NMR Studies of Concentrated Aqueous Electrolytes for Low Temperature Batteries

Concentrated aqueous electrolytes have received much attention in recent years due to their high ionic conductivity, electrochemical window that significantly exceeds the water electrolysis limit, and surprisingly low freezing point. We present here two projects. In collaboration with Fudan University in China (Fei Wang, Ziyue Li) we have investigated phosphoric acid solutions being considered for proton-based batteries capable of operating at low temperature. ³¹P and ¹H NMR self-diffusion coefficient as well as spin-lattice time relaxation measurements were performed on acid concentrations ranging from 1.25 to 16.6 M. The observed maximum in conductivity at the 5.9 M concentration is attributed to changes in solvation structure and transport mechanism in addition to the usual trade-off between solution viscosity and carrier concentration.<br/><br/>In a second collaborative project with Oregon State University (Alexis Scida, David Ji) we examine concentrated (up to 7 M) aqueous lithium chloride solutions from room temperature down to -40C using a combination of self-diffusion measurements and 1D ¹H, ⁷Li, ¹⁷O, and ³⁵Cl NMR spectra. Along with Raman Spectroscopy and first-principles MD simulations we observe a tendency toward less hydration of the Li⁺ ions and more Li⁺ - Cl^- pair formation accompanied by ice-like water clusters upon cooling.