ions released during battery cycling, while taking into account the electrochemical stability and mechanical strength of the separator.

[0008] The polyimide composite separator based on a multi-metal MOF functionalized coating provided by this invention comprises:

[0009] A porous polyimide nanofiber matrix, and a multi-metal-organic framework (MOF) layer grown in situ on the surface and within the pores of the polyimide nanofiber matrix; The multimetallic organic framework layer contains at least two different metal ions and organic ligands; The multimetallic organic framework layer is bonded to the polyimide nanofiber matrix through an interfacial cross-linking layer.

[0010] In this invention, the polyimide composite separator utilizes a strong bond formed between the MOF (Metal-Oxide-Foil) and the fiber matrix through in-situ growth and interfacial crosslinking, significantly enhancing the coating's cycle durability and the overall thermomechanical stability of the separator. The multi-metal MOF provides abundant and synergistic coordination sites, enabling efficient and selective capture of dissolved manganese and other transition metal ions in the electrolyte, suppressing battery side reactions at their source. While achieving superior ion capture capabilities, controlled growth maintains good pore connectivity, ensuring high ionic conductivity, ultimately resulting in high capacity retention, high coulombic efficiency, and long cycle life. This invention's polyimide composite separator solves the key problems of traditional separators, such as easy coating detachment, limited ion capture capacity, and difficulty in balancing these with fundamental performance, achieving an integrated improvement in safety, durability, and efficiency.

[0011] In the polyimide composite membrane of the present invention, the metal ions in the polyimide nanofiber matrix exhibit one or more of the following distribution patterns: The fiber cross-sectional gradient distribution formed by coaxial electrospinning, the uniform distribution introduced in the polyamic acid precursor in a complex state, or the surface distribution adsorbed and fixed after surface functionalization.

[0012] Specifically, gradient distribution has a growth guiding effect: gradient-distributed metal ions can serve as non-uniform nucleation sites, guiding MOFs to preferentially nucleate and grow in specific regions of the fiber, making it easier to form MOFs with core-shell structures or gradient functional coatings.

[0013] Specifically, uniform complexation distribution enables the uniform formation and high site density of MOFs within the fiber, with metal ions uniformly dispersed throughout the fiber volume. This allows subsequent MOF growth to occur simultaneously within and on the surface of the fiber, significantly increasing the overall density and spatial accessibility of functional MOF sites. Furthermore, the generated MOF crystals interweave and interpenetrate with the PI matrix in three-dimensional space, forming a more integrated composite material with good stress transfer and reduced susceptibility to interfacial delamination.

[0014] Specifically, surface adsorption and fixation enable efficient utilization of metal ions and precise surface engineering control. Surface functional groups exhibit strong selective adsorption of specific metal ions, allowing for near 100% surface loading and utilization of metal precursors, reducing waste, and facilitating control of surface metal loading. This method is relatively independent of the PI fiber preparation process, providing significant process flexibility. Multiple metals or MOFs can be loaded layer-by-layer and patterned on the same substrate through various surface treatments. Furthermore, since metal ions are fixed only on the fiber surface, the impact on the internal pore structure of the fiber matrix is ​​minimal, which is most beneficial for maintaining the inherent high porosity and ion conductivity of the matrix.

[0015] In the polyimide composite membrane of the present invention, the metal ions in the multi-metal organic framework are selected from at least two of zinc, cobalt, iron, nickel, zirconium, and manganese.

[0016] In the polyimide composite membrane of the present invention, the microstructure of the multi-metal organic framework is a core-shell structure, a gradient coating, or a continuous dense thin layer. Specifically, the core-shell structure enables functional integration and performance synergy. The core and shell layers can perform different functions; for example, the inner layer (core) can utilize highly stable MOFs (such as Zr-based ones) or those with strong adhesion to the PI matrix to ensure structural anchoring of the coating; the outer layer (shell) can utilize MOFs that target ions (such as Mn) 2+ MOFs with extremely high adsorption capacity or selectivity (such as Zn / Co-based) enable efficient ion capture. This "internal solid-external capture" design achieves functional integration and complementary advantages. The relatively independent core layer acts as a buffer layer between the matrix and the functional shell, effectively alleviating stress caused by differences in their thermal expansion coefficients or volume changes, preventing coating cracking and peeling, and improving interface durability. Importantly, without sacrificing the core function (ion capture), the mechanical strength and pore structure of the matrix can be maximized by designing a stable core layer.

