<i>In-Operando</i> FTIR Spectroscopy Study\non Rapid Polysulfide Binding via Caffeine in Lithium–Sulfur\nBatteries

Lithium–sulfur batteries are limited by the high\nmobility\nof polysulfides in the electrolyte, which allows them to migrate from\nthe cathode to the lithium anode. This is known as polysulfide shuttling\nand simultaneously diminishes the active material and poisons the\nanode. Various cathode additives have been shown empirically to mitigate\nthis problem, although the mechanism is not often ascertained experimentally.\nHerein, we demonstrate for the first time that the small molecule\nadditive caffeine reduces the polysulfide shuttling current in a lithium–sulfur\nbattery and decreases the capacity fade in galvanostatic cycling experiments.\nUsing <i>in-operando</i> Fourier transform infrared (FTIR)\nspectroscopy, we identify reversible shifts in the carbonyl stretching\nfrequencies of caffeine that are strongly correlated with the onset\nof polysulfide formation during both discharging and charging. These\nspectroscopic shifts are consistent with a polar–polar interaction\nbetween polysulfides and the carbonyl groups of caffeine, leading\nto the observed decrease in polysulfide mobility.