Li-ion hopping conduction enabled by associative Li-salt in acetonitrile solutions

Abstract

To date, ionic conduction in nonaqueous electrolytes has been explained through the vehicle-type migration mechanism. Yet, new research hints at another conduction mode: ion-hopping, seen in highly concentrated solutions with multi-coordinating solvents. Our research uncovers that Li-ion hopping conduction also occurs in monodentate acetonitrile (AN) electrolytes, enabled by a highly associative Li-salt. Using techniques like pulse-field gradient NMR, Raman spectroscopy, and dielectric relaxation spectroscopy, we examined AN solutions with lithium trifluoroacetate (LiTFA) and lithium bis(fluorosulfonyl)imide (LiFSI). Results showed that Li-ion diffusion in LiTFA-AN was faster due to an anion-bridge structure formed by the associative nature of LiTFA. In contrast, the LiFSI-AN solution demonstrated slower Li-ion movement. In practical applications, like LiFePO₄ symmetric cells, 4 M LiTFA-AN outperformed 1 M LiTFA-AN in rate performance, despite its lower ionic conductivity. This challenges the belief that associative Li-salts are unsuitable for battery electrolytes and prompts reconsideration of other associative Li-salts.