A cyano covalent organic framework material and a preparation method thereof, a functional separator and a preparation method thereof

Abstract

The application provides a cyanide covalent organic framework material and a preparation method thereof, and a functional diaphragm and a preparation method thereof, and belongs to the technical field of diaphragms.The application provides a cyanide covalent organic framework material, which has a chemical structure as shown in formula I.The COF material provided by the application contains a cyanide functional group, can realize enrichment and acceleration of anion transfer, promote decomposition of lithium salt anions, form a stable SEI rich in LiF, inhibit growth of lithium dendrites, and improve safety and interface stability of a battery; one-dimensional nano open pores of the COF material can significantly improve the lithium ion transfer rate of a compact modification layer, and improve ion conductivity and cycle performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of membrane technology, specifically relating to a cyano-covalent organic framework material and its preparation method, and a functional membrane and its preparation method. Background Technology

[0002] Lithium metal batteries have become a promising next-generation energy storage technology due to their high energy density. However, their development is constrained by uncontrollable lithium dendrite deposition, which leads to shortened battery life and serious safety hazards. Given that the nucleation and growth process of lithium is influenced by the composition and structure of the solid electrolyte interphase (SEI) membrane, constructing a stable LiF-rich SEI through interface engineering is an effective strategy for achieving uniform lithium deposition and suppressing lithium dendrite formation. Since the separator is a key component in direct contact with the electrodes and electrolyte, its functional design is an effective strategy for optimizing the SEI composition at the interface. Slow interfacial charge transfer kinetics are a key bottleneck restricting the induction of stable SEI formation using traditional separator modification methods. The key to solving this problem lies in enhancing the electron supply capacity at the interface. Electron localization effects can accelerate the charge transfer rate, significantly improving the reduction and decomposition efficiency of LiTFSI to induce the formation of a LiF-rich SEI, providing a new approach for stabilizing lithium metal anodes.

[0003] To address the aforementioned issues, a research model with tunable electron localization is urgently needed. Co-fibers (COFs), due to their ordered pore structure and tunable framework properties, have become an ideal platform for studying electron localization effects. With the rapid development of molecular engineering, the degree of electron localization in COFs can be controllably adjusted by introducing tunable electron-withdrawing groups. However, current modification strategies based on electron localization primarily focus on accelerating the initial dissociation of lithium salts, while neglecting the effects on interfacial anions (TFSIs). - The enrichment behavior of lithium dendrites lacks effective regulation, resulting in a limited number of anions participating in subsequent reduction and decomposition reactions despite accelerated dissociation. This makes it difficult to maximize the construction of a LiF-rich SEI, thus hindering the effective suppression of lithium dendrites and affecting the battery's cycle performance. Therefore, improving the method to effectively suppress lithium dendrites and thereby enhance the battery's cycle stability has become a pressing technical challenge in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a cyano-based covalent organic framework material and its preparation method, as well as a functional separator and its preparation method. The cyano-based covalent organic framework material provided by this invention, when used as a modification layer for a separator, can effectively suppress lithium dendrite formation, thereby improving the cycle stability of the battery.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a cyano-covalent organic framework material having the chemical structure shown in Formula I: Formula I.

[0006] This invention also provides a method for preparing the cyano-covalent organic framework material described in the above technical solution, comprising the following steps: (1) Mix 1,3,5-tricarboxymethyl phloroglucinol, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, solvent and catalyst to obtain a mixed solution; (2) The mixed solution obtained in step (1) is subjected to a solvothermal reaction to obtain a cyano-covalent organic framework material.

[0007] Preferably, in step (1), the mass ratio of 1,3,5-tricarboxymethyl phloroglucinol and 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile is (58~65):(100~110).

[0008] Preferably, the solvent in step (1) is dioxane and mesitylene.

[0009] Preferably, the temperature of the solvothermal reaction in step (2) is 100~130℃, and the reaction time is 3~4 days.

