1) Internal factors affecting the safety of lithium-ion batteries 1. Materials. (1) The thermal stability of the cathode material and its reaction activity with electrolyte. Most of the positive electrode materials have good thermal stability when in the discharged state, and exhibit thermal instability when in the charged state. When it is not in contact with the electrolyte, the main cause of the heat generation of the charged cathode material is the transformation of the crystal form. After contact with the organic electrolyte, the charged cathode reacts with the electrolyte to oxidize the electrolyte and release a large amount of heat. (2) The safety of anode materials. The negative electrode material should have good chemical stability, be able to form an SEI film with the electrolyte and not react with the electrolyte, otherwise the large amount of heat generated by the reaction may cause the battery to explode. (3) The composition of the electrolyte. The electrolyte plays the role of transferring charge between the positive and negative electrodes inside the battery. It requires high conductivity and the chemical properties of the electrolyte must be stable to minimize the electrochemical reaction rate between the electrolyte and the active material interface during storage. The self-discharge capacity loss is small. At present, the electrolyte of lithium batteries contains a certain amount of acidic substance HF. The positive electrode material may be dissolved and corroded. When the positive electrode material is dissolved, the internal resistance of the battery will increase, and poor thermal stability may easily cause safety hazards. (4) The choice of diaphragm. When the temperature in the battery cell approaches the melting point of the polymer diaphragm, the polymer matrix ion-conducting polymer film melts, and the holes will be blocked and become a non-porous insulating layer. The internal resistance of the battery rises rapidly, and the current through the battery is limited ; When the temperature continues to rise above the melting point of the polymer film, the polymer film further melts, causing the positive and negative electrodes to communicate directly, causing an internal short circuit, and eventually causing the battery to explode. Therefore, the melting point, thickness and porosity of the diaphragm should be considered in the selection of the diaphragm. 2. The rationality of the battery structure design. Lithium-ion batteries usually adopt safety valves, thermistors, and design protection circuits to protect them. The protection circuit is a PCB circuit composed of a variety of electronic components. It controls the charging and discharging process of the battery to prevent the battery from overcharging and discharging. Each of the components has the possibility of failure. Once the protection circuit fails, the probability of an accident in the battery will be greatly increase. 3. Production process. There are many noteworthy details in the manufacturing process, which can easily cause safety hazards, such as: pole piece burrs, pole powder shedding, etc., which will pierce the diaphragm and cause an internal short circuit; the battery's tab is too long to contact the pole piece or the shell Causes a short circuit; the tabs press on the winding core, resulting in a short circuit of the positive and negative electrodes, causing the battery core to become hot, and in severe cases, it will cause an explosion; the improper amount of the positive and negative electrodes during the production process will cause lithium to be released, and piercing the diaphragm will easily cause internal short circuits . At the same time, the use of secondary batteries should be avoided to avoid safety accidents such as short circuits. (2) External factors that affect the safety of lithium-ion batteries. One is overcharge and overdischarge. Overcharge or overdischarge can easily cause the positive and negative electrode materials or electrolyte in the lithium battery pack
to decompose and release gas and heat, which will cause the battery to swell, and the internal pressure will rise sharply. In severe cases, it will even cause smoke, fire and explosion; the second is external Short circuit. External short circuit The positive and negative electrodes are short-circuited due to improper operation or failure of the protection circuit board. When the short circuit occurs, the battery discharge current is large, causing the battery cell to heat up, and the high temperature will shrink or completely damage the diaphragm inside the battery cell, causing an internal short circuit in the battery, resulting in an explosion ; The third is overheating. Excessive external temperature may cause the deformation of the metal shell, the melting of the internal diaphragm of the battery, the decomposition of the positive and negative materials or the electrolyte, etc., all of which can easily cause the battery to short circuit. u003c/pu003e
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