Direct Observation of Fast Lithium-Ion Diffusion in a Superionic Conductor: Li7P3S11 Metastable Crystal

Li7P3S11 metastable crystal attracts much attention as a solid electrolyte for all-solid-state Li-ion batteries (LIBs) because of its extremely high ionic conductivity at room temperature. Knowing the relationship between the structural framework and dynamics of Li ions will deepen the understanding of lithium-superionic conductors as solid electrolytes in LIBs. However, clarifying the dynamics of Li ions in Li7P3S11 metastable crystal is hindered by the lack of adequate techniques. In this study, we directly observe the fast Li-ion diffusion in Li7P3S11 metastable crystal by using state-of-the-art quasielastic neutron scattering. A signal for the self-diffusion of Li ions is clearly observed at 473 K. The jump diffusion model is used to determine the self-diffusion constant D, mean residence time tau(0), and jump length < l > of the Li ions. We use crystal-structure analysis to successfully identify < l > as the average bond length l(Li -Li)(av) for the nearest-neighbor Li -Li bonds. The Li ions are located within stable regions in the conduction pathways, and the potential barrier is low between them. Consequently, we suggest that the Li ions migrate between stable regions within < l > = 4.3 angstrom in Li7P3S11 metastable crystal.