Safety study of coolants for thermal management of battery systems – Immersion cooling of lithium-ion batteries

Sammanfattning

Recently immersion cooling in cooling liquids has been researched for thermal management of lithium-ion batteries. Previous research has though mainly concerned the liquids cooling capability during battery cycling. Not many experiments are conducted including actual thermal runaway events. Therefore, have the events of thermal runaway of lithium-ion batteries during immersion cooling in hydrocarbon-based cooling fluids been studied during this project. The lithium-ion batteries used are cylindrical 3.7 V Samsung 50 E2 NMC 21700 with a capacity of 5 Ah. The hydrocarbon-based cooling fluids used are the naphthenic NYNAS® PHF 15, and the paraffinic NYNAS® RPHF 21. Both cooling fluids are contributed by Nynas AB. Methods for initiation of thermal runaway are tested by heating of the battery and oil to a temperature of 160o C, overcharging up to 4 C, and needle penetration. The method of needle penetration is declared best fitted for thermal runaway initiation in hydrocarbon-based fluids. The cooling fluids’ ability to prevent thermal runaway propagation are tested in battery modules of seven batteries, with 3 mm spacing between the batteries, while immersed in respective fluid. Reference test is done of module in air. No propagation occurs in either test. The cooling fluids exhibit a good capability to cool both the running battery and the surrounding batteries during the thermal runaway event compared to the test in air. Hot Disc analyses are performed to determine thermal conductivity and specific heat capacity of the cooling fluids. The analyses for PHF give a thermal conductivity of 0.14 W/m·K and a specific heat capacity of 1.41 kJ/kg·K, the RPHF give a thermal conductivity of 0.22 W/m·K and a specific heat capacity of 1.14 kJ/kg·K. The measured values have a deviation to values received from Nynas AB of 20 % for PHF and 38-45 % for RPHF. This makes the analytical results implausible. GC-MS is used to detect contaminations from the batteries in the cooling fluids after the experiments. No contaminations are found. Hydrocarbons and an additive are detected, an antioxidant, 2,6-di-tert-butyl-p-cresol. Thermal conductivity, viscosity, specific heat, and circulation of the fluids are identified as important parameters for the cooling fluids. Proceeding experiments of thermal runaway during immersion cooling in enclosed modules is of interest.