Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for Fast-Charging Lithium-Ion Batteries

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li-O-2 systems). Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm(-3)). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li+ cations to form a fluid polymeric network of anions and Li+ cations.