Bipolar-Type Organic Electrode Material Composed of a Viologen-Naphthalene Diimide-Viologen Triad for Li-Organic Batteries

Abstract

Redox-active organic molecules (ROMs) play a crucial role as electrode materials in the development of future energy storage devices. In this study, we synthesized a bipolar-type ROM, NDI-2Vi, that comprises a triad of an n-type naphthalene diimide (NDI) unit and two p-type viologen (Vi) units, which can reversibly insert/extract the Li cations and Br/TFSI anions, respectively, during the redox reactions in nonaqueous electrolyte. The bipolar redox mechanism allowed NDI-2Vi to always bear certain charges with counterions, effectively preventing undesired dimerization decomposition of the molecules through the Coulombic repulsion. Through ex situ X-ray photoelectron spectroscopy (XPS), its reversible bipolar redox mechanism accompanying dual-ion intercalation with anion exchange was elucidated. The NDI-2Vi electrode delivered a specific capacity of 156.8 mA h g？1, corresponding to 98.5% of its theoretical capacity storing six electrons per molecule. By employing a highly concentrated electrolyte, its cycle stability could be markedly improved due to retarded dissolution of active materials. Finally, kinetic analyses revealed that the NDI-2Vi electrode undergoes rapid capacitive electrochemical reactions, leading to its high rate capability, even in the highly concentrated electrolyte.