Balanced Iodophilicity and Solvophilicity Unlocks Fast Iodine Conversion Chemistry

The shuttling of polyiodides and sluggish redox kinetics greatly hinder the implementation of an aqueous Zn-I₂ battery. Despite numerous catalysts have been implemented to improve the iodine cathode stability, an important aspect, the balance between polyiodide adsorption and interfacial mass transport kinetics at the cathode surface, has been overlooked. It is known that insufficient intermediate trapping ability will cause low iodine utilization and fast capacity decay. However, excessive adsorption of iodine species will block the ion transport and lead to passivation of the catalyst, which is particularly serious under lean-electrolyte and high mass loading conditions. To tackle this challenge, we employ a dual single atomic catalyst encompassing NiN₄P and FeN₄P sites to promote a catalytic interface with well-balanced solvophilicity and iodophilicity. Specifically, the FeN₄ and NiN₄ sites primarily enhance polyiodide immobilization and mass transport, respectively. The P ligands further strengthen these functions by tuning the Fe site from low to medium spin state and creating the anion-rich inner Helmholtz plane at Ni sites. Benefiting from this dual-metal atomic catalyst, the iodine cathode exhibits high cycling stability and ultralow self-discharge rate under a low E/I ratio. This work provides insights into regulating the aqueous halogen cathode interface for long cycle life.