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 nextgeneration 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 have 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 600 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-energy-barrier calculations, the large-scale MLMD simulations sample different diffusion pathways and yield accurate activation barriers for the diffusion in good agreement with experiments. Furthermore, thermal-conductivity measurements and simulations revealed strong anharmonic effects and glasslike 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.