High nitrogen-rich triazine / carbon nanotube composite material, preparation method and application in lithium-sulfur battery

Abstract

The invention discloses a high nitrogen-rich triazine / carbon nanotube composite material, a preparation method and an application in a lithium-sulfur battery. CTF is synthesized through strong acid catalytic polymerization, the CTF is compounded with CNT, a CTF-CNT functional material is prepared, and the surface of a polypropylene diaphragm is coated with the CTF-CNT functional material. According to the preparation method, the defects that a traditional solvothermal method is long in reaction time and small in preparation amount are abandoned, CTF-CNT is synthesized through a strong acid polymerization method, in the composite material, a triazine group of CTF fixes polysulfide through chemical adsorption, CNT constructs a conductive network to improve the electron transmission efficiency, the shuttle effect is synergistically inhibited, and lithium ion conduction is optimized; therefore, the bifunctional diaphragm not only can inhibit shuttling of polysulfide, but also can adjust uniform deposition of lithium dendrites. Experiments show that the electrochemical performance of the CTF-CNT diaphragm containing 10% of CNT is optimal, and in a long cycle test, the CTF-CNT still has the specific capacity of 583 mAh / g after circulating 300 times at the multiplying power of 1C.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur batteries, and in particular to a functionalized diaphragm modified material for solving the polysulfide shuttle effect, regulating dendrite growth, and improving battery performance. The present invention relates to a diaphragm modified material composed of high-nitrogen-rich triazine (CTF) and carbon nanotubes, as well as its preparation and application. Background Art

[0002] Lithium-sulfur batteries (Li-S batteries) are popular due to their high theoretical specific capacity (1675 mAh g - ¹) and high energy density (2600 Whkg - ¹), holds broad application prospects in electric vehicles, wearable electronics, and large-scale energy storage systems. However, practical applications of lithium-sulfur batteries still face numerous challenges, particularly the polysulfide shuttle effect, which severely impacts their cycle life and coulombic efficiency. During charge and discharge, lithium polysulfides dissolve in the electrolyte and migrate to the negative electrode, leading to loss of active materials, increased electrolyte consumption, and increased side reactions, resulting in rapid capacity decay and unstable battery performance.

[0003] In recent years, functionalized membrane modification has been considered an effective strategy to address the shuttle effect. By introducing polar materials or catalytic materials on the surface of traditional polypropylene (PP) membranes, polysulfides can be effectively adsorbed or catalytically converted, thereby slowing down their migration behavior and improving battery stability. Currently, in order to alleviate the polysulfide shuttle effect in lithium-sulfur batteries, researchers have explored a variety of functionalized membranes, including inorganic material adsorption, porous MOF structures, electrochemically active conductive polymers, high specific surface area carbon-based materials, and multi-component composite strategies. Although these methods have achieved certain results in improving battery performance, they still generally have problems such as insufficient conductivity, poor structural stability, or complex preparation.

[0004] Covalent organic frameworks (COFs) are a class of porous crystalline materials constructed by organic molecules through covalent bonds. They have a highly ordered pore structure, a high specific surface area, good chemical stability and designability. In particular, triazine covalent organic frameworks (CTFs) have unique advantages in physical shielding and chemical adsorption of polysulfides due to their rich nitrogen content, strong polarity and abundant functional sites. In addition, the regularly distributed pores in CTFs can provide fast and stable migration channels for lithium ions, which helps to improve the ion conductivity of the battery. However, traditional CTF materials are usually electrical insulators with poor electronic conductivity, which greatly limits their independent application in the field of electrochemical energy storage.

[0005] However, the current synthesis process of related materials still has problems such as complexity, high cost, and unsuitability for large-scale preparation, and research on their actual application performance in lithium-sulfur batteries is still insufficient, especially in the construction of conductive networks, structural stability and electrochemical performance optimization, which still needs further exploration.

[0006] Therefore, the present invention develops a CTF-CNT composite coating with a reasonable structural design, simple process, good conductivity and strong polysulfide inhibition ability. This composite material can be prepared in large quantities and applied for the first time to lithium-sulfur battery separator coating to solve the problems of shuttle effect and performance degradation in lithium-sulfur batteries, which has important scientific research value and application prospects. Summary of the Invention

[0007] To overcome these limitations, the present invention provides a high-nitrogen-rich triazine / carbon nanotube composite material, its preparation method, and its application in lithium-sulfur batteries. This innovative composite design utilizes carbon nanotubes (CNTs) in conjunction with high-nitrogen-rich triazine (CTF). The synergistic effect of CTF and CNTs creates a synergistic effect of chemical adsorption and rapid electron / ion transport. CTF effectively immobilizes polysulfide molecules, while CNTs provide a convenient electron pathway for the reaction, effectively improving reaction rate and material utilization.

