Root-Nodule-Inspired Cobalt Selenide with Sulfur-Doping-Induced Phase Transition for High-Performance Lithium–Sulfur Batteries

The root-nodule system in legumes enables efficient biological nitrogen fixation through symbiotic interactions and hierarchical mass transport. Inspired by this natural architecture, we synthesized a cobalt selenide (CoSe) catalyst supported on carbon nanofibers (CoSe@C) that mimics this root-nodule structure. This unique design promotes rapid electron transport and facilitates efficient catalytic conversion of lithium polysulfides (LiPSs) to Li₂S. Through controlled sulfur doping, the initial hexagonal phase of CoSe (h-CoSe) underwent a phase transition to an orthorhombic structure (o-CoSeS), which exhibited a high-spin state due to an increased density of unpaired electrons in the Co d-orbitals. Density functional theory (DFT) calculations revealed that this electronic configuration enhances orbital hybridization between Co d-orbitals and LiPSs p-orbitals, thereby strengthening LiPS adsorption and accelerating the redox kinetics. When o-CoSeS supported on carbon nanofibers (o-CoSeS@C) was used to modify the separator in lithium-sulfur (Li-S) batteries, the battery delivered an initial discharge capacity of 1509 mAh g^-1 at 0.1 C and maintained an ultralow decay rate of 0.057% per cycle over 1000 cycles at 1 C. The exceptional cycling stability stems from the synergy between the biomimetic hierarchical network, which facilitates mass/charge transport, and the optimized electronic structure of the Co active sites, which boosts catalytic activity. This work proposes a novel biomimetic strategy for designing high-performance catalysts for Li-S batteries and provides atomic-level insights into the regulation of transition-metal electronic states for catalytic optimization.