Towards Stable Metal-I₂ Battery: Design of Iodine-Containing Functional Groups for Enhanced Halogen Bond

The redox chemistries of iodine have attracted tremendous attention for charge storage owing to their high theoretical specific capacity and natural abundance. However, the practical capacity and cycle life are greatly limited by the active mass loss originating from the dissolved iodine species in either non-aqueous or aqueous batteries. Despite intensive progress in physical and physicochemical confinements of iodine species (I₂/I₃ ^-/I^-), less attention has been paid to confining iodine species beyond the host-iodine interface, inhibiting further development of iodine cathodes with high I₂ contents. Here a halogen bond (XB)- enhanced design concept is proposed between I₂ molecules to achieve stable cycling performances, as exemplified by the Na-I₂ battery. The enhanced XB is derived from the incorporation of -B(OH)I₃ groups in highly integrated porous carbon/I₂ cathode (HOCF-BI_n), which can generate extended interactions between -B(OH)I₃ and following I₂ molecules. Due to the strong intermolecular force between I₂ molecules, the HOCF-BI_n cathodes exhibit substantially strengthened I₂/I₃ ^-/I^- confinement, enabling outstanding cycling stability at I₂ loading ranging from 1.8 to 6.2 mg cm^-2. This findings demonstrate a functional group to manipulate XB chemistry within I₂ molecules and polyiodides for stable and low-cost metal-iodine batteries.