Mimicking Na+ ion transport in superionic Na3PS4 solid electrolytes through amorphization

Abstract

Identifying solid electrolytes with superior Na⁺ ion conductivity at room temperature is critical for designing safe and high energy density solid-state batteries with enhanced rate capabilities. Sodium thiophosphate (Na₃PS₄) based solid electrolytes have shown excellent promise with relatively high ionic conductivity. In particular, the orthorhombic γ – Na₃PS₄ phase exhibits superionic behavior (ionic conductivity ∼10–50 mS/cm) compared to that of the cubic (ionic conductivity ∼0.1 mS/cm) and tetragonal (ionic conductivity ∼0.001–0.01 mS/cm) Na₃PS₄ phases at room temperature. However, the reported γ – Na₃PS₄ phase is stable only at high temperatures and, therefore, does not contribute towards improving ionic conductivity at room temperature. In this study, we report ab initio molecular dynamics calculations to gain fundamental insights into the superionic behavior of the γ – Na₃PS₄ phase. These insights were applied to simulate and develop correlations between structure and ionic conductivity in amorphous Na₃PS₄ glassy electrolytes. Our results indicate that the concentration of local structural units in the glasses impact the ionic conductivity. We found out that glasses with a relatively higher concentration of isolated PS₄ and PS₃ units exhibit greater Na⁺ ion diffusivity at temperatures below 500 K. Tuning the concentration of these structural units can be achieved through appropriate heat treatment of the amorphous Na₃PS₄ to achieve high ionic conductivity for novel glass-ceramic type Na₃PS₄ solid electrolytes at room temperature. Overall, our results qualitatively suggest guidelines for achieving superior ionic conductivity in Na₃PS₄ glass-ceramic electrolytes.