Structural tuning of redox-active covalent organic frameworks for high-performance cathodes in sodium-ion batteries

The growing demand for sustainable, high capacity, and cost-effective energy storage has intensified the search for new alternative electrode materials beyond lithium-ion batteries. In this study, we employed dispersion corrected density functional theory (DFT-D3) calculations to investigate the potential of benzimidazole monolayers as cathode materials for sodium-ion batteries (SIBs). Our findings reveal that a single Na ion preferentially adsorbs along the ring structure of benzimidazole with an adsorption energy of -1.74 eV. The benzimidazole functionalised hexaazatriphenylene (BIFHAT) system demonstrates a specific capacity of 478.47 mAh g^-1 and an open circuit voltage (OCV) of 0.35 V while accommodating up to nine Na ions. To further enhance performance, we introduce strategic ring modifications and evaluate five distinct structural variations. Among these, the benzotriazole system exhibits superior sodium storage, supporting up to twelve Na ions with an exceptional capacity of 634.21 mAh g^-1 and an OCV of 0.53 V. The results also indicate that increasing the Na-ion concentration systematically reduces the adsorption energy across all studied configurations. Furthermore, quantum theory of atoms in molecules (QTAIM) analysis confirms that Na ion adsorption with nitrogen or oxygen sites in the considered structures is predominantly governed by noncovalent interactions. These findings highlight the potential of structurally tuned benzimidazole-based systems as promising cathode materials for next generation SIBs, offering an efficient and sustainable alternative for energy storage applications.