Modeling, Validation and Analysis of Degradation Processes of Lithium Ion Polymer Batteries
Type of Degreedissertation
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The lithium ion polymer battery (LiPB) has become one of the most widely used energy storage devices because of its high energy and power densities. However, its performance and life span are limited by the degradation processes, such as capacity and power fade, under various operating conditions. In order to study the degradation mechanisms, an accelerated test consisting of large cycling currents was conducted on LiPB. According to the analyses of impedance spectra, morphologies and compositions of the cycled cells, the results have shown that the degradation is predominantly caused by the side reactions at the anode. The side reactions consume lithium ions and produce deposits that increase the thickness of the solid electrolyte interphase (SEI) and deposit layers. By fitting a semi-empirical degradation model to the experimental data obtained from different numbers of cycles, the changes of the internal parameters are extracted and used to describe the degradation processes caused by the side reactions. In order to better understand the mechanism of the side reactions and predict the degradation of LiPB under various operating conditions, the degradation processes are described using physical principles based on Butler-Volmer and Nernst equations that are integrated into the electrochemical-thermal model. The key parameter for the side reactions used in the model is experimentally determined from the self-discharging behavior of the battery. The model is used to analyze the effects of the loss of ions and active materials on capacity fade. The integrated model is then validated against experimental data obtained from testing the battery under different state of charge (SOC) cycling limits and charging C-rates (1C = 15.7A) and used to study the effects of the operating conditions on degradation mechanisms. In addition to the side reactions, another important degradation process reported in the literature is crack formation and fracture of the anode and cathode particles due to mechanical stresses. A stress model is developed to describe the mechanical stresses caused by a non-uniform distribution of lithium ions and inhomogeneous localized volume changes inside an electrode particle. The stress model is then incorporated into the electrochemical-thermal model, which has been validated by studying the volume change of a cell while charging and discharging. The simulation results show that the electrode particles are under cyclic stress when the cell is being cycled, which may cause crack and fracture.