Dynamic Monkey Bar Mechanism of Superionic Li-ion Transport in LiTaCl₆

The LiTaCl₆ solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm^-1 ). However, the mechanism has been unclear. We train machine-learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on-the-fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li-ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large-amplitude vibrations of Cl^- ions in TaCl₆ ^- enable the fast Li-ion transport by allowing dynamic breaking and reforming of Li-Cl bonds across the space in between the TaCl₆ ^- octahedra. We term this process the dynamic-monkey-bar mechanism of superionic Li⁺ transport which could aid the development of new solid electrolytes for all-solid-state lithium batteries.