New study identifies electron transport as key limiter of Li-air charging
13 May 2013
Researchers at MIT, the University of Pittsburgh, and Sandia National Laboratories have used transmission electron microscope (TEM) imaging to observe the electrochemical oxidation of Li2O2, providing insights into the rate-limiting processes that govern charge in Li–O2 cells. The results of the study, reported in the ACS journal Nano Letters, suggest that electron transport in Li2O2 ultimately limits the oxidation kinetics at high rates or overpotentials.
The TEM technique could help in finding ways to make Li-air batteries—widely seen as important for the future wide-spread adoption of electromobility due to their inherently high energy densities—practical in the near future, the researchers, led by MIT professor Yang Shao-Horn and Pitt professor Scott X. Mao, suggested.
|
|
| Oxidation of Li2O particles. (a) Schematic illustration of the in situ TEM microbattery superimposed over a low-magnification TEM image of a SE-coated Si NW contacting a single Li2O2 particle. (b) Higher-magnification TEM image of the particles in (a), showing a MWCNT bundle contacting two physically separated Li2O2 particles labeled as Particle 1 and Particle 2, respectively. (c−g) Oxidation of Particles 1 and 2 during application of a 10 V potential to the MWCNT/Li2O2 positive electrode against the Si NW negative electrode. Li2O2 close to the MWCNT bundle in Particle 1 is rapidly oxidized (c), before slowing down due to increasingly poor contact between the MWCNT bundle and the Li2O2 particle (d), indicating an electron-transport limitation. Oxidation of Particle 1 resumed when the MWCNT bundle was bent, improving physical contact (e), and oxidation of Particle 2 (f,g) occurred only when Particle 1 came into direct contact with Particle 2 where oxidation also began at the MWCNT/Li2O2 interface. Credit: ACS, Zhong et al. Click to enlarge. |
Despite the promising advantage of Li−O2 batteries, many issues still must be resolved before these batteries can be exploited commercially, including electrolyte instability, poor cycle life and rate capability, and low round-trip efficiencies largely resulting from high over-potentials on charge.
Little is known about the processes that govern the kinetics of Li2O2 electrochemical oxidation on charge, which hinders the development of rechargeable Li−O2 batteries with enhanced performance characteristics for practical use.
...Our objective in this current study is to examine if the electrochemical oxidation kinetics of Li2O2 are ultimately limited by lithium ion diffusion or electronic transport in Li2O2 at very high overpotentials, where the kinetics of nucleation of active sites are faster than mass and electronic transport.
—Zhong et. al.
The new observations show, for the first time, the oxidation of lithium peroxide, the material formed during discharge in a lithium-air battery. At high charging rates, this oxidation occurs mostly at the boundary between the lithium peroxide and the carbon substrate on which it grows during discharge—in this case, multiwall carbon nanotubes used in the battery electrode.
The confinement to this interface, Shao-Horn says, shows that it is the resistance of lithium peroxide to a flow of electrons that limits the charging of such batteries under practical charging conditions.
An electrolyte-coated probe tip serves as the opposing electrode for removing lithium ions during charging, as electrons flow through the nanotube framework to the external circuit. During charging, the lithium peroxide particles shrink beginning at the nanotube-peroxide interface, showing that oxidation occurs where it is easiest to remove electrons.
“The lithium transport can keep up,” Shao-Horn says, which indicates that electron transport could be a critical limit on charging of batteries for electric vehicles.
The rate of lithium peroxide oxidation in these experiments was approximately 100 times faster than the charging time for laboratory-scale lithium-air batteries, and approaches what is needed for practical applications. This demonstrates that if these batteries’ electron-transfer characteristics can be improved, it could allow for much faster charging while minimizing energy losses.
This provides insights into how to design the air electrode. To our knowledge, this is the first direct evidence that electron transport is limiting the charging.
—Yang Shao-Horn
The finding suggests that lithium-air battery performance would improve if electrodes had a high-surface-area structure to maximize contact between lithium peroxide and the carbon required to transport electrons away during charging.
The critical next step will be to measure actual currents during charging, Shao-Horn says.
The work was supported in part by the National Science Foundation, and was performed in part at the Center for Integrated Nanotechnologies at Sandia National Laboratories.
Resources
Li Zhong, Robert R. Mitchell, Yang Liu, Betar M. Gallant, Carl V. Thompson, Jian Yu Huang, Scott X. Mao, and Yang Shao-Horn (2013) In Situ Transmission Electron Microscopy Observations of Electrochemical Oxidation of Li2O2. Nano Letters 13 (5), 2209-2214 doi: 10.1021/nl400731w
I'm not sure what it all means, but it must help to observe nano-meter charging activity.
Posted by: kelly | 13 May 2013 at 01:23 PM