[0010] The present invention also provides a functional diaphragm, comprising a diaphragm and a modification layer stacked together, wherein the modification layer contains a cyano-covalent organic framework material and a binder; The cyano-covalent organic framework material is the cyano-covalent organic framework material described in the above technical solution or the cyano-covalent organic framework material prepared by the preparation method described in the above technical solution.

[0011] Preferably, the thickness of the modified layer is >4 μm.

[0012] Preferably, the thickness of the modification layer is 5~20μm.

[0013] The present invention also provides a method for preparing the functional diaphragm described in the above technical solution, comprising the following steps: 1) A cyano-covalent organic framework material, binder, and organic solvent are mixed to obtain a slurry; 2) Coat the slurry obtained in step 1) onto the surface of the diaphragm to obtain a functional diaphragm.

[0014] Preferably, in step 1), the mass ratio of cyano-covalent organic framework material to binder is 6:4 to 9:1.

[0015] This invention provides a cyano-based covalent organic framework material (COF) with the chemical structure shown in Formula I. The COF material provided by this invention contains cyano functional groups, which can achieve anion enrichment and accelerate charge transfer, promote lithium salt anion decomposition, form a stable LiF-rich SEI, inhibit lithium dendrite growth, and improve battery safety and interface stability. Utilizing the one-dimensional nanopores of the COF material, the lithium-ion transport rate of the modified layer can be significantly improved, enhancing the battery's ionic conductivity and cycle performance. Experimental results show that the functional separator provided by this invention has an ionic conductivity of 1.28 mS / cm. -1 The LFP|2CN-COF@PP|Li battery assembled with this functional separator has an initial reversible capacity of 141.9 mAh g at 1C. -1 It still maintains 132.4 mAh g after 900 cycles. -1 The capacity retention rate reached 94%. Attached Figure Description

[0016] Figure 1 SEM image of the cyano-covalent organic framework material prepared in Example 1; Figure 2 The image shows the XRD pattern of the cyano-covalent organic framework material prepared in Example 1. Figure 3 The image shows the XRD pattern of the covalent organic framework material prepared in Comparative Example 1. Figure 4 Infrared spectra of the cyano-covalent organic framework material prepared in Example 1 and the covalent organic framework material prepared in Comparative Example 1; Figure 5 The image shows the test results of the folding resistance of the functional diaphragm prepared in Example 2. Figure 6 SEM image of the PP membrane in Comparative Example 2; Figure 7 SEM image of the surface of the functional diaphragm prepared in Example 2; Figure 8 SEM image of the cross-section of the functional diaphragm prepared in Example 2; Figure 9 SEM image of the surface of the functional diaphragm prepared in Comparative Example 3; Figure 10 The above are comparison diagrams showing the electrolyte wettability of the functional membranes prepared in Example 2 and Comparative Examples 2-3. Figure 11 The impedance curves of stainless steel symmetrical cells assembled with functional separators prepared in Example 2 and Comparative Examples 2-3 are shown in comparison. Figure 12The graph shows a comparison of the cycle performance of lithium iron phosphate button batteries assembled with the functional separators prepared in Example 2 and Comparative Examples 2-3. Detailed Implementation

[0017] This invention provides a cyano-covalent organic framework material having the chemical structure shown in Formula I: Formula I.

[0018] In this invention, the tilde in the cyano-covalent organic framework material refers to a repeating structure.

[0019] The COF material provided by this invention contains cyano functional groups, which can enrich anions and accelerate charge transfer, promote the decomposition of lithium salt anions, form a stable LiF-rich SEI, inhibit the growth of lithium dendrites, and improve the safety and interface stability of the battery. By utilizing the one-dimensional nano-open pores of the COF material, the lithium-ion transport rate of the modified layer can be significantly improved, thereby enhancing the ionic conductivity and cycle performance of the battery.

