Ion conductivity and the stability of the interface between Na metal and Na₃OCl

Solid-state Li⁺/Na⁺ conductors are used as electrolytes in all-solid-state Li/Na batteries. The ion conductivity and electrode stability of solid electrolytes directly determine battery performances. In addition to the solid electrolyte layer, Na⁺ conductors are also used as additives in electrode composites to enhance the charge–discharge properties. In this study, the ion conductivity and Na metal stability of the Na-rich anti-perovskite Na₃OCl were investigated. Na₃OCl was prepared by controlling the Cl⁻/O²⁻ ratio. Dense pellets of Na₃OCl were prepared by sintering. The conductivities of stoichiometric and Cl-excess Na₃OCl are 9.7 × 10⁻⁶ and 3.0 × 10⁻⁵ S/cm at 240 °C, respectively. The Na⁺ vacancies, which were introduced as the charge compensation of Cl⁻/O²⁻ substitution, are considered to be the origin of the conductivity improvement. Off-stoichiometry of Cl⁻/O²⁻ can also be effective when Na₃OCl is used as the anode composite. The decomposition of Na₃OCl at the Na metal interphase was suggested, indicating that Na₃OCl is unstable with Na. These results contradict the current knowledge on the charge–discharge performance of Na₃OCl anode composites. The present results indicate that the Na⁺ conduction properties and stability in Na₃OCl with high crystallinity are different from the in situ-formed Na₃OCl in previously reported anode composites. In conjunction with its low conductivity, decomposition at the Na metal interphase indicates that the direct use of Na₃OCl as a solid electrolyte is challenging.