Oct 31 2017
Rice University researchers state that high-performance electrodes for lithium-ion batteries can be enhanced by looking closely at their defects — and exploiting them.
An illustration shows the growth of a lithium-deficient phase (blue) at the expense of a Lithium-rich phase (red) in a lithium iron phosphate microrod. 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. (Courtesy of the Mesoscale Materials Modeling Group)
Rice materials researcher Ming Tang and chemists Song Jin at the University of Wisconsin-Madison and Linsen Li at Wisconsin and the Massachusetts Institute of Technology led a study that integrated advanced, in situ X-ray microscopy and modeling to gain a greater understanding into lithium transport in battery cathodes. They discovered that a typical cathode material for lithium-ion batteries, olivine lithium iron phosphate, discharges or takes in lithium ions through a much larger surface area than formerly thought.
“We know this material works very well but there’s still much debate about why,” Tang said. “In many aspects, this material isn’t supposed to be so good, but somehow it exceeds people’s expectations.”
Part of the reason, Tang said, comes from point defects — atoms out-of-place in the crystal lattice — referred to as antisite defects. Such defects are difficult to fully get rid of in the fabrication process. As it turns out, he said, they make real-world electrode materials act very differently from flawless crystals.
That and other findings in a Nature Communications papers could possibly help manufacturers build better lithium-ion batteries that power electronic devices around the world.
The study’s lead authors— Liang Hong of Rice and Li of Wisconsin and MIT — and their colleagues partnered with Department of Energy scientists at Brookhaven National Laboratory to use its robust synchrotron light sources and see in real time what happens within the battery material during its charging. They also applied computer simulations to describe their observations.
One surprise, Tang said, was that microscopic defects in electrodes are a characteristic, not a bug.
People usually think defects are a bad thing for battery materials, that they destroy properties and performance, with the increasing amount of evidence, we realized that having a suitable amount of point defects can actually be a good thing.
Ming Tang, Rice University materials researcher
Within a defect-free, flawless crystal lattice of a lithium iron phosphate cathode, lithium can only travel in one direction, Tang said. Due to this, it is thought that the lithium intercalation reaction can occur over only a fraction of the particle’s surface area.
But the Rice team made an unexpected discovery when examining Li’s X-ray microscopic images: The surface reaction occurs on the large side of his defective, synthesized microrods, which disproves theoretical predictions that the sides would be sedentary because they are parallel to the observed movement of lithium.
The researchers explained that particle defects essentially alter the electrode’s lithium transport properties and enable lithium to jump inside the cathode via more than one direction. That expands the reactive surface area and allows for more efficient exchange of lithium ions between the electrolyte and the cathode.
Since the cathode in this research was made by a common synthesis technique, Tang said, the finding is extremely relevant to practical applications.
What we learned changes the thinking on how the shape of lithium iron phosphate particles should be optimized, assuming one-dimensional lithium movement, people tend to believe the ideal particle shape should be a thin plate because it reduces the distance lithium needs to travel in that direction and maximizes the reactive surface area at the same time. But as we now know that lithium can move in multiple directions, thanks to defects, the design criteria to maximize performance will certainly look quite different.
Ming Tang, Rice University materials researcher
The second unexpected observation, Tang said, is to do with the transport of phase boundaries in the cathode while it is charged and discharged.
“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 divided by an interface, or a phase boundary. How rapidly the lithium can be extracted relies on how quickly the phase boundary travels across a particle, he said.
In contrast to bulk materials, Tang explained, it has been hypothesised that phase boundary movement in small battery particles can be restricted by the surface reaction rate. The researchers were able to offer the first concrete proof 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, 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.
Ming Tang, Rice University materials researcher
The co-authors of the paper 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.