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Argonne researchers use X-rays to understand the flaws of speedy charging

A team at Argonne National Laboratory has used spatially resolved energy dispersive X-ray diffraction to obtain a “movie” of lithiation and delithiation in different sections of a Li-ion battery cell and to quantify lithium gradients that develop in a porous graphite electrode during cycling at a 1C rate (full discharge in 1 hour). An open-access paper on their work is published in the RSC journal Energy and Environmental Science.

The principal problem with fast charging happens during the transport of lithium ions from the positive cathode to the negative anode. If the battery is charged slowly, the lithium ions extracted from the cathode gradually slot themselves between the planes of carbon atoms that make up the graphite anode—a process known as lithium intercalation. When this process is sped up, lithium can end up depositing on the surface of the graphite as metal—i.e., lithium plating. ​

When this happens, the performance of the battery suffers significantly, because the plated lithium cannot be moved from one electrode to the other, explained Argonne battery scientist Daniel Abraham. According to Abraham, this lithium metal will chemically reduce the battery’s electrolyte, causing the formation of a solid-electrolyte interphase that ties up lithium ions so they cannot be shuttled between the electrodes. As a result, less energy can be stored in the battery over time.

Fast charging is very important for electric vehicles. We’d like to be able to charge an electric vehicle battery in under 15 minutes, and even faster if possible. By seeing exactly how the lithium is distributed within the electrode, we’re gaining the ability to precisely determine the inhomogeneous way in which a battery ages.

—Daniel Abraham

To study the movement of lithium ions within the battery, Abraham partnered with postdoctoral researcher Koffi Pierre Yao and Argonne X-ray physicist John Okasinski at the laboratory’s Advanced Photon Source, a DOE Office of Science User Facility. There, Okasinski essentially created a 2D image of the battery—a standard 2032-type coin cell, containing a Gr-based anode and a Li1.03(Ni0.5Co0.2Mn0.3)O2 (NCM523)-based cathode—by using X-rays to image each phase of lithiated graphite in the anode.

By gaining this view, the researchers were able to precisely quantify the amount of lithium in different regions of the anode during charging and discharging of the battery.

In the study, the scientists established that the lithium accumulates at regions closer to the battery’s separator under fast-charging conditions.

To see a particular region selectively in the heart of the battery, the researchers used a technique called energy dispersive X-ray diffraction. Instead of varying the angle of the beam to reach particular areas of interest, the researchers varied the wavelength of the incident light.

By using X-rays, Argonne’s scientists were able to determine the crystal structures present in the graphite layers. Because graphite is a crystalline material, the insertion of lithium causes the graphite lattice to expand to varying degrees. This swelling of the layers is noticeable as a difference in the diffraction peaks, Okasinski said, and the intensities of these peaks give the lithium content in the graphite.

While this research focuses on small coin-cell batteries, Okasinski said that future studies could examine the lithiation behavior in larger pouch-cell batteries, like those found in smartphones and electric vehicles.

The research was supported by DOE’s Office of Energy Efficiency and Renewable Energy (Office of Vehicle Technologies).

Resources

  • Koffi P. C. Yao, John S. Okasinski, Kaushik Kalaga, Ilya A. Shkroba and Daniel P. Abraham (2019) “Quantifying lithium concentration gradients in the graphite electrode of Li-ion cells using operando energy dispersive X-ray diffraction” Energy & Environmental Science doi: 10.1039/C8EE02373E

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