High ionic conducting rare-earth silicate electrolytes for sodium metal batteries

Solid-state sodium-ion batteries (SIBs) are a viable alternative to existing lithium-ion batteries (LIBs) due to the low cost and abundance of sodium and the high safety of using solid-state components. Here, we report novel composite sodium silicate electrolytes exhibiting high ionic conductivity for solid-state SIBs. Rare-earth silicates (3 + x)Na₂O-Gd₂O₃-6SiO₂ (NGS, x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25 mol%, following the composition Na₃GdSi₃O₉), are prepared by the conventional solid-state method. The phase and morphology of the prepared ceramic electrolytes are characterized using powder X-ray diffraction and scanning electron microscopy. The electrical properties of the samples are investigated using impedance spectroscopy, with NGS 0.15 mol% Na₂O, (3.45 Na₂O-Gd₂O₃-6 SiO₂; NGS15) sintered at 1075 °C for 6 h exhibiting the highest ionic conductivity of 7.25 × 10⁻⁴ S cm⁻¹ at 25 °C comparable to that of NASICON electrolytes. Na plating/stripping is conducted to demonstrate the compatibility of the prepared ceramic electrolyte with a sodium metal anode that exhibits exceptional stability for 1000 h at a current density of 0.1 mA cm⁻². A hybrid battery built using a Na anode, an NGS15 ceramic electrolyte with 20 μL of liquid electrolyte on the cathode side, and a Na₃V₂(PO₄)₃ cathode exhibited an initial discharge capacity of 90 mA h g⁻¹ at 0.1C with a capacity retention of 98.01% for 100 charge-discharge cycles, highlighting the potential of the sodium rare-earth silicate as a sodium battery separator and electrolyte.