Antiperovskite K₃OI for K‑Ion Solid State Electrolyte

Discovering new K-ion solid-state electrolytes is crucial for the emerging K-batteries to improve energy density, cycle life, and safety. Here, we present a combined experimental and theoretical study of antiperovskite K₃OI as a K-ion solid-state electrolyte. A solid–solid phase transition at approximately 240 °C induces an increase in ionic conductivity by 2 orders of magnitude. Anion disorder in the I–O sublattice is found to be a potential mechanism for the observed phase transition. The Ba-doped K₃OI sample K_2.9Ba_0.05OI achieves 3.5 mS cm^–1 after the phase transition with a low activation energy of 0.36 eV. Stable cycling of K/K_2.9Ba_0.05OI/K symmetric cells are observed with a low overpotential of 50 mV at 0.5 mA/cm² at 270 °C. This study not only reports K₃OI as a promising K-ion solid-state electrolyte that is compatible with reactive K metal but also improves the understanding of alkali antiperovskite solid-state electrolytes in general.