The PNNL zinc-polyiodide flow battery reduces the battery's size and cost and makes it well suited to store energy in densely populated cities and provides a battery that packs far more energy than any other battery of its kind and size.

The zinc-polyiodide redox flow battery, described in Nature Communications, uses an electrolyte that has more than two times the energy density of the next-best flow battery used to store renewable energy and support the power grid. The energy density is approaching that of a type of lithium-ion battery used to power

portable electronic devices and some small electric vehicles.

"With improved energy density and inherent fire safety, flow batteries could provide long-duration energy storage for the tight confines of urban settings, where space is at a premium," said Imre Gyuk, energy storage program manager at the Department of Energy's Office of Electricity Delivery and Energy Reliability, which funded this research. "This would enhance the resiliency and flexibility of the local electrical grid."

"Another, unexpected bonus of this electrolyte's high energy density is it could potentially expand the use of flow batteries into mobile applications such as powering trains and cars," said the study's corresponding author, Wei Wang, a materials scientist at PNNL.

Both flow and lithium-ion batteries were invented in the 1970s, but only the lithium-ion variety took off at that time. Lithium-ion batteries could carry much more

energy in a smaller space than flow batteries, making them more versatile. As a result, lithium-ion batteries have been used to power portable electronics for many

years. And utilities have begun using them to store the increasing amounts of renewable energy generated at wind farms and solar power facilities.

But the high-energy lithium-ion batteries' packaging can make them prone to overheating and catching fire. Flow batteries, on the other hand, store their active

chemicals separately until power is needed, greatly reducing safety concerns. This feature has prompted researchers and developers to