Influence of complexing additives on the reversible deposition/dissolution of magnesium in an ionic liquid

Abstract

Better together: Electrochemical measurements and quantum chemical calculations are presented, which show that addition of crown ether enhances the reversibility of Mg deposition/dissolution from a MgTFSI2-containing ionic liquid by strongly coordinating to Mg2+, weakening the [Mg2+−TFSI−] interaction and TFSI− decomposition. BH4− is assumed to support this by acting both as coordinating ligand and as water scavenger. Aiming at a fundamental understanding of the synergistic effects of different additives on the electrochemical Mg deposition/dissolution in an ionic liquid, we have systematically investigated these processes in a combined electrochemical and theoretical study, using 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP-TFSI) as the solvent and a cyclic ether (18-crown-6) and magnesium borohydride as additives. Both crown ether and BH4− improve Mg deposition, its reversibility, and cycling stability. The combined presence of both additives and their concentration relative to that of Mg2+ are decisive for more facile and reversible Mg deposition/dissolution. These results and those of quantum chemical calculations indicate that 18-crown-6 can partly displace TFSI− from its direct coordination to Mg2+. Furthermore, the interaction between Mg2+ and directly coordinated TFSI− is weakened by coordination with 18-crown-6, preventing its Mg+-induced decomposition. Finally, Mg deposition is improved by the weaker overall coordination upon Mg2+ reduction to Mg+.