Preparation method of positive electrode material, positive electrode material and lithium-sulfur battery

Abstract

The application provides a preparation method of a positive electrode material applied to a lithium-sulfur battery, and the preparation method comprises the following steps: providing a carbon source and a metal salt solution, wherein metal ions in the metal salt solution comprise at least one of nickel ions, cobalt ions or iron ions; adding the carbon source into a solvent, and heating and stirring to obtain a first dispersion liquid; mixing the first dispersion liquid and the metal salt solution, and heating to obtain a second dispersion liquid; drying the second dispersion liquid to obtain a first precursor; in an inert gas, the first precursor is kept at 800-1000 DEG C for 3-5 h to obtain a solid product, and the solid product is subjected to pickling to obtain a carbon nanotube composite material with metal particles loaded on the inner wall; adding sulfur into the composite material, and heating to make sulfur vapor adhere to the outer wall of the carbon nanotube to obtain the positive electrode material applied to the lithium-sulfur battery. The application further provides a positive electrode material and a lithium-sulfur battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-sulfur batteries, and in particular, to a method for preparing a positive electrode material, a positive electrode material, and a lithium-sulfur battery. Background Art

[0002] At present, lithium-sulfur batteries have a high theoretical capacity (1673 mA h g -1 ) and economical and practical raw material supply, and have broad application prospects in the future. Lithium-sulfur batteries realize the mutual conversion between electrical energy and chemical energy through the cleavage and formation of sulfur-sulfur bonds. During the charging and discharging process of a lithium-sulfur battery, the positive active material sulfur reacts with the negative metal lithium, and the reaction generates lithium polysulfide Li2S x (2<x<8) that is soluble in the electrolyte, and Li2S2 and Li2S that are insoluble in the electrolyte. The dissolved polysulfide undergoes the redox "shuttle effect" between the positive electrode and the negative electrode, which can cause "overcharging" and corrosion and pulverization of the lithium negative electrode, resulting in low coulombic efficiency and serious lithium loss during cycling; while the insoluble Li2S2 and Li2S are unevenly covered on the sulfur positive electrode, leading to poor conductivity of the positive electrode and deteriorating the cycling stability of the battery. Introducing catalysts is an important means to solve the shuttle effect, but due to low catalytic activity, the number of available catalysts is limited, which severely restricts the full exertion of its catalytic activity. Therefore, developing positive electrode materials with high activity and larger catalytic active surface is the key to promoting the practical application of lithium-sulfur batteries. Summary of the Invention

[0003] In view of this, the present application provides a method for preparing a positive electrode material, a positive electrode material, and a lithium-sulfur battery.

[0004] To achieve the above objective, the present application provides a method for preparing a positive electrode material applied to lithium-sulfur batteries, the preparation method comprising: providing a carbon source and a metal salt solution, wherein the metal ions in the metal salt solution comprise at least one of nickel ions, cobalt ions or iron ions; adding the carbon source into a solvent, heating and stirring to obtain a first dispersion; mixing the first dispersion and the metal salt solution, and heating to obtain a second dispersion; drying the second dispersion to obtain a first precursor; in an inert gas, holding the first precursor at 800-1000°C for 3-5h to obtain a solid product, the solid product is subjected to acid washing to obtain a carbon nanotube composite material with metal particles loaded on the inner wall; adding sulfur to the composite material and heating to allow sulfur vapor to attach to the outer wall of the carbon nanotubes, so as to obtain a positive electrode material applied to lithium-sulfur batteries.

[0005] In some possible implementation manners, the mass of the metal particles accounts for 3% to 20% of the mass of the carbon nanotubes, and the mass of the carbon nanotubes accounts for 10% to 30% of the mass of the positive electrode material.

[0006] In some possible implementations, the preparation method further includes: before adding sulfur to the composite material, the composite material undergoes a secondary treatment; specifically: the metal salt solution is added to the composite material to obtain a third dispersion; the third dispersion is dried to obtain a second precursor; in an inert gas, the second precursor is kept at 800-1000℃ for 2-5 hours to obtain a sintered product, wherein the inner and outer walls of the carbon nanotubes in the sintered product are loaded with metal particles.

