In Situ Tracking of Radical Evolution in a Conjugated Covalent Organic Framework for Reversible Sodium Storage

Covalent organic frameworks (COFs) have emerged as promising platforms for sodium-ion storage owing to their tunable redox-active sites and ordered porous architectures. However, the role of radical intermediates in their electrochemical processes remains elusive due to the lack of direct experimental evidence. Herein, we report a highly crystalline β-ketoenamine-linked COF (BT-COF-AA) that delivers exceptional rate capability and ultralong cycling stability over 10 000 cycles at 5 A g^{- 1}. More importantly, in situ electron paramagnetic resonance (EPR) spectroscopy provides time-resolved and direct observation of transient radical intermediates, enabling the elucidation of a sequential sodium-ion storage mechanism. Specifically, Na⁺ ions initially coordinate with nitrogen sites in benzothiadiazole units, triggering localized electron transfer and the formation of stabilized radicals. Subsequent sodiation occurs at carbonyl oxygen sites, generating ketyl radicals accompanied by dynamic electron delocalization across the conjugated framework. This reversible radical evolution establishes a framework-coupled redox pathway that underpins the outstanding electrochemical performance. Combined experimental and theoretical results reveal a radical-mediated charge storage mechanism and provide fundamental insights for the rational design of high-performance organic electrode materials.