Analysis Of Explosion Types Of Lithium Ion Batteries

There are three types of battery cell explosion: external short circuit, internal short circuit, and overcharge. The outside here refers to the outside 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 component fails to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte to vaporize and expanding the battery shell. When the internal temperature of the battery is as high as 135 ℃, the high-quality diaphragm paper will close the pores, terminate or nearly terminate the electrochemical reaction, cause the current to drop sharply and the temperature to drop slowly, thus avoiding the explosion. However, if the closing rate of the pores is too poor, or if the pores are not closed at all, the battery temperature will continue to rise, more electrolyte will vaporize, and finally the battery shell will be broken, or even the battery temperature will be raised to make the material burn and explode. The internal short circuit is mainly caused by the burr of copper and aluminum foil penetrating the diaphragm, or the dendritic crystal of lithium atom penetrating the diaphragm. These tiny needle like metals will cause a micro short circuit. Because the needle is very thin and has a certain resistance value, the current is not necessarily large.

Copper aluminum foil burrs are caused during the production process. The observable phenomenon is that the battery leakage is too fast, and most of them can be screened out by the cell factory or assembly factory. Moreover, because the burr is small, it will sometimes be burnt out, making the battery return to normal. Therefore, the probability of explosion caused by micro short circuit of burr is not high. Such a statement can be seen from the fact that there are often bad batteries with low voltage shortly after charging in each cell factory, but there are few explosions, which is statistically supported. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because, after overcharge, needle like lithium metal crystals are everywhere on the pole piece, puncture points are everywhere, and micro short circuits are occurring everywhere. Therefore, the battery temperature will gradually increase, and finally the high temperature will cause electrolyte gas. In this case, whether the material burns and explodes due to high temperature, or the shell is first broken, causing air to enter and cause fierce oxidation with lithium metal, the explosion ends.

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 battery temperature is not high enough for the material to burn and the generated gas is not enough to break the battery shell, the consumer will stop charging and take the mobile phone out of the door. At this time, the heat generated by numerous micro short circuits slowly increases the battery temperature. 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, and when they throw it away, it explodes.

Based on the above explosion types, we can focus on the prevention of overcharge, the prevention of external short circuit, and the improvement of the safety of the cell. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which has a great relationship with battery system design and battery assembly. The key to improve the safety of battery cells is chemical and mechanical protection, which is closely related to the battery cell manufacturer.