Regulating The Catalytic Conversion Kinetics Of Polysulfides For Li-S Batteries

Lithium-sulfur（Li-S）batteries feature high specific capacity,environmental friendliness,natural abundance,and low cost of sulfur cathode and are considered as one of the most promising alternatives for next-generation energy storage systems.Nonetheless,the practical applications of Li-S batteries still face great challenges,especially the shuttle effect and the sluggish conversion kinetics of polysulfide lead to the irreversible loss of active species,low sulfur utilization,and poor electrochemical performance.To solve the above-mentioned problems,researchers mainly focus on the construction of sulfur host materials,separator modification design,electrolyte optimization,electrode protection,and other aspects.Among them,catalytic materials employed in the additive of sulfur cathode or separator modification simultaneously achieve the ability of chemical adsorption and catalytic conversion toward polysulfides,thus improving the electrochemical performance of Li-S batteries.Herein,we have designed transition metals and metal compounds as separator modification materials or sulfur cathode additives,and combined with experimental characterizations and theoretical calculations to comprehensively analyze the effect of catalytic materials on the shuttling polysulfides,kinetic conversion process,and electrochemical performance.The detailed research contents are as follows:（ⅰ）Well-dispersed cobalt nanoparticles（～0.8 wt%）embedded in nitrogen-doped hierarchical porous carbon（Co@N-HPC）are prepared via high-temperature pyrolysis strategy.The experimental and theoretical studies reveal that Co@N-HPC could not only possess strong chemical affinity to polysulfides but also reduce the Li ion diffusion barrier and promote the Li₂S deposition/dissolution,thus effectively suppressing the shuttle effect and facilitating the polysulfide reaction kinetics.In addition,highly dispersed Co nanoparticles in three-dimensional carbon matrix ensure the exposure of ample polysulfide confining sites and catalytically active sites.Consequently,the Li-S batteries employed with Co@N-HPC functional separators exhibit a superior rate capability(808.4 mAhg^-1 at 10 C),an excellent cycling stability（capacity decay of 0.055%per cycle after 1000 cycles at 4 C）,and an advanced areal capacity retention(5.78 mAh cm^-2 over 100 cycles under an elevated sulfur loading of 7 mg cm^-2).（ⅱ）Fe₃C electrocatalysts are synthesized via freeze-drying and high-temperature method,and are regarded as the sulfur additives.During the reduction process,reactive Li₂S with a unique three-dimensional porous structure is deposited on Fe₃C surface,which not only shortens ion/electron diffusion path but also exposes and retains more active sites in Fe₃C.While for the following oxidation process,Fe₃C also facilitates the reactive Li₂S decomposition to renew electrocatalyst surface and guarantee the retention and exposure of active sites,thus accomplishing the effective contact and continuous conversion between the catalytic active site and sulfur species.Comprehensive experimental and theoretical calculation results demonstrate that even under the high sulfur loading,low temperature and lean electrolyte,Fe₃C electrocatalysts can effectively suppress the shuttle effect and enhance the conversion kinetics of polysulfide,demonstrating an excellent electrochemical performance.Therefore,high areal capacity Li-S batteries(>6 mAh cm^-2)under a wide-temperature range（-10～40℃）are obtained via the as-fabricated Fe₃C combined with carbon nanofiber sulfur cathode.Additionally,with a lean electrolyte/sulfur（E/S）ratio of 8 μL mg^-1 and elevated sulfur loading of 16.1 mg cm^-2,the initial areal capacity reaches up to 15.2 mAh cm^-2 and the excellent capacity of 10 mAh cm^-2 is maintained over 70 cycles.