Unveiling the relationship between polymorphism and ionic conductivity in Na3−xSb1−xWxS4 solid electrolyte for Na-ion batteries

Abstract

Sulphides with general formula Na_3−xSb_1−xW_xS₄ are promising solid state electrolytes for Na-ion batteries (SIBs), thanks to their high conductivity at room temperature, their high malleability and the possibility to synthesise them through wet-chemistry techniques. The materials can crystallize either in a tetragonal or a cubic polymorph as a function of composition, with the highest conductive form being the cubic polymorph (for x ≥ 0.075), for which the Na ion diffusion pathways are expected to be 3D and isotropic. However, the existence of a discrepancy at room temperature between average and local structure is known even for higher substituted systems. Here we investigate the effect of composition and structure on the transport properties of the Na_3−xSb_1−xW_xS₄ system by combining experimental and computational techniques, including synchrotron X-ray diffraction (XRD), pair distribution function (PDF) analysis, bond valence site energy (BVSE) analysis and classical molecular dynamics (MD). The results obtained from long time-scale MD provide new insights into the mechanistic features of ionic transport in this family of sulphide electrolytes and highlight their subtle interplay with the average and local structure of these materials.