An ionic covalent organic framework-functionalized separator and its preparation and application

Abstract

The present invention discloses a separator functionalized by ionic covalent organic frameworks and its preparation and application, which are applicable to the technical field of energy materials for lithium-sulfur batteries. The present invention provides a functional separator, which includes an electrically neutral basic separator and a double modification layer of anion- and cation-type COFs. The double modification layer is a double-layer functional structure on the positive electrode side of the basic separator, and includes an inner layer formed by self-assembly of anion- and cation-type COF nanosheets and a dense outer layer formed by stacking of anion-type COF nanosheets. The double modification layer structure introduced in the present invention realizes the functionalization of the inner and outer layers: the outer anion-type COF dense modification layer can effectively inhibit the shuttle of polysulfides through charge repulsion and physical barrier effects; in the inner layer "anion + cation" ionic COF self-assembly layer, the locally assembled positive and negative charge centers and nanopores greatly promote the desolvation of lithium ions and accelerate the lithium ion conduction, thereby improving the electrochemical performance of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technologies, and particularly to an ionic covalent organic framework functionalized modified separator and its preparation and application. Background Art

[0002] As a new energy storage technology, lithium-sulfur batteries have attracted much attention due to their high theoretical specific capacity (1675 mAh g -1 ) and theoretical energy density (2600 Wh kg -1 ), and have become a research hotspot among industry scholars.

[0003] However, in the practical application research of lithium-sulfur batteries, polysulfides generated by the sulfur cathode dissolve in the electrolyte and diffuse through the separator to the anode, thereby causing the "shuttle effect".

[0004] The specific manifestations are as follows: During the charge and discharge process, the intermediate polysulfides (Li2S x , 4 ≤ x ≤ 8) formed by the sulfur cathode are easily soluble in the organic electrolyte. Driven by the concentration gradient difference, the soluble higher-order polysulfides migrate freely back and forth between the positive and negative electrodes.

[0005] Traditional commercial separators are polyolefin materials with large pores (>100 nm), which can allow the migration of both lithium ions (positive ions) and higher-order polysulfide ions S x 2- (anions), enabling the polysulfide ions S x 2- to react with the lithium metal anode to form insoluble lower-order polysulfides, which not only causes the loss of active substances in the sulfur cathode but also seriously reduces the Coulombic efficiency of the battery.

[0006] To address the chemical behavior of polysulfides in lithium-sulfur batteries, developing an ion-selective separation membrane with lithium ion conduction and the ability to inhibit the "shuttle effect" is the key to achieving high-performance lithium-sulfur batteries.

[0007] Covalent organic framework materials (COFs) are regarded as emerging separator and separator modification materials due to their dense and ordered pores, light weight, stable chemical properties, and structural designability. By regulating the one-dimensional nanopores and functional groups of COFs, the "shuttle effect" can be effectively inhibited.

[0008] However, most of the current COF-modified separator materials are single-modified layers with a single function, and rarely consider the transport behavior of electrolyte lithium salt anions and the conduction efficiency of lithium ions, which will limit the kinetic process of the battery.

[0009] Therefore, it is highly necessary to develop COF functional composite membranes with high lithium-ion conduction performance and the property of suppressing the "shuttle effect". Summary of the Invention

[0010] The object of the present invention is to overcome the above-mentioned shortcomings and deficiencies of the prior art, and provide an ionic covalent organic framework functionalized modified separator and its preparation and application.

[0011] The functional separator of the present invention has a structure of a double-layer modified layer. The inner modified layer is an assembly layer of "anion + cation" ionic COFs, aiming to promote the desolvation process of lithium ions and accelerate lithium-ion conduction.

[0012] The outer modified layer of the present invention is a dense modified layer of anionic COFs, aiming to effectively inhibit the shuttle of polysulfides through the action of charge repulsion and physical barrier.

[0013] The present invention introduces a double-layer modified layer structure, which not only realizes the functional coupling of the inner and outer modified layers, but more importantly, controls the ordered pore size of the modified layer at the sub-nanometer level, effectively inhibiting the "shuttle effect" of polysulfides.

