Researchers at the Swiss Federal Institute of Technology (ETH) in Zurich are analyzing battery materials operation in real time using 3-D movies to gain insight into how to develop better lithium ion batteries.
Visualizing batteries in operation was essentially impossible until recent advances in x-ray tomography. Using world-class facilities at a synchrotron at the Paul Scherrer Institute in Switzerland "we can watch the battery at work," said Vanessa Wood, head of the Laboratory for Nanoelectronics at ETH's Department of Information Technology and Electrical Engineering.
The limited lifetime of electrode materials is attributed to a massive—up to threefold—expansion of the electrode material during charging. During discharge, the materials contract again, but do not reach their original state. Electrode particles break apart, the electrode structure disintegrates, and the fragments loose contact to the rest of the cell.
The researchers needed visual proof in real time.
They used the x-ray tomography setup at the Swiss Light Source, a synchrotron located at the Paul Scherrer Institute in Switzerland for producing electromagnetic radiation of high brightness. The synchrotron's high-resolution x-ray images are then computationally assembled into three-dimensional movies.
The researchers observed the inside of the battery as it charged and discharged over 15 hours. The 3-D movies captured the degradation mechanisms in the battery and quantified the processes occurring within every particle for the thousands of particles in the electrode.
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 x-ray images show that charging destroys the particle structure irreversibly with cracks forming within the particles. The crack-formation is not random with cracks growing at locations where the crystal lattice contains preexisting defects. During discharge, the particle volume decreases but the material does not reach its original state again. The process is therefore is shown to be not completely reversible.
The volume change of the individual particles drives expansion of the entire electrode from 50 microns to 120 microns. However, during discharge, the electrode contracts only to 80 microns. This permanent deformation of the electrode demonstrates that the polymer binder that holds the electrode together is not yet optimized for high volume expansion materials, according to the researchers.
This is critical for battery performance because deformation of the binder causes individual particles to become disconnected from the electrode and the battery looses capacity.
The researchers chose crystalline tin oxide as a model material because it undergoes a series of complex transformations also present in other materials, enabling deeper understanding into the behavior of a variety of battery materials. The insights provide the basis for developing new electrode materials and electrode structures that are tolerant to volume expansion.
The results of the study will be published in the journal Science.