Swollen iPhone battery. How why?

Question

iPhone

I took out the battery from the iPhone 5, it looks like a sealed swollen bag. Why is it swollen, what will happen if it is pierced, can it be used or is it very dangerous. If you pierce lithium itself, will it catch fire?

Answer 1

Why is he bloated

If overdischarged or overused, gas is released. Lithium-polymer batteries (like yours) are inflated, in cylindrical cases, for example, 18650, a special valve is installed that opens the positive contact when the pressure of the gases inside increases.
He will "deflate", the gases will come out. If you do not immediately seal the puncture site, the lithium inside will begin to oxidize and the battery will die very quickly.
you can use it, some pierce (even valves for 18650) and release gas.
If you don't mind using it, you can leave it. Of course, it is better to replace the battery, but it does not represent such a super-danger.
If you pierce the lithium layers themselves and thus create a short circuit, under certain conditions and the proper level of charge, ignition can be achieved. A careful puncture of the shell will not cause fire (unless a match is brought up, of course).