Coupled Cation–Anion Dynamics Enhances Cation Mobility in Room-Temperature Superionic Solid-State Electrolytes

Single-ion conducting solid electrolytes are gaining tremendous attention as essential materials for solid-state batteries, but a comprehensive understanding of the factors that dictate high ion mobility remains elusive. Here, for the first time, we use a combination of the Maximum Entropy Method analysis of room-temperature neutron powder diffraction data, ab initio molecular dynamics and joint-time correlation analysis to demonstrate that the dynamic response of the anion framework plays a significant role in the new class of fast ion conductors, Na11Sn2PnX12 (Pn = P, Sb; X = S, Se). Facile [PX4]3- anion rotation exists in superionic Na11Sn2PS12 and Na11Sn2PSe12, but greatly hindered [SbS4]3- rotational dynamics are observed in their less conductive analogue, Na11Sn2SbS12. Along with introducing dynamic frustration in the energy landscape, the fluctuation caused by [PX4]3- anion rotation is firmly proved to couple to, and facilitate long range cation mobility, by transiently widening the bottlenecks for Na+-ion diffusion. The combined analysis described here resolves the role of the long debated paddle-wheel mechanism, and is the first direct evidence that anion rotation significantly enhances cation migration in rotor phases. The joint time correlation analysis developed in our work can be broadly applied to analyze coupled cation-anion interplay where traditional transition state theory does not apply. These findings deliver important insights into the fundamentals of ion transport in solid electrolytes. Invoking anion rotational dynamics provides a vital strategy to enhance cation conductivity and serves as an additional and universal design principle for fast ion conductors.