Insight into interfacial processes and degradation mechanism in magnesium metal batteries

Abstract Magnesium (Mg) metal batteries are attractive due to their high energy density and low cost. However, the progress of rechargeable Mg batteries is hindered by the limited options of Mg-ion electrolytes, serious electrode passivation and poor cycling performance. Direct visualization of the dynamic processes of electrochemical deposition/stripping of metallic Mg is of great significance for intensive understanding the degradation mechanism. Herein, the deposition/stripping process of Mg is directly tracked by using in situ optical microscope and in situ atomic force microscope. It is shown that the growth of deposited Mg is dynamically slow and the dissolution process is obviously irreversible in magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2)-based electrolyte. Interestingly, it is found that the weak reversibility is mainly caused by the point-contact of the deposited Mg to the electrode. By introducing magnesium borohydride (Mg(BH4)2) to improve the stability of the electrolyte, the nucleation sites significantly increased and closely packed on the electrode during the deposition. Moreover, the connection between deposited Mg and electrode turns into surface-contact mode, in which ion transmission and the reaction reactivity is greatly improved, revealing that the stability of the electrolyte to the electrode could affect the nucleation process and thus change the contact mode. These findings provide direct insights into the electrolyte-dependent reactivity and performance degradation mechanisms in Mg metal batteries.