Investigation of path dependence in commercial lithium-ion cells for pure electric bus applications: Aging mechanism identification

There is a growing need to provide more realistic and accurate State of Health estimations for batteries in electric vehicles. Thus, it is necessary to research various lithium-ion cell aging processes, including cell degradation and related path dependence. This paper focuses on quantitative analyses of cell aging path dependence in a repeatable laboratory setting, considering the influence of duty cycles, depth of discharge (DOD), and the frequency and severity of the thermal cycle, as reflected in pure electric buses operated in Beijing. Incremental capacity analysis (ICA) and differential voltage analysis (DVA) are applied to infer cell degradation mechanisms and quantify the attributions to capacity fade. It was observed that the cells experienced a higher rate of aging at 80% DOD and an accelerated aging at 40 °C in the thermal cycling, as a result of possible loss of active material (LAM) in both electrodes, in addition to the loss of lithium inventory (LLI) and inhibited kinetics. The slight capacity fade from low-temperature extremes likely caused by LLI due to lithium plating, whereas the noticeable fade after the high-temperature excursion was likely caused by LAM and hindrance to kinetics. These results may lead to improved battery management in EV applications.