Lithium-air batteries with around three times the energy capacity by weight of today's lithium-ion batteries could enable electric cars to drive a greater distance on a single charge in the days to come.
However, the technology has a number of challenges, including losing energy as it stores and discharges its charge. If scientists could better understand the rudimentary reactions that happen as the battery charges and discharges electricity, the performance of the battery could be enhanced. One reaction that has not been completely explained is just how oxygen blows bubbles inside a lithium-air battery when it discharges. The bubbles enlarge the battery and develop wear and tear that can result in its failure.
The first step-by-step explanation of how lithium-air batteries develop bubbles has been published in a research paper appearing in Nature Nanotechnology. The research was aided by a first-of-a-kind video that reveals bubbles inflating and later deflating within a nanobattery. Researchers had earlier only observed the bubbles, but not how they were formed.
If we fully understand the bubble formation process, we could build better lithium-air batteries that create fewer bubbles. The result could be more compact and stable batteries that hold onto their charge longer.
Chongmin Wang, Pacific Northwest National Laboratory, Department of Energy
Wang works out of the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility located at PNNL. His co-authors include other PNNL staff and a researcher from Tianjin Polytechnic University in China.
The team's exclusive video may be a silent black-and-white film, but it offers ample action. It shows a grey bubble popping out from the battery's flat surface and growing larger and larger. Later, the bubble deflates, the top turning inside of itself until only a scrunched-up shell remains.
The popcorn-worthy video was taken with an in-situ environmental transmission electron microscope at EMSL. Wang and his colleagues constructed their miniature battery inside the microscope's column. This allowed them to observe as the battery charged and discharged inside.
Video evidence led the team to suggest that as the battery discharges, a sphere of lithium superoxide pops out from the battery's positive electrode and gets coated with lithium oxide. The sphere's superoxide interior then undergoes a chemical reaction that develops lithium peroxide and oxygen. Oxygen gas is discharged and inflates the bubble. When the battery charges, lithium peroxide deforms, and leaves the former bubble to appear like a deflated balloon.
This finding was the emphasis of a Nature News & Views column written by researchers at Korea's Hanyang University, who describe the research as "a solid foundation for future Li-O2 battery designs and optimization."
DOE's Office of Energy Efficiency and Renewable Energy supported this research.
PNNL researchers used an environmental transmission electron microscope to record a first-of-a-kind video that shows bubbles inflating and later deflating inside a tiny lithium-air battery. The video helped researchers develop the first step-by-step explanation of how lithium-air batteries form bubbles. The knowledge could help make lithium-air batteries that are more compact, stable and can hold onto a charge longer. (credit:PNNL)