Composite NASICON (Na3Zr2Si2PO12) Solid-State Electrolyte with Enhanced Na+ Ionic Conductivity: Effect of Liquid Phase Sintering

NASICON-type of solid-state electrolyte, Na₃Zr₂Si₂PO₁₂ (NZSP), is one of the potential solid-state electrolytes for all-solid-state Na battery and Na-air battery. However, in solid-state synthesis, high sintering temperature above 1200 °C and long duration are required, which led to loss of volatile materials and formation of impurities at the grain boundaries. This hampers the total ionic conductivity of NZSP to be in the range of 10^-4 S cm^-1. Herein, we have reduced both the sintering temperature and time of the NZSP electrolyte by sintering the NZSP powders with different amounts of Na₂SiO₃ additive, which provides the liquid phase for the sintering process. The addition of 5 wt % Na₂SiO₃ has shown the highest total ionic conductivity of 1.45 mS cm^-1 at room temperature. A systematic study of the effect of Na₂SiO₃ on the microstructure and electrical properties of the NZSP electrolyte is conducted by the structural study with the help of morphological and chemical observations using X-ray diffraction (XRD), scanning electron microscopy, and using focused ion-beam-time of flight-secondary ion mass spectroscopy. The XRD results revealed that cations from Na₂SiO₃ diffused into the bulk change the stoichiometry of NZSP, leading to an enlarged bottleneck area and hence lowering activation energy in the bulk, which contributes to the increment of the bulk ion conductivity, as indicated by the electrochemical impedance spectroscopy result. In addition, higher density and better microstructure contribute to improved grain boundary conductivity. More importantly, this study has achieved a highly ionic conductive NZSP only by facile addition of Na₂SiO₃ into the NZSP powder prior to the sintering stage.