Battery drum shell explosion reason detailed explanation
Analysis on the cause of battery drum shell and explosion:
I. characteristics of lithium ion battery
Lithium is the smallest and most active metal on the periodic table. Small volume so high capacity density, popular with consumers and engineers. However, the chemical properties are too active, it brings a very high risk. When exposed to air, lithium reacts violently with oxygen and explodes. 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 cells that can be used to store lithium atoms. That way, even if the battery's casing breaks down and oxygen enters, the molecules are too big to fit into these 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 positive lithium atoms lose electrons and are oxidized to lithium ions. Lithium ions travel through the electrolyte to the negative electrode, into the negative electrode storage cell, and gain an electron, reduction to lithium atoms. When discharging, the whole process is reversed. In order to prevent the battery positive and negative poles directly touch and short circuit, the battery will be coupled with a number of thin hole diaphragm paper, to prevent short circuit. Good diaphragm paper can also be in the battery temperature is too high, automatically close the hole, so that lithium ions cannot pass through, in order to self-waste martial arts, to prevent danger.
The lithium cell is overcharged to a voltage higher than 4. After 2V, you start to have side effects. The higher the overcharge voltage, the higher the risk. The lithium cell voltage is higher than 4. After 2V, less than half of the lithium atoms left in the anode material often collapse, causing a permanent drop in battery capacity. If you keep charging, because the negative cell is already full of lithium atoms, subsequent lithium metals will accumulate on the surface of the negative material. These lithium atoms grow dendritic crystals from the negative surface toward the lithium ions. The crystals of the lithium metal pass through the membrane paper, short-circuiting the anode and cathode. Sometimes the battery will explode before the short circuit occurs, because in the process of overcharging, electrolyte and other materials will crack to produce gas, making the battery shell or pressure valve bulge and rupture, let oxygen in and accumulate lithium atoms on the negative electrode surface, and then explode. Therefore, lithium battery charging, must set the upper limit of voltage, in order to take into account the life of the battery, capacity, and safety. The optimal maximum charging voltage is 4. 2 v.
There is also a lower voltage limit when a lithium cell is discharged. When the cell voltage is less than 2. At 4V, some of the material starts to break down. And because the battery will discharge itself, the longer the voltage will be lower, so it is best not to discharge 2. It stops at 4V. Lithium battery from 3. 0V discharges to 2. The energy released during the 4V period accounts for only about 3% of the battery's capacity. So 3. 0V is an ideal cut-off voltage for discharge.
In addition to voltage limitation, current limitation is also necessary when charging and discharging. When the current is too high, lithium ions can accumulate on the material surface before they can enter the storage cell. When these ions gain electrons, they crystallize lithium atoms on the surface of the material, which can be as dangerous as overcharging. If the battery housing breaks, it will explode.
Therefore, the protection of lithium-ion batteries, at least, should include: charging voltage upper limit, discharge voltage lower limit, and current upper limit three. 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, these three protection plate protection is obviously not enough, lithium battery explosions around the world or frequency. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.
Ii. Cause of battery explosion:
1: greater internal polarization!
2: the electrode absorbs water and reacts with the electrolyte to produce gas drum.
3: quality and performance of electrolyte itself.
4: the amount of liquid injection fails to meet the process requirements during the liquid injection.
5: poor sealing performance of laser welding in the installation and preparation process, air leakage, air leakage and leakage measurement.
6: dust. Polar 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, making it difficult to enter the shell.
8: liquid injection sealing problem, poor sealing performance of steel ball leads to gas drum.
9: shell incoming material has a thick shell wall, and shell deformation affects the thickness.
Iii. Explosion type analysis
The cell core explosion can be classified into three types: 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 component fails to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte vaporization, which will stretch the battery shell. When the internal temperature of the battery is high to 135 degrees Celsius, the diaphragm paper of good quality will close the pores, and the electrochemical reaction will be terminated or nearly terminated. The current will drop sharply and the temperature will also drop slowly, thus avoiding explosion. However, poor pore closure rates, or diaphragm paper that does not close at all, can cause the battery temperature to continue to rise, more electrolyte vaporization, and eventually the battery shell burst, or even raise 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 micro-short circuits. Because the needle is very thin and has a certain resistance value, the current is not necessarily very high. Burr of copper and aluminum foil is caused in the production process, the phenomenon that can be observed is that the battery leakage is too fast, most can be screened out by the cell factory or assembly factory. And because the burrs are small, they sometimes burn out, allowing the battery to return to normal. Therefore, the probability of explosion due to burr micro-short circuit is not high.
