shows the strain of the RP and the Celgard membranes kept
at 90 ◦C for over 60 min. Following the increase of the temperature
to 90 ◦C, the strain of the RP membrane in both directions exhibits a low rate of shrinkage of −0.26% (Fig. 3a) and −0.25% (Fig. 3b),
respectively. The Celgard membrane shows an opposite behavior
in two perpendicular directions in that it shrinks by −1.55% in one
direction (Fig. 3c) and expands first by 0.59% and then shrinks in
the other direction (Fig. 3d). After the temperature reaches 90 ◦C
and keeps unchanged, the dimension change of the RP membrane
in both directions is almost ignorable, being −0.003% and −0.03%,
respectively (Fig. 3a and b). To the contrary, the strain of the Celgard
membrane in both directions has a sustained shrinkage of
−0.5% after 270 min (Fig. 3c) and −0.6% after 300 min (Fig. 3d). Such
shrinkagemay lead to the direct contact ofthe positive and negative
electrodes and thus poses a safety issue of a battery