Comparative Study of Mg(CB11H12)2 and Mg(TFSI)2 at the Magnesium/Electrolyte Interface

An essential requirement for electrolytes in rechargeable magnesium-ion (Mg-ion) batteries is to enable Mg plating–stripping with low overpotential and high Coulombic efficiency. To date, the influence of the Mg/electrolyte interphase on plating and stripping behaviors is still not well understood. In this study, we investigate the Mg/electrolyte interphase from electrolytes based on two Mg salts with weakly coordinating anions: magnesium monocarborane (Mg­(CB11H12)2) and magnesium bis­(trifluoromethanesulfonyl)­imide (Mg­(TFSI)2). Cyclic voltammetry and chronopotentiometry of Mg plating–stripping demonstrate significantly lower overpotential in the Mg­(CB11H12)2 electrolyte than in Mg­(TFSI)2 under the same condition. Surface characterizations including X-ray photoelectron spectroscopy and scanning electron microscopy clearly demonstrate the superior chemical and electrochemical stability of the Mg­(CB11H12)2 electrolyte at the Mg surface without noticeable interphase formation. On the other hand, characterizations of the Mg/electrolyte interface in the Mg­(TFSI)2 electrolyte indicate the formation of magnesium oxide, magnesium sulfide, and magnesium fluoride as the interfacial compounds resulting from the decomposition of TFSI– anions because of both chemical reduction by Mg and cathodic reduction during Mg deposition.