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 devised various safety tests to ensure the safety of lithium-ion batteries during 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 short circuits simulating the internal and external short circuit of lithium-ion batteries are common and difficult to pass.
The main reason is that the current speed in both security tests is large. Due to ohm 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, resulting in the appearance of a small positive and negative circle. 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 short time can be controlled as a strong 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 increase in the lithium-ion battery and the temperature will rise again, which can lead to the failure of the diaphragm, serious problems such as short-circuiting of the positive and negative electrodes, leading to lithium ions.
