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 (S²⁻/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, S²⁻ 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_1/3Fe_1/3Mn_1/3O₂, P2/O3 layered Na_0.85Mn_0.5Ni_0.4Fe_0.1O₂, and Na₃V₂(PO₄)₃ cathodes. As a result, SSNIBs with P2/O3 layered Na_0.85Mn_0.5Ni_0.4Fe_0.1O₂ 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.