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

Two iron-semiquinoid framework materials, (HNMe)Fe(Cl dhbq) () and (HNMe)Fe(Cl dhbq)(SO) (Cl dhbq = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (), 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 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 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, 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.