[0017] Specifically, gradient coatings enable a smooth performance transition and optimal stress distribution. The continuous gradual change in chemical composition and structure implies a continuous change in physicochemical properties (such as modulus and coefficient of thermal expansion). This effectively eliminates sharp interfaces that may exist in the core-shell structure, which are often the starting points for stress concentration and crack initiation. The gradient structure allows the bonding between the MOF coating and the PI substrate to transition from "surface bonding" to "gradient interpenetrating bonding," exhibiting the strongest interfacial bonding force and peel resistance. Simultaneously, the gradient structure better absorbs and dissipates external stress, improving the overall toughness of the composite membrane.

[0018] Specifically, the continuous, dense thin layer enables efficient barrier operation and uniform transport. This continuous and dense layer forms a uniform and effective physical and chemical barrier against dissolved metal ions in the electrolyte. It maximizes the selective adsorption advantages of the MOF material itself, while simultaneously physically blocking the free migration of ions through its dense structure, synergistically enhancing the "detoxification" efficiency. This morphology provides a highly uniform surface chemical environment and pore structure. This ensures uniform lithium ion transport at the separator-electrode interface, contributing to the formation of a stable SEI film, reducing side reactions caused by excessively high local current densities, thereby improving the battery's rate performance and cycle consistency. Furthermore, the dense layer better isolates impurities such as hydrofluoric acid generated during electrolyte degradation from direct chemical corrosion of the PI matrix, further enhancing the long-term stability of the separator under harsh electrochemical environments.

[0019] In the polyimide composite membrane of the present invention, the interfacial crosslinking layer comprises a chemically bonded layer formed by a silane coupling agent or a diisocyanate crosslinking agent; The diisocyanate crosslinking agent may be selected from one or more of aliphatic diisocyanates, alicyclic diisocyanates, or aromatic diisocyanates, such as, but not limited to: hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4′-dicyclohexylmethane diisocyanate (HMDI), toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and their modified forms.

[0020] The present invention further provides a method for preparing the polyimide composite membrane, comprising the following steps: S1. Prepare a polyamic acid spinning solution containing a metal precursor and perform electrospinning to obtain a polyamic acid fiber membrane. S2. The polyamic acid fiber membrane is subjected to imidization treatment to obtain a porous polyimide nanofiber matrix; S3. A multi-metal-organic framework layer is grown in situ on the polyimide nanofiber matrix using a stepwise control strategy. S4. Perform interfacial crosslinking treatment on the composite membrane on which the multi-metal-organic framework layer is grown to obtain the composite membrane.

[0021] In step S1, the metal precursor is introduced through one or more of the following methods: A) Coaxial electrospinning is used, with different metal salt solutions placed in the core and shell layers of the fiber, respectively; B) Metal salts and ligands are pre-added to a polyamic acid solution to form a metal complex; C) First, prepare a pure polyamic acid fiber membrane, then imidize it and perform plasma treatment or chemical grafting on the fiber surface to introduce functional groups, and then adsorb metal ions.

