[Py1,4]FSI-NaFSI-Based Ionic Liquid Electrolyte for Sodium Batteries: Na+ Solvation and Interfacial Nanostructure on Au(111)

The NaFSI-[Py-1,Py-4]FSI/Au(111) interface was investigated using cyclic voltammetry (CV) and in situ atomic force microscopy (AFM). Raman spectroscopy was used to evaluate the Na+ solvation in [Py-1,Py-4]FSI. It was found that Na coordinates with three FSI-forming [Na(FSI)(3)](2-). In situ AFM revealed that the interaction of [Py-1,Py-4]FSI with Au(111) is much stronger compared with other ionic liquids measured using the same technique. On applying a potential, a force of similar to 50 nN is required to penetrate through the innermost layer. On addition of low concentration of NaFSI (0.05 M), an insignificant change in the innermost solvation layer was observed, whereas on addition of 0.25 and 0.5 M NaFSI, a significant change in the interfacial structure was noted. The present study clearly shows that Na+ ions vary the ionic liquid/Au(111) interface and could provide insight into the interfacial processes in ionic-liquid-based sodium batteries.