Solvation-Structure Modification by Concentrating Mg(TFSA)₂–MgCl₂–Triglyme Ternary Electrolyte

Abstract

Mg(TFSA)₂/triglyme(G3)-based electrolytes (TFSA: bis (trifluoromethanesulfonyl) amide) are one of candidates for magnesium rechargeable batteries, but the passivation of Mg-metal anode due to the TFSA anion is fatal in practical use. In this work we show that at elevated temperatures around 150 °C a comparable amount of MgCl₂ salt can be dissolved in concentrated Mg(TFSA)₂/G3 solutions, and the passivation of Mg metal is markedly suppressed in such highly concentrated solutions (1 ≤ G3/Mg-salts ≤ 2) in comparison with in the dilute solutions (G3/Mg-salts ≫ 2). By decreasing the amount of G3 solvent, the solvation structure of Mg²⁺ ions is modified in that free TFSA anions are drastically lowered, which would consequently decrease the reactivity of TFSA anions. We also demonstrate that a full-cell using MgCo₂O₄ cathode with the electrolyte of Mg(TFSA)₂/MgCl₂/G3 1:1:2 at 150 °C delivers a cell voltage of ∼2 V versus Mg-metal anode.