Electron-withdrawing group-terminated polyimide covalent organic framework material as well as preparation method and application thereof

Abstract

The invention relates to an electron withdrawing group terminated polyimide covalent organic framework material as well as a preparation method and application thereof. According to the invention, 4, 4 '-(hexafluoroisopropenyl) diphthalic anhydride and a tetra (4-aminophenyl) methane monomer are utilized to prepare a polyamic acid covalent organic framework, and 4-cyanophenyl, 4-aminotrifluorotoluene, 4-nitroaniline and 4-chloroaniline are utilized to perform end capping, so that the electron withdrawing group-terminated polyimide covalent organic framework material is prepared. The rich pore structure of the COF skeleton provides a rapid ion transport channel, the transport of current carriers is promoted, and the electron cloud density of the COF skeleton is adjusted through Lewis acidity formed by an end-capping electron withdrawing group through an induction effect, so that an electron-deficient domain is constructed. When being used in a lithium metal battery system, the material can guide uniform distribution of lithium ions and solve the problem of lithium dendrite growth; when the material is loaded on a lithium-sulfur battery positive electrode material, polysulfide can be pulled to move directionally to complete order reduction conversion reaction, so that the lithium-sulfur battery with long cycle life is obtained.

Description

Technical Field

[0001] This invention relates to the field of functional polymer materials and energy storage technology, and in particular to an electron-withdrawing group-terminated polyimide covalent organic framework material, its preparation method, and its application. Background Technology

[0002] With the global energy structure shifting towards cleaner and lower-carbon energy sources, the demand for high-capacity, long-life, and highly secure energy storage devices is experiencing explosive growth in fields such as new energy vehicles, smart grids, and portable electronic devices. Among these, lithium-sulfur batteries, with their advantages of high theoretical specific capacity of sulfur cathodes (1675mAh / g), abundant resource reserves, low cost, and environmental friendliness, are considered one of the core directions of next-generation high-energy-density energy storage technologies.

[0003] However, the current performance improvement of lithium-sulfur batteries is still limited by the inherent defects of electrode materials. The core problems are concentrated in the following three aspects: (1) Traditional electrode carrier materials lack precise functional group regulation, have weak adsorption capacity for key intermediates in the energy storage process, and are prone to intermediate dissolution and loss, resulting in rapid capacity decay; (2) Most framework materials have disordered pore structure and low crystallinity, which cannot provide regular confined space for ion transport, resulting in low mass transfer efficiency and poor rate performance. In addition, the material itself has insufficient conductivity, which further restricts the electron conduction dynamics; (3) Existing materials have a single surface chemical active site, which makes it difficult to guide the reaction directionally by regulating the electronic structure.

[0004] To address these issues, researchers have attempted to optimize performance through material structure design and functional modification, such as introducing polar groups and constructing ordered channels in framework materials. However, existing solutions still have significant limitations: while some materials incorporate functional groups, this can lead to channel collapse; some crystalline framework materials lack precise electronic structure control, failing to balance "structural stability" and "functional guidance." Therefore, developing a novel framework material that combines high crystallinity, precise electron-withdrawing group modification, and excellent chemical stability is crucial for overcoming the current performance bottlenecks of energy storage devices and is a core challenge that urgently needs to be addressed by those skilled in the art. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide an electron-withdrawing group-terminated polyimide covalent organic framework material, its preparation method, and its application. By designing electron-withdrawing groups to end the polyimide covalent organic framework material, the resulting Lewis acidity precisely adjusts the electron cloud density of the COF framework, constructing electron-deficient domains. Its application in lithium-sulfur battery cathode materials can improve their cycle life.

[0006] First aspect: An electron-withdrawing group-terminated polyimide covalent organic framework material, with the following structural formula:

[0007] Among them, the end-capping group R is an electron-withdrawing group, and its structural formula is as follows: , , and At least one of them; n is the number of repeating units.

[0008] The electron-withdrawing group-terminated polyimide covalent organic framework (PI-COF) material of this invention uses a polyimide organic framework as a substrate. The rich porous structure of the COF skeleton provides rapid ion transport channels, promoting carrier transport. Simultaneously, end-capping groups with different electron-withdrawing intensities are introduced. Through the Lewis acidity formed by the inductive effect, the electron cloud density of the COF skeleton is precisely adjusted. The end-capping agent induces the formation of delocalized electron domains, guiding the desolvation behavior of electrolyte molecular clusters at the COF interface. When loaded onto a metal to form a composite negative electrode, the differential charge distribution of the rich heterogeneous segments of the COF skeleton allows for redox reactions in the electrolyte. This reduction process and its products can construct an SEI film with high ionic conductivity, high mechanical strength, and high stability. This SEI film effectively inhibits the growth of lithium dendrites and the decomposition of the electrolyte, thereby reducing capacity loss and improving the battery's coulombic efficiency, cycle life, and safety.

