Novel Sulfide‐Chloride Solid‐State Electrolytes with Tunable Anion Ratio for Highly Stable Solid‐State Sodium‐Ion Batteries

Abstract Solid‐state Na batteries (SSNBs) are among the most promising next‐generation energy storage devices due to their high energy density, enhanced safety, and cost‐efficiency. Achieving high‐performance SSNBs depends on the development of solid‐state electrolytes (SSEs) with excellent ionic conductivity, wide electrochemical windows, and robust mechanical properties. Sulfide and halide‐based Na SSEs have been widely studied in recent years with their respective strengths and limitations. Herein for the first time, a new family of Na‐Zr‐S‐Cl sulfide‐chloride Na SSEs with tailored anion compositions is explored. A high ionic conductivity of 4.89 × 10 −4 S cm −1 is realized with a Cl‐rich structure, attributed to the unique chloride bridging structure and low Na‐ion migration barrier. Furthermore, by tuning the ratio between sulfur and chloride anions, two different unique structures are obtained with different surface morphology, thermal stability, mechanical properties, and electrochemical stability. The Cl‐deficient structure of Na 2 S‐1.3ZrCl 4 SSEs demonstrates excellent stability for solid‐state Na‐ion batteries, maintaining a high reversible capacity of over 90 mAh g −1 after 600 cycles at 0.1 C. This study offers insights into the design of new Na SSEs, advancing the development of safer and high‐performance SSNBs.