Carbon-coated iron selenide derived from double-framework as an advance anode for Na-ion battery

Owing to the desirable nano-morphology, controllable structure, and ease of preparation, metal-organic frameworks (MOFs) are widely used as the precursors for electrodes in Na-ion battery (NIB). However, MOF structures are prone to fracture and collapse during the reactions. Additionally, MOF-derived electrodes often exhibit a high expansion rate, which negatively impacts the long cyclic capability of NIBs. Herein, we employed a stable covalent-organic framework (COF) as a protective coating for the first time to preserve the MOF structure. A shuttle-like iron selenide (Fe₃Se₄) coated with N-doped carbon (NC) was synthesized using a simple hydrothermal method, surface coating, and subsequent selenizing process. Due to its large specific surface area and well-developed porosity, the double-framework derived Fe₃Se₄/NC electrode provides abundant active sites for Na⁺ storage. The COF and COF-derived NC protect the structure of Fe₃Se₄/NC during synthesis and cyclic process, respectively. The high conductivity of the NC coating enhances the electron/ion conductivity of Fe₃Se₄/NC, thereby beneficial the rate performance. As the material anode for NIB, the Fe₃Se₄/NC electrode exhibits a high initial charging/discharging capacity (425.7/478.4 mAh·g^-1 with an initial Coulombic efficiency of 89.0 %), excellent rate performance (333.5 mAh·g^-1 at 12 A·g^-1), long-durable cycle capability (290.8 mAh·g^-1 after 1000 cycles at 8 A·g^-1) and fast charging ability (143 s). This work provides a novel strategy of "COF on MOF" to prepare high-performance electrode materials for NIB.