Abstract
Understanding the potential thermal hazards of lithium-ion batteries (LIBs) during thermal runaway (TR) is helpful to assess the safety of LIB during storage, transport and use. This paper presents a comprehensive analysis of the thermal runaway (TR) characteristics of type 21700 cylindrical LIBs with a specific energy of 266 W∙h/kg. The batteries with both 30% state of charge (SOC) and 100% SOC were triggered to TR by uniform heating using a flexible heater in a laboratory environment. Three high definition cameras and one high-speed camera were placed to capture TR behavior and flame evolution from different viewpoints. Correlation between the heat release rate (HRR) and the mean flame height of turbulent jet diffusion flame were used to estimate the HRRs of LIBs. Additional characteristics of cell failure (for cells with 100% and 30% SOC) were also noted for comparison, including: number of objects ejected from the cell; sparks and subsequent jet fires. An approach has been developed to estimate the HRRs from TR triggered fires and results compared with previous HRR measurements for type 18650 cylindrical cells with a similar cathode composition.
Highlights
The characterisation of lithium-ion battery (LIB) fires is becoming of increasing importance, not least to the rise in number of electric vehicles (EVs) being introduced over recent years
Two jet fires were reported during single-cell LIB tests [4,6,13], with multiple jet fire events being observed in both single-cell LIB and LIB module tests [9,12, 18]
The duration of combustion was found to be greater than 10 s for 100% state of charge (SOC) cells, with the shortest time difference between the onset of temperature rise and ignition being 3.3 s, which is very short for any intervention to reduce cell temperature to prevent thermal runaway (TR)
Summary
The characterisation of lithium-ion battery (LIB) fires is becoming of increasing importance, not least to the rise in number of electric vehicles (EVs) being introduced over recent years. The vast majority of LIB fires can be traced back to LIB thermal runaway (TR) triggered by mechan ical, electrical and/or thermal abuse [1,2,3], and incorporate complex chemical and physical processes, from decomposition of the electrode materials and the burning of some of them, to the ejection of sparks and flammable gases as well as their combustion. Maximum cell surface temperatures for fully charged, cylin drical, type 18650 LIBs have been reported in the range 311 to 876 C. Measured flame temperatures as high as 1069 C for single cells [8] and 1500 C for a battery module [9] have been reported. There is evidence that observed mass loss rates decrease dramatically during evolution of the flame [4,6,8,9,12]
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