Preparation method of polyoxometalate-based covalent organic framework modified separator, modified separator and lithium-sulfur battery

Abstract

The present invention discloses a preparation method of a polyoxometalate-based covalent organic framework modified separator, the modified separator and a lithium-sulfur battery, belonging to the technical field of batteries. Using 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and (nBu4N)3[CoMo6O18((OCH2)3CNH2)2] (POM) as two monomers as solutes, N,N-dimethylformamide as a solvent, and trifluoroacetic acid as a catalyst, TFPOT-POM solid powder is synthesized to obtain a TFPOT-POM precursor material. Finally, the modified separator is prepared from the TFPOT-POM precursor material, PVDF and Super-P in a defined ratio. TFPOT-POM has an ordered structure and high catalytic activity, which can improve the conduction rate of lithium ions in the separator, thereby improving the charge and discharge efficiency of the battery.

Description

Technical Field

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

[0002] Energy storage is an important part and a key supporting technology of the new energy industry, and has become one of the focuses of international energy science and technology strategic competition. Lithium-sulfur batteries are considered to be one of the most promising next-generation energy storage technologies due to their high energy density and low cost. The theoretical energy density of lithium-sulfur batteries is very high (2600 Wh·kg -1 ). Due to the high natural availability, affordability and environmental friendliness of sulfur, Li-S batteries can meet the growing global energy demand; nevertheless, several important challenges have hindered their commercial application. For decades, efforts have been made to solve the obstacles existing in lithium-sulfur batteries to improve the sulfur electrochemical performance of lithium-sulfur batteries. Therefore, it is of great significance to develop lithium-sulfur batteries with high battery capacity and cycle stability.

[0003] The main problems existing in the positive electrode of lithium-sulfur batteries are as follows: (1) Polysulfides dissolve into the electrolyte. During the cycling process, intermediate long-chain lithium polysulfides (Li2S4 to Li2S8) are easily dissolved into the ether-based electrolyte, which leads to the continuous loss of active substances into the electrolyte, and some of them still dissolve and do not precipitate back to the positive electrode in the form of lithium sulfide at the end of discharge. Therefore, low discharge capacity and rapid capacity decay are often observed during the cycling process; (2) The conductivity of sulfur and lithium sulfide is low. The insulating properties of sulfur and lithium sulfide in terms of electrons and ions result in poor utilization of active substances. During the discharge process, the precipitation of insulating lithium sulfide will also cause passivation of the cathode surface, thereby limiting the actual achievable discharge capacity; (3) Sulfur has a large volume expansion during lithiation. Due to the density difference between sulfur and lithium sulfide (2.03 g·cm -3 and 1.66 g·cm -3 ), sulfur will undergo a volume expansion of about 80% after being fully lithiated into lithium sulfide, which will lead to pulverization and structural damage of the electrode layer.

[0004] On the negative electrode side, there are several issues that need to be overcome before lithium-sulfur batteries can be successfully commercialized: (1) The polysulfide shuttle effect. Long-chain lithium polysulfides (Li2S4 to Li2S8) dissolved in the electrolyte can diffuse to the lithium negative electrode for chemical reduction (instead of electrochemical reduction), forming lower-order polysulfides, and then diffusing back to the sulfur positive electrode to re-oxidize. This polysulfide shuttle effect is essentially an internal short circuit, leading to self-discharge and low Coulombic efficiency during cycling; (2) Non-uniform solid electrolyte interface (SEI). Lithium metal is highly reactive and reacts with the electrolyte to form SEI on the surface. It has ionic conductivity but is electronically insulating. However, in most cases, the SEI is non-uniform and cannot adequately passivate the lithium metal surface, resulting in continuous and unwanted side reactions with the electrolyte. This consumes both lithium metal and electrolyte, leading to poor reversibility and low Coulombic efficiency during repeated plating and stripping processes; (3) Lithium metal dendrite growth. The growth of lithium dendrites causes the SEI to continuously break and reform, further consuming lithium metal and electrolyte. This causes the battery to eventually fail due to electrolyte depletion and high impedance through the thick SEI.

[0005] To circumvent the above challenges, researchers have made great efforts to improve the electrochemical performance of LSBs, including constructing high-quality sulfur hosts, developing functional separators or interlayers, and optimizing electrolyte additives. Among the reported strategies, the design of functional separators is feasible and can effectively inhibit the polysulfide shuttle to a certain extent, promoting the rapid conversion of lithium polysulfides to improve the overall performance of LSBs. Currently, the modified materials can be divided into three categories: (1) Carbon-based modifiers for physically constraining soluble polysulfides, such as carbon nanotubes, graphene, and mesoporous carbon; (2) Polar materials with strong affinity for chemical adsorption of polysulfides, such as heteroatom-containing polymers, metal oxides, and metal-organic frameworks; (3) Transition metal-based materials for chemically adsorbing and catalytically converting polysulfides, such as TiO2-Mxene, porous metal foams, metal-embedded carbon nanosheets, and metal-containing polymers. However, it should be noted that using a densifying agent layer can effectively inhibit the polysulfide shuttle, but it will cover the pore structure of the separator to a certain extent, thus generating unnecessary impedance to the conduction of lithium ions. In addition, when polysulfides are adsorbed on the modified layer and converted into insoluble sulfur, the slow lithium-ion diffusion kinetics may exacerbate their reutilization, resulting in the loss of active substances and further clogging the pores in the separator. Therefore, it is crucial to develop a functional layer on the separator to simultaneously achieve rapid lithium-ion conduction and polysulfide blocking.

