Transport Properties of Water in Salt Electrolytes (WiSE)

Abstract

The emergence of new electrochemical technologies employing WiSE (Water in Salt Electrolytes) have proven the utility of high species concentrations (> 8 M) in enabling aqueous electrodeposition of metals (Li, Zn) with an improved solvent stability window. However, the transport of active species as well as the solvent become important in the WiSE concentration range and is not currently well understood. In order to understand the dynamic interplay of ionic strength and solvation, a transport model is developed using the concentrated solution theory as described by Newman. Steady state predictions suggest a thin boundary layer where the local concentration can exceed the solubility limit and cause precipitation of a salt film on the electrode surface. This effect has been shown in previous electrodeposition literature and could help explain loss of performance in WiSE battery systems over continuous cycling. In order to characterize the transition from aqueous solution to WiSE solutions, traditional experimental methods to determine physical properties (density, viscosity, and conductivity) as well as a a low frequency Electro-hydro-dynamic (EHD) Impedance technique are employed. The physical characterization can then be used to estimate important transport properties such as the cationic transference number (t+) and effective Schmidt number (Sc = ν/ρD) within the predicted mass transfer limited boundary layer as functions of electrolyte concentration within the WiSE regime (Cbulk > 8M).