A composite carbon nanotube material and preparation method and its application in batteries

Abstract

The present invention discloses a composite carbon nanotube material containing sulfur and carbon nanotubes. The carbon nanotubes are curved or spiral in shape, have transition metal nanoparticles at their ends, and are hollow. The composite carbon nanotube material can be used in lithium-sulfur batteries to mitigate sulfur expansion, promote uniform sulfur distribution, reduce polysulfide shuttling, and improve the capacity, rate capability, and cycle stability of lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a composite carbon nanotube material and a preparation method thereof, and application thereof in batteries. Background Art

[0002] Carbon nanotubes, as one-dimensional nanomaterials, are lightweight, feature a perfectly connected hexagonal structure, and possess numerous exceptional mechanical, electrical, and chemical properties. Carbon nanotubes, when constructed as a single sheet, are also known as single-walled nanotubes (SWNTs); those composed of multiple concentric sheets are also known as multi-walled nanotubes (MWNTs). In recent years, with the deepening of research on carbon nanotubes and nanomaterials, their broad application prospects have continued to emerge, particularly as additives in lithium-ion batteries to enhance the conductivity of electrodes (both anodes and cathodes).

[0003] In recent years, lithium-sulfur batteries (Li-S batteries) have become a research hotspot, boasting a theoretical specific capacity of up to 1675 mAh / g and a theoretical specific energy of 2600 Wh / kg (when combined with lithium metal). Their theoretical specific capacity is approximately an order of magnitude higher than that of current commercial lithium batteries. Furthermore, sulfur, with its abundant reserves, non-toxicity, and environmental friendliness, holds great potential for future applications. However, at current technological levels, Li-S batteries still face numerous technical challenges and shortcomings, including low Coulombic efficiency, poor cycling performance, and high self-discharge rates. In particular, sulfur in the cathode of typical Li-S batteries exhibits low electronic conductivity and a high expansion coefficient, which susceptibility to the shuttling effect of soluble polysulfides. This is the case with the diffusion of soluble long-chain polysulfides to the anode, where they are reduced to insoluble Li2S2 / Li2S and short-chain polysulfides. These short-chain polysulfides, driven by a concentration gradient, migrate back to the cathode, continuing their cyclical oxidation process by losing electrons and oxidizing to long-chain polysulfides. This is the primary cause of capacity decay in Li-S batteries. Summary of the Invention

[0004] The present invention addresses the problems in the prior art and discloses a composite carbon nanotube material. The present invention is achieved through the following technical solutions:

[0005] The present invention provides a composite carbon nanotube material. The composite carbon nanotube contains sulfur and carbon nanotube material. The carbon nanotube is curved or spiral. Transition metal nanoparticles are present at the end of the carbon nanotube. The carbon nanotube is a hollow structure.

[0006] The inventors have discovered that the morphological characteristics of hollow carbon nanotubes can be adjusted by adjusting the helicity of the carbon nanotubes to achieve different active sites and specific surface areas, which is beneficial to improving the conductivity and electrolyte absorption rate of the carbon nanotube material; the carbon nanotube diameter is evenly distributed, which can avoid material agglomeration and facilitate dispersion, thereby improving the resistance, rate performance and cycle performance of the battery. The transition metal nanoparticles at the ports of the carbon nanotube material are the nucleation sites for the formation of the carbon nanotube material of the present invention. During the in-situ synthesis of carbon nanotubes, the morphology of the carbon nanotubes can be controlled to be straight, curved, or spiral to varying degrees by controlling the synthesis conditions. The pitch of the spiral is relatively uniform, and the helical angle varies with the number of spiral turns, thereby maintaining the stability of the carbon nanotubes.

[0007] The inventors further studied the compounding of the carbon nanotubes with sulfur to form a composite carbon nanotube material. First, due to the van der Waals force between carbon nanotubes, the expansion of sulfur can be alleviated while improving the conductivity of the material. In particular, carbon nanotubes with an appropriate degree of helicity can further promote the migration of lithium ions. Second, the doped transition metal elements easily combine with polysulfide anions to form metal-sulfur bonds, resulting in the weakening of SS bonds, showing effective metal-sulfur bonding, which plays a role in adsorbing and fixing sulfur atoms, avoiding sulfur agglomeration and uneven dispersion, and reducing the generation of polysulfides. Finally, doping the material with nitrogen elements can not only help improve the degree of helicity of carbon nanotubes, improve conductivity and battery coulombic efficiency, but also form N-Li bonds with lithium polysulfide, achieving adsorption and binding of lithium polysulfide under synergistic action, reducing the shuttle effect of lithium polysulfide.

[0008] As a further solution, the diameter of the carbon nanotubes is 5 nm to 100 nm, which is beneficial for improving the electrical properties of the carbon nanotubes and also facilitates the bending of the carbon nanotube material.

[0009] As a further solution, the diameter of the carbon nanotubes is 20 nm-40 nm.

