With the large-scale expansion of electric vehicles, we see and access to electric vehicles more and more opportunities, but many people still have doubts about the safety of lithium-ion batteries, in fact, after years of technological development, the safety of lithium-ion battery itself has been greatly improved, followed by battery pack security management system,
For example, thermal runaway warning system, rapid fire extinguishing device has made considerable progress in recent years, even if the lithium-ion battery safety incidents, can also be early warning, extinguishing agent heat runaway spread, for the passengers in the car to escape to obtain sufficient time to ensure the safety of passengers and property. The reasons for the thermal runaway of Li-ion batteries can be divided into two main categories: 1) External reasons, such as overcharge, external short circuit, heating and mechanical misuse; 2) internal reasons, such as internal defects (such as metal impurities, etc.), negative and positive materials in the cycle of decline and so on. It is relatively simple to monitor the external causes of the thermal runaway of Li-ion batteries, such as monitoring the voltage and monitoring the surface temperature of the battery, but it is difficult to monitor the internal causes of Li-ion batteries. Recently, Rengaswamy Srinivasan (first author, communication author) of the American Johns Hopkins Applied Physics Laboratory found that by monitoring the internal resistance of Li-ion batteries, the temperature changes in Li-ion batteries could be analyzed at a high resolution, early warning of thermal runaway of Li-ion batteries Rengaswamy Srinivasan the internal resistance of the battery into two parts: 1) impedance amplitude z;
2) Angle J, Rengaswamy Srinivasan study found that the angle J and Li-ion battery capacity relationship is small, but with the battery temperature is very strong correlation, so can be monitored by the change of Angle J to achieve the real-time monitoring of the internal temperature of the Li-ion battery, so that the thermal runaway before the occurrence of the warning. The figure above is a typical AC impedance map of a lithium-ion battery, and the AC impedance of a Li-ion battery consists of two parts: the real part z ' and the imaginary portion Z '. For ease of analysis, the author integrates the real and imaginary parts of the impedance into two parts: the amplitude z= (z ' 2+z ' 2) 1/2, and the angle J between Z ' and Z ', so z ' =z cos (j), Z ' =z sin (j). The amplitude z is related to the size and capacity of Li-ion battery, the larger the battery is, the smaller the z, and the angle J has little relation with the size of the battery.
This means that even though there are two different sizes of batteries, the Z-value gap is larger, but the angle J is still comparable. The battery used in the experiment was two models, from the Japanese soup shallow 50Ah battery and the 5300 type 5.3Ah battery from Boston Battery, the figure below is two kinds of battery ac impedance graph (50%SOC), from the figure can see the real part of Impedance Z ' and imaginary part z ' Have a significant correlation with the capacity of lithium-ion batteries, and the angle J and the capacity of the battery is very small, but with a large temperature correlation, battery temperature change of 50 ℃, the angle J change can reach 20 degrees, so it is good to improve the measurement of the internal temperature by observing the angle of the resolution, and after years of technological progress,
We are now able to perform high resolution (10-3) measurements on the impedance angle J for Li-ion batteries at very low cost, so monitoring the change of angle J is an effective method for monitoring the temperature change inside a Li-ion battery. The following image is the LG 3AH18650 battery through the external heating induced thermal runaway in the process of monitoring data, from the figure can be seen before the start of heating, the battery temperature of 14.3 ℃ (battery surface temperature), Angle J 9.5 degrees, the battery heated to 50 ℃ (battery surface temperature), Angle J raised to 0.96 degrees, after 240s heating the battery temperature reached 117 ℃, Angle J also raised to 0.79 degrees, at this time the battery did not occur thermal runaway. After 55s, the battery temperature further increased to the 128℃ (battery surface temperature), Angle J also quickly reduced to 2.9 degrees, indicating that the temperature inside the battery began to decrease (this may be the internal diaphragm began to melt absorbed part of the heat). Then around 60s battery leakage and eruption, the temperature of the battery continues to rise, while the angle of j is constantly decreasing, indicating that the battery gas leakage took away part of the heat, causing the battery internal temperature drop, but despite the constant change in the internal temperature of the battery, the battery surface temperature is not reflected at all,
The voltage of the Li-ion battery is almost no fluctuation before the battery leakage, which indicates that the monitoring angle j is a more effective method to detect the thermal runaway of Li-ion battery than to monitor the surface temperature and voltage of Li-ion battery. The long-term monitoring of changes in the internal temperature of Li-ion batteries is a very challenging thing, in order to monitor the temperature inside the Li-ion battery There are also a number of methods, such as the way we have previously reported on the battery internal fiber-optic system to monitor the SOC and temperature status ( For lithium-ion batteries with smart eyes), there is also a model method to predict the temperature inside the Li-ion battery (the core temperature of the lithium-ion batteries in thermal runaway), but these methods are either too large or complex, or too complex to calculate, so it is difficult to implement on-board BMS, The method adopted by Rengaswamysrinivasan is to measure the impedance of Li-ion battery in a certain frequency range, and to get the value of angle J can monitor the internal temperature of Li-ion battery with high resolution, which greatly improves its practical value.
