Abstract:All solid-state rechargeable sodium-ion batteries are of key importance for future energy storage applications and novel electrolytes with improved performance are continuously being sought after for such batteries. Here we report large sodium-ion conductivity (>10^–5 S/cm) at ambient temperature in a layered chalcogenide compound of composition Na₂(Ga_0.1Ge_0.9)₂Se_4.95, synthesized as a glass–ceramic composite in the ternary system Na₂Se–Ga₂Se₃–GeSe₂. Alkali-ion-containing crystalline chalcogenides are typically hygroscopic and consequently unstable in an ambient environment. However, in the present study the crystal phase is possibly stabilized against moisture via encapsulation in an inert glass matrix and forms a three dimensionally continuous percolation network for fast sodium-ion conduction. This result demonstrates that such layered chalcogenide compounds with planar pathways for alkali-ion diffusion can serve as viable alternatives to the conventional solid electrolytes.