Exploring superionic conduction in lithium oxyhalide solid electrolytes considering composition and structural factors

Lithium (Li) oxyhalides have emerged as promising solid electrolyte candidates for all-solid-state batteries (ASSBs) due to their superior ionic conductivity and excellent cathode capability. However, the mechanism of Li transport in oxyhalides has remained unclear due to the complex nature of these compounds, which often comprise multiple phases such as crystalline and amorphous structures. Herein, a first-principles study using density functional theory and ab initio molecular dynamic simulation is performed to gain a theoretical insight into the structural behavior and conduction pathway in oxyhalide electrolytes. Li_2.5ZrCl_5.5O_0.5 electrolyte, as a case of study, is comprehensively investigated by considering various phases. It’s revealed that the \({\rm{P}}\bar{3}{\rm{m}}1\) $\mathrm{P}\bar{3}\mathrm{m}1$ phase is energetically more stable and exhibits one order of magnitude higher Li conductivity compared to the C2/m phase among potential crystalline phases (1.63 mS cm⁻¹ versus 0.24 mS cm⁻¹ at room temperature). Interestingly, an amorphous phase with even higher ionic conductivity (~40 mS cm⁻¹) can be generated by creating LiCl-deficient Li-Zr-O-Cl compounds. The remarkably high ionic conductivity suggests that the amorphous structure is likely the dominant conducting pathway, facilitating fast Li transport due the weak bonding between Li and other atoms. Cl anion mobility is also observed in the amorphous phase through mean square displacement (MSD) calculation, although further analyses revealed this to be localized vibration. This study sheds light on the nature of oxychloride structures and promotes a deeper understanding of the superionic conduction mechanism in oxychlorides, which will contribute to the development of advanced solid electrolytes and ASSBs.