Design and Synthesis of Cubic K_3−2xBa_xSbSe₄ Solid Electrolytes for K–O₂ Batteries

Abstract Developing K‐ion conducting solid‐state electrolytes (SSEs) plays a critical role in the safe implementation of potassium batteries. In this work, a chalcogenide‐based potassium ion SSE is reported, K₃SbSe₄, which adopts a trigonal structure at room temperature. Single‐crystal structural analysis reveals a trigonal‐to‐cubic phase transition at the low temperature of 50 °C, which is the lowest among similar compounds and thus provides easy access to the cubic phase. The substitution of barium for potassium in K₃SbSe₄ leads to the creation of potassium vacancies, expansion of lattice parameters, and a transformation from a trigonal phase to a cubic phase. As a result, the maximum conductivity of K_3−2xBa_xSbSe₄ reaches around 0.1 mS cm⁻¹ at 40 °C for K_2.2Ba_0.4SbSe₄, which is over two orders of magnitude higher than that of undoped K₃SbSe₄. This novel SSE is successfully employed in a K–O₂ battery operating at room temperature where a polymer‐laminated K_2.2Ba_0.4SbSe₄ pellet serves as a separator between the oxygen cathode and the potassium metal anode. Effective protection of the K metal anode against corrosion caused by O₂ is demonstrated.