Has always been its high energy discharge stable and people of all ages, and thus established its dominance in the telecommunications, electronics fields. However, the past two years, but it has occurred more than cell phones, laptops bombings, the culprit is the lithium battery. This “diagnosis”, the lithium battery into the debate on the cusp. Why the sudden explosion of the battery will be good, or even kills come from? Improper use or the battery itself is a problem? No matter what, “lithium batteries” seems to have been branded a “suspected bomb” notoriety, how do we deal with it in the end it?
What are the conditions to have explosive
In order to ascertain the cause of battery explosion, we need to be clear - in the end what is the “explosion”?
Explosion in the usual sense, generally refers to the explosion of gunpowder, gas, etc., people always bang explosion impression, ablaze. Beijing Institute of Technology Professor Feng Shun Shan long been engaged in research and ammunition, he told reporters, “the explosion is a chemical reaction in a very short period of sudden and instantaneous release a lot of energy in the process.”
Potentially explosive chemical elements include carbon, nitrogen, oxygen, hydrogen, sulfur, etc. However, not only these elements but also, if it is normal to relieve the reaction process is not sufficient to constitute an explosion. According to experts, the explosion must have three conditions: the reaction process can release a lot of heat, the reaction is very fast, and to produce large amounts of gaseous products. For example, when explosives being large enough external energy effects, such as crashing or encountered when using detonators, will suddenly violent chemical reaction occurs in a short time, high temperature, high pressure gas, the formation of an explosion.
Speaking of explosives, people always immediately think of the name of TNT, which is a high explosive, an organic compound with a benzene ring, the scientific name is called TNT, under normal conditions, TNT yellow powder or flaky scales. Per kilogram TNT explosion can produce 4.2 million joules of energy, which energy is enough to ruin a bridge or a house.
Lithium has the right material conditions constitute explosion
Control the various elements to form the desired explosive, and to analyze the composition of the reaction the internal lithium battery. Internal lithium battery in the end what chemicals it contains? They form explosive enough to do?
Ter laboratory safety testing expert Chinese Ministry Liu said, “Usually people say phone lithium-ion batteries, also known as battery pack or battery pack, which is mainly composed of batteries and protection circuit, in which the positive and negative electrodes of batteries , electrolyte, separator, shell composition. “
Currently a widely used cathode material for lithium cobaltate, lithium cobalt oxide will decompose under certain conditions, the separation of lithium ions, a negative electrode material is graphite, the molecular structure with many small space, which is a lithium-ion storage cells. The electrolyte is typically a mixed organic solvent which is commonly called the electrolyte of lithium hexafluoro phosphate salt. As the function of the insulating film is mainly separated positive and negative, to prevent short-circuit the poles of the contact, the current isolation of the porous polymer film, a material generally used in the film of the polyolefin microporous film is to make the lithium ions can pass through freely. When charging the battery, the lithium ion battery positive electrode through the electrolyte to the cathode movement, embedded in the porous carbon layer, the more negative the embedded lithium ion battery charge capacity is higher. Similarly, when the battery discharges, the anode embedded in the carbon layer of lithium-ion prolapse, and movement back to the cathode. This completes a lithium-ion battery charge and discharge process.
Elements from the inside of the battery, it already contains carbon, oxygen and electrolytes easy to burn, these substances are sealed in a small space, if certain conditions have entirely possible due to chemical reactions release heat and huge gas, it could explode.
But not to say that with these elements, they will explode. Professor, Department of Applied Chemistry, Harbin Institute of Technology, the lithium battery expert Shi Pengfei told reporters that “only under certain environmental conditions, the lithium ion electrolyte, isolate interactions occur between the membrane, complex changes occur until the cause explosions. “
Why high-temperature lithium battery explosion could bring
You can further questioning, in the end what kind of environment conditions require, the internal battery only abnormal situation? Reporter specifically ask Professor Sun Jinhua State Key Laboratory of Fire Science, China University of Technology. Professor Sun said, “If the improper charging, the lithium battery in a short time will produce a lot of heat, it will explode.”
