Battery Drum Shell and Explosion Cause Analysis

introduction

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

    Lithium is the smallest and most reactive metal on the periodic table. Small size so high capacity density, widely welcomed by consumers and engineers. However, the chemical characteristics are too active, which brings a high risk. When exposed to air, lithium metal explodes in a violent oxidation reaction   with oxygen. To improve safety and voltage, scientists have invented materials such as graphite and lithium cobaltate to store lithium atoms. The molecular structure of these materials forms tiny storage grids at the    nanoscale that can be used to store lithium atoms. In this way, even if the battery shell is broken and oxygen enters, the oxygen molecules will be too large to enter these small cells, so that the lithium atoms will not contact with oxygen and avoid explosion. This principle of lithium-ion batteries allows people to     obtain its high capacity density at the same time, but also achieve the purpose of safety. 

    When a lithium-ion battery is charged, the lithium atoms in the positive electrode lose electrons and oxidize to lithium ions. Lithium ions swim to the negative electrode through the electrolyte, enter the negative cell, and obtain an electron, which is reduced to lithium atoms. When discharging, the entire program is reversed. In order to prevent the battery from directly touching the positive and negative electrodes and short circuit, a diaphragm paper with many fine holes will be added to the battery to prevent   short circuit. Good diaphragm paper can also automatically close the fine hole when the battery temperature  is too high, so that lithium ions can not cross, in order to waste the martial arts, to prevent danger.

safeguard procedures

After the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occu r. The higher the overcharge voltage, the higher the risk. After the lithium cell voltage is higher than 4.2V, th  e number of lithium atoms remaining in the positive electrode material is less than half, and the storage cell   often breaks down, so that the battery capacity is permanently reduced. If you continue to charge, because t he negative cell has been filled with lithium atoms, subsequent lithium metal will accumulate on the surface   of the negative material. These lithium atoms will grow dendritic crystals from the negative surface in the dir  ection of the lithium ions. These lithium metal crystals will pass through the diaphragm paper and short-circu it the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs, beca use during the overcharge process, materials such as electrolyte will crack to produce gas, making the batte ry housing or pressure valve burst and burst, allowing oxygen to go in and react with lithium atoms piled on t he surface of the negative electrode, and then explode. Therefore, when charging lithium batteries, it is nec   essary to set the upper voltage limit to take into account the life, capacity, and safety of the battery. The mo  st ideal charging voltage is up to 4.2V.

Lithium battery should also have a lower voltage limit when discharging. When the cell voltage is below 2.4V, part of the material will begin to be destroyed. And because the battery will self-discharge the longer the voltage will be lower, therefore, it is best not to stop at 2.4V when discharging. During the period from 3.0V discharge to 2.4V, the energy released by the lithium battery only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal cut-off voltage for discharge.

When charging and discharging, in addition to the limitation of voltage, the limitation of current is also necessary. When the current is too large, lithium ions have no time to enter the storage cell and will gather on the surface of the material. When these lithium ions gain electrons, they crystallize lithium atoms on the surface of the material, which, like overcharging, can be dangerous. If the battery case breaks, it will explode.

Therefore, the protection of lithium-ion batteries, at least include: charging voltage upper limit, discharge voltage lower limit, and current upper limit three. In general, in the lithium battery pack, in addition to the lithium battery core, there will be a protection plate, which mainly provides these three protections. However, these three protections of the protection board are obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, the cause of the battery explosion must be more carefully analyzed

Battery explosion cause

1, the internal polarization is large!

2, the pole sheet absorbs water, reacts with the electrolyte gas drum.

3, the quality of the electrolyte itself, performance problems.

4, the injection amount of liquid can not meet the process requirements.

5, in the installation process, laser welding welding sealing performance is poor, gas leakage, gas leakage detection.

6, dust, pole dust is the first easy to lead to micro short circuit, the specific reasons are unknown.

7, the positive and negative electrode plate is thicker than the process range, and it is difficult to enter the shell.

8, injection sealing problem, steel ball sealing performance is not good leading to gas drum.

9. The shell wall is too thick for incoming materials, and the shell deformation affects the thickness.

Explosion type analysis

The types of pool core explosion can be summarized into three types: external short circuit, internal short circuit and overcharge. The outside refers to the outside of the battery cell, including the short circuit caused by the poor insulation design of the battery pack.                                                                                                           

When there is a short circuit 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 to vaporize, and the battery shell will be extended. When the internal temperature of the battery is high to 135 degrees Celsius, the good quality diaphragm paper will close the fine hole, the electrochemical reaction will terminate or nearly terminate, the current will drop sharply, and the temperature will slowly drop, thus avoiding the explosion. However, the closing rate of the fine hole is too poor, or the diaphragm paper that the fine hole will not close at all will allow the battery temperature to continue to rise, more electrolyte vaporization, and finally the battery shell will burst, and 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 through the diaphragm, or the dendritic crystals of lithium atoms through the diaphragm. These tiny, needle-like pieces of metal can cause micro-short circuits. Because the needle is very thin and has a certain resistance value, the current is not necessarily very large. Aluminum foil burrs are caused in the production process, and the observable phenomenon is that the battery leakage is too fast, and most of them can be screened out by the battery cell factory or the assembly plant. Moreover, because the burrs are small, they can sometimes be burned off, making the battery return to normal. Therefore, the probability of explosion caused by the burr micro-short circuit is not high.

