Investigation on temperatures and gases emitted during thermal abuse tests of commercial Li-ion 18650 cells

Increased use of lithium-ion batteries (LIBs) has shown the safety limits of these devices, especially when subjected to electrical, mechanical, or thermal abuse. Thermal abuse is caused by exposure of the cell to temperatures above or below the operating temperatures indicated by the safety window (i.e., specific temperature and voltage range). This abuse leads to the degradation of the internal components of the cell with consequent release of gases and vapors by venting, and the generation of heat which, when not exchanged efficiently, triggers thermal runaway (TR) with consequent fire or explosion. To characterize the different phases, in this work thermal abuse tests are conducted on Lithium Cobalt Oxide (NCR) 18650 cells. Abuse tests were performed in a stainless-steel tubular reactor, in inert gas (N2), heated up to 240°C at constant heating rate (2, 5 and 10°C/min). The analysis of gas emitted was conducted through a Fourier transform infrared spectroscope (FT-IR Spectrum 3, Perkin Elmer). The comparison between the results obtained at the three heating rates shown difference in terms of temperature and time for each event and the quantities of gases emitted. In a Li-ion cell heated by the slower heating rate the venting and the onset occurred a lower temperature (133.4 and 177.4°C) compared to the higher heating rate (197.1 and 221.9°C). The durations of catastrophic events are also not comparable, with longer durations for higher heating rate (such as TR of 125s vs 102s for 2°C/min). The intermediate test, 5°C/min, shows results closest to fast heating. The gases produced are similar in terms of composition, essentially composed by hydrofluoric acid (HF), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and the electrolytic solvents dimethyl carbonate (DMC), ethylene carbonate (EC) and diethyl carbonate (DEC), trend over time, greater release during venting and TR, and estimated concentration, HF maximum value above 400ppm while for the other gas percentages below 4%.