Scientists see pathways of lithium in electric car batteries while charging at high speed

22-07-2020

Researchers measure how lithium moves inside batteries for electric vehicles while charging. The data shows the obstacles to overcome to be able to charge them faster.

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To make electric cars competitive against swiftly refueling diesel and petrol cars, scientists are looking for ways to charge the vehicle batteries in just 10 minutes, the amount of time it takes to refuel a conventional car. The hurdles to overcome are due to the high energy density of the lithium ion batteries used in electric cars. “When charging, lithium goes from one electrode to the other. In each electrode there are particles in which the lithium rests following charge, providing the battery with a means to stably store energy. However, the particles also present obstacles to lithium trying to get beyond them to other particles”, explains Donal Finegan, scientist at the National Renewable Energy Laboratory (NREL) in the US and corresponding author of the article. “You can make an analogy with running: If lithium ran on a straight and open running track, it would be extremely quick, but if it ran in a forest, it would be slowed down by having to go around the trees. This difference between a straight path and path around obstacles is known as ‘tortuosity’”, he adds.

Until today, the distribution of lithium within electrodes during fast charging and the behavior of the electrode material at different depths was not well understood. “Currently, ID15A at the ESRF is the leading beamline in the world for high-speed and high-spatial resolution X-ray diffraction measurements”, says Finegan.

At the beamline, Finegan, together with scientists from University of Oslo, University College London, Massachusetts Institute of Technology, MAX IV and the ESRF, measured diffraction from 100 spatial points every second during the process of fast charging and discharging of a lithium-ion battery, in conditions reproducing those of commercial battery usage. “We got a full depth profile of each battery electrode in less than half a second”, he says.

These data visualized phase heterogeneities such as lithium plating on the graphite negative electrode while charging. Graphite is used as electrode material to stabilise the lithium ions when storing charge. When charging the battery too rapidly, the lithium sits on top of the graphite rather than resting inside it. This is known as ‘lithium plating’ and is much less stable and increases degradation and the risk of failure. “We need to quantify the limitations of graphite electrodes and understand the conditions that lead to lithium plating”, says Finegan.

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The different parts of the battery (left) and a schematic image of how the experiment was carried out (right). Credits: Finegan, D.P., et al, Energy and Environmental Science, 8 July 2020. DOI: 10.1039/d0ee01191f

High spatial resolution made it possible to observe, for the first time, a region where empty graphite and graphite full of lithium coexist next to each other, but only where lithium plates. “This is a very surprising, never-seen before event”, adds Finegan. If graphite is in its full lithiated state, it does not allow lithium to access it and be stabilized in it, which could cause problems for future charging.

“Our data is very valuable for theoreticians that model how graphite behaves, and how lithium plating occurs during fast charging. We will continue to test batteries in an array of conditions and architectures to see what works best in order to decrease its charging time”, he concludes.

"Thanks to ESRF-EBS Extremely Brilliant Source improving by several fold the beamline temporal resolution, real time characterization of working devices has become a staple experiment of ID15A. This means, for example, studying batteries during even faster charging and in greater detail than before", says Stefano Checchia, scientist at ID15A and co-author of the publication.

Reference:

Finegan, D.P., et al, Energy and Environmental Science, 8 July 2020. DOI: 10.1039/d0ee01191f

Text by Montserrat Capellas Espuny