Lithium-sulfur battery composite positive electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of secondary battery positive electrode materials. The invention relates to a cathode material, in particular to a lithium-sulfur battery composite cathode material and a preparation method thereof. The product provided by the invention comprises hollow porous carbon microspheres and elemental sulfur loaded in the hollow porous carbon microspheres, the elemental sulfur is distributed in the internal cavities and the outer surfaces of the hollow porous carbon microspheres; on the basis of the total mass of the lithium-sulfur battery composite positive electrode material, the mass percentage content of the elemental sulfur is 75-80%; the porosity of the hollow porous carbon microspheres is 50-55%; mesopores and macropores with pore size distribution of 2-50nm are formed in the surfaces of the hollow porous carbon microspheres; the particle size distribution is 0.5-15 [mu] m; the thickness distribution of the shell layer is 50 to 500 nm; wherein the mass ratio of the sulfur content in the inner cavity to the outer surface is (8-10): 1; in addition, the carbon microspheres are doped with nitrogen elements and silicon elements.

Description

Technical Field

[0001] This invention belongs to the technical field of secondary battery cathode materials. More specifically, it relates to a lithium-sulfur battery composite cathode material and its preparation method. Background Technology

[0002] Lithium-sulfur batteries use metallic lithium as the negative electrode and elemental sulfur as the positive electrode active material. Their working principle is based on a reversible, multi-step electrochemical reaction between sulfur and lithium ions (generating a series of lithium polysulfides, Li₂S₂). x (x=1~8). However, this reaction process also leads to some severe technical challenges for lithium-sulfur batteries, seriously restricting their commercialization process. These challenges are mainly concentrated on the sulfur cathode side: Insulation properties of sulfur and its discharge products (Li2S2 / Li2S): Elemental sulfur and the final discharge product lithium sulfide have extremely low electronic conductivity, which severely hinders electron transport, resulting in low utilization of active materials, poor rate performance, and high polarization.

[0003] The huge volume change during charging and discharging: When sulfur is completely discharged and converted into Li2S, the volume expansion rate is as high as 80%. This huge volume change will destroy the integrity of the positive electrode structure, leading to electrode pulverization and detachment of active material from the conductive network, thereby aggravating capacity decay.

[0004] Therefore, developing a novel composite cathode material and its preparation method, aiming to enhance the synergistic confinement effect of polysulfides through reasonable structural design while maintaining high sulfur content and high loading, effectively buffering volume changes, and maintaining its own excellent electronic conductivity, is of great significance for promoting the practical application of high-performance lithium-sulfur batteries. Summary of the Invention

[0005] The technical problem this invention aims to solve is the need for existing lithium-sulfur battery cathode materials to maintain high conductivity and buffer against material pulverization caused by volume changes during battery use. Based on these challenges, this invention provides a lithium-sulfur battery composite cathode material and its preparation method.

[0006] The purpose of this invention is to provide a composite cathode material for lithium-sulfur batteries.

[0007] Another objective of this invention is to provide a method for preparing a composite cathode material for lithium-sulfur batteries.

[0008] The above-mentioned objective of this invention is achieved through the following technical solution: A lithium-sulfur battery composite cathode material, comprising: Hollow porous carbon microspheres, and elemental sulfur loaded in the hollow porous carbon microspheres; The elemental sulfur is distributed in the internal cavity and outer surface of the hollow porous carbon microspheres; Based on the total mass of the lithium-sulfur battery composite cathode material, the mass percentage of elemental sulfur is 75-80%. The porosity of the hollow porous carbon microspheres is 50-55%.

