Conjugated terminal group polyimide covalent organic framework material as well as preparation method and application thereof

Abstract

The invention relates to a conjugated end group polyimide covalent organic framework material as well as a preparation method and application thereof. Two monomers including pyromellitic dianhydride and tetra (4-aminophenyl) porphyrin are used for preparing a polyamic acid covalent organic framework, 4-ethynyl aniline, 4-naphthylacetylene aniline and 4-(phenylacetylene) aniline are heated for condensation cyclization and end capping based on the polyamic acid covalent organic framework, and the conjugated end group polyimide covalent organic framework material (PI-COF) is prepared. The rich pore structure of the COF skeleton provides a rapid ion transmission channel, the transport of carriers is promoted, the electron cloud density of the COF skeleton is adjusted by the pi electron conjugation effect of the end-capping group, and a specifically distributed heterogeneous interface is constructed, so that the mobility of electrons is regulated and controlled; the material is loaded on a lithium-sulfur battery positive electrode host material to construct a confinement catalytic system, and can anchor polysulfide and catalyze the polysulfide to complete a reduced-order conversion reaction, so that the lithium-sulfur battery with high capacity and long cycle life is obtained.

Description

TECHNICAL FIELD

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

[0002] With the depletion of non-renewable resources such as oil and the worsening of the greenhouse effect, the development of new energy is imminent. With the vigorous development of new energy vehicles, unmanned aerial vehicles and other electronic devices, the requirements for energy storage devices are also increasing. At present, due to the limitation of the theoretical specific capacity of the positive electrode material of the traditional commercial lithium ion battery, its energy density has approached the theoretical limit, and it is difficult to meet the increasing requirements of emerging electronic devices for long endurance, long service life and light weight.

[0003] In order to meet the market demand for energy storage devices, several new battery systems have become the focus of global research, such as zinc ion batteries, sodium ion batteries and lithium-sulfur batteries. Although lithium-sulfur batteries have the advantage of high theoretical energy density, the industrialization process of lithium-sulfur batteries is still hindered by several key technical problems: (1) the "shuttle effect" of the intermediate product polysulfide in the positive electrode side leads to continuous loss of active material and rapid capacity decay; (2) the insulating nature of the sulfur positive electrode and its discharge product causes slow reaction kinetics; (3) the large volume change during charging and discharging easily damages the electrode structure; (4) the continuous consumption of electrolyte during charging and discharging leads to loss of active material; (5) uncontrollable lithium dendrite growth on the negative electrode side poses a serious safety hazard.

[0004] Loading high-performance materials on the electrode material to anchor catalytic polysulfides, regulate the flow of electrons and build a stable three-dimensional structure is one direction to solve the above problems, and the use of which high-performance material to load the electrode to achieve good results is a problem that needs to be solved by those skilled in the art. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a conjugated end group polyimide covalent organic framework material and a preparation method and application thereof. By introducing a conjugated end group into the polyimide covalent organic framework material, the electron cloud density of the COF skeleton is adjusted using the pi electron conjugation effect of the end-capping group, and a specific distribution of heterojunction interfaces is constructed to regulate the flow of electrons. The application of the conjugated end group polyimide covalent organic framework material to the positive electrode material of lithium-sulfur batteries can improve the cycle life thereof.

[0006] First aspect: A conjugated end group polyimide covalent organic framework material, the structural formula of which is:

[0007] wherein the end-capping group R is a conjugated group, and the structural formula thereof is , and n is the number of repeating units.

[0008] The conjugated end group polyimide covalent organic framework material (PI-COF) of the present application takes polyimide organic framework as a substrate, has excellent structural stability, and the highly porous COF skeleton ensures smooth and rapid transport of ions inside the electrode material, while introducing a conjugated end group, which adjusts the electron cloud density of the COF skeleton by using the π electron conjugation effect. The composite negative electrode formed by loading on the metal constructs a specific distribution of heterojunction interface, thereby regulating the flowability of electrons and inhibiting the growth of lithium dendrites. At the same time, due to the charge differentiation produced by the distribution of the main chain and the hetero-chain segment of the side chain in the COF skeleton, the electrolyte can be induced to undergo oxidation and reduction, and the SEI film generated by the reaction has high ionic conductivity, high mechanical strength and high stability, which can effectively prevent the continuous decomposition of the electrolyte, reduce the capacity loss, and improve the coulombic efficiency, cycle life and safety of the battery.

