Treatment method of acidified carbon nanotube and application of acidified carbon nanotube in lithium-sulfur battery

Abstract

The invention discloses a treatment method of acidified carbon nanotubes and application of the acidified carbon nanotubes in a lithium-sulfur battery, and belongs to the technical field of lithium-sulfur batteries. Hydrophilic functional groups such as carboxyl and hydroxyl are introduced to the surfaces and defect sites of the carbon nanotubes through the strong acid oxidation effect of concentrated nitric acid, the phenomenon that hydrophobicity is prone to agglomeration is improved, the dispersion stability of the carbon nanotubes in a polar solvent and a base material is improved, the used raw materials are low in cost and easy to obtain, operation is easy and convenient, and the method is suitable for industrial large-scale production. The acidified carbon nanotube obtained by the invention is used as a positive electrode material and a diaphragm material of a lithium-sulfur battery, so that the lithium-sulfur battery is high in first discharge specific capacity, high in coulombic efficiency and good in capacity retention ratio. The invention solves the problems of shuttle effect and performance degradation in the lithium-sulfur battery and improves the performance of the battery through the acidified carbon nanotube treatment method which is simple in treatment process and has polysulfide inhibition capability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-sulfur battery technology, and specifically relates to a method for treating and applying acidified carbon nanotubes. Background Technology

[0002] Energy is the cornerstone of human survival and development. With the increasing global population and national economies, the demand for energy is constantly rising. However, the increasingly serious environmental pollution problem poses a huge threat to sustainable development. Therefore, finding green and environmentally friendly clean energy sources and developing a rechargeable battery system with high energy density, long cycle life, and low economic cost has become a current research hotspot. Lithium-ion batteries are currently the best commercially viable rechargeable battery system. However, with the popularization of new energy vehicles and the rapid development of safer and more efficient electronic devices, existing lithium-ion batteries can no longer meet the needs of the public, necessitating the development of a new battery system with higher specific energy.

[0003] Lithium-sulfur batteries, due to their high theoretical specific capacity (1675 mAh / g) and energy density (2600 Wh / kg), have broad application prospects in electric vehicles, wearable electronic devices, and large-scale energy storage systems. However, lithium-sulfur batteries still face many challenges in practical applications, especially the shuttle effect of polysulfides, which severely affects the cycle life and coulombic efficiency of the batteries. During charging and discharging, lithium polysulfides dissolve in the electrolyte and migrate to the lithium anode, leading to the loss of active materials, accelerated electrolyte consumption, and the generation of side reactions, resulting in rapid capacity decay and unstable battery performance.

[0004] The unique structure of carbon nanotubes (CNTs) determines their special properties and applications. As a novel carbon-based material that combines nanoscale dimensions with macroscopic material properties, CNTs have broken the boundaries of traditional material mechanics, electrical, and thermal properties with their unique coaxial tubular structure, large van der Waals attraction, huge specific surface area, and strong carbon-carbon covalent interactions, rapidly becoming the core of research and the focus of industrial applications in the field of nanomaterials. However, untreated carbon nanotubes suffer from the drawbacks of aggregation, contain a large number of impurities, and are highly inert and hydrophobic, thus limiting their performance and applications.

[0005] Acidification treatment introduces oxygen-containing functional groups, such as carboxyl and hydroxyl groups, into the surface and defect sites of carbon nanotubes. This introduction makes the carbon nanotube surface hydrophilic, improving its dispersion stability in water or ethanol and preventing aggregation. Simultaneously, the oxidative etching process during acidification creates pores or defects in the nanotube walls, increasing the specific surface area and enhancing the adsorption capacity for polysulfides. Acidification can also etch the outer walls of multi-walled carbon nanotubes, creating open-type carbon nanotubes or arrays. Their unique hollow tubular structure and open ports are more conducive to the transport and migration of substances such as lithium ions and electrons.

