Ionic structure and transport properties of KF–NaF–AlF3 fused salt: a molecular dynamics study

Abstract

We used the first-principles molecular dynamics simulations combined with the interatomic potential molecular dynamics to study the ionic structure and transport properties of KF–NaF–AlF₃ fused salt. Simulation results show that the ionic structure of KF–NaF–AlF₃ fused salt is principally dominated by the distorted five-coordinated [AlF₅]²⁻ and six-coordinated [AlF₆]³⁻ groups. When melting to a liquid, a part of the six-coordinated [AlF₆]³⁻ group dissociated into the four-coordinated [AlF₄]⁻ and five-coordinated [AlF₅]²⁻ groups. Four, five and six-coordinated aluminum–fluoro complexes coexist in KF–NaF–AlF₃ fused salt, while the tetrahedral [AlF₄]⁻ groups are relatively rare. The content of the bridging fluorine atom is relatively small, about 5–11%, which indicates that the polymerization degree of the ionic structure of the KF–NaF–AlF₃ fused salt system is lower. The KF–NaF–AlF₃ fused salt has better liquidity and ionic conductivity due to the high self-diffusion coefficients of all particles in the fused salt system. KF can effectively break the F atom bridges, which reduces the polymerization degree of the ionic structure of the fused salt system and increases its ionic conductivity.