A composite cathode for a lithium-sulfur battery and a lithium-sulfur battery

Abstract

The present invention belongs to the field of lithium-sulfur secondary batteries, and relates to a lithium-sulfur battery composite cathode and a lithium-sulfur battery. The composite cathode includes a positive current collector, a carbon-sulfur composite material layer located on the positive current collector, and a composite barrier layer located on the carbon-sulfur composite material layer. The composite barrier layer is composed of a TiO2-graphene layer close to the carbon-sulfur composite material layer and an SiO2-carbon nanotube layer located on the TiO2-graphene layer. The thickness of the TiO2-graphene layer is less than the thickness of the SiO2-carbon nanotube layer. The setting of a specific composite barrier layer can effectively inhibit the polysulfide shuttle effect, effectively prevent the loss of the sulfur cathode material, ensure that more active sulfur participates in the electrochemical reaction, prevent side reactions between the electrode material and the electrolyte, reduce the increase of the interfacial impedance, and will not excessively increase the thickness of the electrode, affect the lithium-ion transport efficiency or cause an increase in inactive components, thereby improving the energy density and capacity retention rate of the battery.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-sulfur secondary batteries, and particularly relates to a composite cathode for a lithium-sulfur battery and a lithium-sulfur battery. The composite cathode can effectively inhibit the polysulfide shuttle effect and improve the cycle stability and energy density of the battery. Background Art

[0002] With the progress of new energy vehicle technology and its huge market potential, modern society is extremely eager for advanced high-energy storage devices with low cost, high safety and environmental friendliness. The key to the development of electric vehicles lies in the battery. At present, great progress has been made in the lithium iron phosphate battery and the lithium-ion battery technology with ternary cathode materials widely used. However, due to the limitation of the theoretical lithium storage capacity of the cathode and anode materials themselves, it is very difficult to make a major breakthrough in the energy density of currently commercial traditional lithium-ion batteries. Lithium metal has become the preferred anode material in rechargeable battery systems due to its low density, high electronegativity and high theoretical specific capacity (3860 mAh g -1 ). Compared with the current mainstream ternary NCM lithium-ion battery, the battery with a sulfur cathode has a theoretical specific energy as high as 2600 Wh kg -1 , which is more than 10 times that of the currently widely used lithium-ion battery. At the same time, sulfur has the advantages of rich natural reserves, high theoretical specific capacity and green pollution-free, and can be selected as the cathode material to form a lithium-sulfur battery system with a lithium anode. The commercial lithium-sulfur battery energy device will be a great progress in the development of human science and technology.

[0003] Although lithium-sulfur batteries have significant theoretical advantages, their practical applications still face many challenges. Firstly, the initial product of lithium-sulfur battery discharge, long-chain polysulfides, is soluble in common ether-based electrolytes, providing a medium for the dissolution, diffusion, and shuttling of polysulfides between electrodes. This shuttling effect leads to the occurrence of redox side reactions and the corrosion of the lithium metal anode, greatly affecting the Coulombic efficiency and specific capacity retention rate of the lithium-sulfur battery system. In recent years, researchers have explored various methods to inhibit the diffusion and shuttling of lithium polysulfides. For example, CN118017049A discloses an interlayer material for lithium-sulfur batteries. The interlayer material includes a conductive base layer and a catalytic layer attached to at least part of the surface of the conductive base layer. The catalytic layer includes nanostructured oxides and manganese oxides. The nanostructured oxides are attached to the conductive base layer, and the manganese oxides are attached to at least part of the surface of the nanostructured oxides. There is a heterostructure interface layer between the manganese oxides and the nanostructured oxides. The improvement of catalytic activity is achieved by forming a heterostructure between the manganese oxides and the nanostructured oxides, and the oxides have higher stability than sulfides. Utilizing the conductivity of the conductive matrix and the good anchoring ability of the catalytic layer for polysulfides, polysulfide intermediates are effectively captured while accelerating their redox kinetics, thereby alleviating the shuttling effect of polysulfides. CN116845352A discloses a preparation method of a covalent organic framework-reinforced gel electrolyte for lithium-sulfur batteries. By using the gel electrolyte, the amount of electrolyte used is greatly reduced, the dissolution of lithium polysulfides into the electrolyte is inhibited, and at the same time, high ionic conductivity and lithium ion transference number are ensured. CN111370667A discloses a three-dimensional ordered porous sulfur-loaded material for the positive electrode of a lithium-sulfur battery. By combining the template method and the solvent-induced method, macropores are uniformly and orderly distributed in the positive electrode sulfur-loaded material, and these ordered macropores are connected by a framework containing rich micropores and mesoporous structures. In this porous conductive framework, polar sites zinc sulfide and single-atom active sites Co-N-C are widely distributed to fix the polysulfide intermediate products generated during the electrochemical reaction, so as to weaken the problem of rapid capacity decay caused by its "shuttling effect". CN117335094A discloses a preparation method of a high-entropy single-atom catalyst-modified separator. Different soluble metal salts are mixed and heat-treated with a carbon-nitrogen compound to obtain a precursor, and then the metal atoms on the precursor are migrated to a nitrogen-doped carbon carrier by high-temperature argon treatment to obtain a high-entropy single-atom catalyst, which can be used as a modifier material for the separator in a lithium-sulfur battery. The prepared high-entropy single-atom catalyst-modified separator greatly improves the wettability of the separator for the electrolyte, accelerates the kinetics of the sulfur redox reaction in the lithium-sulfur battery, and inhibits the shuttling effect.

