Phase Behavior of Ionic Liquid–LiX Mixtures: Pyrrolidinium Cations and TFSI– Anions – Linking Structure to Transport Properties

The thermal phase behavior and ionic conductivity of mixtures of N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids (PY1RTFSI where R = 4 or 5 for butyl or pentyl) with LiTFSI have been examined as model systems for ionic liquid-based lithium battery electrolytes. Several mixed salt crystalline phases form. The ionic conductivity variability of the mixtures correlates well with the reported phase behavior. The crystal structures for the (1-x) PY1RTFSI-(x) LiTFSI (x = 0.67) (or 1/2 PY1RTFSI/LiTFSI with R = 4 or 5) phases have been determined. These phases are isostructural, consisting of ionic planar layers formed by Li+···TFSI– linkages in which the uncoordinated PY1R+ cations reside in cavities within the layers. These structures have been used to aid in correlating Raman data with cation–anion coordination in the IL-LiTFSI binary salt mixtures.