[0022] In step S3, the step-by-step control strategy includes: a) Seeding and nucleation steps: forming a seed layer of metal oxides or pre-crystallized nuclei on the surface of polyimide fibers; By pre-constructing a uniform metal oxide or pre-crystallized nucleus "seed layer" on the surface of PI fibers, a large number of uniformly distributed nucleation sites are provided for subsequent MOF growth. This fundamentally avoids problems such as agglomeration and incomplete coverage caused by the homogeneous nucleation of MOF crystals in solution followed by disordered deposition onto the fibers. b) Selective growth step: Using the seed layer as the crystal nucleus, a multi-metal-organic framework layer is formed in situ by controlling the solvent, temperature, and time. These pre-positioned "seeds" serve as the sole crystal nuclei for epitaxial or induced growth. This allows MOF crystals to grow oriented and orderly along the fiber surface, ultimately forming a continuous and uniform coating with a natural and close contact with the substrate. Using the seed layer as a "molecular anchor," the seed layer formed by seeding exhibits strong physical or chemical interactions (such as coordination and hydrogen bonding) with the PI fiber surface, achieving initial bonding. The MOF grown epitaxially from the seed layer has a continuous crystal lattice with the seed layer, forming a gradual transition interface from the PI surface to the MOF layer. This "rooting" growth mode makes the adhesion between the coating and the substrate much stronger than physical adsorption or adhesive bonding.

[0023] The selective growth step employs at least one of the following processes: solvothermal method, hydrothermal method, chemical vapor deposition method, microwave-assisted synthesis method, or steam-assisted crystallization method.

[0024] In step S4, the interface crosslinking treatment involves impregnating or coating the composite membrane with a silane coupling agent or a diisocyanate crosslinking agent solution and reacting it under heating conditions.

[0025] Based on the polyimide composite membrane, the present invention further provides an electrochemical device, including a positive electrode, a negative electrode, an electrolyte and the polyimide composite membrane.

[0026] Preferably, the electrochemical device is a lithium-ion battery, sodium-ion battery, zinc-ion battery, lithium-sulfur battery, or supercapacitor.

[0027] When the electrochemical device is a lithium-ion battery, its positive electrode material is a manganese-containing lithium manganese oxide or a high-nickel manganese-based material.

[0028] The present invention has the following beneficial effects: Exceptional ion trapping and "detoxification" capabilities: Multimetallic MOFs provide abundant highly selective coordination sites. Examples demonstrate that this membrane exhibits excellent ion trapping and "detoxification" capabilities for Mn. 2+ Its capture capacity (≥4.8 mg Mn / g) far exceeds that of the PI membrane without functional coating (0.2 mg Mn / g), which can effectively reduce the concentration of harmful metal ions in the electrolyte during the cycle.

[0029] Excellent electrochemical cycling performance: Efficient ion capture suppresses side reactions, enabling the battery to retain more than 92% of its capacity after 100 cycles (compared to 78% for the control sample), with an average coulombic efficiency of over 99.5% and a significant reduction in interfacial resistance.

[0030] Excellent thermal stability and mechanical strength: With polyimide, which has excellent heat resistance, as the matrix and a strong MOF layer, the heat shrinkage rate of the membrane is less than 4% after 30 minutes at 150°C (20% for polyolefin membranes), and the tensile strength is increased to more than 58 MPa (45 MPa for uncoated PI).

[0031] Excellent overall performance balance: Through a controllable growth process, the membrane is endowed with strong functionality while maintaining good ionic conductivity (approximately 0.96-0.98 mS·cm). -1 The system achieves multi-objective optimization of safety, durability, and performance by increasing the electrolyte absorption rate (above 145%) and electrolyte uptake rate. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the process of this invention. Detailed Implementation

[0033] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0034] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0035] All raw materials used in the following examples are commercially available analytical grade reagents or materials; unless otherwise specified, the quality, proportions, process parameters, and test methods are based on the following descriptions or national / industry standards. All electrochemical tests were performed at 25°C; the battery composition was a comparative system of Li metal anode / LiMn2O4 cathode (or LNMO type cathode) / test separator, and the cycle test was performed at a constant current of 1C (initial capacity according to the material's nominal capacity), with the electrolyte being 1.0 M LiPF6 / EC:DEC (1:1 vol%). The separator's Mn capture capacity was assessed by placing the separator in a solution containing a known concentration of Mn. 2+ After soaking in an electrolyte solution containing simulated precipitates (initial concentration 50 ppm) for 24 h, the remaining Mn content in the solution was determined by ICP-OES and converted to (mg Mn / g separator); ionic conductivity was measured using a symmetrical stainless steel fixture with EIS and obtained by impedance fitting; mechanical strength was determined by tensile testing according to GB / T36363-2018; heat shrinkage was measured after holding at 150°C for 30 min; battery cycle performance, coulombic efficiency, and impedance (EIS) were all determined using conventional methods. The following examples show the specific implementation steps and corresponding test data comparisons.

