Due to the limitations of materials, battery technology, and manufacturing processes, lithium ion batteries have always had a significant fire risk during their use. The safety of lithium ion batteries under collision and crush has always been a focus of attention.
Generally, the fundamental reason for a fire caused by external forces on a lithium ion battery is that an internal short circuit occurs inside the battery due to severe deformation and damage, resulting in severe electrochemical reactions and high heat generation inside the battery, ultimately leading to thermal runaway and combustion and explosion of the battery.
Researchers have obtained the failure behavior of cylindrical lithium ion batteries under different mechanical external forces through mechanical integrity experiments and numerical simulations. Some researchers have discovered the thermal runaway behavior of the 18650 lithium ion battery under various crush methods, and found that even if the battery is not damaged or broken under compression, it may still develop into a catastrophic accident. Many research reports focus more on the basic theory of thermal runaway combustion of lithium ion batteries, while relatively few studies have been conducted on the safety testing of lithium ion batteries during crush.
This article intends to conduct systematic extrusion experiments on common lithium ion batteries. The effects of different crush conditions on the fire safety of lithium ion batteries were studied.
1.Test
18650 battery，50% SOC，No.1,2,3
The four crush speeds tested were 50 mm/min, 100 mm/min, 200 mm/min, and 400 mm/min, respectively, and the crush deformation variables were 10%, 20%, 30%, 40%, and 50%, respectively. Squeeze the battery at the set constant crush speed. When the battery shape variable reaches the set value, stop squeezing, and observe the battery.
During the experiment, the temperature sensor K-type thermocouple was fixed to the positive electrode of the lithium ion battery. Use a customized temperature collection device to collect temperature data measured by thermocouples. Record the temperature change of the positive electrode during the experiment.
2.Results
2.1 Effect of crush deformation
When the crush deformation is lower than 20%, there is no significant change in the surface temperature of the positive electrode after No 1 is extruded. When the crush deformation reaches 30%, the surface temperature of the positive electrode increases significantly, and the temperature increases faster. The temperature reaches the maximum value within 90 seconds and then slowly decreases.
The temperature change on the positive electrode surface of No 2 is similar to that of No.1. When the crush deformation is lower than 20%, the temperature change on the positive electrode surface is not significant. When the extrusion deformation is 30%, the temperature of the positive electrode increases, with a maximum temperature of about 37 ℃. The temperature decreases slowly after a long exothermic time. When the extrusion deformation is 40%, the temperature of the positive electrode of the battery increases rapidly, and the maximum temperature rises to about 57 ℃, and a large amount of electrolyte flows out of the positive electrode. When the deformation variable is set to 50% for extrusion, the temperature change trend of the positive electrode of the battery is similar to that when the deformation variable is 40%. The maximum temperature rises to about 63 ℃ while a large amount of electrolyte flows out.
The temperature change on the positive electrode surface of No 3 is significantly different from that of 1 and 2. When the crush deformation is lower than 10%, there is no significant change in the surface temperature of the positive electrode. When the crush deformation is 20%, the temperature of the positive electrode increases significantly and the maximum temperature reaches about 47 ℃, with a small amount of electrolyte flowing out. When the crush deformation is 30% and 40%, the positive electrode temperature rises slightly and then slowly decreases. After loosening the crush head, the temperature rises again. When the crush deformation is 50%, the battery will break after being squeezed, and the temperature of the positive electrode will rapidly rise to about 95 ℃. After that, the temperature of the released crush head will rise again.
Lithium cobalt oxide positive electrode batteries do not exhibit significant temperature anomalies when the crush deformation is within 20%, and only when the crush deformation is 30% dry can they easily generate heat and heat, creating a certain fire hazard. The positive electrode temperature of a ternary material positive electrode battery is significantly increased when the sensitive deformation variable of the battery is 20%, and the maximum positive electrode temperature in the crush experiment can rise to about 95 ℃, which is higher than the 66 ℃ of the lithium cobalt oxide positive electrode battery, indicating that the ternary material positive electrode battery has greater fire safety hazards when subjected to external crush. The crush results under different deformation variables indicate that the crush experimental deformation variable for evaluating the fire safety of lithium ion batteries should be set to ≥ 30%.
