Iron metal-organic framework@covalent-organic framework heterostructure nanofilms: synergistic ion-electron highways...

Advancing filtering lithium-ion capacitors (FLICs) urgently require enhancing anodic bulk-phase ion-electron transport kinetics to achieve the dual functionality of AC-line filtering and high-capacity lithium-ion storage. Herein, a novel iron metal-organic framework@covalent-organic framework (Fe-MOF@COF_TD) heterostructure nanofilm electrode was designed to leverage continuous one-dimensional mesoporous channels and a laminar mesoporous-microporous architecture for enhanced Li⁺ transport, while utilizing interface-contact-induced charge redistribution, interfacial Co²⁺ chelating bonds, and metal orbital coupling within the Fe-MOF layer to ensure efficient electron transmission. Notably, four-coordinated Fe(II) constructs an ordered 2D layered mesopore-micropore architecture, while six-coordinated Fe(III) leads to disordered aggregation and higher transport tortuosity, thus endowing the Fe(II)-based heterostructure with superior Li⁺ diffusion kinetics and electrochemical performance over the Fe(III)-based counterpart. The optimized Fe(II)-MOF@COF_TD//COF_TD FLIC device exhibits good energy storage properties, such as a high volumetric energy density of 47.6 mWh cm^-3 at 5.3 W cm^-3 and retained 45.8 mWh cm^-3 at 19.8 W cm^-3, ~80% capacitance retention after 4000 cycles. Moreover, this device also delivers excellent AC-line filtering capability, featuring a volumetric filtering capacitance of 2.62 F cm^-3 and a phase angle of -70.5° at 120 Hz. This work provides a feasible heterostructure-engineering strategy for developing dual-function FLICs with simultaneously enhanced energy storage and AC-filtering performances.