[0009] The beneficial effects of the above technical solution are as follows: The above technical solution utilizes hollow porous carbon microspheres to construct an electron transport network, loading elemental sulfur onto the hollow porous carbon microspheres, enabling electrons to be rapidly transported to each sulfur particle through the carbon skeleton, greatly overcoming the barrier caused by the insulation of sulfur itself, and improving the utilization rate and rate performance of active materials. The hollow structure provides an important buffer space for the volume expansion of sulfur during charging and discharging, effectively absorbing stress and preventing the electrode material from pulverizing and the structure from collapsing due to drastic volume changes, thereby ensuring the structural integrity and long cycle life of the electrode. To ensure high energy density, a significant amount of elemental sulfur is required. Under this high loading, some of the sulfur is distributed on the surface and some inside the carbon microspheres. The elemental sulfur on the surface can quickly provide initial capacity and serve as the "frontline" for internal sulfur reactions. Porosity control plays a crucial balancing role. If the porosity is too low, although the strength of the carbon layer structure can be significantly improved, it is not conducive to ion transport, resulting in increased ion impedance. Therefore, an appropriate lower limit of porosity is the basis for ensuring sufficient ionic conductivity of the product when elemental sulfur is loaded both inside and outside. However, if the porosity is too high, the strength of the carbon layer will decrease significantly, and during the expansion of the cathode material, it will be impossible to effectively limit the stress generated by the expansion of the corresponding amount of elemental sulfur inside.

[0010] Furthermore, the surface of the hollow porous carbon microspheres has mesopores and macropores with a pore size distribution of 2-50 nm.

[0011] The aforementioned pore size provides a smooth channel for electrolyte wetting and rapid diffusion of lithium ions, further reducing the ion transport density at the interface and thus improving the charge-discharge rate of high current. In particular, it facilitates the full and uniform penetration of elemental sulfur into the internal cavity of carbon microspheres under capillary action during product processing, thereby preventing its local accumulation on the surface.

[0012] Furthermore, the hollow porous carbon microspheres have a particle size distribution of 0.5-15 μm.

[0013] If the particle size is too small, it is easy to agglomerate, which is not conducive to the uniform coating of electrode slurry; while if the size is too large, it will lead to an excessively long diffusion path of lithium ions. More importantly, the mechanical strength of excessively large particles will be significantly reduced, and the buffering effect on volume expansion will be weakened.

[0014] Furthermore, the shell thickness distribution of the hollow porous carbon microspheres is 50-500 nm.

[0015] A shell thickness of 50-500 nm ensures that the carbon spheres have sufficient mechanical strength to withstand the volumetric expansion stress of sulfur during cycling, preventing breakage. At the same time, the carbon wall at this thickness has low electron transport resistance, ensuring excellent overall conductivity.

[0016] Furthermore, the portion of elemental sulfur distributed in the internal cavity is the first portion; the portion of elemental sulfur distributed on the outer surface is the second portion. The mass ratio of the first part to the second part is 8-10:1.

[0017] The beneficial effects of the above technical solution are as follows: By confining sulfur primarily within the carbon microspheres, sufficient contact between sulfur and the conductive carbon substrate is ensured, preventing the active material from becoming disconnected from the conductive network, thus guaranteeing a high utilization rate of the active material. The elemental sulfur inside is physically confined by the centrally controlled carbon microspheres, which greatly inhibits the dissolution and shuttle effect of polysulfides, and is the fundamental guarantee of long cycle life. The external elemental sulfur is directly exposed to the electrolyte, providing a rapid reaction interface, which helps reduce polarization and contributes to high initial capacity and excellent rate performance.

[0018] Furthermore, the hollow porous carbon microspheres are doped with nitrogen and silicon elements.

[0019] The introduced nitrogen-containing functional groups are polar and can form strong chemical bonds with polysulfides, firmly "anchoring" them to the carbon skeleton and strongly suppressing the shuttle effect from the root. The doping of nitrogen can enhance its effect and further improve the physical strength of the carbide skeleton.

[0020] A method for preparing a lithium-sulfur battery composite cathode material, the specific preparation steps of which include: Hollow porous carbon microspheres are available; Hollow porous carbon microspheres and elemental sulfur are mixed and heated to 180-200℃ in an inert atmosphere and kept at that temperature for 10-12 hours to load elemental sulfur onto the hollow porous carbon microspheres. Cooling yields the lithium-sulfur battery composite cathode material.

