Exploring Cationic Substitutions in NaAlCl₄ and Na₂ZnCl₄ with Density Functional Theory and Machine Learning

NaAlCl₄ is an established solid electrolyte in high-temperature Na-based battery systems,[1] but its ionic conductivity is not sufficient for room-temperature applications.[2] To address this shortcoming, we explored the substitution of various elements into the structure of NaAlCl₄ with density functional theory and machine learning methods to identify and evaluate stable substitution options.<br/>The computationally demanding vibrational contributions to the most promising substitutions were obtained with the assistance of on-the-fly machine-learned potentials, which expedite the phonon calculations by at least one order of magnitude at a minor error of 0.7±1 meV/atom at room temperature.<br/>Isovalent substitutions were found to be most favorable, with potassium and silver being promising choices as substitutes for sodium and gallium for aluminum, yielding a stability below 4 meV/atom compared to their respective ternary chlorides. Evaluations of the configurational space of these substitutions indicate large disorder, with the exchange of Al with Ga resulting in a full solid solution.<br/>Substitutions with ions of differing charges usually did not result in stable structures, with one notable exception for the ion pair Al³⁺ and Zn²⁺. Based on the lattice of Na₂ZnCl₄, a stable structure type with separate layers for the differently-charged cations and a significant amount of Na vacancies was found.<br/>The ionic conductivity of the substituted structure was evaluated using MD simulations assisted by the general-purpose machine learning model MACE-MP-0,[3] and the substitution of Al³⁺ and vacancies into Na₂ZnCl₄ was found to significantly enhance the mobility of the Na⁺ ions in the structure.<br/>In conclusion, our investigation may assist the fast, reliable discovery and evaluation of novel fast Na conductors and other materials by inclusion of thermodynamic properties and machine learning.<br/>References:<br/>[1] S. Hikari, “ZEBRA Batteries”, Springer New York, New York (2014), pp. 2165-2169<br/>[2] J. Park, J. P. Son, et al. <i>ACS Energy Lett.</i>, 7, pp. 3293-3301 (2022)<br/>[3] I. Batatia, P. Benner, et al. <i>arXiv:2401.00096</i>