Ferrocene-based metal-organic frameworks with dual redox sites for advanced sodium-ion battery anodes

Conventional sodium-ion battery (SIB) anodes are still lagging in capacity, reaction kinetics, and cycling stability, which motivates the development of high-performance anodes for advancing grid-scale energy storage. Herein, we report a ferrocene-based metal-organic framework (NF-MOF) anode with dual redox sites, synthesized on carbon cloth (CC) via a facile solvothermal method. This innovative design yields a binder-free NF-MOF@CC anode that integrates the redox activity of ferrocene (Fc/Fc⁺) with the Ni⁰/Ni²⁺ redox couple. The prudent design ensures intimate interfacial contact between NF-MOF and CC, eliminating binder-induced resistance and accommodating Na⁺-induced strain through hierarchical porosity. As a result, NF-MOF@CC shows rapid capacitive-dominated Na⁺ storage and high diffusion coefficients, delivering a high reversible capacity of 705 mAh g^-1 at 0.1 A g^-1 and achieving a 3.9-time higher capacity than its powder counterpart. Ex situ structural and simulation studies confirm reversible Ni and Fe redox reactions with minimized lattice distortion, while electrostatic potential mapping identifies optimal Na⁺ adsorption sites. Moreover, full-cell configurations paired with Na₃V₂(PO₄)₃ or NaNiFeMn₂O₆ cathodes achieve remarkable cyclability, retaining 95.1 % (200th cycle) and 95.3 % (500th cycle) capacity, respectively, alongside high energy densities (310 Wh kg^-1 at 3160 W kg^-1). This work pioneers a dual-redox MOF design strategy, offering a universal platform for robust, high-capacity SIB anodes.