Experimental study on Na⁺ conductivity in NaAlBr4 and atomic-scale investigation of Na+ conduction

The ionic conduction properties of Li/Na metal halides have been extensively studied, with recent attention turning towardsAl-based systems. However, limited studies have focused on alkali Al bromides. In this study, we explored the Na+ conductionproperties of NaAlBr4 . Conductivity measurements at 30 °C revealed a Na + conductivity of 1.2 × 10 −5 S/cm, surpassing thatof isostructural NaAlCl4 threefold. Molecular dynamics (MD) simulations to elucidate the conduction mechanisms revealedthat Na + conduction was not observed in stoichiometric NaAlBr4 , which has high formation energies of Na + vacancies andinterstitials (0.88 eV and 0.73 eV, respectively). Nevertheless, a conductivity of 1.2 × 10−5 S/cm was observed. The activationenergy for ion conduction was experimentally determined as 0.43 eV, and the migration energies were calculated as 0.26 eV(Na + vacancies) and 0.16 eV (Na + interstitials) by MD simulations. These discrepancies in ion conduction were partiallyexplained by the role of transient defects enriched via ball milling in facilitating Na + conduction on the particle surface,offering insights into the complex ion conduction of ball-milled NaAlBr4 .