Is My Battery Safe After Extended Heat Exposure?

[abstracted6]

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?

 

[Drakkith]

I believe the cathode in the cells has overheated and produced oxygen, which has expanded and tripped a safety feature in the cells. (They are designed to do that instead of just exploding)
See here under the Safety Requirements section. http://en.wikipedia.org/wiki/Lithium-ion_battery#Disadvantages