[0009] When the PI-COF is loaded on the positive electrode host material of the lithium-sulfur battery, the conjugated domain is formed at the end of the COF skeleton and nearby, constructing a confined catalytic system. On the one hand, the system efficiently anchors polysulfides through strong physical and chemical action, and on the other hand, it provides a rich electron environment for the reduction reaction of polysulfides by virtue of the dynamic rearrangement of π electron cloud, significantly reducing the energy barrier of the liquid-solid conversion step, thereby synergistically inhibiting the shuttle effect and improving the reaction kinetics.

[0010] The second aspect is: A preparation method of the conjugated end group polyimide covalent organic framework material of the first aspect, comprising the following steps: mixing pyromellitic dianhydride and tetra(4-aminophenyl)porphyrin with a first solvent, and heating to perform nucleophilic acylation reaction under an inert gas atmosphere to obtain a polyamide acid covalent organic framework; mixing the polyamide acid covalent organic framework with an end-capping agent, and heating to perform end-capping reaction and condensation cyclization reaction to obtain the conjugated end group polyimide covalent organic framework material; The first solvent is a mixture of o-dichlorobenzene and n-butanol; The end-capping agent is at least one of 4-ethynylaniline, 4-naphthalene ethynyl aniline and 4-(phenyl ethynyl) aniline.

[0011] The present application synthesizes polyamic acid covalent organic framework (PAA-COF) by using pyromellitic dianhydride (PMDA) and tetra(4-aminophenyl) porphyrin (TAPP) as monomers, and the PAA-COF shows excellent rigidity. Meanwhile, by using 4-ethynylaniline, 4-naphthalene ethynylaniline, and 4-(phenyl ethynyl) aniline as capping agents, a conjugated cyclic polyimide covalent organic framework is formed through condensation and capping reaction. The material has good conductivity, and when it is used as an electrode material, the transmission rate of electrons can be significantly improved, a high-efficiency electron conductive network is constructed, and the battery has excellent rate performance. The large conjugation and rigid structure of porphyrin can maintain integrity in long-term charge and discharge cycles to resist performance degradation caused by material collapse.

[0012] As a preferred solution, the conjugated end-capped polyimide covalent organic framework material is mixed with a second solvent for 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.

[0013] The role of dichloromethane is to remove small organic molecules and oligomers. Dichloromethane is an organic solvent with moderate 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.

[0014] 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 reactions may use metal catalysts or produce salt byproducts (such as anhydrides, pyridine salts, etc.). Ethanol has good solubility and carrying capacity for such ionic impurities, and the mutual solubility of ethanol and water can also remove trace amounts of water that may exist in the COF channels.

[0015] Mixed use: The boiling point of the mixed solvent is between the two, which can improve the extraction efficiency. More importantly, using dichloromethane first to remove most of the organic impurities, and then using ethanol to remove polar residues and moisture, is a kind of gradient purification idea, which can maximize the cleanliness of the COF channels.

[0016] High proportion of ethanol (such as ethanol:dichloromethane=4:1 or higher): The polarity of the mixed solvent is too strong, and although the skeleton of the polyimide COF is stable, the high-polarity 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 channel collapse or deformation.

[0017] High proportion of dichloromethane (such as ethanol: dichloromethane = 1:4 or lower): the mixed solvent is too weak in polarity, and is seriously insufficient in solubility and carrying capacity for catalyst metal ions, salt by-products and trace moisture.

[0018] By Soxhlet extraction and vacuum drying, a conjugated end-capped polyimide covalent organic framework material with high purity and similar molecular weight can be obtained.

