Yes J85, this is an extract from a Japanese patent application that highlights the industrial dilemma of separator structural integrity.
"In this case, the general objective of an effective battery separator is to have low air resistance and at the same time a very small pore size in order to provide a material that drastically reduces any possibility of electrodes touching, but has the ability to control the movement of electrolyte from one location in the battery cell to the other (i.e., to close the circuit to generate the necessary power). Currently, these properties have not been effectively provided simultaneously. For example, Celgard has introduced and sells expanded film battery separators with very small pore sizes, which are very good in the above-mentioned respects, but the corresponding air resistance for such materials is very high, thus limiting the overall effectiveness of such separators. In contrast, duPont has marketed a nanofiber nonwoven membrane separator that provides very low air resistance, but its pore size is very large. Furthermore, the overall mechanical strength exhibited by these two materials is very limited. Celgard separators have good strength in the longitudinal direction, but almost zero strength in the transverse (cross) direction. Such low transverse strength, at least in the respects mentioned above, requires very delicate handling in manufacturing. The DuPont material is somewhat better, except that its strength is considerably lower in both directions, and has a higher transverse strength than the Celgard material. In fact, the DuPont product is closer to an isotropic material (a material with roughly the same strength in both the longitudinal and transverse directions), and therefore is a more reliable material to handle than the Celgard type. However, the measured tensile strength of the DuPont separator is actually quite low, and thus users are still forced to handle and install the material carefully in manufacturing. Similarly, the dimensional stability of these prior art battery separators is quite questionable due to these tensile strength problems, and over long periods of time when in a rechargeable battery cell, the material may undesirably lose structural integrity.."
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