In a study led by materials scientist Ming Tang and chemists Song Jin at the University of Wisconsin-Madison and Linsen Li at Wisconsin and the Massachusetts Institute of Technology which used X-ray spectroscopy and modelling to gain insight into lithium transport in battery cathodes, it was found that the common cathode material for lithium-ion batteries, olivine lithium iron phosphate, releases or takes in lithium ions through a much larger surface area than previously 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."
According to Tang point while these antisite defects, where atoms are misplaced in the crystal lattice, are impossible to eliminate in the fabrication process they help make real-world electrode materials behave very differently from perfect crystals – which could potentially help manufacturers develop better lithium-ion batteries.
"People usually think defects are a bad thing for battery materials, that they destroy properties and performance," he said. "With the increasing amount of evidence, we realised that having a suitable amount of point defects can actually be a good thing."
Inside a defect-free, perfect crystal lattice of a lithium iron phosphate cathode, lithium can only move in one direction, Tang said. Because of this, it is believed the lithium intercalation reaction can happen over only a fraction of the particle's surface area.
However, the team found that when they analysed the surface reaction it was taking place on the large side of imperfect, synthesized microrods, which goes counter to theoretical predictions that the sides would be inactive because they are parallel to the perceived movement of lithium.
The researchers said that particle defects fundamentally change the electrode's lithium transport properties and enable lithium to hop inside the cathode along more than one direction increasing the reactive surface area and allowing for more efficient exchange of lithium ions between the cathode and electrolyte.
Because the cathode in this study was made by a typical synthesis method, Tang said, the finding could be highly relevant to practical applications.
"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,” he explained.
Another interesting observation, Tang said, has to do with the movement of phase boundaries in the cathode as 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 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.
It has been predicted that phase boundary movement in small battery particles can be limited by the surface reaction rate and the researchers were able to provide concrete evidence for this surface reaction-controlled mechanism, for the first time.
"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."
