A diaphragm and its preparation method and application

Abstract

The present invention discloses a diaphragm and its preparation method and application. The diaphragm includes a stacked polymer layer and a covalent organic framework layer; wherein the polymer layer includes polar polymer nanofibers and an ionic polymer coated on the surface of the polar polymer nanofibers; the covalent organic framework layer includes an ion-functionalized covalent organic framework material; the ionic groups of the ionic polymer and the ion-functionalized covalent organic framework material have the same or different charges. The present invention constructs a double-layer diaphragm by combining a polar polymer with an ionic group with a covalent organic framework with an ionic group, thereby regulating the surface charge of the diaphragm, thereby regulating the lithium ion transmission behavior in the electrolyte, inhibiting the growth of lithium dendrites, and facilitating the preparation of high-performance lithium metal batteries without lithium dendrites.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery separators, and in particular relates to a separator and a preparation method and application thereof. Background Art

[0002] With the energy crisis and environmental pollution becoming increasingly serious, renewable clean energy and energy storage systems have become urgent needs in contemporary society. Lithium batteries stand out among other batteries due to their light weight, low cost, high energy density, long cycle life, and low self-discharge rate. The key components of a lithium battery include a positive electrode, a negative electrode, a separator, and an electrolyte. The separator, placed between the positive and negative electrodes, primarily prevents direct contact and short circuits, while also providing a diffusion path for ion migration.

[0003] Lithium metal anodes suffer from a serious problem of dendrite growth. The increased surface area of ​​dendrites triggers side reactions, leading to consumption of lithium active materials and electrolyte, formation of dead lithium, and volume expansion, ultimately reducing the battery's Coulombic efficiency and even piercing the separator, causing a short circuit. The dendrite phenomenon consists of a nucleation and growth process, which occur in tandem and are accompanied by electrolyte consumption. There are two main approaches to suppressing dendrites: one is to slow down the nucleation process. Research has found that dendrite nucleation is closely related to the migration of lithium ions and anions. Increasing lithium ion migration and reducing anion migration can effectively prolong dendrite nucleation time. The other is to inhibit dendrite growth, promoting uniform lithium deposition and thus suppressing dendrite growth. To date, the main strategies to suppress dendrite growth and improve the performance of lithium metal batteries include constructing an artificial SEI, manipulating the anode or cathode structure, and modifying the electrolyte composition. These strategies primarily focus on the electrodes and electrolyte, while less research has focused on modifying the separator structure to regulate ion transport and lithium deposition behavior. Although the separator does not directly participate in the reaction inside the battery, its structure and characteristics determine the performance of the battery. The uniform and rapid transport of ions in the separator is crucial to suppressing the growth of lithium dendrites.

[0004] Current commercial polyolefin separators have the following main problems: (1) low melting point, poor thermal stability, and easy thermal shrinkage, which can cause short circuits between the positive and negative electrodes; (2) low porosity and poor electrolyte wettability, resulting in low separator ion conductivity and difficulty in improving battery performance; (3) uneven lithium ion transmission, low lithium ion migration number, and severe dendrite growth at the lithium negative electrode, which affects battery safety performance. Patent CN118546356A discloses a method for preparing a water-soluble aromatic polyamide separator, and Patent CN118546411A discloses a highly heat-resistant and flame-retardant terminal aldehyde silicone oil-modified chitosan lithium ion battery separator. These polar polymer-based separators have good thermal stability and lyophilicity, but need to be improved in regulating the uniform transmission of lithium ions. CN110739427A discloses a covalent organic framework material for lithium battery separator coatings, and CN114976476A discloses a covalent organic framework-based separator with excellent wettability and a high lithium ion transference number, enabling uniform lithium deposition. However, there is less research on ionizing separator materials to regulate lithium ion transport behavior. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a diaphragm and its preparation method and application, so as to solve the problem that the current diaphragm materials cannot effectively inhibit the growth of lithium dendrites.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] In a first aspect, the present invention provides a membrane comprising a stacked polymer layer and a covalent organic framework layer; wherein the polymer layer comprises polar polymer nanofibers and an ionic polymer coated on the surface of the polar polymer nanofibers; the covalent organic framework layer comprises an ion-functionalized covalent organic framework material; the ionic groups of the ionic polymer and the ion-functionalized covalent organic framework material have the same or different charges.

[0008] Preferably, the mass ratio of the polymer layer to the covalent organic framework layer is (15-30):1.

[0009] Preferably, the thickness of the polymer layer is 10-60 μm; the thickness of the covalent organic framework layer is 2-25 μm; and the grafting rate of the ionic groups in the ion-functionalized covalent organic framework material is ≥10%.

[0010] Preferably, the polar polymer nanofiber includes one or more of para-aramid, meta-aramid, polyimide, poly(p-phenylenebenzobisoxazole) fiber, and poly(2,5-dihydroxy-1,4-phenylenepyridin-diimidazole) fiber.

[0011] Preferably, the ionomer is obtained by copolymerizing a compound having an ionic group with a crosslinking agent.

