An electron-withdrawing group terminated polyimide covalent organic framework material and a preparation method and application thereof

Abstract

The present application relates to an electron-withdrawing group terminated polyimide covalent organic framework material and a preparation method and application thereof. The present application utilizes 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and tetra(4-aminophenyl)methane monomers to prepare a polyamide acid covalent organic framework, and utilizes 4-aminobenzonitrile, 4-amino-trifluorotoluene, 4-nitroaniline and 4-chloroaniline to perform termination, thereby preparing an electron-withdrawing group terminated polyimide covalent organic framework material. The rich pore structure of the COF framework provides a fast ion transmission channel, promotes the transport of carriers, the Lewis acidity formed by the end-capped electron-withdrawing group adjusts the electron cloud density of the COF framework through an inductive effect, and an electron-deficient domain is constructed. In a lithium metal battery system, it can guide the uniform distribution of lithium ions, solve the problem of lithium dendrite growth; when loaded on a lithium-sulfur battery positive electrode material, it can lead the directional movement of polysulfides to complete the down-order conversion reaction, thereby obtaining a lithium-sulfur battery with long cycle life.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of functional polymer materials and energy storage technology, in particular to an electron-withdrawing group terminated polyimide covalent organic framework material and a preparation method and application thereof. BACKGROUND

[0002] With the transformation of global energy structure to clean and low-carbon, the demand for high-capacity, long-life and high-safety energy storage devices in the fields of new energy vehicles, smart grids and portable electronic devices is growing explosively. Among them, lithium-sulfur batteries are considered as one of the core directions of the next generation of high-energy density energy storage technology due to their high theoretical specific capacity (1675 mAh / g) of sulfur positive electrode, abundant resource reserves, low cost and environmental friendliness.

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

[0004] To solve the above problems, researchers try to optimize the performance by material structure design and functional modification, such as introducing polar groups and constructing ordered pores in the framework material, but the existing schemes still have obvious limitations: some materials may lead to pore collapse after introducing functional groups; some crystalline framework materials lack precise electronic structure regulation, and cannot balance "structural stability" and "functional orientation". Therefore, developing a new type of framework material with high crystallinity, precise electron-withdrawing group modification and excellent chemical stability has become a key to breaking through the performance bottleneck of current energy storage devices, and is also a core problem that needs to be solved by technical personnel in the field. SUMMARY

[0005] Therefore, the purpose of the present application is to provide an electron-withdrawing group terminated polyimide covalent organic framework material and a preparation method and application thereof. By designing electron-withdrawing group termination in the polyimide covalent organic framework material, the Lewis acidity precisely regulates the electron cloud density of the COF skeleton, and an electron-deficient domain is constructed. The application in lithium-sulfur battery positive electrode material can improve its cycle life.

[0006] In a first aspect,

[0007] An electron-withdrawing group terminated polyimide covalent organic framework material has the following structural formula:

[0008]

[0009] wherein the end-capping group R is an electron-withdrawing group, and the structural formula is 、 、 and at least one; n is the number of repeating units.

[0010] The electron-withdrawing group end-capped polyimide covalent organic framework material (PI-COF) of the application takes polyimide organic framework as a substrate, and the rich pore structure of the COF framework provides a fast ion transmission channel, promoting the transport of carriers. Different end-capping groups with different electron-withdrawing strengths are introduced, the Lewis acidity formed by the induction effect precisely adjusts the electron cloud density of the COF framework, the end-capping agent induces the formation of a delocalized electron domain, and the desolvation behavior of electrolyte molecular clusters is guided at the COF interface. The composite negative electrode formed by loading on the metal can make the electrolyte undergo an oxidation-reduction reaction due to the rich heterogeneous segment distribution of the COF framework, which differentiates the charge, and the reduction process and product can construct a SEI film with high ionic conductivity, high mechanical strength and high stability. The SEI film can effectively inhibit the growth of lithium dendrites and the decomposition of electrolyte, reduce the capacity loss, and improve the coulombic efficiency, cycle life and safety of the battery.

[0011] When the PI-COF is loaded on the positive host material of the lithium-sulfur battery, the electron-withdrawing group constructs an electron-deficient domain at the end and near the region of the COF framework. As a Lewis acid site, the electron-deficient domain realizes efficient anchoring of polysulfides through strong chemical adsorption, guiding their directional migration to the inside of the host channel; at the same time, the electron-deficient domain as an efficient electron trap can polarize the S-S bond, significantly reducing its conversion reaction energy barrier, thereby effectively catalyzing the S-S bond breaking / forming of polysulfides, synergistically inhibiting the shuttle effect and improving the reaction kinetics.