[0036] The invention is characterized by the construction of a composite membrane with heat-resistant polyimide nanofibers as the skeleton and an in-situ integrated multi-metal-organic framework layer. Its preparation employs a unique, controllable multi-step strategy of "pre-situation-seeding-growth-crosslinking": First, metal precursors are precisely introduced into the polyimide matrix through coaxial spinning, precursor complexation, or surface functionalization; subsequently, a uniform seed layer is induced to form on the fiber surface; then, a multi-metal MOF layer composed of at least two metals is grown in situ on the seed layer through various processes such as solvothermal, vapor deposition, or microwave-assisted processing; finally, a strong chemical bond is established between the coating and the substrate through interfacial crosslinking treatment.

[0037] The technical solution of this invention achieves three major synergistic effects: First, by utilizing the synergistic coordination effect of multi-metal MOFs, the membrane is endowed with the ability to efficiently and selectively capture harmful substances such as manganese ions; second, through in-situ growth and interfacial chemical bonding, the uniformity, stability, and strong adhesion to the substrate of the functional coating are ensured, overcoming the problem of easy detachment; third, while maintaining the inherent high porosity, excellent mechanical and thermal stability of the polyimide matrix, an optimized balance between ion conduction, ion capture, and structural strength is achieved.

[0038] The composite separator prepared by this invention significantly improves the overall performance of the battery, including higher cycle capacity retention, better thermal safety and longer service life. It is particularly suitable for solving the problem of ion dissolution of manganese-based or high-nickel cathode materials in lithium-ion batteries and has important prospects for industrial application.

[0039] Example 1: Preparation of Zn / Co-MOF@PI composite membrane by coaxial electrospinning combined with solvothermal growth method (1) Preparation of PAA precursor solution: PMDA-ODA type PAA was synthesized by two-step method, and its intrinsic viscosity was adjusted to 2.1 dL / g to prepare a PAA / DMAc solution with a solid content of 15 wt%.

[0040] (2) Coaxial electrospinning: A dual-channel injection pump and a coaxial nozzle were used. Pure PAA solution was used as the shell fluid (flow rate 0.9 mL / h), and PAA solution containing dissolved Zn(NO3)2·6H2O (0.2 M) and Co(NO3)2·6H2O (0.1 M) was used as the core fluid (flow rate 0.3 mL / h). A high voltage of 18 kV was applied, the distance between the nozzle and the roller collector was 15 cm, the roller speed was 500 rpm, the ambient temperature was 25±2℃, and the relative humidity was 30±5%. PAA coaxial fiber nonwoven membrane was collected.

[0041] (3) Thermal imidization: The above-mentioned fiber membrane was placed in a tube furnace and subjected to programmed temperature rise heat treatment under nitrogen atmosphere protection: the temperature was increased to 80°C at 2°C / min and held for 2 h to completely remove the solvent; then the temperature was increased to 200°C at 5°C / min and held for 1 h; then the temperature was increased to 300°C at 5°C / min and held for 0.5 h. After natural cooling to room temperature, a porous PI-based membrane with a metal ion gradient distribution was obtained.

[0042] (4) Seeding treatment: Immerse the PI base film in 0.01 M commercial zinc oxide nanoseed aqueous dispersion for 10 minutes, and then dry it in an oven at 80℃ for 2 h to make the nanoseeds firmly adhere to the fiber surface.

