Abstract
Separator integrity is an important factor in preventing internal short circuit in lithium-ion batteries. Local penetration tests (nail or conical punch) often produce presumably sporadic results, where in exactly similar cell and test set-ups one cell goes to thermal runaway while the other shows minimal reactions. We conducted an experimental study of the separators under mechanical loading, and discovered two distinct deformation and failure mechanisms, which could explain the difference in short circuit characteristics of otherwise similar tests. Additionally, by investigation of failure modes, we provided a hypothesis about the process of formation of local “soft short circuits” in cells with undetectable failure. Finally, we proposed a criterion for predicting onset of soft short from experimental data.
Highlights
Porous membrane separators, such as polyethylene (PE), polypropylene (PP) and polypropylene-polyethylenepolypropylene (PP/PE/PP) made of polyolefin, are widely used in the lithium-ion batteries for EVs11–13
For dry processed PE and trilayer separators, the strength in diagonal direction (DD) and transverse direction (TD) is in the same order, which is much lower than that in machine direction (MD)
The failure surfaces in both MD and TD loading were perpendicular to the loading direction, while the fracture surface had a 45° angle in DD loading which made it still perpendicular to TD
Summary
Porous membrane separators, such as polyethylene (PE), polypropylene (PP) and polypropylene-polyethylenepolypropylene (PP/PE/PP) made of polyolefin, are widely used in the lithium-ion batteries for EVs11–13 They can be manufactured by cold and hot stretch of precursor films with different stretch rates and ratios, followed by annealing until achieving a required porosity[12,14,15,16,17,18]. The so called wet and dry processes of polyolefin bring significant differences. We observed a significant difference in the size of short circuit area under these two failure modes. One implication of this finding is in explanation of the extent of electrochemical and thermal reactions when each of these modes is activated. The findings explain the so called “soft short” versus “hard short” in the cells subjected to mechanical abuse conditions
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