A conjugated end-capped polyimide covalent organic framework material, and a preparation method and application thereof

Abstract

The present application relates to a kind of conjugated end group polyimide covalent organic framework material and its preparation method and application.The present application utilizes phthalic anhydride and four (4-amino phenyl) porphyrin two monomers to prepare polyamide acid covalent organic framework, and based on this, using 4-ethynyl aniline, 4-naphthalene acetylene aniline, 4- (phenyl acetylene) aniline heating is condensed and cycled and terminated, and a conjugated end group polyimide covalent organic framework material (PI-COF) is prepared.The rich pore structure of COF skeleton provides fast ion transmission channel, promotes the transport of carrier, and the π electron conjugation effect of end group adjusts the electron cloud density of COF skeleton, constructs specific distribution of heterojunction interface, to regulate the flowability of electron;Load it on lithium-sulfur battery positive host material, construct limited catalytic system, can anchor polysulfide and catalyze it to complete the reduction order conversion reaction, to obtain high capacity, long cycle life lithium-sulfur battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of functional polymer materials and energy storage technology, and particularly relates 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 π electron conjugation effect of the end-capping group is used to adjust the electron cloud density of the COF skeleton, 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:

[0007] A conjugated end group polyimide covalent organic framework material, the structural formula of which is:

[0008]

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

[0010] The conjugated end group polyimide covalent organic framework material (PI-COF) of the present application takes polyimide organic framework as the base, has excellent structural stability, the highly porous COF skeleton ensures the smooth and rapid transport of ions inside the electrode material, and the conjugated end group is introduced to adjust 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, inhibiting the growth of lithium dendrites, and at the same time, benefiting from 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 to 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 electrode host material of the lithium-sulfur battery, the conjugated domain is formed at the end of the COF skeleton and nearby, and a confined catalytic system is constructed, which on the one hand efficiently anchors polysulfides through strong physical and chemical action, and on the other hand provides a rich electron environment for the reduction reaction of polysulfides by virtue of the dynamic rearrangement of π electron cloud, thereby significantly reducing the energy barrier of the liquid-solid conversion step, and synergistically inhibiting the shuttle effect and improving the reaction kinetics.

[0012] The second aspect is:

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

[0014] Mixing pyromellitic dianhydride and tetra(4-aminophenyl)porphyrin with a first solvent, and heating to perform nucleophilic acylation reaction under inert gas atmosphere to obtain a polyimide acid covalent organic framework;

[0015] Mixing the polyimide 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;

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

[0017] The end-capping agent is at least one of 4-ethynylaniline, 4-naphthalene ethynyl aniline and 4-(phenyl ethynyl) aniline.

[0018] 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.

[0019] 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.

[0020] 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.

[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 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[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 pyromellitic dianhydride and the tetra(4-aminophenyl)porphyrin are mixed with the first solvent, a catalyst is further 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.

[0028] 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.

[0029] 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.

[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-polymer, and reduced 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 is accelerated: the intermediate polyamide acid rapidly undergoes ring closure to form imide bonds, promoting molecular weight growth.

[0033] 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.

[0034] End-capping reaction: The electron-withdrawing end-capping agent reacts with the end of the growing chain, controlling the molecular weight and introducing conjugated end-group.

[0035] 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.

[0036] If the reaction time is too long: It may lead to excessive growth or partial degradation (although polyimide is stable, the conjugated end-group 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 reaction: Make the remaining amide acid completely cyclize, ensuring high molecular weight and chemical stability.

[0039] Promote complete end-capping: Higher temperature may promote the final reaction of the end-capping agent with the chain end, ensuring the density of electron-withdrawing groups.

[0040] Structural annealing: Slowly heating makes the COF lattice further ordered, eliminating internal stress.

[0041] Rising rate of temperature impact: Slowly heating avoids temperature shock causing 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-time high-temperature treatment has the following effects:

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

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

[0045] Stabilization: Treatment above battery operating temperature 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: It may lead to partial decomposition or pore collapse.

[0048] As a preferred solution, the molar ratio of the phthalic anhydride 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.

[0049] 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 prepared conjugated end-group polyimide covalent organic framework material can be completely capped, and the amount of end-capping agent is less.

[0050] Third aspect:

[0051] An application of the conjugated end-group polyimide covalent organic framework material of the first aspect is used as a 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 capping region anchors polysulfides through π electron conjugation and catalyzes the completion of the reduction conversion reaction. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 A synthesis schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0053] Figure 2 A structure schematic diagram of the PI-COF prepared in the embodiments of the present application.

[0054] Figure 3 A TEM image of the PI-COF-1 prepared in Example 1.

[0055] Figure 4 An infrared spectrum of the PI-COF-1 prepared in Example 1.

