Ionic Transport Enhancement from Schottky Defects Formation in NaTaCl6 Halide Solid Electrolytes

Abstract

All-solid-state sodium batteries (ASSSB) are increasingly recognized as a viable technology for future energy storage, driven by the abundance and cost advantages of sodium as well as its promising energy density. Halide-based electrolytes such as NaTaCl6 have shown potential due to their compatibility with high-voltage cathodes. However, most efforts to improve their sluggish ionic transport have focused on inducing amorphization via mechanochemical processes, leaving the direct impact of defect formation relatively unexplored. Here, we demonstrate that introducing Schottky defects of Na and Cl vacancies in NaTaCl6 substantially raises its ionic conductivity, from 1.28 × 10-5 S/cm to 4.77 × 10-4 S/cm without inducing amorphization. Comprehensive experimental analyses and first-principles calculations reveal that these vacancies diversify the local Na coordination environments, thereby lowering energy barriers for ion migration. Furthermore, while high-energy ball milling effectively promotes partial amorphization, it also triggers unregulated defect formation, resulting in a wide range of conductivities. Our findings highlight the importance of defect engineering as both an alternative and complementary strategy to amorphization, offering a new route to designing high-performance sodium- based solid electrolytes.