Sulfur-Bridged Dual Fe-N₄ Sites to Boost Lithium-Sulfur Battery Performance

Despite the growing interest in d-block single-atom catalysts (SACs) for lithium-sulfur (Li-S) batteries, the localized nature of their 3d orbitals imposes a substantial energy barrier for activating sulfur intermediates during redox reactions. This limitation stems from the low mobility of electrons in 3d orbitals, which hampers effective hybridization with the 2p orbitals of lithium polysulfides (LiPS). Here, we report a single-atom iron catalyst anchored on a nitrogen-doped porous carbon scaffold, featuring a unique axial dual-layer Fe-N₄ structure bridged by a sulfur atom (Fe-SNC), designed to overcome these limitations. The axial configuration, stabilized by the S bridge, enables delocalization of Fe 3d orbitals and promotes dynamic electron transfer between the dual Fe-N₄ layers, thereby enhancing bidirectional Li-S redox kinetics. LiPS adsorption tests, density functional theory (DFT) calculations, symmetric-cell cyclic voltammetry, and potentiostatic deposition/dissolution experiments confirm the superior catalytic activity of Fe-SNC. When applied in Li-S cells, the S@Fe-SNC cathode achieves an initial capacity of 1550 mAh g^-1 at 0.1C and 724 mAh g^-1 at 3C, retaining 546 mAh g^-1 after 1200 cycles at 3C. Under high-sulfur-loading (7.5 mg cm^-2) and lean-electrolyte conditions (E/S = 4.7 μL mg^-1), it reaches excellent areal performance at 8.0 mAh cm^-2. DFT results attribute the high activity to efficient interlayer electron transfer and an asymmetric dynamic spin evolution across the dual Fe-N₄ layers. This asymmetric spin distribution facilitates enhanced d-p orbital hybridization, particularly π-type coupling between Fe 3d_xz/yz and S 2p orbitals, thereby improving conductivity, intermediate binding strength, and overall reaction kinetics.