Remarkable properties of Na2M3Cl8 compounds (M= Mg, Zn, Ca, Sr) as solid-state electrolytes. A theoretical study.

Abstract

Advanced atomistic computations have been applied to investigate the structural, electronic, and transport properties of the unexplored chloride compounds Na2M3Cl8 (M = Mg, Zn, Ca, Sr). Lattice parameters computed using density functional theory and force field methods closely match reported values, particularly when compared to Na2Mg3Cl8. Electronic structure analysis confirms that Na2M3Cl8 exhibits semiconductor characteristics with a large energy gap of ~5 eV, except for Na2Zn3Cl8, which results from the 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 exhibit the lowest NaCl Schottky defect energies, making them candidates for efficient Na-ion transport. These compounds possess 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 mScm-1 and 3.29 mScm-1, respectively, comparable to leading superionic solid-state electrolytes. 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.