Tri(Thiophene‐Nitrile) Covalent Organic Frameworks With Low HOMO/LUMO Donor/Acceptor for High‐Potential‐Plateau Zinc Batteries

Abstract

ABSTRACT Covalent organic frameworks (COFs) featuring desirable redox‐active sites have become competitive cathode materials for aqueous zinc‐organic batteries (ZOBs). However, multi‐active COFs, albeit with high capacity, are often confined to their sloping and low redox potential (<0.8 V) caused by high molecular orbital energy levels (HOMO/LUMO) of active moieties. Here we report a capacity‐voltage trade‐off‐breaking design of COFs by pairing low‐HOMO trithiophene donor (−6.04 eV) with low‐LUMO trinitrile acceptor (−3.82 eV) via robust olefin linkages (TN‐COF), using high‐LUMO triazine acceptor (−1.75 eV) as the counterpart (TA‐COF). The tri(thiophene‐nitrile) donor‐acceptor enables low HOMO/LUMO energy levels (−3.98/−5.94 eV) for TN‐COF cathode, thus unlocking a flat and high redox potential of 1.2 V compared to TA‐COF (0.9 V). An 18‐electron CF 3 SO 3 − /NH 4 + (de)coordination process is activated per hexagonal tri(thiophene‐nitrile) unit in TN‐COF cathode with ultralow activation energy (0.17 eV). This facilitates 98.2% utilization of C−S/C≡N active sites to liberate high capacity of 335 mAh g −1 (vs. 80.1%/268 mAh g −1 for TA‐COF). The ideal combination of high voltage and capacity gives TN‐COF cathode superior energy density (402 Wh kg −1 ) and ultralong life (70,000 cycles). This finding widens the design philosophy of high‐voltage‐capacity COFs via HOMO/LUMO energy engineering for advanced ZOBs.