[0019] 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.

[0020] As a preferred solution, after the pyromellitic dianhydride and the tetra(4-aminophenyl)porphyrin are mixed with the first solvent, a catalyst is added; the catalyst is isoquinoline, and the molar ratio of the isoquinoline to the pyromellitic dianhydride 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 solution, the temperature of the nucleophilic acylation reaction is 120-130℃, and the time is 2-3h; the end-capping reaction and the condensation cyclization reaction consist of 4 stages, stage one is to increase the temperature 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 to increase the temperature from 180℃ to 200℃ at a rate of 2℃ / min, and the temperature of stage four is 200℃, and the time is 12-14h.

[0022] The purpose of stage one is controllable pre-polymerization and initial cyclization. Starting from 120℃, the monomers and end-capping agents are gradually dissolved and undergo polycondensation. Slow heating ensures a gentle start of the reaction, avoids local overheating or rapid gelation, and is conducive to the orderly arrangement of molecular chains, laying a foundation for subsequent crystallization.

[0023] 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-polymer, and reduced crystallinity of the final COF. Slow heating: unnecessarily prolongs the process time.

[0024] The purpose of stage two is main body polycondensation and cyclization. 180℃ is an efficient temperature for imidization reaction, at which: Dehydration cyclization is accelerated: the intermediate polyamide acid rapidly undergoes ring closure to form imide bonds, promoting molecular weight growth.

[0025] Dynamic covalent repair: the formation of polyimide COF often involves reversible reactions, and constant temperature provides sufficient time for bond breaking-recombination, repairing defects and improving crystallinity.

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

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

[0028] If the reaction time is too long: it may lead to excessive growth or partial degradation (although the polyimide is stable, the conjugated end-group may be sensitive), energy consumption increases, and the benefits are limited.

[0029] The purpose of stage three is post-curing and crystallization perfection. At this temperature: Drive residual reaction: complete the ring closure of residual amide acid to ensure high molecular weight and chemical stability.

[0030] Promote complete end-capping: higher temperature may promote the final reaction of the end-capping agent with the chain end to ensure the density of electron-withdrawing groups.

[0031] Structural annealing: slow heating to further order the COF lattice and eliminate internal stress.

[0032] Rising rate of temperature impact: slow heating avoids temperature shock leading to violent boiling of solvent or local overheating of materials, ensuring smooth transition.

[0033] The purpose of stage four is deep curing and crystallization optimization. Long-time high-temperature treatment has the following effects: Reaction completion: polycondensation and ring closure reach equilibrium to achieve the highest possible molecular weight and crosslinking degree.

[0034] Maximize crystallinity: long-time annealing promotes grain growth and defect repair to increase specific surface area.

[0035] Stabilization: treatment above battery operating temperature ensures material thermal stability.

[0036] If the reaction time is insufficient: insufficient crystallinity, poor channel regularity, affecting ion transport and active site exposure.

[0037] If the reaction time is too long: it may lead to partial decomposition or pore collapse.

[0038] As a preferred solution, the molar ratio of the pyromellitic dianhydride to the tetrakis(4-aminophenyl)porphyrin is 3:1. This ratio is the optimal molar ratio, and the conjugated end-group polyimide covalent organic framework material prepared has the best structural stability.

[0039] As a preferred solution, the molar ratio of the end-capping agent to the pyromellitic dianhydride is 1.5-2.0:1.0. This ratio is the optimal molar ratio, and the conjugated end-group polyimide covalent organic framework material prepared can be completely end-capped, and the amount of end-capping agent used is less.

[0040] A third aspect: The application of the conjugated end-capped polyimide covalent organic framework material of the first aspect is used for the positive electrode material of a lithium-sulfur battery. The PI-COF constructs a confined catalytic system, and the spatial structure and surface characteristics thereof provide key sites and active sites for the conversion reaction of polysulfides. The end-capped region anchors the polysulfides and catalyzes the polysulfides to complete the reduction conversion reaction through the π electron conjugation effect. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 The synthesis schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0042] Figure 2 The structure schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0043] Figure 3 The TEM image of the PI-COF-1 prepared in Example 1.