[0012] Preferably, the ion-functionalized covalent organic framework material includes one or more of a sulfonate-functionalized flower-shaped hollow covalent organic framework material, a quaternary ammonium salt-functionalized flower-shaped hollow covalent organic framework material, and a carboxyl-functionalized flower-shaped hollow covalent organic framework material.

[0013] In a second aspect, the present invention provides a method for preparing a diaphragm, comprising the following steps:

[0014] Providing a polar polymer nanofiber membrane; soaking the polar polymer nanofiber membrane in a mixed solution containing a compound with an ionic group, a crosslinking agent, and an initiator; then sandwiching the soaked polar polymer nanofiber membrane between two substrates to perform an intra-membrane copolymerization reaction to obtain a polymer membrane with an ionic polymer coated on the fiber surface;

[0015] Providing a covalent organic framework material containing a double bond; mixing the covalent organic framework material containing a double bond with a thiol compound having an ionic group and a photoinitiator, and obtaining an ion-functionalized covalent organic framework material through a thiol-ene click reaction;

[0016] Providing an ion-functionalized covalent organic framework material dispersion comprising an ion-functionalized covalent organic framework material, a binder, and a solvent;

[0017] The vacuum filtration method is adopted to deposit the ion functionalized covalent organic framework material dispersion on the polymer film coated with the ionic polymer on the fiber surface to obtain the diaphragm.

[0018] Preferably, the mass ratio of the compound with an ionic group to the cross-linking agent is (6-9):1, and the molar ratio of the compound with an ionic group to the initiator is (50-150):1; the cross-linking agent is one or more of polyethylene glycol diacrylate, polypropylene glycol diacrylate, and polybutylene glycol diacrylate; and the initiator is one or more of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, dibenzoyl peroxide, and N,N-dimethylaniline.

[0019] Preferably, the polar polymer nanofiber membrane is immersed in the mixed solution for 10 to 120 seconds; the copolymerization temperature is 60 to 100°C, and the reaction time is 10 to 36 hours.

[0020] Preferably, the preparation method of the polar polymer nanofiber membrane comprises the following steps: mixing polar polymer nanofibers, a strong base, a proton transfer agent and a solvent to obtain a polar polymer nanofiber dispersion; coating the polar polymer nanofiber dispersion on the surface of a substrate, and obtaining a polar polymer nanofiber membrane by a phase inversion method.

[0021] Preferably, the thiol compound with an ionic group is selected from one or more of sodium 2-mercaptoethane sulfonate, sodium 3-mercaptopropane sulfonate, cysteamine hydrochloride, 3-aminopropanethiol hydrochloride, 2-mercaptoacetic acid, 3-mercaptopropionic acid, and 2-mercaptobutyric acid; the photoinitiator is selected from one or more of 2,2-dihydroxymethylpropionic acid and 2,2-dimethoxy-2-phenylacetophenone; and the mass ratio of the covalent organic framework material containing a double bond to the ionic compound containing a thiol group and the photoinitiator is 1: (15~20): 1.

[0022] Preferably, the mass ratio of the ion-functionalized covalent organic framework material to the binder is (3-10):1.

[0023] Preferably, in the vacuum filtration method, the negative pressure formed by vacuum filtration is 0.02~0.05 MPa.

[0024] In a third aspect, the present invention further provides an application of the above-mentioned diaphragm in a lithium battery, wherein the polymer layer of the diaphragm is arranged to face the positive electrode, and the covalent organic framework layer of the diaphragm is arranged to face the negative electrode of the lithium battery.

[0025] The beneficial effects of the present invention are:

[0026] The present invention combines a polar polymer with ionic groups with a covalent organic framework with ionic groups to construct a double-layer membrane, thereby regulating the surface charge of the membrane, thereby adjusting the lithium ion transport behavior in the electrolyte and inhibiting the growth of lithium dendrites, which is conducive to the preparation of high-performance lithium dendrite-free lithium metal batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an SEM image of the surface of the polymer layer of the double-layer diaphragm prepared in Example 1;

[0028] Figure 2 This is a SEM image of the surface of the covalent organic framework layer of the double-layer membrane prepared in Example 1;

[0029] Figure 3 This is an SEM image of the cross section of the double-layer diaphragm prepared in Example 1;

[0030] Figure 4 The Li||Li symmetric cell assembled with Example 1 and commercial PE separator was tested at 0.5 mA / cm 2 The current density is 0.5 mAh / cm 2 Lithium deposition curve obtained by capacity cycling;

[0031] Figure 5 The specific capacity-cycle number curves of lithium iron phosphate batteries assembled with Example 1, Example 2, and a commercial PE separator were obtained by cycling 1000 times at a cycle rate of 1C;

[0032] Figure 6 This is the chronoamperometric curve of the double-layer diaphragm prepared in Example 1. DETAILED DESCRIPTION

[0033] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0034] It should be noted that, unless otherwise specified in this application, except for the covalent organic framework material containing a double bond, which needs to be synthesized according to the literature method, the remaining raw materials used and the raw materials for synthesizing the covalent organic framework material containing a double bond are all commercially available products. The covalent organic framework material containing a double bond can be prepared according to the synthesis scheme in the document Chem. Commun., 2021, 57, 7362-7365.