[0007] In some possible implementations, the sulfur accounts for 40% to 90% of the mass percentage of the cathode material.

[0008] In some possible implementations, the carbon source includes at least one of melamine, glucose, or chitosan; the metal salt includes at least one of nickel chloride, cobalt chloride, ferric chloride, nickel nitrate, cobalt nitrate, ferric nitrate, nickel acetate, cobalt acetate, or ferric acetate.

[0009] In some possible implementations, the concentration of the metal salt solution is 10~20 g·L. -1 The concentration of the first dispersion is 10~20 g·L. -1 .

[0010] In some possible implementations, the mass ratio of the composite material to the sulfur is (5~60):(40~95).

[0011] This application also provides a cathode material for use in lithium-sulfur batteries. The cathode material includes carbon nanotubes, metal particles supported on the inner wall of the carbon nanotubes, and sulfur supported on the outer wall of the carbon nanotubes. The metal particles include at least one of cobalt, iron, or nickel.

[0012] In some possible implementations, the outer wall of the carbon nanotube is also loaded with the metal particles.

[0013] This application also provides a lithium-sulfur battery, including the aforementioned positive electrode material.

[0014] In this application, the prepared cathode material exhibits several advantages. First, due to the difference in electron distribution inside and outside the carbon nanotubes, the metal particles loaded inside the carbon nanotubes can further promote electron enrichment on the surface of the carbon nanotubes, reducing the work function of the catalyst and lowering the energy barrier of the catalytic reaction in lithium-sulfur batteries. This also extends the catalytically active surface to the carbon nanotube surface region adjacent to the metal particles, resulting in a catalytic material with high catalytic activity and a large catalytic surface area, composed of both carbon nanotubes and metal particles. This cathode material also facilitates the full adsorption and rapid catalytic conversion of lithium polysulfides in lithium-sulfur batteries, enhancing reaction kinetics, reducing the coverage of insulating products from lithium polysulfides, and improving sulfur utilization. Second, the metal particles loaded inside the carbon nanotubes are adequately protected by the surface carbon layer, reducing the passivation of the metal particle catalyst by lithium polysulfides. This results in a catalyst with good stability and high catalytic activity, thereby improving the charge-discharge specific capacity and cycle stability of lithium-sulfur batteries. This cathode material meets the practical needs of applied lithium-sulfur batteries and has guiding significance for promoting the development of high-performance lithium-sulfur batteries. Attached Figure Description

[0015] Figure 1 This is an X-ray diffraction pattern of the cathode material prepared in Example 1 of this application.

[0016] Figure 2 This is a scanning electron microscope image of the cathode material prepared in Example 1 of this application.

[0017] Figure 3 This is a transmission electron microscope image of the cathode material prepared in Example 1 of this application.

[0018] Figure 4 This is a scanning electron microscope image of the cathode material prepared in Example 2 of this application.

[0019] Figure 5 This is a scanning electron microscope image of the cathode material prepared in Comparative Example 2 of this application.

[0020] Figure 6 This is a transmission electron microscope image of the cathode material prepared in Comparative Example 2 of this application.

[0021] Figure 7 The graphs show the cycle performance test results of the cathode materials prepared in Example 1 and Comparative Example 2 of this application.

[0022] Figure 8 This is a scanning electron microscope image of the lithium-sulfur battery disassembled in Example 1 of this application, showing the lithium sulfide deposited cathode material.

[0023] Figure 9 The image shows a scanning electron microscope image of the lithium-sulfur battery disassembled in Comparative Example 1 of this application, and the lithium sulfide deposited cathode material.

