Improved Stability and Rate Capability of Ionic Liquid Electrolyte with High Concentration of LiFSI

Ionic liquid (IL) electrolytes offer a number of advantages over conventional carbonate-based electrolytes but commonly suffer from poor performance at high charge and discharge rates due to their relatively low ionic conductivity and high viscosity. While increasing the lithium salt content of an IL electrolyte beyond optimal levels exacerbates both of those characteristics, this work demonstrates the surprising kinetic capabilities of highly concentrated IL electrolytes. Results presented herein show that NMC-811 half-cells cycled in PYR 13 FSI with a high-concentration of LiFSI exhibit superior rate performance as compared to lower-concentration IL solutions. Furthermore, extended cycling shows that higher LiFSI concentrations promote enhanced stability during long-term cycling, significantly outperforming the capabilities of a conventional organic electrolyte. Transference number calculations, differential capacity analyses, and electrochemical impedance spectroscopy were used to illuminate the underlying mechanisms contributing to the performance improvements at high concentrations, ultimately revealing the significance of LiFSI molarity in the formation of a robust and conductive solid-electrolyte interphase layer capable of promoting rapid lithium-ion transport as well as stable long-term cycling. The results of this study highlight the unique capability of IL electrolytes to enable successful implementation of a challenging high-energy electrode material.