Researchers directly visualize formation and disappearance of Li-O2 reaction products; insights to support development of rechargeable lithium-air batteries
The rechargeable Li−air (Li-O2) battery represents a conceptually attractive energy storage device for electric vehicle applications due to its high theoretical energy storage capacity (earlier post); however, among the obstacles to commercialization is a lack of fundamental understanding of the reactions involved.
Researchers from MIT, Oak Ridge National Laboratory and Lawrence Berkeley Laboratory now have developed a new technique to observe in situ the oxygen reduction and oxygen evolution reactions in Li-O2 batteries. Utilizing a special all solid-state cell design and ambient pressure X-ray photoelectron spectroscopy (APXPS), they directly visualized the formation and disappearance of Li-O2 reaction products (namely Li2O2) on an LixV2O5 surface as a function of applied battery potential.
In a paper published in Scientific Reports (an online, open access primary research publication from the publishers of Nature), they describe the redox of oxygen on the surface of a mixed electronic and Li+ ionic conductor, LixV2O5, using a specially designed, all solid-state Li-ion battery to eliminate parasitic reactions between oxygen reduction/evolution reaction intermediates and aprotic electrolytes used in conventional Li-O2 batteries reported to date.
Under ultra-high vacuum (UHV), they found that lithium intercalated into LixV2O5 while molecular oxygen was reduced to form lithium peroxide on LixV2O5 in the presence of oxygen upon discharge.
Interestingly, the oxidation of Li2O2 began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O2 cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O2 chemistry.—Lu et al.
Further studies will be needed to elucidate the physical origin to low overpotentials of Li2O2 electrochemical oxidation, the researchers noted. They noted three differences between Li2O2 formed in their study and those formed in Li-O2 batteries with aprotic electrolytes.
Li2O2 particles formed on the surface of LixV2O5 are extremely thin. The average thickness of maximum Li2O2 coverage on the LixV2O5 surface was estimated to be ~0.7 nm. This is in contrast to Li2O2 particles having sizes in the range of 100 to 1000 nm formed in Li-O2 batteries with aprotic electrolytes.
The surfaces of Li2O2 are free of carbonate species formed on the surface of the LLTO/LiPON/LixV2O5 cell, which is in contrast to the coverage of Li2CO3-like species on Li2O2 particles even with ether-based electrolytes.
Very thin Li2O2 particles may have stoichiometry and electronic properties considerably different from Li2O2 particles.
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. Although the device used in the study was designed purely for research, not as a practical battery design in itself; if replicated in a real cell such designs could improve the longevity of lithium-air batteries, the authors suggested.
The observational method this team developed could have implications for studying reactions far beyond lithium-air batteries, Yang Shao-Horn, the Gail E. Kendall Associate Professor of Mechanical Engineering and Materials Science and Engineering at MIT and the senior author of the paper, says. This research, she says, “points to a new paradigm of studying reaction mechanisms for electrochemical energy storage. We can use this technique to study a large number of reactions,” she says. “It allows us to look at a large number of different electrochemical energy-related processes.”
The work was partly funded by the National Science Foundation and the US Department of Energy.
Yi-Chun Lu, Ethan J. Crumlin, Gabriel M. Veith, Jonathon R. Harding, Eva Mutoro, Loïc Baggetto, Nancy J. Dudney, Zhi Liu & Yang Shao-Horn (2012) In Situ Ambient Pressure X-ray Photoelectron Spectroscopy Studies of Lithium-Oxygen Redox Reactions. Scientific Reports 2, Article number: 715 doi: 10.1038/srep00715