Li-Ion Diffusion Correlations in LiAlGeO₄: Quasielastic Neutron Scattering and Ab Initio Simulation

We investigated the impact of Li stoichiometry and host flexibility on Li+ diffusion processes in LiAlGeO4 at the microscopic level using quasielastic neutron scattering (QENS) and ab initio molecular dynamics (AIMD) simulations. Using sufficiently long AIMD trajectories, we could simulate the observed QENS signal and identify the localized dynamics of Li in crystalline LiAlGeO4. Such information is vital to identify the bottleneck of diffusion processes and design materials for battery application. Our AIMD simulations in LiAlGeO4 reveal that the Li+ conductivity can be significantly improved by manipulating the Li stoichiometry and/or host flexibility via amorphization. We determined that excess Li stoichiometry enhances the Coulomb repulsion of neighboring Li sites and softens the host structure to enable faster Li+ diffusion along the hexagonal c-axis. In the amorphous structure, random orientations of AlO4 and GeO4 polyhedral units create a wide distribution of intersite distances and significantly soften the host structure, greatly enhancing the Li+ diffusion. The simulations are used to understand the nature of diffusion, especially the role of the host structure, the possible hopping pathways, and the diffusion behavior in various structures of LiAlGeO4.