Cellulose nanofiber composite diaphragm modified by covalent organic framework as well as preparation method and application of cellulose nanofiber composite diaphragm

Abstract

The invention relates to a covalent organic framework modified cellulose nanofiber composite diaphragm which is characterized in that cellulose nanofiber is used as a matrix, an amide type or amine type covalent organic framework layer is grown on the surface in situ, the loading capacity of a covalent organic framework is regulated and controlled, and a hierarchical pore structure with gradient pore diameter is constructed; the preparation method comprises the following steps: synthesizing a covalent organic framework precursor bonded with an imine bond on the surface of cellulose nanofiber in situ by a one-pot method under an acidic mixed solution and a heating condition, and regulating and synchronously reducing to convert into an amine type covalent organic framework; or after the imine bond bonded covalent organic framework is synthesized in two stages, the imine bond bonded covalent organic framework is selectively converted into an amide type covalent organic framework through oxidation modification; the obtained cellulose nanofiber compound coated with the covalent organic framework is subjected to the processes of film forming, solvent replacement, supercritical drying and the like, and the composite diaphragm is obtained; the diaphragm has excellent electrolyte wettability, high liquid absorption rate, good ionic conductivity and thermal stability, and can promote uniform transmission of lithium ions and inhibit growth of lithium dendrites.

Description

Technical Field

[0001] This invention belongs to the field of battery separator material technology, and relates to a covalent organic framework modified cellulose nanofiber composite separator, its preparation method and application, and particularly to a cellulose nanofiber composite separator containing amide and amine covalent organic frameworks, its preparation method and application. Background Technology

[0002] In lithium-ion battery systems, the separator, as a key component, plays a crucial role in battery performance and safety. The separator not only effectively separates the positive and negative electrodes to prevent short circuits but also needs excellent electrolyte adsorption and conductivity to ensure efficient lithium-ion migration, thereby maintaining normal battery operation and cycle stability. An ideal battery separator should simultaneously possess excellent thermal stability, mechanical strength, and electrolyte wettability. However, currently widely used polyolefin separators are prone to thermal shrinkage or melting at high temperatures, posing a risk of internal short circuits and safety accidents. Furthermore, their low surface energy and poor electrolyte wettability further limit the battery's high-rate and long-cycle performance. In addition, these separators are mainly derived from non-renewable fossil resources, making it difficult to meet the demands of green, low-carbon, and sustainable development.

[0003] In recent years, cellulose-based separators have attracted much attention due to their renewable, biodegradable, and environmentally friendly properties. As a natural polymer material, cellulose possesses excellent mechanical strength and thermal stability. Separators made from cellulose can not only improve battery safety but also alleviate the environmental problems caused by traditional petroleum-based separators. In particular, cellulose nanofibers (CNFs), with their unique one-dimensional nanostructure, high aspect ratio, and excellent mechanical properties, exhibit ideal separator substrate characteristics. Through structural design and surface modification, CNF-based separators can further improve electrolyte wettability and ion conductivity, thereby significantly improving battery charge-discharge efficiency and cycle life. However, existing CNF-based separators still face many challenges in practical applications. For example, due to limitations in wet molding processes and the inherent properties of cellulose, separators are prone to uneven pore distribution or asymmetrical double-sided structures, leading to uneven lithium-ion flux distribution and intensified dendrite growth, thus affecting battery safety and stability. Meanwhile, traditional cellulose fiber membranes suffer from problems such as excessively large pore size and poor flexibility; while membranes constructed from overly dense nanocellulose may restrict ion transport, making it difficult to balance mechanical stability and high ionic conductivity. Therefore, how to design and fabricate a novel cellulose-based lithium-ion battery membrane that combines environmental friendliness, excellent electrolyte wettability, high thermal stability, and superior ion conductivity has become a current research hotspot and challenge in this field. Summary of the Invention

[0004] In view of this, in order to solve the problem that the traditional nanocellulose membrane has low porosity and non-uniform pore structure due to the preparation process, which affects its suitability as an ideal battery membrane for high-energy battery systems, the present invention provides a covalent organic framework modified cellulose nanofiber composite membrane, its preparation method and application. The composite membrane uses cellulose nanofiber as a matrix, and an amide-type or amine-type covalent organic framework layer is grown in situ on its surface. By controlling the covalent organic framework loading, a multi-level pore structure with gradient pore size is constructed. The prepared composite membrane has excellent electrolyte wettability, liquid absorption and high ionic conductivity.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing a covalently organic framework-modified cellulose nanofiber composite membrane includes the following steps:

[0007] S1. Preparation of a suspension of cellulose nanofiber composites coated with an imine-type covalent organic framework: 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added to an aqueous suspension of cellulose nanofibers (CNF) and stirred until homogeneous. The mass ratio of CNF to TAPB was 2~8:1. After stirring and mixing, a solution of 1,3,5-tricarboxyphenyl (TFB) acetic acid pre-activated with an acid mass ratio of 1:1 to 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added. The mixture was reacted at room temperature and washed to allow the imine-type covalent organic framework to be generated in situ and coated on the surface of the cellulose nanofibers, thus obtaining a suspension of cellulose nanofiber composites coated with an imine-type covalent organic framework. The mass ratio of the total mass of TAPB and TFB, which serve as the covalent organic framework, to the mass of CNF was 0.25~1:1.

[0008] S2. Functionalization of the linker bond: The cellulose nanofiber composite suspension coated with animine-type covalent organic framework obtained in step S1 is mixed with anhydrous N,N-dimethylformamide (DMF), acid, and oxidant. The amount of oxidant is 1 to 3 equivalents of the molar amount of the imine groups in the covalent organic framework. The mixture is reacted under an inert atmosphere for 5 to 15 hours to oxidize the imine groups in the covalent organic framework to amide groups. After the reaction is completed, the mixture is washed to obtain a cellulose nanofiber composite suspension coated with an amide-type covalent organic framework.

