What is this part called? And is this the issue for my phone dieing?

For the past 3 days now, my phone jumps from having charge, to no charge at all so it powers off and says 0%… It is random what percentage until it powers off it has been 60% , 56%, 28%, etc…

About 4 weeks ago I did replace the battery to resolve another issue which partially worked… So it shouldn’t be the battery unless I was given a faulty one.

However, I’ve opened up my phone and there is a yellow strip in it which is un-sticking , and so is the black cover over the battery .

• I was wondering what are these names of the phone called? ( The black cover and the yellow strip)
• Is this probably the issue for the phone to die all of a sudden?
• And is this something I can buy a replacement part and repair this myself?

Any suggestions will be greatly appreciated.

Many thanks!

Ichigo111