[0024] Figure 10 The image shows a scanning electron microscope image of the lithium-sulfur battery disassembled in Comparative Example 2 of this application, and the lithium sulfide deposited cathode material. Detailed Implementation

[0025] Embodiments of the present invention are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0026] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0027] In lithium-ion batteries, a well-designed cathode is crucial for achieving high specific capacity and cycle stability. From physical confinement to chemical adsorption and catalytic conversion, modifying or doping cathode materials to increase adsorption sites or introducing catalysts effectively regulates the conversion process of lithium polysulfides, enabling rapid and uniform deposition of lithium polysulfides and suppressing the shuttle effect. Traditional catalysts face problems such as difficulty in dispersion, easy agglomeration, and excessively large particle size, which severely limit their catalytic activity in actual catalytic processes. Therefore, developing cathode materials with high activity and larger catalytically active surfaces is key to promoting the practical application of lithium-sulfur batteries.

[0028] This application provides a method for preparing a cathode material for lithium-sulfur batteries, the method comprising the following steps: S1. A carbon source and a metal salt solution are provided, wherein the metal ions in the metal salt solution include at least one of nickel ions, cobalt ions, or iron ions.

[0029] In some embodiments, the metal salt includes at least one selected from nickel chloride, cobalt chloride, ferric chloride, nickel nitrate, cobalt nitrate, ferric nitrate, nickel acetate, cobalt acetate, or ferric acetate. The metal salt provides a source for the metal particle catalyst.

[0030] The carbon source includes at least one of melamine, glucose, or chitosan.

[0031] S2. Add the carbon source to the solvent, heat and stir to obtain the first dispersion.

[0032] In some embodiments, the solvent includes one of organic solvents such as deionized water, ethanol, methanol, and propanol.

[0033] A carbon source is added to a solvent and heated and stirred to ensure that the carbon source is fully dispersed in the solvent, resulting in a uniformly dispersed first dispersion. In some embodiments, the heating temperature is 60–90°C, and the heating time is 1–24 h to ensure sufficient dispersion of the carbon source. In some embodiments, the concentration of the first dispersion is 10–20 g·L⁻¹. -1 If melamine is used as the carbon source, the concentration of the melamine solution is 10~20 g·L. -1 , such as 10 g·L -1 12 g·L -1 15 g·L -1 Or 20 g·L -1 In this embodiment, the concentration of the first dispersion refers to the molar ratio of the carbon source to the volume ratio of the solvent.

[0034] S3. Mix the first dispersion and the metal salt solution and heat to obtain the second dispersion.

[0035] After the first dispersion and the metal salt solution are mixed, they are heated and stirred to ensure that the first dispersion and the metal salt solution are fully and evenly mixed.

[0036] In some embodiments, the concentration of the metal salt solution is 10-20 g·L. -1 , such as 10 g·L -1 12 g·L -1 15g·L -1 17 g·L -1 Or 20 g·L -1 .

[0037] S4. Dry the second dispersion to obtain the first precursor.

[0038] In some embodiments, the drying process can be achieved by heating in a water bath, oven, or other methods to completely evaporate the solvent in the second dispersion, thereby obtaining a carbon source precursor powder loaded with a metal salt, i.e., the first precursor. In some embodiments, the heating temperature is 60-90°C, and the time is 1-24 hours.

[0039] In other embodiments, the solid and liquid components in the second dispersion can be separated by methods such as evaporating part of the solvent, filtration, centrifugation, or recrystallization to obtain a solid product. The separated solid product is then dried by freeze-drying or oven-drying to obtain the first precursor.

[0040] S5. In an inert gas atmosphere, the first precursor is kept at 800-1000℃ for 3-5 hours to obtain a solid product. The solid product is then acid-washed to remove external and unreacted metal particles, resulting in a carbon nanotube composite material with metal particles loaded on the inner wall.

[0041] Under the aforementioned high-temperature conditions, metal ions in the metal salt are reduced to elemental metals, and the carbon matrix dissolves and precipitates within the metal particles, gradually growing into carbon nanotubes. The metal particles, acting as catalytic growth agents, remain on the inner wall of the carbon nanotubes during growth, resulting in a carbon nanotube composite material with metal particles loaded on its inner wall. Both carbon nanotubes and metal particles possess excellent electrical conductivity. The carbon nanotubes construct a three-dimensional conductive network, shortening the electron and lithium-ion transport paths and rapidly providing electrons and lithium ions for the conversion of sulfur, the active substance in the electrochemical process.

