Preparation method of zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery, modified diaphragm and lithium-sulfur battery

Abstract

The invention discloses a preparation method of a zwitter-ion covalent organic framework modified diaphragm for a lithium-sulfur battery, the modified diaphragm and the lithium-sulfur battery, and belongs to the technical field of batteries, three monomers of bromophenanthridine, 4, 4 '-biphenyl dicarboxaldehyde and trialdehyde phloroglucinol are adopted as solutes, and a mixed solution of o-dichlorobenzene and benzyl alcohol is adopted; the preparation method comprises the following steps: synthesizing ETB-COF solid powder under the conditions of taking PVDF (Polyvinylidene Fluoride) as a solvent and taking acetic acid as a catalyst, then dispersing the ETB-COF into a sodium hydrogen sulfite solution, introducing HSO3 <-> into the ETB-COF to prepare an HSO3 <->-ETB-COF precursor material, and finally preparing the HSO3 <->-ETB-COF precursor material, PVDF and Super-P according to a limited proportion to obtain the modified diaphragm. The HSO3-ETB-COF has an ordered structure, so that the conduction rate of lithium ions in the diaphragm can be improved, and the charge-discharge efficiency of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a preparation method of an amphoteric ion covalent organic framework modified separator for a lithium-sulfur battery, a modified separator, and a lithium-sulfur battery. Background Art

[0002] In the context of the rapid development of advanced electronic devices, electric vehicles, and smart grids, the demand for high-efficiency and powerful energy storage devices (especially batteries) is increasing continuously. Lithium-ion batteries (LIBs) have stood out among many commercially available energy storage devices due to their high energy density, excellent rate performance, and long service life. However, LIBs technology is based on a lithiation-delithiation mechanism and uses insertion compounds as the cathode and anode materials, which limits their energy density and charge storage capacity. In addition, currently commercialized lithium-ion battery cathode materials generally have problems such as high cost, scarce resources, or high toxicity. Therefore, it is crucial to develop new electrode materials that are rich in resources and environmentally friendly.

[0003] Lithium-sulfur batteries (LSBs) have attracted extensive attention in the academic and industrial fields due to their high theoretical energy density of 2600 Wh / kg and ultra-high theoretical discharge capacity of 1675 mAh / g. However, despite the outstanding advantages of LSBs, LSBs still face multiple challenges in commercial applications, mainly including the following aspects: (1) Low conductivity: Sulfur has poor conductivity, resulting in low electron conduction efficiency during the charge and discharge process of LSBs. This limits the power density and charging speed of the battery; (2) "Shuttle effect" of polysulfides: During the charge and discharge process, polysulfides (Li 2 S x , x = 1~8) will dissolve in the electrolyte, causing them to migrate between the cathode and anode. This phenomenon not only reduces the energy efficiency of the battery but may also lead to capacity decay and decreased cycle stability of the battery; (3) Stability of the electrolyte: The electrolyte of LSBs is prone to decomposition under high voltage and high temperature conditions, resulting in a decline in battery performance. In addition, the interaction between the electrolyte and polysulfides may affect the overall performance of the battery; (4) Short cycle life: Due to the dissolution and precipitation of polysulfides, LSBs are prone to capacity decay after multiple charge and discharge cycles, resulting in a short cycle life. This is an important obstacle in commercial applications; (5) Structural stability: The electrode materials of LSBs will undergo volume changes during the charge and discharge process, which may lead to the destruction of the electrode structure and a decline in battery performance.

[0004] In response to the existing problems of current LSBs, domestic and foreign scholars have optimized them from several aspects, such as the structure and composition of the positive electrode material, the optimization of the electrolyte, the modification of the separator material, and the optimization of the negative electrode / interface. Among them, the design and selection of the separator are crucial for the overall performance of the battery. Although traditional polymer separators have good mechanical strength and chemical stability, they have weak lithium-ion conductivity and sulfur retention ability. Therefore, it is particularly important to develop new multifunctional separator materials to achieve better battery performance. The best modified materials for the separator should have characteristics such as light weight, high catalytic activity, and high specific surface area. Covalent organic frameworks (COFs) have advantages such as clear structure, large specific surface area, periodic molecular arrangement, uniform pore distribution, and rich functional groups, making them an ideal choice for separator modification materials. The COF material coated on the positive side of the separator not only acts as a physical barrier to prevent polysulfides from diffusing from the positive side to the negative side but also further accelerates the electron transfer to the active material. This design strategy can not only inhibit the "shuttle effect" of polysulfides but also achieve a long cycle life for LSBs. It can also improve the electron conductivity of the positive sulfur electrode and the utilization rate of the active material.

[0005] Given the good electrochemical performance of COF-modified separators, COF modification layers with various structures have attracted extensive attention. However, the surface structures of most COFs have low polarity and it is difficult to have chemical interactions with highly polar polysulfides. Therefore, COFs relying only on physical adsorption easily reach the adsorption saturation of polysulfides, resulting in unsatisfactory electrochemical performance. In this regard, the surface treatment of COFs is carried out by adjusting the building blocks and doping to obtain local polar structures and rich active sites. Reported studies have shown that some elements (such as O, N, P, F, etc.) and some functional groups (such as -OH, COOH, -SO 3 H, etc.) can have chemical interactions with polysulfides and show strong anchoring and catalytic effects on them. They can effectively inhibit the migration of polysulfides without affecting the conduction of lithium ions. Therefore, we designed and synthesized a stable ternary porous cationic organic framework and then modified it into a cationic framework through ion exchange with HSO 3 to obtain HSO 3 -ETB-COF. This material is used to modify the separator of lithium-sulfur batteries. The cationic sites can accept electrons from the cathode and transfer them to polysulfides to promote their decomposition. At the same time, the cationic sites can receive electrons from polysulfides and transfer them to the anode during charging to promote the oxidation of polysulfides.

