In the past September, we have been screened by a mobile phone, of course not the iPhone7, but another Samsung mobile phone - Note7. Note7 is not because its strong performance has attracted our attention, but because it has many battery fire explosions at home and abroad, and because Samsung treats domestic and foreign users differently, Samsung has stopped globally. Production and sales have opened the recall. For the strong performance of an electronic device can not hide its safety problems, today we will study the cause and mechanism of thermal runaway of lithium-ion batteries. For lithium-ion batteries, thermal runaway is the most serious safety accident, which can cause lithium-ion batteries to ignite or even explode, directly threatening the safety of users. The thermal runaway of lithium-ion batteries is mainly due to the fact that the internal heat generation is much higher than the heat dissipation rate. A large amount of heat is accumulated inside the lithium-ion battery, causing a chain reaction, which causes the battery to ignite and explode. There are many factors that cause thermal runaway, which are generally divided into two categories, internal factors and external factors. The internal factors are mainly: the battery production defect leads to the internal short circuit; the improper use of the battery causes the internal lithium dendrite to cause a short circuit between the positive and negative electrodes. The external factors are mainly: external factors such as extrusion and acupuncture cause a short circuit of the lithium ion battery; external short circuit of the battery causes the internal heat accumulation of the battery to be too fast; excessive external temperature causes decomposition of the SEI film and the positive electrode material. Krishna Shah of the University of Texas at Arlington analyzed the thermal runaway phenomenon of lithium-ion batteries and established a prediction mechanism for thermal runaway of lithium-ion batteries, which has important reference significance for the safety design of lithium-ion batteries. . Related research shows that the thermal runaway process of lithium-ion battery is mainly composed of the following reaction: SEI membrane decomposition, electrolyte and binder react, electrolyte and cathode active material decompose. The factors affecting the thermal runaway of lithium-ion batteries can be divided into two, one is the heat production rate inside the battery, and the other is the heat dissipation rate of the lithium-ion battery. Conventional thermal analysis tools generally assume that the heat production of lithium-ion batteries is uniform throughout the volume. Therefore, these tools analyze that thermal runaway is independent of the thermal conductivity of the battery, which is different from the actual situation of lithium-ion batteries. Therefore, the prediction results are also inaccurate. Studies have shown that even with this large thermal gradient inside the 26650 battery, traditional methods and tools cannot accurately predict the thermal state inside and outside the battery. To solve the above problem, Krishna Shah added thermal conductivity parameters to the traditional lithium electrothermal analysis model, resulting in a dimensionless parameter, the thermal runaway number (TRN). First, Krishna Shah established an equation relationship between battery temperature and heat and heat dissipation, as shown below. For the heat generating function Q(T) in the formula, the Taylor expansion is performed at the temperature T0, and the high-order term is ignored to obtain the following formula. Then the formula needs to go through a complicated mathematical solution process. If you can't read it, you can't introduce it to everyone. Let's see the result directly. The final derivation gives the following results The above formula must be met throughout the entire operating range to ensure that thermal runaway does not occur. The formula combines the internal heat transfer kr of the battery, the heat dissipation μ1 of the battery surface, the heat generation rate parameter β of the battery, and the radius of the battery. The heat generation rate parameter β of the battery and the heat dissipation and thermal conductivity coefficient of the battery are the key parameters for controlling the thermal runaway of the lithium ion battery. By increasing the β value, the TRN value also increases, and when the TRN>1, the battery will occur. Thermal runaway, and TRN<1 is the battery, there will be no thermal runaway. It should be noted that β is not a fixed value, but increases with increasing temperature, so TRN will also increase. . The heat dissipation of the battery is mainly composed of two steps, the heat conduction inside the battery and the heat convection outside the battery. Therefore, under the premise of β, it is necessary to adjust the thermal conductivity kr of the battery and the surface heat dissipation parameter μ1 to ensure TRN<1, thereby ensuring Battery safety. For example, when β = 6000 W/m3K, the safe range of kr and μ1 is as shown in the figure above. Through the work of Krishna Shah, we can use the TRN formula to calculate the thermal safety factor of lithium-ion batteries in the safety design of batteries. The β and kr values ​​can be measured by the corresponding experiments, according to the β and kr values ​​of different materials. The R value of the battery and the surface heat dissipation μ1 can be adjusted to ensure TRN<1, ensuring the safety of the lithium ion battery.
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