Influence of Complexing Additives on the Reversible Deposition / Dissolution of Magnesium in an Ionic Liquid

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 Mg$^{2+}$ 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 Mg$^{2+}$. Furthermore, the interaction between Mg$^{2+}$ 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 Mg$^{2+}$ reduction to Mg$^{+}$.