Thermal runaway is the most serious safety accident of lithium-ion batteries. The electric energy and chemical energy stored in the lithium-ion battery are released in a short period of time, making the temperature inside the lithium-ion battery even reach 900°C or more. At the same time, the electrolyte is in thermal runaway. , The large amount of gas generated from the decomposition of active materials will cause the pressure inside the battery to rise sharply, and even cause the explosion of the lithium ion battery. In order to ensure the safety of lithium-ion batteries, we usually design an explosion-proof valve on the battery shell, which can be destroyed in time when the pressure is too high, release the internal pressure of the battery, and prevent the battery from exploding during thermal runaway.
Regarding the 18650 battery, the explosion-proof valve is designed in the upper cover of the battery. The explosion-proof valve also has the function of a circuit breaker. When the internal pressure of the battery rises to a certain level, the explosion-proof valve acts to cut off the current loop. When the pressure rises further, the explosion-proof valve structure is destroyed, releasing the pressure inside the battery to prevent the battery from exploding. Previously, it was important for us to understand the design of the explosion-proof valve from the principle. Due to the structural design of the upper cover of the 18650 battery, it is difficult for us to directly see the action of the explosion-proof valve in the process of thermal runaway. Recently, Donal P. Finegan of the City College of London (first Author) and Paul R. Shearing (corresponding author) used a high-speed camera to photograph the movement of the explosion-proof valve of the 18650 battery cover from different manufacturers during thermal runaway, and restored the whole process of the action of the 18650 battery's explosion-proof valve in thermal runaway.
Related research shows that once thermal runaway occurs, thermal runaway may spread to the entire 18650 battery within 2s. Therefore, in order to better observe the entire thermal runaway process, more than 1000 X-ray photos of the 18650 battery must be taken per second. The explosion time of the battery is less than 0.01s, which requires a higher shooting speed. In order to meet this demanding requirement, DonalP.Finegan used equipment from the European Synchrotron Radiation Laboratory to photograph the action of the 18650 battery explosion-proof valve. The equipment used to photograph the arcing process of the fuse, and its resolution reached The level of millions of shots per second.
The test conducted a total of 5 18650 batteries from LG, Panasonic, Samsung and Sanyo and one 18650 battery with a new design of double explosion-proof valves (as shown in the table below). The positive and negative materials of the 5 batteries were NMC and NCA respectively. , LMO and LCO are mixed to verify the impact of different systems and battery structures on the safety of explosion-proof valves.
The structure design of the 18650 battery cover and explosion-proof valve of LG, Panasonic, Sanyo and Samsung is shown in the figure below. From the figure, we can notice that all the explosion-proof valves are connected to the pad through a raised structure (the tab is welded to the pad Above), when the internal pressure of the battery rises to a certain level, the explosion-proof valve will deform and separate from the pad, thereby cutting off the current loop. At the same time, we can also observe that all explosion-proof valve structures have a ring of nicks. When the pressure inside the battery continues to rise to a certain value, the explosion-proof valve is destroyed and the battery is released to prevent the battery from exploding. In addition, the battery cover types of LG, Panasonic and Sanyo also contain PTC structure (positive temperature coefficient material). When the temperature of the battery rises, the resistance of the PTC material increases sharply, thereby preventing the current from continuing to increase. It adopts a rate-type design (20A discharge), so the PTC structure is not designed.
The picture below shows the pressure relief process of two LG batteries and Sanyo batteries taken by a high-speed camera (2000 frames per second). From the pictures a and b below, it can be noticed that although LG-S3 and LG-B4 have the same Explosion-proof valve structure design, but because the LG-B4 battery has a center pin inside, the two batteries behave differently in thermal runaway. Regarding S3, since there is no center pin support, the battery core collapses toward the center. Driven by the fast flowing gas, a large number of active materials on the battery core are torn off and sprayed to the outside of the battery with the air flow (1.4545 in the figure a below) s). The B4 battery has a center pin in the center of the cell, so under the use of air flow, the center pin moves to one side, which reduces the active material tearing on the side of the winding pin, but causes the electrode on the other side. Increased tearing.
From the above analysis, it is not difficult to see that the center pin inside the 18650 battery cell has a significant impact on the battery's behavior in thermal runaway. From the following figures b and c, we can notice that under the influence of the gas generated by the thermal runaway, LG-B4 and The center needle of Sanyo battery has moved up obviously. The existence of the center needle prevents the battery core from collapsing to the center, provides enough diffusion channels for gas diffusion, and reduces the positive and negative active materials carried out during the gas eruption. Reduce the risk of thermal runaway spreading to surrounding batteries.
