Design of Sodium Chalcohalide Solid Electrolytes with Mixed Anions for All‐Solid‐State Sodium‐Ion Batteries

Solid‐state sodium‐ion batteries (SSNIBs) have emerged as a promising alternative to lithium‐ion systems for grid‐scale energy storage, owing to sodium's abundance and the improved safety of solid‐state designs. Among various solid‐state electrolytes (SSEs), halide‐based Na + SSEs offer high electrochemical stability but are limited by low ionic conductivity and poor thermal stability. Herein, a novel class of sodium hafnium chalcohalide SSEs is reported with a dual‐anion (S 2− /Cl − ) framework, with a high ionic conductivity of 4.5 × 10 −4 S cm −1 . The incorporation of sulfur enhances Na⁺ mobility by reducing the migration barrier through increased anion polarizability and expanded diffusion pathways. Additionally, S 2 − contributes to stronger interatomic bonding, leading to higher cohesive energy density, improved thermal stability, and mechanical robustness. These SSEs exhibit minimal sulfur oxidation and excellent chemical/electrochemical interface stability with different cathode materials, such as O3‐layered NaNi 1/3 Fe 1/3 Mn 1/3 O 2 , P2/O3 layered Na 0.85 Mn 0.5 Ni 0.4 Fe 0.1 O 2 , and Na 3 V 2 (PO 4 ) 3 cathodes. As a result, SSNIBs with P2/O3 layered Na 0.85 Mn 0.5 Ni 0.4 Fe 0.1 O 2 employing the sodium hafnium chalcohalide SSEs demonstrate outstanding cycling performance, achieving a capacity retention of 88.5% after 200 cycles at 0.1 C. This study establishes a new design strategy for high‐performance SSEs, demonstrating that mixed‐anion frameworks offer a viable route to overcome the intrinsic limitations of single‐anion electrolytes in next‐generation SSNIBs.