In-depth analysis of the SEI thickness growth over the lifetime of Li-ion cells from an automotive battery

Developing a detailed understanding of the ageing evolution in automotive lithium-ion cells is key to batteries with long service life. In this study, the SEI composition and growth rate of graphite negative electrodes from cycle-aged large-format automotive lithium-ion cells is investigated as important parameters in physics-based ageing models. In this work, automotive cells were cycle-aged in a duty cycle protocol with intermittent reference parameter test. The SEI layers formed at different state-of-health were characterized by X-ray photoelectron spectroscopy sputter depth profiling and electron microscopy on electrode cross-sections. The SEI growth over time showed a linear relationship. However, the growth rates derived from the two techniques differed considerably and major differences in SEI thicknesses were found between edge and center regions of electrode sheets. In an attempt to facilitate access to relevant experimentally derived model parameters, a lab-scale setup build from Li-ion cell components from an automotive battery was developed that could reproduce the SEI evolution and growth behavior.