Comparison of thermal runaway initiation methods for a cylindrical Li-ion cell

Li-ion batteries have become an important part of our daily life in applications as different as cell phones and electric ferries. High energy density is one of the factors for the successful market penetration of this technology. However, when stressed beyond their design limits, Li-ion batteries can start self-heating and reach thermal runaway, releasing combustible and unhealthy gases, catching fire or even explode. In so-called propagation tests, battery modules or installations are tested for their ability to prevent propagation of thermal runaway between cells or modules. Thermal runaway is initiated in one cell using a chosen initiation method. An important topic for propagation tests is: How should thermal runaway be initiated so that it best resembles a realistic field failure? The Norwegian Defence Research Establishment (FFI) has investigated the safety of a cylindrical Li-ion cell with iron phosphate based chemistry and capacity in the 30–60 Ah range. During these studies, several initiation methods have been used. Results from the various experiments highlight some of the differences between initiation methods. This report summarizes the differences and provides a background for choosing a suitable initiation method. Single cells were forced into thermal runaway using various methods of external heating (nozzle heaters, flexible heating sheet, infrared radiation heating, adiabatic heating) or by generating internal short circuits (internal heating element, nail penetration). The cell behaviour was observed and categorized according to hazard severity levels. The cells revealed a large variation in cell behaviour, both for different initiation methods and for identical methods. All hazard severity levels between 4 (major leakage or vent) and 7 (energetic failure) were observed, and mass losses ranged from 15 to 86%. This variation shows that abuse tests or propagation tests without repetitions can give a misleading impression of the potential hazards of the battery. It also shows that a single successful safety test example is not sufficient evidence for considering a cell as safe. Repeated testing is necessary to reveal all possible cell behaviours. Out of the tested methods for initiating thermal runaway, internal heating element was the method that gave least variation in cell behaviour. This method generally did not produce the worst-case cell behaviour observed for many of the external heating techniques. Module developers and safety evaluators should be aware of the possibility for sidewall rupture when using cells with rigid walls. Ruptures can cause the ventilation gases to be released in unintended directions. The results also clearly demonstrated the flammability of the released gases and the possibility for ignition. Additionally, the results exemplified that cell wall temperature measurements cannot be regarded as a reliable pre-warning parameter for thermal incidents.