Covalent Organic Nanosheets with a Tunable Electronic Structure to Achieve Unprecedented Stability and High-Performance in Sodium-Ion Batteries

This study develops a new type of fluorinated covalent organic nanosheets (CONs) as anode materials for sodium-ion batteries by incorporating an electron-withdrawing benzothiadiazole (BT) unit and F atom into the framework. These modifications lead to a reduced bandgap and electron density, generating strong permanent dipoles that increased Na⁺ accessible sites within the self-assembled solid-state structure. To elucidate the effect of these electronic changes, the Na⁺ storage performance of fluorinated D/A-CON-10-F is compared to that of nonfluorinated D/A-CON-10. The reduced electron density in D/A-CON-10-F weakens its interaction with Na⁺, yet enhances ion and charge carrier conductivities, leading to improved electrochemical performance. Notably, D/A-CON-10-F exhibits a reversible discharge capacity of ≈637 mA h g⁻¹ at 100 mA g⁻¹, maintaining structural stability over 5000 cycles with excellent rate capability. These results demonstrate that dipole engineering in CONs effectively enhances charge transport and long-term stability, offering a promising strategy for next-generation sodium-ion battery anodes.