[0006] In existing technologies, carbon nanotubes are acidified using a mixed acid of sulfuric acid and nitric acid. For example, patent CN120209644A discloses a functional composite material for lithium-ion battery separators and its production process. By forming a functional protective layer on the surface of a polyolefin separator, the reliability and safety of lithium-ion batteries are improved, and the conductivity of the polyolefin separator is significantly improved, thereby enhancing the overall performance of the lithium-ion battery. Another example is patent CN112542579B, which discloses a conductive Janus film, its preparation method, and its application in the preparation of lithium-sulfur battery cathodes, which can significantly improve the discharge specific capacity, electrochemical performance, and cycle performance of lithium-sulfur batteries. However, using a mixed acid of concentrated sulfuric acid and concentrated nitric acid to acidify carbon nanotubes still has the following drawbacks: 1) Concentrated sulfuric acid in the mixed acid has strong dehydration and high exothermic properties. The strong dehydration may exacerbate the "etching" of the carbon nanotube walls, leading to a shortening of the carbon nanotube length and the formation of a large number of structural defects, which is not conducive to the subsequent reaction. Meanwhile, concentrated sulfuric acid acidification usually requires heating, which can easily damage or deform the carbon nanotube structure, or even cause dangerous side reactions, resulting in high operational risks; 2) Mixed acid treatment may introduce sulfate ions, which may adsorb sulfur-containing functional groups. For the electrode, the residual sulfur component will cause self-discharge and contaminate the electrolyte. For the catalytic pathway, sulfur may become a catalytic active site that will produce side reactions, which is not conducive to the catalytic reaction and interferes with the subsequent electrochemical application of lithium-sulfur batteries; and the cost of subsequent waste liquid treatment using mixed acid is high; 3) Mixed acid treatment may increase the degree of defects, increase the volume expansion of the electrode during charging and discharging, and reduce the conductivity of carbon nanotubes and the structural integrity of the electrode.

[0007] Therefore, there is an urgent need to develop an acidified carbon nanotube treatment method that is simple to process, can alleviate the volume expansion of electrodes during charging and discharging, and has the ability to suppress polysulfides, in order to solve the shuttle effect and performance degradation problems in lithium-sulfur batteries and improve battery performance. Summary of the Invention

[0008] This invention addresses the technical problems of performance degradation in lithium-sulfur batteries, as well as the damage to sidewalls, dangerous operation, complex post-processing, and significant environmental pollution caused by existing acidified carbon nanotube treatment processes. It proposes a method for treating acidified carbon nanotubes and its application in lithium-sulfur batteries.

[0009] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0010] In a first aspect, the present invention provides a method for treating acidified carbon nanotubes, comprising the following steps:

[0011] (1) After drying the carbon nanotubes, add them to the nitric acid solution and stir ultrasonically to obtain an acidified carbon nanotube solution;

[0012] (2) The solution obtained in step (1) is filtered by suction. The precipitate is washed and filtered until the pH of the filtrate is 5-6, and the desired filter cake is obtained.

[0013] (3) After drying the filter cake obtained in step (2), it is ground to obtain acidified carbon nanotubes.

[0014] The drying in step (1) is vacuum drying at a temperature of 50-60℃ for 4-6 h; the solid-liquid ratio of the carbon nanotubes and nitric acid is 0.25-3:50-200 g / mL, and the mass percentage of the nitric acid solution is 68-69.2%; the ultrasonication time is 2-4 h, and the stirring time is 2-4 h.

[0015] The drying in step (3) is vacuum drying at a temperature of 50-60℃; the grinding is done manually first, and then ball milled in a ball mill for 2-3 hours.

[0016] The present invention provides acidified carbon nanotubes obtained by the aforementioned processing method.

[0017] This invention provides the application of the aforementioned acidified carbon nanotubes in the positive electrode of a lithium-sulfur battery.

[0018] Secondly, the present invention provides a positive electrode sheet for a lithium-sulfur battery, comprising a positive electrode active material prepared from the above-mentioned acidified carbon nanotubes, the preparation method comprising the following steps:

[0019] (1) After mixing sublimed sulfur and acidified carbon nanotubes, S@ACNT composite material was obtained by melt-infusion sulfur.

[0020] (2) After mixing S@ACNT composite material, conductive carbon black (Super P) and PVDF, a solvent is added and the mixture is ball-milled twice. The resulting slurry is coated on carbon-coated aluminum foil current collector and dried to obtain the positive electrode sheet of lithium-sulfur battery.

[0021] The mass ratio of sublimed sulfur to acidified carbon nanotubes in step (1) is 70-75:30-25; the molten sulfur filling is carried out under vacuum or inert atmosphere, the heating temperature is 150-155℃, and the holding time is 12-14 h.

[0022] In step (2), the mass ratio of S@ACNT composite material, conductive carbon black and PVDF is 7-8:1-2:1; the solvent is N-methylpyrrolidone; the rotation speed of the secondary ball mill is 300-350 rpm and the time is 3-4 h; the drying is vacuum drying at a temperature of 50-60℃ and a time of 10-12 h.

[0023] This invention provides the application of the aforementioned acidified carbon nanotubes in the preparation of lithium-sulfur battery separators.

[0024] The present invention also provides a lithium-sulfur battery separator, which comprises a polypropylene base film and a functional coating coated on the surface. The functional coating comprises the aforementioned acidified carbon nanotubes and a binder. The preparation method of the lithium-sulfur battery separator includes the following steps: dissolving acidified carbon nanotubes and PVDF in a solvent and stirring evenly to obtain a mixed slurry; coating the slurry onto the surface of the polypropylene base film by vacuum filtration; and drying to obtain the lithium-sulfur battery separator.