[0006] Defects and deficiencies of the prior art: So far, the modification of lithium-sulfur batteries can be mainly divided into the following aspects: (1) Electrode material modification: By adding conductive agents (such as carbon nanotubes, graphene, etc.) to improve the conductivity of the sulfur electrode, thereby enhancing the electron conduction efficiency. Or composite sulfur with conductive materials (such as carbon materials) to form a composite electrode to improve the conductivity and structural stability of the electrode; (2) Electrolyte optimization: Develop electrolytes with higher stability and better compatibility to reduce the dissolution of polysulfides and improve the cycle performance of the battery; (3) Improvement of functionalized separators: Modify the separator (such as coating with conductive materials or adding hydrophilic materials) to improve the lithium-ion conductivity and reduce the polysulfide shuttle effect. Use porous materials as separators to improve the lithium-ion conduction efficiency and the overall performance of the battery. However, many modification methods involve multi-step synthesis processes, increasing the complexity of production. Moreover, the additives required for some separator modification processes are relatively expensive, thus increasing the manufacturing cost of the battery. At the same time, a certain degree of modification of the separator will have a certain impact on the lithium and sulfur transport rates, thus restricting the performance of the battery. Some modifiers will cause problems such as battery safety and stability, such as thermal runaway and short circuit. Summary of the Invention

[0007] Aiming at the problems existing in the prior art, this application proposes a preparation method, a modified separator and a lithium-sulfur battery of an amphoteric ion covalent organic framework modified separator for lithium-sulfur batteries.

[0008] The preparation method of the amphoteric ion covalent organic framework modified separator for lithium-sulfur batteries includes the following steps: S1. Synthesize ETB-COF Add ethidium bromide (EB), 4,4'-biphenyldicarboxaldehyde (BPDA) and phloroglucinol trialdehyde (TFP) into a Pyrex tube for mixing to obtain a mixed monomer. Then add a mixed solution of o-dichlorobenzene and benzyl alcohol into the above Pyrex tube, ultrasonically vibrate for 2 - 4 min, add 0.1 mL of acetic acid as a reaction catalyst into the Pyrex tube, and continue ultrasonically vibrate for 1 - 5 min; then repeat the freeze-vacuum process of the Pyrex tube three times, seal it with butane gas by flame, and obtain a red-brown powdery solid product after heating; next, add deionized water and tetrahydrofuran solution alternately for centrifugal washing 3 - 5 times until the color of the washing solution is clear; put the washed solid product into a vacuum at 100 °C for drying for 10 - 12 h, and finally obtain a dark red solid powder ETB-COF; S2. Prepare HSO 3 - -ETB-COF Disperse 100 mg of ETB-COF in 80 - 100 mL of 0.1 mol / L sodium bisulfite solution, stir at room temperature for 20 - 24 h, then filter, wash with deionized water 2 - 3 times, and finally dry under vacuum to obtain HSO 3 - -ETB-COF precursor material; S3. Preparation of modified separator (1) Dry milling: Mix the HSO 3 - -ETB-COF precursor material, PVDF and Super-P, and ball mill for 30 - 40 min to obtain a mixture; (2) Wet milling: Add NMP solution to the mixture and ball mill for 1 - 2 h to obtain a slurry; (3) Uniformly coat the slurry on the surface of PP with a four-sided sample preparation device of 40 - 50 mm, and then place it in a vacuum oven at 60 °C and dry for 12 - 16 h to obtain a separator; (4) Cut the separator into circular pieces with a diameter of 16 mm to obtain a modified separator.

[0009] In step S1, the mass ratio of ethidium bromide (EB), 4,4'-biphenyldicarboxaldehyde (BPDA) and phloroglucinol trialdehyde (TFP) is (3 - 7):2:2; In step S1, the volume ratio of o-dichlorobenzene and benzyl alcohol is (1 - 2):1; In step S1, the dosage relationship between ethidium bromide (EB) and o-dichlorobenzene is: (5 - 15) mg / ml; In step S1, the concentration of acetic acid is 6 mol / L; In step S1, the heating time is 120 h and the heating temperature is 120 °C; In step S2, the vacuum drying temperature is 80 - 100 °C and the drying time is 24 - 36 h; In step S3(1), the weight ratio of the HSO 3 - -ETB-COF precursor material, PVDF and Super-P is 7:1:2; A zwitterionic covalent organic framework modified separator for lithium-sulfur batteries obtained by the above preparation method.

[0010] A lithium-sulfur battery comprising a zwitterionic covalent organic framework modified separator for lithium-sulfur batteries obtained by the above preparation method.

[0011] Compared with the prior art, the present invention has the following advantages: 1. A zwitterionic covalent organic framework ETB-COF was synthesized by a solvothermal method, and HSO 3 - -ETB-COF was prepared through an ion exchange process. ETB-COF and HSO 3 - -ETB-COF exhibit good stability and high specific surface area, and as a separator modification material for Li-S batteries, they can enhance the stability of the batteries.

