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)₆^4- 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 pK_a, highlighting the importance of the water structure at the interphase of WiSE for multivalent electrolytes.