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

Abstract

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 6/7Li and 31P solid-state NMR. NMR line shapes and spin–lattice relaxation times (T1) 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 31P-T1 and 31P MAS line shapes. Further, we applied the 31P–31P dipolar correlation experiment to examine the 31P 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.