Study On Preparation And Electrochemical Properties Of S@TiO₂/PPy Composite For Lithium-sulfur Battery Cathode Material

Lithium-sulfur battery is promising to replace the lithium-ion battery in the field of energy storage and conversion due to its high specific capacity and specific energy,and green non-polution characteristics,etc.However,sulfur’s inherent defects have been still hindering its commercial application,such as low conductivity,volume expansion during charge/discharge,shuttle effect caused by soluble long chain polysulfides,etc.To further improve the lithium-sulfur battery’s electrochemical properties,in this thesis,the S/PPy,S@TiO2 and S@TiO2/PPy composite were designed to study the effects of different components and structures on the electrochemical performance of the battery,respectively.The major research contents and results of this thesis are as follows:(1)Polypyrrole nanowires with a diameter of 80 nm and polypyrrole nanotubes of 180 nm were respectively prepared by a chemical oxidation method.Electrochemical properties results showed that the polypyrrole nanowires with high specific surface area exhibited better performance.The initial discharge capacity of S/PPy-wire was 1232.3 mAh g-1 and the capacity still retain 56.3%after 100 cycles,at a current density of 0.2 C.While the initial discharge capacity of S/PPy-tube were 1195.1 mAh g-1,and the capacity only retain 43.0%at the same current density.(2)The TiO2 hollow spheres with a diameter of 220 nm was prepared by RF resin spheres by using a sacrificial template method.We further investigated the effects of hollow spheres’morphology and sulfur’s content on electrochemical performance.The results showed that the 60 wt.%S@TiO2composite with a core-shell structure possessed the best electrochemical properties.The initial discharge capacity of S@TiO2 electrode was 732.2 mAh g-1 and the capacity still retain 453.2 mAh g-1 after 100 cycles,at a current density of 0.5 C.(3)S@TiO2/PPy composite were prepared by sulfur encapsulated into TiO2 hollow spheres and then loaded on a PPy-wires matrix together.The S@TiO2/PPy cathodes for lithium-sulfur batteries delivered an excellent electrochemical performance with a high initial specific capacity of 1385.9 mAh g-1 at 0.2 C and a low decay rate of 0.047%per cycle in 500 cycles at 1 C.The enhanced property of S@TiO2/PPy composites maybe derived from a synergistic reaction on sulfur between TiO2 hollow spheres and conductive polypyrrole nanowires,which played a vital role on inhibiting the shuttle effect and improving the conductivity of sulfur cathodes through encapsulating sulfur active materials,chemical adsorbing polysulfides and mixing the conductive component.Through the research of this thesis,we obtained a lithium-sulfur battery with S@TiO2/PPy composite cathode in high specific capacity and long cycle properties at 1 C,which provided an experimental basis for solving the cathode’s problem of lithium-sulfur battery.