Covalent triazine framework polymer nanosheets for lithium-sulfur battery cathode materials

Abstract

The present invention belongs to the technical field of lithium-sulfur batteries, and discloses a covalent triazine framework polymer nanosheet for a positive electrode material of a lithium-sulfur battery, wherein a hexaazatriphenylhexanecarbonitrile monomer is polymerized on the surface of sodium chloride by high-temperature triazine, and then sodium chloride crystals are removed to obtain a covalent triazine framework polymer nanosheet product, and the positive electrode material of a lithium-sulfur battery is obtained after sulfur is melted. The preparation method of the present invention is simple, low in cost, has a high product yield and a uniform structure, and the obtained material itself has a porous structure and rich active sites, and can be used in lithium-sulfur batteries to promote the rapid conversion of lithium polysulfide, effectively inhibit the shuttle effect, and improve the rate performance and cycle performance of lithium-sulfur batteries.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium-sulfur batteries and relates to a covalent triazine framework polymer nanosheet used for positive electrode materials of lithium-sulfur batteries. Background Art

[0002] With the continuous development and progress of today's society, the development and utilization of clean energy has become a key direction in related fields. After the exploration of a large number of scientific researchers, many green and environmentally friendly new energy sources have been found, such as solar energy, wind energy and hydrogen energy. However, these clean energy sources have certain requirements for the environment in practical applications, and will also cause an imbalance between power supply and demand, which greatly hinders their practical application. Therefore, it is necessary to find a new type of energy storage device to effectively convert, store and utilize it. In recent years, lithium-sulfur batteries (Li-S) have been considered as one of the most promising energy storage systems due to their high theoretical capacity (1675 mAh / g), high energy density (2600 mAh / g) and environmental friendliness. In addition, elemental sulfur has the advantages of low cost, no pollution, and abundant reserves on the earth.

[0003] Despite these advantages, the practical application of lithium-sulfur batteries is still restricted by many problems, such as the shuttle effect of the intermediate product lithium polysulfide and the poor conductivity of sulfur. In order to solve the above problems, various materials with large specific surface area and good conductivity are used as sulfur carriers, such as graphene, carbon spheres, carbon nanotubes (CNTs), metal organic frameworks, and covalent organic frameworks. However, the poor adsorption of lithium polysulfide by these materials is not enough to inhibit the sulfur loss of lithium-sulfur batteries.

[0004] In summary, it is necessary to find a simple and efficient preparation method to synthesize materials with special structures and ensure that the material has a large yield and good performance to meet its application as a positive electrode material for lithium-sulfur batteries. Summary of the invention

[0005] In view of the above technical problems, the present invention provides a covalent triazine framework polymer nanosheet as a lithium-sulfur battery positive electrode material, aiming to achieve rapid conversion of lithium polysulfide and efficient inhibition of the shuttle effect, and improve the energy density and charge-discharge cycle performance of the lithium-sulfur battery positive electrode.

[0006] In order to solve the technical problem, the present invention adopts the following technical solution:

[0007] A method for preparing a covalent triazine framework polymer nanosheet for a lithium-sulfur battery positive electrode material, comprising the following steps:

[0008] Step 1, adding cyclohexanone and diaminomaleonitrile to acetic acid, heating and refluxing reaction; washing with hot acetic acid after the reaction is completed to obtain a crude product containing hexaazatriphenylhexacarbonitrile HATCN; ultrasonically dispersing the crude product in a nitric acid solution, continuing to heat and reflux reaction; after the reaction is completed, adding ice water thereto, refrigerating and standing; washing the obtained product with deionized water, vacuum drying, and obtaining a once purified HATCN;

[0009] Step 2, dissolving the once purified HATCN in an acetonitrile solution by ultrasonication, and heating under reflux for reaction; after the reaction, washing with acetonitrile and vacuum drying to obtain a second purified HATCN;

[0010] Step 3, adding the secondary purified HATCN to acetonitrile and stirring with a magnetic stirrer until dissolved, then adding sodium chloride and stirring with a stirring paddle until uniformly dispersed; drying the resulting mixture to fully evaporate the acetonitrile to obtain a solid mixture of HATCN and sodium chloride;