[0009] S3. The composite membrane is prepared by vacuum filtration. The cellulose nanofiber composite with amide-type covalent organic framework obtained in step S2 is ultrasonically dispersed in water to obtain a suspension. The suspension is then vacuum filtered using a sand core funnel equipped with a polytetrafluoroethylene filter membrane to obtain a wet covalent organic framework-coated cellulose nanofiber composite membrane.

[0010] S4. Carbon dioxide supercritical drying of the membrane: The wet covalent organic framework-coated cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement. After separation, it is subjected to supercritical carbon dioxide drying to obtain an amide-type covalent organic framework-coated cellulose nanofiber composite membrane.

[0011] A method for preparing a covalently organic framework-modified cellulose nanofiber composite membrane includes the following steps:

[0012] S1′ Preparation of amine-type covalent organic framework-coated cellulose nanofiber composites: 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added to an aqueous suspension of cellulose nanofibers (CNF) and stirred until homogeneous. The mass ratio of CNF to TAPB was 2~8:1. After stirring and mixing, a mixed aqueous solution containing 10 mL of 1,3,5-tris(4-aminophenyl)benzene (TAPB) at a mass ratio of 1:1 and 10 mL of formic acid to acetic acid at a ratio of 1:0.5~1.5 was added. The mixture was reacted at 60~90℃ for 24~72 hours to generate and coat the surface of the cellulose nanofibers with amine-type covalent organic frameworks. After the reaction, the mixture was washed to obtain a suspension of amine-type covalent organic framework-coated cellulose nanofiber composites. The mass ratio of the total mass of TAPB and TFB as covalent organic frameworks to CNF was 0.25~1:1.

[0013] S2′, The composite membrane is prepared by vacuum filtration. The cellulose nanofiber composite with amine covalent organic framework coated in step S1′ is ultrasonically dispersed in water to obtain a suspension. The suspension is then vacuum filtered using a sand core funnel equipped with a polytetrafluoroethylene filter membrane to obtain a wet covalent organic framework coated cellulose nanofiber composite membrane.

[0014] S3′, Supercritical carbon dioxide drying of the membrane: The wet covalent organic framework-coated cellulose nanofiber composite membrane obtained in step S2′, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement. After separation, it is subjected to supercritical carbon dioxide drying to obtain an amine-containing covalent organic framework-coated cellulose nanofiber composite membrane.

[0015] Furthermore, the cellulose nanofiber composite coated with the covalent organic framework obtained in step S1 is washed sequentially with deionized water, acetone, and N,N-dimethylformamide (DMF) for three rounds. This stepwise washing utilizes the differences in solubility of different solvents for acids / inorganic salts / small molecule monomers / oligomers to achieve stepwise removal. Water preferentially removes acid residues, acetone promotes dehydration and removes some organic impurities, and DMF further removes oligomers and residual water, ensuring the purity of the composite and the repeatability of subsequent processes.

[0016] Further, in step S2, the oxidant is oxone, and the amount used is 2 to 2.25 equivalents of the molar amount of imine groups in the covalent organic framework, with a reaction time of 10 to 12 hours. This equivalent amount and time window setting are mainly used to synergistically overcome the diffusion limitation inside the COF pores and ensure sufficient conversion: Since the COF is coated on the CNF surface and the imine sites are located inside the pores, the oxidant needs to gradually diffuse into the pores. The outer sites will preferentially consume the oxidant. Insufficient oxidant or too short a reaction time can easily lead to incomplete internal oxidation and the formation of a conversion gradient. Using 2 to 2.25 equivalents can compensate for diffusion and reaction losses and maintain the effective oxidant concentration in the system. Controlling the reaction time to 10 to 12 hours can provide sufficient time for the oxidant to penetrate and gradually react in the shell / pores, so that the imine bond to amide bond can achieve a high degree of conversion and a more uniform functional group distribution. At the same time, it avoids excessive oxidation caused by the continuous action of the oxidant due to excessive reaction time, increased skeleton defects, or adverse effects on the cellulose matrix, thereby taking into account both structural integrity and membrane performance stability.

[0017] Further, in step S4, the cellulose nanofiber composite coated with an amide-type covalent organic framework is washed sequentially with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and water, for three rounds of washing. Sodium thiosulfate is used to quench residual oxidizing species and remove inorganic residues, tetrahydrofuran is used to remove moderately polar organic byproducts and residual reagents, n-hexane is used to remove hydrophobic impurities, acetone is used for further desolvation and to promote displacement, and finally, water washing is used to restore the aqueous phase environment and remove soluble salt residues, thereby reducing the impact of impurities on the electrochemical stability and consistency of the membrane.

[0018] Furthermore, in step S1′, the ratio of formic acid to acetic acid solution is 4.0. Formic acid and 4.0 Acetic acid; the reaction temperature in step S1′ is 75~85℃, and the reaction time is 45~50 hours; by controlling the concentration and ratio of formic acid and acetic acid, acetic acid can dominate the rapid and reversible condensation and structural self-repair of imine COF on the CNF surface, improving the coating uniformity, while formic acid provides stronger acidity and reducing ability, slowly converting imine bonds into amine bonds under heating conditions; this invention selects a matching window of 75~85℃ and 45~50 hours to ensure that acidic components and reactants diffuse fully inside the shell and promote the reaction to tend to be complete, while avoiding excessive degradation of the cellulose skeleton due to higher temperature or longer time, which would affect the film strength and pore structure stability.