[0006] Polyoxometalates are a class of poly-metal-oxygen cluster compounds, which have the characteristics of diverse shapes, high negative charges, strong redox capabilities, and easy grafting. With their good proton transfer ability and catalytic activity, they have been widely used in the field of electrochemistry. Density functional theory (DFT) calculations show that the redox potential of POMs is adjustable, which enables the selection and design of suitable POMs for specific battery systems. Although researchers have connected POMs with carriers such as polymers, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), or graphene through in-situ synthesis, impregnation synthesis, and mechanical grinding synthesis, some remarkable achievements have been made in the fields of energy storage / conversion and electrocatalysis.

[0007] Defects and deficiencies of the prior art:

[0008] There are still some problems to be solved urgently at present, such as the single function of the carrier, difficult functionalization, easy aggregation, difficult to form a stable composite material with POMs, and defects such as easy loss and short service life during application. When polyoxometalate materials are applied in batteries, inevitable aggregation will bury the effective active sites of polyoxometalates, thus limiting their adsorption or catalytic function for LiPSs. Although POMs can catalyze the reduction of elemental sulfur and the oxidation of Li2S, the reduced POMs substances after lithiation are easily dissolved in the electrolyte during the charge-discharge process, inevitably leading to the loss of POMs catalysts on the positive electrode side. This makes it difficult for such POMs materials to meet the requirements of an increasingly energy-demanding society. Summary of the Invention

[0009] Aiming at the problems existing in the prior art, this application proposes a preparation method of a polyoxometalate-based covalent organic framework modified separator, a modified separator, and a lithium-sulfur battery.

[0010] The preparation method of the polyoxometalate-based covalent organic framework modified separator includes the following steps:

[0011] S1. Synthesize TFPOT-POM

[0012] Place 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and polyoxometalate (POM) in a Pyrex tube to obtain a mixed monomer, then add N,N-dimethylformamide solution, ultrasonically vibrate, then add a catalyst and ultrasonically vibrate, then repeat the freeze-pump process three times, seal with butane gas, heat to obtain a solid product, and then wash and vacuum dry to obtain TFPOT-POM;

[0013] S2. Prepare the modified separator

[0014] Mix TFPOT-POM, PVDF and Super-P, and ball mill for 1 - 1.5 h to obtain a mixture; add an N-methylpyrrolidone solution to the mixture and ball mill for 1 - 2 h to obtain a slurry; coat the slurry on the surface of PP, dry it under vacuum, and cut the obtained separator into circular pieces with a diameter of 16 mm to obtain a modified separator.

[0015] In step S1, the mass ratio of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and polyoxometalate (POM) is (2 - 6):25;

[0016] The molecular formula of the polyoxometalate (POM) described in step S1 is ( n (Bu4N)3[CoMo6O 18 ((OCH2)3CNH2)2];

[0017] In step S1, the dosage relationship between 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and N,N-dimethylformamide is: (1 - 8) mg / mL;

[0018] The time of ultrasonic oscillation described in step S1 is 3 - 5 min;

[0019] The catalyst described in step S1 is trifluoroacetic acid, and the dosage relationship between trifluoroacetic acid and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) is 0.001 - 0.0025 mL / mg; the mass fraction of the trifluoroacetic acid is not less than 99%;

[0020] The heating temperature described in step S1 is 100 - 140 °C, and the heating time is 100 - 140 h;

[0021] The specific process of washing described in step S1 is: alternately add acetone and tetrahydrofuran solution and centrifuge and wash 3 - 5 times;

[0022] The vacuum drying temperature described in step S1 is 80 - 120 °C, and the vacuum drying time is 10 - 12 h;

[0023] In step S2, the weight ratio of TFPOT-POM, PVDF and Super-P is 7:1:2;

[0024] The vacuum drying temperature described in step S2 is 60 - 80 °C, and the vacuum drying time is 12 - 16 h.

[0025] A polyoxometalate-based covalent organic framework modified separator obtained by the above preparation method.