[0008] Carbon nanotubes have excellent electrical conductivity, high mechanical strength and a unique one-dimensional structure, and can construct an efficient electron transport network. The composite material prepared by the present invention combines high-nitrogen-rich triazine CTF with carbon nanotubes, which not only makes up for the conductivity defects of CTF, but also enhances the mechanical stability and interface bonding ability of the material. In this composite material, the polar triazine groups of CTF interact with carbon nanotubes to produce a synergistic effect: CTF is responsible for capturing and fixing polysulfides, and carbon nanotubes provide a fast electron transmission path. High-nitrogen-rich triazine CTF forms a strong chemical bond with polysulfide molecules by virtue of the polarity of the nitrogen group, effectively inhibiting its diffusion; at the same time, the nitrogen group can combine with lithium ions to uniformly transport lithium ions in the regular channels of triazine CTF, so as to achieve the effect of regulating the uniform deposition of lithium dendrites.

[0009] The superiority of the composite material was fully demonstrated through comprehensive material characterization, electrochemical performance testing, and analysis of the separator's lithium ion conduction mechanism and catalytic performance. The composite material of the invention can be prepared in large quantities and is used for the first time in lithium-sulfur battery separator coatings.

[0010] In order to solve the above technical problems, the present invention adopts the following technical solutions: A method for preparing a high-nitrogen-rich triazine / carbon nanotube composite material comprises the following steps: (1) Add carbon nanotubes (CNTs) to 1,4-dicyanobenzene (DCB), grind and mix in a ball mill, add trifluoromethanesulfonic acid (CF3SO3H), grind evenly, seal in a Pyrex tube, seal the tube with a vacuum flame under liquid nitrogen cooling, and heat to react for a period of time; (2) After the reaction of step (1), the mixture is cooled, washed, and vacuum-dried to obtain an orange high nitrogen-rich triazine / carbon nanotube CTF-CNT precursor with AB stacking. (3) The obtained CTF-CNT precursor was evenly ground in a ball mill and then transferred to a tube furnace. The temperature was raised to 350°C at 5°C / min under a nitrogen atmosphere and calcined for 2 hours to obtain highly crystalline black block CTF-CNT with AA stacking. The black powder was then ball milled in a planetary ball mill for 5 hours to obtain the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT.

[0011] Furthermore, in step (1), the molar ratio of 1,4-dicyanobenzene DCB to trifluoromethanesulfonic acid CF3SO3H is 2:(1-2), and the mass fraction of carbon nanotubes in 1,4-dicyanobenzene DCB is 5%-20%, preferably 10%.

[0012] Furthermore, in step (1), the mixture is ground and mixed in a ball mill for 30-60 minutes, the reaction temperature is 250° C., the reaction time is 12-15 hours, and the heating rate is 5° C. / min.

[0013] Furthermore, the mixing of DCB and CF3SO3H in step (1) needs to be carried out under the protection of an inert gas, and the reaction product needs to be cooled in liquid nitrogen for 10 minutes to terminate the reaction and form the material.

[0014] Furthermore, in step (2), after the reaction is completed and cooled, the mixture is washed three times with deionized water and ethanol in sequence, and the vacuum drying temperature is 60-80° C. and the vacuum drying time is 8-12 h.

[0015] Furthermore, in step (3), the calcination temperature is 350° C. and the calcination time is 2-3 hours.

[0016] Furthermore, the ball milling time in the planetary ball mill in step (3) is 5-8 hours.

[0017] The present invention also provides a high-nitrogen-rich triazine / carbon nanotube CTF-CNT composite material prepared by the preparation method.

[0018] The present invention also provides an application of the high nitrogen-rich triazine / carbon nanotube CTF-CNT composite material in a lithium-sulfur battery. The high nitrogen-rich triazine / carbon nanotube composite material is made into a functionalized lithium-sulfur battery diaphragm. The specific preparation method is as follows: the CTF-CNT composite material is coated on the surface of a commercial diaphragm, the coating thickness is 5-20 μm, and the loading amount is 0.2-2.0 mg / cm², preferably 0.2-0.3 mg / cm 2 The commercial separator is a polypropylene (PP) film (Celgard 2400). The coating material is a high-nitrogen triazine / carbon nanotube composite material and a binder (PVDF) in a mass ratio of 9:1. The solvent is N-methylpyrrolidone (NMP). After coating, it is vacuum-dried at 60°C for 12 hours.

