Approaching Electrochemical Limits of Mg x Cl y z+ Complex-Based Electrolytes for Mg Batteries by Tailoring the Solution Structure

The future demand for energy storage requires the development of next generation batteries, e.g. based on magnesium (Mg). Mg as anode material offers great advantages such as low costs and a high volumetric capacity compared to state-of-the-art anodes. However, the lower standard potential of Mg∣Mg 2+ (−2.36 V vs SHE) compared to Li∣Li + (−3.04 V vs SHE) or Li + intercalation/deintercalation into/from graphite (≈−2.95 V vs SHE) emerges the need for high voltage cathodes and suitable electrolytes to achieve competitive cell energy values. The oxidative stabilities of less than 3.5 V vs Mg∣Mg 2+ for most of those electrolytes which enable Mg electrodeposition/-dissolution is too low to facilitate needed high-voltage Mg-based batteries. In this study, we therefore investigate the limits of oxidative stability of a commonly used Mg(TFSI) 2 - and MgCl 2 -based electrolyte by variation of solvents (ethers and ionic liquids) and salt ratios. Further on, we highlight the underlying reasons for the oxidative stability limits.