That may be hard unless you look at the NiMH chemistry. But even then your looking at 5 off AA size to do it. Maybe someone else knows of something?
There is always 2 cell LiPO at 7.4v regulated down to 6v. But personaly (I use them alot in my R/C stuff) I wouldn't use them to
vape with. And of course they are square in shape.