# Investigation into the Fire Hazards of Lithium-Ion Batteries under Overcharging
## 1 Introduction  
With the continuation of environmental problems such as global warming, greenhouse gas emissions, and sea-level rise caused by extensive use of fossil fuels, LIBs have been widely used as a power source for electronic devices including laptops, mobile phones, electrical vehicles (EV), and hybrid electrical vehicles (HEV) due to their high energy density, stable performance, long life, and other excellent properties [1; 2; 3; 4; 5]. However, fires and explosions caused by LIBs have often been reported. From 24 August 2016 to 10 October 2016, there were many accidents induced by Samsung Note 7 phones, which forced Samsung to recall Note 7 all over the world and lost it billions of dollars. Moreover, a fire occurred in a BYD electric car resulting from the spontaneous combustion of the battery, and a Tesla Model S charging at a Tesla Supercharger in Norway caught fire on 1 January 2016. All of these incidents have raised serious concerns about the safety of LIBs. Therefore, it is necessary to conduct further research on the fire hazards of LIBs so that we can use them better.  
Commonly, accidents were the result of overcharging LIBs; this problem has attracted many researchers' attention. [PERSON] et al. [6] compared the thermal runaway behaviors of two widely used commercial LIBs (LCO/LMO) at different states of charge including overcharge by Accelerating RateCalorimetry (ARC) measurements. Their results show that an LIB tends to exhibit thermal runaway behavior at overcharge state and the LIB based on LCO is found to be more thermally unstable than the LMO cell. [PERSON] et al. (2017) investigated the overcharge behaviors of a large battery (32 Ah prismatic cell) by closely monitoring the internal/external cell temperature during the whole charge process. Their analysis revealed that lithium plating is one of the major causes responsible for thermal runaway during overcharge. [PERSON] et al. (2018) conducted experiments on discharged, partially charged, fully charged, and overcharged cells with ARC to observe the thermal behaviors and gas release behaviors of cells during heating. They found that the thermal risk would increase with the growth of state of charge (_SOC_), and there were more gases released for overcharged cells than for normal ones. There are numerous articles associated with the effects of overcharge on the electrochemical stability, aging process, and electrode materials of LIB ([PERSON], 2010; [PERSON] and [PERSON], 2011; [PERSON] and Stein, 2012; [PERSON] and [PERSON], 2013). [PERSON] et al. investigated the effects of slight overcharge (110% _SOC_) on the electrochemical performance of high-power batteries to identify the capacity decay mechanism. Their work revealed the abuse tolerance behaviors and capacity decay mechanism of a battery under overcharge with the help of X-ray Diffraction (XRD) and a Scanning Electron Microscope (SEM). [PERSON] et al. studied the variation of electrode materials and separator after overcharge treatment by SEM. Furthermore, they explored the Differential Scanning Calorimetry (DSC) curves of LIBs with different overcharge conditions. However, there are few reports related to the fire hazards of overcharged LIB under external thermal abuse, and the overcharge condition scaled by cut-off voltage is also scarcely discussed.  
In this work, we researched the fire hazards of two widely used commercial LIBs, NMC and LFP, under overcharge conditions scaled by cut-off voltage (4.2 V, 4.5 V, 4.8 V, and 5.0 V). Specific parameters including cell voltage, surface temperature, flame temperature, and heat flux were measured. The results were comparatively analyzed to provide basic knowledge on performance-based fire safety design.