Sodium Ion Transport Mechanisms in Antiperovskite Electrolytes Na₃OBr and Na₄OI₂: An in Situ Neutron Diffraction Study

Na-rich antiperovskites are recently developed solid electrolytes with enhanced sodium ionic conductivity and show promising functionality as a novel solid electrolyte in an all solid-state battery. In this work, the sodium ionic transport pathways of the parent compound Na₃OBr, as well as the modified layered antiperovskite Na₄OI₂, were studied and compared through temperature-dependent neutron diffraction combined with the maximum entropy method. In the cubic Na₃OBr antiperovskite, the nuclear density distribution maps at 500 K indicate that sodium ions hop within and among oxygen octahedra, and Br^- ions are not involved. In the tetragonal Na₄OI₂ antiperovskite, Na ions, which connect octahedra in the ab plane, have the lowest activation energy barrier. The transport of sodium ions along the c axis is assisted by I^- ions.