${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ 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 ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal is hindered by the lack of adequate techniques. In this study, we directly observe the fast Li-ion diffusion in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ 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 ${l^{\mathrm{av}}}_{\mathrm{Li}─\mathrm{Li}}$ for the nearest-neighbor $\mathrm{Li}─\mathrm{Li}$ 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 $⟨l⟩=4.3\AA$ in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal.

• Received 23 June 2015

DOI:https://doi.org/10.1103/PhysRevApplied.4.054008