[0019] Furthermore, after the reaction in step S1′ is completed, the cellulose nanofiber complex coated with the covalent organic framework is washed with deionized water, acetone and deionized water in sequence for three rounds. This sequence reduces the impact of residual acid and organic matter on subsequent processing by first removing acid, then removing organic impurities and then returning to the aqueous phase, thereby improving the dispersibility and film stability of the complex.

[0020] Furthermore, the stirring speed in steps S1 and S1′ is 500-800 rpm to ensure uniform mixing of monomer molecules with CNF particles that have differences in specific gravity, and subsequently uniform nucleation and coating. The stirring speed in step S2 is 50-100 rpm. Using a higher stirring speed in the condensation stage helps to quickly eliminate local concentration gradients and promote the uniform distribution of monomers near the CNF backbone, allowing COF to preferentially nucleate at multiple points on the fiber surface and form a uniform coating layer, while inhibiting the self-aggregation of the solution phase. Reducing the stirring speed in the bond conversion stage can reduce mechanical shear damage to the formed COF shell, avoid peeling of the coating layer or damage to the pore structure, and retain necessary convection to maintain the macroscopic mass transfer of the conversion reaction, thereby taking into account both structural integrity and conversion uniformity.

[0021] Further, in steps S3 and S2′, the cellulose nanofiber composite coated with a covalent organic framework is ultrasonically dispersed in water to obtain a suspension. The ultrasonic duration is 5-20 minutes, the ultrasonic power is 600W, and the duty cycle is 75%, i.e., 3 seconds of operation followed by 1 second of rest. This ultrasonic window is used to dissociate the weak aggregation and entanglement between the composite fibers, making the suspension system uniform and stable, thereby achieving a smoother spread and a more consistent pore distribution during vacuum filtration. Insufficient ultrasonication can lead to residual agglomeration causing membrane defects and local densification, while excessive ultrasonication may cause excessive fiber cutting or damage to the continuity of the COF coating layer. This time control can improve the membrane thickness, pore structure uniformity, and mechanical reliability.

[0022] Furthermore, in steps S4 and S3′, the wet covalent organic framework-coated nanocellulose composite membrane is placed in anhydrous ethanol for solvent replacement for 2 to 4 hours. Through the slight swelling of the membrane by anhydrous ethanol and the effective replacement of water in the pores, the surface tension and capillary pressure in the subsequent drying process can be reduced, and the network shrinkage caused by the reformation of hydrogen bonds between cellulose nanofibers can be inhibited, thereby improving the porosity of the composite membrane and maintaining the hierarchical pore structure.

[0023] Furthermore, the wet covalently organic framework-coated nanocellulose composite membrane was clamped in a nylon film and placed in a supercritical drying autoclave. After being maintained at 40–50°C and 8–9.5 MPa for 4–5 hours, the pressure was slowly released at a rate of 0.5–1.0 MPa per minute. The nylon film clamping was used to limit the warpage and shrinkage of the membrane during the replacement and drying process, improving dimensional stability and product consistency. Drying in the supercritical range near the carbon dioxide critical point can eliminate capillary forces at the liquid-gas interface, promoting supercritical drying. The membrane is fully permeated and the ethanol in the pores is extracted, thereby maximizing the integrity of the macroporous / mesoporous structure composed of COF micropores and CNF; at the same time, a smooth transition from wet to dry state is achieved by appropriate holding time and slow pressure release, avoiding pore compression damage or structural collapse caused by pressure gradient.

[0024] A covalent organic framework modified cellulose nanofiber composite membrane was prepared by a method for preparing cellulose nanofiber composite membranes.

[0025] Application of the covalent organic framework modified cellulose nanofiber composite separator in lithium-ion batteries and lithium metal battery separators.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. The method for preparing a covalently organic framework-modified cellulose nanofiber composite separator disclosed in this invention effectively solves the problems of low porosity and uneven pore structure caused by the traditional nanocellulose separator due to its preparation process by in-situ coating cellulose nanofibers with amide-type or amine-type covalently organic frameworks to form a multi-level pore size composite separator. Based on the high specific surface area and abundant modifiable functional groups of the covalently organic framework, a covalently organic framework layer is uniformly grown on the surface of cellulose nanofibers in an aqueous system, optimizing the uniformity of the separator's pore distribution. Furthermore, by controlling the covalently organic framework loading, a balance is achieved between porosity and mechanical strength. This design significantly improves the uniformity of lithium-ion flux distribution, promotes uniform lithium metal deposition, inhibits lithium dendrite growth, and simultaneously increases porosity and electrolyte absorption rate, thereby improving battery cycle performance and safety.

[0028] 2. The method for preparing covalently organic framework-modified cellulose nanofiber composite membranes disclosed in this invention involves modifying the COF backbone to prepare membranes containing imine / amide / amine COFs coated with cellulose nanofibers. The polar groups CONH- and –NH– introduced into the covalent organic framework significantly improve the wettability of the membrane surface to the organic electrolyte, allowing the electrolyte to fully wet the hierarchical channels and maintain a high liquid uptake rate, thereby improving the effective electrolyte loading and ion conductivity of the membrane.

[0029] 3. To enhance the adaptability of the separator to high-energy battery systems, the preparation method of the covalent organic framework-modified cellulose nanofiber composite separator disclosed in this invention focuses on a strategy of tunable bonding groups of the covalent organic framework: on the one hand, imine-type covalent organic frameworks are converted into amide-type covalent organic frameworks containing amide groups (–CONH–) through oxidative modification; on the other hand, amine-type covalent organic frameworks are directly constructed in an acidic mixed solution by changing the synthesis conditions. By controlling the transition between imine-type and amide-type bonding structures, the synergistic coordination of carbonyl oxygen, amino nitrogen, and other groups is enhanced, thereby strengthening ionic conductivity and selectivity for lithium-ion transport. This functionalized design optimizes the ion transport pathway and improves the battery's kinetic performance and cycle stability.

