A Dual-Ion Battery Cathode via Oxidative Insertion of Anions in a Metal-Organic Framework

A redox-active metal-organic framework, Fe-2(dobpdc) (dobpdc(4-) = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), is shown to undergo a topotactic oxidative insertion reaction with a variety of weakly coordinating anions, including BF4- and PF6-. The reaction results in just a minor lattice contraction, and a broad intervalence charge-transfer band emerges, indicative of charge mobility. Although both metal- and ligand-based oxidations can be accessed, only the former were found to be fully reversible and, importantly, proceed stoichiometrically under both chemical and electrochemical conditions. Electrochemical measurements probing the effects of nanoconfinement on the insertion reaction revealed strong anion size and solvent dependences. Significantly, the anion insertion behavior of Fe-2(dobpdc) enabled its use in the construction of a dual-ion battery prototype incorporating a sodium anode. As a cathode, the material displays a particularly high initial reduction potential and is further stable for at least 50 charge/discharge cycles, exhibiting a maximum specific energy of 316 Wh/kg.