Geomimetic Thermosynthesis in Heterogeneous Structural Complexes of In Situ Growing Imine-Based COF on MXene for Enhanced Sodium Ion Storage

Covalent organic frameworks (COFs) have gained significant attention as next-generation electrode materials for energy storage, owing to their chemical versatility, ecofriendliness, and cost-effectiveness. However, their practical application in energy storage systems is hindered by challenges such as insufficient exposure of functional groups for sodium storage and poor ion/electron transport kinetics. In this work, we developed an organic-inorganic heterojunction structure by in situ growth of an imine-based COF on the surface of MXene, which was employed as an anode material for sodium-ion batteries. This heterojunction design enhances sodium ion and electron transport, while the porous COF layer maximizes the exposure of active sites. In situ FT-IR and Raman spectroscopy analyses reveal that the C=N and C=C functional groups in the COF@D-Ti₃C₂T _<i>x</i> electrode enable reversible sodium-ion storage. Furthermore, the flexible hydrogen bonds between the COF and MXene layers effectively mitigate volume expansion during cycling, improving the structural stability and long-term cycling performance. As a result, the COF@D-Ti₃C₂T _<i>x</i> composite electrode delivers a remarkable reversible capacity of 401.6 mA h g^-1 after 300 cycles at 0.1 C. This work not only introduces a novel synthesis strategy for imine-based COFs but also explores sodium-active reaction units and organic-inorganic heterojunction designs, offering new insights for advancing rechargeable battery technologies.