Uneven Stripping Behavior, an Unheeded Killer of Mg Anodes

Uniform magnesium (Mg) plating/stripping under high areal capacity utilization is critical for the practical application of Mg-metal anodes in rechargeable Mg batteries. However, the failure of the Mg-metal anode when cycling under a practical areal capacity (>4 mA h cm^-2 ), is of rising concern. The mechanism behind these failures remains controversial. In this work, it is illustrated that the initial plating Mg can be undoubtedly uniform in a wide range of current densities (≤5 mA cm^-2 ) and under a practical areal capacity (6 mA h cm^-2 ). However, an unusual self-accelerating pit growth is observed in the Mg stripping side under moderate current densities (0.1-1 mA cm^-2 ), which severely deteriorates the anode integrity and subsequent Mg plating uniformity, causing failure of the Mg-metal anode or short circuit of the battery. The stripping morphology depends on the applied current density, as non-uniformity versus the current density displays a volcano plot during the stripping process. Through in situ spectroscopy, it is illustrated that this current-dependent behavior is determined by the evolution of chlorine-containing complex ions near the interface. This research reminds that the plating/stripping process of the Mg-metal anode must be considered comprehensively for practical Mg-metal batteries.