In daily life, lithium batteries can be seen almost everywhere, ranging from electric vehicles to computer motherboard batteries. Lithium batteries have become an indispensable part of people. We often see various safety accidents caused by lithium batteries. The most impressive phenomenon is the explosion and fire caused by lithium batteries.
1. Avoid short circuit and overcharge.
Almost all safety accidents caused by lithium batteries are caused by short circuit. We know that when the positive and negative electrodes of the battery are connected to each other in an abnormal path under the condition of very small resistance, that is, when we often say short circuit, very large current and heat will be generated in the battery. The generated heat and excessive electric energy release will not only seriously damage the battery life, but also for the lithium battery sealed, A certain pressure will be generated inside the battery, resulting in a sudden increase in the internal pressure of the battery. Due to the very active chemical characteristics of lithium ions, the shell will burst and burn eventually. Due to the chemical characteristics of lithium-ion battery, when we overcharge (overcharge) the battery, the negative electrode of lithium battery cannot embed more lithium ions, resulting in the precipitation of lithium ions as metal lithium on the surface of the negative electrode, resulting in the occurrence of dendrite lithium phenomenon. When the dendrite lithium grows to a certain extent, it will pierce the diaphragm, resulting in internal short circuit and damage of isolation membrane, There will also be internal short circuit, resulting in safety accidents. Therefore, in our daily use of lithium battery, we should try to avoid short circuit or overcharge. However, for most digital products, their internal charging circuit will be equipped with corresponding protection IC to avoid overcharge of lithium battery. When the protection circuit detects that the lithium battery has reached the full power state, it will automatically cut off the charging circuit. However, it is still not recommended to connect the mobile phone or other devices to the charger in the power on state for a long time. After all, no one wants to bet with a small charging protection chip with his mobile phone or other devices or even personal safety.
2. Piercing is also dangerous.
Compared with short circuit and overcharge, piercing lithium battery is also a very unwise practice. If the lithium battery is pierced by any hard object, its internal lithium ion will directly react with oxygen in the air, and violent combustion will also occur. For lithium batteries or other electronic products with lithium batteries inside, we usually see eye-catching trash can signs. If the lithium battery is discarded at will, it will not only cause environmental pollution, but also be prone to fire in the process of waste treatment.
3. Avoid high temperature and fire baking. High temperature or fire burning will also lead to explosion and combustion of lithium battery.
Especially in the hot summer or in the car exposed to the sun for a long time, the ambient temperature of lithium battery will be higher than its normal storage temperature. Therefore, when we use the equipment containing lithium-ion battery, we should try to avoid using it in the environment of too high temperature for a long time. Although the explosion and combustion time of lithium battery is relatively long when it is directly roasted by fire, there will still be a sudden increase in its internal pressure, which is what we often call bulging. When our mobile phone battery or other lithium-ion battery is bulging, we should immediately cut off the power supply and stop using it to prevent further increase of internal pressure and replace it with a new battery.
Battery explosion generally occurs in the following situations:
1. Overcharge explosion
The protection circuit is out of control or the detection cabinet is out of control, so that the charging voltage is greater than 5V, resulting in electrolyte decomposition, violent reaction inside the battery, rapid rise of internal pressure of the battery and battery explosion.
2. Overcurrent explosion
When the protection circuit is out of control or the detection cabinet is out of control, the charging current is too large, causing lithium ions to be embedded too late. However, lithium metal is formed on the surface of the electrode sheet, penetrating the diaphragm, and the positive and negative electrodes are directly short circuited, resulting in explosion (rarely).
3. Explosion during ultrasonic welding of plastic shell
When ultrasonic welding plastic shell, due to equipment reasons, its ultrasonic energy is transferred to the battery cell. The ultrasonic energy is very important to melt the internal diaphragm of the battery, and the positive and negative electrodes are directly short circuited, resulting in explosion.
4. Explosion during spot welding
During spot welding, excessive current causes serious internal short circuit and explosion. In addition, during spot welding, the positive connecting piece is directly connected with the negative electrode, so that the positive and negative electrodes will explode after direct short circuit.
5. Over discharge explosion
Over discharge or over-current discharge (above 3C) of the battery is easy to dissolve and deposit the negative copper foil on the diaphragm, resulting in direct short circuit of the positive and negative electrodes and explosion (rarely).
6. Explosion during vibration drop
The internal pole pieces of the electric core are misaligned due to violent vibration or falling of the electric core, resulting in direct and serious short circuit and explosion (rarely occurs).
After the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the risk. When the voltage of the lithium cell is higher than 4.2V, less than half of the lithium atoms remain in the cathode material. At this time, the storage cell often collapses, resulting in a permanent decrease in the battery capacity. If charging is continued, since the storage cell of the negative electrode has been filled with lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the surface of the negative electrode in the direction of lithium ions. These lithium metal crystals will pass through the diaphragm paper and short circuit the positive and negative electrodes. Sometimes the battery will explode before the short circuit occurs, because in the overcharge process, electrolyte and other materials will crack to produce gas, which makes the battery shell or pressure valve bulge and crack, allowing oxygen to enter and react with lithium atoms accumulated on the surface of the negative electrode, and then explode.
Therefore, when charging lithium battery, the upper voltage limit must be set to take into account the service life, capacity and safety of the battery at the same time. The optimal upper limit of charging voltage is 4.2V. There should also be a lower voltage limit when the lithium cell is discharged. When the cell voltage is lower than 2.4V, some materials will begin to be damaged. Because the battery will discharge itself, the longer it is discharged, the lower the voltage will be. Therefore, it is best not to stop at 2.4V during discharge. During the period from 3.0V to 2.4V, the energy released by lithium battery accounts for only about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. During charging and discharging, in addition to voltage limitation, current limitation is also necessary. When the current is too large, lithium ions do not have time to enter the storage cell and will accumulate on the material surface.
After these lithium ions obtain electrons, lithium atoms will crystallize on the surface of the material, which, like overcharge, will cause danger. If the battery case breaks, it will explode. Therefore, the protection of lithium-ion batteries should include at least three items: upper charging voltage limit, lower discharge voltage limit and upper current limit. In general, there is a protective plate in the lithium battery pack except the lithium battery cell, which mainly provides these three protections. However, these three protections of the protection board are obviously not enough, and lithium battery explosions are still frequent all over the world. To ensure the safety of the battery system, the cause of battery explosion must be analyzed more carefully.
1. Large internal polarization;
2. The pole piece absorbs water and reacts with the electrolyte, resulting in gas drum;
3. Quality and performance of electrolyte itself;
4. During liquid injection, the liquid injection volume cannot meet the process requirements;
5. The sealing performance of laser welding in the assembly process is poor, and there is air leakage when measuring air leakage;
6. Dust and electrode dust are easy to cause micro short circuit first;
7. The anode and cathode plates are thicker than the process range, so it is difficult to enter the shell;
8. Liquid injection sealing problem, poor sealing performance of steel ball, resulting in air drum;
9. The shell wall is too thick for shell incoming materials, and the shell deformation affects the thickness;
10. Too high ambient temperature is also the main cause of explosion.
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