NaMCl6 (M = Nb and Ta): A New Class of Sodium-Conducting Halide-Based Solid Electrolytes

Highly conductive solid electrolytes (SEs) are a prerequisite for developing all-solid-state batteries. Recently, Li-ion conducting halide-based SEs have attracted enormous attention owing to their electrochemical, mechanical, and transport properties. Despite the success of Li-ion conducting halides, the research on the Na analogue remains sparse, and the reported ionic conductivities are limited. In this study, we demonstrate the significantly improved ionic conductivity in NaM5+Cl6 (M5+ = Nb, Ta) by extending the existing structural framework of the Na-ion conducting halides Na3M3+Cl6 (M3+ = Y, Er) and Na2ZrCl6. Transport analysis reveals that the cation M rules the activation energies of the resulting phases. Given that M is pivotal, a further fine-tuning of M in NaMCl6 leads to a high ionic conductivity of 0.1 mS cm–1 at 30 °C, owing to a beneficial inductive effect and enhanced migration entropy. These discoveries will provide more design strategies for Na-ion conducting halide-based SEs.