[0014] The present invention uses a solvothermal method to synthesize an anionic COF material (TpPa-SO3H) and a cationic COF material (TpTG Cl ), and introduces Li + and TFSI – into the anionic skeleton and the cationic skeleton respectively through an ion exchange strategy to prepare an anionic COF (TpPa-SO3Li) and a cationic COF (TpTG-TFSI). Then, the two ionic COFs are exfoliated into two-dimensional nanosheets by ultrasonic dispersion method and used for the modification of the lithium-sulfur battery separator.

[0015] The separator modified layer has a double-layer composite structure. The inner layer is a self-assembly layer of anionic TpPa-SO3Li and cationic TpTG-TFSI mixed in a certain mass ratio, and the outer layer is a dense modified layer composed of stacked anionic TpPa-SO3Li.

[0016] Among them, in the inner layer structure, the COF nanosheets with opposite charges achieve a near-neutral structure and sub-nanometer pore size through self-assembly, which not only promotes the conduction of lithium ions, but also can effectively inhibit the shuttle of polysulfides;

[0017] The outer layer anionic COF nanosheet modified layer further inhibits the shuttle of polysulfides through electrostatic repulsion. The double-layer separator modification is achieved by the doctor blade method, and the thickness of each layer is effectively controlled.

[0018] The present invention is realized through the following technical solutions:

[0019] An ionic covalent organic framework material-functionalized separator, which simultaneously has the characteristics of promoting lithium ion conduction and inhibiting the "shuttle effect", comprises the following preparation steps:

[0020] (1) Preparation of ionic COFs material: Add a mixed solvent of mesitoylbenzene triol, triaminoguanidine hydrochloride, dioxane and water into a pressure-resistant tube and disperse it evenly by ultrasonic wave. First, quickly freeze it in a liquid nitrogen bath; then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature. After 2-4 cycles of the "freezing-degassing-thawing" process, heat the sealed pressure-resistant tube at 100-120 °C for reaction. After the reaction is completed, wash and dry it with a certain amount of N,N-dimethylacetamide, deionized water and acetone respectively to obtain a cationic COF material;

[0021] Add a mixed solution of 2,5-diaminobenzenesulfonic acid, mesitoylbenzene triol, dioxane, mesitylene and acetic acid into a pressure-resistant tube and disperse it evenly by ultrasonic wave. First, quickly freeze it in a liquid nitrogen bath, then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature. After 2-4 cycles of the "freezing-degassing-thawing" process, heat the sealed pressure-resistant tube at 100-120 °C for reaction. After the reaction is completed, wash and dry it with a certain amount of dioxane and acetone respectively to obtain an anionic COF material.

[0022] (2) Ion exchange reaction: Add the cationic COF material and LiTFSI into a flask according to a mass ratio of 1:10-1:20, use water as a solvent, and react at 50-70 °C for 24-72 hours. After the reaction is completed, wash it with ethanol and dry it at 80-100 °C for 6-10 hours to obtain a cationic COF containing TFSI - of.

[0023] Similarly, add the anionic COF material and lithium acetate into a flask according to a mass ratio of 1:10-1:20, use water as a solvent, and react at 50-70 °C for 24-72 hours. After the reaction is completed, wash it with ethanol and dry it at 80-100 °C for 6-10 hours to obtain an anionic COF containing Li + of.

[0024] (3) Double-layer diaphragm modification: First, the anion- and cation-type COFs, Super P, and polyvinylidene fluoride after ion exchange are ground evenly, dispersed in N-methylpyrrolidone, and stirred evenly to obtain the inner layer slurry. It is coated onto the polypropylene diaphragm with a scraper of a certain thickness and dried to obtain the inner layer modified composite diaphragm. Then, the anion-type COF, Super P, and polyvinylidene fluoride after ion exchange are dispersed in N-methylpyrrolidone, and stirred evenly to obtain the outer layer slurry. It is coated onto the inner layer modified composite diaphragm with a scraper of a certain thickness to obtain the double-layer modified composite diaphragm. Finally, the double-layer modified composite diaphragm is dried and cut into circular pieces with a mold to obtain the functionalized modified diaphragm for lithium-sulfur batteries.

