Structural Evidence for High Ionic Conductivity of Li7P3S11 Metastable Crystal

Electrical conductivity, time-of-flight neutron diffraction (TOF-ND), and synchrotron X-ray diffraction (SXRD) measurements were carried out for (Li7P3S11)-Li-7 metastable crystal and ((Li2S)-Li-7)(70)(P2S5)(30) glass. Reverse Monte Carlo modeling based on the TOF-ND and SXRD data showed three-dimensional structures of 7Li7P3S11 metastable crystal and ((Li2S)-Li-7)(70)(P2S5)(30) glass. A detailed polyhedral analysis of the three-dimensional atomic configuration yielded the spatial distribution of [LiS4] tetrahedra ([LiS4] units) and S-4 tetrahedra (without Li+ ion; [S-4] units) in (Li7P3S11)-Li-7 metastable crystal and ((Li2S)-Li-7)(70)(P2S5)(30) glass. When an ac-[S-4] unit is recognized as a fully acceptable unit of the Li+ ion, the coordination number of ac-[S-4] units around a [LiS4] unit for (Li7P3S11)-Li-7 metastable crystal was estimated to be 3.9 on average, whereas it was 1.9 for ((Li2S)-Li-7)(70)(P2S5)(30) glass. Such structural characteristics in (Li7P3S11)-Li-7 metastable crystal and ((Li2S)-Li-7)(70)(P2S5)(30) glass are deeply associated with electrochemical properties.