[0030] 4. The preparation method of the covalent organic framework modified cellulose nanofiber composite membrane disclosed in this invention employs ethanol solvent replacement combined with supercritical carbon dioxide drying technology. The membrane is soaked in ethanol to swell and then filtered, which helps remove residual moisture from the composite membrane, reduces hydrogen bonding of cellulose bonds, and increases porosity. The low surface tension and good solvent compatibility of ethanol effectively reduce capillary forces within the pores during drying. Ethanol removal under supercritical conditions results in extremely low capillary forces, avoiding shrinkage of the fiber pore structure and collapse of COF micropores caused by thermal drying, thereby significantly improving the porosity and electrolyte absorption rate of the composite membrane.

[0031] 5. The covalent organic framework-modified cellulose nanofiber composite membrane disclosed in this invention uses cellulose nanofibers as a matrix, on which an amide-type or amine-type covalent organic framework (COF) coating layer is grown in situ. By controlling the COF loading, a gradient pore size hierarchical pore structure is constructed, consisting of COF micropores and CNF framework macropores / mesopores, achieving a balance of high porosity, structural integrity, and continuous ion channels. The preparation method includes: firstly, synthesizing imine-type COF in situ on the surface of cellulose nanofibers; then, selectively converting imine bonds to amide bonds under controlled oxidation conditions to obtain an amide-type COF coating layer with strong dipole sites and higher ion selectivity, while maintaining the COF stacking crystallization characteristics and pore structure stability during the conversion process, avoiding excessive oxidation leading to framework defects and pore structure collapse; or, achieving a coupling process of imine generation and reduction conversion under acidic mixed solution and heating conditions to directly construct an amine-type COF coating layer, thereby obtaining stronger anion binding capacity. The resulting COF-coated cellulose nanofiber composite was then subjected to vacuum filtration to form a membrane, followed by ethanol solvent replacement and supercritical carbon dioxide drying. This process suppressed pore shrinkage caused by drying capillary forces, maintained the membrane's porosity and pore size gradient distribution, and ensured the continuous coating of the COF layer on the fiber surface and the integrity of crystal stacking. The resulting composite membrane exhibits excellent electrolyte wettability and liquid absorption capacity, high ionic conductivity and lithium-ion transference number, and good thermal stability. It can promote uniform lithium-ion transport, reduce interfacial transport impedance, and inhibit lithium dendrite growth, making it suitable for lithium-ion batteries and lithium metal battery separators.

[0032] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0034] Figure 1 From left to right, the images are of cellulose nanofibers coated with amine / imine / amide covalent organic frameworks prepared in Example 6, Comparative Example 2, and Example 2 of the present invention, and an image of a cellulose nanofiber membrane coated with amine / imine / amide covalent organic frameworks.

[0035] Figure 2(a) is a comparison diagram of stress-strain curves of cellulose nanofiber composite membranes coated with imine-type covalent organic frameworks in the embodiments of the present invention. ImineCOF@CNF-0.25 represents the stress-strain curve of Comparative Example 1, ImineCOF@CNF-0.5 represents the stress-strain curve of Comparative Example 2, ImineCOF@CNF-0.75 represents the stress-strain curve of Comparative Example 3, ImineCOF@CNF-1.0 represents the stress-strain curve of Comparative Example 4, and CNF represents the stress-strain curve of Comparative Example 5.

[0036] Figure 2 (b) is a comparison diagram of stress-strain curves of cellulose nanofiber composite membranes coated with amide-type covalent organic frameworks in the embodiments of the present invention. AmideCOF@CNF-0.25 represents the stress-strain curve of Example 1, AmideCOF@CNF-0.5 represents the stress-strain curve of Example 2, AmideCOF@CNF-0.75 represents the stress-strain curve of Example 3, AmideCOF@CNF-1.0 represents the stress-strain curve of Example 4, and CNF represents the stress-strain curve of Comparative Example 5.

[0037] Figure 2 (c) is a comparison diagram of stress-strain curves of cellulose nanofiber composite membranes coated with amine-type covalent organic frameworks in the embodiments of the present invention. AmineCOF@CNF-0.25 represents the stress-strain curve of Example 5, AmineCOF@CNF-0.5 represents the stress-strain curve of Example 6, AmineCOF@CNF-1.0 represents the stress-strain curve of Example 7, and CNF represents the stress-strain curve of the comparative example.

[0038] Figure 2 (d) is a comparison diagram of stress-strain curves of cellulose nanofiber composite membranes coated with imine / amide / amine covalent organic frameworks in the embodiments of the present invention. ImineCOF@CNF-0.5 represents the stress-strain curve of Comparative Example 2, AmideCOF@CNF-0.5 represents the stress-strain curve of Example 2, AmideCOF@CNF-0.5 represents the stress-strain curve of Example 6, and CNF represents the stress-strain curve of Comparative Example 5.

[0039] Figure 3 These are comparison graphs of the diaphragm at different temperatures. AmineCOF@CNF represents Example 6, ImineCOF@CNF represents Comparative Example 2, and AmindCOF@CNF represents Example 2.

