When a battery is exposed to direct sunlight or intense heat, gas buildups can occur and cause the battery to swell or, even worse, blow up. Apple's new design would include "special battery pouches" with the intention of avoiding fire hazards:
Apple's patent application outlines a battery cell which includes a weakness for relieving pressure. This battery cell may include a jelly roll comprising layers which are wound together, including a cathode with an active coating, a separator, and an anode with an active coating. The jelly roll may also include a first conductive tab coupled to the cathode and a second conductive tab coupled to the anode.
It's likely that the jelly roll would be inside a flexible pouch and the two tabs would protrude from it so they could connect to the battery. The pouch would have pre-determined weak spots, and these areas would create holes to relieve pressure should the battery need to do so. I dunno how I feel about my battery making holes in itself, but it does beat having my computer explode in my lap. [Patently Apple via 9 to 5 Mac]