Remarkable properties of Na ₂ M ₃ Cl ₈ compounds (M = Mg, Zn, Ca, and Sr) as solid-state electrolytes: a theoretical study

Advanced atomistic computations have been applied to investigate the structural, electronic, and transport properties of the unexplored chloride compounds Na₂M₃Cl₈ with M = Mg, Zn, Ca, and Sr. Lattice parameters computed using density functional theory and force field methods closely match reported values, particularly when compared to experimental values of Na₂Mg₃Cl₈. Electronic structure analysis confirms that Na₂M₃Cl₈ exhibits semiconductor characteristics with a large energy gap of ∼5 eV, except for Na₂Zn₃Cl₈, which results from a hybridization of [NaCl] trigonal prismatic and [MCl₂] octahedral components. Defect energetics computations reveal that NaCl Schottky defects are the predominant defect type, characterized by low formation energy, which promotes sodium vacancy formation and extensive Na-ion migration. Notably, Na₂Ca₃Cl₈ and Na₂Sr₃Cl₈ contain the lowest NaCl Schottky defect energies, making them candidates for efficient Na-ion transport. These compounds exhibit excellent conductivity properties, with activation energies as low as 0.20 eV for Na₂Ca₃Cl₈ and 0.15 eV for Na₂Sr₃Cl₈, along with outstanding room-temperature conductivities of 3.78 mS cm^-1 and 3.29 mS cm^-1, respectively, comparable to leading superionic solid-state electrolytes. This study extends prior work on Na-Mg-Cl systems by revealing how divalent cation substitution (Ca, Sr, Zn) modulates defect energetics, octahedral distortion, migration pathways, and uncovering non-monotonic trends that deepen the understanding of structure-transport relationships in halide SSEs. Given these remarkable theoretical findings, experimental validation is crucial to assess the stability and practical applicability of Na₂M₃Cl₈ compounds, particularly Na₂Ca₃Cl₈ and Na₂Sr₃Cl₈, for their integration into next-generation Na-ion battery technology.