[0044] Figure 4 The infrared spectrum of the PI-COF-1 prepared in Example 1.

[0045] Figure 5 The cycle performance curve of the lithium-sulfur battery prepared in Example 1. DETAILED DESCRIPTION

[0046] Various exemplary embodiments of the present application will now be described in detail, which should be considered to be illustrative of the present application and should not be construed to limit the present application, and are understood to be a more detailed description of certain aspects, features and embodiments of the present application.

[0047] It should be understood that the terms used in the present application merely describe specific embodiments and are not intended to limit the present application. In addition, for the numerical range in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any stated value or stated range of values, and any other stated value or stated range of values within the stated range is also included within the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0048] 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 documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In the case of conflict between the present specification and any document incorporated herein by reference, the present specification will control.

[0049] Many modifications and variations of the specific embodiments of the application can be made without departing from the scope or spirit of the application, which will be apparent to those skilled in the art. Other embodiments 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 are illustrative only.

[0050] As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” and the like are open-ended terms that are intended to denote the inclusion of elements or steps without excluding other elements or steps.

[0051] A conjugated end group polyimide covalent organic framework material has a structure as shown in the following formula:

[0052] wherein the end capping group R is a conjugated group, and the structure of the end capping group R is at least one of the following formulas: and n is the number of repeating units.

[0053] Specifically, it can be one of formulas I-III.

[0054]

[0055]

[0056] A preparation method of a conjugated end group polyimide covalent organic framework material, as shown in the following formula: Figure 1 includes the following steps: Pyromellitic dianhydride (PMDA) and tetra(4-aminophenyl) porphyrin (TAPP) are vacuum dried at 60°C for 12h to remove water and impurities therefrom, then mixed with a first solvent, a catalyst is added, and a nucleophilic acylation reaction is carried out under an inert gas atmosphere to obtain a polyamic acid covalent organic framework (PAA-COF).

[0057] The polyamic acid covalent organic framework is mixed with an end capping agent, heated to carry out an end capping reaction and a condensation cyclization reaction, then mixed with a second solvent after the reaction is completed, and subjected to Soxhlet extraction and vacuum drying to obtain a conjugated end group polyimide covalent organic framework material (PI-COF).

[0058] The molar ratio of pyromellitic dianhydride to tetra(4-aminophenyl) porphyrin is 3:1.

[0059] The end capping agent is at least one of 4-ethynylaniline, 4-naphthalene ethynylaniline, and 4-(phenyl ethynyl) aniline; and the molar ratio of the end capping agent to pyromellitic dianhydride is 1.5-2.0:1.0. ​​

[0060] The catalyst is isoquinoline, and the molar ratio of isoquinoline to pyromellitic dianhydride is 1:20.

[0061] The first solvent is a mixture of o-dichlorobenzene and n-butanol, and the volume ratio of o-dichlorobenzene to n-butanol is 9.6-10.0:1.0.

[0062] 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.

[0063] The temperature of the nucleophilic acylation reaction is 120-130℃, and the time is 2-3h; the end-capping reaction and the condensation cyclization reaction consist of four stages, the temperature of stage one is increased from 120℃ to 180℃ at a rate of 2℃ / min, the temperature of stage two is 180℃, and the time is 6-8h, the temperature of stage three is increased from 180℃ to 200℃ at a rate of 2℃ / min, the temperature of stage four is 200℃, and the time is 12-14h; the temperature of the Soxhlet extraction is 60-70℃, and the time is 12-13h; the temperature of the vacuum drying is 80-90℃, and the time is 12-13h.

[0064] Figure 2 It is a structural schematic diagram of PI-COF.

[0065] Application of a conjugated end-group polyimide covalent organic framework material in lithium metal batteries and lithium-sulfur batteries.

