Rechargeable batteries that feature an aqueous, potassium-based electrolyte have been sitting on battery designers’ drawing boards for years. Compared with popular lithium-ion batteries, which depend on an electrolyte solution of expensive lithium salts in flammable organic solvents, aqueous potassium-ion batteries (AKIBs) could offer advantages in cost and safety. But the AKIBs reported until now have not worked very well. They don’t store enough energy to be practical, because of the difficulty in finding cathodes, anodes, and electrolytes that work well together. And they don’t last long, because the electrodes degrade as potassium ions move in and out during charging cycles. To overcome these problems, Yong-Sheng Hu and Yaxiang Lu of the Chinese Academy of Sciences and coworkers made an AKIB with a perylene diimide–based anode, an iron-manganese Prussian blue–type cathode, and a highly concentrated KCF3SO3 aqueous electrolyte. The battery’s overall performance tops all previous efforts. It operates reliably over an 80°C temperature range, exhibits an energy density of 80 W h/kg, and retains nearly 75% of its charge capacity for more than 2,000 charging cycles (Nat. Energy 2019, DOI: 10.1038/s41560-019-0388-0). Battery specialist Lauren E. Marbella of Columbia University describes the work as “an important step forward,” noting that the new battery is “a promising model” from which to build future designs.