Direct Link Between Structure, Dynamics and Thermodynamics in Molten Salts

The strongly increased use of molten salts in the energy industry necessitates knowledge of their physicochemical properties on a microscopic scale to guide the development of new technology. Here, we focus on the eutectic LiCl–KCl mixture and unravel the links between structure, dynamics, and thermodynamics and investigate both mixing and temperature effects. In the mixture, for K–Cl, an elongation of the ionic bond length is accompanied by faster ion dynamics and lower Gibbs energy of activation; the opposite is true for Li–Cl. This leads to the counter-intuitive result of retarded dynamics of the lighter ion, while the heavier ion diffusion is accelerated. In contrast, higher temperatures lead to a shortening of the cation–anion distances accompanied by faster dynamics despite an increase in the Gibbs energy of activation. Ionic bond breaking happens on the picosecond timescale and is suggested to proceed through an associative substitution mechanism because of a negative entropy of activation.