[0019] The separator prepared by the present invention can not only inhibit the shuttling of polysulfides, but also regulate the uniform deposition of lithium dendrites. The effect of the functionalized separator in inhibiting the shuttling of polysulfides was explored by assembling lithium-sulfur batteries, and the effect of the functionalized separator in regulating the growth of lithium dendrites was explored by assembling lithium-lithium symmetric batteries.

[0020] The present invention also provides a lithium-sulfur battery, which uses a sulfur / carbon nanotube composite cathode material (sulfur content 69.7 wt%) as the positive electrode active material, a lithium metal sheet as the negative electrode, an electrolyte containing 1M lithium bistrifluoromethanesulfonyl imide (LiTFSI) and 1% lithium nitrate (LiNO3) in a 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solution, and a CTF-CNT coating as the separator. The preparation method is as follows: 1) Sublimed sulfur and CNT material are uniformly mixed in a certain proportion, and a sulfur / carbon nanotube composite cathode material is obtained by melt-filling sulfur under a vacuum or inert gas atmosphere. The heating temperature is 155°C, and the holding time is 12-18 hours. The CNT mass ratio of the sulfur / carbon nanotube composite cathode material is 30%; 2) The positive electrode sheet preparation method is as follows: sulfur / carbon nanotube composite positive electrode material, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 80:10:10, ball-milled for 2 hours using nitrogen methyl pyrrolidone (NMP) as a solvent, mixed evenly to form a slurry, and then evenly coated on the current collector (coated on a carbon foil current collector) with a sulfur loading of 2.0 mg / cm². After vacuum drying, the lithium-sulfur battery positive electrode sheet is prepared; 3) The CTF-CNT composite material and the binder are mixed in a certain proportion, using nitrogen methyl pyrrolidone as the solvent. The mixture is evenly mixed to form a slurry, which is then coated on a polypropylene (PP) film. The loading of the CTF-CNT composite material on the PP film is about 0.2 mg / cm². After drying at room temperature, a multifunctional composite membrane is obtained.

[0021] 4) Assemble the lithium-sulfur battery positive electrode sheet, lithium negative electrode, multifunctional composite separator, electrolyte and battery shell into a lithium-sulfur battery.

[0022] Preferably, the positive electrode is an S / CNT composite positive electrode plate, and the electrolyte consists of a lithium-containing electrolyte, a non-aqueous organic solvent, and lithium nitrate. The lithium-containing electrolyte is lithium bistrifluoromethanesulfonyl imide (LiTFSI) with a concentration of 1 mol / L, and the non-aqueous organic solvent is a 1:1 volume ratio mixture of dioxolane (DOL) and ethylene glycol dimethyl ether (DME), with 1% by mass of lithium nitrate added. The positive electrode plate is a sulfur-carbon composite material.

[0023] After 300 cycles at 1 C, the specific capacity is 583 mAh g -1 It can still maintain a specific capacity of 662 mAh / g at a rate of 3 C. After rate cycling, it can still maintain a specific capacity of 828 mAh / g at a rate of 0.5 C.

[0024] Beneficial effects of the present invention: The present invention provides a method for preparing a novel CTFs composite material, and applies it to lithium-sulfur battery separator materials: the present invention abandons the defects of the traditional solvent thermal method, which has a long reaction time and a small amount of preparation, and synthesizes a nitrogen-rich triazine CTF with simple operation and short reaction time by a strong acid polymerization method. The preparation method of the covalent organic framework in the present invention has solvent-free reaction conditions, a simple process, and can be prepared in large quantities. It is compounded with carbon nanotubes to prepare a separator modified material with excellent performance. When used as a lithium-sulfur battery separator material, the composite material has obvious effects in inhibiting polysulfide shuttling, improving lithium ion conduction, and inhibiting the uniform deposition of lithium dendrites. The composite material can be prepared in large quantities and applied to lithium-sulfur battery separator coatings for the first time, providing important scientific basis and technical support for the design and development of the next generation of high-performance lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 This is the TGA curve of the sulfur / carbon nanotube positive electrode of the present invention; Figure 2 SEM images of CTF and CTF-CNT of the present invention show the effect of CNT embedding degree on pore structure. Figure 3 XRD patterns of CTF, CTF-CNT, and CNT.