[0025]

[0026]

[0027] Preferably, the reaction molar ratios of mesitoylphloroglucinol to triaminoguanidine hydrochloride and triaminoguanidine hydrochloride to 2,5-diaminobenzenesulfonic acid are 1:1 and 1:1.5, respectively.

[0028] Preferably, the ultrasonic time in step (1) is 5 - 10 mins, the reaction temperature is 100 - 120 °C, the time is 48 - 72 h, and the drying condition is a vacuum environment at 80 - 100 °C. The cationic COF material obtained by the reaction of mesitoylphloroglucinol (Tp) with triaminoguanidine hydrochloride (TG Cl ) is named TpTG Cl ; the anionic COF material obtained by the reaction of mesitoylphloroglucinol (Tp) with 2,5-diaminobenzenesulfonic acid (Pa-SO3H) is named TpPa-SO3H.

[0029] Preferably, the concentrations of LiTFSI and lithium acetate in the solution in step (2) are about 120 - 200 g·L -1 . Among them, the cationic COF ion-exchanged from TpTG Cl is named TpTG-TFSI; the anionic COF ion-exchanged from TpPa-SO3H is named TpPa-SO3Li.

[0030] Preferably, in the preparation of the inner layer slurry, the total mass of anionic and cationic COF nanosheets and Super P is 80-120 mg. The mass ratio of polyvinylidene fluoride to the total mass of the above mixture is 0.1-0.4. In the preparation of the outer layer slurry, the total mass of anionic COF nanosheets and Super P is 80-120 mg. The mass ratio of polyvinylidene fluoride to the total mass of the above mixture is 0.1-0.4. The grinding time is 0.5-1 h, the stirring time is 4-8 h, and the drying condition of the modified composite separator is vacuum at 60-80 °C.

[0031] In step (3), the separator modified by the inner layer slurry is named TpTG-TFSI@TpPa-SO3Li / Celgard (abbreviated as TT@TP / Celgard), and the separator successively modified by the inner and outer layer slurries is named TpPa-SO3Li / TpTG-TFSI@TpPa-SO3Li / Celgard (abbreviated as TP / TT@TP / Celgard).

[0032] The above preparation method prepares a separator functionalized and modified with ionic covalent organic frameworks.

[0033] The composite separator modified with ionic covalent organic frameworks of the present invention can be applied to the separator material of lithium-sulfur batteries.

[0034] The present invention has the following advantages and effects compared with the prior art:

[0035] The present invention adopts a double-modification layer strategy based on ionic COFs, controls the ordered pore size of the modification layer at the sub-nanometer level, and effectively inhibits the "shuttle effect" of polysulfides.

[0036] Among them, the inner modification layer is a self-assembled layer of COF nanosheets with opposite charges, making the stacking structure of COF nanosheets tighter. The positive charge center N in the cationic COF nanosheets + can also act as a Lewis acid site to attract solvent molecules and promote the desolvation and conduction of lithium ions.

[0037] The sulfonic acid anions in the anionic COF nanosheets aggregate negative charges and play an electrostatic repulsion role to repel polysulfides.

[0038] The inner and outer modification layers play different roles respectively, diversifying the functionality of the selective separation membrane.

[0039] Based on the above, the separator functionalized and modified with ionic covalent organic frameworks improves the cycle stability and specific capacity of the battery to a certain extent.

[0040] Applying the above functionalized separator to a lithium-sulfur battery, at a current density of 0.5C (C = 1675 mAh g-1 After cycling 100 cycles under the conditions of -1 , the capacity can still be maintained at 867.8 mAh g, indicating that the present invention significantly improves the electrochemical performance of lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic structural diagram of ionic COFs obtained in Examples 1-3 of the present invention and Comparative Example 2; among them, a is TpPa-SO3Li; b is TpTG-TFSI.

[0042] Figure 2 It is an infrared spectrogram of the species involved in the synthesis reactions in Examples 1-3 of the present invention and Comparative Example 2.

[0043] Figure 3 It is an XRD spectrum of the COFs involved in Examples 1-3 of the present invention and Comparative Example 2.

