Study on Microscopic Properties of Molten NaF-AlF3-CaF2/LiF/KF Using First-Principles Molecular Dynamics.

Abstract

This study employs first-principles molecular dynamics (FPMD) simulations combined with the Voronoi tessellation method to explore the microstructure, transport properties, electronic properties, and Raman spectra of the NaF-AlF₃-CaF₂/LiF/KF systems with varying cryolite ratios, additive types, and concentrations. The results indicate that Na⁺, Ca²⁺, Li⁺, and K⁺ exist in a free state in the molten salts, while Al³⁺ forms complex ion groups in the form of [AlF_x]^3-x with F^-, and free F^- also exists in the molten salts. In the NaF-AlF₃-CaF₂ system, the average Al-F distance is slightly shorter than that in the other two systems, while the Al-F coordination number is higher in NaF-AlF₃-LiF. Cryolite ratios and additive concentrations have little effect on the average Al-F distance. The diffusion abilities of different ions follow the order: Li⁺ > Na⁺ > F^- > Al³⁺, with the diffusion ability of K⁺ being close to that of Li⁺. In Al-F chemical bonds, both ionic and covalent bonds coexist and double bonds can be observed in certain transient structures. The Al-F complex ion groups mainly consist of [AlF₄]^-, [AlF₅]^2-, and [AlF₆]^3-, with the concentration of [AlF₆]^3- being higher in the NaF-AlF₃-LiF system compared to the other two systems. This study establishes the relationship between the microscopic properties and the composition of aluminum electrolytes, demonstrating the suitability of FPMD combined with Voronoi tessellation for probing the microstructural properties of aluminum electrolytes.