Construction of a Few-Layered COF@CNT Composite as an Ultrahigh Rate Cathode for Low-Cost K-Ion Batteries.

Potassium-ion batteries (PIBs) are attracting great interest for large-scale energy storage owing to the abundant resources and low redox potential of K + /K. However, the large volume changes and slow kinetics caused by the larger ionic radius of K + for cathode materials remain a critical challenge for PIBs. Herein, we construct few-layered covalent organic frameworks integrated with carboxylated carbon nanotubes (DAAQ-COF@CNT) as cathode materials for PIBs. The synthesized DAAQ-COF@CNT features numerous active sites, a stable conductive framework, and an appropriate surface area with nanopores, which can render high electrical conductivity, shorten the ion/electron diffusion distance, and accelerate K + diffusion. In consequence, the DAAQ-COF@CNT delivers a high reversible capacity of 157.7 mAh g -1 at 0.1 A g -1 , an excellent rate capability of 111.2 mAh g -1 at 1 A g -1 , and a long cycling stability of 77.6% capacity retention after 500 cycles at 0.5 A g -1 . The integrated characterization of ex situ X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and theoretical simulation discloses that the storage mechanism of DAAQ-COF@CNT is based on the reversible reaction between electroactive C═O groups and K + during two successive steps. This work provides a promising high-performance cathode material for PIBs and encourages the development of new types of covalent organic frameworks for PIBs.