[0010] As a further solution, the pitch of the carbon nanotubes is 90nm-100nm. A suitable pitch not only ensures the stability of the carbon nanotubes after curling, but also prevents the conductive material from agglomerating and becoming difficult to disperse, which affects the battery's internal resistance. A pitch that is too large prevents the conductive material from further reducing the distance between particles after mixing with the active material, resulting in increased binder usage and increased battery internal resistance.

[0011] In the present invention, the pitch of the carbon nanotube refers to the straight-line distance between two points corresponding to the center of the plane of the helical coil of two adjacent helices.

[0012] As a further solution, the helix angle of the carbon nanotubes is 30°-120°.

[0013] The helical angle of the carbon nanotube in the present invention refers to the bending angle formed when the nearly straight carbon nanotube deviates from the straight direction and begins to bend during its growth process, starting from the tube end. The spiral bending angles that appear in sequence can be expressed as θ1, θ2, θ3...

[0014] As a further solution, the aspect ratio of the carbon nanotubes is 3 to 1000. If the aspect ratio is too small, the performance is similar to that of spherical granular conductive agents, and the performance of the conductive battery is reduced; if the aspect ratio is too large, the nanotube structure is easily unstable.

[0015] As a further solution, the ratio of the particle size of the transition metal nanoparticles to the diameter of the carbon nanotube material is (1-1.5):(1-1.5). The similar diameters of the two allow the transition metal particles to be stably connected to the ends of the carbon nanotubes, promoting the formation of a longer spiral morphology through stable chemical bonding.

[0016] As a further embodiment, the transition metal nanoparticles include one or more transition metals.

[0017] As a further embodiment, the transition metal includes one or more of iron, cobalt, nickel, copper, and manganese.

[0018] As a further embodiment, the transition metal is iron. Iron is preferred as the transition metal because it is better than other metals such as cobalt and nickel, as iron can bind to four nitrogen atoms to form covalent bonds, while the latter can bind to two nitrogen atoms at most.

[0019] A second aspect of the present invention is to provide an additive material suitable for batteries, comprising the composite carbon nanotube material.

[0020] A third aspect of the present invention is to provide a method for preparing a composite carbon nanotube material, comprising the following steps:

[0021] S1: dissolving the carbon source, nitrogen source, and transition metal salt, mixing them uniformly, drying them, and calcining them under inert gas to obtain A1 material;

[0022] S2: mixing the A1 material obtained in S1 with an electrophilic reagent and a protonic acid solution and drying the mixture to obtain the A2 material;

[0023] S3: uniformly mixing the A2 material obtained in S2 with a sulfur source, and heating the mixture at a constant temperature of 150° C. or above under an inert gas atmosphere for 6-15 hours to obtain a composite carbon nanotube material.

[0024] In the method of the present invention, a transition metal salt, a carbon source, and a nitrogen source are calcined at high temperature, making it easier to obtain non-helical, relatively short, straight carbon nanotubes (CNTs). Subsequently, treatment with an electrophilic reagent and a protonic acid facilitates the production of curved or helical CNT materials. During high-temperature calcination, the carbon source reduces the transition metal to form transition metal nanoparticles. The cyclic nitrogen source provides cyclic carbon-nitrogen fragments, while the non-cyclic nitrogen source also chelates at high temperatures to form cyclic carbon-nitrogen fragments. Subsequently, the transition metal and carbon-nitrogen form coordination bonds, adsorbing more carbon-nitrogen fragments that migrate toward the transition metal nanoparticles and gradually grow. Other irregularly structured fragments or particles may also be formed during this process. In step S2, the electrophilic reagent and protonic acid mixed treatment further breaks and reorganizes the unformed particles, chains, and irregular cyclic fragments, leading to the stable formation of nanotubes with a certain degree of helicity, or even to the formation of helical shapes with varying degrees of stability. In addition, in step S2, the defect sites in the tube are filled with fragments, and the metal complexation effect causes the originally separated tubes to be combined together, making the tubes longer and more densely packed and uniform in shape. The comprehensive coordination of multiple bond energies causes the carbon nanotubes to cross-link and form a stable structure. In step S3, the A2 material is mixed with a sulfur source and heated to obtain a composite carbon nanotube material. During this process, the spiral carbon nanotubes provide a larger specific surface area, pores and mechanical properties, which slows down the expansion of sulfur under the wrapping of the uniformly morphological carbon nanotubes. The adsorption of transition metals on sulfur can fix sulfur while preventing sulfur agglomeration, and nitrogen doping can adsorb polysulfides formed during the later charge and discharge process of the battery, preventing the polysulfides from moving to the negative electrode side, thereby improving the performance advantages of the composite carbon nanotube material in the battery.

[0025] As a further solution, the transition metal salt in S1 includes at least one of transition metal chloride, transition metal nitrate, transition metal acetate, and transition metal fluoride.

