In daily life, lithium batteries are almost everywhere, from electric cars 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 thing is the explosion and fire caused by lithium batteries.
1. Avoid short circuit and overcharge.
Most of the safety accidents caused by lithium batteries are caused by short circuits. We know that when the positive and negative electrodes of the battery are connected to each other in an abnormal path with a very low resistance, which is often referred to as a short circuit, a very large current and heat are generated inside the battery, and the heat and excessive power are generated. The release will not only lead to severe damage to the battery life, but for sealed lithium batteries, a certain pressure will be generated inside the battery, which will cause a sudden increase in the internal pressure of the battery, and because the lithium ion's chemical properties are very active, it will eventually Cases of bursting and burning of the housing occur. Due to the chemical characteristics of lithium-ion batteries, when we overcharge (overcharge) the battery, the lithium battery negative electrode cannot insert more lithium ions, resulting in the precipitation of lithium ions as metallic lithium on the surface of the negative electrode, resulting in dendritic lithium The emergence of lithium, when the dendritic lithium grows to a certain degree, it will pierce the separator, causing a short circuit inside the battery, and the separator will be damaged. The internal short circuit will also occur, which will cause a safety accident. Therefore, in our daily use of lithium batteries, we should try to avoid short circuits or overcharges. However, for most digital products, the internal charging circuit will be equipped with a corresponding protection IC to avoid overcharging of lithium batteries. When the protection circuit detects that the lithium battery has reached a fully charged state, it will automatically cut off the charging circuit. However, it is still not recommended to connect the mobile phone or other equipment to the charger under the power supply for a long time. After all, no one wants to use their mobile phone or other equipment or even personal safety to make a bet with a small charging protection chip.
2. Piercing is equally dangerous.
Compared to short circuit and overcharge, piercing a lithium battery is also a very unwise approach. If a lithium battery is pierced by any hard object, the lithium ions inside it will directly react with oxygen in the air, and the phenomenon of violent combustion will also occur. For the lithium battery itself or other electronic products that contain lithium batteries, we usually see a prominent trash can sign. If the lithium battery is discarded at will, it will not only cause environmental pollution, but also be more prone to fires and other situations during garbage disposal.
3. Avoid high temperature and fire grilling or burning will also cause the lithium battery to explode and burn.
Especially in the hot summer or in the car exposed for a long time, the ambient temperature of the lithium battery will be higher than its normal storage temperature. Therefore, when we use equipment containing lithium-ion batteries, we should try to avoid prolonged use in environments with excessively high temperatures. Although lithium batteries have a relatively long time to explode and burn when they are directly fired, but There will still be a sudden increase in internal pressure, which is what we often say. When our mobile phone battery or other lithium-ion batteries bulge, we should immediately cut off the power and stop using it to prevent the internal pressure from increasing further and replace the battery with a new one.
There are several situations in which a battery explosion occurs:
1. Overcharge explosion
The runaway of the protection circuit or the runaway of the detection cabinet caused the charging voltage to be greater than 5V, which caused the electrolyte to decompose, a violent reaction occurred inside the battery, the battery internal pressure rose rapidly, and the battery exploded.
2. Overcurrent explosion
The runaway of the protection circuit or the runaway of the detection cabinet makes the charging current too large to cause the lithium ions to be inserted too late, and forms lithium metal on the surface of the pole piece, penetrates the diaphragm, and causes a short circuit between the positive and negative electrodes to cause an explosion (rarely).
3． Explosion when ultrasonic welding plastic case
When ultrasonic welding a plastic case, the ultrasonic energy is transferred to the battery core due to equipment reasons. The ultrasonic energy is very large, which causes the internal membrane of the battery to melt, and the positive and negative electrodes are directly short-circuited, causing an explosion.
4． Explosion during spot welding
Excessive current during spot welding caused an explosion inside the serious short circuit. In addition, the positive electrode connection piece was directly connected to the negative electrode during spot welding, which caused the positive electrode and the negative electrode to explode after a short circuit.
5． Over-explosion
The battery over-discharge or over-current discharge (above 3C) easily dissolves and deposits the negative electrode copper foil on the separator, which causes the positive and negative electrodes to short-circuit directly and cause an explosion (rarely).
6. Explosion when dropped by vibration
When the battery cell is severely vibrated or dropped, the internal pole piece of the battery cell is dislocated, and it is directly short-circuited and exploded (rarely).
After the lithium battery is overcharged to a voltage higher than 4.2V, it will start to have side effects. The higher the overcharge voltage, the higher the danger. After the voltage of the lithium battery cell is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half. At this time, the storage cell often collapses, causing the battery capacity to drop permanently. If you continue to charge, since the storage cell of the negative electrode is already filled with lithium atoms, 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 toward the lithium ions. These lithium metal crystals will pass through the separator paper, shorting the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharge process, materials such as the electrolyte will crack to produce gas, which will cause the battery case or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the negative electrode surface. And then exploded.
Therefore, when charging a lithium battery, the upper limit of the voltage must be set so that the battery life, capacity, and safety can be taken into account at the same time. The optimal upper limit of the charging voltage is 4.2V. There is also a lower voltage limit when the lithium battery is discharged. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. And because the battery will self-discharge, the voltage will be lower the longer it is discharged, so it is best not to stop at 2.4V during discharge. During the period when the lithium battery is discharged from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. When charging and discharging, in addition to the voltage limitation, the current limitation is also necessary. When the current is too large, lithium ions have no time to enter the storage cell, and will collect on the surface of the material.
After these lithium ions acquire electrons, lithium atom crystals will be generated 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 must include at least: the upper limit of the charging voltage, the lower limit of the discharge voltage, and the upper limit of the current. In general, in addition to the lithium battery cell, there will be a protection plate in the lithium battery pack. This protection plate mainly provides these three protections. However, these three protections of the protection board are obviously insufficient, and the global lithium battery explosion incidents are still frequent. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.
1. Large internal polarization;
2. The pole piece absorbs water and reacts with the electrolyte.
3. The quality and performance of the electrolyte;
4. The amount of injection during the injection does not meet the process requirements;
5. The laser welding has poor sealing performance during the assembly process, and leaks when measuring leaks;
6. Dust, pole piece dust is easy to cause micro short circuit first;
7. The positive and negative plates are thicker than the process range, and it is difficult to insert the shell;
8. Injecting sealing problem, poor sealing performance of steel ball causes air drum;
9. The shell material is too thick, and the shell deformation affects the thickness;
10.Excessive ambient temperature is also the main cause of the explosion