Coupling Coordination Structures and Superionic Lithium Conduction in Amorphous Oxyhalide Solid-State Electrolytes

Amorphous oxyhalide solid-state electrolytes (SSEs) hold great promise for achieving all-solid-state batteries (ASSBs) due to their superionic conductivity and high-voltage stability. However, their fundamental understanding of atomic-scale structures and ion conduction mechanisms remains unclear. Herein, we reveal the general "volcano-type" relationship between ionic conductivity and inorganic lithium salt concentration in tantalum-based amorphous oxyhalide SSEs (TaCl₅-xLi₂O). Lithium salt concentration modulates the lithium-ion concentration, dual-anion (O/Cl) framework structure, and precipitation of LiCl impurities, collectively determining the ionic conductivity. Based on this finding, the optimized amorphous TaCl₅-0.5Li₂O achieves a high ionic conductivity of 7.27 × 10^-3 S cm^-1 at 25 °C. Structural analysis further reveals that the existence of multiple Ta-O-Cl polyhedra and oligomers contributes to disordered lithium coordination environments. The weak interactions between lithium ions and the dual-anion framework contribute to low migration energy barriers, establishing an energetically flat three-dimensional lithium-ion migration network. Furthermore, the TaCl₅-0.5Li₂O-based ASSBs achieve good rate performance and cycling stability over 600 cycles. These findings provide fundamental insights into the mechanistic correlation between the coordination structures and the ionic conduction in amorphous oxyhalide SSEs.