Unveiling the structure and ion dynamics of amorphous Na3−xOHxCl antiperovskite electrolytes by first-principles molecular dynamics

Abstract

Sodium oxyhalide and hydroxyhalide antiperovskites are promising solid-state electrolytes (SSEs) because of their low melting point and rapid synthesis and, as such, they are becoming competitive with respect to other systems. While the structure and the mechanism underlying the ion dynamics are increasingly well understood in crystalline antiperovskites, their amorphous counterpart lacks precise structural characterization, hampering any conclusive insight into their properties. In this work, we resort to first-principles molecular dynamics within the Car–Parrinello scheme to assess the structure and ion dynamics of amorphous Na_3−xOH_xCl (with x = 0, 0.5, and 1) antiperovskites at a quantitative level. We obtain a detailed structural description of these amorphous systems, unveiling the mechanism inherent to the dynamics of Na ions, the role of H atoms, and the resulting ionic conductivity. Our results demonstrate that the structure of amorphous Na₃OCl significantly differs from its crystal phase, showing very limited intermediate-range order and a short-range order mainly driven by four-fold Na atoms. Our results reveal that there is no evidence of phase separation in the amorphous Na_3−xOH_xCl, unlike the previous conjectured model of glassy Li₃OCl. The amorphous structure of Na₃OCl features remarkable Na ion dynamics and ionic conductivity, rivaling that of defective crystalline phases and highlighting its potential as a promising solid-state electrolyte. In hydroxylated models, the presence of hydroxyl OH⁻ anions plays a crucial role in the mobility of Na ions. This is facilitated by the rapid rotation of O–H bonds and paddlewheel-type mechanisms, leading to enhanced ion mobility in the amorphous Na_3−xOH_xCl systems. This work provides unprecedented physical and chemical insight into the interplay between the structure, bonding, and ion transport in amorphous sodium-rich oxyhalide and hydroxyhalide antiperovskites, paving the way to their practical realization in next-generation SSEs.