Impact of the metal centre (Al3+, Fe3+) on the post-synthetic lithiation of functionalized MIL-53s and the electrochemical properties of lithiated derivatives

Abstract

Metal–organic frameworks (MOFs) combining both organic and inorganic redox-active moieties have recently drawn interest in the field of electrochemical energy storage. Here we focused our attention on MIL-53(M) (M = Al, Fe) analogues based on 2,5-dioxo-1,4-benzenedicarboxylate, as this ligand was already found to present an interesting electrochemical activity based on the quinone/phenolate redox couple in the solid state. We described here our attempts to chemically lithiate the title solids. Various synthetic paths were explored, and the resulting solids were characterized by a broad set of techniques, including X-ray diffraction, MAS NMR spectroscopy, transmission electron microscopy, inductively coupled plasma-atomic emission spectroscopy and total X-ray scattering experiments, among others. We showed that although the lithiation was accompanied by a loss of the long-range order whatever the synthetic conditions and the trivalent cation, the reactivity strongly differed for M = Al and Fe. Eventually, the electrochemical extraction/uptake of Li⁺ in the lithiated derivatives was evaluated in Li-half cells. Although their storage capacities are moderate, we found that the presence of even a minor amount of M³⁺ cations not only impacts the working potential of the ligand but also improves their long term capacity retention.