[0021] Furthermore, the specific preparation steps also include: Hollow porous carbon microspheres and elemental sulfur are mixed and heated to 180-200℃ at a rate of 0.3-0.5℃ / min in an inert atmosphere. The mixture is then held at 0.2-0.3MPa for 10-12 hours to load elemental sulfur onto the hollow porous carbon microspheres.

[0022] By slowly raising the temperature, the surface sulfur is prevented from melting too quickly and blocking the pore channels, ensuring that the internal pores have enough time to be filled with sulfur, which is conducive to achieving an ideal internal and external sulfur distribution ratio. External pressure can overcome capillary resistance and force the molten sulfur into the finer channels and cavities, further ensuring the sulfur loading rate and uniformity, and reducing the residue of external sulfur, so that more sulfur is protected inside the carbon sphere.

[0023] Furthermore, the hollow porous carbon microspheres provided include: Using silica microspheres as a hard template; A carbon-containing organic polymer layer is coated onto the surface of the rigid template to obtain a precursor; The precursor is carbonized at high temperature and then cooled to room temperature to obtain carbonized material; The carbide material is chemically etched to remove the hard template, resulting in hollow carbon microspheres; Hollow carbon microspheres were treated with an activator to obtain hollow porous carbon microspheres.

[0024] Furthermore, the hollow porous carbon microspheres also include: Using silica microspheres as a hard template; A carbon-containing organic polymer layer is coated onto the surface of the rigid template to obtain a precursor; The organic polymer layer is a polyacrylonitrile coating layer; Under inert gas protection, the precursor is heated to 800-1000℃ at a rate of 2-4℃ / min and carbonized for 3-4 hours, then cooled to room temperature to obtain carbonized material. The carbide material is chemically etched to remove the hard template, resulting in hollow carbon microspheres; Hollow carbon microspheres are mixed and impregnated with a sodium silicate-potassium hydroxide composite solution, dried, and then activated by heating at 780-850℃ in an inert atmosphere. After cooling, hollow porous carbon microspheres are obtained. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8-10.

[0025] The hard template method is the most reliable method for preparing hollow microspheres with highly controllable morphology and size. By controlling the particle size of the silica template and the polymer coating thickness, the particle size and shell thickness of the final carbon microspheres can be precisely controlled, ensuring batch-to-batch consistency and structural designability. Sodium silicate decomposes at high temperatures, and its sodium component may assist in activation. More importantly, it enables the doping of silicon (Si) elements. The combination of potassium hydroxide and sodium silicate enables the simultaneous completion of "deep pore formation" and "N / Si co-doping" in a "one-step" process. Detailed Implementation

[0026] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0027] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0028] Example 1 Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 3% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 60r / min for 24h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 800°C in an argon atmosphere at a rate of 2°C / min, and carbonized at this temperature for 3 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 780°C at a rate of 5°C / min in an argon atmosphere for 80 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 50%, a surface pore size distribution of 2-32 nm, and a shell thickness distribution of 50-400 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. Preparation of composite cathode materials: Based on the total mass of the composite cathode material, elemental sulfur and hollow porous carbon microspheres were prepared with a mass percentage of 75% for elemental sulfur and the remainder being hollow porous carbon microspheres. The two are mixed and added to a high-pressure reactor. Under nitrogen protection, the temperature is raised to 180°C at a rate of 0.3°C / min. The mixture is then kept at 0.2 MPa for 10 hours to load elemental sulfur onto the hollow porous carbon microspheres. After natural cooling to room temperature, the material is discharged to obtain the composite cathode material. Through the above preparation process, the heating rate, holding temperature, and pressure are controlled to allow elemental sulfur to be distributed on the outer surface and in the internal cavity. The portion of elemental sulfur distributed in the internal cavity is the first portion; the portion of elemental sulfur distributed on the outer surface is the second portion. The mass ratio of the first part to the second part is 8:1.

