Sulfide-Bridged Covalent Quinoxaline Frameworks for Lithium‐Organosulfide Batteries

The chelating ability of quinoxaline cores and the redox activity of organo-sulfide bridges in layered covalent-organic frameworks (COFs) offer dual active sites for reversible lithium (Li)-storage. The designed COFs combining these properties feature disulfide and polysulfide-bridged networks showcasing an intriguing lithium-storage mechanism, which can be considered as a lithium-organo-sulfide (Li-OrS) battery. The experimental-computational elucidation of three quinoxaline COFs containing systematically enhanced sulfur atoms in sulfide bridging demonstrates fast kinetics during Li interactions with the quinoxaline core. Meanwhile, bilateral covalent bonding of sulfide bridges to quinoxaline core enables a redox-mediated reversible cleavage of the sulfur-sulfur bond and the formation of covalently anchored lithium-sulfide chains or clusters during Li-interactions, accompanied by a marked reduction of Li-polysulfide (Li-PS) dissolution into the electrolyte, a frequent drawback of lithium-sulfur (Li-S) batteries. The electrochemical behavior of model compounds mimicking the sulfide linkages of the COFs and operando Raman studies on the framework structure unravels the reversibility of the profound Li-ion-organo-sulfide interactions. As a result, nearly 84% retention of specific capacity was observed at 100 mA/g for the polysulfide-bridged COF after 500 charge-discharge cycles. Thus, integrating redox-active organic-framework materials with covalently anchored sulfides enables a stable Li-OrS battery mechanism which shows benefits over a typical Li-S battery.