Thermal runaway and fire behavior of large lithium iron phosphate batteries

Research Background

Lithium-ion batteries have been widely used due to their high energy density and good cycle performance. In recent years, the capacity of batteries has gradually increased. Understanding the thermal runaway characteristics and fire behavior of large-scale lithium-ion batteries is important for their fire prevention and control. significance. In this study, a 326Ah large-scale lithium iron phosphate battery was selected, and a series of combustion experiments were carried out to systematically study the combustion process, flame shape, thermal runaway expansion characteristics, the effect of flame on the thermal runaway process of the battery, and the law of combustion heat production.

Work introduction

Wang Qingsong’s research group from the University of Science and Technology of China selected 326Ah large-scale lithium iron phosphate batteries and carried out a series of combustion experiments, revealing the combustion characteristics of large-scale lithium-ion power batteries and filling the gap in the industry. When the battery is locally heated, based on the temperature of each surface of the battery, the propagation process of thermal runaway inside the single battery can be clearly observed, and the larger the battery size, the more obvious this process is. The combustion and mass loss process of large batteries can be divided into multiple stages. In the most violent thermal runaway stage, the combustibles in the battery are rapidly consumed, forming a violent columnar jet fire; flame combustion can cause thermal runaway of the battery to occur earlier, but the The battery surface temperature peaks have little effect. Studies have shown that compared with other low-capacity single cells, the tested large single cells not only have higher energy density, but also have smaller combustion heat per unit capacity. The results were published in the journal Renewable and Sustainable Energy Reviews . Doctoral student Mao Binbin is the first author of the paper, and Wang Qingsong is the corresponding author.

 

Description of content

Due to its extremely high safety and good cycle performance, lithium iron phosphate batteries have been widely used in electric vehicles and energy storage power stations. The research object of this paper is a prismatic lithium iron phosphate battery, the positive electrode material is lithium iron phosphate, and the negative electrode material is graphite, and its combustion fire characteristics are studied, which can provide data support for the safety early warning and fire extinguishing design of the subsequent battery system.

Based on the ISO9705 full-size room combustion test bench and the ISO5660 cone calorimeter and other combustion instruments, a medium-sized battery combustion chamber with a size of 1.8m×1.8m×2m was designed and built. In the combustion experiment chamber, the largest side of the battery was heated by a heating plate to excite it to thermal runaway combustion; the combustion phenomenon was recorded by DV, and the temperature and voltage changes were monitored by thermocouple and charge-discharge cycler respectively; based on gas analyzer measurement Based on the oxygen consumption method, the battery combustion heat release rate (HRR) was obtained.

Figure 1 shows the temperature, mass, voltage and heat release rate curves of a fully charged battery during the combustion process. Since the battery cannot spontaneously ignite, the battery is manually ignited after the pressure relief valve is opened and the electrolyte is leaked. Based on the pressure relief valve opening event and the battery quality curve, the battery combustion process can be divided into four stages. Figure 2 shows the battery combustion phenomenon. The combustion process of the 326Ah large-scale power battery is very violent. In the stage III, when the battery is rapidly thermally runaway, a large amount of combustible gas is ejected, forming a very violent columnar flame, as shown in Figure 2(g).

Figure 2. The combustion process of a fully charged battery, with the heating start time as time 0, the pressure relief valve is opened at 1459s, the battery is manually ignited at 1484s, and the flame at 2214s is a columnar intense combustion flame.

The thickness of this battery sample is 7 cm; based on the temperature curves of the heated surface, side center and back surface of the battery, a very obvious thermal runaway expansion process is observed. This study also carried out two experiments without artificial ignition to analyze the effect of flame combustion on the thermal runaway history of the battery; the results show that flame combustion can advance the thermal runaway time point of the battery, but it has an impact on the maximum temperature of the battery surface after thermal runaway. smaller.
In this study, the combustion heat production of the tested 326Ah large-scale lithium iron phosphate battery was compared with the small-capacity battery studied by the predecessors, as shown in Table 1. The 326Ah large battery in this study has the highest specific capacity, and its normalized peak HRR and combustion heat per unit capacity are relatively small, showing superior thermal safety. In addition, after removing the highest and lowest values, the average mass loss ratio of each battery was 24.5%±3.1%, and the normalized average total combustion heat was 19.9±4.9MJ kg ^-1 , showing good regularity. This shows that, based on the industrial database and experimental experience, after estimating the mass ratio of the combustibles in the battery and the combustion heat lost per unit mass, the combustion heat of the battery can be predicted only by weighing the initial mass of the battery. The above empirical formula can avoid dangerous and costly combustion experiments, and has important guiding significance for the safety assessment of lithium-ion batteries.

Conclusion

 This paper studies the combustion characteristics of a 326Ah large lithium iron phosphate power battery , which fills the gap of the current lack of combustion characteristics of lithium-ion batteries above 100 ampere-hours. The core conclusions are as follows:
1) The combustion process of this large battery can be divided into 4 stages. Stage III is the most violent thermal runaway stage. It can be observed that Violent columnar jet fire, the peak combustion heat release rate can reach 88.78kW;
2) When the battery is locally heated, there is a thermal runaway expansion process inside the battery cell, and the larger the battery volume, the more obvious this phenomenon is;
3) Combustion The flame can accelerate the spread of the thermal runaway of the battery, but it has little effect on the highest temperature on the battery surface;
4) This paper compares the combustion heat of the tested large single battery with the previous small battery, and the results show that the large single battery It has a high mass specific capacity, and its unit combustion heat production is low (the heat production per unit capacity and the peak heat release rate per unit surface area are both low); this shows that from the perspective of combustion heat production, the large-scale single cell Chemicalization is also one of the favorable directions for the development of the battery industry. Binbin Mao, Chaoqun Liu, Kai Yang, Shi Li, Pengjie Liu, Mingjie Zhang, Xiangdong Meng, Fei Gao, Qiangling Duan, Qingsong Wang* , Jinhua Sun. Thermal runaway and fire behaviors of a 300 Ah lithium ion battery with LiFePO ₄ as cathode, Renewable and Sustainable Energy Reviews , 2021, DOI:10.1016/j.rser.2021.110717