Dynamical Origin of Ionic Conductivity for Li7P3S11 Metastable Crystal As Studied by 6/7Li and 31P Solid-State NMR

To examine the dynamical origin of high ionic conductivity of (Li2S)(70)(P2S5)(30) glass ceramic obtained by annealing (Li2S)(70)(P2S5)(30) glass, we applied Li-6/7 and P-31 solid-state NMR NMR line shapes and spin-lattice relaxation times (T-1) were measured as a function of temperature. The results showed that dynamics of the PS4 tetrahedra and P2S7 ditetrahedra units in (Li2S)(70)(P2S5)(30) glass ceramic is not appreciable at temperatures below ca. 310 K, where the ionic conductivity is low. At higher temperatures, however, significant motion especially for the P2S7 ditetrahedra unit is apparent in both of P-31-T-1 and P-31 MAS line shapes. Further, we applied the P-31-P-31 dipolar correlation experiment to examine the P-31 line width, which is reduced by motion at higher temperatures. It was shown that the line width of the P2S7 unit is attributable to the distribution of local structures of and around the P2S7 ditetrahedra unit. With these, we concluded that the significant motional fluctuation of the P2S7 ditetrahedra unit at above 310 K allows facile diffusive motion of lithium ions, leading to the high ionic conductivity.