What are the main reasons for the explosion of lithium batteries?
Characteristics of lithium-ion batteries
Lithium is the smallest and most reactive metal on the periodic table. Due to its small size and high capacity density, it is widely welcomed by consumers and engineers. However, the chemical properties are too active, which brings a very high risk. When lithium metal is exposed to air, it will undergo a violent oxidation reaction with oxygen and explode. 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 creates tiny nanoscale storage lattices 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 tiny storage cells, so that the lithium atoms will not come into contact with oxygen and avoid explosion.
This principle of lithium-ion battery enables people to achieve the purpose of safety while obtaining its high capacity density. When a lithium-ion battery is charged, the lithium atoms of 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 a lithium atom. When discharging, the whole procedure is reversed. In order to prevent the positive and negative electrodes of the battery from directly touching and short-circuiting, a separator paper with many pores will be added to the battery to prevent short-circuiting. A good separator paper can also automatically close the pores when the temperature of the battery is too high, so that lithium ions cannot pass through, so as to waste martial arts 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 cell is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half, and the storage cell often collapses at this time, resulting in a permanent decrease in the battery capacity.
If charging is continued, since the storage cell of the negative electrode is already full of 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 in the direction of the lithium ions. 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 overcharge process, the electrolyte and other materials will crack to generate gas, causing the battery shell or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. And then explode.
Therefore, when charging a lithium battery, the upper voltage limit must be set, so that the battery life, capacity, and safety can be taken into account at the same time. The ideal upper limit of the charging voltage is 4.2V. There is also a lower voltage limit when the lithium battery is discharged. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. And because the battery will self-discharge, the longer the discharge, the lower the voltage will be. Therefore, it is best not to put it at 2.4V to stop when discharging. During the period of lithium battery discharge from 3.0V to 2.4V, the energy released 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, the lithium ions have no time to enter the storage cell and will accumulate on the surface of the material. After these lithium ions gain electrons, lithium atoms will crystallize on the surface of the material, which is as dangerous as overcharging. If the battery case breaks, it will explode. Therefore, the protection of lithium-ion batteries should 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. Generally, in the lithium battery pack, in addition to the lithium battery cells, there will be a protective plate, which mainly provides these three protections. However, these three protections of the protective plate are obviously not enough, and the explosion of lithium batteries is still frequent around the world.
To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.
Lithium battery explosion reasons
1. The internal polarization is large;
2. The pole piece absorbs water and reacts with the electrolyte;
3. The quality and performance of the electrolyte itself;
4. When injecting liquid, the amount of liquid injection cannot meet the process requirements;
5. In the assembly process, the sealing performance of laser welding and welding is poor, and there is air leakage. Leakage and leakage detection;
6. Dust, pole piece dust is easy to cause micro-short circuit first;
7. The positive and negative plates are thicker than the process range, and it is difficult to enter the shell;
8. The problem of liquid injection and sealing, the poor sealing performance of steel balls leads to air bulging;
9. The incoming shell material has a thick shell wall, and the shell deformation affects the thickness;
Analysis of Lithium Battery Explosion Types
The types of battery cell explosions can be classified into three types: external short circuit, internal short circuit, and overcharge.
The external part here refers to the external part of the cell, including the short circuit caused by the poor insulation design inside the battery pack. When a short circuit occurs outside the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte to vaporize and expanding the battery shell. When the internal temperature of the battery is as high as 135 degrees Celsius, the separator paper of good quality will close the pores, the electrochemical reaction will be terminated or almost terminated, the current will drop sharply, and the temperature will also drop slowly, thereby avoiding explosion. However, if the pore closing rate is too poor, or the separator paper with pores that will not close at all, the temperature of the battery will continue to rise, more electrolyte will vaporize, and finally the battery casing will be broken, and even the temperature of the battery will be raised to the point where The material burns and explodes.