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.

Surface Forces and Structure in a Water-in-Salt Electrolyte

“…Recently, atomic force microscope (AFM) measurements were reported by Ichii et al involving a similar WiS electrolyte containing a mixture of LiTFSI and lithium bis­(penta-fluoro­ethyl­sulfonyl)­imide (LiBETI). In those experiments, layering was observed with smaller structural dimensions than in our present measurements . Potential causes for these differences may lie in the different WiS electrolyte composition studied and the impact of the AFM tip geometry and chemistry.…”

“…In those experiments, layering was observed with smaller structural dimensions than in our present measurements. 36 Potential causes for these differences may lie in the different WiS electrolyte composition studied, and the impact of the AFM tip geometry and chemistry. It is also important to note that the AFM experiments output a 'resonant frequency shift' of the AFM cantilever, compared to the direct surface normal force (and therefore interaction free energy) measured in SFB.…”

Tip Charge Dependence of Three-Dimensional AFM Mapping of Concentrated Ionic Solutions

“…The present results were discussed in the context set by 3D AFM, however, the conclusions also apply for other AFM measurements performed in liquid [40,42,43,[61][62][63]. In general, neutral tips are recommended for generating force maps with a close relationship with the particle density distribution.…”

Atomic-Scale Structural Analysis on the Interfaces between Molten Gallium and Solid Alloys by Atomic Force Microscopy

“…That is, atomic-resolution AFM imaging was achieved in molten Ga. Although many studies on atomic-resolution AFM analysis in liquid environments have been reported, − no AFM studies in molten metal have been performed. This is the first result showing that AFM has an atomic resolution in molten metal.…”

“…Quartz tuning fork (QTF) sensors have also been used as the AFM force sensors for investigations in liquids. They have also imaged surface topography on the atomic/molecular resolution , and have characterized interfacial solvation structures of aqueous solutions, ionic liquids, , and a polymeric liquid . Since the deflection of the QTF sensors is electrically detected using the piezoelectricity of the quartz, QTF sensors do not require optical transparency for the liquids.…”