In recent decades, promising technological advancements in clean energy production and green transportation, driven by the depletion of oil resources, energy security issues, environmental challenges, and associated health concerns, have moved the world closer to a sustainable energy outlook. However, this approach has led to a high reliance on electricity and the need for optimized electricity storage systems. Among different options, lithium battery systems have gained considerable attention, particularly for electric vehicles, owing to their superior properties, such as high energy density, fast charging capacity, and long lifespan, making them compatible with both stationary and mobile applications. Nevertheless, the safe and long-term operation of lithium battery systems depends on their working temperature, as the aging process of batteries accelerates with the temperature rise, and at critical temperatures, the exothermic reactions within battery cells might lead to thermal runaway and explosion. This necessitates employing a suitable strategy to regulate the temperature within lithium batteries, which could be quite challenging due to design restrictions related to geometry, coolant selection, cost, weight, and battery system size, especially at fast charging/discharging rates. Hence, it is essential to initially have a thorough understanding of how the system operates under different working conditions and then employ an effective strategy to improve its performance. 

In this framework, the present thesis first investigates the thermal behavior of a single cylindrical battery cell with varying geometrical parameters for the jelly roll. The study is based on a mathematical model predicting the temperature field within the cell to identify design considerations at the cell level to minimize thermal issues for the battery thermal management system. The best balance between thermal concerns and capacity was found for 21700 cylindrical cells, wherein the optimum thicknesses for the positive active material, the negative active material, the positive current collector, and the negative current collector were 180, 34, 21, and 20 μm, respectively. The thesis then shifts focus to the module-level study, evaluating the performance of air-based battery thermal management systems that meet many design criteria for battery applications but present challenges due to the low thermal conductivity of air as a coolant medium. The thermofluid characteristics of the air-based cooling system under discussion were investigated using computational fluid dynamics (CFD) simulations and compared to free cross-flow heat exchangers. The study suggests the  k-kl-ω transition model as a computationally efficient and fairly accurate turbulence model for such heat exchangers. Moreover, it was determined that under certain conditions, two-dimensional models of free cross-flow heat exchangers could replace computationally demanding three-dimensional models for wall-bounded cross-flow heat exchangers intended for battery cooling. These findings serve as a basis for developing a novel approach for modeling the performance of large air-cooled battery systems, termed the simplified modeling approach.

The simplified modeling approach consists of three sub-models, including a CFD model to simulate heat and flow characteristics around a cell in a periodic flow region, a set of approximate equations to determine the heat transfer rate for each row along the battery module, and an analytical model to predict the temperature field within individual cells. The employment of these sub-models, along with their independent functioning, significantly reduces computing costs. This model was employed to investigate cell spacing within an air-cooled battery module. At a constant mass flow rate to the system, the study suggests that maintaining transverse and longitudinal center-to-center distances of 1.7D and 0.9D between the cells, respectively, results in a fair balance between the maximum temperature rise and temperature gradient within the module. Following this, the model was combined with an empirical capacity degradation model to study how cell spacing and cooling conditions affect the number of cycles a battery module can operate. According to the study, proper cell spacing may extend the lifetime of the battery module by up to 55%. However, this life cycle extension comes at the cost of greater power consumption, which significantly raises cyclical costs, especially in densely packed battery modules. To address this issue, splitter plates were integrated into the design of densely packed battery modules. It was observed that splitter plates with lengths comparable to wake size could mitigate the maximum temperature rise and capacity degradation process within the batteries without causing extra cyclical costs.