Preparation And The Electrochemical Properties Of S@C@TiO₂@C

With the increasing severity of energy and environmental issues,the demand for clean renewable energy such as solar energy is increasing day by day.Lithium-sulfur batteries are the most promising next-generation energy storage systems because of their high specific capacity and energy density.However,the poor conductivity of sulfur,the volume expansion during charge and discharge,and the dissolution of polysulfide lead to low efficiency and poor stability of the battery,which limits the practical application of lithium-sulfur batteries.In order to solve the problems encountered in lithium-sulfur batteries,researchers often combine sulfur with other conductive materials and materials with sulfur-retaining effects in an attempt to break the limitations in the practical use of lithium-sulfur batteries.Carbon materials are favored by researchers because of their low cost and good electrical conductivity.In addition,due to its unique structure and properties,metal oxide materials can provide abundant polar active sites for the adsorption of polysulfides,effectively suppress the dissolution of polysulfides,and alleviate the poor stability of cells caused by the shuttle effect,and are widely used in lithium-sulfur battery.Therefore,carbon and metal oxides are expected to make full use of the advantages of both of them to comprehensively improve the performance of lithium-sulfur batteries.In this paper,the common TiO₂ and MnO₂ were selected as the research object.By compounding with carbon materials,a variety of nano-composite materials with core-shell structure were prepared and used in lithium-sulfur cathode materials,which greatly improved the cycle stability and rate performance.The main research results are as follows:（1）Using SiO₂ as a template,dopamine hydrochloride as a carbon source and tetrabutyl titanate as a titanium source,the hollow C@TiO₂ was obtained through carbonization and etching steps,and then combined with sublimed sulfur to obtain S@C@TiO₂ cathode material.It was found that the two porous structures of the material increase the specific surface area and reluctantly make contact with the electrolyte.At the same time,the conductive carbon layer provides an effective way for electron/ion transmission and reduces the the battery resistance.Moreover,the TiO₂ layer interacts with lithium polysulfide during the entire cycle,which can enhance the suppression of the shuttle effect,thereby improving the performance of the lithium-sulfur battery.After 100 cycles at 1C,it still maintains a specific capacity of nearly 80%,displaying a good cycle stability.（2）The hollow structure C@MnO₂ was prepared from KMnO₄ as a raw material,and was then combined with elemental sulfur to obtain S@C@MnO₂.Through relevant characterization and electrochemical performance test,it can be proved that owing to the joint action between the conductive carbon layer and MnO₂ with sulfur-retaining effect,the material greatly improves the conductivity of the battery,and at the same time it plays a very good inhibitory effect on the dissolution of lithium polysulfide during the cycle,thereby increasing battery performance.There still retains 70%of the specific capacity after 100 cycles at 1C.Thus,it is an excellent lithium-sulfur battery anode composite material.（3）Based on the above research results,a hollow C@TiO₂@C nanostructure with a three-layered core-shell structure was successfully synthesized and composited with sulfur to prepare S@C@TiO₂@C composite.The electrochemical performance test found that it has a first-round Coulomb efficiency of up to 99%and excellent rate performance,and has a specific capacity of517.2 mA h g^-1 at 1C after 200 cycles.The excellent electrochemical performance comes from its unique three-layer composite structure,which not only acts as a sturdy protective shell,effectively relieving the mechanical stress caused by the volume expansion caused by sulfur in the charge-discharge process,and greatly enhances the stability of the material in the circulation process.Integrated with the improve the conductivity provided by double carbon layer,the S@C@TiO₂@C play an important role in enhancing the full battery performance.The new three-layered sulfur cathode composite material is expected to become an ideal cathode material for lithium-sulfur batteries,and promote the further development of the industry.