[0004] However, due to the weak interaction of conventional materials, the adsorption capacity for polysulfides is limited, and a high degree of suppression of the shuttle effect cannot be achieved. Simply setting a barrier layer on the surfaces of the cathode and anode materials may affect lithium-ion transport and generate more defects, leading to a short-circuit risk, which in turn affects the performance of the lithium-sulfur battery. In addition, the introduction of a thicker barrier layer or more inactive carrier materials will also reduce the specific energy of the cathode itself. Therefore, it is necessary to develop a composite cathode for lithium-sulfur batteries. If the structure or composition of the cathode layer can be simply adjusted to effectively block the polysulfide shuttle effect, thereby improving the cycle stability, energy density, and safety of the lithium-sulfur battery, and enhancing the commercial application prospects of the lithium-sulfur battery. Summary of the Invention

[0005] To solve the defects in the prior art, the present invention provides a composite cathode for a lithium-sulfur battery and a lithium-sulfur battery. The composite cathode includes a cathode current collector, a carbon-sulfur composite material layer on the cathode current collector, and a composite barrier layer on the carbon-sulfur composite material layer. The composite barrier layer is composed of a TiO2-graphene layer close to the carbon-sulfur composite material layer and a SiO2-carbon nanotube layer on the TiO2-graphene layer. The setting of the composite barrier layer can effectively block the diffusion of polysulfides. First, the TiO2-graphene layer, as the first barrier, has high chemical stability and ion selectivity, which can prevent the diffusion of polysulfides from the cathode to the electrolyte, while graphene provides good electrical conductivity and mechanical strength to ensure smooth electron transport paths. SiO2 has excellent chemical stability and hydrophobicity, which can effectively prevent the penetration of polysulfides, and carbon nanotubes provide additional conductive channels to ensure the efficient transport of lithium ions. The setting of this layer further enhances the blocking effect on polysulfides. At the same time, the double-layer barrier layer material has good chemical stability and mechanical strength, can form a stable electrode / electrolyte interface in the battery, effectively prevent side reactions between the electrode material and the electrolyte, reduce the increase in interfacial impedance, thereby improving the electrochemical performance of the battery, and further improving the cycle stability and Coulomb efficiency of the battery.

[0006] To achieve the above object of the present invention, the present invention provides a composite cathode for a lithium-sulfur battery. The composite cathode includes a cathode current collector, a carbon-sulfur composite material layer on the cathode current collector, and a composite barrier layer on the carbon-sulfur composite material layer. The composite barrier layer is composed of a TiO2-graphene layer close to the carbon-sulfur composite material layer and a SiO2-carbon nanotube layer on the TiO2-graphene layer.

[0007] Further, the thickness of the TiO2-graphene layer is 0.05 - 5 μm, the thickness of the SiO2-carbon nanotube layer is 0.1 μm - 5 μm, and the thickness of the TiO2-graphene layer is less than that of the SiO2-carbon nanotube layer. The setting of the specific thickness of the composite barrier layer can first ensure that the barrier layer can effectively block the diffusion of polysulfides. This composite barrier layer can achieve the complete adsorption and catalysis of polysulfides without excessively increasing the thickness of the positive electrode layer and without affecting the lithium ion transport efficiency. At the same time, the reasonable thickness control of the present invention keeps the electrode / electrolyte interface in good contact, ensuring the high-efficiency operation of the battery.

[0008] Further, the thickness of the carbon-sulfur composite material layer is 200 - 500 μm, and the mass ratio of nano sulfur to conductive carbon black in this layer is 5 - 10:1. The content of sulfur in the positive electrode of the lithium-sulfur battery of the present invention is relatively high, the specific capacity is relatively high, and the battery has a relatively large energy density.

[0009] Further, the mass ratio of TiO2 to graphene in the TiO2-graphene layer is 1:2 - 5, and the mass ratio of SiO2 to carbon nanotubes in the SiO2-carbon nanotube layer is 2 - 5:1. The TiO2-graphene layer close to the carbon-sulfur composite material layer has a higher carbon content and can achieve rapid charge transport. In contrast, the SiO2-carbon nanotube layer far from the carbon-sulfur composite material layer has a lower carbon content, but the carbon nanotubes can also achieve charge transport along specific paths, and the lower carbon content of this layer can also improve the insulation performance between the positive and negative electrodes to a certain extent.

