Highly Crystalline and Robust Donor-Acceptor Type Covalent Organic Frameworks for Long-life Sodium-Ion Battery Cathodes.

Covalent organic frameworks (COFs) hold great potential in sodium-ion battery cathodes. However, most reported COF-based electrodes show unsatisfying capacity and rate performance due to their limited redox site density, low crystallinity, and poor conductivity. Herein, a highly crystalline and robust donor-acceptor type COF with abundant redox active sites is developed by the polymerization of donor unit benzo[1,2-b:3,4-b″:5,6-b″']trithiophene-2,5,8-tricarbaldehyde) (BTT) and acceptor unit s-indacene-1,3,5,7(2H,6H)-tetrone (ICTO) (denoted as BTT-ICTO) for cathodic Na storage. The BTT-ICTO-graphene composites (BTT-ICTO@G) synthesized by in situ growth have a loose sheet structure with rough surfaces, contributing to the improved conductivity and active site utilization of BTT-ICTO. Benefiting from the robustness of BTT-ICTO linked by ethylene bonds, the BTT-ICTO@G cathodes exhibit a high capacity of 325 mAh g at 0.1 A g with a high active site utilization of 80%, excellent rate performance of 190 mAh g at 5.0 A g, and exceptional cycle performances of 196 mAh g over 10 000 cycles at 2.0 A g with only 0.0015% decay per cycle. These properties make the BTT-ICTO@G cathodes among the best-reported COF-based sodium-ion battery cathodes. In addition, in situ Raman, ex situ Fourier transform infrared, and theoretical calculations disclose the reaction pathway of Na storage.