[0026] As a further solution, the carbon source in S1 includes at least one of a biomass carbon source, carbon powder, graphene and activated carbon.

[0027] As a further solution, the nitrogen source in S1 includes one or more nitrogen-containing heterocycles, guanidine salts and their derivatives, preferably a nitrogen source that can form a six-membered ring, such as melamine, to promote the formation of more pyridine-type nitrogen, adsorb polysulfides, inhibit the shuttling of polysulfides, and enhance battery performance.

[0028] As a further embodiment, the electrophilic reagent solution in S2 includes at least one of aluminum chloride, aluminum sulfate, boron trifluoride, sulfur trioxide, ferric bromide, titanium bromide, tin chloride, zinc chloride, ferric chloride, and molybdenum pentachloride.

[0029] As a further embodiment, the protonic acid solution in S2 includes at least one of formic acid, trifluoroacetic acid, oxalic acid, hydrochloric acid, sulfuric acid, citric acid, camphorsulfonic acid, and benzoic acid. Without the combined action of an electrophilic reagent and a protonic acid, relatively short, linear carbon nanotubes are more likely to form. Adding S2 increases the helicity of the carbon nanotubes. Each turn of the highly helical carbon nanotubes forms a new topological active site, improving conductivity, specific surface area, and porosity.

[0030] As a further solution, S1 may further include other non-metallic sources in addition to being uniformly mixed with the carbon and nitrogen sources. These other non-metallic sources may be one or more of a phosphorus and fluorine source. P and F readily form complex bonds with transition metals, further increasing active sites, promoting nanotube formation, and increasing material porosity. They can also form sulfur-containing chemical bonds with polysulfide anions, further reducing polysulfide formation and comprehensively improving ion transport, conductivity, and safety of the interface layer.

[0031] As a further embodiment, the phosphorus source is an organic or inorganic substance containing phosphorus, and the fluorine source is an organic or inorganic substance containing fluorine. Typically, but not limitatively, for example, the phosphorus source is at least one of triphenylphosphine, tetraphenylphosphine bromide, 1-butyl-3-methylimidazolium sodium hexafluorophosphate, phosphine, sodium phosphate, sodium phosphite, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphite, and phosphoric acid; and the fluorine source is at least one of lithium fluoride, ammonium fluoride, sodium fluoride, potassium fluoride, aluminum fluoride, magnesium fluoride, calcium fluoride, tetrabutylammonium fluoride, ammonium fluoroborate, tetrabutylammonium fluoroborate, and polyvinylidene fluoride.

[0032] As a further solution, the molar ratio of carbon, nitrogen, other non-metallic and transition metal elements in the carbon source, nitrogen source, other non-metallic source and transition metal salt is (5-20):(50-200):(5-50):(1-5).

[0033] As a further embodiment, the molar ratio of carbon, nitrogen, other non-metallic elements, and transition metal elements in the carbon source, nitrogen source, other non-metallic source, and transition metal salt is (5-20):(50-120):(5-50):1. At this ratio, the generated carbon nanotubes have a suitable helical structure, good ionic and electronic conductivity at the negative electrode interface, and excellent overall performance. When the relative ratio of nitrogen is less than 50, the electronic conductivity is insufficient and the nanotubes tend to be more linear. When the relative ratio exceeds 120, the tubes are too long and dense, the porosity decreases, and lithium ion transport begins to be gradually inhibited. The electronic conductivity is better, but the ionic conductivity decreases slightly.

[0034] As a further solution, the mass ratio of the A1 material, the electrophilic reagent solution and the protonic acid solution in S2 is 1:(5-10):(10-200).

[0035] As a further solution, the calcination temperature in S1 is 500° C.-800° C. and the calcination time is 2 h-10 h. If the calcination temperature is too high, the metal-nitrogen bond will be destroyed and the performance will be degraded.

[0036] As a further solution, the calcination temperature in S1 is maintained at 550°C for 2 hours, and then the temperature is further increased to 750°C and maintained for 2 hours. Calcination at staged temperatures can enable the nucleation reaction and tube growth reaction to proceed more fully.

[0037] As a further solution, the treatment temperature in S2 is 10°C-100°C, and the reaction time is 1h-20h.

[0038] As a further solution, the sulfur source in S3 includes one or more of lithium sulfide, sodium sulfide, potassium sulfide, calcium sulfide, tin sulfide, aluminum sulfide, magnesium sulfide, iron sulfide, and elemental sulfur; preferably, the sulfur source is elemental sulfur, and the elemental sulfur is one or more of sublimated sulfur, crystalline sulfur, nanosulfur, or molten sulfur; more preferably, the sulfur source is sublimated sulfur.

[0039] As a further solution, the mass ratio of the A2 material and the sulfur source (calculated as sulfur S) in S3 is (1-9):(1-9); preferably, the mass ratio is (2-5):(5-8), and most preferably, the mass ratio is 3:7.

