Lithium-ion batteries have become the batteries of portable electronics. This kind of battery has high capacity density and competitive price, and is expected to remain the mainstream of the market in the next few years. But lithium batteries have always hidden the danger of explosion. Due to the wider and wider application, explosion incidents also emerge in an endless stream. In fact, lithium-ion batteries can be made very safe through correct battery system design and battery level determination. At present, the explosion-proof circuit and explosion-proof battery technology are mature, and explosion incidents should be less and less.
Lithium-ion battery characteristics
Lithium is the smallest and most active metal on the chemical periodic table. Due to its small size and high capacity density, it is widely welcomed by consumers and engineers. However, the chemical properties are too active, which brings a very high risk. When lithium metal is exposed to air, it will explode due to a violent oxidation reaction with oxygen. 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 tiny storage lattices at the nanometer level, which can be used to store lithium atoms. In this way, even if the battery casing is broken and oxygen enters, the oxygen molecules are too large to enter these tiny storage cells, so that the lithium atoms will not come into contact with oxygen to avoid explosion. This principle of lithium-ion batteries enables people to achieve the purpose of safety while obtaining high capacity density.
When a lithium-ion battery is charged, the lithium atoms at the positive electrode will lose electrons and be oxidized to lithium ions. 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 a lithium atom. When discharging, the whole procedure is reversed. In order to prevent the positive and negative electrodes of the battery from directly touching and short-circuiting, a separator paper with many pores is added inside the battery to prevent short-circuiting. A good separator paper can also automatically close the pores when the battery temperature is too high, so that lithium ions cannot pass through, so as to abolish martial arts and prevent danger.
  Safeguard
After the lithium battery cell is overcharged to a voltage higher than 4.2V, it will start to produce side effects. The higher the overcharge voltage, the higher the risk. After the lithium cell voltage is higher than 4.2V, the remaining lithium atoms in the positive electrode material are less than half. At this time, the storage cell will often collapse, causing a permanent decline in battery capacity. If continue to charge, because the memory location of negative pole has been filled with lithium atom, follow-up lithium metal can be piled up on negative pole material surface. These lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of lithium ions. These lithium metal crystals will pass through the separator paper and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will be cracked to produce gas, which will cause the battery case or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. And then explode. Therefore, when charging a lithium battery, the upper voltage limit must be set so that the life, capacity, and safety of the battery can be taken into account at the same time. The ideal charging voltage upper limit is 4.2V. There is also 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 destroyed. And because the battery will self-discharge, the longer it is placed, the lower the voltage will be. Therefore, it is best not to stop at 2.4V when discharging. During 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 the voltage limit, the current limit is also necessary. When the current is too large, lithium ions will not have time to enter the storage cells and will accumulate on the surface of the material. After these lithium ions gain electrons, lithium atom crystals will be generated on the surface of the material, which is as dangerous as overcharging. In case the battery case breaks, it will explode. Therefore, the protection of lithium-ion batteries must include at least three items: the upper limit of the charging voltage, the lower limit of the discharging voltage, and the upper limit of the current. In a general lithium battery pack, in addition to the lithium battery core, there will be a protective plate, which mainly provides these three protections. However, these three protections of the protective board are obviously not enough, and 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.
Analysis of Explosion Types
The types of battery cell explosion can be classified into three types: external short circuit, internal short circuit, and overcharge. The outside here refers to the outside of the cell, including the short circuit caused by poor internal insulation design of 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 case. When the internal temperature of the battery is as high as 135 degrees Celsius, the good-quality separator 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, thereby avoiding the explosion. However, if the pore closure rate is too poor, or the separator paper whose pores will not be closed at all, the temperature of the battery will continue to rise, more electrolyte will vaporize, and finally the battery casing will be broken, and the battery temperature will even be raised to a high level. The material burns and explodes, and the internal short circuit is mainly caused by the burrs of copper foil and aluminum foil piercing the diaphragm, or the dendrites of lithium atoms piercing the diaphragm. These tiny needle-like metals can cause micro-short circuits. Since the needle is very thin and has a certain resistance value, the current may not be very large. The copper and aluminum foil burrs are caused during the production process. The observable phenomenon is that the battery leaks too fast, and most of them can be screened out by the battery factory or assembly factory. Moreover, due to the tiny burrs, they are sometimes blown, 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 supported by statistics from the fact that there are often bad batteries with low voltage shortly after charging in each battery cell factory, but explosions rarely occur. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharging. Because, after overcharging, needle-shaped lithium metal crystals are everywhere on the pole piece, puncture points are everywhere, and micro-short circuits are happening everywhere. Therefore, the temperature of the battery will gradually increase, and finally the high temperature will gas 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 the lithium metal, it will all end in an explosion. However, the explosion caused by the internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the temperature of the battery is not high enough to burn the material and the gas generated is not enough to break the battery casing, the consumer will stop charging and take the mobile phone out. At this time, the heat generated by numerous micro-short circuits slowly increases the temperature of the battery, and after a period of time, the explosion occurs. The common description of consumers is that when they pick up the mobile phone, they find that the mobile phone is very hot, and it explodes after throwing it away.
