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“When you take heat out of water, it turns into ice,” he said. “And when you take lithium out of these particles, it forms a different lithium-poor phase, like ice, that coexists with the initial lithium-rich phase.” The phases are separated by an interface, or a phase boundary. How fast the lithium can be extracted depends on how fast the phase boundary moves across a particle, he said.
Lithium iron phosphate microrods undergo phase transformation in a battery electrode during charging. Rice University researchers led a study that found defects in a common cathode material for lithium-ion batteries can potentially improve performance over perfect electrodes by allowing for lithium transport over much more surface area than previously thought possible.
Unlike in bulk materials, Tang explained, it has been predicted that phase boundary movement in small battery particles can be limited by the surface reaction rate. The researchers were able to provide the first concrete evidence for this surface reaction-controlled mechanism, but with a twist.
“We see the phase boundary move in two different directions through two different mechanisms, either controlled by surface reaction or lithium bulk diffusion,” he said. “This hybrid mechanism paints a more complicated picture about how phase transformation happens in battery materials. Because it can take place in a large group of electrode materials, this discovery is fundamental for understanding battery performance and highlights the importance of improving the surface reaction rate.”
The paper’s co-authors are graduate student Fan Wang of Rice, Jun Wang, Yuchen-Karen Chen-Wiegart and Jiajun Wang of Brookhaven National Laboratory, Kai Xiang and Yet-Ming Chiang of MIT, and Liyang Gan, Wenjie Li and Fei Meng of the University of Wisconsin-Madison. Tang is an assistant professor of materials science and nanoengineering at Rice.
The research was supported by the U.S. Department of Energy (DOE) Office of Basic Energy Science, the National Science Foundation (NSF), a University of Wisconsin-Madison WEI Seed Grant and the Vilas Research Travel Awards. Research was also conducted at the Department of Energy’s Brookhaven and Argonne national laboratories. The Texas Advanced Computing Center at the University of Texas at Austin and the National Energy Research Scientific Computing Center funded by the DOE and the Big-Data Private-Cloud Research Cyberinfrastructure funded by the NSF and Rice provided computing resources.
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