Microscopic Mechanisms of Superionic Na-ion Conductivity in Crystalline and Amorphous NaMOCl4 (M = Nb, Ta) Solid Electrolytes

Sodium-ion solid electrolytes offer a sustainable route toward next-generation batteries, but few match the performance of lithium counterparts. Halide-based NaMOCl4 (M = Nb, Ta) has recently emerged as a promising analogue to LiMOCl4, yet its structure-transport relationships remain unclear due to poor crystallinity in experiments. Here, we combine density functional theory and machine-learned molecular dynamics to reveal that crystalline NaMOCl4 exhibits negligible room-temperature conductivity, with high activation barriers arising from vacancy-mediated diffusion below an order-disorder transition. Above this transition, rotational and translational motion of the MO2/2Cl4 chains creates new Na sites and enhances transport. In contrast, the amorphous phase inherently supports facile, three-dimensional Na diffusion through dynamic framework flexibility. These results show that ordered crystalline phases hinder ionic transport, while disorder – either thermally induced or structural – facilitates it, revising prior assumptions from the Li system and providing design principles for high-conductivity Na halide electrolytes.