Here we present fluorine-free pyrrolidinium- and imidazolium-based ionic liquids and electrolytes with flexible oligoether anions. The electrolytes offer promising physicochemical and transport properties that can potentially be used in lithium batteries, especially at elevated temperatures.

Abstract

A new class of fluorine-free ionic liquids (ILs) and electrolytes based on aliphatic flexible oligoether anions, 2-(2-methoxyethoxy)acetate (MEA) and 2-[2-(2-methoxyethoxy)ethoxy]acetate (MEEA), coupled with pyrrolidinium and imidazolium cations is introduced. For the ILs with MEEA anions, Li⁺ conducting electrolytes are created by doping the ILs with 30 mol % of LiMEEA. The structural flexibility of the oligoether functionality in the anion results in glass transition temperatures (T_g ) as low as −60 °C for the neat ILs and the electrolytes. The imidazolium-based ILs and electrolytes reveal better thermal stabilities but higher T_g and lower electrochemical stabilities than the corresponding pyrrolidinium-based analogues. All neat ILs show comparable transport properties for the cations and these decrease by the addition of lithium salt – the pyrrolidinium-based electrolyte being affected the most.