Impact of concurrent vibration and charge cycling on the performance and aging of 21700 NMC811 lithium-ion cells

Electric vehicle (EV) battery packs contain numerous lithium-ion cells which are subjected to various simultaneous stresses, including charge cycling, temperature variations, vibration from road inputs and the powertrain, and calendar ageing. Lithium-ion battery degradation mechanisms are well understood but there is a gap in knowledge considering vibration and the effect of the simultaneous stress combinations upon cell performance. Failure to understand this could result in poor EV life, performance or range. This study considers the simultaneous effects of charge cycling, vibration and calendar ageing through experimental work upon commercial 21700 NMC lithium-ion cells. These cells are subjected to charge cycling at high C-rates of 0.7C charge and 1.0C discharge and vibration to the automotive SAE J2380 standard, representing vehicle life. Differential Voltage Analysis (DVA), Electrochemical Impedance Spectroscopy (EIS), Equivalent Circuit Modelling (ECM) and voltage profile fitting techniques are used to provide a robust approach to understanding changes in cell performance and health. Three key findings emerge from this research: Firstly, the vibration representing three vehicle lifetimes has no detrimental effect on cell performance. Secondly, the cells that are charge cycled exhibit reduced State of Health (SOH) due to charge cycling induced degradation. Thirdly, the cells that are simultaneously charge cycled and vibrated exhibit a slightly smaller reduction in SOH (up to 0.33%) compared with charge cycled cells. This suggests that EV cells are tolerant of real-world EV use and that the presence of vibration during operation may be associated with slightly improved capacity retention, although further investigation is required.