[0042] Meanwhile, metal particles loaded inside carbon nanotubes can further promote electron enrichment on the surface of carbon nanotubes, reducing the work function of the catalyst and lowering the energy barrier of the catalytic reaction in lithium-sulfur batteries. This also extends the catalytically active surface of the metal particles to the adjacent surface region of the carbon nanotubes, resulting in a catalytic material composed of carbon nanotubes and metal particles with high catalytic activity and a large catalytic surface area. This catalytic material can further promote the adsorption of polysulfides and catalyze the rapid electrochemical conversion of polysulfides to low-valence lithium sulfides, thereby reducing the "shuttle effect" of polysulfides, improving the utilization rate of the cathode material, and ultimately obtaining a high-capacity lithium-sulfur battery.

[0043] Metal particles are loaded onto the inner wall of carbon nanotubes. The metal particles loaded inside the carbon nanotubes are fully protected by the surface carbon layer, which avoids excessive reaction between the metal particles and lithium polysulfides. This maintains the high catalytic activity and good stability of the metal particle catalyst and improves the cycle stability of lithium-sulfur batteries.

[0044] In some embodiments, the heating temperature can be 800°C, 900°C, or 1000°C, and the heating and holding time can be 3h, 4h, or 5h. This heating temperature and time allow the metal particles in the first precursor to catalyze the formation of carbon nanotubes from the carbon source.

[0045] In some embodiments, the metal particles account for 3% to 20% of the mass of the carbon nanotubes, such as 3%, 5%, 7%, 10%, 13%, 15%, 18%, or 20%; the metal particles account for 0.1% to 10% of the mass of the cathode material, such as 0.1%, 0.5%, 1%, 2%, 5%, 8%, 9%, or 10%. Within this range, the metal particles can have a better catalytic effect.

[0046] In some embodiments, the solid product is further treated with an acid solution to obtain a carbon nanotube composite material with metal particles loaded on its inner wall. The acid solution includes hydrochloric acid, dilute sulfuric acid, dilute nitric acid, or acetic acid, etc. The acid solution can remove unreacted elemental metals and oxides from the product after high-temperature sintering, thereby reducing impurities in the composite material. In this step, the metal particles on the inner wall of the carbon nanotubes are not removed.

[0047] In some embodiments, the inert gas may be argon, helium, or nitrogen.

[0048] S6. Perform secondary processing on the composite material obtained in step S5.

[0049] Specifically: (1) A metal salt solution is added to the composite material to obtain a third dispersion. The metal salt solution can be the same metal salt as the metal salt solution in step S3 or multiple different metal salts. In this step, the metal salt solution again provides a source for the metal particles.

[0050] (2) Dry the third dispersion to obtain the second precursor.

[0051] This step uses the method described in step S4 to process the third dispersion to obtain a second precursor of metal salt loaded onto the composite material.

[0052] (3) In an inert gas, the second precursor is kept at 800-1000℃ for 2-5 hours to obtain the pre-product.

[0053] At the aforementioned high temperature, the metal salt is converted into a metallic element and loaded onto the outer wall of the carbon nanotube, thus obtaining a preform of carbon nanotubes with metal particles loaded on both the inner and outer walls.

[0054] S7. Sulfur is added to the preform and heated to allow sulfur vapor to adhere to the outer wall of the carbon nanotubes, thereby obtaining a cathode material for use in lithium-sulfur batteries.

[0055] In this step, the preform and active sulfur can be thoroughly mixed using mechanical mixing, sulfur melting, and solvent methods. After the preform and sulfur are uniformly mixed, under heating conditions, sulfur forms sulfur vapor, enters the preform, and recrystallizes into sulfur particles loaded onto the outer wall of carbon nanotubes, thus obtaining a cathode material for lithium-sulfur batteries. In the cathode material, metal particles are loaded onto the inner and outer walls of the carbon nanotubes, while sulfur particles and some metal particles are co-loaded onto the outer wall of the carbon nanotubes. The cathode material prepared in this application is simple, controllable, and low-cost, which is beneficial for the industrialization of lithium-sulfur batteries.

