Lithium-air batteries are looked to by many as a very high-energy density next-generation energy storage solution for electric vehicles. However, the technology has several holdups, including losing energy as it stores and releases its charge.The reaction mechanisms are, in general, not well understood.One reaction that hasn’t been fully explained is how oxygen blows bubbles inside a lithium-air battery when it discharges. The bubbles expand the battery and create wear and tear that can cause it to fail. Now, researchers from Pacific Northwest National Laboratory (PNNL) have provided the first step-by-step explanation of how lithium-air batteries form bubbles. The paper is published in the journal Nature Nanotechnology.
The research was aided by a first-of-a-kind video that shows bubbles inflating and later deflating inside a nanobattery. Researchers had previously only seen the bubbles, but not how they were created.
The performances of a Li–O2 battery depend on a complex interplay between the reaction mechanism at the cathode, the chemical structure and the morphology of the reaction products, and their spatial and temporal evolution; all parameters that, in turn, are dependent on the choice of the electrolyte. In an aprotic cell, for example, the discharge product, Li2O2, forms through a combination of solution and surface chemistries that results in the formation of a baffling toroidal morphology. In a solid electrolyte, neither the reaction mechanism at the cathode nor the nature of the reaction product is known.
Here we report the full-cycle reaction pathway for Li–O2 batteries and show how this correlates with the morphology of the reaction products. Using aberration-corrected environmental transmission electron microscopy (TEM) under an oxygen environment, we image the product morphology evolution on a carbon nanotube (CNT) cathode of a working solid-state Li–O2 nanobattery and correlate these features with the electrochemical reaction at the electrode. We find that the oxygen-reduction reaction (ORR) on CNTs initially produces LiO2, which subsequently disproportionates into Li2O2 and O2. The release of O2 creates a hollow nanostructure with Li2O outer-shell and Li2O2 inner-shell surfaces. Our findings show that, in general, the way the released O2 is accommodated is linked to lithium-ion diffusion and electron-transport paths across both spatial and temporal scales; in turn, this interplay governs the morphology of the discharging/charging products in Li–O2 cells.
|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.|
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, corresponding author,
Wang works out of EMSL, the Environmental Molecular Sciences Laboratory, 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 unique video was captured with an in-situ environmental transmission electron microscope at EMSL. Wang and his colleagues built their tiny battery inside the microscope’s column. This enabled them to watch as the battery charged and discharged inside.
Video evidence led the team to propose that as the battery discharges, a sphere of lithium superoxide jets out from the battery’s positive electrode and becomes coated with lithium oxide. The sphere’s superoxide interior then goes through a chemical reaction that forms lithium peroxide and oxygen. Oxygen gas is released and inflates the bubble. When the battery charges, lithium peroxide decomposes, and leaves the former bubble to look like a deflated balloon.
This finding was the focus 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.”
This research was supported by DOE’s Office of Energy Efficiency and Renewable Energy.
Langli Luo, Bin Liu, Shidong Song, Wu Xu, Ji-Guang Zhang, Chongmin Wang (2017) “Revealing the reaction mechanisms of Li-O2 batteries using environmental transmission electron microscopy,” Nature Nanotechnology doi: 10.1038/nnano.2017.27.
Yang-Kook Sun and Chong S. Yoon (2017) “Lithium-oxygen batteries: The reaction mechanism revealed,” Nature Nanotechnology doi: 10.1038/nnano.2017.40