2.2 Impact of crush speed
Set the shape variable to 30%. The temperature of the positive electrode surface of each battery varies with time at different crush rates.
The surface temperature of No. 1 positive electrode increases rapidly after being crush. During the experiment, it was observed that a large amount of electrolyte flowed out. When the crush speed is 100 mm/min, 200 mm/min, and 400 mm/min, the maximum temperature is about 68 ℃, 81 ℃, and 71 ℃, respectively. Crush speed has little effect on the temperature rise of 1. Different crush speeds have the same deformation rate, and the temperature rise of the positive electrode is basically the same.
The battery capacity increases from 2.2 Ah in No.1 to 2.6 Ah in No.2, making it more sensitive to the crush speed. The temperature rise of the positive electrode surface further increases with the increase of crush speed. When the crush speed increases from 100 mm/min to 400 mm/min, the maximum temperature of the positive electrode surface increases from about 65 ℃ to about 95 ℃, accompanied by a large amount of electrolyte outflow.
The temperature change of No. 3 positive electrode surface is similar to that of No. 2. The greater the crush speed, the higher the temperature rise. When the crush speed is 50 mm/min
At 100 mm/min and 400 mm/min, the maximum temperature is about 65 ℃, 78 ℃, and 106 ℃, respectively, which is higher than the maximum temperature of No. 2 under the same experimental conditions. At a pressing speed of 200 mm/min, No.3 exploded, producing a large amount of white smoke inside the membrane and electrolyte ejected from the positive electrode.
The experimental results of different crush speeds show that the higher the crush speed, the higher the temperature rise of the lithium ion battery, the greater the fire risk. Among the three types of lithium ion batteries, the ternary material positive electrode battery exhibits the greatest temperature rise with increasing crush speed, and the maximum temperature that can be reached is the highest, indicating the greatest risk. From the experimental comparison of different crush speeds, it is suggested that the crush speed should be ≥ 200mm/min in the evaluation of the fire safety of lithium ion batteries, in order to better observe the differences between different lithium ion batteries. At the same time, the temperature change on the surface of lithium ion batteries can be used as an important basis for judging the fire safety, especially if the surface temperature can rise to above 100 during extrusion, there is a greater risk of combustion and explosion for lithium ion batteries.
3.Conclusion
Lithium ion batteries are prone to internal short circuits, which can lead to ignition, combustion, and even explosion under external pressure. In order to study the fire risk of lithium ion batteries under crush conditions, three types of lithium ion batteries were selected to carry out crush ignition experiments under different crush deformation variables and different crush speeds. The following conclusions were drawn: When the crush deformation of lithium cobalt oxide positive electrode batteries is within 20%, there is no significant abnormal risk of battery temperature; It is only when the crush deformation reaches 30% that it is easy to generate heat and heat, resulting in certain fire hazards.
When the shape variation of the ternary material positive electrode battery is 20%, there is a significant temperature rise that reaches a maximum temperature that is significantly higher than that of the lithium cobalt oxide positive electrode battery. When subjected to external compression, there is a greater potential fire safety hazard. As the crush speed increases, the temperature rise of lithium ion batteries increases, and the risk of fire increases.
Compared to lithium cobalt oxide positive electrode batteries, ternary material positive electrode batteries achieve higher temperatures with greater temperature rise as the crush speed increases, and are more prone to combustion and explosion.
When evaluating the fire safety of lithium ion batteries through crush experiments, the shape variable should be set to 30%, and the crush speed should be set to 200 mm/min. At the same time, the temperature change on the surface of lithium ion batteries can be an important basis for judging the fire safety.