[0020] To address the technical problems of poor cycle stability caused by unstable negative electrode interface and lithium dendrite growth during the charging and discharging process of lithium metal batteries, this invention uses cyano-based covalent organic framework materials to achieve anion enrichment and accelerate charge transfer, promote anion decomposition to form a stable LiF-rich SEI, accelerate lithium ion transport and inhibit lithium dendrite growth, thereby achieving high ionic conductivity and stable long cycle life of the battery.

[0021] A high dielectric constant can increase the dissociation degree of lithium salts, thereby achieving anion enrichment and satisfying one of the necessary conditions for LiF formation. The strongly electron-withdrawing cyano group not only improves the dielectric constant of the material but also effectively induces electron localization. Therefore, combining the dual advantages of high dielectric constant and electron localization, cyano-functionalized COFs were designed as membrane modification materials to regulate interfacial chemistry. This cyano-functionalized COF integrates three synergistic mechanisms: I. High dielectric constant enhances lithium salt dissociation; II. Enhanced electron localization accelerates the catalytic decomposition of anions, promoting the formation of stable LiF-rich SEIs; III. The cyano-functionalized COF provides a rapid and directional ion transport channel, achieving uniform lithium ion flux and effectively suppressing lithium dendrite growth.

[0022] This invention also provides a method for preparing the cyano-covalent organic framework material described in the above technical solution, comprising the following steps: (1) Mix 1,3,5-tricarboxymethyl phloroglucinol, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, solvent and catalyst to obtain a mixed solution; (2) The mixed solution obtained in step (1) is subjected to a solvothermal reaction to obtain a cyano-covalent organic framework material.

[0023] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0024] In this invention, 1,3,5-tricarboxymethyl phloroglucinol (Tp), 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, solvent and catalyst are mixed to obtain a mixed solution.

[0025] In this invention, the preferred mass ratio of 1,3,5-tricarboxymethyl phloroglucinol to 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile is (58~65):(100~110). As one embodiment, the mass ratio of 1,3,5-tricarboxymethyl phloroglucinol to 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile can be 59:(100~110), 60:(100~110), 61:(100~110), 62:(100~110), 63:(100~110), or 64:(100~110), and can also be 63:105.4.

[0026] In this invention, the solvent is preferably dioxane and mesitylene; the volume ratio of dioxane to mesitylene is preferably 1:(1~2), more preferably 1:1. This invention does not have a particular limitation on the amount of solvent used, as long as all raw materials are completely dissolved. As one embodiment, the mass ratio of 1,3,5-tricarboxymethyl phloroglucinol to the volume ratio of the solvent can be (58~65) mg:(1~8) mL, or it can be 63 mg:3 mL.

[0027] In this invention, the catalyst is preferably an aqueous solution of acetic acid; the concentration of the aqueous solution of acetic acid is preferably 5~7 mol / L, more preferably 6 mol / L; the mass ratio of 1,3,5-tricarboxymethyl phloroglucinol to the volume ratio of the catalyst is preferably (58~65) mg: (0.3~0.6) mL, more preferably (58~65) mg: 0.5 mL.

[0028] In this invention, the preferred method for mixing the 1,3,5-tricarboxymethyl phloroglucinol, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, solvent, and catalyst is to ultrasonically mix the 1,3,5-tricarboxymethyl phloroglucinol, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, and solvent, and then add the catalyst dropwise.

[0029] In this invention, the ultrasonic mixing time is preferably 8-15 minutes. As one embodiment, the ultrasonic mixing time can be 10 minutes. This invention does not impose any particular limitation on the ultrasonic mixing power; any ultrasonic mixing power well-known to those skilled in the art can be used.

[0030] The present invention does not impose any special limitation on the dripping rate; dripping operations familiar to those skilled in the art can be used.

[0031] After mixing, the present invention preferably performs post-processing on the product obtained by mixing to obtain a mixed solution.

[0032] In this invention, the post-processing preferably includes first freezing, followed by vacuuming, nitrogen purging, thawing, and second freezing in sequence.

[0033] In this invention, the first freezing is preferably carried out in liquid nitrogen. Performing a first freezing after mixing ensures that the liquid is not removed during the subsequent vacuuming process, thereby removing dissolved oxygen (O2) and moisture from the reaction system.

