Assessment of safety characteristics for Li-ion battery cells by abuse testing

Licentiate thesis, 2014

Despite the many advantages with lithium-ion batteries there can be disadvantages in form of safety issues. In an abuse situation a Li-ion cell can undergo a thermal runaway releasing excessive heat, flammable and toxic gas emissions and eventually accompanied by dissembling/explosion and fire. Abuse tests are a method for assessment of the safety characteristics of Li-ion batteries. Results on cells and electrolytes from abuse testing by overcharge, short circuiting, external heating and fire test are presented and discussed. The thermal runaway was studied by external heating of various commercial Li-ion cells with cylindrical and pouch packaging. Cells with lithium cobalt based oxides showed a thermal runaway at around 200 °C with a maximum rate of temperature increase of about 5000 °C/min. Cells with lithium iron phosphate (LFP) showed significantly lower reactivity but a thermal runaway did still occur for some cells. Short circuit and overcharge tests of LFP pouch cells showed in most cases temperature increases below about 100 °C. Fires did not occur in these tests except one unexpected fire during an overcharge test. The fire tests show that the reactivity of Li-ion cells in a fire are dependent on the state of charge (SOC), however, the total heat release shows a low SOC dependence. Toxic emissions of hydrogen fluoride (HF), phosphorous oxyfluoride (POF3) and phosphorus pentafluoride (PF5) were studied by FTIR in fire experiments conducted on electrolytes and cells. The results are extrapolated to obtain an estimate of the possible emissions from a fire in an electric vehicle.