Solvation structure on water-in-salt/mica interfaces and its molality dependence investigated by atomic force microscopy

Abstract

Water-in-salt electrolytes (WiSEs), which contain salt with extremely high concentration (>20 mol kg−1), are attracting much interest as new electrolytes for energy storage systems, especially for lithium-ion batteries (LIBs), because they are considered to be suitable for developing safer systems. Structural analysis on WiSE/solid interfaces would provide beneficial information for developing LIBs. Whereas the interfacial structures of diluted aqueous electrolytes (less than 1 mol kg−1) have been intensively studied by high-resolution atomic force microscopy (AFM), those of WiSEs have not. In this study, we demonstrated AFM investigations on WiSE/mica interfaces, where FM-AFM utilizing a quartz tuning fork sensor was used. We studied the surface topography on atomic resolution and the interfacial solvation structure by frequency shift (Δf)–distance curve measurement and two-dimensional Δf mapping. In addition, we investigated the salt molality dependence of the solvation structure and compared the AFM results with Raman spectra.