Identifying the Role of the Cationic Geometric Configuration in Spinel Catalysts for Polysulfide Conversion in Sodium–Sulfur Batteries

An AB₂X₄ spinel structure, with tetrahedral A and octahedral B sites, is a paradigmatic class of catalysts with several possible geometric configurations and numerous applications, including polysulfide conversion in metal-sulfur batteries. Nonetheless, the influence of the geometric configuration and composition on the mechanisms of catalysis and the precise manner in which spinel catalysts facilitate the conversion of polysulfides remain unknown. To enable controlled exposure of single active configurations, herein, Co_td²⁺ and Co_oh³⁺ in Co₃O₄ catalysts for sodium polysulfide conversion are in large part replaced by Fe_td²⁺ and Fe_oh³⁺, respectively, generating FeCo₂O₄ and CoFe₂O₄. Through an examination of electrochemical activation energies, the characterization of symmetric cells, and theoretical calculations, we determine that Co_oh³⁺ serves as the active site for the breaking of S-S bonds, while Co_td²⁺ functions as the active site for the formation of S-Na bonds. The current study underlines the subtle relationship between activity and geometric configurations of spinel catalysts, providing unique insights for the rational development of improved catalysts by optimizing their atomic geometric configuration.