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 Sc 3+ - and Yb 3+ -doped Na 3 Zr 2 Si 2 PO 12 solid electrolyte for application in solid-state Na-ion batteries. A significant improvement of Na-ion conductivity in Na 3 Zr 2 Si 2 PO 12 has been achieved through crystal engineering and microstructure refinement. The presence of the monoclinic-ZrO 2 impurity phase adversely affecting the Na-ion conductivity is eliminated using the cubic-ZrO 2 precursor at the place of monoclinic-ZrO 2 in the conventional solid-state reaction method. Utilizing cubic-ZrO 2 also refined the microstructure with thin and microcrack-free grain boundaries. A replacement of 16.5 at. % of Zr 4+ by Sc 3+ in Na 3 Zr 2 Si 2 PO 12 enhances the room-temperature total ionic conductivity from 0.61 to 0.96 mS·cm –1 . Replacing 11.11 at. % of Sc 3+ by Yb 3+ further improves the room-temperature ionic conductivity to 1.62 mS·cm –1, which is >2.5 times higher than that of bare Na 3 Zr 2 Si 2 PO 12 . 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 Sc 3+ /Yb 3+ -doped Na 3 Zr 2 Si 2 PO 12 measured employing Na metal as electrodes confirms Na-ion conduction. Furthermore, a very low current density (∼10 –7 A/cm 2 ) in the cyclic-voltammetry profile of the Na|solid-electrolyte|Na cell demonstrates the suitability of Sc 3+ /Yb 3+ -doped Na 3 Zr 2 Si 2 PO 12 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.