[0040] Figure 4 This is a contact angle diagram of the diaphragm. AmineCOF@CNF represents Example 6, ImineCOF@CNF represents Comparative Example 2, and AmindCOF@CNF represents Example 2. Detailed Implementation

[0041] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0042] Example 1

[0043] An amide-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0044] S1. Weigh 8.81 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 4.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm at room temperature (25 °C) for 48 hours. After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and N,N-dimethylformamide to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0045] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 were added to a mixture of 10 mL anhydrous DMF and 0.5 mL acetic acid, and 45.0 mg of Oxone was added. The mixture was stirred at 100 rpm for 10 h under a nitrogen atmosphere. After the reaction was completed, the precipitate was collected and washed three times each with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and deionized water to obtain amide-linked covalent organic framework / cellulose nanofibers.

[0046] S3. Add the amide-linked covalent organic framework / cellulose nanofibers obtained in step S2 to 500 mL of deionized water and disperse under ultrasonic conditions at 500 W for 5 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel equipped with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amide-linked covalent organic framework / cellulose nanofiber composite membrane.

[0047] S4. The wet amide-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying autoclave. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain the amide-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmindCOF@CNF-0.25.

[0048] Example 2

[0049] An amide-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0050] S1. Weigh 17.62 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 8.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and N,N-dimethylformamide to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0051] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 were added to a mixture of 10 mL anhydrous DMF and 0.5 mL acetic acid, and 90.0 mg of Oxone was added. The mixture was stirred at 100 rpm for 10 h under a nitrogen atmosphere. After the reaction was completed, the precipitate was collected and washed three times each with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and deionized water to obtain amide-linked covalent organic framework / cellulose nanofibers.

[0052] S3. Add the amide-linked covalent organic framework / cellulose nanofibers obtained in step S2 to 500 mL of deionized water and disperse under ultrasonic conditions of 500 W for 10 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel equipped with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amide-linked covalent organic framework / cellulose nanofiber composite membrane.

[0053] S4. The wet amide-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying autoclave. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain the amide-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmindCOF@CNF-0.5.

[0054] Example 3

[0055] An amide-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0056] S1. Weigh 26.43 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 12.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and N,N-dimethylformamide to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0057] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 were added to a mixture of 10 mL anhydrous DMF and 0.5 mL acetic acid, and 135.0 mg of Oxone was added. The mixture was stirred at 100 rpm for 10 h under a nitrogen atmosphere. After the reaction was completed, the precipitate was collected and washed three times each with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and deionized water to obtain amide-linked covalent organic framework / cellulose nanofibers.

[0058] S3. Add the amide-linked covalent organic framework / cellulose nanofibers obtained in step S2 to 500 mL of deionized water and disperse under ultrasonic conditions at 500 W for 15 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amide-linked covalent organic framework / cellulose nanofiber composite membrane.

[0059] S4. The wet amide-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying autoclave. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain the amide-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmindCOF@CNF-0.75.

[0060] Example 4

[0061] An amide-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0062] S1. Weigh 35.25 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 16 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and N,N-dimethylformamide to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0063] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 were added to a mixture of 10 mL anhydrous DMF and 0.5 mL acetic acid, and 180.0 mg of Oxone was added. The mixture was stirred at 100 rpm for 10 h under a nitrogen atmosphere. After the reaction was completed, the precipitate was collected and washed three times each with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and deionized water to obtain amide-linked covalent organic framework / cellulose nanofibers.

[0064] S3. Add the amide-linked covalent organic framework / cellulose nanofibers obtained in step S2 to 500 mL of deionized water and disperse under ultrasonic conditions at 500 W for 15 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amide-linked covalent organic framework / cellulose nanofiber composite membrane.

[0065] S4. The wet amide-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying autoclave. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain the amide-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmindCOF@CNF-1.0.

[0066] Example 5

[0067] An amine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0068] S1. Weigh 8.81 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of a mixed aqueous solution of 4.0 M formic acid and 4.0 M acetic acid containing 4.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm at 80 °C for 48 hours. After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain amine-linked covalent organic framework / cellulose nanofibers.

[0069] S2. The amine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 20 minutes to obtain a uniform suspension. 100 mL of the suspension is then filtered under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0070] S3. The wet amine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an amine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmineCOF@CNF-0.25.

[0071] Example 6

[0072] An amine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0073] S1. Weigh 17.62 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Subsequently, slowly add 10 mL of a mixed aqueous solution of 4.0 M formic acid and 4.0 M acetic acid containing 8.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm at 80 °C for 48 hours. After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain amine-linked covalent organic framework / cellulose nanofibers.

[0074] S2. The amine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 20 minutes to obtain a uniform suspension. 100 mL of the suspension is then filtered under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0075] S3. The wet amine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an amine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmineCOF@CNF-0.5.

[0076] Example 7

[0077] An amine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0078] S1. Weigh 35.25 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Subsequently, slowly add 10 mL of a mixed aqueous solution of 4.0 M formic acid and 4.0 M acetic acid containing 16.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm at 80 °C for 48 hours. After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain amine-linked covalent organic framework / cellulose nanofibers.

[0079] S2. The amine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 20 minutes to obtain a uniform suspension. 100 mL of the suspension is then filtered under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet amine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0080] S3. The wet amine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain the amine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as AmineCOF@CNF-1.0.

[0081] Comparative Example 1

[0082] An imine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0083] S1. Weigh 8.81 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 4.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0084] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 5 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet imine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0085] S3. The wet imine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an imine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as ImineCOF@CNF-0.25.

[0086] Comparative Example 2

[0087] An imine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0088] S1. Weigh 17.62 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 8.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0089] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 10 minutes to obtain a uniform suspension. 100 mL of the suspension is then filtered under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet imine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0090] S3. The wet imine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an imine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as ImineCOF@CNF-0.5.

