iPhone 4 Teardown
The battery is glued into place and needs to be pried away, which will often deform the battery. So long as you do not puncture the casing, it should be ok to flatten it back into shape and reinsert it into the system. If you puncture the casing, you should get a new battery or risk excessive heat and possibly a risk of explosion.
How did you manage to get the battery out so cleanly?