[0056] In some embodiments, the composite material and active sulfur are mixed at a mass ratio of (5-60):(40-95) and heat-treated in an inert gas at a temperature of 90-300°C for 1-24 hours. Under these conditions, the sulfur is sufficiently converted into sulfur vapor. This ratio range also applies to preforms.

[0057] In some embodiments, carbon nanotubes account for 0.1% to 30% of the mass of the cathode material. Within this range, carbon nanotubes not only provide support in the cathode material but also improve its conductivity.

[0058] In some embodiments, the diameter of the carbon nanotubes is 100-200 nm, and the specific surface area of ​​the carbon nanotubes is 200-500 m². 2 g -1 The pore size of carbon nanotubes ranges from 0.5 to 100 nm.

[0059] In some embodiments, the sulfur is active nano-sulfur particles with a particle size of 10-100 nm. The active sulfur accounts for 40%-90% of the mass of the cathode material to ensure the energy density of the cathode material in lithium-sulfur batteries. For example, the active sulfur accounts for 40%, 50%, 60%, 80%, or 90% of the mass of the cathode material.

[0060] In some embodiments, the mass ratio of carbon nanotubes, active sulfur, and metal particles in the cathode material is 40:56:4. Within this range, the cathode material possesses good catalytic activity, conductivity, and high energy density.

[0061] This application also provides a cathode material comprising carbon nanotubes, metal particles supported on the inner wall of the carbon nanotubes, and sulfur supported on the outer wall of the carbon nanotubes. The metal particles include at least one of cobalt, iron, or nickel. The carbon nanotubes form a conductive network through which electrons and ions can pass. The carbon nanotubes and metal particles together form a catalytic material, enabling the catalytically active region of the catalyst to be fully extended to the surface of the carbon matrix. This further promotes the adsorption of polysulfides and catalyzes the electrochemical reaction of polysulfides to rapidly convert them into low-valence lithium sulfides, thereby reducing the "shuttle effect" of polysulfides and improving the utilization rate of the cathode material, thus obtaining a lithium-sulfur battery with high capacity and high stability.

[0062] In some embodiments, the outer wall of the carbon nanotubes is also loaded with the metal particles to give the cathode material catalytic activity.

[0063] This application also provides a lithium-sulfur battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator and the electrolyte are located between the positive electrode and the negative electrode.

[0064] The positive electrode sheet includes a current collector and a coating material disposed on the surface of the current collector. The coating material includes a positive electrode material of a lithium-sulfur battery, a binder, and a conductive agent. The positive electrode material of the lithium-sulfur battery, the binder, and the conductive agent are dispersed in a solvent (e.g., N-methylpyrrolidone) in a ratio of 8:1:1 and uniformly mixed to obtain a dispersion. The dispersion is then coated on the current collector, dried, and sliced ​​to obtain the positive electrode sheet.

[0065] The binder includes at least one of PVDF, CMC, SBR, sodium alginate, or potassium alginate. The conductive agent includes at least one of CNT, SP, GN, CMK-3, mesoporous carbon, and carbon fiber. The current collector includes at least one of copper foil, aluminum foil, carbon-containing aluminum foil, stainless steel foil, carbon cloth, and carbon paper.

[0066] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the invention. Unless otherwise stated, reagents, software, and instruments involved in the following embodiments that are not specifically described are all conventional commercially available products or open-source materials.

[0067] Example 1 2 g of cobalt chloride was added to 20 mL of deionized water to obtain a cobalt chloride solution. 2 g of melamine was added to 60 mL of deionized water and heated at 50°C with stirring until homogeneous, yielding a second dispersion. The resulting cobalt chloride solution was added to the first dispersion to obtain the second dispersion. The mixture was kept at 80°C for 6 hours, the water in the second dispersion was evaporated, and the mixture was dried to obtain a metal salt-loaded melamine precursor powder, i.e., the first precursor.