[0010] Further, the carbon-sulfur composite material layer is composed of elemental sulfur, conductive carbon black and a binder, and the preparation method is as follows:

[0011] Add elemental sulfur, conductive carbon black, and binder into an organic solvent, mix evenly to obtain a positive electrode slurry, coat the positive electrode slurry on a positive electrode current collector, and vacuum dry at 80 - 100 °C to form a carbon-sulfur composite material layer on the surface of the positive electrode current collector, thus obtaining it; the mass ratio of elemental sulfur, conductive carbon black, and binder is 5 - 10:1:0.5 - 1.

[0012] Further, the preparation method of the TiO2-graphene layer is as follows:

[0013] Add TiO2 particles, graphene nanosheets, and binder into an organic solvent, mix evenly to obtain a slurry, coat the above slurry on the carbon-sulfur composite material layer, and vacuum dry at 100 - 150 °C, thus obtaining it; the mass ratio of TiO2 particles, graphene nanosheets, and binder is 1:2 - 5:0.5 - 1.

[0014] Further, the preparation method of the SiO2-carbon nanotube layer is as follows:

[0015] Add SiO2 particles, carbon nanotubes, and a binder into an organic solvent, mix well to obtain a slurry, coat the above slurry on the TiO2-graphene layer, and dry it in air at 100-150 °C to obtain; the mass ratio of the SiO2 particles, carbon nanotubes, and binder is 2-5:1:0.5-1.

[0016] Furthermore, the same organic solvent and binder are used in the preparation of the carbon-sulfur composite layer, TiO2-graphene layer, and SiO2-carbon nanotube layer. Using the same binder can ensure good bonding between the layers. The binder forms a uniform interface between the layers, enhancing the adhesion between the layers and preventing the occurrence of interlayer peeling or delamination. At the same time, it can also reduce the defects between the interfaces of each layer, ensure seamless connection between the materials of each layer, and reduce the interface impedance. Preferably, the binder is one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose-styrene butadiene rubber, and polyvinyl alcohol, and the organic solvent is one of N-methylpyrrolidone, acetonitrile, and acetone.

[0017] Another object of the present invention is to provide a lithium-sulfur battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is a composite positive electrode, which includes a positive electrode current collector, a carbon-sulfur composite layer located on the positive electrode current collector, and a composite barrier layer located on the carbon-sulfur composite layer. The composite barrier layer is composed of a TiO2-graphene layer close to the carbon-sulfur composite layer and a SiO2-carbon nanotube layer located on the TiO2-graphene layer.

[0018] The present invention also provides a method for preparing a composite positive electrode of a lithium-sulfur battery, which includes the following steps:

[0019] 1) Prepare the carbon-sulfur composite layer: Add elemental sulfur, conductive carbon black, and a binder with a mass ratio of 5-10:1:0.5-1 into an organic solvent, mix well to obtain a positive electrode slurry, coat the positive electrode slurry on the positive electrode current collector aluminum foil, and vacuum dry it at 80-100 °C to form a carbon-sulfur composite layer with a thickness of 200-500 μm on the aluminum foil surface;

[0020] 2) Prepare the TiO2-graphene layer: Add TiO2 particles, graphene nanosheets, and a binder with a mass ratio of 1:2-5:0.5-1 into an organic solvent, mix well to obtain a slurry, coat the above slurry on the carbon-sulfur composite layer, and vacuum dry it at 100-150 °C to form a TiO2-graphene layer with a thickness of 0.05-5 μm on the carbon-sulfur composite layer;

[0021] 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and a binder with a mass ratio of 2 - 5:1:0.5 - 1 are added to an organic solvent, and the mixture is uniformly mixed to obtain a slurry. The above slurry is coated on the TiO2-graphene layer and dried in air at 100 - 150 °C to form a SiO2-carbon nanotube layer with a thickness of 0.1 μm - 5 μm, which is thicker than the TiO2-graphene layer, on the TiO2-graphene layer;

[0022] The binder is one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose-styrene butadiene rubber, and polyvinyl alcohol, and the organic solvent is one of N-methylpyrrolidone, acetonitrile, and acetone. The preparation method of this composite cathode adopts a simple slurry coating process. Each layer of material is uniformly mixed and then coated on the positive electrode current collector or the previous layer of material, followed by drying treatment. This process is simple and easy to implement, suitable for large-scale industrial production, and reduces the manufacturing cost. The same binder and dispersion solvent can ensure that each layer of slurry has similar rheological properties, so that each layer of material can be evenly distributed during the coating process, avoiding problems such as local accumulation or uneven thickness.

[0023] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0024] 1. The double-layer composite barrier layer with a specific composition can effectively inhibit the polysulfide shuttle effect, effectively prevent the loss of sulfur cathode material, ensure that more active sulfur participates in the electrochemical reaction, and improve the energy density and capacity retention rate of the battery. Titanium dioxide and silicon dioxide in the composite barrier layer can effectively prevent the diffusion of polysulfides from the positive electrode to the electrolyte, greatly inhibiting the polysulfide shuttle effect. Graphene and carbon nanotubes both have excellent electrical conductivity, and can significantly improve the overall electrical conductivity of the electrode without affecting the lithium ion transport, improving the cycle stability, Coulomb efficiency and rate performance of the battery;

[0025] 2. The introduction of the composite barrier layer forms a stable electrode / electrolyte interface, which can effectively prevent side reactions between the electrode material and the electrolyte, reduce the increase of interface impedance, and thus improve the electrochemical performance of the battery. The design of the two specific thicknesses enables the electrode / electrolyte interface to maintain good contact, ensuring that the barrier layer can effectively block the diffusion of polysulfides without overly increasing the thickness of the electrode, affecting the lithium ion transport efficiency or causing an increase in inactive components and affecting the specific energy of the battery.

