Thermal-electrochemical Coupled Modeling of a Lithium-ion Cell

A multi-dimensional thermal and electrochemical coupled model is developed for Li-ion batteries. The model is capable of predicting the cell internal temperature distribution as well as the average cell temperature evolution. Numerical simulations are performed for a large-size Li-ion cell for electric vehicle applications. Comparisons between the coupled and decoupled model predictions indicate the importance of thermalelectrochemical coupling for accurate prediction of the thermal and electrochemical behaviors of Li-ion batteries. Numerical results show that large temperature gradients develop along the cell height, causing severe non-uniformity in both electrode reaction rate and electrolyte concentration distributions. Evolutions of cell potential and temperature are found to be greatly affected by the thermal environment and the cell aspect ratio. INTRODUCTION Performance and safety are two primary considerations in the design of advanced lithium-ion batteries. The performance of a lithium-ion battery can be greatly influenced by the thermal environment, and its thermal behavior is in turn determined by the electrochemical and chemical processes occurring inside the cell during charge and discharge. Significant temperature gradients may develop when a lithium-ion cell is scaled up for electric vehicle (EV) and hybrid electric vehicle (HEV) applications and, thus current and temperature distributions become two-dimensional. Information on the temperature distribution is essential to capture the hot spot that may trigger thermal runaway thus leading to the failure of a battery. While experiments are necessary to study the underlying physical chemistry of materials used in lithium-ion batteries and evaluate the battery performance, mathematical models based on first principles are useful in providing a fundamental understanding of the internal transport phenomena and in aiding the optimization of battery design. Fuller et al. first developed an isothermal electrochemical model for a dual-insertion lithium-ion cell and the model predictions were compared to the experimental data with good agreement. Botte et al. recently extended it to a nonisothermal one with an exothermic side reaction taken into account. However, both models are virtually one-dimensional and assumed a uniform cell temperature. Most recently, Baker and Verbrugge performed a two-dimensional asymptotic analysis to a simplified lithium-ion cell system assuming secondary current distribution and linear kinetics. A general thermal-electrochemical coupled model has been developed for battery systems based on the previously developed micro-macroscopic modeling approach. The model is multi-dimensional and capable of predicting the temperature distribution inside a cell as well as the overall cell temperature evolution. The model has been applied to simultaneously simulate thermal and electrochemical behaviors of a Ni-MH cell. In the present work, a two-dimensional thermal-electrochemical study is performed by applying this generic model to the lithium-ion system. Solid-state species diffusion is taken into account. As a first step, no side reactions of chemical nature are considered for simplicity. Numerical simulations are performed for a large-size lithium-ion cell intended for EV and HEV applications. Importance of thermal-electrochemical coupling will be illustrated by comparing the coupled and decoupled model predictions. Finally, two-dimensional behaviors of the cell will be numerically explored. PHYSICAL MODEL System Description The lithium-ion cell of interest consists of the negative electrode current collector (Cu), negative electrode (LixC6), separator, positive electrode (LiyMn2O4), and the positive electrode current collector (Al), as schematically shown in Fig. 1. The electrolyte is a solution of lithium salt in a non-aqueous solvent. Electrochemical reactions occurring at the electrode/electrolyte interfaces are as follows: Composite positive electrode Liy-xMn2O4+xLi+xe      →        ← discharge