Such a statement, can often charge from the inside of the cell factory, the voltage is low soon after 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, punctures 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 be the electrolyte gas. In this case, whether the temperature is too high to make the material burn and explode, or the shell is broken first, so that the air enters and the lithium metal has intense oxidation, 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 before the battery temperature is high enough to burn the material and generate enough gas to break the battery's casing, consumers stop charging and take their phones outside. At this time, the heat generated by numerous micro-short circuits slowly increases the battery temperature, and after a period of time, the explosion occurs. 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 preventing overcharge, preventing external short circuit, and improving 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 the cell manufacturer.
Iv. Design specifications
As 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 much higher than one in 100 million. Therefore, battery system design, there must be more than two security lines. A common mistake is to use adaptor directly to the adaptor pool. This will overcharge the protection of the task, completely to the battery pack on the protection plate. Although the failure rate of protective plate is not high, but, although the failure rate is low to one in a million, the global still can have explosion accident happening every day on probability.
Battery systems such as over charge, over discharge, over current are respectively to provide two safety protection, each protection failure rate if it is one in 10,000, two protection can be reduced to one in 100 million failure rate. Common battery charging system block diagram is as follows, including Charger
and battery pack two major parts. Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor Adaptor The adapter converts alternating current to direct current, and the charging controller limits the maximum current and voltage of direct current. The battery pack consists of two major parts, the protection plate and the cell, and a PTC to limit the maximum current.
Text box: adapter ac variable dc text box: charging controller current limiting pressure limiting text box: Charger
text box: protective plate overcharge, pass flow such as protective text box: battery text box: current limiting text box: battery core with mobile phone battery system as an example, the overcharge protection system using the charger output voltage is set in 4. About 2V, to achieve the first layer of protection, so that even if the protection plate on the battery pack fails, the battery will not be overcharged and dangerous. The second protection is the overcharge protection function on the protection plate, which is generally set as 4. 3 v. In this way, the protection plate is usually not responsible for cutting off the charging current, only when the charger voltage is unusually high, only need to act. Overcurrent protection is the responsibility of the protection plate and current limiting plate, which are also two protection to prevent overcurrent and external short circuit. Because overdischarge only occurs when an electronic product is used. Therefore, the general design is by the electronic products circuit board to provide the first to the protection, the battery pack on the protection board provides the second protection. When the electronic device detects a power supply voltage below 3. At 0V, the machine should shut down automatically. If this function is not designed when the product is designed, the protection plate will be electrically lowered to 2. At 4V, close the discharge circuit.
In short, battery system design must provide two electronic protection against overcharge, over discharge and over current respectively. The protection plate is the second protection. Remove the guard plate and charge it. If the battery explodes, it means the design is poor.
Although the above method provides two kinds of protection, consumers often buy non-original chargers to charge the chargers after the chargers break down, and the chargers often remove the charging controllers to reduce the costs based on the cost considerations. As a result, bad money drives out good money and there are many inferior chargers on the market. This makes overcharge protection lose its first and most important line of defense. And overcharge is the most important factor that causes battery explosion, accordingly, inferior charger can be called the prime criminal that is battery explosion incident.
Of course, not all battery systems use this approach. 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 bar. This is because laptops typically have charging controllers built into the computer, giving consumers only one adapter. Therefore, the extra battery pack of notebook computer must have a charging controller to ensure the safety of the extra battery pack when charging with the adapter. In addition, the use of car cigarette lighter charging products, sometimes will be charging controller in the 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 divided according to whether the cell can be short-circuit and overcharge. Because, before the battery explodes, if lithium atoms accumulate on the surface of the material, the explosion will be more powerful. What's more, the protection against overcharging is often limited to one line of defense by consumers using inferior chargers. Therefore, the resistance of the cell to overcharging is more important than the resistance to external short circuit.
Comparing the safety of aluminum shell cell and steel shell cell, aluminum shell has higher safety advantages than steel shell.