[0029] Example 2 Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 9 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 4% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 70r / min for 24h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 900°C in an argon atmosphere at a rate of 3°C / min, and carbonized at this temperature for 3.5 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 810°C at a rate of 5°C / min in an argon atmosphere for 90 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:9, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 52%, a surface pore size distribution of 2-46 nm, and a shell thickness distribution of 50-440 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. Preparation of composite cathode materials: Based on the total mass of the composite cathode material, and with the mass percentage of elemental sulfur at 78% and the remainder being hollow porous carbon microspheres, elemental sulfur and hollow porous carbon microspheres were prepared. The two were mixed and added to a high-pressure reactor. Under nitrogen protection, the temperature was raised to 190°C at a rate of 0.4°C / min. The mixture was then kept at 0.25 MPa for 11 hours to load elemental sulfur onto the hollow porous carbon microspheres. After natural cooling to room temperature, the material was discharged to obtain the composite cathode material. Through the above preparation process, the heating rate, holding temperature, and pressure are controlled to allow elemental sulfur to be distributed on the outer surface and in the internal cavity. The portion of elemental sulfur distributed in the internal cavity is the first portion; the portion of elemental sulfur distributed on the outer surface is the second portion. The mass ratio of the first part to the second part is 9.2:1.

[0030] Example 3 Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 10 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 5% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 80r / min for 24h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 1000°C in an argon atmosphere at a rate of 4°C / min, and carbonized at this temperature for 4 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 850°C at a rate of 5°C / min in an argon atmosphere for 100 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:10, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 55%, a surface pore size distribution of 2-50 nm, and a shell thickness distribution of 50-500 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. Preparation of composite cathode materials: Based on the total mass of the composite cathode material, and with the mass percentage of elemental sulfur at 80% and the remainder being hollow porous carbon microspheres, elemental sulfur and hollow porous carbon microspheres were prepared. The two are mixed and added to a high-pressure reactor. Under nitrogen protection, the temperature is raised to 200°C at a rate of 0.5°C / min. The mixture is then kept at 0.3 MPa for 12 hours to load elemental sulfur onto the hollow porous carbon microspheres. After natural cooling to room temperature, the material is discharged to obtain the composite cathode material. Through the above preparation process, the heating rate, holding temperature, and pressure are controlled to allow elemental sulfur to be distributed on the outer surface and in the internal cavity. The portion of elemental sulfur distributed in the internal cavity is the first portion; the portion of elemental sulfur distributed on the outer surface is the second portion. The mass ratio of the first part to the second part is 10:1.

[0031] Example 4 The difference between this embodiment and Embodiment 1 is as follows: Sodium silicate was not added, and all other conditions remained unchanged.

[0032] Example 5 The difference between this embodiment and Embodiment 1 is as follows: Hollow porous carbon microspheres with a particle size distribution of 0.1-20 μm were selected by sieving, while keeping other conditions unchanged.

[0033] Example 6 The difference between this embodiment and Embodiment 1 is as follows: Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 3% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 60r / min for 24h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 800°C in an argon atmosphere at a rate of 2°C / min, and carbonized at this temperature for 3 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 880°C at a rate of 5°C / min in an argon atmosphere for 90 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:9, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 60%, a surface pore size distribution of 2-75 nm, and a shell thickness distribution of 50-400 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. The remaining conditions remain basically unchanged.

