The principle of lithium battery ballooning and gas explosion

First, lithium ion battery characteristics

Lithium is the smallest and most active metal on the chemical periodic table. Small in size and high in capacity density, it is widely welcomed by consumers and engineers. However, the chemical properties are too active and pose a very high risk. When exposed to air, lithium metal can explode with intense oxidation of oxygen. 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 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.

This principle of lithium-ion batteries allows people to achieve high-density density while achieving safety. When a lithium ion battery is charged, the lithium atom of the positive electrode loses electrons and oxidizes to lithium ions. Lithium ions swim through the electrolyte to the negative electrode, enter the cell of the negative electrode, and obtain an electron, which is reduced to a lithium atom. When discharging, the entire program is reversed. In order to prevent the battery from being short-circuited by direct contact between the positive and negative terminals, a separator paper with a large number of fine holes is added to the battery to prevent short circuit. Good diaphragm paper can also automatically close the pores when the battery temperature is too high, so that lithium ions cannot pass through, so as to prevent the danger from happening.

Protective measures: When the lithium battery cell 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 is. After 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 direction of 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 it exploded.

Therefore, lithium batteries, be sure to set the upper limit of voltage, can simultaneously to the battery life, capacity, and security. The ideal maximum charging voltage of 4.2 V. Lithium batteries discharge voltage will be lower limit. When the batteries voltage is below 2.4 V, part of the material will begin to be destroyed. And because the battery self-discharge, put the more the longer the voltage is low, therefore, discharge when had better not to 2.4 V. Lithium battery discharge from 3.0 V to 2.4 V during this period, the release of energy accounted for only about 3% of the battery capacity. Therefore, is an ideal discharge cut-off voltage of 3.0 V. Charge and discharge, in addition to the limit of voltage and current limits can also be necessary. When the current is too large, lithium ions to enter the store, would gather in the material surface. After these lithium ion for electronic, can produce lithium atoms in the material surface crystallization, this has to do with the overcharge, will cause the risk. In the event of a broken battery case, 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 board, 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, the battery must be the cause of the explosion, a more careful analysis.

Second, the battery explosion reasons:

1, internal polarization is larger;

2, water absorption, react with the electrolyte gas drum;

3, electrolyte itself, the quality of the performance issues;

4, when liquid injection liquid quantity cannot meet the technical requirements;

5, preparation process of laser welding sealing performance is poor, leak. Measuring air leak test;

6, dust, dust sheet first can cause micro short circuit, for reasons unknown;

7, it’s a process from the cathode scope thick, hard into the shell;

8, filler sealing problem, steel ball seal performance is bad to cause gas drum;

9, the incoming exist shell wall thick, shell thickness deformation effect;

Third, explosion type analysis

Battery core explosion type can be summarized as external short circuit, internal short circuit and overcharge 3 kinds.

The external refers to the batteries of the external, contains a battery pack and poor internal insulation design caused by short circuit. When external short-circuited batteries, electronic components and failed to cut off the circuit, batteries inside will produce heat, caused part of the electrolyte vaporization, the battery shell support. When high battery internal temperature to 135 degrees Celsius, good quality paper, diaphragm will close pores, electrochemical reaction termination or near end, current, temperature is falling slowly, then avoid the explosion occurred. But the pores close rate is poor, or pores do not close the diaphragm paper, can let the battery temperature continues to rise, more electrolyte vaporization, finally the battery shell burst, and even to increase the battery temperature to make the material combustion and explosion.

The internal short circuit is mainly caused by the burr of the copper foil and the aluminum foil piercing the diaphragm, or the dendrite of lithium atoms piercing the diaphragm. 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. Copper and aluminum foil burrs are caused during the production process. The observed phenomenon is that the battery leaks too quickly, and most of them can be screened by the battery factory or assembly plant. Moreover, since the burrs are small, they are sometimes blown off, so that the battery returns to normal. Therefore, the probability of an explosion caused by a burr micro-short circuit is not high. In this way, it is possible to get a bad battery with a low voltage shortly after charging in each battery factory, but there are few explosion events and statistical support. Therefore, the explosion caused by the internal short circuit is mainly caused by overcharge. Because, after overcharging, the lenticular lithium metal crystals are everywhere on the pole piece, and the piercing point is everywhere, and micro short circuits occur everywhere. Therefore, the battery temperature will gradually increase, and finally the high temperature will be the electrolyte gas. In this case, whether the temperature is too high, the material burns and explodes, or the outer shell is broken first, so that the air enters and the lithium metal is violently oxidized, which is the end of the explosion.

However, such an explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the battery temperature is not high enough for the material to burn and the gas generated is not enough to break the battery casing the consumer terminates charging and takes the mobile phone out. At this time, the heat generated by the numerous micro short circuits slowly increases the temperature of the battery, and after a period of time, the explosion occurs. The common description of consumers is that when the phone is picked up, the phone is found to be very hot, and it will explode after being thrown away.

In combination with the above types of explosions, we can focus on the prevention of overcharge, the prevention of external short circuits, and the safety of battery cells. Among them, overcharge prevention and external short circuit prevention are electronic protection, which has a great relationship with battery system design and battery assembly. The focus of battery safety improvement is chemical and mechanical protection, which has a great relationship with battery manufacturing plants.

The page contains the contents of the machine translation.