[0068] The first precursor powder was transferred to a tube furnace at 800°C and held at that temperature for 3 hours in an argon atmosphere to obtain a solid product of carbon nanotubes with cobalt particles loaded on the inner wall. The solid product was washed with 3M hydrochloric acid for 1 hour, then washed with deionized water and ethanol and filtered until the supernatant was neutral, yielding a filter residue. This residue was dried at 80°C for 12 hours to obtain a carbon nanotube composite material with cobalt particles loaded on the inner wall. The mass ratio of cobalt to carbon in the composite material was 15:85.

[0069] The composite material and sulfur powder were uniformly mixed at a mass ratio of 3:7 and then kept at 155°C for 12 hours under argon protective gas to obtain the positive electrode material for lithium-sulfur batteries.

[0070] Example 2 The difference between Example 2 and Example 1 is that, before mixing the composite material and sulfur, the composite material was further treated by taking 100 mg of the composite material and adding 2 mg mL of sulfur. -1 The solution of cobalt chloride was stirred until homogeneous to obtain a third dispersion. The third dispersion was stirred in a water bath at 60°C and the solvent was evaporated to dryness to obtain the second precursor.

[0071] The second precursor powder was transferred to a tube furnace at 800°C and held at that temperature for 2 hours in an argon atmosphere to obtain a carbon nanotube composite material with cobalt particles loaded on both the inner and outer walls. The mass ratio of cobalt particles to carbon nanotubes was 15:85. The remaining steps were the same as in Example 1.

[0072] Example 3 The difference between Example 3 and Example 1 is that the metal salt solution is a ferric chloride solution, while the other steps are the same as in Example 1.

[0073] Example 4 The difference between Example 4 and Example 2 is that the metal salt solution is a ferric chloride solution, while the other steps are the same as in Example 2.

[0074] Example 5 The difference between Example 5 and Example 1 is that the metal salt solution is a nickel chloride solution, while the remaining steps are the same as in Example 1.

[0075] Example 6 The difference between Example 6 and Example 2 is that the metal salt solution is a nickel chloride solution, while the other steps are the same as in Example 2.

[0076] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the solid product was washed with 3M hydrochloric acid for 6 hours to obtain carbon nanotubes with no cobalt particles loaded on the inner wall. The remaining steps were the same as in Example 1.

[0077] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that the solid product was washed with 3M hydrochloric acid for 6 hours to obtain carbon nanotubes without cobalt particles loaded on the inner wall. Before mixing the carbon nanotubes and sulfur, the composite material was further treated by taking 100 mg of carbon nanotubes and adding 5 mg / mL... -1 (To ensure that the content of cobalt loaded on carbon nanotubes in this embodiment is the same as in Example 2) The cobalt chloride solution was stirred until homogeneous to obtain a third dispersion. The third dispersion was stirred in a 60°C water bath and the solvent was evaporated to dryness to obtain a second precursor.

[0078] The second precursor powder was transferred to a tube furnace at 800°C and held at that temperature for 2 hours in an argon atmosphere to obtain a carbon nanotube composite material with cobalt particles loaded only on the outer wall. The mass ratio of cobalt particles to carbon nanotubes was 15:85. The remaining steps were the same as in Example 1.

[0079] See Figure 1 This application performs X-ray diffraction testing on the cathode material prepared in Example 1. Figure 1 The spectrum showed characteristic peaks (111), (200) and (220), which are characteristic peaks of elemental cobalt. This indicates that metallic cobalt was prepared in the cathode material.

[0080] See Figure 2 This application also performs scanning electron microscopy tests on the cathode material prepared in Example 1, from... Figure 2It is clearly visible that a carbon nanotube structure is formed in the cathode material, and cobalt particles (white irregular blocks in the figure) are loaded inside the carbon nanotubes.

[0081] See Figure 3 This application also performs transmission electron microscopy tests on the cathode material prepared in Example 1, from... Figure 3 It is clearly visible that a carbon nanotube structure outline is formed in the cathode material, and cobalt particles (black irregular blocks in the figure) are loaded inside the carbon nanotubes.

[0082] See Figure 4 This application also subjected the cathode material prepared in Example 2 to scanning electron microscopy testing, from... Figure 4 It is clearly visible that a carbon nanotube structure has been formed in the cathode material, and the cobalt metal particles ( Figure 4 The white, spherical structure is loaded onto the outer wall of the carbon nanotube, while the inner wall of the carbon nanotube also contains some cobalt metal particles.

