Unlocking the mechanism of anharmonic lattice dynamics in ionic conductor β-Li3VO4

Abstract

Anharmonic lattice dynamics (ALD) has proven to be a promising approach for the development of advanced superionic conductors for solid-state batteries. However, the relationship between ALD and ion diffusion remains poorly understood due to the coupling between lattice dynamics and the potential energy surface. In this study, we demonstrate that in β-Li₃VO₄, the enhanced ALD of O_I atoms is coupled with the motion of Li_II ions, resulting in increased activation and diffusion of Li_II ions as temperature increases. Rietveld refinement analysis of the high-temperature X-ray diffraction (HTXRD) patterns indicates that ALD primarily involves Li_II and O_I atoms, with thermal vibration factors increasing significantly with temperature. In situ Raman spectroscopy combined with first-principles calculations reveals that three phonon modes associated with Li_II–O_I vibrations exhibit strong anharmonicity. Among these, one mode is linked to the activation of Li ions, while the other two are associated with the diffusion process. Based on these observations, we propose an atomic-scale mechanism to describe the ALD process. Our findings provide deeper insights into how ALD enhances ion diffusion and support the idea of precisely controlling ion mobility in superionic conductors through phonon engineering.