Transition-region d-Electron count: The overlooked hidden variable behind catalysis and dendrite suppression in...

For lithium‑sulfur batteries (LSBs), transition region metals offer d electron counts from 1 to 10, making them an ideal platform to study electronic configuration and catalytic performance. This study selects zinc and cobalt because their 3d orbitals lie at two extremes: Zn²⁺ has a fully filled [Ar]3d¹⁰ configuration, while Co²⁺ has an unsaturated [Ar]3d⁷ configuration. Such extreme differences reveal a long overlooked hidden variable: the transition region d electron count. Using Zinc(II) Bis(trifluoromethanesulfonyl)imide (Zn(TFSI)₂) and Cobalt bis(trifluoromethylsulfonyl)imide (Co(TFSI)₂) as model additives for LSBs, we demonstrate that the decisive factor is not the metal type but the 3d orbital occupancy. This hidden variable leads the two systems toward distinct pathways in catalyzing polysulfide conversion and suppressing lithium dendrites.In the d¹⁰ system (Zn²⁺), an electric field drives Zn²⁺ to react with polysulfides, forming an in situ ZnS nanocatalytic interface. This interface accelerates sulfur redox and suppresses shuttling, but the fully filled d orbital causes strong electron localization, offering limited dendrite suppression. In the d⁷ system (Co²⁺), the higher d band center induces strong orbital hybridization, generating ultrafine CoS nanocrystals with abundant defects for targeted polysulfide capture. A reversible Co²⁺/Co³⁺ redox cycle lowers the reaction barrier and accelerates conversion kinetics. Moreover, the unsaturated 3d⁷ configuration enables an electron/ion dual conductive interface, significantly inhibiting lithium dendrites. The d¹⁰ system is far inferior in dendrite suppression. Thus, the transition region d electron count is the overlooked hidden variable for LSBs, and understanding it enables rational design of high performance additives.