On the Discrepancy between Local and Average Structure in the Fast Na Ionic Conductor NaSbWS.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS by WS in NaSbWS leads to a very high ionic conductivity of 41 mS cm at room temperature. While pristine NaSbS crystallizes in a tetragonal structure, the substituted NaSbWS crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse Sb NMR experiments that the short-range order of NaSbWS remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine NaSbS, quasi-elastic neutron scattering and solid-state Na nuclear magnetic resonance measurements revealed drastically improved Na diffusivity and decreased activation energies for NaSbWS. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.