[0066] The conjugated end-group polyimide covalent organic framework material (PI-COF) is mixed and ground with sulfur powder, put into 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, and then a third solvent is added, and stirred thoroughly until a uniform paste slurry is formed; the slurry is coated on an aluminum foil current collector with a doctor blade, vacuum dried to volatilize the third solvent, cut into an electrode sheet with a diameter of 15mm (active material loading of about 1mg) to obtain a PI-COF / S composite positive electrode.

[0067] The mass ratio of sulfur powder to PI-COF is 7.0-7.3:3.

[0068] The mass ratio of the mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder is 7.0-7.3:2:1.

[0069] The third solvent is N-methyl pyrrolidone (NMP), and the amount used is 19 times the mass of PVDF.

[0070] The heating temperature for vacuum drying to volatilize the third solvent is 40-80℃, and the heating time is 6-12h.

[0071] A lithium-sulfur battery includes a PI-COF / S composite positive electrode.

[0072] Example 1 1. Synthesis of conjugated end-capped polyimide covalent organic framework (PI-COF-1) (1) Weigh 9.81 mg (0.045 mmol) of pyromellitic dianhydride (PMDA) and 10.86 mg (0.015 mmol) of tetra(4-aminophenyl)porphyrin, and place them in a vacuum drying oven, and dry them at 60°C for 12 hours to remove water and impurities in the raw materials.

[0073] (2) Transfer the dried PMDA and tetra(4-aminophenyl)porphyrin to a Schlenk tube, add a first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0), and then add a catalyst isoquinoline 0.29 mg. Under an argon inert gas atmosphere, heat to 120°C, and keep the temperature constant for 2.5 hours to carry out a nucleophilic acylation condensation reaction to obtain a polyamic acid covalent organic framework (PAA-COF); (3) Add 10.53 mg (0.09 mmol) of 4-ethynylaniline 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 hours (stage two), heat to 200°C at 2°C / min (stage three), and keep the temperature constant at 200°C for 12 hours (stage four).

[0074] (4) Mix the reaction product with a second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), Soxhlet extract at 60°C for 12.5 hours, and then vacuum dry at 80°C for 12.5 hours to obtain a 4-ethynylbenzene-terminated polyimide covalent organic framework material (PI-COF-1).

[0075] Figure 3 The TEM image of PI-COF-1 shows that PI-COF-1 has a sheet-like structure, mainly in the form of a two-dimensional plane and a multi-dimensional stacked structure, which is highly consistent with the theoretical simulation of the polyimide framework. 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.

[0076] Figure 4 The infrared spectrum of PI-COF-1 shows that the infrared spectrum of PI-COF-1 has strong characteristic double peaks at 1772 cm -1 and about 1725 cm -1 , which are respectively 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.

[0077] 2. Preparation of lithium-sulfur battery (1) Sulfur powder, PI-COF-1 were weighed with a mass ratio of 7:3, mixed and ground 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 with a mass ratio of 7:2:1 and mixed, and then N-methyl pyrrolidone (NMP) was added with a mass of 19 times that of PVDF, and stirred thoroughly until a uniform paste slurry was formed; the slurry was coated on an aluminum foil current collector with a doctor blade, vacuum dried at 60 °C for 8 h, cut into a diameter of 13 mm electrode (active material loading about 1.2 mg), and PI-COF-1 / S composite positive electrode was obtained.

[0078] (2) PI-COF-1 / S composite positive electrode and lithium metal sheet were transferred to an argon-filled glove box for assembly of a coin cell battery. The coin cell battery 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.

[0079] Figure 5 The cycle performance curve of the prepared lithium-sulfur battery.

[0080] 3. Preparation of PI-COF-1-based solid-state electrolyte 120 mg of PI-COF powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed, 6 mL of NMP was added, and ultrasonic treatment was performed for 40 min until the mixture was uniform, forming a uniform thick slurry; the slurry was uniformly coated in a polytetrafluoroethylene mold, first vacuum dried at 60 °C for 8 h to remove the solvent, then heated to 100 °C for 6 h to completely remove the impurities, and a PI-COF-1-based solid-state electrolyte film with a thickness of about 60 μm was obtained, ready for use.

