Using an in situ liquid transmission electron microscopy technique that can visualize chemical reactions occurring in liquid environments, researchers from the University of Illinois at Chicago and Argonne National Laboratory have discovered a new reason lithium-oxygen batteries—which promise up to five times more energy than the lithium-ion batteries that currently power electric vehicles and cell phones—tend to slow down and die after just a few charge/discharge cycles. They report their findings in the journal Nano Energy.

What we were able to see for the first time is that lithium peroxide develops in the liquid electrolyte of lithium-oxygen batteries, and is a contributor to the slow down and ultimate death of these batteries. This is a newly discovered reason why these promising batteries have such a steep drop off in efficiency and yield after relatively few charge/discharge cycles.

—Reza Shahbazian-Yassar, associate professor of mechanical and industrial engineering in the University of Illinois at Chicago College of Engineering and lead author

Lithium-oxygen batteries have been tantalizing to battery researchers for years because of their potential high energy density. But they tend to slow down and stop working relatively quickly compared to other batteries. One of the reasons for this loss of power is that a byproduct of the chemical reactions that take place inside the battery—lithium peroxide (Li₂O₂—builds up on the electrodes of the battery. The coated electrodes can no longer function efficiently and chemical reactions that produce energy ultimately stop.

But now, Shahbazian-Yassar and his colleagues, using a new transmission electron microscopy technique developed by UIC engineering graduate students Kun He and Yifei Yuan, have demonstrated at the nanometer level that lithium peroxide also forms in the battery’s liquid electrolyte component, further slowing chemical reactions.

During discharge, the nucleation of Li₂O₂ is observed at the carbon electrode/electrolyte interface, and the following growth process exhibits Li⁺ diffusion-limited kinetics. Nucleation and growth of Li₂O₂ are also observed within the electrolyte, where there is no direct contact with the carbon electrode indicating the existence of non-Faradaic disproportionation reaction of intermediate LiO₂ into Li₂O₂. The growth of Li₂O₂ isolated in the electrolyte exhibits O₂^- diffusion-limited kinetics. Li₂O₂ at the carbon electrode surface and isolated in the electrolyte are both active upon charging and gradually decomposed.

For Li₂O₂ particles rooted at the carbon electrode surface, the decomposition starts at the electrode/Li₂O₂ interface indicating electron-conduction limited charge kinetics. For Li₂O₂ isolated within the electrolyte, surprisingly, a side-to-side decomposition mode is identified indicating the non-Faradaic formation of dissolvable O₂^-, whose diffusion in the electrolyte controls the overall charge kinetics.

—He et al.

Shahbazian-Yassar said that knowing that lithium peroxide is building up in the electrolyte itself is a very important finding.

Now, we can start to come up with ideas and designs that either prevent this from happening or do something to maintain the proper functioning of the electrolyte so it doesn’t interfere with the battery’s operation, and we can use the new liquid microscopy technique to see if we are moving in the right direction.

—Reza Shahbazian-Yassar

So far, lithium-oxygen batteries have only existed as lab-based prototypes, with mass-produced lithium-oxygen batteries for public or commercial use still a long way off, Shahbazian-Yassar said.

Kun He, Yifei Yuan, Tara Foroozan and Boao Song of the UIC College of Engineering; and Xuanxuan Bi, Khalil Amine and Jun Lu of Argonne National Laboratory are co-authors on the paper.

This research was funded in part by grant 1620901 from the Division of Materials Research at the National Science Foundation, and by subcontract 4J-30361 from Argonne National Laboratory.

Resources

• Kun He, Xuanxuan Bi, Yifei Yuan, Tara Foroozan, Boao Song, Khalil Amine, Jun Lu, Reza Shahbazian-Yassar (2018) “Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries,” Nano Energy, Volume 49, Pages 338-345 doi: 10.1016/j.nanoen.2018.04.046.