[0011] Step 4, adding trifluoromethanesulfonic acid into a test tube in a liquid nitrogen low temperature environment, and then adding the solid mixture obtained in step 3, evacuating the system and sealing it with a flame; transferring the system to a tube furnace, calcining it under an inert atmosphere, so that HATCN polymerizes triazine on the surface of sodium chloride crystals to obtain CTF-HATCN;

[0012] Step 5: remove the sodium chloride particles, collect the product and freeze-dry it, and then ultrasonically disperse and peel the obtained CTF-HATCN to obtain a covalent triazine framework polymer nanosheet for a lithium-sulfur battery positive electrode material, which is recorded as CTF-HATCN nanosheet.

[0013] Preferably, in step 1, when preparing a crude product containing HATCN, the ratio of cyclohexanone, diaminomaleonitrile and acetic acid is 0.4 g:1.1 g:15-20 mL, the reaction temperature of the heating reflux reaction is 90-110 ° C, and the reaction time is 4-6 h.

[0014] Preferably, in step 1, the concentration of the nitric acid solution is 20-30%, the reaction temperature for continued heating and reflux reaction is 90-110°C, the reaction time is 2-4 h, the volume ratio of the nitric acid solution to ice water is 1:1-3, the refrigerated standing temperature is 0-10°C, and the time is 12-24 h.

[0015] Preferably, the ratio of acetonitrile in step 2 to cyclohexanone in step 1 is 350-450 mL: 0.4 g, the reaction temperature of the heating reflux reaction is 80-90 °C, and the reaction time is 2-3 h.

[0016] Preferably, in step 3, the ratio of the secondary purified HATCN to sodium chloride is 0.1 g:10-20 g, and the drying temperature is 60°C-80°C and the time is 20-24 h.

[0017] Preferably, the ratio of trifluoromethanesulfonic acid in step 4 to the secondary purified HATCN in step 3 is 20-30 µL: 0.1 g, the inert atmosphere is argon or nitrogen, the calcination temperature is 400°C-600°C, and the insulation time is 16-20 h.

[0018] Preferably, in step 5, the method for removing the sodium chloride particles is to dissolve the sodium chloride in ultrapure water and collect the product after vacuum filtration.

[0019] The covalent triazine framework polymer nanosheets prepared by the present invention can be used for positive electrode materials of lithium-sulfur batteries. The application method is: CTF-HATCN nanosheets are mixed with sublimated sulfur, and the mixture is ground evenly, then loaded into a reactor and placed in a nitrogen atmosphere, and heated for reaction to obtain positive electrode active materials of lithium-sulfur batteries.

[0020] Preferably, the ratio of CTF-HATCN nanosheets to sublimated sulfur is 0.3 g: 0.7-1 g, the heating reaction temperature is 150-160 °C, and the reaction time is 8-10 h.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The monomer HATCN used in the present invention has a large molecular weight and a stable structure. The polymerized CTF-HATCN can form an ordered macroporous triazine skeleton with rich pore structure and abundant reactive active sites. At the same time, NaCl is added as a template to form a uniform sheet structure after simple peeling. The uniform sheet stacking can effectively catalyze the conversion of lithium polysulfide, reduce the loss of active substances, and inhibit the shuttle effect of polysulfide, thereby ultimately improving the electrochemical performance of lithium-sulfur batteries.

[0023] 2. The lithium-sulfur battery positive electrode material prepared by the present invention is applied to lithium-sulfur batteries, which show higher discharge specific capacity and better cycle performance. The first discharge specific capacity at a rate of 0.2 C is 953.41 mAh / g, and the discharge specific capacity after 200 cycles is 626.03 mAh / g. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 FTIR images of the hexaazatriphenylhexacarbonitrile (HATCN) monomer obtained in step 2 of Example 1 and the covalent triazine framework polymer nanosheet (CTF-HATCN) obtained in step 5;

[0025] Figure 2 This is a scanning electron microscope image of the CTF-HATCN nanosheet obtained in Example 1;

[0026] Figure 3 This is a transmission electron microscopy image of the CTF-HATCN nanosheet obtained in Example 1;

[0027] Figure 4 This is a scanning electron microscope image of the covalent triazine framework polymer obtained in Example 2;

