There is consensus that one of the keys to electric vehicle (EV) acceptance will be fast charging. The ability to gain 200-300 miles of EV range in 10-20 minutes of charging time makes it possible to travel cross-country using nothing but electrons. But, there is more to fast charging than simple stuffing more electricity into a battery pack.
The lithium ion battery packs that are used to power EVs consist of a positive electrode (cathode) that is usually made from metal oxides that can trap lithium ions within their structures through a process called intercalation. The negative electrode (anode) is typically sheets of carbon graphite material that can accommodate the lithium ions between their layers. When a lithium ion battery is discharged, lithium ions travel from the graphite anode, through a liquid electrolyte, to the cathode. When the same battery is charged, lithium ions move from the cathode and are taken up between the layers of graphite. It’s important to note that under ordinary circumstances, there is no metallic lithium in the battery cell—just ions of the element.
Rate Depends on the Graphite
The rate at which the lithium ion battery can be charged depends upon the ability of the graphite anode to take up lithium ions. At slow charging rates (hours), the lithium ions travel into the bulk of the anode material and are safely stored there. At high charging rates (minutes), direct deposition of metallic lithium onto the surface of the graphite becomes thermodynamically favorable. This nucleation of lithium on the anode surface has several undesirable outcomes. First, the metallic lithium doesn’t plate onto the carbon evenly, but instead forms needle-like, spiky, dendritic crystals that can grow long enough to reach the cathode, shorting out the cell and potentially causing a fire. In addition, the lithium ions that plate onto the anode surface are not stored in the graphite planes, reducing the “inventory” of lithium ions and thus the charge capacity of the battery. The exposed metallic lithium also can react with the electrolyte, forming a solid electrolyte interface (SEI) that isolates and removes more lithium from potential chemical reactions and further reducing the battery’s performance and life.
The key to determining the maximum rate of fast charging is to determine the exact conditions under which lithium metal nucleation occurs on the surface of the graphite anode. The problem has been how to detect the nucleation in situ. Both nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques have been used in an attempt to determine the presence of metallic lithium, but neither technique has been successful at determining the spatial distribution of the plated lithium metal on the surface of the graphite anode.
During normal charging (a) an SEI forms between the partially-lithiated graphite surface of the anode. As the graphite layer becomes filled with lithium ions (b), conditions favoring lithium plating begin to occur. The plated lithium metal (c) produces a new SEI that reduces the inventory of lithium ions. Lithium acetylide species form (d) which can be detected by Raman spectroscopy and serve as a marker for the formation of plated lithium metal. ( Image source: Reprinted (adapted) with permission from (Lithium Acetylide: A Spectroscopic Marker for Lithium Deposition During Fast Charging of Li-Ion Cells, Marco-Tulio Fonseca Rodrigues, Victor A. Maroni, David J. Gosztola, Koffi P. C. Yao, Kaushik Kalaga, Ilya A. Shkrob, and Daniel P. Abraham, ACS Applied Energy Materials Article ASAP, DOI: 10.1021/acsaem.8b01975). Copyright (2018) American Chemical Society.")
A New Method to Detect Nucleation
Now, researchers at Argonne National Laboratory have demonstrated the use of Raman spectroscopy as a means to detect lithium metal nucleation and to map the spatial distribution of the metallic lithium on the surface of the anode. “Raman spectroscopy is a vibrational spectroscopy,” Daniel Abraham, senior scientist at Argonne explained to Design News. “We have a light that shines onto a sample—in this case the light happens to be a laser—and it actually makes the chemical bonds inside the molecules vibrate. You can think of molecules and crystals as a system of balls connected by springs—under certain conditions you can actually set the balls and springs to vibrate. Certain vibrational frequencies correspond to certain chemical compounds,” he added.
“When the lithium ion cell is charged at extremely high rates, there is an excess of lithium ions at the surface of the graphite anode electrode, so before the lithium ions can diffuse through the thickness of the electrode, the graphite at the top surface fills up with lithium and the excess amount of lithium plates onto the graphite,” Abraham told us.
“The thing about lithium metal is that it is not actually Raman-active, it can’t be seen in the Raman spectra” said Abraham. The Argonne team had noted that when they fast-charged a lithium ion cell, a Raman peak corresponding to a compound called lithium acetylide (e.g., Li-C≡C-X and Li2C2) was observed. These species yield an intense Raman band in a spectral region (1800 to 1900 cm-1) free of other spectral features and could therefore serve as a unique marker for the occurrence of Li deposition. “What we found is that this acetylide peak only forms when there are these tiny little lithium nuclei on the surface of the graphite electrode,” said Abraham. The presence of lithium metal also enhances the Raman peak through a process called surface-enhanced Raman scattering, so the far stronger than expected acetylide Raman peak is also an indication that lithium metal nucleation is occurring, according to Abraham.
Looking to Work in Real-Time
At present the Argonne team is doing the Raman spectroscopy after the battery cells have been charged—the next step is to do the same kinds of measurements in real-time. The goal is to be able to watch the nucleation sites grow by shooting a laser through a window in the cell and looking for the presence of acetylide in the Raman spectra. “If we can see the presence of an acetylide peak, it tells us that there is active lithium nucleation present,” Abraham told us.
“From a practical perspective, the kinds of things that could be interesting are, how the morphology of the graphite particles effects the lithium plating, or how an electrode could be designed so that lithium plating does not occur on the surface,” said Abraham. “If the lithium ions can diffuse quickly through the electrode, you have much less likelihood of plating during fast charging.”
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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