New Insights to Self-Aggregation in Ionic Liquid Electrolytes for High-Energy Electrochemical Devices

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

Abstract

Some cations of ionic liquids (ILs) of interest for high-energy electrochemical storage devices, such as lithium batteries and supercapacitors, have a structure similar to that of surfactants. For such, it is very important to understand if these IL cations tend to aggregate like surfactants since this would affect the ion mobility and thus the ionic conductivity. The aggregation behaviour of ILs consisting of the bis(trifluoromethanesulfonyl) imide anion and different N-alkyl-N-methyl-pyrrolidinium cations, with the alkyl chain varied from C3H7 to C8H17, was extensively studied with NMR and Raman methods, also in the presence of Li+ cations. H-2 NMR spin-lattice and spin-spin relaxation rates were analyzed by applying the "two step" model of surfactant dynamics. Here we show that, indeed, the cations in these ILs tend to form aggregates surrounded by the anions. The effect is even more pronounced in the presence of dissolved lithium cations.