Unravelling Fast-Charging Degradation in NMC/Gr Pouch Cells: Lithium Plating and SEI Properties

With the increasing adoption of fast charging technology in the electric vehicle market, understanding its impact on battery performance and longevity is critical. In this study, 1.8 Ah LiNi0.6Mn 0.2Co 0.2O2/graphite pouch cells were charged at various charging rates (0.5C, 2C, 4C, and 6C) to investigate the degradation mechanisms. Our results showed that well-designed NMC/Gr pouch cells could achieve over 1000 cycles with 2C charging rate, while only around 500 with 4C and 6C charging rates. Fast charging effects on NMC and graphite electrodes were obtained through a series of post-mortem characterizations, including electrochemical impedance spectroscopy (EIS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Though higher charging rates cause pulverization of NMC secondary particles, the primary underlying mechanism driving the fast-charging related performance fading lies in the graphite anode, where lithium plating and LiF-rich SEI formation result in Li inventory loss and impedance growth. The postmortem results suggest that the formation of a LiF-rich SEI, which exacerbates anode impedance and some irreversible Li+ ion loss, is likely driven by the substantial decomposition of PF6- during fast charging, an effect often overlooked in smaller laboratory-scale studies.