Glass formation and ionic transport in the mechanosynthesized Na₂S-SnS₂ system

Sulphide solid-state electrolytes have been receiving growing attention due to their high ionic conductivity and their suitable mechanic properties associated with their mechanical softness, in particular for glasses and glass ceramics. The effect of glass network formers, as SiS 2 , GeS 2 and SnS 2 , on the properties needs investigation. This study shows that mechanochemical synthesis enables the formation of amorphous phase in the x Na₂S - (100-x) SnS₂ system (20 ≤ x ≤ 70 mol%), defining a broad glass-forming domain governed by the balance between network connectivity and alkali-induced depolymerization. Structural characterization by X-ray diffraction confirms complete amorphization, while thermal analysis reveals a stable glassy state with T g ~ 240 °C and ΔT ~ 43 °C for x = 66. Ionic conductivity exhibits a strong compositional dependence, increasing by nearly three orders of magnitude with Na₂S content and reaching 1.5 × 10 −5 S.cm −1 at room temperature for 66 Na₂S – 34 SnS₂. Two distinct transport regimes are identified, with a transition near 50 mol% Na₂S, attributed to progressive network depolymerization and the emergence of percolating Na + conduction pathways. Activation energies decrease from 0.60 to ~0.40–0.43 eV across the series. Comparison with crystalline Na₂SnS₃ highlights the critical role of structural disorder, with the amorphous phase exhibiting significantly enhanced ionic transport. The optimal 66 Na₂S - 34 SnS₂ glass combines high ionic conductivity with negligible electronic conductivity (~ 10 −9 S.cm −1 ), confirming predominantly ionic transport. Electrochemical measurements indicate limited stability against Na-based electrodes, emphasizing interfacial constraints despite favorable bulk transport. These results demonstrate that Na₂S acts both as a network modifier and a charge carrier source, establishing structure-transport relationships relevant to sulphide-based solid electrolytes.