[0026] A lithium-sulfur battery comprising a polyoxometalate-based covalent organic framework modified separator.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. The polyoxometalate-based covalent organic framework TFPOT-POM is synthesized by a solvothermal method. By improving the catalytic ability of the covalent organic framework, the prepared TFPOT-POM exhibits good stability and high catalytic activity. As a separator modification material for Li-S batteries, it can enhance the stability of the battery.

[0029] 2. TFPOT-POM has an ordered porous structure, which can improve the stability of polyoxometalate in the electrolyte, thereby improving the charge and discharge efficiency of the battery; in addition, due to the presence of transition metals in the structure, the battery assembled with the TFPOT-POM modified intermediate layer has a higher ionic conductivity (1.99 mS·cm -1 ). The modified intermediate layer based on TFPOT-POM can not only inhibit the shuttle of polysulfides, but also accelerate the battery reaction process by catalytically oxidizing polysulfides through transition metals; the TFPOT-POM battery also exhibits excellent electrochemical performance, with an initial specific capacity of 844 mAh·g at 1 C -1 , and a specific capacity of 643 mAh·g after 500 cycles -1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic diagram of the synthesis process of the polymer TFPOT-POM of the present invention;

[0031] Figure 2 is the XRD pattern of the polymer obtained in Example 1 of the present invention;

[0032] Figure 3 is the XRD pattern of the polymer obtained in Comparative Example 1;

[0033] Figure 4 is the XRD pattern of the polymer obtained in Comparative Example 2;

[0034] Figure 5 is the XRD pattern of the polymer obtained in Comparative Example 3;

[0035] Figure 6 is the XRD pattern of the polymer obtained in Comparative Example 4;

[0036] Figure 7 is the nitrogen adsorption-desorption analysis and pore size distribution diagram of the polymer obtained in Example 1 of the present invention, wherein, Figure 7 (a) is the N2 adsorption-desorption isotherm diagram of TFPOT-POM, Figure 7 (b) is the pore size distribution diagram of TFPOT-POM;

[0037] Figure 8 This is the XPS diagram of the polymer obtained in Example 1 of the present invention. Among them, Figure 8 (a) is the C 1s spectrum of TFPOT-POM; Figure 8 (b) is the N 1s spectrum of TFPOT-POM; Figure 8 (c) is the O 1s spectrum of TFPOT-POM; Figure 8 (d) is the Co 2p spectrum of TFPOT-POM before and after adsorbing Li2S6; Figure 8 (e) is the Mo 3d spectrum of TFPOT-POM before and after adsorbing Li2S6; Figure 8 (f) is the S 2p spectrum of TFPOT-POM after adsorbing Li2S6;

[0038] Figure 9 This is the high-resolution scanning electron microscopy diagram of the polymer obtained in Example 1 of the present invention. Among them, Figure 9 (a) is the scanning electron microscopy diagram of TFPOT-POM / PP; Figure 9 (b) is the scanning electron microscopy diagram of the polymer TFPOT-POM / PP after cycling; Figure 9 (c) is the cross-sectional scanning diagram of the TFPOT-POM / PP separator;

[0039] Figure 10 This is the catalytic performance diagram of the Li-S battery prepared by the present invention. Among them, Figure 10 (a) is the CV diagram of the TFPOT-POM and PP batteries; 10 (b) is the EIS diagram of the TFPOT-POM battery; Figure 10 (c) is the conductivity diagram of the TFPOT-POM and PP batteries; Figure 10 (d) is the ion transference number diagram of the TFPOT-POM battery; Figure 10 (e) is the deposition experiment diagram of the TFPOT-POM modified battery; Figure 10 (f) is the symmetric battery CV diagram of the TFPOT-POM / PP and PP modified batteries;

[0040] Figure 11 This is the performance diagram of the Li-S battery prepared by the present invention. Among them Figure 11 (a) is the three-cycle CV diagram of the modified separator prepared with TFPOT-POM obtained in Example 1; 11 (b) is the EIS diagram of the TFPOT-POM and PP batteries; 11 (c) is the rate performance diagram of the TFPOT-POM modified battery; 11 (d) is the charge-discharge curve diagram of the TFPOT-POM modified battery; 11 (e) is the 500-cycle long-term cycling performance diagram of the TFPOT-POM and PP modified batteries at 1 C. Detailed implementation manners

[0041] The present invention will be further described below in conjunction with embodiments. The instruments and equipment used in the embodiments of the present invention and their models are: Chenhua CHI660E electrochemical workstation and CT2001A Blue Electric battery test system.

[0042] Measure the cyclic voltammogram of the Li-S battery in the voltage range of 1.7 - 2.8 V at 0.1 mV·s -1 The electrochemical impedance, the cycle stability at 1 C and the rate performance of the Li-S battery at different current densities were measured on the CT2001A Blue Electric battery test system.

