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Analysis on the causes of battery shell and explosion:

1、 Lithium ion battery characteristics

Lithium is the smallest and most active metal on the chemical periodic table. Small size and high capacity density are popular with consumers and engineers. However, if the chemical properties are too active, it will bring high risks. When lithium metal is exposed to the air, it will react violently with oxygen and explode. In order to improve safety and voltage, scientists have invented materials such as graphite and lithium cobalt to store lithium atoms. The molecular structure of these materials forms a nanoscale small storage lattice, which can be used to store lithium atoms. In this way, even if the battery shell breaks and oxygen enters, the oxygen molecules will be too large to enter these small cells, so that lithium atoms will not contact with oxygen and avoid explosion. This principle of lithium-ion battery enables people to achieve its high capacity density and safety at the same time.

When a lithium-ion battery is charged, the lithium atom of the cathode will lose its electrons and be oxidized to lithium ion. The 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 the positive and negative poles of the battery, 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 pores when the battery temperature is too high, so that lithium ions cannot pass through, so as to waste martial arts and prevent danger.

protective measures

When the lithium battery cell is overcharged to a voltage higher than 4.2V, it will start to have side effects. The higher the overcharge voltage, the higher the danger. When the voltage of the lithium cell is higher than 4.2V, the number of lithium atoms left in the cathode material is less than half. At this time, the storage cell will often collapse, causing a permanent decrease in battery capacity. If you continue to charge, because the storage cell of the negative electrode is full of lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the negative surface to the direction of lithium ion. These lithium metal crystals will pass through the diaphragm paper and short circuit the positive and negative poles. Sometimes the battery will explode before the short circuit occurs. This is because during overcharging, materials such as electrolyte will crack to produce gas, which will cause the battery shell or pressure valve to swell and crack, allowing oxygen to enter and react with lithium atoms accumulated on the negative surface, and then explode. Therefore, when charging the lithium battery, it is necessary to set the upper voltage limit to take into account the battery life, capacity and safety. The optimal upper limit of charging voltage is 4.2V.

There should also be a lower voltage limit when the lithium battery is discharged. When the cell voltage is lower than 2.4V, some materials will start to be damaged. Because the battery will self-discharge, the voltage will be lower as the battery is discharged for a long time, so it is better not to discharge it until it reaches 2.4V. During the period from 3.0V 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 discharge cut-off voltage.

In addition to voltage limitation, current limitation is also necessary during charging and discharging. When the current is too large, lithium ions will gather on the material surface before entering the cell. After these lithium ions gain electrons, they will produce lithium atom crystals on the surface of the material, which will cause danger like overcharge. If the battery shell breaks, it will explode.

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

2、 Cause of battery explosion:

1: The internal polarization is large!

2: The electrode absorbs water and reacts with the electrolyte.

3. The quality and performance of the electrolyte itself.

4: The liquid injection volume cannot meet the process requirements.

5: In the assembly process, the sealing performance of laser welding is poor, and gas leakage and gas leakage are not measured.

6: Dust and polar dust are easy to cause micro short circuit at first, and the specific reason is unknown.

7: The positive and negative electrode plates are thicker than the process range, and it is difficult to enter the shell.

8: There is a problem of liquid filling and sealing, and the poor sealing performance of the steel ball leads to air bubbles.

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

3、 Explosion type analysis

The types of battery cell explosion can be divided into external short circuit, internal short circuit and overcharge. The exterior here refers to the exterior of the cell, including the short circuit caused by poor insulation design inside 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, which will cause partial electrolyte vaporization and expand the battery shell. When the internal temperature of the battery is as high as 135 ℃, the high-quality diaphragm paper will close the pores, the electrochemical reaction will be terminated or nearly terminated, the current will drop suddenly, and the temperature will also drop slowly, thus avoiding the explosion. However, the closing rate of the pores is too poor, or the diaphragm paper that the pores will not close at all will make the battery temperature continue to rise, more electrolyte will vaporize, and finally the battery shell will be broken, and even the battery temperature will be raised to make the material burn and explode.

