Analysis Of The Causes Of Lithium Battery Bulging And Explosion
May 16, 2025Analysis Of The Causes Of Lithium Battery Bulging And Explosion
The working principle of lithium-ion batteries
Lithium is the smallest and most active metal on the chemical periodic table. It is popular with consumers and engineers because of its small size and high capacity density. However, its chemical properties are too active, which brings extremely high danger. When lithium metal is exposed to air, it will produce a violent oxidation reaction with oxygen and explode. In order 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 nano-scale tiny storage grids that can be used to store lithium atoms. In this way, even if the battery shell is broken and oxygen enters, the oxygen molecules are too large to enter these tiny storage grids, so that the lithium atoms will not come into contact with oxygen and avoid explosion. This principle of lithium-ion batteries allows people to achieve safety while obtaining its high capacity density.
Lithium is the smallest and most active metal on the chemical periodic table. It is popular with consumers and engineers because of its small size and high capacity density. However, its chemical properties are too active, which brings extremely high danger. When lithium metal is exposed to air, it will produce a violent oxidation reaction with oxygen and explode. In order 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 nano-scale tiny storage grids that can be used to store lithium atoms. In this way, even if the battery shell is broken and oxygen enters, the oxygen molecules are too large to enter these tiny storage grids, so that the lithium atoms will not come into contact with oxygen and avoid explosion. This principle of lithium-ion batteries allows people to achieve safety while obtaining its high capacity density.
When a lithium-ion battery is charged, the lithium atoms in the positive electrode lose electrons and oxidize to lithium ions. The lithium ions travel to the negative electrode through the electrolyte, enter the storage cell of the negative electrode, and obtain an electron to be reduced to lithium atoms. When discharging, the whole process is reversed. In order to prevent the positive and negative electrodes of the battery from directly touching and short-circuiting, a diaphragm paper with many pores is added to the battery to prevent short-circuiting. Good diaphragm paper can also automatically close the pores when the battery temperature is too high, so that lithium ions cannot pass through and prevent danger.
When the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the danger. When the lithium battery cell voltage is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half, and the storage cell often collapses at this time, causing a permanent decrease in the battery capacity. If charging continues, since the storage cell of the negative electrode is already full of lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendrites from the negative electrode surface toward the lithium ions. These lithium metal crystals will pass through the diaphragm paper, causing the positive and negative electrodes to short-circuit. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will decompose to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the negative electrode surface, and then explode. Therefore, when charging lithium batteries, it is necessary to set an upper voltage limit to take into account the battery life, capacity, and safety at the same time. The most ideal upper limit of charging voltage is 4.2V.
There is also a lower voltage limit when discharging lithium batteries. When the battery voltage is lower than 2.4V, some materials will begin to be damaged. Because the battery will self-discharge, the longer it is discharged, the lower the voltage will be. Therefore, it is best not to stop discharging at 2.4V. During the period when the lithium battery is discharged from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage.
When charging and discharging, in addition to voltage limitation, current limitation is also necessary. When the current is too large, lithium ions will not have time to enter the storage cell and will gather on the surface of the material. After these lithium ions obtain electrons, lithium atoms will crystallize on the surface of the material, which is dangerous just like overcharging. If the battery shell breaks, it will explode.
Therefore, the protection of lithium-ion batteries must at least include: upper limit of charging voltage, lower limit of discharging voltage, and upper limit of current.
In general, in addition to the lithium battery cell, there will be a protection board in the lithium battery pack, which mainly provides these three protections. However, these three protections of the protection board are obviously not enough, and lithium battery explosions are still frequent around the world. To ensure the safety of the battery system, a more careful analysis of the causes of battery explosions is necessary.
The types of battery core explosions can be summarized as external short circuits, internal short circuits and overcharge. The external here refers to the outside of the battery cell, including short circuits caused by poor insulation design inside the battery pack.
When a short circuit occurs outside the battery cell and the electronic components fail to cut off the circuit, high heat will be generated inside the battery cell, causing part of the electrolyte to vaporize and expand the battery shell. When the temperature inside the battery reaches 135 degrees Celsius, a good quality diaphragm paper will close the pores, the electrochemical reaction will terminate or almost terminate, the current will drop sharply, and the temperature will slowly drop, thereby avoiding the explosion. However, if the pore closing rate is too poor, or the pores of the diaphragm paper are not closed at all, the battery temperature will continue to rise, more electrolyte will vaporize, and finally the battery shell will be broken, and even the battery temperature will be raised to the point where the material burns and explodes.