[0009] When PI-COF is loaded onto the host material of lithium-sulfur battery cathode, electron-withdrawing groups construct electron-deficient domains at and near the ends of the COF backbone. These electron-deficient domains, acting as Lewis acid sites, achieve efficient anchoring of polysulfides through strong chemisorption, guiding their directional migration into the host pores. Simultaneously, these electron-deficient domains act as efficient electron traps, polarizing SS bonds and significantly lowering the energy barrier for conversion reactions. This effectively catalyzes the breaking / formation of SS bonds in polysulfides, synergistically suppressing the shuttle effect and enhancing reaction kinetics.

[0010] The second aspect: A method for preparing an electron-withdrawing group-terminated polyimide covalent organic framework material as described in the first aspect includes the following steps: 4,4'-(hexafluoroisopropene)phthalic anhydride and tetra(4-aminophenyl)methane were mixed with a first solvent and heated under an inert gas atmosphere to carry out a nucleophilic acylation reaction to obtain a polyamic acid covalent organic framework. The polyamic acid covalent organic framework is mixed with a capping agent and heated to carry out a capping reaction and a condensation cyclization reaction to obtain the electron-withdrawing group-capped polyimide covalent organic framework material. The first solvent is a mixture of o-dichlorobenzene and n-butanol; The capping agent is at least one of 4-aminobenzonitrile, 4-aminotrifluorotoluene, 4-nitroaniline and 4-chloroaniline.

[0011] This invention synthesizes a polyamic acid covalent organic framework (PAA-COF) using 4,4'-(hexafluoroisopropene)phthalic anhydride and tetra(4-aminophenyl)methane as monomers. PAA-COF has a three-dimensional network structure. At the same time, 4-aminobenzonitrile, 4-aminotrifluorotoluene, 4-nitroaniline, and 4-chloroaniline are used as end-capping agents to construct a highly ordered polyimide covalent organic framework through condensation and substitution reactions. The crystal structure of this material is formed by the periodic and orderly arrangement of imide rings and aromatic rings in space through covalent bonds. The stable three-dimensional structure can maintain its integrity during long-term battery charge-discharge cycles, preventing performance degradation caused by material collapse.

[0012] As a preferred embodiment, the method further includes the following steps: mixing the electron-withdrawing group-terminated polyimide covalent organic framework material with a second solvent, and performing Soxhlet extraction and vacuum drying; the second solvent is a mixture of ethanol and dichloromethane, wherein the volume ratio of ethanol to dichloromethane is 1.0:1.0-1.3.

[0013] The role of dichloromethane: to remove small organic molecules and oligomers. Dichloromethane is a moderately polar, low-boiling-point (39.6℃) organic solvent. It can effectively dissolve most organic monomer raw materials, intermediates, and unreacted oligomers (low molecular weight polyimides) used in synthesis. However, dichloromethane has extremely poor solubility for fully polymerized, highly cross-linked crystalline polyimide COF, and exhibits a certain selectivity for the molecular weight of the product COF, thus avoiding product loss.

[0014] The role of ethanol: to remove highly polar impurities and ionic residues. Ethanol is a highly polar, water-soluble protic solvent. Its main purpose is to remove impurities that dichloromethane cannot effectively handle. COF synthesis reactions may use metal catalysts or produce salt byproducts (such as acid anhydrides, pyridinium salts, etc.). Ethanol has a good ability to dissolve and carry away these ionic impurities. Furthermore, ethanol and water are miscible, which can also remove trace amounts of moisture that may be present in the COF pores.

[0015] Mixed use: The boiling points of the mixed solvents are between those of the two solvents, which can improve extraction efficiency. More importantly, dichloromethane is used first to remove most of the organic impurities, and then ethanol is used to remove polar residues and water. This is a gradient purification approach that can ensure the cleanliness of the COF channels to the greatest extent.

[0016] High proportions of ethanol (e.g., ethanol:dichloromethane = 4:1 or higher): The mixed solvent is too polar. Although the polyimide COF backbone is stable, the high polarity of ethanol may cause a slight swelling effect on some incompletely closed ring-shaped imide segments or localized areas of the backbone through hydrogen bonding and other interactions, especially under prolonged high-temperature Soxhlet extraction. This may slightly damage crystallinity, leading to partial collapse or deformation of the pores.

[0017] High proportion of dichloromethane (e.g., ethanol:dichloromethane = 1:4 or lower): The mixed solvent has too weak polarity, resulting in a severe lack of ability to dissolve and carry catalyst metal ions, salt byproducts, and trace amounts of water.

[0018] High-purity electron-withdrawing group-terminated polyimide covalent organic framework materials can be obtained through Soxhlet extraction and vacuum drying.