[0012] The second aspect is:

[0013] A preparation method of the electron-withdrawing group end-capped polyimide covalent organic framework material of the first aspect, comprising the following steps:

[0014] 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and tetra(4-aminophenyl) methane are mixed with a first solvent, and a nucleophilic acylation reaction is carried out under an inert gas atmosphere to obtain a polyimide acid covalent organic framework;

[0015] The polyimide acid covalent organic framework is mixed with an end-capping agent, and an end-capping reaction and a condensation cyclization reaction are carried out to obtain the electron-withdrawing group end-capped polyimide covalent organic framework material;

[0016] The first solvent is a mixture of o-dichlorobenzene and n-butanol.

[0017] The capping agent is at least one of 4-aminobenzonitrile, 4-amino-trifluorotoluene, 4-nitroaniline and 4-chloroaniline.

[0018] The present application synthesizes polyamide acid covalent organic framework (PAA-COF) with 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and tetrakis(4-aminophenyl) methane as monomers, PAA-COF has a three-dimensional grid structure, and 4-aminobenzonitrile, 4-amino-trifluorotoluene, 4-nitroaniline and 4-chloroaniline are used as capping agents to construct a highly ordered polyimide covalent organic framework through condensation and substitution reaction. The crystal structure of the material is formed by periodic ordering of imide rings and aromatic rings in space through covalent bonds, and the stable three-dimensional structure can maintain integrity in long-term battery charging and discharging cycles, preventing performance degradation caused by material collapse.

[0019] As a preferred solution, the method further comprises the steps of mixing the electron-withdrawing group-capped polyimide covalent organic framework material with a second solvent, performing Soxhlet extraction and vacuum drying; the second solvent is a mixture of ethanol and dichloromethane, and the volume ratio of ethanol to dichloromethane is 1.0:1.0-1.3.

[0020] The role of dichloromethane is to remove organic small molecules and oligomers. Dichloromethane is an organic solvent with medium polarity and low boiling point (39.6℃). It can effectively dissolve most of the organic monomer raw materials, intermediates and oligomers (low molecular weight polyimides) used in synthesis, while dichloromethane has poor solubility for fully polymerized and highly cross-linked crystalline polyimide COF, and has a certain selectivity for the molecular weight of the product COF, which can avoid product loss.

[0021] The role of ethanol is to remove high-polarity impurities and ion residues. Ethanol is a high-polarity, water-soluble protic solvent. Its main target is to remove impurities that cannot be effectively treated by dichloromethane. COF synthesis reaction may use metal catalysts or produce salt byproducts (such as anhydride, pyridine salt, etc.). Ethanol has good solubility and carrying capacity for such ionic impurities, and the mutual solubility of ethanol and water can also take away the trace amount of water that may exist in the COF channel.

[0022] Mixed use: the boiling point of the mixed solvent is between the two, which can improve the extraction efficiency. More importantly, dichloromethane is used to remove most of the organic impurities first, and then ethanol is used to remove the polar residues and moisture, which is a kind of gradient purification idea, which can maximize the cleanliness of the COF channel.

[0023] High proportion of ethanol (e.g. ethanol: dichloromethane = 4:1 or higher): the mixed solvent is too polar, the skeleton of the polyimide COF is stable, but the high polarity of ethanol may have a weak swelling effect on some incomplete ring-closed imide segments or local skeleton through hydrogen bonding, etc., especially under the long-term action of high-temperature Soxhlet extraction. This may slightly damage the crystallinity, causing partial pore collapse or deformation.

[0024] High proportion of dichloromethane (e.g. ethanol: dichloromethane = 1:4 or lower): the mixed solvent is too weak, and the solubility and carrying capacity of the catalyst metal ions, salt by-products and trace moisture are seriously insufficient.

[0025] Through Soxhlet extraction and vacuum drying, high-purity electron-withdrawing group-terminated polyimide COF materials can be obtained.

[0026] As a preferred solution, 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.

[0027] As a preferred solution, after the 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and the tetrakis(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'-(hexafluoroisopropylidene) diphthalic 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.

[0028] As a preferred solution, the temperature of the nucleophilic acylation reaction is 120-130℃, and the time is 2-3h; the termination reaction and the condensation cyclization reaction consist of 4 stages, stage one is heated from 120℃ to 180℃ at a rate of 2℃ / min, the temperature of stage two is 180℃, and the time is 6-8h, stage three is heated from 180℃ to 200℃ at a rate of 2℃ / min, the temperature of stage four is 200℃, and the time is 12-14h.

[0029] The purpose of stage one is controllable pre-polymerization and initial cyclization. Starting from 120℃, the monomers and termination agents are gradually dissolved and undergo polycondensation. Slow heating ensures a gentle start of the reaction, avoiding local overheating or rapid gelation, which is conducive to the orderly arrangement of molecular chains, laying the foundation for subsequent crystallization.