[0028] Figure 5 Atomic force microscopy image of CTF-HATCN nanosheets obtained in Example 1 ( Figure 5 (a)) and the corresponding thickness data ( Figure 5 (b));

[0029] Figure 6 This is a nitrogen adsorption-desorption curve of the co-CTF-HATCN nanosheets obtained in Example 1;

[0030] Figure 7 This is the pore size distribution diagram of the co-CTF-HATCN nanosheet obtained in Example 1;

[0031] Figure 8 This is a cycle performance diagram of the target product obtained in Example 1 as a positive electrode material for a lithium-sulfur battery;

[0032] Fig. 9 This is the cyclic voltammetry linear curve of the target product obtained in Example 1 as a positive electrode material for a lithium-sulfur battery. DETAILED DESCRIPTION

[0033] The following is a detailed description of the embodiments of the present invention. The following embodiments are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments. The experimental methods used in the following embodiments are conventional methods unless otherwise specified; the reagents and materials used in the following embodiments are commercially available unless otherwise specified; the battery performance tests in the following embodiments are all conducted using the LAND test system.

[0034] The centrifuge used in the following examples is Anke TGL-10B produced by Shanghai Anting Scientific Instrument Factory, the magnetic stirrer is RT-10 multi-point magnetic stirrer produced by Guangzhou Yike Laboratory Technology Co., Ltd., the calcining furnace is OTF-1200X produced by Hefei Kejing Material Technology Co., Ltd., the scanning electron microscope is Zeiss Supra 40 produced in Germany, and the transmission electron microscope is JEOL-F2010 produced in Japan. The drugs used in the following examples were used directly after purchase without any treatment.

[0035] Example 1

[0036] In this embodiment, the covalent triazine framework polymer nanosheets are prepared according to the following steps:

[0037] Step 1. Add 0.4 g of cyclohexanone and 1.1 g of diaminomaleonitrile to 15 mL of acetic acid, heat and reflux at 100 °C for 4 h; after the reaction, wash with 80 °C hot acetic acid for several times to obtain a crude product containing hexaazatriphenylhexacarbonitrile (HATCN); ultrasonically disperse the crude product in 20 mL of 30% nitric acid solution, continue to heat and reflux at 100 °C for 2 h; after the reaction, add 40 mL of ice water, put it in a 0 °C refrigerator and let it stand for 12 h; the obtained product is washed with deionized water and vacuum dried to obtain once purified HATCN.

[0038] Step 2: Dissolve the once purified HATCN in 400 mL of acetonitrile solution by ultrasonication, and heat under reflux at 80°C for 2 hours; after the reaction, wash with acetonitrile and vacuum dry to obtain a second purified high-purity HATCN.

[0039] Step 3: Add 0.1 g of the secondary purified HATCN to 15 mL of acetonitrile and stir magnetically until dissolved, then add 20 g of sodium chloride and stir with a stirring paddle until evenly dispersed; dry the mixture in an oven at 80 °C for 24 h to fully evaporate the acetonitrile to obtain a solid mixture of HATCN and sodium chloride.

[0040] Step 4: Add 20 µL of trifluoromethanesulfonic acid into a test tube in a liquid nitrogen low-temperature environment, then add the solid mixture obtained in step 3, evacuate the system and seal it with a flame; transfer the system to a tube furnace and calcine it at 400 °C for 20 h in an argon atmosphere to polymerize HATCN on the surface of sodium chloride crystals to obtain CTF-HATCN.

[0041] Step 5: Dissolve sodium chloride in ultrapure water and collect the product after vacuum filtration, freeze-dry, and then ultrasonically disperse and peel the obtained CTF-HATCN to obtain CTF-HATCN nanosheets used as positive electrode materials for lithium-sulfur batteries.

[0042] Example 2

[0043] This example prepares a covalent triazine framework polymer according to the following steps:

[0044] Step 1. Add 0.4 g of cyclohexanone and 1.1 g of diaminomaleonitrile to 15 mL of acetic acid, heat and reflux at 100 °C for 4 h; after the reaction, wash with 80 °C hot acetic acid for several times to obtain a crude product containing hexaazatriphenylhexacarbonitrile (HATCN); ultrasonically disperse the crude product in 20 mL of 30% nitric acid solution, continue to heat and reflux at 100 °C for 2 h; after the reaction, add 40 mL of ice water, put it in a 0 °C refrigerator and let it stand for 12 h; the obtained product is washed with deionized water and vacuum dried to obtain once purified HATCN.