[0012] 2. ETB-COF and HSO 3 - -ETB-COF have an ordered structure, which can improve the conduction rate of lithium ions in the separator, thereby enhancing the charge-discharge efficiency of the battery. In addition, due to the presence of charged bisulfite groups in the structure, the battery assembled with the modified middle layer composed of HSO 3 - -ETB-COF has a higher ionic conductivity (2.8 mS·cm -1 ). Moreover, the modified middle layer based on HSO 3 - -ETB-COF can not only inhibit the shuttle of polysulfides but also promote the diffusion of Li + through electrostatic interaction. The HSO 3 - -ETB-COF battery also exhibits excellent electrochemical performance, with an initial specific capacity of 1000 mAh·g -1 at 1 C and a specific capacity of 754.6 mAh·g -1 after 500 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram of the synthesis process of the polymer HSO 3 - -ETB-COF of the present invention; Figure 2 is the XRD pattern of the polymers obtained in Example 1 and Example 4 of the present invention, wherein, Figure 2 (a) is the PXRD pattern, space-filling model, and AA stacking model of ETB-COF calculated based on the P1 space group; Figure 2 (b) is the PXRD pattern of ETB-COF and HSO 3 - -ETB-COF; Figure 3 is the nitrogen adsorption-desorption analysis and pore size distribution diagram of the polymers obtained in Example 1 and Example 4 of the present invention, wherein, Figure 3 (a) is ETB-COF and HSO 3- -ETB-COF's N 2 Adsorption - desorption isotherm diagram, Figure 3 (b) is for ETB - COF and HSO 3 - -ETB-COF pore size distribution diagram; Figure 4 This is the XPS diagram of the polymers obtained in Example 1 and Example 4 of the present invention. Among them, Figure 4 (a) is for ETB - COF and HSO 3 - -ETB-COF's X - ray photoelectron spectroscopy; Figure 4 (b) is for ETB - COF and HSO 3 - -ETB-COF's C1s spectrum; Figure 4 (c) is for ETB - COF and HSO 3 - -ETB-COF's N 1s spectrum; Figure 4 (d) is for EBT - COF and HSO 3 - -ETB-COF's O 1s spectrum; Figure 4 (e) is for EBT - COF's Br1s spectrum; Figure 4 (f) is for HSO 3 - -ETB-COF's S1s spectrum; Figure 5 This is the high - resolution scanning electron microscopy diagram of the polymer obtained in Example 6 of the present invention. Among them, Figure 5 (a) is the scanning electron microscopy diagram of ETB - COF / PP, Figure 5 (b) is the scanning electron microscopy diagram of the polymer HSO 3 - -ETB-COF / PP; Figure 5 (c) is for ETB - COF / PP and HSO 3 - Bending test of ETB - COF / PP separator; Figure 6 This is the performance diagram of the Li - S battery prepared by the present invention. Among them, Figure 6 (a) is for ETB - COF and HSO 3 - -ETB-COF battery's CV diagram; Figure 6 (b) ETB - COF and HSO 3 - -ETB-COF conductivity diagram; Figure 6 (c) is for ETB - COF and HSO 3 -EIS diagram of the -ETB-COF battery; Figure 6 (d) is HSO 3 - Example migration number diagram of the -ETB-COF battery; Figure 6 (e) is ETB-COF and HSO 3 - Rate performance diagram of the -ETB-COF modified battery; Figure 6 (f) is HSO 3 - Discharge-charge curve diagram of the -ETB-COF modified battery at different current densities of 0.1, 0.2, 0.5, 1, and 2 C; Figure 6 (g) is ETB-COF and HSO 3 - Long cycle performance diagram of the -ETB-COF modified battery at 1 C for 500 cycles; Figure 7 is the HSO obtained in Example 6 3 - Small multiple scanning electron microscope diagram of the modified separator prepared with -ETB-COF; Figure 8 Scanning electron microscope diagram of the modified separator in Comparative Example 1; Figure 9 Scanning electron microscope diagram of the modified separator in Comparative Example 2; Figure 10 Scanning electron microscope diagram of the modified separator in Comparative Example 3. Detailed implementation mode

[0014] The concept and technical effects of the present invention will be clearly and completely described below in conjunction with the embodiments and the drawings to fully understand the purpose, features, and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0015] The following further illustrates the present invention in conjunction with the embodiments. The instruments and equipment used in the embodiments of the present invention and their models are: Shanghai Chenhua Electrochemical Workstation (Chenhua CHI660E) and Blue Electric Battery Test System (CT2001A).

[0016] Measure the cyclic voltammetry curve of the Li-S battery in the voltage range of 1.7 - 2.8 V at 0.1 mV·s -1 At this time. Electrochemical impedance, measure the cyclic stability at 1 C and the rate performance of the Li-S battery at different current densities in the Blue Electric Battery Test System (CT2001A).

[0017] The assembly process of the Li-S battery of the present invention includes the following steps: 1. Preparation of C / S composite material Prepare the C / S composite material using the traditional melt diffusion method; first, mix Super-P powder and sulfur powder with a mass ratio of 2.5:7.5 and grind them thoroughly in a mortar. Then, put the mixture into a reaction kettle lined with polytetrafluoroethylene and heat it at 155 °C for 24 h under the atmosphere of argon protection. Finally, collect the product after cooling to room temperature to obtain the C / S composite material.