[0019] As a preferred embodiment, the Soxhlet extraction temperature is 60-70℃ and the time is 12-13h; the vacuum drying temperature is 80-90℃ and the time is 12-13h.

[0020] As a preferred embodiment, after the 4,4'-(hexafluoroisopropene)phthalic anhydride and the tetra(4-aminophenyl)methane are mixed with the first solvent, a catalyst is also added; the catalyst is isoquinoline, and the molar ratio of the isoquinoline to the 4,4'-(hexafluoroisopropene)phthalic anhydride is 1:20; in the first solvent, the volume ratio of the o-dichlorobenzene to the n-butanol is 9.6-10.0:1.0.

[0021] As a preferred embodiment, the nucleophilic acylation reaction is carried out at a temperature of 120-130℃ for 2-3 hours; the end-capping reaction and condensation cyclization reaction consist of four stages: in stage one, the temperature is increased from 120℃ to 180℃ at a rate of 2℃ / min; in stage two, the temperature is 180℃ for 6-8 hours; in stage three, the temperature is increased from 180℃ to 200℃ at a rate of 2℃ / min; and in stage four, the temperature is 200℃ for 12-14 hours.

[0022] The goal of Stage 1 is controlled prepolymerization and initial cyclization. Starting at 120°C, the monomers and capping agents gradually dissolve and undergo condensation polymerization. Slow heating ensures a gentle start to the reaction, avoiding localized overheating or rapid gelation, which is beneficial for the orderly arrangement of molecular chains and lays the foundation for subsequent crystallization.

[0023] If the temperature is too low or the temperature rises too quickly, it may lead to uneven reaction, a wide molecular weight distribution, poor prepolymer order, and ultimately a decrease in the crystallinity of COF. If the temperature rises too slowly, it unnecessarily prolongs the process time.

[0024] The purpose of stage two is the main polycondensation and cyclization. 180℃ is the most efficient temperature for imidization reactions; at this temperature: Accelerated dehydration cyclization: The polyamic acid intermediate rapidly closes its ring to form an imide bond, driving molecular weight growth.

[0025] Dynamic covalent repair: The formation of polyimide COF often involves reversible reactions. Isothermal conditions provide sufficient time for bond breakage and recombination, repairing defects and improving crystallinity.

[0026] The capping reaction proceeds: the electron-withdrawing capping agent reacts with the end of the growing chain to control the molecular weight and introduce electron-withdrawing groups.

[0027] If the reaction time is insufficient: the reaction is incomplete, the molecular weight is low, the end-capping rate is low, the COF crystallization is incomplete, and the specific surface area and porosity decrease.

[0028] If the reaction time is too long, it may lead to overgrowth or partial degradation (although polyimide is stable, electron-withdrawing groups may be sensitive), increased energy consumption, and limited benefits.

[0029] The purpose of stage three is to achieve post-curing and perfect crystallization. At this temperature: Drive residual reaction: Completely cyclize residual ammonium acid to ensure high molecular weight and chemical stability.

[0030] Promoting complete end-capping: Higher temperatures may promote the final reaction between the end-capping agent and the chain end, ensuring the density of electron-withdrawing groups.

[0031] Structural annealing: Slow heating further orders the COF lattice and eliminates internal stress.

[0032] Effect of heating rate: Slow heating avoids temperature shocks that could cause violent boiling of the solvent or localized overheating of the material, ensuring a smooth transition.

[0033] The purpose of stage four is to achieve deep curing and optimized crystallization. Prolonged high-temperature treatment has the following effects: Complete reaction: bringing condensation and cyclization into equilibrium to obtain the highest possible molecular weight and degree of crosslinking.

[0034] Maximizing crystallinity: Long-term annealing promotes grain growth and defect repair, increasing specific surface area.

[0035] Stabilization: Processed at temperatures above the battery operating temperature to ensure the thermal stability of the material.

[0036] If the reaction time is insufficient: the crystallinity is insufficient, the pore regularity is poor, which affects ion transport and exposure of active sites.

[0037] If the reaction time is too long, it may lead to partial decomposition or pore collapse. However, polyimide has good heat resistance, so the impact is usually minimal.

[0038] As a preferred embodiment, the molar ratio of 4,4'-(hexafluoroisopropene)phthalic anhydride to tetrakis(4-aminophenyl)methane is 3:1. This ratio is the optimal molar ratio, resulting in electron-withdrawing group-terminated polyimide covalent organic framework materials with the best structural stability.

[0039] As a preferred embodiment, the molar ratio of the capping agent to the 4,4'-(hexafluoroisopropene)phthalic anhydride is 1.5-2.0:1.0. This ratio is the optimal molar ratio, resulting in a fully capped electron-withdrawing group-capped polyimide covalent organic framework material with a relatively small amount of capping agent required.

