Modeling Voltage Decay During Calendar-Life Aging

Degradation in lithium-ion cells results from internal mechanisms whose effects can be partially observed through input/output data. In particular, we notice that when a cell experiences calendar aging, its voltage decays over time along with its capacity/power. This work aims to establish a mathematical model to describe these observations. Since calendar-life aging involves both reversible and irreversible aspects, we combine a simple solid-electrolyte-interface (SEI) growth model to describe irreversible lithium loss and a redox-shuttle model to describe reversible loss. We also propose a reduced-order model framework for fast computations. The models are parameterized using six-month self-discharge data gathered at different ambient temperatures from a commercial cell. We find that while SEI growth leads to permanent capacity loss and some voltage decrease, the redox-shuttle can dominate voltage decay during self-discharge. Therefore, it is important to include a redox-shuttle component in a model of voltage decay during calendar-life aging.