Reversible Anion Storage in a Metal-Organic Framework for Dual-Ion Battery Systems

Abstract

Metal-organic frameworks (MOFs) exhibit a crystalline structure composed of well-dispersed metal centers separated by organic linker molecules. The aim of this work is to understand and utilize the redox-activity of the metal ion nodes and organic linker molecules in order to employ MOFs in the field of battery applications. In this study, a copper-based MOF has been synthesized with a radical anionic linker using 7,7,8,8-tetracyanoquinodimethane (TCNQ). The Cu(TCNQ) MOF shows a reversible reaction with PF6− anions of the electrolyte when applied as positive electrode material in a lithium metal cell. Cyclic voltammetry and constant current cycling studies reveal a different capacity retention behavior depending on the potential range of charge/discharge cycles. In situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy measurements indicate that Cu(TCNQ) undergoes a conversion reaction with the PF6− anions going along with the formation of a new crystalline phase at a potential of 3.75 V vs. Li/Li+. The observed reversible storage of anions might open a new door for the application of MOFs as anion host material for dual-ion batteries. As a result of this work, for dual-ion batteries, two classes of cathode materials have to be regarded in the future, “conversion” and “insertion” materials.