[0040] Third aspect: An application of a polyimide covalent organic framework material with electron-withdrawing groups as described in the first aspect is disclosed for use as a cathode material in lithium-sulfur batteries. It provides a confined space and transformation site for polysulfide reactions on the substrate, and the electron-withdrawing groups construct electron-deficient domains, thereby guiding the directional movement of polysulfides to complete the down-order conversion reaction.

[0041] Fourth aspect: An application of a polyimide covalent organic framework material with electron-withdrawing groups as described in the first aspect is disclosed for use as a protective layer in lithium metal batteries. Utilizing its dense framework structure and good interfacial compatibility, a stable SEI interface is formed, thereby suppressing the uncontrolled growth of lithium dendrites.

[0042] In other embodiments, PI-COF can also be prepared as a solid electrolyte, using its excellent ion conduction properties to replace traditional liquid electrolytes, thereby significantly improving battery safety. Attached Figure Description

[0043] Figure 1 This is a schematic diagram illustrating the synthesis of PI-COF prepared in an embodiment of the present invention.

[0044] Figure 2 This is a schematic diagram of the structure of PI-COF prepared in an embodiment of the present invention.

[0045] Figure 3 This is a TEM image of PI-COF-1 prepared in Example 1.

[0046] Figure 4 The infrared spectrum is that of PI-COF-1 prepared in Example 1.

[0047] Figure 5 The image shows the cycle performance curve of the lithium-sulfur battery prepared in Example 1. Detailed Implementation

[0048] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0049] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0050] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0051] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0052] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0053] An electron-withdrawing group-terminated polyimide covalent organic framework material has the following structure:

[0054] Among them, the end-capping group R is an electron-withdrawing group, and its structural formula is as follows: , , and At least one of them; n is the number of repeating units.

[0055] Specifically, it can be one of formulas I-IV.

[0056]

[0057]

[0058]

[0059] A method for preparing an electron-withdrawing group-terminated polyimide covalent organic framework material, such as... Figure 1As shown, it includes the following steps: 4,4'-(hexafluoroisopropene)phthalic anhydride and tetra(4-aminophenyl)methane were vacuum dried at 60 °C for 12 h to remove moisture and impurities. They were then mixed with a first solvent, a catalyst was added, and the mixture was heated under an inert gas atmosphere to carry out a nucleophilic acylation reaction to obtain a polyamic acid covalent organic framework.

[0060] Polyamic acid covalent organic frameworks were mixed with end-capping agents and heated to carry out end-capping and condensation cyclization reactions. After the reaction was completed, the mixture was mixed with a second solvent and subjected to Soxhlet extraction and vacuum drying to obtain electron-withdrawing group-terminated polyimide covalent organic framework materials.

[0061] The molar ratio of 4,4'-(hexafluoroisopropene)phthalic anhydride to tetrakis(4-aminophenyl)methane is 3:1.

[0062] The end-capping agent is at least one of 4-aminobenzonitrile, 4-aminotrifluorotoluene, 4-nitroaniline and 4-chloroaniline; the molar ratio of the end-capping agent to 4,4'-(hexafluoroisopropene)phthalic anhydride is 1.5-2.0:1.0.

[0063] The catalyst is isoquinoline, and the molar ratio of isoquinoline to 4,4'-(hexafluoroisopropene)phthalic anhydride is 1:20.

[0064] The first solvent is a mixture of o-dichlorobenzene and n-butanol, with a volume ratio of o-dichlorobenzene to n-butanol of 9.6-10.0:1.0.

[0065] The second solvent is a mixture of ethanol and dichloromethane, with a volume ratio of ethanol to dichloromethane of 1.0:1.0-1.3.

[0066] The nucleophilic acylation reaction was carried out at a temperature of 120-130℃ for 2-3 hours. The end-capping and condensation cyclization reactions consisted of four stages: in stage one, the temperature was increased from 120℃ to 180℃ at a rate of 2℃ / min; in stage two, the temperature was 180℃ for 6-8 hours; in stage three, the temperature was increased from 180℃ to 200℃ at a rate of 2℃ / min; and in stage four, the temperature was 200℃ for 12-14 hours. The Soxhlet extraction was carried out at a temperature of 60-70℃ for 12-13 hours. The vacuum drying was carried out at a temperature of 80-90℃ for 12-13 hours.

[0067] Figure 2 This is a schematic diagram of a PI-COF structure.

[0068] Application of an electron-withdrawing group-terminated polyimide covalent organic framework material in lithium metal batteries and lithium-sulfur batteries.

