Scanning electron microscope image of a cathode particle on a microelectrode. Image: Jinhong Min, Li+ Research Group, University of Michigan.
Scanning electron microscope image of a cathode particle on a microelectrode. Image: Jinhong Min, Li+ Research Group, University of Michigan.

Rather than being solely detrimental, cracks in the positive electrode of lithium-ion batteries can reduce battery charge time, say researchers at the University of Michigan. 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," said Yiyang Li, assistant professor of materials science and engineering, and corresponding author of a paper on this work in Energy and Environmental Science. “Unfortunately, if the cracks are removed, the battery particles won’t be able to charge quickly without the extra surface area from those cracks.

"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 these 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 is 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 characterization 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 who works in Li's lab. "Battery scientists know that the cracking occurs but have not measured how such cracking affects the charging speed."

Measuring the charging speed of individual cathode particles was key to discovering the upside to cracking cathodes. Li and Min accomplished this by inserting the particles into a device that is typically used by neuroscientists to study how individual brain cells transmit electrical signals.

"Back when I was in graduate school, a colleague studying neuroscience showed me these arrays that they used to study individual neurons," Li said. “I wondered if we can also use them to study battery particles, which are similar in size to neurons.”

Each array is a custom-designed, 2cm-by-2cm chip containing up to 100 microelectrodes. After scattering some cathode particles in the center of the chip, Min used a needle around 70 times thinner than a human hair to move single particles onto their own electrode on the array. Once the particles were in place, Min could simultaneously charge and discharge up to four individual particles at a time on the array, allowing him to measure 21 particles in this particular study.

This experiment revealed that the cathode particles' charging speeds did not depend on their size. Li and Min think that the most likely explanation for this unexpected behavior is that larger particles actually behave like a collection of smaller particles when they crack. Another possibility is that the lithium ions move very quickly in the grain boundaries – the tiny spaces between the nanoscale crystals comprising the cathode particle. Li thinks this is unlikely unless the battery's electrolyte – the liquid medium in which the lithium ions move – penetrates these boundaries, forming cracks.

The benefits of cracked materials are important to consider when designing long-lived batteries with single-crystal particles that don't crack. To charge quickly, these particles may need to be smaller than today's cracking cathode particles. The alternative is to make single-crystal cathodes with different materials that can move lithium faster, but those materials could be limited by the supply of necessary metals or have lower energy densities, Li said.

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.