[0044] Figure 4 It is an optical photograph of the separators obtained in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention;

[0045] Among them, a is the commercial PP separator Celgard; b is the single-layer modified separator TT@TP / Celgard; c is the double-layer functionalized modified separator TP / TT@TP / Celgard; d is the optical photograph of TP / TT@TP / Celgard after folding in half; e is the optical photograph of TP / TT@TP / Celgard after folding in half twice.

[0046] Figure 5 It is a comparison diagram of the electrolyte wettability of the separators obtained in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention; a is the commercial Celgard; b is TT@TP / Celgard; c is TP / TT@TP / Celgard.

[0047] Figure 6 It is an SEM image of the surface of the separators obtained in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention; a is the commercial Celgard; b is TT@TP / Celgard; c is TP / TT@TP / Celgard.

[0048] Figure 7 It is an SEM image of the cross section of the separators obtained in Comparative Example 2 and Example 1 of the present invention; the left is TT@TP / Celgard; the right is TP / TT@TP / Celgard.

[0049] Figure 8 It is an impedance test curve of the lithium-sulfur battery based on the separators obtained in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0050] Figure 9It is a comparison chart of the cycling performance of lithium-sulfur batteries with diaphragms obtained from Example 1, Comparative Example 1, and Comparative Example 2 of the present invention at a rate of 0.5C; a is the comparison chart of the cycling performance in the first 100 cycles; b is the comparison chart of the cycling performance in the first 1000 cycles. Detailed implementation manners

[0051] The present invention will be further described in detail below in conjunction with specific embodiments.

[0052] As Figures 1-9 shown, the present invention discloses a preparation method of a diaphragm functionalized and modified by an ionic covalent organic framework; it will be specifically described through the following examples and comparative examples.

[0053] Example 1

[0054] Step (1) Synthesis of COFs material

[0055] Add 140.58 mg of triaminoguanidine hydrochloride, 210.4 mg of trimesoylbenzene triol, 10 mL of dioxane, and 3 mL of deionized water into a pressure-resistant tube, ultrasonicate for 10 mins, and mix and disperse evenly. First, quickly freeze in a liquid nitrogen bath, then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature. After 3 cycles of the "freezing-degassing-thawing" process, heat the sealed pressure-resistant tube at 100-120 °C for reaction. After the reaction, wash it with 30 mL of N,N-dimethylacetamide, 30 mL of deionized water, and 30 mL of acetone respectively, and dry it under vacuum at 80 °C overnight to obtain the cationic COF material TpTG Cl .

[0056] Add 282.3 mg of 2,5-diaminobenzenesulfonic acid and 210.4 mg of trimesoylbenzene triol into a mixed solution of 2 mL of dioxane, 8 mL of mesitylene, and 2 mL of 6M acetic acid, ultrasonicate for 10 mins, and mix and disperse evenly. First, quickly freeze in a liquid nitrogen bath; then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature. After 3 cycles of the "freezing-degassing-thawing" process, heat the sealed pressure-resistant tube at 100-120 °C for reaction. After the reaction, wash it with 30 mL of dioxane and 30 mL of acetone respectively, and dry it under vacuum at 80 °C overnight to obtain the anionic COF material TpPa-SO3H. To meet the requirement of the COFs sample amount needed in the example, step (1) needs to be repeated in multiple groups.

[0057] Step (2) Ion exchange reaction

[0058] Take 600 mg of TpTG Cl and 6 g of LiTFSI and add them to a flask, and prepare a 30 mL solution with water as the solvent, and reflux at 50 °C for 72 h.

[0059] 600 mg of TpPa-SO3H and 6 g of lithium acetate were added to a flask, and a 30 mL solution was prepared using water as the solvent. The mixture was refluxed at 50 °C for 72 h.

[0060] After the reaction was completed, the above reaction products were washed with 30 mL of deionized water and 30 mL of ethanol respectively, and dried under vacuum at 80 °C overnight to obtain cationic TpTG-TFSI and anionic TpPa-SO3Li respectively.

[0061] Step (3) Preparation of the slurry

[0062] Preparation of the inner layer slurry: 30 mg of TpTG-TFSI, 30 mg of TpPa-SO3Li, 30 mg of Super P and 10 mg of polyvinylidene fluoride were ground evenly, dispersed in 0.8 mL of N-methylpyrrolidone, and stirred magnetically for 6 h to obtain the inner layer slurry.

