Concentration effects on dynamic fluctuations in structure and thermodynamic properties of LiCl–AlCl3 molten Salt: Insights from ab initio molecular dynamics

Abstract

The short–range and mid–range structural features, as well as thermodynamic properties of a LiCl–AlCl3 mixed molten salt at 873 K, were investigated as a function of AlCl3 concentration using ab initio molecular dynamics methods. In all solutions, the short-range structure of Al3+ reveals a regular tetrahedral [AlCl4]– unit. These tetrahedral units exist in isolation within dilute AlCl3 solution and polymerize into dimers or trimers through Al–Cl–Al bridge bonds in concentrated solutions. The dynamic dissociation analysis of Al3+ and Li+ structures indicates that the Al–Cl bond considerably stronger than Li–Cl bond. The polymerization of the [AlCl4]– units leads to the dissociation of the Li+ coordination shells into freely isolated cations. Furthermore, we examined changes in density, self-diffusion coefficient, and ionic conductivity with varying AlCl3 concentration in terms of thermodynamic properties. The conductivity and self-diffusion coefficient of Li+ showed a noticeable peak at 50 % concentration. This peak may be associated with the aggregation of [AlCl4]– units and the dissociation of Li+ coordination shells.