Investigation of Path-Dependent Degradation in Lithium-Ion Batteries

Models that predict battery lifetime require knowledge of the causes of degradation and operating conditions that accelerate it. Batteries experience two ageing modes: calendar ageing at rest and cyclic ageing during the passage of current. Existing empirical ageing models treat these as independent, but degradation may be sensitive to their order and periodicity - a phenomenon that has been called "path dependence". This experimental study of path dependence probes whether interactions between ageing conditions can impact a battery's state of health. Groups of graphite/NCA 18650 lithium-ion cells were exposed to load profiles consisting of similar proportions of calendar and cyclic ageing applied in various orders. Load profiles at higher C-rates exhibited path dependence, which differential voltage analysis correlates with increased anode degradation. These results suggest that more accurate ageing models should include the possible coupling between calendar and cyclic ageing modes.