[0034] The present invention does not have any special limitations on the first freezing operation, as long as it ensures that the liquid will not be drawn away during the subsequent vacuuming process.

[0035] The present invention does not impose any special limitations on the vacuuming operation; any operation known to those skilled in the art can be used.

[0036] The present invention does not have any special limitations on the nitrogen gas purging operation; a nitrogen atmosphere is sufficient.

[0037] This invention does not impose any particular limitation on the thawing operation; any thawing operation well-known to those skilled in the art can be used. As one embodiment, the thawing operation can be performed using a hair dryer. This invention does not impose any particular limitation on the thawing operation; complete thawing is sufficient.

[0038] In this invention, the second freezing is preferably carried out in liquid nitrogen. The invention does not impose any particular limitations on the operation of the second freezing, as long as it ensures that the liquid is not removed during the subsequent vacuuming process.

[0039] In this invention, the vacuuming, nitrogen purging, thawing, and second freezing processes are preferably repeated multiple times, more preferably three times. This repeated vacuuming, nitrogen purging, thawing, and second freezing process removes excess air and moisture.

[0040] After obtaining the mixed solution, the present invention performs a solvothermal reaction on the mixed solution to obtain a cyano-covalent organic framework material.

[0041] In this invention, the temperature of the solvothermal reaction is preferably 100-130°C; the time of the solvothermal reaction is preferably 3-4 days; and the solvothermal reaction is preferably carried out under vacuum conditions. As one embodiment, the temperature of the solvothermal reaction can be 120°C; and the time of the solvothermal reaction can be 72 hours.

[0042] After the solvothermal reaction is completed, the present invention preferably performs post-processing on the product obtained by the solvothermal reaction to obtain a cyano-covalent organic framework material.

[0043] In this invention, the post-treatment is preferably performed by washing, solvent replacement and vacuum drying in sequence.

[0044] In this invention, the detergent used for washing is preferably N,N-dimethylformamide; and the number of washing cycles is preferably 3.

[0045] In this invention, the solvent replacement is preferably performed sequentially using tetrahydrofuran and acetone. This invention does not impose any particular limitations on the solvent replacement operation; it is sufficient to continue until the solution becomes clear.

[0046] In this invention, the vacuum drying temperature is preferably 55~90℃; the vacuum drying time is preferably 12~24h. As one embodiment, the vacuum drying temperature can be 60℃, 70℃, or 80℃; the vacuum drying time can be 16h, 18h, or 20h. This invention uses vacuum drying to remove solvents.

[0047] The preparation method provided by this invention is simple.

[0048] The present invention also provides a functional diaphragm, comprising a diaphragm and a modification layer stacked together, wherein the modification layer contains a cyano-covalent organic framework material and a binder; The cyano-covalent organic framework material is the cyano-covalent organic framework material described in the above technical solution or the cyano-covalent organic framework material prepared by the preparation method described in the above technical solution.

[0049] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0050] The functional diaphragm provided by this invention includes a diaphragm; the material of the diaphragm is preferably polypropylene. This invention does not have a specific limitation on the molecular weight of the polypropylene; any polypropylene well-known to those skilled in the art can be used. In this invention, the diaphragm is a base membrane.

[0051] The present invention does not have a special limitation on the thickness of the diaphragm; any diaphragm well known to those skilled in the art can be used.

[0052] The functional diaphragm provided by the present invention further includes a modification layer; the modification layer contains a cyano-covalent organic framework material and a binder; the binder is preferably polyvinylidene fluoride (PVDF).

[0053] In this invention, the thickness of the modification layer is preferably >4 μm, more preferably 5~20 μm. As one embodiment, the thickness of the modification layer can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, or 19 μm. Limiting the thickness of the modification layer to the above range further improves the lithium-ion permeability of the modification layer, thereby enhancing the ionic conductivity and cycle performance of the battery.