[0035] An embodiment of the present invention provides a diaphragm, comprising a stacked polymer layer and a covalent organic framework layer; wherein the polymer layer comprises polar polymer nanofibers and an ionic polymer coated on the surface of the polar polymer nanofibers; the covalent organic framework layer comprises an ion-functionalized covalent organic framework material; the ionic groups of the ionic polymer and the ion-functionalized covalent organic framework material have the same or different charges.

[0036] In some preferred embodiments, the mass ratio of the polymer layer to the covalent organic framework layer is (15-30):1.

[0037] In some preferred embodiments, the thickness of the polymer layer is 10-60 μm; the thickness of the covalent organic framework layer is 2-25 μm; and the grafting rate of the ionic groups in the ion-functionalized covalent organic framework material is ≥10%, more preferably 15%-80%.

[0038] In some preferred embodiments, the polar polymer nanofibers include one or more of para-aramid, meta-aramid, polyimide, poly(p-phenylene benzobisoxazole) fibers, and poly(2,5-dihydroxy-1,4-phenylene pyridinium diimidazole) fibers; the ionic polymer is obtained by copolymerizing a compound with an ionic group with a crosslinking agent; the compound with an ionic group includes but is not limited to one or more of 3-sulfopropyl acrylate potassium salt, β-(acryloyloxy)propionic acid, and propyleneoxyethyltrimethylammonium chloride; the ion-functionalized covalent organic framework material includes one or more of sulfonate-functionalized flower-shaped hollow covalent organic framework material, quaternary ammonium salt-functionalized flower-shaped hollow covalent organic framework material, and carboxyl-functionalized flower-shaped hollow covalent organic framework material.

[0039] The present invention also provides a method for preparing the above-mentioned diaphragm, comprising the following steps:

[0040] S1. Preparation of Polar Polymer Nanofiber Membranes

[0041] S1.1 Mix polar polymer nanofibers, a strong base, a proton transfer agent, and a solvent, and stir at 40-100°C for 1-12 h to obtain a polar polymer nanofiber dispersion. The strong base is selected from one or more of potassium tert-butoxide, potassium hydroxide, sodium hydroxide, and lithium hydroxide; the proton transfer agent is selected from one or more of methanol, ethanol, n-butanol, and water; and the solvent is selected from one or more of dimethyl sulfoxide, 1-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. The mass ratio of the polar polymer nanofibers, the strong base, and the proton transfer agent is 1:(1-2):(1-2); and the mass ratio of the polar polymer nanofibers to the solvent is 1:(17-95).

[0042] S1.2 Coating the polar polymer nanofiber dispersion onto a substrate surface to form a polar polymer nanofiber membrane by a phase inversion method; wherein the coating method is blade coating, spin coating, or spray coating, and the coating thickness is preferably 100-300 μm; the substrate is a smooth substrate, preferably glass, steel plate, polytetrafluoroethylene plate, or roller. The solvent used for the phase inversion is selected from one or more of deionized water, ethanol, isopropanol, ethyl acetate, methanol, and n-hexane.

[0043] S2. Preparation of polymer membranes

[0044] The polar polymer nanofiber membrane is immersed in a mixed solution containing a compound with an ionic group, a cross-linking agent, an initiator and a solvent and allowed to stand for 10 to 120 seconds. The soaked polar polymer nanofiber membrane is then sandwiched between two substrates and copolymerized at 60 to 100°C for 10 to 36 hours to coat the surface of the polar polymer nanofiber membrane with the ionic polymer, thereby obtaining a polymer membrane with the fiber surface coated with the ionic polymer. The crosslinking agent is one or more of polyethylene glycol diacrylate, polypropylene glycol diacrylate, and polybutylene glycol diacrylate; the initiator is one or more of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, dibenzoyl peroxide, and N,N-dimethylaniline; the solvent is one or more of dimethyl sulfoxide, 1-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide; the mass ratio of the compound having an ionic group to the crosslinking agent is (6-9):1, and the molar ratio of the compound having an ionic group to the initiator is (50-150):1.

[0045] S3. Preparation of Ion-Functionalized Covalent Organic Frameworks

[0046] A covalent organic framework material containing double bonds is provided; the covalent organic framework material containing double bonds is mixed with a thiol compound having ionic groups and a photoinitiator, and the ionic groups are modified on the covalent organic framework via a thiol-ene click reaction to obtain an ion-functionalized covalent organic framework material. The thiol compound having ionic groups is selected from one or more of sodium 2-mercaptoethane sulfonate, sodium 3-mercaptopropane sulfonate, cysteamine hydrochloride, 3-aminopropanethiol hydrochloride, 2-mercaptoacetic acid, 3-mercaptopropionic acid, and 2-mercaptobutyric acid; the photoinitiator is selected from one or more of 2,2-dihydroxymethylpropionic acid and 2,2-dimethoxy-2-phenylacetophenone; and the mass ratio of the covalent organic framework material containing double bonds to the thiol compound having ionic groups and the photoinitiator is 1:(15-20):1. The conditions for the thiol-ene click reaction are: irradiating a mixture of a covalent organic framework material containing double bonds, a thiol compound with an ionic group, and a photoinitiator under ultraviolet light in the 365 nm band for 1 to 5 hours.