[0026] 3. In addition, the setting of the composite barrier layer can also prevent the loss of sulfur cathode active material, ensure that more active sulfur participates in the electrochemical reaction, and improve the energy density and capacity retention rate of the battery. In addition, the composite barrier layer can also reduce the impact of the volume expansion of the electrode material on the battery structure and further extend the service life of the battery. Detailed implementation mode

[0027] To make the technical problems solved by the present invention, the technical solutions adopted, and the achieved technical effects clearer, the technical solutions of the embodiments of the present invention will be further described in detail below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0028] It should be noted that referring to "embodiment" in this article means that the specific features, structures, or characteristics described in connection with the embodiment may be included in at least one embodiment of the present application. The phrase appears at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0029] The experimental methods in the following embodiments are all conventional methods unless otherwise specified. The materials, reagents, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0030] Among them, the performance test of the lithium-sulfur battery in the present invention is carried out by assembling a 2032 lithium-sulfur button battery. Specifically: the prepared composite cathode is cut into small round pieces with a diameter of 12 mm, and then a commercial lithium sheet of the same size is used as the anode, the electrolyte is a solution of 1,3-dioxolane / ethylene glycol dimethyl ether (DOL / DME, volume ratio 1:1) containing 1 mol lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 0.1 mol / L lithium nitrate (LiNO3), and the separator is Celgard 2400, and a 2032 button battery is assembled. The specific assembly method is as follows: place the composite cathode into the positive electrode case, add 100 mL of electrolyte, place the separator, then place the anode on the separator, add a gasket and a spring washer and the negative electrode case in sequence on the anode, and finally press the positive electrode case and the negative electrode case together under a pressure of 50 MPa to obtain a 2032 button battery.

[0031] Example 1

[0032] A lithium-sulfur battery composite cathode, the preparation method is as follows:

[0033] 1) Prepare a carbon-sulfur composite material layer: Add elemental sulfur, conductive carbon black, and binder polyvinylidene fluoride with a mass ratio of 5:1:0.5 to the organic solvent N-methylpyrrolidone, mix well to obtain the positive electrode slurry, coat the positive electrode slurry on the positive electrode current collector aluminum foil, and vacuum dry at 80 °C to form a 200-μm-thick carbon-sulfur composite material layer on the surface of the aluminum foil;

[0034] 2) Preparation of TiO2-graphene layer: TiO2 particles, graphene nanosheets, and binder polyvinylidene fluoride with a mass ratio of 1:2:0.5 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the carbon-sulfur composite material layer and dried in vacuum at 100 °C to form a 0.05-μm-thick TiO2-graphene layer on the carbon-sulfur composite material layer;

[0035] 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and binder polyvinylidene fluoride with a mass ratio of 2:1:0.5 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the TiO2-graphene layer and dried in air at 100 °C to form a 0.1-μm-thick SiO2-carbon nanotube layer on the TiO2-graphene layer.

[0036] The lithium-sulfur battery using this composite cathode was tested. The initial discharge specific capacity at a rate of 0.5C was 1159 mAh g -1 , and the capacity retention rate was 92.2% after 800 cycles; the initial discharge specific capacity at a rate of 2C was 970 mAh g -1 , and the capacity retention rate was higher than 85% after 1000 cycles.

[0037] Example 2

[0038] A lithium-sulfur battery composite cathode was prepared as follows:

[0039] 1) Preparation of carbon-sulfur composite material layer: Elemental sulfur, conductive carbon black, and binder carboxymethyl cellulose-styrene-butadiene rubber with a mass ratio of 10:1:1 were added to the organic solvent acetone, mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on the positive electrode current collector aluminum foil and dried in vacuum at 100 °C to form a 500-μm-thick carbon-sulfur composite material layer on the aluminum foil surface;

[0040] 2) Preparation of TiO2-graphene layer: TiO2 particles, graphene nanosheets, and binder carboxymethyl cellulose-styrene-butadiene rubber with a mass ratio of 1:5:1 were added to the organic solvent acetone, mixed evenly to obtain a slurry. The above slurry was coated on the carbon-sulfur composite material layer and dried in vacuum at 150 °C to form a 4.5-μm-thick TiO2-graphene layer on the carbon-sulfur composite material layer;

[0041] 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and binder carboxymethyl cellulose-styrene-butadiene rubber with a mass ratio of 5:1:1 were added to the organic solvent acetone, mixed evenly to obtain a slurry. The above slurry was coated on the TiO2-graphene layer and dried in air at 150 °C to form a 5-μm-thick SiO2-carbon nanotube layer on the TiO2-graphene layer.