[0081] 4. Preparation of PI-COF-1 negative electrode protective layer 25 mg of PI-COF powder was weighed and 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, oxygen value <0.01 ppm), 35 μL of the dispersion liquid was uniformly dropped on the surface of a lithium metal sheet with a diameter of 15 mm, and the lithium metal sheet was placed on a 60 °C heating platform for heating for 6 h to completely volatilize the solvent, and after cooling to room temperature, a modified lithium metal negative electrode with a PI-COF-1 protective layer on the surface was obtained.

[0082] Example 2 1. Synthesis of conjugated end-capped polyimide covalent organic framework material (PI-COF-2) (1) Weigh 9.81 mg (0.045 mmol) of pyromellitic dianhydride (PMDA) and 10.86 mg (0.015 mmol) of tetra(4-aminophenyl)porphyrin, and place them in a vacuum drying oven at 60°C for 12 h to remove water and impurities in the raw materials.

[0083] (2) Transfer the dried PMDA and tetra(4-aminophenyl)porphyrin 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 0.29 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 polyamic acid covalent organic framework (PAA-COF); (3) Add 21.87 mg (0.09 mmol) of 4-naphthalene acetylene aniline 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).

[0084] (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 4-nitroaniline-terminated polyimide covalent organic framework material (PI-COF-2).

[0085] 2. Preparation of lithium-sulfur battery (1) Weigh sulfur powder and PI-COF-2 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 12 h. After natural cooling, a composite sulfur positive electrode material is obtained. Weigh 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, then 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 disc with a diameter of 13 mm (active material loading about 1.2 mg), and obtain a PI-COF-2 / S composite positive electrode.

[0086] (2) PI-COF-2 / S composite cathode and lithium metal sheet were transferred to the glove box filled with argon for the assembly of button cells. The button 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.

[0087] 3. Preparation of PI-COF-2-based solid-state electrolyte 120 mg of PI-COF-2 powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed and added to 6 mL of NMP. The mixture was uniformly mixed by ultrasonic treatment for 40 min to form a uniform viscous slurry. The slurry was uniformly coated in a polytetrafluoroethylene mold. First, the solvent was removed by vacuum drying at 60°C for 8 h, and then the impurities were completely removed by vacuum drying at 100°C for 6 h. A PI-COF-2-based solid-state electrolyte film with a thickness of about 60 μm was obtained and ready for use.

[0088] 4. Preparation of PI-COF-2 anode protective layer 25 mg of PI-COF-2 powder was dispersed in 12 mL of NMP by ultrasonic treatment 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 heated on a 60°C heating table for 6 h to completely volatilize the solvent. After cooling to room temperature, a modified lithium metal anode with a PI-COF-2 protective layer on the surface was obtained.

[0089] Example 3 1. Synthesis of conjugated end-capped polyimide covalent organic framework material (PI-COF-3) (1) 9.81 mg (0.045 mmol) of pyromellitic dianhydride (PMDA) and 10.86 mg (0.015 mmol) of tetra(4-aminophenyl)porphyrin were placed in a vacuum drying box and dried at 60°C for 12 h to remove water and impurities in the raw materials.

[0090] (2) The dried PMDA and tetra(4-aminophenyl)porphyrin were transferred to a Schlenk tube, and the first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0) and catalyst isoquinoline 0.29 mg were added. Under an argon atmosphere, the temperature was raised to 120°C, and the nucleophilic acylation condensation reaction was carried out at a constant temperature for 2.5 h to obtain polyamide acid covalent organic framework (PAA-COF); (3) To PAA-COF, add 4-(phenylethynyl)aniline 17.37 mg (0.09 mmol). Follow the procedure for temperature ramping: ramp at 2 °C / min to 180 °C (Phase One), hold at 180 °C for 6 h (Phase Two), ramp at 2 °C / min to 200 °C (Phase Three), hold at 200 °C for 12 h (Phase Four).

