Numerical Analysis of performance and life of lithium ion battery systems using physics based models

Abstract

Due to the improvement over the last 20 years, lithium-ion batteries start to be used in hybrid electric vehicles, electric vehicle (EV), and energy storage system (ESS) that require high power and high energy. In general, since the systems in EV and ESS require long life and safety for more than 10 years, it is necessary to optimize the battery systems to satisfy the demands. In the design process of the battery systems, experimental processes are essential to evaluate performance and life of the cells and the battery pack systems. But empirical test needs high cost for considering numerous design parameters. Also, experimental tests for life cycle take a long time, which is not affordable for the commercial products. Therefore, there is a high demand for approaches using numerical models to predict the performance and life of the battery cells and the pack systems.<br/> This dissertation proposed physics based models of lithium ion battery cell and pack systems and analyzes the performance and life behaviors in various operating conditions. The battery model is developed based on the multi-scale multi-dimensional (MSMD) model framework and calculates electrochemical reactions, lithium diffusion and charge conservation in electrode and electrolyte, and capacity fade mechanism due to SEI layer growth in coupled manners. The model can solve distributions of temperature and electrical potentials that occur over the cell volume. <br/> By using the 3D cell model based on MSMD model framework, the degradation rates of a small cell and a large cell, having the same electrode design, is compared to present effects of cell size on degradation. It is identified that degradation is accelerated in large cells due to high temperature from heat generated in current collectors and a long charge time. In addition, the effects of cell formats on performance and life of LIB cells is investigated. The 3D cell models for a pouch cell, a cylindrical cell, and a prismatic cell are developed and simulated for cycle operations. The model results show that the cylindrical cell with discrete tabs which has relatively long electric paths in metal current collectors shows higher electrical resistance than the pouch cell and the prismatic cell and degrades faster than the others. But when these cells have continuous tabs, it is observed that degradation rates of all cells are similar, implying that the effects of cell formats and tab designs on degradation are significant.<br/> We developed a resistance added lumped (RAL) cell model that can carry accurate and fast prediction of behaviors of large format LIB cells compared to the full 3D cell models. The RAL cell model is a cell domain sub-model in the MSMD model framework, and implements the electrical resistance and thermal resistance of cells, which is determined by the cell design. The RAL cell model is verified by comparison to the 3D cell model and the lumped cell model and it is shown that the RAL cell model can predict the behavior of the large format LIB cells more accurately than the lumped cell model and also the calculation time is significantly faster than the 3D cell model.<br/> Finally, a battery pack model is developed for the life analysis of the battery pack systems which have multiple cells. To consider phenomena in the battery pack system, the MSMD model framework is extended to the pack domain, which calculates the electrical networks between cells and the heat transfer network including convective cooling flows. The pack system model adopts the RAL cell model as a cell domain sub-model to model two battery packs with 32 cells in 4P8S configuration in different connection structures. The cycle simulations for the both battery packs are carried out to investigate responses of the battery packs with a different electric connection in imbalanced situations. <br/> <br/> Keywords : Lithium ion battery (LIB)