Analysis of battery swollen up and explosion reasons:
1. Characteristics of lithium ion battery
Lithium is the smallest and most active metal on the chemical periodic table. Small volume, high volume density, widely welcomed by consumers and engineers. However, the chemical characteristics are too active, which brings great danger. When exposed to air, lithium metal will explode with intense oxidation of oxygen. To improve safety and voltage, scientists have invented materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms nanoscale small storage lattices that can be used to store lithium atoms. In this way, even if the lithium ion battery shell ruptures and oxygen enters, the oxygen molecules will be too large to enter these tiny storage cells, so that lithium atoms will not contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to achieve high capacity density and safety at the same time.
When lithium ion batteries are charged, the lithium atoms of positive electrodes will lose electrons and become lithium ions. Lithium ion swims to the negative electrode through the electrolyte, enters the storage cell of the negative electrode, and obtains an electron, which is reduced to lithium atom. When discharging, the whole procedure is reversed. In order to prevent short circuit caused by direct contact between positive and negative electrodes, a kind of diaphragm paper with many holes will be added to the lithium ion battery to prevent short circuit. Good diaphragm paper can also automatically close the fine holes when the lithium ion battery temperature is too high, so that lithium ions can not pass through, in order to self-discard martial arts, to prevent danger.
The lithium ion battery core will start to produce side effects when the charge exceeds 4.2V. The higher the overcharging voltage, the higher the risk. When the voltage of the lithium core is higher than 4.2V, the number of lithium atoms remaining in the cathode material is less than half. At this time, the storage cell often collapses, causing a permanent decline in lithium ion battery capacity. If the charge is continued, subsequent lithium metals will accumulate on the surface of the negative material because the storage cell of the negative electrode is filled with lithium atoms. These lithium atoms will grow dendrites from the surface of the negative electrode to lithium ions. These lithium metal crystals will pass through the separator paper to make the positive and negative poles short circuited. Sometimes the lithium ion battery explodes before the short circuit occurs. This is because in the process of overcharging, materials such as electrolyte will crack to produce gas, causing the lithium ion battery shell or pressure valve to swell and burst, allowing oxygen to react with lithium atoms accumulated on the surface of the negative electrode, and then explode. Therefore, when recharging lithium batteries, it is necessary to set a voltage ceiling, so as to take into account the life, capacity and safety of the batteries at the same time. The ideal charge voltage limit is 4.2V.
The discharge voltage of the lithium core should also have a lower voltage limit. When the core voltage is lower than 2.4V, some materials will start to be destroyed. Because the lithium ion battery will self-ithium ion ldischarge, the longer the discharge time, the lower the voltage. Therefore, it is better not to put 2.4V to stop discharging. The lithium ion battery releases only about 3% of the lithium ion battery capacity during the period from 3.0V to 2.4V. Therefore, 3.0V is an ideal discharge cut-off voltage.
In addition to voltage limitation, current limitation is also necessary. When the current is too large, the lithium ion will not enter the storage cell and will aggregate on the surface of the material. When these lithium ions acquire electrons, lithium atoms will crystallize on the surface of the material, which is as dangerous as overcharging. If the lithium ion ibattery shell breaks, it will explode.
Therefore, the protection of lithium-ion batteries should include at least three items: the upper limit of charging voltage, the lower limit of discharging voltage and the upper limit of current. In general, besides the lithium ion battery core, there will be a protective plate in the lithium ion battery pack, which mainly provides these three kinds of protection. However, the protection of these three protection is obviously not enough, and the global lithium ion battery explosion is still frequent. To ensure the safety of the lithium ion battery system, we must make a more careful analysis of the cause of the lithium ion battery explosion.
Two. Reasons for lithium ion battery explosion:
1: internal polarization is bigger!
2: the polar piece absorbs water and reacts with the electrolyte.
3: the quality and performance of the electrolyte itself.
4: when the liquid injection time, the injection volume can not meet the technological requirements.
5: the laser welding process in the assembly process is poor in sealing performance, leak and leak detection.
6: dust, polar piece of dust first easily lead to short circuit, the specific reasons are unknown.
7: the positive and negative plates are thicker than the process area and difficult to enter the shell.
8: the sealing problem of liquid injection, the poor sealing performance of steel ball results in bumps.
