SOC-Dependency of the Time Constants and Polarizations of Li-ion Batteries

This paper presents an analysis of the time constants of Li-ion batteries. Our experiments were conducted on batteries with positive electrode material of NMC811. The time constants were determined using a distribution function of relaxation times (DRT) from the impedance spectra. Batteries’ internal processes can be characterized using their time constants and associated polarizations. Therefore, they allow for the evaluation of Li-ion batteries for safety and performance, as well as other energy storage technologies. In this study, we investigated the effect of temperature and State of Charge (SOC) on the time constants and polarizations of eight cylindrical cells. After the initial cycling of the cells, their EIS (Electrochemical Impedance Spectroscopy) data were collected at different temperatures from -20 to +60° C and SOCs ranging from 100% to 0%. The EIS data were processed to determine the time constants. We identified four dominant peaks in the medium to low-frequency range, assigned to contact resistance, Solid Electrolyte Interphase (SEI), charge transfer (CT), and diffusion. Additionally, two dominant peaks were observed in the high-frequency range. Next, we studied the SOC-dependency of the polarizations and representative time constants of the processes. The representative time constants were defined as the local maxima. The charge transfer kinetics (CT) and diffusion processes showed strong SOC dependencies. For example, the time constant of CT dropped from 1s at 0% SOC to 7ms at 50% SOC and increased to about 65ms at 100% SOC. During this cycle, its polarization changed from 27 mΩ to 0.3 mΩ and 4.9 mΩ, respectively. In contrast, the time constant and polarization of the high-frequency processes showed very small variations with SOC levels.