Analysis of common causes and types of explosions in lithium-ion batteries

1. There are serious quality problems in raw materials and incoming materials, and the most important thing is the quality of the electrolyte; 2. The pole piece absorbs water, and the water reacts with the electrolyte, resulting in a large amount of gas and heat; 3. The amount of liquid injection is too small, and the internal resistance of charging and discharging Larger, affecting lithium ion transmission; 4. Cover plate welding and sealing process is poor, sealing performance is poor, there is air leakage, external moisture enters; 5. Internal micro short circuit, mainly due to dust burrs, mostly from the cutting of pole pieces And the burr of the cutter; 6. If the current is too large, the negative electrode cannot accept more lithium ions, forming dendrites, which pierce the diaphragm and short-circuit; 7. The battery is overcharged, and the material structure collapses, and lithium dendrites form the piercing diaphragm. Explosion type analysis Li-ion battery explosion type, it is mainly divided into short circuit and overcharge, short circuit is divided into internal short circuit and external short circuit. Let's talk about the external short circuit first. The principle is very simple. A conductive material connects the positive and negative electrodes at the same time from the outside of the battery. When an external short circuit occurs, the current increases and the battery starts to generate heat. When the heat increases, the electrolyte will decompose, a large amount of gas will appear, and the battery will swell. We said that many diaphragms have the function of closing pores. When a certain temperature is reached, the diaphragm will close the pores and prevent the transmission of lithium ions, thereby blocking the chemical reaction, the current drops, and the temperature slowly drops, thus preventing the battery from exploding. Of course, the above is only based on a good quality diaphragm. The main reason for the internal short circuit is that the burrs of the copper foil and the aluminum foil pierce the diaphragm, or the dendrites of lithium atoms pierce the diaphragm. These tiny needle-like metals can cause micro short circuits. Because it is very thin and has a certain resistance value, the current is not necessarily very large. The copper and aluminum foil burrs can be caused during the production process. Observable phenomena are battery leakage and fast self-discharge, most of which can be detected by a short circuit tester. Moreover, due to the small burrs, they will sometimes be burned, causing the battery to return to normal. Therefore, the probability of explosion caused by burr micro-short circuit is not high. Burrs mainly come from slitting, and laser cutting has few burrs. There are more sources of dendrite, which can be caused by high currents, lithium evolution, and so on. Explosion caused by overcharging is relatively common. After the battery is overcharged, needle-like lithium metal crystals are everywhere on the pole pieces, the puncture points are everywhere, and micro short circuits are occurring everywhere. Therefore, the battery temperature will gradually rise, and finally the high temperature will cause the electrolyte to gas. In this case, whether the temperature is too high to cause the material to burn and explode, or the outer shell is first broken, causing the air to enter and oxidize the lithium metal, it is an explosion! ! ! However, the explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. We use common mobile phone battery analysis, and there is another situation. When the battery temperature is not high enough to burn the material and the gas generated is not enough to break the battery casing, we stop charging and take the mobile phone out. At this time, the heat generated by the numerous micro-short circuits gradually increased the battery temperature, and after a period of time, it exploded. The common description of consumers is that when they pick up the phone, they find that the phone is very hot, and it explodes after throwing it away. Although many of them now have automatic power-off functions, there is still a warm reminder, try not to charge for a long time. Regardless of whether the protection mechanism is charging or discharging, we have a measurement standard, and usually use voltage to stop. Take the ternary battery with lithium nickel cobalt manganate and graphite anode as an example. When the charging voltage of the battery is higher than 4.2V, the danger will come quietly. The higher the voltage, the higher the danger. Because when the voltage is higher than 4.2V, the number of lithium atoms in the positive electrode material decreases, and the structure of the stored atoms collapses, and it is permanent and irreversible. If you continue to charge, then when the negative electrode is full of lithium ions, lithium ions will generate lithium metal with electrons on the surface of the negative electrode, which is what we call dendrites. These dendrites will pierce the diaphragm and short-circuit the positive and negative electrodes. Overcharge will also increase the temperature. If the temperature exceeds 180°C, the electrolyte will decompose, a large amount of gas and heat will appear, and the shell will expand. After rupture, oxygen enters and reacts violently with lithium metal and will explode. Therefore, the upper limit of the charging voltage is a protective measure. There is also a lower voltage limit when discharging lithium-ion batteries. When the cell voltage is lower than 2.4V, some materials will begin to be destroyed. And because the battery will self-discharge, the longer it is left, the lower the voltage will be. Therefore, it is best not to stop when the battery is discharged to 2.4V. During the period of lithium ion battery discharge from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery capacity. Usually, our ternary battery chooses 3.0V as an ideal discharge cut-off voltage. So not over-discharging is another protective measure. In addition to the charge cut-off voltage and discharge cut-off voltage just mentioned, we also use current to protect the battery, which is the rate performance that we often pay attention to. The so-called full charge in a few minutes uses special materials, such as graphene. It can be simply understood as your teacup, the kind with a mesh tea divider. If you pour the water slowly, it will not block at all and the water will not splash out. But if you change a big faucet, you will find that the Internet is blocked and there is no time to get in. The important manifestation of the damage of high current to the battery is that when the current is too large, lithium ions will not have time to enter the storage cell and will accumulate on the surface of the material. After these lithium ions gain electrons, lithium atoms will crystallize on the surface of the material, which is the same as overcharging and can cause danger. If the battery casing ruptures, it will explode. Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: How to maintain the power lithium-ion battery in winter?