Complex and Nonlinear Resistance Changes of NMC/Gr cells during ageing

Li-ion battery cell degradation has a strong impact on vehicle performance through performance degradation and deviation from original control calibration. Ageing leads to complex changes in resistance, arising from various contributions. This results in non-uniform resistance changes with temperature and state-of-charge (SoC) as well as altered time-based dynamic responses to current application. This study examines in detail the complex resistance changes with ageing and the dependencies on temperature, SoC and time. The paper evaluates data from a 9-month NMC/Gr cell aging study at 45°C, using both frequency and time-based methods. Electrochemical impedance spectroscopy with Distribution of Relaxation Times is employed for frequency analysis, while time domain data is extracted from monthly Hybrid Pulse Power Characterization tests using model fitting approaches. The combination of the frequency and time based data allows identification of the individual resistance features that change with ageing (e.g. ohmic, charge transfer and diffusion resistances). Frequency analysis is conducted at various voltages and temperatures to illustrate relative resistance changes, while time domain data includes multiple SoC points to depict monthly resistance trends with SoC. The results indicates several significant results. Resistance decreases substantially in early life followed by a slow subsequent increase. At low temperatures, resistance stays significantly lower even at end-of-life relative to beginning-of-life, while at high temperatures the opposite is observed. This results from a combination of charge transfer decrease and ohmic increase. Time constants of all resistances also significantly increase with aging, altering dynamic response. These findings suggest that simple metrics like a single number state-of-power are not sufficient for measuring aging, and a more sophisticated approach incorporating modeling and efficiency optimization is needed.