[0047] S4. Preparation of Ion-Functionalized Bilayer Membranes

[0048] S4.1 Mixing an ion-functionalized covalent organic framework material, a binder, and a solvent to obtain a uniformly dispersed ion-functionalized covalent organic framework material dispersion. The binder is selected from one or more of polyvinylidene fluoride, sodium carboxymethyl cellulose, polyacrylic acid, polyvinyl alcohol, hyperbranched polyethyleneimine, polyamide imide, ammonium polyphosphate, sulfonated polyetheretherketone, polyvinyl pyrrolidone, perfluorosulfonic acid-polytetrafluoroethylene copolymer, aramid nanofibers, and cellulose nanofibers; the solvent is selected from one or more of dimethyl sulfoxide, 1-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide; and the mass ratio of the ion-functionalized covalent organic framework material to the binder is (3-10):1.

[0049] S4.2 Using the vacuum filtration method, under a negative pressure of 0.02~0.05 MPa, the ion-functionalized covalent organic framework material dispersion is evenly dispersed on the surface of the polymer membrane coated with the ionic polymer on the fiber surface to form a covalent organic framework layer, and then an ion-functionalized double-layer membrane is obtained.

[0050] The polymer layer in the ion-functionalized double-layer separator prepared by the present invention is formed by coating an ionic polymer on the surface of polar polymer fibers, which can hinder the migration of anions and promote the selective migration of lithium ions. The covalent organic framework layer has a uniform porous structure and charged groups, which further promotes the uniform and rapid migration of lithium ions and inhibits the growth of lithium dendrites. Therefore, the double-layer separator can effectively improve the overall performance of lithium batteries when used in lithium batteries.

[0051] The present invention is described in detail below with reference to specific embodiments.

[0052] Example 1

[0053] An ion-functionalized double-layer membrane consists of a polymer layer and a covalent organic framework layer, wherein the polymer layer is composed of para-aramid and a copolymer of potassium 3-sulfopropyl acrylate and polyethylene glycol diacrylate, and the covalent organic framework layer is composed of a flower-shaped hollow covalent organic framework material functionalized with the binder polyvinylidene fluoride and sodium 2-mercaptoethane sulfonate; the thickness of the polymer layer is 40±5 μm, and the thickness of the covalent organic framework layer is 10±0.5 μm.

[0054] The preparation method of the ion-functionalized double-layer diaphragm comprises the following steps:

[0055] (1) Para-aramid, potassium tert-butoxide, anhydrous methanol, and dimethyl sulfoxide were mixed in a mass ratio of 3:3:3:91 and stirred at 80 °C for 1 h to obtain a para-aramid nanofiber dispersion. The para-aramid nanofiber dispersion was then scraped onto a smooth glass surface with a film thickness of 200 μm and phase-converted in deionized water to obtain an aramid wet film.

[0056] (2) The aramid wet film obtained in step (1) is immersed in a mixed solution of potassium salt of 3-sulfopropyl acrylate, polyethylene glycol diacrylate and azobisisobutyronitrile for about 10 seconds, wherein the mass ratio of potassium salt of 3-sulfopropyl acrylate and polyethylene glycol diacrylate is 7.5:1, and the molar ratio of potassium salt of 3-sulfopropyl acrylate and azobisisobutyronitrile is 100:1. The film is then sandwiched between two parallel smooth glass pieces and reacted at 80°C for 12 hours. After washing with anhydrous ethanol, a polymer layer of an ionic polymer containing sulfonate groups coated on the surface of the aramid nanofiber is obtained.

[0057] (3) Referring to the synthesis scheme in Chem. Commun., 2021, 57, 7362-7365, 1,3,5-tris(4-aminophenyl)benzene and 2,5-divinylbenzene-1,4-benzenedicarboxaldehyde were selected in a molar ratio of 2:3 to synthesize a covalent organic framework material containing a double bond. The covalent organic framework material containing a double bond, sodium 2-mercaptoethane sulfonate, and 2,2-dihydroxymethylpropionic acid were mixed in a mass ratio of 1:15:1 and irradiated under ultraviolet light at 365 nm for 1.5 h. After washing with dimethyl sulfoxide and anhydrous ethanol, a covalent organic framework material with a sulfonate grafting rate of 45% was obtained.

[0058] (4) The sulfonate-grafted covalent organic framework material obtained in step (3), polyvinylidene fluoride, and 1-methyl-2-pyrrolidone are mixed, wherein the mass ratio of the sulfonate-grafted covalent organic framework material to polyvinylidene fluoride is 3:1, to obtain a uniform dispersion of the covalent organic framework material. The covalent organic framework material is uniformly dispersed on the surface of the polymer layer base membrane obtained in step (2) by vacuum filtration at a negative pressure of 0.04 MPa, forming a sulfonate-functionalized covalent organic framework layer on the surface of the polymer membrane layer; and at the same time, a double-layer membrane comprising an ion polymer layer and an ion-functionalized covalent organic framework layer is obtained.