[0025] The mass ratio of acidified carbon nanotubes to PVDF is 4-5:1, and the solvent is N-methylpyrrolidone; the carbon nanotube loading on the lithium-sulfur battery separator is 0.1-0.3 mg / cm³. 2 The drying process is vacuum drying, with a temperature of 40-60℃ and a time of 10-12 hours.

[0026] Thirdly, the present invention also provides a lithium-sulfur battery, the lithium-sulfur battery comprising the above-mentioned lithium-sulfur battery positive electrode or the above-mentioned lithium-sulfur battery separator.

[0027] The beneficial effects of this invention are:

[0028] 1. This invention provides a simple and effective method for treating acidified carbon nanotubes with polysulfide suppression capabilities. The acidification process, using concentrated nitric acid, introduces hydrophilic functional groups such as carboxyl and hydroxyl groups onto the surface and defect sites of the carbon nanotubes, improving their tendency to aggregate and enhancing their dispersion stability in polar solvents and matrix materials. This provides a foundation for composite material preparation and functional modification. Furthermore, the acidification method removes amorphous carbon and other impurities from the surface during the acidification process, increasing the specific surface area and providing more active sites, thereby improving performance in adsorption, catalysis, and energy storage applications.

[0029] 2. The acidified carbon nanotubes obtained in this invention, when used as a separator material for lithium-sulfur batteries, suppress polysulfide shuttle, enhance lithium-ion and electron conduction, and effectively alleviate polysulfide dissolution during lithium-sulfur battery charging and discharging. Battery cycle tests show that batteries assembled using the acidified carbon nanotubes of this invention have high initial discharge specific capacity, high coulombic efficiency, and good capacity retention. When used as a cathode material for lithium-sulfur batteries, they can alleviate volume expansion, maintain the integrity of the electrode structure, and no large number of cracks and pore structures appear on the electrode surface before and after cycling. When used as a separator material for lithium-sulfur batteries, they can have a high discharge specific capacity and maintain good capacity retention.

[0030] 3. The acidified carbon nanotube treatment method provided by this invention uses inexpensive and readily available raw materials and reagents. The operation method is simple and controllable, and the product can be obtained using only procedures such as ultrasound and stirring. It does not require complex reaction steps or expensive reagents. The processing time is short, saving energy and costs, and is environmentally friendly, making it suitable for large-scale industrial production. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 SEM images of the acidified carbon nanotubes prepared in Example 4 and Comparative Example 1.

[0033] Figure 2 The images show the FTIR spectra of the acidified carbon nanotubes prepared in Example 4 and Comparative Example 1.

[0034] Figure 3 The image shows the RAMAN diagrams of the acidified carbon nanotubes prepared in Example 4 and Comparative Example 1.

[0035] Figure 4 SEM images of the batteries assembled in Example 3 and Comparative Example 1 before and after cycling.

[0036] Figure 5 Long-cycle test diagrams of the batteries assembled in Examples 1-5 and Comparative Example 1.

[0037] Figure 6 The graphs show the rate test results of the batteries assembled in Examples 1-5 and Comparative Example 1.

[0038] Figure 7 Impedance test diagrams of the batteries assembled in Examples 3 and 5 and Comparative Example 1.

[0039] Figure 8 CV diagrams of the batteries assembled in Examples 3 and 5 and Comparative Example 1. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Example 1

[0042] A method for preparing a lithium-sulfur battery based on S@ACNT cathode assembly, the specific steps of which are as follows:

[0043] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 60°C for 6 h, 0.25 g of carbon nanotubes were weighed and added to a beaker containing 50 mL of concentrated nitric acid with a mass fraction of 68%. The mixture was sonicated for 2 h and then magnetically stirred for 2 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration was repeated until the pH of the filtrate was about 6. The obtained filter cake was then stored. The filter cake was then placed in a vacuum oven at 60°C for overnight drying. Afterward, it was manually ground in an agate mortar until no large particles were found. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 3 h. The resulting black powder was the acidified carbon nanotube ACNT-1.

[0044] (2) Preparation of sulfur / carbon cathode: 3.75 g of sublimed sulfur and 1.25 g of acidified carbon nanotube ACNT-1 composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155℃ for 12 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 1 g of S@ACNT, 0.125 g of Super P and 0.125 g of PVDF were mixed with 1 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60℃ for 12 h to dry. The positive electrode prepared above is cut into a 12 mm diameter electrode sheet as the positive electrode, and a lithium sheet is used as the negative electrode.

[0045] (3) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was assembled into a coin cell.