[0063] Preparation of the outer layer slurry: 60 mg of TpPa-SO3Li, 30 mg of Super P and 10 mg of polyvinylidene fluoride were ground evenly, dispersed in 0.8 mL of N-methylpyrrolidone, and stirred magnetically for 6 h to obtain the outer layer slurry.

[0064] Step (4) Preparation of the composite separator

[0065] Modification of the inner layer separator: A dry and clean commercial polypropylene separator (with a thickness of 25 μm) was selected. The inner layer slurry from step (3) was evenly coated on the commercial separator using a 30 μm blade on an automatic coater, and dried under vacuum at 80 °C overnight to obtain a singly modified composite separator TT@TP / Celgard.

[0066] Modification of the outer layer separator: The outer layer slurry from step (3) was evenly coated on the singly modified composite separator TT@TP / Celgard using a 35 μm blade on an automatic coater, and dried under vacuum at 80 °C overnight to obtain a doubly modified composite separator TP / TT@TP / Celgard.

[0067] The doubly modified separator obtained in this example was cut into round pieces for assembling lithium-sulfur button batteries (full cells) for constant current charge-discharge cycling tests and battery impedance tests.

[0068] Example 2

[0069] Based on steps (1) and (2) in Example 1, the same ion-exchanged COFs were synthesized.

[0070] Preparation of Slurry: Grind 40 mg of TpTG-TFSI, 40 mg of TpPa-SO3Li, 10 mg of Super P, and 10 mg of polyvinylidene fluoride evenly, disperse them in 0.8 mL of N-methylpyrrolidone, and stir magnetically for 6 h to obtain the inner-layer slurry; Grind 40 mg of TpPa-SO3Li, 40 mg of Super P, and 20 mg of polyvinylidene fluoride evenly, disperse them in 0.8 mL of N-methylpyrrolidone, and stir magnetically for 6 h to obtain the outer-layer slurry.

[0071] Preparation of Composite Separator: Select a dry and clean commercial polypropylene separator (with a thickness of 25 μm), take the above-mentioned inner-layer slurry, and evenly coat it on the commercial separator using a 30-μm blade on an automatic coater, and dry it under vacuum at 80 °C overnight to obtain a single-layer modified composite separator TT@TP / Celgard. Take the above-mentioned outer-layer slurry, and evenly coat it on the single-layer modified composite separator TT@TP / Celgard using a 35-μm blade on an automatic coater, and dry it under vacuum at 80 °C overnight to obtain a double-layer modified composite separator TP / TT@TP / Celgard.

[0072] Cut the double-layer modified separator obtained in this example with a mold into round pieces for assembling lithium-sulfur button batteries (full cells) for constant current charge-discharge cycle tests and battery impedance tests.

[0073] Example 3

[0074] Based on steps (1) and (2) in Example 1, synthesize the same ion-exchanged COFs.

[0075] Preparation of Slurry: Grind 25 mg of TpTG-TFSI, 25 mg of TpPa-SO3Li, 25 mg of Super P, and 25 mg of polyvinylidene fluoride evenly, disperse them in 0.8 mL of N-methylpyrrolidone, and stir magnetically for 6 h to obtain the inner-layer slurry; Grind 70 mg of TpPa-SO3Li, 20 mg of Super P, and 10 mg of polyvinylidene fluoride evenly, disperse them in 0.8 mL of N-methylpyrrolidone, and stir magnetically for 6 h to obtain the outer-layer slurry.

[0076] Preparation of composite separator: Select a dry and clean commercial polypropylene separator (with a thickness of 25 μm). Take the above-mentioned inner layer slurry and evenly coat it on the commercial separator using a 30-μm doctor blade on an automatic coater, and then dry it overnight in a vacuum at 80 °C to obtain a single-layer modified composite separator TT@TP / Celgard. Take the above-mentioned outer layer slurry and evenly coat it on the single-layer modified composite separator TT@TP / Celgard using a 35-μm doctor blade on an automatic coater, and then dry it overnight in a vacuum at 80 °C to obtain a double-layer modified composite separator TP / TT@TP / Celgard.

