Electron Microscopy Investigation of Graphite Electrodes from Fast-Charged Lithium-Ion Cells

Lithium-ion batteries (LIBs) capable of safe and reliable cycling at a fast-charge rate (i.e., >2C or <30 minutes to full charge) will have a direct impact on today's consumer technology by improving the utility of portable electronics and reducing the barrier for widespread adoption of electric vehicles. However, at high charge rates, the long-term battery performance suffers because of heterogeneous reactivity across both electrodes (e.g., lithium metal plating on the anode surface, fracture of the cathode particles, etc.) [1]–[3]. A fundamental study of the processes responsible for battery degradation is a necessary step toward designing next-generation LIBs and is a critical research priority for the U.S. Department of Energy. Here, we demonstrate that high charging currents lead to chemical, structural, and morphological changes in the graphite anode that spans the atomic-scale to the bulk-scale. As the integrity of the graphite host is critical to lithium-ion cell performance, our findings regarding the degradation mechanism provide the essential knowledge to enable the development of viable fast-charging LIBs.Our strategies to study the graphite anode degradation are based on advanced analytical electron microscopy and diffraction. We focus on the modifications that occur in the material when subjected to charging rates up to 6C (i.e., 10 minutes to full charge). The characterization is performed post-mortem across different length-scales on as-prepared electrodes and electrodes harvested from cycled cells (represented in Figure 1). At the bulk-scale, we use scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to probe the morphological and chemical changes in cross-sections of the graphite anode samples. At the nano- and atomic-scale, we use advanced transmission electron microscopy (TEM) and diffraction techniques to reveal changes in both the nano-scale chemistry and the graphite microstructure. Key findings from this investigation include the morphological changes to the graphite particles at the micron-scale and the partial-filling of the inter- and intra-particle gaps with solid electrolyte interphase (SEI) compounds at the meso-scale. Further, at the nano- to atomic-scale, we observe greater disorder and amorphous character in the graphite near the internal pore surfaces when compared to that of the bulk graphite regions. The nature of the lattice disorder, and the characterization methods employed to examine this disorder, will be discussed. Additional supporting evidences of graphite disorder and phase change from Raman spectroscopy and X-ray diffraction studies will also be presented.Acknowledgements: SP acknowledges support from the U.S. Department of Energy Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the U. S. Department of Energy under contract number DE‐SC0014664. This work was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. DA and MTFR acknowledge support from DOE's Vehicle Technologies Office. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.References:[1] C. Heubner, K. Nikolowski, S. Reuber, M. Schneider, M. Wolter, and A. Michaelis, "Recent Insights into Rate Performance Limitations of Li-ion Batteries," Batter. Supercaps, Nov. 2020.[2] M.-T. Fonseca Rodrigues et al., "Lithium Acetylide: A Spectroscopic Marker for Lithium Deposition During Fast Charging of Li-Ion Cells," ACS Appl. Energy Mater., vol. 2, no. 1, pp. 873–881, Jan. 2019.[3] Y. Yang et al., "Quantification of Heterogeneous Degradation in Li-Ion Batteries," Adv. Energy Mater., vol. 9, no. 25, p. 1900674, Jul. 2019.Figure 1