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 towards Al-based systems. However, limited studies have focused on alkali Al bromides. In this study, we explored the Na⁺ conduction properties of NaAlBr₄. Conductivity measurements at 30 °C revealed a Na⁺ conductivity of 1.2 × 10⁻⁵ S/cm, surpassing that of isostructural NaAlCl₄ threefold. Molecular dynamics (MD) simulations to elucidate the conduction mechanisms revealed that Na⁺ conduction was not observed in stoichiometric NaAlBr₄, which has high formation energies of Na⁺ vacancies and interstitials (0.88 eV and 0.73 eV, respectively). Nevertheless, a conductivity of 1.2 × 10⁻⁵ S/cm was observed. The activation energy 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 partially explained 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.