[0054] The present invention also provides a method for preparing the functional diaphragm described in the above technical solution, comprising the following steps: 1) A cyano-covalent organic framework material, binder, and organic solvent are mixed to obtain a slurry; 2) Coat the slurry obtained in step 1) onto the surface of the diaphragm to obtain a functional diaphragm.

[0055] This invention mixes cyano-covalent organic framework materials, binders, and organic solvents to obtain a slurry.

[0056] In this invention, the preferred mass ratio of the cyano-covalent organic framework material to the binder is 6:4 to 9:1. As one embodiment, the mass ratio of the cyano-covalent organic framework material to the binder can be 7:3.

[0057] In this invention, the organic solvent is preferably N-methylpyrrolidone. There is no particular limitation on the amount of organic solvent used, as long as the raw material is completely dissolved.

[0058] In this invention, the mixing of the cyano-covalent organic framework material, binder, and organic solvent is preferably carried out under magnetic stirring conditions; the magnetic stirring time is preferably 2-5 hours; and the magnetic stirring speed is preferably 210-300 rpm. As one embodiment, the magnetic stirring time can be 3 hours; and the magnetic stirring speed can be 290 rpm.

[0059] After obtaining the slurry, the present invention coats the slurry onto the surface of the diaphragm to obtain a functional diaphragm.

[0060] In this invention, the coating is preferably performed using a doctor blade; the distance between the doctor blade and the surface of the diaphragm is preferably 5~20μm.

[0061] After coating, the present invention preferably performs vacuum drying on the coated product to obtain a functional diaphragm. The present invention does not have specific limitations on the vacuum drying operation; vacuum drying methods well-known to those skilled in the art, which remove the solvent, are sufficient.

[0062] As one implementation method, the vacuum drying temperature can be 55°C; the vacuum drying time can be 8 hours.

[0063] The functional separator fabrication process provided by this invention has the advantages of being simple and easy to scale up, and is suitable for button battery applications. It can significantly improve the ionic conductivity and cycle stability of lithium metal batteries.

[0064] The functional separator provided by this invention solves the problems of low ionic conductivity and unstable battery interface after assembly of existing commercial PP separators. The base membrane of the functional separator is a polypropylene microporous membrane and the modification layer is a dense coating layer, which not only has excellent ionic conductivity, but also enhances the interface stability of the electrode / electrolyte of lithium metal batteries.

[0065] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0066] Example 1 Cyano-covalent organic framework materials have the chemical structure shown in Formula I: Formula I; The preparation method of the cyano-covalent organic framework material is as follows: 63.0 mg of 1,3,5-tricarboxymethyl phloroglucinol and 105.4 mg of 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile were mixed in a hard-shell ampoule. Dioxane (1.5 mL) and mesitylene (1.5 mL) were added to the ampoule in a 1:1 volume ratio. The ampoule containing the mixed solution was sonicated for 10 min to ensure homogeneity. Then, 0.5 mL of 6M acetic acid aqueous solution was added dropwise. The ampoule containing the mixed solution was then placed in liquid nitrogen for freezing. To remove excess air and moisture from the ampoule that could affect the reaction, the ampoule was subjected to three [unspecified treatments]. The process of vacuuming, nitrogen purging, thawing, and freezing was repeated several times. After removing impurities, the sample was vacuumed again using an oil pump. The ampoule was then vacuum-sealed while ensuring a vacuum inside. It was then placed in a constant temperature oven and heated at 120°C for 72 hours for a solvothermal reaction. After the reaction was complete, the sample was washed three times with N,N-dimethylformamide and then replaced with tetrahydrofuran and acetone in sequence. Finally, it was dried in a vacuum drying oven at 80°C for 12 hours to obtain an orange powdery cyano-covalent organic framework material, denoted as 2CN-COF.

[0067] Figure 1 The image shows a SEM image of the cyano-covalent organic framework material prepared in Example 1.

[0068] from Figure 1 It can be seen that 2CN-COF exhibits a flower-like structure.