[0030] If the temperature is too low or the heating is too fast: it may lead to uneven reaction, wide molecular weight distribution, poor order of pre-polymers, and decreased crystallinity of the final COF. Slow heating: unnecessarily prolongs the process time.

[0031] The purpose of stage two is main body polycondensation and cyclization. 180℃ is an efficient temperature for imidization reaction, at which:

[0032] Dehydration cyclization acceleration: Rapid ring closure of polyamic acid intermediates forms imide bonds, driving molecular weight growth.

[0033] Dynamic covalent repair: The formation of polyimide COFs often involves reversible reactions. Constant temperature provides sufficient time for bond breaking-recombination, repairing defects and improving crystallinity.

[0034] End capping reaction: Electrophilic end capping agents react with the end of the growing chain, controlling molecular weight and introducing electron-withdrawing groups.

[0035] If the reaction time is insufficient: Incomplete reaction, low molecular weight, low end capping rate, COF crystallization is not perfect, specific surface area and porosity decrease.

[0036] If the reaction time is too long: May lead to excessive growth or partial degradation (although polyimides are stable, electron-withdrawing groups may be sensitive), increased energy consumption, and limited benefits.

[0037] The purpose of stage three is post-curing and crystallization perfection. At this temperature:

[0038] Drive residual reactions: Complete cyclization of residual amic acid to ensure high molecular weight and chemical stability.

[0039] Promote complete end capping: Higher temperatures may promote the final reaction of end capping agents with chain ends, ensuring electron-withdrawing group density.

[0040] Structural annealing: Slow heating allows the COF lattice to further order and eliminate internal stress.

[0041] Rising rate of temperature: Slow heating avoids temperature shock, which can cause solvent to boil violently or materials to overheat locally, ensuring smooth transition.

[0042] The purpose of stage four is deep curing and crystallization optimization. Long-term high-temperature treatment has the following effects:

[0043] Reaction completion: Allows polycondensation and cyclization to reach equilibrium, achieving the highest possible molecular weight and crosslinking degree.

[0044] Crystallinity maximization: Long-term annealing promotes grain growth and defect repair, increasing specific surface area.

[0045] Stabilization: Treatment at temperatures higher than battery operating temperatures ensures material thermal stability.

[0046] If the reaction time is insufficient: Insufficient crystallinity, poor channel regularity, affecting ion transport and active site exposure.

[0047] If the reaction time is too long: May cause partial decomposition or pore collapse. However, polyimides are heat-resistant, so the impact is usually minimal.

[0048] As a preferred solution, the molar ratio of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride to tetra(4-aminophenyl)methane is 3:1. This ratio is the optimal molar ratio, and the electron-withdrawing group-terminated polyimide covalent organic framework material prepared has the best structural stability.

[0049] As a preferred solution, the molar ratio of the capping agent to the 4,4'-(hexafluoroisopropylidene)diphthalic anhydride is 1.5-2.0:1.0. This ratio is the optimal molar ratio, and the electron-withdrawing group-terminated polyimide covalent organic framework material prepared can be completely capped, and the amount of capping agent used is less.

[0050] Third aspect:

[0051] An application of the electron-withdrawing group-terminated polyimide covalent organic framework material of the first aspect is used as a positive electrode material for lithium-sulfur batteries. It provides a confined space and a conversion site for polysulfide reactions for the substrate, and the electron-withdrawing group capping constructs an electron-deficient domain, thereby attracting the directional movement of polysulfides to complete the reduction conversion reaction.

[0052] Fourth aspect:

[0053] An application of the electron-withdrawing group-terminated polyimide covalent organic framework material of the first aspect is used as a protective layer for lithium metal batteries. By utilizing its dense framework structure and good interface compatibility, a stable SEI interface is formed, thereby inhibiting the uncontrollable growth of lithium dendrites.

[0054] In other embodiments, the PI-COF can also be prepared into a solid-state electrolyte, which replaces the traditional liquid electrolyte by utilizing its excellent ion conduction performance, thereby significantly improving the safety of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 A synthesis schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0056] Figure 2 A structure schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0057] Figure 3 A TEM image of the PI-COF-1 prepared in Example 1.

[0058] Figure 4 An infrared spectrum of the PI-COF-1 prepared in Example 1.

[0059] Figure 5 A cycle performance curve of the lithium-sulfur battery prepared in Example 1. DETAILED DESCRIPTION

[0060] The following detailed description of various exemplary embodiments of the application will not be considered to be limiting of the application, but rather a description of certain aspects, features, and embodiments of the application.

