First-Principles Calculation of Ion Structures and Transport Properties of NaF–KF–AlF3 Low-Temperature Electrolytes at Various Temperatures

Abstract

The research and application of low-temperature aluminum electrolysis technology have become pivotal for the aluminum industry. In this study, first-principles molecular dynamics simulation (FPMD) was employed to investigate the ionic microstructure and transport properties of NaF–KF–AlF₃ molten salt with a cryolite ratio of 1.4 and NaF content of 40 mol %, across different temperatures. The computational results indicated that [AlF₄]^– and [AlF₅]^2– complexes predominated in the NaF–KF–AlF₃ molten salt, with Raman spectroscopy calculations indicating frequencies of 616 cm^–1 for [AlF₄]^–, 552 cm^–1 for [AlF₅]^2–, and 500 cm^–1 for [AlF₆]³⁻. The low proportion of bridging F ions indicated a relatively low degree of overall polymerization. In the temperature range of 780–850°C, increased temperature led to increased free F_f ions, while also intensifying the decomposition and aggregation reactions of Al−F ionic groups in the molten salt. Furthermore, the diffusion coefficients for all ions increased. Between 780 and 800°C, the diffusion capabilities of the ions followed the order: K⁺ > Na⁺ > F^– > Al³⁺, while between 820 and 850°C, the order shifted to K⁺ > F^– > Na⁺ > Al³⁺. The electrical conductivity of the molten salt ranged from 1.2−1.5 S/cm, and its viscosity was in the range of 1.1−1.6 mPa-s.