I can't speak to the 50W, but the 20W istick uses an 18650 form factor LiPo. I was talking with a redditor about it. He tore his down and looked up the part number of the battery. That would explain the slight difference in length of the cell also.
One of the reasons people are scared of LiPos is the fact that R/C guys use them to the ragged edge of the envelope with regards to discharge and charge rates. LiPos dream about the way we treat batteries in terms of dis- and re-charge. I'll leave any further discussion of this to people that know better, I know I have no qualms using them after discussing this with R/C nerds.
I R Dat Nerd...
and i do use mine to the ragged edge...
and most people don't consider the fact that young children do so as well... and often have more battery knowledge than most of the user base here...
and...
that's one reason that people abuse the snot out of them to make them vent on youtube... by charging a 4 volt cell to 12 volts with straight car battery power... so the potential of outright abuse is understood... and sometimes for fun and sensationalism.
if it was a nimh rechargeable, instead of a small burst with a hot flame, it would be a mild explosion with heated, corrosive acid projectiles... and there's a lot less potential energy capabilities in a nimh than any comparably sized lithium cell of any chemistry...
consider the fact that instead of a lipo with a sealed pouch, the batteries we use have a hard, metal shell, and it's obvious the potential for explosion is higher with our cells. it's the heat that's the kicker in li-pos..