Aqueous Synthesis of Na _3‐2x Sb _1‐x W _x S _4‐x I _x Chalcogenide Solid‐State Electrolytes With Ultrahigh Ionic Conductivity

ABSTRACT Chalcogenide solid‐state electrolytes (SSEs) offer a promising path for safe and high‐energy‐density all‐solid‐state sodium‐ion batteries. However, their large‐scale implementation remains limited by stringent fabrication conditions and an incomplete understanding of their chemical degradation mechanisms. In this work, a facile and scalable aqueous synthesis strategy was developed to prepare Na _2.8 Sb _0.9 W _0.1 S _3.9 I _0.1 SSEs, which exhibit the highest ionic conductivity (10.3 mS cm ⁻¹ ) reported for wet‐chemistry‐derived SSEs. DFT calculations and molecular dynamics simulations reveal that aliovalent W/I co‐doping facilitates Na ⁺ transport kinetics since it introduces abundant vacancies into SSEs and mitigates electrostatic confinement, thereby allowing localized Na ⁺ ions to hop into interconnected, isotropic 3D fast‐ion transport channels. Contrary to common sense, oxygen rather than moisture is found to be the primary driving force for the chemical degradation of Na _2.8 Sb _0.9 W _0.1 S _3.9 I _0.1 SSEs, due to the higher chemical potential for oxidation of SSEs and the lower activation barrier for the oxygen‐involved decomposition. A Na ₃ Sn|SSE|TiS ₂ full cell, where SSE is Na _2.8 Sb _0.9 W _0.1 S _3.9 I _0.1 , demonstrates excellent cycling performance, i.e., a capacity retention of 127 mAh g ⁻¹ after 1100 cycles at 0.1 A g ⁻¹ . Thus, this work provides a new pathway toward high‐performance all‐solid‐state sodium‐ion batteries.