Development Of High Performance Li-S Batterycathode With High Sulfur Content And Suppressed Shuttle Effect

To overcome the global enviromental and energy challenges,the automobile industry is required to develop more convenient,safer and environmentally benign alternatives;as a result,it is expected that the traditional fuel vehicles would be replaced by the hybrid electric vehicles and/or electric vehicles.Nevertheless,the traditional lead-acid batteries and lithium ion batteries fail to satisfy the increasing demand on the energy density,the power density and the security standard.Lithium-sulfur batteries are considered as the next-generation battery system,attributing to its high energy density,low cost,environmental friendliness and high energy conversion efficiency.At present,carbon or oxides/sulfides are widely adopted as host materials of sulfur to synthesize the cathode materials in Li-S batteries.However,the sulfur content in those composites are usually low,typically below 80%,with narrow cavities in such host materials,sulfur solids tend to be randomly deposited outside of the cavities,resulting in undesirable dissolution and severe shuttling of lithium polysulfides during the cyling process.In this thesis,high performance Li-S cathodes with high sulfur content were developed.In general,sulfur microspheres were in-situ encapsulated within a metal-or carbon-based permeable nanoshell;owing to their structural and compositional merits,the technological challenges which hamper the development of high-performance Li-S batteris(e.g.,the shuttle effect and the poor electrical conductivity of sulfur)coud be alleviated or circumvented.The contributions are specified as the three parts attached below.In the first project,the S@Ag/SnOx composites with different particle sizes and morphologies were synthesized by a chemical plating method,where the sulfur was encapsulated within a porous and Ag-based core-shell,and the content of sulfur of S@Ag composites was 89%.The shell of the S@Ag/SnOx composites with ca.67 nm in thickness was formed from numerous well connected Ag or SnOx nanoparticles with small sizes(?12 nm),as revealed by the SEM?TEM?XPS characterization and analysis.The electrochemical performance and in situ UV-vis spectroscopy results displayed that the S@Ag/SnOx composites released an initial charge capacity of 1231 mAh g-1 at 0.1 C and the leakage of the polysulfides would be effectively restrained by the inorganic shell which works as a physical barrier in the Li-S cells.In the second project,a wet chemical method was developed to in-situ synthesize core-shell structure S@Co(OH)2 composite with 80%sulfur content.The measurement of the electrochemical performance revealed that the initial discharge capacity of S@Co(OH)2 composite was 1052 mAh g-1 at 0.1 C and a capacity of 746 mAh g-1 was retained after 120 cycles.The density functional theory(DFT)was also operated to understand the interaction between the Co(OH)2 nanosheets and the polysulfides,which confirmed the dual-role of the Co(OH)2 nanoshell in the electrode.That is,it can not only work as a physical barrier to inhibit the leakage of the polysulfides,but also catalyze the decomposition of the polysulfides during the discharge/charge process,further suppressing the shuttle effect.Furthermore,when the S@Co(OH)2 cathode is intercalated with an interlayer of carbon nanofibers(CNF,between the electrodes and separator),the discharge capacity of the Li-S batteries could be significantly enhanced to 1485 mAh g-1.While at 2 C,the S@Co(OH)2 electrode displayed a reversible capacity of 606 mAh g-1 after 1000 cycles,which revealed an improved cycling stability and low capacity decay rate.Last but not least,submicron sulfur sphere@carbon nanofibers(S@CNFs)composite was synthesized,which was used together with the Fenugreek Gum(FG,a new water soluable binder with abundant polar groups),in Li-S cells.The ductility of FG is as high as 32%,which could help to tolerate the volume expansion of the S electrode.The electrochemical measurements,SEM and XPS characterizations demonstrated that there exist strong chemical interactions between O-groups in the FG binder and the sulfur species,which can form strong Li-O/S-O bonds that could prevent the lithium polysulfides from leaking out of the CNFs and thus inhibit the shuttle effect.The as-constructed S@CNFs electrode with FG binder delivers an initial capacity of 900 mAh g-1 at 2C and a reversible capacity of 410 mAh g'1 after 1300 cycles,with a low capacity decay of 0.04%per cycle and capacity retention of 45.6%.In summary,a general in-situ coating method was developed which enables coating of a metal-based porous thin layer with different compositions on sulfur particles,leading to sulfur composites with core-shell structure,high sulfur content(>80%)an high electrical chemical performance in Li-S batteris;and when working togerther with a polar water soluable binder such as FG,their electrical performance could be further enhanced owing to the strong chemical interaction between the binders and polysulfides.This thesis provides a general approach to the development of Li-S cathodes with high sulfur content and suppressed shuttle effect.