Enhanced Densification and Ionic Conductivity of Mg Doped - Na₃Zr₂Si₂PO₁₂ Solid Electrolyte - Synthesized via Solution Combustion Method

Abstract

Sodium-ion batteries (SIBs) are promising alternative to lithium-ion batteries due to sodium’s abundance, low cost, and favourable electrochemical properties. However, the use of liquid electrolytes poses challenges such as leakage, flammability, and low electrochemical stability. As a solution, the NASICON-type solid-state electrolyte, Na3Zr2Si2PO12 (NZSP), emerges as a potential candidate for all-solid-state Na batteries, aiming to replace traditional liquid electrolytes used in Na-ion batteries, offering a safer and more stable alternative. However, conventional solid-state synthesis routes often suffer from high sintering temperatures, formation of insulating secondary phases, and poor ionic conductivity. In this study, magnesium (Mg²⁺) was doped at the Zr⁴⁺ site of NZSP and synthesized via solution combustion method to enhance phase stability, reduce secondary phase formation, and improve densification and conductivity. The rationale behind choosing Mg doping in NZSP will be discussed in detail in the present paper. NZSP powders were doped with Mg at varying concentrations (0.05, 0.1, 0.15, and 0.2 wt.%) and sintered at temperatures between 1150°C and 1200°C. The 0.1 wt % Mg-doped sample sintered at 1175°C achieved optimal performance, exhibiting a relative density of 96% and the highest total ionic conductivity of 6.77 × 10⁻⁴ S cm⁻¹ at room temperature. Phase analysis confirmed minimal ZrO₂ and Na₃PO₄ impurity formation at this concentration. DC polarization measurements further confirmed the dominant ionic conduction behaviour, with negligible electronic contribution. Also, the optimized sample exhibited low activation energy, indicating efficient sodium-ion transport through the NZSP lattice. However, doping beyond 0.15 wt.% resulted in increased formation of insulating ZrO₂ and Na₃PO₄ phases, which negatively impacted conductivity. This study establishes that Mg doping serves as an effective sintering aid and structural stabilizer in NZSP, enabling the fabrication of dense, phase-pure ceramics with enhanced ionic conductivity.