Magnesium rechargeable batteries (MRBs) are one of the promising next-generation large-scale energy storage technologies, owing to the remarkable properties of magnesium metal, i.e. large volumetric capacity, large natural abundance, low cost, and high stability under ambient atmosphere. Since the successful achievement of the prototype MRB by Aurbach in 2000, many researchers have focused on and indeed developed many different electrode and electrolyte materials, all aiming at materialization of the practical MRBs. Transition metal oxide-based spinel compounds, MgM₂O₄ (M = Ni, Co, Mn, Cr), are attracted much attention as high voltage cathode active materials. Recent theoretical and experimental studies suggested that these compounds require high operation temperature due to extremely large diffusion barrier of Mg²⁺ ions in the oxide-based lattice. As conventional organic solvent based electrolytes are highly volatile and flammable at higher temperature, thermally stable electrolytes are urged for. To this end, we focused on ionic liquids (ILs) as both thermally and electrochemically stable solvents. The electrochemical characteristics of IL-based electrolytes in MRBs have been studied so far, however the electrolytes which support (quasi-)reversible magnesium deposition / dissolution are very limited case.

We herein studied the reactivity of Mg metal in a series of pyrrolidinium-based ILs with different anion structures, bis(trifluoromethanesulfonyl)amide (TFSA), bis(fluoromethanesulfonyl)amide (FSA), bis(pentafluoroethanesulfonyl)amide (BETA), dicyanamide (DCA), and 4,5-dicyano-2-trifluoromethyl imidazolate (TDI). TFSA and TDI anions are found to be stable toward Mg metal even at 100 °C, while the other anions are severely decomposed and/or reacted with Mg metal even though no current is applied. The Mg deposition/ dissolution behavior of Grignard reagent-IL mixtures clearly supports this static stability – highly reversible in TFSA and TDI based, while poor in FSA, BETA, and DCA based electrolyte solutions. Intrinsic thermal and electrochemical stability of anions is likely reason for this observation. The electrolyte solutions composed of TFSA and TDI based ILs and corresponding Mg salts are however inactive electrochemically, probably due to strong association of Mg²⁺ and anions, making Mg-species presented in the IL-solutions neutral Mg(anion)₂ and/or aggregate [Mg(anion)_n]. This situation can be solved simply by addition of ether into the solutions. The electrolyte solutions composed of stable Mg salt, ether, and corresponding ILs support Mg deposition during the cathodic scan and dissolution during the reverse anodic scan. In particular case, equimolar mixture of Mg(TFSA)₂ and appropriate glymes dissolved in ILs showed quasi-reversible Mg deposition/dissolution behavior, and excellent anodic stability of > 4 V vs. Mg²⁺/Mg was achieved even at 100 °C on Pt and Al electrodes. This electrolyte is compatible with high voltage oxide-based spinel active materials while conventional Mg(TFSA)₂/glyme electrolyte solutions are decomposed during charging. The performance of such Mg complex-IL based electrolytes are insufficient yet, however the design concept of electrolyte materials will open possibilities of electrolytes applicable to 3 V class MRBs.

Acknowledgement: This work is partly supported by ALCA-SPRING project of Japan Science and Technology Agency (JST).