Direct Growth of Na‐Ion Conducting Na ₃ O ₁₅ Si ₆ Y Solid Glass Electrolyte With Reduced Interfacial Resistance for Safer Room‐Temperature Sodium–Sulfur Pouch Cells

ABSTRACT The stability of the solid electrolyte–electrode interface is critical for solid‐state batteries. Traditionally developed solid electrolytes are prone to high interfacial resistance due to the way the electrolyte is applied to the electrodes, and no effective method has yet been devised to resolve this issue. Here, we report a solid‐vapor growth method for directly growing a Na + ‐conducting solid electrolyte on a sodium metal anode, thereby minimizing interfacial resistance. As a result, the solid electrolyte (i.e., Na 3 O 15 Si 6 Y) exhibits a reduced interfacial resistance of approximately 6.2 Ω.cm 2 , accompanied by a room‐temperature Na‐ion conductivity of ∼1.6 mS·cm − 1 . Ab initio molecular dynamics (AIMD) simulations suggest that Na + ions are the primary charge carriers in the Na 3 O 15 Si 6 Y system and experience a low activation energy barrier (∼0.25 eV) for Na‐ion diffusion. Larger bottleneck sizes contribute to the low activation energy barrier and high ionic conductivity of Na 3 O 15 Si 6 Y. A full cell comprising a Na metal anode with an as‐grown solid electrolyte and a sulfur cathode operates for over 140 cycles, with an average Coulombic efficiency of ∼99.0%. Moreover, a 3 × 3 cm 2 pouch cell delivers ∼450 mAh·g −1 reversible capacity and demonstrates the stability and scalability of solid vapor‐grown electrolytes for advanced solid‐state battery applications.