A great deal of the research on lithium batteries goes into figuring out how to keep them together for the greatest number of charge-discharge cycles. Unfortunately, the active compounds in these batteries that give the greatest energy storage capacity or power output, also tend to be those compounds that come unglued under stress.
Taking high-resolution 3D movies with X-ray tomography (somewhat like the CAT scans used on human subjects), researchers at the Swiss Light Source, a mecca for seeing the unseeable, have witnessed the expansion and contraction of the internal structure of lithium-ion batteries, while the batteries are operating.
Stanford University’s Dr. Cui has explained that the expansion and contraction of batteries leads to their eventual failure, but until now, there has been no real-time observation of these internal reactions. Martin Ebner, a Ph.D. candidate at the Laboratory for Nanoelectronics in the Department of Information Technology and Electrical Engineering (D-ITET) at ETH (Eidgenössische Technische Hochschule) and Professor Vanessa Wood, head of the Laboratory for Nanoelectronics at D-ITET, used “spectrally pure” and intense x-rays to take a non-invasive look into the “complex electrochemical and mechanical degradation” leading to shortened battery life.
ETH-Professor Marco Stampanoni, a faculty member at the Institute for Biomedical Engineering at D-ITET, runs the tomographic x-ray microscopy beamline at the Swiss Light Source, the synchrotron facility at the Paul Scherrer Institute. Ebner and Wood had Stampanoni focus on batteries, acquiring high-resolution x-ray images that were later assembled into three-dimensional movies that show the flexing and disintegration of the crystalline strutures making up electrodes. Even better, researchers were able to quantify the relevant factors from the 15 hours of image collection.
According to ETH, “The data illustrate that tin oxide (SnO) particles expand during charging due to the influx of lithium ions causing an increase in particle volume. The scientists demonstrate that material lithiation happens as a core-shell process, progressing uniformly from the particle surface to the core. The material undergoing this reaction expands linearly with the stored charge. The x-ray images show that charging destroys the particle structure irreversibly with cracks forming within the particles. ‘This crack-formation is not random,’ emphasizes Ebner. Cracks grow at locations where the crystal lattice contains pre-existing defects. During discharge, the particle volume decreases; however, the material does not reach its original state again; the process is therefore not completely reversible.”
For instance, an electrode that was originally 50 micrometers in one dimension may swell to 120 micrometers under charge. During discharge, it may contract to 80 micrometers, a permanent deformation causing the polymer binder to allow release of individual particles from the electrode. A loss that is repeated on every charge/discharge cycle, and that lead to final battery failure.
Researchers chose crystalline tin oxide as a model material because its transformations are also present in other materials, with insights providing, “the basis for developing new electrode materials and electrode structures that are tolerant to volume expansion.”
Wood thinks using amorphous or nanostructured materials instead of crystalline ones will provide opportunities to avoid this internal self-destruction. “On the quest for new materials, one must also bear in mind that they are only of industrial interest if they can be produced in large quantities at a low cost. However, amorphous and nanostructured materials offer a sufficient playground for innovation.”
Amorphous and nano-materials may offer economic advantages for development, being less expensive to process than crystalline structures overall. This quest for longer-lasting, less-costly battery materials alternatives will eventually pay off in reliable, affordable sources of energy storage.
The team’s work can be seen in, “Visualization and quantification of electrochemical and mechanical degradation in Lithium ion batteries,” published online in Science Express, 17th October 2013.