Molecular mobility in mixed “water-in-salt” solutions of LiOAc and KOAc according to NMR data

Abstract

Abstract Diffusion coefficients of ions and water have been measured in two- and three-component concentrated (“water-in-salt”) aqueous solutions of KOAc and LiOAc, which were proposed as new electrolytes for water-based Li-ion batteries. It was demonstrated that the diffusion coefficient for acetate anion is greater than for lithium cation one in the KOAc-containing solutions and the difference grows under increasing concentration of potassium acetate. Water diffusion is also faster comparing with lithium cation and acetate anion in all solutions studied. Temperature dependences of spin-lattice relaxation rates (1/T1) of 1H, 7Li, and 39K nuclei have been measured for both ions and water. The dependences do not reach their maxima for the most samples, and only for acetate anion in sample IV (31.9 moles KOAc – 8.0 moles LiOAc – H2O) it turned out to be possible the reliable calculation of the rotational correlation time τc. Comparison of the translational (via D) and rotational (via τc) mobility of the acetate anion near the eutectic point showed that the Stokes-Einstein relation is valid for this solution only in a small high-temperature part of the studied range, but not for the lower temperatures.