Immersion cooling has been widely investigated in battery thermal management due to its high cooling efficiency; however, the influence of coolant properties on the thermal behavior and temperature uniformity of large-capacity energy storage battery modules remains unclear. In this study, a three-dimensional numerical model is developed to investigate the thermal performance of an immersion-cooled battery module consisting of 52 prismatic cells. The cooling performance of silicone oil (SO), synthetic hydrocarbon (SH), and two synthetic esters (SE) with different viscosities is systematically compared under various discharge rates and volumetric flow rates. The battery thermal model was validated through single-cell experiments under natural air convection conditions. The research results indicate that at a 0.5C discharge rate, the 30 cSt SE achieves a reduction in maximum battery pack temperature of 6.3% and 7.0% compared to SO and SH, respectively. Furthermore, the maximum temperature difference is significantly reduced by 22.9% and 25.4% under the same conditions. Due to differences in the inherent properties and flow heat transfer characteristics of the coolant, at a volumetric flow rate of 12 L/min, the 30 cSt SE resulted in a 15.8% reduction in module temperature difference compared to the 20 cSt SE. To further evaluate the internal thermal balance of the battery module, two thermal uniformity indicators were introduced to quantify the consistency of the highest temperature of individual cells and the internal temperature difference. Considering both the temperature performance and thermal uniformity at the module level, from a heat dissipation performance perspective, the 30 cSt SE demonstrates significant potential for thermal management of large-scale prismatic battery packs.

Driven by the increasing energy density of battery energy storage systems, immersion cooling (IC) has emerged as a promising approach for mitigating thermal runaway (TR) hazards. In this study, overcharge-induced TR tests were conducted on commercial 314 Ah lithium iron phosphate batteries in an accelerating rate calorimeter to compare their thermal, pressure, mass loss, and gas venting responses under air cooling (AC) and static ester-based immersion cooling. For the two cells tested, internal short circuit onset occurred at 1150 s under AC and 1377 s under IC, while TR was triggered at 1232 and 1404 s, respectively. The peak surface temperature decreased from 422.4 °C under AC to 302.4 °C under IC, and the maximum surface temperature difference was reduced by approximately 31%. The maximum chamber pressure rise rate decreased from 3.12 to 1.68 kPa/s, although a higher late-stage cumulative pressure was observed under IC within the sealed ARC chamber. Battery mass loss decreased from 1014.2 g (18.27%) under AC to 845.2 g (15.24%) under IC. In addition, the CO₂ fraction in the post-cooling gas mixture increased from 30.4% to 38.4%, while the H₂ fraction decreased from 43.6% to 36.9%. Based on the modified Le Chatelier calculation, the estimated lower explosive limit increased from 6.16% to 7.18%, suggesting lower composition-based ignitability under the adopted assumptions. Overall, the tested static ester-based immersion cooling configuration delayed TR evolution, reduced peak thermal response and mass loss, and moderated the transient pressure rise under the present experimental conditions. These findings provide experimental reference data for the thermal-safety design of large-capacity battery energy storage systems.