Comparative Study of Mg(CB 11 H 12 ) 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 behavior 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). The cyclic voltammetry and chronopotentiometry of Mg plating-stripping demonstrate significantly lower overpotential in Mg(CB11H12)2 electrolyte than that 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 Mg(CB11H12)2 electrolyte at the Mg interface without noticeable interphase formation. On the other hand, surface characterizations of the Mg/electrolyte interface in 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 due to both chemical reduction by Mg and cathodic reduction during Mg deposition.