[0043] The assembly process of the Li-S battery of the present invention includes the following steps:

[0044] 1. Preparation of C / S composite

[0045] The C / S composite was prepared using a conventional melt diffusion method; first, Super-P powder and sulfur powder with a mass ratio of 25:75 were mixed and thoroughly ground in a mortar. Then, the mixture was placed in a reaction kettle lined with polytetrafluoroethylene and heated at 155 °C for 24 h under an argon-protected atmosphere. Finally, the product was collected after cooling to room temperature to obtain the C / S composite;

[0046] 2. Preparation of conventional positive electrode

[0047] The C / S composite, carbon black, and PVDF with a mass ratio of 8:1:1 were mixed and ground evenly, and then 1-methyl-2-pyrrolidone (NMP) was added and stirred thoroughly to form a homogeneous slurry. The slurry was evenly coated on the aluminum foil using a coater and then dried at 60 °C for 24 h. After drying, the aluminum foil was cut into circular electrodes using a slicer to obtain the required electrodes;

[0048] 3. Assembly of the battery

[0049] Before assembly, all components related to the battery were prepared: a CR2023 type button battery was selected as the battery model; a commercial metallic lithium sheet with a diameter of 16 mm was used as the electrode; the electrolyte was composed of a mixed solution of 1 wt% LiNO3 and 1 M LiTFSI in DOL / DME, where the volume fraction of DOL and DME was 1:1; the modified film prepared in the embodiment was cut into a diaphragm disc with a diameter of 16 mm.

[0050] The assembly of the button battery was carried out in an argon glove box to ensure that the moisture and oxygen content in the glove box were both less than 1 ppm; the assembly steps were as follows: first, the prepared sulfur positive electrode material was placed in the positive electrode case, and then an appropriate amount of electrolyte was dropped on the surface of the sulfur positive electrode, and the ratio of it to sulfur was 24 μL·mg -1; Next, add the cut diaphragm, and drop 40 μL of electrolyte solution on it again. Then, put in a lithium metal sheet. In addition, add a stainless-steel gasket, a shrapnel, and a negative electrode case. Finally, use a button battery encapsulation machine for encapsulation. After encapsulation, let it stand for 24 h for subsequent testing.

[0051] The preparation process of the polyoxometalate (POM) of the present invention was synthesized according to the literature Angew. Chem. Int. Ed. 2023, 62, 32, e202305239. The specific preparation process is as follows:

[0052] Dissolve 20.7 mmol of sodium molybdate dihydrate in 12 mL of water, add 31.0 mmol of an aqueous solution of 6.0 M HCl, stir vigorously at room temperature for 1 - 2 minutes, then add 10.4 mmol of a 10 mL aqueous solution of tetrabutylammonium bromide, stir for 10 min, filter to collect the precipitate, and wash it successively with 20 mL of water, 20 mL of ethanol, 20 mL of acetone, and 20 mL of ether. Dissolve 4.78 g of the crude product in 35 mL of acetonitrile and store it at -10 °C for 24 hours to obtain (nBu4N)4[Mo8O 26 , filter to obtain transparent, colorless, block-shaped crystals, and dry for 12 hours. The crystals lose transparency during drying; put 1.31 mmol of (nBu4N)4[Mo8O 26 , 3.47 mmol of (HOCH2)3CNH2, and 1.77 mmol of Co(OAc)2·4H2O in a two-necked flask, add 75 mL of acetonitrile, stir and heat at 85 °C for 48 h, then add 1.5 mL of 30% hydrogen peroxide dropwise to the mixture, and continue the reaction overnight. After cooling, remove the white precipitate by centrifugation, and then obtain the green crystal polyoxometalate (POM) by bottle-to-bottle diffusion of ether from the supernatant.

[0053] Example 1

[0054] Synthesize TFPOT-POM

[0055] Put 8.8 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT), 55 mg of (nBu4N)3[CoMo6O 18((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 2 mL of N,N-dimethylformamide solution was added to the above Pyrex tube, and ultrasonic oscillation was carried out for 5 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and ultrasonic oscillation was continued for 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. It was heated at 100 °C for 140 h to obtain a light green powdery solid product. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately three times until the color of the washing solution was clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h to finally obtain a light green solid powder TFPOT-POM with a yield of 70.5%. The schematic diagram of the synthesis process of TFPOT-POM is as shown in Figure 1 shown.

[0056] Example 2

[0057] Synthesis of TFPOT-POM

[0058] 4.6 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT), 55 mg of (nBu4N)3[CoMo6O 18 ((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 0.5 mL of N,N-dimethylformamide solution was added to the above Pyrex tube, and ultrasonic oscillation was carried out for 4 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and ultrasonic oscillation was continued for 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. It was heated at 140 °C for 100 h to obtain a light green powdery solid product. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately five times until the color of the washing solution was clear. The washed solid product was placed in a vacuum and dried at 80 °C for 10 h to finally obtain a light green solid powder TFPOT-POM with a yield of 65.3%.

