Superconjugated Anthraquinone Carbonyl-Based Covalent Organic Framework as Anode Material for High-Performance Aqueous Ammonium-Ion Batteries.

Covalent organic frameworks (COFs) have recently been emerged as promising anode candidates for aqueous ammonium ion batteries (AAIBs), but suitable COFs need to be developed and the energy storage mechanisms need to be further investigated. Here we report an anthraquinone carbonyl-containing COF as anode for AAIBs, which exhibits a high specific capacity of 141 mAh g-1 at 0.1 A g-1 and good cycling stability with 90% retention after 8000 cycles at 6 A g-1. We find that the anthraquinone carbonyls (denoted as C=O1) with superconjugated structure in the COF can form hydrogen bonds with NH4+ ions, thus serving as optimal sites and providing the major contribution to NH4+ ion storage, whereas another type of carbonyls (denoted as C=O2) in the same COF suffers from enol-keto tautomerism, which prevents the formation of hydrogen bonds and is therefore unfavourable for NH4+ ion storage. This is further verified by the theoretical calculations where the hydrogen bonding rate of C=O1 groups is much faster than that of C=O2. This work not only provides insights into understanding the mechanism of NH4+ ion storage, but also offers new ideas for the design of advanced COF-based materials for AAIBs.