There are some reasons why lithium batteries explode.

The type of cell explosion can be classified as external short circuit, internal short circuit and overcharge. The outer part refers to the outer part of the cell, including the short circuit caused by poor insulation design inside the battery pack.

Lithium is the smallest and most active metal on the periodic table. Small volume so high capacity density, widely welcomed by consumers and engineers. But too much chemical activity can be dangerous. When exposed to air, lithium can explode in a violent oxidation reaction with oxygen. To improve safety and voltage, scientists have developed materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms nano-sized, tiny storage cells that can be used to store lithium atoms. That way, even if the battery shell breaks and oxygen enters, the molecules are too big to fit into the tiny cells, preventing the lithium atoms from coming into contact with oxygen and exploding. The principle of lithium ion battery enables people to obtain its high capacity density and achieve the purpose of safety at the same time.

When a lithium-ion battery is charged, the positively charged lithium atoms lose electrons and oxidize to lithium ions. The lithium ions swim through the electrolyte to the negative electrode, where they enter a cell, where they gain an electron and are reduced to lithium atoms. Discharge, the whole process reversed. In order to prevent the battery positive and negative directly touch and short circuit, the battery will be added with a number of thin holes diaphragm paper, to prevent short circuit. Good diaphragm paper can also be in the battery temperature is too high, automatically close the hole, let the lithium ion can not pass through, in order to waste martial art, to prevent the danger.

Protection measures

When a lithium battery is overcharged to a voltage higher than 4.2v, side effects can begin. The higher the overcharge voltage, the higher the risk. When the battery voltage is higher than 4.2v, the number of lithium atoms left in the anode material is less than half, and the cell often collapses, causing a permanent drop in battery capacity. If you keep charging, because the anode's cell is already full of lithium atoms, subsequent lithium metals will accumulate on the anode's surface. These lithium atoms grow dendritic crystals from the negative surface in the direction of the lithium ions. These lithium metal crystals will pass through the diaphragm paper, causing the positive and negative poles to short circuit. Sometimes the battery will explode before the short circuit occurs. This is because in the over-charging process, the electrolyte and other materials will crack and produce gas, making the battery shell or pressure valve bulge and crack, letting the oxygen go in and react with the lithium atoms accumulated on the negative electrode surface, and then explode. 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 considered at the same time. The optimal upper limit of charging voltage is 4.2v.

When the lithium cell discharge must also have the voltage lower limit. When the cell voltage is below 2.4v, some of the material begins to break down. Since the battery will discharge itself, the longer the discharge, the lower the voltage will be. Therefore, it is better not to put 2.4v to stop the discharge. Between 3.0v and 2.4v, lithium batteries release only about 3% of their capacity. Therefore, 3.0v is an ideal cut-off voltage for discharge.

In addition to voltage limitation, current limitation is also necessary during charging and discharging. When the current is too high, the lithium ions can accumulate on the surface of the material before they have time to enter the cell. When these ions gain electrons, they create crystals of lithium atoms on the material's surface, which, like overcharging, can be dangerous. In case the battery case breaks, it will explode.

Therefore, the protection of lithium-ion batteries should include at least three items: upper limit of charging voltage, lower limit of discharging voltage and upper limit of current. In general, lithium battery pack, in addition to lithium battery core, there will be a protective plate, this protective plate is mainly to provide these three protection. However, the protection of the three protection plate is obviously not enough, lithium battery explosion is still frequent worldwide. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.

Cause of battery explosion

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 cannot meet the process requirements.

5. Poor sealing performance of laser welding in the installation and preparation process, air leakage, air leakage and leakage measurement.

6. Dust. Extremely thin dust is easy to cause micro-short circuit in the first place, and the specific reason is unknown.

7. The anode and cathode plates are thicker than the process range and difficult to be put into the shell.

8, injection sealing problem, poor sealing performance of steel ball lead to gas drum.

9. The incoming material of the shell is thick, and the shell deformation affects the thickness.

Explosion type analysis

The type of cell explosion can be classified as external short circuit, internal short circuit and overcharge. The outer part refers to the outer part of the cell, including the short circuit caused by poor insulation design inside the battery pack.

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, poor pore closure, or diaphragm paper with no pore closure at all, will cause the battery temperature to continue to rise, causing more electrolyte to vaporize, eventually bursting the battery housing and even raising the battery temperature enough to cause the material to 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 is low on the bad battery, but there are few explosions, 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. As a result, the battery temperature will gradually rise, and eventually the high temperature will release the electrolyte 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 current to direct current, and the charging controller limits the maximum current and voltage of the direct current. The battery pack consists of two parts, the protection plate and the cell, and a PTC to limit the maximum current.

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 risk. 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 the electronic product circuit board to provide the first to the protection, the battery pack on the protection board provides a second protection. 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 short, the battery system must be designed to provide two electronic protection against overcharge, over discharge and over current. The protective plate is the second 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 the chargers are broken, and the chargers often remove the charging controller to reduce the cost due to cost considerations. 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, therefore, inferior charger can be called the culprit of battery explosion.

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 ditch

If electronic safeguards fail, the last line of defence 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 explodes, if there are lithium atoms piled up on the surface of the material, the explosion will be more powerful. Also, overcharging protection is often reduced to a defensive line by consumers using faulty chargers, so the cell's ability to withstand overcharging is more important than its ability to withstand an external short circuit.

Comparison of safety between aluminum case and steel case the aluminum case has higher safety advantages than steel case.

The page contains the contents of the machine translation.