Theoretical study on structural stability and ion transport in crystalline and amorphous phases of antiperovskite electrolyte Na3OCl

Abstract

The current Li-ion battery risks battery damage such as swelling caused by temperature change or leakage caused by external force since it uses a liquid electrolyte solution. There has been a resurgence of research interest in Na-ion battery chemistries in recent years because of its potential cost advantages. The main advantages of solid-state electrolytes (SSEs)are that they do not corrode, combust, leak or cause short internal circuit-like their liquid counterparts. Moreover, solid electrolytes are inert toward metallic Li and act as a separator, helping resist dendrite growth. Different classes of electrolytes with different microstructures and characteristics exhibit specific advantages, so we cannot identify whether crystalline or amorphous electrolytes are preferable to inorganic solid electrolytes. We provide a overview of structure and electrochemical properties of Na3OCl, highlighting its unique antiperovskite structural stability and the key factors that contribute to its excellent ion transport property. The structural phase transition of the high-symmetry cubic phase of antiperovskite Na3OCl is investigated by computing the phonon band structures of 14 different polymorphs with distinct types of ONa6 octahedral tilting. The resulting P-T phase diagram shows that, at high temperature and low pressure, the high-symmetry cubic structure with Pm3m symmetry is the most stable phase. At low temperature and high pressure, on the other hand, the monoclinic structure with P21/m symmetry becomes the most stable phase. The energy barriers falling in the range from 0.30 to 0.34 eV are not much different in phases. While the crystalline structure and ion dynamics of sodium oxyhalide and hydroxyhalide systems have been well-studied, thelack of precise characterization of their amorphous counterparts hinders understanding their atomic-scale properties. In this research, molecular dynamics simulations were conducted using first-principles within the Car-Parrinello scheme to investigate the structure and ion dynamics of amorphous Na3-xOHxCl (x= 0, 0.5, 1) antiperovskites. The results showed that the amorphous Na3OCl structure is significantly different from its crystalline form, with limited intermediate-range order and short-range order dominated by four-fold Na atoms. However, the amorphous Na3OCl did not show evidence of phase separation unlike previous models of glassy Li3OCl. Remarkable Na ion dynamics and high ionic conductivity were observed in the amorphous Na3OCl, indicating its potential as a promising SSE that rivals those of defective crystalline phases. Hydroxyl OH anions played a crucial role in enhancing ion mobility and efficient transport in the amorphous Na3-xOHxCl systems. Overall, this study provides a better understanding of the interplay between structure, bonding, and ion transport in amorphous sodium-rich oxyhalide and hydroxyhalide antiperovskites, which may lead to their practical use in next-generation SSEs.