Abstract: A novel model considering electrochemistry, battery aging and heat transfer is developed for the design and optimization of battery thermal management system (BTMS) to ensure efficient and durable operation of batteries. The multiphysics behaviors in different working cycles of BTMSs are analyzed and compared. It is found that solid electrolyte interphase (SEI) formation inside the aged battery pack leads to a higher heat generation rate. The reversible heat generation rate became smaller and smaller during the cycling due to SEI formation and Li+ reduction inside the battery. However, the irreversible heat generation rate increased with the increasing working cycles. Meanwhile, the increment in irreversible heat generation was much higher than the decrement in reversible heat generation. Thus, the total heat generation rate became larger and larger during the cycling. Therefore, the maximum temperature and maximum temperature difference in 1000 cycles of BTMS were higher than those of original case by 2.54K, 2.15K, 1.93K and 2.34K, 2.04K, 1.85K, respectively. Such large data deviation on maximum temperature and maximum temperature difference caused by capacity fade will definitely affect BTMS design. Without considering the battery aging effect, 0.05m/s were enough for BTMSs to meet the requirements for the maximum temperature and maximum temperature difference. However, BTMSs were unable to control the temperature of battery pack after 1000 cycles to reach these requirements under the investigated inlet velocity when considering the effect of capacity fade. Thus, the optimization schemes were proposed for BTMS to ensure the effective thermal management for battery pack over long-term battery cycling. It is found that the addition of Al₂O₃ nanoparticles with different volume fractions could always enhance the cooling performance of BTMS. Furthermore, as the volume fraction of nanoparticles increased, BTMS with nanofluid was more effective in controlling thermal behaviors of battery pack. Thus, the maximum temperature and maximum temperature difference in 1000 cycles, the maximum temperature and maximum temperature difference were decreased by 1.24K, 0.98K, 0.86K and1.09K, 0.88K, 0.79K for water-1% Al2O3, 1.92K, 1.56K, 1.36K and 1.63K, 1.52K, 1.27K for water-3% Al2O3, 2.64K, 2.2K, 1.94K and 2.29K, 2.02K, 1.83K for water-5% Al2O3. For the BTMS optimized operation strategy based on battery heat generation, it can be observed that this method was more effective in controlling thermal behaviors and reducing battery capacity fade in all the working cycles, with achieving a significant effect on improving pressure loss and increasing battery discharging potential. For the aged battery pack in 1000 working cycles, the maximum temperature and maximum temperature difference were decreased by 5.98K, 4.17K, 3.04K and 4.27K, 2.79K, 1.81K after using the operation strategy.

Key words: li-ion battery, battery thermal management system, battery aging, heat transfer, numerical simulation