Li-O2 batteries are considered promising candidates for next-generation energy storage devices owing to their high theoretical energy density (∼3500 Wh kg−1).However, their practical applications are hindered by their large overpotential,low rate capability, and poor cycling stability. The key to improve the performance of Li-O2 batteries is to find effective cathode catalysts that increase the kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Ni−bis(dithiolene) can be regarded as an inorganic analogue of tetrathiafulvalene (TTF).The research groups of Profs. Jing-Lin Zuo, Jing Ma, Ping He have recently successfully synthesized a bimetallic redox active complex by simultaneously incorporating Ni–bis(dithiolene) and Co–porphyrin units as the ORR/OER bifunctional cathode catalysts for Li-O2 batteries.
Fig.1. Bimetallic redox active metal complexes as bifunctional catalysts for Li-O2 batteries.
The bimetallic COF was synthesized by the assembly of Ni(bded)2 (bded = bis[1,2-di(4-formylphenyl)ethylene-1,2-ditholate]) and5,10,15,20-tetrakis(4-aminophenyl)-porphinatocobalt. Meanwhile, the nonmetallic or single metallic isostructural COFs were also synthesized by replacing Ni(bded)2 with its organic analog 2,3,6,7-tetra (4-formylphenyl)-tetrathiafulvalene (TTF-TBA) or using free-based porphyrin linkers 5,10,15,20-tetrakis (4-aminophenyl) porphinato. The synergistic combination of Ni–bis(dithiolene) and Co–porphyrin resulted in a bimetallic Ni/Co-COF with a high BET surface area (116 m2 g−1), fairly good electrical conductivity (1.18 × 10−4 S m−1), excellent chemical stability, and bifunctional ORR/OER activity as the Li-O2 battery cathode. Li-O2 batteries with Ni/Co-COF cathodes have low discharge/charge potential gap (1.0 V) and stable cycling for 200 cycles at a current density of 500 mA g−1, which is significantly better than that of nonmetal or single metal-based isostructural COFs. Furthermore, the important role of Ni and Co centers in promoting the ORR/OER and inducing uniform Li2O2 deposition was supported by control experiments and DFT calculations. This work will be helpful to design new electrocatalysts for aprotic Li-O2 batteries and beyond.
Fig.2. Electrochemical performance of Li-O2 batteries using different COF cathode catalysts.
The work has been published in Science Advances (DOI: 10.1126/sciadv.adf2398) with the title of Covalent Organic Frameworks with Ni–Bis(dithiolene) and Co–Porphyrin Units as Bifunctional Catalysts for Li–O2 Batteries. The co-first authors of the paper are Si-Wen Ke, Wei Li and Yuming Gu. Profs Shuai Yuan and Jian Su participated in this work. The above research work has been supported by the National Key Research and Development Program of China (2018YFA0306004, 2019YFC0408303), the National Natural Science Foundation of China (No. 21875099, 22033004, 21873045, 21922508, 22179059, and U1801251), and the Natural Science Foundation of Jiangsu Province (BK20220765, BK20220928). We are grateful to the High Performance Computing Centre of Nanjing University for providing the IBM Blade cluster system.