Internal short circuits are mainly caused by burrs on copper and aluminum foils penetrating the diaphragm, or dendrites of lithium atoms penetrating the diaphragm. These tiny needle-like metals will cause micro short circuits. Since the needles are very thin and have a certain resistance value, the current may not be very large. The burrs on copper and aluminum foils are caused during the production process. The observable phenomenon is that the battery leaks too quickly, and most of them can be screened out by the battery cell factory or assembly plant. Moreover, because the burrs are small, they may be burned off sometimes, so that the battery returns to normal. Therefore, the probability of explosion caused by burr micro-short circuit is not high.
This statement can be statistically supported by the fact that there are often bad batteries with low voltage shortly after charging in each battery factory, but there are few explosions. Therefore, the explosion caused by internal short circuit is mainly caused by overcharging. Because after overcharging, the electrode is full of needle-shaped lithium metal crystals, piercing points are everywhere, and micro short circuits occur everywhere. Therefore, the battery temperature will gradually rise, and finally the high temperature will gasify the electrolyte. In this case, whether the temperature is too high to cause the material to burn and explode, or the shell is broken first, allowing air to enter and violently oxidize with lithium metal, it will end in explosion.
However, this explosion caused by internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that the battery temperature is not high enough to cause the material to burn, and the gas generated is not enough to break the battery shell, and the consumer stops charging and takes the mobile phone out. At this time, the heat generated by numerous micro short circuits slowly raises the battery temperature, and after a period of time, the explosion occurs. Consumers often describe that they find their phones very hot when they pick them up, and they explode after they throw them away.
Based on the above types of explosions, we can focus on three aspects of explosion prevention: prevention of overcharging, prevention of external short circuits, and improvement of battery cell safety. Among them, overcharging prevention and external short circuit prevention belong to electronic protection, which are closely related to battery system design and battery assembly. The key to improving battery cell safety is chemical and mechanical protection, which is closely related to battery cell manufacturers.
Since there are hundreds of millions of mobile phones in the world, to achieve safety, the failure rate of safety protection must be less than one in 100 million. Because the failure rate of circuit boards is generally much higher than one in 100 million. Therefore, when designing a battery system, there must be more than two safety lines of defense. A common design error is to use a charger (adaptor) to directly charge the battery pack. In this way, the responsibility of overcharging protection is completely handed over to the protection board 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 is still a chance that explosion accidents will occur every day around the world.
If the battery system can provide two safety protections for overcharging, over-discharging, and overcurrent, respectively, if the failure rate of each protection is one in ten thousand, the two protections can reduce the failure rate to one in 100 million. The block diagram of a common battery charging system is as follows, which includes two major parts: the charger and the battery pack. The charger also includes two parts: the adapter (Adaptor) and the 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 board and the battery cell, as well as 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 set at about 4.2V to achieve the first level of protection. In this way, even if the protection board on the battery pack fails, the battery will not be overcharged and dangerous. The second line of protection is the overcharge protection function on the protection board, which is generally set to 4.3V. In this way, the protection board does not have to be responsible for cutting off the charging current. It only needs to act when the charger voltage is abnormally high. Overcurrent protection is the responsibility of the protection board and the current limiter, which are also two lines of protection to prevent overcurrent and external short circuit. Since overdischarge only occurs during the use of electronic products. Therefore, the general design is that the circuit board of the electronic product provides the first protection, 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 as low as 2.4V.
In short, when designing a battery system, two electronic protections must be provided for overcharging, overdischarging, and overcurrent. The protection board is the second protection. If the battery explodes when the protection board is removed, it means that the design is bad.
Although the above method provides two protections, consumers often buy non-original chargers to charge after the charger breaks down, and charger manufacturers often remove the charging controller to reduce costs based on cost considerations. As a result, bad money drives out good money, and many low-quality chargers appear on the market. This makes overcharging protection lose the first and most important line of defense. Overcharging is the most important factor causing battery explosions, so low-quality chargers can be regarded as the culprit of battery explosions.
Of course, not all battery systems use the solution shown in the figure above. In some cases, the battery pack will also have a charging controller design. For example, many notebook computer external battery sticks have a charging controller. This is because notebook computers generally have the charging controller built into the computer, and only provide consumers with an adapter. Therefore, the external battery pack of a notebook computer must have a charging controller to ensure the safety of the external battery pack when charging with an adapter. In addition, products that use a car cigarette lighter to charge sometimes have the charging controller built into the battery pack.
If all electronic protection measures fail, the last line of defense will be provided by the battery cell. The safety level of the battery cell can be roughly divided into levels based on whether the battery cell can survive external short circuits and overcharges. Because before the battery explodes, if lithium atoms accumulate on the surface of the material inside, the explosion will be more powerful. Moreover, the protection against overcharging is often left with only one line of defense because consumers use inferior chargers. Therefore, the battery cell's ability to resist overcharging is more important than its ability to resist external short circuits.
Comparing the safety of aluminum shell batteries with steel shell batteries, aluminum shells have a higher safety advantage than steel shells.
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