Modulating charge transport in MOFs with zirconium oxide nodes and redox-active linkers for lithium sulfur batteries

Abstract

Despite the promises of high specific energy density and low cost, lithium sulfur (Li-S) batteries still face major challenges in long-term cyclability due to polysulfide dissolution. Metal–organic frameworks (MOFs) have unique advantages as porous sulfur hosts due to their high chemical and structural tunability. Herein, we demonstrate that MOFs consisting of functionalized metal oxide nodes and redox-active linkers influence charge transport and enhance Li-S cycling. We show that anthraquinone-based Zr-MOF composite cathodes display higher maximum capacity and longer cycle life compared to sulfur/carbon electrodes. Lithiating the terminal ligands and bridging hydroxides in the Zr6(μ3-O)4(μ3-OH)4 nodes further improves battery performance at high charge rates. We hypothesize that the additional redox sites within the linkers and the presence of lithiated clusters permit greater electrochemical accessibility to the polysulfides by providing additional electron and ion conduction pathways. Moreover, the pore structure plays a critical role in available conduction pathways. Our efforts demonstrate the utility of metal oxide clusters for modulating charge transport and provide structure–function relationships to direct design of novel materials for energy storage devices.