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, less than half of the lithium atoms remain in the cathode material. At this time, the storage cell often collapses, resulting in a permanent decline in the battery capacity. If charging is continued, since the storage 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 dendritic crystals from the negative surface to the direction of lithium ions. These lithium metal crystals will pass through the diaphragm paper and short circuit the positive and negative electrodes. Sometimes, the battery will explode before the short circuit occurs. This is because during the overcharge process, electrolyte and other materials will be cracked to produce gas, which makes the battery shell or pressure valve bulge and burst, allowing oxygen to enter and react with lithium atoms accumulated on the negative surface, and then explode. Therefore, when charging the lithium battery, the upper voltage limit must be set to take into account the battery life, capacity and safety. 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. Since 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 during discharge. During the period from 3.0V to 2.4V, the energy released by the 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 do not have time to enter the storage cell and will gather on the surface of the material. After these lithium ions acquire electrons, lithium atoms will crystallize on the surface of the material, which is the same as overcharge, which will cause danger. If the battery case breaks, it will explode.
Therefore, the protection of lithium-ion batteries should at least include three items: upper limit of charging voltage, lower limit of discharge voltage, and upper limit of current. In general, 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 protection 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.