[0059] Example 2

[0060] An ion-functionalized double-layer membrane consists of a polymer layer and a covalent organic framework layer, wherein the polymer layer is composed of para-aramid and a copolymer of potassium 3-sulfopropyl acrylate and polyethylene glycol diacrylate, and the covalent organic framework layer is composed of a flower-shaped hollow covalent organic framework material functionalized with binder polyvinylidene fluoride and cysteamine hydrochloride; the polymer layer thickness is 20±5 μm, and the covalent organic framework layer thickness is 2.5±0.5 μm.

[0061] The preparation method of the ion-functionalized double-layer diaphragm comprises the following steps:

[0062] (1) Para-aramid, potassium tert-butoxide, anhydrous methanol, and dimethyl sulfoxide were mixed in a mass ratio of 1:1:1:10 and stirred at 60 °C for 2 h to obtain a para-aramid nanofiber dispersion. The para-aramid nanofiber dispersion was then scraped onto a smooth glass surface with a film thickness of 100 μm and phase-inverted in deionized water to obtain an aramid wet film.

[0063] (2) The aramid wet film obtained in step (1) is immersed in a mixed solution of potassium salt of 3-sulfopropyl acrylate, polyethylene glycol diacrylate and azobisisobutyronitrile for about 60 seconds, wherein the mass ratio of potassium salt of 3-sulfopropyl acrylate and polyethylene glycol diacrylate is 7:1, and the molar ratio of potassium salt of 3-sulfopropyl acrylate and azobisisobutyronitrile is 50:1. The film is then sandwiched between two parallel smooth glass pieces and reacted at 70°C for 16 hours. After washing with anhydrous ethanol, a polymer layer of an ionic polymer containing sulfonate groups coated on the surface of the aramid nanofiber is obtained.

[0064] (3) Referring to the synthesis scheme in Chem. Commun., 2021, 57, 7362-7365, 1,3,5-tris(4-aminophenyl)benzene and 2,5-divinylbenzene-1,4-benzenedicarboxaldehyde were selected in a molar ratio of 2:3 to synthesize a covalent organic framework material containing a double bond. The covalent organic framework material containing a double bond, cysteamine hydrochloride, and 2,2-dimethoxy-2-phenylacetophenone were mixed in a mass ratio of 1:17:1 and irradiated under ultraviolet light at 365 nm for 3 h. After washing with dimethyl sulfoxide and anhydrous ethanol, a covalent organic framework material with a quaternary ammonium salt grafting rate of 30% was obtained.

[0065] (4) The quaternary ammonium salt grafted covalent organic framework material obtained in step (3), polyvinylidene fluoride, and N, N-dimethylformamide are mixed, wherein the mass ratio of the quaternary ammonium salt grafted covalent organic framework material to polyvinylidene fluoride is 4:1, to obtain a uniform dispersion of the covalent organic framework material. The covalent organic framework material is uniformly dispersed on the surface of the polymer layer base film obtained in step (2) by vacuum filtration under a negative pressure of 0.03 MPa, forming a quaternary ammonium salt functionalized covalent organic framework layer on the surface of the polymer film layer, and at the same time, obtaining a double-layer membrane comprising an ionic polymer layer and an ionic functionalized covalent organic framework layer.

[0066] Example 3

[0067] An ion-functionalized double-layer membrane consists of a polymer layer and a covalent organic framework layer, wherein the polymer layer is composed of meta-aramid and a copolymer of propyleneoxyethyltrimethylammonium chloride and polypropylene glycol diacrylate, and the covalent organic framework layer is composed of a flower-shaped hollow covalent organic framework material functionalized with the binder sodium carboxymethyl cellulose and 2-mercaptoacetic acid; the thickness of the polymer layer is 30±5 μm, and the thickness of the covalent organic framework layer is 5±0.5 μm.

[0068] The preparation method of the ion-functionalized double-layer diaphragm comprises the following steps:

[0069] (1) Meta-aramid, potassium hydroxide, anhydrous ethanol, and N, N-dimethylformamide were mixed in a mass ratio of 1:1:1:50 and stirred at 80 °C for 12 h to obtain a meta-aramid nanofiber dispersion. The meta-aramid nanofiber dispersion was then scraped onto a smooth glass surface with a film thickness of 300 μm. Phase inversion occurred in isopropyl alcohol to obtain an aramid wet film.

[0070] (2) The aramid wet film obtained in step (1) is immersed in a mixed solution of propylene glycol trimethyl ammonium chloride, polypropylene glycol diacrylate, and dimethyl azobisisobutyrate for about 120 seconds, wherein the mass ratio of propylene glycol trimethyl ammonium chloride and polypropylene glycol diacrylate is 9:1, and the molar ratio of propylene glycol trimethyl ammonium chloride and azobisisobutyronitrile is 125:1. The film is then sandwiched between two parallel smooth glass plates and reacted at 90°C for 10 hours. After washing with anhydrous ethanol, a polymer layer of an ionic polymer containing a quaternary ammonium salt coated on the surface of the aramid nanofiber is obtained.

