Insights into the Transport and Thermodynamic Properties of a Bis(fluorosulfonyl)imide-Based Ionic Liquid Electrolyte for Battery Applications

Ionic liquid electrolytes (ILEs) have become popular in various advanced Li-ion battery chemistries because of their high electrochemical and thermal stability and low volatility. However, because of their relatively high viscosity and poor Li+ diffusion, it is thought large concentration gradients form, reducing their rate capability. Herein, we utilize operando Raman microspectroscopy to visualize ILE concentration gradients for the first time. Specifically, using lithium bis(fluorosulfonyl)imide (LiFSI) in N-propyl-N-methylpyrrolidinium FSI, its apparent diffusion coefficient, lithium transference number, thermodynamic factor, ionic conductivity, and resistance of charge transfer against lithium metal were isolated. Furthermore, the analysis of these concentration gradients led to insights into the bulk structure of ILEs, which we propose are composed of large, ordered aggregates.

Automated summary

Ionic liquid electrolytes (ILEs) are popular in advanced Li-ion battery chemistries due to their high stability and low volatility. However, their high viscosity and poor Li+ diffusion lead to concentration gradients, reducing their rate capability. In this study, operando Raman microspectroscopy was used to visualize ILE concentration gradients for the first time, providing insights into their structure.