As a development trend in the future automotive industry – electric vehicles. Its biggest safety hazard is the explosion, fire and combustion of the power battery. In fact, incidents of electric vehicle fires and combustion are not uncommon. The global incidents of electric vehicle fires also include Zotye spontaneous combustion, Volanda collision fires several days later, and coupe fires. These accidents have aroused widespread concern about the safety performance of lithium-ion batteries used in electric vehicles.
With the popularization of new energy vehicles represented by battery power. The safety performance of power batteries is becoming increasingly important. From the perspective of the battery itself, it is a carrier of energy, and there are inherent safety factors in different electrochemical systems. Different capacities. The differences in manufacturing processes, as well as the environment and degree of use, have a significant impact on the safety of batteries.
1 Environmental Test
The safe use of lithium-ion batteries under different environmental conditions is crucial. Among all environmental factors. The temperature has the greatest impact on the charging and discharging performance of lithium batteries. The interface between electrodes/electrolytes is considered the heart of lithium batteries. If the temperature drops, the reaction rate of the electrode will also decrease: assuming that the voltage of the lithium battery remains constant, the discharge current will decrease. The power output of lithium batteries will also decrease. When the temperature rises, the output power of lithium batteries will increase. Temperature also affects the transmission speed of electrolyte, with an increase in temperature accelerating and a decrease in temperature slowing down: the charging and discharging performance of lithium batteries will also be affected. But if the temperature is too high, it will disrupt the chemical balance inside the lithium battery. We conducted routine environmental tests using a 20 Ah single lithium-ion battery as an example:
a) Firstly. In a test chamber with an ambient temperature of (20 ± 5) ℃ at room temperature. Discharge the lithium-ion battery at a current of 1 IV A until the discharge termination voltage is 2.0 V. At this time, test this set of batteries. The capacity value should not be lower than the rated value or exceed 110% of the capacity value. The result of the capacity test is 21 Ah, which is 105% of the rated value, indicating that the lithium-ion battery is within the qualified range at room temperature of (20 ± 5) ℃.
b) Secondly, under fully charged lithium-ion batteries, store them in a low-temperature test chamber at (-20 ± 2) ℃ for 20 hours. Discharge at a current of 1 IVA under environmental conditions of (-20 ± 2) ℃. The capacity of the lithium-ion battery should not be lower than 70% of the rated value until the discharge termination voltage is 2. O V. After measurement, the battery capacity is 20 Ah, which is the rated value, indicating that there is no change in its capacity value at a low temperature of 20 ℃.
C) Finally. Under fully charged lithium-ion batteries, store them in a high-temperature test chamber at (55 ± 2) ℃ for 5 hours, and then discharge them at a current of 1 IV A under (55 ± 2) ℃ until the discharge termination voltage is 2.0 V. The capacity should not be lower than 95% of the rated value. The actual measured capacity value is 20 Ah. At a high temperature of 55 ℃, the capacity value remains unchanged.
According to the above experimental instructions. Although theoretically, the environment has an impact on lithium-ion batteries. But the experimental results indicate that this group of lithium-ion batteries operates under temperature changes. The capacity value has not changed significantly. Lithium ion battery packs are used as power batteries for electric vehicles. It is achieved by N small volume individual lithium-ion batteries in series and parallel to provide energy to the entire car. So. For the safety of lithium-ion batteries used in automobiles. The safety of individual lithium-ion batteries is crucial.
2.Safety Test
The single cell battery of lithium-ion batteries is a relatively complex electrochemical system composed of components such as positive electrode, negative electrode, separator, and electrolyte. The level of its safety depends on the “bottom plate effect” of the entire component. Only by achieving individual safety can the overall performance of the battery be safely improved after series and parallel connection. So how can we ensure the safety of individual lithium-ion batteries? We need to verify through the following safety tests (safety performance tests are destructive tests. For safety reasons, the tested battery should be placed in an explosion-proof chamber):
a) Over-discharge
Lithium ion batteries are stored in explosion-proof chamber at room temperature (20 ± 5) ℃. Discharge at a current of 1 IV A until the voltage of the battery reaches 0 V (if there is an electronic protection circuit, the discharge electronic protection circuit should be temporarily removed). Lithium ion batteries should not explode, catch fire, or leak liquid, as shown in Figure 1. Through testing, the above situation did not occur in lithium-ion batteries. The purpose of this experiment is to assess the driving range of electric vehicles beyond the specified battery capacity. Safety performance of lithium-ion batteries under extreme discharge conditions.
b) Overcharging
There are two ways to conduct overcharging tests on lithium-ion batteries:
1) Lithium ion batteries are charged in an explosion-proof chamber at room temperature (20 ± 5) ℃ with a current of 3 IV A. The test is stopped when the battery voltage reaches 5 V or the charging time reaches 90 minutes (one of the conditions is prioritized);
2) Charge with a current of 9 IV A, and stop the test when the voltage of the lithium-ion battery reaches 10 V. When the specified voltage is reached, lithium-ion batteries should not explode or catch fire. The lithium-ion battery did not explode or catch fire. This experiment is to assess the charging of lithium-ion batteries for electric vehicles. When the charging device loses control or accidents occur due to human factors. Causing high current overcharging. The safety performance of lithium-ion batteries in this state.