Unlocking high-performance organic cathodes: tailoring active group densities in covalent frameworks for aqueous zinc ion batteries

Aqueous zinc ion batteries (AZIBs) are a promising energy storage technology due to their cost-effectiveness and safety. Nowadays, organic materials with sustainable and designable structures are of great interest as AZIB cathodes. However, small molecules in organic cathode materials face dissolution problems and suboptimal cycle life, while large molecules suffer from low theoretical capacity due to inert carbon skeletons. Here, we designed two covalent organic framework materials (BB-COF and TB-COF) with the same structure and number of energy storage groups to investigate the correlation between the densities of active sites and electrochemical performance. Our study concludes that the electrochemical behavior of organic conjugates-based energy storage materials doesn't exhibit a linear correlation with active sites quantity. Adjusting active sites densities is crucial for material advancement. BB-COF and TB-COF, with dual active sites (C=O and C=N), exhibit distinct characteristics. TB-COF, with denser active groups, shows higher initial capacity (222 mAh g^-1). Conversely, BB-COF, featuring a larger conjugated ring diameter, exhibits superior rate performance and enduring cycle stability. It even maintains stable cycling for 2,000 cycles at -40 ℃. More deeply, in-situ electrochemical quartz crystal microbalance (EQCM) reveals the energy storage mechanism of BB-COF storing H⁺ first and then Zn²⁺.