Characterization methods and modelling for Li-ion batteries: entropy, impedance, pressure, diffusivity & temperature swings induced aging

Doctoral thesis, 2022

Lithium-ion batteries are commonly used for energy storage systems, and temperature is one key impact factor on the cell performance and lifetime. In literature, the focus has been on the ambient temperature of the test condition, and there is little discussion about how the temperature swing during cycling affects the battery lifetime. In this study, lithium-ion cells are cycled with the same current but different temperature swings for more than two years. The results show that the cells cycled with a high temperature swing aged faster. Moreover, pure thermal cycling does not introduce significant aging to the cell.

During the study, a series of characterization methods were also developed, including a physics-based circuit model, a convenient method to measure the entropic coefficient, an on-line impedance measurement technique, an effective reference performance test procedure, and methods to measure the cell
pressure and thickness change during cycling. A selection of results are that the physics-based circuit model could predict key quantities, such as overpotential, concentrations etc., with less than 0.05% deviation compared with a state-of-art model. Furthermore, the on-line impedance measurement technique managed to extract the battery pack impedance between 0.01 Hz and 5 Hz based on CAN signals. In addition, the cell pressure and thickness change during cycling of a commercial cell were found to be up to 60 kPa and 150 μm.