Electrochemical-thermal analysis of large-sized lithium-ion batteries: Influence of cell thickness and cooling strategy in charging

Abstract

With the transition to electric propulsion accelerating, lithium-ion batteries (LIBs) are essential due to their high energy density and efficiency. However, higher energy requirements and rapid charging pose significant safety and performance challenges, particularly for large cells. This research uniquely investigates the thermal and electrochemical responses of large-sized prismatic LiFePO4 cells under various cell thicknesses (ranging from 11.3 mm to 90 mm), charging conditions (ranging from 0.3C to 1C), and cooling strategies. The model reveals that higher charging rates markedly increase temperature and variability within the cells. Charging a 45 mm-thick cell at a 1C rate and with natural air cooling results in a temperature rise of 24.3K and a deviation of 6.18K. In contrast, charging at 0.3C results in 5.4K and 1.17K, respectively. Despite the smaller heat exchange surface, bottom-cooling is most efficient for thick cells due to significant anisotropic heat conduction. Single-side cooling becomes more effective for thinner cells. A critical cell thickness for such transition exists at around 22.5 mm. Double-side cooling consistently outperforms single-side cooling, significantly reducing temperature deviations across various cell thicknesses. Best cooling performance is seen in the case of the thinnest cell (11.3 mm) with double-side cooling, with only 3K temperature rise and 1K deviation, at 1C charging. Additionally, improved thermal management during charging slightly increases heat generation of the cell, due to the higher resistance at lower temperatures, elevating ohmic heat generation from separators and electrodes. Polarization heat from the negative electrode is identified as the dominant heat source, driven by increased overpotential at higher temperatures due to elevated Li ion concentration at the electrode surface. The findings provide crucial insights into the thermal and electrochemical behaviors of large-sized LIBs, offering valuable guidance for the design and management of these LIB systems to enhance safety, performance, and longevity in electric vehicle applications, addressing a pressing need in the field.