Lithium battery explosion: the importance of lithium battery overcharge protection
"Lithium battery" is a kind of battery with lithium metal or lithium alloy as positive / negative material and non-aqueous electrolyte solution.
After lithium ions obtain electrons, lithium atoms will crystallize on the surface of materials, which is as dangerous as overcharge. If the battery case breaks, it will explode. Therefore, the protection of lithium-ion battery should at least include: upper limit of charging voltage, lower limit of discharge voltage and upper limit of current. Generally, in addition to the lithium battery cell, there is a protection board in the lithium battery pack, which mainly provides the three protections.
The three protections of the protection board are obviously not enough, and the global lithium battery explosions are still frequent. This is also the reason why the aviation industry has banned carrying large capacity lithium batteries on board.
The causes of lithium battery explosion include the following:
1. The internal polarization is large;
2. The electrode absorbs water and reacts with the electrolyte;
3. The quality and performance of electrolyte itself;
4. The injection volume can not meet the process requirements;
5. In the assembly process, the sealing performance of laser welding is poor, and there is air leakage when measuring the air leakage;
6. Firstly, dust and electrode dust are easy to cause micro short circuit;
7. The positive and negative plates are thicker than the process range, so it is difficult to put them into the shell;
8. The sealing problem of liquid injection and the poor sealing performance of steel ball lead to air drum;
9. The shell wall is too thick in the incoming material, and the shell deformation affects the thickness;
10. The high temperature outside is also the main cause of the explosion.
The main explosion types of lithium batteries are as follows:
The types of lithium battery explosion can be classified into three types: external short circuit, internal short circuit and overcharge. The external part here refers to the external part of the cell, including the short circuit caused by poor internal insulation design of the battery pack. When there is a short circuit outside the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte to vaporize, which will support the battery shell and lead to explosion.
When the internal temperature of the battery is as high as 135 ℃, the good separator paper will close the pores, the electrochemical reaction will be terminated or nearly terminated, the current will drop suddenly, and the temperature will drop slowly, thus avoiding the explosion.
The internal short circuit is mainly caused by the burr of copper foil and aluminum foil penetrating the diaphragm, or the dendritic crystal of lithium atom penetrating the diaphragm.
These fine needle like metals can cause micro short circuits. Because the needle is very thin and has a certain resistance value, the current is not necessarily very large. The burr of copper and aluminum foil is caused by the production process. It can be observed that the leakage of battery is too fast, and most of them can be screened out by cell factory or assembly factory. Moreover, due to the small burr, sometimes it will be burnt out, making the battery return to normal. Therefore, the probability of explosion caused by burr micro short circuit is not high. This view can be seen from the fact that there are bad batteries with low voltage soon after charging in all cell factories, but there are few explosions, which is supported by statistics. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge.
Overcharge explosion is also a common occurrence of inferior batteries.
After overcharge, needle like lithium metal crystals are everywhere on the electrode piece of lithium battery, and puncture points are everywhere, and micro short circuit occurs everywhere. Therefore, the temperature of the battery will gradually increase, and finally the high temperature will cause the electrolyte gas to evaporate. In this case, whether the material burns and explodes when the temperature is too high, or the shell is broken first, causing the air to go in and the lithium metal to undergo intense oxidation, it is the end of the explosion. However, the explosion caused by internal short circuit caused by overcharge does not necessarily occur at the time of charging. It is possible that when the temperature of the battery is not high enough for the material to burn and the gas produced is not enough to support the battery shell, the consumer will stop charging and take the mobile phone out. At this time, the heat generated by many micro short circuits slowly increases the temperature of the battery. After a period of time, the explosion occurs. The common description of consumers is that when they pick up the mobile phone, they find it very hot, throw it away and explode.
The focus of lithium battery explosion is to pay attention to the prevention of overcharge, the prevention of external short circuit and the improvement of cell safety. Overcharge prevention and external short circuit prevention belong to electronic protection, which has a great relationship with battery system design and battery assembly. The focus of cell safety improvement is chemical and mechanical protection, which has a great relationship with cell manufacturers.