Highly Crystalline Covalent Organic Framework Nanosheets with Multiple Redox-Active Sites and Cation-π Effect for Safe Potassium-Ion Batteries

Abstract

Covalent organic frameworks (COFs) with multiple and highly accessible redox-active sites, along with excellent stability exhibit great potential for application in potassium-ion batteries (KIBs). However, most COF-based KIBs exhibit low capacities and slow kinetics. Moreover, the safety issues of KIBs remain unaddressed. Here, we demonstrate fully sp2 carbon-conjugated COF (BTT-ICTO) nanosheets with excellent quality and large sizes, which can be employed for the fabrication of high-capacity highly safe KIBs using optimal flame-retarded electrolytes. The unique and ultrathin structures of these COF nanosheets greatly promote the exposure of the redox-active sites and accelerate electrochemical kinetics. In addition, the cation-π effect can simultaneously provide additional sites for K+ storage. The optimal electrolytes not only guarantee intrinsic safety of KIBs, but also have excellent compatibility with the COF nanosheets. As the anode of KIBs, the COF nanosheets deliver an unprecedented reversible capacity up to 566 mAh g-1 at 0.1 A g-1 and an excellent rate capacity of 310 mAh g-1 at 5.0 A g-1. Additionally, the COF nanosheets have also exhibit decent cycling performance during 3200 charging/discharging cycles (427 mAh g-1 at 1.0 A g-1 after cycling). The overall performances have set a new record for COF-based KIBs.