[0091] Comparative Example 3

[0092] An imine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0093] S1. Weigh 26.43 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 12.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm for 48 hours at room temperature (25 °C). After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0094] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 15 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet imine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0095] S3. The wet imine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an imine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as ImineCOF@CNF-0.75.

[0096] Comparative Example 4

[0097] An imine-linked covalent organic framework / cellulose nanofiber composite membrane and its preparation method, comprising the following steps:

[0098] S1. Weigh 35.25 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and add it to 10 mL of an aqueous suspension of cellulose nanofibers with a concentration of 5 mg / mL. Stir at 1000 rpm for 30 min. Then, slowly add 10 mL of an aqueous acetic acid solution (8.75 M) containing 16.0 mg of 1,3,5-tricarboxyphenyl (TFB) to the above suspension. Stir at 800 rpm at room temperature (25 °C) for 48 hours. After the reaction is complete, collect the precipitate and wash it three times each with deionized water, acetone, and deionized water to obtain imine-linked covalent organic framework / cellulose nanofibers.

[0099] S2. The imine-linked covalent organic framework / cellulose nanofibers obtained in step S1 are added to 500 mL of deionized water and dispersed under ultrasonic conditions at 500 W for 20 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet imine-linked covalent organic framework / cellulose nanofiber composite membrane.

[0100] S3. The wet imine-linked covalent organic framework / cellulose nanofiber composite membrane obtained in step S2, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane is separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying reactor. It is kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain an imine-linked covalent organic framework / cellulose nanofiber composite membrane, denoted as ImineCOF@CNF-1.0.

[0101] Comparative Example 5

[0102] The comparative example contains only cellulose nanofibers and no covalent organic framework is added to the membrane. The specific preparation process is as follows:

[0103] S1. Take 10 mL of a 5 mg / mL aqueous suspension of cellulose nanofibers and add the cellulose nanofibers to 490 mL of deionized water. Disperse the nanofibers under ultrasonic conditions at 500 W for 5 minutes to obtain a uniform suspension. Take 100 mL of the suspension and filter it under reduced pressure through a sand core funnel fitted with a polytetrafluoroethylene filter membrane with a pore size of 0.1 μm and a diameter of 50 mm to obtain a wet pure cellulose nanofiber membrane.

[0104] S2. The wet pure cellulose nanofiber membrane, together with the filter membrane, was placed in anhydrous ethanol for solvent replacement and kept for 4 hours. Subsequently, the composite membrane was separated from the filter membrane, clamped with a nylon film, and placed in a supercritical carbon dioxide drying autoclave. It was kept at 40~50℃ and 8.0~9.5 MPa for 4~5 hours to obtain a cellulose nanofiber composite membrane, denoted as CNF.

[0105] Comparison of Implementation Results

[0106] The physical and electrochemical properties of various cellulose nanofiber membranes prepared in Examples 1-7 and Comparative Examples 1-5 were compared. Ionic conductivity and lithium-ion transference number were tested after the cellulose nanofiber membranes were further assembled into batteries. A CR2032 battery was used as the standard test battery, and 50 μL of electrolyte was added. The electrolyte consisted of 1 M LiPF6 and ethyl carbonate (EC) and ethyl carbonate (EMC) (EC:EMC = 1:1 wt). The results are shown in Table 1 below.

[0107] Table 1. Performance of various cellulose nanofiber membranes in the examples and comparative examples.

[0108]

[0109] The test data from Examples 1–4, Examples 5–7, and Comparative Examples 1–4 and Comparative Example 5 show that when the COF content ratio is the same or similar, the porosity (approximately 65%–76%) and liquid absorption rate (approximately 168%–243%) of the membranes in each group are generally on the same order of magnitude, and the difference is not enough to explain the significant differentiation in ionic conductivity and lithium-ion transference number alone. However, the amide-type membranes exhibit higher ionic conductivity and lithium-ion transference number at all content ratios. In particular, at a content ratio of 0.5, the amide-type membrane (Example 2) has an ionic conductivity of 2.41 mS / cm and a lithium-ion transference number of 0.75, which is significantly better than the corresponding imine-type membrane (Comparative Example 2: 1.65 mS / cm, 0.64), amine-type membrane (Example 6: 1.52 mS / cm, 0.69), and pure CNF membrane (Comparative Example 5: 0.61 mS / cm, 0.51). This indicates that the decisive advantage of this invention mainly stems from the differences in interfacial ion selectivity and transport impedance caused by the differences in COF functional groups: the imine bond (–C=N–) has a certain polarity, which can improve electrolyte wetting and enhance transport, but for Its coordination / solvation regulation ability and its binding effect on anions are limited, therefore σ and The improvement is limited; the amine group (–NH–) can more significantly inhibit anion migration through hydrogen bonding / acid-base interactions, making... Compared to imine systems, it is superior, but due to the lack of a strong carbonyl dipole site, it is less effective for... The lack of stability and continuous migration channels in the solvation structure makes it prone to "single-point coordination," which limits the continuity of migration. Therefore, its conductivity improvement is not as good as that of the amide system. The amide group also has a strong C=O dipole pair. The reversible coordination ability and the hydrogen bond-dipole microenvironment formed by the N–H / carbonyl synergistic process can promote both The solvated phase within the channels stabilizes and facilitates hopping migration, reduces interfacial transport impedance, and more readily adsorbs and binds anions, thereby improving... The contribution ratio is adjusted so that the highest ionic conductivity and lithium-ion transference number can still be achieved even when the difference between porosity and liquid absorption rate is not significant, thus demonstrating better electrochemical transport performance and application value. Figure 1 From left to right, the images are of cellulose nanofibers coated with amine / imine / amide covalent organic frameworks prepared in Example 6, Comparative Example 2, and Example 2 of this invention, and an image of a cellulose nanofiber membrane coated with amine / imine / amide covalent organic frameworks.

