Rice engineers discover overabundance of intentional defects may cause battery cathodes to fail.
Intentional defects in batteries have given Rice College scientists a window into the hazards of pushing lithium-ion cells too far.
New simulations by Rice supplies scientist Ming Tang and graduate pupil Kaiqi Yang, detailed within the Journal of Supplies Chemistry A, reveals an excessive amount of stress in extensively used lithium iron phosphate cathodes can open cracks and shortly degrade batteries.
The work extends latest Rice analysis that demonstrated how placing defects in particles that make up the cathode may enhance battery efficiency by as much as two orders of magnitude by serving to lithium transfer extra effectively.
However the lab’s subsequent modeling examine revealed a caveat. Below the stress of speedy charging and discharging, defect-laden cathodes threat fracture.
“The conventional picture is that lithium moves uniformly into the cathode, with a lithium-rich region that expands smoothly into the cathode’s center,” stated Tang, an assistant professor of supplies science and nanoengineering at Rice’s Brown Faculty of Engineering.
However X-ray photographs taken at one other lab confirmed one thing else. “They saw a fingerlike boundary between the lithium-rich and lithium-poor regions, almost like when you inject water into oil,” he stated. “Our question was, what causes this?”
The foundation of the issue seems to be that stress destabilizes the initially flat boundary and causes it to turn into wavy, Tang stated. The change within the boundary form additional will increase the stress stage and triggers crack formation. The examine by Tang’s group reveals that such instability will be elevated by a standard kind of defect in battery compounds known as antisites, the place iron atoms occupy spots within the crystal the place lithium atoms must be.
“Antisites can be a good thing, as we showed in the last paper, because they accelerate the lithium intercalation kinetics,” Tang stated, “But here we show a countereffect: Too many antisites in the particles encourage the moving interface to become unstable and therefore generate more stress.”
Tang believes there’s a candy spot for the variety of antisites in a cathode: sufficient to reinforce efficiency however too few to advertise instability. “You want to have a suitable level of defects, and it will require some trial and error to figure out how to reach the right amount through annealing the particles,” he stated. “We think our new predictions might be useful to experimentalists.”
Reference: “Three-dimensional phase evolution and stress-induced non-uniform Li intercalation behavior in lithium iron phosphate” by Kaiqi Yanga and Ming Tang, 19 December 2019, Journal of Supplies Chemistry A.
The U.S. Division of Power (DOE) supported the analysis. Simulations had been carried out on supercomputers on the Texas Superior Computing Middle on the College of Texas and DOE’s Nationwide Power Analysis Scientific Computing Middle.