Preparation and characterization of new solid electrolytes Na3−xZn1−xAl1+xS4†

Sulfide solid electrolytes, known for their high ionic conductivity and formability, are key materials for the practical use of all-solid-state sodium batteries. In this study, new sulfide solid electrolyte materials, Na_3−xZn_1−xAl_1+xS₄ (x ≤ 0.2), were prepared via a self flux synthesis route, using reagents such as Na₂S, Zn, Al, and S. The new materials were characterized using X-ray powder diffraction, Raman spectroscopy, and electrochemical impedance spectroscopy. Na_3−xZn_1−xAl_1+xS₄ formed a solid solution up to x = 0.2 and crystallized with a β-Ca₃Ga₂N₄-type structure. As the Al content increased, the number of sodium vacancies also increased, resulting in improved ionic conductivity. Among Na_3−xZn_1−xAl_1+xS₄ samples, Na_2.9Zn_0.9Al_1.1S₄ exhibited the highest ionic conductivity of 4.5 × 10⁻⁶ S cm⁻¹ at 25 °C and lowest activation energy of 32 kJ mol⁻¹. Furthermore, the Na_2.9Zn_0.9Al_1.1S₄ phase was relatively stable when exposed to humid air, which facilitated its practical use in all-solid-state sodium batteries.