Locally concentrated ionic liquids (LCILs), which are mixtures of ionic liquids (ILs) and non solvating diluents, are promising candidates for next-generation battery electrolytes.[ 1] The liquid nanostructure and dynamics of LCILs at interfaces have not previously been investigated, meaning the mechanism by which diluents improve electrochemical performance is unresolved.
In this work, the nanostructure and dynamics of LCILs comprised of 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP) with the low viscosity diluent 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1 (w/w) have been studied and compared to the pure IL. The electrochemical windows (ECWs) of the pure ILs are, weakly affected by added diluent on graphite electrode. Atomic force microscopy (AFM) force shows that the interfacial nanostructure of ILs immediately adjacent to graphite electrode is retained with added diluent, even as the potential is changed, which explains the consistent ECWs. These results confirm that fluorinated diluents like TFTFE do not coordinate with the IL cation and anion but rather physically separate the ions while preserving local ion structures. Video-rate AFM shows that long-lived near-surface nanostructures for HMIM FAP and the 2:1 (w/w) HMIM FAP:TFTFE persist on graphite. 2:1 (w/w) HMIM FAP:TFTFE mixture diffuses more rapidly than pure HMIM FAP on graphite.
These outcomes provide valuable insights for a wide range of IL applications in interface sciences, including electrolytes, catalysts, lubricants, and sensors.