Theoretical investigation on local structure and transport properties of NaFAlF3 molten salts under electric field environment

• [AlF6]3− groups are the dominant specie in NaFAlF3 molten salts at CR ≥ 2.6
• AlF bonds have ionic characters as well as partial covalently characters.
• Electric field has no significant impact on the local structure characters.
• The change of CR can significantly affect structure and transport properties.

The effect of electric field and molecular ratio CR (NaF/AlF3) on basic structure and transport properties of NaFAlF3 molten salts were investigated by molecular dynamics simulations with the Buckingham potential model. The [AlF6]3− groups are the dominant specie in NaFAlF3 molten salts at CR ≥ 2.6, and followed by the [AlF5]2− groups, while CR ≤ 2.4, [AlF5]2− groups are the protagonists up to 40%. In NaFAlF3 system, with the increase of CR, the proportion of Fb decreases slightly and the percentage of Ff increases dramatically. The AlF bonds have ionic characters as well as partial covalently characters due to the hybridization of F-2p and Al-3s, 3p orbitals. The order of ion diffusion ability follows as Na+ > F− > Al3+. Adding more NaF can break some F bridges of structure networks and decrease the polymerization degree of NaFAlF3 molten salts, the viscosity reduces and ionic conductivity increases as a consequence. The calculated results of ionic conductivity are in agreement with the experimental results. Electric field has no significant impact on the local structure characters, while transport properties are not. The change of CR (NaF/AlF3) can significantly affect these characters of both the structure and transport.