[0056] Figure 5 A cycle performance curve of the lithium-sulfur battery prepared in Example 1. DETAILED DESCRIPTION

[0057] Various exemplary embodiments of the present application will now be described in detail, which should not be considered as limiting the present application, but rather as a description of certain aspects, features and embodiments of the present application.

[0058] It should be understood that the terms described in the present application are only for describing the specific embodiments, and are not used 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 intermediate value in the stated range, and any other stated value or intermediate value in the stated range, is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0059] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as those commonly understood by one of ordinary skill in the art to which this application pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the 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 is not an admission that it is prior art with respect to the present application.

[0060] Many modifications and variations of the present application described in the specific embodiments of the application can be made by those skilled in the art without departing from the spirit or scope of the application. 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.

[0061] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" and the like are open-ended terms that are intended to mean "including but not limited to".

[0062] A conjugated end group polyimide covalent organic framework material has a structure as shown in the following formula:

[0063]

[0064] wherein the end capping group R is a conjugated group, and has a structure as shown in at least one of the following formulas: and n is the number of repeating units.

[0065] Specifically, it can be one of formulas I-III.

[0066]

[0067]

[0068] 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:

[0069] 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).

[0070] ​​The polyamide acid covalent organic framework is mixed with a capping agent, heated to perform a capping reaction and a condensation cyclization reaction, 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).

[0071] The molar ratio of pyromellitic dianhydride and tetrakis(4-aminophenyl)porphyrin is 3:1.

[0072] The capping agent is at least one of 4-ethynylaniline, 4-naphthylethynylaniline, and 4-(phenylethynyl)aniline; the molar ratio of the capping agent to pyromellitic dianhydride is 1.5-2.0:1.0.

[0073] The catalyst is isoquinoline, and the molar ratio of isoquinoline to pyromellitic dianhydride is 1:20.

[0074] 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.

[0075] 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.

[0076] The temperature of the nucleophilic acylation reaction is 120-130℃, and the time is 2-3h; the 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.

[0077] Figure 2 It is a structural schematic diagram of the PI-COF.

[0078] The conjugated end group polyimide covalent organic framework material is applied in lithium metal batteries and lithium-sulfur batteries.

[0079] The conjugated end group polyimide covalent organic framework material (PI-COF) is mixed and ground with sulfur powder, put into a reaction kettle, heated in a 155℃ blast drier 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 fully stirred to form a uniform paste slurry; 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 piece with a diameter of 15mm (active material loading of about 1mg), and a PI-COF / S composite positive electrode is obtained.

[0080] The mass ratio of sulfur powder to PI-COF is 7.0-7.3:3.

[0081] The mass ratio of mixed material, superconducting carbon black (Super P) and polyvinylidene fluoride (PVDF) binder is 7.0-7.3:2:1.

[0082] The third solvent is N-methyl pyrrolidone (NMP), and the amount is 19 times the mass of PVDF.

[0083] The heating temperature for the third solvent to volatilize by vacuum drying is 40-80℃, and the heating time is 6-12h.

[0084] A lithium-sulfur battery comprising a PI-COF / S composite positive electrode.

[0085] Example 1

[0086] 1. Synthesis of conjugated end-capped polyimide covalent organic framework material (PI-COF-1)

[0087] (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 box, dry at 60℃ for 12h to remove water and impurities in the raw materials.

[0088] (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℃, and keep the temperature constant for 2.5h to carry out nucleophilic acylation condensation reaction to obtain polyamide acid covalent organic framework (PAA-COF);

[0089] (3) Add 4-ethynylaniline 10.53 mg (0.09 mmol) to PAA-COF. Heat according to the procedure: heat to 180℃ at 2℃ / min (stage one), keep the temperature constant at 180℃ for 6h (stage two), heat to 200℃ at 2℃ / min (stage three), and keep the temperature constant at 200℃ for 12h (stage four).

[0090] (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 12.5h, and then vacuum dry at 80℃ for 12.5h to obtain 4-ethynylbenzene-terminated polyimide covalent organic framework material (PI-COF-1).

[0091] Figure 3For the TEM image of PI-COF-1, it can be seen that PI-COF-1 presents a sheet structure, mainly in two-dimensional plane and multi-dimensional stacked structure, which is highly consistent with the theoretical simulation of the polyimide framework. There is no agglomeration or phase separation 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.

[0092] 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 at 1772 cm -1 and about 1725 cm -1 , respectively belonging to the asymmetric and symmetric stretching vibration of the imide ring C=O. The above evidence fully proves that the polyimide COF material with complete structure is successfully synthesized by the preparation method of the present application.