[0061] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Additionally, the use of the term "about" in relation to a value or a range of values is intended to include each individual intermediate value and each smaller range that falls within the range of values. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0062] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, preferred methods and materials are described. All publications mentioned in this specification are herein incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any reference in this specification is not intended as an admission that the reference is prior art, but rather that the reference is part of the technical literature that is relevant to A person of ordinary skill in the art.

[0063] Many modifications and variations of this application can be made in the light of the above teachings without departing from the spirit and scope thereof. Other implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples given are exemplary only.

[0064] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended terms that mean including, but not limited to.

[0065] An electron-withdrawing group-terminated polyimide covalent organic framework material has a structure of:

[0066]

[0067] wherein the terminal group R is an electron-withdrawing group, and has a structure of at least one of , , and ; n is the number of repeating units.

[0068] Specifically, it can be one of formulas I-IV.

[0069]

[0070]

[0071]

[0072] A method for preparing an electron-withdrawing group-terminated polyimide covalent organic framework material, such as... Figure 1 As shown, it includes the following steps:

[0073] 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.

[0074] 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.

[0075] The molar ratio of 4,4'-(hexafluoroisopropene)phthalic anhydride to tetrakis(4-aminophenyl)methane is 3:1.

[0076] 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.

[0077] The catalyst is isoquinoline, and the molar ratio of isoquinoline to 4,4'-(hexafluoroisopropene)phthalic anhydride is 1:20.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] Figure 2 This is a schematic diagram of a PI-COF structure.

[0082] Use of an electron-withdrawing group terminated polyimide covalent organic framework material in lithium metal batteries and lithium-sulfur batteries.

[0083] The electron-withdrawing group terminated polyimide covalent organic framework material (PI-COF) is mixed and ground with sulfur powder, placed in a reaction kettle, and heated in a blast drier at 155℃ for 12h to obtain a mixed material; the mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder are mixed, a third solvent is added, and the mixture is stirred thoroughly until a uniform paste slurry is formed; the slurry is coated on an aluminum foil current collector using a doctor blade, vacuum dried to volatilize the third solvent, cut into an electrode sheet (active material loading about 1mg) with a diameter of 15mm, and a PI-COF / S composite positive electrode is obtained.

[0084] The mass ratio of sulfur powder to PI-COF is 7.0-7.3:3.

[0085] The mass ratio of the mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder is 7.0-7.3:2:1.

[0086] The third solvent is N-methyl pyrrolidone (NMP), and the amount used is 19 times the mass of PVDF.

[0087] The heating temperature for vacuum drying to volatilize the third solvent is 40-80℃, and the heating time is 6-12h.

[0088] A lithium-sulfur battery comprising a PI-COF / S composite positive electrode.

[0089] Example 1

[0090] 1. Synthesis of an electron-withdrawing group terminated polyimide covalent organic framework material (PI-COF-1)

[0091] (1) 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) 19.98mg (0.045mmol) and tetra(4-aminophenyl)methane 5.7mg (0.015mmol) are weighed and placed in a vacuum drying box, dried at 60℃ for 12h to remove water and impurities in the raw materials.

[0092] (2) The dried 6FDA and tetra(4-aminophenyl)methane are transferred to a Schlenk tube, a first solvent (o-dichlorobenzene 2.4mL + n-butanol 0.25mL, volume ratio 9.6:1.0) is added, and a catalyst isoquinoline 0.35mg is added. Under an argon gas atmosphere, the temperature is raised to 120℃, and the nucleophilic acylation condensation reaction is carried out at a constant temperature for 2.5h to obtain a polyamide acid covalent organic framework (PAA-COF).

[0093] (3) To the PAA-COF, 4-aminobenzonitrile 10.62 mg (0.09 mmol) was added. The temperature was raised according to the procedure: 2 °C / min to 180 °C (phase one), 6 h at 180 °C (phase two), 2 °C / min to 200 °C (phase three), 12 h at 200 °C (phase four).

[0094] (4) The reaction product was mixed with a second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), Soxhlet extracted at 60 °C for 12.5 h, followed by vacuum drying at 80 °C for 12.5 h to obtain the 4-aminobenzonitrile-terminated polyimide covalent organic framework material (PI-COF-1).

[0095] Figure 3 For the TEM image of PI-COF-1, it can be seen that PI-COF-1 presents a sheet-like structure, mainly in two-dimensional plane and multi-dimensional stacked structure, which is highly consistent with the theoretical simulation of the polyimide framework, and no agglomeration or phase separation is observed in the entire field of view, indicating that the polyimide segments are uniformly arranged in the covalent framework, forming a highly regular two-dimensional porous structure.