[0034] Example 7 The difference between this embodiment and Embodiment 1 is as follows: Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 7 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 3% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 60r / min for 18h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 780°C in an argon atmosphere at a rate of 2°C / min, and carbonized at this temperature for 3 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 780°C at a rate of 5°C / min in an argon atmosphere for 80 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 50%, a surface pore size distribution of 2-32 nm, and a shell thickness distribution of 45-360 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. The remaining conditions remain basically unchanged.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: Preparation of hollow porous carbon microspheres: Polyacrylonitrile was dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL; Silica microspheres were added to a polyacrylonitrile solution and ultrasonically dispersed at a frequency of 80 kHz for 20 min to obtain a silica dispersion; wherein the amount of silica microspheres used was 3% of the mass of the polyacrylonitrile solution. The silica dispersion was kept at 60℃ and stirred at 60r / min for 24h, then centrifuged to separate the filter cake, which was then washed three times with anhydrous ethanol and dried to obtain the precursor. The precursor was heated to 800°C in an argon atmosphere at a rate of 2°C / min, and carbonized at this temperature for 3 hours. It was then cooled to room temperature to obtain the carbonized material. After mixing the carbon material with an excess of 5% hydrofluoric acid by mass, the mixture was subjected to continuous ultrasonic reaction at an ultrasonic frequency of 60kHz and a temperature of 60℃ for 24 hours to remove silicon dioxide by etching with hydrofluoric acid. After filtration, washing and drying, and sieving, hollow carbon microspheres were obtained. Hollow carbon microspheres and sodium silicate-potassium hydroxide composite solution were mixed at a mass ratio of 1:5 and allowed to stand for 12 hours at room temperature. After filtration and drying, the mixture was heated to 880°C at a rate of 5°C / min in an argon atmosphere for 90 minutes to activate the reaction. After cooling to room temperature, the mixture was washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:3, and the amount of water used in this solution is 80% of the total mass of the composite solution. By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 32%, a surface pore size distribution of 0.5-25 nm, and a shell thickness distribution of 50-400 nm were obtained. After sieving, particles with a particle size distribution of 0.5-15 μm were obtained. The remaining conditions remain basically unchanged.

[0036] Of course, since the porosity or pore size distribution of the carbon microspheres has been adjusted in the above embodiments or comparative examples, the difficulty of percolating with elemental sulfur will increase. It is understandable that in order to achieve the corresponding distribution of elemental sulfur, the percolation time and pressure can be reasonably adjusted, which will not be described in detail here.

[0037] The performance tests conducted on the products obtained from the above embodiments or comparative examples are as follows: Specific test methods and results are presented below. The composite cathode material, conductive agent Super P, and binder PVDF were dispersed into a cathode slurry in the presence of NMP solvent at a mass ratio of 8:1:1. The slurry was then uniformly coated onto aluminum foil using a doctor blade coater. The coated electrode was then transferred to a vacuum oven at 80°C and dried for 12 hours to completely remove the solvent and moisture.

[0038] The dried electrode sheet is punched into a small round sheet with a diameter of 14 mm using a punching machine to obtain the positive electrode sheet; Accurately weigh the mass of each electrode and calculate the actual mass of the active material sulfur (S) in each electrode; Lithium metal sheet is used as the negative electrode and Celgard 2325 membrane is used as the separator; The electrolyte formulation is as follows: 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixed solvent of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) (volume ratio 1:1), with 2 wt% LiNO3 additive added. The positive electrode, separator, negative electrode, electrolyte, positive electrode shell, negative electrode shell, gasket and other components are assembled into a CR2032 button cell. Let the battery stand for 24 hours to allow the electrolyte to fully soak in; First charge / discharge capacity test: At a temperature of 25℃, at a rate of 0.1C (1C is calculated based on the mass of the active material S obtained above), and within a voltage window of 1.7-2.8V, charge and discharge were performed. The first discharge and charge specific capacities were recorded in mAh / g. The first coulombic efficiency was calculated based on the formula: first coulombic efficiency = (first charge capacity / first discharge capacity) * 100%. The test results are shown in Table 1. Table 1: Results of the first charge-discharge test Ratio and cycle performance testing: At a temperature of 25℃ and a voltage window of 1.7-2.8V, the battery was subjected to constant current charge-discharge cycles at 0.1C and 0.3C respectively. After 100 cycles, the capacity retention rate of the battery was tested. The detailed test results are shown in Table 2. Table 2: Capacity Retention Test Results As can be seen from the test results in Tables 1 and 2, the product obtained by this invention has excellent initial coulombic efficiency and stable performance during long-term cycling.

