On the Discrepancy between Local and Average Structure in the Fast Na⁺ Ionic Conductor Na_2.9Sb_0.9W_0.1S₄

Abstract

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.