Why Do Lithium Batteries Explode?
1. Characteristics of Lithium Battery
Lithium is the smallest and most active metal on the chemical periodic table. Because of its small size and high capacity density, it is widely used by consumers and engineers. However, the chemical properties are too active, which brings extremely high risks. When lithium metal is exposed to the air, it will react violently with oxygen and then explode. In order to improve safety and voltage, scientists invented materials like graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms a small nano-scale storage grid for lithium atoms. Then, even if the battery shell ruptures and oxygen enters, the oxygen molecules are too large to enter these small storage cells, so that the lithium atoms will not come into contact with the oxygen and avoid explosion.
When a lithium-ion battery is charged, the lithium atoms in the anode lose electrons and are oxidized to lithium ions. Lithium ions swim to the cathode through the electrolyte, enter the storage cell of the cathode, and obtain an electron, which is reduced to lithium atoms. When discharging, the whole procedure is reversed. To prevent the anode and cathode of the battery from directly touching and short-circuiting, add a diaphragm paper with many pores to the battery. 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 danger can be prevented.
To ensure the safety of the battery system, we msut analyze the causes of the lithium battery explosions more carefully.
2. Reasons for Explosions of Lithium Batteries
(1) Large internal polarization.
(2) The pole piece absorbs water and reacts with the electrolyte.
(3) There are some problems in the quality and performance of the electrolyte.
(4)The amount of liquid injected cannot meet the technological requirements when injecting liquid.
(5) In the assembly process, laser welding has poor sealing performance and air leakage.
(6) Dust on pole piece is likely to cause micro short circuit, the specific reason is unknown.
(7) The positive and negative pole pieces are thicker than the process range, making it difficult to enter the shell.
(8) The problem of liquid injection sealing, poor sealing performance of the steel ball leads to air bulging.
(9)The shell wall is too thick in the incoming shell material. And the shell deformation affects the thickness.
3. Measures for Avoiding Lithium Battery Explosions
If the lithium battery cell is overcharged to a voltage higher than 4.2V, there will be side effects. 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 anode material is less than half. At this time, the cell often collapses, causing a permanent decrease in battery capacity.
(1) Upper voltage limit
If continue charging, since the cell of the cathode is already filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the cathode material. These lithium atoms will grow dendrites from the surface of the cathode toward the direction of the lithium ions. These lithium metal crystals will pass through the separator paper and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. Because during the overcharging process, the electrolyte and other materials will crack and produce gas. And it causes the battery shell or pressure valve to swell and rupture. Next, the oxygen enters and reacts with the lithium atoms accumulated on the surface of the negative electrode and then exploded. Therefore, when charging a lithium battery, we must set the upper limit of the voltage. So we can balance the battery life, capacity, and safety. The most ideal upper limit of the charging voltage is 4.2V.
(2) Lower voltage limit
There is also a lower voltage limit when discharging lithium batteries. When the cell voltage is lower than 2.4V, some materials will begin to get destroyed. On the other hand, the battery will self-discharge, the longer it lasts, the lower the voltage will be. Therefore, it is best to stop discharging before the voltage turns to 2.4V. When the lithium battery is discharged 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.
(3) Upper current Limit
When charging and discharging, in addition to the voltage limit, the current limit is also necessary. When the current is too large, lithium ions have no 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. That is the same as overcharging, which is very dangerous. If the battery case ruptures, it will explode.
Therefore, the protection of lithium-ion batteries must include at least 3 points:
- upper limit of the charging voltage
- lower limit of the discharge voltage
- upper limit of the current
In the general lithium battery pack, in addition to the lithium battery core, there will be a protective board. And the board mainly provides these three protections. However, these three protections boards are obviously not enough, and there are still frequent explosions of lithium batteries around the world.
4. Analysis on the Types of Lithium Battery Explosions
The types of battery cell explosion can be classified as: external short circuit, internal short circuit, and overcharge.
External Short Circuit
The external part here refers to the outside of the battery cell, including short circuits caused by poor internal insulation design of the battery pack. When a short circuit occurs on the outside of the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell. And it will cause part of the electrolyte to vaporize and expand the battery shell. When the internal temperature of the battery reaches 135℃, a good quality diaphragm paper will close the pores. Then, the electrochemical reaction will be terminated or almost terminated. Besides, the current will drop sharply with the temperature droping slowly, thus avoiding an explosion. However, the diaphragm paper with the poor pore closure rate or the unclosed pores will cause the rising battery temperature. And more electrolyte will vaporize. Finally the battery shell will be broken. What’s worse, the battery temperature will be high enough to make the material burns and explodes.
Internal Short Circuit
The internal short circuit is mainly because the diaphragm is pierced by the burrs of the copper foil and aluminum foil, or the dendritic crystals of lithium atoms. These tiny needle-like metals can cause micro short circuits. Since the needle is very thin and has a certain resistance value, the current is not necessarily large. The copper and aluminum foil burrs are caused by the production process. The battery leaks too fast, and most of them can be screened by the battery cell factory or the assembly factory. However, the small burrs sometimes can be burned, causing the battery to return to normal. Therefore, the probability of explosion caused by burr micro-short circuit is not high. This can be testified by the fact that there are often bad batteries with low voltage shortly after charging in various battery cell factories, but there are few explosions. Thus, the explosion caused by an internal short circuit is mainly because of overcharging.
Here’s the reason: After overcharge, there are needle-like lithium metal crystals everywhere on the pole piece. Also, the puncture point is everywhere wiht the micro short circuit. Therefore, the battery temperature will gradually rise, which will finally cause the electrolyte to gas. The following situations will end up with an explosion:
- The temperature is too high to cause the material to burn and explode.
- The outer shell is first broken, withthe air entering and oxidizing the lithium metal.
Overcharge
But the explosion for overcharging does not necessarily occur at the time of charging. It’s possible that when the battery temperature is not high enough to burn the material and the gas generated is not enough to break the battery casing, the consumer stop charging and take the mobile phone out. At this time, the heat generated by numerous micro-short circuits slowly raises the battery temperature. And it explodes after a period of time.
As we have known about the causes of battery explosions, now we should prevent the overcharge, external short circuits, and improve cell safety. Among them, overcharge prevention and external short circuit prevention belong to electronic protection. And that has a great relationship with battery system design and battery assembly. The focus of battery cell safety enhancement is chemical and mechanical protection. And it has a great connection with battery cell manufacturers.