[0071] (3) Referring to the synthesis scheme in Chem. Commun., 2021, 57, 7362-7365, a covalent organic framework material containing a double bond was synthesized using 1,3,5-tris(4-aminophenyl)benzene and 2,5-bis(allyloxy)terephthalaldehyde in a molar ratio of 2:3. The covalent organic framework material containing a double bond, 2-mercaptoacetic acid, and 2,2-dihydroxymethylpropionic acid were mixed in a mass ratio of 1:18:1 and irradiated under ultraviolet light at 365 nm for 5 h. After washing with dimethyl sulfoxide and anhydrous ethanol, a covalent organic framework material with a carboxyl grafting rate of 65% was obtained.

[0072] (4) The carboxyl-grafted covalent organic framework material obtained in step (3), sodium carboxymethyl cellulose, and N, N-dimethylacetamide are mixed, wherein the mass ratio of the carboxyl-grafted covalent organic framework material to the sodium carboxymethyl cellulose is 6:1, to obtain a uniform dispersion of the covalent organic framework material. The covalent organic framework material is then uniformly dispersed on the surface of the polymer layer base film obtained in step (2) by vacuum filtration at a negative pressure of 0.05 MPa, forming a carboxyl-functionalized covalent organic framework layer on the surface of the polymer film layer. At the same time, a double-layer membrane comprising an ionic polymer layer and an ionic functionalized covalent organic framework layer is obtained.

[0073] Example 4

[0074] An ion-functionalized double-layer membrane consists of a polymer layer and a covalent organic framework layer, wherein the polymer layer is composed of polyimide and a copolymer of β-(acryloyloxy)propionic acid and polybutylene glycol diacrylate, and the covalent organic framework layer is composed of a covalent organic framework material grafted with a binder polyacrylic acid and 3-aminopropanethiol hydrochloride; wherein the polymer layer thickness is 55±5 μm, and the covalent organic framework layer thickness is 15±0.5 μm.

[0075] The preparation method of the ion-functionalized double-layer diaphragm comprises the following steps:

[0076] (1) Polyimide, sodium hydroxide, n-butanol, and N, N-dimethylacetamide were mixed in a mass ratio of 1:2:2:40 and stirred at 60 °C for 12 h to obtain a polyimide nanofiber dispersion. The polyimide nanofiber dispersion was then scraped onto a smooth glass surface with a film thickness of 250 μm and phase-inverted in n-hexane to obtain a polyimide wet film.

[0077] (2) The wet film obtained in step (1) was immersed in a mixed solution of β-(acryloyloxy)propionic acid, polybutylene glycol diacrylate and dibenzoyl peroxide for about 90 seconds, wherein the mass ratio of β-(acryloyloxy)propionic acid to polyethylene glycol diacrylate was 6.5:1, and the molar ratio of β-(acryloyloxy)propionic acid to dibenzoyl peroxide was 75:1. The wet film was then sandwiched between two parallel smooth glass plates and reacted at 100°C for 24 hours. After washing with anhydrous ethanol, a polymer layer of a carboxyl-containing ionic polymer coated on the surface of the polyimide nanofiber was obtained.

[0078] (3) Referring to the synthesis scheme in Chem. Commun., 2021, 57, 7362-7365, a covalent organic framework material containing a double bond was synthesized using 1,3,5-tris(4-aminophenyl)benzene and 2,5-bis(allyloxy)terephthalaldehyde in a molar ratio of 2:3. The covalent organic framework material containing a double bond, 3-aminopropanethiol hydrochloride, and 2,2-dihydroxymethylpropionic acid were mixed in a mass ratio of 1:19:1 and irradiated under ultraviolet light at 365 nm for 1 h. After washing with dimethyl sulfoxide and anhydrous ethanol, a covalent organic framework material with a quaternary ammonium salt grafting rate of 10% was obtained.

[0079] (4) The quaternary ammonium salt grafted covalent organic framework material obtained in step (3), polyvinyl alcohol, and 1-methyl-2-pyrrolidone are mixed, wherein the mass ratio of the quaternary ammonium salt grafted covalent organic framework material to the polyvinyl alcohol is 4:1, to obtain a uniform dispersion of the covalent organic framework material. The covalent organic framework material is uniformly dispersed on the surface of the polymer layer base film obtained in step (2) by vacuum filtration under a negative pressure of 0.02 MPa, forming a quaternary ammonium salt functionalized covalent organic framework layer on the surface of the polymer film layer, and at the same time, a double-layer membrane comprising an ionic polymer layer and an ionic functionalized covalent organic framework layer is obtained.

[0080] Figure 1 This is an SEM image of the polymer layer surface of the double-layer membrane prepared in Example 1. It can be seen that the surface of the polymer layer is smooth. The polymer layer can improve the thermal stability of the membrane, and can effectively hinder the migration of anions, accelerate the desolvation process during lithium ion transmission, and realize the selective migration of lithium ions.