[0069] Comparative Example 1 Based on Example 1, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile was replaced with biphenyl diamine without a cyano group, while the other steps remained unchanged, to obtain a covalent organic framework material, denoted as BD-COF.

[0070] The XRD pattern of the cyano-covalent organic framework material prepared in Example 1 is shown below. Figure 2 As shown; the XRD pattern of the covalent organic framework material prepared in Comparative Example 1 is shown below. Figure 3 As shown.

[0071] from Figure 2 and 3It can be seen that a strong and sharp characteristic diffraction peak appears at 2θ=3.4°, corresponding to the (100) crystal plane diffraction of the COF framework; a weaker characteristic diffraction peak is observed at 2θ=6.0°, belonging to the (110) crystal plane diffraction, further confirming the periodicity and orderliness of the molecular arrangement within the COF layers; a broadened diffraction peak appears at 2θ=26.8°, which corresponds to the π-π stacking effect along the (001) crystal plane between the two-dimensional layers of COF; the test results of the COF material were refined using Pawley, and the refined simulated diffraction peaks completely overlapped with the experimentally measured diffraction peaks. The slight difference and extremely low goodness of fit prove this point: the R of 2CN-COF wp =3.71%, R p =2.8%; R of BD-COF wp =5.21%, R p =3.44%. In summary, the synthesized COF has good crystallinity.

[0072] Figure 4 The images show the infrared (FT-IR) spectra of the cyano-covalent organic framework material prepared in Example 1 and the covalent organic framework material prepared in Comparative Example 1.

[0073] from Figure 4 It can be seen that all COF samples exhibit the characteristic vibrational peaks of the β-ketoenamine structure: at 1618 cm⁻¹. -1 The strong absorption peak at 1579 cm⁻¹ corresponds to the stretching vibration of the C=O bond in the β-ketoenamine structure. -1 The characteristic peak at 1494 cm⁻¹ is attributed to the tensile vibration of the C=C bond; furthermore, the peak at 1494 cm⁻¹... -1 With 1443cm -1 The absorption peak at 2200 cm⁻¹ is a characteristic skeletal vibration peak of the aromatic ring, confirming the formation of a stable β-keto-enamine linkage structure during the reaction; the 2CN-COF sample showed an absorption peak at 2200 cm⁻¹. -1 A distinct absorption peak was observed at all locations, which is a characteristic stretching vibration peak of -C≡N, confirming that the cyano functional group has been successfully grafted into the COF backbone.

[0074] Example 2 A functional membrane consists of a membrane and a modification layer stacked together, wherein the modification layer contains the cyano-covalent organic framework material and PVDF of Example 1; The diaphragm is a Celgard 2500 PP diaphragm; The thickness of the modified layer is 18 μm; The method for preparing the functional diaphragm is as follows: 70 mg of cyano-covalent organic framework material and 30 mg of PVDF were added to 2 mL of NMP and stirred at 290 rpm for 3 h to obtain a slurry. Then, take 2 mL of slurry and use a scraper on an automatic coating machine to evenly coat the slurry onto the diaphragm. Then, place it in a vacuum drying oven and dry it at 55°C for 8 hours under vacuum conditions to evaporate and remove the NMP solvent. Finally, use a manual slicer to cut out a functional diaphragm with a diameter of 19 mm, which is 2CN-COF@PP.

[0075] Figure 5 The image shows the test results of the folding resistance of the functional diaphragm prepared in Example 2.

[0076] from Figure 5 It can be seen that the functional diaphragm maintains its structural integrity after repeated folding, without cracking or other phenomena.

[0077] Comparative Example 2 Based on Example 2, the modification layer is omitted, i.e., Celgard2500 PP membrane, which is PP.

[0078] SEM image of the PP membrane in Comparative Example 2 is shown below. Figure 6 As shown.

[0079] from Figure 6 It can be seen that the surface of the Celgard2500 PP membrane has a large number of micron-sized pore structures.