[0059] Example 3

[0060] Synthesis of TFPOT-POM

[0061] 6.7 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT), 55 mg of (nBu4N)3[CoMo6O 18((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 1 mL of N,N-dimethylformamide solution was added to the above Pyrex tube, and ultrasonic oscillation was carried out for 3 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and ultrasonic oscillation was continued for 3 min. Then, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. After heating at 140 °C for 100 h, a light green powdery solid product was obtained. Next, acetone and tetrahydrofuran solutions were added successively and centrifugally washed alternately three times until the color of the washing solution became clear. The washed solid product was placed in a vacuum and dried at 100 °C for 10 h to finally obtain a light green solid powder TFPOT-POM with a yield of 62.2%.

[0062] Example 4

[0063] Preparation of TFPOT-POM

[0064] 10 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and 55 mg of (nBu4N)3[CoMo6O 18 ((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 4 mL of N,N-dimethylformamide solution was added to the above Pyrex tube, and ultrasonic oscillation was carried out for 3 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and ultrasonic oscillation was continued for 3 min. Then, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. After heating at 100 °C for 120 h, a light green powdery solid product was obtained. Next, acetone and tetrahydrofuran solutions were added successively and centrifugally washed alternately three times until the color of the washing solution became clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h to finally obtain a light green solid powder TFPOT-POM with a yield of 68.2%.

[0065] The X-ray diffraction pattern (XRD) of the TFPOT-POM powder obtained in Example 1 is as Figure 2 shown. The crystal structure of TFPOT-POM was analyzed, and the main peak of the diffraction peak was located at 5.7° corresponding to the diffraction of the (100) crystal plane. The diffraction intensity of the main peak was relatively high, indicating that the TFPOT-POM material has high crystallinity.

[0066] Comparative Example 1

[0067] 8.8 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and 55 mg of (nBu4N)3[CoMo6O 18((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 2 mL of o-dichlorobenzene solution was added to the above Pyrex tube, and it was ultrasonically shaken for 5 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and it was continuously ultrasonically shaken for 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times, flame-sealed with butane gas, and heated to obtain a light green powdery solid product. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately three times until the color of the washing liquid became clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h to finally obtain a light green solid powder TFPOT-POM with a yield of 59.4%. The obtained XRD diffraction peaks are as Figure 3 shown.

[0068] Comparative Example 2

[0069] 8.8 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT), 55 mg of (nBu4N)3[CoMo6O 18 ((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 2 mL of mesitylene solution was added to the above Pyrex tube, and it was ultrasonically shaken for 5 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and it was continuously ultrasonically shaken for 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times, flame-sealed with butane gas, and heated to obtain a light green powdery solid product. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately three times until the color of the washing liquid became clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h to finally obtain a light green solid powder TFPOT-POM with a yield of 62.3%. The obtained XRD diffraction peaks are as Figure 4 shown.

[0070] Comparative Example 3

[0071] 8.8 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT), 55 mg of (nBu4N)3[CoMo6O 18((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 2 mL of 1,4-dioxane solution was added to the above Pyrex tube, and it was ultrasonically shaken for 5 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and it was ultrasonically shaken for another 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. After heating, a light green powdery solid product was obtained. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately three times until the color of the washing solution was clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h, and finally a light green solid powder TFPOT-POM was obtained with a yield of 58.9%. The obtained XRD diffraction peaks are as Figure 5 shown.

[0072] Comparative Example 4

[0073] 8.8 mg of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TFPOT) and 55 mg of (nBu4N)3[CoMo6O 18 ((OCH2)3CNH2)2] (POM) was added to a Pyrex tube and mixed to obtain a mixed monomer. Then, 2 mL of acetonitrile solution was added to the above Pyrex tube, and it was ultrasonically shaken for 5 min. 0.01 mL of trifluoroacetic acid was added as a reaction catalyst to the Pyrex tube, and it was ultrasonically shaken for another 3 min. After that, the Pyrex tube was subjected to the freeze-pump process three times and flame-sealed with butane gas. After heating, a light green powdery solid product was obtained. Next, acetone and tetrahydrofuran solutions were added in turn and centrifugally washed alternately three times until the color of the washing solution was clear. The washed solid product was placed in a vacuum and dried at 100 °C for 12 h, and finally a light green solid powder TFPOT-POM was obtained with a yield of 50.8%. The obtained XRD diffraction peaks are as Figure 6 shown.

[0074] It can be seen from the above Example 1 and Comparative Examples 1-4 that different reaction solvents have a significant impact on the crystallinity and yield of the product.

