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

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.