Design and Synthesis of Cubic K_{3-2 x} Ba_x SbSe₄ Solid Electrolytes for K-O₂ Batteries

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, K₃ SbSe₄ , 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 K₃ SbSe₄ 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_{3-2 x} Ba_x SbSe₄ reaches around 0.1 mS cm^-1 at 40 °C for K_2.2 Ba_0.4 SbSe₄ , which is over two orders of magnitude higher than that of undoped K₃ SbSe₄ . This novel SSE is successfully employed in a K-O₂ battery operating at room temperature where a polymer-laminated K_2.2 Ba_0.4 SbSe₄ 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.