I. Lithium-ion battery characteristics
Lithium is the smallest diameter and the most active metal on the chemical periodic table. Small volume so high capacity density, popular with consumers and engineers. However, the chemical properties are too active, it brings a high degree of danger. When exposed to air, lithium metal can explode in a violent oxidation reaction with oxygen. In order to improve safety and voltage, scientists have invented materials such as graphite and lithium cobaltate to store lithium atoms.
The molecular structure of these materials, forming a nano-level fine storage lattice, can be used to store lithium atoms. In this way, even if the battery shell ruptures and oxygen enters, the oxygen molecules will be too large to enter these tiny storage compartments, making the lithium atoms not come into contact with oxygen and preventing an explosion.
This principle of lithium-ion batteries makes it possible to obtain its high capacity density while also achieving safety. When a lithium-ion battery is charged, the lithium atoms at the positive electrode lose electrons and oxidize to lithium ions. The lithium ions swim through the electrolyte to the negative electrode, enter the storage compartment of the negative electrode, and gain an electron and are reduced to lithium atoms. When discharging, the whole procedure is reversed.
To prevent the positive and negative electrodes of the battery from directly touching and shorting out, a diaphragm paper with many fine holes is added to the battery to prevent shorting out. A good diaphragm paper can also automatically close the fine holes when the battery temperature is too high, so that lithium ions can not cross, in order to self-destruct and prevent dangerous occurrences.
Protective measures: lithium-ion battery cells overcharge to a voltage higher than 4.2V, will begin to appear after the vice use. The higher the overcharge voltage, the higher the danger will follow. After the voltage of lithium cell is higher than 4.2V, the number of lithium atoms left in the positive electrode material is less than half, and the storage cell will collapse at this time, causing a permanent drop in battery capacity. If charging continues, as the storage compartment 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 dendritic crystals from the surface of the negative electrode in the direction of the lithium ions. These lithium metal crystals can pass through the diaphragm paper and short-circuit the positive and negative electrodes.
Sometimes the battery explodes before the short circuit occurs, because in the overcharge process, the electrolyte and other materials will be cracked gas, making the battery shell or pressure valve bulge 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 lithium-ion batteries, it is important to set the upper voltage limit in order to take into account the battery life, capacity, and safety. The ideal charging voltage limit is 4.2 V. There is also a lower voltage limit when discharging lithium cells. When the cell voltage is below 2.4V, some materials will start to be destroyed. Because the battery will self-discharge, the longer you put the voltage will be lower, therefore, it is best not to discharge to 2.4V before stopping. Lithium-ion battery from 3.0V to 2.4V discharge period, 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 high, lithium ions do not have time to enter the storage compartment and will collect on the material surface. These lithium ions gain electrons and crystallize lithium atoms on the surface of the material, which can be dangerous in the same way as overcharging. In case the battery case breaks, it can explode.
Therefore, the protection of lithium-ion batteries should include at least three items: upper limit of charging voltage, lower limit of discharging voltage, and upper limit of current. General lithium-ion battery pack, in addition to lithium-ion battery cells, there will be a protection plate, this protection plate is important to supply these three protections. However, the protection board of the three protection is obviously not enough, the global lithium-ion battery explosion is still frequently spread.
To ensure the safety of the battery system, the cause of the battery explosion must be more carefully analyzed:
Second, the battery explosion causes :
1, a large internal polarization.
2, the pole piece of water absorption, and electrolyte reaction gas drum.
3, the quality of the electrolyte itself, performance problems.
4、Injecting liquid when the amount of liquid does not meet the process requirements.
5、Poor sealing performance of laser welding in the assembly process, air leakage. Measurement of gas leakages.
6、Dust, pole piece dust first easy to cause micro short circuit, the specific reason is unknown.
7、The positive and negative electrode pieces are thicker than the process range, and it is difficult to enter the shell.
8, liquid injection sealing problems, steel ball sealing performance is not good, resulting in gas drum.
9, shell material shell wall thickness, shell deformation affects the thickness.
Third, the type of explosion analysis
Battery cell explosion can be summarized as a class of external short circuit, internal short circuit, and overcharge three.
Here the external refers to the outside of the cell, including the short circuit caused by poor insulation design inside the battery pack. When a short circuit occurs outside the cell and the electronic components fail to cut off the circuit, high heat will occur inside the cell, causing some of the electrolyte to vaporize and expand the battery case. When the internal temperature of the battery reaches 135 degrees Celsius, good quality diaphragm paper, will close the pores, the electrochemical reaction is terminated or nearly terminated, the current plunges, the temperature also slowly drop, and thus prevented the explosion.
However, the pore closure rate is too poor, or the pores simply will not close the diaphragm paper, will allow the battery temperature to continue to rise, more electrolyte vaporization, and finally the battery shell will burst, or even raise the battery temperature to make the material burn and explode.
Internal short circuit is important because the burr of copper and aluminum foil through the diaphragm, or dendritic crystals of lithium atoms through the diaphragm caused by. These tiny needles of metal can cause micro-shorts. Since the needles are very thin and have a certain resistance value, the current may not be very high. The copper and aluminum foil burrs are caused during the production process and can be observed as the battery leaks too quickly, mostly screened by the cell factory or assembly plant.
Moreover, due to the small size of the burr, sometimes it will be burned off and the battery will be back to normal. Therefore, the chance of explosion caused by micro-short circuit of burr is not high. Such a claim can be supported statistically by the fact that there are often defective batteries with low voltage soon after charging within each cell factory, but few explosions occur. Therefore, the internal short circuit caused by the explosion, or importantly, caused by overcharging. Because, after overcharging the poles are covered with needle-like lithium metal crystals, puncture points are everywhere, and micro-short circuits are occurring everywhere. Therefore, the battery temperature will gradually increase, and finally high temperature will electrolyte gas. This situation, whether the temperature is too high so that the material burns and explodes, or the casing is first propped up, so that the air goes in and lithium metal occurs intense oxidation, are explosive end.
But overcharge triggered by internal short circuit caused by this explosion, does not necessarily occur at the time of charging. It is possible that the battery temperature is not yet high enough to make the material burn, the emergence of gas is not enough to burst the battery shell, the consumer will terminate the charge, take the phone out. At this point the heat from the numerous micro-shorts that occur slowly raises the temperature of the battery, and only after a period of time does the explosion occur. The common description of consumers are found to be hot when they pick up the phone and explode after throwing it away. Comprehensive above the type of explosion, we can focus on the explosion-proof prevention of overcharging, external short circuit prevention, and enhance the safety of the three aspects of the battery cell. Among them, overcharge prevention and external short circuit prevention belong to the electronic protection, and battery system design and battery pack with a greater relationship. The focus of cell safety improvement is chemical and mechanical protection, which is more related to the battery cell manufacturer.
