Microstructural Densification of NASICON Solid Electrolytes Toward High-Performance Solid-State Sodium Batteries

NASICON-type oxide ceramics are widely recognized as promising sodium solid electrolytes due to their superior ionic conductivity and thermal stability. However, their practical deployment is often limited by intrinsic porosity and suboptimal pellet density, which facilitate dendrite penetration. While high-temperature sintering is typically required for densification, achieving near-theoretical densities remains a challenge. Herein, an efficient densification strategy is reported, using Na₂TeO₃ (NTO) as a low-melting point (710°C) functional densifier. Unlike previously reported additives that liquefy near 1000°C, the early-stage melting of NTO initiates liquid-phase sintering at lower temperatures, providing an extended thermal window for particle rearrangement and precise grain boundary engineering. Optimization studies reveal that the addition of 3 wt.% NTO yields a relative density of 97%, facilitating a high room-temperature critical current density of 6 mA cm^-2. Symmetric cell evaluations demonstrate ultra-stable sodium plating/stripping for over 1500 h at 1 mA cm^-2, outperforming previously reported densification strategies. Furthermore, full cells utilizing Na₃V₂(PO₄)₃ cathode exhibit a discharge capacity of 102 mAh g^-1 at 0.1C, along with excellent rate capability and capacity retention. This work establishes a scalable strategy for engineering high-density oxide electrolytes, bridging the gap between material design and high-performance, dendrite-resistant solid-state battery architectures.