[0096] Figure 4 For the infrared spectrum of PI-COF-1, it can be seen that the infrared spectrum of PI-COF-1 appears strong characteristic doublet peaks at 1781 cm -1 and about 1720 cm -1 , respectively, which are attributed to the asymmetric and symmetric stretching vibrations of the imide ring C=O. The above evidence fully proves that a polyimide COF material with complete structure is successfully synthesized by the preparation method of the present application.

[0097] 2. Preparation of lithium-sulfur battery

[0098] (1) Sulfur powder, PI-COF-1 were weighed according to a mass ratio of 7:3, ground and mixed in a mortar for 30 min, and then the mixed powder was transferred to a hydrothermal reaction kettle, which was sealed and placed in a 155 °C air oven for 12 h. After natural cooling, a composite sulfur positive electrode material was obtained. The above mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder were weighed according to a mass ratio of 7:2:1 and mixed, and then N-methyl pyrrolidone (NMP) was added, which was 19 times the mass of PVDF, and the mixture was stirred thoroughly until a uniform paste slurry was formed; the slurry was coated on an aluminum foil current collector using a doctor blade, vacuum dried at 60 °C for 8 h, cut into a disc with a diameter of 13 mm (active material loading about 1.2 mg), and a PI-COF-1 / S composite positive electrode was obtained.

[0099] (2) PI-COF-1 / S composite cathode and lithium metal sheet were transferred to the glove box filled with argon for the assembly of the coin cell. The coin cell model was CR2032, the separator was polypropylene microporous membrane Celgard2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0100] Figure 5 The cycle performance curve of the prepared lithium-sulfur battery.

[0101] 3. Preparation of PI-COF-1-based solid-state electrolyte

[0102] 120 mg of PI-COF-1 powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed and added to 6 mL of NMP, and ultrasonic treatment was performed for 40 min until the mixture was uniformly mixed to form a uniform thick slurry. The slurry was uniformly coated in a polytetrafluoroethylene mold, and first dried at 60°C for 8 h to remove the solvent, and then heated to 100°C for 6 h to completely remove impurities, to obtain a PI-COF-1-based solid-state electrolyte film with a thickness of about 60 μm, which was ready for use.

[0103] 4. Preparation of PI-COF-1 anode protective layer

[0104] 25 mg of PI-COF powder was dispersed in 12 mL of NMP and ultrasonic treatment was performed for 30 min to form a stable dispersion liquid. In an argon glove box (water and oxygen values <0.01 ppm), 35 μL of the dispersion liquid was uniformly dropped and coated on the surface of a lithium metal sheet with a diameter of 15 mm. The lithium metal sheet was placed on a heating platform at 60°C and heated for 6 h to completely volatilize the solvent. After cooling to room temperature, a modified lithium metal anode with a PI-COF-1 protective layer on the surface was obtained.

[0105] Example 2

[0106] 1. Synthesis of electron-accepting group-terminated polyimide covalent organic framework material (PI-COF-2)

[0107] (1) 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) 19.98 mg (0.045 mmol) and tetra(4-aminophenyl)methane 5.7 mg (0.015 mmol) were weighed and placed in a vacuum drying box, and dried at 60°C for 12 h to remove water and impurities in the raw materials.

[0108] (2) The dried 6FDA and tetrakis(4-aminophenyl)methane were transferred to a Schlenk tube, the first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0) was added, and then the catalyst isoquinoline 0.35 mg was added. Under the inert atmosphere of argon, the temperature was raised to 120°C, and the nucleophilic acylation condensation reaction was carried out for 2.5 h at constant temperature to obtain polyamic acid covalent organic framework (PAA-COF);

[0109] (3) 4-aminotrifluorotoluene 14.49 mg (0.09 mmol) was added to PAA-COF. The temperature was raised according to the procedure: 2°C / min to 180°C (stage one), constant temperature at 180°C for 6 h (stage two), 2°C / min to 200°C (stage three), and constant temperature at 200°C for 12 h (stage four).

[0110] (4) The reaction product was mixed with the second solvent (ethanol and dichloromethane mixed in a volume ratio of 1.0:1.2), and Soxhlet extraction was carried out at 60°C for 12.5 h, followed by vacuum drying at 80°C for 12.5 h to obtain 4-aminotrifluorotoluene-terminated polyimide covalent organic framework material (PI-COF-2).

[0111] 2. Preparation of lithium-sulfur battery

[0112] (1) Sulfur powder and PI-COF-2 were weighed in a mass ratio of 7:3 and ground in a mortar for 30 min, and then the mixed powder was transferred to a hydrothermal reaction kettle, which was sealed and placed in a 155°C air oven for 12 h. After natural cooling, a composite sulfur positive electrode material was obtained. The above-mentioned mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder were weighed in a mass ratio of 7:2:1 and mixed, and then PVDF was added in an amount of 19 times the mass of N-methyl pyrrolidone (NMP), and the mixture was stirred thoroughly until a uniform paste-like slurry was formed. The slurry was coated on an aluminum foil current collector using a doctor blade, vacuum dried at 60°C for 8 h, cut into a disc with a diameter of 13 mm (active material loading about 1.2 mg), and a PI-COF-2 / S composite positive electrode was obtained.

