Interfacial processes of Mg anodes for magnesium-sulfur batteries: An EIS study

• Development of distinct Equivalent Circuit Models for Mg anodes with native, artificial and in situ SEI in the presence/absence of sulfur species. • Adsorption layer formation during OCV independent from Mg surface layers. • Mg 2+ desolvation provokes current-dependent plating/stripping asymmetry. • Sulfur species compete in reduction process with Mg plating and alter charge transfer characteristics and surface layer formation. • a-SEI coated Mg foil and AZ31 Mg alloy most promising in terms of performance, thickness and cost. The development of magnesium batteries strongly relies on the use of a Mg metal anode and its benefits of high volumetric capacity, reduction potential, low cost and improved safety, however, to date, it still lacks sufficient cycling stability and reversibility. Along with the electrolyte selection, the interfacial processes can be affected by the anode itself applying electrode engineering strategies. In this study, six different Mg anode approaches – namely bare Mg metal, Mg foil with an organic and inorganic artificial solid electrolyte interphase, Mg alloy, Mg pellet and a tape-casted Mg slurry – are selected to be investigated by means of electrochemical impedance spectroscopy in Mg|Mg and Mg|S cells. While a plating/stripping overpotential asymmetry was observed and assigned to the desolvation during Mg plating, the impedance spectra of stripping and plating hardly differ for all applied anodes. In contrast, the sulfur species significantly influence the impedance response by altering the surface layer composition. By systematic process assignment of the gained spectra in Mg|Mg and Mg|S cells, specific equivalent circuit models for different anodes and cell conditions are derived. Overall, the study aims to give valuable insights into the interfacial processes of Mg anodes to support their further development toward long-lasting Mg batteries.