I think this must be a 5th gen ipod class and the gap is larger. I have a 6th gen ipod classic, the plastic open tool can't insert in!
The battery(thick) I bought from ifixit has problem: the cable is not same length as the original one. Be carefully.
Insert a plastic opening tool into the seam between the front and back of the iPod.
This is not possible for ipod 6th gen, I think ifitit may did for 5th gen?