Enhanced Lithium-Ion Storage with Nitrogen-Enriched Triazine Covalent Organic Frameworks via Mechanical Exfoliation

초록

Covalent organic frameworks (COFs) often demonstrate limited electrochemical performance due to the stacking tendency of their 2D-extended structures, particularly in triazine COFs (T-COFs), where strong pi-pi interactions cause an eclipsed arrangement. This stacking effect conceals the active sites inside the 1D channels, hindering efficient ion transport, especially under high-charge/discharge conditions. The slow movement of Li+ ions through these channels results in the poor utilization of redox-active sites, thereby diminishing both capacity and rate capabilities. In this work, we introduce a facile method to improve the electrochemical properties of COFs when used as anode materials in lithium-ion batteries. A 2D-COF was synthesized by the nucleophilic substitution of cyanuric chloride with melamine, followed by mechanical exfoliation into 2D few-layer nanosheets using a simple mechanical grinding technique. These bulk and exfoliated COFs were characterized using FTIR, XPS, TGA, FESEM, and DRS. Photophysical studies indicate that both bulk and E-COFs exhibited low band gap values of 3.31 and 4.25 eV, respectively, showing their semiconducting nature. Electrochemical tests reveal that the exfoliated COF delivers a maximum capacity of 847 mAh g-1, with a Coulombic efficiency of 72% at 0.2 A g-1. The shortened Li+ diffusion pathways in exfoliated COF led to significantly enhanced redox site utilization and faster lithium storage kinetics, in contrast to the diffusion-limited behavior seen in bulk COF.

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