Modeling, simulation and experimental investigation of the thermal and electrochemical behavior of a LiFePO4-based lithium-ion battery

Kurzfassung

In this thesis the modeling and simulation of a LiFePO4-based lithium-ion battery was presented. For a better understanding of the behavior of the cell, a model was developed which allows a detailed insight into occurring processes during operation. Modeling framework. A multi-scale approach was used to describe physico-chemical processes occurring on different time and length scales. It includes transport processes within active materials and in a liquid electrolyte. Electron-transfer processes on particle surfaces were considered which were coupled with incorporation of lithium ions into the active materials. Structural information of a repeat unit was resolved. This enables the prediction of temperature variation within the whole battery across the windings during operation. Besides kinetic effects, the model includes thermodynamic information about the active materials of both electrodes. It was separated into an enthalpy and entropy contribution as a function of lithium concentration within the active material. Furthermore, a complex intercalation mechanism was proposed for the cathode material. It describes the phase transition between LiFePO4 and FePO4. The results were separated into two parts: the macro-model is based on global kinetics and describes the electrochemical and the thermal behavior of a complete battery. The micro-model focuses on the influence of the phase transition on the electrochemical performance. Experimental work. For model parameterization and validation, experiments were performed using a LiFePO4-based lithium-ion battery of the company A123 systems with a nominal capacity of 2.3 Ah. The focus of investigation was the electrochemical performance (charge curves, discharge curves, electrochemical impedance spectra) and the heat production and temperature variation under various conditions (current and ambient temperatures). Conclusion. The main focus of this work was to describe and understand the electrochemical performance and the temperature behavior of a LiFePO4-based lithium-ion battery under different ambient temperatures and applied currents. The developed macro-model was shown to be sufficient for reproducing discharge and charge curves and thermal behavior of the battery very well, while impedance spectra could not be fully reproduced. The developed micro-model contains additional detailed information on the elementary kinetic level regarding intercalation reactions and phase transition. Based on this model, it could be shown that the interface of the LiFePO4−FePO4 system acts as buffer for lithium ions. Compared to previous publications, for the first time this work shows the contributions of thermodynamics and kinetics at different ambient temperatures and current requirements. For the first time discharge / charge curves and impedance spectra of a full LiFePO4-battery are shown comparatively and interpreted on the base of thermodynamic and kinetic effects of the elementary kinetic micro-model and of the global kinetic macro-model. Outlook. This work is a base for further investigations. A coupling of both models over all scales is strongly recommended. This would enable the prediction of temperature distribution and electrochemical performance based on elementary kinetics. The current model with its elementary reactions forms a base for including further side reactions, such as SEI formation or lithium plating. This will lead to a better understanding of aging and degradation behavior of a battery cell.