[0110] As shown in Figure 2, the tensile stress-strain curves reveal that the pure CNF membrane exhibits the highest ultimate tensile strength. With increasing mass fraction of the covalent organic framework in ImineCOF@CNF-0.25, ImineCOF@CNF-0.5, ImineCOF@CNF-0.75, and ImineCOF@CNF-1.0, the ultimate tensile strength of the composite membrane gradually decreases, but remains comparable to that of the CNF membrane within the 0.25–0.5 range. The AmindCOF@CNF and AmineCOF@CNF systems show a similar trend. Under the same mass ratio of 0.5, the tensile strength of AmindCOF@CNF-0.5 is higher than that of ImineCOF@CNF-0.5 and AmineCOF@CNF-0.5, indicating that the amide-type covalent organic framework, while maintaining the hierarchical porous structure and electrochemical performance, has a relatively small weakening effect on the cellulose nanofiber skeleton. This invention, by introducing a covalent organic framework and constructing a gradient pore structure, still maintains the good tensile strength and toughness of the composite separator, meeting the mechanical reliability requirements of lithium battery winding and assembly.

[0111] Figure 3 shows the thermal stability test results of the PP and the separators prepared in Examples 2, 6, and Comparative Example 2 at 80℃, 120℃, and 160℃. At 80℃, the PP separator slightly curled, while the other samples maintained their intact morphology. When the temperature increased to 120℃ and 160℃, the pure PP separator showed significant shrinkage, curling, and even near-melting, while the AmineCOF@CNF-0.5, ImineCOF@CNF-0.5, and AmindCOF@CNF-0.5 composite separators of Examples 2, 6, and Comparative Example 2 maintained their original circular size and flat appearance without significant deformation. This indicates that compared to PP, these composite separators have superior heat resistance and dimensional stability, significantly improving battery safety under high-temperature conditions.

[0112] As shown in Figure 4, the contact angle test results indicate that, under the same addition amounts (Examples 2, 6, and Comparative Example 2), the COF@CNF membranes prepared in this invention all exhibit significantly better wetting performance than traditional PP membranes. At 0 s, the contact angle of the PP membrane surface is approximately 50°, and the droplets show obvious bulging, indicating poor wettability to the electrolyte. In contrast, the initial contact angles of the AmineCOF@CNF-0.5, ImineCOF@CNF-0.5, and AmideCOF@CNF-0.5 membranes are approximately 13°, 16°, and 12°, respectively, and the droplets spread rapidly, indicating that the introduction of COF can significantly improve the affinity of the membrane surface for the electrolyte. Further observation of the contact angle changes at 10 s revealed that the contact angles of all three COF@CNF membranes rapidly decreased to approximately 0°, indicating complete droplet spreading and rapid absorption by the membrane. In contrast, the PP membrane maintained a relatively large contact angle of approximately 46°, showing limited improvement in wettability. These results clearly demonstrate that the addition of COF can construct abundant polar functional groups and enhance the surface energy of the membrane, thereby significantly improving the membrane's wettability and absorption capacity for the electrolyte.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a covalently organic framework-modified cellulose nanofiber composite membrane, characterized in that, Includes the following steps: S1. Preparation of a suspension of cellulose nanofiber composites coated with an imine-type covalent organic framework: 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added to an aqueous suspension of cellulose nanofibers (CNF) and stirred until homogeneous. The mass ratio of CNF to TAPB was 2~8:

1. After stirring and mixing, a solution of 1,3,5-tricarboxyphenyl (TFB) acetic acid pre-activated with an acid mass ratio of 1:1 to 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added. The mixture was reacted at room temperature and washed to allow the imine-type covalent organic framework to be generated in situ and coated on the surface of the cellulose nanofibers, thus obtaining a suspension of cellulose nanofiber composites coated with an imine-type covalent organic framework. The mass ratio of the total mass of TAPB and TFB, which serve as the covalent organic framework, to the mass of CNF was 0.25~1:

1. S2. Functionalized linker conversion: The cellulose nanofiber composite suspension coated with animine-type covalent organic framework obtained in step S1 is mixed with anhydrous N,N-dimethylformamide (DMF), acid and oxidant. The amount of oxidant is 1 to 3 equivalents of the molar amount of imine groups in the covalent organic framework. The mixture is reacted under an inert atmosphere for 5 to 15 hours to oxidize the imine groups in the covalent organic framework to amide groups. After the reaction is completed, the mixture is washed to obtain the cellulose nanofiber composite suspension coated with an amide-type covalent organic framework. S3. The composite membrane is prepared by vacuum filtration. The cellulose nanofiber composite with amide-type covalent organic framework obtained in step S2 is ultrasonically dispersed in water to obtain a suspension. The suspension is then vacuum filtered using a sand core funnel equipped with a polytetrafluoroethylene filter membrane to obtain a wet covalent organic framework-coated cellulose nanofiber composite membrane. S4. Carbon dioxide supercritical drying of the membrane: The wet covalent organic framework-coated cellulose nanofiber composite membrane obtained in step S3, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement. After separation, it is subjected to supercritical carbon dioxide drying to obtain an amide-type covalent organic framework-coated cellulose nanofiber composite membrane.

