Overview

With more accident caused by lithium-ion battery happens, People are more concerned about battery thermal run away, as the thermal run away occurring in one cell may spread heat to other cells, leading to shut-down of the whole battery system.

Traditionally we will trigger thermal run away by heating, pinning or overcharging during tests. However, these methods can neither control thermal runaway in a specified cell, nor can they easily be implemented during tests of battery systems. Recently people are developing new method to trigger thermal runaway. The Propagation test in new IEC 62619: 2022 is an example, and it is estimated that this method will be a wide use in the future. This article is to introduce some new methods that are under research.

Laser Radiation:

Laser radiation is to heat a small area with high energy laser pulse. The heat will be conducted inside the material. Laser radiation is widely used in the areas of material processing, like welding, connecting and cutting. Commonly there are kinds of laser as follows:

• CO₂ laser: carbon dioxide molecular gas laser
• Semiconductor laser: Diode laser made of GaAs or CdS
• YAG laser: Sodium laser made of yttrium aluminium garnet
• Optical fiber: laser made of glass fiber with rare earth element

Some researchers use laser of 40W, 1000nm wave length and 1mm diameter to test on different cells.

Having a CT scan on the not triggered cell, it can be found that there’s no structural influence except for the hole on the surface. It means laser is directional, and high-power, and the heating area is precise. Therefore laser is a good way for testing. We can control the variable, and calculate the input and output energy accurately. Meanwhile laser has the advantages of heating and pinning, like fast heating, and more controllable. Laser has more advantages like:

• It can trigger thermal runaway and will not heat the neighbor cells. This is good for thermal contact performance

•  It can stimulate internal shortage

• It can input less energy and heat in shorter time to trigger thermal runaway, which makes the test well under control.

Thermite Reaction:

Thermite reaction is to make Aluminum to react with metallic oxide in high temperature, and aluminum will transfer into aluminum oxide. As the enthalpy of formation of aluminum oxide is very low ( -1645kJ/mol), therefore it will generate much heat. Thermite material is quite available, and different formula can generate different amount of heat. Researchers therefore start testing with 10Ah pouch with thermite.

Thermite can easily trigger thermal runaway, but the thermal input is not easy to control. Researchers are seeking to design a thermal reactor that is sealed and able to concentrate heat.

High-power Quartz Lamp:

Theory: Place a high-power quartz lamp under a cell, and separate the cell and the lamp with a plate. The plate need to be drilled with a hole, so as to guarantee energy conduct.

The test shows that it needs a very high power and long time to trigger thermal runaway, and thermal is ranged not evenly. The reason may be that quartz light is not directional light, and the too much heat loss makes it hardly trigger thermal runaway precisely. Meanwhile energy input is not exact. The ideal thermal runaway test is to control the triggering energy and lower surplus input value, to decrease the influence to test result. Therefore we can draw the conclusion that quartz lamp is not useful for now.

Conclusion:

Comparing to traditional method of triggering cell thermal runaway (like heating, overcharge and penetrating), laser propagation is a more effective way, with smaller heating area, lower input energy and shorter trigger time. This is contributed to a high effective energy input on the limited area. This method has been introduced by IEC. We can expect that many countries will take this method into consideration. However it raise high requirement on laser devices. It requires appropriate laser source and radiation-proof devices. At present there are not enough cases for thermal runaway test, this method is still needed verification.