[0027] Figure 4 Fourier infrared transform images of CTF, CTF-CNT, and CNT.

[0028] Figure 5 X-ray photoelectron spectroscopy XPS images of CTF, CTF-CNT, and CNT.

[0029] Figure 6 BET images of the CTF-CNT-10 material, specific surface area, and pore size of the CTF material prepared in Example 1 (a) CNT, (b) CTF-CNT-10.

[0030] Figure 7 The rate performance of CTF-CNT-5, CTF-CNT-10, and CTF-CNT-20 at 0.1-3C.

[0031] Figure 8 This is the rate performance of CTF-CNT and CTF at 0.1-3C.

[0032] Figure 9 Long-cycle performance of batteries containing CTF-CNT coating and batteries containing CTF coating, (a) 1C long-cycle performance comparison; (b) CV curve shows that the CTF-CNT redox peak current density is the largest, ΔE=281 mV. Figure 10 Comparison images of the battery rate performance and deposition SEM in Application Example 2, where (ad) are symmetric battery rate performance; (ef) are comparison images of lithium deposition SEM.

[0033] Among them, the horizontal coordinate of the obtained XRD pattern is the diffraction angle (2θ), and the vertical coordinate is the diffraction peak intensity (Intensity); the horizontal coordinate of the obtained nitrogen isothermal adsorption and desorption curve is the relative pressure (Relative pressure), and the vertical coordinate is the nitrogen adsorption amount (Volume adsorbed); the obtained X-ray absorption fine structure spectrum (XAFS) diagram, the horizontal coordinate is the absorption energy (Energy), and the vertical coordinate is the absorption intensity (Normalized date); the obtained symmetric battery performance diagram, the horizontal coordinate is time (Time), and the vertical coordinate is voltage (Voltage); the obtained cycle performance diagram, the horizontal coordinate is the cycle number (Cycle number), and the vertical coordinate is the specific capacity (Specific capacity).

[0034] It should be noted that, except Figure 7 In addition, the CTF-CNT in the figures refers to CTF-CNT-10. DETAILED DESCRIPTION

[0035] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, all other embodiments obtained by ordinary technicians in this field without creative work based on the embodiments of the present invention are within the scope of protection of the present invention.

[0036] Example 1 The preparation method of the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT in this embodiment is as follows: To 1,4-dicyanobenzene (DCB) (2 mmol, 0.256 g) were added 5% (0.0128 g), 10% (0.0256 g), and 20% (0.0512 g) carbon nanotubes, which were ground and mixed in a ball mill for 30 min. Trifluoromethanesulfonic acid (CF3SO3H) (1 mmol, 0.15 g) was added, and the mixture was evenly ground and sealed in a Pyrex tube. The tube was cooled to -196 °C with liquid nitrogen and then sealed with a vacuum flame (vacuum degree ≤ 10 - ³ Pa), heated to 250°C at 5°C / min for 12 hours. The product was then cooled in liquid nitrogen for 10 minutes to terminate the reaction and form the material. After cooling, it was washed three times with deionized water and ethanol, followed by vacuum drying at 60°C for 12 hours, yielding an orange CTF-CNT precursor with an AB stacking pattern. The resulting CTF-CNTs were uniformly ground in a ball mill and transferred to a tube furnace. Calcinated in a nitrogen atmosphere at 5°C / min to 350°C for 2 hours, this yielded highly crystalline black CTF-CNT blocks with an AA stacking pattern. Subsequently, the product was ball milled in a planetary ball mill for 5 hours to yield black CTF-CNT powders (labeled as CTF-CNT-5, CTF-CNT-10, and CTF-CNT-20, depending on the carbon nanotube content).

[0037] For comparison, this example also performs the synthesis of CTF, and the specific method is as follows: 2 mmol of 1,4-dicyanobenzene (DCB, 0.256 g) and 1 mmol of trifluoromethanesulfonic acid (CF3SO3H, 0.15 g) were placed in an argon glove box and ground in an agate mortar for 20 minutes until uniformly mixed. The mixture was transferred to a Pyrex glass tube, cooled to -196°C with liquid nitrogen, and then sealed with a vacuum flame (vacuum degree ≤ 10 - ³ Pa).