[0046] (4) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is a polypropylene PP membrane (Celgard 2325), and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0047] Example 2

[0048] A method for preparing a lithium-sulfur battery based on S@ACNT cathode assembly, the specific steps of which are as follows:

[0049] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 60°C for 6 h, 1 g of carbon nanotubes was weighed and added to a beaker containing 100 mL of concentrated nitric acid with a mass fraction of 68%. The mixture was sonicated for 2 h and then magnetically stirred for 2 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was about 6. The filter cake was then stored. The filter cake was then placed in a vacuum oven at 60°C for overnight drying. Afterward, it was manually ground in an agate mortar until no large particles were found. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 3 h. The resulting black powder was the acidified carbon nanotubes ACNT-2.

[0050] (2) Preparation of sulfur / carbon cathode: 7.5 g of sublimed sulfur and 2.5 g of acidified carbon nanotube ACNT-2 composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155℃ for 12 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 2 g of S@ACNT, 0.25 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60℃ for 12 h to dry. The positive electrode prepared above is cut into a 12 mm diameter electrode sheet as the positive electrode, and a lithium sheet is used as the negative electrode.

[0051] (3) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) lithium salt and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was added to assemble a coin cell.

[0052] (4) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is a polypropylene PP membrane (Celgard 2325), and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0053] Example 3

[0054] A method for preparing a lithium-sulfur battery based on S@ACNT cathode assembly, the specific steps of which are as follows:

[0055] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 60°C for 6 h, 2 g of carbon nanotubes were weighed and added to a beaker containing 150 mL of concentrated nitric acid with a mass fraction of 68%. The mixture was sonicated for 2 h and then magnetically stirred for 2 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was about 6. The filter cake was then stored. The filter cake was then placed in a vacuum oven at 60°C for overnight drying. Afterward, it was manually ground in an agate mortar until no large particles were found. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 3 h. The resulting black powder was the acidified carbon nanotubes ACNT-3.

[0056] (2) Preparation of sulfur / carbon cathode: 7.5 g of sublimed sulfur and 2.5 g of acidified carbon nanotube ACNT-3 composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155℃ for 12 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 2 g of S@ACNT, 0.25 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60℃ for 12 h to dry. The positive electrode prepared above is cut into a 12 mm diameter electrode sheet as the positive electrode, and a lithium sheet is used as the negative electrode.

[0057] (3) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was assembled into a coin cell.

[0058] (4) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is a polypropylene PP film (Celgard 2325), and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0059] Example 4

[0060] A method for preparing a lithium-sulfur battery based on S@ACNT cathode assembly, the specific steps of which are as follows:

[0061] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 60°C for 6 h, 3 g of carbon nanotubes were weighed and added to a beaker containing 200 mL of concentrated nitric acid with a mass fraction of 68%. The mixture was sonicated for 4 h and then magnetically stirred for 4 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was about 6. The filter cake was then stored. The filter cake was then placed in a vacuum oven at 60°C for overnight drying. Afterward, it was manually ground in an agate mortar until no large particles were found. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 3 h. The resulting black powder was the acidified carbon nanotubes ACNT-4.

[0062] (2) Preparation of sulfur / carbon cathode: 7.5 g of sublimed sulfur and 2.5 g of acidified carbon nanotube ACNT-4 composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155℃ for 12 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 2 g of S@ACNT, 0.25 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60℃ for 12 h to dry. The positive electrode prepared above is cut into a 12 mm diameter electrode sheet as the positive electrode, and a lithium sheet is used as the negative electrode.

[0063] (3) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was assembled into a coin cell.

[0064] (4) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is a polypropylene PP membrane (Celgard 2325), and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0065] The acidified carbon nanotubes prepared in this embodiment were characterized and analyzed. The specific results are as follows:

[0066] from Figure 1 The SEM images show that the acid-treated carbon nanotubes did not exhibit obvious carbon nanotube breakage, shortening, or damage; however, the nanotube diameter was slightly thinner. The interwoven network structure is more dense, providing richer transport channels for lithium ions and electrons, thus improving the utilization rate of the active material. Furthermore, long nanotubes were observed in both treated and untreated carbon nanotubes, indicating that acid treatment only introduces surface defects.

[0067] from Figure 2 The FTIR images show that carbon nanotubes and acidified carbon nanotubes are located at 1800-2800 cm⁻¹. -1 The region was observed at 3400 cm. -1 A strong hydroxyl peak was observed in the region, and the presence of the carbonyl functional group was observed in the 1200-1600 cm⁻¹ region, proving that the carbon nanotube surface was successfully acidified. The hydroxyl group increases the hydrophilicity of the material by affecting the double-layer capacitance, while the carbonyl group can reduce the charge transfer resistance and accelerate the movement of ions and electrons.