[0093] 2. Preparation of lithium-sulfur battery

[0094] (1) Sulfur powder, PI-COF-1 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 reaction kettle, sealed and 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 mixed material, superconducting carbon black (SuperP) and polyvinylidene fluoride (PVDF) binder were weighed according to the mass ratio of 7:2:1 and mixed, and then PVDF was added. The mass of N-methyl pyrrolidone (NMP) is 19 times, and the slurry is fully stirred until a uniform paste is formed; the slurry is coated on an aluminum foil current collector with a doctor blade, vacuum dried at 60°C for 8h, cut into a diameter = 13mm electrode piece (active material loading about 1.2mg), and a PI-COF-1 / S composite positive electrode is obtained.

[0095] (2) PI-COF-1 / S 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 is CR2032, the separator is polypropylene microporous membrane Celgard2400, 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.

[0096] Figure 5 The cycle performance curve of the prepared lithium-sulfur battery.

[0097] 3. Preparation of PI-COF-1-based solid-state electrolyte

[0098] Take 120 mg of PI-COF 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-1-based solid-state electrolyte film with a thickness of about 60 μm, ready for use.

[0099] 4. Preparation of PI-COF-1 anode protective layer

[0100] 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, and cool to room temperature to obtain a modified lithium metal anode with a PI-COF-1 protective layer on the surface.

[0101] Example 2

[0102] 1. Synthesis of conjugated end-capped polyimide covalent organic framework material (PI-COF-2)

[0103] (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.

[0104] (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 polyamide acid covalent organic framework (PAA-COF);

[0105] (3) Add 4-naphthalene acetylene aniline 21.87 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).

[0106] (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-naphthalene acetylene-terminated polyimide covalent organic framework material (PI-COF-2).

[0107] 2. Preparation of lithium-sulfur battery

[0108] (1) Sulfur powder and PI-COF-2 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 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-2 / S composite positive electrode was obtained.

[0109] (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. 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.

[0110] 3. Preparation of PI-COF-2-based solid-state electrolyte

[0111] 120 mg of PI-COF-2 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-2-based solid-state electrolyte film with a thickness of about 60 μm was obtained and ready for use.

[0112] 4. Preparation of PI-COF-2 negative electrode protection layer

[0113] Take 25 mg of PI-COF-2 powder and disperse it in 12 mL of NMP, and ultrasonically treat it for 30 min to form a stable dispersion; in an argon glove box (water, oxygen <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 table and heat it for 6 h to completely volatilize the solvent, and after cooling to room temperature, obtain a modified lithium metal negative electrode with a PI-COF-2 protective layer on the surface.

[0114] Example 3

[0115] 1. Synthesis of conjugated end-capped polyimide covalent organic framework material (PI-COF-3)

[0116] (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 oven, and dry them at 60°C for 12 h to remove water and impurities in the raw materials.

[0117] (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 polyamide acid covalent organic framework (PAA-COF);

[0118] (3) Add 4-(phenylethynyl)aniline 17.37 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).

[0119] (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 a 4-(phenylethynyl)phenyl-terminated polyimide covalent organic framework material (PI-COF-3).

[0120] 2. Preparation of a lithium-sulfur battery

[0121] (1) Sulfur powder and PI-COF were weighed in 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 reactor. After sealing, it was 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 (Super P) and polyvinylidene fluoride (PVDF) binder were weighed in a mass ratio of 7:2:1 and mixed, and then PVDF was added. The mass of N-methyl pyrrolidone (NMP) was 19 times that 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 positive electrode sheet with a diameter of 13 mm (active material loading of about 1.2 mg), and a PI-COF-3 / S composite positive electrode was obtained.

[0122] (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.

[0123] 3. Preparation of PI-COF-3-based solid-state electrolyte

[0124] PI-COF-3 powder and 60 mg of lithium hexafluorophosphate (LiPF6) were weighed, 6 mL of NMP was added, and the mixture was ultrasonically treated for 40 min until it was uniformly mixed to form a uniform viscous slurry. The slurry was uniformly coated in a polytetrafluoroethylene mold, and the solvent was initially removed by vacuum drying at 60 °C for 8 h, and then the temperature was increased to 100 °C for vacuum drying for 6 h to completely remove impurities. A PI-COF-3-based solid-state electrolyte film with a thickness of about 60 μm was obtained and was ready for use.

[0125] 4. Preparation of PI-COF-3 negative electrode protective layer

[0126] PI-COF-3 powder was weighed and dispersed in 12 mL of NMP, and ultrasonically treated 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. The lithium metal sheet was heated on a 60 °C heating table for 6 h to completely volatilize the solvent, and then cooled to room temperature. A modified lithium metal negative electrode with a PI-COF-3 protective layer on the surface was obtained.