[0038] The sealed tube was placed in a muffle furnace and heated at 5°C / min to 250°C for 12 hours. After cooling to room temperature, the tube was crushed and the orange solid was collected. It was ultrasonically washed with deionized water (3 × 50 mL) and anhydrous ethanol (3 × 50 mL) for 30 minutes, followed by vacuum drying at 60°C for 12 hours to obtain a CTF precursor with an AA stacking structure. The CTF-AB was transferred to a tube furnace and heated at 5°C / min to 350°C under a nitrogen atmosphere (flow rate of 100 mL / min). It was calcined for 2 hours and cooled naturally to obtain a highly crystalline orange-red CTF with an AA stacking structure. It was then ball-milled in a planetary ball mill for 5 hours to obtain an orange-red CTF powder.

[0039] Example 2 The preparation method of the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT in this embodiment is as follows: 10% (mass fraction) carbon nanotubes (CNTs) (0.0256 g) were added to 1,4-dicyanobenzene (DCB) (2 mmol, 0.256 g) and ground in a ball mill for 30 min. Trifluoromethanesulfonic acid (CF3SO3H) (1.2 mmol, 0.18 g) was added and ground uniformly. The mixture was sealed in a Pyrex tube and sealed in a vacuum flame under liquid nitrogen cooling. The temperature was raised to 250°C at 5°C / min for 13 h. After cooling, the mixture was washed three times with deionized water and ethanol, and dried in a vacuum oven at 60°C for 12 h to obtain an orange CTF-CNT precursor with AB stacking. The obtained CTF-CNTs were uniformly submerged in the ball mill and transferred to a tube furnace. The temperature was raised to 350°C at 5°C / min for 2.5 h under nitrogen atmosphere to obtain highly crystalline black block CTF-CNTs with AA stacking. The mixture was then ball milled in a planetary ball mill for 5 h to obtain black CTF-CNT powder.

[0040] Example 3 The preparation method of the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT in this embodiment is as follows: 10% (mass fraction) carbon nanotubes (CNTs) (0.0256 g) were added to 1,4-dicyanobenzene (DCB) (2 mmol, 0.256 g) and ground in a ball mill for 30 min. Trifluoromethanesulfonic acid (CF3SO3H) (1.5 mmol, 0.225 g) was added and ground uniformly. The mixture was sealed in a Pyrex tube and sealed in a vacuum flame under liquid nitrogen cooling. The temperature was raised to 250°C at 5°C / min for 14 h. After cooling, the mixture was washed three times with deionized water and ethanol, and dried in a vacuum oven at 80°C for 8 h to obtain an orange CTF-CNT precursor with AB stacking. The obtained CTF-CNTs were uniformly submerged in the ball mill and transferred to a tube furnace. The temperature was raised to 350°C at 5°C / min for 3 h under nitrogen atmosphere to obtain highly crystalline black block CTF-CNTs with AA stacking. The mixture was then ball milled in a planetary ball mill for 5 h to obtain black powder CTF-CNTs.

[0041] Example 4 The preparation method of the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT in this embodiment is as follows: 10% (mass fraction) carbon nanotubes (CNTs) (0.0256 g) were added to 1,4-dicyanobenzene (DCB) (2 mmol, 0.256 g) and ground in a ball mill for 30 min. Trifluoromethanesulfonic acid (CF3SO3H) (2 mmol, 0.3 g) was added and ground uniformly. The mixture was sealed in a Pyrex tube and sealed in a vacuum flame under liquid nitrogen cooling. The temperature was raised to 250°C at 5°C / min for 15 h. After cooling, the mixture was washed three times with deionized water and ethanol, and dried in a vacuum oven at 70°C for 10 h to obtain an orange CTF-CNT precursor with AB stacking. The obtained CTF-CNTs were uniformly mixed in the ball mill and transferred to a tube furnace. The temperature was raised to 350°C at 5°C / min for 2 h under nitrogen atmosphere to obtain highly crystalline black block CTF-CNTs with AA stacking. The mixture was then ball milled in a planetary ball mill for 5 h to obtain black powder CTF-CNTs.

[0042] Application Example 1 In this application example, the CTF-CNT composite material prepared in Example 1 is used as a diaphragm coating in the field of lithium-sulfur batteries. The specific steps are as follows: (1) Preparation of CTF-CNT functionalized membrane: First, a CTF-CNT composite and a polyvinylidene fluoride (PVDF) binder were thoroughly mixed in a 9:1 mass ratio. Using N-methylpyrrolidone (NMP) as a solvent, the materials were ball-milled to form a slurry at a speed of 350 r / min for 2 hours. The mixed slurry was then evenly coated onto a polypropylene separator (Celgard 2400) using a coater. A doctor blade was then used to apply the slurry onto the separator surface, controlling the coating thickness to 5-20 μm, preferably 16 μm, with a coating mass loading of 0.2 mg / cm² to ensure good mechanical stability and polysulfide suppression. After coating, the separator was dried in a vacuum drying oven at 60°C for 12 hours to remove any residual solvent. The resulting separator was then cut into 16 mm diameter pieces, resulting in an integrated functionalized modified separator. This method is simple and suitable for mass production.