[0068] from Figure 3 Raman spectroscopy reveals that both carbon nanotubes and acidified carbon nanotubes exhibit similar properties at 1350 cm⁻¹. -1 and 1580 cm -1 Characteristic peaks appear at these points, corresponding to amorphous carbon (D peak) and graphitized carbon (G peak), respectively. The intensity ratio of the D peak to the G peak can be used to assess the degree of graphitization (or disorder) of nitrogen-doped carbon. IT of CNTs and acidified CNTs D / I G The ratios were 1.140 and 1.244, respectively, indicating that the disorder of the acidified CNT material was better than that of CNT, which was conducive to the incorporation of more heteroatoms.

[0069] Example 5

[0070] A method for preparing a lithium-sulfur battery based on an ACNT separator and an S@ACNT cathode assembly, the specific steps of which are as follows:

[0071] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 60°C for 6 hours, 2 g of carbon nanotubes were weighed and added to a beaker containing 150 mL of concentrated nitric acid with a mass fraction of 68%. The mixture was sonicated for 3 hours and then magnetically stirred for 3 hours to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was about 6. The filter cake was then stored. The filter cake was then dried overnight in a vacuum oven at 60°C. After that, it was manually ground in an agate mortar until there were no large particles. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 3 hours. The resulting black powder was the acidified carbon nanotubes (ACNTs).

[0072] (2) Preparation of acidified carbon nanotube membrane ACNTP: 25 mg of acidified carbon nanotube material ACNT and 5 mg of polyvinylidene fluoride (PVDF) binder were magnetically stirred with 25 mL of N-methylpyrrolidone (NMP) as solvent to make the solution uniformly mixed. A polypropylene PP membrane (Celgard 2325) was laid flat on the surface of a sand filter. After wetting the membrane surface with ethanol, 10 mL of N-methylpyrrolidone (NMP) was added. 3 mL of the solution was pipetted into the sand filter for pump filtration. Then the membrane was placed in a vacuum dryer at 60 °C for 12 h to remove the solvent residue. The membrane prepared above was cut into membranes with a diameter of 16 mm to finally obtain acidified carbon nanotube membrane ACNTP.

[0073] (3) Preparation of sulfur / carbon positive electrode: 7.5 g of sublimed sulfur and 2.5 g of acidified carbon nanotube ACNT composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155℃ for 12 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 2 g of S@ACNT, 0.25 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60℃ for 12 h to dry. The positive electrode obtained above was cut into a 12 mm diameter electrode sheet as the positive electrode and a lithium sheet as the negative electrode.

[0074] (4) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) lithium salt and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was added to assemble a coin cell.

[0075] (5) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is an acidified carbon nanotube membrane ACNTP, and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0076] Example 6

[0077] A method for preparing a lithium-sulfur battery based on an ACNT separator and an S@ACNT cathode assembly, the specific steps of which are as follows:

[0078] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 50°C for 5 h, 2 g of carbon nanotubes were weighed and added to a beaker containing 150 mL of concentrated nitric acid with a mass fraction of 68.5%. The mixture was sonicated for 2 h and then magnetically stirred for 2 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was 5.5. The filter cake was then stored. The filter cake was then dried overnight in a vacuum oven at 55°C. After that, it was manually ground in an agate mortar until no large particles were found. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 2.5 h. The resulting black powder was the acidified carbon nanotubes (ACNTs).

[0079] (2) Preparation of acidified carbon nanotube membrane ACNTP: 20 mg of acidified carbon nanotube material ACNT and 5 mg of polyvinylidene fluoride (PVDF) binder were magnetically stirred with 25 mL of N-methylpyrrolidone (NMP) as solvent to make the solution uniformly mixed. A polypropylene PP membrane (Celgard 2325) was laid flat on the surface of a sand filter. After wetting the membrane surface with ethanol, 10 mL of N-methylpyrrolidone (NMP) was added. 3 mL of the solution was pipetted into the sand filter for pump filtration. Then the membrane was placed in a vacuum dryer at 50 °C for 11 h to remove the solvent residue. The membrane prepared above was cut into membranes with a diameter of 16 mm to finally obtain acidified carbon nanotube membrane ACNTP.

[0080] (3) Preparation of sulfur / carbon positive electrode: 7 g of sublimed sulfur and 3 g of acidified carbon nanotube ACNT composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 150℃ for 14 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 2 g of S@ACNT, 0.5 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 300 rpm and the ball milling time was 3.5 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 50℃ for 11 h to dry. The positive electrode obtained above was cut into a 12 mm diameter electrode sheet as the positive electrode and a lithium sheet as the negative electrode.

[0081] (4) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) lithium salt and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was added to assemble a coin cell.

