Pore Engineering of Covalent Organic Frameworks Boosts Chlorine Confinement and Electrochemical Performance in Li─Cl2 Batteries.

The development of high-energy-density Li─Cl2 batteries is hindered by insufficient Cl2 storage in cathodes. Although porous host materials have been preliminarily explored, the effect of pore size on Cl2 confinement and electrochemical behavior still remains unclear. Herein, two novel covalent organic frameworks (COFs) with distinct pore sizes, namely TH-COF (mesoporous, 2.7 nm) and HH-COF (microporous, 0.9 nm), were fabricated by reacting triphenylene-2,3,6,7,10,11-hexacarboxylic acid with 3- and 6-connected amines, respectively, to serve as a model system for elucidating the pore-size effect in Li-Cl2 batteries. Owing to its smaller pore size and resultant stronger spatial confinement, the microporous HH-COF enables superior Cl2 capture and markedly enhanced battery performance, as exemplified by a high capacity of 4500 mAh g-1, a high current density of 10 000 mA g-1, and a Coulombic efficiency (CE) above 94% for each of the 500 cycles, outperforming its mesoporous TH-COF counterpart and all previously reported electrodes. Density functional theory calculations reveal stronger host-guest interactions between Cl2 and the microporous HH-COF than its mesoporous counterpart TH-COF. This study not only clarifies the pivotal role of pore-size engineering in Li-Cl2 batteries but also establishes a rational design principle for developing high-performance Cl2 host cathodes.