80% of accidents of lithium-ion battery are caused by short circuit. After short circuit, battery fire and explosion accidents occur frequently. The safety problems of power lithium battery in the newspaper are pushed to the forefront of public opinion again. The more serious consequences of short circuit are related to the phenomenon of "thermal runaway".
The thermal stability of battery material has always been an important factor in the safety of power lithium battery. Compared with the negative material, the energy density and power density of the positive material are low. The thermal reaction between the positive material and the electrolyte is also considered as the main inducement for the development of thermal runaway battery. Therefore, the key to power lithium battery is to find the cathode material with good thermal stability.
In essence, "thermal runaway" is a positive energy feedback cycle process: the increased temperature will cause the system to become hot, and the system will become hot and raise the temperature, which in turn will make the system hotter. Thermal runaway is a very common phenomenon, from concrete curing to star explosion, there may be thermal runaway.
The causes of thermal runaway of lithium-ion battery are as follows:
1. Often overcharge.
2. Unauthorized modification of shell.
3. The ambient temperature is over 60 ° C.
4. The tear of the diaphragm separating the negative electrode and the positive electrode of lithium-ion battery will lead to short circuit, which often causes thermal collapse.
Cobalt oxide chemicals in lithium batteries are involved in the "runaway heat" reaction When the chemical is heated to a certain temperature, it starts to heat itself and then develops into fire and explosion. In some cases, the pressure released by the organic electrolyte will cause the cell to break. If it is exposed to high temperature or sparks, it may also burn.
The probability of thermal runaway is related to the base number of lithium batteries. The production of lithium batteries in China, Japan and South Korea is increasing year by year, especially in the field of mobile phones / laptops, which are widely used, battery accidents seem to occur more frequently. From 2006 to 2011, many large-scale electronic enterprises have had relevant events. Since 2012, there have been few small-scale electronic products, but there are common reports of accidents in large-scale applications, such as aircraft, which shows the following phenomena.
The phenomenon and intensity of thermal runaway are related to the size, configuration and number of cells of lithium battery. There are only a few lithium cells in a small lithium battery pack, so the chance of thermal runaway from the problematic cell to other cells is relatively low. The Boeing 787's huge battery packs are another story: they're packed in sealed metal boxes that don't drain waste heat. When one cell is hot enough to ignite the electrolyte, the rest of the cells follow quickly.
When the battery is charged, the surface deposition of lithium metal is very easy to form branched lithium dendrites, which pierce the diaphragm and cause a direct short circuit between the positive and negative poles. Moreover, lithium metal is very active and can react directly with electrolyte to release heat, and its melting point is very low. Even if the surface lithium dendrite does not pierce the diaphragm, as long as the temperature is slightly higher, lithium metal will dissolve, thus causing short circuit. The higher the temperature of redox thermal reaction is, the weaker the oxidation ability is, the stronger the oxidation ability of positive material is, the stronger the reaction will be, and the more prone to safety accidents.
All lithium-ion batteries should be replaced every 36 months or so. Moreover, when the power drops to 20%, you should charge it. Excessive discharge will damage the lithium battery, thus increasing the possibility of "thermal runaway" and other accidents.