Lithium-ion (Li-ion) batteries have been widely used in portable devices and electric vehicles as power sources due to their high energy density, long lifespan, no memory effect, and low self-discharge rate. However, when overheating occurs, the performance, reliability, durability, and safety of Li-ion batteries can be seriously deteriorated.
Overheating of Li-ion batteries typically takes place during normal high-rate discharges and abnormal discharges such as short circuits. For high-rate discharges, the parasitic heat generation of Li-ion batteries can accelerate the capacity fading. In abnormal discharges, the overheating issue becomes more severe, and the battery temperature could even reach the threshold for exothermic reactions and trigger thermal runaway.
In this thesis, phase change material (PCM)-based and heat pipe-based battery thermal management (BTM) systems were designed to dissipate the heat generated by cylindrical and prismatic batteries respectively during normal discharges. The proposed PCM-based BTM system was developed through innovatively embedding PCM cores in the cylindrical battery centers. Compared to conventional PCM-based BTM systems using PCM externally to batteries, the proposed design consumes less PCM while achieves better cooling effects. For prismatic batteries, ultra-thin heat pipes were sandwiched between the batteries to manage their temperature rises. The implementation of heat pipes notably alleviates the heat accumulation in battery packs, and the cooling performance can be further enhanced when an evaporative cooling strategy is applied to dissipate the heat.
Regarding the more severe overheating issues caused by abnormal discharges, internal and external short circuit experiments were performed on Li-ion batteries, and a modified electrochemical-thermal coupling model was developed to simulate the short circuits. Good correlations were found between experimental and numerical results for non-thermal runaway batteries. For batteries with thermal runaways, the model can predict whether and when the thermal runaway will occur. Besides, strategies that can hinder thermal runaway and its propagation in battery packs were also investigated and verified through experiments.