Exploring sulfur solubility in ionic liquids for the electrodeposition of sulfide films with their electrochemical reactivity toward lithium

Metal binary sulfides (TiS2, FeS2), in either powder or thin film forms, were the first studied Li insertion electrodes for rechargeable lithium batteries, with thin films made mainly by sputtering. Here we exploit the equilibrium solubility of molecular sulfur into ionic liquids at its melting point (120 °C), which we estimated to be at a maximum level of 80 mM by both electrochemical and microwave studies, to prepare thin films of both Co9S8 and FeSx showing initial capacities of 559 mAh g−1 and 708 mAh g−1 versus lithium in coin cells, respectively. We demonstrate that the growth of Co9S8 films involves the reaction of soluble sulfur with the electrodeposited Co metallic layer, while the formation of FeSx films enlists a precipitation between the reduced Fe(II) cations and the electrochemically produced Sxy− species in the ionic liquid bath. Such findings, namely the solubility of sulfur into ionic liquids, open opportunities to electrodeposit sulfur-based compounds as well as capture sulfur from various media enabling a better environment.