Improving ionic conductivity of von-Alpen-type NASICON ceramic electrolytes via magnesium doping

NASICON (sodium superionic conductor) compounds have attracted considerable attention as promising solid electrolyte materials for advanced sodium (Na)-based batteries. In this study, we investigated the improvement in ionic conductivities of von-Alpen-type NASICON (vA-NASICON) ceramic electrolyte by introducing magnesium (Mg²⁺) as a heterogeneous element. The optimal Mg-doped vA-NASICON exhibited a high ionic conductivity of 3.64 × 10⁻³ S cm⁻¹, that was almost 80% higher than that of un-doped vA-NASICON. The changes in the physicochemical properties of vA-NASICONs through Mg introduction were systematically analyzed, and their effects on the ionic conductivities of vA-NASICON were studied in detail. When the optimal ratio of Mg²⁺ was used in the synthetic process, the relative density (96.6%) and grain boundary ion conductivity were maximized, which improved the total ionic conductivity of vA-NASICON. However, when Mg²⁺ was introduced in excess, the ionic conductivity decreased because of the formation of an undesired Na_xMg_yPO₄ secondary phase. The results of this study are expected to be effectively applied in the development of advanced sodium-based solid electrolytes with high ionic conductivities.