The electrochemical and transport processes within a lithium-ion battery lead to a non-linear and highly dynamic current/voltage behavior . Understanding this behavior is a requirement for system design, operation control and state diagnosis. The combination of experiments and modeling allows a combined empirical and knowledge-based insight.
We present the experimental characterization as well as the development and parameterization of an equivalent circuit model of commercial 20 Ah lithium-ion pouch cells. The experimental characterization consisted of activation cycles, charge and discharge curves with different C-rates, internal resistance tests and electrochemical impedance spectroscopy (EIS). A total of 18 cells from the same production batch was investigated. Figure 1 shows the comparison of the discharge capacity of the cells, as well as an exemplary Nyquist plot.
With help of the experimental results we developed an equivalent circuit model, as shown in Figure 2. The model consists of an R(RC) series of circuit elements. Charge/discharge experiments at different C-rates as well as electrochemical impedance spectroscopy were used as basis for model parameterization. Open-circuit voltage is calculated as function of Li stoichiometry in the active materials rather than as function of global charge throughput. This allows a physically-based description of electrode resistances as function of stoichiometry. The necessary electrode balancing parameters are obtained from experimental quasi-OCV curves (C/20-rate) through mathematical optimization . This allowed to identify the chemistry as graphite at the anode and a blend of NMC and LMO at the cathode.
The parameterized model shows a good agreement with the original experimental data in the time domain under various operating conditions, including full cycles (cf. Figure 2), half cycles and dynamic load profiles. R. Xiong, “Battery management algorithm for electric vehicles”, 1st ed., Singapore, [Beijing]: Springer; China Machine Press, 2020.
 M. Mayur, M. C. Yagci, S. Carelli, P. Margulies, D. Velten, and W. G. Bessler, “Identification of stoichiometric and microstructural parameters of a lithium-ion cell with blend electrode”, Phys. Chem. Chem. Phys. 21, 23672-23684 (2019), DOI: 10.1039/c9cp04262h.