[0069] Electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF) was mixed and ground with sulfur powder, and then placed in a reaction vessel and heated in a forced-air dryer at 155℃ for 12 hours to obtain a mixed material. The mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder were mixed, and a third solvent was added. The mixture was stirred thoroughly until a uniform paste was formed. The paste was coated onto an aluminum foil current collector with a scraper, and vacuum dried to evaporate the third solvent. The paste was then cut into electrodes with a diameter of 15 mm (active material loading of about 1 mg) to obtain the PI-COF / S composite cathode.

[0070] The mass ratio of sulfur powder to PI-COF is 7.0-7.3:3.

[0071] The mass ratio of the mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder is 7.0-7.3:2:1.

[0072] The third solvent is N-methylpyrrolidone (NMP), used in an amount 19 times the mass of PVDF.

[0073] Vacuum drying is performed at a heating temperature of 40-80℃ for 6-12 hours to allow the third solvent to evaporate.

[0074] A lithium-sulfur battery comprising a PI-COF / S composite cathode.

[0075] Example 1 1. Synthesis of electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF-1) (1) Weigh 19.98 mg (0.045 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) and 5.7 mg (0.015 mmol) of tetra(4-aminophenyl)methane, place them in a vacuum drying oven, and vacuum dry at 60°C for 12 h to remove moisture and impurities from the raw materials.

[0076] (2) The dried 6FDA and tetra(4-aminophenyl)methane were transferred to a Schlenk tube, and the first solvent (2.4 mL of o-dichlorobenzene + 0.25 mL of n-butanol, volume ratio 9.6:1.0) was added, followed by 0.35 mg of isoquinoline catalyst. Under an inert argon atmosphere, the temperature was raised to 120 °C and the reaction was kept at a constant temperature for 2.5 h to carry out a nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF); (3) Add 10.62 mg (0.09 mmol) of 4-aminobenzonitrile to PAA-COF. Proceed with the temperature program: increase the temperature to 180℃ at 2℃ / min (stage 1), hold at 180℃ for 6 h (stage 2), increase the temperature to 200℃ at 2℃ / min (stage 3), and hold at 200℃ for 12 h (stage 4).

[0077] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), extracted with Soxhlet at 60°C for 12.5 h, and then vacuum dried at 80°C for 12.5 h to obtain 4-aminobenzonitrile-terminated polyimide covalent organic framework material (PI-COF-1).

[0078] Figure 3 The TEM image of PI-COF-1 shows that PI-COF-1 has a sheet-like structure, mainly consisting of two-dimensional planar and multi-dimensional stacked structures, which is highly consistent with the theoretically simulated polyimide framework. No aggregation or phase separation was observed throughout the field of view, indicating that the polyimide segments are uniformly arranged within the covalent framework, forming a highly regular two-dimensional porous structure.

[0079] Figure 4 The infrared spectrum of PI-COF-1 shows that the infrared spectrum of PI-COF-1 is at 1781 cm⁻¹. -1 Approximately 1720cm -1 Strong characteristic double peaks were observed at the point, attributed to the asymmetric and symmetric stretching vibrations of the C=O ring of the imide ring, respectively. This evidence fully confirms that the method of this invention successfully synthesized a structurally complete polyimide COF material.

[0080] 2. Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and PI-COF-1 in a mortar at a mass ratio of 7:3 and grind and mix them for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder in a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the PI-COF-1 / S composite cathode.

[0081] (2) The PI-COF-1 / S composite cathode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0082] Figure 5 The graph shows the cycle performance of the prepared lithium-sulfur battery.

[0083] 3. Preparation of PI-COF-1 based solid electrolyte Weigh 120 mg of PI-COF-1 powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and sonicate for 40 min until the mixture is homogeneous and forms a uniform viscous slurry. Coat the slurry evenly into a polytetrafluoroethylene mold, first vacuum dry at 60 °C for 8 h to remove the solvent, and then heat to 100 °C and vacuum dry for 6 h to completely remove impurities, to obtain a PI-COF-1 based solid electrolyte membrane with a thickness of about 60 μm, for later use.

[0084] 4. Preparation of PI-COF-1 negative electrode protective layer Weigh 25 mg of PI-COF powder and disperse it in 12 mL of NMP. Sonicate for 30 min to form a stable dispersion. In an argon glove box (water and oxygen values ​​< 0.01 ppm), take 35 μL of the dispersion and uniformly drop it onto the surface of a lithium metal sheet with a diameter of 15 mm. Place the lithium metal sheet on a 60 °C heating table and heat for 6 h to allow the solvent to evaporate completely. After cooling to room temperature, a modified lithium metal anode with a PI-COF-1 protective layer on the surface is obtained.

[0085] Example 2 1. Synthesis of electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF-2) (1) Weigh 19.98 mg (0.045 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) and 5.7 mg (0.015 mmol) of tetra(4-aminophenyl)methane, place them in a vacuum drying oven, and vacuum dry at 60°C for 12 h to remove moisture and impurities from the raw materials.