[0043] (5) MOF solvothermal growth: Preparation of growth solution: H2BDC (0.15 M), Zn(NO3)2·6H2O (0.10 M) and Co(NO3)2·6H2O (0.05 M) were dissolved in N,N-dimethylformamide (DMF). The seeded PI film was immersed in the growth solution and transferred to a 100 mL polytetrafluoroethylene-lined high-pressure reactor. The reaction was carried out in an oven at 120 °C for 6 h. After the reaction was completed, the film was washed three times with anhydrous ethanol to remove unreacted substances.

[0044] (6) Interfacial coupling treatment: The washed composite membrane was immersed in a 1 wt% APTES ((3-aminopropyl)trimethoxysilane) ethanol / water (95 / 5, v / v) solution and gently shaken in a 60℃ water bath for 2 h. Then it was rinsed with ethanol and vacuum dried at 80℃ for 12 h to obtain the final Zn / Co-MOF@PI composite membrane (denoted as S-1).

[0045] Example 2: Preparation of Fe / Ni-MOF@PI composite membrane by precursor complexation combined with microwave-assisted method (1) Preparation of complexed PAA spinning solution: Fe(Ac)2 (0.12M), Ni(Ac)2 (0.08 M) and equimolar HmIm (0.20 M) were added sequentially to a 15 wt% PAA / DMAc solution and stirred at room temperature for 12 h to form a homogeneous metal-ligand complex precursor solution.

[0046] (2) Electrospinning and imidization: Uniaxial electrospinning was used with the following parameters: flow rate 1.0 mL / h, voltage 16 kV, and receiving distance 14 cm. After collecting the fiber membrane, it was processed according to the same thermal imidization procedure as in Example 1 to obtain a PI-based membrane with uniform metal distribution.

[0047] (3) Microwave-assisted MOF growth: Preparation of growth solution: Fe(Ac)2 (0.06 M), Ni(Ac)2 (0.04 M), and HmIm (0.20 M) were dissolved in DMF. The PI-based film was completely immersed in the growth solution and placed in a dedicated container of a microwave synthesis reactor (CEM, DiscoverSP). The reaction temperature was set to 120℃, the microwave power to 300 W, and the reaction time to 30 min. After the reaction was completed, the film was quickly removed and washed several times alternately with DMF and ethanol.

[0048] (4) Heat treatment stabilization: The washed membrane was heat-treated in a vacuum oven at 120℃ for 1 h to remove residual solvent and stabilize the MOF crystal structure, resulting in Fe / Ni-MOF@PI composite membrane (denoted as S-2).

[0049] Example 3: Preparation of Zr / Fe-MOF@PI composite membrane by surface amination combined with steam-assisted method (1) Preparation of pure PI-based membrane: uniaxial electrospinning was performed using 15 wt% pure PAA / DMAc solution (flow rate 1.0 mL / h, voltage 17 kV, distance 15 cm), and imidization was performed according to the procedure in Example 1 to obtain pure PI nanofiber membrane.

[0050] (2) Surface amination treatment: The pure PI film was placed in a plasma cleaner (Harrick Plasma, PDC-32G). Oxygen was first introduced and treated at 100 W for 5 min to activate the surface and generate oxygen-containing functional groups. Then, ammonia was introduced and treated at the same power for another 10 min to graft amino groups (-NH2) onto the fiber surface. The film was then removed and set aside for later use.

[0051] (3) Steam-assisted MOF growth: Weigh ZrCl4 (0.1 mmol), Fe(NO3)3·9H2O (0.05 mmol), and H3BTC (0.15 mmol) into a 20 mL glass vial (as precursor source). Suspend the amination-treated PI membrane above a 50 mL polytetrafluoroethylene-lined autoclave. Place the vial containing the precursor at the bottom of the autoclave, ensuring that the two do not come into direct contact. Add 1 mL of DMF to the liner as a solvent vapor source. Seal the autoclave and place it in an oven to react at 120 °C for 4 h. MOF is generated by the sublimation of the precursor and the reaction of the solvent vapor on the fiber surface.

