Effects of Covalency on Anionic Redox Chemistry in Semiquinoid-Based Metal–Organic Frameworks
The results demonstrate the feasibility and versatility of metal-organic frameworks as energy storage materials for a wide range of battery chemistries.
Abstract
Two iron-semiquinoid framework materials, (H<sub>2</sub>NMe<sub>2</sub>)<sub>2</sub>Fe<sub>2</sub>(Cl<sub>2</sub> dhbq)<sub>3</sub> (<b>1</b>) and (H<sub>2</sub>NMe<sub>2</sub>)<sub>4</sub>Fe<sub>3</sub>(Cl<sub>2</sub> dhbq)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub> (Cl<sub>2</sub> dhbq<sup><i>n</i>-</sup> = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (<b>2-SO</b><sub><b>4</b></sub>), 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 <b>1</b> 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 <b>2-SO</b><sub><b>4</b></sub> 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, <b>1</b> 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.