[0091] (4) Mix the reaction product with a second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), Soxhlet extract at 60 °C for 12.5 h, followed by vacuum drying at 80 °C for 12.5 h to obtain the methyl 4-aminobenzoate end-capped polyimide covalent organic framework material (PI-COF-3).

[0092] 2. Preparation of lithium-sulfur battery (1) Weigh sulfur powder and PI-COF 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 12 h. After natural cooling, a composite sulfur positive electrode material is obtained. Weigh the above-mentioned mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder in a mass ratio of 7:2:1 and mix them, then 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 13 mm diameter electrode (active material loading about 1.2 mg), and obtain a PI-COF-3 / S composite positive electrode.

[0093] (2) Transfer the PI-COF-3 / S composite positive electrode and lithium metal sheet to an argon-filled glove box for the assembly of 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 acetyl phosphate), with a volume ratio of EC / DMC / EMC of 1:1:1.

[0094] 3. Preparation of PI-COF-3-based solid-state electrolyte Weigh 120 mg of PI-COF-3 powder and 60 mg of lithium hexafluorophosphate (LiPF6), add 6 mL of NMP, and ultrasonically treat for 40 min until the mixture is uniform. A uniform viscous slurry is formed. Uniformly coat the slurry in a polytetrafluoroethylene mold, first vacuum dry at 60 °C for 8 h to remove the solvent, then heat to 100 °C for 6 h to completely remove impurities, and obtain a PI-COF-3-based solid-state electrolyte film with a thickness of about 60 μm, ready for use.

[0095] 4. Preparation of PI-COF-3 negative electrode protection layer Take 25 mg of PI-COF-3 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 evenly 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 stage and heat for 6 h to completely volatilize the solvent, and after cooling to room temperature, obtain a modified lithium metal anode with a PI-COF-3 protective layer on the surface.

[0096] Comparative Example 1 1. Synthesis of unblocked polyimide covalent organic framework material (1) Take 9.81 mg (0.045 mmol) of pyromellitic dianhydride (PMDA) and 10.86 mg (0.015 mmol) of tetra(4-aminophenyl)porphyrin, and place them in a vacuum drying box, and dry at 60°C for 12 h to remove water and impurities in the raw materials.

[0097] (2) Transfer the dried PMDA and tetra(4-aminophenyl)porphyrin to a Schlenk tube, add a first solvent (o-dichlorobenzene 2.4 mL + n-butanol 0.25 mL, volume ratio 9.6:1.0), and then add a catalyst, isoquinoline 0.29 mg. Under an argon inert gas atmosphere, heat to 120°C, and keep the temperature constant for 2.5 h to perform a nucleophilic acylation condensation reaction to obtain a polyamic acid covalent organic framework (PAA-COF); (3) Temperature program: heat at 2°C / min to 180°C (stage one), keep the temperature constant at 180°C for 6 h (stage two), heat at 2°C / min to 200°C (stage three), and keep the temperature constant at 200°C for 12 h (stage four).

[0098] (4) Mix the reaction product with a second solvent (ethanol and dichloromethane mixed at a volume ratio of 1.0:1.2), and Soxhlet extract at 60°C for 12.5 h, and then vacuum dry at 80°C for 12.5 h to obtain an unblocked polyimide covalent organic framework material.

[0099] 2. Preparation of lithium-sulfur battery (1) Take sulfur powder and un-capped polyimide covalent organic framework material by mass ratio 7:3, mix in a mortar for 30 min, then transfer the mixed powder to a hydrothermal reactor, seal and place in a 155 °C air oven for 12 h. After natural cooling, a composite sulfur positive electrode material is obtained. Take the above mixed material, super conductive carbon black (Super P) and polyvinylidene fluoride (PVDF) binder by mass ratio 7:2:1 and mix, then add N-methyl pyrrolidone (NMP) with 19 times the mass of PVDF, fully stir to form a uniform paste slurry; use a spatula to coat the slurry on an aluminum foil current collector, vacuum dry at 60 °C for 8 h, cut into a 13 mm diameter electrode sheet (active material loading about 1.2 mg), to obtain a composite positive electrode.

