Overcharging lithium-ion battery cells to above 4.2V will begin to produce side effects. The higher the overcharge voltage, the greater the danger. When the voltage exceeds 4.2V, less than half of the lithium atoms remain in the positive electrode material. At this point, the storage cells often collapse, causing permanent capacity loss. If charging continues, since the negative electrode storage cells are already full of lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow into dendritic crystals from the negative electrode surface in the direction from which the lithium ions originated. These lithium metal crystals can penetrate the separator, causing a short circuit between the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs because during overcharging, the electrolyte and other materials decompose, producing gas that causes the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the negative electrode surface, leading to an explosion.
Therefore, when charging lithium-ion batteries, it is essential to set an upper voltage limit to balance battery life, capacity, and safety. The ideal upper charging voltage limit is 4.2V. Lithium-ion battery cells also have a lower voltage limit during discharge. When the cell voltage drops below 2.4V, some materials begin to deteriorate. Furthermore, because batteries self-discharge, the voltage decreases the longer they are discharged; therefore, it's best not to discharge to 2.4V before stopping. During the discharge process from 3.0V to 2.4V, the energy released by a lithium-ion battery only accounts for about 3% of its capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. Besides voltage limitations, current limitations are also necessary during charging and discharging. If the current is too high, lithium ions will not have enough time to enter the storage cells and will accumulate on the material surface.