[0040] The fourth aspect of the present invention is to provide a positive electrode sheet, an electrode assembly, a battery cell, an electrochemical device or an electrical equipment having the sulfur-composite carbon nanotubes and the sulfur-composite carbon nanotubes prepared by the preparation method.

[0041] As a further embodiment, the battery cell is a lithium battery, sodium battery, potassium battery, zinc battery, or magnesium battery. The battery cell includes a positive electrode, a negative electrode, a separator, and the novel electropolymerizable ionic liquid. The negative electrode may comprise an alkali metal, an alkaline earth metal, a carbon material containing carbon as a constituent element, a silicon material containing silicon as a constituent element, a tin material containing tin as a constituent element, a carbon-silicon composite material containing carbon as a constituent element, or a lithium-containing transition metal nitride. The separator may be selected from polyethylene, polypropylene, PP / PE, PP / PE / PP separators, ceramic separators, and rubber-coated separators.

[0042] As a further solution, the electrochemical device can be used in terminal consumer products, and the terminal consumer products applied for include but are not limited to mobile phones, laptops, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, and portable printers.

[0043] As a further embodiment, the electrochemical device can be used in electrical equipment, including large-scale electrical equipment and small-scale electrical equipment, wherein small-scale electrical equipment includes end-consumer products, wearable electronic devices, or mobile electronic devices; large-scale electrical equipment includes transportation electrical equipment. Transportation electrical equipment includes, but is not limited to, automobiles, motorcycles, power-assisted bicycles, buses, subways, high-speed trains, airplanes, and ships; wearable electronic devices or mobile electronic devices include, but are not limited to, stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, drones, motors, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0044] The characteristics and beneficial effects of the present invention are:

[0045] (1) The carbon nanotube material of the present invention can adjust its helical structure. The straight, curved or helical carbon nanotubes synthesized in situ can exert different electrical properties. The transition metal is distributed at the tube ends and the tube diameter is evenly distributed, which is beneficial to improving the conductivity and lithium ion transport performance of the material.

[0046] (2) The carbon nanotubes and sulfur compound in the present invention greatly improve the coulombic efficiency and cycle performance of lithium-sulfur batteries and reduce the self-discharge rate: First, due to the van der Waals force between carbon nanotubes, the expansion of sulfur can be alleviated while improving the conductivity of the material. In particular, carbon nanotubes with a suitable degree of helicity can further promote the migration of lithium ions; secondly, the doped transition metal elements easily combine with polysulfide anions to form metal-sulfur bonds, resulting in the weakening of SS bonds, showing effective metal-sulfur bonding, playing a role in adsorbing and fixing sulfur atoms, avoiding sulfur agglomeration and uneven dispersion, and reducing the generation of polysulfides; finally, the doping of nitrogen elements in the material can not only help improve the degree of helicity of carbon nanotubes, improve conductivity and battery coulombic efficiency, but also form N-Li bonds with lithium polysulfide, achieving adsorption and binding of lithium polysulfide under synergistic action, reducing the shuttle effect of lithium polysulfide. With the mutual cooperation of carbon nanotube structure and doping elements, the high performance of lithium-sulfur batteries can be achieved.

[0047] (3) The method for preparing sulfur-composite carbon nanotubes in the present invention is simple, efficient, and conducive to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments or descriptions 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.

[0049] Figure 1 Transmission electron microscopy (TEM) images of carbon nanotube materials (500 nm, 100 nm) corresponding to Example 1 and high-magnification (2 nm) transmission electron microscopy (HTEM) images of the carbon nanotube ends corresponding to Example 1;

[0050] Figure 2 a is a transmission electron microscopy (TEM) image of the carbon nanotube material corresponding to Example 7; Figure 2 b is a transmission electron microscopy (TEM) image of the carbon nanotube material corresponding to Example 9;

[0051] Figure 3 This is a scanning electron microscope (SEM) image of the carbon nanotube material corresponding to Comparative Example 1;

[0052] Figure 4 This is a transmission electron microscope (TEM) image of the carbon nanotube material corresponding to Comparative Example 2;

[0053] Figure 5 The XPS test results corresponding to nitrogen in carbon nanotube materials are shown in FIG1, wherein ① corresponds to Example 1; ② corresponds to Example 6; ③ corresponds to Example 4;

[0054] Figure 6 a is the SEM-EDS distribution diagram of sulfur element in the composite carbon nanotube material corresponding to Example 1; Figure 6 b is the SEM-EDS distribution diagram of sulfur element in the composite carbon material corresponding to Comparative Example 1; Figure 6 c is the SEM-EDS distribution diagram of sulfur element in the positive electrode sheet after 50 cycles of the battery corresponding to Example 1; Figure 6 d is the SEM-EDS distribution diagram of sulfur element in the positive electrode sheet after 50 cycles of the battery corresponding to Comparative Example 1. DETAILED DESCRIPTION

[0055] To facilitate understanding of the composite carbon nanotubes of the present invention, the composite carbon nanotubes of the present invention will be described in more detail below, with examples of the present invention provided. However, the scope of the present invention is not limited thereby.

