Redox-active molecules for aqueous electrolytes of energy storage devices: A review on fundamental aspects, current progress, and prospects

The increasing demand for aqueous energy storage (AES) solutions with high energy density, enlarged voltage windows, and extended cycling stability has spurred the development of advanced electrolytes. Redox-active molecules hold the promise for formulating aqueous electrolytes with enhanced electrochemical performance. In this review, we provide a comprehensive overview of established and recently reported studies on redox electrolytes for AES devices. Delving into mechanisms at both molecular and micrometer scales, this review covers the fundamental principles governing the electrolytes, encompassing their physicochemical properties, ion solvation behavior, interfacial modulation, and transport mechanisms. We present an overview of the redox properties of various compounds from different families. While irreversible electron/mass transfer processes can facilitate the passivation of solid electrolyte interfaces, particular attention is given to the reversible redox electrolyte in enhancing the energy performance of AES systems. Redox-active molecules are categorized based on their ability to improve the cycling stability of electrodes, increase the voltage windows of electrolytes, and enhance the energy density of cells. High solubility and reversible redox behavior have been achieved via the molecular design. Trade-offs between the shuttling effect and electrolyte modification as well as controversies on molecular solubility are discussed. By examining these aspects, the review aims to stimulate advanced research in redox-active molecules for AES technologies.