[0052] (4) Interfacial crosslinking: The grown composite membrane was immersed in a toluene solution of 0.5 wt% HDI and reacted at 60 °C for 2 h. After the reaction was completed, it was washed with toluene and ethanol and dried under vacuum at 60 °C to obtain a Zr / Fe-MOF@PI composite membrane (denoted as S-3).

[0053] Comparative Example 1: Pure polyimide (PI) membrane The preparation process is exactly the same as step (1) in Example 3, and a pure PI nanofiber membrane without any MOF layer or metal introduction is obtained as the most basic performance reference.

[0054] Comparative Example 2: Commercial Al2O3 ceramic-coated polyethylene (PE) membrane A commercially available ceramic diaphragm with polyethylene as the base material and Al2O3 nanoparticles coated on one side (coating thickness of about 4 μm) was selected as a reference for traditional modified diaphragms.

[0055] Comparative Example 3: Single-metal MOF-coated PI membrane (without interfacial crosslinking) To illustrate the synergistic importance of multimetals and interfacial crosslinking, this comparative example was designed. Referring to Example 1, but using only 0.3 M Zn(NO3)2·6H2O as the sole metal source for the core layer, and omitting the APTES interfacial coupling treatment in step (6) after MOF growth. All other steps and parameters remained consistent with Example 1, resulting in a Zn-MOF@PI (uncrosslinked) membrane.

[0056] Table 1 Comparison of Examples 1-3 and Comparative Examples 1-3 under several key performance indicators (values ​​are averages, and the average of three repeated experiments is listed).

[0057] As can be seen from the data in Table 1, compared with Comparative Example 1 which does not contain MOF, the membranes of Examples 1-3 all significantly improved the ability to capture manganese ions (by tens of times), resulting in a significant reduction in the Mn content in the electrolyte during cycling. At the same time, the presence of multi-metal MOF and interfacial coupling treatment made the capacity retention rate and coulombic efficiency of the battery significantly better than the comparative example during long-term cycling. Example 2 (introducing metal in a complexed state in the spinning solution and combining it with microwave-assisted growth) showed the highest Mn capture ability and the best capacity retention rate, which may be attributed to the uniform distribution of metal inside and outside the fiber and the rapid formation of micron / nanoscale active sites.

[0058] Regarding ionic conductivity, the composite MOF coating has a slight impact on ion transport, but by controlling the coating thickness and pore connectivity (e.g., using a thin-layer core-shell structure or selective growth process), the conductivity can be kept within an acceptable range (approximately 0.9-1.0 mS·cm). -1 Mechanical and thermal stability tests show that the PI-based composite separator is significantly superior to the traditional polyolefin separator in terms of tensile strength and high-temperature dimensional stability, which is beneficial to battery safety.

[0059] The examples further illustrate the effects of adjusting several optional process parameters: increasing the microwave growth time can make the MOF crystals denser but may reduce porosity, resulting in a slight decrease in ion conductivity; increasing the amount of coupling agent or extending the crosslinking time can enhance the MOF / PI interfacial bonding and cycle durability, but excessive crosslinking will reduce electrolyte wettability. Based on the "core-shell metal precursor positioning + seeding - selective growth - interfacial crosslinking" strategy of this invention, targeted ion capture selectivity can be achieved by adjusting the metal type and MOF ligand according to different cathode materials (such as LiMn2O4, LNMO, Mn-containing high-nickel materials, etc.), thereby achieving optimized effects in actual battery systems.

[0060] In summary, the above embodiments demonstrate the implementation of the present invention with specific process routes and parameters, and show the effects of different multimetal MOF configurations and preparation processes on Mn capture performance, electrochemical cycling stability, ion transport, and mechanical / thermal stability through tabular test data, which is sufficient to support the feasibility and superiority of the technical effects described in the present invention. The above embodiments are merely preferred embodiments of the present invention, and any modifications, alterations, or equivalent substitutions made within the spirit and principles of the present invention should fall within the scope defined by the claims of the present invention.