[0056] Example 1:

[0057] According to the molar ratio of carbon, nitrogen and transition metal elements of 10:100:1, a certain amount of sucrose, dicyandiamide and ferric nitrate nonahydrate were weighed and dissolved in anhydrous ethanol, and then dried to obtain a solid powder. The solid powder was then transferred to a tube furnace and calcined at 550°C for 2 hours under a nitrogen atmosphere. The temperature was continued to rise and maintained at 750°C for 2 hours. Aluminum chloride solution and trifluoroacetic acid solution (mass ratio 1:10:200) were added to the precursor material obtained after cooling to room temperature and mixed evenly. After heating at 45°C for 20 hours, the carbon nanotube material was filtered and dried.

[0058] The carbon nanotube material and sublimed sulfur were mixed (by mass ratio of 3:7) and heated at 160° C. for 6 hours under inert atmosphere to obtain the final composite carbon nanotube material.

[0059] Example 2:

[0060] The ferric nitrate nonahydrate in Example 1 was replaced with cobalt nitrate in an equal molar amount of metal, and the other conditions were the same.

[0061] Example 3:

[0062] The ferric nitrate nonahydrate in Example 1 was replaced with nickel nitrate in an equal molar amount of metal, and the other conditions were the same.

[0063] Example 4:

[0064] The dicyandiamide in Example 1 was replaced by melamine, and the other procedures were the same.

[0065] Example 5:

[0066] The molar ratio of carbon, nitrogen and transition metal elements in Example 1 was changed from 10:100:1 to 20:50:5, and the other components were the same.

[0067] Example 6:

[0068] The molar ratio of carbon, nitrogen and transition metal elements in Example 1 was changed from 10:100:1 to 20:200:1, and the other ratios were the same.

[0069] Example 7:

[0070] The mass ratio of the precursor material, aluminum chloride solution, and trifluoroacetic acid solution in Example 1 was changed from 1:10:200 to 1:5:200, and the other ratios were the same.

[0071] Example 8:

[0072] In Example 1, the trifluoroacetic acid was replaced with dilute hydrochloric acid, and the other steps were the same.

[0073] Example 9:

[0074] The heating at 45° C. for 20 h in Example 1 was replaced by heating at 45° C. for 10 h, with the other steps being identical.

[0075] Example 10:

[0076] Triphenylphosphine was additionally added to Example 1, and the molar ratio of carbon, nitrogen, phosphorus, and transition metal elements was 10:100:5:1. Other conditions were the same.

[0077] Example 11:

[0078] Tetrabutylammonium fluoride was additionally added to Example 1, and the molar ratio of carbon, nitrogen, fluorine, and transition metal elements was 10:100:5:1. Other conditions were the same.

[0079] Comparative Example 1:

[0080] The ferric nitrate nonahydrate was removed from Example 1, and the other parts were the same.

[0081] Comparative Example 2:

[0082] The aluminum chloride was removed from Example 1, and the other parts were identical.

[0083] Comparative Example 3:

[0084] The molar ratio of carbon, nitrogen and metal elements in Example 1 was changed from 10:100:1 to 20:40:5, and the other components were the same.

[0085] Comparative Example 4:

[0086] Conventional sulfur-carbon composite materials are used.

[0087] Battery preparation:

[0088] The above-mentioned composite carbon nanotube material, binder polyvinylidene fluoride (PVDF), and conductive carbon black were mixed in NMP at a mass ratio of 8:1:1 to prepare a slurry, which was coated on aluminum foil and dried to obtain a positive electrode sheet. The positive electrode sheet was cut into small discs and assembled with lithium metal to form a button battery. The electrolyte was a 1,3-dioxolane (DOL) and dimethoxymethane (DME) solution (volume ratio 1:1). Constant current charge and discharge tests were carried out at room temperature.

[0089] 0.1C / 0.1C represents the charge and discharge procedure of 0.1C charging and 0.1C discharging; 3C / 0.3C first-cycle discharge capacity retention rate represents the retention rate of the first-cycle discharge capacity of a charge and discharge procedure of 3C charging and 0.3C discharging relative to the first-cycle discharge capacity of 0.3C / 0.3C, which is used to test the rate performance of the battery under 3C; the corresponding capacity retention rate of the Nth cycle is calculated with the 0.3C first-cycle capacity retention rate as 100%.