[0086] (2) The dried 6FDA and tetra(4-aminophenyl)methane were transferred to a Schlenk tube, and the first solvent (2.4 mL of o-dichlorobenzene + 0.25 mL of n-butanol, volume ratio 9.6:1.0) was added, followed by 0.35 mg of isoquinoline catalyst. Under an inert argon atmosphere, the temperature was raised to 120 °C and the reaction was kept at a constant temperature for 2.5 h to carry out a nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF); (3) Add 14.49 mg (0.09 mmol) of 4-aminotrifluorotoluene to PAA-COF. Proceed with the temperature program: increase the temperature to 180℃ at 2℃ / min (stage 1), hold at 180℃ for 6 h (stage 2), increase the temperature to 200℃ at 2℃ / min (stage 3), and hold at 200℃ for 12 h (stage 4).

[0087] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), extracted with Soxhlet at 60°C for 12.5 h, and then vacuum dried at 80°C for 12.5 h to obtain 4-aminotrifluorotoluene-terminated polyimide covalent organic framework material (PI-COF-2).

[0088] 2. Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and PI-COF-2 at a mass ratio of 7:3 and grind and mix them in a mortar for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder at a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the PI-COF-2 / S composite cathode.

[0089] (2) The PI-COF-2 / S composite cathode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0090] 3. Preparation of PI-COF-2 based solid electrolyte Weigh 120 mg of PI-COF-2 powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and sonicate for 40 min until the mixture is homogeneous and forms a uniform viscous slurry. Coat the slurry evenly into a polytetrafluoroethylene mold, first vacuum dry at 60 °C for 8 h to remove the solvent, and then heat to 100 °C and vacuum dry for 6 h to completely remove impurities, to obtain a PI-COF-based solid electrolyte membrane with a thickness of about 60 μm, for later use.

[0091] 4. Preparation of PI-COF-2 negative electrode protective layer Weigh 25 mg of PI-COF powder and disperse it in 12 mL of NMP. Sonicate for 30 min to form a stable dispersion. In an argon glove box (water and oxygen values ​​< 0.01 ppm), take 35 μL of the dispersion and uniformly drop it onto the surface of a lithium metal sheet with a diameter of 15 mm. Place the lithium metal sheet on a 60 °C heating table and heat for 6 h to allow the solvent to evaporate completely. After cooling to room temperature, a modified lithium metal anode with a PI-COF-2 protective layer on the surface is obtained.

[0092] Example 3 1. Synthesis of electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF-3) (1) Weigh 19.98 mg (0.045 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) and 5.7 mg (0.015 mmol) of tetra(4-aminophenyl)methane, place them in a vacuum drying oven, and vacuum dry at 60°C for 12 h to remove moisture and impurities from the raw materials.

[0093] (2) The dried 6FDA and tetra(4-aminophenyl)methane were transferred to a Schlenk tube, and the first solvent (2.4 mL of o-dichlorobenzene + 0.25 mL of n-butanol, volume ratio 9.6:1.0) was added, followed by 0.35 mg of isoquinoline catalyst. Under an inert argon atmosphere, the temperature was raised to 120 °C and the reaction was kept at a constant temperature for 2.5 h to carry out a nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF); (3) Add 12.42 mg (0.09 mmol) of 4-nitroaniline to PAA-COF. Proceed with the temperature program: increase the temperature to 180℃ at 2℃ / min (stage 1), hold at 180℃ for 6 h (stage 2), increase the temperature to 200℃ at 2℃ / min (stage 3), and hold at 200℃ for 12 h (stage 4).

[0094] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), extracted with Soxhlet at 60°C for 12.5 h, and then vacuum dried at 80°C for 12.5 h to obtain 4-nitroaniline-terminated polyimide covalent organic framework material (PI-COF-3).

[0095] 2. Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and PI-COF-3 at a mass ratio of 7:3 and grind and mix them in a mortar for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder at a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the PI-COF-3 / S composite cathode.

[0096] (2) The PI-COF-3 / S composite cathode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0097] 3. Preparation of PI-COF-3 based solid electrolyte Weigh 120 mg of PI-COF powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and sonicate for 40 min until the mixture is homogeneous and forms a uniform viscous slurry. Coat the slurry evenly into a polytetrafluoroethylene mold, first vacuum dry at 60 °C for 8 h to remove the solvent, and then heat to 100 °C and vacuum dry for 6 h to completely remove impurities, to obtain a PI-COF-3 based solid electrolyte membrane with a thickness of about 60 μm, for later use.

