Overpotential-Regulated Stable Cycling of a Thin Magnesium Metal Anode

To obtain high energy density for magnesium (Mg)-metal batteries, a promising low-cost energy storage technology, a thin Mg-metal anode of tens of micrometers must be used. However, the Coulombic efficiency (CE) and the anode utilization rate (AUR) of thin Mg metal are far from sufficient to sustain a long cycle life. This drawback is closely related to the morphological instability during galvanostatic cycling. In this work, we observed that the morphological evolution of Mg metal can be controlled with a pre-applied overpotential. With a properly pre-applied overpotential (e.g., -0.5 V), we show that the average AUR and the average CE of thin Mg metal (16 μm, equivalent to 6 mA h cm^-2) in a Mg/Mo asymmetric cell can be substantially improved from 29.8 to 74.8% and from 97.7 to 99.5%, respectively, under a practical current density of 2 mA cm^-2. These advances can theoretically improve the energy density and cycle life of Mg-S batteries to more than 1000 W h kg^-1 and 100 cycles, respectively. This work deepens our understanding of the morphological and compositional evolution of Mg metal during stripping and plating processes and suggests a facile and effective method to substantially improve the cycling stability of thin Mg metal.