High-Voltage, Room-Temperature Liquid Metal Flow Battery Enabled by Na-K vertical bar K-beta ''-Alumina Stability

Flow batteries are a compelling grid-scale energy storage technology because the stored energy is decoupled from the system power. Aqueous redox flow batteries (RFBs), however, are limited by low open-circuit voltages (OCVs). Replacing the aqueous negative electrolyte (negolyte) with liquid alkali metals-of which Na-K, a room-temperature liquid metal alloy, is attractive-would increase the OCV considerably. However, a suitable solid electrolyte has not been reported for Na-K. Here we show that K-beta ''-alumina is a selective and robust K+ ion conductor in contact with Na-K, to which it is stable with minimal exchange of Na. We report the cycling of cells with OCVs of 3.1-3.4 V employing aqueous and nonaqueous positive electrolytes (posolytes), and power density tests showing promising maximum power densities of 65 mW cm(-2) at 22 degrees C and >100 mW cm(-2) at 57 degrees C, ohmically limited by 330-mu m K-beta ''-alumina membranes. Further development of Na-K vertical bar K-beta ''-alumina batteries could unlock cost-effective energy storage.