Effect of Interaction among Magnesium Ions, Anion, and Solvent on Kinetics of the Magnesium Deposition Process

To clarify the effects of anion species and solvents on the Coulombic efficiency and polarization of magnesium deposition/dissolution reactions, the anode/electrolyte interfacial behavior of magnesium tetrakis­(hexafluoroisopropyloxy) borate (Mg­[B­(HFIP)₄]₂) and magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA)₂) was investigated and compared in triglyme and 2-methlytetrahydrofuran (2-MeTHF). When using triglyme, which has strong interaction with magnesium ions, decomposition of [B­(HFIP)₄]⁻ in Mg­[B­(HFIP)₄]₂/triglyme was hard to occur because of the high reduction stability of the uncoordinated [B­(HFIP)₄]⁻ anion, resulting in significantly higher Coulombic efficiency and smaller polarization than Mg­(TFSA)₂/triglyme. When 2-MeTHF was used as the solvent, magnesium deposition/dissolution reactions occurred in the Mg­[B­(HFIP)₄]₂/2-MeTHF electrolyte but not in the Mg­[TFSA]₂/2-MeTHF electrolyte. This is because the coordinated [B­(HFIP)₄]⁻ anion in Mg­[B­(HFIP)₄]₂/2-MeTHF is stable at the magnesium deposition potential. However, the reductive stability of the coordinated [B­(HFIP)₄]⁻ anion is inferior to that of the uncoordinated [B­(HFIP)₄]⁻ anion, resulting in the Mg­[B­(HFIP)₄]₂/2-MeTHF Coulombic efficiency being lower than that of Mg­[B­(HFIP)₄]₂/triglyme. Our results indicate that solvents that could not be used with Mg­(TFSA)₂ are suitable in weakly coordinating anion electrolytes, such as Mg­[B­(HFIP)₄]₂. Controlling the interaction between magnesium ions and anions by selecting suitable anions and solvents is essential for designing new electrolytes for magnesium rechargeable batteries.