I had the same issue while trying to replace an iPhone 6s battery myself. I unhooked the battery and removed the screen and began to remove the adhesive but it broke. I used a tweezer to try to remove the battery and I guess I punctured it. The battery hissed and a stream of smoke and fumes released from the battery. I quickly took the phone outside when the phone cooled off 20 something minutes later I was able to remove the swollen battery from the back casing. Everything still looked intact so I decided to put the phone back together to see if it would still function. Luckily everything still works.