The Influence of Cation Structure on the Chemical–Physical Properties of Protic Ionic Liquids

In this study we investigated the influence of five different cations on the physical–chemical properties of protic ionic liquids (PILs) based on bis­(trifluoromethanesulfonyl)­imide (TFSI^–). We showed that the viscosities, ionic conductivities, densities, and thermal properties of these PILs are strongly affected by the structure of the protic cation. Furthermore, the influence of the cation structure on the lithium coordination was investigated by Raman spectroscopy for all investigated PIL-based electrolytes for lithium ion batteries (LIBs). This investigation clearly demonstrates that the lithium average coordination number in PIL-based electrolytes is strongly affected by (ring) size and the number of protons on the cation structure and, more importantly, it might be significantly lower (more than 60%) than that of electrolytes containing aprotic ionic liquids (AILs). Electrochemical performances of these PIL-based electrolytes were then also investigated to address some conclusions on their applicability for LIBs.