[0113] (2) The PI-COF-2 / S composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of a coin cell battery. The coin cell battery was model CR2032, the separator was polypropylene microporous membrane Celgard 2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), with a volume ratio of EC / DMC / EMC of 1:1:1.

[0114] 3. Preparation of PI-COF-2-based solid-state electrolyte

[0115] Take 120 mg of PI-COF-2 powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and ultrasonically treat for 40 min to form a uniform viscous slurry. Uniformly coat the slurry in a polytetrafluoroethylene mold, first dry at 60°C for 8 h to remove the solvent, then heat to 100°C for 6 h to completely remove impurities, to obtain a PI-COF-based solid-state electrolyte film with a thickness of about 60 μm, ready for use.

[0116] 4. Preparation of PI-COF-2 negative electrode protective layer

[0117] Take 25 mg of PI-COF powder and disperse it in 12 mL of NMP, and ultrasonically treat 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 coat it on the surface of a lithium metal sheet with a diameter of 15 mm. Place the lithium metal sheet on a 60°C heating platform and heat for 6 h to completely volatilize the solvent. After cooling to room temperature, a modified lithium metal negative electrode with a PI-COF-2 protective layer on the surface is obtained.

[0118] Example 3

[0119] 1. Synthesis of electron-withdrawing group-terminated polyimide covalent organic framework material (PI-COF-3)

[0120] (1) Take 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) 19.98 mg (0.045 mmol) and tetra(4-aminophenyl)methane 5.7 mg (0.015 mmol), and place them in a vacuum drying box, dry at 60°C for 12 h to remove water and impurities in the raw materials.

[0121] (2) Transfer the dried 6FDA and tetra(4-aminophenyl)methane to a Schlenk tube, add the first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0), and then add the catalyst isoquinoline 0.35 mg. Under an argon inert gas atmosphere, heat to 120°C, and keep the temperature constant for 2.5 h to carry out nucleophilic acylation condensation reaction to obtain polyamide acid covalent organic framework (PAA-COF);

[0122] (3) Add 4-nitroaniline 12.42 mg (0.09 mmol) to the PAA-COF. Heat according to the procedure: heat to 180°C at 2°C / min (stage one), keep the temperature constant at 180°C for 6 h (stage two), heat to 200°C at 2°C / min (stage three), and keep the temperature constant at 200°C for 12 h (stage four).

[0123] (4) The reaction product was mixed with a second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), Soxhlet extracted 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).

[0124] 2. Preparation of lithium-sulfur battery

[0125] (1) Sulfur powder and PI-COF-3 were weighed at a mass ratio of 7:3, ground and mixed in a mortar for 30 min, and then the mixed powder was transferred to a hydrothermal reaction kettle. After sealing, it was placed in a 155 °C air oven for reaction for 12 h. After natural cooling, a composite sulfur positive electrode material was obtained. The above-mentioned mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder were weighed at a mass ratio of 7:2:1 and mixed, and then N-methyl pyrrolidone (NMP) was added, which was 19 times the mass of PVDF. After stirring thoroughly, a uniform paste slurry was formed. The slurry was coated on an aluminum foil current collector using a doctor blade, vacuum dried at 60 °C for 8 h, cut into a disc with a diameter of 13 mm (active material loading about 1.2 mg), and a PI-COF-3 / S composite positive electrode was obtained.

[0126] (2) The PI-COF-3 / S composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of a coin cell. The coin cell model was CR2032, the separator was a polypropylene microporous membrane Celgard 2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0127] 3. Preparation of PI-COF-3-based solid electrolyte

[0128] 120 mg of PI-COF powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed and added to 6 mL of NMP. After ultrasonic treatment for 40 min, a uniform viscous slurry was formed. The slurry was uniformly coated in a polytetrafluoroethylene mold. After preliminary removal of the solvent at 60 °C for 8 h, the temperature was increased to 100 °C for 6 h of vacuum drying to completely remove impurities. A PI-COF-3-based solid electrolyte film with a thickness of about 60 μm was obtained and ready for use.