[0083] See Figure 5 This application also includes scanning electron microscopy testing of the cathode material prepared in Comparative Example 2, from... Figure 5 It is clearly visible that a carbon nanotube structure has been formed in the cathode material, and the cobalt metal particles ( Figure 5 Medium-sized spherical particles were loaded onto the outer wall of carbon nanotubes. This indicates that a cathode material loaded onto the outer wall of carbon nanotubes was obtained in Comparative Example 2.

[0084] See Figure 6 This application also conducted transmission electron microscopy tests on the cathode material prepared in Example 2, from... Figure 6 It is clearly visible that a carbon nanotube structure outline has been formed in the cathode material, and the metallic cobalt particles ( Figure 6 The black, spherical structure is loaded onto the outer wall of the carbon nanotube. This further illustrates that the cathode material loaded onto the outer wall of the carbon nanotube was obtained in Comparative Example 2.

[0085] This application also describes the preparation of lithium-sulfur batteries using the cathode materials obtained in Examples 1-6 and Comparative Examples 1-2. The specific preparation steps are as follows: The positive electrode material of the lithium-sulfur battery, along with the binder PVDF and conductive carbon black, was uniformly dispersed in the solvent N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1. The mixture was stirred until uniformly dispersed, forming a slurry. This slurry was coated onto a current collector and dried in a 60°C oven to form a positive electrode sheet for later use. The positive electrode sheet was then assembled into a coin cell in an argon-filled glove box in the following order: positive electrode shell, gasket, positive electrode sheet, electrolyte, separator, electrolyte, lithium sheet, gasket, spring sheet, and negative electrode shell. The electrolyte on both sides of the separator was 20 μL, with the electrolyte solvent being a 1:1 volume ratio mixture of ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL). The lithium salt was 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the additive was 2% LiNO3. Subsequently, the lithium-sulfur battery was compacted using a coin cell sealing machine with the positive electrode shell at the bottom and the negative electrode shell at the top for testing.

[0086] The test results are shown in Table 1, which examines the initial discharge capacity and capacity retention rate after 1000 cycles at a test temperature of 25℃, a charge / discharge current of 1.67A / g (1C), a charge / discharge voltage range of 1.7-2.8V, and a cycle count of 1000.

[0087] Table 1. Retention rate of lithium-sulfur batteries prepared in Examples 1-6 and Comparative Examples 1 to 2 after 1000 cycles See Figure 7 As shown in Table 1, under the same conditions, compared to Comparative Examples 1 and 2, the lithium-sulfur battery prepared with the cobalt-loaded cathode material on the inner wall of carbon nanotubes in Example 1 exhibits superior initial discharge capacity and coulombic efficiency after 1000 cycles compared to Comparative Examples 1 and 2. This indicates that cobalt-loaded on the inner wall of carbon nanotubes possesses higher and more stable charge-discharge specific capacity and cycle stability. This also indirectly demonstrates that in Example 1, cobalt-loaded on the inner wall of carbon nanotubes can enhance the catalytic activity of the carbon nanotube surface.

[0088] In conjunction with Example 2, compared with Comparative Example 1 and Comparative Example 2, it can be shown that cobalt metal loaded on both the inner and outer walls of carbon nanotubes also has a higher and more stable charge-discharge specific capacity.

[0089] As shown in Table 2, Examples 3-6 all exhibit higher discharge specific capacity and capacity retention compared to Comparative Example 1. This indicates that loading one of the metal particles—cobalt, iron, or nickel—on the inner wall of the carbon nanotubes enhances the catalytic activity of the cathode material.

[0090] See Figures 8 to 10This application also conducts lithium sulfide deposition tests on lithium-sulfur batteries using Examples 1 and Comparative Examples 1 to 2 as positive electrode materials. After assembling the lithium-sulfur batteries, they are discharged at a constant voltage to 2.08V. The batteries are then disassembled, and positive electrode materials are scraped from the current collector. The lithium sulfide deposited on the positive electrode surface is then tested using a scanning electron microscope.

