# A Review on the Thermal Hazards of the Lithium-Ion Battery and the Corresponding Countermeasures
## 1 Introduction  
In light of the steadily increasing energy demands and the consensus regarding the reduction of pollution, humans have paid great attention to the development of new energy such as solar energy, wind energy, tidal energy, lithium-ion battery (LIB) and fuel cell [1; 2; 3; 4; 5]. Advantages including high energy density, less pollution, stable performance and long-life cycle compared to many alternatives have made LIBs the dominant power sources for electrical applications [6; 7]. Especially, with the approaching era of electric vehicles (EVs), it is foreseeable that the use of LIB will be more common in the future. However, behind the boom, there exist challenges that must be faced. As a result of the high energy density of LIBs, they are sensitive to abusive conditions such as high temperature, crashing, overcharge, over-discharge and short-circuit, etc. [8; 9; 10]. Moreover, the typical components of a battery, e.g., plastic packing, separator and electrolyte are combustible. Thereby, the accidents induced by LIBs are frequently reported and can occur in a range of applications from mobile telephones, to EVs and even airplanes [11; 12; 13; 14]. Especially with the increasing energy density, e.g., the popularity of LiNi\({}_{0.8}\)Mn\({}_{0.1}\)Co\({}_{0.1}\) (NMC811), the problems will be exacerbated [15; 16]. The abusive conditionsdescribed above will destroy the original stable structure of battery, which triggers chain reactions inside battery and eventually leads to thermal runaway. Among, thermal runaway of a battery typically manifests as high temperature, gas ejection, violent combustion and even explosion, which will lead to catastrophic results. Regarding the thermal hazards of the LIB, much research has been directed toward enhancing the inherent safety of battery and improving the thermal management to prevent thermal failure. For the former, it mainly focuses on enhancing the fire-retardant performance of battery components including the cathode, anode, electrolyte, additives and separator [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. As for the latter, it includes the inclusion of battery safety devices, thermal management of LIBs during usage, thermal hazards warning and firefighting should a hazard occur. The thermal management of the LIB can usually be categorized into several types including air, liquid and phase change material (PCM) based systems [28; 29; 30; 31; 32; 33]. Prior to the occurrence of a thermal hazard, LIBs are prone to experiencing evident increases in temperature and may also release representative gases such as CO, H\({}_{2}\) and SO\({}_{2}\). Therefore, battery safety devices such as vents can be used to terminate the deterioration of failure, meanwhile a temperature sensor or gas sensor can also be applied to warn to the hazard [34; 35; 36; 37]. Finally, if the thermal hazards are accompanied by severe combustion, it is necessary to fight the fire to impede its progression. Until now, researchers have paid great attention to firefighting methods and extinguishers so that the flame or combustion of the LIB can be extinguished effectively [38; 39; 40; 41]. Although much work has been done on the thermal hazards of the LIB, comprehensive summaries on the thermal hazards of batteries involving battery components, a single battery and a battery pack are scarce. Besides, most researchers focus on the thermal management of battery during normal usage, countermeasures that suppress the thermal hazard require further study. The current work provides a comprehensive review on the thermal hazards of battery, and related thermal hazard prevention techniques. The Section 1 partially summarizes the safety-related LIB incidents that have occurred in recent years. The Section 2 is regarding the common causes of battery thermal hazard and the respective mechanism. The Section 3 demonstrates the thermal hazards of the LIB, involving typical battery components, a single battery and a battery pack.