[0098] 4. Preparation of PI-COF-3 negative electrode protective layer Weigh 25 mg of PI-COF powder and disperse it in 12 mL of NMP. Sonicate for 30 min to form a stable dispersion. In an argon glove box (water and oxygen values ​​< 0.01 ppm), take 35 μL of the dispersion and uniformly drop it onto the surface of a lithium metal sheet with a diameter of 15 mm. Place the lithium metal sheet on a 60 °C heating table and heat for 6 h to allow the solvent to evaporate completely. After cooling to room temperature, a modified lithium metal anode with a PI-COF-3 protective layer on the surface is obtained.

[0099] Example 4 1. Synthesis of electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF-4) (1) Weigh 19.98 mg (0.045 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) and 5.7 mg (0.015 mmol) of tetra(4-aminophenyl)methane, place them in a vacuum drying oven, and vacuum dry at 60°C for 12 h to remove moisture and impurities from the raw materials.

[0100] (2) The dried 6FDA and tetra(4-aminophenyl)methane were transferred to a Schlenk tube, and the first solvent (2.4 mL of o-dichlorobenzene + 0.25 mL of n-butanol, volume ratio 9.6:1.0) was added, followed by 0.35 mg of isoquinoline catalyst. Under an inert argon atmosphere, the temperature was raised to 120 °C and the reaction was kept at a constant temperature for 2.5 h to carry out a nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF); (3) Add 11.48 mg (0.09 mmol) of 4-chloroaniline to PAA-COF. Increase the temperature according to the program: increase the temperature to 180℃ at 2℃ / min (stage 1), hold at 180℃ for 6 h (stage 2), increase the temperature to 200℃ at 2℃ / min (stage 3), hold at 200℃ for 12 h (stage 4).

[0101] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), extracted with Soxhlet at 60°C for 12.5 h, and then vacuum dried at 80°C for 12.5 h to obtain 4-chloroaniline-terminated polyimide covalent organic framework material (PI-COF-4).

[0102] 2. Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and PI-COF-4 at a mass ratio of 7:3 and grind and mix them in a mortar for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder at a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the PI-COF-4 / S composite cathode.

[0103] (2) The PI-COF-4 / S composite cathode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0104] 3. Preparation of PI-COF-4 based solid electrolyte Weigh 120 mg of PI-COF-4 powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and sonicate for 40 min until the mixture is homogeneous and forms a uniform viscous slurry. Coat the slurry evenly into a polytetrafluoroethylene mold, first vacuum dry at 60 °C for 8 h to remove the solvent, and then heat to 100 °C and vacuum dry for 6 h to completely remove impurities, to obtain a PI-COF-based solid electrolyte membrane with a thickness of about 60 μm, for later use.

[0105] 4. Preparation of PI-COF-4 negative electrode protective layer Weigh 25 mg of PI-COF powder and disperse it in 12 mL of NMP. Sonicate for 30 min to form a stable dispersion. In an argon glove box (water and oxygen values ​​< 0.01 ppm), take 35 μL of the dispersion and uniformly drop it onto the surface of a lithium metal sheet with a diameter of 15 mm. Place the lithium metal sheet on a 60 °C heating table and heat for 6 h to allow the solvent to evaporate completely. After cooling to room temperature, a modified lithium metal anode with a PI-COF-4 protective layer on the surface is obtained.

[0106] Comparative Example 1 1. Synthesis of uncapped polyimide covalent organic framework materials (1) Weigh 19.98 mg (0.045 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) and 5.7 mg (0.015 mmol) of tetra(4-aminophenyl)methane, place them in a vacuum drying oven, and vacuum dry at 60°C for 12 h to remove moisture and impurities from the raw materials.

[0107] (2) The dried 6FDA and tetra(4-aminophenyl)methane were transferred to a Schlenk tube, and the first solvent (2.4 mL of o-dichlorobenzene + 0.25 mL of n-butanol, volume ratio 9.6:1.0) was added, followed by 0.35 mg of isoquinoline catalyst. Under an inert argon atmosphere, the temperature was raised to 120 °C and the reaction was kept at a constant temperature for 2.5 h to carry out a nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF); (3) Heat up according to the program: heat up to 180℃ at 2℃ / min (stage one), keep at 180℃ for 6 hours (stage two), heat up to 200℃ at 2℃ / min (stage three), keep at 200℃ for 12 hours (stage four).

[0108] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane in a volume ratio of 1.0:1.2), extracted with Soxhlet at 60°C for 12.5 h, and then vacuum dried at 80°C for 12.5 h to obtain uncapped polyimide covalent organic framework material.

[0109] 2. Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and uncapped polyimide covalent organic framework material at a mass ratio of 7:3 and grind and mix them in a mortar for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder at a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the composite cathode.