[0042] The lithium-sulfur battery using this composite cathode was tested. At a rate of 0.5C, the initial discharge specific capacity was 1241 mAh g -1 , and the capacity retention rate was 98.3% after 800 cycles; at a rate of 2C, the initial discharge specific capacity was 1016 mAh g -1 , and the capacity retention rate was higher than 90% after 1000 cycles.

[0043] Example 3

[0044] A composite cathode for a lithium-sulfur battery is prepared as follows:

[0045] 1) Prepare the carbon-sulfur composite material layer: Add elemental sulfur, conductive carbon black, and binder polytetrafluoroethylene with a mass ratio of 8:1:1 to organic solvent acetonitrile, mix well to obtain the positive electrode slurry, coat the positive electrode slurry on the positive electrode current collector aluminum foil, and vacuum dry at 90 °C to form a 300-μm-thick carbon-sulfur composite material layer on the aluminum foil surface;

[0046] 2) Prepare the TiO₂-graphene layer: Add TiO₂ particles, graphene nanosheets, and binder polytetrafluoroethylene with a mass ratio of 1:4:1 to organic solvent acetonitrile, mix well to obtain the slurry, coat the above slurry on the carbon-sulfur composite material layer, and vacuum dry at 120 °C to form a 1.5-μm-thick TiO₂-graphene layer on the carbon-sulfur composite material layer;

[0047] 3) Prepare the SiO₂-carbon nanotube layer: Add SiO₂ particles, carbon nanotubes, and binder polytetrafluoroethylene with a mass ratio of 4:1:1 to organic solvent acetonitrile, mix well to obtain the slurry, coat the above slurry on the TiO₂-graphene layer, and air dry at 120 °C to form a 3-μm-thick SiO₂-carbon nanotube layer on the TiO₂-graphene layer.

[0048] The lithium-sulfur battery using this composite cathode was tested. At a rate of 0.5C, the initial discharge specific capacity was 1193 mAh g -1 , and the capacity retention rate was 96.1% after 800 cycles; at a rate of 2C, the initial discharge specific capacity was 992 mAh g -1 , and the capacity retention rate was higher than 88% after 1000 cycles.

[0049] Example 4

[0050] A composite cathode for a lithium-sulfur battery is prepared as follows:

[0051] 1) Prepare the carbon-sulfur composite material layer: Add elemental sulfur, conductive carbon black, and binder polyvinyl alcohol with a mass ratio of 6:1:1 to organic solvent N-methylpyrrolidone, mix well to obtain the positive electrode slurry, coat the positive electrode slurry on the positive electrode current collector aluminum foil, and vacuum dry at 100 °C to form a 400-μm-thick carbon-sulfur composite material layer on the aluminum foil surface;

[0052] 2) Preparation of TiO₂-graphene layer: TiO₂ particles, graphene nanosheets, and binder polyvinyl alcohol with a mass ratio of 1:3:0.8 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the carbon-sulfur composite material layer and dried in vacuum at 140 °C to form a 3.5-μm-thick TiO₂-graphene layer on the carbon-sulfur composite material layer;

[0053] 3) Preparation of SiO₂-carbon nanotube layer: SiO₂ particles, carbon nanotubes, and binder polyvinyl alcohol with a mass ratio of 4:1:0.8 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the TiO₂-graphene layer and dried in air at 140 °C to form a 4-μm-thick SiO₂-carbon nanotube layer on the TiO₂-graphene layer.

[0054] The lithium-sulfur battery using this composite cathode was tested. The initial discharge specific capacity at a rate of 0.5C was 1172 mAh g -1 , and the capacity retention rate was 95.8% after 800 cycles; the initial discharge specific capacity at a rate of 2C was 985 mAh g -1 , and the capacity retention rate was higher than 86% after 1000 cycles.

[0055] Comparative Example 1

[0056] A lithium-sulfur battery cathode was prepared as follows:

[0057] 1) Preparation of carbon-sulfur composite material layer: Elemental sulfur, conductive carbon black, and binder polyvinyl alcohol with a mass ratio of 6:1:1 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on the positive electrode current collector aluminum foil and dried in vacuum at 100 °C to form a 400-μm-thick carbon-sulfur composite material layer on the aluminum foil surface;

[0058] 2) Preparation of TiO₂-graphene layer: TiO₂ particles, graphene nanosheets, and binder polyvinyl alcohol with a mass ratio of 1:3:0.8 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the carbon-sulfur composite material layer and dried in vacuum at 140 °C to form a 3.5-μm-thick TiO₂-graphene layer on the carbon-sulfur composite material layer.

[0059] The lithium-sulfur battery using this composite cathode was tested. The initial discharge specific capacity at a rate of 0.5C was 1153 mAh g -1 , and the capacity retention rate was 54.6% after 800 cycles; the initial discharge specific capacity at a rate of 2C was 960 mAh g -1 , and the capacity retention rate was 42% after 1000 cycles.