9: the shell material is thick and the shell deformation affects the thickness.
Three. Explosion type analysis
The type of lithium ion battery core explosion can be classified into three types: external short circuit, internal short circuit and overcharge. The outer part of the lithium ion battery refers to the outer part of the core, which includes the short circuit caused by poor insulation design of the lithium ion battery pack.
When the short circuit occurs outside the core and the electronic components fail to cut off the circuit, high heat will occur inside the core, which will cause some electrolytes to vaporize and expand the lithium ion battery shell. When the temperature inside the lithium ion battery reaches 135 degrees Celsius, the good quality diaphragm paper will close the fine hole, terminate or nearly terminate the electrochemical reaction, and the current will drop sharply and the temperature will drop slowly, thus avoiding the explosion. However, the poor closure rate of fine holes, or the diaphragm paper that the fine holes will not close at all, will make the lithium ion battery temperature continue to rise, more electrolytes will vaporize, and finally the lithium ion battery shell will burst, or even raise the lithium ion battery temperature to make the material burn and explode.
The internal short circuit is mainly caused by the burr of copper foil and aluminium foil breaking through the diaphragm, or the dendritic crystal of lithium atom breaking through the diaphragm. These tiny needled metals can cause short circuit. Because the needle is very thin and has a certain resistance value, the current will not be very large. The burr of copper and aluminium foil is caused by the production process. It can be observed that the leakage of lithium ion battery is too fast. Most of the burrs can be detected by the core factory or assembly plant. Moreover, because the burr is fine, sometimes it will be burned, so that the lithium ion battery will return to normal. Therefore, the probability of burglings caused by short circuit burr is not high.
In this way, it can be said that there are often bad batteries with low voltage shortly after charging, but few explosions occur, which is supported by statistics. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because there are needle-like lithium crystals everywhere on the overcharged rear pole sheet, puncture points are everywhere, and micro-short circuits occur everywhere. Therefore, the lithium ion battery temperature will gradually increase, and finally the high temperature will be the electrolyte gas. In this case, whether the material is burned and exploded due to excessive temperature or the shell is broken first, which causes the air to enter and the lithium metal to oxidize intensely, it is the end of the explosion.
But the explosion caused by overcharging caused by internal short circuit does not necessarily happen at the time of charging. It is possible that consumers will stop charging and take their mobile phones out before the temperature of the lithium ion battery is high enough to make the material burn and produce enough gas to break the lithium ion battery shell. At this time, the heat generated by numerous micro-short circuits slowly increases the temperature of the lithium ion battery, and after a period of time, the explosion occurs. The common description of consumers is that when picking up a cell phone, the cell phone is found to be very hot and exploded after being thrown away.
Based on the above types of explosion, we can focus on the prevention of overcharge, external short circuit, and enhance the safety of the core. Overcharge prevention and external short circuit prevention belong to electronic protection, which is closely related to lithium ion battery system design and lithium ion battery assembly. The emphasis of the core safety enhancement is chemical and mechanical protection, which is related to the lithium ion battery core manufacturer.
Four. Design specifications
Since there are hundreds of millions of mobile phones in the world, the failure rate of security protection must be less than one in 100 million to achieve safety. As a result, the failure rate of printed circuit boards is generally far higher than 1/100000000. Therefore, when designing the lithium ion battery system, there must be more than two safety lines. The common mistake is to use the adaptor to charge the lithium ion battery pool directly. In this way, the overloaded protective task is completely handed over to the protective panel on thelithium ion battery pack. Although the failure rate of protective panels is not high, even if the failure rate is as low as one in a million, the probability of explosion accidents will occur everyday in the world.
If the lithium ion battery system can provide two safety protections for overcharge, overdischarge and overcurrent respectively, the failure rate of each protection can be reduced to one in 100,000 if the failure rate of each protection is one in 10,000. The common lithium ion battery charging system block diagram is as follows, including two parts of charger and lithium ion battery pack. The charger also includes two parts: adapter (Adaptor) and charge controller. The adapter converts AC to DC, while the charging controller limits the maximum current and voltage of DC. The lithium ion battery pack contains two parts of the protection panel and the lithium ion battery core, and a PTC to limit the maximum current.