[0039] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A composite cathode material for lithium-sulfur batteries, characterized in that, include: Hollow porous carbon microspheres, and elemental sulfur loaded in the hollow porous carbon microspheres; The elemental sulfur is distributed in the internal cavity and outer surface of the hollow porous carbon microspheres; Based on the total mass of the lithium-sulfur battery composite cathode material, the mass percentage of elemental sulfur is 75-80%. The porosity of the hollow porous carbon microspheres is 50-55%.

2. The lithium-sulfur battery composite cathode material according to claim 1, characterized in that, The hollow porous carbon microspheres have mesopores and macropores with a pore size distribution of 2-50 nm on their surface.

3. The lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The hollow porous carbon microspheres have a particle size distribution of 0.5-15 μm.

4. The lithium-sulfur battery composite cathode material according to claim 3, characterized in that, The shell thickness of the hollow porous carbon microspheres is distributed in the range of 50-500 nm.

5. The lithium-sulfur battery composite cathode material according to claim 1, characterized in that, The portion of elemental sulfur distributed within the internal cavity is the first portion; the portion of elemental sulfur distributed on the outer surface is the second portion. The mass ratio of the first part to the second part is 8-10:

1.

6. The lithium-sulfur battery composite cathode material according to claim 1, characterized in that, The hollow porous carbon microspheres are doped with nitrogen and silicon.

7. A method for preparing a lithium-sulfur battery composite cathode material as described in any one of claims 1-6, characterized in that, The specific preparation steps include: Hollow porous carbon microspheres are available; Hollow porous carbon microspheres and elemental sulfur are mixed and heated to 180-200℃ in an inert atmosphere and kept at that temperature for 10-12 hours to load elemental sulfur onto the hollow porous carbon microspheres. Cooling yields the lithium-sulfur battery composite cathode material.

8. The method for preparing a lithium-sulfur battery composite cathode material according to claim 7, characterized in that, The specific preparation steps also include: Hollow porous carbon microspheres and elemental sulfur are mixed and heated to 180-200℃ at a rate of 0.3-0.5℃ / min in an inert atmosphere. The mixture is then held at 0.2-0.3MPa for 10-12 hours to load elemental sulfur onto the hollow porous carbon microspheres.

9. The method for preparing a lithium-sulfur battery composite cathode material according to claim 7, characterized in that, The hollow porous carbon microspheres provided include: Using silica microspheres as a hard template; A carbon-containing organic polymer layer is coated onto the surface of the rigid template to obtain a precursor; The precursor is carbonized at high temperature and then cooled to room temperature to obtain carbonized material; The carbide material is chemically etched to remove the hard template, resulting in hollow carbon microspheres; Hollow carbon microspheres were treated with an activator to obtain hollow porous carbon microspheres.

10. The method for preparing a lithium-sulfur battery composite cathode material according to claim 9, characterized in that, The hollow porous carbon microspheres also include: Using silica microspheres as a hard template; A carbon-containing organic polymer layer is coated onto the surface of the rigid template to obtain a precursor; The organic polymer layer is a polyacrylonitrile coating layer; Under inert gas protection, the precursor is heated to 800-1000℃ at a rate of 2-4℃ / min and carbonized for 3-4 hours, then cooled to room temperature to obtain carbonized material. The carbide material is chemically etched to remove the hard template, resulting in hollow carbon microspheres; Hollow carbon microspheres are mixed and impregnated with a sodium silicate-potassium hydroxide composite solution, dried, and then activated by heating at 780-850℃ in an inert atmosphere. After cooling, hollow porous carbon microspheres are obtained. In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8-10.

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