(585e) Interfacial Structure in Mono- and Divalent Water-in-Salt Electrolytes

Water is an environmentally friendly and nonflammable alternative to conventional organic solvents used in electrolytes. However, its practical use is limited by its low thermodynamic stability window. Recently, highly concentrated salts (~20 m) in water (water-in-salt electrolytes, WiSEs) have proven viable candidates for electrolytes by displaying unusually high stability. Due to the suppressed hydrogen evolution reaction and changes in water activity, WiSEs present exciting opportunities for aqueous lithium and zinc batteries, CO₂ reduction, and similar electrosynthesis devices. Studies show that WiSEs are molecularly heterogeneous with water-rich cation and non-aqueous anion domains. It is hypothesized that these domains alter the stability window, however, fundamental understanding of ion structure at these interfaces, particularly for the case of divalent electrolytes is lacking.

In this talk, we bridge this gap by investigating ion and water structuring under confinement with a surface forces apparatus. We verify adsorbed layer thicknesses, ion/water domain sizes, and long-range electrostatic decay length for more well-studied concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) systems, before studying how these structural signatures are changed in solutions of Zn(TFSI)₂ as well as mixtures of the two salts. We reveal that small additions of the divalent salt to the monovalent system disrupts both the short- and long-range structure. We further correlate the confined interfacial structure with bulk structure using Wide Angle X-ray Scattering and Raman spectroscopy and double-layer capacitance measurements. Our future work involves extending this by studying how the observed fluid structure changes as a function of applied potential to link structure with performance, including in situ electroanalytical diagnostic measurements.