August 5, 2014
New Method Provides Nanoscale Details Of Electrochemical Reactions In Electric Vehicle Battery Materials
Using a new method to track the electrochemical reactions in a common electric vehicle battery material under operating conditions, scientists at the U.S. Department of Energy's Brookhaven National Laboratory have revealed new insight into why fast charging inhibits this material's performance. The study also provides the first direct experimental evidence to support a particular model of the electrochemical reaction. The results, published August 4, 2014, in Nature Communications, could provide guidance to inform battery makers' efforts to optimize materials for faster-charging batteries with higher capacity.
"Our work was focused on developing a method to track structural and electrochemical changes at the nanoscale as the battery material was charging," said Brookhaven physicist Jun Wang, who led the research. Her group was particularly interested in chemically mapping what happens in lithium iron phosphate—a material commonly used in the cathode, or positive electrode, of electrical vehicle batteries—as the battery charged. "We wanted to catch and monitor the phase transformation that takes place in the cathode as lithium ions move from the cathode to the anode," she said.
Getting as many lithium ions as possible to move from cathode to anode through this process, known as delithiation, is the key to recharging the battery to its fullest capacity so it will be able to provide power for the longest possible period of time. Understanding the subtle details of why that doesn't always happen could ultimately lead to ways to improve battery performance, enabling electric vehicles to travel farther before needing to be recharged.
X-ray imaging and chemical fingerprinting
Many previous methods used to analyze such battery materials have produced data that average out effects over the entire electrode. These methods lack the spatial resolution needed for chemical mapping or nanoscale imaging, and are likely to overlook possible small-scale effects and local differences within the sample, Wang explained.
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).
References: Scientific paper: "In operando tracking phase transformation evolution of lithium ion phosphate with hard X-ray microscopy" DOI: 10.1038/ncomms5570