Impact of Multivalent Cations on Interfacial Layering in Water-In-Salt Electrolytes

Water-in-salt electrolytes (WiSEs) are of interest for use as aqueous multivalent electrolytes due to their potential to address reversibility and passivation concerns common in multivalent batteries. In this work, the impact of the addition of multivalent cation salts, including Zn(TFSI)2, Mg(TFSI)2, Ca(TFSI)2, and Al(TFSI)3 on the double layer behavior in LiTFSI WiSE is investigated. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is utilized to observe the potential-dependent double-layer composition. TFSI– is enriched at relatively positive potentials for LiTFSI WiSE, and water is enriched at negative potentials for mixed electrolytes containing Mg2+ and Ca2+, but this shift does not hold for mixed electrolytes containing Zn2+ or Al3+. Ultramicroelectrode (UME) voltammetry shows confinement of a probe molecule Fe(CN)64– at the interphase in the presence of Mg2+ and Ca2+, an effect that is eliminated by the addition of 1.75 and 1.25 mM of Zn2+ or Al3+, respectively, to LiTFSI WiSE. Atomic force microscope (AFM) measurements show the presence of smaller interlayer distances at positive potentials relative to those seen without the presence of Zn2+. These effects are correlated to cation pKa, highlighting the importance of the water structure at the interphase of WiSE for multivalent electrolytes.