Sticky thick clear fluid under battery on my S4

So, I've noticed a few months ago while taking out my battery that it was sticky on the back side (back side looking at it when fitted in the phone) and the inner backside of the phone was sticky too. I cleaned it up with bunch of paper towels and popped the battery back in.

Since then, I've checked it and cleaned it for a couple of times, and it was sticky on the same place every time. The thick clear sticky fluid is about the fingerprint size. Battery life seems alright (quite worse than when it was new, but the phone is 20 months+ old so it didn't surprise me).

Has the battery gone bad? Is it unsafe and in any way hazardous to keep using it? It doesn't leak out anywhere, it's just that spot between the phone and the fitted battery.

Thanks for the help!