Thermal Design Methodology For Traction Lithium-ion Batteries

Lithium-ion batteries have been extensively used as traction power sources due to their advantages such as higher energy density and longer cycle life, over other types of secondary batteries. However, thermal problems associated with the traction lithium-ion batteries, including non-uniform temperature distribution and internal hot spots/zones, adversely affect the performance, durability and safety of the batteries used in electric vehicles. As one method to address these thermal problems, thermal design of the single cell is important to improve the thermal performance of batteries, and to reduce the complexity and cost of battery thermal management system.Focusing on the thermal issues at single cell level(rather than battery module or pack) under normal condition(rather than thermal runaway), this thesis conducts systematic research on thermal parameter estimation, thermal modeling and thermal design of traction lithium-ion batteries. The estimated thermal parameters are critical inputs for thermal modeling, and thermal design is based on the validated thermal model.The methods of battery thermal parameter estimation can be divided into two categories, the time-domain method and the frequency-domain method. In the time-domain method, the battery is heated with an electric heater and the temperature responses are recorded with attached thermocouples at strategically-chosen locations. The heat conduction is simulated using a time-domain thermal model containing thermal parameters. Using optimization techniques, these thermal parameters are adjusted step by step till the difference between the simulated and the experimental temperature responses at the corresponding locations reaches a minimum. This method permits the simultaneous and in-situ estimation of multiple thermal parameters, including the specific heat capacity and anisotropic thermal conductivities. In the frequency-domain method, the electric heater heats the battery using sine power at different frequencies, while the phase angles of temperature at strategically-chosen locations are measured. The anisotropic thermal conductivities can be estimated through fitting the Bode plot of phase angle to experiment results.With the estimated thermal parameters as reliable inputs, a multi-dimensional thermo-electrical model for laminated lithium-ion batteries is developed. This model is validated using the measured temperatures at multiple locations on the battery surface. Simulation results reveal that the heat flux between the tab and the battery core significantly affects the temperature distribution of the battery. Based on several assumptions justified from the simulation results, an analytical solution to the planar temperature distribution of the core is deduced using the integral transform technique. The analytical solution agrees well with the simulation results. Finally, another thermo-electrical model is proposed for a spirally wound battery to investigate the thermal characteristics of battery with different configurations.Using the analytical and numerical solutions to the thermal model, the design of laminated lithium-ion batteries is optimized to minimize the temperature rise while achieving uniform thermal distribution. The analytical solution is used to investigate the influence of the core aspect ratio and the tab displacement on battery thermal performance. Furthermore, a systematic thermal design method is proposed based on the coupling of numerical solution and optimization algorithium. This method is applied to optimize the dimensions of laminated lithium-ion batteries. A method to determine the maximum capacity of large-format batteries is proposed based on the Pareto front in terms of thermal performance. It should be noted that the thermal design method is not only applicable to the laminated traction batteries, but also for the batteries of spirally and prismatically wound configurations or for batteries for energy storage usage.