Electrochemical Properties of a La-Doped Na₅YSi₄O₁₂ Ceramic Electrolyte

Solid-state sodium batteries operating at ambient temperatures are promising alternatives to liquid electrolyte batteries, offering improved safety and higher energy density. Among solid-state electrolytes, the ceramic Na-conducting electrolyte Na₅YSi₄O₁₂ (N5) stands out due to its high Na-ion conductivity, wide electrochemical stability window, and compatibility with high-energy-density cathodes. However, achieving high ionic conductivity at room temperature remains a challenge. In this study, the effect of La doping on the electrochemical properties of an N5 ceramic electrolyte is systematically investigated. By optimizing the molar ratio of La₂O₃, La is successfully doped at the Y-site, leading to lattice parameter expansion, as confirmed by Rietveld refinement of the XRD data. Scanning electron microscopy analysis reveals significant changes in grain boundary morphology in La-doped Na₅YSi₄O₁₂, resulting in an increase in grain boundary ionic conductivity from 1.6 × 10^-4 S cm^-1 of the pristine one to 4.3 × 10^-4 S cm^-1. Additionally, La doping preserves the low activation energy (0.19 eV) and high electrochemical stability window (up to 8 V vs Na/Na⁺). La-doped N5 demonstrates superior cycling stability for Na deposition/stripping, maintaining performance for over 1000 h at 15 μA cm^-2, compared to only 118 h for the pristine one. This study indicates that La-doped N5 is a promising solid electrolyte for all-solid-state Na batteries.