Nanoscale relaxation in "Water-in-Salt" and "Water-in-Bisalt" electrolytes

Gonzalz, M. A.*; Borodin, O.*; Kofu, Maiko   ; Shibata, Kaoru  ; Yamada, Takeshi*; Yamamuro, Osamu*; Xu, K.*; Price, D. L.*; Saboungi, M.-L.*

"Water-in-salt" (WIS) and "water-in-bisalt" (WIBS) electrolytes have recently been developed for Li-ion batteries. We report Quasielastic Neutron Scattering (QENS) measurements on solutions of a WIS electrolyte at two concentrations, and a WIBS electrolyte at one concentration. The data were Fourier transformed to obtain experimental intermediate scattering functions (ISFs) and compared with corresponding quantities obtained from Molecular Dynamics (MD) simulations. Both QENS and MD ISFs could be fitted well by a single stretched exponential function to obtain apparent translational diffusion coefficients for the water molecules. The QENS values agree well with the MD simulations for the solutions of a WIS electrolyte at two concentrations, but MD simulations predict a slower relaxation of water compared to QENS for the WIBS electrolyte. Comparison of the incoherent and coherent scattering reveals much faster water dynamics compared with structural relaxation of the ionic framework.