In-Plane Exposed Metal Sites in Covalent Organic Frameworks for High-Performance Lithium Sulfur Batteries

Covalent organic frameworks (COFs), featuring structural tunability and high porosity, have attracted great interest and are being developed into metal-COFs (M-COFs) by introducing metal atoms to boost their electrocatalytic performance. However, current M-COFs in Li-S batteries are fully metal coordinated in-plane environments, showing less exposed metal sites due to eclipsed (AA/AA') stacking modes. Here, a bottom-up molecular design strategy is proposed to construct M-COFs with in-plane exposed metal sites by precisely incorporating pyridine N atom pairs into the COF framework. Comprehensive experimental characterizations and theoretical calculations demonstrate that the in-plane exposed active sites significantly reduce activation barriers for polysulfides conversion, thereby achieving excellent rate performance (1412 mAh g^-1 at 0.5 C, 842 mAh g^-1 at 5 C) and cycle stability (0.027% capacity fade per cycle over 2000 cycles at 1 C).