Zirconia-free fine-grained NASICON-type solid electrolyte prepared from Mg2+, Y3+ co-doping zirconia precursors

The increasing demand for all-solid-state batteries with high energy density and enhanced safety has attracted attention to the development of high-performance solid-state electrolytes. NASICON-structured Na₃Zr₂Si₂PO₁₂ is considered as a promising solid-state electrolyte material due to its superior thermal and chemical stability. However, its practical application is constrained by the presence of monoclinic ZrO₂ (m-ZrO₂) impurity phase and low ionic conductivity at room temperature. This study presents the synthesis of fine-grained (0.5–0.7 μm) NASICON-type materials free from m-ZrO₂ impurity phase via a solid-state method, and using Mg²⁺ and Y³⁺ co-doped zirconia precursor as a substitute for conventional zirconia. The elimination of the m-ZrO₂ impurity phase and the implementation of Mg²⁺ and Y³⁺ co-doping strategy simultaneously optimized the bulk and grain boundary structure, reduced resistance, and enlarged the bottleneck size of Na⁺ ion transport channels. AC impedance spectroscopy analysis revealed that the room temperature conductivity of Na_3.32Mg_0.16Zr_1.84Si₂PO₁₂ doped with 8 mol% MgO increased from 0.43 mS cm⁻¹ to 1.10 mS cm⁻¹, and further increased to 2.32 mS cm⁻¹ upon substitution of MgO with 2 mol% Y₂O₃ in Na_3.32Mg_0.12Y_0.08Zr_1.84Si₂PO₁₂. This study presents a feasible approach to enhance the ionic conductivity of NASICON-structured solid-state electrolytes through the regulation of multiple factors.