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 dhbqn– = deprotonated
2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO4), 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-SO4 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.