Lithium-air batteries promise to store four times more energy than conventional batteries. Unfortunately, exactly what goes inside advanced lithium-air batteries as they charge and discharge has always been impossible to observe directly.
Now, a new technique developed by Massachusetts Institute of Technology researchers promises to change that, allowing study of this electrochemical activity as it happens.
A lack of understanding of how lithium reacts with oxygen has hindered the development of practical lithium-air batteries, but this type of battery offers the prospect of storing up to four times as much energy as today’s lithium-ion batteries for a given weight, and so could be a key enabling technology for energy storage, among other uses. Most existing lithium-air batteries suffer from large energy losses during charging and discharging, and have been unable to successfully sustain repeated cycles.
This new method for studying the reactions of such batteries in detail could help researchers in their quest to design better batteries. Such improvements to lithium-air batteries could potentially enhance round-trip efficiency (energy retention between charge and discharge) and cycle life (the ability to charge and discharge a battery many times).
This study showed that using metal oxides as the oxygen electrode could potentially enable a lithium-air battery to maintain its performance over many cycles of operation. The device used in this study was designed purely for research, not as a practical battery design in itself; if replicated in a real cell, Lu says, such designs could greatly improve the longevity of lithium-air batteries.
A solid-state lithium-air battery (highlighted in orange) is positioned inside a test chamber at the advanced light dource (ALS) at Lawrence Berkeley national laboratory, in preparation for its testing using X-ray photoelectron microscopy.
The reactions that take place inside a conventional lithium-air battery are complex. Finding out what really happens during charging and discharging required the use of a special kind of high-intensity X-ray illumination at one of only two facilities in the world capable of producing such an experiment: the advanced light source (ALS) at the Lawrence Berkeley national laboratory (LBNL) in California.
That facility made it possible to study the electrochemical reactions taking place at the surface of electrodes, and to show the reactions between lithium and oxygen as the voltage applied to the cell was changed.
The tests used a novel solid-state version of a lithium-air battery made possible via collaboration with Nancy Dudney and colleagues at Oak Ridge National Laboratory (ORNL). When discharging, such batteries draw in some lithium ions to convert oxygen into lithium peroxide. Using ALS, researchers were able to produce detailed spectra of how the reaction unfolds, and show that this reaction is reversible on metal oxide surfaces. The observational method this team developed could have implications for studying reactions far beyond lithium-air batteries and explore different electrochemical energy-related processes.