Nanoscale Relaxation in “Water-in-Salt” and “Water-in-Bisalt” Electrolytes

“Water-in-salt”
(WIS) and “water-in-bisalt”
(WIBS) electrolytes have recently been developed for Li-ion batteries,
combining the safety and environmental friendliness of aqueous electrolytes
with a larger operating window made possible by a solid-electrolyte
interphase. We report quasielastic neutron scattering (QENS) measurements
on solutions of a WIS electrolyte at two concentrations, 13.9 and
21 m (molal) lithium bis­(trifluoromethane)­sulfonimide
LiTFSI in H2O/D2O and a WIBS electrolyte at
(21 m LiTFSI + 7 m lithium triflate
(LiOTf)) in H2O/D2O. 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 13.9 and 21 m solutions, 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, consistent with
the nanodomain picture where the lithium diffusion occurs through
the tortuous water domain around the slower relaxing ionic matrix,
leading to highly non-Gaussian water motion.