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 LiCoO 2 /graphite pouch cells fabricated with five different battery separator membranes-16μm Al 2 O 3 -coated polyethylene (PE), 16μm polypropylene (PP), 16μm polyethylene (PE), 12μm polyethylene (PE) and 7μm 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.

An Li4Ti5O12 || LiNi1/3Mn1/3Co1/3O2 lithium-ion pouch cell has been subjected to an overcharge early in its cycle-life and kept cycling it up to 1500 cycles afterwards. We report on the non-invasive experimental verifications we conducted to corroborate our initial findings obtained via incremental capacity analysis. First, we used incremental capacity analysis on the negative electrode during overcharge to bring evidence of electrolyte reduction below 1 V vs. Li/Li+. Second, we used X-ray computerized tomography (CT scan) to demonstrate that gas bubbles were trapped between layers of positive and negative electrodes, thereby disabling the ionic conduction pathway between the two and causing capacity fade. Third, we administered a massage to the pouch cell to establish that the gas bubbles could be displaced, thereby recovering about 60% of the faded capacity. Finally, we proposed a new approach based on the study of the open-circuit voltage to describe quantitatively the gains of active materials that led to the partial capacity recovery.