[0080] SEM image of the surface of the functional diaphragm prepared in Example 2 is shown below. Figure 7 As shown, the SEM image of the cross-section of the functional diaphragm prepared in Example 2 is as follows. Figure 8 As shown.

[0081] from Figures 7-8 As can be seen, after coating with 2CN-COF functionalized coating material, the large number of micron-level pore structures of the original PP membrane are completely covered, and the functionalized coating has a large number of interwoven pore structures, which ensures sufficient contact between the electrolyte and the coating material, thereby reducing the interfacial resistance and ultimately improving the ionic conductivity. The cross-sectional SEM image of the functional membrane shows that the thickness of the functional membrane is about 18μm, and the coating material is tightly bonded to the PP surface.

[0082] Comparative Example 3 Based on Example 2, the cyano-covalent organic framework material was modified to the covalent organic framework material of Comparative Example 1, while other conditions remained unchanged, to obtain a functional membrane, denoted as BD-COF@PP.

[0083] SEM image of the surface of the functional membrane prepared in Comparative Example 3 is shown below. Figure 9 As shown.

[0084] from Figure 9As can be seen, after being coated with BD-COF material, a uniform coating was formed on the surface of the original PP membrane.

[0085] The wettability comparison diagrams of the electrolytes (containing 1 mol / L LiTFSI main salt, an equal mass ratio of DOL and DME mixed solvents, and 2 wt% LiNO3) of the functional membranes prepared in Example 2 and Comparative Examples 2-3 are shown below. Figure 10 As shown.

[0086] from Figure 10 It can be seen that the initial contact angle of the PP membrane is 45.7°, while the contact angle of the 2CN-COF@PP membrane significantly decreases to 16.6°. After 10 seconds, the contact angle of the 2CN-COF@PP membrane rapidly decreases to 5.4°, while the PP membrane remains at 37.1°. The improved electrolyte wettability of the functional membrane of this invention originates from the electron-rich region of the COF material and the Li + The strong adsorption between the membranes is evident; similarly, BD-COF@PP also exhibits a significantly lower contact angle of 15.4° compared to PP membranes. These results indicate that the functionalized modification layer can effectively improve electrolyte wettability, which is crucial for enhancing lithium-ion transport kinetics.

[0087] The functional separators prepared in Example 2 and Comparative Examples 2-3 were assembled into stainless steel symmetric batteries and lithium iron phosphate button batteries, and their ionic conductivity and constant current charge-discharge cycle tests were conducted.

[0088] The stainless steel symmetrical battery consists of a positive electrode shell + stainless steel sheet + separator + stainless steel sheet + gasket + negative electrode shell. All battery assembly operations are carried out in a glove box filled with high-purity argon gas to prevent side reactions caused by contact between the electrodes and electrolyte and air. The electrolyte volume is 55 microliters. The stainless steel sheet is 0.5 mm thick and 15.5 mm in diameter. The gasket is 15.4 mm in diameter and 0.2 mm thick. The lithium iron phosphate battery consists of a positive electrode shell + lithium iron phosphate positive electrode sheet + separator + negative electrode + negative electrode shell. The preparation method is as follows: NMP, 90wt% LFP, 5wt% conductive carbon black, and 5wt% PVDF are added to a dispersion container. The components are thoroughly mixed by high-speed shear stirring (2000 r / min, stirring time 60 min) to prepare a uniform, stable positive electrode slurry without significant agglomeration. The obtained slurry is coated onto aluminum foil and vacuum heated at 80℃ for 12 hours to remove NMP. After cooling to room temperature, it is cut into 14mm diameter discs, then vacuum heated at 110℃ for 5 hours to remove moisture. The discs are then placed in a glove box for subsequent testing and application. The resulting lithium iron phosphate positive electrode sheet has an active material surface loading of approximately 4 mg / cm². -2During assembly, the electrodes are stacked sequentially. The electrolyte volume is 55 microliters. The negative electrode is a lithium metal sheet with a thickness of 500µm and a diameter of 15mm.

