Enhanced ionic conductivity of a Na₃Zr₂Si₂PO₁₂ solid electrolyte with Na₂SiO₃ obtained by liquid phase sintering for solid-state Na⁺ batteries

Abstract

NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) is supposed to be one of the best potential solid electrolytes with the characteristics of high ionic conductivity and safety for use in solid-state sodium batteries. Many methods have been used to enhance the ionic conductivity of NZSP, among which liquid phase sintering is a simple and rapid method. However, the transport mechanism of sodium ions in a NZSP electrolyte obtained by liquid phase sintering is not clear, and its application in solid-state batteries has not been confirmed. In this study, we synthesized NZSP with Na₂SiO₃ additives by liquid phase sintering to reduce the sintering temperature and improve the ionic conductivity. NZSP with 5 wt% Na₂SiO₃ (NZSP-NSO-5) achieves the highest ionic conductivity of 1.28 mS cm^-1 and the lowest activation energy of 0.21 eV. Furthermore, the DFT study proves the Na⁺ diffusion mechanism and the decline in activation energy after addition. Lastly, the Na/Na₃V₂(PO₄)₃ battery with a Na₂SiO₃-added NZSP solid electrolyte exhibits a remarkably extended cycling capacity of 96.6% capacity retention after being cycled at 0.1 C 100 times. The liquid phase sintering with addition of low melting point salt compounds to electrolyte powder represents a rapid and straightforward technique for improving other ceramic electrolytes.