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.