The internal short circuit is mainly caused by the burr of copper foil and aluminum foil penetrating the diaphragm, or the dendritic crystal of lithium atom penetrating the diaphragm. These tiny needle-like metals will cause micro-circuit. Because the needle is very thin and has a certain resistance value, the current may not be very large. The burr of copper and aluminum foil is caused during the production process. The observed phenomenon is that the battery leakage is too fast, and most of them can be screened out by the core factory or assembly factory. Moreover, due to the fine burr, sometimes it will be burnt off, making the battery return to normal. Therefore, the probability of explosion caused by burr micro short circuit is not high.

Such a statement can be seen from the inside of each cell factory that there are often bad batteries with low voltage shortly after charging, but there are few explosions, which is supported statistically. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because, after overcharge, there are needle-like lithium metal crystals everywhere, puncture points everywhere, and micro-circuit everywhere. Therefore, the battery temperature will gradually increase, and finally the high temperature will cause 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, which makes the air enter and the lithium metal oxidize violently, it is the end of the explosion.

However, the explosion caused by 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 generated is not enough to break the battery shell, consumers will stop charging and take their mobile phones out. At this time, the heat generated by many micro short circuits will slowly increase the temperature of the battery. After a period of time, the explosion will occur. The common description of consumers is that when they pick up the mobile phone, they find it very hot, and it explodes after throwing it away.

Based on the above explosion types, we can focus on the prevention of overcharge, the prevention of external short circuit, and the improvement of core safety. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which are closely related to battery system design and battery assembly. The focus of cell safety improvement is chemical and mechanical protection, which has a great relationship with battery cell manufacturers.

4、 Design specification

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 1 in 100 million. Because the failure rate of circuit board is generally much higher than 1/100 million. Therefore, when designing the battery system, there must be more than two safety lines. The common wrong design is to use the charger directly to charge the battery. In this way, the responsibility of overcharge protection 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, the probability of explosion accidents will occur every day in the world.

If the battery system can provide two safety protection for overcharge, overcharge and overcurrent, the failure rate of each protection can be reduced to one in ten thousand, and the failure rate can be reduced to one in ten million. The block diagram of common battery charging system is as follows, including charger and battery pack. The charger also includes adapter and charging controller. The adapter converts AC power into DC power, and the charging controller limits the maximum current and maximum voltage of DC power. The battery pack consists of two parts, the protection plate and the battery core, and a PTC to limit the maximum current.

Text box: adapter AC/DC text box: charging controller current-limiting and voltage-limiting text box: charger text box: protective text box for overcharge, over-discharge and over-current of the protection board: battery pack text box: current-limiting text box: battery cell Take 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 even if the protection board on the battery pack fails, The battery will not be overcharged and cause danger. 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 cut off the charging current. It only needs to act when the charger voltage is abnormally high. Overcurrent protection is taken care of by the protection plate and current limiter, which are also two protection measures to prevent over-current and external short circuit. Overdischarge only occurs in 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 power supply voltage is lower than 3.0V, it should automatically shut down. If this function is not designed when the product is designed, the protection board will close the discharge circuit when the voltage is low to 2.4V.

In short, when designing the battery system, it is necessary to provide two layers of electronic protection for overcharge, over-discharge and over-current. The protection plate is the second protection. Remove the protective plate and charge it. If the battery will explode, it means that the design is poor.

Although the above methods provide two protections, consumers often buy non-original chargers to charge after the charger breaks down. 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 makes overcharge protection lose the first and most important line of defense. And overcharge is the most important factor that causes the battery explosion. Therefore, the inferior charger can be called the prime culprit of the battery explosion.

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

The last line of defense

If the electronic protection measures fail, the last line of defense will be provided by the cell. The safety level of the cell can be roughly divided according to whether the cell can pass the external short circuit and overcharge. Before the battery explodes, if there are lithium atoms accumulated on the material surface, the explosion power will be greater. Moreover, the protection against overcharge is often left with only one line of defense because consumers use inferior chargers. Therefore, the ability of the cell to resist overcharge is more important than the ability to resist external short circuit.

Safety comparison between aluminum shell and steel shell has a high safety advantage over steel shell.

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