Improved Solid Electrolyte Interface on Lithium Metal Using High-Sulfur-Content Polymers

Abstract

Recent studies in lithium-sulfur batteries have demonstrated the need to balance suppressing the lithium polysulfide effect with protecting lithium metal anodes. On one hand, polysulfides - intermediates of lithium’s reaction with sulfur that are soluble in compatible electrolytes - react with and shuttle between the cathode and the anode, contributing to undesired self-discharge and capacity fading. On the other hand, those same polysulfides, working in conjunction with lithium nitrate (LiNO3, a common electrolyte additive), help form a stable SEI and suppress lithium dendrite formation by limiting unfavorable electrolyte decomposition reactions. The objective, then, is to find electrolyte additives that can protect lithium metal like polysulfides do, but without the self-discharge and capacity fading issues. We explore high-sulfur content polymers to accomplish this task. In this study, we specifically experiment with high-sulfur-content polymers based on trichloropropane (TCP). TCP-based sulfur polymers are synthesized via a facile emulsion method, with CTAB serving a phase transfer catalyst between TCP and lithium/sodium polysulfides. The sulfur content in the polymers can be highly controlled and can be as high as 90 wt%. Upon reacting with lithium, these polymers form both inorganic polysulfides and organic Li-S-C species, which improves the mechanical stability of the SEI. A lithium deposition efficiency of >99% can be achieved for 400 cycles using these high-sulfur-content polymers. Sulfur cathodes and lithium metal anodes could potentially move battery technology into the 21st century, but challenges still remain to realize their full potential. High-sulfur content polymers mimic the positive effects that lithium polysulfides have on the SEI of lithium metal, while also mitigating self-discharge and capacity fading.