Combining the above types of explosions, we can focus on the three aspects of explosion protection: the prevention of overcharge, the prevention of external short circuits, and the improvement of battery 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 battery safety improvement is chemical and mechanical protection, which has a lot to do with battery cell manufacturers.
design specification
Since there are hundreds of millions of mobile phones in the world, to achieve safety, the failure rate of security 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 the battery system, there must be more than two safety lines of defense. A common mistake is to use the adapter to directly charge the battery pack. In this way, the overcharge protection task 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, explosion accidents will still happen every day in the world. If the battery system can provide two safety protections for overcharge, overdischarge, and overcurrent respectively, if the failure rate of each protection is 1 in 10,000, the two protections can reduce the failure rate to 1 in 100 million. The block diagram of a common battery charging system is as follows, including two parts: a charger and a battery pack. The charger also includes two parts: an adapter (Adaptor) and a charge controller. The adapter converts the AC to DC, and the charge controller limits the maximum current and voltage of the DC. The battery pack consists of two parts, the protection board and the battery core, and a PTC to limit the maximum current.
The battery cell takes 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. In this way, even if the protection board on the battery pack fails, the battery will not be overcharged. Dangerous. The second protection is the overcharge protection function on the protection board, which is generally set to 4.3V. In this way, the protection board is usually not responsible for cutting off the charging current, and only needs to act when the voltage of the charger is abnormally high. The overcurrent protection is in charge of the protection board and the current limiter, which are also two protections to prevent overcurrent and external short circuit. Because over-discharge will only happen in the process of electronic products being used. 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 the product is not designed with this function, 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 overcharge, overdischarge, and overcurrent. Among them, the protective plate is the second protection. Remove the protective plate and charge it. If the battery explodes, it means poor design.
Although the above method provides two protections, consumers often buy non-original chargers to charge after the charger is broken, and charger manufacturers, based on cost considerations, often remove the charge 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 overcharging is the most important factor causing battery explosion, therefore, inferior charger can be regarded as the culprit of battery explosion.
Of course, not all battery systems use the solution shown in the above figure. In some cases, there will also be a charge controller design in the battery pack. For example: Many notebook computers have charge controllers for external battery sticks. This is because notebook computers generally have charge controllers built into the computer, and only provide consumers with an adapter. Therefore, the external battery pack of the notebook computer must have a charge controller to ensure the safety of the external battery pack when being charged by an adapter. In addition, products that are charged by car cigarette lighters sometimes have a charge controller built into the battery pack.
the last line of defense
If all electronic protection measures fail, the last line of defense will be provided by the batteries. The safety level of the battery can be roughly classified according to whether the battery can pass through an external short circuit and overcharge. Because, before the battery explodes, if there are lithium atoms accumulated on the surface of the material inside, the explosion will be more powerful. Moreover, overcharge protection is often left with only one line of defense because consumers use inferior chargers. Therefore, the ability of the battery to resist overcharge is more important than the ability to resist external short circuits. If, an external short circuit does not explode the cell, you can get a star. And the battery that will not explode when overcharged can get two stars. Then the safety level of the battery cell has four levels from zero star to three stars. The batteries made in Japan and Taiwan can usually reach the highest level of three stars, while the batteries made in China are uneven.
The method of anti-external short circuit of the battery usually includes two measures: the use of high-quality diaphragm materials and the use of pressure valves. Among them, the high-quality separator works best, and more than 99% of the batteries will not explode when the external short circuit occurs. Pressure valves have side effects. For example, when the battery is overcharged, if the pressure valve ruptures too early, it will allow oxygen to enter and cause an explosion. In addition, the accuracy of Chinese-made pressure valves is very unreliable. Less than 10% of the pressure valves produced by China Aluminum Shell Factory will work within the pressure range declared by the industry, and the reliability is only 1% or 2%. Therefore, for the assembly plant, the best way to ensure that the battery will not explode in a short circuit is to do experiments directly. As long as the battery is fully charged, and then short-circuited in an explosion-proof box, the ability to withstand short-circuit can be determined.
The anti-overcharge method of the battery cell is very complicated, and the chemical formula, mechanical properties of the casing, and supporting electronic components must be considered. Generally, when the battery is overcharged to a certain voltage, the additives start to react. On the one hand, the internal resistance is increased, and on the other hand, the electric energy is converted into heat energy, so as to reach the safety level of 1C/12V overcharging for 6 hours without explosion. For batteries used in series and in parallel, supporting electronic components or sophisticated pressure valves are sometimes added.
At present, about half of the batteries made in China can pass the short-circuit test, but most of them cannot pass the overcharge test. In terms of electronic protection, the most vulnerable line of defense is overcharge protection, and the explosion caused by overcharge is also the most serious. Therefore, the anti-overcharge ability of the battery cell is the most important thing for the relevant industry when selecting the battery cell. For a single battery, at least it must be able to reach 1C/6V overcharge for 6 hours without exploding. For batteries used in series, at least 6 hours can be overcharged by 1C/12V. Only in this way can the safety of the battery pack be ensured in the case of consumers using inferior chargers.
Notes to Consumers
If the lithium-ion battery system is properly designed, there are two electronic protections for overcharge, overdischarge, and overcurrent, and the battery cell can also pass the short circuit test and overcharge test. Then, the security should be safe, and consumers can use it with peace of mind.
When buying a charger, it is best to buy the original charger. When buying a non-original charger, don't trust the output voltage value marked by the manufacturer. Use the ammeter to measure the output voltage of the charger, and it can only be used if the voltage is lower than 4.3V. When you must use a charger with a voltage higher than 4.3V, it is best to plug it into the end of the phone to charge. Never charge a non-original battery pack on a non-original charging stand, most explosions happen this way.