[0081] Figure 2This is an SEM image of the surface of the covalent organic framework layer of the double-layer membrane prepared in Example 1. It can be seen that one of the surfaces of the aramid fiber is covered with a flower-shaped covalent organic framework material. The covalent organic framework layer has a uniform porous hollow structure and an ion-functionalized framework, which can effectively promote the rapid and uniform migration of lithium ions and inhibit the growth of lithium dendrites.

[0082] Figure 3 This is an SEM image of the cross section of the double-layer membrane prepared in Example 1. It can be seen that the obtained membrane has a polymer layer and a covalent organic framework layer stacked together, and the polymer layer has a three-dimensional network structure constructed by nanofibers.

[0083] Figure 4 The Li||Li symmetric cells assembled with Example 1 and commercial PE separators were tested at 0.5 mA / cm 2 The current density is 0.5 mAh / cm 2 From the lithium deposition curve obtained by the capacity cycle, it can be seen that the Li||Li symmetric battery assembled in Example 1 has a stable voltage and a small overvoltage value, indicating that the double-layer separator provided by the present invention has a good effect of inhibiting the growth of lithium dendrites.

[0084] Figure 5 The specific capacity-cycle number curves of lithium iron phosphate batteries assembled with Example 1, Example 2 and commercial PE membrane were obtained by cycling 1000 times at a cycle rate of 1C; it can be seen that the batteries assembled with Example 1 and Example 2 have relatively high coulombic efficiency and cycle stability, which is attributed to the selection of lithium ions and accelerated migration process by the ion-functionalized double-layer membrane.

[0085] Figure 6 The chronoamperometric curves for the double-layer separator prepared in Example 1 are shown in the inset, with the electrochemical impedance spectroscopy before and after polarization. The assembled battery model is CR2032, with both the positive and negative electrodes being lithium metal. The electrolyte solvent composition is DEC / EC (diethyl carbonate / ethylene carbonate) = 1:1 (volume ratio), containing 1 mol / L LiPF6. The lithium ion transference number, which represents the ratio of lithium ion mobility to total ion mobility, was measured according to the Bruce-Vincent method and calculated using formula (1):

[0086]

[0087] where t Li+ is the lithium ion migration number, ΔV is the polarization voltage (V), I o is the initial current (A), R o is the interfacial impedance of the battery before polarization, I s is the steady-state current (A), R sThe calculated lithium ion migration number for Example 1 is 0.71, which is greater than that of commercial PE separators (<0.4). This indicates that the double-layer separator provided by the present invention can promote the selective and rapid migration of lithium ions, effectively improving the cycle and capacity performance of lithium batteries.

[0088] In summary, the present invention provides an ion-functionalized double-layer membrane consisting of a polymer layer and a covalent organic framework layer, which has the following advantages:

[0089] (1) In the ion-functionalized double-layer separator provided by the present invention, the polymer layer carries positively and negatively charged ionic groups. These charged functional groups can promote the rapid desolvation of lithium ions and accelerate the migration of lithium ions, thereby increasing the lithium ion flux of the separator; or provide a large number of anion adsorption sites, inhibiting the migration of anions and thereby selectively and rapidly transporting lithium ions. The uniform and porous framework structure of the covalent organic framework layer can promote the uniform deposition of lithium. At the same time, its abundant ionic groups can enhance the affinity between the separator and the electrolyte, effectively bind anions, and synergistically promote the rapid migration of lithium ions.

[0090] (2) The ion-functionalized double-layer membrane provided by the present invention, when the polymer layer is negatively charged and the covalent organic framework layer is positively charged, the asymmetric membrane with opposite charges can effectively achieve selective lithium ion transport and promote uniform lithium ion deposition. The charged groups contained in the covalent organic framework material in the covalent organic framework layer have opposite electrical properties to those of the ionic polymer in the polymer layer. Through the synergistic effect between the different functional layers, the lithium ion migration number of the membrane is increased, ion accumulation is eliminated, the uniform distribution of lithium ions is promoted, and the growth of lithium dendrites is inhibited.

[0091] (3) The type and content of the charged groups contained in the double-layer diaphragm provided by the present invention are adjustable and evenly distributed on the surface of the diaphragm, which can effectively interact with the electrolyte and promote the selective and rapid migration of lithium ions.

[0092] (4) The double-layer separator provided by the present invention utilizes polar polymer nanofibers with high heat resistance. The resulting separator has excellent thermal stability and can effectively avoid thermal shrinkage at high temperatures, resolving the thermal shrinkage problem of commercial polyolefin separators, thereby avoiding safety issues such as battery combustion and explosion caused by thermal runaway. Therefore, the double-layer separator provided by the present invention can effectively improve the cycle capacity and safety of lithium batteries when used in lithium batteries.

[0093] (5) The preparation method provided by the present invention is simple to operate, easy to prepare on a large scale and for continuous production, does not require the addition of special equipment, is less affected by ambient temperature and humidity, and the structure, porosity, and thickness of the diaphragm are easily controlled without the addition of a pore-forming agent. For example, the porosity of the polymer layer can be controlled by adjusting the concentration of the polar polymer nanofiber dispersion, and the porosity of the covalent organic framework layer can be controlled by adjusting the mass ratio of the binder to the covalent organic framework material.

