Vacancy mediated fast sodium-conduction in halide sodalites: A theoretical study

All-solid-state sodium-ion batteries (ASSSIBs) are viewed as a potential alternative to Li-ion batteries due to the advantages in safety and cost. Na-ion electrolytes are the key component of ASSSIBs and required to possess low electronic conductivity, high ionic conductivity, wide electrochemical stable window, and good electrode compatibility. Framework structures, like Na8(AlSiO4)6X2 (X = Cl, Br and I), receive broad research interest for catalysis, ion exchange and adsorption due to the interesting open structure. However, little attention is paid to such types of minerals for superionic conductors. In this investigation, a first-principles study using density functional theory and ab initio molecular dynamic simulations are carried out to predict electronic structure, thermodynamic interface stability and Na-ionic conductivity of Na8(AlSiO4)6X2, suggesting wide bandgaps (>6 eV) but extremely low ionic conductivity in perfect Na8(AlSiO4)6X2. By introducing artificial Na vacancy, three-dimensional diffusional pathway forms in the structure and generates significantly improved sodium conduction while maintaining the energetic and thermal stability, demonstrating the potential for ASSSIBs.