[0077] Cut the double-layer modified separator obtained in this example into round wafers for assembling lithium-sulfur button batteries (full cells) for constant current charge-discharge cycle tests and battery impedance tests.

[0078] Comparative Example 1

[0079] Cut the commercial polypropylene separator into round wafers for assembling lithium-sulfur button batteries (full cells) for constant current charge-discharge cycle tests and battery impedance tests.

[0080] Comparative Example 2

[0081] Cut the single-layer modified separator TT@TP / Celgard obtained in step (3) of Example 1 into round wafers for assembling lithium-sulfur symmetric button batteries (full cells) for constant current charge-discharge stability cycle tests and battery impedance tests.

[0082] Use the full cells assembled in the above examples and comparative examples for constant current charge-discharge cycle tests and battery impedance tests. The constant current charge-discharge cycle tests show that after 1000 cycles at 0.5C, the capacity retention rates of the initial specific capacities of the batteries assembled with commercial separators are the lowest, which are 659.5 mAh g -1 and 27.9% ( Figure 9 ).

[0083] Compared with the commercial separator battery, the TT@TP / Celgard battery has a higher initial specific capacity of 777.8 mAh g -1 . The TP / TT@TP / Celgard battery has significantly improved cycle stability under the same test conditions, and the initial specific capacity and capacity retention rate of the battery are 1014.0 mAh g -1 and 47.4% respectively. This is mainly due to the physical barrier of the synthesized double-layer COF nanopores and the electrostatic interaction of the negatively charged skeleton. It can be obtained from the battery impedance test ( Figure 8) Compared with commercial diaphragms, the impedance of the single-layer modified TT@TP / Celgard is smaller; this is attributed to the decoupling effect of the self-assembled layer on lithium ions-lithium salt anions. More importantly, in contrast, the double-layer modified TP / TT@TP / Celgard has the smallest impedance, which is more conducive to the catalytic conversion of sulfides.

[0084] At room temperature, the impedance of lithium-sulfur batteries based on different diaphragm matches was tested, as well as the cycle performance comparison at a rate of 0.5C, as shown in Table 1.

[0085] Table 1 Electrochemical characterization of lithium-sulfur batteries based on different diaphragm matches at room temperature:

[0086]

[0087] In summary, the diaphragm functionalized by the ionic covalent organic framework of the present invention significantly improves the cycle stability of the battery and reduces the impedance of the battery, all of which are attributed to the double-layer modification strategy: the outer layer modification meets the basic requirements of physical barrier and charge repulsion for polysulfides, and the inner layer modification promotes the desolvation and transport of lithium ions. The double-layer modification strategy greatly improves the functionality of the diaphragm, and the whole preparation process does not involve harsh reaction conditions such as high temperature and high pressure, which has a certain feasibility and universality.

[0088] More specifically, the double modification layer of the present invention is a double-layer functional structure on the positive electrode side of the base diaphragm, including an inner layer formed by the self-assembly of anion- and cation-type COF nanosheets and a dense outer layer formed by the stacking of anion-type COF nanosheets. The double modification layer structure introduced in the present invention realizes the functionalization of the inner and outer layers: the outer anion-type COF dense modification layer can effectively inhibit the shuttle of polysulfides through charge repulsion and physical barrier effects; in the inner layer "anion + cation" ionic COF self-assembled layer, the locally assembled positive and negative charge centers and nanopores greatly promote the desolvation of lithium ions and accelerate the lithium ion conduction, thereby improving the electrochemical performance of the lithium-sulfur battery.

[0089] Although the above content describes the present invention in conjunction with the drawings, the present invention is not limited to the above-mentioned optimal embodiments. The above embodiments are merely illustrative and not restrictive, and any equivalent or deformation carried out without departing from the purpose of the present invention falls within the protection scope of the present invention.

