Determining the SoH of Lithium-Ion Batteries based on the Differential Impedance Analysis

Impedance spectroscopy is suitable for online battery diagnostics and state-of-health (SoH) estimation. Conventionally, impedance spectroscopy analysis is realized by modeling the impedance spectrum measurement with equivalent circuits. However, finding an appropriate global model can be time-consuming and challenging due to the overlap of the phenomena during the investigation. This paper proposes a novel approach for the SoH assessment based on the differential impedance analysis (DIA). The DIA extracts the charge transfer resistance and the double-layer capacitance from the impedance spectrum without relying on a global model. Furthermore, the time constant formed by the charge transfer resistance and double-layer capacitance is demonstrated to have the highest correlation for assessing the SoH of a battery. For the validation, a commercial lithium-ion battery cell of type Sony US18650VT3 is used and cycled 400 times. The results show a systematic correlation between the model parameters and the number of cycles, which forms the basis of a novel online method of SoH assessment. Moreover, the most suitable parameter to determine the SoH is the time constant τ at a frequency of 56 Hz with a coefficient of determination equal to 0.875.