[0110] (2) The composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0111] Comparative Example 2 Preparation of lithium-sulfur batteries (1) Weigh sulfur powder and carbon nanotubes as cathode carriers in a mortar at a mass ratio of 7:3 and grind and mix them for 30 min. Then transfer the mixed powder to a hydrothermal reactor, seal it, and place it in a 155℃ forced-air oven for 12 h. After natural cooling, the composite sulfur cathode material is obtained. Weigh the above mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder in a mass ratio of 7:2:1 and mix them. Add N-methylpyrrolidone (NMP) at a mass ratio of 19 times that of PVDF and stir thoroughly until a uniform paste is formed. Coat the paste onto an aluminum foil current collector with a scraper, dry it under vacuum at 60℃ for 8 h, and cut it into electrode sheets with a diameter of 13 mm (active material loading of about 1.2 mg) to obtain the composite cathode.

[0112] (2) The composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of button batteries. The button battery model was CR2032, the separator was a polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (acetylated ethyl methyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0113] The lithium-sulfur batteries prepared in Examples 1-4 and Comparative Examples 1-2 were subjected to electrochemical performance tests on the Xinwei testing system. The voltage window for the tests was 1.7~2.8V. The test results are shown in Table 1.

[0114] Table 1 Performance test results of lithium-sulfur batteries prepared in Examples 1-4 and Comparative Examples 1-2

[0115] As can be seen from Table 1, the lithium-sulfur battery assembled in the embodiments of the present invention has significantly improved the first-cycle discharge specific capacity and capacity retention rate compared with the lithium-sulfur battery assembled in the comparative example.

[0116] The above embodiments are merely illustrative of several implementations of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the invention patent. For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. An electron-withdrawing group-terminated polyimide covalent organic framework material, characterized in that, The structural formula is: Among them, the end-capping group R is an electron-withdrawing group, and its structural formula is as follows: , , and At least one of them; n is the number of repeating units.

2. A method for preparing an electron-withdrawing group-terminated polyimide covalent organic framework material as described in claim 1, characterized in that, Includes the following steps: 4,4'-(hexafluoroisopropene)phthalic anhydride and tetra(4-aminophenyl)methane were mixed with a first solvent and heated under an inert gas atmosphere to carry out a nucleophilic acylation reaction to obtain a polyamic acid covalent organic framework. The polyamic acid covalent organic framework is mixed with a capping agent and heated to carry out a capping reaction and a condensation cyclization reaction to obtain the electron-withdrawing group-capped polyimide covalent organic framework material. The first solvent is a mixture of o-dichlorobenzene and n-butanol; The capping agent is at least one of 4-aminobenzonitrile, 4-aminotrifluorotoluene, 4-nitroaniline and 4-chloroaniline.

3. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 2, characterized in that, It also includes the following steps: The electron-withdrawing group-terminated polyimide covalent organic framework material is mixed with a second solvent and subjected to Soxhlet extraction and vacuum drying; the second solvent is a mixture of ethanol and dichloromethane, wherein the volume ratio of ethanol to dichloromethane is 1.0:1.0-1.

3.

4. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 3, characterized in that, The Soxhlet extraction temperature is 60-70℃ and the time is 12-13h; the vacuum drying temperature is 80-90℃ and the time is 12-13h.

5. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 2, characterized in that, After the 4,4'-(hexafluoroisopropene)phthalic anhydride and the tetra(4-aminophenyl)methane are mixed with the first solvent, a catalyst is also added; the catalyst is isoquinoline, and the molar ratio of the isoquinoline to the 4,4'-(hexafluoroisopropene)phthalic anhydride is 1:20; in the first solvent, the volume ratio of the o-dichlorobenzene to the n-butanol is 9.6-10.0:1.

0.

6. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 5, characterized in that, The nucleophilic acylation reaction is carried out at a temperature of 120-130℃ for 2-3 hours. The end-capping reaction and condensation cyclization reaction consist of four stages: in stage one, the temperature is increased from 120℃ to 180℃ at a rate of 2℃ / min; in stage two, the temperature is 180℃ for 6-8 hours; in stage three, the temperature is increased from 180℃ to 200℃ at a rate of 2℃ / min; and in stage four, the temperature is 200℃ for 12-14 hours.

7. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 2, characterized in that, The molar ratio of 4,4'-(hexafluoroisopropene)phthalic anhydride to tetrakis(4-aminophenyl)methane is 3:

1.

8. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 7, characterized in that, The molar ratio of the capping agent to the 4,4'-(hexafluoroisopropene) phthalic anhydride is 1.5-2.0:1.

0.

9. An application of the electron-withdrawing group-terminated polyimide covalent organic framework material as described in claim 1, characterized in that, Positive electrode material for lithium-sulfur batteries.

10. An application of the electron-withdrawing group-terminated polyimide covalent organic framework material as described in claim 1, characterized in that, Used as a protective layer for lithium metal batteries.

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