In this way, it can be said that there are often bad batteries with low voltage shortly after charging in each cell factory, but there are few explosions, which are statistically supported. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharge. Because, after overcharging, the pole plate is full of needle-like lithium metal crystals, the puncture point is everywhere, and micro-short circuits are occurring everywhere. Therefore, the temperature of the battery will gradually increase, and finally 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 first burst, so that the air goes into the lithium metal to undergo intense oxidation, it is an explosion.

However, this explosion caused by an internal short circuit caused by overcharge does not necessarily occur at the time of charging. It is possible that when the temperature of the battery is not high enough for the material to burn and the gas produced is not enough to burst the battery case, the consumer will stop charging and take the phone out. At this time, the heat generated by many micro-short circuits slowly increases the temperature of the battery, and after a period of time, an explosion occurs. The common description of consumers is that when they pick up the phone, they find the phone is very hot, and it explodes after throwing it away.

Based on the above types of explosion, we can focus on the prevention of overcharge, the prevention of external short circuit, and the improvement of cell safety. 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 focus of cell safety improvement is chemical and mechanical protection, which has a great relationship with the battery cell manufacturer.

Design specifications

Since there are hundreds of millions of mobile phones in the world, to achieve security, the failure rate of security protection must be less than one in 100 million. Because the failure rate of the circuit board is generally much higher than one in 100 million. Therefore, when the battery system is designed, there must be more than two safety lines of defense. A common design mistake is to use the charger (adaptor) directly to the charging pool. In this way, the overcharge protection task is completely handed over to the protection plate on the battery pack. Although the failure rate of the protection board is not high, even if the failure rate is as low as one in a million, there will be an explosion accident every day in the world.

If the battery system can provide two safety protections for overcharge, overdischarge, and overcurrent, the failure rate of each protection can be reduced to one in 100 million if the failure rate of each protection is 1 in 10,000. The common battery charging system box diagram is as follows, including two parts: charger and battery pack. The charger also includes an adapter (Adaptor) and a charging controller two parts. The adapter converts alternating current to direct current, and the charge controller limits the maximum current and voltage of direct current. The battery pack consists of two parts: a protection plate and a battery core, and a PTC to limit the maximum current.

  Taking the mobile phone battery system as an example, the overcharge protection system uses the charger output voltage to be 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 dangerous. The second protection is the overcharge protection function on the protection board, which is generally set at 4.3V. In this way, the protection board usually does not have to be responsible for cutting off the charging current, only when the charger voltage is abnormally high, it needs to act. Overcurrent protection is responsible for the protection plate and current limiting plate, which are also two types of protection to prevent overcurrent and external short circuit. Since overdischarge will only occur during the process of electronic products being used. Therefore, the general design is to provide the first protection by the circuit board of the electronic product, and the protection board on the battery pack provides the second protection. When the electronic product detects that the supply voltage is lower than 3.0V, it should automatically shut down. If the product is not designed for this function, the protection board will close the discharge circuit at a low voltage of 2.4V.

In short, when the battery system is designed, two electronic protections must be provided for overcharge, overdischarge, and overcurrent. The protection plate is the second protection. Take the shield off and charge it. If the battery explodes, it's a bad design.

Although the above method provides two protections, because consumers often buy non-original chargers to charge after the charger breaks, and charger manufacturers, based on cost considerations, often remove the charging controller to reduce costs. As a result, bad money drives out good money, and many inferior chargers appear on the market. This deprives overcharge protection of its first and most important line of defense. Overcharge is the most important factor in the explosion of the battery, therefore, the inferior charger can be called the culprit of the battery explosion.

Of course, not all battery systems adopt the scheme shown above. In some cases, the battery pack will also have a charge controller design. For example, many laptop computers have a charge controller in addition to the battery rod. This is because notebook computers generally have the charge controller built into the computer and only give the consumer an adapter. Therefore, the external battery pack of the notebook computer must have a charging controller to ensure the safety of the external battery pack when charging with the adapter. In addition, the use of car cigarette lighter charging products, sometimes the charge controllerwill be made in the battery pack

The last line of defense

If electronic protection fails, the last line of defense will be provided by the battery cell. The safety level of the cell can be roughly distinguished according to whether the cell can be separated by external short circuit and overcharge. Because before the battery explodes, if there are lithium atoms inside the surface of the material, the explosion will be more powerful. Moreover, the protection of overcharge is often due to the use of inferior chargers by consumers and only one line of defense, because of this, the ability of the cell to resist. Overcharge is more important than the ability to resist external short circuit.

Aluminum shell cell and steel shell cell safety comparison Aluminum shell relative to steel shell has a high safety advantage.