Abstract
The large-size single cell is a promising development trend for the lithium-ion battery (LIB) industry. LiFePO4 is one of the favoured cathode materials, which is believed to have better safety features, while relatively little is known about the ejection of flammable gases and hot sparks during thermal runaway (TR). Similarly, the self-ignition mechanism, i.e., ignition of the released flammable gases by the ejected hot sparks, is also unclear. Relatively little is understood about the thermal radiation hazards of the jet flame. A series of gas analysis and combustion tests were conducted on a 280 Ah large-size cell with LiFePO4 as cathode. In the unignited, or delayed ignition tests, involving cells with 100%, 75% and 30% state of charge (SOC), the composition and concentration of the gaseous products during TR were measured. The observation of the delayed ignition in one of the 100% SOC tests also led to caution about the gas explosion of LIB. This was followed by the quantified measurements of time-dependent flame heights, heat release rates and thermal radiation hazards. The self-ignition of the ejected electrolyte vapour and flammable gases by the ejected scorching sparks were captured and analysed. Two cycles of jet flames were observed and attributed to the successive TR of the two jelly rolls connected in parallel inside the cell. The evolution of the cell surface temperatures, heat release and mass loss rates were measured, which aided the analysis and can also support model validation by other researchers. The previously developed Multi-point Source (MPS) Radiation Model was adapted with the support of some measurements to estimate the thermal radiation hazards of battery. The adapted simple model can potentially be used to estimate thermal radiation hazards in practical applications where measurements are lacking.
Self-ignition of the large-size LFP cell and the subsequent flame radiation hazard assessment. [Display omitted]
•Self-ignition mechanisms of the large-size LFP cell were investigated.•A radiation model for the jet flame of the large-size LFP cell was developed.•A method is proposed to assess the radiation hazards of the LIB jet flame.