Na-Ion Solvation and High Transference Number in Superconcentrated Ionic Liquid Electrolytes: A Theoretical Approach

Ionic liquids have been extensively studied for developing next-generation Li- and Na-ion batteries because they are potentially safer replacements for conventional organic solvents in liquid electrolytes. Some recent work has drawn our attention to high lithium and sodium salt concentration ionic liquid systems which demonstrate high alkali-metal-ion transference numbers and support cycling at high current densities. Here we present an in-depth theoretical study of ion dynamics in a concentrated room-temperature ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C(3)mpyr][FSI]) with a sodium salt (NaFSI), focusing on how the solvation structure of a Na ion changes with salt concentration and how this affects its dynamics. These findings will help to understand the behavior of superconcentrated ionic liquid materials that will guide experimentalists in optimizing ionic liquid-based electrolyte materials.