[0094] It should be noted that the above embodiments all belong to the same inventive concept, and the description of each embodiment has its own focus. For any details not described in individual embodiments, reference may be made to the description in other embodiments.

[0095] The above-described embodiments merely illustrate the implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A diaphragm, characterized in that: It includes a polymer layer and a covalent organic framework layer arranged in a stacked manner; The polymer layer comprises polar polymer nanofibers and an ionic polymer coated on the surface of the polar polymer nanofibers; the covalent organic framework layer comprises an ion-functionalized covalent organic framework material, and the ion-functionalized covalent organic framework material is an ion-functionalized flower-shaped hollow covalent organic framework material; the ionic groups of the ionic polymer and the ion-functionalized covalent organic framework material have the same or different charges; The polar polymer nanofibers include one or more of para-aramid, meta-aramid, polyimide, poly(p-phenylene benzobisoxazole) fiber, and poly(2,5-dihydroxy-1,4-phenylene pyridinium diimidazole) fiber.

2. The diaphragm according to claim 1, characterized in that The mass ratio of the polymer layer to the covalent organic framework layer is (15-30):

1.

3. The diaphragm according to claim 1, wherein The thickness of the polymer layer is 10-60 μm; the thickness of the covalent organic framework layer is 2-25 μm; and the grafting rate of the ionic groups in the ion-functionalized covalent organic framework material is ≥10%.

4. The diaphragm according to claim 1, characterized in that The ionic polymer is obtained by copolymerizing a compound with an ionic group and a cross-linking agent; The ion-functionalized covalent organic framework material includes a sulfonate-functionalized flower-shaped hollow covalent organic framework material, a quaternary ammonium salt-functionalized flower-shaped hollow covalent organic framework material, and a carboxyl-functionalized flower-shaped hollow covalent organic framework material.

5. A method for preparing a diaphragm according to any one of claims 1 to 4, characterized in that: The following steps are involved: Providing a polar polymer nanofiber membrane; immersing the polar polymer nanofiber membrane in a mixed solution containing a compound having an ionic group, a cross-linking agent, and an initiator; Then, the polar polymer nanofiber membrane after soaking is sandwiched between two substrates to perform intra-membrane copolymerization reaction to obtain a polymer membrane with the fiber surface coated with the ionic polymer; Providing a covalent organic framework material containing a double bond; mixing the covalent organic framework material containing a double bond with a thiol compound having an ionic group and a photoinitiator, and obtaining an ion-functionalized covalent organic framework material through a thiol-ene click reaction; Providing an ion-functionalized covalent organic framework material dispersion comprising an ion-functionalized covalent organic framework material, a binder, and a solvent; The ion-functionalized covalent organic framework material dispersion is deposited on the polymer film coated with the ionic polymer on the surface of the fiber by a vacuum filtration method to obtain the diaphragm.

6. The method for preparing a diaphragm according to claim 5, wherein: The mass ratio of the compound with an ionic group to the cross-linking agent is (6-9):1, and the molar ratio of the compound with an ionic group to the initiator is (50-150):1; the cross-linking agent is one or more of polyethylene glycol diacrylate, polypropylene glycol diacrylate, and polybutylene glycol diacrylate; the initiator is one or more of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, dibenzoyl peroxide, and N,N-dimethylaniline; and the compound with an ionic group is one or more of 3-sulfopropyl acrylate potassium salt, β-(acryloyloxy)propionic acid, and propyleneoxyethyltrimethylammonium chloride.

7. The method for preparing a diaphragm according to claim 5, wherein: The polar polymer nanofiber membrane is immersed in the mixed solution for 10 to 120 seconds; the temperature of the copolymerization reaction is 60 to 100° C., and the reaction time is 10 to 36 hours.

8. The method for preparing a diaphragm according to claim 5, wherein: The mercapto compound with an ionic group is selected from one or more of sodium 2-mercaptoethane sulfonate, sodium 3-mercaptopropane sulfonate, cysteamine hydrochloride, 3-aminopropanethiol hydrochloride, 2-mercaptoacetic acid, 3-mercaptopropionic acid, and 2-mercaptobutyric acid; the photoinitiator is selected from one or more of 2,2-dihydroxymethylpropionic acid and 2,2-dimethoxy-2-phenylacetophenone; The mass ratio of the covalent organic framework material containing double bonds to the mercapto compound with ionic groups and the photoinitiator is 1: (15-20):

1.

9. The method for preparing a diaphragm according to claim 5, wherein: The mass ratio of the ion-functionalized covalent organic framework material to the binder is (3-10):1; In the vacuum filtration method, the negative pressure formed by vacuum filtration is 0.02~0.05 MPa.

10. Use of the separator according to any one of claims 1 to 4 in a lithium battery, characterized in that: The polymer layer of the separator is arranged facing the positive electrode.

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