[0075] The nitrogen isothermal adsorption measurement of the TFPOT-POM prepared in Example 1 was carried out at 77 K to evaluate the permanent porosity and specific surface area of TFPOT-POM. In Figure 7 a, the nitrogen desorption isotherm shows an H3-type hysteresis loop, which indicates a type-IV adsorption isotherm, suggesting the presence of a microporous structure, which is a characteristic of a layered structure. It was calculated that the (BET) specific surface area of TFPOT-POM is 40.44 m² / g. In addition, the pore size distribution (PSD) of TFPOT-POM was calculated using non-local density functional theory (NLDFT) (see Figure 7b) It shows that the average pore size of TFPOT-POM is 0.81 nm.

[0076] Figure 8 is the X-ray photoelectron spectrum of the TFPOT-POM material prepared in Example 1, which confirms the valence states and compositions of the elements in the TFPOT-POM structure. In the XPS spectrum of TFPOT-POM, the C1s spectrum ( Figure 4 ) shows three peaks at 284.8 eV, 286.1 eV, and 287.1 eV, corresponding to C-C (including C=C and C-C), carbon-heteroatom bonds (C-O and C-N), and C=N bonds, respectively. The N1s spectrum ( Figure 8 b) shows characteristic peaks at 398 eV and 402 eV, corresponding to primary amines and quaternary amines, respectively. A peak is shown at 400.2 eV, corresponding to the C=N bond, indicating the formation of an imine bond. A peak located at 399.7 eV can be clearly observed in TFPOT-POM, which corresponds to the nitrogen atom in the triazine ring. To study the adsorption of polysulfides by TFPOT-POM, the XPS spectrum of TFPOT-POM after adsorbing Li2S6 was tested. The analysis of the high-resolution Co 2p XPS spectrum ( Figure 8 c) shows that a peak position shift occurred after the adsorption of Li2S6, indicating an interaction between TFPOT-POM and LiPSs. Similar peak position shifts can also be observed in the Mo3d and O 1s XPS spectra. In TFPOT-POM, the peak position of Co 2p after adsorption shifted by 0.6 eV. The peak position of Mo 3d after adsorption shifted by 0.4 eV ( Figure 8 d). The presence of oxygen atoms in the TFPOT-POM monomer helps to further adsorb polysulfides. In the O 1s spectrum, the C-O-C bond shifted towards a lower binding energy direction, and a Li-O bond appeared ( Figure 8 e). In addition, the S 2p spectra of TFPOT-POM after adsorbing Li2S6 were also compared. Obviously, when using TFPOT-POM as a catalyst, an additional increase appeared in the peak of elemental sulfur (S0). At the same time, the S in TFPOT-POM T -1 (162.1 eV) and S0 B (162.8 eV) ( Figure 8 f) peaks shifted towards a higher binding energy direction. These results clearly show that TFPOT-POM exhibits strong electrostatic adsorption, chemical catalysis, and conversion effects on LiPSs.

[0077] Example 5

[0078] Preparation of modified separator

[0079] (1)Dry grinding: Mix the TFPOT-POM prepared in Example 1 with PVDF and Super-P in a weight ratio of 7:1:2, and ball mill for 1 h to obtain a mixture;

[0080] (2)Wet grinding: Add NMP solution to the mixture and ball mill for 1 h to obtain a slurry;

[0081] (3)Coat the slurry evenly on the surface of PP with a 40-mm four-sided sample preparation device, and then place it in a vacuum oven at 60 °C to dry for 12 h to obtain a separator;

[0082] (4)Cut the separator into circular pieces with a diameter of 16 mm, which is the TFPOT-POM modified separator.

[0083] Example 6

[0084] Preparation of modified separator

[0085] (1)Dry grinding: Mix the TFPOT-POM prepared in Example 1 with PVDF and Super-P in a weight ratio of 7:1:2, and ball mill for 1.5 h to obtain a mixture;

[0086] (2)Wet grinding: Add NMP solution to the mixture and ball mill for 2 h to obtain a slurry;

[0087] (3)Coat the slurry evenly on the surface of PP with a 40-mm four-sided sample preparation device, and then place it in a vacuum oven at 80 °C to dry for 16 h to obtain a separator;

[0088] (4)Cut the separator into circular pieces with a diameter of 16 mm, which is the TFPOT-POM modified separator.

[0089] Figure 9 Figure 5a and 5b are the scanning electron microscope images of TFPOT-POM / PP prepared in Example 5. The TFPOT-POM / PP composite catalyst was prepared on the PP sub-catalyst by the coating method. To verify the cycle stability of the modified separator, the separator after long cycling was subjected to scanning electron microscope testing. It can be seen from Figure 9 Figures 5a and 9b that there is no obvious change in the TFPOT-POM / PP separator before and after the comparison cycle. Moreover, the voids of the original PP separator have been completely covered by these two composite separators. Through the cross-section Figure 9 Figure 5c, it can be seen that the thickness of the composite coating is 2.81 μm.

