Frustration-driven Na⁺ migration in Na₃OBH₄

Strategies to improve ionic conductivity in solid electrolytes focus predominantly on the static interactions between polyanions and cations, while the dynamic behavior of polyanions has been largely overlooked. In this work, we employ machine-learning molecular dynamics (MLMD) to explore the coupling between Na^+ migration and BH_4^- reorientation in Na_3OBH_4. The results show that polyanion reorientations occur at rates about four orders of magnitude higher than Na^+ migrations and exhibit only weak temporal correlation with them, indicating that Na^+ transport does not follow a paddle-wheel mechanism. Analysis of the atomistic energy landscape reveals that BH_4^- reorientation induces localized potential fluctuations that split the Na^+ energy states, raising the initial-state energy while lowering the transition-state energy and thereby generating low-barrier migration pathways. Frequency analyses further identify an optimal relation between the fluctuation frequency and Na^+ vibrational modes that enhances low-temperature ionic diffusivity. These results confirm dynamic frustration as a central factor in polyanion-assisted cation transport, providing design guidelines for complex hydride electrolytes.