Dynamic Bifunctional Sites on a COF Enable Efficient Immobilization and Conversion of Iodine Species Toward Li‐Iodine Batteries

ABSTRACT The thermodynamic instability of iodine cation (I + ) and shuttle effect of polyiodide in the two‐electron Li‐iodine (Li‐I 2 ) batteries remain an unresolved bottleneck. The design and preparation of an advanced cathode capable of effectively anchoring and activating iodine species is a desirable but highly challenging target to overcome these issues. In this study, we strategically synthesized a pyridine‐functionalized COF (BPY‐COF‐HI) cathode that enables highly reversible multivalent transition of iodine (I − /I 0 /I + ) within Li‐I 2 batteries. The pyridine sites reversibly switch between protonated state (NH + ) and neutral state (N), allowing them to anchor I 3 − via electrostatic interactions and activate I + via halogen bonding, respectively. Benefiting from this dynamic bifunctional regulation driven by the single pyridine site, a carbon‐nanotube‐integrated composite cathode (BPY‐COF@CNT‐HI) delivers a high‐voltage discharge plateau at 3.58 V corresponding to the reversible I + /I 0 redox and achieves a gravimetric energy density of 642 Wh kg I −1 at 0.3 A g −1 . Remarkably, the cathode maintains ultralong cycling stability over 8000 cycles at 2.0 A g −1 with an exceptionally low capacity fade of 0.0055% per cycle. This result widens perspectives for designing high‐performance cathodes for Li‐I 2 batteries with two‐electron redox chemistry.