Three levels of safety issues of lithium power battery system (2): Trigger

In the above 'Three Levels of Lithium Power Battery System Safety Issues (1): Evolution'The concept of

3. Lithium power battery safety accident triggering

After the evolution process, battery accidents will enter the 'triggering' stage. Generally, after entering the triggering phase, the energy inside the lithium-ion power battery will be released in an instant. This process is irreversible and uncontrollable, which is also called thermal runaway. After thermal runaway, the battery heats up violently, the temperature can be as high as 1000 ℃, and phenomena such as smoke, fire and explosion can be observed.

Of course, from the broad definition of 'safetyFor example, thermal runaway does not necessarily occur after a battery collision; and if the insulation failure of the battery pack causes high-voltage electric shocks, and the battery leakage produces peculiar smells that cause discomfort to the vehicle personnel, the battery will not experience thermal runaway. In the safety design of the power battery system, the above situations need to be considered. Thermal runaway is the most common cause of safety accidents, and it is also a unique feature of lithium-ion power battery safety accidents. Therefore, a special introduction is made here with thermal runaway as the core.

A large number of experimental studies have shown that the battery after thermal runaway may not necessarily cause smoke, fire and explosion at the same time, or it may not happen, depending on the thermal runaway of the battery material The mechanism. Figure 4, Figure 5 and Table 2 show the thermal runaway mechanism of a 25 A·h lithium ion power battery with a ternary positive electrode/PE matrix ceramic separator/graphite negative electrode. Figure 4 shows the temperature and voltage curve in the adiabatic thermal runaway experiment of this lithium-ion power battery. According to the characteristics of the thermal runaway temperature change, the thermal runaway process is divided into 7 stages. In different stages, battery materials have undergone different changes. Figure 5 explains the changes in battery materials at various stages through a series of pictures.

Table 2 lists the seven phased characteristics and corresponding mechanisms of the thermal runaway of this lithium-ion power battery. Combining Figure 5 and Table 2, can explain the battery smoke, fire and explosion after thermal runaway. In the case of smoke, in stage V, if the internal temperature of the battery is lower than the melting temperature of the anode current collector aluminum foil of 660°C, the cathode coating of the battery will not spray out with the gas generated by the reaction, and what is observed at this time will be white. If the internal temperature of the battery is higher than 660℃, the positive electrode current collector aluminum foil will melt, and the positive electrode coating of the battery will spray out a large amount of gas generated by the reaction. At this time, black smoke will be observed. For fires, fires in thermal runaway accidents are generally caused by the ignition of the electrolyte and its decomposition products. Therefore, starting from Phase II, the electrolyte leaked from the safety valve may be ignited and cause fire.

From the perspective of the 3 elements of the combustion reaction (combustibles, oxygen, and ignitors), the combustibles are mainly electrolyte; there is insufficient oxygen in the battery, so the electrolyte A fire needs to be leaked out; the main cause of ignition is that the temperature of the emitted gas is higher than its flash point. In the case of an explosion, the explosion is generally manifested as an impact caused by the instantaneous diffusion of high-pressure gas. The battery has conditions for the accumulation of high-pressure gas, and the safety valve is the key to timely release of the high-pressure gas accumulation. If the safety valve body can be opened before the battery shell ruptures and release enough high-pressure gas generated in the thermal runaway process, the battery will not explode; if the safety valve body cannot be opened in time, an explosion accident may occur.

Classification of accident triggering

There are many reasons for the triggering of lithium-ion power battery thermal runaway accidents. According to the characteristics of the triggering, it can be divided into three types: mechanical triggering, electrical triggering and thermal triggering. As shown in Figure 6, the three types of trigger forms have certain internal connections. Generally, mechanical triggering will cause a short circuit and cause electrical triggering, while electrical triggering heat generation causes thermal triggering, and thermal runaway caused by thermal triggering is the core of accident triggering. The mechanism analysis of other trigger forms is inseparable from the research on the thermal trigger mechanism.

Mechanical triggering includes squeezing, acupuncture, drop, etc. The main feature is the deformation of the battery under force;

Electrical triggering includes external short circuit and internal short circuit. , Overcharge, overdischarge, etc. The main feature is that there is current flow during the triggering process;

The thermal triggering includes abnormal heating, flame heating, etc. The main feature is that the battery continues to absorb heat from the environment and the temperature rises.

The safety test standard specifies various accident triggering factors obtained from accident analysis in detail. The probability of triggering accidents in batteries that have passed the safety test standards is also greatly reduced. However, because the actual working conditions are very complicated, the cause of the accident may be different from the situation stipulated in the safety test standard. This explains why the various power battery systems listed in Table 1 have passed the safety test standards, and accidents may still occur.

The promotion of lithium battery industry chain enterprises, lithium grid (li-b.cn) welcomes contributions.