Novel inorganic ionic liquids possessing low melting temperatures and wide electrochemical windows: Binary mixtures of alkali bis(fluorosulfonyl)amides

Thermal properties of alkali bis(fluorosulfonyl)amides, MFSI (M = Li, Na, K, Rb, Cs), have been investigated. Binary phase diagrams of LiFSI–KFSI and NaFSI–KFSI systems have been constructed. Eutectic point for LiFSI–KFSI is 338 K at (xLi, xK) = (0.45, 0.55) and, that for NaFSI–KFSI is 330 K at (xNa, xK) = (0.45, 0.55). The electrochemical window of the eutectic LiFSI–KFSI is as wide as 6.0 V at 348 K with the cathode limit being lithium metal deposition. The electrochemical window of the eutectic NaFSI–KFSI is 5.0 V at 340 K with sodium metal deposition at the cathode limit. These new inorganic ionic liquids are highly promising for various electrochemical applications.