Rather than being solely detrimental, cracks in the positive electrode of lithium-ion batteries reduce battery charge time, research done at the University of Michigan shows.
This runs counter to the view of many electric vehicle manufacturers, who try to minimize cracking because it decreases battery longevity.
"Many companies are interested in making 'million-mile' batteries using particles that do not crack. Unfortunately, if the cracks are removed, the battery particles won't be able to charge quickly without the extra surface area from those cracks," said Yiyang Li, assistant professor of materials science and engineering and corresponding author of the study published in Energy & Environmental Science. "On a road trip, we don't want to wait five hours for a car to charge. We want to charge within 15 or 30 minutes."
The team believes the findings apply to more than half of all electric vehicle batteries, in which the positive electrode—or cathode—is composed of trillions of microscopic particles made of either lithium nickel manganese cobalt oxide or lithium nickel cobalt aluminum oxide.
Theoretically, the speed at which the cathode charges comes down to the particles' surface-to-volume ratio. Smaller particles should charge faster than larger particles because they have a higher surface area relative to volume, so the lithium ions have shorter distances to diffuse through them.
However, conventional methods couldn't directly measure the charging properties of individual cathode particles, only the average for all the particles that make up the battery's cathode. That limitation means the widely accepted relationship between charging speed and cathode particle size was merely an assumption.
"We find that the cathode particles are cracked and have more active surfaces to take in lithium ions—not just on their outer surface, but inside the particle cracks," said Jinhong Min, a doctoral student in materials science and engineering working in Li's lab. "Battery scientists know that the cracking occurs but have not measured how such cracking affects the charging speed."
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