Lithium ion battery characteristics
Lithium is the smallest and most active metal on the chemical periodic table. Small volume, high volume density, widely welcomed by consumers and engineers. However, the chemical characteristics are too active, which brings great danger. When exposed to air, lithium metal will explode with intense oxidation of oxygen. 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 tiny nanoscale storage lattices that can be used to store lithium atoms. In this way, even if the battery shell ruptures, oxygen enters, because the oxygen molecule is too big to enter these small storage cells, so that lithium atoms will not contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to obtain high capacity density, but also to achieve safety purposes.
Electrical explosion protection test
When lithium ion batteries are charged, the lithium atoms of positive electrodes will lose electrons and become lithium ions. Lithium ions swim through the electrolyte to the anode, enter the anode cell, and obtain an electron, which is reduced to lithium atoms. When discharging, the whole procedure is reversed. In order to prevent the battery’s positive and negative electrodes from directly touching and short-circuiting, the battery will be coupled with a diaphragm paper with many holes to prevent short-circuiting. Good diaphragm paper can also be in the battery temperature is too high, automatically close the pores, so that lithium ions can not penetrate, to self-waste martial arts, to prevent danger.
protective measures
The lithium battery core will start to produce side effects when the charge exceeds 4.2V. The higher the overcharging voltage, the higher the risk. When the lithium cell voltage is higher than 4.2V, the number of lithium atoms left in the cathode material is less than half, and the cell often collapses, causing a permanent decline in battery capacity. If the charge continues, subsequent lithium metal will accumulate on the surface of the anode material because the cell of the anode is already filled with lithium atoms. These lithium atoms will grow dendrites from the surface of the negative electrode to lithium ions. These lithium metal crystals will pass through the separator paper to make the positive and negative poles short circuited. Sometimes the battery explodes before the short circuit occurs, because during the overcharge process, materials such as electrolyte will crack to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to react with lithium atoms deposited on the negative surface, and then explode. Therefore, when charging lithium batteries, it is necessary to set the voltage limit, so as to take into account the battery life, capacity, and safety. The ideal charge voltage limit is 4.2V. The discharge voltage of the lithium core should also have a lower voltage limit. When the core voltage is lower than 2.4V, some materials will start to be destroyed. And because the battery will self-discharge, the longer the discharge voltage will be lower, therefore, it is better not to discharge 2.4 V before stopping. The lithium battery releases only about 3% of the battery capacity during the period from 3.0V to 2.4V. Therefore, 3.0V is an ideal discharge cut-off voltage.
In addition to voltage limitation, current limitation is also necessary. When the current is too large, the lithium ion will not enter the storage cell and will aggregate on the surface of the material. When these ions get electrons, they produce lithium atom crystals on the surface of the material, which, like overcharging, can be dangerous. If the battery shell breaks, it will explode.
Therefore, the protection of lithium-ion batteries should include at least three items: charge voltage upper limit, discharge voltage lower limit, and current upper limit. In general, in addition to lithium battery batteries, there will be a protective plate, this plate is mainly to provide these three protections. However, the protection of these three protection is obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, we must make a more careful analysis of the cause of the battery explosion.
Explosion type analysis
The type of battery core explosion can be classified into three types: external short circuit, internal short circuit and overcharge. The exterior here refers to the exterior of the battery, including the short circuit caused by poor insulation design inside the battery pack.
When a short circuit occurs outside the battery and the electronic components fail to cut off the circuit, high heat will occur inside the battery, resulting in partial vaporization of electrolyte, which will expand the battery shell. When the temperature inside the battery reaches 135 degrees Celsius, good quality diaphragm paper closes the pores, the electrochemical reaction terminates or nearly terminates, the current suddenly drops, and the temperature slowly drops, thus avoiding explosion. However, poor pore closure, or diaphragm paper with no pore closure, will keep the battery temperature rising, more electrolyte vaporization, and eventually burst the battery shell, or even raise the battery temperature to make the material burn and explode the internal short circuit mainly because the burr between copper and aluminum foil breaks through the diaphragm, or lithium. The dendrites of atoms break through the diaphragm. These tiny needled metals can cause short circuit. Because the needle is very thin and has a certain resistance value, the current will not be very large.
Copper and aluminum foil burrs are caused by the production process, and the observed phenomenon is that the battery leakage is too fast, most of which can be detected by the battery factory or assembly plant screen. Moreover, because the burr is fine, sometimes it will be burned, so that the battery will return to normal. Therefore, the probability of burglings caused by short circuit burr is not high. Such a view can be obtained from the battery factory often charged within a short time, the voltage on the low side of the bad battery, but few explosions, statistical support. Therefore, the explosion caused by internal short circuit is still mainly caused by explosion.