The safety of lithium-ion batteries is a priority for each of our production companies, especially in areas related to the safety of our lives and properties, such as electric vehicles and new energy vehicles. Safety is the top priority. 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 case of abuse. Therefore, how to ensure that lithium-ion batteries can pass the safety test through the battery structure design? Ensuring the safety of the battery in use is a problem that we need to consider.
In view of the safety risks that lithium-ion batteries may encounter in actual use, we have designed safety tests such as extrusion, acupuncture, short circuit, overcharge and over-discharge, and high and low temperature. Among the many safety tests, the squeeze, acupuncture, and external short-circuit tests that simulate internal and external short-circuit of lithium-ion batteries are the most common and the most difficult to pass.
The main reason is that the instantaneous current in these two safety tests is too large. Due to factors such as ohmic impedance, a large amount of heat is generated inside the lithium-ion battery within a period of time. Due to the limitation of the structure of the lithium-ion battery, this heat cannot be quickly diffused. To the outside of the battery, the temperature of the lithium-ion battery is too high, which triggers the decomposition and combustion of the active material and the electrolyte, resulting in thermal runaway.
Take the common square battery in electric vehicles as an example. Due to the structural design, the heat generated in each part of the battery diffuses at different speeds, so obvious temperature gradients are generated in the plane direction and thickness direction of the battery. When the current is high, because the heat generated inside the battery, especially in the middle of the battery cell, cannot be diffused well, the temperature inside the battery cell will rise sharply [1], causing safety issues.
In the squeeze test, as the degree of battery deformation increases, the positive and negative current collectors will be torn first and slip along the 45-degree failure line. The active material will also enter the 45-degree failure line. The degree of diaphragm deformation continues to increase, and the diaphragm eventually reaches the point of failure, causing short circuits between the positive and negative electrodes [2]. The short circuits between the positive and negative electrodes caused by extrusion are mainly dotted short circuits [3], so they will occur at the short-circuit points. Very large current, the concentrated release of heat, causes the temperature of the short-circuit point to rising sharply, so it is easy to cause thermal runaway.
The acupuncture experiment is also a method for simulating the internal short circuit of the lithium-ion battery. The basic principle is to use a metal needle to slowly insert into the lithium-ion battery at a certain speed, thereby causing the internal short circuit of the lithium-ion battery. At this time, the power of the entire lithium-ion battery is released through the short-circuit point. Related studies have shown that when an internal short-circuit occurs, up to about 70% of the energy is released through the short-circuit point within the 60s [4], and this part of the heat is eventually converted into Thermal energy, because the generated heat cannot be diffused in time, so that the instantaneous temperature of the short-circuit point can reach more than 1000 ℃, causing thermal runaway [5].
Compared with the above-mentioned extrusion and acupuncture experiments, the external short circuit test appears to be relatively mild. The external short circuit test is to connect a lithium-ion battery to a fixed-value resistor, and the power of the lithium-ion battery is released through the resistor. The size of the short-circuit current can be controlled according to the size of the fixed value resistor, from tens of amperes to hundreds of amperes, or even thousands of amperes. Because large currents will accumulate a large amount of heat in the lithium-ion battery in a short period of time, it may cause lithium-ion The battery is thermally out of control.
The ability to pass the short-circuit test is mainly affected by the magnitude of the short-circuit current. The larger the short-circuit current, the faster the heat generation rate of the lithium-ion battery and the heat diffusion rate of the lithium-ion battery will not change much, so it means As more heat accumulates inside the lithium-ion battery, the temperature rises more, which may cause serious problems such as diaphragm shrinkage, short circuit of the positive and negative electrodes, and then cause thermal runaway of the lithium-ion battery.
The factor that affects the short-circuit current of lithium-ion batteries is mainly the resistance of the short-circuit resistance and is also affected by factors such as the internal resistance and state of charge of the lithium-ion battery. AkosKriston in the Netherlands and others have discovered through research on a variety of lithium-ion batteries, In the process of short circuit of the lithium-ion battery, the current change is mainly divided into the following parts. The discharge current of the battery in area 1 can reach 274C. This part is mainly driven by the discharge of the electric double layer and the diffusion layer of the lithium-ion battery. In area 2, The discharge rate of lithium-ion batteries can reach 50-60C. The main limiting factor for this part of the current is material diffusion. Due to the accumulation of heat, the thermal runaway of the battery may occur in this area. In area 3, as the driving force decreases, the discharge current of the battery also gradually decreases [6].
AkosKriston's research found that the main factor that affects the results of short-circuit test is the ratio of the short-circuit resistance value to the internal resistance of the lithium-ion battery, which is even greater than the internal resistance of the lithium-ion battery and the state of charge of the battery. . It can be seen that the closer the resistance of the short-circuit resistance is to the lithium-ion battery, the more likely the lithium-ion battery will be thermally out of control. Only when the resistance of the short-circuit resistance is 9-12 times the internal resistance of the lithium-ion battery, the lithium-ion battery pass the short-circuit safety test.
In fact, this is not difficult to understand. In the process of short-circuit discharge, the heat is mainly generated by the short-circuit resistance of the external circuit and the internal resistance of the battery. According to the Joule heating formula P=I2R, the heating power is proportional to the resistance range under the same current. In the case of certain battery energy, the part with a large resistance will naturally generate more heat.
From the above analysis, it is not difficult to see that the factors that affect the safety test results of lithium-ion batteries are essentially the heat generation rate and heat dissipation rate. By means of safety protection design, the heat generation rate during the safety test is reduced, or when necessary In this case, cutting the current and preventing the continued heat generation can effectively prevent the thermal runaway of the lithium-ion battery.
The second is to improve the heat dissipation rate of lithium-ion batteries. Through the structural design of lithium-ion batteries, the heat dissipation rate can be improved, which can effectively avoid the excessively high temperature of lithium-ion batteries, especially at the battery pack level. Corresponding heat dissipation means need to be equipped. When the ion battery is out of control, it can quickly dissipate heat to ensure that it does not cause a chain reaction.
