The safety of lithium-ion batteries is our priority, especially in the fields related to the safety of our lives and property, such as passenger cars. In order to ensure the safety of lithium-ion battery, people have designed a variety of safety tests to ensure the safety of lithium-ion battery in case of abuse. Therefore, from the perspective of battery structure design optimization, how to pass this series of safety tests is a new problem that we need to consider.
In view of the safety risks that lithium-ion batteries may encounter in practical use, we designed safety tests such as extrusion, acupuncture, short circuit, overcharge and discharge, high and low temperature. Among many safety tests, extrusion, acupuncture and external short circuit tests simulating internal and external short circuits of lithium-ion batteries are the most conventional and difficult to pass. The main reason is that the instantaneous current in the two safety tests is too large. Due to ohmic impedance and other factors, a large amount of heat is generated in the lithium-ion battery for a period of time. Limited by the structure of the lithium-ion battery, these heat can not quickly diffuse to the outside of the battery, resulting in the high temperature of the lithium-ion battery, resulting in the decomposition and combustion of active substances and electrolyte, Cause heat to get out of control.
Taking the square battery commonly used in electric vehicles as an example, due to the structural design, the diffusion speed of heat generated in each part of the battery is different, so there will be an obvious temperature gradient in the plane direction and thickness direction of the battery. Especially in case of high current, the heat generated in the battery, especially in the middle of the cell, cannot be well diffused, Therefore, the temperature inside the cell will rise sharply, resulting in safety problems.
In the extrusion test, with the increase of the deformation degree of the battery, the positive and negative current collectors will be torn first, slip along the 45 degree failure line, and the active material will also be damaged.It will enter into the 45 degree failure line. With the continuous increase of diaphragm deformation, the diaphragm finally reaches the failure point, causing the occurrence of positive and negative short circuit. The positive and negative short circuit caused by extrusion is mainly point short circuit. Therefore, a very large current will be generated at the short circuit point, and the heat will be released intensively, causing the temperature at the short circuit point to rise sharply. Therefore, it is easy to cause heating out of control.
Nail penetration experiment is also a method used to simulate the internal short circuit of lithium-ion battery. Its basic principle is to use a metal needle to slowly insert into the interior of lithium-ion battery at a certain speed, resulting in the internal short circuit of lithium-ion battery. At this time, the power of the whole lithium-ion battery is released through the short circuit point. Relevant studies show that when the internal short circuit occurs, At most, about 70% of the energy will be released through the short-circuit point within 60s, and this part of the heat will eventually be converted into heat energy. Because the generated heat cannot be diffused in time, the instantaneous temperature of the short-circuit point can reach more than 1000 ℃, resulting in uncontrolled heating.
Compared with the above extrusion and acupuncture experiments, the external short circuit test is relatively mild. The external short-circuit test is to connect the lithium-ion battery to a fixed resistance, and the power of the lithium-ion battery is released through the resistance. The short-circuit current can be controlled according to the fixed resistance, from tens of amps to hundreds of AMPS, or even thousands of amps. Due to the large current, a large amount of heat will accumulate in the lithium-ion battery in a short time, which may lead to the thermal runaway of the lithium-ion battery.
Whether the short-circuit test can be passed is mainly affected by the short-circuit current. The larger the short-circuit current is, the faster the heat generation rate of lithium-ion battery will be, and the heat diffusion rate of lithium-ion battery will not change much. Therefore, it means that more heat will be accumulated in the lithium-ion battery and the temperature will rise more, which may lead to the contraction of diaphragm, Serious problems such as short circuit of positive and negative electrodes, which leads to thermal runaway of lithium-ion battery.
The main factor affecting the short-circuit current of lithium-ion battery is the resistance value of short-circuit resistance, followed by factors such as internal resistance and charge state of lithium-ion battery. Researchers in the Netherlands found that during the short-circuit process of lithium-ion battery, the current change is mainly divided into the following parts, and the discharge current of area 1 battery can reach 274c, This part is mainly driven by the discharge of electric double layer and diffusion layer of lithium-ion battery. In area 2, the discharge rate of lithium-ion battery can reach 50-60c. The main limiting factor of this part of current is material diffusion. Due to the accumulation of heat, the thermal runaway of battery may occur in this area. In region 3, the discharge current of the battery gradually decreases with the decrease of the driving force.
Their research also found that the main factor affecting the short-circuit test results is the ratio of short-circuit resistance to the internal resistance of lithium-ion battery, which is even greater than the internal resistance of lithium-ion battery and the charge state of battery. It can be seen that the closer the resistance value of the short-circuit resistance is to the lithium-ion battery, the more prone the lithium-ion battery is to thermal runaway. The lithium-ion battery can pass the short-circuit safety test only when the resistance value of the short-circuit resistance is more 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-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 heat formula P = I2R, when the current is the same, the heating power is directly proportional to the resistance range. When the battery energy is certain, the part with large resistance will naturally generate more heat.
From the above analysis, it is not difficult to see that the factors affecting the safety test results of lithium-ion batteries are mainly the heat generation rate and heat dissipation rate in essence. Reducing the heat generation rate during the safety test process by means of safety protection design, or cutting the current and preventing the continuous heat generation if necessary can effectively avoid the thermal runaway of lithium-ion batteries. The second is to improve the heat dissipation rate of lithium-ion battery. By improving the heat dissipation rate through the structural design of lithium-ion battery, the temperature of lithium-ion battery can be effectively avoided. Especially at the battery pack level, corresponding heat dissipation means need to be equipped to quickly dissipate heat in case of thermal runaway of some Lithium-Ion batteries, so as to ensure no chain reaction.