Reversibility of a High-Voltage, Cl--Regulated, Aqueous Mg Metal Battery Enabled by a Water-in-Salt Electrolyte

Rechargeable Mg batteries are a promising post-Li-ion battery technology, but their development has been critically hampered by the passivating nature of Mg, particularly in aqueous solutions. Due to a quick dismissal of its reversibility, the use of Mg anodes in aqueous electrolytes has been overlooked, and most researchers focus on 1 nonaqueous systems instead. In this work, reversible, aqueous Mg battery chemistry has been realized for the first time, via the conversion of its impermeable passivation film to a conductive metallic oxide complex, facilitated by Cl- regulation and the suppression of the hydrogen evolution reaction using a MgCl2 water-in-salt (WIS) electrolyte. When coupled with copper hexacyanoferrate as the cathode, the full battery exhibits an impressive voltage plateau of 2.4-2.0 V and a stability of over 700 cycles with a Coulombic efficiency of up to 99% at 0.5 A g(-1). Mg dissolution and deposition have proven reversible in the aqueous MgCl2 WIS electrolyte.