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 realizedviasolvothermal 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 the Na(+)storage capacity can be improved by increasing the charge carrier conductivityviathe inclusion of BT units, while the 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 exhibiting highly enhanced reversible discharge capacity (similar to 450 mA h g(-1)) after 30 cycles at a scan rate of 100 mA g(-1). To the best of our knowledge, this discharge capacity is among the best values reported so far for organic electrodes prepared without thermal treatment.