Microstructure and the Plating/Stripping Behavior of Magnesium Metal Negative Electrode for Rechargeable Magnesium Batteries

Abstract

Recently, rechargeable magnesium (Mg) battery systems attract lots of attention owing to their promising theoretical energy density, low cost from the natural abundance of Mg, and less dendritic growth of Mg during charging. Nevertheless, rechargeable Mg battery systems suffer from sluggish reaction kinetics on the electrodes and poor cycle life, which greatly limit their practical competitiveness. Most studies focus on the development of novel electrolytes and their feasibility for rechargeable Mg batteries, while the plating/stripping behavior of Mg metal negative electrode during the charge/discharge cycle is relatively less studied.Therefore, in this research, the platting/stripping behavior of the Mg metal electrode and the resultant surface microstructure after cycling were investigated in an all phenyl complex (APC) (PhMgCl + AlCl3 in THF) electrolyte. A Swagelok-type Mg||Mg symmetric cell was used in this study with an air-free transfer system to prevent the charged/discharged electrode from oxidation when moving to a scanning electron microscope (SEM) to observe the surface microstructure.When discharging the pristine pure Mg electrode, the stripping was found to be highly localized with numerous discharge holes on the electrode surface. Interestingly, in the subsequent charging stage, the platted Mg preferentially deposited along the circumferences of the discharge holes, which might be related to the distribution of Mg species right after discharge and the surface conditions of the discharged surface. Later discharge was found to preferentially strip the platted Mg and leave marks around the discharge holes. In addition to cycling on a pure Mg electrode, the platting/stripping behavior and the resultant microstructure of Mg metal deposited on a copper (Cu) current collector were also studied and will be discussed for potential anode-free Mg battery applications. Regardless of the platting/stripping conditions, no dendritic Mg was found during the charge/discharge cycles. Figure 1