1. Incorrect charge and discharge leads to overcharge or overdischarge of the battery. If you continue to charge the battery after it reaches the saturation state, it is an overcharge. When the battery discharge voltage reaches the battery discharge termination voltage, it is called overdischarge. Overcharging or overreleasing will destroy the internal structure of the cell, leading to serious polarization of the electrode sheet, so that SEI film, electrolyte and other decomposition reaction and gas. Happened when lithium ion battery charger, excess lithium ions will be released from the positive active material, the positive crystal structure was destroyed, its oxygen, appear easily and lithium can deposit on the surface of carbon anode materials, lithium dendrite formation, the dendrite is easy to puncture the batteries diaphragm, lead to internal short circuit, appeared a lot of heat energy and fuel more internal chemical reaction, cause the battery to bulge, And even burst into flames.

　　2. Inappropriate storage or use environment. Lithium-ion batteries convert chemical energy into electricity through electrochemical reactions of internal compounds. Chemical reactions are affected by temperature, and the degree of side reactions and reaction products are dependent on temperature. When the battery is stored or used in a high temperature environment, the SEI film inside the battery and the electrolyte will decompose because of high temperature, and the decomposed material will also react with the positive and negative electrodes to produce gas. Especially when the battery is charged beyond the recommended temperature range, a large amount of lithium ions will be released from the active material of the battery. When the amount of lithium ions is increased, the stability of the crystal structure will be destroyed, and the safety of the battery will be reduced.

　　3. Mechanical abuse of batteries. Impact or extrusion is a typical mechanical way to trigger thermal runaway, which may cause damage to battery structure and protection panel function. Lead to battery internal or external short circuit, battery mechanical damage generally occurs in a very short time, battery structure by mechanical damage and lead to short circuit cell, and then cause thermal runaway. If the function of the battery protection board is damaged, the battery is more likely to cause overcharge and overdischarge without protection.

　　4. The battery factory has defects in the production process. Such as dust particles mixed inside the cell in the manufacturing process, burr is too large, uneven winding, uneven coating process problems, will cause the quality defects of lithium ion battery itself, buried hidden safety risks for lithium ion battery. Such quality problems can cause internal microshorting, which may occur slowly, and we will not be able to assess when the battery will become thermal runaway.

　　5. Battery core design defects, including insufficient negative gram capacity, thinner diaphragm used to reduce battery volume, insufficient diaphragm allowance design to completely isolate positive and negative electrode pieces, and no insulation tape designed for pole ears, etc. Cell design defects may not be detected in the early stage, and when the battery is assembled, after repeated use, due to various external factors such as cell phone structure extrusion, the stability of the internal structure of the battery will decrease and cause a variety of side reactions, internal micro short circuit, etc., which will lead to battery bulging, even fire and explosion. At present, qualified mobile phone batteries are designed with overcharge, overrelease, short circuit and other basic protection functions, and this kind of protection function failure is generally difficult to occur, therefore, mobile phones in normal and reasonable circumstances of use is difficult to occur battery bulging, fire or explosion. However, many manufacturers are pursuing curved screen, more functions, higher performance and lighter design of mobile phones. At the same time, they blindly improve battery capacity and constantly squeeze battery space, which leads to battery design defects, which is a very important safety hazard at present. In short, the battery bulge is very dangerous and unstable, such as the development of this situation, easy to cause the battery thermal out of control, light will lead to battery rupture, electrolyte overflow, fire, explosion. If the mobile phone battery is bulging, the user should stop using the mobile phone immediately and contact the after-sales department for repair and replacement.

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