Asymmetric composition of ionic cluster and correlated transference number in water-in-salt electrolytes*

The recent research breakthrough on “water-in-salt” electrolytes opens up exciting new avenues for expanding the electrochemical window of aqueous electrolytes. Subsequent work from the electrolyte community found the solvation environment of ions in the “water-in-salt” systems dictates ion mobility. In this work, the crosslinked heterogeneous ion and water domains were captured by molecular dynamics (MD) simulations and small-angle X-ray scattering techniques. The asymmetric composition of ionic clusters composed of more TFSI^- ion than Li⁺ ion was first observed in the water-in-salt electrolyte. The decay of the Li-TFSI association correlation function is faster than that of the residence correlation function for the large percolated ionic clusters with asymmetric composition, which implies that the Li⁺ ion can hop through the TFSI^- ions in the ionic cluster. Furthermore, a reasonably high correlated transference number (i.e., ~0.32) can be maintained even in 20 m electrolytes due to a weak negative correlation between the motion of cations and anions featuring heterogeneous ionic regions.