Tuning the Reversibility of Mg Anodes via Controlled Surface Passivation by H2O/Cl– in Organic Electrolytes

Abstract

Developing a new generation of battery chemistries is a critical challenge to moving beyond current Li-ion technologies. In this work, we introduce a surface-science-based approach for understanding the complex phenomena controlling the reversibility of Mg anodes for Mg-ion batteries. We identify the profound impact of trace levels of H2O (≤3 ppm) on the kinetics of Mg deposition and determine that passive films of MgO and Mg(OH)2 are formed only after Mg deposition ceases, rather than continuously during Mg reduction. We also find that Cl– inhibits passivation through the formation of adsorbed Cl– (Mg–Cl(ad)) and/or MgCl2 on the surface, as well as through a dynamic competition with H2O in the double layer. This surface-science-based approach goes well beyond Mg anodes, highlighting the need for more in-depth understanding of electrolyte chemistries before a new generation of efficient and reversible battery technologies can be realized.