The effect of fluorine-containing SEI on aging at the anode–separator interface in lithium-ion batteries

Aging processes at the anode–separator contact surface (“interface”) critically affect the lifetime and safety of lithium-ion batteries. Nevertheless, the role of the important fluorine–containing anode passivation layers (“solid electrolyte interphases”, SEI) under application-oriented conditions remains insufficiently understood. In this study, industrial lithium-ion cells with graphite anodes and NMC 811-based cathodes were subjected to long-term cycling and high charging rates. X-ray computed tomography (CT) combined with post-mortem scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), as well as ion exchange chromatography, Fourier transform infrared spectroscopy (FTIR), and depth-resolved X-ray photoelectron spectroscopy (XPS), was employed to investigate SEI composition as a function of cycling temperature and cycle number. The results demonstrate that cycling temperature has a significantly stronger impact on cell aging than the number of completed cycles. Elevated temperatures promote formation of fluorine containing phases within the SEI, in particular LiF, accompanied by very likely dendrite-induced blocking of electrolyte transport pathways at the anode–separator interface. Furthermore, the fraction of irreversible capacity associated with fluorine–containing SEI-compounds (LiF and Li x PO y F z ) was quantified, revealing a temperature-dependent contribution to interfacial degradation. These findings provide mechanistic insight into temperature-driven SEI evolution and highlight the critical role of LiF in aging processes of industrial lithium-ion batteries. • Cycling temperature has a stronger impact on aging than cycle number. • Elevated temperatures promote formation of fluorine-containing SEI. • Fluorine-containing SEI accumulates at the anode–separator interface. • Fluorine-containing SEI formation correlates with cumulative irreversible capacity rise. • Interfacial fluorine-containing SEI contributes to charge transport limitation and impedance growth.