[0090] Table 1 Test results of the embodiments of the present invention and comparative examples

[0091]

[0092] The electrical performance of the materials obtained in Examples 1 to 11 of the present invention in the battery is significantly better than that of the batteries in Comparative Examples 1 to 4. The main reason is that, firstly, the carbon nanotube material in the present invention has a curled, curved or spiral morphology, such as Figure 1-Figure 2 As shown. Since there is a van der Waals force between carbon nanotubes, the expansion of sulfur can be alleviated while improving the conductivity of the material. In particular, carbon nanotubes with a suitable degree of helicity can further promote the migration of lithium ions. Secondly, the doped transition metal elements easily combine with polysulfide anions to form metal-sulfur bonds, resulting in the weakening of SS bonds, showing effective metal-sulfur bonding, which plays a role in adsorbing and fixing sulfur atoms, avoiding sulfur agglomeration and uneven dispersion, and reducing the generation of polysulfides. Finally, doping nitrogen elements in the material can not only help to improve the degree of helicity of carbon nanotubes, improve conductivity and battery coulombic efficiency, but also form N-Li bonds with lithium polysulfide, and achieve adsorption and binding of lithium polysulfide under synergistic effect, reducing the shuttle effect of lithium polysulfide. Therefore, Examples 1 to 11 of the present invention have better electrical properties. In Comparative Example 1, because no transition metal salt (such as Figure 3 ), no nucleation sites can be formed at the port, so carbon nanotubes cannot be formed in the end, and the battery performance is greatly reduced. By comparing the distribution of sulfur in the composite carbon nanotube material prepared in Example 1 and Comparative Example 1 ( Figure 6 a and Figure 6 b) It can be seen that the sulfur distribution in the material of Example 1 is significantly more uniform, while the sulfur in the material of Comparative Example 1 is significantly agglomerated, further proving that the helical carbon nanotubes of the composite carbon nanotube material prepared in the embodiment have obvious advantages in terms of sulfur fixation and distribution; Comparison of the sulfur distribution in the positive electrode sheet of the battery assembled with the composite materials prepared in Example 1 and Comparative Example 1 after 50 cycles ( Figure 6 c and Figure 6d) As can be seen, the sulfur distribution in the electrode of Comparative Example 1 is loose and uneven, indicating partial sulfur loss. This also confirms that sulfur may dissolve into the electrolyte in the form of lithium polysulfides during battery charge and discharge, further demonstrating that Example 1 exhibits stronger sulfur adsorption and reduces sulfur detachment from the positive electrode. A comparison of Examples 1 and 3 reveals that the varying bonding abilities between metal particles affect the nucleation rate and anisotropic growth rate of carbon nanotubes. Metal selection further promotes the helical shape of carbon nanotubes. Iron is a preferred transition metal, compared to cobalt and nickel, as iron can form covalent bonds with four nitrogen atoms, while cobalt and nickel typically bind two. This leads to superior battery performance in Example 1. A comparison of Example 1 with Example 4 reveals that a six-membered ring nitrogen source, such as melamine, is preferred, as it promotes the formation of more pyridinic nitrogen, adsorbs polysulfides, inhibits polysulfide shuttling, and enhances battery performance. A comparison of Example 1 with Example 5 reveals that a reduction in nitrogen relative to the transition metal still reduces the generation of active sites, thereby reducing battery performance. By comparing Example 1 with Example 6, it was found that as the amount of nitrogen relative to the transition metal increased, the battery performance decreased. This may be due to the excessive formation of carbon nanotubes, which reduced the porosity of the material and gradually began to inhibit lithium ion transmission. Although the electronic conductivity was better, the ionic conductivity began to decrease, thereby reducing the overall performance of the battery.

[0093] In the method of the present invention, we added an electrophilic reagent solution and a protonic acid solution, and found that the mixed treatment of the electrophilic reagent solution and the protonic acid can further promote the formation of irregular ring fragments such as particles, chains, five-membered or seven-membered rings that are not completely formed into tubes through the breaking and recombination of chemical bonds, so that the carbon nanotubes can form a curved shape with a certain degree of helicity, and even form a helical shape with varying degrees of stability, such as Figure 1 As shown; on the other hand, the defect sites in the carbon nanotubes can be filled with fragments, and the metal complexation effect causes the originally separated tubes to be combined together, making the tubes longer and the morphology more dense and uniform, and the comprehensive coordination of multiple bonds can make the carbon nanotubes cross-linked to form a stable structure, as shown Figure 2 As shown. By comparing Example 1 with Example 7, it is found that reducing the electrophilic reagent solution will lead to poor performance. The electrophilic reagent acts as an inducer to promote the grafting of nanotubes and the integrity of longer structures. Reducing the electrophilic reagent weakens the recombination effect, reduces the tubular helicity, and deteriorates the electrical properties. Figure 2 a. Further comparison of Example 1 and Example 8 shows that the electrical performance of Example 1 is better than that of Example 8. We believe that this is because the fluorine polarity of trifluoroacetic acid is strong, which is conducive to promoting the reorganization of covalent bonds and the formation of multiple helices. Further comparison of Example 1 and Example 9 shows that the electrical performance of Example 1 is better than that of Example 9. The reason may be that after the time of co-treatment with electrophilic reagent and protonic acid is shortened, some fragments have not had time to migrate to form a complete helical tube, such as Figure 2b. The formation of highly helical carbon nanotubes cannot be separated from the electrophilic reagent and the proton acid. We can also verify this through Comparative Example 2. After the electrophilic reagent is removed in Comparative Example 2, it does not interact with the proton acid, which is not conducive to the formation of long helical nanotubes. Figure 4 The reactive nanotubes do not form a stable helical structure, so the active sites are greatly reduced and the battery performance is reduced.

