Lowering of Proton and Deuteron Mean Kinetic Energy in the LiTFSI Water-in-Salt Electrolyte System

Water-in-salt electrolytes have emerged as promising materials for energy storage devices, significantly extending the electrochemical stability window of water through confinement within a salt matrix. While the structure and distribution of water molecules in these systems is becoming increasingly better characterized, the molecular nature and energetics of water present a greater challenge. Measurement of the quantum kinetic energy of light atoms is a sensitive probe of their environment, reflecting the potential experienced by the atoms and inclusive of their zero-point energy. It is found that the mean kinetic energy of the proton and deuteron in the archetypal water-in-salt electrolyte system, LiTFSI-H₂O, decreases as a function of salt concentration. This indicates that while the disruption of the hydrogen bond network of water is known to lead to an increasing OH stretching frequency, other components to the quantum kinetic energy must decrease to result in the overall lower measured average.