(Invited) Quantificationof Heterogeneous, Irreversible Lithium Plating in Extreme Fastcharging of Li-Ion Batteries

There is an ever-increasing demand for rechargeable batteries for electric vehicles, which primarily use lithium-ion batteries (LIB). One of the major proposed improvements in using LIBs for vehicles is in reducing the long recharging times (order of hours), to make it comparable to the 5-10 minute refueling times in gasoline-powered vehicles. Therefore, extreme fast charging (XFC) has been defined for LIBs, with a target to bring the charging time down to 15 minutes or less [1]. However, XFC of LIB is associated with several problems, the most prominent of which are a large loss of battery capacity over cycling and safety issues [2]. Therefore, an understanding of the how local, irreversibly plated lithium affects the local SOC of the cathode and anode, as well as tying the contributions from individual loss mechanisms quantitatively over the cell to the overall capacity fade of the cell during fast charging is necessary to design the battery for better safety and consistent performance. Towards addressing this issue, high energy X-ray diffraction (XRD) is employed, which helps build on the existing understanding of lithium plating in two ways. Firstly, XRD provides a way to quantify the amount of Li plating, as well as other loss mechanisms, and tie them to the global cell performance after XFC cycling. Second, XRD is an in-situ technique, allowing to characterize the entire battery in the fully assembled condition. Thus, local heterogeneities in the cathode and anode can be studied and correlated to heterogeneities in Li plating. In this work, sub-mm-scale XRD is used to quantify Li plating across different single layer pouch cells (3 mAh/cm2 specic capacity, with graphite anode and NMC cathode), where the charging rate and protocol are systematically varied (4C to 9C). The cells are cycled through hundreds of XFC cycles and studied in the discharged state. At the local level, the characteristics of plated lithium crystallites such as the preferred crystallographic orientations and size of plated Li on graphite are studied. Additionally, the regions with local lithium plating are correlated with the local SOC (lithium occupancy) and loss of active surface area in the cathode and anode, in the discharged state. Finally, the capacity fade of the cycled cells is correlated to the amount of dead Li, with separated contributions from irreversibly plated Li, Li trapped in graphite as LixC (which cannot be reversibly extracted from the anode) and reaction of plated Li with the electrolyte. Based on this knowledge of the properties of lithium plating and the conditions that favor it, as well as it's effect on overall battery performance, new approaches towards designing batteries can be realized, such that irreversible Li plating is minimized. This step will in turn help to guide the rational design of the next generation of XFC capable LIBs with a consistent and safe performance. References [1] T. R. Tanim, E. J. Dufek, M. Evans, C. Dickerson, A. N. Jansen, B. J. Polzin, A. R. Dunlop, S. E. Trask, R. Jackman, I. Bloom, et al. Extreme fast charge challenges for lithium-ion battery: Variability and positive electrode issues. Journal of The Electrochemical Society, 166(10):A1926{A1938, 2019. [2] A. Tomaszewska, Z. Chu, X. Feng, S. O'Kane, X. Liu, J. Chen, C. Ji, E. Endler, R. Li, L. Liu, et al. Lithium-ion battery fast charging: A review. eTransportation, 1:100011, 2019.