Determination of the ion-conduction properties of Na3OBr and its dominant defect species

Abstract

Antiperovskite materials have attracted significant attention as solid electrolytes for all-solid-state Li⁺/Na⁺-ion batteries because of their high conductivity and electrochemical stability. However, the synthesis of Na-rich antiperovskites (such as Na₃OBr and Na₃OCl) is challenging because it involves the purification of Na₂O. Although the conductivity of Na₃OBr has been extensively studied, the pellet microstructure is not described in sufficient detail, despite its significant impact on the conductivity of such systems. In this study, the Na₂O₂ impurities in commercial Na₂O were reduced by Na metal using a 4 h purification process. Subsequently, non-doped, Ca²⁺-doped, and Br⁻-rich Na₃OBr were fabricated using pure Na₂O. Rietveld refinement and ab initio calculations confirmed Br⁻–O²⁻ site mixing (rather than Schottky defects) to be the predominant defect species in undoped Na₃OBr. Rietveld refinement confirmed the doping of Ca²⁺ ions into Na⁺ sites and excess Br⁻ ions into O²⁻ sites, with the introduction of Na⁺ vacancies for charge compensation. Uniaxial pressing produced Na₃OBr pellets containing several pores, which remained after sintering. However, hot pressing caused effective pellet densification, producing Na₃OBr pellets with a high relative density. The conductivity of Na₃OBr decreased on Ca²⁺ doping and increased on substituting the O²⁻ sites with excess Br⁻. The conductivities of sufficiently dense hot-pressed pellets were measured in a glovebox containing negligible O₂/H₂O, eliminating the influence of pellet pores and Na₃OBr oxidation/hydration on the results. Therefore, these results represent the intrinsic conduction features of Na₃OBr.