An Anion-Concentrating Layer for a Large-Capacity Al Anode Enabled by an Amorphous 2D Covalent Organic Framework

Rechargeable aluminum batteries suffer from dendritic anode growth and capacity decay due to severe concentration polarization caused by sluggish ion transport kinetics, particularly under high-capacity conditions. To address this issue, we developed an amorphous covalent organic framework (COF) coating layer engineered for anion-concentrating functionality. The conformational flexible imine linkages in the amorphous COF enable adaptive trapping of Al 2 Cl 7 − , storing them as crystalline AlCl 3 and ligand-bonded AlCl 4 − within the mesopores. This mechanism enriches electrochemical active species to buffer the concentration polarization, and promotes the formation of highly active Al 3 Cl 10 − species at the surface, thereby accelerating interfacial kinetics to guide homogeneous deposition. As a result, dendrite-free aluminum deposition is achieved even at an ultrahigh areal capacity of 10 mAh cm −2 under the protection of the COF. The half-cell with the COF exhibits an exceptional cycling time of over 6000 h with a coulombic efficiency of 99.9%, while symmetric cells demonstrate long-term stability over 7500 h with a low overpotential of 36 mV. Full cells with the graphite cathode also maintain stability over 5000 cycles under a large current density of 1.2 A g −1 . This work presents an effective coating strategy for regulating electrodeposition in metal anode batteries, highlighting a promising path toward practical applications.