[0043] (2) Preparation of sulfur / carbon composite positive electrode sheet: Sublimed sulfur and CNT composite materials with a mass ratio of 7:3 were weighed and placed in a mortar. The mixture was then ground thoroughly to mix well. The mixed material was then placed in a glass tube and evacuated with a vacuum pump until the vacuum level in the glass tube reached below 10 mbar and maintained for 10 minutes. The tube was then sealed with an acetylene torch. The sealed glass tube was placed in an oven at 155°C for 12 hours. After cooling, a sulfur / carbon nanotube composite material (sulfur content 69.7 wt%, by Figure 1 , TGA verification). The obtained sulfur / carbon nanotube composite material, Super P and PVDF were mixed in a mass ratio of 8:1:1, and nitrogen methyl pyrrolidone (NMP) was used as a solvent. The materials were mixed evenly by ball milling to form a slurry. The ball milling speed was 350 r / min and the ball milling time was 2 hours. The mixed slurry was evenly coated on the carbon-coated aluminum foil current collector using a coater, and then transferred to a vacuum oven at 60°C for 12 hours. The coater blade height was 200 μm, and the obtained electrode sulfur loading was 2 mg / cm 2 Roll compaction was performed to a compaction density of 1.6 g / cm³. The prepared positive electrode was cut into 12 mm diameter sheets as the positive electrode and the lithium sheet as the negative electrode.

[0044] (3) Preparation of electrolyte: button cells were assembled by adding an electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) lithium salt and 1% lithium nitrate in a 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio of 1:1) solvent.

[0045] (4) Battery assembly: positive electrode: sulfur / carbon composite electrode (diameter 12 mm); negative electrode: lithium metal sheet (thickness 200 μm); separator: CTF-CNT functionalized separator; electrolyte: 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0046] Figure 1 This is the thermogravimetric curve (TGA) of the sulfur / carbon nanotube composite material prepared in the present invention. It can be seen from the figure that the ratio of the sulfur / CNT composite material in the positive electrode sheet after heating is approximately 7:3, which is consistent with the 7:3 ratio before heating.

[0047] The scanning electron microscope image of the CTF-CNT composite material prepared in Example 1 is as follows: Figure 2 As shown, it can be seen that the material has a block structure and the CTF and CNT are very tightly combined.

[0048] The XRD patterns of the CTF-CNT-10 material and the CTF material prepared in Example 1 are as follows: Figure 3 As shown in the figure, the XRD pattern of CTF shows strong peaks, indicating the formation of a crystalline network with hexagonal stacking pores. The peak at 7.29° is the in-plane reflection (100), the three peaks at 12.55°, 14.52° and 19.11° are attributed to the (110), (200) and (210) reflections respectively, and the peak at 26.46° corresponds to the (002) crystal plane, whose interlayer distance is 3.4 Å. It is worth noting that the intensity of the (100) peak gradually decreases with the increase of CNT content. For two-dimensional layered materials, the (100) peak often reflects the order of the material interlayer arrangement or the specific direction of the crystal. The weakening of the peak intensity means that the material interlayer arrangement tends to be disordered, which is caused by the interlayer spacing of the graphite in the CNT.

[0049] Figure 4 The Fourier infrared absorption spectra of CTF-CNT-10 material and CTF material prepared in Example 1 are shown at 1501 cm -1 The peak is attributed to the carbon-nitrogen (C=N) bond. It is worth noting that with the increase of CNT content, the peak of the carbon-nitrogen bond gradually weakens, indicating that the CNT ratio gradually masks the redox active sites, which will lead to the weakening of the material's ability to bind lithium ions. However, all materials maintain the same characteristic peak type, and with the increase of CNT content, the interface resistance of the battery during transfer will decrease, which is more conducive to the transmission of lithium ions.

