The sodium-rich antiperovskites (NaRAPs) with composition Na3OB (B=Br, Cl, I, BH4, etc.) are a family of materials that has recently attracted great interest for application as solid electrolytes in sodium metal batteries. Non-Arrhenius ionic conductivities have been reported for these materials, the origin of which is poorly understood. In this work, we combined temperature-resolved bulk and local characterisation methods to gain an insight into the origin of this unusual behaviour using Na3OBr as a model system. We first excluded crystallographic disorder on the anion sites as the cause of the change in activation energy; then identified the presence of a poorly crystalline impurities, not detectable by XRD, and elucidated their effect on ionic conductivity. These findings improve understanding of the processing-structure-properties relationships pertaining to NaRAPs and highlight the need to determine these relationships in other materials systems, which will accelerate the development of high-performance solid electrolytes.
Poorly crystalline impurities consisting of Na2O, Na2O2, and NaOH were detected in Na3OBr antiperovskite synthesised with commercially available Na2O. These impurities form a eutectic mixture which melts at around 250 °C, reducing the activation energy for Na+ conduction from 0.47 eV to 0.43 eV.
Keywords: non-Arrhenius behaviour; poorly crystalline impurities; sodium antiperovskites; solid sodium-ion conductors; solid-state batteries.
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