[0018] 2. Preparation of conventional positive electrode Mix the C / S composite material, carbon black, and PVDF with a mass ratio of 8:1:1 and grind them evenly. Subsequently, add 1-methyl-2-pyrrolidone (NMP) and stir well to form a uniform slurry. Use a coater to evenly coat the slurry on the aluminum foil, and then dry it at 60 °C for 24 h. After drying, use a slicer to cut the aluminum foil into circular electrodes to obtain the required electrodes.

[0019] 3. Assembly of the battery Before assembly, prepare all components related to the battery. Select the CR2023 type button battery as the battery model; use a commercial lithium metal sheet with a diameter of 16 mm as the electrode. The electrolyte consists of a mixed solution of 1 wt% LiNO 3 and 1 M LiTFSI in DOL / DME, with a volume fraction of 1:1. Cut the modified film into a diaphragm disc with a diameter of 16 mm.

[0020] The assembly of the button battery needs to be carried out in an argon glove box to ensure that the moisture and oxygen content in the glove box are both less than 1 ppm. The assembly steps are as follows: first, put the prepared sulfur positive electrode material into the positive electrode case; then, drop an appropriate amount of electrolyte on the surface of the sulfur positive electrode, and the ratio of the amount of electrolyte to sulfur is 24 μL·mg -1 . Then, add the cut diaphragm and drop 40 μL of electrolyte on it again, and then put in the lithium metal sheet. In addition, add a stainless steel gasket, a spring piece, and a negative electrode case. Finally, use a button battery encapsulation machine for encapsulation, and let it stand for 24 h after encapsulation for subsequent testing.

[0021] Example 1 Synthesis of ETB-COF 30 mg of ethidium bromide (EB), 12 mg of 4,4'-biphenyldicarboxaldehyde (BPDA), and 12 mg of tri-formylphloroglucinol (TFP) were mixed and added into a Pyrex tube. Then, 2 ml of a mixed solution of o-dichlorobenzene and benzyl alcohol with a volume ratio of 2:1 was added into the Pyrex tube. After ultrasonic oscillation for 4 min, 0.1 ml of 6 mol / L acetic acid was added and ultrasonic oscillation was continued for 3 min after thorough mixing with the mixed monomers. Subsequently, the Pyrex tube was subjected to the freeze-pump-thaw process three times, flame-sealed with butane gas, and heated at 120 ºC for 120 h to obtain a red-brown powdery solid product. Deionized water and tetrahydrofuran solution were successively added and the mixture was centrifuged at high speed and washed alternately 5 times. After the washing solution became clear, the washed solid sample was placed in a vacuum drying oven and dried at 100 ºC for 12 h to finally obtain a dark red solid powder ETB-COF with a yield of 76 %. Example 2 Synthesis of ETB-COF 10 mg of ethidium bromide (EB), 4 mg of 4,4'-biphenyldicarboxaldehyde (BPDA), and 4 mg of tri-formylphloroglucinol (TFP) were mixed and added into a Pyrex tube. Then, 3 ml of a mixed solution of o-dichlorobenzene and benzyl alcohol with a volume ratio of 2:1 was added into the Pyrex tube. After ultrasonic oscillation for 2 min, 0.1 ml of 6 mol / L acetic acid was added and ultrasonic oscillation was continued for 1 min after thorough mixing with the mixed monomers. Subsequently, the Pyrex tube was subjected to the freeze-pump-thaw process three times, flame-sealed with butane gas, and heated at 120 ºC for 120 h to obtain a red-brown powdery solid product. Deionized water and tetrahydrofuran solution were successively added and the mixture was centrifuged at high speed and washed alternately 3 times. After the washing solution became clear, the washed solid sample was placed in a vacuum drying oven and dried at 100 ºC for 10 h to finally obtain a dark red solid powder ETB-COF with a yield of 70.5 %. Example 3 Synthesis of ETB-COF 40 mg of ethidium bromide (EB), 16 mg of 4,4'-biphenyldicarboxaldehyde (BPDA), and 16 mg of tri-formylphloroglucinol (TFP) were mixed and added into a Pyrex tube. Then, 4 ml of a mixed solution of o-dichlorobenzene and benzyl alcohol with a volume ratio of 2:1 was added into the Pyrex tube. After ultrasonic oscillation for 4 min, 0.1 ml of 6 mol / L acetic acid was added after sufficient mixing with the mixed monomers, and ultrasonic oscillation was continued for 1 min. Then, the Pyrex tube was subjected to the freeze-vacuum process three times, flame-sealed with butane gas, and heated at 120 ºC for 120 h to obtain a red-brown powdery solid product. Deionized water and tetrahydrofuran solution were successively added for high-speed centrifugal washing three times. After the washing solution became clear; the washed solid sample was placed in a vacuum drying oven and dried at 100 ºC for 12 h to finally obtain a dark red solid powder ETB-COF with a yield of 68.1%; Example 4 Preparation of HSO 3 - -ETB-COF 100 mg of ETB-COF prepared in Example 1 was dispersed in 80 ml of 0.1 M sodium bisulfite solution and stirred at room temperature for 24 h. Then, the mixed solution was filtered, washed twice with deionized water, and finally dried in vacuo at 80 ºC for 24 h to obtain HSO 3 - -ETB-COF; HSO 3 - The schematic diagram of the synthesis process of HSO Figure 1 is shown as follows.

[0022] Example 5 Preparation of HSO 3 - -ETB-COF 100 mg of ETB-COF prepared in Example 1 was dispersed in 100 ml of 0.1 M sodium bisulfite solution and stirred at room temperature for 20 h. Then, the mixed solution was filtered, washed three times with deionized water, and finally dried in vacuo at 100 ºC for 36 h to obtain HSO 3 - -ETB-COF.