[0090] Assembly of Li-S battery

[0091] 1. Preparation of C / S composite material

[0092] To prepare the C / S composite material using the traditional melting diffusion method, first, Super-P powder and sulfur powder with a mass ratio of 2.5:7.5 are mixed and thoroughly ground in a mortar. Then, the mixture is placed in a reaction kettle lined with polytetrafluoroethylene and heated at 155 °C for 24 h under an argon-protected atmosphere. Finally, after cooling to room temperature, the product is collected to obtain the C / S composite material.

[0093] 2. Preparation of the conventional positive electrode

[0094] 80% by mass of the above-prepared C / S composite material, 10% carbon black, and 10% PVDF are respectively mixed and ground evenly. Subsequently, 1-methyl-2-pyrrolidone (NMP) is added and stirred thoroughly to ensure the formation of a homogeneous slurry. The slurry is evenly coated on the aluminum foil using a coater and then dried at 60 °C for 24 h. After drying, the aluminum foil is cut into circular electrodes using a slicing machine to obtain the required electrodes.

[0095] 3. Assembly of the battery

[0096] Before assembly, all components related to the battery are prepared. The CR2023 type button battery is selected as the battery model. The electrode uses a commercial lithium metal sheet with a diameter of 16 mm. 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% LiNO3 and 1 M LiTFSI as the solutes; the HSO3 prepared in Example 6 - -ETB-COF / PP modified separator, cut into a diameter of 16 mm as the separator disc.

[0097] 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, the prepared sulfur positive electrode material is placed in the positive electrode case, then the electrolyte is dropped on the surface of the sulfur positive electrode, and its ratio to sulfur is 24 μL·mg -1 , then the cut separator disc is added, and 40 μL of the electrolyte is dropped on it again. Subsequently, the lithium metal sheet is placed, and in addition, a stainless steel gasket, a spring piece, and a negative electrode case are added. Finally, it is encapsulated using a button battery encapsulation machine, and after encapsulation, it is left standing for 24 h for subsequent testing.

[0098] Figure 10 For the analysis of the catalytic performance of the composite battery, the electrochemical catalytic performance of TFPOT-POM as a separator material for lithium-sulfur (Li-S) batteries was evaluated. First, within a voltage window of 1.7 - 2.8 V, typical cyclic voltammetry (CV) measurements were carried out on the material to analyze their electrochemical behavior at a scanning rate of 0.1 mV·s -1 . At Figure 10The CV curves of POT-POM in a show the characteristic redox peaks of the Li-S battery. During the scanning process, the two obvious reduction peaks at 2.25 V and 2.01 V correspond to the conversion of S8 to soluble long-chain polysulfides (LiPSs), which are then further reduced to the final products (Li2S2 and Li2S). During the scanning process, the two oxidation peaks at 2.35 V and 2.43 V represent the conversion of Li2S to Li2Sn (2 ≤ n ≤ 8), and finally back to S8. This is attributed to the reduced polarization and the accelerated reaction kinetics.

[0099] From Figure 10 the electrochemical impedance spectroscopy (EIS) data shown in b, the bulk resistance (Rb) values of TFPOT-POM and PP separators are 3.35 Ω and 4.71 Ω, respectively, and the corresponding ionic conductivities ( Figure 10 c) are 1.32 and 1.99 mS·cm -1 . The enhanced ionic conductivity of TFPOT-POM may be attributed to the catalytic conversion promoted by the aldehyde monomer and the oxygen atoms in the polyacid, which is beneficial to Li + transport. The Li + transference number reflects the interaction with the lithium salt anion and the polysulfide anion, and is calculated based on the chronoamperometry and EIS data ( Figure 10 d). The Li + transference numbers of TFPOT-POM and PP are 0.987 and 0.648, respectively. These results indicate that the active oxygen atoms in TFPOT-POM anchor the dissociation of the lithium salt and the anions in the polysulfide, thus accelerating Li + transport. Since the nucleation and dissolution of Li2S usually have the highest energy barrier in the charge-discharge reactions of lithium-sulfur (Li-S) batteries, they are the rate-limiting steps in the redox process. To further clarify the role of TFPOT-POM in the solid-liquid conversion of polysulfides, potentiostatic charge-discharge experiments were carried out to study the electrochemical conversion kinetics of Li2S ( Figure 10 e). Specifically, the potentiostatic discharge curves of Li2S deposition show that the TFPOT-POM separator (542 s, 84.5 mAh·g -1 ) has a much earlier nucleation and growth response time (638 s) and a higher deposition capacity (51.46 mAh·g -1 ), indicating that the presence of oxygen atoms improves the conversion efficiency from polysulfides to Li2S and accelerates the deposition kinetics of Li2S. In addition, symmetric cells using TFPOT-POM and PP separators with an electrolyte containing Li2S6 were also tested ( Figure 10f). Due to poor conductivity and weak interaction with LiPSs, the CV curve of the PP separator shows a low current density and large polarization, resulting in slow conversion kinetics of LiPSs. In contrast, TFPOT-POM exhibits a higher current density and lower polarization, showing excellent catalytic activity for polysulfide conversion, which is attributed to its strong adsorption ability for polysulfides and the high catalytic activity of polyacid.

