Characterization of co-fired sodium-ion conductive Na₂Ni₂TeO₆ and Na₂Zn₂TeO₆ with honeycomb layer structure

We investigated the reactivity of P2-type honeycomb layered oxides Na₂Ni₂TeO₆ (NNTO) and Na₂Zn₂TeO₆ (NZTO) co-fired at the temperature from 500 °C to 800 °C. From X-ray diffraction measurements, it was found that the reaction between NNTO and NZTO is unremarkable at the temperature below 700 °C. However, when annealed at 800 °C, they formed the solid-solution phase without any secondary phases. The NNTO and NZTO composite pellets co-fired at 800 °C showed sodium-ion conductivity well above 10^-4 S cm^-1 at room temperature, indicating that the solid-solution phase of NNTO and NZTO has good ionic conductivity. A maximum room temperature conductivity of 7.4 × 10^-4 S cm^-1 was confirmed at the mixing ratio NNTO: NZTO = 0.5 : 1.5. These results can be applied to the fabrication of all-solid-state batteries using NNTO as the cathode active material and NZTO as the solid electrolyte via a simple co-sintering process.