So a battery to one of my phones got left in the car (just the battery) for an extended period of time during a Texas summer. The sides of the battery expanded by a couple millimeters. But I can compress the sides with my fingers as if there was a pocket of air in between the outer tape (with markings/info on it) and whatever the next layer under is. I poked a small hole in the side and it sort of "deflated" and a strange smell asserted itself. My curiosity want's to know what actually expanded under the heat?