1. Short Circuiting: When a lithium ion battery is overcharged (even during normal charging or discharging) with cobalt lithium, the lithium ion will be accumulated in the cathode and form dendrite to penetrate the membrane. Interior short circuit will take place.
2. Form heavy current: The exterior and interior short circuit will cause over-heavy current with hundreds amps.
i. When exterior short circuit, because the exterior load is too low, the cell will discharge with heavy current for a moment. A great of power will be consumed in internal resistance and release lots of heat.
ii. The main reason of interior short circuit is the membrane is penetrated. The heavy current is caused in the interior. The temperature ascends making melt down the membrane. The area of short circuit enlarges, which causes other problems.
3. Where the gas releases from: The lithium ion cell must adopt the organic electrolytic solution which the decomposition voltage is over 2V in order to reach the operating high-voltage of single electric core with a voltage between 3 ～ 4.2V(operating voltage of nickel-hydrogen cell and nickel-chromium cell is 1.2V, operating cell of plumbic acid is 2V). However the organic electrolytic solution will be electrolyzed under the condition of heavy current and high temperature. When electrolyzing, the gas will arise, which causes the interior pressure to climb caused the pressure is too strong even the shell will be broken through.
4. How does the burning take place: The heating comes from heavy current? In the meanwhile, the oxide of anode lithium will arise oxidizing reaction and the melt lithium will be isolated under the situation of high voltage (over 5V). Under the situation that the gas causes the shell to break, the lithium directly contacts with the air, which causes to burn and ignite the electrolytic solution as well. The intense flame will happen and the gas will expand rapidly, which finally causes explosion.
5. Whether the polymer cell has the safety problems or not? The differences between polymer cell and lithium ion cell are: the electrolytic solution of polymer cell is colloidal state and semisolid state and the one of lithium ion cell is liquid state. Therefore, the flexible package can be used for polymer cell. The shell can be broken earlier when the interior gas arises, which can avoid from gathering much gas and expanding intensively. However the electrolytic solution of polymer cell has not basically settled the problems of safety. Adopt the cobalt acid lithium and organic electrolytic solution, the electrolytic solution is the colloidal state, which the much fiercer burning will happen. The burning is the most important problem of polymer cell’s safety.
