Computational Study of Mechanism of Fast Ionic Transport in Li₃YCl₆ ( LYC): Role of Anharmonicity & Host Dynamics

Understanding the mechanisms of ionic transport in solid electrolytes (SEs) is key to further tailoring their ionic conductivity and finding accurate descriptors for high throughput predictions. The link between vibrational dynamics and ionic diffusion offers a promising avenue to explore ion transport beyond static structural descriptors. However, strong anharmonic effects in SEs suggest the potential breakdown of the harmonic phonon picture and the importance of anharmonicity for facile ion migration. In this study, we investigate the role of vibrational modes, soft bonding, and strong anharmonicity in lithium ion migration in Li₃YCl₆ (LYC), a halide lithium ion conductor that has gained interest for its wide electrochemical stability window, low electronic conductivity, and good stability towards oxide cathodes. The crystal structure of LYC is characterized by a rigid hexagonal close packed anion sublattice and a flexible disordered cation sub-lattice. We use a combination of ab initio molecular dynamics (AIMD), lattice dynamics, and nudged elastic band (NEB) methods to reveal strong lattice anharmonicity and vibrational coupling between the modes of the mobile and rigid sub-lattices. Ab initio molecular dynamics (AIMD) simulations are performed over different temperatures in order to understand the diffusive dynamics. By quantifying the similarities between phonon eigenvectors and AIMD trajectories, soft phonon modes that may be associated with lithium diffusion are identified. The anharmonic nature of these phonon modes is confirmed by mapping the potential energy surface. We demonstrate how the lithium dominated anharmonic modes enables in-plane lithium hopping while large amplitude in-plane motion of the rigid sub-lattice assists out of plane lithium migration.