We often see news reports of smart phone battery exploding or burning, so some people are worried about the safety of batteries, as we know, smart phone batteries usually use lithium polymer batteries, also named Li-polymer battery and Lipo battery.
The explosion or burning of the Lipo battery cells is caused by the violent chemical reactions inside the batteries, producing a great deal of heat and gas. There must be some external conditions that cause a violent chemical reaction inside the battery.
1. The battery is seriously overcharged.
The lithium-ion battery’s chemical system determines that when the battery is overcharged, LiCoO2 inside the battery becomes Co3O4. Co3O4 is a strong oxidant that can react with the electrolyte to produce gas (O2). Oxygen continues to react with the electrolyte to produce water (H2O), which can continue to react with Li and electrolytes. . . Therefore, the inside of the cell will generate heat and a large of gas. This reaction will increase as the voltage rises. Generally, when the voltage of cell over 4.2V, this reaction occurs, and when the voltage of cell reaches near 5V, with the increase of the internal temperature of cell and the gas pressure, most of the cells will burn (polymer cells) or explode (liquid hard package) due to the internal short circuit.
2. The cell is heated to a high temperature.
When the cell is heated to a high temperature, the electrolyte will vaporize and decompose at the same time. The internal pressure of the cell will increase. The separator of cell may melt, causing the internal short circuit, then the cell starts to burn. And at a certain temperature, the cathode will will undergo an autocatalytic reaction when the cell is fully charged, this will also lead cells to burn. The higher the cell voltage, the shorter the time for high-temperature resistance.
3. The battery is short-circuited.
When the battery is fully charged, and this battery is suddenly short-circuited, this may cause a safety accident on the battery. There are two cases of the short circuit. 1) The battery is external short-circuited. For liquid hard-packed cells, the electrolyte may decompose and vaporize due to the short-circuit high temperature , which may cause the internal pressure of the cell to be larger and then explosion. For soft packaging batteries, under the action of an external short circuit, it will bulge, or split, does not burn in general. 2) The internal short circuit of the cell. When the cell is fully charged, due to various reasons (such as pierced by a sharp object, struck violently), when the cathode and anode inside the cell are short-circuited, the temperature rises violently, causing a violent reaction inside the cell. At this time, if the cell is hard packaging, it will explode, and the polymer soft packaging cell will burn.
To solve the above-mentioned problems, the protection of the lipo cell is mainly by use a protection board, a PTC, a special charger or a fuse. The protection board protects the batteries from over-charging, over-discharging, and short-circuit. The PTC protection will cut off currents when the batteries are under high current and high temperature. The special charger is the charging the battery at limited current and voltage, and the fuse is less used now，its main function is to completely cut off the connection between the cell and the external circuit when the cell voltage or current is abnormal, ensuring that the cell will not be work again.
Overall, as long as we use the lipo battery correctly, it is safe.
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