Title: Multi-Physics Numerical Analysis of Single-phase Immersion Cooling for Thermal Management of Li-Ion Batteries

Abstract: Battery thermal management systems (BTMSs) are critical for efficient and safe operation of lithium-ion batteries (LIBs), especially for fast charging/discharging applications that generate significant heating within the cell. Forced immersion cooling, where a dielectric fluid flows in direct contact with the LIB cells, is an effective cooling approach. But because of its complex nature, a thorough understanding of the underlying physics - including the coupled electrochemical, thermal, fluid, and mechanical effects - is required before immersion cooling will see wide adoption into commercial systems. In this work, to investigate the performance of a LIB subjected to forced immersion cooling, we develop a fully coupled modeling approach that solves the detailed electrochemical model in conjunction with the thermal-fluid transport models for both the cell and fluid domain. After calculating the electrochemical and thermal responses, we also estimate the mechanical stresses within the cell generated due to the ion diffusion and temperature rise that impact reliability. To assess the effectiveness of forced immersion cooling, we evaluate several different configurations for a cylindrical 18650 battery cell under varying cell discharge rates. We compare forced immersion cooling for two liquids (deionized water and mineral oil) at three different fluid mass flow rates. The results highlight the strong cross-coupling of the electrochemical and heat transfer phenomena. By comparing results across fluids and flow rates, we define a new metric that can be used to compare the cooling capacity considering different flow parameters. Overall, this study provides insights that will be useful in the design of immersion cooling-based BTMSs including, for example, the selection of forced immersion cooling specifications, such that the temperature is controlled without significant capacity loss.