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.

液流电池是一种引人注目的网格规模储能技术，因为所存储的能量与系统电源分离。但是，水性氧化还原液流电池（RFB）受低开路电压（OCV）的限制。用液态碱金属（其中Na-K（一种室温液态金属合金）是有吸引力的）代替含水负电解液（电解质）会大大提高OCV。然而，尚未报道用于Na-K的合适的固体电解质。在这里，我们证明K-β″-氧化铝是一种选择性且坚固的K ⁺与Na-K接触的离子导体，它对Na的交换稳定，而Na的交换最少。我们报告细胞的循环与采用3.1-3.4的V的OCV水性和非水性正电解质（posolytes），和功率密度测试结果，显示有希望的65毫瓦厘米的最大功率密度^-2在22℃和> 100毫瓦厘米^-2在57°C，由330-μmK-β''-氧化铝膜欧姆限制。Na-K |K-β''-氧化铝电池的进一步开发可以释放具有成本效益的储能。