Explosion or combustion of large-capacity polymer cells is caused by a violent chemical reaction inside the cell, which generates a large amount of heat and gas. To cause a strong chemical reaction inside the cell, necessary external conditions must be met:
1. The battery is severely overcharged. The chemical system of the lithium ion battery determines that when the battery is overcharged, the LiCoO2 inside the battery becomes Co3O4. Co3O4 is a strong oxidant and can react with the electrolyte to generate gas (O2). . Oxygen continues to react with the electrolyte to produce water (H2O), which in turn can continue to react with Li and the electrolyte. . . In this way, the inside of the cell generates heat and a large amount of gas. This reaction will increase with the increase of the voltage. Generally speaking, when the cell voltage exceeds 4.2V, the reaction occurs and the cell voltage reaches When approaching 5V, most of the cells will burn (polymer cells) or explode (liquid hard pack) due to internal short circuits and the like as the internal temperature of the cells increases and the gas pressure acts. The specific reaction process is as follows:
2. The battery core is heated by high temperature. When the battery core is heated by high temperature, the electrolyte will vaporize and decompose at the same time. The internal pressure of the battery core increases. The isolation film inside the battery core may melt and cause internal short circuit of the battery core to cause combustion. When the material is fully charged, at a certain temperature, an autocatalytic reaction occurs and combustion occurs. The higher the cell voltage, the shorter the time to withstand high temperatures. The following figure shows the Joule thermal characteristics of different materials:
3. The battery core is short-circuited. When the battery core is fully charged, it is suddenly short-circuited. This may cause a safety accident in the battery core. There are two cases of short circuit:
1) The external short circuit of the battery core. For the liquid hard-packaged battery core, the internal electrolyte decomposes and vaporizes due to the increase of the temperature of the battery core, which may cause the internal pressure of the battery core to be too large and explode, and some of the batteries are due to various other reasons. The internal pressure is already very large, and the battery may explode under the action of short circuit and high temperature. For flexible packaging batteries, under the action of an external short circuit, it will bulge or crack. Generally, there is no burning phenomenon.
2) The internal short circuit of the battery core, when the battery is fully charged, for various reasons, when the anode and cathode of the battery core are short-circuited, the temperature rises sharply, causing a violent reaction inside the battery core. At this time, if it is hard-shell-packed The battery will explode, and the cells in the polymer flexible package will burn.
To solve the above problems, the protection board, PTC, special charger and fuse are mainly used to protect the battery core. The protection board protects the over-charge, over-discharge and over-current of the battery. The PTC protects the battery from the off-current under the condition of short-circuit and large current when the battery is overheated. The dedicated charger is correct. The charging current and voltage of the battery are limited, and the fuse is currently used less. Its main function is to completely cut off the connection between the battery and the external circuit when the voltage or current of the battery is abnormal, so as to ensure that the battery core will not be used again. In order to ensure that the battery will not cause serious safety accidents. Therefore, the reliability of these protective devices directly affects the safe use of the cells, which should be paid enough attention.
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