Coupled Cation–Anion\nDynamics Enhances Cation\nMobility in Room-Temperature Superionic Solid-State Electrolytes

Single-ion conducting solid electrolytes are gaining\ntremendous\nattention as essential materials for solid-state batteries, but a\ncomprehensive understanding of the factors that dictate high ion mobility\nremains elusive. Here, for the first time, we use a combination of\nthe Maximum Entropy Method analysis of room-temperature neutron powder\ndiffraction data, ab initio molecular dynamics, and joint-time correlation\nanalysis to demonstrate that the dynamic response of the anion framework\nplays a significant role in the new class of fast ion conductors,\nNa11Sn2PnX12 (Pn = P, Sb; X = S,\nSe). Facile [PX4]3– anion rotation exists\nin superionic Na11Sn2PS12 and Na11Sn2PSe12, but greatly hindered [SbS4]3– rotational dynamics are observed in\ntheir less conductive analogue, Na11Sn2SbS12. Along with introducing dynamic frustration in the energy\nlandscape, the fluctuation caused by [PX4]3– anion rotation is firmly proved to couple to and facilitate long-range\ncation mobility, by transiently widening the bottlenecks for Na+-ion diffusion. The combined analysis described here resolves\nthe role of the long-debated paddle-wheel mechanism, and is the first\ndirect evidence that anion rotation significantly enhances cation\nmigration in rotor phases. The joint-time correlation analysis developed\nin our work can be broadly applied to analyze coupled cation–anion\ninterplay where traditional transition state theory does not apply.\nThese findings deliver important insights into the fundamentals of\nion transport in solid electrolytes. Invoking anion rotational dynamics\nprovides a vital strategy to enhance cation conductivity and serves\nas an additional and universal design principle for fast ion conductors.