Super‐High Sodium‐Ion Conductivity of Na _2.9 Sb _0.9 W _0.1 S ₄ at Low Pressures by Systematic Pressure and Temperature Treatments

ABSTRACT Sodium‐ion solid electrolytes with high ionic conductivity at low mechanical pressures are essential for the practical implementation of solid‐state batteries. We investigated the combined influence of pressure and temperature on the conductivity of the sulfide‐based sodium electrolyte Na _2.9 Sb _0.9 W _0.1 S ₄ . A series of thermo‐mechanical protocols were applied in which the pellets were first subject to high‐pressure compaction (≤ 664 MPa) and subsequently annealed at 250°C for 1 h under a constant pressure (97 MPa). Electrochemical impedance spectroscopy (EIS) was performed to monitor the ionic conductivity. The most effective protocol (664 MPa → 250°C, 97 MPa) yielded a record superionic conductivity of 44.7 mS cm ⁻¹ at pressures ≥ 18 MPa. After one week under constant pressure (97 MPa), and subsequent relaxation to 1.3 MPa, the conductivity remained at 13.1 mS cm ⁻¹ . Lower‐pressure treatment also caused high conductivity (9.1 mS cm ⁻¹ at 1.3 MPa), demonstrating the material's robustness. Powder X‐ray diffraction confirmed a pressure‐induced transition from tetragonal to cubic modifications of Na _2.9 Sb _0.9 W _0.1 S ₄ , correlating with the conductivity enhancement. The results demonstrate that systematic pressure and temperature treatments afford super‐high sodium‐ion conductivity in Na _2.9 Sb _0.9 W _0.1 S ₄ at pressures compatible with industrial battery manufacturing, highlighting its promise as a solid electrolyte for next‐generation sodium‐ion batteries.