DELL battery beetle
UPDATE: @sgstair pointed me to the digi where this part is listed: search.digikey.com/us/en/products/EYP-2MT102B/P10936-ND/2...
It's no conspiracy behind, this is thermal cut-off to protect user from battery explosion.
I don't know what is the purpose for this small pocket with some chemical compound built-in into each element of Li-ion battery for DELL notebook. But I have a guess: it looks like this is time bomb. I suppose in presence of current compound slowly dissolves conductor and at predefined time it just disappears and battery stop working.
It seems that this is what happened to the battery I disassembled because all cells when directly connected to the charger demonstrates a lot of reserve.