[0089] The stainless steel symmetrical cells assembled with the functional separators prepared in Example 2 and Comparative Examples 2-3 were tested for impedance after being kept at 30-90℃ for 1 hour. The impedance curves are shown in the comparison graphs below. Figure 11 As shown.

[0090] from Figure 11 As can be seen from the calculations, the ionic conductivity of the PP membrane, the BD-COF@PP functional membrane, and the 2CN-COF@PP functional membrane are 0.37, 0.84, and 1.28 mS / cm, respectively. -1 It is evident that functionalizing the 2CN-COF material coating can significantly improve the ionic conductivity of the functional membrane. This improvement is attributed to the excellent electrolyte wettability and ordered pore structure of the functionalized COF layer, thereby achieving more efficient lithium-ion transport.

[0091] The lithium iron phosphate button batteries assembled with the functional separators prepared in Example 2 and Comparative Examples 2-3 were charged and discharged in a voltage range of 2.5-4V. The cycle performance comparison graph at 1C rate is shown below. Figure 12 As shown.

[0092] from Figure 12 It can be seen that the LFP|2CN-COF@PP|Li battery assembled with the functional separator in Example 2 has an initial reversible capacity of 141.9 mAh g at 1C. -1 It still maintains a capacity of 132.4 mAh g after 900 cycles. -1 The capacity retention rate reached 94%, indicating that the 2CN-COF@PP functional membrane can achieve Li+ retention by stabilizing the electrode interface. + The battery with the functional separator in Comparative Example 3 failed after approximately 552 stable cycles, while the battery with the functional separator in Comparative Example 2 had significantly poorer long-cycle performance and failed immediately after 370 cycles.

[0093] In summary, the long-cycle performance of lithium iron phosphate batteries using cyano-COF modified functional separators is significantly better than that of batteries using non-cyano-COF modified separators and ordinary separators. This indicates that cyano-COF modified separators can construct a stable electrode / electrolyte interface, suppress lithium dendrite growth, and promote lithium-ion transport in the battery, thereby improving the battery's cycle life and significantly enhancing ionic conductivity and cycle performance.

[0094] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cyano-covalent organic framework material having the chemical structure shown in Formula I: Equation I.

2. The method for preparing the cyano-covalent organic framework material according to claim 1, comprising the following steps: (1) Mix 1,3,5-tricarboxymethyl phloroglucinol, 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile, solvent and catalyst to obtain a mixed solution; (2) The mixed solution obtained in step (1) is subjected to a solvothermal reaction to obtain a cyano-covalent organic framework material.

3. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of 1,3,5-tricarboxyloyl phloroglucinol to 4,4'-diamino-[1,1'-biphenyl]-3,3'-dicarboxynitrile is (58~65):(100~110).

4. The preparation method according to claim 2, characterized in that, The solvents in step (1) are dioxane and mesitylene.

5. The preparation method according to claim 2, characterized in that, The temperature of the solvothermal reaction in step (2) is 100~130℃, and the reaction time is 3~4 days.

6. A functional diaphragm, comprising a diaphragm and a modifying layer stacked together, wherein the modifying layer contains a cyano-covalent organic framework material and a binder; The cyano-covalent organic framework material is the cyano-covalent organic framework material of claim 1 or the cyano-covalent organic framework material prepared by the preparation method of any one of claims 2 to 5.

7. The functional diaphragm according to claim 6, characterized in that, The thickness of the modified layer is >4μm.

8. The functional diaphragm according to claim 7, characterized in that, The thickness of the modified layer is 5~20μm.

9. A method for preparing the functional diaphragm according to any one of claims 6 to 8, comprising the following steps: 1) A cyano-covalent organic framework material, binder, and organic solvent are mixed to obtain a slurry; 2) Coat the slurry obtained in step 1) onto the surface of the diaphragm to obtain a functional diaphragm.

10. The preparation method according to claim 9, characterized in that, In step 1), the mass ratio of cyano-covalent organic framework material to binder is 6:4 to 9:1.

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