[0060] Comparative Example 2

[0061] A cathode for a lithium-sulfur battery is prepared as follows:

[0062] 1) Prepare a carbon-sulfur composite layer: Add elemental sulfur, conductive carbon black, and binder polyvinyl alcohol with a mass ratio of 6:1:1 into an organic solvent N-methylpyrrolidone, mix well to obtain a cathode slurry, coat the cathode slurry on a cathode current collector aluminum foil, and dry it under vacuum at 100 °C to form a 400-μm-thick carbon-sulfur composite layer on the aluminum foil surface;

[0063] 2) Prepare a SiO₂-carbon nanotube layer: Add SiO₂ particles, carbon nanotubes, and binder polyvinyl alcohol with a mass ratio of 4:1:0.8 into an organic solvent N-methylpyrrolidone, mix well to obtain a slurry, coat the above slurry on the carbon-sulfur composite layer, and dry it in air at 140 °C to form a 4-μm SiO₂-carbon nanotube layer on the carbon-sulfur composite layer.

[0064] The lithium-sulfur battery using this composite cathode is tested. The initial discharge specific capacity at a rate of 0.5C is 1139 mAh g -1 , and the capacity retention rate is 48.7% after 800 cycles; the initial discharge specific capacity at a rate of 2C is 937 mAh g -1 , and the capacity retention rate is 36.5% after 1000 cycles.

[0065] Comparative Example 3

[0066] A composite cathode for a lithium-sulfur battery is prepared as follows:

[0067] 1) Prepare a carbon-sulfur composite layer: Add elemental sulfur, conductive carbon black, and binder polyvinyl alcohol with a mass ratio of 6:1:1 into an organic solvent N-methylpyrrolidone, mix well to obtain a cathode slurry, coat the cathode slurry on a cathode current collector aluminum foil, and dry it under vacuum at 100 °C to form a 400-μm-thick carbon-sulfur composite layer on the aluminum foil surface;

[0068] 2) Prepare a SiO₂-carbon nanotube layer: Add SiO₂ particles, carbon nanotubes, and binder polyvinyl alcohol with a mass ratio of 4:1:0.8 into an organic solvent N-methylpyrrolidone, mix well to obtain a slurry, coat the above slurry on the carbon-sulfur composite layer, and dry it in air at 140 °C to form a 4-μm SiO₂-carbon nanotube layer on the carbon-sulfur composite layer.

[0069] 3) Preparation of TiO₂-graphene layer: TiO₂ particles, graphene nanosheets, and binder polyvinyl alcohol with a mass ratio of 1:3:0.8 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the SiO₂-carbon nanotube layer and dried under vacuum at 140 °C to form a 3.5-μm-thick TiO₂-graphene layer on the carbon-sulfur composite material layer.

[0070] The lithium-sulfur battery using this composite cathode was tested. The initial discharge specific capacity at a rate of 0.5C was 1148 mAh g -1 , and the capacity retention rate was 83.1% after 800 cycles; the initial discharge specific capacity at a rate of 2C was 902 mAh g -1 , and the capacity retention rate was 67% after 1000 cycles.

[0071] Comparative Example 4

[0072] A lithium-sulfur battery composite cathode was prepared as follows:

[0073] 1) Preparation of carbon-sulfur composite material layer: Elemental sulfur, conductive carbon black, and binder carboxymethyl cellulose-styrene-butadiene rubber with a mass ratio of 6:1:1 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on the positive electrode current collector aluminum foil and dried under vacuum at 100 °C to form a 400-μm-thick carbon-sulfur composite material layer on the aluminum foil surface;

[0074] 2) Preparation of TiO₂-carbon nanotube layer: TiO₂ particles, carbon nanotubes, and binder polyvinyl alcohol with a mass ratio of 1:6:1 were added to the organic solvent N-methylpyrrolidone, mixed evenly to obtain a slurry. The above slurry was coated on the carbon-sulfur composite material layer and dried under vacuum at 140 °C to form a 6-μm-thick TiO₂-carbon nanotube layer on the carbon-sulfur composite material layer;

[0075] 3) Preparation of SiO₂-graphene layer: SiO₂ particles, graphene, and binder polytetrafluoroethylene with a mass ratio of 6:1:0.8 were added to the organic solvent acetonitrile, mixed evenly to obtain a slurry. The above slurry was coated on the TiO₂-carbon nanotube layer and dried in air at 120 °C to form a 1-μm-thick SiO₂-graphene layer on the TiO₂-carbon nanotube layer.

[0076] The lithium-sulfur battery using this composite cathode was tested. The initial discharge specific capacity at a rate of 0.5C was 1087 mAh g -1 , and the capacity retention rate was 65.9% after 800 cycles; the initial discharge specific capacity at a rate of 2C was 913 mAh g -1 , and the capacity retention rate was 56.9% after 1000 cycles.

