Designing High Ionic Conducting NASICON-type Na₃Zr₂Si₂PO₁₂ Solid-Electrolytes for Na-Ion Batteries

Abstract

The present work investigates the synthesis and characteristics of a Na super-ionic conductor -type Sc3+- and Yb3+-doped Na3Zr2Si2PO12 solid electrolyte for application in solid-state Na-ion batteries. A significant improvement of Na-ion conductivity in Na3Zr2Si2PO12 has been achieved through crystal engineering and microstructure refinement. The presence of the monoclinic-ZrO2 impurity phase adversely affecting the Na-ion conductivity is eliminated using the cubic-ZrO2 precursor at the place of monoclinic-ZrO2 in the conventional solid-state reaction method. Utilizing cubic-ZrO2 also refined the microstructure with thin and microcrack-free grain boundaries. A replacement of 16.5 at. % of Zr4+ by Sc3+ in Na3Zr2Si2PO12 enhances the room-temperature total ionic conductivity from 0.61 to 0.96 mS·cm–1. Replacing 11.11 at. % of Sc3+ by Yb3+ further improves the room-temperature ionic conductivity to 1.62 mS·cm–1, which is >2.5 times higher than that of bare Na3Zr2Si2PO12. The strategic approach used to raise the ionic conductivity in the current work can be applied to other materials, paving way toward realizing high-performance solid-electrolytes for viable and economic Na-ion batteries. A room-temperature conductivity of 1.51 mS·cm–1 for Sc3+/Yb3+-doped Na3Zr2Si2PO12 measured employing Na metal as electrodes confirms Na-ion conduction. Furthermore, a very low current density (∼10–7 A/cm2) in the cyclic-voltammetry profile of the Na|solid-electrolyte|Na cell demonstrates the suitability of Sc3+/Yb3+-doped Na3Zr2Si2PO12 as a solid-electrolyte for Na-ion batteries. A detailed analysis of these materials has been performed, and the possible reasons for the conductivity enhancement are discussed.