Preparation And Properties Of Composite Cathode For Lithium Sulfur Batteries

Rechargeable lithium–sulfur?Li-S?batteries are one of the most promising energy conversion and storage technologies owing to their high theoretical capacity?1675 mAh g-1?and energy density?2500 kW kg-1?,high abundance of sulfur,and low environmental impact.However,the insulating nature of sulfur and its discharge product?lithium sulfides?,the adverse shuttle effect of polysulfides,and the large volumetric expansion of sulfur during discharge have plagued the widespread practical utilization of Li-S batteries.This thesis focused on improving cycling performance of sulfur cathode.For this purpose,nitrogen doped porous carbon materials with manganese oxidecathode materials coating were prepared as the sulfur hosts for cathodes of Li-S battery.Main contents are as follows:?1?Preparation and Properties of NHPC@MnO₂/S cathodes.N-doped hollow porous carbon spheres?NHPC?synthesis followed an in suit coating strategy with melamineformaldehyde resin?MF?as the carbon and nitrogen source,with SiO₂ as the template.After template removal,NHPC was obtained.After loading MnO₂ nanosheets onto the surface of NHPC,the NHPC@MnO₂ were applied as high performance sulfur hosts for Li-S battery.The physical characterizations of samples were studied by means of XRD,TGA,FESEM,TEM,etc and electrochemical performance was tested.N-doping of carbon and deposition of the inherently polar MnO₂ promote chemical binding of the host with sulfur and its reduction product,polysulfides.Proper N-doping can improve the electron conductivity of carbon,and the nanosheet structure facilitates electron transport through MnO₂.Attributed to these advantages,the NHPC@MnO₂/S cathodes with a high sulfur content?70 wt %?exhibit an excellent cycle stability?93% retention within 100 cycles?at 0.5 C and an ultra-low capacity decay rate of 0.07 % per cycle over 500 discharge-charge cycles at 2 C.?2?Preparation and Properties of NMWCNTs@MnO₂/S cathodes.N-doped multiwalled carbon nanotube?NMWCNTs?were obtained by modification of MWCNTs.NMWCNTs@MnO₂ were then prepared by an in suit growth of MnO₂ nanosheets on the surface of NMWCNTs,and evaluated as high performance sulfur hosts for Li-S battery.The physical characterizations of samples were studied by means of XRD,TGA,FESEM,TEM,etc.Compared with MWCNTs/S and NMWCNTs /S cathodes,electrochemical performance was improved effectively,especially for cycling stability.The NMWCNTs@MnO₂/S cathodes with a high sulfur content?61 wt %?exhibit an excellent cycle stability?92% retention within 100 cycles?at 0.1 C,which was obviously superior to MWCNTs/S and NMWCNTs /S cathodes.To summarize,our work demonstrates the synergistic effect of MnO₂ nanostructure and Ndoped carbon nanospheres for enhanced performance of lithium sulfur battery cathodes.