Molecular Engineering of Covalent Organic Nanosheets for High-Performance Sodium-Ion Batteries

The bandgap-dependent performance of covalent organic nanosheets (CONs) as sodium-ion battery anode materials was probed by inclusion of electron-deficient benzothiadiazole (BT) units into their network. Conjugation of BT units with electron-rich moieties afforded low-bandgap materials, and a self-assembled CON morphology with a large number of insertion sites for Na+ ions was realized via solvothermal Stille cross-coupling. The bandgap dependence of Na+ storage capacity was probed by the synthesis and characterization of large-bandgap CONs, which were subsequently compared to low-bandgap CONs in terms of electrochemical behavior. Four different CONs were investigated in total to reveal that Na+ storage capacity can be improved by increasing charge carrier conductivity via the inclusion of BT units, while surface area can be controlled by maintaining the material backbone. The electrode with a solvothermally prepared low-bandgap CON demonstrated stable rate capability and cycling performance while highlighting highly enhanced reversible discharge capacity (~450 mAh/g) after 30 cycles at a scan rate of 100 mA/g. To the best of our knowledge, this discharge capacity is among the best values reported so far for organic electrodes prepared without thermal treatment.