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

Two iron-semiquinoid framework materials, (H₂NMe₂)₂Fe₂(Cl₂ dhbq)₃ (1) and (H₂NMe₂)₄Fe₃(Cl₂ dhbq)₃(SO₄)₂ (Cl₂ dhbq = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO), 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 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.