[0077] It can be seen from the test results of the embodiments and comparative examples of the present invention that the composite cathode of the lithium-sulfur battery prepared based on the present invention has good stability. The lithium-sulfur battery using this composite cathode has a high discharge specific capacity and a high capacity retention rate. It can be seen from the comparison of the discharge specific capacity after 1000 cycles of the electrode at the 2C rate that the setting of the specific composite barrier layer of the present invention can significantly improve the cycling performance of the lithium-sulfur battery. It can be seen that the presence of this barrier layer can achieve almost complete adsorption of polysulfides, greatly reducing the dissolution and migration of polysulfides in the electrolyte, which is beneficial to improving the cycling performance of the lithium-sulfur battery. It can be seen from the comparison of the comparative examples that although the composite cathode with only one barrier layer or the composite cathode with the SiO2-carbon nanotube layer set first and then the TiO2-graphene layer has a high initial discharge capacity, the battery capacity retention rate is low after long-term cycling, that is, the stability of its barrier layer is poor and it cannot achieve long-term effective adsorption of polysulfides.

[0078] The above has introduced a composite cathode of a lithium-sulfur battery and a lithium-sulfur battery in detail. The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, its architecture form can be flexible and changeable, and a series of products can be derived. Just making several simple deductions or substitutions should be regarded as belonging to the patent protection scope determined by the claims submitted by the present invention.

Claims

1. A method for preparing a composite positive electrode for a lithium-sulfur battery, characterized in that: It consists of the following steps: 1) preparing a carbon-sulfur composite material layer: adding elemental sulfur, conductive carbon black and a binder in a mass ratio of 5 to 10:1:0.5 to 1 to an organic solvent, mixing to obtain a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector aluminum foil, and vacuum drying at 80 to 100° C. to form a carbon-sulfur composite material layer with a thickness of 200 to 500 μm on the surface of the aluminum foil; 2) preparing a TiO2-graphene layer: adding TiO2 particles, graphene nanosheets, and a binder in a mass ratio of 1:2-5:0.5-1 to an organic solvent, mixing to obtain a slurry, coating the slurry on the carbon-sulfur composite material layer, and vacuum drying at 100-150° C. to form a 0.05-5 μm thick TiO2-graphene layer on the carbon-sulfur composite material layer; 3) preparing a SiO2-carbon nanotube layer: adding SiO2 particles, carbon nanotubes and a binder in a mass ratio of 2 to 5:1:0.5 to 1 to an organic solvent, mixing to obtain a slurry, coating the slurry on the TiO2-graphene layer, and drying in air at 100 to 150° C. to form a SiO2-carbon nanotube layer on the TiO2-graphene layer that is thicker than the TiO2-graphene layer and has a thickness of 0.1 μm to 5 μm; The TiO2-graphene layer and the SiO2-carbon nanotube layer constitute a composite barrier layer, and the binder and organic solvent used in preparing the carbon-sulfur composite material layer, the TiO2-graphene layer and the SiO2-carbon nanotube layer are the same.

2. The method for preparing a lithium-sulfur battery composite positive electrode according to claim 1, characterized in that: The thickness of the TiO2-graphene layer is 0.05-4.5 μm, and the thickness of the SiO2-carbon nanotube layer is 0.1 μm-5 μm.

3. The method for preparing a lithium-sulfur battery composite positive electrode according to claim 1, characterized in that: The thickness of the carbon-sulfur composite material layer is 300-500 μm.

4. The method for preparing a lithium-sulfur battery composite positive electrode according to claim 3, characterized in that: The thickness of the carbon-sulfur composite material layer is 300-400 μm.

5. The method for preparing a composite positive electrode for a lithium-sulfur battery according to claim 1, characterized in that: The binder used in preparing the positive electrode active material layer, TiO2-graphene layer and SiO2-carbon nanotube layer is one of polyvinylidene fluoride, polytetrafluoroethylene, carboxymethyl cellulose-styrene butadiene rubber and polyvinyl alcohol, and the organic solvent is one of N-methylpyrrolidone, acetonitrile and acetone.

6. A method for preparing a composite positive electrode for a lithium-sulfur battery, characterized in that: It consists of the following steps: 1) preparing a sulfur positive electrode active material layer: adding elemental sulfur, conductive carbon black and binder polyvinylidene fluoride in a mass ratio of 5:1:0.5 to an organic solvent N-methylpyrrolidone, mixing well to obtain a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector aluminum foil, and vacuum drying at 80° C. to form a 200 μm thick sulfur positive electrode active material layer on the surface of the aluminum foil; 2) preparing a TiO2-graphene layer: adding TiO2 particles, graphene nanosheets, and a binder polyvinylidene fluoride in a mass ratio of 1:2:0.5 to an organic solvent N-methylpyrrolidone, mixing to obtain a slurry, coating the slurry on the sulfur cathode active material layer, and vacuum drying at 100° C. to form a 0.05 μm thick TiO2-graphene layer; 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and binder polyvinylidene fluoride in a mass ratio of 2:1:0.5 were added to an organic solvent N-methylpyrrolidone, mixed to obtain a slurry, and the slurry was coated on the TiO2-graphene layer and dried in air at 100°C to form a 0.1μm SiO2-carbon nanotube layer on the TiO2-graphene layer; The lithium-sulfur battery with the composite cathode has a first-cycle discharge capacity of 1159 mAh g at a rate of 0.5C. -1 After 800 cycles, the capacity retention rate is 92.2%; the first cycle discharge capacity at a rate of 2C is 970mAh g -1 , the capacity retention rate is higher than 85% after 1000 cycles.

