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

Abstract

All-solid-state sodium batteries (ASSSB) are emerging as a viable energy storage technology due to their cost-effectiveness and high energy density. Halide-based electrolytes 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. We demonstrate that introducing Schottky defects, specifically Na and Cl vacancies in NaTaCl6, significantly increases ionic conductivity to 4.77 x 10-4 S/cm without amorphization. Comprehensive experimental analyses and first-principles calculations reveal that these vacancies diversify the local Na environments, thereby lowering energy barriers. 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 for designing high-performance sodium-based solid electrolytes.