New PNNL zinc-polyiodide redox flow battery offers 2x energy density of next-best system; potential for mobile applications
Researchers at Pacific Northwest National Laboratory (PNNL) have developed a new zinc-polyiodide redox flow battery offering more than two times the energy density of the next-best flow battery used to store renewable energy and support the power grid.
Lab tests revealed the demonstration battery discharged 167 Wh l-1 of electrolyte. In comparison, zinc-bromide flow batteries generate about 70 Wh l-1, vanadium flow batteries can create between 15 and 25 Wh l-1, and standard lithium iron phosphate batteries could put out about 233 Wh l-1. The team calculated that their new battery theoretically could discharge even more—up to 322 Wh l-1—if more chemicals were dissolved in the electrolyte. An open access paper on the work appears in Nature Communications.
Like other flow batteries, the zinc-polyiodide battery produces power by pumping liquid from external tanks into the battery’s stack, a central area 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, Zn2+, and negatively charged iodide ion, I-.
|Flow batteries produce power by pumping electrolytes from external tanks into a central stack. PNNL’s new zinc-polyiodide flow battery’s electrolytes carry positively charged zinc ions and the negatively charged ions iodide and polyiodide.
When the battery is charged, one of the tanks also holds another negative ion, polyiodide, I3-. When power is needed, the two liquids are pumped into the central stack. 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.
|Both flow and lithium-ion batteries were invented in the 1970s, but only the lithium-ion variety took off at that time.
|Li-ion batteries had a much higher volumetric energy density than flow batteries, making them more versatile. As a result, lithium-ion batteries have been used to power portable electronics for many years.
|However, the high-energy Li-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 take a serious second look at flow batteries.
To test the feasibility of their new battery concept, Wei and his PNNL colleagues created a small battery on a lab benchtop. They mixed the electrolyte solution, separating a black zinc-polyiodide liquid and a clear zinc-iodide liquid in two glass vials as miniature tanks. Hoses were connected between the vials, a pump and a small stack.
They put the 12-watt-hour capacity battery through a series of tests, including determining how different concentrations of zinc and iodide in the electrolyte affected energy storage. The demonstration battery put out far more energy for its size than today’s most commonly used flow batteries: the zinc-bromide battery and the vanadium battery. PNNL’s zinc-polyiodide battery also had an energy output that was about 70% that of a lithium iron phosphate battery.
PNNL’s zinc-polyiodide battery is also safer because its electrolyte isn’t acidic like most other flow batteries. It’s nearly impossible for the water-based electrolyte to catch fire and it doesn’t require expensive materials that are needed to withstand the corrosive nature of other flow batteries.
Another advantage of PNNL’s new flow battery is that it can operate in extreme climates. The electrolyte allows it to work well in temperatures as cold as -4 ˚F \ and as warm as +122 degrees. Many batteries have much smaller operating windows and can require heating and cooling systems, which cut into a battery’s net power production.
One problem the team encountered was a build-up of metallic zinc dendrites that grew from the central stack’s negative electrode and went through the membrane. The team reduced the dendrite buildup by adding alcohol to the electrolyte solution.
Managing zinc dendrite formation will be a key in enabling PNNL’s zinc-polyiodide battery to be used in the real world. Wei and his colleagues will continue to experiment with different alcohols and other additives and use advanced instruments to characterize how the battery’s materials respond to those additives. The team will also build a larger, 100-watt-hour model of the battery for additional testing.
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.—Wei Wang, corresponding author
(Lichtenstein-based nanoFlowcell is brining flow-battery vehicle concepts to the Geneva Motor Show. Earlier post.)
Researchers characterized the new battery's chemical interactions using a variety of advanced instruments—including nuclear magnetic resonance, Raman spectroscopy, mass spectroscopy and more — at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science national user facility at PNNL. This research was funded by the Department of Energy's Office of Electricity Delivery and Energy Reliability.
Bin Li, Zimin Nie, M. Vijayakumar, Guosheng Li, Jun Liu, Vincent Sprenkle, Wei Wang (2015) “Ambipolar Zinc-Polyiodide Electrolyte for High Energy Density Aqueous Redox Flow Battery,” Nature Communications doi: 10.1038/ncomms7303