External short circuit performance of Graphite-LiNi1/3Co1/3Mn1/3O2 and Graphite-LiNi0.8Co0.15Al0.05O2 cells at different external resistances

This study aims at analyzing the response of Li-ion cells and at identifying the hazards and governing phenomena from hard to soft external short circuit conditions. 10 Ah pouch cells and 4.5 mAh coin cells were short circuited while synchronized current, potential and temperature signals, audio, IR and visual video recordings were registered. The anode, cathode and separator harvested from the cells were characterized by Scanning Electron Microscopy, micro X-ray Computed Tomography and 3D-profilometry. The complex short circuit behavior obtained can be described by 3 regions: In the first region 274C-rate is observed which is mainly governed by the cell's double and diffusion layer discharge. In the second region, the current drops significantly to 50–60C-rate where mass transport becomes the current limiting factor. The maximum temperature (77–121 °C) is reached and cell rupture, venting and electrolyte leakage may occur. In the final, third region the current decline continues due to the decaying electromotive force. The normalized external/internal resistance ratio is found to be the main influential factor on current and hazards rather than the external resistance or the capacity of the cell. The implications on the relevance and fitness-for-purpose of external short circuit test in standards are outlined.