Typically, defects in particular chemical formulas are considered a bad thing. But researchers at Rice University have discovered that defects can actually be used to an advantage in future battery chemistries to improve how quickly the device can charge.
An illustration shows a battery’s cathode undergoing phase transition from iron phosphate to lithium iron phosphate LFP during charging. Simulations by Rice University scientists showed that adding defects--distortions in their crystal lattices--could help batteries charge faster. (Image source: Kaiqi Yang, Rice University)
Scientists from the university’s Brown School of Engineering discovered through simulations that placing specific defects in cathodes of a lithium-based battery design can improve its performance. This finding could lead to charging that’s two orders of magnitude faster than current batteries.
The team--led by Ming Tang, Rice assistant professor of materials science and nanoengineering--worked specifically with lithium iron phosphate-based cathodes, focusing on the phase transition the cathode makes from iron phosphate to lithium iron phosphate when charging. Researchers discovered that by adding defects, called antisites, to the crystal lattices of the cathodes could help them to charge faster. “We think we can turn defects into friends, not enemies, for better energy storage,” Tang said.
Antisites typically aren’t something researchers hope to see when designing this type of battery, which is why the research is unique. These defects form when atoms sit on sites of the lattice that should be occupied by lithium, typically limiting lithium movement inside the crystal lattice, researchers said. This is why scientists consider them detrimental to how the battery performs.
Tang and his team turned this idea on its head, however, discovering that in the case of lithium iron phosphate, the antisites actually create detours within the cathode. These detours allow lithium ions to reach the reaction front over a wider surface, thus improving the charge or discharge rate of the batteries.
Optimizing material defects
Basically, when lithium is inserted into the cathode, it changes from a lithium-poor phase to a lithium-rich one, he said. If the surface reaction kinetics are slow, then lithium can only be inserted into lithium iron phosphate within a narrow surface region around the phase boundary, which researchers call the “road,” Tang said. This limits the battery’s recharging speed.
“However, antisite defects can make lithium insertion take place more uniformly across the surface, and so the boundary would move faster and the battery would charge faster,” he said.
Researchers also noted that they can use certain methods to control the concentration of defects, such as annealing the material, or heating without burning it. This would allow them to add other optimal properties to the battery design, such as using larger cathode particles than nanoscale crystals to be used to help improve energy density and reduce surface degradation.
“An interesting prediction of the model is that this optimal defect configuration depends on the shape of the particles,” said Tang. “We saw that facets of a certain orientation could make the detours more effective in transporting lithium ions. Therefore, you will want to have more of these facets exposed on the cathode surface.”
Researchers published a paper on their work in the Nature journal Computational Materials. They believe that their model is applicable not just to the type of cathode with which they worked, but also can be applied as a general strategy to improve phase-changing battery compounds. “For structural materials like steel and ceramics, people play with defects all the time to make materials stronger,” said Tang. “But we haven’t talked much about using defects to make better battery materials. Usually, people see defects as annoyances to be eliminated.”
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.