New State-Diagram of Aqueous Solutions Unveiling Ionic Hydration, Antiplasticization, and Structural Heterogeneities in LiTFSI-H2O.

Abstract

Here, we report a new state-diagram for aqueous solutions based on concentration-dependent glass-transition temperatures of concentrated and ice freeze-concentrated solutions. Different from the equilibrium phase diagram, this new state-diagram can provide comprehensive information about the hydration numbers of solutes, nonequilibrium vitrification/cold-crystallization, and vitrification/devitrification processes of aqueous solutions in three distinct concentration zones separated by two critical water-content points of only functions of the hydration number. Based on this new state-diagram, we observe the comparable hydration ability of LiTFSI to LiCl and an atypical concentration-dependent cold-crystallization behavior of the LiTFSI-H2O system. These results unveil the negligible hydration ability of TFSI- in a water-rich solution, characterize the antiplasticizing effect of water induced by the strengthened Li+-TFSI--H2O interaction when only hydration water and confined water are present, and confirm the increasing fraction of water-rich domains with the decrease in water content when the cation and anion become incompletely hydrated on average. These results highlight the novel water-content-mediated interactions among the anion, cation, and H2O for LiTFSI-H2O.