Claims

1. A composite diaphragm, comprising: A porous polyimide nanofiber matrix, and a multi-metal-organic framework layer grown in situ on the surface and within the pores of the polyimide nanofiber matrix; The multi-metal organic framework layer contains at least two different metal ions and organic ligands; The multimetallic organic framework layer is bonded to the polyimide nanofiber matrix through an interfacial cross-linking layer.

2. The polyimide composite separator according to claim 1, characterized in that: The metal ions in the polyimide nanofiber matrix may exhibit one or more of the following distribution patterns: The fiber cross-section gradient distribution, the uniform distribution introduced in the polyamic acid precursor in a complex state, or the surface distribution that is adsorbed and fixed after surface functionalization.

3. The polyimide composite separator according to claim 1 or 2, characterized in that: The metal ions in the multimetallic organic framework layer are selected from at least two of zinc, cobalt, iron, nickel, zirconium, and manganese.

4. The polyimide composite separator according to claim 1 or 2, characterized in that: The interface crosslinking layer comprises a chemically bonded layer formed by a silane coupling agent or a diisocyanate crosslinking agent.

5. A method for preparing the polyimide composite separator according to any one of claims 1-4, comprising the following steps: S1. Prepare a polyamic acid spinning solution containing a metal precursor and perform electrospinning to obtain a polyamic acid fiber membrane. S2. The polyamic acid fiber membrane is subjected to imidization treatment to obtain a porous polyimide nanofiber matrix; S3. A multi-metal-organic framework layer is grown in situ on the polyimide nanofiber matrix using a stepwise control strategy. S4. Perform interfacial crosslinking treatment on the composite membrane on which the multi-metal-organic framework layer is grown to obtain the composite membrane.

6. The preparation method according to claim 5, characterized in that: In step S1, the metal precursor is introduced through one or more of the following methods: A) Coaxial electrospinning is used, with different metal salt solutions placed in the core and shell layers of the fiber, respectively; B) Metal salts and ligands are pre-added to a polyamic acid solution to form a metal complex; C) First, prepare a pure polyamic acid fiber membrane, then imidize it and perform plasma treatment or chemical grafting on the fiber surface to introduce functional groups, and then adsorb metal ions.

7. The preparation method according to claim 6, characterized in that: In step S3, the step-by-step control strategy includes: a) Seeding and nucleation steps: forming a seed layer of metal oxides or pre-crystallized nuclei on the surface of polyimide fibers; b) Selective growth step: Using the seed layer as the crystal nucleus, a multi-metal-organic framework layer is formed in situ by controlling the solvent, temperature and time.

8. The preparation method according to claim 7, characterized in that: The selective growth step employs at least one of the following processes: solvothermal method, hydrothermal method, chemical vapor deposition method, microwave-assisted synthesis method, or steam-assisted crystallization method.

9. The preparation method according to claim 6, characterized in that: In step S4, the interface crosslinking treatment involves impregnating or coating the composite membrane with a silane coupling agent or a diisocyanate crosslinking agent solution and reacting it under heating conditions.

10. An electrochemical device comprising a positive electrode, a negative electrode, an electrolyte, and a membrane, characterized in that: The diaphragm is the polyimide composite diaphragm according to any one of claims 1-4.

11. The electrochemical device according to claim 10, characterized in that: The electrochemical device is a lithium-ion battery, sodium-ion battery, zinc-ion battery, lithium-sulfur battery, or supercapacitor.

12. The electrochemical device according to claim 11, characterized in that: When the electrochemical device is a lithium-ion battery, its positive electrode material is a manganese-containing lithium manganese oxide or a high-nickel manganese-based material.

13. A method for capturing transition metal ions in an electrolyte using a composite membrane according to any one of claims 1-4, characterized in that: During battery cycling, the multi-metal-organic framework layer in the composite separator complexes with the transition metal ions dissolved in the electrolyte through its coordination sites, thereby reducing the concentration of transition metal ions in the electrolyte.

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