High-Voltage, Room-Temperature Liquid Metal Flow Battery Enabled by Na-K|K-β″-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-β″-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°C and >100 mW cm⁻² at 57°C, ohmically limited by 330-μm K-β″-alumina membranes. Further development of Na-K|K-β″-alumina batteries could unlock cost-effective energy storage. Flow batteries are a compelling grid-scale energy storage technology because the stored energy is decoupled from the system power. Conventional flow batteries have aqueous solutions on both sides, and thus are constrained in voltage by water splitting (∼1.5 V). Replacing the negative side with a liquid metal would yield a much higher voltage flow battery, benefiting energy density, power density, and efficiency. As a room-temperature liquid metal, Na-K is attractive. However, a suitable solid electrolyte has not been reported for Na-K since the well-studied Na-β″-alumina ceramic fractures on contact. Here we show that K-β″-alumina is a selective and robust K⁺ ion conductor in contact with Na-K. We report the cycling of cells with open-circuit voltages of 3.1–3.4 V. Our maximum power densities are limited, as expected, by the K-β″-alumina membranes, but can be improved with thin membranes. Further development of Na-K|K-β″-alumina batteries could unlock cost-effective energy storage. Na-K is a room-temperature liquid metal that could unlock a high-voltage flow battery. We show that K-β″-alumina solid electrolyte is stable to Na-K and selectively transports K⁺. We report the cycling of cells with OCVs of 3.1–3.4 V employing aqueous and nonaqueous posolytes, and maximum power densities of 65 mW cm⁻² at 22°C, ohmically limited by 330-μm K-β″-alumina membranes.