Cracking in Li

just_super/iStock

By subscribing, you agree to our Terms of Use and Policies You may unsubscribe at any time.

Researchers across the globe are in hot pursuit of further battery technology, primarily aiming to increase energy density and reduce charging times. In a significant advancement, a study by a team of scientists at the University of Michigan has found that the cracks in the positive electrode of lithium-ion batteries can help to reduce battery charge time.

The perception of cracks, usually known to shorten battery lifespans, was solved using a neuroscience-inspired technique. This contradicts the beliefs of many electric car makers, who believe that cracking reduces 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, in a statement from the university.

The study was published in the journal Energy and Environmental Sciences.

The researchers estimate their study will apply to more than half of all electric car batteries in which the positive electrode (or cathode) is made up of billions of tiny particles formed 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."

Traditional techniques, however, could not directly detect the charging characteristics of individual cathode particles, only the average of all the particles that make up the battery's cathode. Because of this constraint, the commonly believed link between charging speed and cathode particle size was only 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. Battery scientists know that the cracking occurs but have not measured how much cracking affects the charging speed," said Jinhong Min, a doctoral student in the materials science and engineering department involved with the project.

Measuring the charging speed of individual cathode particles was critical to determining the benefit of cracking cathodes, which the team performed by introducing the particles into a device that neuroscientists routinely use to analyze how individual brain cells communicate electrical impulses.

The team designed arrays, which is a 2-by-2-centimeter devices with up to 100 microelectrodes. A needle 70 times finer than a human hair was used to move single particles onto their electrodes on the array after dispersing several cathode particles in the center of the device. They could charge and discharge up to four individual particles at a time on the array after the particles were in place, and this specific investigation monitored 21 particles.

The experiment found that the charging rates of cathode particles were independent of their size. The most plausible explanation for this surprising behavior is that bigger particles behave like a cluster of smaller particles when they break. Another theory is that lithium ions travel very fast in the grain boundaries, which are the microscopic crevices between the nanoscale crystals that make up the cathode particle. Li believes this is improbable unless the electrolyte in the battery—the liquid medium in which the lithium ions move—penetrates these limits and forms fissures.

According to the team, the advantages of cracked materials must be considered when constructing long-lasting batteries with single-crystal particles that do not break. These particles may need to be smaller than today's damaging cathode particles to charge fast. "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," said Li.

As the transition towards EVs is gathering pace across the world, the research hopes to shed light on manufacturers' beliefs and efforts to reduce cracking to ensure battery durability.

Abstract

Polycrystalline Li(Ni,Mn,Co)O2 (NMC) secondary particles are the most common cathode materials for Li-ion batteries. During electrochemical (dis)charge, lithium is believed to diffuse through the bulk and enter (leave) the secondary particle at the surface. Based on this model, smaller particles would cycle faster due to shorter diffusion lengths and larger surface-area-to-volume ratios. In this work, we evaluate this widespread assumption by developing a new high-throughput single-particle electrochemistry platform using the multi-electrode array from neuroscience. By measuring the reaction and diffusion times for 21 individual particles in liquid electrolytes, we find no correlation between the particle size and either the reaction or diffusion times, which is in stark contrast to the prevailing lithium transport model. We propose that electrochemical reactions occur inside secondary particles, likely due to electrolyte penetration into cracks. Our high-throughput, single-particle electrochemical platform further opens new frontiers for robust, statistical quantification of individual particles in electrochemical systems.