[0127] Comparative Example 1

[0128] 1. Synthesis of unblocked polyimide covalent organic framework material

[0129] (1) Take 9.81 mg (0.045 mmol) of pyromellitic dianhydride (PMDA) and 10.86 mg (0.015 mmol) of tetrakis(4-aminophenyl)porphyrin, and place them in a vacuum drying oven at 60°C for 12 hours to remove water and impurities from the raw materials.

[0130] (2) Transfer the dried PMDA and tetrakis(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 gas atmosphere, heat to 120°C and maintain the temperature for 2.5 hours to carry out nucleophilic acylation condensation reaction to obtain polyamic acid covalent organic framework (PAA-COF).

[0131] (3) Temperature program: heat to 180°C at 2°C / min (stage one), maintain the temperature at 180°C for 6 hours (stage two), heat to 200°C at 2°C / min (stage three), and maintain the temperature at 200°C for 12 hours (stage four).

[0132] (4) Mix the reaction product with the second solvent (ethanol and dichloromethane mixed in a volume ratio of 1.0:1.2), and perform Soxhlet extraction at 60°C for 12.5 hours, followed by vacuum drying at 80°C for 12.5 hours to obtain the unblocked polyimide covalent organic framework material.

[0133] 2. Preparation of lithium-sulfur battery

[0134] (1) Take sulfur powder and unblocked polyimide covalent organic framework material in a mass ratio of 7:3, and grind and mix them in a mortar for 30 minutes. Then transfer the mixed powder to a hydrothermal reaction kettle, seal it, and place it in a 155°C air oven for 12 hours. 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. 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 hours, cut into a disc with a diameter of 13 mm (active material loading about 1.2 mg), and obtain a composite positive electrode.

[0135] (2) Transfer the composite positive electrode and lithium metal sheet to an argon-filled glove box to assemble a coin cell battery. The coin cell battery 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 being 1:1:1.

[0136] Comparative Example 2

[0137] Preparation of lithium-sulfur battery

[0138] (1) Sulfur powder, positive electrode carrier carbon nanotube were weighed according to the mass ratio of 7:3, mixed and ground in a mortar for 30 min, and then the mixed powder was transferred to a hydrothermal reaction kettle, 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.

[0139] (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.

[0140] The lithium-sulfur batteries prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to electrochemical performance test on a new Wei test system, and the test voltage window was 1.7~2.8V. The test results are shown in Table 1.

[0141] Table 1 Performance test results of lithium-sulfur batteries prepared in Examples 1-3 and Comparative Examples 1-2

[0142]

[0143] 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.

[0144] 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 of preparing a conjugated endcapped polyimide covalent organic framework material, characterized in that, comprising the steps of: mixing pyromellitic dianhydride and tetra(4-aminophenyl)porphyrin with a first solvent, heating to perform nucleophilic acylation reaction under inert gas atmosphere to obtain polyamic acid covalent organic framework; mixing the polyamic acid covalent organic framework with a capping agent, heating to perform capping reaction and condensation cyclization reaction to obtain the conjugated end-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-ethynylaniline, 4-naphthalene ethynylaniline and 4-(phenyl ethynyl) aniline.

2. The method for preparing the conjugated end-group polyimide covalent organic framework material according to claim 1, characterized in that, further comprising the steps of: mixing the conjugated end-capped polyimide covalent organic framework material with a second solvent to perform 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.

3. The method of claim 2, wherein the covalent organic framework material is prepared by the reaction of a polyimide precursor and a covalent organic framework precursor. 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 conjugated end-group polyimide covalent organic framework material according to claim 1, characterized in that, After mixing the pyromellitic dianhydride and the tetra(4-aminophenyl)porphyrin with the first solvent, a catalyst is further 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.

5. The method of claim 4, wherein the covalent organic framework material is prepared by the reaction of a polyimide precursor and a covalent organic framework precursor. The temperature of the nucleophilic acylation reaction is 120-130℃, and the time is 2-3h; the capping reaction and the condensation cyclization reaction consist of 4 stages, stage one temperature is increased from 120℃ to 180℃ at a rate of 2℃ / min, stage two temperature is 180℃, and the time is 6-8h, stage three temperature is increased from 180℃ to 200℃ at a rate of 2℃ / min, and stage four temperature is 200℃, and the time is 12-14h.

6. The method of claim 1, wherein the covalent organic framework material is a conjugated endcapped polyimide. The molar ratio of the pyromellitic dianhydride to the tetra(4-aminophenyl)porphyrin is 3:

1.

7. The method for preparing the conjugated end-group polyimide covalent organic framework material according to claim 6, characterized in that, The molar ratio of the capping agent to the pyromellitic dianhydride is 1.5-2.0:1.

0.

8. Use of a conjugated endcapped polyimide covalent organic framework material prepared according to the method of any one of claims 1 to 7. A positive electrode material for a lithium-sulfur battery.

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