Drum case of lithium battery
Shell characteristics
Lithium, atomic number 3, atomic weight 6.941, is the lightest alkali metal element. 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 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.
protective measures
When the overcharge voltage of lithium batteries is 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 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 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 grow dendritic crystals from the negative surface toward the direction of lithium ions. These lithium crystals will pass through the diaphragm paper and short the positive and negative electrodes. Sometimes the 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 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 optimal upper limit of charging voltage is 4.2V. There is also a lower voltage limit when lithium cores are discharged. When the core voltage is less than 2.4V, some materials will begin to be destroyed. Because the battery will self-discharge, the longer the discharge time, the lower the voltage. Therefore, it is better not to put 2.4V to stop discharging. During the period from 3.0V discharge to 2.4V discharge, the energy released by lithium batteries only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. In addition to voltage limitation, current limitation is also necessary when charging and discharging. 电流过大时，锂离子来不及进入储存格，会聚集 于材料表面。
When these lithium ions acquire electrons, lithium atoms will crystallize on the surface of the material, which is as dangerous as overcharging. If the battery case 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 battery core, there will be a protective plate in the lithium battery pack, which mainly provides these three kinds of protection. However, these three protections are obviously insufficient, and the explosion of lithium batteries is still a frequent occurrence in the world. In order to ensure the safety of battery system, the cause of battery explosion must be analyzed more carefully.
The explosion cause
1. Large internal polarization;
2. The electrode sheet absorbs water and reacts with the electrolyte to form gas drum;
3. Quality and performance of electrolyte itself;
4. During the infusion, the infusion volume fails to meet the process requirements;
5. Poor sealing performance of laser welding in the installation and preparation process, and air leakage during air leakage measurement;
6. Dust and extremely thin dust are easy to cause micro-short circuit at first;
7. The anode and cathode plates are thicker than the process range, making it difficult to enter the shell;
8. The sealing problem of liquid injection and the poor sealing performance of steel ball lead to gas drum;
9. The incoming material of the shell is thick, and shell deformation affects the thickness;
High ambient temperature outside is also the main cause of the explosion.
The explosion type
Analysis of explosion types types of cell explosion can be summarized as external short circuit, internal short circuit, and overcharge. The external part refers to the external part of the cell, including the short circuit caused by poor insulation design inside the power pool group. When a short circuit occurs outside the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte to vaporize and stretch the battery housing. When the internal temperature of the battery is up to 135 degrees Celsius, the diaphragm paper of good quality will close the pore, the electrochemical reaction will be terminated or almost terminated, the current will drop sharply, and the temperature will also drop slowly, thus avoiding explosion. However, the poor pore closure rate, or the diaphragm paper that the pore will not close at all, will cause the battery temperature to continue to rise, more electrolyte vaporization, and finally the battery shell burst, or even increase the battery temperature to make the material burn and explode. The internal short circuit is mainly caused by the burr of copper foil and aluminum foil piercing the diaphragm, or the dendritic crystal of lithium atoms piercing the diaphragm.
These fine, needle-like metals can cause a micro-short circuit. Because the needle is very thin and has a certain resistance value, the current is not necessarily very large. The burr of copper and aluminum foil is caused in the production process, and the observable phenomenon is that the battery leakage is too fast, most of which can be screened out by the cell factory or assembly factory. And because the burrs are small, they can sometimes burn out, allowing the battery to return to normal. Therefore, the probability of explosion due to burr short circuit is not high. Such a statement can be from the cell factory inside are often charging soon, the voltage on the low side of the bad battery, but there are few incidents of explosion, statistical support. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge.
Because, after overcharging, you have acicular lithium crystals all over the plate, puncture points all over the place, micro-shorts all over the place. Therefore, the battery temperature will gradually increase, and finally high temperature will electrolyze the liquid gas. In this case, whether the temperature is too high to cause the material to burn and explode, or the shell is first broken, so that the air goes in and the lithium metal is violently oxidized, is the end of the explosion. However, the explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It's possible that consumers stop charging their phones and take them out before the battery is hot enough to burn the material and produce enough gas to burst the battery's casing. At this time, the heat generated by numerous micro-short circuits will slowly increase the battery temperature, and after a period of time, the explosion will occur. The common description of consumers is that they pick up the phone and find it hot, throw it away and it explodes.
