To improve upon those methods, the Brookhaven team used a combination of full- field, nanoscale-resolution transmission x-ray microscopy (TXM) and x-ray absorption near-edge spectroscopy (XANES) at the National Synchrotron Light Source (NSLS), a DOE Office of Science User Facility that provides beams of high-intensity x-rays for studies in many areas of science. These x-rays can penetrate the material to produce both high-resolution images and spectroscopic data-a sort of electrochemical "fingerprint" that reveals, pixel by pixel, where lithium ions remain in the material, where they've been removed leaving only iron phosphate, and other potentially interesting electrochemical details.
The scientists used these methods to analyze samples made up of multiple nanoscale particles in a real battery electrode under operating conditions (in operando). But because there can be a lot of overlap of particles in these samples, they also conducted the same in operando study using smaller amounts of electrode material than would be found in a typical battery. This allowed them to gain further insight into how the delithiation reaction proceeds within individual particles without overlap. They studied each system (multi-particle and individual particles) under two different charging scenarios-rapid (like you'd get at an electric vehicle recharging station), and slow (used when plugging in your vehicle at home overnight).
Insight into why charging rate matters
These animated images of individual particles, taken while the electrode is charging, show that lithiated (red) and delithiated (green) iron phosphate phases co-exist within individual particles. This finding directly supports a model in which the phase transformation proceeds from one phase to the other without the existence of an intermediate phase.
|Contact: Karen McNulty Walsh|
DOE/Brookhaven National Laboratory