Temperature-Dependent Time Constants of Li-ion Batteries

We investigate the effect of temperature on the time constants of Li-ion batteries (LIBs). Using the distribution of relaxation times (DRT), the time constants of three cylindrical Li-ion cells were determined. EIS (Electrochemical Impedance Spectroscopy) was conducted on the cells, and the measured impedance spectra were analyzed using DRT. The DRT analysis is usually formulated as a Ridge Regression optimization problem. While the regression tuning parameter has a significant impact on the results, the studies on selecting this parameter are very limited. This letter proposes novel cost functions to select the optimal regressions parameters. The cost functions include (i) Discrepancy, (ii) Cross-Discrepancy, and (iii) the Sum of Squared Errors. The first two criteria exploit the Kramers-Kronig relations, and they quantify the discrepancy of the reconstructed impedance spectra using only its real, imaginary, or both components. The last criterion quantifies the errors in real and imaginary components of the data from the reconstructed EIS. The method was applied to the impedance spectra of Li-ion cells at low and high temperatures and different state of charges (SOCs). We identified the time constants of the cells using the proposed criteria for different test conditions.