Intensifying D-Orbitals Energy Level Splitting of Local Co Atoms in CoO Lattice for Accelerated Iodine Redox Kinetics

Modulating the electronic structure of catalysts to maximize their power holds the key to address the challenges faced by zinc-iodine batteries (ZIBs), including the shuttle effect and slow redox kinetics at the iodine cathode. Herein, oxygen vacancies is innovatively introduced into CoO lattice to create high-spin-state Co active sites in nonstoichiometric CoO_1₋x nanocrystals supported by carbon nanofibers (H-CoO_1₋x/CNFs). This simple strategy intensifies crystal field splitting of Co 3d orbitals, optimizing the spin-orbital coupling between Co 3d orbitals and iodine species. The resulting enhanced availability of more unpaired electrons in non-degenerate e_g orbitals facilitates faster electron donation/acceptance during iodine redox reactions, thus improved reaction kinetics. Therefore, the assembled ZIBs employing H-CoO_1₋x/CNFs/I₂ cathode acquires a narrower overpotential gap (37 mV), higher initial capacity (203.0 mAh g^‒1), and better cycling stability (96.0% capacity retention after 2200 cycles at 0.5 A g^‒1) compared to the CoO/CNFs/I₂ cathode without experiencing defect engineering (109 mV/192.6 mAh g^‒1/74.7% after 1000 cycles). This work opens new avenues for maximizing the potential power of cathode host catalysts, making immediate contributions to the advancement of aqueous halogen batteries.