Lithium-ion battery safety is our priority issues, especially in passenger cars and other areas related to the safety of our lives and property, safety is the most important.In order to ensure the safety of lithium-ion batteries, people have designed A variety of safety tests to ensure the safety of lithium-ion batteries in the abuse of the case, so how to design the battery structure to ensure that lithium-ion batteries can pass the safety test, so as to ensure the safety in use, is the issue we need to consider.
For lithium-ion batteries in actual use may encounter security risks, we designed extrusion, acupuncture, short circuit, overcharge and over discharge, high and low temperature and other safety tests in a large number of safety tests, analog lithium-ion battery The occurrence of internal short circuit and external short circuit squeeze, acupuncture and external short circuit test is the most common and most difficult to pass the safety test.The reason, mainly because of these two safety tests in the instantaneous current is too large, due to ohm Impedance and other factors make a large amount of heat generated within the lithium-ion battery within a period of time, subject to the structure of the lithium-ion battery, these heat can not quickly spread to the outside of the battery, resulting in lithium-ion battery temperature is too high, causing the decomposition of active substances and electrolytes Burning, causing thermal runaway.
Taking the common square battery in EV as an example, due to the structural design, the heat generated in various parts of the battery is diffused at different speeds, and thus a significant temperature gradient is generated in the plane direction and the thickness direction of the battery. In particular, When the current is high, the heat generated inside the battery, especially in the middle of the batteries can not diffuse well. Therefore, the temperature inside the batteries rises sharply, causing safety problems.
In the extrusion test, as the degree of deformation of the battery increases, the positive and negative current collectors will first be torn and slip along the 45-degree failure line, and the active material will also enter the 45-degree failure line as Diaphragm deformation increased, the diaphragm eventually reached the point of failure, causing the occurrence of positive and negative short circuit, squeeze caused by short-circuit is mainly the short point-based, so short circuit will have a very large current, heat Concentrated release, causing a sharp rise in the temperature of the short circuit point, it is easy to cause thermal runaway.
Acupuncture experiments are also a method for simulating short circuits in lithium-ion batteries. The basic principle is to use a metal needle to slowly insert into the interior of a lithium-ion battery at a certain speed, thereby causing an internal short circuit in the lithium-ion battery. When the entire lithium-ion battery power is discharged through the short circuit point, the relevant research shows that when the short circuit occurs, up to about 70% of the energy within 60s through the release of short-circuit point, this part of the heat are eventually converted to heat, as a result of The heat generated can not be spread in time, so that the short-circuit point instantaneous temperature can reach more than 1000 ℃, causing thermal runaway.
Compared to the above extrusion and needle punching test, the outer short circuit test appears to be relatively gentle. The external short circuit test is to connect a lithium ion battery to a set resistance, and the power of the lithium ion battery to be discharged through a resistor. The size of the resistor can control the size of the short circuit current from tens of amperes to hundreds of amperes or even thousands of amperes. Since large current will accumulate a large amount of heat within a short time in a lithium-ion battery, it may cause thermal runaway of the lithium-ion battery .
Short-circuit test can be mainly affected by the size of the short-circuit current, the greater the short-circuit current, the faster lithium-ion battery heat generated, and lithium-ion battery thermal diffusion rate will not have much change, it means Accumulation of more internal lithium-ion battery heat, the temperature rise more, may lead to diaphragm contraction, short-circuit and other serious problems, which led to thermal runaway lithium-ion batteries.
Lithium-ion battery short-circuit current is mainly due to the short-circuit resistance of the resistance, followed by lithium-ion battery internal resistance and state of charge and other factors, the Netherlands AkosKriston et al. Through a variety of lithium-ion batteries were found and found , In the process of short-circuit lithium-ion battery, the current changes are divided into several parts, the discharge current of batteries in Area 1 up to 274C, this part of the lithium-ion battery mainly by the electric double layer and diffusion layer driven discharge in the region 2 , Lithium-ion battery discharge rate up to 50-60C, the main limiting factor in this part of the current diffusion of material, due to the accumulation of heat in this region may occur thermal runaway of the battery in the region 3, with the driving force Decline, the battery discharge current is gradually declining.
Akos Kriston's study found that the main factor affecting the short circuit test results is the ratio of the short-circuit resistance to the internal resistance of the lithium-ion battery, which is even greater than the effect of the internal resistance of the lithium-ion battery and the state of charge of the battery on the experimental results Large. Can see that the short-circuit resistance and lithium-ion battery closer to the more likely lithium-ion battery thermal runaway, only when the resistance of short-circuit resistance is more than 9-12 times the lithium-ion battery resistance, lithium Ion battery to pass the short circuit safety test.In fact, it is not hard to understand, in the process of short circuit discharge, the heat mainly by the short circuit resistance of the external circuit and the battery internal resistance generated, according to Joule heat equation P = I2R, the same current Under the heating power and the resistance is proportional to the battery energy in the case of certain, the larger part of the resistance will naturally produce more heat.
It is not difficult to see from the above analysis, the factors that affect the safety test results of lithium-ion batteries are essentially heat production rate and heat dissipation rate. By means of safety protection design, the heat production rate during safety testing is reduced or, if necessary Case, cut the current, to prevent the continued heat production can effectively avoid the thermal runaway lithium-ion battery.Second, to improve the lithium-ion battery cooling rate through lithium-ion battery structure design, improve the cooling rate, can effectively avoid the lithium ion Battery temperature is too high, especially in the battery level, you need to be equipped with the appropriate means of cooling, thermal runaway in some lithium-ion batteries, the rapid cooling to ensure that does not cause a chain reaction.
