If stuff is build for single cell li-ion at 3.7V, combining 2 cells at 2.4V is going to be a bit much. I also believe cells in series are slightly more brittle

If stuff is build for single cell li-ion at 3.7V, combining 2 cells at 2.4V is going to be a bit much. I also believe cells in series are slightly more brittle, because of the potential for uneven discharge.