Preparation Of Functional Cathode Host Matreials For High Performance Lithium Sulfur Batteries

Sulfur cathodes combined with lithium anodes could provide a capacity of 1675mAh g^-1,which affords times higher energy density than the commercial lithium ion batteries.Nevertheless,their attainability for further application of Li-S batteries is seriously questioned,especially for the difficulty of sluggish kinetics and shuttle effects.Based on the issues mentioned above,this thesis designs four sulfur host materials with chemical adsorption capacity to lithium polysulfides through doping and compositing with polar materials.The design intentions focus on understanding and improving the factors that affect the reaction kinetics of lithium sulfur batteries,and put forward new ways to improve the reaction kinetics,providing new design concept for the preparation of lithium sulfur batteries.A nori-derived carbon(ANOC)via carbonization and subsequent activation is prepared as sulfur host.The obtained ANOC has abundant nitrogen and oxygen,which provide multiple polar sites to anchor polysulfides.Furthermore,the 3D layered hierarchical structure of the ANOC provides an electron transfer network to perform a high discharge capacity even at a high discharge rate.The result demonstrates that even with a high sulfur content of 81.2 wt%,the cathode still reveals a capacity of 657 mAh g^-1 after 100 cycles at 0.2 C.A hollow MoO₂ sphere anchored on nitrogen-doped graphene is synthesized as sulfur host.The nitrogen-doped graphene could provide a conductive network and ensure a high sulfur loading.Meanwhile,the C-O-Mo bond between MoO₂ and N-doped graphene explores a pathway for electron transfer.The aforesaid synergistic effect would enhance the kinetics of lithium sulfur batteries profitably.The discharge capacity could reach 615 mAh g^-1 at the current of 2 C,and retain 82%after 500 cycles at 1 C.BiOCl is employed as sulfur host material in Li-S batteries,which could integrate suppression of shuttle effect and promotion kinetics redox reactions of lithium polysulfides.The polar BiOCl performs robust chemical adsorption ability to polysulfides,and the electrocatalytic activity of BiOCl would accelerate the fragmentation of polysulfides into shorter chains.The battery with BiOCl/G-S cathode reveals a high capacity of 1414 mAh g^-1 at the current density of 0.1 C and the low capacity decay rate of 0.007%during 2000 cycles at 2 C.PTCDI/G is employed as redox mediator to accelerate the conversion process between polysulfides and sulfur.PTCDI has redox reaction ability,and the product serves as reducing and oxidizing agent in facilitating the process of both charging and discharging.Moreover,PTCDI has strong adsorption ability with polysulfides to restrain shuttle effect,resulting in promotional kinetics and cycle stability.Therefore,a high initial capacity of 1496 mAh g^-1 at 0.05 C and slight capacity decay of 0.009%per cycle at 5 C over 1500 cycles is achieved with PTCDI/G host.