Triiodide Anion as a Magnesium-ion Transporter for Low Overpotential Battery Cycling in Iodine-Containing Mg(TFSI)2 Electrolyte

Magnesium iodide (MgI₂) solid-electrolyte interface (SEI) layers have previously been shown to protect Mg metal anodes from passivation through products formed during Mg(TFSI)₂ electrolyte decomposition (TSFI = trifluorosulfonimide). MgI₂ formed in situ from small quantities of I₂ added to the electrolyte shows a drastic decrease in the overpotential for magnesium deposition and stripping. In this work, a MgI₂ SEI layer was created in an ex situ fashion and then the electrochemical characteristics of this MgI₂ SEI layer were probed both alone and with small quantities of I₂ or Bu₄NI₃ additives to identify the electroactive species. Chronopotentiometry (CP) and cyclic voltammetry (CV) show that the MgI₂ SEI alone is insufficient for low overpotential magnesium cycling. I(3d) XPS data show that I₃^- is formed within the SEI layer, which can serve as the electroactive species when ligated with Mg²⁺ for low overpotential (<50 mV at 0.1 mA cm^-2 current density) cycling. Moreover, Raman shifts at 110 and 140 cm^-1 are consistent with I₃^- formation, and these signatures are observed before and after CP experiments. The Mg⁰ deposition curves in the CV with additives are consistent with diffusive species. Finally, electrochemical impedance spectroscopy (EIS) shows that there is a large decrease in the charge-transfer resistance within the SEI when either I₂ or Bu₄NI₃ additives are used, which supports a solvating effect that facilitates magnesium deposition and stripping.