[0050] Figure 5The XPS spectra of the CTF-CNT-10 and CTF materials prepared in Example 1 show that the peak at 284.8 eV of the composite material is attributed to the -C=N bond in the triazine ring. The C 1s spectrum shows a distinct characteristic peak at 285.5 eV. In the CTF and CNT composite material CTF-CNT-10, the peak in this region is due to the sp 3 Hybridized carbon atoms. This happens because of the generation of surface defects. Strong acid treatment often destroys the structure of carbon nanotubes, resulting in surface defects or irregular edge structures.

[0051] Figure 6 This is the nitrogen adsorption and desorption diagram of the material prepared in Example 1. It can be seen from the figure that the CTF-CNT material ( Figure 6 b) with 506m 2 / g specific surface area and 1.75nm pore size. The larger specific surface area can expose more redox active sites in the material.

[0052] Figure 7 The rate test of CTF obtained with different carbon nanotube contents (5%, 10%, 20%) in Example 1 was measured, and the performance tests at different rates (0.1C, 0.2C, 0.5C, 1C, 2C, 3C) showed that the carbon nanotube composite coating with a mass fraction of 10% in the CTF had the best rate performance. Even at a rate of 3C, it could still maintain a specific capacity of 662mAh / g. After rate cycling, it could still maintain a specific capacity of 828mAh / g at a rate of 0.5C. Therefore, the composite material with a carbon nanotube content of 10% had the best rate performance.

[0053] The battery cycle performance test was carried out at a voltage range of 1.7 to 2.8 V. The battery rate performance and cycle performance at 1 C are shown in Figure 2. Figure 8 , as shown in 9.

[0054] like Figure 8 The figure shows the CTF-CNT prepared in Example 1. The figure shows that the capacity of CTF-CNT at different current densities is higher than that of CTF. For lithium-sulfur batteries, higher rate performance generally indicates that the material has good conductivity, which can effectively support the rapid transport of ions and electrons while suppressing the shuttling effect of polysulfides and preventing capacity decay.

[0055] like Figure 9The CTF-CNT prepared in Example 1 is shown in Figure (a). The long cycle performance of the battery containing the CTF-CNT coating is compared in detail with that of the battery containing the CTF coating. As can be seen from (a), the battery containing the CTF-CNT coating still has 583 mAh g after 300 cycles at 1 C. -1 The specific capacity is significantly greater than that of pure CTF separator (534 mAh g -1 ) and PP separator (422 mAh g -1 ) batteries. CTF-CNT long-cycle batteries have good performance, and the batteries can maintain high capacity and low capacity decay during multiple charge and discharge processes.

[0056] Figure 9 (b) shows the CV curve of CTF-CNT, which shows that it has a typical oxidation peak and a double reduction peak. The high voltage reduction peak at 2.32 V indicates that the sulfur element is first reduced to long-chain lithium polysulfide (Li2S x , 4≤x≤8), while the low-voltage reduction peak at 2.05V corresponds to the further reduction of long-chain lithium polysulfides to solid-state Li2S2 / Li2S. At the same time, the oxidation peak at 2.33 V is consistent with the oxidation of Li2S2 / Li2S and its conversion to elemental sulfur. Compared with CTF-PP and PP, the CV curve of CTF-CNT shows a larger current density and a smaller potential difference (ΔE=281 mV), indicating that it has better redox conversion kinetics. This shows that the electrochemical performance of CTF-CNT has been significantly improved by the uniform composite of CTF and CNT.

[0057] Application Example 2 In this application example, the CTF-CNT composite material prepared in Example 1 is used as a diaphragm coating in a lithium-lithium symmetrical battery to protect the lithium metal negative electrode. The specific steps are as follows: A CTF-CNT-functionalized membrane was prepared according to the method described in Application Example 1. Lithium sheets with a diameter of 14.5 mm were placed on either side of the membrane. A button cell was assembled using an electrolyte consisting of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1% lithium nitrate in a solvent of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) (1:1 volume ratio).