Based on the above explosion types, we can focus on the prevention of overcharge, the prevention of external short circuit, and improve the safety of the cell. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which is closely related to battery system design and battery pack installation. The emphasis of cell safety improvement is chemical and mechanical protection, which is closely related to cell manufacturers.
The design specification
Because 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 circuit board failure rate is generally well above one in 100 million. Therefore, battery systems must be designed with more than two lines of defense. A common mistake is to use an adaptor directly to the charging pool. This will overcharge the protection task, completely to the battery pack on the protection board. Although the failure rate of protective plate is not high, but, even if the failure rate is low to one in a million, the global probability still has explosion accident every day. If the battery system can be over charge, over discharge, over current are respectively to provide two safety protection, each protection failure rate if is one in ten thousand, two protection can be reduced to one in one hundred million.
Common battery charging system block diagram is as follows, including charger and battery pack two major. The charger includes an Adaptor and a charging controller. The adapter converts alternating to direct current, and the charging controller limits the maximum current and voltage of the direct current. The battery pack contains two parts, the protection plate and the cell, and a PTC to limit the maximum current. Ac adapter dc role: the electrical controller current limit voltage limit. Charger role: protection plate over charge, over discharge, over current and other protection.
Battery action: current limiting chip. Take the cell phone battery system as an example, the overcharge protection system USES the charger output voltage set at about 4.2v to achieve the first layer of protection, so that even if the protection board on the battery pack fails, the battery will not be overcharged and will not be in danger. The second protection is the overcharge protection function on the protection plate, which is generally set at 4.3v. In this way, the protection plate is usually not responsible for cutting off the charging current, only when the charger voltage is abnormally high, only need to act. Overcurrent protection is by the protection plate and current limiting plate to be responsible for, this is also two protection, to prevent overcurrent and external short circuit. Because overdischarge can only occur in the process of electronics being used. Therefore, the general design is provided with the first protection by the circuit board of the lithium ion battery pool, and the second protection by the protection board on the battery pack. The device should shut down automatically when it detects a power supply voltage below 3.0v. If this feature is not designed when the product is designed, the guard will close the discharge circuit when the voltage is lowered to 2.4v.
In general: the battery system design, must overcharge, over discharge, and over current to provide two electronic protection. Take the shield off and charge it. If the battery explodes, it's bad design. Although the above method provides two layers of protection, consumers often buy non-original chargers to charge the chargers after they are broken, and the chargers manufacturers, based on cost considerations, often remove the charging controller to reduce the cost. As a result, bad money drives out good money and there are many inferior chargers on the market. This leaves overcharging without its first and most important line of defense. And overcharge is the most important factor that causes battery explosion, because this, inferior charger can be called the culprit that is battery explosion incident. Of course, not all battery systems use the scheme shown above. In some cases, the battery pack will also have a charging controller design.
For example, many laptop computers have a charging controller attached to a battery pack. This is because laptops typically have a charging controller built into the computer, giving the consumer only one adapter. Therefore, the external battery pack of notebook computer must have a charging controller to ensure the safety of the external battery pack when using the adapter to charge. In addition, the product that USES car cigarette lighter to charge, sometimes also can make charge controller in battery pack.
Last line of defense: if electronic defenses fail, the last line of defense is provided by the cell. The safety level of the cell can be roughly classified according to whether the cell can be short-circuited and overcharged. Because, before the battery explosion, if there are lithium atoms stacked on the surface of the material, the explosion will be more powerful. Also, overcharge protection is often reduced to a defensive line by consumers using faulty chargers, so the cell's ability to withstand overcharge is more important than its ability to withstand an external short circuit. Compared with the steel case, the aluminum case has higher safety advantages.
