Defining Thermal Runaway in Lithium Ion Batteries

Continuing our blog series on the hazards of Lithium Ion Batteries, we now discuss the inception of Thermal Runaway. Understanding the inception point of Thermal Runaway helps us to understand the likelihood and severity of Lithium Ion Battery-induced fires. Fire Protection Engineers should have a basic understanding of Thermal Runaway when assessing the hazards of Lithium Ion Battery storage and use.

Thermal Runaway can be generally described as the condition where Arrhenius internal heat generation exceeds cooling capacity. The term “Arrhenius” refers to the self-propagating nature of exothermic chemical reactions that cause an exponential increase in temperature. Lithium Ion Battery Thermal Runaway specifically refers to the uncontrolled heating of a Lithium Ion cell due to this Arrhenius heat generation. The end result is combustion of the internal cell components, which can lead to ignition of surrounding materials or thermal runaway in adjacent cells.

Chemical decomposition is the primary exothermic reaction that causes Thermal Runaway in Lithium Ion Cells. The rate of chemical decomposition within the cell determines whether the cell will go into Thermal Runaway. The rate of heat production is governed by the Activation Energy and Pre-Exponential energy content within the cell. Different materials have different activation energies. The electrolyte, usually comprised of Dimethyl Carbonate or a similar substance, contains the most potential chemical energy and is therefore the biggest factor in determining when Thermal Runaway occurs. Normally we can ignore the heat contribution from decomposition of the electrodes, separator, or SEI. The activation energy for chemical decomposition of Dimethyl Carbonate is reached at about 80 degrees Celsius. Therefore, the low end of the “danger zone” for many kinds of Lithium Ion Batteries is often considered to be around 80 degrees Celsius.

Once the chemical decomposition of the electrolyte begins, the cell will heat itself until combustion unless cooling can keep pace with the heat production. The time until combustion occurs can be seconds, or it can be months in the future. In future blog posts we will explore the factors involved in predicting the time of Thermal Runaway.