[0100] To more deeply investigate the effect of the COF-modified separator on the performance of lithium-sulfur batteries, CR2025 coin cells were assembled and typical cyclic voltammetry (CV) measurements were carried out at a scanning rate of 0.1 mV·s -1 in the voltage window of 1.7 - 2.8 V in the first three cycles (as Figure 11 a) to analyze the electrochemical behavior. The CV curves of the second and third cycles overlap, indicating good redox reversibility of the TFPOT-POM separator during charge and discharge processes. As Figure 11 shown in b, the charge transfer resistance (Rct) of the TFPOT-POM modified separator battery is significantly lower than that of the PP separator, indicating that the charge transfer performance and redox kinetics are improved. This modification also brings unique oxidation behavior, enhancing the electrochemical reaction kinetics and reducing the energy barrier. As Figure 11 shown in c, the TFPOT-POM modified separator exhibits excellent rate performance. The discharge capacities at 0.1, 0.2, 0.5, 1 and 2 C are 1033, 933, 823, 727 and 618 mAh·g -1 . When the current density is restored to 0.1 C, the capacity can be restored to 912 mAh·g -1 . The charge-discharge curves in Figure 11 d show that even at a high rate of 2 C, the TFPOT-POM battery still exhibits two typical discharge platforms.

[0101] Long-term cycling tests were carried out on lithium-ion batteries with TFPOT-POM and PP separators, and the results are as Figure 11 shown in e. At a rate of 1 C, the TFPOT-POM battery underwent 500 cycles. The initial capacity was 844 mAh·g -1 , and after 500 cycles, the capacity remained at 643 mAh·g -1 , and the corresponding capacity decay rate was 0.047%. In contrast, the capacity decay rate of the PP separator after 500 cycles was 0.076%.

[0102] 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 utilize the present invention. Nor is any limitation imposed on the present invention by the sequence of the embodiments and the specific operations. Any replacement 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 polyacid-based covalent organic skeleton modified diaphragm, characterized in that: The following steps are involved: S1. Synthesis of TFPOT-POM 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine, namely TFPOT, and polyoxometalate, namely POM, are placed in a Pyrex tube and mixed to obtain a mixed monomer, and then N,N-dimethylformamide solution is added, ultrasonically shaken, and then a catalyst is added and ultrasonically shaken, and then a freezing vacuum process is repeated three times, flame-sealed with butane gas, heated to obtain a solid product, and then washed and vacuum-dried to obtain TFPOT-POM; The molecular formula of the polyoxometalate, namely POM, is ( n Bu4N)3[CoMo6O 18 ((OCH2)3CNH2)2]; S2. Preparation of modified diaphragm TFPOT-POM, PVDF and Super-P were mixed and ball-milled for 1-1.5 hours to obtain a mixture; N-methylpyrrolidone solution was added to the mixture and ball-milled for 1-2 hours to obtain a slurry; the slurry was coated on the surface of PP, vacuum-dried, and the obtained diaphragm was cut into discs with a diameter of 16 mm to obtain a modified diaphragm.

2. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The mass ratio of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine, ie, TFPOT, and polyoxometalate, ie, POM, in step S1 is (2-6):

25.

3. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The dosage of 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine, namely TFPOT, and N,N-dimethylformamide in step S1 is (1-8) mg / mL, and the ultrasonic oscillation time is 3-5 min.

4. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The catalyst in step S1 is trifluoroacetic acid, and the dosage of trifluoroacetic acid and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine, namely TFPOT, is 0.001-0.0025 mL / mg; the mass fraction of the trifluoroacetic acid is not less than 99%.

5. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The heating temperature in step S1 is 100-140° C., and the heating time is 100-140 hours.

6. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The specific process of washing in step S1 is: adding acetone and tetrahydrofuran solution in sequence and washing by alternating centrifugation for 3-5 times; the temperature of vacuum drying is 80-120° C., and the time of vacuum drying is 10-12 hours.

7. The method for preparing a polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The weight ratio of TFPOT-POM, PVDF and Super-P in step S2 is 7:1:

2.

8. The method for preparing the polyacid-based covalent organic skeleton modified diaphragm according to claim 1, characterized in that: The vacuum drying temperature in step S2 is 60-80° C., and the vacuum drying time is 12-16 hours.

9. A polyacid-based covalent organic skeleton modified diaphragm obtained by the preparation method according to any one of claims 1 to 8.

10. A lithium-sulfur battery comprising the polyacid-based covalent organic skeleton modified diaphragm according to claim 9.

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