The origin of high Na⁺ ion conductivity in Na₁₊Zr₂SiP_3-O₁₂ NASICON materials

Due to the high sodium ion conductivity, sodium super ionic conductors (NASICONs) are among the most promising candidates as solid electrolytes in solid state batteries and have therefore gained enormous attention in recent years. Previous experimental and computational investigations show excellent sodium ion conductivity for Na₁₊Zr₂SiP_3-O₁₂ with x = 2-2.5. In order to elucidate the conductivity maximum at high substitution levels, we investigate the influence of the cation environment on the site energies of sodium ions and the correlated migration using density functional theory. The results reveal that the site energy strongly depends on electrostatic effects. In addition, an increasing fraction of sodium and silicon ions enhances the correlated migration due to the increasing coulombic repulsion of sodium ions and opening of the bottleneck along the migration path. Based on the results, we generate an energy model to predict configurational and migration energy in subsequent Kinetic Monte Carlo simulations. We show that sodium ions are trapped due to introduced silicon ions but percolate in the system for x≥2.0, which greatly increases the ionic conductivity.