Organic Anion Immobilization by COF‐1 Enables Ultrathin SEI for Long‐Life Mg Metal Anodes

Mg metal anode has attracted considerable interest for the high theoretical capacity and low electrochemical potential, yet its electrochemical performance is compromised by parasitic electrolyte decomposition and the consequent surface passivation. In this work, an ultrathin solid electrolyte interphase (SEI) is generated on the Mg metal surface, facilitated by the anion immobilization effect of covalent organic framework‐1 (COF‐1), which significantly enhances the stability of the Mg metal anode. The COF‐1 nanoparticles, featuring unique benzene and B 3 O 3 ring structures with high specific surface area, remain uniformly dispersed in the liquid electrolyte and preferentially bind with dissociated TFSI − anions. Cryo‐TEM, TOF‐SIMS, and XPS characterizations confirm significant suppression of anion‐derived decomposition products, which leads to the formation of a thin SEI layer of 5 nm. Notably, semiquantitative analysis reveals that COF‐1 incorporation promotes the proportion of MgO nanoparticles within the SEI, effectively blocking electron tunneling to prevent parasitic reactions, while maintaining favorable Mg 2+ transport kinetics. Consequently, this engineered electrolyte enables the Mg metal anode to achieve exceptional electrochemical performance, including the Coulombic efficiency of ≈98.6% and stable cycling for ≈800 cycles. This work provides fundamental insights into interfacial design principles of the metal anodes.