2. A method for preparing a covalently organic framework-modified cellulose nanofiber composite membrane, characterized in that, Includes the following steps: S1′ Preparation of amine-type covalent organic framework-coated cellulose nanofiber composites: 1,3,5-tris(4-aminophenyl)benzene (TAPB) was added to an aqueous suspension of cellulose nanofibers (CNF) and stirred until homogeneous. The mass ratio of CNF to TAPB was 2~8:

1. After stirring and mixing, a mixed aqueous solution containing 10 mL of 1,3,5-tris(4-aminophenyl)benzene (TAPB) at a mass ratio of 1:1 and 10 mL of formic acid to acetic acid at a ratio of 1:0.5~1.5 was added. The mixture was reacted at 60~90℃ for 24~72 hours to generate and coat the surface of the cellulose nanofibers with amine-type covalent organic frameworks. After the reaction, the mixture was washed to obtain a suspension of amine-type covalent organic framework-coated cellulose nanofiber composites. The mass ratio of the total mass of TAPB and TFB as covalent organic frameworks to CNF was 0.25~1:

1. S2′, The composite membrane is prepared by vacuum filtration. The cellulose nanofiber composite with amine covalent organic framework coated in step S1′ is ultrasonically dispersed in water to obtain a suspension. The suspension is then vacuum filtered using a sand core funnel equipped with a polytetrafluoroethylene filter membrane to obtain a wet covalent organic framework coated cellulose nanofiber composite membrane. S3′, Supercritical carbon dioxide drying of the membrane: The wet covalent organic framework-coated cellulose nanofiber composite membrane obtained in step S2′, together with the filter membrane, is placed in anhydrous ethanol for solvent replacement. After separation, it is subjected to supercritical carbon dioxide drying to obtain an amine-containing covalent organic framework-coated cellulose nanofiber composite membrane.

3. The method for preparing the cellulose nanofiber composite membrane as described in claim 1, characterized in that, In step S1, the cellulose nanofiber composite coated with a covalent organic framework is washed sequentially with deionized water, acetone, and N,N-dimethylformamide (DMF) for three rounds. In step S2, the oxidant is Oxone, and the amount of oxidant used is 2 to 2.25 molar equivalents of the imine groups in the covalent organic framework. The reaction time is 10 to 12 hours to ensure that the oxidant diffuses fully in the COF channels and achieves uniform amidation transformation from the outside to the inside, while avoiding excessive oxidation and framework damage caused by excessive amount or time. After the reaction in step S2, the cellulose nanofiber composite coated with an amide-type covalent organic framework is washed sequentially with 10% sodium thiosulfate solution, tetrahydrofuran, n-hexane, acetone, and water for three rounds.

4. The method for preparing the cellulose nanofiber composite membrane as described in claim 2, characterized in that, In step S1′, the ratio of formic acid to acetic acid solution is 4.

0. Formic acid and 4.0 Acetic acid is used, with the ratio of formic acid to acetic acid controlled at 1:1 to ensure that acetic acid can catalyze the rapid and reversible condensation of COF and the slow reduction of formic acid. In step S1′, the reaction temperature is controlled at 75~85℃ and the reaction time is controlled at 45~50 hours to ensure that the thermodynamic driving and kinetic processes for the continuous, uniform and controllable formation of COF are compatible and that the reduction reaction is sufficient. At the same time, it avoids insufficient reaction leading to internal and external differences or excessive reaction causing degradation of the cellulose matrix. After the reaction in step S1′ is completed, the cellulose nanofiber complex coated with the amine-type covalent organic framework is washed sequentially with deionized water, acetone and deionized water, for three rounds of washing.

5. The method for preparing the cellulose nanofiber composite membrane according to claims 1-2, characterized in that, During the mixing process of steps S1 and S1′, the stirring speed is controlled at 500~800 rpm to ensure the uniformity of mixing of monomer molecules with CNF particles that have differences in specific gravity, and subsequently uniform nucleation and coating. During the mixing process of step S2, the stirring speed must be reduced to 50~100 rpm to ensure that the shear force does not damage the diffusion ability of the formed material to penetrate into the hollow microstructure and to ensure the stability and uniformity of the transformation process.

6. The method for preparing the cellulose nanofiber composite membrane according to claims 1-2, characterized in that, In steps S3 and S2′, the covalent organic framework-coated cellulose nanofiber composite is ultrasonically dispersed in water to obtain a suspension. The ultrasonic power is controlled at 600W and the duty cycle is 75%, i.e., working for 3 seconds and stopping for 1 second. The ultrasonic duration is 5 to 20 minutes to ensure the uniformity of the composite fiber dispersion and improve the smoothness and pore structure uniformity of the filtration film, while avoiding excessive ultrasonication that could damage the fibers or coating layer.

7. The method for preparing the cellulose nanofiber composite membrane according to claims 1-2, characterized in that, In steps S4 and S3′, the wet covalently organic framework-coated cellulose nanofiber composite membrane is immersed in anhydrous ethanol for solvent replacement for 2–4 hours. The wet covalently organic framework-coated cellulose nanofiber composite membrane is then clamped in a nylon film and placed in a supercritical drying vessel. After maintaining the vessel at 40–50 °C and 8–9.5 MPa for 4–5 hours, the pressure is slowly released at a rate of 0.5–1.0 MPa per minute to ensure supercritical drying. The ethanol in the pores is fully replaced and discharged to eliminate capillary forces, thereby maintaining the integrity of the multi-level pore structure and avoiding rapid pressure release that could cause pore collapse or membrane cracking.

8. The covalently organic framework-modified cellulose nanofiber composite membrane prepared by the method of preparing the cellulose nanofiber composite membrane according to any one of claims 1 to 2.

9. The application of the covalent organic framework modified cellulose nanofiber composite separator as described in claim 8 in lithium-ion batteries and lithium metal battery separators.

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