Fluorine-free ionic liquids (ILs) with saccharinate anion coupled to ether-functionalized pyrrolidinium/imidazolinium cations exhibit promising thermal, transport, and electrochemical properties. The imidazolinium-based ILs offer higher electrochemical stability, while the pyrrolidinium-based ILs display superior ion transport. The electrolytes show lower overpotentials and outstanding Li stripping/plating performance at elevated temperature.

Abstract

Fluorine-free ionic liquids (ILs) and electrolytes based on ether-functionalized pyrrolidinium or imidazolinium cations coupled with a “greener”, non-basic, and hydrolytically stable saccharinate (Sac) anion, are herein presented with their thermal, transport and electrochemical properties. The thermal stability, glass transition temperature, and electrochemical stability of the imidazolinium based ILs surpasses the pyrrolidinium IL, while the latter offer better (ion) transport properties. Ether-functionalization of the IL cation improves the transport properties with negligible effects on the thermal and electrochemical stabilities. The Li⁺ conducting electrolytes – created by adding 10 mol % of lithium saccharinate (LiSac) to the neat (BMMIm)(Sac) and (C₂₀₁MMIm)(Sac) ILs show an as low initial overpotential as ±0.05 V and outstanding Li stripping/plating performance over 100 hours at 60 °C for the latter, but a very large polarization interfacial resistance, 4730 Ω cm², impeding the kinetics of stripping/plating, even at these elevated temperatures for the former. Hence the rather modest modification has an enormous impact in practice.