Is there fire suppression in some phone batteries, and if so, how does it work?

I recently saw a rugged smartphone, three or four years old, with a ~5000 mAh battery, suffer a major impact to its back. This wasn’t a fall, it was a fast impact with a heavy object. Immediately there was this loud hissing sound, and clouds of what looked like white and very-light-brown smoke came out of every aperture in the sides of the phone. The "smoke" was not acrid, did not disturb my (quite sensitive) lungs, and smelled only like slightly peculiar burning, not even strongly plasticine. There were absolutely no flames visible at all, the phone was warm and not hot to the touch within 30 minutes, and although the back of the phone had meltage visible, much of it was intact.

To me the above looked a lot like some sort of fire suppression system I had never heard of, which perhaps released a whole lot of CO2 or perhaps something else, to prevent anything more dangerous from happening. Anyone know if this is true, and if so, how it works?