[0129] 4. Preparation of PI-COF-3 negative electrode protection layer

[0130] Take 25 mg of PI-COF powder, disperse in 12 mL of NMP, ultrasonic treatment for 30 min to form a stable dispersion; In the argon glove box (water, oxygen value <0.01 ppm), take 35 μL of the dispersion and evenly drop coat on the surface of lithium metal sheet with a diameter of 15 mm, place the lithium metal sheet on the 60 ℃ heating table and heat for 6 h, so that the solvent is completely volatilized, and after cooling to room temperature, the modified lithium metal negative electrode with PI-COF-3 protective layer on the surface is obtained.

[0131] Example 4

[0132] 1. Synthesis of electron-withdrawing group terminated polyimide covalent organic framework material (PI-COF-4)

[0133] (1) Take 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) 19.98 mg (0.045 mmol) and tetra(4-aminophenyl) methane 5.7 mg (0.015 mmol), and place them in a vacuum drying box, dry at 60 ℃ for 12 h to remove water and impurities in the raw materials.

[0134] (2) Transfer the dried 6FDA and tetra(4-aminophenyl) methane to a Schlenk tube, add the first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0), and then add the catalyst isoquinoline 0.35 mg. Under the atmosphere of argon inert gas, heat to 120 ℃, and keep the temperature constant for 2.5 h to carry out nucleophilic acylation condensation reaction to obtain polyamide acid covalent organic framework (PAA-COF);

[0135] (3) Add 4-chloroaniline 11.48 mg (0.09 mmol) to the PAA-COF. Heat according to the procedure: heat to 180 ℃ at 2 ℃ / min (stage one), keep the temperature constant at 180 ℃ for 6 h (stage two), heat to 200 ℃ at 2 ℃ / min (stage three), and keep the temperature constant at 200 ℃ for 12 h (stage four).

[0136] (4) Mix the reaction product with the second solvent (ethanol and dichloromethane mixed in a volume ratio of 1.0:1.2), Soxhlet extract at 60 ℃ for 12.5 h, and then vacuum dry at 80 ℃ for 12.5 h to obtain 4-chloroaniline terminated polyimide covalent organic framework material (PI-COF-4).

[0137] 2. Preparation of lithium-sulfur battery

[0138] (1) Sulfur powder, PI-COF-4 were weighed according to the mass ratio of 7:3, ground and mixed in a mortar for 30 min, then the mixed powder was transferred to a hydrothermal reactor, sealed and placed in a 155 °C air oven for 12 h. After natural cooling, the composite sulfur positive electrode material was obtained. The above-mentioned mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder were weighed according to the mass ratio of 7:2:1 and mixed, and then N-methyl pyrrolidone (NMP) was added, which was 19 times the mass of PVDF. After stirring thoroughly, a uniform paste slurry was formed. The slurry was coated on an aluminum foil current collector using a doctor blade, vacuum dried at 60 °C for 8 h, cut into a disc with a diameter of 13 mm (active material loading about 1.2 mg), and a PI-COF-4 / S composite positive electrode was obtained.

[0139] (2) The PI-COF-4 / S composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for the assembly of a coin cell. The coin cell model was CR2032, the separator was polypropylene microporous membrane Celgard 2400, and the electrolyte was 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0140] 3. Preparation of PI-COF-4-based solid-state electrolyte

[0141] 120 mg of PI-COF-4 powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed and added to 6 mL of NMP. After ultrasonic treatment for 40 min, a uniform viscous slurry was formed. The slurry was uniformly coated in a polytetrafluoroethylene mold. After preliminary removal of the solvent at 60 °C for 8 h, the temperature was increased to 100 °C for 6 h of vacuum drying to completely remove impurities. A PI-COF-based solid-state electrolyte film with a thickness of about 60 μm was obtained and ready for use.

[0142] 4. Preparation of PI-COF-4 negative electrode protective layer

[0143] 25 mg of PI-COF powder was dispersed in 12 mL of NMP and ultrasonically treated for 30 min to form a stable dispersion. In an argon glove box (water and oxygen values <0.01 ppm), 35 μL of the dispersion was uniformly dropped onto the surface of a lithium metal sheet with a diameter of 15 mm. The lithium metal sheet was heated on a 60 °C heating table for 6 h to completely evaporate the solvent. After cooling to room temperature, a modified lithium metal negative electrode with a PI-COF-4 protective layer on the surface was obtained.

[0144] Comparative Example 1

[0145] 1. Synthesis of unblocked polyimide covalent organic framework material

[0146] (1) Take 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) 19.98 mg (0.045 mmol) and tetrakis(4-aminophenyl)methane 5.7 mg (0.015 mmol), and place them in a vacuum drying oven, dry at 60 °C for 12 h to remove water and impurities in the raw materials.