7. A method for preparing a composite positive electrode for a lithium-sulfur battery, characterized in that: It consists of the following steps: 1) preparing a sulfur positive electrode active material layer: adding elemental sulfur, conductive carbon black and a binder carboxymethyl cellulose-styrene-butadiene rubber in a mass ratio of 10:1:1 to an organic solvent acetone, mixing them evenly to obtain a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector aluminum foil, and vacuum drying at 100° C. to form a 500 μm thick sulfur positive electrode active material layer on the surface of the aluminum foil; 2) preparing a TiO2-graphene layer: adding TiO2 particles, graphene nanosheets, and a binder carboxymethyl cellulose-styrene-butadiene rubber in a mass ratio of 1:5:1 to an organic solvent acetone, mixing to obtain a slurry, coating the slurry on the carbon-sulfur composite material layer, and vacuum drying at 150° C. to form a 4.5 μm thick TiO2-graphene layer on the sulfur positive electrode active material layer; 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and a binder carboxymethyl cellulose-styrene-butadiene rubber in a mass ratio of 5:1:1 were added to an organic solvent acetone, mixed to obtain a slurry, and the slurry was coated on the TiO2-graphene layer and dried in air at 150°C to form a 5μm SiO2-carbon nanotube layer on the TiO2-graphene layer; The lithium-sulfur battery with the composite cathode has a first-cycle discharge capacity of 1241 mAh g at a rate of 0.5C. -1 After 800 cycles, the capacity retention rate is 98.3%; the first cycle discharge capacity at a rate of 2C is 1016mAh g -1 , the capacity retention rate is higher than 90% after 1000 cycles.

8. A method for preparing a composite positive electrode for a lithium-sulfur battery, characterized in that: It consists of the following steps: 1) preparing a sulfur positive electrode active material layer: adding elemental sulfur, conductive carbon black and binder polytetrafluoroethylene in a mass ratio of 8:1:1 to an organic solvent acetonitrile, mixing well to obtain a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector aluminum foil, and vacuum drying at 90° C. to form a 300 μm thick sulfur positive electrode active material layer on the surface of the aluminum foil; 2) preparing a TiO2-graphene layer: adding TiO2 particles, graphene nanosheets, and a binder polytetrafluoroethylene in a mass ratio of 1:4:1 to an organic solvent acetonitrile, mixing to obtain a slurry, coating the slurry on the sulfur cathode active material layer, and vacuum drying at 120° C. to form a 1.5 μm thick TiO2-graphene layer; 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and binder polytetrafluoroethylene in a mass ratio of 4:1:1 were added to an organic solvent acetonitrile, mixed to obtain a slurry, and the slurry was coated on the TiO2-graphene layer and dried in air at 120°C to form a 3μm SiO2-carbon nanotube layer on the TiO2-graphene layer; The lithium-sulfur battery using the composite cathode has a first-cycle discharge capacity of 1193 mAh g at a rate of 0.5C. -1 After 800 cycles, the capacity retention rate is 96.1%; the first cycle discharge capacity at a rate of 2C is 992mAh g -1 , the capacity retention rate is higher than 88% after 1000 cycles.

9. A method for preparing a composite positive electrode for a lithium-sulfur battery, characterized in that: It consists of the following steps: 1) preparing a sulfur positive electrode active material layer: adding elemental sulfur, conductive carbon black and binder polyvinyl alcohol in a mass ratio of 6:1:1 to an organic solvent N-methylpyrrolidone, mixing well to obtain a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector aluminum foil, and vacuum drying at 100° C. to form a 400 μm thick sulfur positive electrode active material layer on the surface of the aluminum foil; 2) preparing a TiO2-graphene layer: adding TiO2 particles, graphene nanosheets, and a binder polyvinyl alcohol in a mass ratio of 1:3:0.8 to an organic solvent N-methylpyrrolidone, mixing to obtain a slurry, coating the slurry on the sulfur cathode active material layer, and vacuum drying at 140° C. to form a 3.5 μm thick TiO2-graphene layer; 3) Preparation of SiO2-carbon nanotube layer: SiO2 particles, carbon nanotubes, and binder polyvinyl alcohol in a mass ratio of 4:1:0.8 were added to an organic solvent N-methylpyrrolidone, mixed to obtain a slurry, and the slurry was coated on the TiO2-graphene layer and dried in air at 140°C to form a 4μm SiO2-carbon nanotube layer on the TiO2-graphene layer; The lithium-sulfur battery using this composite cathode has a first-cycle discharge capacity of 1172 mAh g at a rate of 0.5C. -1 After 800 cycles, the capacity retention rate is 95.8%; the first cycle discharge capacity at a rate of 2C is 985mAh g -1 , the capacity retention rate is higher than 86% after 1000 cycles.

10. A lithium-sulfur battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The positive electrode is the composite positive electrode according to any one of claims 1 to 5.