Microstructure Evolution of Electrodeposited Magnesium on Copper Current Collector during Cycles for Anode-Free Rechargeable Magnesium Battery

Rechargeable magnesium (Mg) batteries attract lots of interest owing to their high theoretical volumetric energy density and low cost from the natural abundance of Mg. Additionally, Mg is less prone to dendrite growth during charging, which improves the battery safety. However, rechargeable Mg battery systems suffer from sluggish reaction kinetics on the electrodes and poor cycle life, greatly limiting their practical competitiveness. While most studies of rechargeable Mg batteries focus on the development of electrolytes and their feasibilities, the microstructure evolution of the Mg metal anodes during cycles is not fully unveiled. Furthermore, when using Mg metal as the anode, the excessive Mg may limit the overall energy density of the cells. Recently, anode-free battery systems were introduced to further improve the battery capacity. In this report, we investigated anode-free rechargeable magnesium batteries by electrodepositing Mg on a copper (Cu) current collector in an all phenyl complex (APC) electrolyte. Afterward, the microstructure evolution of the electrodeposited Mg after discharge/charge cycles was studied. The electrodeposited Mg on the Cu current collector with a current density of 2.25 mA/cm2 and a capacity of 1.5 mAh/cm2 shows a uniform distribution of Mg grains with an average grain size of 1.14 ± 0.06 μm. Subsequent Mg stripping with half the deposited capacity (0.75 mAh/cm2) was conducted at various current densities. Stripping with intermediate current density (0.75 mA/cm2) results in non-uniform discharge holes and possible local exposure of the Cu current collector, which could deteriorate the anode integrity. In contrast, the stripped electrodeposited Mg at relatively large (2.25 mA/cm2) and small (0.25 mA/cm2) discharge current densities show dense and uniform discharged holes on the surface. After prolonged discharge/charge cycles at a relatively large current density (2.25 mA/cm2), the electrodeposited Mg still exhibits a uniform surface morphology. In addition to cycling the electrodeposited Mg, a Mg-sulfur (Mg-S) full cell was also assembled with an anode of electrodeposited Mg on Cu and a sulfur/graphite cathode to explore the potential of anode-free rechargeable Mg-S battery. Figure 1