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

Abstract

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.