[0023] The X-ray diffraction patterns (XRD) of the ETB-COF powder obtained in Example 1 and HSO 3 - -ETB-COF prepared in Example 4 are shown as follows, and the ETB-COF and HSO Figure 2 were analyzed 3 -- The crystal structure of ETB-COF was determined, and the structure was modeled and simulated using Materials Studio software. The XRD pattern of ETB-COF shows that the diffraction peaks are located at 3.45° and 26.3°, corresponding to the diffraction of the (100) and (001) crystal planes, respectively ( Figure 2 a). The XRD data were processed using the Pawley refinement technique. The space group is P1, with lattice constants a = 30.08 Å, b = 30.65 Å, c = 4.10 Å, α = β = 90, γ = 120°, and the fitting parameters Rwp = 10.58% and Rp = 7.59%. The comparison between the experimental and calculated diffraction patterns shows that ETB-COF has a 2D network structure and an AA stacking structure. Compared with ETB-COF, HSO 3 - -ETB-COF can still maintain high crystallinity after substituting the bisulfite ion ( Figure 2 b).

[0024] The ETB-COF prepared in Example 1 and the HSO 3 - -ETB-COF prepared in Example 4 were subjected to N 2 adsorption experiments at 77 K as Figure 3 shown. The N 2 adsorption isotherm (Figure 3a) shows that both ETB-COF and HSO 3 - -ETB-COF exhibit both type I and type IV isotherms, indicating that there are both micropores and mesopores in the structures of ETB-COF and HSO 3 - -ETB-COF. In addition, the specific surface areas of ETB-COF and HSO 3 - -ETB-COF were calculated to be 289.21 m 2 ·g -1 and 179.99 m 2 ·g -1 respectively. Non-local density functional theory (NLDFT) was used to calculate the pore size distributions of ETB-COF and HSO 3 - -ETB-COF. The results show that the pore sizes of ETB-COF and HSO 3 - -ETB-COF are 4.64 nm and 1.68 nm, respectively (Figure 3b).

[0025] Figure 4 This is the ETB-COF prepared in Example 1 and the HSO 3 -X-ray photoelectron spectroscopy of the two materials, ETB-COF, confirmed the valence states and compositions of the elements in the ETB-COF structure. Peaks of C 1s and N 1s were observed in the XPS spectrum of ETB-COF (Figure 4a). Among them, the peaks of C 1s, N 1s, and O 1s were attributed to carbon and nitrogen elements in the ETB-COF framework. For ETB-COF, the C 1s peak could be divided into 283.5 eV (C=C) and 285.4 eV (C=N) (Figure 4b). The N 1s peak was divided into 398.7 eV (-NH-), 400 eV (C=N), and 401.1 eV (-N 3 - =) (Figure 4c). The O 1s peak was divided into 530.8 eV and 532.4 eV (Figure 4d), with the former attributed to the carbonyl double bond (C=O) and the latter attributed to water in the air (H-O-H). The appearance of the Br 1s peak (Figure 4e). By comparing the positions of the C 1s and N 1s peaks in ETB-COF and the ETB-COF polymer (Figure 4a - d), we found that the positions of these peaks changed slightly, indicating strong interactions between the COF and bisulfite ions after implanting the polymer. In addition, the polymer HSO 3 - -ETB-COF showed S 2p peaks at 169.1 eV (S 2p1 / 2) and 166.9 (S 2p3 / 2) (Figure 4f). 3 - -ETB-COF and ETB-COF polymer, we found that the positions of these peaks changed slightly, indicating strong interactions between the COF and bisulfite ions after implanting the polymer. In addition, the polymer HSO + =). The O 1s peak was divided into 530.8 eV and 532.4 eV (Figure 4d), with the former attributed to the carbonyl double bond (C=O) and the latter attributed to water in the air (H-O-H). The appearance of the Br 1s peak (Figure 4e). By comparing the positions of the C 1s and N 1s peaks in ETB-COF and the ETB-COF polymer (Figure 4a - d), we found that the positions of these peaks changed slightly, indicating strong interactions between the COF and bisulfite ions after implanting the polymer. In addition, the polymer HSO 3 - -ETB-COF and ETB-COF polymer, we found that the positions of these peaks changed slightly, indicating strong interactions between the COF and bisulfite ions after implanting the polymer. In addition, the polymer HSO 3 - -ETB-COF showed S 2p peaks at 169.1 eV (S 2p1 / 2) and 166.9 (S 2p3 / 2) (Figure 4f).

[0026] Example 6 Preparation of Modified Separator (1) Dry milling: Mix the HSO 3 - -ETB-COF prepared in Example 4 with PVDF and Super-P in a weight ratio of 7:1:2, and ball mill for 30 min to obtain a mixture; (2) Wet milling: Add NMP solution to the mixture and ball mill for 1 h to obtain a slurry; (3) Uniformly coat the slurry on the surface of PP using a 40-mm four-sided sample preparation device, and then place it in a vacuum oven at 60 °C for 12 h to obtain a separator; (4) Cut the separator into circular pieces with a diameter of 16 mm, which is HSO 3 --ETB-COF / PP modified separator.

[0027] Replace HSO in step (1) of Example 6 with the ETB-COF prepared in Example 1 3 - -ETB-COF, and repeat the above steps to prepare the modified separator to obtain the ETB-COF / PP modified separator.

