Ionic Conductivities of Potassium Antiperovskites K₃OX (X = Cl, Br, and I): Bottleneck Tuning vs Anion Polarizability

In recent years, antiperovskites have emerged as promising solid-state electrolytes due to their compositional flexibility, easy processing, scalable synthesis, and high ion selectivity. Our group has recently reported K3OI, which is thermodynamically stable against strongly reducing potassium metal. Such an irreducible property is desirable, but K3OI still suffers from low ionic conductivity at room temperature. Conventional methods to improve ionic conductivity of solid-state electrolytes include increasing the polarizability of immobile anions or creating more free volume for mobile cations to travel by, for example, widening the bottleneck. Therefore, we present a systematic study of K3OX (X═Cl, Br, and I) to investigate the influence of bottleneck tuning versus anion polarizability on K-ion conductivity. Cubic potassium halide antiperovskites K3OX (X═Cl, Br, I) were synthesized to test our hypothesis that a decrease in anion radius and subsequent increase in free volume will lead to higher ionic conductivity in comparison to an increase in anion polarizability. Moreover, it was observed that K3OCl goes through a phase transition at 393 K from an orthorhombic structure (low T) to a cubic structure (high T). The cubic phase was stabilized by halide mixing with bromide, which exhibited the lowest activation energy of all synthesized compounds. These findings are supported by bond valence site energy analysis and experimental results, which show that the ionic conductivity of K3OCl is the highest, whereas the lowest ionic conductivity of K3OBr is more than 3 orders of magnitude less.