As the mainstream power source of electric vehicles, lithium-ion power batteries have the characteristics of high specific energy. At present, automotive power batteries mostly use a large number of small-capacity batteries to be connected in series and parallel to meet high-energy requirements. In this way, the safety problem of the automotive power battery system is no longer just the safety problem of the battery cells, but the safety problem of the battery pack. The automobile power battery accidents that have occurred in recent years are all caused by a large amount of heat generated by a battery cell in the battery pack after thermal runaway, which causes the surrounding battery cells to heat up and produce thermal runaway. In this way, the problem of thermal runaway in the battery pack is the main concern of the safety of the battery pack.
If the mechanism of thermal runaway spread is explored, the spread of thermal runaway can be effectively blocked, and the thermal runaway can be confined to the battery cell, and the harm can be minimized. At present, there are not many researches on the thermal runaway spread of batteries. After the battery safety issue has attracted attention in recent years, some scholars have conducted experiments and simulation studies on the thermal runaway spread of power batteries. Research on effective prevention and control technologies for battery thermal runaway spread is also being carried out. This work will focus on the two aspects of thermal runaway spread mechanism and modeling research, and thermal runaway spread safety prevention and control technology. The research status will be reviewed, and the development direction of thermal runaway spread related research will be discussed.
1 Influencing factors of thermal runaway spread
There are many factors that affect the propagation characteristics of thermal runaway. The first is the thermal runaway characteristics of the battery itself, such as the battery thermal runaway characteristic temperature, energy release rate, etc.; secondly, the heat dissipation conditions of the batteries and the heat transfer conditions between the batteries, as mentioned above, Heat transfer is an important reason for the thermal runaway spread of the battery pack, so the heat transfer characteristics are also an important factor that directly affects the rate of thermal runaway spread; in addition, the battery will emit high-temperature gas and particle mixtures when thermal runaway occurs. These gases are flammable and extremely Fire is prone to occur. These high-temperature sprays and the flames generated by the burning of the sprays will heat the surrounding batteries, thereby accelerating the process of thermal runaway. In addition, the electrical connection between the batteries will also affect the spread of thermal runaway.
FENG et al. [1] analyzed the heat flow path during the thermal runaway propagation of the square shell battery series module, as shown in Figure 1. The analysis results show that the heat transfer of the thermal runaway battery through the shell to the next battery is the main heat transfer path. , The heat transfer through the pole connecting piece is relatively very small, and at the same time, more heat is transferred back to the previous battery or dissipated to the environment. COLEMAN et al. [3], KIZILEL et al. [2] researches on the thermal runaway propagation of cylindrical batteries have shown that when the battery spacing increases, the thermal runaway propagation will be suppressed. This is because the heat transfer between the batteries is reduced, and the battery is The heat transfer of the environment increases. The heat flow path analysis has a certain guiding effect on the design of the suppression scheme for thermal runaway propagation. According to the aforementioned thermal runaway propagation mechanism, the determination of thermal runaway propagation depends on whether the highest temperature of the next battery reaches the thermal runaway trigger temperature. Therefore, whether each heat transfer path causes local hot spots in the next battery may be more important. In this regard, further research and analysis are needed.
The influence of battery thermal runaway ejection and the flame generated by its combustion on the spread of thermal runaway is related to the environment where the module is located and the battery type. Research by Liu et al. [4] showed that the energy released by combustion is almost three times the energy stored in the battery. The research of FENG et al. [5] showed that the flame generated by the thermal runaway nozzle in an open environment has little effect on the spread of thermal runaway. This is because the spray and flame are directly above the thermal runaway battery and do not directly touch the phase. Adjacent battery. However, some literatures [6-8] conducted experimental studies on the thermal runaway propagation of cylindrical batteries. It is shown that when the high-temperature substances ejected by the thermal runaway battery directly contact the surrounding batteries, it will promote the occurrence of thermal runaway propagation.
Due to the internal short circuit during the thermal runaway of the battery, for a battery pack in parallel, after a battery is thermally runaway, the battery connected in parallel with it will discharge to it, causing its temperature to rise further, which may accelerate the spread of thermal runaway. .
2 Current status of research on prevention and control technology for thermal runaway spread
At present, the existing research on the prevention and control of thermal runaway is from the perspective of modules or battery packs, mainly through the means of thermal management, to prevent thermal runaway from spreading between batteries, so as to prevent thermal runaway of one battery in the battery pack. After that, it gradually spread to the surrounding battery. For square shell batteries, the prevention and control technology prevents the front-end surface temperature of adjacent batteries from reaching the thermal runaway trigger temperature TRonset [10], which can suppress the spread of thermal runaway.
Conventional battery thermal management systems can be divided into active cooling and passive cooling from the perspective of whether they rely on energy input to drive. Active cooling usually uses fans or pumps to circulate the coolant. Passive cooling does not rely on external energy for the cooling system. It depends on the battery. The cooling medium in the room absorbs heat; and the Denelon modified refractory insulation felt developed by Guangzhou Lvyuan Environmental Protection Material Co., Ltd. has a lower cost and a better effect in inhibiting the spread of heat.
GZLV modified fire-resistant insulation felt has the following functions in different stages of thermal runaway:
1. The high thermal conductivity at room temperature has a low impact on the thermal management of the battery pack.
2. When the heat spreads, the phase change material absorbs heat and absorbs the energy generated by the heat spread.
3. After the phase change material fails to absorb heat, the composite material still has super heat insulation (0.02W/m.k), which inhibits heat spread.
