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

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.