K 3 SbS 4 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 K 3 SbS 4 with a low-activation energy of 0.27 eV. W-doped K 3-x Sb 1-x W x S 4 (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 2.92 Sb 0.92 W 0.08 S 4 exhibits the highest conductivity of 1.4×10 -4  S cm -1 at 40 °C, which is among the best reported potassium-ion conductors at ambient temperature. In addition, K 2.92 Sb 0.92 W 0.08 S 4 is electrochemically stable with long-chained potassium polysulfide of K 2 S 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.