Unleashing energy storage ability of aqueous battery electrolytes

Electrolytes make up a large portion of the volume of energy storage devices, but they often do not contribute to energy storage. The ability of using electrolytes to store charge would promise a significant increase in energy density to meet the needs of evolving electronic devices. Redox-flow batteries use electrolytes to store energy and show high energy densities, but the same design cannot be applied to portable or microdevices that require static electrolytes. Therefore, implementing electrolyte energy storage in a non-flow design becomes critical. This review summarizes the requirements for a stable and efficient electrolyte and diverse redox-active species dissolved in aqueous solutions. More importantly, we review the pioneering works using static electrolyte energy storage in the hope that it will pave a new way to design compact and energy-dense batteries.