[0028] Figure 5 For the ETB-COF / PP and HSO prepared in Example 6 3 - -ETB-COF / PP, the scanning electron microscope images of ETB-COF / PP and HSO were prepared on the PP separator by the coating method 3 - -ETB-COF / PP composite separator. Figures 5a and 5b show the scanning electron microscope images of ETB-COF / PP and HSO 3 - -ETB-COF / PP separator. It can be seen that the voids of the original PP separator have been completely covered by these two composite separators. To test the adhesion strength of ETB-COF and HSO 3 - -ETB-COF separator on the PP separator, bending tests were carried out on the ETB-COF / PP and HSO 3 - -ETB-COF / PP isolation layer. As can be seen from Figure 5c, there are no creases on the folded separator, it has completely returned to its original state, and no powder has fallen off, indicating that the separator prepared by this method has good bending strength.

[0029] From Figure 7 it can be seen that by scanning with a small-magnification scanning electron microscope, the cracking of the HSO 3 - -ETB-COF / PP modified separator is very small. The smaller the cracking, the more uniform the coating of the separator material and the better the effect.

[0030] Comparative Example 1 (1) Dry grinding: Mix ETB-COF and HSO 3 - -ETB-COF with PVDF and Super-P respectively in a weight ratio of 6:1:3, and ball mill for 30 min; (2) Wet grinding: Add NMP solution to the mixture ground in the previous step, and ball mill for 1 h to obtain a slurry.

[0031] (3) The uniformly mixed slurry was evenly coated on the surface of PP with a 40-mm four-sided sample maker, and then placed in a vacuum oven at 60 °C for drying for 12 h; (4) The separator obtained in the previous step was cut into circular pieces with a diameter of 16 mm, which was the modified separator. The scanning electron micrograph of the obtained modified separator is as Figure 8 shown.

[0032] Comparative Example 2 (1) Dry grinding: ETB-COF and HSO 3 - -ETB-COF were respectively mixed with PVDF and Super-P according to a weight ratio of 5:1:4, and ball milled for 30 min; (2) Wet grinding: NMP solution was added to the mixture ground in the previous step, and ball milled for 1 h to obtain a slurry; (3) The uniformly mixed slurry was evenly coated on the surface of PP with a 40-mm four-sided sample maker, and then placed in a vacuum oven at 60 °C for drying for 12 h; (4) The separator obtained in the previous step was cut into circular pieces with a diameter of 16 mm, which was the modified separator. The scanning electron micrograph of the obtained modified separator is as Figure 9 shown.

[0033] Comparative Example 3 (1) Dry grinding: ETB-COF and HSO 3 - -ETB-COF were respectively mixed with PVDF and Super-P according to a weight ratio of 4:1:5, and ball milled for 30 min; (2) Wet grinding: NMP solution was added to the mixture ground in the previous step, and ball milled for 1 h to obtain a slurry; (3) The uniformly mixed slurry was evenly coated on the surface of PP with a 40-mm four-sided sample maker, and then placed in a vacuum oven at 60 °C for drying for 12 h; (4) The separator obtained in the previous step was cut into circular pieces with a diameter of 16 mm, which was the modified separator. The scanning electron micrograph of the obtained modified separator is as Figure 10 shown.

[0034] It can be seen from the above Example 6 and Comparative Examples 1-3 that different separators prepared by adjusting the ratios of HSO 3 - -ETB-COF, PVDF and Super-P. When HSO 3 - -ETB-COF, PVDF and Super-P are mixed at a weight ratio of 7:1:2, the prepared separator has relatively better flatness and the smallest degree of cracking.

[0035] Assembly of Li-S battery 1. Preparation of C / S composite Prepare the C / S composite using the traditional melt diffusion method. First, mix Super-P powder and sulfur powder with a mass ratio of 2.5:7.5 and grind them thoroughly in a mortar. Then, put the mixture into a reaction kettle lined with polytetrafluoroethylene and heat it at 155 °C for 24 h under an argon-protected atmosphere. Finally, collect the product after cooling to room temperature to obtain the C / S composite.

[0036] 2. Preparation of conventional cathode Mix 80% by mass fraction of the above-prepared C / S composite, 10% carbon black, and 10% PVDF thoroughly and grind them evenly. Subsequently, add 1-methyl-2-pyrrolidone (NMP) and stir well to ensure a uniform slurry is formed. Use a coater to evenly coat the slurry on aluminum foil and then dry it at 60 °C for 24 h. After drying, use a slicer to cut the aluminum foil into circular electrodes to obtain the required electrodes.

[0037] 3. Assembly of the battery Before assembly, prepare all components related to the battery. Select the CR2023 type button battery as the battery model. Use a commercial lithium metal sheet with a diameter of 16 mm as the electrode. The electrolyte uses 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) with a volume fraction of 1:1 as the solvent, 1 wt% LiNO 3 and 1 M LiTFSI as the solute; the HSO 3 - -ETB-COF / PP modified separator prepared in Example 6, cut into a circular separator with a diameter of 16 mm.

[0038] The assembly of the button battery needs to be carried out in an argon glove box to ensure that the moisture and oxygen content in the glove box are both less than 1 ppm. The assembly steps are as follows: First, put the prepared sulfur cathode material into the cathode shell, then drop the electrolyte on the surface of the sulfur cathode, and the volume ratio of the electrolyte to sulfur is 24 μL·mg -1 , then add the cut circular separator and drop 40 μL of electrolyte on it again. Subsequently, put in the lithium metal sheet. In addition, add a stainless steel gasket, a spring piece, and the negative electrode shell. Finally, use a button battery encapsulation machine for encapsulation. After encapsulation, let it stand for 24 h for subsequent testing.

