Inhibition of self-aggregation in ionic liquid electrolytes for high-energy electrochemical devices

Kunze M, Paillard E, Jeong S, Appetecchi GB, Schönhoff M, Winter M, Passerini S

Abstract

Some ionic liquids (ILs) based on pyrrolidinium cations, which are of interest for high-energy electrochemical storage devices, such as lithium batteries and supercapacitors, have a structure similar to that of surfactants and tend to form local aggregates. The investigated ILs consist of the bis(trifluoromethanesulfonyl)imide (TFSI) anion and pyrrolidinium-based cations having a methyl side chain and an ether (ethylmethoxy- or ethylethoxy-) side chain. For such, it is very important to understand if these IL cations tend to aggregate like surfactants because this would affect the ion mobility and thus the ionic conductivity. The aggregation behavior of these ILs was extensively studied with NMR and Raman methods also in the presence of Li+ cations and compared with that of corresponding ILs having no ether group on the cation side chain. 2H NMR spin-lattice and spin-spin relaxation rates were analyzed by applying the "two step" model of surfactant dynamics. Here we show that whereas the ILs based on pyrrolidinium cations without ether functions tend to form aggregates in which the cations are surrounded by the anions, especially in the presence of lithium cations, those with an ether side chain are not aggregated.