Low Solvating Power of Acetonitrile Facilitates Ion Conduction: A Solvation–Conductivity Riddle

Acetonitrile (AN) electrolyte solutions display uniquely high ionic conductivities, of which the rationale remains a long-standing puzzle. This research delves into the solution species and ion conduction behavior of 0.1 and 3.0 M LiTFSI AN and propylene carbonate (PC) solutions via Raman and dielectric relaxation spectroscopies. Notably, LiTFSI–AN contains a higher fraction of free solvent uncoordinated to Li ions than LiTFSI–PC, resulting in a lower viscosity of LiTFSI–AN and facilitating a higher level of ion conduction. The abundant free solvent in LiTFSI–AN is attributed to the lower Li-solvation power of AN, but despite this lower Li-solvation power, LiTFSI–AN exhibits a level of salt dissociation comparable to that of LiTFSI–PC, which is found to be enabled by TFSI anions loosely bound to Li ions. This work challenges the conventional notion that high solvating power is a prerequisite for high-conductivity solvents, suggesting an avenue to explore optimal solvents for high-power energy storage devices.