[0094] We also added a phosphorus source and / or a fluorine source during the preparation process. Although phosphorus and fluorine are not necessary for tubular formation, they easily form coordinated bonds with transition metals, further increasing active sites and promoting the formation of denser tubular structures. This can also reduce battery polarization and internal resistance. Comparing Example 1 with Example 10, Example 10 exhibits superior electrical performance to Example 1. This is because phosphorus not only easily forms coordinated bonds with transition metals, but P doping can also form PS bonds, which can increase the effective adsorption of polysulfides without sacrificing lithium ion diffusion. Therefore, the addition of an appropriate amount of phosphorus can comprehensively improve the ion transport and conductivity of the interface layer. By comparing Example 1 with Example 11, we found that the rate performance and cycle performance of the battery of Example 11 were superior to those of Example 1, demonstrating that the addition of fluorine can indeed further improve battery performance.

[0095] Comparisons of Examples 1-11 with Comparative Example 4 reveal that the electrical performance of the batteries of Examples 1-11 is superior to that of Comparative Example 4. This is believed to be because conventional sulfur-carbon composite materials lack nitrogen-doped helical carbon nanotubes, making it difficult to fully utilize the advantages of carbon nanotubes and nitrogen doping for sulfur cathode materials.

[0096] From the XPS test results of carbon nanotube materials, we can see that Figure 5 As shown in Figure 2, the peaks at 398.4 eV, 399.7 eV, 401.1 eV and 402.1 eV indicate that the N in the material exists in the form of pyridinium-N, pyrrole-N, graphite-N and pyridinium-N-oxide, respectively. Figure 5 ①, Comparative Example 1) shows the highest pyrrolic-N and lowest pyridinic-N content, while the transition metal Fe-doped sample has a relatively higher pyridinic nitrogen content ( Figure 5 In ③, Example 4), pyridinic nitrogen has the strongest ability to adsorb polysulfides, mainly because the lone pair of electrons on the pyridinic nitrogen N does not participate in conjugation, and easily forms a coordination bond with the lithium in the polysulfide, thereby inhibiting the shuttling of polysulfides. Co-doping with other non-metallic elements (P, F, etc.) can further enhance the binding energy with polysulfides; and when the degree of graphitization is high, it has a promoting effect on electronic conductivity within a certain range. Example 6 ( Figure 5Although the pyridinic nitrogen content in ② is the highest, it will cause a decrease in graphitic carbon and weaken the electronic conductivity of the material. Therefore, the combined content of pyridinic nitrogen and graphitic nitrogen in Example 4 is conducive to exerting the relatively strongest adsorption of lithium polysulfide and conductive effects, making the battery performance better than that of other examples.

[0097] In summary, the application of the bent or highly helical carbon nanotubes with a certain helicity in the present invention in combination with sulfur in batteries can improve the electrical performance of the batteries.

[0098] It should be noted that 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 in the scope of protection of the present invention.

Claims

1. A method for preparing a composite carbon nanotube material, characterized in that: The steps include: S1: dissolving the carbon source, nitrogen source, and transition metal salt, mixing them uniformly, drying them, and calcining them under inert gas to obtain A1 material; S2: mixing the A1 material obtained in S1 with an electrophilic reagent and a protonic acid solution and drying the mixture to obtain the A2 material; S3: uniformly mixing the A2 material obtained in S2 with a sulfur source, and heating the mixture at a constant temperature of 150° C. or above under an inert gas atmosphere for 6-15 hours to obtain a composite carbon nanotube material; The electrophilic reagent in S2 includes at least one of aluminum chloride, aluminum sulfate, boron trifluoride, sulfur trioxide, ferric bromide, titanium bromide, tin chloride, zinc chloride, ferric chloride, and molybdenum pentachloride; The protonic acid solution in S2 comprises at least one of formic acid, trifluoroacetic acid, oxalic acid, hydrochloric acid, sulfuric acid, citric acid, camphorsulfonic acid, and benzoic acid; The treatment temperature in S2 is 10°C-100°C, and the reaction time is 1h-20h; The composite carbon nanotube material contains sulfur and carbon nanotube material. The carbon nanotube is bent or spiral. Transition metal nanoparticles are present at the ends of the carbon nanotube. The carbon nanotube is a hollow structure.

