Probing lithium-ion induced micro-environment changes in pyrrolidinium-based mono-cationic and di-cationic ionic liquids

Abstract

Recent advancements in lithium-ion battery technology have focused on integrating lithium salts into ionic liquid (IL) electrolytes to overcome some of the limitations associated with traditional organic liquid electrolytes. However, this proposition brings complexities because lithium salts can have a profound impact on the microscopic structural organization of ILs. In this work, we investigate the structural organization and diffusion dynamics of pyrrolidinium-based monocationic ionic liquids (MILs) and dicationic ionic liquids (DILs) in the absence and presence of lithium salt by employing time-resolved fluorescence spectroscopy (TRFS), nuclear magnetic resonance (NMR) spectroscopy, and molecular dynamics (MD) simulation studies. Our findings reveal that the coordination of Li⁺ ions with the anions of both MILs and DILs leads to a change in the structural arrangement of the nonpolar regions of the given media. Quite interestingly, a significantly more pronounced perturbation in the nano-structural organization of MILs as compared to DILs upon the introduction of Li⁺ ions has been observed in the current investigations. Through meticulous data analysis, it has been elucidated that the differential impact of lithium salt on the DIL as compared to the MIL stems from the unique structural organization of the DIL. The findings of this study are expected to provide valuable insights into the design and formulations of optimized electrolytes for lithium-ion battery applications.