[0058] like Figure 10 The figure shows the rate capability of the lithium-lithium symmetric battery composed of the functionalized separator prepared by CTF-CNT-10 prepared in Example 1 at different current densities (from 1 to 4 mA cm -2Compared with the original PP separator, the symmetric cell using the CTF-CNT separator exhibited a smaller overpotential during the repeated plating-stripping process, especially at high current densities (greater than 3, 4 mA cm -2 ). The CTF-CNT separator accelerated the ion transfer kinetics, thereby improving the deposition behavior of lithium metal. -2 At a current density of 1.5 GHz, the symmetric cell using the CTF-CNT separator showed a small voltage hysteresis of about 20 mV and a long cycling stability within 1400 h, as shown in Figure 2. Figure 10 (b) At the same time, after cycling at a high current density, we return to 0.5 mA cm -2 The symmetrical cell with the original PP separator can still maintain stable cycling for 1000 times with an overpotential of about 25 mV, while the symmetrical cell with the original PP separator shows severe voltage fluctuation after only 600 h, indicating harmful dendrite formation, as shown in Figure 10 (c). The symmetrical cell was characterized by SEM at 0.5 mA cm -2 The Li deposition morphology after 20 cycles is as follows: Figure 10 (d) Li deposition on the surface of Li foil with pristine PP separator shows tortuous needle-like Li deposition, as shown in Figure 10 (e) shows that this is attributed to the formation of bulk Li dendrites. In contrast, the lithium metal obtained from the battery with CTF-CNT separator exhibits a uniform, dense and smooth morphology, and no dendritic Li is observed. In summary, the use of CTF-CNT coated separators can significantly inhibit the shuttling of polysulfides and enable the uniform deposition of lithium dendrites on the lithium metal anode, as shown in Figure 2. Figure 10 (f) shown.

[0059] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high nitrogen-rich triazine / carbon nanotube composite material, characterized in that: The following steps are involved: (1) Add carbon nanotubes (CNTs) to 1,4-dicyanobenzene (DCB), grind and mix in a ball mill, add trifluoromethanesulfonic acid (CF3SO3H), grind evenly, seal in a Pyrex tube, seal the tube with a vacuum flame under liquid nitrogen cooling, and heat to react for a period of time; (2) After the reaction of step (1), the mixture is cooled, washed, and vacuum-dried to obtain an orange high nitrogen-rich triazine / carbon nanotube CTF-CNT precursor with AB stacking. (3) The obtained CTF-CNT precursor was evenly ground in a ball mill and then transferred to a tube furnace. The temperature was raised to 350°C at 5°C / min under a nitrogen atmosphere and calcined for 2 hours to obtain highly crystalline black block CTF-CNT with AA stacking. The black powder was then ball milled in a planetary ball mill for 5 hours to obtain the high nitrogen-rich triazine / carbon nanotube composite material CTF-CNT.

2. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: In the step (1), the molar ratio of 1,4-dicyanobenzene DCB to trifluoromethanesulfonic acid CF3SO3H is 2:(1-2), and the mass fraction of carbon nanotubes in 1,4-dicyanobenzene DCB is 5%-20%.

3. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: In the step (1), the mixture is ground and mixed in a ball mill for 30-60 minutes, the reaction temperature is 250° C., the reaction time is 12-15 hours, and the heating rate is 5° C. / min.

4. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: In step (1), the mixing of DCB and CF3SO3H needs to be carried out under the protection of an inert gas, and the reaction product needs to be cooled in liquid nitrogen for 10 minutes to terminate the reaction and form the material.

5. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: After the reaction in step (2) is completed and cooled, the mixture is washed with deionized water and ethanol three times in sequence. The vacuum drying temperature is 60-80°C and the vacuum drying time is 8-12 h.

6. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: In step (3), the calcination temperature is 350° C. and the calcination time is 2-3 hours.

7. The method for preparing the high nitrogen-rich triazine / carbon nanotube composite material according to claim 1, characterized in that: The ball milling time in the planetary ball mill in step (3) is 5-8 hours.

8. A high nitrogen-rich triazine / carbon nanotube (CTF-CNT) composite material prepared according to the preparation method of any one of claims 1 to 7.

9. Use of the high nitrogen-rich triazine / carbon nanotube (CTF-CNT) composite material according to claim 8 in a lithium-sulfur battery, characterized in that: A high-nitrogen-rich triazine / carbon nanotube composite material was made into a functional separator for lithium-sulfur batteries. The specific preparation method was as follows: the CTF-CNT composite material was coated on the surface of a commercial separator with a coating thickness of 5-20 μm and a loading of 0.2-2.0 mg / cm². The commercial separator was a polypropylene (PP) film (Celgard 2400). The mass ratio of the coating material to the binder (PVDF) was 9:

1. The solvent was N-methylpyrrolidone (NMP). After coating, the membrane was vacuum-dried at 60°C for 12 hours.

10. The use according to claim 8, characterized in that: After 300 cycles at 1 C, the specific capacity is 583 mAh g -1 , it can still maintain a specific capacity of 662 mAh / g at a rate of 3 C, and after rate cycling, it can still maintain a specific capacity of 828 mAh / g at a rate of 0.5 C.

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