The safety of lithium-ion batteries is our top priority, especially in areas related to the safety of our lives and property, such as passenger cars. To ensure the safety of lithium-ion batteries, people have developed various safety tests to ensure the safety of lithium-ion batteries from abuse. Therefore, from the point of view of improving the battery design, passing this safety test is a new thing that we have to consider.
Considering the safety risks of lithium-ion batteries that can be encountered in practice, we have developed safety tests such as extrusion, acupuncture, short circuit, transfer and discharge, high and low. In many safety tests, extrusion, acupuncture and external small circuit simulating the internal and external short circuit of lithium-ion batteries are common and difficult to pass. The main reason is that the current application and security tests are both large. Due to ohmic impedance and other factors, a lot of heat is generated in lithium-ion batteries over time. Due to the structure of the lithium-ion battery, these temperatures cannot be easily dissipated outside the battery, resulting in excessive temperature of the lithium-ion battery, resulting in the decomposition and combustion of the active material and the electrolyte. , causing the temperature to get out of control.
Taking the square battery that is commonly used in electric cars as an example, due to the design, the speed of heat generated in each part of the battery is different, so there will be a clear temperature gradient distance and direction of flight and direction of the thickness of the battery. Especially in high current conditions, the heat generated in the battery, especially between the cells, cannot be distributed properly. Therefore, the temperature inside the cell will rise significantly, leading to storage problems.
Extrusion test
In the extrusion test, with the increase in the degree of battery deformation, the positive and negative collectors will tear first, will fall around the 45 failure mark, and the active material will be damaged. It will enter the 45 degree landing line. With the continuous increase in the deformation of the diaphragm, the diaphragm eventually reaches the point of failure, causing the occurrence of positive and negative shortness. Bad and bad short circuit caused by extrusion is the main short circuit. Therefore, a large amount of water will be made to make it short and warm, making the temperature in the short place rise well. Therefore, it is easy to control the heat.
Nail penetration test
The fingernail penetration test is also a method used to measure the internal shortness of lithium-ion batteries. Its basic principle is to use a metal needle to slowly insert into the lithium-ion battery at a certain speed, resulting in an internal shorting of the lithium-ion battery. At this time, the power of the entire lithium-ion battery is released through a small circuit. An important study shows that when a short circuit occurs, as much as 70% of the energy will be released from the short circuit within 60 seconds, and this part of the heat will eventually become heat energy. Because the heat generated cannot be released in time, the instantaneous temperature of the short area can reach more than 1000 ℃, resulting in uncontrolled heating.
External short circuit test
Compared with extrusion and acupuncture above, the external short circuit test is smaller. An external short circuit test involves connecting a lithium-ion battery to a fixed resistor, and the power of the lithium-ion battery is released through the resistor. A small circuit current can be controlled as a static resistance, from a few tens of amperes to hundreds of amperes, even thousands of amperes. Due to the large current, a lot of heat accumulates in the lithium-ion battery in a short time, which can lead to the burning of the lithium-ion battery.
The success of the short circuit test depends on the short circuit current. While the current is short-circuit current, the rapid generation of lithium-ion batteries will be, and the heat release of lithium-ion batteries will not change much. Therefore, it means that more heat will accumulate in the lithium-ion battery and the temperature will rise again, which can lead to diaphragm contraction, serious problems such as short circuits of positive and negative electrodes, and – cause lithium ion battery thermal runaway.
The main factor affecting the short circuit of the lithium-ion battery is the resistance value of the short circuit, followed by factors such as internal resistance and the condition of the lithium-ion battery charge. Researchers in the Netherlands found that during the short cycle of the lithium-ion battery, the current change is divided into the following areas, and the battery’s breakdown of area 1 can reach 274c, this part is common through the termination of the double electric current and the diffusion layer of the lithium-ion battery. In zone 2, the discharge rate of lithium-ion batteries can reach 50-60c. The main obstacle to this aspect of the present is the spread of resources. Due to the increase in temperature, battery discharge may occur in this area.
The research also showed that the main factor affecting the short circuit test results is the size of the short circuit resistance of the lithium-ion battery, which is even greater than the resistance of the battery of debt of the battery. It can be seen that the shorter the resistance value of the runner in the lithium-ion battery, the more powerful the lithium-ion battery is. A lithium-ion battery can pass the short-cycle safety test only when the resistance value of the short-cycle resistance is greater than 9-12 times of the internal resistance of the lithium-ion battery. In fact, it is not difficult to understand that in the process of short-term interruption, the temperature is usually caused by the interruption of the external output and the interruption of the battery. According to the Joule heat formula P = I2R, when the current is the same, the heating power falls in the range. When the battery power is clear, the larger part will make it brighter.
From the above analysis, it is not difficult to see that the factors affecting the safety test results of lithium-ion batteries are the highest of heat generation and heat rate. Reduce the amount of heat generation during the safety check process by setting safety precautions, or cut off the power and prevent the continuous heat generation if necessary can avoid overheating of the lithium-battery ion properly. The second is to improve the discharge rate of lithium-ion batteries. By improving the heat dissipation rate through the design of the lithium-ion battery, the temperature of the lithium-ion battery can be effectively avoided. Especially at the battery level, the corresponding heat dissipation method must be provided to quickly remove the heat if the light source of some Lithium-ion batteries is running, to ensure that
