The Possible Role of Thermite Reactions in Thermal Runaway of Li-ion Cells with Layered Cathodes

It is often observed that some runaway Li-ion cells with layered cathode materials become much hotter internally than existing thermal runaway models predict. Further, metals originally in the positive active material (such as Co, Ni, and Mn) are often found in cells whose temperatures became very high. It has been postulated that the formation of metals can be attributed to reduction of rock salt species (MO, where M is the metal), or the reaction of lithiated active material (LiMO2) with CO2. We propose an alternate process for formation of metals that also results in very high cell temperatures, namely thermite reactions between the Al positive electrode current collector and the positive active material. These reactions are highly exothermic, in contrast with the reactions of MO and LiMO2 mentioned. In this paper the thermodynamics of thermite reactions are presented. Incorporating thermite reactions in runaway models will likely improve temperature prediction of Li-ion cells in thermal runaway.