This passage summarizes the mechanism of thermal runaway and preventive measures, which have been applied in the design and manufacturing of all battery systems. However, in practice, different material systems have different chemical characteristics, so the thermal runaway mechanism of the battery core is different. Different system designs will also lead to different system-level risks and different solutions.
Among them, the explosion is the power battery system is more common harm performance, caused by the impact, is more serious, not only will cause property loss and environmental damage, or even cause personal injury or life danger.
Possible causes of combustion or explosion of a power battery system are:
The exothermic side reaction of the power battery (cell) leads to thermal runaway and ignites the electrolyte and other combustible substances;
1、The local connection resistance of the high voltage circuit of the power battery system is too large, and when a large current flows through it, the temperature rises to the ignition point temperature, igniting the combustible material inside the power battery system.
2、External short circuit: the combustion explosion caused by the external short circuit of the battery below 1Ah is not common. It is usually swollen or burnt off the pole ear, and the combustion is caused by the external short circuit of the power battery or large capacity mobile phone battery. Some users mistakenly use batteries – metal outer ear insulation, don’t consider when smoking will appear at the scene of the installed battery, combustion of abnormal situation, power battery structure fixed measures also common failures (ill-considered), in the process of users, the battery pack battery external insulation protective film damage caused by the swaying or cables damaged skin, eventually lead to short circuit burning, The internal temperature of the power battery system continues to rise, reaching the ignition point temperature, igniting the internal combustible material.
3、Internal short circuit: structure or process defects lead to pole ear insertion, too little diaphragm wrapping surplus, burr, etc. Polar ear insertion often occurs in the structure of the rear battery and internal parallel, the insulation of the pole ear is not in place, and it is easy to lead to internal acute short circuit and combustion or explosion in the subsequent product protection plate or the use process, but now this situation is relatively rare, in addition to external force factors.
According to the analysis of the use of electric vehicles, the probability of the first situation is high and the risk coefficient is also high. The thermal runaway caused by the exothermic side reaction of the battery cell is the main reason for the combustion or explosion of the power battery system.
The main exothermic reactions in lithium-ion batteries are:
The temperature range of ESI film decomposition is 90~120℃.
The reaction temperature of negative electrode and electrolyte reaches above 120℃;
Electrolyte decomposition, the temperature is about 200℃;
The reaction between the positive electrode and the electrolyte is accompanied by the decomposition of the positive electrode and the precipitation of oxygen. The temperature range is 180~500℃.
The negative electrode reacts with the binder at about 240 degrees.
Core thermal runaway (combustion, explosion) is the root cause of the exothermic side reaction inside the cell lead to heat accumulation, the external heat exchange rate of the cell is less than the rate of heat accumulation, the temperature continues to rise, directly to the ignition point temperature, causing combustion and explosion.
The thermal process inside the cell follows the conservation of energy: Qp = Qe + Qa
In the formula, Qp is the heat generated by various negative reactions inside the cell, Qe is the heat exchanged between the cell and the environment, namely the heat dissipation, and Qa is the heat absorbed and accumulated by the telecom itself. If QE ≥ QP, QA is negative or zero, the temperature inside the cell will not rise and thermal runaway will not occur.
It can be seen from the above analysis that if the exothermic side reaction inside the cell can not be blocked, the temperature inside the telecommunication will rise until the thermal runaway event occurs.
If you want to reduce the risk of battery explosions, you can take the following steps:
1、Take necessary protective measures to reduce the occurrence probability of external sudden factors. For example, select professional battery testing equipment to conduct professional battery tests such as overcharge, over-discharge, overheating, short circuit, extrusion, puncture and so on. Among them, it is recommended to use the battery charge and discharge tester to test the overcharge and over-discharge of the battery. Battery short circuit test is recommended to use the battery internal and external tester; Battery extrusion, puncture is recommended to use a needle testing machine. Of course, according to the different size of the battery, it is necessary to choose the appropriate size of the battery test machine, if the battery pack test needs to use the walk-in battery test machine.
2、Blocking the positive feedback process of exothermic side reactions, such as using Bunding fuse process in PACK module, or adding PTC material between positive and negative electrode materials and fluid collector;
3、reduce the heat generated by exothermic side reactions, such as the choice of lithium iron phosphate cathode material, change the organic solvent composition of the electrolyte, etc.;
4、improve the ignition point temperature, such as adding flame retardant materials in the electrolyte, the selection of ceramic diaphragm, etc.;
5、 improve the heat dissipation capacity, avoid heat accumulation, such as Lilang battery using efficient liquid cooling design, there are individual solutions to the whole battery, immersed in the coolant.
The thermal runaway mechanism and preventive measures summarized above have been implemented in the design and manufacturing of all battery systems. However, in practice, different material systems have different chemical characteristics, so the thermal runaway mechanism of the battery core is different. Different system designs will also lead to different system-level risks and solutions.
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