[0082] (5) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is an acidified carbon nanotube membrane ACNTP, and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0083] Example 7

[0084] A method for preparing a lithium-sulfur battery based on an ACNT separator and an S@ACNT cathode assembly, the specific steps of which are as follows:

[0085] (1) Preparation of acidified carbon nanotubes: After drying an appropriate amount of carbon nanotubes (CNTs) in a vacuum oven at 55°C for 4 h, 2 g of carbon nanotubes were weighed and added to a beaker containing 150 mL of concentrated nitric acid with a mass fraction of 69.2%. The mixture was sonicated for 4 h and then magnetically stirred for 4 h to obtain an acidified carbon nanotube solution. This solution was then filtered using a sand filter. After the nitric acid was removed, the filter cake was washed with deionized water. The filtration process was repeated until the pH of the filtrate was 5. The filter cake was then stored. The filter cake was then dried overnight in a vacuum oven at 50°C. After that, it was manually ground in an agate mortar until there were no large particles. The powder was then transferred to a ball mill and ball-milled in a planetary ball mill for 2 h. The resulting black powder was the acidified carbon nanotubes (ACNTs).

[0086] (2) Preparation of acidified carbon nanotube membrane ACNTP: 23 mg of acidified carbon nanotube material ACNT and 5 mg of polyvinylidene fluoride (PVDF) binder were magnetically stirred with 25 mL of N-methylpyrrolidone (NMP) as solvent to make the solution uniformly mixed. A polypropylene PP membrane (Celgard 2325) was laid flat on the surface of a sand filter. After wetting the membrane surface with ethanol, 10 mL of N-methylpyrrolidone (NMP) was added. 3 mL of the solution was pipetted into the sand filter for pump filtration. Then the membrane was placed in a vacuum dryer at 40 °C for 10 h to remove the solvent residue. The membrane prepared above was cut into membranes with a diameter of 16 mm to finally obtain acidified carbon nanotube membrane ACNTP.

[0087] (3) Preparation of sulfur / carbon cathode: 7.3 g of sublimed sulfur and 2.8 g of acidified carbon nanotube (ACNT) composite material were weighed into a ball mill jar and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 153 °C for 13 h to obtain sulfur / acidified carbon nanotube composite material S@ACNT. 1.75 g of S@ACNT, 0.25 g of Super P and 0.25 g of PVDF were mixed with 3 mL of N-methylpyrrolidone (NMP) as solvent and ball milled to make a slurry. The ball milling speed was 330 rpm and the ball milling time was 3 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 55 °C for 10 h to dry. The positive electrode prepared above is cut into a 12 mm diameter electrode sheet as the positive electrode, and a lithium sheet is used as the negative electrode.

[0088] (4) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) lithium salt and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was added to assemble a coin cell.

[0089] (5) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is an acidified carbon nanotube membrane ACNTP, and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0090] Comparative Example 1

[0091] A method for fabricating a lithium-sulfur battery based on S@CNT cathode assembly, the specific steps of which are as follows:

[0092] (1) Preparation of sulfur / carbon cathode: 7.5 g of sublimed sulfur and 2.5 g of carbon nanotube (CNT) composite material were weighed into a ball mill jar at a mass ratio of 75:25 and ball milled in a planetary ball mill for 3 h to obtain a light yellow powder. The obtained powder was transferred to a polytetrafluoroethylene (PTFE) reactor and placed in an oven at 155 °C for 12 h to obtain sulfur / carbon nanotube composite material S@CNT. 2 g of S@CNT, 0.25 g of Super P and 0.25 g of PVDF were mixed and ball milled with 3 mL of N-methylpyrrolidone (NMP) as solvent to make a slurry. The ball milling speed was 350 rpm and the ball milling time was 4 h. The uniformly mixed slurry was coated onto a carbon-coated aluminum foil current collector and then transferred to a vacuum oven at 60 °C for 12 h to dry. The positive electrode prepared above is cut into a 12 mm diameter sheet to serve as the positive electrode, and a lithium sheet is used as the negative electrode.

[0093] (2) Preparation of electrolyte: The electrolyte containing 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1% lithium nitrate in 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) (volume ratio 1:1) solvent was assembled into a coin cell.

[0094] (3) Assembly of lithium-sulfur battery: The positive electrode is a sulfur / carbon electrode (12 mm in diameter), the negative electrode is a lithium metal sheet, the separator is a polypropylene PP membrane (Celgard 2325), and the electrolyte is 1M LiTFSI + 1% LiNO3 DOL / DME (1:1 v / v).

[0095] Example of implementation effect 1

[0096] The electrochemical performance of the acidified carbon nanotubes prepared in Example 3 and the carbon nanotubes prepared in Comparative Example 1 was tested, as follows:

[0097] To verify the electrochemical performance of acidified carbon nanotubes in the battery, scanning electron microscopy (SEM) images of the electrodes before and after 100 cycles were used. Figure 4 As shown, before cycling, the electrode surface is dense and smooth with almost no cracks, indicating a strong bond between the acidified carbon nanotubes and the active material. After cycling, the electrode surface structure becomes more porous, with a few cracks and a porous electrode structure. This porous structure not only accelerates electron and ion transport and promotes redox reaction kinetics, but also effectively copes with electrode volume expansion, providing support for high rate and high sulfur loading, thereby improving battery capacity and cycle stability.

