t2 occupancy as a descriptor for polysulfide conversion on spinel oxides

Transition metal oxides with partially filled d orbitals serve as effective polysulfide conversion catalysts in lithium-sulfur batteries. Nonetheless, a predictive descriptor to elucidate their structure-activity relationship is still lacking. To fill this gap, here we investigate the occupancy of t2 orbitals as a possible descriptor for the polysulfide conversion activity of spinel oxides. The electrochemical characterizations reveal a volcano relationship between t2 occupancy and the polysulfide conversion activity of spinel oxides. The theoretical calculations indicate that the t2 occupancy effect is associated with the ability of tetrahedral cations to cleave S-S bonds and form Li-S bonds. As such, Mn0.5Co0.5Cr2O4, with an optimized average t2 occupancy of 2.88, is designed and demonstrates promising polysulfide conversion activity among representative oxides. This work elucidates that t2 occupancy serves as an effective descriptor for evaluating the polysulfide conversion activity of spinel oxides. With the catalytic mechanism proposed, the current study prompts further exploration of the application of t2 occupancy as a descriptor for predicting the polysulfide conversion efficiency of other catalytic systems beyond the spinel structure.