Spectroscopic Studies of Solution Structure and Ion Conduction in Concentrated Aqueous Electrolytes

Abstract

Aqueous Li-ion batteries have been spotlighted because they are potentially low in cost, safe, and environmentally friendly. However, they suffer from low energy density due to limited cell voltage. Concentrated aqueous electrolytes have been attracting alternatives to aqueous electrolytes because they enable a wide electrochemical stability window. Although many studies have reported excellent cell performance, employing concentrated aqueous electrolytes, the solvation structure and dynamic role of water in concentrated aqueous electrolytes still remain elusive. To reveal solution structure and Li-ion transport mechanism in concentrated aqueous electrolytes, we employed dielectric relaxation spectroscopy, and pulsed-field gradient NMR. We found concentrated aqueous electrolytes still contain a non-negligible amount of bulk water which shows fast rotational dynamics, not interacting with ions. Our experimental results support that concentrated aqueous electrolytes have a heterogeneous solvation environment, and the presence of a microscopic water-rich region with high mobility could facilitate the vehicular migration of Li-ions through highly viscous electrolytes. The molecular understanding of concentrated aqueous electrolytes in this study would shed some light on designing solvation structures to overcome the limit of dilute LIB electrolytes.