Development of Silicate-Type Sodium Superionic Conductors for Advanced Sodium Metal Batteries

This thesis investigates silicate-type solid electrolytes, which were synthesized using conventional solid-state methods. The study examines the impact of doping with various rare-earth metals, adding excess Na, selecting different precursors, and applying varied sintering conditions on the phase formation and electrochemical properties of these silicate electrolytes. Powder X-ray Diffraction (PXRD) was employed to optimize phase formation, identifying the presence of secondary phases within the electrolytes. The microstructure of the solid electrolytes was analyzed using Scanning Electron Microscopy (SEM), and phase separation was explored through Energy Dispersive X-ray (EDX) analysis to understand elemental distribution. AC Electrochemical Impedance Spectroscopy (EIS) was utilized to measure the ionic conductivity of solid electrolytes. All the silicate electrolytes synthesized in this thesis demonstrated enhanced phase formation, with an increase in ionic conductivity from sodium gadolinium silicates (NGS) to sodium samarium silicates (NSS) and further to Pr and Nd-doped sodium gadolinium silicates (NGPS/NGNS). Among these, the Pr and Nd-doped sodium gadolinium silicates, Na₅Gd₀.₉Pr₀.₁Si₄O₁₂ and Na₅Gd₀.₉Nd₀.₁Si₄O₁₂, exhibited the highest conductivity of 1.74 and 1.55 mS cm⁻¹, respectively. These compositions also showed excellent Na-ion area-specific resistance of 29.47 and 22.88 Ω cm² at 25 °C, with the highest critical current density of 0.5 mA cm⁻². Silicates also exhibited low electronic conductivity of 9.47 x 10-10 S cm-1 at RT with an electrochemical stability window up to 9 V vs. Na+/Na. Sodium gadolinium silicates (NGS) demonstrated exceptional compatibility with the sodium anode for nearly 3000 cycles. Additionally, these gadolinium silicates exhibited outstanding capacity retention of 98% after 100 charge-discharge cycles during hybrid battery performance with the optimized liquid electrolyte.