The NCA/graphite cylindrical cells were calendar aged and cycle-aged between SOC 20-50 %, 35-65 %, 60-90 % and 20-95 % until end of test (EOT) is reached (e.g. 1000 cycles). The impedance behavior of the calendar and cycle aged cells were measured. Each electrode is carefully harvested once reached the EOT and electrochemically characterized using symmetric cells by means of EIS.
A physics-based impedance model is employed to fit the experimental EIS of symmetrical cells using a least square fitting tool in COMSOL® Multiphysics 5.4. It is based on the pseudo two-dimensional (P2D) model [2] incorporating intercalation kinetics, mass and charge transport based on concentrated solution porous electrode theory. The model additionally accounts for double layer effect and aging mechanisms. Using the model, useful aging parameters were parameterized, and the variation of the aging parameters with different aging history was validated with the support of scattered electron microscopy (SEM) technique. Among the aging mechanisms described, an increase in the local contact resistance between the active material and the conductive carbon, and decrease in particle size due to particle cracking and further film formation due to SEI are considered. The impedance model parameters from both electrodes are then linked to the aging process observed on experimental full cell.
Figure 1 shows the fitting and experimental curves for graphite Nyquist plot, in which we observe a good fitting specifically at low frequencies. Although the model predicts a second semicircle in the middle requency range, the experimental curve also shows a time constant element which can be considered as a truncated semicircle in the same region.
[1] T. G. Zavalis, M. Klett, M. H. Kjell, M. Behm, R. W. Lindström, G. Lindbergh, Electrochemica Acta, 110 (2013) 335-348.
[2] J. Newman and W. Tiedemann, AIChE J., 21, 25–41 (1975).