While lithium battery brings convenience to people’s life, its safety is also concerned by everyone. The violent combustion of electrolyte will penetrate the metal plate, and the explosion limit of pyrolysis gas is higher than that of hydrocarbons. The thermal runaway reaction of lithium battery is easy to cause dangerous events, and its high temperature characteristic is an important and basic parameter of fire hazard. Therefore, it is necessary to explore the impact of low-voltage environment on the thermal runaway release temperature of lithium batteries to reduce the risk of their use.
1. Overview of lithium battery Thermal Runaway
The thermal runaway process of lithium battery includes two forms: primary explosion and combustion explosion. In the primary explosion stage, the internal temperature of the lithium battery will increase. If the pressure increases, the pressure relief port of the lithium battery will be opened and smoke will be continuously released. During the combustion explosion process, the lithium battery will have a severe injection problem. A large amount of luminous high-temperature substances will be sprayed from the positive pole of the battery, and secondary combustion will be carried out at the same time.
In the low voltage environment, the smoke released during the thermal runaway primary explosion of the lithium battery takes a long time, will not burn after the injection is completed, and the brightness of the ejected material will also drop. In this paper, thermal runaway reaction experiments were carried out on multiple lithium batteries in various pressure environments of 30kPa, 60kPa and 101kPa to study the influence of pressure on the temperature of thermal runaway release of battery positive electrode.
2.Effect of Low Pressure Environment on Thermal Runaway of Battery
2.1 Impact of low pressure environment on the temperature of lithium ion battery cell
In the process of thermal runaway combustion and explosion of lithium battery, when the battery charge is different, the drop of ambient pressure will lead to the increase of the response temperature of the battery body. Therefore, the greater the pressure, the greater the probability of lithium battery explosion. At the same time, the combustion temperature of the lithium battery body will also decrease with the increase of the charge, and this process will not be affected by the pressure environment. When the charge is 100%, 50% and 0%, the maximum temperature of the lithium battery will increase with the increase of the pressure. The lower the pressure is, the lower the ignition and explosion risk of the lithium battery is. In addition, the maximum temperature of the lithium battery under different pressures will increase with the increase of the charge.
2.2 Impact of low pressure environment on the temperature of lithium ion battery injection port
In the thermal runaway reaction, the temperature at the outlet of the positive end of the battery is the temperature released by the thermal runaway reaction. The energy source of the thermal runaway release mainly includes the following contents: First, the energy released by the secondary combustion reaction of the combustible and flammable substances generated by the thermal runaway in a variety of pressure environments. Second, the high-temperature materials and energy released by the thermal runaway reaction inside the lithium battery. Through repeated experiments in various pressure environments of 30kPa, 60kPa and 101kPa, it can be found that the low pressure environment will have a variety of effects on the ambient temperature caused by thermal runaway of lithium batteries, which can be divided into the following aspects:
(1) 30kPa pressure environment
In a low pressure environment of 30kPa, the thermal runaway release temperature measured at the positive jet outlet of the battery is mainly divided into five peaks: 1~4 high temperature peaks are concentrated during the thermal runaway reaction of seven batteries; peaks 1 and 2 occur during the first and second thermal runaway reactions, with narrow peak width and short duration of high temperature; Part of the peak values 3 and 4 appeared in the third to seventh thermal runaway reaction stage of lithium battery, where the temperature of the peak value 3 was lower, while the peak value 4 occurred in the period when the thermal runaway reaction of lithium ion battery was completed. The temperature of the peak value 4 was close to that of the peak value 5, and the temperature was higher; The peak value 5 appears after the full reaction of thermal runaway is completed. At this time, the residual combustibles ejected from the lithium ion and the battery skin are burning. Therefore, the released temperature is high, the width of the peak value is large, and the high temperature lasts for a long time.
Therefore, the number of temperature peaks released during the thermal runaway phase of lithium batteries in a 30kPa low pressure environment is small, and the width and number of high temperature peaks are lower than those in a 60kPa environment, so the high temperature risk of thermal runaway of batteries is lower than that in a 60kPa Yuli environment. In addition, the oxygen concentration in the 30kPa low pressure environmental culture is low, which can inhibit the internal violent thermal runaway reaction of the lithium ion battery to a certain extent, making it difficult for the organic gas combustible substances generated by the thermal runaway reaction to conduct the secondary combustion reaction in the low pressure environment. It is found that the thermal runaway exothermic reaction of the internal materials of the 30kPa potassium ion battery is the reason for the high temperature heat generated in the low pressure environment.
(2) 60kPa pressure environment
The thermal runaway reaction of the battery forms a combustion and explosion process by spraying bright and high temperature materials, and then produces a high temperature peak. When the pressure environment is 60kPa, the lithium ion battery has 7 thermal runaway reactions. Through the detection of the ambient temperature of the lithium battery injection port, it is found that there are 5 thermal runaway high temperature peaks in the reaction, of which 1 to 4 high temperature peaks are mainly concentrated in the seventh thermal runaway reaction. The lithium ion high temperature peak value 5 appears after the runaway reaction of seven batteries is completed. In this stage, the potassium ion battery sprays combustibles and plastic skins to form a high temperature peak through combustion reaction. The temperature of peaks 2, 3 and 4 is about 800 ℃, and the maximum temperature is about 1100 ℃. The peak width is narrow. Compared with the pressure environment of 101kPa, seven thermal runaway reactions occurred in the environment of 60kPa: the temperature peak of the lithium-ion battery injection port is four, and the analysis shows that the pyrotechnic substances injected by the thermal runaway reaction in Lok Lok have a low impact on the high temperature. If the peak value is higher than 600 ℃, the width will narrow. Therefore, the duration of high temperature generated by thermal runaway reaction in a low pressure environment of 60kPa is shortened. At the same time, the high temperature release reaction of heat runaway in a pressure environment of 60kPa is safer than that under normal pressure, so the oxygen concentration in this environment is lower than that under normal pressure. The heat released at high temperature mainly releases energy through the thermal reaction of the materials inside the battery, and the other part comes from the combustible gas generated by the thermal runaway reaction inside the battery.
(3) 101kPa pressure environment
Under the pressure environment of 101 kPa, that is, under normal pressure, the battery will undergo thermal runaway reaction in turn under certain conditions. Lithium ion batteries have experienced seven severe thermal runaway reactions in a low-voltage environment. During the battery reaction, the high temperature reaction peak will be captured at a position about 30mm above it, and the environmental high temperature peak caused by the dramatic jet combustion reaction will be generally evenly distributed in the range of 1-7 times of thermal runaway reaction. All the high temperature peak temperatures are far more than 600 ℃, the minimum temperature is about 800 ℃, and the maximum temperature can reach 1100 ℃. At the same time, the peak width of lithium ion thermal reaction release high temperature in 101 kPa is large, so the release temperature lasts for a long time. Seven out of control thermal reactions in 101 kPa environment will release high temperature energy and have a continuous high temperature impact on the surrounding environment. In addition, the sources of high temperature released by thermal runaway reaction of lithium battery in the environment with atmospheric pressure of 101kPa include the following elements: intense thermal reaction takes place inside the lithium ion battery to release energy; Flammable gas is formed in lithium battery through reaction, and secondary combustion reaction is carried out in normal pressure environment to release heat.