Revealing the Role of the Hierarchical Structure of Artificial Graphite on State of Charge Heterogeneity During Li-Ion Battery Fast Charge

Fast charging of lithium-ion batteries (LIBs) has been identified by the U.S. Department of Energy as a critical hurdle to the wider market adoption of electric vehicles. Charging in ten minutes or fewer without losses in battery safety, performance, or lifetime remains elusive due primarily to limitations at the negative electrode under these extreme non-equilibrium conditions. Graphite, used widely as the negative electrode material, suffers from degradation such as Li-plating during fast charging. Degradation is exacerbated by state of charge (SoC) heterogeneity resulting from simultaneous kinetic processes: electrolyte concentration polarization, reaction kinetics, and solid diffusion.

Significant advances have been made in electrode, electrolyte, and charging protocol design that have unlocked greater fast charging capabilities [1]. However, uncertainties remain, especially for artificial graphite (AG) which has gained increasing commercial use. The precise link between AG material properties and fast charging performance remains unknown due to the complexity of connecting micro- and nano- scale properties with system performance. We address this by coupling synchrotron operando X-ray diffraction (XRD), having high spatial and temporal resolution, with phase-field modeling to elucidate the relationship between hierarchical structure, kinetic processes, and SoC heterogeneity. Using these approaches, we explore the evolution of Li_xC₆ phases under increasing charging currents in a three-electrode pouch cell with a Li-metal counter electrode and uniform pressure applied. We observe complex dynamics enabling the existence of three phases simultaneously. By implementing a current interrupt at different ensemble levels of lithiation, we also investigate the relaxation dynamics of Li gradients. A novel implementation of a phase-field model explains the observed results by accounting for intra-particle Li transfer between graphitic domains, indicating an unexpected structural limitation unlike what is observed in natural graphite. These findings highlight the impact of graphitic carbon microstructure on battery performance and suggest tuning the internal structure of AG particles during synthesis as an additional approach for enabling fast charging.

[1] Li, Zhe, et al. A review of lithium deposition in lithium-ion and lithium metal secondary batteries. J. Power Sources, 254, 168-182 (2014).