Claims

1. A preparation method of an ionic covalent organic framework-functionalized separator, characterized in that, It includes the following steps: (1) Preparation steps of ionic COFs materials: Add the mixed solvent of aldehyde monomer, guanidine monomer, dioxane and water into a pressure-resistant tube, and disperse it evenly by ultrasonic treatment; After ultrasonic dispersion, first quickly freeze it in a liquid nitrogen bath; then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature; after 2 - 4 cycles of the "freezing - degassing - thawing" process, heat and react the sealed pressure-resistant tube at 100 - 120 °C; after the reaction is completed, wash and dry it with N,N-dimethylacetamide, deionized water and acetone respectively to obtain cationic COF materials; Add the mixed solution of sulfonic acid group monomer, aldehyde group monomer and dioxane, mesitylene and acetic acid into a pressure-resistant tube, and disperse it evenly by ultrasonic treatment; first quickly freeze it in a liquid nitrogen bath, then evacuate the pressure-resistant bottle, fill it with argon and seal it, and thaw it to room temperature; after 2 - 4 cycles of the "freezing - degassing - thawing" process, heat and react the sealed pressure-resistant tube at 100 - 120 °C; after the reaction is completed, wash and dry it with dioxane and acetone respectively to obtain anionic COF materials; (2) Ion exchange reaction steps: Add the cationic COF material and lithium bis(trifluoromethanesulfonyl)imide into a flask at a mass ratio of 1:10 to 1:

20. Using water as a solvent, react at 50 - 70 °C for 24 - 72 hours. After the reaction, wash with ethanol and dry at 80 - 100 °C for 6 - 10 hours to obtain the cationic COF containing TFSI - ; Add the anionic COF material and lithium acetate to the flask at a mass ratio of 1:10 to 1:

20. Using water as the solvent, react at 50 - 70 °C for 24 - 72 hours. After the reaction, wash with ethanol and dry at 80 - 100 °C for 6 - 10 hours to obtain the anionic COF containing Li + ; (3) Double-layer diaphragm modification steps: First, grind the anion and cationic COFs, conductive carbon black and polyvinylidene fluoride after ion exchange evenly, disperse them in N-methylpyrrolidone, stir evenly to obtain an inner layer slurry; coat the inner layer slurry onto a polypropylene diaphragm with a scraper and dry it to obtain an inner layer modified composite diaphragm; Then, disperse the anion type COF, Super P and polyvinylidene fluoride after ion exchange in N-methylpyrrolidone, stir evenly to obtain an outer layer slurry; coat the outer layer slurry onto the inner layer modified composite diaphragm with a scraper to obtain a double-layer modified composite diaphragm; Finally, dry the double-layer modified composite diaphragm, cut it into circular pieces with a mold to obtain a functionalized modified diaphragm for lithium-sulfur batteries; The guanidine monomer is triaminoguanidine hydrochloride; The sulfonic acid group monomer is 2,5-diaminobenzenesulfonic acid; The aldehyde monomer is mesitoylphloroglucinol.

2. The preparation method of the separator functionalized and modified by the ionic covalent organic framework according to claim 1, wherein, The reaction molar ratios of the aldehyde monomer to the guanidine monomer and the sulfonic acid group monomer are 1:1 and 1:1.5 respectively.

3. The preparation method of the separator functionalized and modified with the ionic covalent organic framework according to claim 1, characterized in that, The ion exchange reaction described in step (2) is to exchange the anion groups of the cationic COF material and the cation groups of the anionic COF material into TFSI - and Li + .

4. The preparation method of the separator functionalized and modified by the ionic covalent organic framework according to claim 1, characterized in that, The inner layer slurry in step (3) is obtained by mixing the anion and cationic COFs, Super P, polyvinylidene fluoride and N-methylpyrrolidone after ion exchange, while the outer layer slurry is obtained by mixing the anion type COF, Super P, polyvinylidene fluoride and N-methylpyrrolidone after ion exchange.

5. The preparation method according to claim 1, characterized in that, The thicknesses of the inner and outer dense modification layers in step (3) are 5 - 9 μm and 4 - 8 μm respectively.

6. A functionalized coating diaphragm, comprising a base diaphragm and a double-layer modification layer, wherein the double-layer modification layer is on the positive electrode side of the base diaphragm; characterized in that The functionalized coating diaphragm is obtained by using the preparation method described in any one of claims 2 - 5.

7. The composite diaphragm modified by the ionic covalent organic framework according to claim 6 is applied to the diaphragm material of a lithium-sulfur battery.

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