Amorphous Sulfo‐Halide Solid Electrolytes With Enhanced Anion Dynamics for Highly Stable All‐Solid‐State Sodium Batteries

ABSTRACT The applicability and long‐term stability of all‐solid‐state Na‐ion batteries (ASSNIBs) depend on maintaining a robust ionic transport interface between electrolytes and electrodes. However, most sodium‐ion solid‐state electrolytes (SSEs) exhibit insufficient ionic conductivity and rigid particle contacts. In this work, a binary amorphous SSE was designed by incorporating high‐polarizability and low‐electronegativity S ²⁻ anions into halide SSEs, featuring mixed S/Cl anionic framework and enhanced flexibility and reorientation dynamics of the anion sublattice, rendering the SSEs both high ionic conductivity (up to 2.12 mS cm ⁻¹ at 30°C) and excellent formability (reduction of 63.3% of the Young's modulus to 2.2 GPa and 46% of the hardness to 2.02 GPa). Multi‐scale characterizations and theoretical calculations reveal that the unique S/Cl mixed‐anion framework promotes the structural amorphization and microscopically optimizes the sodium‐ion potential energy landscape by modulating the Na ⁺ coordination environment through local polyhedral distortion. Furthermore, the assembled ASSNIBs maintain c.a . 100% coulombic efficiency over long‐term cycling, with a capacity retention of 91% after 270 cycles at 0.2C and 80% for more than 1400 cycles at 1C. Therefore, these findings provide new insights for the development of high‐performance binary SSEs for ASSNIBs and general rules for the design of other ionic SSEs.