Regulating the Spin-State of Cobalt in Three-Dimensional Covalent Organic Frameworks for High-Performance Sodium-Iodine Rechargeable Batteries

Although the catalytic activity is heavily reliant on the electronic structure of the catalyst, understanding the impact of electron spin regulation on electrocatalytic performance is still rarely investigated. This work presents a novel approach involving the single-atom coordination of cobalt (Co) within metalloporphyrin-based three-dimensional covalent organic frameworks (3D-COFs) to facilitate the catalytic conversion for sodium-iodine batteries. The spin state of Co is modulated by altering the oxidation state of the porphyrin-centered Co, achieving optimal catalysis for iodine reduction. Experimental results demonstrate that Co^II and Co^III are incorporated into the 3D-COFs, exhibiting spin ground states of S=1/2 and S=0, respectively. The low spin state of Co^III is favorable to hybridize with the sp 3d orbitals of I₃ ^-, thus facilitating the conversion of I₃ ^- to I^-. Density-functional theory (DFT) calculations further reveal that the presence of Co^III enhances iodide adsorption and accelerates the formation of NaI in 3D-COFs-Co^III, thereby promoting its rapid kinetic behaviors. Notably, the I₂@3D-COFs-Co^III cathode achieves a high reversible capacity of 227.7 mAh g^-1 after 200 cycles at 0.5 C and demonstrates exceptional cyclic stability, exceeding 2000 cycles at 10 C with a minor capacity fading rate of less than one 0.01 % per cycle.