Analyze the various parameters of lithium batteries and the necessity of various protections in detail from the principle

　　Lithium is the smallest and most active metal element on the chemical periodic table. Because of its small size, lithium has a high capacity density, so it is widely welcomed by consumers and engineers. However, the chemical properties are too active and will bring extremely high risks. When lithium metal is exposed to the air, it will react violently with oxygen and explode.

　　In order to improve safety and voltage, scientists use materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms a small nano-scale 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 battery enables people to achieve the goal of safety while obtaining its high capacity density.

　　When a lithium-ion battery is charged, the lithium atoms in the positive electrode lose electrons and are 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 positive and negative poles of the battery 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 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 lithium atoms remaining in the positive electrode material is less than half. At this time, the cell often collapses, causing a permanent decrease in battery capacity. If you continue to charge, since the cell of the negative electrode is already 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.

　　Microscopic photo of lithium dendrites

　　These lithium metal crystals will pass through the separator paper, short-circuiting 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, then explode.

　　Therefore, when charging a lithium battery, the upper limit of the voltage 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 charging voltage for ternary lithium batteries is 4.20V, and the most ideal upper limit of charging voltage for lithium iron phosphate batteries is 3.65V.

　　There is also a lower voltage limit when discharging lithium batteries. Let's take a ternary lithium battery as an example. When the cell voltage is lower than 2.4V, some materials will begin to be destroyed. Because 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. 3.0V is the ideal discharge cut-off voltage of a ternary lithium battery, and 2.0V is the ideal discharge cut-off voltage of a lithium iron phosphate battery.

　　When charging and discharging, in addition to the voltage limitation, the charging current must be limited. 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.

　　Therefore, the protection of lithium-ion batteries must include at least three items: the upper limit of the charging voltage, the lower limit of the discharge 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 board, which 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 around the world. To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.