[0039] Figure 6 For the performance analysis of polymer batteries, to study the electron transfer kinetics between redox products and electrodes, in the potential range of 1.7 to 2.8 V, at 0.1 mV·s -1The scanning rate measured the CV curves of different separator cells. As Figure 6 shown in (a), the CV curves of the HSO 3 - -ETB-COF / PP and ETB-COF / PP separator cells showed stronger peak intensity and wider peak width than those of the PP separator cell, indicating a more complete redox reaction occurred. The single peak at 2.41 V originated from the oxidation of LiPSs to elemental sulfur. The reduction of S 8 to S 4 2- (solid-liquid phase transition) corresponded to the first reduction peak at 2.23 V, while the further reduction of S 4 2- to S 2 2- / S 2- (liquid-solid phase transition) resulted in the second reduction peak at 1.95 V. Compared with the ETB-COF / PP separator cell, the CV curve of the HSO 3 - -ETB-COF / PP cell showed a tendency of right-shifted reduction peak and left-shifted oxidation peak. The polarization voltage of the HSO 3 - -ETB-COF / PP cell was 0.448 V, which was lower than 0.497 V of ETB-COF / PP, respectively. It should be noted that the CV curves of the cells with HSO 3 - -ETB-COF / PP and ETB-COF / PP separators showed that there were two different oxidation peaks, which could be well maintained during cycling. This indicated that Li 2 S / Li 2 S 2 had good decomposition kinetics and excellent electrochemical reversibility. Electrochemical impedance spectroscopy (EIS) was used to further evaluate the conductivity. The conductivity of Li 3 - with HSO + -ETB-COF was 0.71 mS·cm -1 , which was better than the conductivity of ETB-COF (0.45 mS·cm -1 ), as shown in Figure 6 (b). This result indicated that HSO 3 - -ETB-COF had good selective permeability to Li + ions. When the lithium ions dissolved in the electrolyte diffused to the end of the separator under the action of the concentration gradient, they would first be physically blocked at the positive electrode. Due to ETB-COF / PP and HSO 3 -The -ETB-COF / PP separator has an ordered porous structure, allowing the electrolyte to fully penetrate it. LiPS can be evenly dispersed in its structure and come into full contact with COF.

[0040] To gain a deeper understanding of the excellent performance of the battery, EIS tests were conducted to measure the electrochemical impedance spectra (EIS) of LSBs assembled with different separators. The high-frequency semi-circle on the real axis of the EIS spectrum is related to the battery ohmic resistance Rs caused by the electrolyte. The mid-frequency semi-circle determines the charge transfer resistance Rct at the electrode / electrolyte interface, and the low-frequency linear Warburg region is related to the diffusion of lithium ions within the electrode. According to Figure 6 the EIS spectra shown in (c), the Rct of the LSB assembled with the HSO 3 - -ETB-COF / PP separator is 66 Ω, much lower than the 81 Ω of the ETB-COP / PP separator battery. The minimum Rct indicates that in the battery assembled with the HSO 3 - -ETB-COF / PP separator, while ensuring the normal passage of lithium ions, the separator maintains an effective barrier to the diffusion of LiPS. In addition, the improved wettability of the composite separator enhances the interfacial compatibility between the separator and the electrode interface, as well as the retention rate of the electrolyte in the separator. The I-t curve and Nyquist curve before and after battery polarization were measured using lithium / lithium symmetric batteries to obtain the transference number of lithium ions (D Li + ), which reflects the migration amount before and after battery repolarization. For HSO 3 - -ETB-COF / PP, the transference number of Li + is 0.941 Figure 6 as shown in (d), while the transference number of ETB-COF is only 0.901. Obviously, HSO 3 - -ETB-COF / PP provides a higher D Li + than the ETB-COF / PP separator, which strongly confirms that HSO 3 - -ETB-COF / PP is beneficial to Li + diffusion.

[0041] To further investigate the effect of the COF-modified separator on the performance of Li-S batteries, coin cells containing HSO 3 - -ETB-COF / PP and ETB-COF / PP were assembled. In the current density range of 0.1 - 2 C, the discharge specific capacities of the batteries assembled with HSO 3 --ETB-COF / PP, ETB-COF / PP, and pure PP separators assembled LSBs' rate performance, as Figure 6 shown in (e). At current ratios of 0.1, 0.2, 0.5, 1, and 2 C, the average specific capacity of the HSO 3 - -ETB-COF / PP separator batteries are 1126 mAh·g -1 , 977.9 mAh·g -1 , 848.4 mAh·g -1 , 765 mAh·g -1 , and 665 mAh·g -1 respectively. Even at a high current ratio of 2 C, the capacity retention rate of the HSO 3 - -ETB-COF / PP separator battery is much higher than that of the ETB-COF / PP separator battery, which means that the HSO 3 - -ETB-COF / PP film rich in bisulfite ion functional groups improves the rate performance of the battery simultaneously. In addition, after the rate is restored to 0.1 C, the stable capacity of the HSO 3 - -ETB-COF / PP separator is restored to 994 mAh·g -1 respectively. The galvanostatic charge-discharge curves of the HSO 3 - -ETB-COF / PP separator batteries with different ratios are shown in Figure 6 (f). The results show that even at a high current density of 2 C, there are still two typical discharge platforms in the HSO 3 - -ETB-COF / PP battery. It is worth noting that at 0.1 C, the battery assembled with the HSO 3 - -ETB-COF / PP separator presents a stable platform with a polarization voltage of 0.17 V, while the polarization voltage of the ETB-COF / PP battery is 0.22 V, indicating that the COF containing bisulfide ions is more conducive to suppressing polysulfides, thus providing a larger specific capacity and lower polarization. These results of the galvanostatic charge-discharge test are very consistent with the data obtained from the CV test. In addition, the long-term cycling of the batteries with different separators is shown in Figure 6 (g). The initial discharge capacities of the HSO 3 - -ETB-COF / PP and ETB-COF / PP separators at 1 C are 1000 and 848 mAh·g -1After 500 cycles, the discharge capacities of the two diaphragms were maintained at 754.6 and 521 mAh·g, respectively. -1 HSO 3 - The capacity decay rates of the HSO-ETB-COF / PP and ETB-COF / PP diaphragm batteries per cycle were as low as 0.049% and 0.0771%, respectively, and the Coulombic efficiency was close to 100%, indicating that the batteries had good cycle stability. These results further indicated that the COF with rich functional groups had strong chemical binding force and enhanced redox kinetics.

