Lithium battery case characteristics
Lithium, atomic number 3, atomic weight 6.941, is the lightest alkaline metal raw material. 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 nanoscale fine storage grid that can be used to store lithium atoms. In this way, even if the battery casing is broken and oxygen enters, the oxygen molecules are too large to enter these fine cells, so that the lithium atoms do not come into contact with oxygen to avoid explosion.
Protective measures taken by the battery
When the lithium battery is overcharged to a voltage higher than 4.2V, it will start to have side effects. The higher the overcharge voltage, the higher the risk. When the lithium battery voltage is higher than 4.2V, the amount of lithium atoms remaining in the positive electrode material is less than half. At this time, the storage cell often collapses, causing a permanent drop in battery capacity. If the charging continues, since the cell of the negative electrode is already filled with lithium atoms, the subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms grow dendrites from the surface of the negative electrode toward the lithium ions. These lithium metal crystals pass through the separator paper, shorting 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 and generate gas, causing the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms deposited on the surface of the negative electrode. Then exploded.
Therefore, when charging a lithium battery, it is necessary to set the upper voltage limit to take into account the battery life, capacity, and safety. The optimal charging voltage is limited to 4.2V. The lithium battery has a lower voltage limit when it is discharged. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. Since the battery will self-discharge, the voltage will be lower for a longer period of time. Therefore, it is best not to put it at 2.4V to stop. During the period from the 3.0V discharge to the 2.4V lithium battery, the energy released is only about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. In charge and discharge, in addition to the voltage limit, current limitation is also necessary. When the current is too large, lithium ions do not reach the cell and accumulate on the surface of the material.
When these lithium ions obtain electrons, crystals of lithium atoms are generated on the surface of the material, which is dangerous as overcharge. In case the battery case is broken, it will explode. Therefore, the protection of the lithium ion battery must include at least three factors: the upper limit of the charging voltage, the lower limit of the discharging voltage, and the upper limit of the current. In the general lithium battery pack, in addition to the lithium battery core, there will be a protective plate, which is mainly to provide these three protections. However, these three protections of the protection board are obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, a more careful analysis of the cause of the battery explosion must be performed.