Different types and thicknesses of commercially available separators have been studied for their impact on rechargeable lithium cell safety. We conducted the study by testing LiCoO2/graphite pouch cells fabricated with five different battery separator membranes-16 mu m Al2O3-coated polyethylene (PE), 16 mu m polypropylene (PP), 16 mu m polyethylene (PE), 12 mu m polyethylene (PE) and 7 mu m polyethylene (PE). Cell safety was investigated by means of thermal ramp, overcharge, internal short circuit, and external short circuit tests. In addition, the physical properties of the separators were also characterized. The correlation between the material properties and the failure mechanisms of the battery cells were studied. It was found that separator thickness or cell energy density is not necessarily the key factor for every cell failure. Instead, other separator properties such as melt temperature, puncture resistance, dimensional stability, and shutdown feature play a more critical role in some cell failures. (C) 2019 The Electrochemical Society.