K₃SbS₄ as a Potassium Superionic Conductor with Low Activation Energy for K-S Batteries

Solid-state K-ion conducting electrolytes are key elements to address the current problems in K secondary batteries. Here, we report a sulfide-based K-ion conductor K3SbS4 with a low-activation energy of 0.27 eV. W-doped K_3-xSb_1-xW_xS₄ (x=0.04, 0.06, 0.08, 0.10 and 0.12) compounds were also explored for increasing vacancy concentrations and improving ionic conductivity. Among them, K_2.92Sb_0.92W_0.08S₄ exhibits the highest conductivity of 1.4x10^-4 S cm^-1 at 40 °C, which is among the best reported potassium-ion conductors at ambient temperature. In addition, K_2.92Sb_0.92W_0.08S₄ is electrochemically stable with long-chained potassium polysulfide of K₂S_x. A room-temperature solid potassium-sulfur (K-S) battery system has therefore been successfully demonstrated, which is the first K-S battery prototype using non-commercial inorganic-based electrolyte to block the polysulfide shuttle.