(Invited) The Role of Internal Short Circuit during the Thermal-Induced Thermal Runaway Process of Lithium-Ion Battery
Authors: Ren Dongsheng , Feng Xuning , Hsu Hungjen , Lu Languang , He Xiangming , Ouyang Minggao 
Published: February 2020

Lithium-ion batteries, with high energy density and long cycle life, are the most widely used power sources for electric vehicles. However, the large-scale application of lithium-ion batteries suffer greatly from safety problems, such as thermal runaway (TR). TR is an electrochemical-thermal coupled process accompanied with fire and explosion, threatening the human lives and properties. Experimental and modeling effort have been paid to reveal the TR mechanism and provide guidance for the design of safer lithium-ion batteries.
ISC usually occurs when separator fails due to thermal shrinkage or mechanical crush, and plays an important role in the TR process. The massive ISC caused by mechanical abuse, such as nail penetration and crush, will directly trigger TR. However, the heat generation from ISC varies greatly during the thermal-induced TR process. ISC can initiate TR immediately, but would also occur much earlier than TR and release little electric energy. The role of ISC during the thermal-induced TR process requires further investigation.
In this study, the relationship between ISC and TR is investigated based on the adiabatic TR tests on commercial pouch 24Ah batteries. For battery at 100%SOC, instant voltage drop from 3.5V to around 1V, which can be considered as the indicator of ISC, happened at 180.9oC, 343s prior to TR at 213.3oC, as illustrated in Figure 1 (a) and (b). Same phenomena were also observed in the TR tests on batteries at 50% and 75%SOC, which showed ISC (rapid voltage drop) at 178.8oC and 171.2oC (957s and 1053s prior to TR), respectively. Furthermore, ISC released little heat during the TR process of the 24Ah battery, as the temperature rate showed no increase when ISC occurred. A battery at 0% SOC was heated to 180oC and then cooled down to reveal the ISC mechanism. The 0% SOC battery showed severe deformation and ruptured pouch, and severe shrinkage at the edge and cracks in the middle were found on the separator after dissembling the battery, as shown in Figure 1 (c). That is the cause of ISC during the thermal-induced TR process of the 24Ah battery. However, the battery was totally dried out with no observable electrolyte, resulting in a large internal resistance, which can explain the little heat generation from ISC.
DSC tests on battery components at 100% SOC were then conducted to reveal the TR mechanism. As shown in Figure 1 (d), sample Cathode+Anode+Electrolyte (Ca+An+Ele) with all the components mixed together exhibited a drastic exothermic peak of 90.34W·g−1 at 265 °C, close to the battery TR temperature (213.3oC). The heat generation of sample Ca+An+Ele was 1550.0 J·g−1 in total. Small amount of heat generation was detected in sample Ca+Ele, while sample An+Ele and sample Ca +An exhibited significant heat generation (926.0 J·g−1 and 1446.8 J·g−1, respectively), indicating that the exothermic reactions between anode and electrolyte and those between cathode and anode are the dominated heat sources during thermal-induced TR process.
In conclusion, ISC was found to occur prior to TR but generate little heat during the thermal-induced TR process of a 24Ah battery. The ISC was caused by thermal shrinkage of separator, while the release of electric energy from ISC was limited by the large internal resistance due to electrolyte drying out. The exothermic reactions between anode and electrolyte and those between cathode and anode turned out to be the dominated heat sources during TR process, according to the DSC tests on battery components. Future work will focus on liquid-nitrogen-ceased TR of battery at 100%SOC and post-mortem analysis of the battery components to better reveal the ISC and TR mechanism.

Figure 1