Chlorine-Rich Na_6-PS_5-Cl₁₊: A Promising Sodium Solid Electrolyte for All-Solid-State Sodium Batteries

Developing argyrodite-type, chlorine-rich, sodium-ion, solid-state electrolytes with high conductivity is a long-term challenge that is crucial for the advancement of all-solid-state batteries (ASSBs). In this study, chlorine-rich, argyrodite-type Na_6-PS_5-Cl₁₊ solid solutions were successfully developed with a solid solution formation range of 0 ≤ x ≤ 0.5. Na_5.5PS_4.5Cl_1.5 (x = 0.5), displaying a highest ionic conductivity of 1.2 × 10^-3 S/cm at 25 °C, which is more than a hundred times higher than that of Na₆PS₅Cl. Cyclic voltammetry and electrochemical impedance spectroscopy results demonstrated that the rich chlorine significantly enhanced the ionic conductivity and electrochemical stability, in addition to causing a reduction in activation energy. The Na_5.5PS_4.5Cl_1.5 composite also showed the characteristics of a pure ionic conductor without electronic conductivity. Finally, the viability of Na_5.5PS_4.5Cl_1.5 as a sodium electrolyte for all-solid-state sodium batteries was checked in a lab-scale ASSB, showing stable battery performance. This study not only demonstrates new composites of sodium-ionic, solid-state electrolytes with relatively high conductivity but also provides an anion-modulation strategy to enhance the ionic conductivity of argyrodite-type sodium solid-state ionic conductors.