08 February 2012
New characteristics in lithium iron phosphate?
MIT researchers have discovered new characteristics in lithium iron phosphate that pave the way for even more effective battery materials.
Martin Bazant, associate professor of Chemical Engineering and Mathematics, led the research and delivered results that show the material behaves quite differently than had previously been thought.
Lithium iron phosphate has become one of the most promising materials for rechargeable batteries because of its stability and ability to deliver a lot of power at once. Despite this widespread interest, the reason for the material's charging and discharging characteristics have remained unclear. It was widely believed that while being charged or discharged, the bulk material separated into different phases with very different concentrations of lithium. This phase separation was believed to limit the material's power capacity. However, Bazant's new research - detailed in a paper appearing in ACS Nano - shows that under many real world conditions, this separation never happens.
Bazant's new theory predicts that above a critical level, the reaction is so fast that the material loses its tendency for the phase separation that happens at lower power levels. Just below the critical current, the material passes through a new 'quasi solid solution' state, where it, doesn't have time to complete the phase separation. According to Bazant, these characteristics help explain why the material is so good for rechargeable batteries.
Previous analysis of lithium iron phosphate had examined its behaviour at a single point in time, ignoring the dynamics of its behaviour. Bazant and postdoc Daniel Cogswell studied how the material changes while in use, either while charging or discharging a battery – and its changing properties over time turned out to be crucial in understanding its performance. This new approach revealed what Bazant describes as 'a whole new phenomenon' and one that could be important for understanding the performance of many battery materials.
Researchers had previously thought that lithium gradually soaks into the particles from the outside in, producing a shrinking core of lithium poor material at the centre. What the MIT found was quite different. At low current, the lithium formed straight parallel bands of enriched material within each particle and the bands travelled across the particles as they were charged up. At higher electric current levels, there was no separation at all, either in bands or in layers; instead, each particle soaked up the lithium all at once, transforming almost instantaneously from lithium poor to lithium rich.
The new finding also helps explain lithium iron phosphate's durability. When there are stripes of different phases present, the boundaries between those stripes are a source of strain that can cause cracking and a gradual degradation in performance. But when the whole material changes at once, there are no such boundaries and thus less degradation. According to Bazant, that is an unusual finding. "Usually, if you're doing something faster, you do more damage, but in this case it's the opposite," he said. Similarly, he and Cogswell predict that operating at a slightly higher temperature would actually make the material last longer, which runs counter to typical material behaviour.
In addition to seeing how the material changes over time, understanding how it works involved looking at the material at scales that others had not examined. While much analysis had been done at the level of atoms and molecules, it turned out that the key phenomena could only be seen at the scale of the nanoparticles themselves, Bazant says — many thousands of times larger. "It's a size dependent effect," he noted.
Bazant's theoretical analysis could lead to a broader understanding not only of lithium iron phosphate, but also of other materials that may undergo similar changes. The findings could enable material scientists to develop new structures and compounds that ultimately lead to batteries that have longer life and higher energy density. This is what is required if battery technology is to be used in high power application such as electric vehicles. Bazant concluded: "[This has been] one of the most interesting scientific puzzles I've ever encountered. It took five years to figure this out."
Author
Chris Shaw
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