[0098] Example 2 of implementation results

[0099] The battery performance of the batteries assembled in Examples 1-5 and Comparative Example 1 was tested, as follows:

[0100] Long-cycle performance testing: The long-cycle stability of the sulfur cathode was obtained based on the specific capacity after continuous charge and discharge at the same current density. After battery assembly, the batteries were placed in a constant temperature chamber at 30 ℃ for 10-12 h to allow the electrolyte to fully wet the positive and negative electrodes and separator inside the battery. After resting, all batteries were first pre-cycled 3 times at a current density of 0.1 C to activate the batteries, and then the current density was converted to the required level for continued long-cycle performance testing. The test temperature was 30 ℃, the operating voltage range was 1.7-2.8V, and the standard specific capacity parameter was 1675 mAh / g.

[0101] The results are as follows Figure 5 As shown, the battery assembled with carbon nanotubes had an initial discharge capacity of 1111 mAh / g, which decreased to 412 mAh / g after 300 cycles at a current density of 0.3 C. The batteries assembled with acidified carbon nanotubes prepared in Examples 1-4 all showed significantly improved discharge capacities. Specifically, Example 3 had an initial discharge capacity of 1570 mAh / g, which decreased to 537 mAh / g after 300 cycles at a current density of 0.3 C. The battery assembled with the acidified carbon nanotube separator prepared in Example 5 had an initial discharge capacity of 1593 mAh / g, and maintained a discharge capacity of 659 mAh / g after 300 cycles at a current density of 0.3 C, demonstrating a significant improvement in long-cycle performance.

[0102] Rate testing: Battery rate performance is obtained based on the specific capacity under continuous charge and discharge at different current densities. After assembly, the batteries are placed in a constant temperature chamber at 30°C for 10-12 hours to allow the electrolyte to fully wet the positive and negative electrodes and the separator. After resting, all batteries are first activated by 5 cycles at a current density of 0.1 C, and then the required current densities are used for further rate performance testing. The test temperature is 30°C, the operating voltage range is 1.7-2.8 V, and the standard specific capacity parameter is 1675 mAh / g.

[0103] Figure 6 The figure shows the rate tests of the batteries assembled in Examples 1-5 and Comparative Example 1. The performance tests at different rates (0.1 C, 0.3 C, 0.5 C, 1 C, 2 C, 3 C) show that the ACNTP prepared in Example 5 has the best rate performance, followed by the ACNT-3 prepared in Example 3. Even at a rate of 3 C, ACNTP can still maintain a specific capacity of 750 mAh / g. After rate cycling, it can still maintain a specific capacity of 1265 mAh / g when returning to a rate of 0.1 C, which shows good rate performance.

[0104] Example of implementation effect 3

[0105] Battery performance tests were conducted on the batteries assembled in Examples 3 and 5, as well as Comparative Example 1, as follows:

[0106] Electrochemical impedance spectroscopy (EIS) tests the electrodes by applying a sinusoidal voltage (current) disturbance, controlling the voltage (current) of the electrodes to alternate according to a sinusoidal pattern. Because it can measure up to 10... -2 Hz~10 6 The spectrum is generated within a frequency range of Hz, so compared to other conventional electrochemical characterization methods, this test can obtain information on electrochemical kinetics and electrode interface structure. After the battery is assembled, impedance testing is performed on an electrochemical workstation, and the measured impedance is the impedance before cycling. After the test, the battery is removed, and a long-cycle test is performed at a current density of 0.1 C. After 50 cycles, the test ends, and impedance testing is performed on the electrochemical workstation, and the measured impedance is the impedance after cycling.

[0107] Figure 7 Electrochemical impedance spectroscopy (EIT) was measured before and after cycling for Examples 3, 5, and Comparative Example 1. The impedance spectra all consisted of a semicircle in the high-frequency region and a straight line in the low-frequency region. The semicircle in the high-frequency region represents the charge transfer impedance (Rct) inside the electrode, while the straight line in the low-frequency region represents the Warburg diffusion impedance, which is related to the diffusion coefficient of ions on the electrode surface. The charge transfer resistances of the three materials were 66.43 Ω, 27.06 Ω, and 6.11 Ω, respectively. ACNTP exhibited a smaller charge transfer resistance, indicating that this material possesses better conductivity and a shorter migration path, implying faster Li... + Migration and electron diffusion rates can reduce electrode polarization and improve the electrochemical cycle stability of the battery.

