Operando Raman and ex situ characterization of an iron-based conductive MOF as a negative electrode in Li-ion batteries

Abstract

An iron-based electrically conductive metal–organic framework (MOF), Fe-HHTP, formed by the coordination of Fe cations and the organic ligand HHTP (2,3,6,7,10,11-hexahydroxytriphenylene), has been synthesized, characterized, and explored as a potential negative electrode for lithium-ion batteries (LIBs). Galvanostatic cycling experiments, with a lower cut-off voltage of 0.1 V vs. Li/Li⁺, reveal that Fe-HHTP undergoes a significant activation process, resulting in an increase in specific capacity up to 1142 mAh g⁻¹ after ∼220 cycles at a current of 355 mA g⁻¹. Operando Raman spectroscopy during the first lithiation, complemented by ex situ techniques at different cycling stages, demonstrates that both the organic ligands and metal centers actively contribute to Li⁺ storage. Initially, lithiation occurs primarily at the MOF boundaries, while continuous cycling induces a structural transformation that allows greater accessibility to the inner regions of Fe-HHTP. These results provide a better understanding of the electrochemical and structural behavior of pristine MOFs that allow tailoring their properties for direct application in energy storage devices without the need for high-temperature processes.