First-Principles Molecular Dynamics Study of a NaF–AlF 3 Molten Salt with a Low Molecular Ratio

This paper reports-the results of the first-principles molecular dynamics calculations of the NaF–AlF 3 electrolyte molten salt system with a low molecular ratio (1.3). The local ion structures of the NaF–AlF 3 molten salt system mainly included four-coordination [AlF 4 ] – , five-coordination [AlF 5 ] 2– , and a few six-coordination [AlF 6 ] 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 [AlF 4 ] – and five-coordination [AlF 5 ] 2– structures accounted for more than 90%, while six-coordination [AlF 6 ] 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°, 114°, and 171°. The Al–F bond has ionic properties, but because F-2 p is hybridized with Al-3 s and 3 p 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–AlF 3 molten salt was Na + > F – > Al 3+ . 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–AlF 3 low-temperature aluminum electrolyte system.