Delocalization-Driven Activation of Inert Imine Linkages in Covalent Organic Frameworks for Reversible Magnesium...

Imine-linked covalent organic frameworks (COFs) are promising cathodes for rechargeable magnesium batteries (RMBs), yet their abundant imine linkages (C═N) typically remain electrochemically inert, serving merely as structural connectors. Here, we reveal that the redox inactivity of imine linkages arises from insufficient π-electron delocalization, and we demonstrate that enhancing local delocalization can switch these bonds into highly reversible redox centers. Through precise fluorine substitution in a triazine-based COF, we achieve localized π-delocalization without disrupting the overall conjugated framework. This delocalization-driven activation lowers the LUMO level and stabilizes the reduced state of imine linkages (C-N^-), enabling a reversible C═N ⇄ C-N^- conversion-a function never realized before in magnesium batteries. The activated imine sites become kinetically preferred Mg²⁺ migration channels, cutting the diffusion barrier by half (from 1.61 to 0.81 eV) and switching the transport pathway from triazine to imine. Consequently, the optimized COF cathode delivers a high specific capacity of 203.6 mAh g^-1, outstanding rate capability, and exceptional cycling stability (72.9% retention after 9000 cycles). This work establishes π-delocalization engineering as a general strategy and a redox switch to unlock latent redox functions in organic frameworks, providing a mechanistic blueprint for activating inert bonds in multivalent energy storage systems beyond magnesium batteries.