2. The method for preparing the composite carbon nanotube material according to claim 1, characterized in that: The transition metal salt includes at least one of transition metal chloride, transition metal nitrate, transition metal acetate, and transition metal fluoride; The carbon source in S1 includes at least one of a biomass carbon source, carbon powder, graphene and activated carbon; The nitrogen source in S1 includes one or more of nitrogen-containing heterocycles, guanidine salts and their derivatives; The sulfur source in S3 includes one or more of lithium sulfide, sodium sulfide, potassium sulfide, calcium sulfide, tin sulfide, aluminum sulfide, magnesium sulfide, iron sulfide, and elemental sulfur.

3. The method for preparing the composite carbon nanotube material according to claim 1, wherein: The sulfur source is elemental sulfur, and the elemental sulfur is one or more of sublimated sulfur, crystalline sulfur, nano sulfur or molten sulfur.

4. The method for preparing the composite carbon nanotube material according to claim 1, wherein: The sulfur source is sublimed sulfur.

5. The method for preparing the composite carbon nanotube material according to claim 1, characterized in that: In addition to being uniformly mixed with the carbon source and the nitrogen source, the S1 also includes other non-metallic sources, wherein the other non-metallic sources are one or more of a phosphorus source and a fluorine source; The phosphorus source is an organic or inorganic substance containing phosphorus, and the fluorine source is an organic or inorganic substance containing fluorine; The molar ratio of carbon, nitrogen, other non-metallic and transition metal elements in the carbon source, nitrogen source, other non-metallic source and transition metal salt is (5-20):(50-200):(5-50):(1-5); The mass ratio of the A1 material, the electrophilic reagent solution and the protonic acid solution in S2 is 1:(5-10):(10-200); The mass ratio of the A2 material and the sulfur source calculated as sulfur S in the S3 is (1-9):(1-9).

6. The method for preparing the composite carbon nanotube material according to claim 5, characterized in that: The phosphorus source is at least one of triphenylphosphine, tetraphenylphosphine bromide, 1-butyl-3-methylimidazolium sodium hexafluorophosphate, phosphine, sodium phosphate, sodium phosphite, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphite, and phosphoric acid; the fluorine source is at least one of lithium fluoride, ammonium fluoride, sodium fluoride, potassium fluoride, magnesium fluoride, calcium fluoride, tetrabutylammonium fluoride, ammonium fluoroborate, tetrabutylammonium fluoroborate, and polyvinylidene fluoride.

7. The method for preparing the composite carbon nanotube material according to claim 5, characterized in that: The molar ratio of carbon, nitrogen, other non-metallic and transition metal elements in the carbon source, nitrogen source, other non-metallic sources and transition metal salt is (5-20):(50-120):(5-50):

1.

8. The method for preparing the composite carbon nanotube material according to claim 5, characterized in that: The mass ratio of the A2 material and the sulfur source calculated as sulfur S in the S3 is (2-5):(5-8).

9. The method for preparing the composite carbon nanotube material according to claim 5, characterized in that: The mass ratio of the A2 material and the sulfur source calculated as sulfur S in the S3 is 3:

7.

10. The method for preparing the composite carbon nanotube material according to claim 1, characterized in that: In the S1, the calcination temperature is 500° C.-800° C., and the calcination time is 2 h-10 h.

11. The method for preparing a composite carbon nanotube material according to claim 1, wherein: In the S1, the calcination temperature is maintained at 550° C. for 2 hours, and then the temperature is further increased to 750° C. and maintained for 2 hours.

12. The composite carbon nanotube material obtained by the preparation method according to any one of claims 1 to 11, characterized in that: The diameter of the carbon nanotube is 5nm-100nm.

13. The composite carbon nanotube material according to claim 12, characterized in that: The diameter of the carbon nanotube is 20nm-40nm.

14. The composite carbon nanotube material according to claim 12, characterized in that: The pitch of the carbon nanotubes is 90nm-100nm.

15. The composite carbon nanotube material according to claim 12, characterized in that: The helical angle of the carbon nanotubes is 30°-120°.

16. The composite carbon nanotube material according to claim 12, characterized in that: The aspect ratio of the carbon nanotubes is not less than 3.

17. The composite carbon nanotube material according to claim 12, characterized in that: The ratio of the particle size of the transition metal nanoparticles to the diameter of the carbon nanotubes is (1-1.5):(1-1.5); The transition metal nanoparticles include one or more transition metals.

18. The composite carbon nanotube material according to claim 12, characterized in that: The transition metal includes one or more of iron, cobalt, nickel, copper and manganese.

19. The composite carbon nanotube material according to claim 12, characterized in that: The transition metal is iron.

20. An additive material suitable for batteries, characterized in that: The composite carbon nanotube material comprises the composite carbon nanotube material according to any one of claims 12 to 19.

21. A positive electrode sheet, an electrode assembly, a battery cell, an electrochemical device or an electrical equipment comprising the composite carbon nanotube material according to any one of claims 12 to 19 and the composite carbon nanotubes prepared by the preparation method according to any one of claims 1 to 11.

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