On the Discrepancy between Local and Average Structure in the Fast Na + Ionic Conductor Na 2.9 Sb 0.9 W 0.1 S 4 .

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS ₄ ^3- by WS ₄ ^2- in Na _2.9 Sb _0.9 W _0.1 S ₄ leads to a very high ionic conductivity of 41 mS cm ^-1 at room temperature. While pristine Na ₃ SbS ₄ crystallizes in a tetragonal structure, the substituted Na _2.9 Sb _0.9 W _0.1 S ₄ 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 ¹²¹ Sb NMR experiments that the short-range order of Na _2.9 Sb _0.9 W _0.1 S ₄ 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 Na ₃ SbS ₄ , quasi-elastic neutron scattering and solid-state ²³ Na nuclear magnetic resonance measurements revealed drastically improved Na ⁺ diffusivity and decreased activation energies for Na _2.9 Sb _0.9 W _0.1 S ₄ . 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.