Explanations for Sun Jinhua, Liu said further added, “is charging lithium battery cathode materials, such as lithium cobalt oxide or lithium manganese oxide, the state is unstable, it will decompose, releasing oxygen under high temperatures. Released oxygen and organic solvent electrolyte reacts to produce a lot of heat. Additionally, under certain voltage is applied, a solvent and an electrolyte itself may also react and emit a lot of heat, causing security problems. “
The heat generated is how? Internal battery in the end what changes have occurred? We can not be seen with the naked eye. But the researchers use certain techniques, but can clearly “see” the process of change within the battery. Dr. Wang Qingsong China University of Technology, the use of advanced micro-calorimeter for thermal reaction of lithium batteries were studied experimentally, the study found, lithium battery cathode active material, a negative electrode active material, electrolyte, such as under normal use and abuse will occur exothermic reaction.
Sun Jinhua said, “heat release, while the reaction gas pressure inside the system was continuously increases, causing the thermal expansion of the battery, the security measures in case of failure, including the potential risk of explosion.”
In addition, the phone call will cause the battery to heat for a long time, but also cause internal circuit and the earpiece fever, if you happen to use a counterfeit battery, could easily cause an explosion. Some would call when the phone is charging, when charging the phone battery will generate heat would have, if we continue to use it to call, then the heat will quickly upgrade, can easily lead to danger.
Crystal cut separation membrane can lead to an internal short circuit detonated
The chemical reaction within the battery is a slow process, the resulting heat accumulation is also a gradual increase process. In another case, a sudden increase in the heat instantaneously, that is shorted.
Under normal conditions, the insulating film inside the battery positive and negative electrodes spaced apart, forming a short circuit is not, however, in the case of over-charging, the insulating film may be cut, it may touch the poles.
Isolation between positive and negative film is how it was scratched? Liu said, “In the case of over-charging, lithium-ion cathode fewer and fewer are going to go negative, while the negative electrode of lithium-ion is increasing. To a certain extent, the negative electrode of lithium ions overflow generated dendritic crystals, crystal growth spread inside the battery, and then cut through the isolation membrane until the contact with the negative, so there has been a short circuit inside the battery. “this is happening because of a short circuit in the battery, so you can call an internal short circuit. At this time, instantaneous release of a large amount of heat, electrolyte boiling, “boom” is heard on the bursting of the battery case.
”Breakdown” phenomenon without a conductor can cause a short circuit
Crystal scratched isolation membrane, actually played the role of “conductor” of communicating the positive and negative. In fact, even with no conductor, in the case of over-charging, the positive and negative poles may be in direct contact, this is called “breakdown”, the same short-circuit may occur.
It should be the concept of over-charging for clarification. The so-called “overcharging”, some people think that is super-long charge, in fact, this is a misunderstanding. Sun Jinhua said, “does not refer to overcharge the charging time is too long, but rather refers to the range of the standard lithium battery voltage exceeds the normal work.” In general, lithium-ion battery voltage is between 3.7 volts to 4.2 volts when charging, the voltage will be higher poles, if to continue to charge 4.2 volts, which is over-charged. If the voltage is high to a certain extent, it will form a “breakdown.”
According to experts, when the poles of the voltage is high enough, a strong electric field in the middle of the insulating material will lose its insulation properties, a conductor, thereby connecting the positive and negative poles, which is a dielectric breakdown. “It is not difficult to understand, even if there are two electrodes in the air, if the voltage is very large, it may also occur breakdown, the naked eye is the formation of a spark.” Therefore, when the voltage of the lithium battery poles when large enough, may be directly connected to the positive and negative poles entirely, this is actually a special kind of short circuit.
