Pyrrolidinium Ionic Liquid Electrolyte with Bis(trifluoromethylsulfonyl)imide and Bis(fluorosulfonyl)imide Anions: Lithium Solvation and Mobility, and Performance in Lithium Metal–Lithium Iron Phosphate Batteries

Abstract

A mixture of n-methyl-n-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [PYR13][TFSI] and n-propyl-n-methylpyrrolidinium bis(fluorosulfonyl)imide, [PYR13][FSI] ionic liquids (ILs) is investigated for lithium–metal batteries. Specifically, the relation among conductivity, solvation structure, and Li⁺ mobility is investigated in a Li/IL/IL type ternary mixture. Li⁺ anion coordination numbers with both [TFSI] and [FSI] in the ternary mixtures are derived from Raman analysis. The Li⁺ transference number was measured by a combination of potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques. The electrochemical stability and transport property of the developed ternary mixture were confirmed with Li–Li symmetrical and Li–LiFePO₄ cells. The ternary system exhibited improved rate capability compared to binary parent electrolytes as well as the state-of-the art carbonate-based electrolyte at −20 °C, and slightly better cycling stability at 25 °C. This study demonstrates the flexibility in tailoring physical properties and the Li⁺ solvation environment by ternary Li/IL/IL mixtures for enhanced battery performance.