3.Comparison and analysis of nail penetration thermal runaway test
Comparing the results of the six groups of tests, the batteries were damaged, but the severity of the tests was different. According to the different experimental characteristics, the experimental characteristics of nail penetration can be divided into three categories.
Category I: For example, the batteries in Group 1 and Group 5 only discharge electrolyte, without smoke and explosion;
Class II: For example, although the battery in Group 3 and Group 6 did not explode, a small amount of smoke and electrolyte were emitted from the pressure relief valve of the battery;
Category III: For example, the battery exploded and produced a large amount of smoke in the test of group 2 and group 3, which caused severe damage. It can be found that the result of thermal runaway caused by nail penetration is more random, but it has caused serious damage to the battery.
In the first type of test results, the battery has no obvious change after the prick just penetrated into the battery. After a period of time, the electrolyte began to flow out slowly, accompanied by a pungent smell, and the electrolyte flowed out after the battery was punctured. In the first group of tests and the fifth group of tests, about 120s and 80s after the battery was punctured, the accumulated electrolyte could be seen. Later, as the test proceeded, the temperature continued to rise and the plastic skin of the battery around the bayonet melted. The electrolyte itself is corrosive and will destroy the objects around the battery. If the electrolyte flows in the battery pack, the conductivity of the electrolyte will also cause external short circuit of other batteries.
The voltage of group 1 and group 5 needles dropped rapidly after piercing into the battery, and then dropped slowly to 0 V in continuous fluctuation. The surface temperature of the battery in group 1 and group 5 increased rapidly after nail penetration. The temperature measuring point on the surface of the battery in the two groups of tests reached 90~100 ℃, and then began to decline, but the temperature of the battery in the fifth group of tests decreased faster than that in the first group of tests.
For the second type of results, the fourth group of tests and the sixth group of tests, the battery produced a small amount of smoke and discharged the electrolyte. For example, in the fourth group of tests, the smoke was ejected from the punctured position at the bottom of the battery at the 9th second after the bayonet penetrated into the battery. Compared with the fourth group of tests, the battery in the sixth group of tests spewed a small amount of smoke and electrolyte at the 24th second after the nail penetrated into the battery. In the fourth and sixth group of tests, the time for the battery to emit smoke shall not exceed 5s. In these two groups of tests, the battery voltage dropped rapidly after being punctured and dropped to 0 V within 90 s.
In the two groups of tests, the temperature measurement point on the battery surface reached the maximum value between 80 and 110 seconds after the nail penetration, which was generally in the range of 110 to 130 ℃, and then the temperature decreased slowly. Compared with the first type of test, the battery voltage in the second type of test dropped faster and the surface temperature of the battery was higher, which indicated that the second type of test produced more reaction heat, joule heat and polarization heat, and the smoke emitted showed more side reaction heat.
In the third type of test, the second and third group of tests produced very serious explosions. The second group of tests began to emit smoke 3 seconds after being punctured, and the explosion sound and more smoke could be heard 5 seconds later. Then the battery continued to emit smoke, and the electrolyte immediately exploded and splashed onto the inner wall of the test chamber. For the third group of tests, the battery did not change within 15s after the battery was punctured, and then began to emit a small amount of smoke. At the 34th second after the battery was punctured, the battery suddenly exploded violently, the test chamber was filled with smoke instantly, and the electrolyte splashed onto the inner wall of the test chamber.
In the nail penetration test of the second and third groups, the change of battery voltage decreased rapidly after being punctured, and decreased to 0V within 30s after being punctured. The surface temperature of the second and third groups of batteries increased rapidly after nail penetration. In the two groups of tests, between 50 and 150 seconds after the nail penetration, the temperature of the battery surface reached the maximum value between 160 and 200 ℃, and then the temperature decreased slowly. It can be seen that the more intense the reaction of the battery after nail penetration, the higher the temperature that can be reached. Among the three types of tests, the battery voltage in the third type of test dropped the fastest and the temperature rose the highest, so the reaction heat, coking heat and polarization heat generated the most, while the thermal runaway reaction in the third type of test was the most severe, indicating that the third type of test produced the most side reaction heat.
Through the analysis of the above three types of test results, it can be found that the influence of nail penetration on the severe thermal runaway of cylindrical lithium iron phosphate battery has a certain randomness, and the cause of randomness is related to the random contact interface formed after the battery is punctured. The good contact between the needle and the electrode unit inside the battery, as well as the number of electrode units participating in the discharge after the battery is punctured, will cause different severe thermal runaway situations. It can be seen that once the battery is damaged by nail penetration, it will not only damage the battery itself, but also affect the objects around the battery if the battery explodes.
In this paper, at the initial room temperature of 20 ℃ With a Φ 5 mm tungsten steel needle, six groups of needle tests were carried out on the cylindrical 32650 lithium phosphate iron battery in the state of full charge (SOC-1) to observe the changes of battery voltage and surface temperature during the needle. According to the test results, it can be found that the severe thermal runaway caused by nail penetration is random, and the random contact interface between the needle and the damaged electrode unit leads to the randomness of the severe thermal runaway. The corresponding characteristics of thermal runaway battery include electrolyte outflow, smoke emission and explosion.
After nail penetration, the voltage of the battery will drop to 0 V. The voltage of the battery that only flows out of the electrolyte will drop slowly, and the voltage of the battery that explodes or emits smoke will drop rapidly. Nail penetration will cause the temperature of the battery to rise. The more intense the thermal runaway reaction of the battery is, the faster and higher the temperature will rise. The damage to lithium iron phosphate battery caused by nail penetration is fatal. It is suggested that in the future research and development and use of the battery, the cylindrical battery structure design should be able to prevent nail penetration, or nail penetration does not harm, and the external of the battery should also be protected to prevent the battery from being punctured.