First-Principles Molecular Dynamics Study of a NaF-AlF3 Molten Salt with a Low Molecular Ratio

This paper reports-the results of the first-principles molecular dynamics calculations of the NaF-AlF3 electrolyte molten salt system with a low molecular ratio (1.3). The local ion structures of the NaF-AlF3 molten salt system mainly included four-coordination [AlF4](-), five-coordination [AlF5](2-), and a few six-coordination [AlF6](3-) structures. The first peak of the radial distribution function of the Al-F ion pair was the highest, and the interactions between Al and F ions were strong, which made the molten salt structure complex. The average coordination number of the Al-F ion pair was 4.53. The coordination distribution of the ion structure was that four-coordination [AlF4](-) and five-coordination [AlF5](2-) structures accounted for more than 90%, while six-coordination [AlF6](3-) structures accounted for less than 10%. The F ion type distribution was mainly terminal fluorine F-t, while bridge fluorine F-b and free fluorine F-f accounted for less.The angular distribution of the Al-F-Al bond was 94 & DEG;, 114 & DEG;, and 171 & DEG;. The Al-F bond has ionic properties, but because F-2p is hybridized with Al-3s and 3p orbitals, Al-F ions also have covalent properties. The average Mulliken bond population of Al-F was 0.29, which is much larger than that of F-F and Na-F. Therefore, the covalent interactions between Al-F ion pairs were strong. The order of ion diffusion ability in the NaF-AlF3 molten salt was Na+ > F- > Al3+. The viscosity of the molten salt was 1.479 mPa s, and the ionic conductivity was 1.306 S/cm. These calculation results provide data support for the application of the NaF-AlF3 low-temperature aluminum electrolyte system.