Low melting alkali-based molten salt electrolytes for solvent-free lithium-metal batteries

Developing advanced electrolytes is indispensable for next-generation lithium-metal batteries (LMBs). Unfortunately, the best electrolytes to date are volatile flammable liquids, which pose safety hazards, or solid-state inorganics, which have poor mechanical properties and resistive electrode/electrolyte interfaces. In this study, we report solvent-free inorganic molten salts—mixtures of alkali-based bis(fluorosulfonyl)amide salts—as electrolytes for LMBs that combine the nonvolatility and safety of solids with the improved electrode/electrolyte interfaces and conductivity of liquids. Li_0.3K_0.35Cs_0.35FSA ternary molten salts with a low melting transition of ∼45°C show higher conductivities and higher oxidative stabilities, support higher current densities, and have improved cycling compared to nonvolatile ionic liquids and solid-state polymer and inorganic conductors. They show excellent compatibility with both Li metal anodes (Coulombic efficiency ∼99.8%) and high-voltage cathodes (no oxidation up to 6 V) without corrosion of the aluminum current collector. Solvent-free molten salt electrolytes provide a new class of electrolytes for a wide range of next-generation battery chemistries.