Diagnosis of Li-ion battery degradation based on resistive and diffusion-related transient voltage changes at early stage of discharge

Lithium-ion batteries degradation can be classified into capacity reduction and increase of the internal resistance. Accurate estimation of the degradation is important to determine the economic value and the remaining useful life of the battery. This paper proposes a method which analyzes the transient voltage response at the early stage of discharge, starting from the fully charged state. An equivalent circuit composed by a series resistance and a Warburg element is adopted to fit the transient voltage. The Warburg element is modeled as a transmission line composed by a ladder of resistances and capacitances. The method is validated with data obtained before and after aging cycles performed at 1.4 C charge and 2.0 C discharge rates, at 25 degrees C, 10 degrees C and -5 degrees C. The loss of cyclable lithium is deemed to be the main reason behind the battery degradation at all the tested temperatures. The dependency of the Warburg parameter to the remaining cell capacity is the same regardless of the aging temperature, suggesting that estimation of the Warburg parameter provides information about the state of health of the cell. On the other hand, the relationship between the capacity reduction and the increase of resistance has a temperature-dependent slope with low increase at lower temperatures where lithium plating is likely to occur. Accordingly, a relatively low resistance increase is deemed to be the sign of lithium plating.