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 a lithium cell is higher than 4.2V, less than half of the lithium atoms remain in the cathode material. At this time, the storage cell often collapses, causing permanent capacity loss of the battery. If you continue to charge, since the storage cell of the negative electrode has been filled with lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendritic crystals from the negative surface to the direction of lithium ions. These lithium metal crystals will pass through the diaphragm and short circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs, because in the overcharge process, electrolyte and other materials will decompose to produce gas, which makes the battery shell or pressure valve bulge and crack, allowing oxygen to enter and react with lithium atoms accumulated on the surface of the negative electrode, and then explode.
Therefore, when charging the lithium battery, the upper voltage limit must be set, so that the service life, capacity and safety of the battery can be taken into account at the same time. The optimal upper limit of charging voltage is 4.2V. There should also be a lower voltage limit when the lithium battery cell is discharged. When the cell voltage is lower than 2.4V, some materials will begin to be damaged. Because the battery will discharge itself, the longer it is discharged, the lower the voltage will be. Therefore, it is best not to stop at 2.4V when discharging. During the discharge period from 3.0V to 2.4V, the energy released by lithium battery only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage. During charging and discharging, in addition to voltage limitation, current limitation is also necessary. When the current is too large, lithium ions will gather on the surface of the material before entering the storage cell.After these lithium ions acquire electrons, lithium atoms will crystallize on the surface of the material, which, like overcharge, will cause danger. In case the battery shell breaks, it will explode. Therefore, the protection of lithium-ion batteries should include at least three items: the upper limit of charging voltage, the lower limit of discharging voltage, and the upper limit of current. Generally, in the lithium battery pack, except for the lithium battery cell, there will be a protective plate, which mainly provides these three protections. However, these three protections of the protective plate are obviously insufficient. Lithium battery explosions are still frequent around the world. To ensure the safety of the battery system, the cause of battery explosion must be analyzed more carefully.
