1.Classification of thermal runaway accident triggering
There are many triggering reasons for the thermal runaway accident of lithium-ion power battery. According to the characteristics of triggering, it can be divided into three categories: mechanical trigger, electrical trigger and thermal trigger. The three types of trigger forms have certain internal relations. Generally, mechanical trigger will cause short circuit and electric trigger, while electric trigger heat generation will cause thermal trigger, and thermal runaway caused by thermal trigger is the core of accident trigger. The mechanism analysis of other triggering forms cannot be separated from the study of thermal triggering mechanism.
Mechanical trigger includes crush, nail penetration, drop, etc., which is mainly characterized by deformation of the battery under force; Electric triggering includes external short circuit, internal short circuit, overcharge, over-discharge, etc. The main feature is that there is current flow during the triggering process; Thermal triggering includes abnormal heating, flame heating, etc. The main feature is that the battery continuously absorbs the heat in the environment and the temperature rises. The safety test standard specifies the detailed accident triggering factors obtained from accident analysis. The probability of triggering accidents of batteries that have passed the safety test standard has also been greatly reduced. The cause of accident triggering may be different from that specified in the safety test standard. This explains why the power battery system that has passed the safety test standard may still have accidents.
2.Expansion of thermal runaway in battery system
2.1 Hazards of uncontrolled thermal runaway expansion
After the thermal runaway is triggered, the heat released after the local monomer thermal runaway spreads to the surrounding, which may heat the surrounding batteries and cause the thermal runaway of the surrounding batteries, also known as the “expansion” of the thermal runaway in the battery pack. The energy released by the thermal runaway of the single battery is limited, but if the chain reaction causes the expansion of the thermal runaway, the energy of the entire battery pack will be released through the thermal runaway, which will cause great harm. For the power battery system of a 60kW • h pure electric vehicle, if all the monomers release all the energy due to uncontrolled expansion of heat, it will be equivalent to releasing 90kg TNT equivalent energy. In other words, once thermal runaway expansion occurs, it will cause great harm. Therefore, people need to prevent the expansion of thermal runaway, and limit the thermal runaway to some monomers.
2.2 Mechanism of thermal runaway expansion
From the perspective of energy conservation, when the heating power caused by the thermal runaway expansion of the surrounding battery of the thermal runaway monomer is greater than its own heat dissipation power, the temperature of the heated surrounding battery will rise, and then the thermal runaway trigger will occur. In the battery module, there are three possible main paths for heat transfer in the process of thermal runaway expansion:
1) Heat conduction between adjacent battery shells;
2) Heat conduction through the battery pole;
3) The burning of the surrounding battery caused by the single battery fire.
The two paths of shell heat conduction and pole heat conduction mainly act between adjacent cells, which is easy to analyze and control. For square cells, when the contact between the shell and the shell is good, the heat conduction through the shell is much greater than that of the pole. For cylindrical battery modules, the heat transfer between monomer and monomer may also need to consider the influence of thermal radiation. However, fire baking can act on adjacent batteries as well as surrounding battery system accessories, so it will be more complex and difficult to assess the harm it causes to the battery system.
Some studies have shown that the heat released by the battery burning on fire is higher than the heat released by the simple heat runaway when the battery is not on fire. After a fire occurs, the flame generally attaches around the thermal runaway battery valve body. At the same time, because the temperature of the outer flame of the flame is the highest, the battery and accessories in the opening direction of the valve body are most heated. In addition, from the design point of view, the battery system itself has a certain degree of airtightness, and the high-temperature gas generated by thermal runaway can not diffuse in time, and may also heat the surrounding batteries.
2.3 Contradiction between prevention and design
According to the mechanism of thermal runaway expansion, we can design a targeted scheme to prevent thermal runaway expansion.
First of all, it is necessary to prevent the occurrence of flame. The direction of flame generation can be guided by the design of the injection direction of the valve body; Fire extinguishing agent can also be added to extinguish the fire. Of course, the power battery system has passed the safety test standard. The probability of flame occurrence has been reduced; At the same time, the good sealing of the power battery system makes the internal oxygen content of the battery system insufficient, which is not conducive to the formation and development of flame.
Secondly, the impact of high-temperature gas diffusion on other components of the battery system should be considered. Some batteries have systems that can discharge high-temperature gases in time.
At the same time, the heat transfer path between the cells should be properly blocked, such as the thermal insulation layer between the single cells. It should be noted that in thermal management, air gaps may be reserved between battery shells for air cooling and separate adjacent batteries. However, in the process of thermal runaway expansion, the thermal runaway battery expands, and the air gap will disappear due to the expansion of the battery. At this time, the heat transfer between the battery and the battery is still fast. It is not feasible to prevent the uncontrolled expansion of heat by simply reserving air gap.
In addition, the internal heat dissipation of the battery system can be enhanced after the monomer thermal runaway is triggered; Discharge the battery around the faulty battery; Fill phase change materials between batteries to absorb heat and other methods to inhibit the expansion of thermal runaway.
However, there are certain contradictions between the design of preventing runaway expansion and the design of other functions of the battery system. The method of blocking the heat transfer path may lead to the intensification of the non-uniformity of the internal temperature of the battery pack, which is in contradiction with the design goal of temperature consistency in the thermal management design of the battery pack. In addition, adding fire extinguishing, exhaust, heat insulation and other measures will reduce the specific energy of the battery system and increase the design cost of the battery system. How to reasonably configure security measures to prevent the occurrence of thermal runaway expansion, while taking into account the performance indicators and design costs of the battery system, is one of the important issues in the battery system security design.
The existing lithium-ion power battery has passed the safety standard test, and also has corresponding safety measures in the battery system. The safety of its power battery system has been greatly improved. However, although the hazards caused by existing safety accidents are limited, with the improvement of the specific energy of lithium-ion power batteries, the hazards caused by a single safety accident will increase; The large-scale popularization of electric vehicles will also increase the frequency of safety accidents. Relevant manufacturers must pay attention to the safety of lithium-ion power battery system, and must not reduce production costs by sacrificing the safety of battery system. Because a safety accident happens every day, which endangers the life and property safety of consumers, and of course also means the loss of the reputation of enterprises’ products.