Liquid Immersion Thermal Management of Lithium-ion Batteries

Abstract

The imperative requirement for the transportation sector to reduce its greenhouse gas emissions has placed greater importance on the more widespread adoption of electric vehicles. The extension of their driving range and faster charging times have been identified as key criteria in order to achieve this aim, necessitating battery packs of increased energy and power density. This will place greater emphasis on the battery thermal management system, as the lithium-ion cells which form these battery packs are highly temperature-sensitive, having a narrow optimum operating temperature range to maximise performance and minimise degradation. While a number of methods of battery thermal management have been both proposed and implemented, many are unable to meet the requirements of maintaining the cells within this temperature range while limiting uneven degradation that arises from cell-to-cell thermal inhomogeneity. Liquid immersion cooling has been identified as a promising method of thermal management, in which increased heat transfer rates can be achieved by the direct contact of a dielectric fluid with the cells. However, limited studies have been conducted on this method, particularly on the application of two-phase conditions which offer even greater rates of heat transfer through the phase change process. This study investigates the immersion cooling of lithium-ion cells in a dielectric fluid under both natural convection and preheated liquid immersion conditions by experimental and numerical means. Individual cells and module arrangements are subjected to charging and discharging processes to examine their spatial and temporal thermal profiles, as well as their electrical performance. The required temperature limits are maintained for individual cells under both immersion conditions due to the high rates of heat transfer from their surface. Greater thermal performance is observed under the preheated conditions when phase change is established, providing increased heat transfer through disturbance of the thermal boundary layer by the rising vapour bubbles and more uniform temperature conditions through the fluid's latent heat. Improved electrical performance is also noted under these conditions, with greater electrochemical reaction efficiency leading to increased usable capacity and a reduction in charging times. The boiling process is localised to the electrode terminal surfaces, the regions of greatest heat flux, with the vapour bubbles increasing in nucleation and departure frequency for greater charge and discharge currents. When arranged in a module, however, analysis of the thermal profiles of the cells reveals significant axial temperature variations across their surface under natural convection immersion conditions, arising from high fluid agitation induced by the instability of the cells' thermal boundary layers. Consequently, substantial temperature differences are observed across the module. In contrast, excellent cell and module thermal homogeneity occurs under the preheated immersion conditions due to the phase change process. Slightly improved thermal performance is observed for reduced inter-cell spacings within the module arrangement under these conditions, as the interaction of the rising vapour bubbles from adjacent cells disturbs the thermal boundary layer, augmenting heat transfer.