The Anti-Explosion Technical Knowledge of Lithium- Ion Battery You Should Know About
Li-ion battery characteristics
Lithium is the smallest and most active metal on the chemical periodic table. Because of its small size and high capacity density, it is widely welcomed by consumers and engineers. However, the chemical properties are too active, which brings extremely high risks. When lithium metal is exposed to air, it will react with oxygen violently 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 a nano-level small storage grid that 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 small storage cells, so that lithium atoms will not come into contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to achieve safety while achieving high capacity density. When a lithium ion battery is charged, the lithium atom in the positive electrode loses electrons and is oxidized to lithium ions. Lithium ions swim to the negative electrode through the electrolyte, enter the storage cell of the negative electrode, 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, a diaphragm paper with many pores is added to the battery to prevent short-circuiting. A good diaphragm paper can also automatically close the pores when the battery temperature is too high, so that lithium ions cannot pass through, so as to waste martial arts and prevent danger.
After 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 risk. When the voltage of the lithium battery cell is higher than 4.2V, the number of remaining lithium atoms in the positive electrode material is less than half. At this time, the storage cell often collapses, causing a permanent decrease in battery capacity. If you continue to charge, since the cell of the negative electrode has been filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of the 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 crack 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 surface of the negative electrode. And then exploded. Therefore, when charging a lithium battery, the upper voltage limit must be set so that the battery life, capacity, and safety can be taken into account at the same time. The most ideal upper limit of the charging voltage is 4.2V. There is also a lower voltage limit when discharging lithium batteries. When the cell voltage is lower than 2.4V, some materials will begin to be destroyed. Also, since the battery will self-discharge, the longer it is left, the lower the voltage will be. Therefore, it is best not to stop when the battery is discharged to 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 accumulate on the surface of the material. After these lithium ions obtain electrons, they will produce lithium atom crystals on the surface of the material, which is the same as overcharging, which is dangerous. If the battery casing breaks, it will explode. Therefore, the protection of lithium-ion batteries must include at least three items: upper limit of charging voltage, lower limit of discharge voltage, and upper limit of current. In general, in a lithium battery pack, in addition to the lithium battery core, there will be a protective board. This protective board mainly provides these three protections. However, these three protections of the protection board are obviously not enough, and there are still frequent explosions of lithium batteries worldwide. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.