Electrochemical impedance spectroscopy testing on the Advanced Technology Development Program lithium-ion cells

The US DOE Advanced Technology Development (ATD) Program is investigating electrochemical impedance spectroscopy (EIS) as a new measure of cell degradation. As part of the program, the Idaho National Engineering and Environmental Laboratory (INEEL) is aging 18650-size cells using cycle-life tests developed under the Partnership for a New Generation of Vehicles, which has been superseded by FreedomCAR in January 2002. At the beginning of life and every four weeks thereafter, cycle-life testing is interrupted for reference performance tests (RPT) to assess the capacity and power fade rates. The EIS impedance is measured at 60% state-of-charge over a range of frequencies at each RPT. The resulting Nyquist plots show that the two semi-circles, representing the anode and cathode impedance growth, are poorly resolved due to the influence of the high frequency capacitive tail and the low-frequency Warburg impedance. However, impedance growth is clearly visible at the trough frequency of the second semi-circle. The magnitude at this frequency is comparable to the standard measure of cell degradation, which is based on the percent-fade in power as a function of test time while delivering 300 Wh. The percent growth in EIS magnitude at the trough frequency highly correlates with the power fade. This suggests that the EIS test is a useful alternate measure of cell degradation.