Much of the nanotechnology-related work going on today for improving Lithium-ion (Li-ion) batteries has focused on developing nanostructured silicon to replace graphite in the anodes of the next generation Li-ion batteries.
While this work has been encouraging, another line of research has taken a different tack. Instead of just replacing the graphite in the anodes, researchers have sought to determine why the degradation of Li-ion batteries’ storage capacity occurs in the first place.
Two years ago, I covered work conducted at Ohio State University in conjunction with both Oak Ridge National Laboratory and the National Institute of Standards and Technology that employed every microscopy tool researchers could get their hands on in the search for nanoscale phenomena that would cause this degradation. The results showed that the material from which the electrodes in Li-ion batteries are made coarsen over time; the lithium ions that need to go between the positively and negatively charged electrodes become increasingly unavailable for charge transfer.
Now researchers at the U.S. Department of Energy’s Brookhaven National Laboratory have developed a new imaging technique that allows them to observe lithium-ion reactions in real time with at the nanoscale precision.
Critical to the new imaging technique is transmission electron microscopy (TEM), which has been used to fabricate the “world’s smallest” battery. In the Brookhaven work, which was published in the journal Nature Communications (“Tracking lithium transport and electrochemical reactions in nanoparticles”), the TEM is modified with an in-situ electrochemical cell that can operate inside the TEM. This novel design gives researchers the combination of live imaging found with in-situ techniques and the spatial resolution and nanoscale precision of TEM.
The design of the modified TEM may be novel, but it’s not overly complex. “The entire setup for the in-situ TEM measurements was assembled from commercially available parts and was simple to implement," said Brookhaven Lab physicist and lead author Feng Wang in a press release. "We expect to see a widespread use of this technique to study a variety of high-energy electrodes in the near future,” says Wang.
The new imaging technique allowed the researchers to observe the lithium ion reaction that occurs across iron fluoride (FeF2) nanoparticles. They watched the lithium ions move quickly across the surface of the nanoparticles and then observed the compounds being broken down into different regions in a layer-by-layer process—all in real time. The Brookhaven team saw that the lithium-ion reaction leaves in its wake a trail of new molecules.
“Although many questions remain regarding the true mechanisms behind this conversion reaction, we now have a much more detailed understanding of electron and lithium transport in lithium-ion batteries,” said Brookhaven physicist and study coauthor Jason Graetz in the release. “Future studies will focus on the charge reaction in an attempt to gain new insights into the degradation over time that plagues most electrodes, allowing for longer lifetimes in the next generation of energy storage devices.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.