[0108] Cyclic voltammetry (CV) is used to characterize the redox kinetics and reversibility of the sulfur cathode at different voltages. Based on the current versus voltage curves during charge and discharge, information such as the number of redox peaks, potential, current intensity, and peak spacing are analyzed to determine the degree of reversibility and reaction mechanism of the redox reaction. After the battery is assembled, it is tested on a CV electrochemical workstation at room temperature, with a voltage range of 1.7–2.8 V and a scan rate of 0.1 mV / s.

[0109] Figure 8 The CV curves of Examples 3, 5, and Comparative Example 1 were measured. The high voltage reduction peak at 2.25 V indicates that elemental sulfur was first reduced to long-chain lithium polysulfides (Li₂S₂).x The low-voltage reduction peak at 2.02 V (where x ≤ 8) corresponds to the further reduction of long-chain lithium polysulfides to solid-state Li₂S₂ / Li₂S. Simultaneously, the oxidation peak at 2.38 V corresponds to the re-oxidation of Li₂S₂ / Li₂S to elemental sulfur. Compared to CNT and ACNT, the CV curve of ACNTP shows a higher current density and a smaller potential difference (ΔE = 0.300 V), indicating superior redox conversion kinetics, which is beneficial for reducing battery polarization and improving the battery's electrochemical performance.

[0110] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for treating acidified carbon nanotubes, characterized in that, Includes the following steps: (1) After drying the carbon nanotubes, add them to the nitric acid solution and stir ultrasonically to obtain an acidified carbon nanotube solution; (2) The solution obtained in step (1) is filtered by suction. The precipitate is washed and filtered until the pH of the filtrate is 5-6, and the desired filter cake is obtained. (3) After drying the filter cake obtained in step (2), it is ground to obtain acidified carbon nanotubes.

2. The method for treating acidified carbon nanotubes according to claim 1, characterized in that: The drying in step (1) is vacuum drying at a temperature of 50-60℃ for 4-6 h; the solid-liquid ratio of the carbon nanotubes and nitric acid is 0.25-3:50-200 g / mL, and the mass percentage of the nitric acid solution is 68-69.2%; the ultrasonication time is 2-4 h, and the stirring time is 2-4 h.

3. The method for treating acidified carbon nanotubes according to claim 2, characterized in that: The drying in step (3) is vacuum drying at a temperature of 50-60℃; the grinding is done manually first, and then ball milled in a ball mill for 2-3 hours.

4. Acidified carbon nanotubes obtained by the processing method according to any one of claims 1-3.

5. The application of the acidified carbon nanotubes according to claim 4 in the positive electrode of a lithium-sulfur battery.

6. A positive electrode sheet for a lithium-sulfur battery, characterized in that, The positive electrode active material comprising the acidified carbon nanotubes as described in claim 4 is prepared by a method comprising the following steps: (1) After mixing sublimed sulfur and acidified carbon nanotubes, S@ACNT composite material was obtained by melt-infusion sulfur. (2) After mixing S@ACNT composite material, conductive carbon black and PVDF, add solvent and ball mill twice. The resulting slurry is coated on carbon-coated aluminum foil current collector and dried to obtain lithium-sulfur battery positive electrode sheet.

7. The lithium-sulfur battery positive electrode sheet according to claim 6, characterized in that: The mass ratio of sublimed sulfur to acidified carbon nanotubes in step (1) is 70-75:30-25; the molten sulfur filling is carried out under vacuum or inert atmosphere, the heating temperature is 150-155℃, and the holding time is 12-14 h. In step (2), the mass ratio of S@ACNT composite material, conductive carbon black and PVDF is 7-8:1-2:1; the solvent is N-methylpyrrolidone; the rotation speed of the secondary ball mill is 300-350 rpm and the time is 3-4 h; the drying is vacuum drying at a temperature of 50-60℃ and a time of 10-12 h.

8. The application of the acidified carbon nanotubes according to claim 4 in the preparation of lithium-sulfur battery separators.

9. A lithium-sulfur battery separator, characterized in that, The lithium-sulfur battery separator comprises a polypropylene base film and a functional coating applied to its surface. The functional coating comprises the acidified carbon nanotubes and binder as described in claim 4. The preparation method of the lithium-sulfur battery separator includes the following steps: dissolving acidified carbon nanotubes and PVDF in a solvent and stirring evenly to obtain a mixed slurry; coating the slurry onto the surface of the polypropylene base film by vacuum filtration; and drying to obtain the lithium-sulfur battery separator.

10. The lithium-sulfur battery separator according to claim 9, characterized in that: The mass ratio of acidified carbon nanotubes to PVDF is 4-5:1, and the solvent is N-methylpyrrolidone; the carbon nanotube loading on the lithium-sulfur battery separator is 0.1-0.3 mg / cm³. 2 The drying process is vacuum drying, with a temperature of 40-60℃ and a time of 10-12 hours.

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