Hierarchical relaxation and the microscopic origin of fast Li+ ions transport in Li7La3Zr2O12

Abstract

Superionic conductors maintain the structural order of crystals while allowing ions within them to move with liquid-like mobility. Li7La3Zr2O12 (LLZO) is a representative example with high thermal stability, a wide electrochemical window, and fast Li+ ion transport. Despite its technological importance, the microscopic origin of its superionic behavior remains insufficiently understood, particularly the role of collective ion motion. In this work, we employ large-scale molecular dynamics simulations based on a deep neural-network derived potential to investigate the structural and dynamical evolution of undoped LLZO across a broad temperature range. The simulations reveal that several dynamical properties of Li+ ions in LLZO resemble those of glass-forming liquids. A characteristic temperature near the Tammann temperature marks the point at which Li+ ion vibrations deviate from harmonic behavior and cooperative hopping begins to emerge, a change accompanied by enhanced dynamic heterogeneity, as reflected in an increase in the Debye–Waller parameter and a peak in the non-Gaussian parameter. By identifying string-like cooperative motion, we establish a direct link between local vibrational processes, structural relaxation, and long-range ion transport. Furthermore, analysis of the vibrational density of states reveals that the excess low-frequency modes originate from mobile Li+ ions and are closely linked to the onset of cooperative dynamics.