Post-mortem analysis of calendar aged large-format lithium-ion cells: Investigation of the solid electrolyte interphase

Although the growth of the solid electrolyte interphase is considered one of the most important degradation phenomena of lithium-ion cells, the mechanism is not yet fully understood. In this work, we present a detailed post-mortem analysis of calendar aged large-format graphite/Li(Ni1/3Mn1/3Co1/3)O2-based lithium-ion cells. X-ray photoelectron spectroscopy depth profiling reveals a distinct coherence of the growth of the solid electrolyte interphase with the phases of the lithiated graphite. Since the graphite phases are in direct correlation with the state of charge and the anode potential, the thickness of the SEI resulting from calendar aging is determined by the storage state of charge. The composition of the SEI has been analyzed as mainly organic near to the electrolyte and more inorganic towards the carbon active material. The same dependency as of the state of charge on the SEI thickness is found for the capacity retention and for the amount of irreversibly lost lithium. Additionally, gas is formed during the aging period and trapped in between the electrodes, leading to associated inhomogeneous lithium plating.