Atomistic Characterisation of Li+ Mobility and Conductivity in Li7-xPS6-xIx Argyrodites from Molecular Dynamics Simulations, Solid-State NMR, and Impedance Spectroscopy

The atomistic mechanisms of Li+ ion mobility/conductivity in Li7-xPS6-xIx 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, Li-7 and I-127 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 PS4 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 Li7-xPS6-xIx with 0.97 <= x <= 1.00. Within this range, molecular dynamics simulations predict Li+ conductivity at ambient conditions to vary considerably.