[0091] Figures 8 to 10 The images shown are scanning electron microscope (SEM) images of the cathode materials disassembled in Example 1, Comparative Example 1, and Comparative Example 2, respectively. Compared to Comparative Example 1 and Comparative Example 2, the lithium sulfide deposited on the surface of the cathode material in Example 1 is more uniform, exhibiting a three-dimensional island-like growth pattern outside the carbon nanotubes. Some lithium sulfide extends outward from the carbon nanotubes in a three-dimensional structure, with less coverage of sites on the metal particles and the outer wall of the carbon nanotubes, indicating that the insulating product of lithium sulfide does not cover the sites on the outer wall of the carbon nanotubes. In contrast, in Comparative Example 1 and Comparative Example 2, lithium sulfide completely covers the surface of the carbon nanotubes and cobalt particles, resulting in dense deposition on the surfaces of the carbon nanotubes and cobalt particles. This dense deposition can passivate the catalyst, leading to a slow catalytic reaction and thus reducing the charge-discharge specific capacity and cycle stability. This also demonstrates that cathode materials formed by loading metal particles onto the inner wall of carbon nanotubes, or by loading metal particles onto both the inner and outer walls of carbon nanotubes and combining them with sulfur, can improve the charge-discharge specific capacity and cycle stability of lithium-sulfur batteries.

[0092] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention should not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a cathode material for lithium-sulfur batteries, characterized in that, The preparation method includes: A carbon source and a metal salt solution are provided, wherein the metal ions in the metal salt solution include at least one of nickel ions, cobalt ions, or iron ions; The carbon source is added to the solvent, heated and stirred to obtain a first dispersion; The first dispersion and the metal salt solution are mixed and heated to obtain the second dispersion; The second dispersion was dried to obtain the first precursor; In an inert gas environment, the first precursor is kept at 800-1000℃ for 3-5 hours to obtain a solid product. The solid product is then acid-washed to obtain a carbon nanotube composite material with metal particles loaded on the inner wall. The carbon nanotube composite material is further subjected to a secondary treatment, specifically: the metal salt solution is added to the carbon nanotube composite material to obtain a third dispersion; the third dispersion is dried to obtain a second precursor; the second precursor is kept at 800-1000℃ for 2-5 hours in an inert gas to obtain a pre-product, wherein the inner and outer walls of the carbon nanotubes in the pre-product are loaded with metal particles. Sulfur is added to the preform and heated to allow sulfur vapor to adhere to the outer wall of the carbon nanotubes, thereby obtaining a cathode material for lithium-sulfur batteries, wherein the sulfur accounts for 40% to 90% of the mass of the cathode material.

2. The method for preparing the cathode material as described in claim 1, characterized in that, The metal particles account for 3% to 20% of the mass of the carbon nanotubes, and the carbon nanotubes account for 10% to 30% of the mass of the cathode material.

3. The method for preparing the cathode material as described in claim 1, characterized in that, The carbon source includes at least one of melamine, glucose, or chitosan; the metal salt includes at least one of nickel chloride, cobalt chloride, ferric chloride, nickel nitrate, cobalt nitrate, ferric nitrate, nickel acetate, cobalt acetate, or ferric acetate.

4. The method for preparing the cathode material as described in claim 1, characterized in that, The concentration of the metal salt solution is 10~20 g·L. -1 The concentration of the first dispersion is 10~20 g·L. -1 .

5. The method for preparing the cathode material as described in claim 1, characterized in that, The mass ratio of the composite material to the sulfur is (5~60):(40~95).

6. A cathode material for use in lithium-sulfur batteries, characterized in that, The cathode material is obtained by the method of preparing cathode material according to any one of claims 1 to 5. The cathode material includes carbon nanotubes, metal particles loaded on the inner wall of the carbon nanotubes, and sulfur and the metal particles loaded on the outer wall of the carbon nanotubes. The metal particles include at least one of cobalt, iron, or nickel.

7. A lithium-sulfur battery, characterized in that, Including the cathode material as described in claim 6.

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