Communicating the positive and negative metal shavings cause a short circuit
In the case of over-charging or discharging, may lead to the accumulation of heat or cause a short circuit, then, if nothing is done, that is placed in normal circumstances, a lithium battery explosion will happen?
The answer is yes, there is also the risk that may occur, the Japanese company’s notebook burning is an example. When the company’s battery explosion, battery manufacturers initially remained silent, tight-lipped, and later under repeated requests from the media, it felt compelled to respond. Finally, manufacturers explained that, because the battery batteries contain metal shavings, the result of external short circuit caused the battery, so it sparked an explosion.
Manufacturer’s interpretation is reasonable? Without charging the battery itself, without short-circuit discharge will occur? Sun Jinhua analysis said, “If the batteries contains metal shavings, may lead to a short circuit completely, even in the normal state, may also lead to the positive and negative connections. “
Visible, the production process of the battery must also be highly restrictive. If the manufacturing process is rough, the assembly process appears some metal burrs, etc. can stay hidden membrane folds, one unfortunately, will make positive and negative poles to a close contact.
Battery reserve a “gas outlet”
For the problem of increased pressure within the battery, lithium batteries must have an internal pressure to ease security devices, which need to mention the pressure relief valve.
"Pressure relief valve means in the battery casing set a relatively thin section, which can effectively prevent the battery accidentally burst, it’s like a pressure cooker of the valve as well." Liu Road.
If the pressure relief valve is designed, when the internal gas then expands moderately relief valve opens, the gas can be smoothly discharged from the relief valve, but with a portion of the energy. If the pressure relief valve design is unreasonable, when the pressure increases would not open, then the internal pressure increases, increasing the danger at any time explode.
It should be noted that the pressure relief valve is not more “sensitive” as possible, if it is very easy to open the case, the outside air will come. Because the internal electrolyte boiling point, melting point is low, it is easy to react with the air, this time, there will also burn or explode.
"Fuse" can effectively prevent a short circuit
In daily life, some have used the phone for a long time, and still use the battery damage, and some young people like all kinds of metal ornaments, keys, etc. together with the phone, do you not know this would short-circuit the left risks. In another case, if the battery is subjected to a collision or impact, it may happen to cause a short circuit within the battery, the same explosion.
Short-circuit problem is very troubling, so be sure need protection circuit within the battery. Daily equivalent circuit protection circuit fuse. Liu said, “a qualified protection circuit should have such a function: When the detected charging voltage is too high, or when the battery is fully charged, you can cut off the charging circuit when the detected current or charge current is too large, you can limit. current or cut off the current. “
Such as mobile phones and laptop use lithium products, should also be charged and discharged its internal limit circuit. For example YD / T 1591 “mobile handset chargers and interface requirements and test methods” stipulates that “mobile phone charging current should be limited within the specified range, when the phone detects that the charging voltage is too high, you should cut off the charging circuit.” It is understood that some battery manufacturers are working on lithium battery protection circuit and more, increasing the safety factor of the battery.
Greatly reduce the risk of a new generation of lithium battery explosion
As the liquid electrolyte easily release heat, some battery manufacturers began to study other electrolytes to replace it. Has now successfully developed a lithium-ion polymer battery.
A liquid electrolyte and a lithium-ion battery type, the polymer lithium ion battery instead places the solid polymer electrolyte, the polymer may be “dry” and can be “colloidal”, the most current product the use of polymer gel electrolyte.
Since the solid electrolyte instead of a liquid electrolyte, polymer lithium-ion battery will not leak occurred, combustion and explosion risks are greatly reduced. Liu said, “polymer lithium-ion batteries can also be done using polymer cathode material, the energy will be higher than the current liquid lithium-ion battery more than 50%.” In addition, the polymer lithium-ion battery voltage, charge and discharge cycle life and other aspects than lithium-ion batteries has increased. Based on these advantages, the next generation of lithium-ion polymer battery known as lithium-ion batteries.
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