[0045] Step 2: Dissolve the once purified HATCN in 400 mL of acetonitrile solution by ultrasonication, and heat under reflux at 80°C for 2 hours; after the reaction, wash with acetonitrile and vacuum dry to obtain a second purified high-purity HATCN.

[0046] Step 3: Add 20 µL of trifluoromethanesulfonic acid into the test tube in a liquid nitrogen low-temperature environment, and then add 0.1 g of HATCN obtained in step 2, evacuate the system and seal it with a flame; transfer the system to a tube furnace and calcine it at 400 °C for 20 h in an argon atmosphere to polymerize the HATCN triazine to obtain CTF-HATCN.

[0047] Step 4: subjecting the obtained CTF-HATCN to ultrasonic dispersion and exfoliation.

[0048] Figure 1 FTIR images of the hexaazatriphenylhexanecarbonitrile (HATCN) monomer and covalent triazine framework polymer nanosheet (CTF-HATCN) obtained in Example 1 indicate that the monomer has completed triazine polymerization to form a covalent triazine framework polymer with a high degree of polymerization.

[0049] Figure 2 and Figure 3 The scanning electron microscope image and transmission electron microscope image of the CTF-HATCN nanosheet obtained in step 5 of Example 1 respectively show that the CTF-HATCN nanosheet has a uniform single-layer sheet structure, and the sheet size is about 1 µm.

[0050] Figure 4 This is a scanning electron microscope image of the sample obtained after ultrasonic stripping in step 4 of Example 2. It can be seen that there is no NaCl as a template and the polymer has an obvious block structure.

[0051] Figure 5 This is an atomic force microscope image of the CTF-HATCN nanosheet obtained in step 5 of Example 1 ( Figure 5 (a) in the figure) and the corresponding thickness data ( Figure 5 (b) ), it can be seen that the thickness of a single layer is about 10 nm.

[0052] Figure 6and Figure 7 The nitrogen adsorption-desorption curve and pore size distribution of the CTF-HATCN nanosheet obtained in step 5 of Example 1 show that the specific surface area of ​​the obtained polymer nanosheet is about 200 cm 3 / g, and the pore size is about 2~3nm.

[0053] The sublimated sulfur and the CTF-HATCN nanosheets obtained in Example 1 were mixed in a mass ratio of 7:3 and ground evenly, then loaded into a reactor, placed in a nitrogen atmosphere, and reacted at 155°C for 8 h to obtain a positive electrode active material for a lithium-sulfur battery. The obtained black active material was ground, mixed with a conductive agent SuperP and a binder PVDF in a mass ratio of 7:2:1, NMP was added, and the mixture was fully stirred into a slurry and evenly coated on one side of the aluminum foil surface, vacuum dried at 55°C for 12 h, and cut into electrode original sheets with a diameter of 12 mm. The sulfur loading was 1 mg / cm 2 about.

[0054] The electrode plates were placed in a glove box filled with argon gas, and button cells were assembled in the order of negative electrode shell, lithium plate, diaphragm, electrolyte, positive electrode plate, gasket, spring plate, and positive electrode shell. The battery was sealed using a tablet press. After the obtained battery was left to stand for 12 hours, a cycle performance test was performed on the LAND test system.

[0055] Figure 8 This is a cycle performance diagram of the target product obtained in Example 1 as a positive electrode material for a lithium-sulfur battery. It can be seen that: when the current density is 0.2 A / g, the first-cycle discharge specific capacity of the material as the positive electrode of a lithium-sulfur battery is 953 mAh / g, the first-cycle coulomb efficiency is 99.63%, and the reversible specific capacity of 517 mAh / g is still maintained after 500 cycles, indicating that the material obtained in this example has good cycle performance as the negative electrode of a lithium-ion battery.

[0056] Fig. 9 This is the cyclic voltammetry linear curve of the target product obtained in Example 1 as a positive electrode material for a lithium-sulfur battery.

