It’s no secret that one of the keys to electrification of the transportation system is fast-charging. Although the range of the latest electric vehicles (EVs) can be 300 miles or more per charge, the general consensus is that traveling long distances across the country will require fast charging—adding 200-300 miles of range in as little as 15 to 20 minutes. High power DC chargers that can pump 150 kilowatts (kW) or more into a lithium ion EV battery pack are now available, however how the battery cells react to high power and current levels can be problematic.
Commercially available lithium ion batteries use a porous graphite negative electrode (anode) and a metal oxide positive electrode (cathode). A liquid electrolyte allows passage of lithium ions between the two electrodes during charging and discharging. During charging, lithium ions are inserted (intercalated) between the carbon atoms of the graphite anode. During discharge the lithium ions are extracted from the graphite and intercalated into the structure of the cathode. At charge/discharge cycling rates of less than 1C (full discharge of the battery capacity in 1 hour), lithium ion batteries can undergo hundreds of cycles with very little degradation in capacity.
X-ray diffraction peaks collected during charge and discharge of the cell at a 1C rate. Panels (a) to (e) correspond to layers L0 to L4, respectively. The diffraction intensities have been normalized by the initial intensity of the graphite in each layer and shown on the same scale. The red color corresponds to charge and the blue color corresponds to the discharge of the cell. Different peaks in this plot correspond to different “stages” of graphite lithiation. (Image source: [K. P. C. Yao, J. S. Okasinski, K. Kalaga, I. A. Shkrob and D. P. Abraham, Energy Environ. Sci., 2019, Advance Article ,DOI: 10.1039/C8EE02373E] Published by The Royal Society of Chemistry.)
Degradation at High Charge Rates
If the battery pack is charged at a rate higher than 1C however, the capacity, cycle life, and thermal stability of lithium ion batteries degrades. Theoretical studies suggest that this degradation at high charge rates results from lithium plating at the surface of the anode, and a gradient of lithium ion concentration in the graphite electrode itself.
The gradient of lithium that forms in the anode is dependent, among other factors, on a complex interaction of lithium diffusion coefficients, the rate of intercalation, the thickness of the anode, and its porosity and tortuosity. Graphite has particularly high tortuosity, primarily because of its pancake-shaped flakes that form into layers. Moving lithium ions into out-of-alignment planes of graphite can make it difficult for significant intercalation to occur. In addition, the potential for lithium intercalation into the graphite is extremely close to that which caused lithium plating. If the lithium has a difficult time moving into the graphite anode, it may instead be deposited as a metal on the graphite particles, particularly near the anode surface.
The resulting gradient of lithium ions in the graphite electrode (highest concentration near the electrolyte surface, lower concentration moving through the thickness of the anode) also can cause a polarization off the cell, allowing the cutoff voltage to be reached before the lithium has been fully extracted from the cathode, and thus reducing the battery capacity. The gradient can also cause structural stresses within the graphite, causing fracture. It has been observed that the higher the charging current flowing through the cell, the steeper these lithium gradients can be within the graphite anode.
Hard to See
Although the concentration gradients of lithium ions in graphite anodes have been extensively modelled, there has been little direct experimental observation and quantification of the phenomenon. This was the goal of recent research at Argonne National Laboratory, where fast DC charging is under examination as a way to hasten EV acceptance. By using energy dispersive X-ray diffraction (EDXRD), ANL researchers were able to spatially resolve lithium concentration profiles during charging and discharging of a lithium ion cell at a rate of 1C. As explained to Design News by Argonne Senior Scientist Daniel Abraham, as lithium intercalates into the graphite crystal, it fills layers between the graphene sheets, yielding distinctive, ordered LixC6 phases. Previous attempts to visualize lithium ion gradients in situ in graphite electrodes used optical microscopy. The method could yield a qualitative insight in the succession of LixC6 phase that form, but has difficulty in separating the contribution from mixed phases, particularly when the composition varies across graphite particles.
According to Abraham, Argonne’s new method yields a qualitative insight into the succession of LixC6 phases. In the approach using X-rays, the concentration profiles of the LixC6 phases correspond to their bulk averages, integrating over the individual particles and quantifying the fraction of each ordered phase in these particles. Through this quantification it is possible to determine the exact phase composition of each probed layer in a solid matrix. Because only the ordered LixC6 phases contribute to the X-ray diffraction signal, lithium ions are not detected in the electrolyte and/or potential disordered solid compounds in the matrix. Using the EDXRD technique, the succession of LixC6 phases formed during charging and discharging has been studied with a spatial resolution of 20 μm and a time resolution of one minute.
The ANL work has been published in the journal Energy and Environmental Science. One unexpected observation that there is a strong asymmetry in the character of the lithium concentration between charging and discharging, which suggests that the complex character of phase transitions during the high-rate delithiation may involve less ordered phases, according to Argonne researchers.
This research has resulted in a useful tool that will not only help in the understanding of how lithium interacts with graphite anodes, but will also serve an important role in developing more effective battery components and materials. According to Argonne’s Abraham, “Our approach provides insights into the electrochemical processes in practical lithium-ion cells. The data can be used for the design of high-performance electrodes that behave more uniformly during operation and for the validation and improvement of cell models.”
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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