Lithium fluoride dissolution equilibria in cyclic alkylcarbonates and water

The solubility of lithium fluoride in ethylene carbonate (EC), propylene carbonate (PC) and water is studied by conductimetry and by electrochemical impedance spectroscopy to explain the nature of species in solution in these solvents. The observed molar conductivity Λ values of LiF in alkylcarbonates are explained by the presence of free ions Li+ and F−, ion pairs LiF and higher ionic aggregates, “symmetrical” triple ions (Li2F+ and LiF2−), and quadrupoles (Li2F2). The experimental Λ values could be fitted taking into account higher ion aggregations. Results indicate that for PC and EC, the formation of quadrupoles predominates with increasing salt concentration, whereas for aqueous solutions free ion formation predominates at low concentration, then dipole formation becomes favourable. In all solvents studied, triple ions formation is negligible. The tendency of LiF to form quadrupoles is essentially due to particular properties of the F− ion, which is a critical member of the halides. Results of this study indicate that LiF is structured in symmetrical aggregates whose dipole moment is null and with Li2F2 as a basic unit.