[0042] The above embodiments are only partial implementation schemes of the present invention, but the protection scope of the present invention is not limited thereto. For example, the various aspects and implementation schemes of the present invention disclosed herein are only examples of the specific ways to make and use the present invention. Nor is any limitation imposed on the present invention by the order of the embodiments and the specific operations. Any substitution and change that can be easily thought of by any scientific research personnel familiar with the technical field of the present invention within the technical scope disclosed by the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a zwitterionic covalent organic framework modified diaphragm for lithium-sulfur batteries, characterized in that: The following steps are involved: S1. Synthesis of ETB-COF Add bromophenanthridine, 4,4'-biphenyldicarboxaldehyde and trialdehyde phloroglucinol into a Pyrex tube and mix to obtain a mixed monomer, then add a mixed solution of o-dichlorobenzene and benzyl alcohol into the Pyrex tube, ultrasonically shake for 2-4 minutes, add 0.1 mL of acetic acid with a concentration of 6 mol / L into the Pyrex tube, and continue ultrasonically shaking for 1-5 minutes; then repeat the freezing and vacuuming process of the Pyrex tube three times, flame seal it with butane gas, and obtain a red-brown powdery solid product after heating; then add deionized water and tetrahydrofuran solution in turn and alternately centrifuge and wash for 3-5 times until the color of the washing liquid becomes clear; the washed solid product is vacuum dried to finally obtain dark red solid powder ETB-COF; S2. Preparation of HSO3 - -ETB-COF Disperse 100 mg of ETB-COF in 80-100 mL of 0.1 mol / L sodium bisulfite solution, stir at room temperature for 20-24 h, filter, wash with deionized water 2-3 times, and finally vacuum dry to obtain HSO3 - -ETB-COF precursor materials; S3. Preparation of modified diaphragm (1) Dry grinding: HSO3 - -ETB-COF precursor material, PVDF and Super-P are mixed and ball-milled for 30-40 min to obtain a mixture; (2) Wet grinding: Add NMP solution to the mixture and ball mill for 1-2 h to obtain a slurry; (3) The slurry is evenly coated on the PP surface using a 40-50 mm four-sided sampler, and then vacuum dried to obtain a diaphragm; (4) The diaphragm was cut into discs with a diameter of 16 mm to obtain a modified diaphragm.

2. The method for preparing the zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery according to claim 1, characterized in that: The mass ratio of bromophenanthridine, 4,4'-biphenyldicarboxaldehyde and trialdehyde phloroglucinol in step S1 is (3-7):2:

2.

3. The method for preparing the zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery according to claim 1, characterized in that: The volume ratio of o-dichlorobenzene to benzyl alcohol in step S1 is (1-2):

1.

4. The method for preparing the zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery according to claim 1, characterized in that: The dosage ratio of ethidium bromide and o-dichlorobenzene in step S1 is: (5-15) mg / ml.

5. The method for preparing the zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery according to claim 1, characterized in that: In step S1, the heating time is 120 h, the heating temperature is 120 ºC; the drying temperature is 100 ºC, and the drying time is 10-12 h.

6. The method for preparing the zwitterionic covalent organic framework modified diaphragm for lithium-sulfur battery according to claim 1, characterized in that: The vacuum drying temperature in step S2 is 80-100°C, and the drying time is 24-36 h.

7. The method for preparing a zwitterionic covalent organic framework modified diaphragm for lithium-sulfur batteries according to claim 1, characterized in that: In step S3 (1), HSO3 - -The weight ratio of ETB-COF precursor material, PVDF and Super-P is 7:1:

2.

8. The method for preparing a zwitterionic covalent organic framework modified diaphragm for lithium-sulfur batteries according to claim 1, characterized in that: The drying temperature in step S3 (3) is 60°C and the drying time is 12-16 h.

9. A modified diaphragm, prepared by the method for preparing a zwitterionic covalent organic framework modified diaphragm for lithium-sulfur batteries according to claim 1.

10. A lithium-sulfur battery, comprising a diaphragm prepared by the method for preparing a zwitterionic covalent organic framework modified diaphragm for a lithium-sulfur battery according to claim 1.

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