Effects of Covalency on Anionic Redox Chemistry in Semiquinoid-Based Metal–Organic Frameworks

• Published in: Journal of the American Chemical Society Vol. 142; no. 5; pp. 2653 - 2664
• Main Authors: Ziebel, Michael E, Gaggioli, Carlo Alberto, Turkiewicz, Ari B, Ryu, Won, Gagliardi, Laura, Long, Jeffrey R
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.02.2020
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b13050

Two iron–semiquinoid framework materials, (H2NMe2)2Fe2(Cl2 dhbq)3 (1) and (H2NMe2)4Fe3(Cl2 dhbq)3(SO4)2 (Cl2 dhbq n– = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO 4 ), are shown to possess electrochemical capacities of up to 195 mAh/g. Employing a variety of spectroscopic methods, we demonstrate that these exceptional capacities arise from a combination of metal- and ligand-centered redox processes, a result supported by electronic structure calculations. Importantly, similar capacities are not observed in isostructural frameworks containing redox-inactive metal ions, highlighting the importance of energy alignment between metal and ligand orbitals to achieve high capacities at high potentials in these materials. Prototype lithium-ion devices constructed using 1 as a cathode demonstrate reasonable capacity retention over 50 cycles, with a peak specific energy of 533 Wh/kg, representing the highest value yet reported for a metal–organic framework. In contrast, the capacities of devices using 2-SO 4 as a cathode rapidly diminish over several cycles due to the low electronic conductivity of the material, illustrating the nonviability of insulating frameworks as cathode materials. Finally, 1 is further demonstrated to access similar capacities as a sodium-ion or potassium-ion cathode. Together, these results demonstrate the feasibility and versatility of metal–organic frameworks as energy storage materials for a wide range of battery chemistries.