In order to avoid over discharge or over charge of the battery due to improper use, a triple protection mechanism is set in the single lithium ion battery. First, the switch element is used. When the temperature inside the battery rises, its resistance value rises accordingly. When the temperature is too high, the power supply will automatically stop; Second, select appropriate separator materials. When the temperature rises to a certain value, the micron sized micropores on the separator will automatically dissolve, so that lithium ions cannot pass through and the internal reaction of the battery stops; The third is to set a safety valve (i.e. the air vent at the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve will automatically open to ensure the safety of the battery.

　　Sometimes, although the battery itself has safety control measures, the control fails due to some reasons. If there is no safety valve or gas can not be released through the safety valve, the internal pressure of the battery will rise sharply and cause explosion.

　　Generally, the total energy stored by lithium-ion batteries is inversely proportional to their safety. With the increase of battery capacity, the battery volume is also increasing, and its heat dissipation performance becomes worse, which will greatly increase the possibility of accidents. For lithium-ion batteries for mobile phones, the basic requirement is that the probability of safety accidents should be less than one in a million, which is also the minimum standard acceptable to the public. For large capacity lithium-ion batteries, especially for automobiles, it is particularly important to use forced cooling.

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