Frequency domain non-linear characterization and analysis of lithium-ion battery electrodes

As an electrochemical system, the current voltage relationship of a lithium-ion battery is non-linear. The accuracy of conventional models for lithium-ion cells always suffer from non-linear dynamic behavior, especially at low SoC levels and high current levels, which limits practical applications of battery models in reality. In-depth understanding and characterization of the non-linear relationship can provide valuable insight to improve the accuracy of battery modeling. In this paper, a frequency domain non-linearity characterization approach using odd random-phase multisines signals is performed on a three-electrode experimental set-up of a commercial 5Ah cylindrical 21700 cell. This allowed the distortions towards the full-cell voltage to be separated into the corresponding electrodes and identify the nature of non-linearity as odd or even order. The results demonstrate that the even order non-linearity from the cathode is the main contributor towards the full-cell voltage of the lithium-ion battery while the non-linearity from the anode starts to dominate at very low (∼2%) state-of-charge.