New simulations by Rice materials scientist Ming Tang and graduate student Kaiqi Yang, detailed in the Journal of Materials Chemistry A, show that too much stress concentration in widely-used lithium iron phosphate cathodes can open cracks and quickly degrade batteries.

The work extends recent Rice research that demonstrated how putting defects in particles that make up the cathode could improve battery performance by up to two orders of magnitude by helping lithium move more efficiently.

The lab’s subsequent modeling study has revealed a caveat. Under the pressure of rapid charging and discharging, defect-laden cathodes risk fracture.

At left, a 3D model by Rice University materials scientists shows a phase boundary as a delithiating lithium iron phosphate cathode undergoes rapid discharge. At right, a cross-section shows the “fingerlike” boundary between iron phosphate (blue) and lithium (red). Rice engineers found that too many intentional defects intended to make batteries better can in fact degrade their performance and endurance. (Credit: Mesoscale Materials Science Group/Rice University)

The root of the problem appears to be that stress destabilizes the initially flat boundary and causes it to become wavy, Tang said. The change in the boundary shape further increases the stress level and triggers crack formation.

The study by Tang’s group shows that such instability can be increased by a common type of defect in battery compounds called antisites, where iron atoms occupy spots in the crystal where lithium atoms should be.

Antisites can be a good thing, as we showed in the last paper, because they accelerate the lithium intercalation kinetics. But here we show a countereffect: Too many antisites in the particles encourage the moving interface to become unstable and therefore generate more stress.

—Ming Tang

Tang believes there’s a sweet spot for the number of antisites in a cathode: enough to enhance performance but too few to promote 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. We think our new predictions might be useful to experimentalists.

—Ming Tang

The US Department of Energy (DOE) supported the research. Simulations were performed on supercomputers at the Texas Advanced Computing Center at the University of Texas and DOE’s National Energy Research Scientific Computing Center.

Resources

• Kaiqi Yang and Ming Tang (2020) “Three-dimensional phase evolution and stress-induced non-uniform Li intercalation behavior in lithium iron phosphate” J. Mater. Chem. A doi: 10.1039/C9TA11697D