Disentangling Cation-Polyanion Coupling in Solid Electrolytes: Which Anion Motion Dominates Cation Transport?

TL;DR

This study uses advanced simulations to dissect how different polyanion motions—translation, rotation, vibration—affect cation mobility in solid electrolytes, revealing that each mode can dominate cation transport depending on conditions.

Contribution

It introduces a novel combination of constraint algorithms and machine-learning molecular dynamics to quantitatively analyze polyanion effects on cation diffusion in superionic conductors.

Findings

Anion translation, rotation, and vibration each can directly facilitate cation diffusion.

Strong coupling between anion translation/vibration and cation mobility was identified.

Different anion motions dominate cation transport depending on temperature and frequency conditions.

Abstract

Lithium and sodium solid electrolytes feature polyanion frameworks and highly mobile cations. Understanding and quantifying the impact of polyanion dynamics on cations will help us to unravel the complex role that anion play in superionic conductors. However, no experimental or computational method can directly extract this information, as polyanion dynamics are always coupled with other factors that affect ion mobility. Here, we present the pioneering study that combines constraint algorithm and machine-learning molecular dynamics to quantitatively reveal the effects of polyanion translation, rotation, and vibration on cation mobility across a diverse material class. Ultralong-time, large-scale machine-learning molecular dynamics simulations with selective constraints on each anion motion mode unequivocally yield results at near room and elevated temperatures. In sharp contrast to the previous understanding that facile anion rotation primarily facilitates cation transport, the strong coupling between anion translation and vibration with cation diffusion has been unraveled for the first time; we find that translation, rotation, and vibration can each directly drive superionicity, with one typically dominant in each class of materials. Anion rotation dominates cation transport when the rotation frequency matches the cation hopping frequency, whereas anion translation prevails at higher and vibration at lower rotation frequencies. The impact of anion dynamics on cation diffusion becomes more prominent at lower temperatures.