Exploring Cationic Substitutions in the Solid Electrolyte NaAlCl₄ with Density Functional Theory

NaAlCl₄ is an established solid electrolyte in high-temperature Na-based battery systems, but its ionic conductivity is not sufficiently high for room-temperature applications. We employ density functional theory and thermodynamic corrections to evaluate the efficacy of various elements for substitution, utilizing on-the-fly machine-learned potentials to accelerate the required phonon calculations by 1 order of magnitude at a minor error of -0.7 ± 1.0 meV/atom. All investigated isovalent substitutions are favorable within 4 meV/atom, with potassium and silver as substitutes for sodium and gallium as a substitute for aluminum. The most promising aliovalent substitution was identified for Zn on the tieline between NaAlCl₄ and Na₂ZnCl₄. The structure of latter, with aluminum ions replacing zinc, yields a structure with separate layers for the differently charged cations and vacancies for potential Na conduction. Our investigation may pave the way for more reliable discovery of new Na conductors by inclusion of thermodynamic properties.