[0100] (2) Transfer the composite positive electrode and lithium metal sheet to an argon-filled glove box for the assembly of a button cell, the button cell model is CR2032, the separator is a polypropylene microporous membrane Celgard 2400, and the electrolyte is 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC is 1:1:1.

[0101] Comparative Example 2 Preparation of lithium-sulfur batteries (1) Take sulfur powder and positive electrode carrier carbon nanotubes by mass ratio 7:3, mix in a mortar for 30 min, then transfer the mixed powder to a hydrothermal reactor, seal and place in a 155 °C air oven for 12 h. After natural cooling, a composite sulfur positive electrode material is obtained. Take the above mixed material, super conductive carbon black (Super P) and polyvinylidene fluoride (PVDF) binder by mass ratio 7:2:1 and mix, then add N-methyl pyrrolidone (NMP) with 19 times the mass of PVDF, fully stir to form a uniform paste slurry; use a spatula to coat the slurry on an aluminum foil current collector, vacuum dry at 60 °C for 8 h, cut into a 13 mm diameter electrode sheet (active material loading about 1.2 mg), to obtain a composite positive electrode.

[0102] (2) Transfer the composite positive electrode and lithium metal sheet to an argon-filled glove box for the assembly of a button cell, the button cell model is CR2032, the separator is a polypropylene microporous membrane Celgard 2400, and the electrolyte is 1 mol / L LiPF6 (lithium hexafluorophosphate) / EC (ethylene carbonate) + DMC (dimethyl carbonate) + EMC (ethyl acetyl phosphate), wherein the volume ratio of EC / DMC / EMC is 1:1:1.

[0103] The lithium-sulfur batteries prepared by the examples 1-3 and the comparative examples 1-2 were tested on a Neware test system, and the voltage window of the test was 1.7-2.8V, and the test results were shown in Table 1.

[0104] Table 1 Performance test results of the lithium-sulfur batteries prepared by the examples 1-3 and the comparative examples 1-2

[0105] From Table 1, it can be seen that the first circle discharge specific capacity and capacity retention rate of the lithium-sulfur batteries assembled by the examples of the present application were obviously improved compared with the lithium-sulfur batteries assembled by the comparative examples.

[0106] 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 the limitation of 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 all within the protection scope of the present application.

Claims

1. A conjugated end-group polyimide covalent organic framework material, characterized in that, The structural formula is: Among them, the end-capping group R is a conjugated 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 the conjugated end-group polyimide covalent organic framework material as described in claim 1, characterized in that, Includes the following steps: Pyromellitic dianhydride and tetra(4-aminophenyl)porphyrin 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 conjugated end-group 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 conjugated end-group polyimide covalent organic framework material according to claim 2, characterized in that, It also includes the following steps: The conjugated end-group 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 conjugated end-group 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 conjugated end-group polyimide covalent organic framework material according to claim 2, characterized in that, After the pyromellitic dianhydride and the tetra(4-aminophenyl)porphyrin are mixed with the first solvent, a catalyst is also added; the catalyst is isoquinoline, and the molar ratio of isoquinoline to the pyromellitic dianhydride is 1:20; in the first solvent, the volume ratio of o-dichlorobenzene to n-butanol is 9.6-10.0:1.

0.

6. The method for preparing the conjugated end-group 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 conjugated end-group polyimide covalent organic framework material according to claim 2, characterized in that, The molar ratio of pyromellitic dianhydride to tetrakis(4-aminophenyl)porphyrin is 3:

1.

8. The method for preparing the conjugated end-group polyimide covalent organic framework material according to claim 7, characterized in that, The molar ratio of the capping agent to the pyromellitic dianhydride is 1.5-2.0:1.

0.

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

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