Na₆MCl₈ rock-salt compounds with M = Mg, Ca, Ba, Zn, Sr as components for solid-state sodium ion batteries

We investigate a new series of rock-salt type structures, Na₆MCl₈ with M = Mg, Ca, Ba, Zn and Sr using advanced atomistic simulations. Calculated results show a direct relationship between the size of the M²⁺ cation and lattice parameters as well as the defect formation energy variation. The NaCl Schottky defect type is highly favourable, and the Na₆BaCl₈ structure possesses the lowest values of defect formation energies. These structures are predicted to be mechanically stable and ductile, implying their compatibility with possible use as electrodes/electrolytes. The Na₆MCl₈ structures exhibit semiconductor characteristics with an energy gap ranging between 4.1-4.6 eV, which differs from the previous value of Na₆MgCl₈. A 3D migration pathway is identified in each rock-salt structure. Despite the small variation of the Na diffusivity and conductivity at 250 K within the structures considered, the Na₆BaCl₈ is characterized by the highest conductivity at 250 K, while the Na₆MgCl₈ structure has the highest conductivity and diffusivity values. The outstanding properties predicted for a Na ion battery suggest future development of synthetic strategies for their actual preparation.