There is a lot of technical information available about chemical reactions and 'outgassing' and about how batteries work internally.
From over a decade of experience, having repaired over 250,000 devices, we know there a number of common causes of battery-swell.
There are a few poss reasons why a battery might expand, bulge or swell up like a balloon including:
- liquid exposure to battery terminals, mainboard or charging port
- short circuit due to physical damage of device
- being bent or crushed
- puncture - caution risk of fire