2x3,7 is 7,4V
But DC motors can handle that over-voltaging... They'll get a little hotter but if you don't stress them hard for too long no problem.
Li ions require an intelligent charger, which costs much more then a normal one.
If you would decide not to use the Li ions, don't use NiCd but use NiMh, they're pretty much the same but NiMh have no memory effect.
wikipedia it for more info.