Critical Role of the Interphase at Magnesium Electrodes in Chloride-Free, Simple Salt Electrolytes

Magnesium (Mg) batteries are a potential beyond lithium-ion technology but currently suffer from poor cycling performance, partly due to the interphase formed when magnesium electrodes react with electrolytes. The use of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) electrolytes would enable high-voltage intercalation cathodes, but many reports identify poor Mg plating/stripping in the electrolyte solution due to a passivating interphase. Here, we have assessed the Mg plating/stripping mechanism at bulk Mg electrodes in a Mg(TFSI)₂-based electrolyte by cyclic voltammetry, ex situ Fourier-transform infrared spectroscopy, and electron microscopy and compared this to the cycling of a Grignard-based electrolyte. Our studies indicate a nontypical cycling mechanism at Mg surfaces in Mg(TFSI)₂-based electrolytes that occurs through Mg deposits rather than the bulk electrode. Fourier-transform infrared spectroscopy demonstrates an evolution in the interphase chemistry during conditioning (repeated cycling) and that this is a critical step for stable cycling in the Mg(TFSI)₂-tetraglyme (4G) electrolyte. The fully conditioned electrode in Mg(TFSI)₂-4G is able to cycle with an overpotential of <0.25 V without additional additives such as Cl^- or BH₄^-.