Vibrational dynamics and the interplay of Li-ion mobility and thermal transport in LiAlSiO4

Abstract

The development of high-performance solid-state electrolytes is critical for advancing next-generation lithium-ion batteries with enhanced safety and energy density. In this study, the role of soft phonons and host lattice dynamics in governing Li⁺ ion diffusion and thermal transport has been investigated for both crystalline β-eucryptite (LiAlSiO₄) and its amorphous analogue. Using a combination of ab initio and machine-learned molecular dynamics (MLMD) simulations, it was revealed that the crystalline phase undergoes a temperature-driven superionic transition above 700 K, facilitated by polyhedral rotations and low-energy phonon modes. In contrast, the amorphous phase exhibits high ionic conductivity at significantly lower temperatures, owing to its disordered structure and abundant low-energy librational vibrations. Constrained molecular dynamics simulations and phonon spectral analyses highlighted the critical role of polyhedral motions in enabling Li⁺ migration. We also found that compared to the conventional nudged elastic band calculations, the large-scale MLMD simulations sample different diffusion pathways and yield accurate activation barriers for diffusion, in good agreement with experiments. Furthermore, thermal-conductivity measurements and simulations revealed strong anharmonic effects and glass-like behaviour in the superionic regime. These findings provide fundamental insights into the interplay between structural order, phonon dynamics, and transport properties, offering design principles for efficient solid-state electrolytes.