Liquid microstructure and dynamics of LiCl-LiF-based fluxes: an ab initio molecular dynamics study

In this work, ab initio molecular dynamics (AIMD) simulations were performed to investigate the liquid microstructure and dynamics of LiCl-LiF-based refining fluxes, with particular attention to the effects of the LiCl/LiF ratio and the addition of AlF₃ or CaF₂ on the microstructure and ionic diffusion kinetics of the melt. The results show that the equimolar 62 wt% LiCl-38 wt% LiF composition has the lowest weighted-average diffusion coefficient of 1.188 Ų ps^-1 and the highest estimated viscosity of 1.035 cP. For the binary LiCl-LiF fluxes, increasing the LiCl/LiF ratio enhances overall ion diffusion and reduces viscosity, indicating that Li⁺ mobility is strongly affected by the balance between Li-F and Li-Cl coordination. The addition of AlF₃ to the LiCl-LiF matrix forms strong Al-centered coordination units, redistributes fluoride species, and changes Li-F/Li-Cl coordination environment, thereby suppressing F^- and Li⁺ diffusion in the ternary melt. In contrast, CaF₂ introduces Ca-centered coordination units and modifies the redistribution of F^- and Cl^-, leading to reduced F^-, Cl^-, and Li⁺ diffusivities, similar to the transport behavior caused by decreasing the LiCl/LiF ratio. These results reveal a composition-dependent relationship between structure and transport in LiCl-LiF-based refining fluxes, and provide a basis for understanding transport-property regulation in multicomponent fluoride-chloride melts.