[0147] (2) Transfer the dried 6FDA and tetrakis(4-aminophenyl)methane to a Schlenk tube, add the first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0), and then add the catalyst isoquinoline 0.35 mg. Under an argon gas atmosphere, heat to 120 °C, and keep the temperature constant for 2.5 h to carry out nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF);

[0148] (3) Temperature program: heat to 180 °C at 2 °C / min (stage one), keep the temperature constant at 180 °C for 6 h (stage two), heat to 200 °C at 2 °C / min (stage three), and keep the temperature constant at 200 °C for 12 h (stage four).

[0149] (4) Mix the reaction product with the second solvent (ethanol and dichloromethane mixed in a volume ratio of 1.0:1.2), Soxhlet extract at 60 °C for 12.5 h, and then vacuum dry at 80 °C for 12.5 h to obtain the unblocked polyimide covalent organic framework material.

[0150] 2. Preparation of lithium-sulfur battery

[0151] (1) Take sulfur powder and unblocked polyimide covalent organic framework material in a mass ratio of 7:3, grind and mix in a mortar for 30 min, then transfer the mixed powder to a hydrothermal reaction kettle, seal it, and place it in a 155 °C air oven for reaction for 12 h. After natural cooling, a composite sulfur positive electrode material is obtained. Take the above-mentioned mixed material, superconducting carbon black (SuperP), and polyvinylidene fluoride (PVDF) binder in a mass ratio of 7:2:1 and mix them, add N-methyl pyrrolidone (NMP) in an amount of 19 times the mass of PVDF, and stir thoroughly until a uniform paste-like slurry is formed. Use a spatula to coat the slurry on an aluminum foil current collector, vacuum dry at 60 °C for 8 h, cut into a pole piece with a diameter of 13 mm (active material loading about 1.2 mg), and obtain a composite positive electrode.

[0152] (2) Transfer the composite positive electrode and lithium metal sheet to an argon-filled glove box to assemble a button cell. The button cell model is CR2032, the separator is polypropylene microporous membrane Celgard 2400, and the electrolyte is 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetylphosphonate), wherein the volume ratio of EC / DMC / EMC is 1:1:1.

[0153] Comparative Example 2

[0154] Preparation of lithium-sulfur battery

[0155] (1) Sulfur powder, positive electrode carrier carbon nanotube were weighed according to the mass ratio of 7:3, ground and mixed in a mortar for 30 min, and then the mixed powder was transferred to a hydrothermal reaction kettle, which was sealed and placed in a 155℃ air oven for reaction for 12 h. After natural cooling, the composite sulfur positive electrode material was obtained. The above mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder were weighed according to the mass ratio of 7:2:1 and mixed, and then N-methyl pyrrolidone (NMP) was added, which was 19 times the mass of PVDF. Stir well until a uniform paste slurry is formed; use a spatula to coat the slurry on an aluminum foil current collector, vacuum dry at 60℃ for 8h, cut into a diameter = 13mm electrode piece (active material loading about 1.2mg), and obtain a composite positive electrode.

[0156] (2) The composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for assembly of a button cell. The button cell model was CR2032, the separator was polypropylene microporous membrane Celgard2400, and the electrolyte was 1mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC was 1:1:1.

[0157] The lithium-sulfur batteries prepared by Examples 1-4 and Comparative Examples 1-2 were subjected to electrochemical performance testing on a new Wei test system, and the voltage window for testing was 1.7~2.8V. The test results are shown in Table 1.

[0158] Table 1 Performance test results of lithium-sulfur batteries prepared by Examples 1-4 and Comparative Examples 1-2

[0159]

[0160] As can be seen from Table 1, the first circle discharge specific capacity and capacity retention rate of the lithium-sulfur battery assembled according to the examples of the present application are obviously improved compared with the lithium-sulfur battery assembled according to the comparative examples.

[0161] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as limiting the scope of the patent. For ordinary skilled in the art, without departing from the concept of the present application, a number of variations and improvements can be made, which are within the scope of protection of the present application.

Claims

1. A method for preparing an electron-withdrawing group-terminated polyimide covalent organic framework material, 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.

2. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 1, 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.

3. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 2, 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.

4. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 1, 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.

5. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 4, 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.

6. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 1, characterized in that, The molar ratio of 4,4'-(hexafluoroisopropene)phthalic anhydride to tetrakis(4-aminophenyl)methane is 3:

1.

7. The method for preparing the electron-withdrawing group-terminated polyimide covalent organic framework material according to claim 6, characterized in that, The molar ratio of the capping agent to the 4,4'-(hexafluoroisopropene) phthalic anhydride is 1.5-2.0:1.

0.

8. The application of an electron-withdrawing group-terminated polyimide covalent organic framework material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, Positive electrode material for lithium-sulfur batteries.

9. The application of an electron-withdrawing group-terminated polyimide covalent organic framework material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, Used as a protective layer for lithium metal batteries.

Images