GZLV modified fire-resistant insulation felt
The product is made of a composite of silica and ceramic fiber felt, with nano-scale voids inside the product, which can slow down heat conduction, provide the lowest heat conduction value, and have excellent thermal shock resistance. Nano-silica fiber felt is prepared by solution air spinning technology. The fiber felt can fully rebound after being compressed by 70%, and can withstand 7250 times of its own weight without being broken. It still has 1000 compression cycles. Very good resilience. More importantly, this nano-silica fiber felt can maintain good flexibility in a 1500°C butane flame and liquid nitrogen.
Based on this material, a thermally responsive, ultra-strong, ultra-thin (1 mm) flexible battery refractory and thermal insulation felt composite material was prepared by filling with phase change materials, which is used to prevent the thermal runaway propagation between cells in the battery pack. The phase change material stored in the nanofiber mat has reliable thermal conductivity under normal conditions and high thermal sensitivity at high temperatures. The high temperature generated by thermal runaway will cause the vaporization of the phase change material, along with absorbing a large amount of heat and releasing a large amount of extinguishing agent. After the phase change material is released, the remaining Denali modified refractory insulation felt has an ultra-low thermal conductivity of less than (0.02W/mk), which can continue to prevent heat transfer from a starting unit to an adjacent unit and prevent system level The heat is out of control. Therefore, the battery pack with this kind of modified fire-resistant insulation felt is able to perform normal thermal management under normal operating temperature, and has a high ability to block thermal runaway under abnormal conditions. In addition, it has the characteristics of mass production, good processing performance, adjustable trigger temperature, etc., and can be used to manufacture a series of advanced, safe and durable modified fire-resistant insulation felts. Its application areas can even be extended to tank emergency materials, space detection and fire-fighting equipment, etc.
3 Conclusion and outlook
The main influencing factors for the thermal runaway spread of power batteries are heat transfer, electrical connection, and fire of ejected objects. For square shell and soft pack batteries, heat transfer may be the most important influencing factor. When thermal runaway occurs in a square-shell battery, the spread of thermal runaway occurs because the temperature of the front surface of the battery reaches the thermal runaway trigger temperature TRonset [10]. In order to simulate the process of thermal runaway propagation, thermal runaway propagation models of different dimensions can be established. Among them, the three-dimensional model has the highest simulation accuracy, the most available information, and the largest amount of calculation.
In terms of thermal management, most of the current researches are still conducted from the perspective of temperature control under normal operating conditions, and there are not too many studies on the suppression of thermal runaway propagation. Among several thermal management methods, liquid cooling, phase change cooling, and emergency cooling may be more effective methods to suppress the spread of thermal runaway. While considering the management effect, the thermal management system must also consider its impact on the efficiency, cost increase, and complexity of the battery pack. Fire safety design is currently mainly simplified experimental research, lacking strong data support. Therefore, it is necessary to provide more theoretical research basis for fire protection design from the perspective of battery thermal runaway combustible gas composition and flow.
In order to achieve efficient thermal runaway blocking between batteries, ultra-low thermal conductivity materials are a good choice, but the selection of ultra-low thermal conductivity materials must meet the high temperature resistance (> 700 ℃, especially for the NCM811 battery temperature resistance must exceed 1000℃) prerequisite. In recent years, in order to solve the problem of mileage anxiety, the energy density of single cells has become higher and higher, and the heat released by thermal runaway has also become higher and higher. In order to delay or block the spread of thermal runaway, pure heat insulation materials are also becoming thicker and thicker, so this additional material in turn weakens the battery's cruising range. In addition, as the heat conduction between batteries is strictly blocked, thermal management under normal operation has become a new challenge. These seemingly contradictory problems are cleverly resolved by phase change filling.
First of all, a large-scale all-inorganic flexible felt was manufactured in this paper to achieve high temperature resistance and low thermal conductivity. The reason for choosing silicon oxide here is that the nanofiber felt has good flexibility and high temperature resistance above 1200°C. Denalon's modified refractory insulation felt not only guarantees low bulk density, but also can effectively block heat convection compared with thick fibers.
Secondly, phase change adsorption is used to realize the integration of heat-triggered heat absorption, fire extinguishing and heat insulation functions. Based on the porous properties of GZLV modified refractory insulation felt, we adsorb a large amount of phase change materials.
Finally, during the thermal runaway of the lithium-ion battery, the thermally triggered phase change filling material is released, which effectively reduces the temperature and inhibits the burning of the battery assembly. GZLV modified fire-resistant insulation felt has ultra-low thermal conductivity and provides continuous thermal protection for other batteries.
In short, the rapid propagation of thermal runaway is the main problem facing the expansion and runaway of safety issues in large lithium-ion battery modules. However, thermal runaway blocking technology that does not introduce negative effects is still a huge challenge. This paper presents the concept of ultra-thin modified refractory insulation felt. Experiments have proved that the fire-resistant insulation felt has heat-triggered switchable thermal physical properties because of the synergistic effect of the flexible GZLV modified fire-resistant insulation felt and the phase change material. Under thermal runaway conditions, the fire-resistant insulation felt shield can simultaneously produce multiple functions such as cooling, extinguishing, and heat insulation. Therefore, the 1 mm thick modified refractory insulation felt successfully suppressed the thermal runaway propagation between the fully-charged 50 Ah lithium-ion batteries with a transient thermal shock of up to 53 kW, and produced up to 512 ℃ temperature difference between adjacent batteries.