From power grids to consumer electronics, batteries power today's world. But current batteries have limited life, can take up considerable space depending on their wattage, and are prone to overheating and catching fire. Many batteries have much smaller operating windows and can require heating and cooling systems, which cut into a battery's net power production. Redox flow batteries, while promising, have been plagued by low-energy-density, large size, and high cost.

PNNL's advanced high-energy density aqueous zinc polyiodide flow battery meets all these challenges by efficiently converting chemical energy to electrical energy without the use of expensive catalysts or membranes. By storing its active chemicals separately until power is needed, it greatly reduces safety concerns. The battery uses a highly soluble iodide/triiodide redox couple that has more than two times the energy density of the next-best flow battery, and its energy density approaches that of a type of lithium-ion battery used to power portable electronic devices and some small electric vehicles.

The battery produces power by pumping liquid from external tanks holding aqueous electrolytes into a stack, where the liquids are mixed. When the battery is fully discharged, both tanks hold the same electrolyte solution: a mixture of the positively charged zinc ions and negatively charged iodide ion. But when the battery is charged, one of the tanks also holds another negative ion, polyiodide. Inside the stack, zinc ions pass through a selective membrane and change into metallic zinc on the stack's negative side. This process converts energy that's chemically stored in the electrolyte into electricity.

The battery may be used to store renewable energy and support the power grid, provide long-term energy storage for the tight confines of urban settings, and potentially enable mobile applications such as powering trains and cars.