In Situ UV–Vis Analysis of Polysulfide Shuttling in Ionic Liquid-Based Li-FeS2 Batteries

Herein, a combination of in situ and ex situ UV–Vis spectroscopy is used to study the formation and shuttling of polysulfides (PSs) in Li-FeS2 batteries. We find three key results. First, PS shuttling significantly decreases (∼4×) when an ionic liquid electrolyte (1.0 M LiFSI in PYR14TFSI; ILE) is used in place of a conventional organic electrolyte (1.0 M LiTFSI in 1:1 DOL:DME). Furthermore, when the Li salt anion in the ILE is changed from FSI– to TFSI–, PS shuttling is effectively suppressed. Second, the total amount of PSs formed using the ILE is approximately double the amount shuttled. This result has implications for battery aging as solubility of PSs could conceivably increase upon extended storage. Third, the majority of PS shuttling occurs during the first cycle. Together, these findings provide important insights for understanding and thus preventing PS shuttling in Li-FeS2 batteries.