(Invited) Thermal Stability and Thermal Energy Measurements for a Lithium-Ion Pouch Cell with Different State of Health

Abstract

Lithium-ion batteries have developed into one of the most popular secondary batteries on the market today due to high voltage, long lifetime and high energy density. However, lithium-ion cells have issues related to safety. Since the development of an internal short in a lithium-ion battery at present cannot be detected [1, 2], the physical reactions of a lithium-ion battery must be managed by a robust battery design. Lithium-ion batteries used in the Norwegian maritime sector need to be approved by thermal propagation tests as verification of design [3]. During a propagation test, one or several lithium-ion cells are forced into thermal runaway either by heating, overcharge or nail penetration. The acceptance criterion is either; no spread of thermal runaway between cells in a module, or between modules. Despite these rules, several fires have been reported. Of the more well-known are the fire/gas explosion onboard the electric car ferry ‘MF Ytterøyningen’ [4] and the fire on board ‘MS Brim’ [5].A commercial 64 Ah lithium-ion pouch cell was studied by forcing thermal runaway with nail penetration, heating and overcharge to evaluate the energy release based on the abuse method. Two cells aged to 80% state of health (SoH) were also studied with accelerating rate calorimetry and nail penetration to see the effect of cell ageing on thermal stability, critical temperature, energy release and jet flame severity. These methods were combined with diagnostics and cell disassembly to relate the differences found in degradation and safety to the diagnostic data. The safety results will be highlighted in this presentation.The uncycled cells safety properties were rated by fire and mass losses. All cells caught fire, and the mass loss ranged from 38.8% for nail penetration, 55.4% for heat ramp and 72.3% for overcharge. Based on mass loss only, overcharge is the most severe abuse method. Contrary to this, energy released to the solid surroundings during thermal runaway (internal thermal energy) decreased with increasing mass loss. Mass loss could be a key feature in explaining energy release and abuse method severity, see Figure 1. As a result, an uncycled cell in a battery module may withstand the heat from a neighboring thermal runaway during overcharge but may be driven into thermal runaway by a nail penetration test.Cyclic ageing at 25 °C to 80% SoH had a minor impact on abuse severity and mass loss. However, cycled cells are found to lose more electrical energy than internal thermalenergy. Cell electrical energy loss is not directly related to a change in the cell's internal thermal energy. Thermal runaway temperature was found to decrease for a cycled cell, from 204°C to 182°C.Dependent on abuse method, a false sense of security may result from the propagation test. Possibly, a module could show no cell-to-cell propagation with the overcharge method but be driven into thermal runaway propagation by a nail penetration. A decrease in thermal stability combined with a change in internal thermal energy could be hazardous and might be an end-of-life criteria. It is of vital importance to develop diagnostic tools capable of detecting a potentially hazardous drop in thermal stability for aged lithium-ion cells.Acknowledgements: This work was part of the BattMarine project (281005), funded by the Research Council of Norway and Norwegian industry.