Catalysis In Li-S Batteries: Study Of Mechanism And Construction Of Materials

Lithium-sulfur（Li-S）batteries show great promise for next-generation energy storage devices due to their theoretically high energy density(～2600 Wh kg^-1),low cost and environmental friendliness.However,the shuttle effect caused by the intermediate lithium polysulfide intermediates（Li₂S_n,3≤n≤8）,leads to large electrolyte demand,low utilization of sulfur active materials,and short cycling life,which is one of the main bottlenecks to the practical use of Li-S batteries.With a purpose to improve the cycling life of Li-S batteries,this thesis focuses on the study of catalysis mechanism,screening appropriate catalysts to accelerate the conversion of Li₂S_n and reduce their accumulation in the electrolyte,basically suppressing the shuttle effect.The shuttling of soluble lithium polysulfides between the electrodes leads to serious capacity fading and excess use of electrolyte,which is the main bottlenecks for practical use of Li-S batteries.Here selective catalysis is proposed as a fundamental remedy for the consecutive solid-liquid-solid sulfur redox reactions.The proof-of-concept In-based catalyst targetedly slows down the solid-liquid conversion,dissolution of elemental sulfur to polysulfides,while accelerates the liquid-solid conversion,deposition of polysulfides into insoluble Li₂S,which basically reduces accumulation of polysulfides in electrolyte,finally inhibiting the shuttle effect.The selective catalysis is revealed,experimentally and theoretically,by changes of activation energies and kinetic currents,modified reaction pathway together with the probed dynamically changing catalyst（Li In S₂ catalyst）,and gradual deactivation of the catalyst.The In-based Li-S battery works steadily over 1000 cycles at 4.0 C and yields an initial areal capacity up to 9.4 m Ah cm^-2 with a sulfur loading of～9.0 mg cm^-2.Catalysis is recently proposed as a proactive strategy to promote the polysulfide conversion and basically restrain the shuttle effect in Li-S batteries.For the catalysis process,the binding between the adsorbates and catalyst surface should be neither too strong nor too weak.Here,we explore a group of main-group metal sulfide catalysts taking advantage of their moderate binding with polysulfide species.The bismuth sulfide is found to present a proper binding energy,which balances the adsorption and desorption of polysulfides,resulting in the superior catalytic performance.The graphene-Bi₂S₃（G-Bi₂S₃）catalyst was prepared though a biomolecule-assisted self-assembly process.The sulfur content can reach as high as 82.8 wt%when the G-Bi₂S₃ hybrids were used as the sulfur host.The shuttle effect of the Bi₂S₃-catalyzed battery is greatly suppressed as the concentration of polysulfides at the the separator/lithium interface detected by in-situ Raman spectra is much lower than that of the Bi₂S₃-free battery.As a result,the Bi₂S₃-catalyzed battery catalysts works steadily with a capacity degradation rate of 0.03%per cycle over 500 cycles at an ultrahigh rate of 5.0 C.When the sulfur loading is increased to 17.6 mg cm^-2,a high initial areal capacity of～21.9 m Ah cm^-2 can be further obtained even with a low electrolyte/sulfur ratio of 7.5μL mg^-1.Ultrasmall vanadium nitride nanoparticles dispersed on porous nitrogen-doped graphene（denoted VN@NG）as a catalytic interlayer are in-situ synthesized to solve the shuttle effect in Li-S batteries.The ultrasmall size of VN particles provide ample triple-phase interfaces（the reactive interfaces among VN nanocatalyst,NG conductive substrate and electrolyte）for accelerating Li PS conversion,which greatly reduces the accumulation of Li PSs in the electrolyte and therefore inhibits the shuttle effect.With the VN nanocatalyst,Li-S batteries have achieved a high capacity retention of 84.5%over 200 cycles at 0.2 C with a high sulfur loading of 7.3 mg cm^-2.