Lithium-ion Battery Safety - Assessment by Abuse Testing, Fluoride Gas Emissions and Fire Propagation

Doctoral thesis, 2017

Lithium-ion batteries offer high energy and power densities as well as long life time but have a more narrow stability window compared to other battery types and contain reactive and flammable materials. In case of overheating the battery cell can release gas (vent) and, at temperatures of about 150-200 °C, a so called thermal runaway can occur, that is a rapid self-heated temperature increase which is typically accompanied with a large release of smoke and gas, fire and a potential cell case explosion or gas explosion. In this work Li-ion battery safety has been studied by abuse tests; external fire, external heating (oven), external short circuit and overcharge. In total fifteen types of commercial Li-ion battery cells with various chemistries and cell capacity sizes ranging from 1.4 Ah to 45 Ah were studied. Cell abuse may have a number of consequences. Emissions of toxic and flammable gases have been studied under external heating and fire tests. During such tests cells can vent long time before/without thermal runaway. In external heating abuse tests, three different vents were identified, two of them occurred before the thermal runaway. In case of a delayed ignition of the vented gases mixed with air in a confined/semiconfined space, a gas explosion can occur. The consequences of a gas explosion might be significantly more severe than those of a cell case explosion (explosion due to extreme cell pressure). In external heating tests, 5 of 11 cells underwent a gas explosion, for all levels of cycle ageing studied (0-300 deep C/2 cycles). Released fluoride gas emissions have been studied by FTIR spectroscopy and gas-washing bottles, a parallel independent method, that verified the total emission amounts. In fire tests, toxic hydrogen fluoride (HF) gas was released with 20-200 mg/Wh of nominal electrical energy capacity, according to data from seven types of commercial Li-ion cells at a state of charge (SOC) in the range of 0-100 %. The HF production rate in limited fire tests was temporarily increased when water mist was applied as a fire fighting medium, however, the total HF emission values were similar with or without water mist. Emission of POF3 was also detected in the fire test, but only for one cell type at 0 % SOC. In external heating tests on the same cell type at 100 % SOC, emissions of HF and POF3 were also detected. The measured amounts indicate that HF might pose a severe and acute toxic threat. Heat spreading from a single Li-ion cell failure to adjacent cells, the cell-to-cell propagation, has been studied both experimentally and by numerical simulations. The use of fire walls between modules can significantly influence the propagation. A general risk assessment for Li-ion is presented and it is concluded that there is a lack of data on failure mechanisms, probability and consequences today. The shut-down principle for future metal-air batteries, has also been studied as an additional safety aspect, utilizing primary (non-rechargeable) commercial Zn-Air cells. Besides offering an additional shut-down technique, it also offers possibilities to reduce electrical hazard voltages, by temporarily bringing the voltage down to zero volt.