[0057] The above are merely exemplary embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a covalent triazine framework polymer nanosheet for a lithium-sulfur battery positive electrode material, characterized in that: The steps include: Step 1, adding cyclohexanone and diaminomaleonitrile to acetic acid, heating and refluxing reaction; washing with hot acetic acid after the reaction is completed to obtain a crude product containing hexaazatriphenylhexacarbonitrile HATCN; ultrasonically dispersing the crude product in a nitric acid solution, continuing to heat and reflux reaction; after the reaction is completed, adding ice water thereto, refrigerating and standing; washing the obtained product with deionized water, vacuum drying, and obtaining a once purified HATCN; Step 2, dissolving the once purified HATCN in an acetonitrile solution by ultrasonication, and heating under reflux for reaction; after the reaction, washing with acetonitrile and vacuum drying to obtain a second purified HATCN; Step 3, adding the secondary purified HATCN to acetonitrile and stirring magnetically until dissolved, then adding sodium chloride and stirring until uniformly dispersed; drying the obtained mixture to fully evaporate the acetonitrile to obtain a solid mixture of HATCN and sodium chloride; Step 4, adding trifluoromethanesulfonic acid to a test tube in a liquid nitrogen low temperature environment, and then adding the solid mixture obtained in step 3, evacuating the system and then flame sealing; transferring the system to a tube furnace, calcining under an inert atmosphere, so that HATCN is triazine polymerized on the surface of sodium chloride crystals to obtain CTF-HATCN; wherein the amount ratio of trifluoromethanesulfonic acid to the HATCN purified twice in step 3 is 20-30 μL: 0.1 g, the inert atmosphere is argon or nitrogen, the calcination temperature is 400°C-600°C, and the insulation time is 16-20 h; Step 5: remove the sodium chloride particles, collect the product and freeze-dry it, and then ultrasonically disperse and peel the obtained CTF-HATCN to obtain a covalent triazine framework polymer nanosheet for a lithium-sulfur battery positive electrode material, which is recorded as CTF-HATCN nanosheet.

2. The preparation method according to claim 1, characterized in that: In step 1, when preparing a crude product containing HATCN, the amount ratio of cyclohexanone, diaminomaleonitrile and acetic acid is 0.4 g:1.1 g:15-20 mL, the reaction temperature of the heating reflux reaction is 90-110 ° C, and the reaction time is 4-6 h.

3. The preparation method according to claim 1, characterized in that: In step 1, the concentration of the nitric acid solution is 20-30%, the reaction temperature of the continued heating reflux reaction is 90-110°C, the reaction time is 2-4 h, the volume ratio of the nitric acid solution to ice water is 1:1-3, the refrigerated standing temperature is 0-10°C, and the time is 12-24 h.

4. The preparation method according to claim 1, characterized in that: The amount ratio of acetonitrile in step 2 to cyclohexanone in step 1 is 350-450 mL: 0.4 g, the reaction temperature of the heating reflux reaction is 80-90 ° C, and the reaction time is 2-3 h.

5. The preparation method according to claim 1, characterized in that: In step 3, the ratio of the secondary purified HATCN to sodium chloride is 0.1 g: 10-20 g, and the drying temperature is 60°C-80°C and the time is 20-24 h.

6. The preparation method according to claim 1, characterized in that: In step 5, the method for removing the sodium chloride particles is to dissolve the sodium chloride with ultrapure water and collect the product after vacuum filtration.

7. A covalent triazine framework polymer nanosheet prepared by the preparation method according to any one of claims 1 to 6.

8. An application of the covalent triazine framework polymer nanosheet according to claim 7 in a positive electrode material for a lithium-sulfur battery, characterized in that: The CTF-HATCN nanosheets are mixed with sublimated sulfur, ground evenly, loaded into a reactor and placed in a nitrogen atmosphere, heated for reaction, and a positive electrode active material for a lithium-sulfur battery is obtained.

9. The use according to claim 8, characterized in that: The dosage ratio of CTF-HATCN nanosheets to sublimated sulfur is 0.3 g: 0.7~1 g, the heating reaction temperature is 150~160 ℃, and the reaction time is 8~10 h.

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