Atomistic characterisation of Li~+ mobility and conductivity in Li_(7-x)PS_(6-x)i_x argyrodites from molecular dynamics simulations, solid-state NMR, and impedance spectroscopy

摘要

The atomistic mechanisms of Li~+ ion mobility/conductivity in Li_(7-x)PS_(6-x)I_x argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, ~7Li and ~(127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177± 2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li~+ ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS_4 tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li_(7-x)PS_(6-x)I_x with 0.97≤×≤ 1.00. Within this range, molecular dynamics simulations predict Li~+ conductivity at ambient conditions to vary considerably.