Large-format lithium-ion (Li-ion) batteries with high energy density for electric vehicles are prone to thermal runaway (or even explosion) under abusive conditions. In this study, overcharge induced explosion behaviors of large-format Li-ion pouch cells with Li[Ni0.8Co0.1Mn0.1]O2 cathode at different current rates (C-rates) (0.5C, 1C, 2C) were inv. Lithium-ion batteryOverchargeExplosion behaviorSafetyDriven by the demands for sustainable, clean energy and reduction of greenhouse gas emission, lithium-ion batteries (LIBs) are becoming an important resource for energy storage. In the field of transportation, electric vehicles (EVs) are developing rapidly and gradually occupy a large portion of the market. Compared with traditional fuel vehicles, EVs use clean energy, have no exhaust emissions, and are very environmentally friendly. To solve the mileage anxiety of users, LIBs have been widely used in the energy storage system of EVs due to their advantages of high energy density, no memory effect and long service life. However, under the dynamic operating conditions of the vehicles, the safety and thermal stability of LIBs are poor due to their limited window of temperature and voltage.With frequent reports of fire and explosion accidents of EVs, the safety of LIBs has become a core issue that requires technology breakthroughs to achieve the further promotion and application of EVs. The triggering methods of thermal runaway can generally be divided into three categories: mechanical abuse (such as collision, puncture, etc.), electrical abuse (such as overcharge, short circuit, etc.) and thermal abuse (such as overheating). Under these abusive conditions, large-format LIBs with high energy density are prone to thermal runaway and lead to hazardous accidents, such as fire and explosion. Whe. 2.1. The 40 Ah Li-ion cellIn this study, three 40 Ah commercially available Li-ion pouch cells with NCM811 cathode and graphite anode were tested; the voltage range of such cell is 3.0–4.2 V. Fig. 1(a) depicts the geometry of the cell (8 mm × 160 mm × 230 mm). The initial mass of the cell is about 650 g. Before the overcharge test, conditioning tests were conducted on the cells using constant current (0.5C, 20 A) and constant voltage (4.2 V) charging mode (CC-CV mode) and constant current (0.5C, 20 A) discharging mode, and repeated three times. The typical CC-CV charging curve of the 40 Ah Li-ion cells is shown in Fig. 1(b).2.2. Experimental settingsAll the tests were conducted in a custom-built laboratory equipped with ventilation and rigorous safety precautions. The cell to be tested is placed on four refractory bricks without restriction on its degrees of freedom. The ambient temperature is 25 °C. Fig. 2(a) illustrates the test platform and the corresponding test instruments. It is assembled with a battery testing system for charging or discharging batteries as well as monitoring the voltage and current, a digital camera (200 fps) for recording the whole experimentation, thermocou. 4.1. Explosion sensitivity and severity of LIBRisk and hazard are two important aspects that characterize Li-ion batteries' safety during overcharging. Based on the experimental results, the characteristic parameters during the explosion of Li-ion pouch cells had been acquired and their explosion behaviors had been discussed in detail. These are the basis of the safety assessment of Li-ion cell explosion. The hazards of the explosion shock wave to the human body and structure are graded respectively according to the pressure value, as shown in Ref. [, ]. Explosion risk and hazard are generally defined by two indicators: sensitivity and severity. Sensitivity is the stability of the material and the likelihood of the explosion of a kind of explosive, which is used to illustrate the probability and difficulty of the occurrence of the explosion. Severity is the consequence of an explosion, which represents the hazards caused by the explosion [, ]. These two indicators can be classified and graded by the characteristic parameters acquired from the experiments.As has been explained above, the energy stored inside the cell decrease with the increase of C-rates, as in Fig. 8(d). Generally, explosion can be divided into two categories according to the velocity of the explosion wave: (1) the process is called deflagration when the velocity is subsonic, and (2) it is called detonation when the velocity is supersonic. A.
