Researchers from the University of Michigan have found that cracks in lithium-ion battery electrodes can help speed up EV charging.
Rather than simply shortening the lifespan of a battery, cracks in the positive electrode of lithium-ion batteries have been found to reduce the charging time of an EV battery.
This discovery runs counter to the view of many prominent EV manufacturers, who aim to minimise cracking because it decreases the longevity of a battery.
“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.
“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 paper, ‘Direct measurements of size-independent lithium diffusion and reaction times in individual polycrystalline battery particles,’ was published in the journal Energy and Environmental Sciences.
The findings apply to more than half of all EV batteries
The team argues that their discovery applies to more than half of all EV batteries, in which the cathode is composed of trillions of microscopic particles made of either lithium nickel manganese cobalt oxide or lithium nickel cobalt aluminium oxide.
In theory, how fast the cathode charges depends on the surface to volume ratio of the particles. Smaller particles should charge faster than larger particles because they have a higher surface area to volume ratio.
Lithium-ions have shorter distances to diffuse through particles with a higher surface area to volume ratio.
The relationship between charging speed and cathode particle size was only assumed
Previously, conventional methods could not measure the charging properties of individual cathode particles. They could only work out the average for all the particles that comprise the battery’s cathode.
This limitation means that the widely accepted relationship between EV 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.”
Measuring individual cathode particles was key
To discover that the cracks in lithium-ion batteries speed up EV charging, the team measured the charging speed of individual cathode particles. This was done 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. I wondered if we can also use them to study battery particles, which are similar in size to neurons,” Li said.
Each array is a custom-designed, 2-by-2 centimetre chip with up to 100 microelectrodes.
Once the cathode particles were scattered at the centre of the chip, the single particles were moved onto their own electrodes by using a needle approximately 70 times thinner than a human hair.
Then the team charged and discharged up to four individual particles at a time. In this study, 21 particles were measured.
EV charging speeds did not depend on the size of the particles
The study found that the cathode particles’ charging speeds did not depend on their size. The most likely explanation for this unexpected behaviour is that larger particles behave like a collection of smaller particles when they crack.
Another possibility is that lithium ions move quickly in the grain boundaries. However, the team believes this is unlikely unless the battery’s electrolyte penetrates these boundaries to cause cracks to form.
Lithium-ion battery cracks speed up EV charging
The benefits of cracked materials are important to consider when designing batteries that do not crack. To speed up EV charging, particles need to be smaller than today’s cracking cathode particles.
Alternatively, scientists can make single-crystal cathodes with different materials that can move lithium faster.
However, those materials could be limited by the supply of necessary metals or have lower energy densities.
