Li-ion rechargeable batteries have been widely used as a reliable energy source for portable electronic devices, but the use in large power and high capacity, such as EV (Electrical Vehicle) and HEV (Hybrid Electrical Vehicle), is limited due to safety concerns associated with the thermal stability. In this paper, the influencing factors on the safety characteristics of Li-ion batteries were analyzed from the materials and the manufacturing process. The key safety problems of the Li-ion packs are further introduced.