Synergistic Ionicity and Phase Regulation Enable Ultrahigh Potassium-Ion Conductivity in KxMg(1+x)/3Sb(2-x)/3O2.

Although layered tellurates show high sodium-ion conductivities, their potassium analogs exhibit a low conductivity of ∼0.0016 mS cm-1 at room temperature. Herein, by synergetic ionicity and phase regulation, we fabricated layered potassium antimonates (KxMg(1+x)/3Sb(2-x)/3O2) of ionic conductivity about 2 orders of magnitude higher than tellurates. Unlike Te-O bonds in K2Mg2TeO6, Sb-O bonds in the antimonate system are predominantly ionic, causing weak interactions with K+ ions and low activation energies for K-ion migration. P2 and P3 phases of KxMg(1+x)/3Sb(2-x)/3O2 exhibit distinct K-ion conductivities, governed by the K stoichiometry and sintering temperature. Consequently, the K0.59Mg0.53Sb0.47O2 sample prepared at 1025 °C delivers the highest K-ion conductivity of 0.101 mS cm-1 at room temperature, a value comparable to that of potassium-containing sulfides. This solid electrolyte demonstrates high reversibility and structural stability upon exposure to humid air, good electrochemical compatibility with K anodes, and holds promise for application in solid-state potassium batteries.