Exploring the origin of ZrO₂ phase in Na₃Zr₂Si₂PO₁₂ ceramic electrolyte

The development of solid-state batteries based on NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) ceramic electrolytes is critically hampered by their unfavorable room temperature ionic conductivity. Moreover, the inadvertent appearance of ionic insulator ZrO₂ phase within NZSP matrix constantly bothers these relevant researches. In this work, via utilizing different kinds of starting materials, it is found that the kinds of Zr source and sintering temperature can regularly affect the relative content of ZrO₂ in the NASICON matrix. More significantly, it reveals that there is no direct correlation between the ZrO₂ content and the room temperature ionic conductivity of NZSP. Moreover, NZSP obtained at 1150 °C via solid-state reaction with Na₂CO₃, ZrO₂, SiO_2, and NH₄H₂PO₄ as starting materials can achieve a room temperature conductivity of 9.1 × 10⁻⁴ S cm⁻¹, which is a relatively high value. The work here offers a thorough understanding of the origin of ZrO₂ phase in NASICON as well as guidance for the mass production of NASICON-type ceramic electrolytes via conventional solid-sate reaction.